Tectonic Evolution of Southeast Asia
Geological Society Special Publications Series Editor
A. J. FLEET
GEOLOGICAL SOCIETY SPECIAL PUBLICATION NO. 106
Tectonic Evolution of Southeast Asia EDITED BY
ROBERT HALL SE Asia Research Group Department of Geology Royal Holloway London University Surrey, UK AND
DEREK BLUNDELL SE Asia Research Group Department of Geology Royal Holloway London University Surrey, UK
1996
Published by The Geological Society London
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Contents HALL, R. & BLUNDELL,D. J. Tectonic evolution of SE Asia: introduction
vii
Part 1: Present-day tectonics MCCAFFREY, R. Slip partitioning at convergent plate boundaries of SE Asia
3
MALOD, J. A. & KEMAL, B. M. The Sumatra margin: oblique subduction and lateral displacement of the accretionary prism
19
RANGIN, C., DAHRIN, D., QUEBRAL,R. & THE MODEC SCIENTIFICPARTY. Collision and strike-slip faulting in the northern Molucca Sea (Philippines and Indonesia): preliminary results of a morphotectonic study
29
RICHARDSON,A. N. & BLUNDELL,D. J. Continental collision in the Banda arc
47
SNYDER,D. B., MILSOM,J. & PRASETYO,H. Geophysical evidence for local indentor tectonics in the Banda arc east of Timor
61
HUGHES, B. D., BAXTER, K., CLARK,R. A. & SNYDER,D. B. Detailed processing of seismic reflection data from the frontal part of the Timor trough accretionary wedge, eastern Indonesia
75
MILSOM, J., KAYE,S. & SARDJONO.Extension, collision and curvature in the eastern Banda arc
85
Part 2: Tectonic development of Southeast Asia METCALFE, I. Pre-Cretaceous evolution of SE Asian terranes
97
PACrd-IAM, G. Cenozoic SE Asia: reconstructing its aggregation and reorganization
123
HALL, R. Reconstructing Cenozoic SE Asia
153
SIMANDJUNTAK,T. O. & BARBER,A. J. Contrasting tectonic styles in the Neogene orogenic belts of Indonesia
185
RICHTER, B. & FULLER, M. Palaeomagnetism of the Sibumasu and Indochina blocks: implications for the extrusion tectonic model
203
STOKES, R. B., LOVATTSMITH,P. E & SOUMPHONPHAKDY,K. Timing of the Shan-Thai-Indochina collision: new evidence from the Pak Lay Foldbelt of the Lao PDR
225
LOVATr SMITH,P. F., STOKES,R. B., BRISTOW,C. & CARTER,A. Mid-Cretaceous inversion in the Northern Khorat Plateau of Lao PDR and Thailand
233
HtrrCHISON, C. S. The 'Rajang accretionary prism' and 'Lupar Line' problem of Borneo
247
OMANG, S. A. K. • BARBER,m. J. Origin and tectonic significance of the metamorphic rocks associated with the Darvel Bay Ophiolite, Sabah, Malaysia
263
NGAH, K., MADON,M. & TJIA, H. D. Role of pre-Tertiary fracture in formation and development of the Malay and Penyu basins
281
TJIA, H. D. & LIEW, K. K. Changes in tectonic stress field in northern Sunda Shelf basins
291
CLENYELL, B. Far-field and gravity tectonics in Miocene basins of Sabah, Malaysia
307
McCOURT, W. J., CROW, M. J., COBBING,E. J. & AMIN, T. C. Mesozoic and Cenozoic plutonic evolution of SE Asia: evidence from Sumatra, Indonesia
321
SAMUEL, M. A. & HARBURV,N. A. The Mentawai fault zone and deformation of the Sumatran Forearc in the Nias area
337
WAKITA,K., SOPAHELUWAKAN,J., MIYAZAKI,K., ZULKARNAIN,I., & MUNASRI.Tectonic evolution of the Bantimala Complex, South Sulawesi, Indonesia
353
vi
CONTENTS
WILSON, M. E. J. & BOSENCE, D. W. J. The Tertiary evolution of South Sulawesi: a record in redeposited carbonates of the Tonasa Limestone Formation
365
BERGMAN, S. C., COFFIELD,D. Q., TALBOT, J. P. • GARRARD,R. A. Tertiary tectonic and magmatic evolution of western Sulawesi and the Makassar Strait, Indonesia: evidence for a Miocene continent-continent collision
391
ALI, J. R., MILSOM, J., FINCH, E. M. & MUBROTO,B. SE Sundaland accretion: palaeomagnetic evidence of large Plio-Pleistocene thin-skin rotations in Buton
431
VROON, P. Z., VAN BERGEN, M. J. & FORDE, E. J. Pb and Nd isotope constraints on the provenance of tectonically dispersed continental fragments in east Indonesia
445
LINTHOUT, K., HELMERS, H., WIJBRANS,J. R. & VAN WEES, J. D. A. M. 4°Ar/a9Ar constraints on obduction of the Seram ultramafic complex: consequences for the evolution of the southern Banda Sea
455
CHARLTON, T. R. Correlation of the Salawati and Tomori Basins, eastern Indonesia: a constraint on left-lateral displacements of the Sorong fault zone
465
MALAIHOLLO,J. E A. & HALL, R. The geology and tectonic evolution of the Bacan region, east Indonesia
483
BAKER, S. & MALAIHOLLO,J. Dating of Neogene igneous rocks in the Halmahera region: arc initiation and development
499
PUBELLIER, M., QUEBRAL,R., AURELIO, M. & RANGIN, C. Docking and post-docking escape tectonics in the southern Philippines
511
CROWHURST, P. V., HILL, K. C., FOSTER, D. A. & BENNETT, A. R Thermochronological and geochemical constraints on the tectonic evolution of northern Papua New Guinea
525
WOPFNER, H. Gondwana origin of the Baoshan and Tengchong terranes of west Yunnan
539
ZI-IOU, Z., LAO, Q., CHEN, H., DING, S. & LIAO, Z. Early Mesozoic orogeny in Fujian, southeast China
549
Index
557
Tectonic evolution of SE Asia: introduction R O B E R T H A L L 1 & D. J. B L U N D E L L
SE Asia Research Group, Department of Geology Royal Holloway, University of London, Egham, Surrey TW20 OEX, UK 1 Fax no. (44) 01784 471780 Email
[email protected]
SE Asia is probably the finest natural geological laboratory in the world yet is still not geologically well known. It is a spectacular region in which the manifestations and processes of plate collision can be observed at present and in which their history is recorded. It is a region that must be understood if we are to understand mountain belts, arc development, marginal basin evolution and, more generally, the behaviour of the lithosphere in collision settings. Furthermore, the region is developing rapidly on the economic front, and a major part of this rapid development is built on natural resources. The geological reasons for the distribution of these resources are therefore of major importance for the inhabitants of the region and for attempts to discover and exploit them. These were some of the thoughts which stimulated the collection of papers (Fig. 1) in this volume. In order to understand the development of this complex region an essential first step is to identify the key features of the active tectonics and determine how plates and sub-plate lithospheric fragments are moving. How successfully can rigid plate tectonics be applied in describing present tectonics? Where are the boundaries between plates? What are the rates at which different parts of the region are moving? The first part of this volume includes a number of papers which deal with these questions, based upon the application of GPS (Global Positioning System) measurements to determining the nature and rates of plate movements and plate boundary zone deformation, results from the BIRPS deep seismic reflection experiment in the Banda arc, the first to cross a modern active margin, and other geophysical and geological studies. MeCaffrey provides a regional overview of recent GPS results and earthquake data bearing on the present plate tectonics. It is ironic that in such an active region the identification of several plates and determination of some important relative plate motions, critical to a full kinematic description, are still very uncertain. McCaffrey discusses the way in which motion is partitioned in obliquely convergent settings, almost the rule in SE Asia. Oblique convergence is commonly inferred in the past for SE Asia (as shown in many of the later papers in the
volume) and is often used as an explanatory tool in orogenic belts elsewhere in the world. He draws attention to the deformation of the upper plate in these convergent settings and emphasizes the importance of a three-dimensional understanding of the process. In this rapidly evolving region, information from slip vectors and geodetic measurements will allow a fourth dimension of time to be included as data accumulate, which will raise the important problem of whether and for how long the present motions can be assumed to extend back into the past. Two well-known areas of oblique convergence, the Sumatra and Philippine Sea plate margins, illustrate the realities of present tectonics, first in identifying small plates or smaller tectonic elements, and second in providing a kinematic description. The increase in obliquity of convergence between Java and north Sumatra has long been considered to result in thrusting normal to the subduction trench and arc-parallel movement on the Sumatran fault. However, as McCaffrey points out, this simple model does not predict some important features, such as subduction west of the Andaman Sea and differences in amounts of extension between north and south Sumatra. Malod & Kemal discuss evidence from marine surveys off Sumatra and propose that this area can be understood as a number of plate slivers between the trench and the Sumatran fault, with variations in the partitioning of movement in different parts of the Sunda forearc. This is explained as the result of differences in coupling between the subducting and overriding plates, possibly reflecting the presence of major structures on the subducting slab, notably an extinct spreading centre on the Indian plate. Further east, the Philippine fault and trench are also considered as the joint expression of partitioning of oblique convergence, but once again applying this simple model presents problems: how does this paired trench-fault system link southwards into the arc-arc collision of the Molucca Sea? Rangin et al. tackle the first important problem of what is actually present in the zone of transition by mapping structures from the south Philippines into Indonesia and determining which features belong
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolution of SoutheastAsia, Geological Society Special Publication No. 106, pp. vii-xiii.
vii
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to which plate, a demanding task. Their use of the technique of multibeam mapping, combined with geophysical evidence, is producing highly detailed maps, in many cases of much higher quality than those available for land areas nearby, and revealing unsuspected structures. The arguments surrounding the tectonics of the Banda arc, and some of the complexities of the collision zone, for example its curvature, age of the Banda Sea, and significance of deep basins such as the Weber trough, also illustrate the difficulties of linking simple kinematic models to reality. Very recently two BIRPS deep seismic reflection profiles have crossed the Banda arc and imaged the deep structure of the central part of the AustraliaSE Asia collision zone. Richardson & Blundell discuss how deep reflectors can be linked to recent seismicity, and their connection to structures on land. They identify two sets of divergent structures in the collision zone: a southern set dipping in the same direction as subduction is related to the subducted Australian margin, whereas a northern
set, in the upper plate, is antithetic, and more recent. They argue that these structures changed orientation and migrated north because buoyant continental crust blocked the subduction zone, although Australia continued to move north. Using the BIRPS images to estimate the volume of material in the collision zone they infer that an earlier Neogene collision event, involving a separate microcontinental fragment or outer margin high, must have preceded the present phase of collision which began at about 2.4 Ma. Snyder e t al. draw attention to unusual features of the BIRPS 'Timor' sector of the Banda orogenic zone, including a Bouguer anomaly and thin sediment cover implying a thicker continental crust than normal beneath the accretionary complex, a very narrow forearc, and presence of the volcano Gunung Api in the backarc. Like Richardson & Blundell, they infer the presence of additional Australian margin crust but suggest it formed either a promontory of the Australian margin or a prebreakup extensional basin within the former pas-
INTRODUCTION sive margin, now inverted and thickened. Also like Richardson and Blundell they deduce that the deformation has advanced across the collision zone, causing crustal fracturing in the backarc region and allowing magmatic uprise and underplating beneath Gunung Api. Hughes et al. have reprocessed part of the Timor section to bring out the details of the Timor trough and the accretionary wedge developed on its northern flank. Australian shelf sediments are clearly imaged, relatively undisturbed, and beneath a low-angle decollement above which are highly deformed sediments stacked in a complex set of thrust sheets indicative of the northward underthrusting of the Australian continent. However, a covering drape of sediment indicates that deformation in this area has been inactive for about 1 Ma. One feature that has bedevilled discussions of the Banda arc is the significance of the deep troughs which parallel the arc, which in a superficial way have characteristics of trenches, but when studied more carefully seem to be in the wrong position. Milsom et al. present evidence that the deformation front between Australia and the Banda arc extends between the Kai islands, in the region of maximum curvature of the arc, rather than following the apparent subduction-related feature of the Aru trough. They conclude that the arc must have been a continuous feature before collision with a curvature acquired recently. The present setting of the region is both the start and the end point in attempts to comprehend its development. The start, because an understanding of what is present provides the first clues of what has happened in the past few millions or tens of millions of years, leaving traces in the slabs beneath the arcs, the arcs themselves, and in the stratigraphic record of the arc regions as well as the Sunda continental shelf further north. The end, because the present is the result of what has happened in the past, and many of the present features of the region may be explicable only in terms of what has now disappeared and can only be inferred. Thus, our understanding of the region and its development is an iterative synthesis, each new step forward requiring a reconsideration of present and past. The second section of the volume therefore begins with three overview papers intended to synthesize earlier observations which may provoke new research improving and extending our present regional understanding. Metealfe reviews the pre-Cretaceous development of SE Asia and east Asia and shows that the region has grown by addition of allochthonous terranes which separated from different parts of Gondwana. Their northwards movement was accompanied by the opening and closing of three successive oceans: the Palaeo-, Meso- and
ix
Ceno-Tethys. Assembly began with the formation of Cathaysia in the Late Devonian-Early Carboniferous and its growth continued within the Palaeo-Tethys in the later Palaeozoic. As the Meso-Tethys opened by Late Carboniferous-Early Permian rifting of the northern margin of Gondwanaland so the Palaeo-Tethys began to close, and so subsequently did the opening of the Ceno-Tethys result in elimination of the MesoTethys. The final stage of the reassembly of Gondwana in Asia is not yet complete but the present complexity of the region, as well as the comprehensive grasp of so many disciplines in earth science required to attempt the task of describing its development, give an indication of Metcalfe's achievement in producing a comprehensible synthesis. A description is an essential step in identifying the driving processes and modelling the development of the region. Reconstructing SE Asia in the Cenozoic requires a description of India-Eurasia collision, the motion of the Philippine Sea plate and the present collision of Australia with eastern Indonesia. Eurasian extrusion models have proved popular with many workers in SE Asia, partly because of the striking similarities of plasticine models to tectonic maps, and partly because they offer a means to explain and link different events. As with many major advances, the extrusion model has also been effective in provoking other new ideas and the search for new evidence. Paekham considers the relationships between the observed geology and different models of the region, reviewing the timing of different events, the evidence from crustal volumes for extrusion, and the relationship between predicted and observed palaeomagnetic measurements. In east Asia and western SE Asia the estimates of rates and amounts of movements predicted by the extrusion model, and those that can be determined, are becoming much closer. The inadequacies of the fit cannot just be explained as weaknesses of the model but also emphasize the variable quality of data and their uncertainties. Packham concludes that a regional understanding of SE Asia requires a better understanding of the timing of deformation, especially uplift, in the Himalayas and central Asia, a clearer picture of whether rotations detected using palaeomagnetic data are regional or local, and much new stratigraphic information, particularly in the eastern part of the region. The key role of palaeomagnetic data in developing regional models is a theme discussed by Hall in an attempt to produce a kinematic model for the whole of Cenozoic SE Asia. At the centre of the region is the island of Borneo from which rotations recorded appear to be in conflict with those predicted by an indentation model. Further east, new
X
R. HALL • D. J. BLUNDELL
data from the Philippine Sea plate confirm longterm clockwise rotations of the whole plate suggested by many earlier studies. The attempt to synthesise these results suggests that, even if Cenozoic extrusion of continental fragments from east Asia is accepted, this has not been the most important driving force in the development of the marginal basins of eastern SE Asia. According to this model, development of marginal basins was linked physically and temporally, and opening appears to be mainly subduction-related rather than indentor-driven. This new model does suggest some possible configurations for the region which are different from those previously accepted and may provoke reconsideration of some evidence, especially the habit of looking for local explanations of tectonic phenomena. The animation which accompanies the paper reminds us that regional events may have causes outside the immediate area, and the manifestation of plate movement changes may propagate gradually across the whole region, as plate boundary changes in one area cause changes in others. Several aspects of the model, such as the proposed ages of some basins, for example the Banda Sea, remain to be tested by ocean drilling, palaeomagnetic work, and stratigraphic and structural studies. The remaining papers in the volume are arranged broadly in geographical order. Simandjuntak & Barber illustrate the variation in orogenic styles from different parts of Indonesia. The present diversity and complexity of tectonic processes in SE Asia may provide keys to the interpretation of other orogenic belts. The differences in the history of deformation within the region may leave traces other then geological structures and Richter & Fuller discuss the still thorny question of the implications of palaeomagnetic data from Sundaland and Indochina. They conclude that different parts of Sundaland and Indochina have deformed in different ways; some parts as small blocks with rotations indicating local deformation, some extrusion-driven rotation, principally in Indochina, and some parts recording the results of deformation dominated by the oblique subduction of the Indian plate. Distinguishing between such areas for the whole SE Asia region is an important task for geologists and palaeomagnetists for the future. Even the most carefully constructed regional kinematic models are totally dependent on basic data revealing the timing of tectonic events and the evidence for them. The papers concerned with areas around the South China Sea suggest the need for revisions of models as well as new interpretations of relationships between effects and supposed causes. In Laos, Stokes et al. argue that suturing of the Shan-Thai and Indochina blocks occurred in
the Late Jurassic and that the Indosinian Orogeny, currently assumed to be of Permian or Triassic age is significantly younger than commonly assumed. Based largely on seismic data, Lovatt Smith et al. suggest regional tilting, compressive folding, reverse faulting and basin inversion in Thailand record important phases of structural development which pre-date the currently assumed Tertiary age of structuring. If correct, these interpretations have implications for hydrocarbon exploration and potential. Hutchison draws attention to the contrast between tectonic models of the South China Sea region and the geology of Borneo and proposes that the term Rajang Group should be more carefully applied. An older turbidite sequence, assigned to the Rajang Group proper, represents an accretionary prism compressed and uplifted between the Schwaner Mountains volcano-plutonic arc and a South China Sea microcontinent during an Eocene orogeny. Similar but younger turbiditic rocks deformed by a Miocene orogeny are interpreted not as deposits of a forearc, but as derived from the eroding and uplifted Rajang Group, and should be separated from it. A further record of the late Mesozoic or early Tertiary subduction setting of the NE Borneo margin is to be found in the large Darvel Bay Ophiolite Complex of Sabah. Mineralogical and geochemical studies by Omang & B a r b e r suggest its formation in a suprasubduction zone environment, but with complexities due to high T-low P deformation along a transform fault. High P-T garnet pyroxenites and amphibolites found as clasts in Miocene rocks were derived from a metamorphic sole underlying the complex, formed during subduction and emplacement of the ophiolite. Within the Sunda shelf are sedimentary basins of the South China Sea and adjacent areas which record a link between east Asian tectonics and the plates beyond the subduction zones bounding SE Asia. The importance of pre-existing structures in controlling tectonic development is often forgotten and Ngah et al. suggest that the Malay, Penyu and West Natuna basins originated in the Late Cretaceous as three rift arms that developed during doming of continental crust above a mantle plume. The hydrocarbon potential of these basins was subsequently influenced by changes in stress patterns which Tjia & Liew argue resulted from the interplay of Eurasian extrusion driven by Indian indentation, and changes in directions and rates of motion of the plates in the Pacific and Indian Oceans. Borneo is situated in the middle of this region at the south side of the South China Sea and ought to record the effects of these changes. The geology of Sabah is therefore of considerable regional interest since, if Borneo has rotated, it is an area where the consequences should be most
INTRODUCTION obvious. Clennell discusses the interplay between large-scale regional plate motions (his 'far-field tectonics') and pre-existing structures and local tectonic influences for the development of the unusual circular basins of Sabah. These papers on sedimentary basins show that we are still some way from clearly linking local and regional tectonics. Tjia and colleagues show that there were reversals in the sense of movement on important faults, that the effects of fault movements differ from area to area, and that there is still uncertainty in the timing of fault movements. Clennell infers that basins in Sabah appear not to record some tectonic events because they were decoupled due to the thicknesses of underlying muds and m61anges. Sumatra is an area where a long history of subduction should be recognizable since the island is usually considered to have been situated above the northward-subducting Indian plate from at least the Mesozoic. Despite this there appear to be distinct periods characterized by igneous activity, separated by intervals with little or none. McCourt et al. use isotopic dating and geochemistry to identify plutonic episodes and their character, which they link to plutonism elsewhere in Sunda margin. Understanding the tectonic significance of the igneous episodes needs to be the next step forward. McCourt et al. speculate that variations in the obliquity of convergence, and collision of allochthonous terranes are implicated, although Neogene and younger strike-slip faulting complicates the picture. It would be useful to consider the Cenozoic history of this margin in the light of known plate motions, although the major uncertainty here is not the motion of the Indian plate but the orientation and position of Sumatra; different regional models show very different configurations for the early Tertiary. The Cenozoic history of this margin is clearly complex as indicated by other papers in the volume and Samuel & H a r b u r y show that this complexity is still far from understood. In their paper, based on detailed studies on land in the Sumatran forearc islands, principally Nias, they interpret the Mentawai fault system not as a strike-slip fault, but as an extensional structure with late contractional reactivation. If correct, at least one of the plate slivers of the forearc proposed by Malod & Kemal either does not exist or is of very recent origin. Samuel & Harbury's work also illustrates the importance of field-based studies in providing a firm stratigrapbic basis for the interpretation of other evidence, such as seismic and marine geophysical evidence, and they infer a long extensional history for the Sumatran forearc, with major extensional structures later reactivated, again with possible links to changes in plate motions such as the angle of convergence.
xi
A good stratigraphic base is fundamental to attempts to interpret the tectonic evolution of the region and Wakita et aL provide an example of how this is achieved by detailed radiolarian studies. The Bantimala Complex and Balangbaru Formation of south Sulawesi record critical events in the accretionary growth of the SE Sunda margin and dating based on micropalaeontological evidence is difficult to obtain and interpret since the turbidite sequences yield few fossils, and these may be reworked. Radiolaria, which can often be assigned to narrow zones, provide an additional means of comparing the ages of different lithological units and suggest the Bantimala Complex and Balangbaru Formation are contemporaneous, requiring a modification of previous tectonic interpretations. From the same region Wilson & Bosenee show how redeposited limestones of the Tonasa Limestone Formation can be used as indicators of tectonic activity. Detailed measured sections, well dated by fossils, illustrate how a very clear palaeogeographic picture can be deduced and linked to larger-scale tectonics. Their results provide a basis for regional interpretation and will be of considerable interest to those exploring for hydrocarbon in the frontier regions of east Indonesia. Sulawesi is currently much less wellknown than it deserves to be, especially considering its large size and critical position at the Eurasia-Australia-Pacific junction, and recent results have shown that simple tectonic models for its development need reconsideration. Bergman et al. present data from west Sulawesi which will need to be incorporated in new models and speculate on possible solutions. Of considerable interest is the evidence, based on isotopic studies, for a magmatic contribution from old Australian-type continental crust to the Tertiary plutonic rocks of west Sulawesi. Bergman et al. also focus attention on the structures around the Makassar Strait. This has previously been widely accepted to be an extensional basin but they interpret it as a foreland basin bounded by converging Neogene thrust belts, with the late Miocene western Sulawesi magmatic arc recording continent-continent collision. The collapse of the orogenic belt is seen as the cause of young extension in the region. The rapid changes predicted by the model are certainly consistent with the variety and speed of tectonic processes currently observed in SE Asia. The Neogene collisions of continental fragments in Sulawesi are a principal cause of its geological complexity and Ali et al, provide some insights into how tectonic models can be tested using palaeomagnetic data. In Buton, large rotations are recorded, but are apparently very local, and were very rapid. This work reinforces the value of, and need for, many more palaeomagnetic studies in SE
xii
R. HALL • D. J. BLUNDELL
Asia in order to separate local from regional motions. Buton is one of several continental fragments which are now being reassembled in SE Asia. These include Australian and Sundaland material but their origin can often only be inferred from indirect arguments, commonly controversial. Vroon et al. suggest that isotopic evidence can contribute to solving this problem and show how different types of continental crust can be characterized by analysis of igneous and sedimentary rocks. They suggest that different parts of east Indonesia have provenances in southern New Guinea, north Australia, Pacific New Guinea and Sundaland, leaving the tectonicians with an additional tool but, in this area, some additional problems to solve. In the midst of the continental fragments of east Indonesia are the deep basins of the Banda Sea, as yet unsampled by the ocean drilling programme, and of uncertain age. Linthout et al. report new isotopic ages from Seram implying Neogene spreading in the southern Banda Sea before ophiolite obduction on Seram in the Late Miocene. These ages are broadly consistent with ages of rocks recovered during recent dredging in the Banda Sea and with the tectonic reconstructions of Hall, although the great depths and low heatflow measurements remain apparent inconsistencies. On the north side of the Banda Sea the Sorong fault system separates Australia from the Philippine Sea and Molucca Sea plates and terminates in the continental fragments of east Sulawesi. The timing of movement on the strike-slip faults has never been clear, although recent work suggests this plate boundary zone became a strike-slip system in the early Miocene. The timing of movements and the distribution of continental crust in this region is of major interest, not least in the search for hydrocarbons, since this is an area of established production, recent discoveries, as well as active exploration, all linked to Australian crust. Based on stratigraphic arguments, Charlton argues that two of the basins, the Salawati basin of western New Guinea and the Tomori basin of eastern Sulawesi, were originally a single sedimentary basin, now separated by latest Miocene to Quaternary movements on the fault, implying a left-lateral displacement of about 900 km. Movement on the fault system is one of the latest complications in the development of east Indonesia; on the south side of the fault system is Australian crust while on the north side are the arc-arc collision of the Molucca Sea and the clockwise-rotating Philippine Sea plate. In the fault zone, which includes several major splays, are fragments of both Philippine Sea and Australian origin and Bacan is one of the islands which includes rocks of both provenances. Bacan therefore offers the possibility of elucidating
some of the history of the plate boundary zone, and establishing the timing of tectonic events. Malaihollo & Hall report new stratigraphic data from Bacan which record the early arc history of the Philippine Sea plate and the arrival of continental crust, providing a basis for distinguishing different tectonic models. Baker & Malaihollo discuss the timing of volcanism in the islands of Halmahera immediately north of Bacan, which records the initiation of subduction of the Molucca Sea plate beneath Halmahera and the development of the present-day arc-arc collision. Volcanism began in the middle to late Miocene and migrated northwards implying that the double subduction system was established between about 15-12 Ma. This evidence still remains to be incorporated in models linking east Indonesia to the Philippines. Pubellier et al. provide further evidence from the southern Philippines critical to linking the two areas and understanding the development of the present tectonic setting described earlier in the volume by Rangin et al. Once again, the theme of partitioning of oblique convergence is emphasized. In addition, there are complications reflecting Neogene changes in plate motions and the complexities of intra-arc deformation. Strain has been partitioned between several orientations of faults, reactivated at different stages as thrusts and wrench faults, as well as subduction zones. Of particular interest is the way in which this development has resulted in intra-arc extension and fragmentation within the Philippines. The interplay of subduction and strike-slip faulting is a theme which appears in many of the papers in the volume. Ancient strike-slip motion is often difficult to demonstrate and quantify and is consequently often neglected. However, there is evidence, traditionally linked to the convergent component of collision, which may be differently interpreted. Crowhurst et al. show that fission track data suggest that the Papuan metamorphic rocks may be interpreted as representing early Neogene extension after arc collision, rather than contraction-related metamorphism. These arguments may be applicable in other parts of SE Asia where fission track and isotopic ages are revealing unsuspected events, very young ages, and short time periods for the very complex tectonic evolution of many parts of the region. The volume concludes with two papers from south China accompanied by interpretations of the timing and significance of events. Wopfner suggests that the Baoshan and Tengchong Blocks in western Yunnan have a Gondwana origin, supported by the presence of Upper Palaeozoic glaciomarine deposits, cold-water faunas and Glossopteris. The terranes separated from Gondwana in the Early Permian and docked with
. ° .
INTRODUCTION Cathaysia in the Late Triassic, although Tertiary strike-slip faulting on the Nujiang Line has juxtaposed the two terranes. Z h o u et al. reinterpret part of the history of SE China, diverging in particular from the traditional practice of relating major unconformities to separate orogenies, and suggesting instead that they record different stages in the evolution of a single orogeny following early Mesozoic collision between the South China and the South China Sea blocks. SE Asia is one of the most exciting regions of the globe for any earth scientist. The size, difficulties and practicalities of the region demand a long-term investment of effort but the rewards are illustrated by the papers in the volume. These give an insight into how the history of the region will be uncovered, as well as providing an overview of present regional tectonics and its development. Because of the rapid rates of movements in many parts of the region new geodetic tools and increasingly refined methods of examining earthquake data mean that realistic and rapid tests of regional and global plate models are possible. The challenge for the future is to examine how far interpretations of these data can be pushed back into the past, to improve kinematic descriptions and models, and to identify the processes which have led to the complexity of the region and which can be applied to older orogenic belts. In these tasks, there remains an important role for the field geologist as well as those developing and applying new technologies. An improvement of our understanding will have benefits for knowledge as well
Xlll
as for the many inhabitants of the region, in helping to develop its resources and mitigate its hazards. This Special Publication arose from a conference on the Tectonic Evolution of SE Asia held at the Geological Society in London in December 1994. In addition to the reasons outlined above for the conference, the London University SE Asia Research Group wished to mark the retirement of Dr A. J. Barber in 1994. Tony Barber initiated several of the studies which are presented as publications in this volume and was instrumental in developing the programme of the SE Asia Research Group over many years, particularly by fieldwork throughout the region. We wish him a long and happy retirement. We thank all of the following who provided reviews of manuscripts: J. R. Ali, M. Allen, M. G. Audley-Charles, A. J. Barber, H. Bellon, S. C. Bergman, J. C. Briden, C. S. Bristow, T. R. Charlton, J. Charvet, B. Clennell, D. Q. Coffield, M. C. Daly, J. E. Dixon, C. Elders, R. Ellam, A. Fortuin, M. Fuller, R. J. Garrard, N. S. Haile, N. A. Harbury, K. C. Hill, A. J. Hurford, C. S. Hutchison, S. J. Kelly, J. Malod, S. J. Matthews, R. McCaffrey, M. Menzies, I. Metcalfe, J. S. Milsom, A. H. G. Mitchell, G. Moore, S. J. Moss, R. J. Murphy, G. J. Nichols, G. Packham, C. D. Parkinson, S. Polachan, M. Pubellier, A. J. Racey, C. Rangin, J-P. Rehault, M. A. Samuel, D. Snyder, P. Styles, R. E. Swarbrick, M. E Thirlwall, E Tongkul, J. J. Veevers, R. von Huene, G. K. Westbrook, H. J. Wensink. We are especially grateful to Diane Cameron who carried out many of the major tasks related to organizing the conference, in addition to her assistance in reviewing and editing the manuscripts. Simon Baker, Steve Moss and Moyra Wilson also provided considerable assistance during the preparation of the volume, for which we are extremely grateful.
Slip partitioning at convergent plate boundaries of SE Asia ROBERT McCAFFREY
Department of Earth & Environmental Sciences, Rensselaer Polytechnic Institute, Troy, N e w York 12180, USA Abstract: The active tectonics of SE Asia can be characterized by the interactions of large, rigid plates separated by broad zones of deformation. The relative motion of these plates across their boundaries is often partitioned in the sense that the normal and shear components occur on different structures. Earthquake slip vectors and geological and geodetic measurements are used to infer the degree to which oblique convergence, which is ubiquitous in SE Asia, is partitioned. The active tectonics of Sumatra, the Himalayan thrust, the Philippines, the New Guinea fold-andthrust belt, the Huon-Finisterre collision, and the San Cristobal trench can be understood in terms of upper plate deformation associated with oblique convergence. Western Java may also exhibit partitioning of oblique subduction. Structures accommodating normal and shear components of the motion are often very close. Arc-parallel strain rates are estimated for forearcs of the region. The arc-parallel deformation of forearcs of the SE Asia region demonstrates that plate convergence, whether normal to structure or not, is a three-dimensionalprocess.
When convergence between two plates is not perpendicular to their boundary, shear stress parallel to the plate boundary results in marginparallel shear strain within both plates. In a collision belt or subduction zone, if one of the plates is weak in shear, then the total slip may be partitioned into shear and thrust components (defined by the plate boundary orientation). Often these shear and normal components of slip across the boundary are accommodated on different geological structures. Oblique convergence is globally much more common than trench-normal convergence and often the obliquity varies along the margin. The deformation in many convergence zones may be understood in terms of such slip partitioning. A widely cited example of these parallel structures is along the San Andreas system, which comprises a strike-slip fault and a parallel fold-and-thrust belt (Mount & Suppe 1987) even though the relative motion between the Pacific and North American plates is only a few degrees from the trend of the San Andreas fault. Throughout SE Asia and the SW Pacific, we see several examples of oblique convergence (Fig. 1), both in subduction and collision settings. Studies of Sumatra led Fitch (1972) to first propose that slip was sometimes accommodated by parallel thrust and strike-slip faults. Since that study, it has been found that partitioning of oblique subduction is globally quite common although it is rarely completely partitioned (Jarrard 1986; McCaffrey 1994). Similar geometries, in which a major strike-slip fault accommodates the boundary-parallel shear
strain, exist in the Philippines (Barrier et al. 1991) and possibly in Irian Jaya (Abers & McCaffrey 1988). Other convergence zones that have clear slip partitioning are the Himalayan thrust (Molnar & Lyon-Caen 1989), the Aegean (Gilbert et al. 1994), New Zealand (Anderson et al. 1993) and the Aleutians (Ekstr(Sm & Engdahl 1989; McCaffrey 1992). McKenzie & Jackson (1983) argued that, if plate motions are driven by ductile shear from below, oblique convergence in continental settings should be completely partitioned, that is, slip will occur only on strike-slip and pure thrust faults. This theory provides a test of crustal dynamics in oblique collision zones but requires detailed knowledge of the deformation. SE Asia is a good region to test such ideas because oblique convergence zones are plentiful and fast. In the past few years much has been done to improve our knowledge of the pattems of crustal deformation in SE Asia. In particular, systematic studies of large earthquakes, geological studies of active faults, and geodetic measurements using the Global Positioning System (GPS), that commenced in the SW Pacific region in 1988, are revealing important results, and surprises. The goal of this paper is to review our knowledge of the crustal deformation in SE Asia with attention to the partitioning of slip between thrust and shear faults in the broad deforming boundaries. First, constraints on the motions of the major plates are discussed in order to put bounds on the convergence geometries and then the deformation within these zones is described.
From Hall, R. & Blundell, D. (eds), 1996, Tectonic Evolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 3-18.
4
R. McCAFFREY
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SLIP PARTITIONING AT CONVERGENT PLATE BOUNDARIES
5
Plate motions
The Philippine Sea plate
The broad kinematic framework of SE Asia, determined by the relative motions of the Pacific, Australian, and Eurasian plates, is described to the first order by NUVEL-1A, the global plate motion solution of DeMets et al. (1994a). In addition, the motions of the Philippine Sea, the SE Asian, and Caroline plates control the large-scale tectonics of the region, and these are discussed below.
The motion of the Philippine Sea plate (PSP) is still poorly constrained due to lack of reliable kinematic data; only subduction zone earthquake slip vectors are available (Ranken et al. 1984; S e n t et al. 1993) and these are probably deflected by local deformation of the leading edges of the upper plates (McCaffrey 1994). Moreover, earthquake slip vectors do not contain information about rates of motion. Despite these handicaps, S e n t et al. (1993) revised estimates of PSP motion by constraining it to comply with the NUVEL-1A global solution. However, this does not improve the tie of the PSP to its neighbouring plates; for example the uncertainty in the latitude of the Pacific-PSP pole is nearly 50 ° (Sent et al. 1993). Ongoing programmes by several groups to measure the motion and deformations of the PSP with GPS will provide the required data to constrain the PSP motion relative to the rest of the world. Recent GPS measurements across Taiwan (Yu & C h e n 1994) show that the PSP and Eurasia converge at azimuth 3070_+ 1 and a rate of 86 _+2 m m a -1, which is in a similar direction but 20% faster than the S e n t et al. (1993) PSP-Eurasia pole predicts. A very long baseline interferometry vector (Matsuzaka et al. 1991) between sites on Japan (North American plate) and the Izu-Bonin arc (PSP) also agrees with the azimuth predicted by the combined PSP-North America Euler pole (Sent et al. 1993; DeMets et al. 1994a) but is 15% faster. Both results suggest that Sent et al. may have underestimated the angular velocities for the PSP relative to Eurasia and North America. In the following, the S e n t et al. (1993) poles are used for the PSP but other poles are tried in the kinematic analyses of forearcs around the PSP.
SE Asia as a separate plate
One of the important regional problems is whether or not SE Asia (SEA) moves independently of Eurasia. This question is linked to collision in the Himalayan region and its answer will provide constraints on the mechanism of continental collision. Recently, the first direct measurements of the relative motion between Australia and Indonesia across the Java trench were made. Five GPS measurements over 4 years on a baseline between Christmas Island (Fig. la) and western Java reveal a relative velocity (67 _+7 m m a-t rate at 011 _+4 ° azimuth) that matches the NUVEL-1A vector for Australia relative to Eurasia (71 _+ 2 m m a -1 rate at 020 _+3 ° azimuth) within 1 cm a-1 (Tregoning et al. 1994) (Fig. l a). This single vector can be interpreted in several ways: that SEA moves slowly if at all relative to Eurasia (within the uncertainties of GPS); that SEA moves relative to Eurasia but the pole of rotation is near western Java; or that elastic strains contaminate the geodetic signal due to the proximity of the GPS sites to the plate boundary. Until more results become available, the author favours the first view - that SEA moves very slowly, 1 cm a -1 or less, northward relative to Eurasia. Because the motion of SEA relative to Eurasia is slow and the pole of rotation is poorly constrained, in the following discussions SEA will be treated as part of Eurasia.
The Caroline plate
Weissel & Anderson (1978) argued for the existence of a separate Caroline plate (CAR)
Fig. 1. Tectonic maps. (a) SE Asia. Thrust faults have barbs on the hanging wall. Heavy black arrows show relative plate motions at boundaries (vectors show upper plate moving relative to lower) using poles given in Table 1. Grey arrows with error ellipses show GPS vectors of Cocos Island (CO) and Christmas Island (CI) relative to Java (Tregoning et aL 1994) (black arrows show expected Australia-Eurasia vector). Small arrows pointing landward of trenches show average slip vector (SV) azimuths of interplate thrust earthquakes along 400 km long segments of the forearcs (lengths of arrows are scaled to number of SV in average). Thick grey lines show outline of broad deforming region between the Indian and Australian plates (DeMets et aI. 1994b). SEA, SE Asian plate; PSP, Philippine Sea plate; ODR, Oki Daito ridge; CAR, Caroline plate; JT, Java trench; BS, Banda Sea; MS, Molucca Sea; PF, Philippine fault; PT, Philippine trench; AS, Andaman Sea; SS, Sunda Strait; NGT, New Guinea trench. Bathymetry contours at 2000 m intervals. (b) Southwest Pacific. Format same as (a). GPS vectors are relative to Pacific plate (Bevis et al. 1995). The large dot north of New Zealand is the pole of rotation of the N. Tonga forearc relative to the Pacific determined from GPS vectors. NH, New Hebrides arc; LB, Lau Basin; LR, Louisville Ridge; SV, slip vector.
6
R. McCAFFREY
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between the Pacific, the PSP, and northern New Guinea (Fig. la). This suggestion was based largely on the morphologies of the Mussau trench and Sorol trough (Fig. 2); the former thought to be a subduction zone and the latter predominantly a left-lateral shear zone with some extension. Rates of motion of CAR relative to surrounding plates are based on the observed increase in sediment thickness away from the Ayu Ridge (Weissel & Anderson 1978) and on inferences about the amount of shortening at the Mussau trench (Hegarty et al. 1983). Such motions are estimated at less than 1 cm a-l. Ranken et al. (1984) and Seno et al. (1993) estimated rotation poles for CAR relative to the Pacific and PSP by requiring convergence along the Mussau trench and a combination of extension and strike-slip at the Sorol trough. Recent earthquake mechanisms along the Mussau trench and Sorol trough (Fig. 2) are not consistent with a simple plate boundary between CAR and the Pacific. The slip vector for one event near the intersection of the Sorol trough and Mussau trench is nearly orthogonal to the expected slip based on the Seno et al. Pacific-CAR pole, Events along the Mussau trench itself indicate some thrusting but mostly right-lateral strike-slip on very steep planes that are consistent with the strike and polarity of the trench.
Thrust earthquakes along the Yap trench have slip vectors that are consistent with either Pacific-PSP or CAR-PSP motion. The alleged Caroline-Pacific plate boundary is not a very compelling feature and may simply be a region where stress concentrates due to locally steep, inherited topography. In this paper it is presumed that the 'Caroline plate' is kinematically part of the Pacific plate. Even if the Caroline plate is a separate block, ignoring it in the kinematic analysis will not be critical because it probably moves slowly relative to surrounding plates.
Deforming boundary zones and slip partitioning By far the most elusive aspect of SE Asian tectonics is the description of the deforming zones between the rigid portions of the major plates described above. One way to view the deformation of the plate boundary zones is in terms of slip partitioning. Before discussing slip partitioning at convergent margins of SE Asia, some basic geometrical concepts are reviewed. The mechanics of slip partitioning are discussed by McCaffrey (1992) and Platt (1993) but this paper concentrates on the kinematic consequences. Most of the inferences
SLIP PARTITIONING AT CONVERGENT PLATE BOUNDARIES drawn here are based on the deflections of slip vectors of interplate earthquakes away from the expected convergence direction between the two plates (Fig. 1 shows orientations of slip vectors and plate vectors along plate boundaries). Earthquake slip vector deflections tell us mostly about the arcparallel component of deformation within the upper plate, because most convergence is at a high angle to the trench normal, arc-normal deformation of the forearc does little to deflect the slip vectors (McCaffrey 1991). If the plate convergence vector and the orientation of the trench are known, then the arc-parallel rate of motion v s of the forearc relative to the upper plate can be inferred from the slip vector based on a simple kinematic relationship (Fig. 3). Moreover, if the obliquity varies along the margin then v s may vary as well, resulting in an arc-parallel strain rate. Note that the arc-parallel strain rate arises from the gradient in obliquity, not from the obliquity itself, and we will see that arc-parallel strain can occur where the obliquity is zero. Hence, even though subduction may be locally perpendicular to the trench and apparently two-dimensional, rapid arc-parallel deformation of the upper plate can occur, producing a threedimensional strain field. Earthquakes used are those that reveal slip between the subducting plate and the overriding forearc. McCaffrey (1994) discusses the selection of data (i.e. trench orientations, plate convergence vectors and slip vectors) needed for this type of analysis. Slip vectors are averaged in 5 0 - 1 5 0 k m long segments of the trenches. This paper uses the Harvard centroid-moment tensor solutions from 1977 through August 1994 and other published solutions for earthquakes prior to 1977. It is assumed that the downgoing plate does not deform rapidly; slip vector deflections require motions of a large percentage of the plate motion rate and it is unlikely that subducting plates are sheared at such high rates. The goal is to infer arc-parallel slip rates v s (Fig. 3) and arc-parallel strain rates (arc-parallel gradients in Vs) in forearcs in the SE Asia region (Table 1). Where available the slip vector derived slip rates are compared to geological or geodetic measurements of slip rates. Sumatra and Java
Fitch (1972) correctly inferred that oblique convergence at Sumatra was partitioned into a component of thrusting at the Java trench that was normal to the trench and a component of arc-parallel shear on the Sumatra fault. In the past few years, detailed investigations utilizing more and better data have refined the kinematics of the Sumatra region. Attempts to treat the Sumatra forearc, between the trench and the Sumatra fault, as a rigid plate (e.g.
7
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Curray 1989) generally failed to explain subduction west of the Andaman Sea, where the trench takes a turn to the NNE (Fig. l a) or the difference in the amount of extension in the Sunda Strait (100 km) (Huchon & Le Pichon 1984; Harjono et al. 1991) and Andaman Sea (400 km) (Curray et al. 1979). These features can be explained if the forearc stretches in addition to being translated northwestward relative to SEA (Diament et al. 1990; McCaffrey 1991). Using a convergence direction at the Java trench of N3°E and a rate of 75 m m a -t, an arc-parallel extensional strain rate of 3-4 × 10-8 a-1 is required of the Sumatra forearc to match the deflections of the slip vectors (McCaffrey 1991). Tregoning et al. (1994) show that the N U V E L - 1 A AustraliaEurasia vector is approximately correct for the Java trench. The NUVEL-1A Australia-Eurasia convergence direction results in a lower inferred arcparallel strain rate (1.8_+0.5 × 10-8 a-I; Fig. 4a, Table 1) than the N3°E direction does because it matches better the slip vectors west of Sumatra. Such arc-parallel stretching of the forearc implies that the slip rate on the Sumatra fault
8
R. McCAFFREY
Table 1. Arc-parallel strain rates for forearcs Trench name
Pole of rotation*
Number of trench segments
Number of slip vectors
Arc-parallel strain rate (x 10-8 a -1)
Correlation coefficient
RMS (mm a-z)
Himalaya Izu Java Mariana New Hebrides N. Ryukyu N. Tonga Philippine S. Ryukyu S. Tonga Sumatra
N IND-EUR S PAC-PSP N AUS-EUR S PAC-PSP N PAC-AUS S PSP-EUR N PAC-AUS S PSP-EUR S PSP-EUR N PAC-AUS N AUS-EUR
13 7 15 10 11 5 4 23 4 15 18
19 67 40 61 124 27 99 140 14 481 102
2.2 _+0.5 0.7 _-!-0.7 -0.1 _+0.4 1.7 + 0.5 12.3 + 2.5 1.3 _+0.3 16.3 _+5.4 2.6 _+ 1.0 0.6 _ 1.6 -0.1 _+0.4 1.8 _+0.5
0.79 0.52 -0.05 0.81 0.96 0.92 0.93 0.60 0.13 0.15 0.84
12 6 12 6 25 2 14 18 7 11 11
* N, Demets et al. (1994a) NUVEL-1A; S, Seno et al. (1993); IND, India; EUR, Eurasia; PAC, Pacific; PSR Philippine Sea plate; AUS, Australia. Trench segments are between 50 and 150 km long. Positive values of strain rates are arc-parallel extrensional, and negative values are compressional.
increases to the N W (McCaffrey 1991), w h i c h is supported by the few direct m e a s u r e m e n t s of slip rate on the fault (Fig. 4a). At its southern end, near 5°S, the slip rate is less than 10 m m a -z (Bellier et al. 1991) but increases to about 10 m m a -1 near the equator and to about 28 m m a -1 near 2.2°N (Sieh et al. 1994) (Fig. 4a). The average gradient in the slip rate on the fault is close to that predicted f r o m slip vector deflections; an increase in 28 m m a -1 over 1200 k m f r o m the S u n d a Strait to 2.2°N gives a strain rate of 2.3 x 10 -8 a -1. As is seen in Fig. 4a, slip vectors and the N U V E L - 1 A Australia-Eurasia pole give approximately the correct slip rate for the Sumatra fault but also predict left-lateral, arc-parallel shear at a rate of 15-20 m m a -1 at the longitude of W Java. A pole of rotation that fits the earthquake slip vectors south of Java, and hence does not require upper plate d e f o r m a t i o n near Java, predicts h i g h e r than o b s e r v e d slip rates for the S u m a t r a fault
(McCaffrey 1991). Therefore, either the earthquake slip vectors are p o o r indicators o f upper plate deformation, there are faults other than the Sumatra fault that a c c o m m o d a t e forearc deformation off Sumatra (Karig et al. 1980; D i a m e n t et al. 1992), or there is left-lateral shear through Java (Dardji et al. 1991; Malod et al. 1996). The constant v s value at Java (Fig. 4a) indicates a near-zero strain rate. T h e Java forearc m a y be m o v i n g relative to the Sunda shelf but, unlike the Sumatra side, not u n d e r g o i n g internal strain. The m e c h a n i s m for the arc-parallel stretching of the Sumatra forearc is unclear but involves some extension on cross-forearc faults, such as in the strait b e t w e e n Java and S u m a t r a ( H u c h o n & Le Pichon 1984; Harjono et al. 1991; M a l o d et al. 1996), and s o m e strike-slip faults crossing the forearc (Karig et al. 1980). T h e largest shallow, strikeslip earthquake to occur in Sumatra in the past 30 years was beneath the forearc and had nodal planes
Fig. 4. Plot of plate convergence obliquity (shaded curves), slip vector obliquity (open circles), arc-parallel forearc slip rates (v s, closed circles), and arc-parallel slip rate of subducting plate (v., thin solid curve) along convergence zones of SE Asia. The horizontal axis is distance in kilometres along the de(ormation front or trench (also labelled with approximate latitude or longitude). Positive distance is the direction faced if the thrust fault dips to the viewer's right (for example, positive distance is west if the fault dips N). The vertical axis is angles in degrees and slip rates in mm a -z. Plate obliquity is inferred from trench-normals, plate convergence directions (poles used are listed in Table 1), and their uncertainties. Thin dashed curves, when present, show obliquity from poles of rotation based on fitting slip vectors at the trench. Slip vector obliquity is the angle between the slip vector and the direction normal to the trench (Fig. 3). Positive values of v s indicate right-lateral shear and negative values are left-lateral. Heavy dark straight lines show the best-fit to the values of v s, the slope of which (dVs/dX) is the arc-parallel strain rate (Table 1; positive slopes show arc-parallel extension, negative slopes show arc-parallel compression). Triangles show geodetic and geological estimates of arc-parallel slip rates, as discussed in text. NZ, New Zealand.
9
SLIP PARTITIONING AT CONVERGENT PLATE BOUNDARIES
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that were rotated by 45 ° relative to the trend of the Sumatra fault (Zwick & McCaffrey 1991). It is likely that more detailed study of the Sumatra forearc structure will find complex deformation, such as in the Aleutian (Geist et al. 1988) and Cascadia (Goldfinger et al. 1992) forearcs.
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R. McCAFFREY vectors tend to remain perpendicular to the thrust front instead of following the convergence direction (Fig. 4b). Kinematically this requires deformation of the overriding plate. The uniform arc-parallel strain rate estimated from slip vectors
(Molnar & Lyon-Caen 1989), or about one-third of the total India-Eurasia motion. Due to the large curvature of the Himalayan front, the obliquity varies by more than 90 ° along it (Fig. 4b). Despite this large variation in obliquity, earthquake slip
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(b) Fig. 5. Fault plane solutions (1962-1984; M s > 5.5), slip vectors, and plate vectors for (a) eastern Indonesia and (b) New Guinea. Size of earthquake symbol is scaled to logarithm of seismic moment. Asterisks show active volcanoes. Small arrows show slip vectors of interplate thrust earthquakes and large arrows show expected plate convergence directions based on (a) Australia-Eurasia and (b) Australia-Pacific (DeMets et al. 1994a). MT, Manokwari trough; BH, Bird's Head; SF, Sorong fault; CB, Cenderwasih Bay; TAF, Tarera-Aiduna fault; MTB, Mamberambo thrust belt; HFTB, Highlands fold-thrust belt; NGT, New Guinea trench; MV, Markham Valley; HF, Huon-Finisterre Range; BP, Bismarck plate; NBT, New Britain trench; PNG, Papua New Guinea.
SLIP PARTITIONINGAT CONVERGENT PLATE BOUNDARIES is 2.2 + 0.5 × 10-8 a-l (Fig. 4a) if the India-Eurasia convergence rate is used and one-third of that (i.e., 0.7 x 10-8 a-l) if two-thirds of India-Eurasia convergence is taken up in Asia. Molnar & Lyon-Caen (1989) estimate 1.0 _+0.5 x 10-8 a -1 (18 _ 9 mm a-t over a distance of 1800 km) for the rate of E-W extension of Tibet. Extension of the Himalayan frontal arc is thought to be driven by gravitational collapse of the overthickened Tibetan Plateau, instead of shearing due to oblique plate convergence. The evidence for this is the correlation of faulting type with elevation. Normal faulting is confined to elevations above 4000 m, strike-slip occurs at all elevations, and thrust faulting is confined to elevations below 4 5 0 0 m (e.g. Molnar et al. 1993). However, because the earthquakes showing rotated slip vectors occur within 200 km of the thrust front, the Himalayan forearc must also be extending along strike at about 10-8 a-1. Therefore, while normal faulting may be confined to high elevations, E-W extension of the upper plate is not. As is the case for the Sumatra forearc, such extension could be accommodated by strike-slip faulting at low elevations instead of normal faulting. Note that the arc-parallel strain rate applies even to the region near 86°E ( - k m 1000) where subduction is perpendicular to the mountain front. (At several other subduction zones in Fig. 4, similarly, the strain rate is not zero even thought the plate obliquity is near zero.) Even though v s ---0, the margin-parallel gradient in v s is non-zero. In the case of normal convergence, strain will occur in the vertical plane containing the convergence vector and the normal to the margin. Since v s represents motion perpendicular to this plane, a gradient in v s results in strain outside of this plane, producing a three-dimensional strain tensor. The force that stretches the forearc along its length arises from the gradient in the arc-parallel shear stress due to the increase in obliquity on both sides of the point where margin-perpendicular convergence occurs (McCaffrey 1992). Hence, subduction at a curved margin is a three-dimensional process even where it may be locally perpendicular to the trench. Banda arc and Timor
Earthquakes suggest that the present stage of the collision of the Australian continental margin with the Banda arc probably involves more rapid convergence at the backarc thrusts than at the Timor trough (Fig. 5a). The Wetar and Flores backarc thrust zones are considerably more energetic seismically than the subduction thrust fault north of the eastern Java trench and Timor trough (McCaffrey 1988); all known large thrust earthquakes in this decade have occurred at the Flores
||
and Wetar thrusts. The M w = 7.9 December 1992 Flores earthquake is the latest and largest of many to occur along the backarc thrusts. Preliminary GPS results show that the slip rate associated with backarc thrusting increases from Bali eastward to about 50 mm a-1 at the eastern end of the Flores thrust and reaches the full expected rate of Australia-Eurasia relative motion (70 mm a-1) at the Wetar thrust north of Timor (Genrich et al. i994). GPS appears to be confirming the inference based on earthquakes (McCaffrey 1988) and crustal structure (Silver et al. 1983) that the eastern Sunda arc is rotating anticlockwise about a pole in eastern Java. Timor appears to be moving northward relative to the Sunda Shelf (SEA) at about the same rate as Australia, suggesting that the Timor trough no longer accommodates subduction (Johnston & Bowin 1981; McCaffrey & Abers 1991; Genrich et al. 1994). The northward motion of the extinct volcanic islands north of Timor is revealed with GPS to be similar to that of southern Timor (Genrich et al. 1994); hence, little of the northward motion of Australia is now accommodated by internal deformation of the Banda arc structure. The island arc structure, bounded by the Timor trough and the Wetar backarc thrust, is rigidly thrusting over the backarc basin. The present geometry is probably quite young; the Wetar thrust exhibits less than 10 km of shortening (McCaffrey & Nabelek 1986), which would take only 0.15 Ma at the present rate of 60 mm a-1. Probably between 3 Ma, when collision began, and 0.15 Ma convergence was in part accommodated by shortening and thickening of the forearc (Snyder et al. 1993). The Sunda arc now shows a gradual transition from subduction of oceanic lithosphere south of Java to a completed accretion of an island arc terrane to a continental margin at Timor. In the process, convergence has jumped from the forearc trench to the backarc thrust. South of Timor, the Timor trough trends about N70°E while the plate convergence vector is probably close to N10°E (Tregoning et al. 1994), resulting in an obliquity of about 30 ° (Fig. 4a). The geometry predicts that the Timor forearc should be under ENE-trending, left-lateral shear and possibly arc-parallel tension. Interplate earthquake slip vectors (Fig. 5a) are too few to confirm this; values of v s inferred from slip vectors range from zero to the full arc-parallel motion rate (Fig. 4a). The few available slip vectors show motions both parallel to plate convergence and perpendicular to the trench (Fig. 5a), suggesting a complex spatial variation in slip partitioning (below we see that this bimodal distribution of slip vectors is common at forearcs with large curvature). Recent earthquakes in the overriding plate suggest that both shearing and
12
R. M c C A F F R E Y
stretching are occurring. In particular, two shallow earthquakes beneath Timor show left-lateral, strikeslip on NE-trending, vertical planes and shallow, N-striking, normal faulting events are found north of central Timor (Fig. 5a) (McCaffrey 1988). GPS results also suggest that slow E - W stretching of the arc may occur (Genrich e t al. 1994).
H i | l a n d g hs
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The island of New Guinea accommodates oblique convergence between the Pacific and Australian plates at a rate of about 12 cm a-1 (Fig. 5b) and azimuth of N70°E. The young, high mountain belt, called the Highlands fold-and-thrust belt, trends at N100°E, so is about 60 ° away from being perpendicular to plate convergence. Large earthquakes suggest that shortening and shear are distributed over much of the island (Abers & McCaffrey 1988; McCaffrey & Abers 1991; Fig. 5b). Earthquakes within the Highlands are predominantly thrust and strike-slip with strikes generally parallel to structure rather than perpendicular to the plate convergence direction (Fig. 6a). Oblique convergence within the Highlands belt of Irian Jaya is therefore largely partitioned. The Mamberambo thrust belt (Fig. 5b) shows many more earthquakes with planes that strike perpendicular to the plate vector, suggesting less partitioning there (Fig. 6b). Recent GPS measurements (Puntodewo et al. 1994) reveal less than 2 cm a-1 of shortening and less than 3 cm a-1 of left-lateral shear between sites at the north and south coasts of E Irian Jaya (at about 140°E) even though nearly 6 cm a -1 of shortening and 10 cm a-1 of left-lateral shear are predicted by Pacific-Australia relative motion. At the longitude of central New Guinea, most of the motion between Australia and the Pacific occurs offshore, probably at the New Guinea trench, contrary to inferences made from geology and seismology. In westernmost New Guinea, plate motion appears to be accommodated in a different manner. GPS sites on the Bird's Head appear to move relative to Australia at nearly the same rate as the Pacific (Puntodewo et al. 1994; Stevens e t al. 1995). In the vicinity of the Bird's Head, the majority of Pacific-Australia motion occurs by large-scale W S W motion of the Bird's Head relative to Australia. The fault (or faults) on which this motion occurs is not clear although in the SW the main boundary is probably the Tarera-Aiduna fault (Hamilton 1979; Abers & McCaffrey 1988). Manuel Pubellier (pers. comm. 1994) suggests that a large, NE-trending shear zone exists landward of the east coast of Cenderwasih Bay (Fig. 5b). If so, it could be among the fastest slipping continental
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shear zones in the world, being at least 7 cm a-l (Stevens e t al. 1995). The Sorong fault, at least where it passes through the Bird's Head, cannot now form the main boundary between the Pacific and Australia because GPS sites on opposite sides
SLIP PARTITIONINGAT CONVERGENT PLATE BOUNDARIES of the fault show little relative motion (Puntodewo et al. 1994). In Papua New Guinea (PNG), the HuonFinisterre (HF) arc terrane is being emplaced onto the Australian continent along a north-dipping thrust fault that crops out in or near the Markham Valley (MV) (Fig. 5b). Slip vectors of thrust earthquakes beneath the HF range north of the valley remain perpendicular to the valley as it swings to a NW trend (Fig. 5b). This rotation of the slip vectors cannot be explained by a single pole of rotation between rigid plates north and south of the valley. Schouten & Benes (1996) argue that the rotation of the slip vectors is caused by rapid deformation south of the New Britain trench and MV. On the bases of geological observations (Abbott et al. 1994), microearthquakes (Kulig et al. 1993), and large earthquakes (Abers & McCaffrey 1994), it is interpreted that the HF range is cut by cross-faults that accommodate relative slip and rotations of blocks of the HF range as it moves southward. In the case of the HF block, slip partitioning is accommodated by faulting at high angles to the trench as in Cascadia and the Aleutians, rather than arc-parallel faults, as in Sumatra. Solomons and N e w Hebrides
The Solomon and New Hebrides trenches form the boundary between the Pacific and Australian plates and, due to their great curvature, are the sites of large variations in convergence obliquity. The deviations of slip vectors from the plate vectors suggest that both forearcs deform significantly and partition the slip between the plates. At the southern end of the New Hebrides trench, west of 173.5°E, slip is partitioned completely so that thrust earthquakes at the trench have their slip vectors perpendicular to the trench and nearly perpendicular to the plate convergence vector (Fig. 7a). Going west around the bend in the trench, the slip vectors remain perpendicular to the trench to 18°S where the trench becomes nearly normal to the plate vector. East of about 173.5°E, the slip vectors rotate so that they form an angle of about 45 ° to the trench and plate vector. The extensional strain rate estimated for the southern New Hebrides forearc from the slip vectors is 12.3 __+2.5 × 10-8 a -1 (Fig. 4c). The Solomon trench becomes nearly parallel to the plate vector at the San Cristobal section (Fig. 7b). Slip vectors for earthquakes between 162 ° and 165°E show two orientations: one nearly perpendicular to the local trend of the trench and the other nearly parallel to the plate vector. If partitioning occurred by a detached forearc sliver moving along the arc, then the events with slip vectors perpendicular to the trench would be closer to the trench
13
(i.e. below the translating sliver) than those showing oblique slip. The uncertainties in the epicentral locations of these events preclude discerning such a pattern. Nevertheless, structures accommodating shearing and thrusting must be within a few tens of kilometres (the uncertainties in epicentres) of each other. Some fault plane solutions (Fig. 7b) show left-lateral, strike-slip faulting on E - W orientated, nearly vertical faults; these faults are consistent with the type needed to partition the slip by a small sliver of the forearc being translated eastward at a high rate relative to the arc.
Tonga trench
According to the NUVEL-1A Pacific-Australia pole of rotation, the Pacific plate subducts beneath the Tonga arc in a direction nearly perpendicular to the trend of the trench throughout most of its length (Fig. lb), although the obliquity gets large at the northern and southern ends (Fig. 4d). Earthquake slip vectors at the Tonga trench from 15°S to 35°S, deviate by 20 ° or less from the NUVEL-1A Pacific-Australia plate vector (Fig. 8a). If this pole applies to the Tonga trench then slip vectors suggest that the southern Tonga forearc translates very slowly (10 mm a-1 on average), left-laterally relative to Australia and does so as a rigid block (Fig. 4d; Table 1). GPS measurements by Bevis et al. (1995) suggest that the northern Tonga forearc rotates rapidly away from Australia as a rigid block (Fig. lb). Slip vectors also suggest that the forearc is rigid as they can be matched throughout most of the length of the island arc with a single pole of rotation at 54°S, 173°E (Fig. 8a). However, the poles of rotation estimated from GPS and from slip vectors are quite distant. The 3 GPS vectors (relative to fixed Pacific plate) from Tonga forearc sites are all matched within 2 mm a-1 in rate and 2 ° in azimuth by a pole at 32.6°S, 178.9°W (Fig. lb), and an angular velocity of-7.1 o Ma (the distance from the pole to the observation points is constrained by the gradient in GPS velocities). This pole falls near the central Tonga trench (Fig. lb) and therefore cannot describe the motion of the entire forearc from north to south because it predicts spreading between the forearc and the Pacific south of 33°S. There is probably a structure within the forearc south of the GPS sites that separates the N and S sections of the forearc. Most probably this boundary is near 24°S where the Louisville Ridge intersects the trench and spreading in the Lau Basin appears to terminate (Fig. lb). The northern Tonga arc goes through a large bend until the trench is nearly parallel to plate
14
R. McCAFFREY
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convergence (Fig. 8b), in the same fashion as the southern New Hebrides, San Cristobal, and southern Marianas arcs. Like the San Cristobal trench, slip vectors at the northern Tonga forearc appear to describe two sets of directions; parallel to the plate vector and perpendicular to the trench (Fig. 8b). The averages of these slip vectors imply
a rapid arc-parallel extensional strain rate (Table 1; Fig. 4d) for the northern Tonga forearc. However, because the trench-normal slip vectors are arcward (SW) of the vectors that parallel plate motion (Fig. 8b), they probably occur either within the subducted slab or in the upper plate and are not indicative of the interplate slip direction. (If this
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(b) Fig. 8. (a) Plot of slip vector azimuths (small dots), GPS vector azimuths (large dots), the azimuth normal to the Tonga trench, the Pacific-Australia plate convergence direction (PAC-AUS), and the directions predicted by a pole of rotation between the north Tonga forearc and the Pacific based on GPS vectors (labelled GPS) (Bevis et al. 1995) and by a pole based on slip vectors (labelled SV). (b) Map of earthquake slip vectors (small, black arrows) beneath the northern Tonga forearc, the Pacific-Australia predicted direction (large, black arrow = 84 mm a-]) and the GPS vector (grey arrow --- 240 mm a-l).
rotation of the slip vectors resulted from a detached sliver, these events should be trenchward, NE, of the vectors that parallel plate motion.) Excluding these southerly trending slip vectors results in a near zero arc-parallel strain rate for the northern Tonga forearc, in agreement with GPS results of Bevis et al. (1995). Slip vectors east of northern New Zealand reveal a large degree of slip partitioning and v s values suggest approximately 30 m m a -l of right-lateral shear across the North Island, in agreement with geodetic measurements (Fig. 4d) (31 +__9 m m a-1 from Walcott 1984). In the South Island the plate vector becomes nearly parallel to the island and most of the motion occurs by oblique slip near the Alpine fault, although there is still a small
Philippines
The Philippines are a second region that Fitch (1972) regarded as revealing slip partitioning between the Philippine trench and the left-lateral Philippine fault. Using the Seno et al. (1993) PSP-Eurasia pole, the Philippine fault and trench system appears to be strongly partitioned (Barrier et al. 1991) (Fig. 4e). A difficulty in the kinematic analysis of the Philippines is the lack of a reliable plate convergence vector because the presence of several faults in the Philippines makes it unlikely that the Philippine islands are rigidly attached to Eurasia. A pole of rotation can be found between the forearc and the PSP that fits the Philippine trench slip vectors and removes a large part of the obliquity (Fig. 4e). Hence, we cannot be sure, on the basis of slip vectors alone, that the Philippines are partitioned. From combined trilateration and GPS, Duquesnoy et al. (1994) estimate a left-lateral slip rate on the Philippine Fault near l l°N of 26 + 10 m m a -1, which is consistent with the estimates of v s (Fig. 4e) based on the Philippine Sea-Eurasia pole of rotation (Seno et al. 1993). Hence, this first reliable slip rate for the Philippine fault supports the case for the Philippine system being largely partitioned. The arc-parallel strain rate for the Philippine forearc is poorly determined as the v s values do not vary smoothly with distance along the trench (Table 1; Fig. 4e).
R y u k y u trench
Slip vectors along the Ryukyu trench generally follow the Seno et al. (1993) PSP-Eurasia pole (Fig. 4f) and very little slip partitioning is evident even where the obliquity is high. If there is any arc-parallel slip of the forearc, it is of the order of 10 m m a-t or less. Near the centre of the Ryukyu trench (near 28°N), slip vectors are about 30 ° away from the plate vector even thought the obliquity is near zero (Fig. 4f). In this region the Oki Daito Ridge hits the trench (Fig. 1a) and probably causes the local deflection of slip vectors (these two points were excluded from estimates of strain rates).
I z u - M a r i a n a trench
Although the convergence direction is poorly known, the Marianas forearc shows strong partitioning in that slip Vectors remain nearly perpendicular to the trench despite large changes in
16
R. McCAFFREY
the obliquity (Figs 4g and 9). As observed at other trenches, at the south end of the Marianas trench there appears to be two groups of slip vector directions; one nearly perpendicular to the trench and another, comprising only 2 events, that is much more oblique (Fig. 9). The values of v s at the Mariana forearc are generally equal to v , (Fig. 4g), indicating that the upper plate absorbs" all of the arc-parallel component of plate motion by shear resulting in rapid arc-parallel extension (Table 1; Fig. 4g). At the Izu arc, the slip vectors are similar to the plate vector and no arc-parallel motion is required.
27 °N
23 °N
Conclusions Earthquake slip vectors at convergent margins of SE Asia are used to infer the degree of partitioning of slip on margin-parallel and margin-normal structures. W h e r e independent geological or geodetic estimates are available, slip vector-derived estimates of arc-parallel forearc slip rates appear accurate. Regions where oblique convergence is strongly partitioned are the Himalaya, Marianas, New Hebrides, Philippines, Sumatra, Markham Valley, and New Zealand. At some strongly curved margins, earthquake slip vectors are bimodally distributed between the convergence direction and the trench-normal direction, suggesting that slip partitioning occurs at small spatial scales. Several forearcs reveal arc-parallel deformation even where subduction is normal to the trench, showing that subduction is a three-dimensional process. I thank Tony Barber for many years of good work and cheer, the organizers of the symposium for allowing me to attend, and J. Milsom and D. Snyder for helpful reviews. Supported by NSF grants EAR-9105050 and EAR-9406917.
19°N
15°N
ll°N 139°E
143°E
147°E
Fig. 9. Map of earthquake slip vectors (small, black arrows) beneath the Izu-Mariana forearc, and the PSP-Pacific (Seno et al. 1993) predicted directions (large, black arrows).
References ABBOTT, L. D., SILVER, E. A. & GALEWSKY, J. 1994. Structural evolution of a modern arc-continent collision in Papua New Guinea. Tectonics, 13, 1007-1034. ABERS, G. & MCCAFFREY,R. 1988. Active deformation in the New Guinea Fold-and-Thrust Belt: seismological evidence for strike-slip faulting and basement-involved thrusting. Journal of Geophysical Research, 93, 13 332-13 354. & 1994. Active arc-continent collision: Earthquakes, gravity anomalies and fault kinematics in the Huon-Finisterre collision zone, Papua New Guinea. Tectonics, 13, 227-245. ANDERSON, H., WEBB, T. & JACKSON, J. 1993. Focal mechanisms of large earthquakes in the South Island of New Zealand: implications for the accommodation of Pacific-Australia motion. Geophysical Journal of the Royal Astronomical Society, 115, 1032-1054.
BARRIER,E., HUCHON,P. & AURELIO,M. 1991. Philippine Fault: A key for Philippine kinematics. Geology, 19, 32-35. BELLtER, O., SEBRIER,M. & PRAMUMIJOYO,S. 1991. La grande faille de Sumatra: geometrie, cinematique et quantite de deplacement raises en evidence par l'imagerie satellitaire. Comptes Rendus de l'Acad~mie des Sciences de Paris, 312, 1219-1226. BEVlS, M., TAYLOR, E W., SCHUTZ, B. E., RECY, J., ISACKS,B. L. ETAL. 1995. Geodetic observations of very rapid convergence and back-arc extension at the Tonga island arc. Nature, 374, 249-251. CURRAY,J. R. 1989. The Sunda Arc: A model for oblique plate convergence. In: VAN HINTE, J. E., VAN WEERING, TJ. C. E. & FORTUXN, A. R. (eds) Proceedings. Symposium Snellius-H Expedition, Jakarta, Nov. 23-28, 1989, Vol. 1, Geology and Geophysics of the Banda Arc and Adjacent Areas. Netherlands Journal of Sea Research, 24, 131-140.
SLIP PARTITIONING AT CONVERGENT PLATE BOUNDARIES --,
HAMILTON, W. 1979. Tectonics of the Indonesian region. USGS Professional Paper, 1078. HARJONO, H., DIAMENT, M., DUBOIS, J. & LARUE, M. 1991. Seismicity of the Sunda Strait: Evidence for crustal extension and volcanological implications. Tectonics, 10, 17-30• HEGARTY, K. A., WEISSEL, J. K. & HAYES, D. E. 1983. Convergence at the Caroline-Pacific plate boundary: Collision and subduction. In: HAYES, D. E. (ed.) The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Part 2. AGU Geophysical Monograph, 27, 349359. HUCHON, P. & LE P~CHON, X. 1984. Sunda Strait and central Sumatra fault. Geology, 12, 668-672. JARRARD, R. D. 1986. Relations among subduction parameters. Reviews of Geophysics and Space Physics, 24, 217-284. JOHNSTON, C. R. & BOWrN, C. O. 1981. Crustal reactions resulting from the mid Pliocene to Recent continent-island arc collision in the Timor region.
MOORE, D. G., LAWVER,L. A., EMMEL,E J., RAITT, R.W., HENRY,M. & KIECKHEFER,R. 1979. Tectonics of the Andaman Sea and Burma. In: WATKINS, J. S., MONTADERT,L. & DICKERSON, P. W. (eds)
Geological and Geophysical Investigations of Continental Margins. AAPG Memoir, 29, 189-198. DARDJI, N., VILLEMIN, T. & RAMPNOUX, J.-E 1991. Cenozoic fault systems and paleostress along the Cimandiri fault zone, west Java Indonesia. In: PRASETYO, H. & SANTOSO(eds) The Silver Jubilee:
Symposium on the dynamics of subduction and its products. Proceedings LIPI Yogjakarta, 233-253. DEMETS, C., GORDON, R. G., ARGUS, D. E & STEIN, S. 1994a. Effects of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions. Geophysical Research Letters, 21, 2191-2194. & VOGT, E 1994b. Location of the AfricaAustralia-India triple junction and motion between the Australian and Indian plates: results from an aeromagnetic investigation of the Central Indian and Carlsberg ridges. Geophysical Journal International, 119, 893-930. DIAMENT, M., DEPLUS, C., HARJONO, H., LARUE, M., LASSAL, O., DUBOtS, J., & RENARD, V. 1990. Extension in the Sunda Strait (Indonesia): a review of the Krakatau programme. Oceanologica Acta, 10, 31-42. --, HARJONO, H., KARTA, K., DEPLUS, C., DAHRIN, D. ErAL. 1992. Mentawai fault zone off Sumatra: a new key to the geodynamics of western Indonesia. Geology, 20, 259-262. DUQUESNOY,TH., BARRIER,E., KASSER,M., AURELIO,M., GAULON, R. ET AL. 1994. Detection of creep along the Philippine Fault: First results of geodetic measurements on Leyte island, central Philippine. Geophysical Research Letters, 21, 975-978. EKSTROM, G. & ENGDAHL,E. R. 1989. Earthquake source parameters and stress distribution in the Adak Island region of the central Aleutian Islands, Alaska. Journal of Geophysical Research, 94, 15 499-15 519. FITCH, T. J. 1972. Plate convergence, transcurrent faults and internal deformation adjacent to southeast Asia and the western Pacific. Journal of Geophysical Research, 77, 4432-4460. GEIST, E. L., CHILDS, J. R. & SCHOLL, D. W. 1988. The origin of summit basins of the Aleutian Ridge: Implications for block rotation of an arc massif. Tectonics, 7, 327-341. GENRICH, J., BOCK, Y., MCCAFFREY, R., CALAIS, E., STEVENS, C. E T A L . 1994. Kinematics of the eastern Indonesian island arc estimated by Global positioning system measurements. EOS, 75, 162. GILBERT, L., KASTENS, K., HURST, K., PARADISSIS, D., VEIS, G. ET AL. 1994. Strain results and tectonics from the Aegean GPS experiment. EOS, 75, 116. GOLDFINGER, C., KULM, L. D., YEATS, R. S., APPLEGATE, B., MACKAY, M. E. & MOORE, G. E 1992. Transverse structural trends along the Oregon convergence margin: Implications for Cascadia earthquake potential and crustal rotations. Geology, 20, 141-144.
17
BMR Journal of Australian Geology & Geophysics, 6, 223-243. KARIG, D. E., LAWRENCE,M. B., MOORE, G. E & CURRAY, J. R. 1980. Structural framework of the fore-arc basin, NW Sumatra. Journal of the Geological Society, London, 137, 77-91. KULIG, C., MCCAFFREY, R., ABERS, G. A. & LETZ, H. 1993. Shallow seismicity of arc-continent collision near Lae, Papua New Guinea. Tectonophysics, 227, 81-93. MALOD, J. A., KARTA, K, BESLIER, M. O. & ZEN, M. T. 1996. From normal to oblique subduction: Tectonic relationships between Java and Sumatra. Journal of SE Asian Earth Science, 12, 85-93. MATSUZAKA,S., TOBITA,n . , NAKAHORI,Y., AMAGAI,J. & SUGIMOTO, Y. 1991. Detection of Philippine Sea plate motion by very long baseline interferometry. Geophysical Research Letters, 18, 1417-1419. MCCAFFREY, R. 1988. Active tectonics of the eastern Sunda and Banda arcs. Journal of Geophysical Research, 93, 15 163-15 182. 1991. Slip vectors and stretching of the Sumatran forearc. Geology, 19, 881-884. 1992. Oblique plate convergence, slip vectors, and forearc deformation. Journal of Geophysical Research, 97, 8905-8915. 1994. Global variability in subduction thrust zoneforearc systems. Pure and Applied Geophysics, 141, 173-224. • & ABERS, G. A. 1991. Orogeny in arc-continent collision: the Banda arc and western New Guinea. Geology, 19, 563-566 --. & NABELEK, J. 1986. Seismological evidence for shallow thrusting north of the Timor trough.
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18
R. McCAFFREY Plateau and its margins. Geophysical Journal
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ENGLAND, P. & MARTINOD, J. 1993. Mantle dynamics, uplift of the Tibetan Plateau, and the Indian monsoon. Review of Geophysics, 31, 357-396. MOUNT, V. S. & SUPPE, J. 1987. State of stress near the San Andreas fault: Implications for wrench tectonics. Geology, 15, 1143-1146. PLATT, J. P. 1993. Mechanics of oblique convergence. Journal of Geophysical Research, 98, 1623916256. PUNTODEWO, S. S. O., MCCAFFREY, R., CALAIS, E., BOCK, Y., RAIS, J. ET AL. 1994. GPS measurements of crustal deformation within the Pacific-Australia plate boundary zone in Irian Jaya, Indonesia. Tectonophysics, 237, 141-153. RANKZN, B., CAm3WELL, R. K. & KARIG, D. E. 1984. Kinematics of the Philippine Sea Plate. Tectonics, 3, 555-575. SCHOUTEN, H., & BENES, V. 1996. Post-Miocene collision of Australia and the Bismarck arc, western equatorial Pacific. Earth and Planetary Science Letters, in press. SENO, T., STEIN, S. & GRIPe, A. 1993. A model for the motion of the Philippine Sea plate consistent with NUVEL-1 and geological data. Journal of Geophysical Research, 98, 17 941-17 948. SIEH, K., ZACHARIASEN, J., BOCK, Y., EDWARDS, L., TAYLOR, E & GANS, P. 1994. Active tectonics of Sumatra. GSA Abstracts with Programs, 26, A-382. SILVER, E. A., REED, D., MCCAEFREY, R., & JOWOD1WIRYO, Y. 1983. Back-arc thrusting in the eastern Sunda Arc, Indonesia: a consequence of arc-continent collision. Journal of Geophysical Research, 88, 7429-7448.
SNYDEr, D. B., PRASETYO, H., TJOKROSAPOETRO, S., BLUNDELL,D. J., BARBER,A. J., RICHARDSON,A. N. &; MILSOM, J. 1993. A deep seismic reflection transect across the youngest mountain range in the world, the active convergent margin between the Australian craton and the Banda volcanic arc near Timor, Indonesia. EOS, 74, 444. STEVENS, C., PUNTODEWO, S. S. O., MCCAFFREY, R., BOCK, Y. t~ CALAIS, E. 1995. Out of the pot and into the frying pan: A Bird's Head's view of tectonic escape. International Union of
Geodesy and Geophysics, XXI General Assembly, p. A38. TREGONING,P., BRUNNER,E K., BOCK, Y,. PUNTODEWO,S. S. O., McCAFFREY, R. ET AL. 1994. First geodetic measurement of convergence across the Java Trench. Geophysical Research Letters, 21, 21352138. WALCOTT, R. I. 1984. The kinematics of the plate boundary zone through New Zealand: a comparison of short- and long-term deformations. Geophysical Journal of the Royal Astronomical Society, 79, 613-633. WEISSEL, J. K. • ANDERSON, R. N. 1978. Is there a Caroline plate? Earth and Planetary Science Letters, 41, 143-158. Yu, S.-B. & CHEN, H.-Y. 1994. Global positioning system measurements of crustal deformation in the Taiwan arc-continent collision zone. Terrestrial, Atmospheric and Oceanic Sciences 5(4), 477-498. ZWICK, P. & MCCAFFREY, R. 1991. Seismic slip rate and direction of the Great Sumatra Fault based on earthquake fault plane solutions. EOS, 72, 201.
The Sumatra margin: oblique subduction and lateral displacement of the accretionary prism JACQUES
ANDRI~ MALOD 1 & BADRUL
MUSTAFA
KEMAL 2
1 GEMCO, URA 718 du CNRS, UPMC, Case 129, 4 pl. Jussieu, 75252 Paris, France 2 Padang University, 25117 Padang, Indonesia Abstract: The Sumatra active margin extends more than 1600 km from northwest to southeast. The almost north-south direction of subduction is normal in front of Java and oblique in front of Sumatra. The transition between the two regimes of subduction occurs south of the Sunda Strait. A structural and stratigraphical study of the forearc domain, based on data collected during a French-Indonesian programme of cooperation in oceanography, confirms that the convergent motion is partitioned into a convergent motion, more or less perpendicular to the trench, and a strike-slip motion parallel to the trench. This latter motion is taken up along two major faults, the Sumatra and the Mentawai faults. At its northern end, the Mentawai fault is attenuated and seems to terminate within the accretionary prism. It is relayed and connected to the Sumatra fault by the Batee fault. This pattern can be explained by a simple model with two sliver plates: the Mentawai and Aceh sliver plates, on the top of which the forearc basin has developed. The accretionary prism itself is moving northwestwards along the Mentawai fault. Because there is no major evidence of extension within the Mentawai plate, we conclude that the motion along the Sumatra fault may occur at a uniform rate south of latitude 3°N. Strike-slip motion along the Mentawai fault may be explained by relatively better coupling between the subducting slab and the upper plate beneath the accretionary prism compared to that beneath the forearc, perhaps because of the large oceanic structures entering the subduction zone.
Oblique subduction induces the partitioning of the convergent motion into two components, parallel and perpendicular to the subduction zone (Fitch 1972; Jarrard 1986; McCaffrey 1991, 1992). This partitioning results from the coupling effect between the dipping oceanic slab and the upper plate lithosphere. This coupling effect can be identified by study of slip vectors at the subduction zone which indicate the true direction of relative movement of the oceanic slab with respect to the forearc (Jarrard 1986; McCaffrey 1992): In the case where the margin is curved, partitioning increases with obliquity (Fig. 1). If the partitioning is not complete the slip vector will be situated between the convergence vector and the direction perpendicular to the margin. The lateral movement results in the formation of a sliver microplate while the movement perpendicular to the margin, mainly taken up by subduction, is also expressed by compressional deformation within the accretionary prism or the forearc. Marine geophysics cruises, in cooperation with Indonesia, allowed a detailed study of the Sumatra margin to be undertaken (Diament et al. 1992; Malod et al. 1993; Mustafa Kemal 1993; Zen Jr. 1993). The object of this paper is to propose a model for the distribution of the movements in the whole forearc area.
~. plate
k
Oceanic plate
Fig. 1. Partitioning of movements during oblique subduction increases with margin curvature. Full partitioning is shown for sake of clarity.
Structures related to the subduction off Sumatra The subducting oceanic lithosphere of the Wharton basin has a variable age and structure (Fig. 2). Its age ranges from more than 110 Ma in the eastern part of the area, along the Java trench, to approximately 50 Ma south of central Sumatra where the Wharton extinct spreading ridge is being subducted
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 19-28.
Upper plate
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19
20
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.
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Fig. 2. The geodynamic framework of Sumatra subduction. Note the Wharton extinct spreading axis and the Investigator fracture zone entering the subduction. The Sumatra and Mentawai faults are the tectonic response to the obliquity of subduction. A schematic distribution of the movements in the forearc is shown. Thick arrows indicate motions with respect to Sumatra mainland. Thin arrows give the relative motion along strike-slip faults. As pointed out in the text, the numerical values are realistic but only indicative, based on published studies. The inset shows the slab geometry indicated by earthquake depths. The stippled areas are high velocity zones reflecting a subducted slab at depths of 400--450 km according to the seismic tomography of Puspito et al. (1993).
THE SUMATRA MARGIN with a velocity of approximately 60-70 mm a-1 (DeMets et al. 1990). The geometry of the dipping oceanic slab can be deduced from the distribution of earthquakes linked to subduction (Fig. 2). The slab curvature follows the island arc of Java and Sumatra and shows some minor rupture zones to the south of Sumatra (Zen Jr. 1993; R. Louat, pers. comm. 1994). Further north in the region of Lake Toba, a major rupture occurs, which is also indicated by seismic tomography (Puspito et al. 1993). This feature corresponds to a marked change in orientation of the continental margin and the subduction trench which follow the shape of the slab and turn to a westward orientation before resuming a NNW direction. The direction of convergence indicated by global kinematic models is 024 ° to 025 ° (DeMets et al. 1990). In fact, the study of slip vectors shows that the subduction is normal beneath Java, where the average of 15 vectors indicates a 003 ° direction (McCaffrey 1991). The Sunda arc therefore allows observation of the change from frontal subduction to oblique subduction (Fig. 2). Subduction becomes increasingly oblique to the margin as the trench turns from E-W to NW-SE west of Java. One can therefore expect that partitioning of the movement will result in an increasing lateral component of motion. The obliquity of subduction is constant along the main linear portion of the Sumatran margin. The lateral component of the movement is taken up here by the great dextral fault of Sumatra, which extends from the south to the north of Sumatra (Katili 1970). McCaffrey (1992) showed that, correlated with the curvature of the margin, slip vectors indicate that the lateral movement increases gradually along the Sumatra fault. Consequently, he predicted a longitudinal extension of the forearc sliver plate between the trench and the Sumatra fault. This model may partly explain the discrepancy between extension at the two ends of the Sumatra fault, from more than 460 km in the Andaman Sea (Curray et al. 1978) compared to only 50-70 km in the Sunda Strait (Huchon & Le Pichon 1984; Lassal et al. 1989; Diament et al. 1992).
Boundaries of forearc microplates The first result of the study of single channel seismic profiles was the discovery that the faulted zone situated between the accretionary prism and the forearc, previously interpreted as a flexure or backthrust (Karig et al. 1978) is linear, has a highly variable tectonic aspect and looks like a flower structure in some places. Therefore, this structure was re-interpreted as the trace of a strikeslip fault zone, the Mentawai fault, which has also
21
accommodated some compression (Diament et al. 1992; Zen Jr. 1993). The Mentawai fault seems to have been active during the Neogene, but it is not possible to identify the relative importance of strike-slip and compressional movements. Furthermore, the amount of lateral motion cannot be measured from seismic profiles.
The Mentawai microplate
The Mentawai fault (Fig. 3) extends from the south of Sumatra up to the south of Nias. Therefore the Sumatra fault and the Mentawai fault bound an elongated sliver microplate, the Mentawai plate, which corresponds to the forearc domain. Evidence for extensional deformation within this plate is weak since no major normal faults are observed to cross the Mentawai plate. There is no significant indication of the extension predicted in previous models (Jarrard 1986; McCaffrey 1992) or that necessary to explain the difference in the amount of extension between the north and south of Sumatra. There remains the possibility of distributed extension by means of small extensional faults. However, recent faults with very small throws are found only in a 50 km long zone southwest of Sumatra where recent seismicity is concentrated (Zen Jr. 1993; Fig. 4). To the north of Nias island, the NNW-SSE striking Batee fault connects the Mentawai fault and the Sumatra fault (Beaudry & Moore 1985; Matson & Moore 1992). On land where it seems not to have been very active in recent time, its dextral strike-slip movement is documented from Spot images (C. Detourbet, pers. comm. 1994). At sea, it appears as a vertical fault with a large normal throw explaining the Nias basin formation (Fig. 5). The transfer of the major part of the movement on the Sumatra fault may in fact occur by means of several faults in the Nias basin (Matson & Moore 1992) or on its southern ~rim (archipelago of Pini island; Fig. 3). Further north, movement would therefore be concentrated on the Sumatra fault, as suggested by the opening of the Andaman Sea north of Sumatra (Fig. 2). Accordingly, to the north of Nias island, the forearc area moves significantly with respect to the main part of Sumatra. The Sumatra fault forms the northern boundary of a pull-apart in the Sunda Strait (Harjono et al. 1988, 1991, 1993; Lassal et al. 1989; Diament et al. 1990). The dextral strike-slip movement is relayed to the south by the Ujung Kulon fracture zone which extends into the accretionary prism (Fig. 6). To the south of the Sunda Strait the Mentawai fault becomes difficult to follow in the accretionary prism. A progressive initiation of the Mentawai fault could explain why it is no longer strongly
22
J . A . MALOD •
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Fig. 3. Structural sketch of the northern margin of Sumatra, with the delineation of the main sliver plates.
evident in the structures to the south of the Sunda Strait. The Mentawai microplate is greater than 1000 km in length (Fig. 2). Its existence shows that the accretionary prism has moved relative to the forearc domain. The southern limit of the Mentawai plate is coincident with the zone where subduction changes from oblique along Sumatra to orthogonal along Java. The Mentawai plate ends to the north of Nias, where the subducting slab is interrupted by a large discontinuity.
The Aceh microplate North of Nias, the Sumatra fault, the northern extension of the Mentawai fault, and the southern extension of the West Andaman fault (Curray et al. 1978; Izart et al. 1994), bound another sliver plate, the Aceh microplate (Fig. 3). The Mentawai fault may have played a role in the past, but its actual prolongation does not currently show evidence of active strike-slip movement. Backthrusting of the accretionary prism over the forearc basin, along the
THE SUMATRA MARGIN
23
NW
SE
3.0
3.5
4.0
4.5 s.d.t.t
t
5 km
Fig. 4. Recent vertical normal faults associated with active seismicity southeast of Sumatra (location on Fig. 2).
southern continuation of the West Andaman fault, is the probable effect of flexural bending of the forearc basement under the accretionary prism, which would explain the deepening of the forearc basin toward the west (Malod et al. 1993) (Fig. 7). In the longitudinal direction, the forearc is made up of large blocks tilted southwards. Some part of the movement is absorbed by internal deformation of the Aceh microplate as indicated by the Banyak Island thrust zone, active in latest Pliocene (Fig. 7) or the presently active Tuba anticline ridge.
of sliver plate. However, deformation within the prism is difficult to study. Northwest of Nias island, several NW-SE fault zones, which are ramifications of the Mentawai fault, cross the accretionary prism (Fig. 7). The tectonic pattern of these faults suggests strike-slip motion (although mud diapirism might be possible). To the north of this zone, the accretionary prism may be a part of the Andaman Island and Burma plate. Where the Mentawai plate is present, south of Nias Island, we recognize three different zones from south to north (Fig. 2), as follows.
Transfers in the accretionary prism along Sumatra
1. Extension and material ablation in the south The displacement towards the northwest of the Mentawai microplate as a single block is related to the extension in the Sunda Strait and perhaps further south within the forearc itself. At the same time, oceanic sediments accreted to the deeper part
The definition of Mentawai and Aceh plates leads to a consideration of the accretionary prism between the trench and the Mentawai fault as a kind
W
-o.s~~atee
I
5 krn
fault
I
Fig. 5. The Batee fault. Note the large normal throw along this vertical fault (location on Fig. 4).
E
24
J.A. MALOD • B. M. KEMAL accretionary prism or the Sumatra mainland to the northeast of the Sumatra fault, and mechanisms of movement partitioning in the prism and the forearc. However, the general tectonic disposition of the Sumatra margin can be integrated into a simple model, which actually involves the transfer of material at the edge of Sumatra.
Model The boundaries, the distribution and motion of the plates and microplates are illustrated on a simplified structural diagram (Fig. 2).
Sumatra fault and Mentawai fault
Fig. 6. Simplified structural sketch of the southern prolongation of the Mentawai and Sumatra faults. The Ujung Kulon fault zone relays the Sumatra fault system to the south of the Sunda Strait pull-apart. The Mentawai fault zone prolongation indicated by a dashed line is not observed in this region.
of the margin must have been displaced towards the northwest. These two mechanisms, of extension in the basement and removal of sediments from the accretionary prism, contribute to the concave shape of the margin south of the Sunda Strait.
2. Transfer in the central part In the linear part of the margin west of Sumatra, the Mentawai plate and the accretionary prism are displaced towards the northwest. There is relative movement between the plate and the prism along the Mentawai fault. The amount of relative motion and its timing during the Neogene cannot be established. 3. Accumulation and relay zone in the north
In the northern part of the region, according to our interpretation, movement was concentrated mainly on the Sumatra fault, while materials of the prism accumulated in several slices west of Simeulue island. This process occurred as if the subduction was locally frontal, in agreement with the local E-W orientation of the oceanic slab, and is consistent with the horsetail shape of the Mentawai fault in this region and its reduced role further north. This description of the Sumatra oblique subduction and transfer process leaves some unanswered questions, such as possible deformation within the
These two faults have different natures (Fig. 8). The Sumatra fault is an intra-continental strike-slip fault and its geometry can be followed on land and at sea (Fig. 6). It coincides with the volcanic arc which suggests a connection between volcanism and tectonics (Bellier & Srbrier 1994), the volcanic zone being a zone of weakness. The Mentawai fault appears more like a boundary between the prism constituted of accreted oceanic sediments and the forearc basin established on a crust of intermediate or continental type as shown by refraction (Kieckhefer et al. 1980) or expanding seismic profiles (Liu & Curray 1988; Fig. 8). This fault is not necessarily vertical and the compressional component may explain its complicated surface pattern. This fault separates the relatively rigid area of the Mentawai microplate, as shown by the lack of major deformation, from the complex accretionary prism, possibly more deformable because of the existence of numerous thrust slices.
The geometry o f the margin Down to a depth o f 20 or 25 km, the subduction contact occurs between the oceanic slab and the accretionary prism (Fig. 8). Coupling along this contact, assuming an active Mentawai strike-slip fault, induces partitioning of the movement that drags the prism towards the northwest. At greater depths beneath the margin, the coupling occurs between the oceanic slab and the lithosphere of the forearc. Consequently, the partitioning of the movement induces two decoupling zones: the Mentawai fault and the Sumatra fault. In a horizontal plane, the relative movements depend on the importance of the partitioning of the movement in each of these two zones. The Mentawai fault will have dextral strike-slip movement if coupling is more efficient under the accretionary prism than under the forearc basin (Fig. 9); this seems to happen along the rectilinear part of the fault and the Mentawai plate. If the
THE SUMATRA MARGIN
3.or =W-_~_AAccretionary ~ ~~j~rism
25
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Fig. 7. Seismic reflection profiles showing deformations around the Aceh microplate (location on Fig. 4): (a) backthrust of the accretionary prism over the Aceh basin; (h) Late Pliocene northward thrust of Banyak Island; (c) possible strike-slip faults in the accretionary prism forming part of a horsetail termination of the Mentawai fault.
partitioning of movement is the same in both regions there will be no relative movement between prism and forearc (Fig. 9). This may be the case to the south of the Sunda Strait and could explain the
poor expression or even the apparent absence of the Mentawai fault in this region. The occurrence of the Mentawai fault and its continuation into the Batee fault indicate that the
26
J.A. MALOD Be; B. M. KEMAL Mentawai fault 0
Sumatra fault
Accretionaryprism l F°rearc coasl i,~ VolcancJarc ~ ~ lOaSl(I ~...... ......................
50
.........
Sumatra
......... :
Fig. 8. Schematic cross-section, without vertical exaggeration, of the Sumatra subduction system.
first case (Fig. 9) is most likely over most pans of the Sumatra forearc. The coupling between the subducting slab and the overriding forearc seems to be more efficient in the prism area than in the forearc basin. This may be the result of several processes including: (1) the accretionary prism may load the downgoing plate and increase the coupling effect; (2) the deeper part of the accretionary prism may have become better consolidated when subruction was more frontal in the EoceneOligocene time; (3) the large structures of the oceanic sea floor
entering the subduction zone, like the Investigator Ridge and the Wharton extinct spreading ridge, may induce a strong coupling between the slab and the accretionary prism. In any case, since the accretionary prism rests directly on the oceanic slab, its lateral transfer is likely to occur provided there is decoupling along the Mentawai fault.
Movement distribution A quantification of motions is possible only for the most recent period of time during which measure-
Sumatra
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No strike-slip
Fig. 9. Sketch showing two possibilities of motion distribution by decoupling: (a) the situation in central and north Sumatra with the forearc and prism moving at different rates; (b) the situation south of the Sunda Strait, where it is proposed that the forearc and prism are moving at the same rate.
THE SUMATRA MARGIN ments have been made. Measurements of the recent displacement on the Sumatra fault have been made essentially by means of satellite photographs (Bellier et al. 1991, 1993; Bellier & S6brier 1994). These measurements indicate a velocity of displacement that grows from 6 _ 4 m m a-t in the south close to the Sunda Strait (Bellier et al. 1991 ), to 19 _+ 2 m m a -1 in the Singkarak region at 1°S (Bellier et al. 1993), and to about 23 mm a -i in the Lake Toba region at 2°30'N (Detourbet et al. 1993). The value of 6 _ 4 m m a -1 in the region close to the Sunda Strait is low and might be explained by recent extensional deformation occurring in the forearc (Fig. 4). Along the Batee fault a value of 11_ 2 mm a-l is proposed (C. Detourbet, pets. comm. 1994). Figure 9 shows a possible distribution of the movements that would explain the observed structures and rates of Fig. 2. In order to give some numerical values 20 mm a -1 is chosen for the main Sumatra fault segment. Accordingly movement on the northern part of the Sumatra fault is fixed to the sum of this value plus the strike-slip component on the Batee fault, which is 11 m m a -t, yielding 31 m m a-1. These numerical values serve mainly to illustrate the model, and should not be regarded as absolute values. In central and north Sumatra movement is distributed between the Mentawai and Sumatra faults with the forearc and prism moving at different rates (Fig. 9a). In the region south of the Sunda Strait we propose that the forearc and prism are moving at the same rate (Fig. 9b). At the the northern end of the Sunda forearc the movement is absorbed in three ways: opening of the Andaman
27
Sea, westward migration of the Andaman arc that overlaps the Indian ocean oceanic crust, and finally in backthrusting of the accretionary prism over the forearc basin belonging to the Aceh microplate.
Conclusions The authors observations suggest that the degree of partitioning of movement may be different beneath the accretionary prism and the forearc domain. In the most representative portion of the Sumatra margin with respect to oblique subduction, partitioning is more efficient in the accretionary prism than in the forearc region. This may result from the importance of the oceanic structures entering the subduction zone beneath Sumatra. The Mentawai microplate movements are now well understood and partly quantified for recent periods, but those of the accretionary prism necessitate further studies, particularly to look at possible internal deformation. Quantification of the movements also needs further investigation. We thank all the people who participated in the data collection, specially the Commandant Soepangat and his crew on the Indonesian R.V. Baruna Jaya IlL This research was conducted during an Indonesian-French cooperative programme in oceanography. We acknowledge the help of Mr. M. Pain, attach6 scientifique at the French Embassy in Jakarta. This study has been funded by IFREMER, the French Embassy in Jakarta, CNRS-INSU (grant ATP 933909) and UniversitE Pierre et Marie Curie in France and BPPT in Indonesia. S. J. Matthews and M. A. Samuel reviewed the paper and made many valuable comments.
References BEAUDRY,D. & MOORE, G. E 1985. Seismic stratigraphy and Cenozoic evolution of west~Sumatra forearc basin. AAPG Bulletin, 69, 742-759. BELLmR, O. & SE;BmER,M. 1994. Relationship between tectonism and volcanism along the Great Sumatran Fault Zone deduced by SPOT image analyses. Tectonophysics, 233, 215-231. , - - , D~TOURBET, C. & PRAMUMIJOYO,S. 1993. Long term dextral slip rate along the Sumatran fault system. Terra Abstracts', 1, 253, EUG VII, 4-8 April Strasbourg. , & PRAMUMnOYO,S. 1991. La grande faille de Sumatra: gEomEtrie, cinEmatique et quantitE de dEplacement mises en Evidence par l'imagerie satellitaire. Comptes Rendus de l'Acadgrnie des Sciences de Paris, 312, Sgrie II, 1219-1226. CURRAY, J. R., MOORE, D. G., LAWVER, L. A., EMMEL, E J., RAITT, R. W., HENRY,M. & KIECKHEFFER,R. 1978. Tectonics of the Andaman Sea and Burma. In: WATKINS,J. S., MONTADERt,L. & DICKENSON, P. W. (eds) Geological and Geophysical Investigations of Continental Margins. AAPG Memoir, 29, 189-198.
DEMETS, C., GORDON, R. G., ARGUS, D. E & STEk~, C. 1990. Current plate motions. Geophysical Journal International, 10L 425-478. DETOUm3ET, C., BELL~r, O. & SEBmER, M. 1993. La caldeira volcanic de Toba et le systbme de faille de Sumatra (IndonEsie) vues par SPOT. Comptes Rendus de l'Acad(mie des Sciences de Paris, 316, 1439-1445. DIAMENT, M., DEPLUS, C., HARJONO, H., LARUE, M., LASSAL, O., DUBOIS, J. & RENARD, V. 1990. Extension in the Sunda Strait (Indonesia): Review of the Krakatau programme. Oceanologica Acta, 10, 31-41. ~, HARJONO,H., KARTA,K., DEPLUS,C., DAHRIN,D., ZEN, M. T. J. & MALOD,J. A. 1992. Mentawai fault zone off Sumatra: A new key to the geodynamics of western Indonesia. Geology, 20, 259-262. FITCH, T. J. 1972. Plate convergence, transcurrent faults, and internal deformation adjacent to Southeast Asia and the Western Pacific. Journal of Geophysical Research, 77, 4432.4460. HARJONO, H., DIAMENT, M., DUBOIS, J. & LARUE, M. 1988. SEisrnicitE du detroit de la Sonde (IndonEsie):
28
J . A . MALOD t~Z B. M. KEMAL
presentation des rEsultats d'un rdseau local. Comptes Rendus de l'Acaddmie des Sciences de Paris, 307, SErie II, 565-571. --, --, , -& ZEN, M. T. 1991. Seismicity of the Sunda Strait: evidence for crustal extension and volcanological implications. Tectonics, 10, 17-30. , SI~BRIER, M. & DIAMENT, M. 1993. Correction and addition to "Seismicity of the Sunda Strait: evidence for crustal extension and volcanological implications" by Hery Harjono, Michel Diament, Jacques Dubois, Michel Larue and Mudaham Taufick Zen. Tectonics, 12, 787-790. HUCHOn, E & LE PICHON, X. 1984. Sunda Strait and Central Sumatra Fault. Geology, 12, 668-672. IZART, A., MUSTAFA KEMAL, B. & MALOD, J. A. 1994. Seismic stratigraphy and subsidence evolution of the northwest Sumatra forearc basin. Marine Geology, 122, 109-124. JARRARD, R. D. 1986. Terrane motion by strike-slip faulting of forearc slivers. Geology, 14, 780-783. KARIG, D. E., SUPARKA,S., MOORE, G. E & HEHANUSSA, E E. 1978. Structure and Cenozoic evolution of the Sunda arc in the central Sumatra region In: Watkins, J. S., MONTADERT, L. & DICKENSON, P. W. (eds) Geological and Geophysical Investigations of Continental Margins. AAPG Memoir, 29, 223-237. KATILI, J. A. 1970. Large transcurrent faults in southeast Asia with special reference to Indonesia. Geologische Rundschau, 59, 581-600. KIECKHEFER, R. M., SHOR JR., G. G., CURRAY, J. R., SUGIARTA, W. & HEHUWAT, F. 1980. Seismic refraction studies of the Sunda trench and forearc basin. Journal of Geophysical Research, 85, 863-889. LASSAL, O., HUCHON,P. & HARJONO,H. 1989. Extension
crustale dans le detroit de la Sonde (IndonEsie), donnEes de la sismique rEflexion (Campagne KRAKATAU). Comptes Rendus de l'Acaddmie des Sciences de Paris, 309, SErie II, 205-212. LIU, C. S. & CURRAV, J. R. 1988. Structure and nature of the forearc basin of west central Sumatra. EOS, 69, 1443. MALOD, J. A., MUSTAFA KEMAL, B., BESLIER, M. O., DEPLUS, C., DIAMENT,M. ETAL. 1993. DEformations du bassin d'avant-arc au nord-ouest de Sumatra. Une rEponse ~ la subduction oblique. Comptes Rendus de l'Acaddmie des Sciences de Paris, 316, SErie II, 791-797. MATSON, R. G. & MOORE, G. F. 1992. Structural influences on Neogene subsidence in the central Sumatra forearc basin. In: WATKINS,J. S., ZHIQIANG, F. ET AL. (eds) Geology and Geophysics of Continental Margins. AAPG Memoir, 53, 157-181. MCCAFFREY, R. 1991. Slip vectors and stretching of the Sumatran forearc. Geology, 19, 881-884. 1992. Oblique plate convergence, slip vectors, and forearc deformation. Journal of Geophysical Research, 97, 8905-8915. MUSTAFA KEMAL, B. 1993. La marge active au NordOuest de Sumatra. Mdcanismes gdodynamiques de tran~fert lids glla subduction oblique. Th~se de l'UniversitE E et M. Curie. Paris. PUSPITO, N. T., YAMANAKA,Y., MIYATAKE,Z., SHIMAZAKI, K. & HIRAHARA, K. 1993. Three-dimensional P-wave velocity structure beneath the Indonesian region. Tectonophysics, 220, 175-192. ZEN JR., M. T. 1993. DEformation de l'avant-arc en rdponse ?tune subduction ?1 convergence oblique. Exemple de Sumatra. Th~se de l'UniversitE P. et M. Curie, Paris.
Collision and strike-slip faulting in the northern Molucca Sea (Philippines and Indonesia): preliminary results of a morphotectonic study CLAUDE
R A N G I N 1, D A H A R T A & THE MODEC
D A H R I N 2, R A Y Q U E B R A L 3
SCIENTIFIC
PARTY
ANNE GAELLE BADER, JEAN PAUL CADET, GLEN CAGLARCAN, BENOIT DEFFONTAINES, CHRISTINE DEPLUS, ERNESTO G. CORPUS, ROBERT HALL, YANN HELLO, JACQUES MALOD, SERGE LALLEMAND, DOMINGO B. LAYUGAN, RI~MY LOUAT, REYNALDO MORALES, KEITH PANKOW, MANUEL PUBELLIER, MICHEL POPOFF, REYNALDO T. RODELAS & TRAMANADI YUDHO
1 URA 1759 CNRS, Ddpartement de Gdotectonique, T 26-0 El, Universitd Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris, France e ITB Bandung, Indonesia 3 Mines and Geosciences Bureau, Manila, Philippines Abstract: A swath mapping, gravity and single channel seismic survey was carried out in the northern Molucca Sea with R.V. L'Atalante. Preliminary interpretation of these data reveals the presence of an almost complete Sangihe arc and forearc. The Miangas-Pujada-Talaud ridge in the central part of the Molucca Sea appears to be a backstop within the Sangihe forearc. East of the ridge very contrasting terranes are separated by a major NW-SE crustal discontinuity interpreted as a left-lateral strike-slip fault. North of 6°N the Philippine Trench inner wall is dissected by NW-SE trending left-lateral strike-slip faults, resulting from the dominantly oblique convergence between colliding arcs. South of 6°N the westward subduction of the buoyant Snellius volcanic plateau, a fragment of the Halmahera arc terrane, has induced the formation of a new plate boundary, the Philippine Trench along what is interpreted as a former strike-slip fault zone. East of the Miangas-Pujada-Talaud ridge there is a wide sedimentary wedge separated from the Snellius Ridge to the south by the major NW-SE crustal discontinuity. The thickness of this wedge cannot be explained by subduction along the very young Philippine Trench. It could be either an accretionary wedge developed at the deformed leading edge of the Sangihe forearc or be part of a former intra-arc basin which was part of the colliding Halmahera arc terrane. There is no clear evidence for accreted oceanic crust belonging to the recently subducted Molucca Sea. The Miangas-Pujada-Talaud ridge is part of the Sangihe forearc, and the ophiolites could represent its basement, uplifted along the outer arc ridge. The dog-leg-shaped Philippine Trench is propagating southward across the fragmented Halmahera arc terrane and its southern segment could reactivate a former strike-slip fault zone.
The collision of the Hahnahera arc (a fragment of the Philippine Sea plate) with the Sangihe arc (Fig. 1) has been described by Silver & Moore (1978), Hamilton (1979), and Moore & Silver (1983). The ophiolites and melanges exposed in Talaud, Mayu and Tifore Islands have been interpreted as fragments of the almost completely subducted Molucca Sea floor formerly separating the two colliding island arcs (Silver & Moore 1978; McCaffrey et al. 1980). The creation of a new plate boundary, the Philippine Trench, east of the collision zone therefore marks a step in the incorporation of the intra-oceanic Halmahera island arc into the Eurasian continental margin,
if the Sangihe arc is considered to be built on the thinned margin o f Eurasia (Rangin et al. 1990a). In this paper the geometry of this collision zone is documented by a study of the g e o m o r p h o l o g y of the northern Molucca Sea, based on large coverage m u l t i b e a m mapping, with gravity and single channel seismic data. This dataset was recorded during a cruise of R.V. L'Atalante in April and May 1994 (MODEC cruise), in co-operation with the I n d o n e s i a n Institute of D e v e l o p m e n t and Technological Research, Jakarta (BPPT) and the Philippines Bureau of Mines and Geosciences, Manila (BMG).
From Hall, R. & Blundell, D. (eds), 1996, Tectonic Evolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 29-46.
29
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COLLISION AND FAULTING IN THE N MOLUCCA SEA The preliminary interpretations of these data and attempts to answer some basic questions are presented here. How does the collision zone between the Molucca Sea basin margins change from frontal to oblique collision, after oceanic crust of the Molucca Sea has been entirely subducted between the two colliding arcs? What is the morphotectonic signature of a newly created plate boundary, the Philippine Trench? Gravity and seismological data from the Molucca Sea were published by Silver & Moore (1978) and McCaffrey et al. (1980) but the only detailed bathymetric map available before the cruise was that of Krause (1966). Seabed mapping should allow a better understanding of the geometry of this diachronous collision and the rapid structural variations expected in the transition zone between collision and oblique subduction. Here, the focus is on the interpretation of newly acquired seabed surface data (bathymetry and reflectivity, 3.5 kHz echo sounder and single channel seismic reflection with a maximum pene-
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31
tration of 2 s twt). Gravity, magnetic, seismic reflection and swath mapping surveys were conducted to map geological structures and the thickness of sediments between the Halmahera and Sangihe arcs and the Philippine and Cotabato Trenches. The position, speed and direction of the ship were given by the GPS (Global Positioning System) navigation system. The preliminary interpretation of the structures in the survey area is based on the morphological and geophysical data gathered on board in real time. A total of 3725 nautical miles were surveyed during the cruise providing a detailed bathymetric map (Fig. 2). L'Atalante is equipped with a SIMRAD EM-12 dual multibeam echo sounder which provides bathymetry and reflectivity imagery of the sea floor. Only the bathymetric data are discussed here. The MODEC cruise also involved continuous gravity and magnetic measurements and six channel seismic recording. The few seismic data presented here are single channel records acquired on board and are not yet reprocessed.
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32
C. RANGIN ET AL.
Geodynamic framework The Molucca Sea is located at the junction of three convergent major lithospheric plates (Fig. 1). The Eurasian plate is bounded by active volcanic arcs (west Mindanao, Sangihe, and north Sulawesi) and includes the marginal basins of the Celebes Sea and South China Sea which opened in Palaeogene times. The northern margin of the Australian plate extends from New Guinea in the east to central Sulawesi in the west. The Philippine Sea plate has a western part formed largely of Palaeogene and older oceanic crust (the West Philippine Basin) which probably formed in a backarc environment. The associated arc currently comprises a major part of the Philippine archipelago, now orientated approximately north-south between Luzon and Mindanao, and extending southwards into the Halmahera arc. Kinematically, the Australian plate is moving northward with respect to Eurasia at a rate of 78 cm a-1. The Philippine Sea plate is moving westwards with respect to Eurasia about a pole of rotation located at approximately 48°N and 157°E (Seno et al. 1993), at a rate which varies from 810 cm a-1 from north to south along the Philippine Trench. The West Philippine Basin therefore subducts along the Philippine Trench more obliquely in the north than in the south. The Philippine fault, parallel to the trench, accommodates part of the lateral component of this movement, assuming a simple model of shear partitioning. As a result the East Philippine crustal fragment is displaced to the north. Th~ recent development (3 to 5 Ma) of subduction at the Philippine Trench is suggested by (1) the presence of a slab extending to less than 200 km depth (Cardwell et al. 1980; McCaffrey 1982); (2) recent arc volcanism in eastern Mindanao (Quebral et al. 1995); and (3) young arc volcanism in Leyte, Samar and Bicol (Aurelio et al. 1991). The formation of this new active margin is interpreted to be the result of a jump in the position of the subduction zone from west to east after the Philippine arc collided with the Eurasian margin. Further south in the Molucca Sea, the Halmahera arc, carried on the Philippine Sea plate, is in frontal collision with the Sangihe arc (McCaffrey et al. 1980). The Philippine Trench terminates at this latitude (Nichols et al. 1990). The collision zone is marked by a thickening of the crust below the basin, mainly deduced from gravity data and interpreted as oceanic crust imbrication in the central part of the basin (Miangas-Talaud-MayuTifore ridge). The oblique movement of the Philippine plate with respect to Eurasia has therefore been interpreted to have resulted in the diachronous closure of the Molucca Sea basin. The
presence of a slab more than 700 km below the Celebes Sea (Cardwell et al. 1980) provides an indication of the extent of the subducted oceanic basin. The Philippine archipelago is the site of the active collision between the Philippine Sea plate carrying an extinct volcanic arc and continental fragments rifted away from the Eurasian margin (Holloway 1982; Rangin et al. 1990a). The present tectonic setting of the Philippines shows an elongated Philippine Mobile Belt, fringed by two recent subduction zones (Lewis & Hayes 1983), and traversed axially by the 2000 km long left-lateral Philippine fault (Willis 1937; Allen 1962; Aurelio et al. 1991). The volcanic arc of Oligocene age, which forms the basement to the eastern part of the mobile belt, is in tectonic contact with the Eurasian margin represented by continental fragments exposed in the western part of this belt, and rifted away from the Eurasian mainland in south China (Holloway 1982; Rangin et al. 1990a). Southeast of Mindanao, the collision between the Halmahera arc and the along-strike equivalent of the western part of Mindanao occurred during the Late Pliocene and Pleistocene (Moore & Silver 1983; Hawkins et al. 1985; Mitchell et al. 1986; Pubellier et al. 1991; Quebral et al. 1995). According to many authors (Roeder 1977; Cardwell et al. 1980; McCaffrey et al. 1980; Moore & Silver 1983; Hawkins et al. 1985), the arc-arc collision of the Molucca Sea should be traceable onshore into Mindanao, and the Philippine fault has been interpreted as the inferred suture. However, geological observations (Mitchell et al. 1986; Pubellier et al. 1991) indicate that the Philippine fault is not a former suture although a possible suture candidate may be traced through the Cotabato basin.
Transect across the Northern Molucca Sea On the basis of the newly acquired bathymetric data (Fig. 2), this paper presents first a summary of the key features and preliminary interpretations of a NE-SW transect across the Molucca Sea south of Mindanao island. This northern transect extends from the Philippine Sea plate to the Celebes Sea immediately south of Mindanao. We then discuss a shorter but complementary transect carried out further south, around the Talaud Islands.
1. Philippine Trench to the Cotabato Trench Various morphostructural provinces are distinguished from east to west along this transect (Fig. 3).
33
COLLISION AND FAULTING IN THE N MOLUCCA SEA
7N
Fig. 3. Shaded image of the sea bottom in the northern most part of the Molucca Sea, south of Mindanao. The main morphological units of the area are shown. DB: Sarangani-Davao Depression; SB: Sangihe forearc Basin; PR: Pujada ridge; CD: Central Depression.
Outer slope of the Philippine Trench Close to 6°N, there is a dog-leg bend of the Philippine Trench. The trench trends north-south in the north and suddenly changes to a mean direction of N N W - S S E further south. North of 6°N, the outer slope shows regularly spaced west-facing fault scarps interpreted as parallel normal faults, down-thrown towards the trench, with vertical offsets which are up to 900 m (Fig. 4). The outer slope is one of the steepest outer slopes observed at a trench anywhere in the world with gradients of up to 11 o to the west. South of 6°N, the outer slope is less steep with an average gradient of 6 ° . Linear sub-parallel scarps trend at 160 ° to 170 ° and are slightly oblique to the north-south trend of the trench. The normal faults dissecting the outer slope vary in orientation from 160 ° to 140 ° and are parallel to the trench axis. In some places the faults are slightly oblique to the trench axis and trend 125 ° (Fig. 4). The very deep (9600+_100 m) V-shaped trench is characteristic of a non-
accretionary convergent margin. Its north-south trend north of 6°N is slightly en echelon. South of 6°N the trench consists of a series of en echelon basins offset along left-lateral faults resembling Riedel fractures, and its depth shallows from north to south by almost 1000 m.
The inner slope of the Philippine Trench The inner slope of the Philippine Trench, which descends eastwards towards the trench, can be divided into three morphological areas in the northern part of the area surveyed: the lower, middle and upper slope regions (Fig. 3). South of 6°N the lower and upper slope areas lose their distinct character and merge with the middle slope. The middle slope is separated from the trench by the Snellius Ridge (Figs 3 and 4).
Lower slope. The lower slope of the upper plate (Figs 3 and 4) is geomorphologically very different
34
C.
RANGIN
ET AL.
~ PHILIPPINE I~i TRENCH
,
\
Outer ~-jSlope
,
5N--
127 E Fig. 4, Bathymetric map of the Philippine Trench Inner wall. Main structures are also indicated. South of 6°N, the complex morphostructural zone between the Snellius Ridge and the Miangas ridge is the middle slope unit.
in the areas north and south of 6°N. North of 6°N it consists of an 8000 m deep terrace. In this area, the scarps at the base of the inner wall that define the front of the margin could be interpreted either as normal or thrust faults. It is very difficult to trace the true plate boundary since no decollement is observed on seismic profiles. South of 6°N a triangular-shaped terrace lies between the Philippine Trench and the middle slope. The major feature observed in this area is a 50 km linear scarp
facing the trench with a 140 ° trend. A vertical offset of more than 2 km is observable along this scarp that could be interpreted as a major normal fault. West of the scarp the lower slope dips gently towards the NE and pinches out along the trench close to 5°30'N. This surface is part of the flank of the Snellius Ridge which broadens southwards.
Middle slope.
The middle slope has an average gradient of 10 ° to the east across the whole of the
COLLISION AND FAULTINGIN THE N MOLUCCA SEA area surveyed. In the northern part of the area, north of 6°N, it is a complex morphostructural zone of interfering structures. The major characteristic of this area is a series of NW-SE trending features, interpreted as a strike-slip faults. One of these fractures appears to be the offshore extension of the Philippine fault (Quebral et al. 1995) exposed in Mindanao (Fig. 5). Discrete 045 ° trending structures are also observable and may represent a fold axial trend. South of 6°N, the orientation of the middle slope break changes from 140 ° to northsouth, and 010 ° close to 5°40'N, and then turns again to 140 ° south of 5°N. This morphology is interpreted as a combination of NW-SE trending left-lateral strike-slip faults and 010 ° trending folds and thrusts. The middle slope break is probably seismically active in the southernmost part of the area since thrusts were observed on seismic profiles. To the west, the middle slope unit is in tectonic contact with the Miangas ridge. At the base of the steep east-facing scarp of this ridge there is a narrow area of broken topography which descends over about 200-300 m to the middle slope (Figs 3 and 4). This large area (Fig. 3) has an average depth of about 3800 m between 5°30'N and 5°50'N and rises to the south and north. A number of lineaments can be seen on the bathymetric and reflectivity maps which mark irregular ridges with principal orientations of 015 °, 140 °, 170 ° and 180 °. The intersection of these ridges outline a number of small rhomb-shaped, flat-bottomed basins.
Upper slope. The upper slope is steeper than the middle slope north of 6°N and absent further south. In its southern part east-verging thrusts were imaged on seismic profiles. Further to the north, lobate bodies with edges which are convex to the east, are interpreted as the manifestation of flat east-verging thrusts crossed by NW-SE trending strike-slip faults (Figs 3 & 4).
Miangas-Pujada ridges This north-south trending bathymetric high is a complex broad feature formed by two ridges which rise from depths of 3000 m on each side (Fig. 5). To the north the western ridge continues into the Pujada peninsula and to the south the eastern ridge widens considerably to form a bank connecting southward with the Talaud Islands (Fig. 2). For most of its length this high is divided by a depression (the Central Depression of Fig. 5). In the north, the Central Depression is little more than an irregular deeper part of a single bathymetrically high area with steep east- and west-facing slopes (Figs 3 and 5). Further south the Central Depression becomes more pronounced and deeper.
35
The Miangas ridge. This eastern ridge remains high along all its length and is emergent on the island of Miangas at 5°36'N. In most of the area surveyed its depth is typically less than 1200 m. The ridge trends broadly towards 160 ° but in detail the eastern face can be seen to be composed of en echelon north- and 140 ° trending segments. It is asymmetrical in cross-section, and the highest points are on its eastern side, with a very steep east slope (average gradient 20% and a much more gently sloping west face (average gradient 9°). This ridge has a consistently high reflectivity, and seismic lines indicate an absence of sediment on most of the upper parts of the ridge and on the very steep east-facing slope. The Miangas ridge is interpreted to be thrust eastwards over the middle slope of the Philippine Trench. This interpretation is supported by preliminary seismicity data obtained in an OBS (Ocean Bottom Seismometer) network installed around Miangas Island during the MODEC Cruise. This temporary network revealed the possible presence of a west-dipping thrust plane below the ridge. This thrust is not observed on seismic profiles due to the steepness of the slope and the absence of sediments. The inferred thrust trace at the western base of the ridge is offset by 140 ° trending lineaments at 5°30'N and 5°40'N, south and north of Miangas Island respectively. These features are interpreted as left-lateral strikeslip faults offsetting the ridge (Fig. 5). The Pujada ridge. The summit of the western ridge deepens southwards, narrows and disappears at 5°05'N (Fig. 3 & 5). South of 5°30'N, the ridge turns east and has an overall orientation parallel to the base of the Sangihe arc (c. 155°). Its western slope becomes somewhat less steep and more irregular although there remains a steep section about half-way up. Its eastern face has a broad, gently east-dipping upper terrace between 2000 m and 2250 m which then descends more steeply, but irregularly, to the well-marked Central Depression with a flat surface at 3700 m. The ridge has a steep west-facing slope. The reflectivity map and the seismic lines show little sediment cover and only a few discontinuous reflectors (Fig. 6); where there are reflections they mainly dip west but no structure can be identified with confidence. It is suggested that these areas are underlain by basement rocks such as ophiolites or arc volcanics. Two principal lineament orientations, 150 ° and 050 ° are parallel to the steeper slopes on the upper part of the ridge. There are no clear offsets on these lineaments and no clear indications that they are faults on seismic lines although the 150 ° direction is close to the orientation of the western margin of the Sangihe trough and inferred axial directions of folds within
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38
C. RANGIN E T A L .
the Sangihe trough (see below). These lineaments may represent a conjugate fault/joint set within the basement. The eastern side of the Pujada Ridge is locally steeper than the western side, but includes a number of irregular terraces which become wider southwards. Medium to low reflectivity and seismic profiles indicate that there is a sediment cover although the thickness is not great (< 1 s twt). Over both the Pujada and Miangas ridges, the free-air anomaly map is well correlated with the bathymetry (Fig. 7). The slope descending from
126 E
the Sangihe arc is characterized by a regular gradient of the free-air anomaly with values decreasing towards the Sangihe forearc basin. The minimum (-80 mGal) is situated at the northern end of this basin at 5°50'N and 125°50'E, above an area of thrusting, where the sedimentary thickness is probably larger. The Pujada and Miangas ridges are underlain by north-south aligned positive anomalies separated by a low corresponding to the Central Depression. The Miangas ridge, which is topographically higher than the Pujada ridge, is
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COLLISION AND FAULTING IN THE N MOLUCCA SEA
associated with a smaller anomaly. Northwards, the two anomalies merge into a single one, as the ridges do. On the eastern edge of the Miangas ridge, a steep gradient (more than 15 mGal/km) can be correlated with its steep eastern flank.
The Sangihe forearc The Sangihe forearc basin (Figs 3 and 5) is located east of the Sangihe volcanic arc and its northern extension in Mindanao, the Sarangani peninsula, and west of prominent ridges (Pujada and Miangas) formed by ultramafic and mafic material (Fig. 3). This basin pinches out to the north in the Davao Gulf (Fig. 5), between the Sarangani and Pujada peninsulas, where we term it the Sarangani-Davao depression (Fig. 3). Between 5°37'N and 5°45'N there is a marked change in orientation of the eastern slope of the Sangihe volcanic arc, from 155 ° in the south to the 015 ° in the north. At this latitude the basin becomes wider and structurally simpler, and has a different orientation.
The Sarangani-Davao depression. With a central plain at depths of about 3200 m, the basin has an arcuate shape parallel to the Sarangani peninsula. Field work in Mindanao (Pubellier et al. 1991) has shown this arcuate peninsula is a large recumbent fold verging towards the east. The SaranganiDavao depression corresponds to the northern extension of the Sangihe Basin and sediments in the basin are folded and thrust in the Davao Gulf. This is related to the active compressive tectonics affecting the island of Mindanao (Pubellier et al. 1991; Quebral et aL 1995). At the western edge of the basin, the slope of the eastern edge of the Sarangani peninsula is very steep (Fig. 5) and is interpreted as the manifestation of a buried thrust. In the central part of the basin, three broadly northsouth trending, arcuate, and west-verging thrust anticlines are well marked in the bathymetry (Figs 3 & 5). Reverse faults or thrusts are observed along their eastern flanks. The free-air anomalies deduced from six gravity profiles across the basin (Fig. 7) show an asymmetric form with a clear minimum (-80 reGal) close to Sarangani shoreline on the west side of Davao Gulf. This anomaly is correlated with the thickness of sediment, suggesting a flexure of the basin due to loading, by overthrusting from the west, of the Sarangani peninsula. The Sangihe basin. Seismic profiles across this basin show at least 2 s twt of well-stratified sediments (Fig. 6). Buried folds with a clear westvergence are observed on seismic profiles, with an axial trend of 155 ° . This direction is parallel to bathymetric features at the basin margins. South of 5°25'N the seabed surface in the basin changes from a flat smooth surface to an irregular surface
39
cut by numerous channels. Silver & Moore (1978) extended their East Sangihe Thrust through this part of the Sangihe trough. Data presented here do not support the presence of any west-verging thrust along the western edge of the forearc basin.
Sangihe backarc and Cotabato Trench East of the Sangihe arc, the survey was extended into the Celebes Sea and along the southeast termination of the Cotabato Trench. At about 5°N the Cotabato Trench bends from north-south to an approximately ENE-WSW orientation parallel to the southern coast of Mindanao. In the Celebes Sea basin the ship track crossed ODP sites 767 and 770 (Fig. 8). South of 4°50'N the feature corresponding to the continuation of Cotabato Trench is a broad flatbottomed area, sloping gently northwards. A large meandering canyon running south to north down to 5°N probably originates from the large island of the Sangihe group indicating the trench dies out morphologically further south. To the east volcanoes are aligned north-south on the western flank of the Sangihe arc. At 4°40'N a conical seamount, probably belonging to the nearby Sangihe volcanic arc, rises 3500 m above the sea floor. Between 4°50'N and 5°20'N the Cotabato Trench bends from an orientation of 000 ° in the south to 120 ° in the north. The trench floor deepens from 5100-5900 m as it approaches the Sangihe arc. At the northern end of the north-south trending section (north of 4°50'N), which is marked by a 100 reGal negative gravity anomaly, a narrow NNE-trending accretionary wedge is observed at the foot of the steep western flank of the Sangihe arC.
At the trench bend there is an intermediate zone which trends 145 ° and is separated from the Sangihe arc by a non-linear ridge and a plateau which could be the physiographic equivalent of an outer arc high. A NNW-SSE trough 5100 m deep, is parallel to these on their landward side. Here, the trench-fill sediment sequence thickens significantly up to 2 s twt. A re-entrant in the wedge associated with bending of an anticline axis is interpreted as the effect of a subducted asperity (ridge or seamount) on the sea bottom. This is supported by the presence of a 1600 m high curved scar observed in the bathymetry of the wedge at 5°20'N, 124°50'E, which could be attributed to the collapse of the margin following the subduction of a seamount. Along the Cotabato active margin, which trends at 120 °, the lower slope consists of a series of anticlines, 20-30 km apm't, with curved axial traces which are concave upslope, forming a wide foldand-thrust belt typical of an accretionary wedge.
40
C. RANGIN ET AL.
The trend of the anticlines is roughly parallel to the trench axis with an en echelon pattern in several places. An outer arc high, 400-500 m in elevation, separates the accretionary complex from what is suspected to be a forearc basin with an upper surface dipping slightly towards the southeast. A large gravity low is located over the accretionary wedge probably reflecting a thick sedimentary prism. The 4600-5500 m deep abyssal plain of the Celebes Sea basin shows several ridges and seamounts sitting on the oceanic crust. On seismic profiles the basin floor has a very rough undulating surface (amplitude of a hundred metres) which possibly reflects the original grain of the oceanic crust overlain by no more than 0.5 s twt thick sediment cover. The first sediments deposited onto the basement were dated at 43 Ma during ODP Leg 124 (Rangin et al. 1990b). In the vicinity of the trench no clear normal faults were observed on seismic lines. The bathymetric map (Fig. 8) shows lineaments, ridges and troughs trending 110 ° in the northwestern part of the area surveyed. An elongated positive feature, 1.5 km above the sea floor and without a magnetic signature, interpreted as a group of seamounts, trends WSW, parallel to the magnetic lineations proposed by Weissel (1980) in the basin. Another major feature, trending northsouth, is more than 100 km long and l0 km wide and rises 1 km above the sea floor.
Ridge and its lower slopes form two terraces (Fig. 9) which occupy a triangular area which is narrow in the north and widens southward. The eastern margin of the West Talaud Bank is bounded by west-verging thrusts following the base of the cliffs of the Talaud ridge where a second OBS temporary network has been deployed (Fig. 9). The West Talaud Bank is thus thrusted towards the west above the undeformed Sangihe trough.
Talaud ridge The Talaud ridge forms a high area between the 2000 m bathymetric contours. Little of the ridge appears on the map (Fig. 9) because of the very shallow depths of the ridge summit. In the northern part of the area surveyed the Taland and Nanusa Islands are part of the same wide ridge, but further to the south the Talaud ridge narrows and swings southeastward. These trends are reflected by the shape of the islands' coastlines. On seismic profiles the ridge lacks any continuous reflectors and a very thin sediment cover was observed above basement rocks, possibly ophiolites or arc volcanic rocks, as indicated by the geology of the Talaud Islands on land (Sukamto et al. 1980; Moore et al. 1981). Both east and west flanks are fault controlled. These are most likely to be thrust faults because the fault contacts are sub-parallel to the curved isobaths.
East Talaud Bank
2. Talaud Islands to Morotai Basin A second transect, orientated NW-SE, south of the Talaud Islands, was surveyed in the deep waters between the Morotai Basin and the Sangihe trough. It was chosen to be perpendicular to the regional morphology. South of 5°N, the Miangas ridge merges into a very shallow area covered by reefs around the Nanusa Islands (Fig. 9). To the east, the Snellius Ridge which flanks the Philippine Trench between 4°50'N and 5°30'N, becomes very shallow south of 4°50'N. Consequently, most of the area at the latitude of the Talaud Islands was not surveyed because it is too shallow. Various morphological provinces were identified from west to east.
Sangihe trough Only a small part of the undeformed Sangihe forearc basin (Sangihe trough on Fig. 9) was surveyed in this area (Fig. 9). The flat part of the trough is much narrower here than in the north. The area deeper than 3500 m is less than 25 km wide. The trough is slightly deeper in its central parts and reaches depths of 4000 m. The West Talaud Bank rises from the Sangihe trough towards the Talaud
The eastern margin of the Talaud ridge is a wide area, named the East Talaud Bank, which connects northwards to the middle slope of the Philippine Trench inner slope(Figs 3, 9, 10). South of 3°55'N, the East Talaud Bank is very wide (about 35 km) and has an irregular surface which is convexupwards. North of 3°55'N, it becomes considerably narrower (about 18 km wide), and forms a northsouth trending feature between depths of 1500 and 3000 m. South of 4°N (Fig. 9), the bank is separated from the Talaud ridge by NW-dipping thrusts. Seismic profiles show that the East Talaud Bank resembles the West Talaud Bank which is interpreted as the deformed margin of the Sangihe Basin to the west. Along the east flank of the bank, a major low angle thrust can be traced southwards from 4°40'N to 4°00'N where it has a north-south trace; it then turns, first towards 140 ° , and then further south towards 030 °. There are at least 3 s twt of sediments in the slice above the thrust plane, which carries the East Talaud Bank eastwards onto the Snellius Ridge, an observation reported previously by Silver & Moore (1978). The Talaud ridge, and the East and West Talaud Banks on each side, are cut by 140 ° scarps that
41
COLLISION AND FAULTING IN THE N MOLUCCA SEA
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could be strike-slip fault zones. This is supported by the evidence of earthquakes in this area with strike-slip focal mechanisms (McCaffrey 1991). The progressive offset of morphological units towards the northwest suggests left-lateral motion on these faults.
Snellius Ridge East of the main thrust separating it from the East Talaud Bank, the Snellius Ridge is a large submarine plateau sloping to the west with a gradient of 3-6 °. South of 3°40'N (Fig. 9) the plateau is dissected by SW-facing fault scarps trending 140 °. These features also dissect the northern margin of the plateau (Fig. 9) and are interpreted as the trace of major strike-slip fault zones (see above). Well
developed reflectors at the top of the Snellius Ridge could be platform carbonates capping the plateau. The positive gravity anomaly over the Snellius Ridge suggests a dense crust compatible with an ophiolitic or arc origin for the basement. In the southern and deepest end of the Snellius Ridge is the Morotai Basin. This basin has an elongate shape with its major axis orientated 070 ° , and has a maximum depth of about 3500 m. Along its northwestern margin, it is thrust westwards onto the Snellius Ridge, and folds trending 075 ° defornl the basin sediments. These structures terminate abruptly to the west at the curved thrust zone where the Morotai Basin margin is thrusted on top of the Snellius Ridge. Further south the basin is filled by more than 1.5 s twt of almost undeformed sediments. There is no evidence for significant
COLLISION AND FAULTINGIN THE N MOLUCCA SEA deformation at the southern side of the basin close to Morotai Island within the area surveyed, although a little further south the sea bottom rises rapidly from depths of more than 3000 m to emerge at the steep fault-controlled coasts of the island. The southwest end of the Snellius Ridge is situated between two convergent thrust fault zones, and is beneath the East Tataud Bank and the Morotai Basin (to the NW and SE respectively). The superficial convergent thrusts in this area are thus compatible with interpretations at depth that show two convergent subduction zones and an inverted U-shape of the subducted Molucca Sea oceanic slab. More precise correlations need to be done between the superficial data collected during the cruise and structures at depth.
Discussion During the MODEC cruise in the northern Molucca Sea it was possible to trace the Sangihe forearc
E 1 2 3 20.
E124
E125
43
continuously over 500 km from north to south. Between the eastern edge of the forearc and the Philippine Trench, three major crustal units or terranes were also mapped almost continuously from north to south (Fig. 10), and are interpreted to be separated by major lithosphere-scale boundaries.
Miangas-Pujada-Talaud unit The Miangas-Pujada-Talaud unit is probably formed by ophiolite slivers and melanges; it can be traced from the Pujada peninsula to the Talaud Islands, with a major offset between Miangas and Talaud north of the Nanusa Islands. The PujadaMiangas ridge is onlapped westward by the sediments of the Sangihe forearc basin; further south at the southern end of the Talaud ridge there are west-vergent thrusts moving the ridge onto the edge of the Sangihe forearc basin. This ridge could be interpreted as the backstop of the Sangihe subduction zone, implying it is underlain by
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Fig. 10. Overall interpretation of the northern Molucca Sea. The shaded area is the Miangas-Pujada-Talaud unit probably formed by ophiolite slivers and melanges. Immediately east of this is the middle slope unit.
44
C. RANGIN ET AL.
Sangihe forearc basement rather than oceanic crust of the subducted Molucca Sea. The MiangasPujada-Talaud unit is separated from the Philippine Trench middle slope terrace by a major discontinuity that could be a lithospheric-scale thrust. Seismic focal mechanisms associated with this thrust are clearly compressive with east-west P axes. This thrust can be traced south at least to 3°N at the foot of the Talaud ridge. Middle slope unit The middle slope unit, lying immediately to the east of the Pujada-Miangas ridge, is formed by low density material as indicated by the free-air gravity data. The middle slope unit can be traced southward into the East Talaud Bank which is thrusted eastwards onto the Snellius Ridge, almost certainly a fragment of the Halmahera arc terrane. North of 5°30'N, this thrust fault approaches the Philippine Trench. A broad gravity low trending north-south, about 100 km long and with minimum values of -260 mGal is centred on the middle slope unit (Fig. 7) and is clearly shifted to the west with respect to the Philippine Trench (-220 mGal). This anomaly cannot therefore be attributed to the classical gravity low associated with deep trenches. The juxtaposition of the low density material of the middle slope unit in the north and the high density material of the Snellius Ridge in the south, could be explained by the presence of a major crustal discontinuity trending 140 ° . The orientation of isogals on the free-air gravity map above the middle slope unit changes from north-south to NW-SE approaching the northern edge of the Nanusa Islands and the Snellius Ridge (Fig. 7). The discontinuity is also marked by a pronounced NW-SE steep gravity gradient. It is also subparallel to the orientation of the Philippine Trench south of 6°N (Fig. 10). Such a gradient and discontinuity is also clear in recent satellite FAA data. This supports the presence of a major crustal-scale fault zone separating two contrasting terranes. The middle slope unit could be composed of accreted sediments, and be part of the Sangihe accretionary wedge, with the Snellius Ridge representing part of the Halmahera arc. In this interpretation the boundary between the colliding arcs would be the thrust at the base of the middle slope and East Talaud Bank. An alternative interpretation is to consider this unit as a part of the Halmahera arc like the Snellius Ridge to the south. The middle slope unit would then represent an intra-arc basin. Such contrasting features are known within the Halmahera arc system to the south (Hall et al. 1988a, b; Nichols & Hall 1991). In this case the major west-dipping thrust located east of the Miangas-Talaud ridge would be the
main tectonic boundary between the Sangihe and Halmahera arcs. The major crustal discontinuity trending I40 ° which offsets the Pujada-Miangas ridge relative to the Talaud ridge is interpreted as a left-lateral strike-slip fault zone. Philippine Trench The dog-leg shaped Philippine Trench has two distinct segments. The north-south trending segment north of 6°N is the trace of the recently established west-dipping subduction zone as indicated by the short subducted slab. The trend of this trench segment is completely independent of the dominant oblique NW-trending structures observed in the inner slope. In contrast, south of 6°N, the N W - S E trending segment of the Philippine Trench is parallel to the interpreted left-lateral strike-slip fault zones observed in the whole forearc area. The southern segment of the Philippine Trench is tentatively interpreted as the trace of a major left-lateral strike-slip fault zone dissecting the Halmahera terrane. The small faults in Riedel position observed along this part of the trench support this hypothesis. However, thrust fault focal mechanisms of seismic events reported by Ranken et al. (1984) along this trench segment reveal this strike-slip fault zone is now turning into a thrust (Nichols et al. 1990). This suggests a migration of the Philippine Trench towards the south along pre-existing strike-slip fault zones. It is also possible that, although focal mechanisms support a thrust interpretation of this fault zone, the main long-term motion is strike-slip, possibly aseismic.
Conclusion The area surveyed in the northern Molucca Sea reveals the presence of two distinct geodynamic settings east of the Miangas-Pujada-Talaud ridge. A major NW-SE trending tectonic boundary separates these two areas. South of 6°N, the Snellius Ridge, a volcanic plateau capped by carbonates, is subducting below the Sangihe forearc terrane. The buoyancy of this plateau, a fragment of the Halmahera arc terrane, has induced deformation in the upper and lower plates (Talaud ridge and Morotai Basin). Its buoyancy also induces incipient subduction along a new plate boundary, the Philippine Trench. This nettectonic feature is developed along a former strikeslip fault zone cross-cutting the Halmahera terrane. North of the major NW-SE crustal discontinuity crossing the area surveyed, a wide accretionary prism is present within the inner slope of the present Philippine Trench. The thickness of this wedge cannot be explained by subduction along
COLLISION AND FAULTING IN THE N MOLUCCA SEA this trench because it is very recent (Cardwell et al. 1980; Quebral et al. 1995). This wedge could be interpreted either as an accretionary wedge developed at the deformed leading edge of the Sangihe forearc or be part of a former intra-arc basin which was part of the colliding Halmahera arc terrane. There is no evidence from gravity data of dense material comparable to the Snellius Ridge below this wedge. If the ridge extended originally north of 6°N, it could have been already subducted to some greater depth, or be considerably displaced left-laterally, as suggested by the presence of a similar buoyant terrane in central Mindanao (Pubellier et al. 1991). This survey also reveals no clear evidence for
45
accreted oceanic crust belonging to the recently subducted Molucca Sea. The M i a n g a s - P u j a d a Talaud Ridge is clearly in a back-stop position for the Sangihe arc, and the ophiolites of this ridge could therefore have very different origin from either the Molucca Sea or the Halmahera arc. This work was possible due to the co-operation programs between France, the Philippines and Indonesia. We want to thank particularly E. Domingo in the Bureau of Mines and Geosciences in Manila and Dr Zen of BPPT in Jakarta. The active co-operation of the L'Atalante crew and Genavir team was particularly appreciated. We thank particularly Captain G. Tredunit for his enthusiasm for the project and his active participation in contributing to the success of this cruise.
References
ALLEN, C. R. 1962. Circum-Pacific faulting in the Philippine-Taiwan region. Journal of Geophysical Research, 67, 4795-4812. AURELIO,M. A., BARRIER,E., RANGIN,C. & MULLER,C. 1991. The Philippine Fault in the Late Cenozoic evolution of the Bondoc-Masbate-Leyte area, central Philippines. Journal of SE Asian Earth Sciences, 6, 221-238. CARDWELL,R. K., ISACKS,B. L. & KARIG,D. E. 1980. The spatial distribution of earthquakes, focal mechanism solutions and subducted lithosphere in the Philippine and northeastern Indonesian islands. In: HAYES, D. E. (ed.) The Tectonic and Geologic Evolution of South-east Asian Seas and Islands. American Geophysical Union Monograph, 23, 1-35.
HALL, R., AUDLEY-CHARLES, M. G., BANNER, E T., HIDAYAT,S. 8,~ TOBING, S. L. 1988a. The basement rocks of the Halmahera region, east Indonesia: a Late Cretaceous-Early Tertiary forearc. Journal of the Geological Society, London, 145, 65-84. & --. 1988b. Late Paleogene-Quaternary geology of Halmahera, eastern Indonesia: initiation of a volcanic island arc. Journal of the Geological Society, London, 145, 577-590. HAMILTON, W. 1979. Tectonics of the Indonesian region. US Geological Survey Professional Paper, 1078. HAWKINS,J. W, MOORE,G. E, VILLAMOR,R., EVANS,C. & WRIGHT, E. 1985. Geology of the composite terrane of east and central Mindanao. In: HOWELL, D. G. (ed.) Tectonostratigraphic Terranes of the CircumPacific Region. Circum-Pacific Council for Energy and Mineral Resources, Earth Sciences Series, 1, 437-463. HOLLOWAY, N. H. 1982. The stratigraphic and tectonic evolution of Reed Bank, North Palawan and Mindoro to the Asian mainland and its significance in the evolution of the South China Sea. AAPG Bulletin, 66, 1357-1383. KRAUSE, D. C. 1966. Tectonics, marine geology and bathymetry of the Celebes-Sulu Sea region. Geological Society of America Bulletin, 77, 813-831.
LEWIS, S. D. & HAYES, D. E. 1983. A geophysical study of the Manila Trench, Luzon, Philippines: forearc basin structural and stratigraphic evolution. Journal of Geophysical Research, 89, 9196-9214. MCCAFFREY, R. 1982. Lithospheric deformation within the Molucca Sea arc-arc collision: evidence from shallow and intermediate earthquake activity. Journal of Geophysical Research, 87, 3663-3678. - 1991. Earthquake and ophiolite emplacement in the Molucca Sea collision zone, Indonesia. Tectonics, 10, 433-453. --, SILVER, E. A. & RAITT, R. W. 1980. Crustal structure of the Molucca Sea collision zone, Indonesia. In: HAYES,D. E. (ed.) The Tectonic and Geologic Evolution of South-east Asian Seas and Islands. American Geophysical Union Monograph, 23, 161-177. MITCHELL, A. H. G., HERNANDEZ, E t~ DE LA CRUZ, A. E 1986. Cenozoic evolution of the Philippine archipelago. Journal of SE Asian Earth Sciences, 1, 3-22. MOORE, G. E & SILVER,E. A. 1983. Collision processes in the northern Molucca Sea In: HAYES,D. E. (ed.) The Tectonic and Geologic Evolution of South-east Asian Seas and Islands. American Geophysical Union Monograph, 27, 360-372. - - - , KADARISMAN,D., EVANS,C. A. & HAWKINS,J. W. 1981. Geology of the Talaud Islands, Molucca Sea collision zone, northeast Indonesia. Journal of Structural Geology, 3, 467-475. NICHOLS, G. J. • HALL, R. 1991. Basin formation and Neogene sedimentation in a backarc setting, Halmahera, eastern Indonesia. Marine and Petroleum Geology, 8, 50-61. , --, MILSOM, J., MASSON,D., PARSON, L. ET AL. 1990. The southern termination of the Philippine Trench. Tectonophysics, 183, 289-303. PUBELLIER,M., QUEBRAL,R., RANGIN,C., DEFFONTAINES, B., MULLER, C., BUTTERLIN,J. MANZANO,J. 1991. The Mindanao, collision zone, a soft collision event within a continuous strike-slip setting, Journal of Southeast Asian Earth Sciences, 6, 239-248. QUEBRAL,R., PUBELLIER,M. & RANGIN,C. 1995. Eastern Mindanao, Philippines: transition zone from a
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collision to strike-slip environment. Tectonics, (in press). RANGIN, C., JOLWET, L. & PUBELHER, M. 1990. A simple model for the tectonic evolution of southeast Asia and Indonesia region for the past 43 m.y. Bulletin de la Soci(t( g~ologique de France, 8 VI, 889-905. , SILVER, E. A., VON BREYMANN,M. T. ET AL. 1990. Proceedings of the Ocean Drilling Program Initial Reports, 124. RANKEN, B., CARDWELL, R. K. & KARIG, D. E. 1984. Kinematics of the Philippine Sea Plate. Tectonics, 3, 555-575. ROEDER, D. 1977. Philippine arc system - collision or flipped subduction zone? Geology, 5, 203-206. SENO, T., STEIN, S. A. & GRIPP, A. E. 1993. A model for the motion of the Philippine Sea plate consistent
with NUVEL-1 and geological data. Journal of Geophysical Research, 98, 17 941-17 948. SILVER, E. A. & MOORE, J. C. 1978. The Molucca Sea collision zone, Indonesia. Journal of Geophysical Research, 83, 1681-1691. SUKAMTO,R., SUWAR~O,N., YUSUP, J. & MONOARFA,M. 1980. Geologic map of the Talaud Islands, 1:250 000. Geological Research and Development Centre, Bandung, Indonesia. WEISSEL,J. K. 1980. Evidence for Eocene oceanic crust in the Celebes basin. In: HAYES, D. E. (ed.) The Tectonic and Geologic Evolution of South-east Asian Seas and Islands. American Geophysical Union Monograph, 23, 37--47. WILLIS, B. 1937. Geologic observations in the Philippine islands. Natural Resources Council of the Philippines Bulletin, 13.
Continental collision in the Banda arc A. N. R I C H A R D S O N 1 & D. J. B L U N D E L L
Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 OEX, UK 1 Present address." Shell Internationale Petroleum Maatschappij B. V., Postbus 162, 2501 A N Den Haag, The Netherlands Abstract: Two deep seismic reflection profiles across the zone of convergence between Australia and the Banda arc east of Timor reveal the structures formed during lithosphere deformation in response to continental collision. Australian continental crust is bent down to the north to form the lower lithospheric plate. The immediately overlying upper plate is made up of a former outlier of the Australian continental shelf, now squeezed between the Australian continent to the south and the Banda arc to the north. The ages of metamorphism and of a regional unconformity in Timor indicate that the outlier began to accrete to the upper plate in the late Miocene. Near the Timor trough, at the junction between the two plates, seismic reflection data show the upper plate to be cut by north-dipping structures interpreted as thrusts in an accretionary prism formed since the arrival of the Australian continental shelf at the collision zone at about 2.5 Ma. The northern part of the collision zone is dominated by structures dipping southwards, antithetic to subduction, which penetrate the lithosphere to depths of at least 50 kin. Earthquake data indicate that these are active thrust faults which divide the upper plate into imbricate slices. The seismic reflection sections, supported by gravity data and seismicity patterns, have been used to develop a model of plate convergence and continental collision which now consists of a 200 km wide zone in which both oceanic and continental material is being shortened, thickened and uplifted.
The Banda arc lies at the intersection of the SE Asian and Indo-Australian plates (Fig. 1). Over the past few million years, Australian continental material has arrived at and collided with the oceanic plate of the Banda Sea in eastern Indonesia. The collision has progressed to such an extent that continental lithosphere has subducted under oceanic lithosphere to a depth of over 100 km. The collision of continental material with the subduction zone has given rise to a 'collision complex'. This forms a ridge, in places rising above sea-level (e.g. Timor), sub-parallel to the volcanic arc. Over the years, three main structural models, resulting mainly from near-surface observations, have been proposed for Timor: (1) The Imbricate Model (Fig. 2a; Fitch & Hamilton 1974; Hamilton 1 9 7 9 ) i s based largely on marine geological and geophysical data (e.g. von der Borch 1979; Silver et al. 1983; Karig et al. 1987). In this model, Timor is interpreted as an accumulation of chaotic material imbricated against the hanging wall of a subduction trench, the Timor trough, and essentially forms a large accretionary prism. (2) The Overthrust Model (Fig. 2b) is probably the oldest model in which Timor was interpreted in terms of Alpine-style thrust sheets (e.g.
(3)
Wanner 1913). This model was dominantly based on surface geology where overthrust sheets of the Timor allochthon are well exposed. Subsequent workers (Carter et al. 1976; Barber et al. 1977; Barber 1979; Haile et al. 1979; Brown & Earle 1983; AudleyCharles 1981, 1986a, b; Price & AudleyCharles 1983, 1987; Harris 1989; AudleyCharles & Harris 1990) have made a clear distinction between allochthonous units of non-Australian origin and parautochthonous units derived from the Australian continent. The Rebound Model (Fig. 2c; Chamalaun & Grady 1978) suggests that the Australian continental margin entered a subduction zone in the vicinity of the Wetar Strait. Subsequently, the oceanic lithosphere detached from the continental part, resulting in the uplift of Timor by isostatic rebound on steep faults.
Combinations of these models have also been proposed (e.g. Charlton 1989; Charlton et al. 1991; Harris 1992) in which the parautochthon is divided into two parts, reflecting the step-wise nature of the collision. The main part, the 'underplated' parautochthon, is thought to have accreted in the early stages of collision by the sequential addition by underplating of imbricate thrust slices of the
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolution of SoutheastAsia, Geological Society Special Publication No. 106, pp. 47-60.
47
48
A. N. RICHARDSON • D. J. BLUNDELL o
o
o
South
0
10 ° 125 °
126 °
127 °
East
128 °
129 °
130 °
Fig. 1. Location map of the BANDA deep seismic reflection survey lines DAMAR and TIMOR, with shot point numbers shown in italic. Earthquake epicentres shown as stars; numbers relate to McCaffrey (1988), table l, letters a, b and c to events of 30 May 1988, 8 August 1988 and 21 May 1991 (Harvard centroid-moment tensor solutions). Barbed line represents the plate boundary with barbs in the direction of subduction. Arrow indicates relative plate motion. Bathymetry in metres.
outermost Australian continental margin to the base of the forearc complex. A second phase of collision is thought to have commenced in the Early Pliocene with the addition to southern Timor of younger parts of the Australian continental margin, the 'frontally accreted' parautochthon.
Data acquisition and processing The British Institutions Reflection Profiling Syndicate (BIRPS) joined with the Indonesian Marine Geological Institute to record two long, multichannel normal-incidence reflection profiles across the Banda arc west of Timor. The two subparallel traverses of the arc were designed to image the deep structure of the central part of the collision zone (Fig. 1). The proximity of the two lines to each other and to Timor and the geologically contiguous Minor Islands to the east increased confidence that observed reflections represent regional structural features consistent with surface
geology. The survey ship used was M/V GECOKappa which towed a 92-channel hydrophone streamer 4.6 km long. The airgun array of 7324 in 3 (120 litres) capacity was 70 m wide, designed to produce 97 bar m peak-to-peak pressure at frequencies of 3-62.5 Hz. Along the western line, 'TIMOR', shots were fired at 50 m intervals with 50 m receiver groups producing records up to 23 s two-way time (TWT) to show reflections at relatively high spatial resolution down to about 45 k m depth. The eastern line, ' D A M A R ' , was located where deep earthquakes are frequent and was therefore recorded to 37 s with 100 m shot intervals. This was designed to compare reflectors, resolved slightly less well than in the T I M O R profile, with earthquake hypocentres and mechanisms down to around 140 km depth. The seabed topography along both profiles is rugged and ranges from < 100 m to >4000 m below sea-level. This produced several sets of strong
CONTINENTAL COLLISION IN THE BANDA ARC NORTH
(a)
SOUTH
Imbricate Model Volcanic Arc
(b)
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Australian Continental Shelf
Overthrust Model Volcanic Arc
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49
normal moveout curves for reflections at two-way travel times longer than around 10 s, equivalent to depths of around 25 km. For greater depths, indirect evidence from refraction experiments in the Timor trough and Banda Sea (Bowin et al. 1980), and from a general knowledge of p-wave velocities in the upper mantle had to be used for the depth conversion.
Interpretation Australian Continental Shelf (TIMOR SP 6400-8000, DAMAR SP 1-750, Figs 3 & 4)
(c)
Rebound Model
~
Volcanic Arc ~ ~
-timer
Australian Continental Shelf
Fig. 2. Models proposed for the structure of Timor (after Barber 1981).
seabed multiples, which partly obscure primary reflections from deeper levels, and a high level of random noise. The multiples were so easily recognized on the brute stack that they could be distinguished from primary reflections passing through them. Attempts to remove these multiples for the final stack sections were largely successful, but at the same time probably degraded the primary reflections. The brute stacks displayed the clearest images of primary reflections, multiples and diffraction patterns, and so were used more than the final stacks in preparing line drawing interpretations. Following the interpretation method of many other deep seismic experiments, discontinuous linear segments of reflections identified as primary on the seismic sections were traced on to a transparent film overlay. These line drawings more clearly delineate the overall structure and are treated as an interpretation of the geometries of the main reflections (Fig. 3). The principal reflections interpreted on the line drawings were depthcorrected and migrated in the plane of the section to give their depth geometry (Fig. 4). The only direct velocity inforhaation available was provided from the moveout analysis of reflected data from the 4.6 km long streamer. This offset was insufficient to discriminate between
The upper 2.1 s TWT beneath the seabed of the continental shelf portions of both sections consist of fairly parallel reflectors interpreted as sedimentary bedding. This is consistent with the tie of the TIMOR line with the Troubadour No. 1 oil exploration well (Fig. 1) which penetrated Recent to Triassic marine sediments lying unconformably on granite encountered at a depth of 3315m, equivalent to 2.16 s TWT. Extensional faults are evident in the upper 3 s TWT of both sections and appear to penetrate the crystalline basement as well as cut the entire sedimentary cover. These faults were probably initiated in the Jurassic when the Australian Northwest Shelf was last stretched (Bradshaw et al. 1988; Powell 1982). In the region of Sumba Island, where subduction is continuing at present, many faults that were originally related to rifting and spreading are still in active extension and are giving rise to earthquakes with extensional mechanisms. Some strong, north-dipping reflectors were also identified in the otherwise transparent upper crust (Fig. 3). These reflectors appear to sole out of the Jurassic faults to cut through the entire upper crust and continue into a reflective zone in the lower crust. The upper continental crust consists of crystalline rocks so it is most likely that these reflectors represent faults, following the interpretation of other deep seismic reflection surveys elsewhere (e.g. Matthews et al. 1990). Between 8 and 12 s TWT at the southern end of the TIMOR section and 7 and 14 s TWT at the southern end of the DAMAR section lies a zone of intense, sub-horizontal, laterally extensive, layered reflections. The base of this zone is consistent with an increase in interval velocity from 6.3-7.3 to over 8 km s-1 at this level on both profiles (Fig. 5) and corresponds well with the normal jump in p-wave velocity across the Moho to 8.1 km s-1 in the upper mantle (Fowler 1990). The velocity structure and character of reflection patterns in this zone is therefore much like that of the lower continental crust imaged by a number of
20
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Fig. 3. Line drawing interpretations of the BANDA seismic sections: (a) Timor; (b) Damar. Major reflectors and zones of high reflectivity are highlighted with dotted lines.
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Fig. 4. (a) Interpretation of the major structures recognized on the TIMOR profile (Fig. 3a) after depth correction and migration, and their association with earthquake data. Earthquake hypocentral locations, projected along the regional strike (073°E), are shown as black dots. Fault plane solutions are shown as hemisphere projections in the vertical plane of the profile. Upper crust: light grey; lower crust dark grey. (b) Interpretation of the major structures recognized on the DAMAR profile (Fig. 3b) after depth correction and migration, and their association with earthquake data.
other deep seismic reflection surveys from other areas (e.g. Mooney & Brocher 1987). The base of this zone of high reflectivity has been identified as the Moho (originally by Fuchs 1969) and it would be reasonable to apply the same interpretation here. This reflectivity within the lower continental crust has been associated with crustal extension, involving basaltic sill emplacement (Serpa & Voogd 1987; Warner 1990) and/or ductile shear zones (Reston 1987). Since this part of the Australian continent became a rifted margin in the Jurassic (Bradshaw et al. 1988; Powell 1982), it would be reasonable to assign the sub-horizontal reflectors in the lower crust to this age. On this basis the crustal thickness at the southern end of both sections is 35-40 km, consistent with a crustal thickness of 30-40 km determined in this area using seismic refraction data (Bowin et al. 1980).
The other observed deformation of the crust is a gentle flexure as it is bowed down beneath the Timor trough, bringing the Moho down to a depth there of 45-50 km.
Subduction Trench (TIMOR SP 6400-6450, D A M A R SP 700-750) Undisturbed, horizontally bedded sediments of 544 m thickness (0.48 s TWT) are present in the Timor Trough at the base of the trench on the TIMOR profile. Using a sedimentation rate of 480 m/Ma in the trough axis obtained from analysis of DSDP-262 (Johnston & Bowin 1981), situated SW of Timor, these sediments accumulated in a period of c. 1.1 Ma during which little convergence
20
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CONTINENTAL COLLISION IN THE BANDA ARC has taken place. However, immediately south of the horizontal sediments, a small, north-dipping reflector observed on TIMOR at SP 6500 is interpreted as a thrust which has created a hanging wall anticline and delaminated the upper 0.6 s TWT of sediments, sliding them 200-400 m to the south. Other thrusts have formed beneath. A similar situation is observed on the DAMAR profile. Thus a stick-slip system of convergence appears to be operating SE of Timor in much the same way as has been observed in the Timor trough by Karig et al. (1987) SW of Timor. Interval velocities in excess of 5 km s-1 are encountered beneath the trench at 3 s and 2 s TWT below the seabed on the TIMOR and DAMAR profiles respectively (Fig. 5). This is in approximate agreement with refraction data (Bowin et al. 1980). Seismic velocities appropriate for granitic basement are thus encountered at about the same depth below the seabed both at the trench and on the continental shelf at the southern end of the two profiles. The crust, measured from the base of the highly reflective region, appears somewhat thicker at the trench than at the southern end of both sections (Fig. 4). Thickening could imply that the crust has shortened on, or just prior, to its arrival at the subduction trench. However, upper crustal shortening through inversion of extensional faults offsetting sedimentary layers is not evident, except within the trench. The zone of high reflectivity, interpreted as representing the lower crust, appears to terminate abruptly just north of the subduction trench in the DAMAR section. If the high reflectivity of the lower crust is interpreted as a structural fabric of Jurassic age, this would be destroyed if it were folded and sheared by compression. In this case, reflectivity would be drastically reduced, which could explain its abrupt termination.
Accretionary Prism (TIMOR SP 5800--6400, D A M A R SP 750-1100) A few kilometres north of the trench, reflectors within 4 s TWT below the seabed are less coherent. This is probably because of the disrupted nature of the sedimentary package. Deeper reflectors tend to be swamped by strong seabed multiples on the brute stack and noise on the final stack. Nevertheless, a series of southward-verging duplex structures is visible on both sections within what appears to be an accretionary prism (Fig. 3). This interpretation is consistent with the results of seismic reflection surveys to the west and southwest of Timor and south of Sumba (Karig et al. 1987; Van tier Werff et al. 1994) and southeast of
53
the Tanimbar Islands (Jongsma et al. 1989; Richardson 1994). Thus there is evidence that a proportion of the sedimentary sequence is accreted to the underside of the developing accretionary prism within a few kilometres of the trench. In this portion of both TIMOR and DAMAR profiles there is a general increase in seismic velocity from south to noah within 4 s TWT below the seabed (Fig. 5). This is probably the result of compaction and de-watering of the sedimentary package as it is broken up and accreted to the upper plate. North-dipping reflectors co-linear with sedimentary reflectors dipping beneath the trench appear to extend only c. 30 km north of the subduction trench on both profiles. North of this, regions of reflectors parallel to sedimentary reflectors dipping beneath the trench appear to occur in discrete 'packages' offset by prominent south-dipping reflectors (Fig. 4). Shallower packages appear to be offset to the north of deeper packages. The upper surface of these packages can be followed downwards in a relatively straight line to the base of the profiles and extrapolated to c. 140 km depth to join the northern (top) surface of the plate defined by earthquake hypocentres which has been defined in this area by McCaffrey (1989). There are three alternative interpretations for the actual physical nature of the reflectors within the packages. The north-dipping reflectors may represent: (1) remnant sedimentary material attached to the lower plate that has escaped accretion to the collision complex and has been carried to great depth; (2) faults or shear zones representing the zone of decoupling between the upper and lower plates; (3) accreted (underplated) sedimentary rocks. This last interpretation was adopted by Clowes et al. (1987) for similar deep reflectors dipping parallel to the subduction zone under Vancouver Island and by Moore et al. (1991) for the eastern Aleutian Islands, but the other two possibilities appear equally plausible. Whichever is preferred, the packages of reflectors may be interpreted as broadly representing the upper surface of the Australian plate beneath the collision complex. Therefore, it does not appear that the slab has detached above about 140 km depth.
Central Collision Complex (TIMOR SP 3200-5800, D A M A R SP 1100-2350) North of the accretionary prism, where the reflectors dip mainly to the north, both profiles pass into a region of predominantly south-dipping reflections. This geometry is similar to that seen on
54
A. N. RICHARDSON & D. J. BLUNDELL
high quality oil industry seismic reflection profiles in the Tanimbar Islands region made available by Union Texas (SE Asia) Inc. (Richardson 1994) and in the forearc region around Sumba, along strike from westem Timor (van der Werff et al. 1994). The transition zone between north-dipping and south-dipping reflectors appears complex in most cases. The south-dipping reflectors represent either sedimentary layers or low-angle faults because they do not have the characteristic shape of diffraction tails. They are unlikely to represent sedimentary layering because they appear fairly continuous to considerable depth whereas there is no sign of wellbedded sediments just below the seabed. On the TIMOR line (SP 32004400, 4700-5100), the patterns of the south-dipping reflectors are reminiscent of large-scale thrust sheets (Fig. 4). Similar features are observed on the seismic reflection profiles around the Tanimbar Islands where south-dipping reflectors terminate upwards in antiform folds. On the DAMAR profile, major south-dipping reflectors appear to offset northdipping packages of reflectors to the north (Fig. 3), consistent with north-vergent thrust faulting involving the subducted plate. In both the BANDA and Union Texas profiles, interval velocity inversions coincide with these south-dipping reflectors (Fig. 5). If a region with a normal velocity gradient were deformed into a stack of thrust sheets, velocity inversions could be expected to develop at the base of some thrust sheets where high velocity material overrides low velocity material. Shallow earthquakes occur in this area and McCaffrey (1988) has calculated hypocentres and focal mechanisms for a number of them (Figs 1 & 4). Because they have been projected up to 60 km onto the profile it is impossible to correlate particular earthquakes with individual reflectors. However, it is clear that there is a reasonable association and the dominantly thrust mechanisms of the earthquakes support the other evidence that the reflectors represent thrusts. The reflectors' appearance, their coincidence with interval velocity inversions and association with thrust mechanism earthquakes all suggest that they represent north-vergent thrusts which appear to reach depths of tens of kilometres, possibly penetrating into the mantle. Similar reflectors observed elsewhere have been interpreted as shear zones and ancient subduction zone traces (Matthews et al. 1990). The crest of the upper thrust sheet at SP 3900 on the TIMOR profile (Fig. 3) is a topographic feature of the seabed and forms the eastward, submarine extension of the island of Kisar. Kisar is offset from the trend of the main archipelago and consists of tightly folded felsic schists and mylonites which
can be correlated with the Aileu Formation of northern Timor (Richardson 1994). The presence of the thrust on the TIMOR profile in alignment supports the idea that Kisar may have been uplifted on an out-of-sequence thrust. However, the structural grain in Kisar runs approximately N-S (Richardson 1994), at right angles to the structural trend of the other islands. It is possible that Kisar has rotated by 90 ° during thrusting, or alternatively that it forms a lateral ramp. The seabed high at SP 3400 on the TIMOR section (Fig. 3) is mid-way between Kisar and the volcanic island of Romang. South-dipping reflectors with velocity inversions (Figs 3a & 5a) visible at SP 3200-3400 are interpreted as thrust faults. Interval velocities below the topographic high at TIMOR SP 3400 are similar to those below SP 3900 but are generally lower than the next topographic high to the north at SP 2900. The crustal thickness appears to be 18-23 km. The topographic high at SP 3400 on the TIMOR section is therefore interpreted to be similar in composition to Kisar and so may be the northernmost occurrence of continental rocks at the surface along this profile. Dating features identified on deep seismic reflection profiles is one of the most intractable problems within the field of deep seismics (Klemperer et al. 1990) unless a reflection can be traced to outcrop or there is other independent evidence. Since none of the south-dipping structures on TIMOR and DAMAR have been identified before, let alone recognised at the surface, dating them must rely on more speculative geometrical observations. Consequently, the cross-cutting geometric relationship of the south-dipping reflectors with the packages of north-dipping reflectors is used as evidence for their age. The packages of north-dipping reflectors are interpreted as representing the upper surface of the Australian plate beneath the collision complex. The amount of subduction is thought to have been extremely limited south of western Timor for the last 0.6 Ma (Karig et al. 1987). Horizontal sediments at the trench axis on the TIMOR profile imply that subduction has been inactive there for the last 1 Ma apart from a few hundred metres of south-vergent thrusting. The shallowest cross-cutting, southdipping structure on the TIMOR and DAMAR profiles cuts the boundary of north-dipping reflectors about 30 km north of the trench axis. Assuming the rate of convergence to be decreasing (Johnston & Bowin 1981), it would have taken this part of the lower plate c. 1.4 Ma to have reached its present location. Hence, this thrust cannot be older than c. 2.4 Ma which is approximately the age of onset of the current collision derived from evidence in the Timor trough south of western Timor (Johnston & Bowin 1981).
CONTINENTAL COLLISION IN THE BANDA ARC The top of the north-dipping lower plate and the deepest of the south-dipping thrusts define the collision orogen as a triangular-shaped wedge (Fig. 4). Sedimentary material is being added to the southern end of the wedge by underplating on north-dipping thrusts. However, the northern portion of the collision orogen is deformed on south-dipping, north-vergent thrust sheets. The exact depth of the Moho is difficult to constrain for this portion of the profiles because the poor resolution of the velocity picks leads to significant variations between adjacent values of interval velocity. However, assigning the base of the crust to the boundary between interval velocities less than and greater than 8000 ms -l, the crust appears to be thickest directly under the topographic highs: SP 4600 (> 46 km), SP 4000 (c. 30 km) and SP 3400 (> 36 km) on the TIMOR section and SP 1600 (c. 50km) and SP 2150 (c. 20 km) on the DAMAR section (Fig. 5). A 2D density model was produced to match a gravity profile along the TIMOR line recorded in a previous survey by Woodside et al. (1989). Although poorly constrained, the model serves to delimit the approximate thickness of the crust in this region. Two end-member versions of the model were produced. In one (Fig. 6a), which defines a minimum value for crustal thickness, the crust of the central collision complex was made 30 km thick, the same as the continental crust of the Australian Northwest Shelf. A reasonable fit
55
between calculated and observed gravity was achieved. In the second (Fig. 6b), crust with 2 6 7 0 k g m -3 density representing the central collision complex was enlarged at the expense of an upper mantle region with 3 1 0 0 k g m -3 density, whilst maintaining the fit of calculated with observed gravity. The second model gives an upper limit for which a reasonable match can be made with the observed gravity data. In this, crust of the central collision complex attains a maximum thickness of nearly 60 km.
Volcanic Arc (TIMOR SP 1700-3200, DAMAR SP 2350-2900) At the northern end of both seismic sections, submarine volcanoes associated with the subductionrelated volcanic islands of the Banda arc form prominent topographic highs. The volcanoes are said to be built on oceanic crust on the basis of seismic refraction data (Bowin et al. 1980). Long sub-horizontal reflectors just below the seabed are interpreted as delineating lava flows (D. Snyder, pers. comm., 1995). The TIMOR profile crosses the main volcanic ridge at SP 2950. Volcanoes at SP 2000 and SP 2350 on the TIMOR profile lie to the north of the main volcanic ridge and have a steepsided, narrow trough between them at SP 2130 (Fig. 3a). This sharp trough lies along strike from the surface expression of the Wetar Thrust visible
Gravityo (mgal) /
•
t: 1::o°o °
"1°°I -200 2670
~
~
i
o
2800
i
2800
50
3300 \,........ lOO
(km) 150 2o0
/ ?
// /.s
3300 150
3
3200
2OO
/ 2~o
2~6
S 1~o Distance
~o
.=b
from Trench(km)
~
-~o
-lOO
loo Distance
3200 ~o
~
fromTrench(km)
-~o
250
Fig. 6. Gravity models to constrain estimates of crustal thickness in the central part of the Banda orogen. Densities of polygons are in kg m-3.
56
A. N. RICHARDSON 8Z; D. J. BLUNDELL
on sonar imagery (Masson et al. 1991). A southdipping reflector reaches the surface at the base of this trough and can be traced 50 km southwards to a depth of c. 20 km. Two earthquakes with thrust mechanisms project on to the TIMOR section within the horizontal range of this reflector (Fig. 4a) although with significantly greater hypocentral depths. It may be significant, however, that the south-dipping nodal planes of both these events dip at an angle close to the migrated dip of the reflector. This reflector is therefore interpreted as evidence for the along-strike extension of the Wetar Thrust, associated with current movement. A number of major, south-dipping reflectors are also present in this portion of the DAMAR section which extend close to the base of the seismic record (Fig. 3b). These structures therefore appear to be the dominant fabric of the upper plate on both profiles. Some shorter north-dipping reflectors are present on both profiles but none are as long or as continuous as the south-dipping reflectors (Fig. 4). The volcanic arc portion of the DAMAR profile includes the greater proportion of earthquakes less than 100 km deep along the transect (Fig. 4b). Most of these earthquakes have thrust mechanisms, with one nodal plane which has a south-dipping component, and plot close to the major reflectors. It is not possible to correlate specific earthquakes with individual reflectors because of the large envelope of uncertainty in hypocentral locations and the distance that they have been projected on to the section. However, the fact that earthquakes with thrust mechanisms are occurring in this region does suggest that the south-dipping reflectors may represent active thrust faults which penetrate the crust and upper mantle to depths of up to 100 km. On the TIMOR profile, the crustal thickness (defined where Win t > 8000 m s-1) varies from c. 12 km to over 20 km beneath the volcanoes. This is abnormally thick for oceanic crust, even including the volcanic edifices. Interval velocities > 8000 m s-1 are attained at depths of 8-19 km below sea-level on the DAMAR profile (Fig. 5b) which implies that the crust is thinner than on the TIMOR profile but still thicker than normal oceanic crust.
The backarc region (TIMOR SP 1-1700, D A M A R SP 2900-3200) A number of marine geophysical surveys have been conducted in the backarc region. From Lombok to Maopora, the backarc region is dominated by south-dipping reflectors beneath the volcanoes, sometimes overlain by an accretionary prism (Silver et al. 1983). Likewise, the south-dipping reflectors in this part of the BANDA profiles are
interpreted as thrusts because of their appearance on seismic profiles. The two most northerly volcanoes on the TIMOR section, at SP 400 and SP 1200, are isolated seamounts built on the oceanic crust of the Banda Sea. The northernmost edifice rises above sea-level to form the active volcano, Gunung Api. It is not clear why this volcano is active so far to the north of the subduction zone. The volcanoes traversed by the TIMOR profile appear to line up with Gunung Api and the seamount in between them (Fig. 1). The NNW-SSE lineation of seamounts suggests that they may have formed over a 'leaky' transform fault (D. Snyder, pers. comm. 1992). The DAMAR profile passes close to one of the seamounts at SP 2600 and directly over another at SP 2800, a few kilometres to the north. The basin areas in between the volcanoes show clear sedimentary layering near the surface and long, gently dipping reflectors down to the depth of the first multiple (Fig. 3). The velocity pick at TIMOR SP 1599 in the centre of one of these basins is interpreted as representing 1840 m of sediments (cf. oceanic lithosphere Layer 1, Vint = 1900-2400m s-1) and 5860 m of volcanics and intrusives (oceanic lithosphere Layers 2 and 3, Vin t - - 4 2 1 7 m s-1) overlying upper mantle material (oceanic lithosphere Layer 4, Vin t -- 8058 m s-1) at 7.7 km below the sea-bed. Most velocity picks at the extreme northern end of the DAMAR profile have velocities typical of the upper mantle (Vin t > 8000 m s-1) occurring 3 . 7 4 km below the seabed. All these velocity structures broadly agree with seismic refraction data (Bowin et al. 1980) and are therefore interpreted as representing normal oceanic crust. The northern part of the DAMAR line appears to traverse rather rough sea floor. However, one of the major south-dipping thrusts reaches the surface in this region and the rough topography may represent a small accretionary prism.
Summary The southern end of both profiles, which traverse the Australian Continental Shelf, is interpreted in a similar manner to deep seismic reflection profiles from other passive continental shelf areas. Wellstratified sediments overlie a largely reflectionfree upper crustal crystalline basement and a highly reflective lower crust. This arrangement is common in continental regions that have undergone extension. Structures at shallow depths under the northern slope of the trench (or the southern part of the collision zone) dip exclusively northwards and are interpreted to represent thrusts and tilted sediments in an accretionary prism. Deeper, north-dipping
CONTINENTAL COLLISION IN THE BANDA ARC structures are interpreted to be related to the interface between the lower and upper plates. This is supported by the extrapolation of this surface to greater depths where it corresponds well with the upper surface of the Australian plate as defined by earthquakes. The northern part of the collision zone is dominated by predominantly south-dipping structures which extend to sub-crustal depth. These are interpreted as thrusts along which the lateral shortening and crustal thickening have taken place. The thrusts are estimated to be no older than 2.4 Ma, the age of initiation of the collision of the Australian margin with the arc, and seem to be active at the present day at the northern end of the collision zone. A maximum crustal thickness of c. 60 km is calculated under the topographically highest part of the collision zone on the basis of calculated seismic interval velocities. If this crust had been constructed from rifted and thinned Australian margin material, the crust may now be more than double its original thickness which would imply over 50% (100-150 km) shortening in the continental portion of the collision zone. The oceanic crust also appears to be anomalously thick beneath the volcanoes on the basis of calculated seismic interval velocities. It appears likely that the oceanic crust has, like the continental crust, been shortened and thickened on predominantly south-dipping thrusts. The fact that these major thrusts are visible beneath and north of the volcanic arc suggests that the collision zone extends north of the non-volcanic arc and is in total about 200 km wide, comprising both continental and oceanic lithosphere, and may have accommodated over 200 km of shortening. The overall, large-scale character of the collision zone is dominated by two sets of divergent struc-
N
57
tures. The southern set is related to the subducting (lower) plate and dips in the same direction as subduction. The northern set, in the upper plate, is antithetic to this. Figure 7 is a cartoon to highlight these primary features.
The deep-seated nature of the orogen In considering the evolution of the collision complex as an orogen, three dates are significant: (1) The Banda allochthon in Timor was apparently uplifted at c. 37 Ma according to K-Ar dating (Sopaheluwakan 1990). (2) Accurate radiometric dating on the Aileu Formation on the north coast of Timor gives an age of metamorphism associated with gradual cooling of 8 Ma (Berry & Grady 1981) and a Mid-Miocene unconformity has been inferred across Timor (Audley-Charles 1968). (3) Pliocene deformation is consistently recorded by sedimentary sequences elsewhere in Timor (Audley-Charles 1968) and the start of the most recent collision has been dated at 2.4 Ma from evidence in the Timor trough (Johnston & Bowin 1981). The extent of crustal material in the central collision complex is defined by seismic velocity and gravity modelling. The crustal material has the form of an inverted triangle about 50 km (37-60) deep and 125 km (135-160) across on the TIMOR profile, which has a cross-sectional area of 3750 km 2. This thickness of crustal material could, conceivably, have been constructed by a number of means. However, it cannot have been constructed entirely from sediments accreted from the Australian plate since 2.4 Ma. Using the plate velocity vectors in DeMets et al. 1990 (Fig. 1),
Australian Continental S Timor Micro-Continent AccretionaryPrism ~li,helf _ Kisaj~ ~, iSubd ,.)., ~lt~ ctionTrench , , ~
Volcanic Arc ~1~ ~
~60 km
Moho ~"
/~5. /
~i~;~ ~ ; ~
0
km
1O0
I
I
I
]
Fig. 7. Cartoon structural cross-section of the Banda orogen based on our interpretation of the BANDA deep seismic profiles. Australian continental shelf sediments are shaded light grey. Highly reflective lower crust is shaded dark grey.
58
A.N. RICHARDSON t~ D. J. BLUNDELL
156 km (65 km/Ma for 2.4 Ma) of convergence normal to the arc could have occurred. This is an upper estimate as it does not take account of decreasing convergence velocity after collision. An upper estimate of the thickness of the sediments between the Australian coast and Timor from the Banda profiles is about 3 km. Assuming that this entire thickness of sediments is added to the growing orogen and that strike-slip movement has not removed material from the normal plane, a cross-sectional area of 468 km 2 would result (3 × 156 km). This is far less than the estimated crustal area of 3750 km 2. A collision event, adding crustal material to the orogen, must therefore have preceded the Pliocene collision event. Nor is it possible that the Banda orogen was constructed entirely from sediments accreted from the Australian Plate since 8 Ma. Using the same argument as above, a sediment volume accumulated over 8 Ma of convergence would amount to only 1560 km 2 in cross-sectional area, which is still substantially less than the estimated 3750 km 2 of crust. There must therefore be an additional piece of crystalline continental crustal material within the collision complex making up a major portion of its bulk. It is thus proposed that a micro-continental fragment, or perhaps an outer margin 'high', lay a few hundred kilometres to the north of the Australian Northwest Shelf and this was incorporated into the collision complex before the arrival of the Australian continental margin. The material separating the micro-continent from the Australian craton was probably oceanic in origin but could have been extremely stretched and attenuated continental lithosphere. This micro-continental fragment collided with the subduction zone at c. 8 Ma in the Miocene. This caused metamorphism
of the Aileu Formation at the leading edge of the micro-continental fragment ( ' m e t a m o r p h o s e d parautochthon' on Timor and Kisar) and folding in the unmetamorphosed rocks. The micro-continent was thrust beneath the forearc region, thus uplifting the Banda Allochthon, and may have induced Late Miocene inversion structures reported from the Timor Sea (MacDaniel 1988; Pattillo & Nicholls 1990). The Australian continental margin arrived at the collision zone 2.4 Ma ago and started downwarping into the subduction zone. The continental crust could not be subducted very far. Subsequently, the Australian continental margin has acted as a bulldozer, shortening and uplifting continental and oceanic parts of the collision zone on deep seated thrusts dipping antithetic to subduction. The oldest thrusts are presumably the more southerly ones. Since then the locus of activity has propagated northwards in a normal fashion until it has reached the backarc region where thrusts are active at the present day. Uplift is currently continuing as evidenced by several hundred metres of elevation of Pliocene coral reef terraces on Alor, Atauro and Wetar (Nishimura & Suparka 1986) and Sumba (Piranzolli et al. 1991). The authors wish to thank Dr D. Snyder for his major efforts in conducting the BANDA survey and supervising the data processing, and to him and their other BIRPS colleagues for ready access to the deep seismic reflection data. BIRPS is supported by the Natural Environment Research Council (NERC) and all BIRPS data are available, at the cost of reproduction, from the Marine Geophysics Programme Manager, British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA. ANR is grateful to NERC for the award of a studentship during the period of this research.
R e f e r e n c e s
AUDLEY-CHARLES, M. G. 1968. The Geology of Portuguese llmor. Geological Society, London, Memoir, 4. 1981. Geometrical problems and implications of large-scale overthrusting in the Banda ArcAustralian Margin collision zone. In: MCCLAY, K. R. & PRICE, N. J. (eds) Thrust and Nappe Tectonics. Geological Society, London, Special Publications, 9, 407-416. - 1986a. Timor-Tanimbar Trough: the foreland basin to the evolving Banda Orogen. In: ALLEN, P. A. 8£ HOMEWOOD, P. (eds) Foreland Basins. International Association of Sedimentology Special Publication, 8, 91-102. - 1986b. Rates of Neogene and Quaternary tectonic movements in the southern Banda Arc based on micropalaeontology. Journal of the Geological Society, London, 143, 161-175. - - & HARRIS,R. A. 1990. Allochthonous terranes of the
South West Pacific and Indonesia. Philosophical Transactions of the Royal Society of London, A331, 571-587. BARBER, A. J. 1979. Structural interpretations of the island of Timor. South East Asian Petroleum Exploration Association Proceedings, 4, 9-12. 1981. Structural interpretations of the island of Timor, eastern Indonesia. In: BARBER, A. J. WIRYOSUJONO,S. (eds) The Geology and Tectonics of Eastern Indonesia. GRDC Special Publication, 2, 183-198. - - - , AUDLEY-CHARLES,M. G. & CARTER, D. J. 1977. Thrust tectonics in Timor. Journal of the Geological Society of Australia, 24, 51-62. BERRY, R. E & GRADY, A. E. 1981. The age of the major orogenesis in Timor. In: BARBER, A. J. & WIRYOSUJONO,S. (eds) The Geology and Tectonics of Eastern Indonesia. GRDC Special Publication, 2, 171-181.
CONTINENTAL COLLISION IN THE BANDA ARC BOWlN, C., PURDY, G. M., JOHNSTON, C., SHOR, G., LAWVER,L., HARTONO,H. M. S. & JEZEK,P. 1980. Arc-continent collision in the Banda Sea region. AAPG Bulletin, 64, 868-918. BRADSHAW, M. T., YEATES, A. N, BEYNON, R. M., BRAKEL,A. T., LANGFORD,R. P., TOTTERDELL,J. M. & YEUNG, M. 1988. Paleogeographic evolution of the North West Shelf region. In: PURCELL,P. G. & PURCELL, R. R. (eds) The North West Shelf Australia. Proceedings of the NW Shelf Symposium 1988, 29-54. BROWN, M. & EARLE, M. M. 1983. Cordierite-bearing schists and gneisses from Timor, Eastern Indonesia: P-T implications of metamorphism and tectonic implications. Journal of Metamorphic Geology, 1, 183-203. CARTER, D. J., AUDLEY-CHARLES,M. G. & BARBER,A. J. 1976. Stratigraphical analysis of island arccontinental margin collision in eastern Indonesia. Journal of the Geological Society, London, 132, 179-198. CHAMALAUN, E H. & GRADY, A. 1978. The tectonic development of Timor: a new model and its implications for petroleum exploration. Australian Petroleum Exploration Association Journal, 18, 102-108. CHARLTON, T. R. 1989. Stratigraphic correlation across an arc--continent collision zone: Timor and the Australian Northwest Shelf. Australian Journal of Earth Science, 36, 264-274. ~, BARBER, A. J. & BARKHAM, S. T. 1991. The structural evolution of the Timor collision complex, eastern Indonesia. Journal of Structural Geology, 13, 489-500. CLOWES, R. M., BRANDON,M. T., GREEN,A. G., YORATH, C. J., SUTHERLANDBROWN, A., KANASEWICH,E. R. & Spencer, C. 1987. LITHOPROBE - Southern Vancouver Island: Cenozoic subduction complex imaged by deep seismic reflections. Canadian Journal of Earth Science, 24, 31-51. DE METS, C., GORDON,R. G., ARGUS,D. E & STEIN, S. 1990. Current plate motions. Geophysical Journal International, 101, 425-478. FITCH, T. J. & HAMILTON,W. 1974. Reply to AudleyCharles & Milsom (1974). Journal of Geophysical Research, 79, 4982. FOWLER, C. M. R. 1990. The Solid Earth. Cambridge University Press. Fucns, K. 1969. On the properties of deep seismic reflectors. Zeitschriftfiir Geophysik, 35, 133-149. HAILE, N. S., BARBER, A. J. & CARTER, D. J. 1979. Mesozoic cherts on crystalline schists in Sulawesi and Timor. Journal of the Geological Society, London, 136, 65-70. HAMILTON,W. 1979. Tectonics of the Indonesian region. US Geological Survey Professional Paper, 1078. HARRIS, R. A. 1989. Processes of aUochthon emplace-
ment, with special reference to the Brooks Range Ophiolite, Alaska and Iimor, Indonesia. PhD thesis, University of London. 1992. Temporal distribution of strain in the active Banda Orogen: a reconciliation of rival hypotheses. Journal of SE Asian Earth Sciences, 6, 373-386. JOmqSTON, C. R. & BOWIN, C. 1981. Crustal reaction
59
resulting from the mid-Pliocene to Recent continent-island arc collision in the Timor region.
Australian Bureau of Mineral Resources Journal of Geology and Geophysics, 6, 223-243. JONGSMA,D., WOODSIDE,J. M., HUSON,W., SUPARKA,S. & KADARISMAN,D. 1989. Geophysics and tentative Late Cenozoic seismic stratigraphy of the Banda Arc-Australian continent collision zone along three transects. Netherlands Journal of Sea Research, 24, 205-229. KARIG, D. E., BARBER, A. J., CHARLTON, T. R., KLEMPERER,S. & HUSSON~,D. M. 1987. Nature and distribution of deformation across the Banda Arc-Australian collision zone at Timor. Geological Society of America Bulletin, 98, 18-32. KLEMPERER, S. L., HOBBS, R. W. & FREEMAN,B. 1990. Dating the source of lower crustal reflectivity using BIRPS deep seismic profiles across the Iapetus Suture. Tectonophysics, 173, 445-454. MACDAmEL, R. P. 1988. The geological evolution and hydrocarbon potential of the western Timor Sea region. Australian Petroleum Exploration Association Journal, 28, 270-284. MASSON, D. G., MILSOM, J., BARBER, A. J., SIKUMBANG, N. & DWlYANTO,B. 1991. Recent tectonics around the island of Timor, eastern Indonesia. Marine & Petroleum Geology, 8, 35-49. MATTHEWS, D. & THE BIRPS GROUP. 1990. Progress in BIRPS deep seismic reflection profiling around the British Isles. Tectonophysics, 173, 387-396. MCCAFFREY, R. 1988. Active tectonics of the eastern Sunda and Banda Arcs. Journal of Geophysical Research, 93, 15 163-15 182, 1989. Seismological constraints and speculations on Banda Arc tectonics. Netherlands Journal of Sea Research, 24, 141-152. MOONEY, W. D. & BROCHER, T. M. 1987. Coincident seismic reflection/refraction studies of the continental lithosphere: a global review.
Geophysical Journal of the Royal Astronomical Society, 89, 1-6. MOORE, J. C., DIEBOLD, J., FISHER, M. A., SAMPLe, J., BROCHER, T. ET AL. 1991. EDGE deep seismic reflection transect of the eastern Aleutian arc-trench layered lower crust reveals underplating and continental growth. Geology, 19, 420-424. NISHIMURA, S. & SUPARKA,S. 1986. Tectonic development of East Indonesia. Journal of SE Asian Earth Science, 1, 45-57. PATT~LLO, J. & NlCnOLLS, P. J. 1990. A tectonostratigraphic framework for the Vulcan graben, Timor Sea region. Australian Petroleum Exploration Association Journal, 30, 27-51. PIRANZOLLI, P. A., RADTKE, U., HANTORO, W. S., JOUANNIC, C., HOANG, C. T., CAUSSE,C. & BOREL BEST, M. 1991. Quaternary raised coral reef terraces on Sumba Island, Indonesia. Science, 252, 18341837. POWELL, D. E. 1982. The North West Australian continental margin. Philosophical Transactions of the Royal Society of London, A305, 45--62. PRICE, N. J. & Atn)LEY-CHARLES, M. G. 1983. Plate rupture by hydraulic fracture resulting in overthrusting. Nature, 306, 572-575.
60
A. N. RICHARDSON •
& -1987. Tectonic collision processes after plate rupture. Tectonophysics, 140, 121-129. RESTON, T. J. 1987. Spatial interference, reflection character and the structure of the lower crust under extension. Results from 2D seismic modelling. Annales Geophysicae, 5 (ser. B), 339-48. RICHARDSON, A. N. 1994. Lithospheric structure and dynamics of the Banda Arc, Eastern Indonesia. PhD thesis, University of London. SERPA, L. 8z VOOGD,B. DE 1987. Deep seismic reflection evidence for the role of extension in the evolution of continental crust. Geophysical Journal of the Royal Astronomical Society, 89, 55-60. SILVER, E. A., REED, D., MCCAFFREY,R. & JOYODIWIRYO, Y. 1983. Back arc thrusting in the eastern Sunda Arc, Indonesia: a consequence of arc-continent collision. Journal of Geophysical Research, 88, 7429-7448. SOPAHELUWAKAN, J. 1990. Ophiolite obduction in the
-
-
Mutis Complex, Timor, eastern Indonesia: an
D. J. BLUNDELL
example of inverted, isobaric, pressure metamorphism. PhD
medium-high
thesis, Free University, Amsterdam. VAN DER WERFF, W., KUSNIDA, D., PRASETYO, H. & WEERING, TJ. C. E. VAN 1994. Origin of the Sumba forearc basin. Marine and Petroleum Geology, 11, 363-374. YON DER BORCH, C. C. 1979. Continent-island arc collision in the Banda Arc. Tectonophysics, 54, 169-193. WANNER,J. 1913. Geologie von West Timor. Geologische Rundschau, 4, 136-150. WARNER, M. 1990. Basalts, water, or shear zones in the lower continental crust? Tectonophysics, 173, 163-174. WOODSIDE, J. M., JONGSMA,D., THOMMERET,M., STRANG VAN HEES, G. & PUNTODEWO. 1989. Gravity and magnetic field measurements in the eastern Banda Sea. Netherlands Journal of Sea Research, 24, 185-203.
Geophysical evidence for local indentor tectonics in the Banda arc east of Timor D A V I D B. S N Y D E R 1, J O H N M I L S O M 2, & H A R D I
PRASETYO 3
1BIRPS, Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK 2 Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK 3 Marine Geological Institute, J1. Dr. Junjunan 236, Bandung 40174, Indonesia Abstract: Two deep seismic reflection profiles and gravity measurements collected across the
Banda arc east of Timor help to characterize crustal and uppermost mantle structures in the region where arc-continent collision is thought to be furthest advanced. Reflectors beneath the Sahul Platform of the Australian shelf indicate geometries consistent with older extensional rift structures overprinted by more recent horizontal shortening. To the north, the negative Bouguer gravity feature associated with the southern parts of the accretionary complex is unusually broad and deep. Post-collisional sediments are thin where this feature is crossed by the seismic lines, implying that older sediments or the crust are anomalously thick. Still further north, the forearc basin is notably narrow near eastern Timor and contains few sediments, most undeformed. The backarc region to the north is remarkable for the presence of a N-S trending line of seamounts culminating in an active volcano, Gunung Api, which is situated 400 km above the WadatiBenioff zone. Reflection profiles across and along the seamount chain indicate underplating at Moho depths and a fault which appears to have acted as a conduit for basalt eruptions at the surface. Collectively, these observations imply anomalously thick and bouyant crust beneath the Banda arc east of Timor, and suggest two possible causes. Either a local promontory in the irregular boundary of the Australian craton was underthrust beneath the volcanic arc and forearc to 50-70 km depths or a Palaeozoic basin similar to the nearby Bonaparte Basin was underthrust and its former crustal structure inverted and thickened to form the buoyant crust. The new seismic reflection data help to locate the anomalously thick crust implied by gravity anomalies by defining the leaky fracture zone containing Gunung Api. It is inferred that the fracture propagates "ahead of the indenting wedge of underthrust, thickened Australian crust.
The Banda arc between Indonesia and Australia possesses some of the best examples of features associated with the collision of an arc and a continental margin (e.g. Hamilton 1979, 1988; McCaffrey 1988), and also displays distinctive segmentation along strike (Silver et al. 1983; Karig et al. 1987; Jongsma et al. 1989; Harris 1991). A pair of recent deep seismic reflection profiles were sited across one of these distinctive segments, immediately to the east of the island of Timor, in order to better characterize structural geometries throughout the lithosphere (Fig. 1). Focal mechanisms of large shallow earthquakes (McCaffrey 1988) and shallow seismic reflection profiles (Silver et al. 1983) and recent GPS measurements (Genrich et al. 1994) indicate that the Wetar thrust, a northward verging thrust on the northern edge of the inactive volcanic arc (Fig. 1), today absorbs most of the 75 mm a-1 (DeMets et al. 1990) relative convergence between Australia and the Banda Sea. In contrast, shallow seismic reflection profiles within the Timor Sea show reflector geometries in
Mesozoic and Cenozoic sediments that are typical of active subduction zone trenches worldwide (Karig et al. 1987). The Timor trough cannot simultaneously absorb plate convergence and transfer the motion of Australia to the volcanic arc. Loci of horizontal shortening have probably migrated northward across the 2 0 0 k m wide convergence zone during the past 2 Ma, with specific faults, active for short periods, acting as mechanisms of strain partitioning (e.g. McCaffrey 1992, 1996). A series of balanced cross-sections of the accretionary complex showed that m a x i m u m horizontal shortening, and hence the most evolved arc-normal convergence, has occurred in eastern Timor (Johnston & Bowin 1981; Harris 1991). Eastern Timor lies midway along the inferred convergence margin of the Australian continent with SE Asia (Fig. 1). Low 3He-4He ratios in volcanic rocks east of Flores were interpreted as due to the subduction and melting of continental crust at c. 150 km depths whereas to the west the
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 61-73.
61
62
D.B. SNYDERETAL
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Fig. 1. Regional location map showing the Archaean blocks on the Australian mainland (shading: ST = Sturt, K = Kimberley), principal landmasses, the 200 m bathymetric contour (short dash) that approximately defines the limit of the continental shelf, the 3 km contour (long dash) that approximates the limits of oceanic crust in the Indian ocean, the new seismic reflection lines (heavy solid lines), older refraction profiles (medium solid lines) with crustal thickness indicated in kilometres, and the 100, 300, and 500 km contours of the Wadati-Benioff zone (Cardwell & Isacks 1978). Large solid dots indicate the transition from subducted continental crust to subducted oceanic crust (to the west) within the subduction zone as determined from He isotope analysis of erupted lavas (Hilton et al. 1993). The vector labelled AUS---~SEA represents the local relative convergence vector between Australia and Eurasia (DeMets et al. 1990). C = Canning Offshore Basin, L = Londonderry High, A = Ashmore Platform, V = Vulcan sub-basin, P = Petrel sub-basin of the Bonaparte Basin, S = Sahul platform, M = Malita Graben, MS = Money Shoal Graben, B = Browse Basin, and WT = Wetar Thrust.
He source was inferred to be subducted oceanic (MORB) rocks (Hilton et al. 1992). The change in isotope ratios coincides with strike-slip faults at the surface (Breen et al. 1989) and crustal-scale fault planes defined by micro-earthquakes (McCaffrey et al. 1985) which are co-linear with the 3 k m bathymetric contour defining the N W margin o f the Australian continental shelf (Fig. 1). Thus defined, the western edge of the subducted continental margin lies c. 300 k m west of the study area. A similar distance along the strike of the volcanic arc
to the east of eastern Timor, the Wadati-Benioff zone (Fig. 1) curves sharply back onto itself to the N W (Cardwell & Isacks 1978). Here the gross Australia-SE Asia convergence vector (DeMets et al. 1990) is almost parallel to the volcanic arc, i m p l y i n g largely strike-slip d i s p l a c e m e n t s on structures parallel to the arc (see also McCaffrey 1996). If eastem Timor represents the most evolved segment of the Banda arc, local anomalously thickened crust or thick crust inherited from a
LOCAL INDENTOR TECTONICS IN THE BANDA ARC former promontory on the Australian shelf margin may be the cause. The current complex tectonic geometries associated with Irian Jaya prevent precise reconstruction of the former continental margin east of present-day Tanimbar, but New Guinea is thought to represent a large indentor wedge of Australian continental crust plowing into and bending the margin of SE Asia (Pigram et al. 1989). The east Timor region may contain a smaller promontory on the western margin of this larger indentor, but one that produced significant localized shortening across the Banda arc. Evidence for a promontory which has already subducted or underthrust the Banda arc must be sought using reflection and refraction profiles, gravity field anomalies and bathymetric observations. This paper describes and discusses this evidence. The variability in crustal structure observed in the unsubducted part of the Australian shelf south of Timor serves as the starting point, and it suggests similar complexity in subducted parts. Inversion of subducted sedimentary basin structures and thickening of sub-basin crust previously thinned during Devonian-Permian rifting provides an additional mechanism that can produce anomalously thick and buoyant crust east of Timor.
Structures of the northern australian shelf The Australian continental shelf south of Timor is covered by a relatively dense network of deep seismic reflection profiles plus gravity, magnetic and bathymetric measurements (e.g. O'Brien et al. 1993; AGSO North West Shelf Study Group 1994). Sedimentary basin structures are well understood, but crustal thickness is poorly constrained. The nearby Australian mainland contains two large Archaean cratons, the Kimberley and Sturt blocks (Fig. 1). Immediately offshore between these two blocks and continuing northwards to the Malita Graben lies the Bonaparte basin with its Petrel and Vulcan sub-basins. It, and the Canning Basin to the southwest, form composite Late Devonian to Early Carboniferous intra-cratonic rift basins with axes perpendicular to the Australian margin and associated with NE-SW extension (AGSO North West Shelf Study Group 1994). Mid-Carboniferous to Early Permian rifting that led to break-up of Gondwanaland and separation of 'Sibumasu' (Sengor 1987) from NW Australia and the opening of 'Neo-Tethys' (Veevers 1988), also initiated a series of generally NE-trending depocentres which constitute the Westralian Superbasin of Yeates et al. (1987). The superbasin's internal platforms and basins are orientated parallel to the continental margin and thus angularly super-
63
imposed on the older radial rift basins. In the study area these later features include the Sahul and Ashmore Platforms, which show little evidence of Mesozoic extension, and the Malita, Vulcan and Browse Basins (Fig. 1). The start of the break-up phase and the subsequent development of the continental margin was recorded as a major Lower-Middle Carboniferous unconformity. MidCarboniferous to Lower Permian basin fill may actually be a sag-phase sequence deposited following break-up and the 100-400% extension thought to have characterized this rift episode (Etheridge & O'Brien 1994). The Sahul Platform occupies c. 30 000 km 2 of the Australian shelf immediately south of eastern Timor. The Malita Graben separates it from offshore parts of the Sturt block of the Australian mainland (Fig. 1). Seismic reflection profiles and drill holes indicate that c. 6000 m of Carboniferous(?) and as much as 10 000 m of later sediments were deposited in the Malita Graben, confirmed by a core of 3300 m of post-Permian sediments on the Sahul Platform (AGSO North West Shelf Study Group 1994). The uniformity of sediment cover on the Sahul Platform suggests thermal subsidence rather than subsidence synchronous with rifting and implies little thinning of this crustal block. To the north, very young structures associated with convergence tectonics overprint older extensional structures (Figs 3 & 4). Although no evidence exists on seismic reflection sections to indicate thicknesses of post-Permian sediments as great as those observed in the Malita Graben, reflectors beneath the Jurassic and Permian levels indicate a possible older basin north of the Sahul Platform and currently underlying the Timor trough (Figs 3 & 4).
Crustal thickness estimates In contrast to the wealth of information available about the upper crustal and basin structure of the Australian continental shelf, its crustal thickness and that of the Banda arc itself are less well documented. Three refraction profiles provide general estimates for the thickness of Australian continental crust in this area, but these show variability and uncertainty commensurate with the basin structure described above (Jacobson et al. 1979). The three profiles were orientated parallel to the shelf margin, and two lie in the Timor trough (Fig. 1). Crustal thickness estimates range from 31-40 km within an area of 500 000 km 2, and profiles 150kin long show crustal thickness variations of 4-8 km (Bowin et al. 1980). A fourth profile within the Banda Sea, east of Wetar (Figs 1 & 2), indicated a typical oceanic crustal thickness of 11 km (Bowin et al. 1980).
64
D. B. SNYDER ET AL.
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By themselves, these thickness estimates are too sparse to characterize the crust, but crustal thicknesses can be extrapolated or inferred over the region by using reflection profiles. Penetrative reflectivity within extended continental margins generally disappears below the Moho (e.g. Matthews 1986). The intersecting reflection and refraction profiles in the study area (Fig. 1) demonstrate that this relationship applies here, at least
locally. By assuming that it applies for the entire northern Australian shelf, a gradual eastward thickening of the crust from about 30-40 km is inferred between eastern Timor and Tanimbar. This pattern of crustal thickening is not locally correlated with the known basin structure unless the Sahul Platform continues to the NE as argued by Charlton et al. (1991) using stratigraphic criteria. Unrecognized basins, similar to the Malita Graben
65
LOCAL INDENTOR TECTONICS IN THE BANDA ARC N
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Fig. 4. Migrated seismic section from the DAMAR profile in the vicinity of the Timor trough. Post-Permian shelf sediments are clearly seen near the sea floor on the inner trough wall, folded beneath the trough floor. Two northward-dipping reflectors indicated (arrows) in the uppermost basement beneath the shelf sediments are similar to the one described in the previous figure. Moho lies at 13-15 s.
66
D. B. SNYDER ET AL.
or Petrel sub-basin, lying north of the Sahul Platform and beneath the Timor trough may explain these relationships, but cannot readily account for the observed regional along-trench crustal thickness variations. The AGSO North West Shelf Study Group (1994) note that one important consequence of the concentration of CarboniferousPermian extension in the lower crust is that extensional structures appear only as subtle reactivation features within the upper crust and basin fill, often displaced or offset from areas of lower crustal thinning. Parts of the Australian shelf overthrust by the Timor accretionary prism may therefore contain additional NE-trending sag basins like the Malita Graben, or more probably, northern parts of the north-trending rifts associated with the Petrel sub-basin.
Gravity field observations and modelling The Australian continental shelf south of Timor is now covered by a relatively dense grid of marine
gravity measurements. To the north, coverage is less complete but recent surveys onshore have mapped the gravity field on nearly all the outer arc islands between Timor and Tanimbar (Richardson 1993). The gravity field of the Banda Sea region is dominated by a steep gradient which forms a continuous arcuate feature running from northern Timor past Tanimbar and the Kai Islands to southern Seram (Bowin et al. 1980). Typically, values of Bouguer gravity increase from the slightly negative, as in the south coast region of Timor, to greater than +150 mGal as at points on the north coast of Timor (Fig. 5). Attempts to model this gradient used profiles crossing the arc at various locations. Of these, the model of McBride & Karig (1987) incorporates measurements from an anomalous region of western Timor where contour trends are locally transverse to, rather than parallel to, the strike of the arc, and the model of Milsom & AudleyCharles (1985) is unsatisfactory because of the lack of seismic control. The model of Jongsma et al. (1989; see also Richardson 1993) (Fig. 6a) was
Fig. 5. Bouguer gravity map of the study area. Reduction density is 2.2 Mg m-3. Contour interval is 10 mGal. Solid lines indicate the (roughly N-S) deep reflection profiles and (roughly E-W) modelled transects. Note the particularly negative values associated with the accretionary complex between the seismic profiles south of the island of Moa and the north-south gravity 'notch' over the shelf immediately to the south. Stippled borders are located near inflection points in east-west transects of the gravity field and define the possible areal extent of an anomalous low-density body in the crust.
LOCAL INDENTOR TECTONICS IN THE BANDA ARC
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67
based on gravity, magnetic and seismic data along a profile which nearly coincides with the TIMOR line (Fig. 2). The model shows thick continental crust in the downgoing plate almost as far north as the north coast of Timor, with low Bouguer anomalies in the south produced by a thick, lowdensity accretionary complex and the high values in the north due to the presence of what was originally sub-oceanic crust and mantle beneath the forearc basin and the volcanic arc. Subduction, rather than limited overthrusting, is hypothesized at the Wetar thrust and a mantle block is therefore shown as virtually isolated by the action of the opposing subduction zones (Fig. 6a). Of the more localized features of the gravity field, one of the most remarkable is the negative free-air and Bouguer anomaly feature centred directly south of the outer arc island of Moa (Figs 2 & 5). This feature was first indicated by measurements on a few widely spaced lines of marine survey, but was later substantiated by free-air gravity anomalies derived from satellite altimetry and measurements on Moa and the adjacent islands. The lowest Bouguer values occur 40 km to the south of the islands and the free-air minimum lies further south in deeper water. Bouguer anomalies, used in preference to free-air anomalies in Fig. 5, are less influenced by bathymetric variations and therefore better at resolving deep structures. On this map the Moa feature appears nearly elliptical, 40 km across from north to south (measured between half amplitude points) and 120 km from east to west, with an amplitude of c. 70 mGal. It is superimposed on a more elongated, lower amplitude feature that just reaches into the southern part of eastern Timor. Directly south of this 'Moa anomaly', the Bouguer anomalies are 30-50 mGal less than on nearby parts of the Australian shelf margin, and NNW of the feature Bouguer anomalies are 30-50 mGal less than in other parts of the forearc basin (Fig. 5). Collectively, these form a NNW trending band of lower gravity. The two deep seismic profiles cross the long axis of the 'Moa anomaly' on either side of its centre, at about the location of the two half-amplitude points. The profiles also lie near the base of local east-west gravity gradients on the Australian shelf (Fig. 5). Other than the discordantly dipping reflections indicated by arrows in Figs 3 & 4, neither seismic section appears to show basins or shelf features significantly different from those imaged on numerous unpublished seismic profile sections further east, where the regional gravity minimum has lower amplitude. These discordant reflectors have geometries consistent with out-of-plane reflections from dipping reflector surfaces (faults?) striking nearly parallel to the seismic profiles and offset by 10-15 km.
68
D . B . SNYDER E T A L
Two gravity profiles orientated along structural strike (Figs 5 & 6b) illustrate the similarity in the along-strike gravity trends along the prism and shelf by eliminating the more dominant northsouth gravity gradients associated with the subduction zone. One simple model indicates that a combination of anomalous, low densities at the base of the crust and at the top of basement can successfully match observed Bouguer values (Fig. 6b). Three sources of low density material are considered here. The first possibility, of a deep post-orogenic slope basin of the type known to occur south of Timor (e.g. Karig et al. 1987), is not supported by the seismic stratigraphy inferred from the seismic profiles (Fig. 3). Similarly, no direct evidence exists for a significantly thicker accretionary complex east of Timor, nor for anomalously lower density material within it, although Woodside et al. (1989) showed it as a 50 km thick tapering wedge. Thickening of the basement of the downgoing crustal layer by thrusts does seem plausible, based on duplex thrusts suggested by lower crustal reflector geometries (Figs 3 & 4) and on the variations in crustal thickness reported from the NW shelf of Australia (Bowin et al. 1980; O'Brien et al. 1993). Because thickening would effectively add low density material at Moho depth, it would have to be very local to produce a gravity field anomaly as localized as that near Moa. If thickening of the downgoing crustal layer is a valid explanation, syn-orogenic thrust duplexing seems more probable than isostatically uncompensated variations in crustal thickness that existed prior to collision. Elsewhere on the Australian shelf such variations are compensated. Thickening due to thrust duplexing could be accomplished as an independent tectonic mechanism or by restoration of crust previously thinned during the development of Palaeozoic sag or rift basins to its normal thickness whilst preserving or thickening the overlying basin strata. The resulting bi-level thickening is illustrated by the density model shown in Fig. 6b. Inherited crustal structure localizing and intensifying the thickening effect of syn-orogenic thrusting seems the most probable mechanism to produce the anomalously low Bouguer anomalies associated with the Australian shelf margin in this region.
Forearc crustal structures The forearc within the study area is unusual in the narrowness of the forearc basin and in the presence of the island of Kisar. Between the two deep seismic profiles the forearc basin is typically only 15-20 km wide and contains only tens of metres of sediment (Fig. 7). The basin appears to not have
acted as a depocentre, suggesting that it has been structurally higher than other parts of the forearc, such as the Savu Sea, enabling along-arc ocean currents to keep the sea floor relatively free of sediments during the past few million years. Recent mapping on the island of Kisar has revealed, from south to north, low-grade metamorphic rocks such as greenstones, amphibolites, and bands of thick mylonitized quartzites interlayered with greenstones and high-grade pelites (Dropkin et al. 1993; Richardson 1993). Some of these rocks bear a strong similarity to the Aileu (Grady & Berry 1977) or Lolotoi/Mutis Complex Formations on Timor and Moa (interpretations of Richardson 1993 and Dropkin et al. 1993 respectively), and are interpreted as dismembered ophiolites thrust onto thinned Australian shelf or oceanic crust or the volcanic arc. Kisar thus is related to the accretionary complex rather than to the volcanic arc, which emphasizes the unusual narrowness of the forearc basin in this segment of the Banda arc. Southward dipping reflections observed on the deep reflection sections (Fig. 7) may represent backthrusts that have translated the forearc (accretionary prism) northward over the forearc basin and thus narrowed it. The entire forearc block from Kisar to Sermata may represent a large rectangular klippe (Fig. 2).
Backarc features: a seamount chain including Gunung Api Anomalous features also occur north of this segment of the volcanic arc. Although the magnetic patterns are very poorly constrained, Lapouille et al. ( 1 9 8 5 ) suggested transform offsets of c. 50 km of anomalies identified as M7-M10 along a NW-SE orientated line passing through Gunung Api. More recent work (e.g. Rehault, pers. comm. 1995) suggests that the anomalies have been incorrectly identified, but the evidence for offset remains valid. The new seismic reflection profiling has revealed a number of previously undocumented sea-floor volcanoes (seamounts) along the TIMOR line and coincident with this inferred transform fault. The northern part of one of these seamounts is shown in Fig. 8. The API cross-line (Fig. 2) showed that between the seamounts lies a low ridge of extruded basalt separating two levels of the Banda Sea ocean floor (Fig. 9). Sediment thicknesses appear to be similar on both sides of the ridge, which therefore coincides with a down-tothe-east offset of 350-400 m in the sea-floor basement. Arcuate reflectors within the ridge suggest either folds and thrusts or primary extrusive structures among the basalt flows. Gunung Api, the seamounts and the inferred ridge basalt flows
LOCAL INDENTOR TECTONICS IN THE BANDA ARC
69
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Fig. 7. Part of the DAMAR seismic reflection section within the forearc basin. This section was migrated but has not been depth converted. Approximately 200 m of relatively undeformed sediment lie within the 10 km wide forearc basin at the centre of the section. Greater sediment accumulations can be observed (arrows) in slope basins both north and south of the forearc basin. Curved arrows indicate possible backthrust.
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Fig. 8. Part of the TIMOR seismic reflection section just south of Gunung Api. This section was migrated and then depth converted using the velocities shown at the right in km s-1. In this depth section the Moho appears as a bright reflection (arrow) at a depth of 10 km where this line crosses the API line. If refraction velocity function M12, of Bowin et al. (1980), shown for comparison at the left, was used instead for the depth conversions, the bright reflection occurs at 14-15 km depths, and no obvious feature is associated with the Moho. Numerous convex-upward reflections (M) are migration noise due to the incomplete removal of the sea-floor multiple before migration and should be ignored.
70
D . B . SNYDER E T AL. TIMOR
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section was migrated using the same velocity function as used for the TIMOR line shown in Fig. 8. The reflection observed at c. 9.3 s (10-15 km depth) on the TIMOR line (see Fig. 8 caption) is not as bright here. A sea-floor ridge separates oceanic crust, overlain by < 200 m of sediments, at two distinct depths. Reflectors beneath the ridge include arcuate and short horizontal segments on the flanks that probably represent submarine basalt flows, possibly later folded (inset).
indicate that the fracture has acted as a m a g m a conduit. Together these features imply a leaky transform fault zone nearly coincident with the T I M O R seismic profile. A bright horizontal reflector is o b s e r v e d on the T I M O R line where it intersects the API line (9.3 s travel time on Fig. 9; 10.5 k m depth on Fig. 8), although only a w e a k w e s t w a r d dipping reflector is observed on the API line (Fig. 9). Nearby refraction studies indicated that the M o h o occurs at 10.3 k m depth (M 12 profile of B o w i n e t al. 1980). If the refraction velocities from the M 1 2 profile are used to migrate and depth convert the reflection section, the bright reflector occurs within the mantle at 1 4 - 1 5 k m d e p t h s and no o b v i o u s r e f l e c t i o n s coincide with the M o h o at 10.3 k m depth. Either the refraction velocities derived along the nearby profile M 1 2 are inappropriate here or else the mantle is m o r e reflective than the oceanic crust along this profile. T h e bright reflections m a y m a r k the location o f m a g m a rather than a change in rock lithologies or mineralogy. M o r e probably the bright reflector represents underplated m a g m a w h i c h produces a thicker crust i m m e d i a t e l y beneath the sea-floor ridge. A regional fracture zone, or leaky transform
fault, orientated nearly normal to the regional plate m a r g i n w o u l d s u g g e s t that the d e f o r m a t i o n associated with plate c o n v e r g e n c e m a y have here propagated further into the overlying plate than elsewhere along the plate boundary. The location of this fracture m a y be due to an older zone of weakness in the B a n d a Sea lithosphere or due to unusually thick or buoyant structures on the subducted plate (e.g. Cross & Pilger 1982).
Discussion:
appropriateness
an indentor
model
of
T h e part o f the B a n d a arc i m m e d i a t e l y east of T i m o r is characterized by several features not generally observed in other parts of the arc. A n active volcano within the backarc basin 400 k m above the W a d a t i - B e n i o f f zone is rare, and here occurs along a line o f seamounts orientated nearly normal to the c o n v e r g e n t margin. Uplift rates are high on both the volcanic arc and forearc accretionary c o m p l e x islands, bringing parts o f eastern T i m o r nearly 3 k m above sea-level. T h e forearc basin is only 10 k m wide where it was crossed by the D A M A R seismic line. A n o m a l o u s l y
LOCAL INDENTOR TECTONICS IN THE BANDA ARC low Bouguer gravity anomalies are observed in the corridor defined by the two deep seismic profiles, anomalies more negative than anywhere else along the Banda arc. Taken together, these features suggest that unusual lithospheric strains are associated with the convergence that occurs within this part of the arc. Explanations proposed involve three major lithospheric plates (e.g. Charlton 1986), but the authors prefer to discuss the local features in terms of a single two-plate boundary. Unusually thick parts of the Australian shelf entering the subduction zone are one possible explanation. Crust not greatly thinned during Jurassic rifting, crust thickened as it entered the modern convergence zone, or unusually thick sedimentary basin fill could all contribute relatively buoyant, low density continental crustal material to this segment of the arc. The greater amount of low density material satisfies the observed gravity anomalies. Its associated buoyancy, compared to oceanic crust, would enhance uplift. A thickened wedge of subducting material resists further subduction by deforming internally to form a core of continental material. This core may partly underthrust and dome the arc, and partly impinge upon and strain the oceanic crust of the backarc (Fig. 10). This form of localized strain is here proposed to have caused the fracture associated with the line of seamounts including Gunung Api. Unsubducted parts of the Australian shelf provide some evidence that thick sedimentary basins
GUNUNG API ~
71
may have underthrust the arc in the area of the deep seismic profiles. About 500 km to the south, more than 6000 m of Palaeozoic and younger sediments occur in the Petrel sub-basin (Schltiter & Fritsch 1985; AGSO North West Shelf Study Group 1994) between the offshore parts of two Archaean blocks (Fig. 1). The northern continuation of the Petrel sub-basin is poorly defined, so that it or a similar Carboniferous-Jurassic basin may continue beneath the Sahul platform or align with the band of lower Bouguer anomalies, including the 'Moa anomaly', that trend nearly orthogonal to the convergence margin (Fig. 5). Crustal thinning associated with the crustal extension that formed the Petrel subbasin and other similar basins to the SW has estimated stretching factors of 100-400% (O'Brien et al. 1993; Etheridge & O'Brien 1994). These estimates are consistent with the lower crustal thicknesses of 31 km modelled at the western ends of the two refraction lines, within the Moa gravity feature, when compared with the 35-39 km thicknesses (Fig. 1) reported at the eastern ends (Bowin et al. 1980). Thinner crust beneath the relatively undeformed shelf in this region is not obviously consistent with the proposed model of anomalously thickened continental crust doming and indenting the arc and backarc, respectively. Thrust duplexing within the lower crust could partly restore the pre-rift crystalline crust's thickness while retaining the full overlying basin's thickness, and result in local mass
DAMAR
BAN
....
Fig. 10. Block diagram showing the proposed local promontory of thickened Australian shelf crust and sediments. This promontory becomes a potential indentor wedge during subsequent convergence by lifting and offsetting the overlying crust across faults inferred to segment the accretionary complex that includes Timor and to propagate northward into the backarc Banda Sea. The model contains elements intended to explain the various observed anomalous features in the bathymetry, gravity field and crustal thickness in the vicinity of the new deep seismic reflection profiles. The cross pattern indicates crystalline basement within the Australian crust.
72
D.B. SNYDER ET AL.
deficits such as that m o d e l l e d for the ' M o a anomaly' (Figs 6b & 10). Shear thrusts within the relatively weak lower crust need not directly underlie the rift basins. O v e r l y i n g these deeper, N N E trending Devonian-Carboniferous basin structures are the NE trending platforms and graben structures, such as the Sahul Platform and Malita Graben, that are associated with the Carboniferous-Permian rifting of Gondwanaland. A small outboard platform, similar to the A s h m o r e and Sahul Platforms (Fig. 1), and effectively forming a promontory on the pre-collisional Australian margin, may have already been subducted and contributed to the low mass a n o m a l y of thicker continental material beneath the Moa gravity anomaly. Insufficent details of crustal thicknesses and structures are available to resolve more clearly the various superposed pre-collision extension-related structures from the recent collisional ones. The coincidence of a north-south orientated boundary between Archaean crustal blocks and the midPalaeozoic Petrel (Bonaparte) Basin with a N E - S W orientated late-Palaeozoic rifted m a r g i n and Neogene convergence zone permits many possibilities for crustal structures. Some of the older structures may nucleate subsequent strain. The presently preferred explanation for high uplift rates on Timor and other anomalous features within the Banda arc immediately east of Timor is
partial restoration of crustal thicknesses beneath a northward continuation of the Petrel sub-basin of the Bonaparte rift by thrust duplexing within the lower crust during recent collision. Dynamically depressed crust is inferred from the gravity field east of Timor. In eastern Timor the thickened crust has rebounded isostatically to produce rapid uplift and neutral gravity anomalies. In both areas, overcompensated crust within the subducted slab has acted as a buoyant mass that jams subduction and translates convergence to the north where it loads and fractures the oceanic lithosphere of the Banda Sea. We would like to especially thank K. Bartram, A. Macfarlane, B. Situmorang, S. Suparka and M. Takim, for their interest and support in organizing the seismic project in Indonesia. The BANDA seismic reflection profiles were acquired by GECO Singapore Pte. and processed by Prakla-Seismos, and are available at the cost of reproduction from the British Geological Survey (Marine Geophysics Programme Manager), West Mains Road, Edinburgh EH9 3LA, UK. This research was done under the auspices of the Indonesian Institute of Sciences (LIPI) and the Natural Environment Research Council (NERC) of the UK. BANDA was funded by the British Natural Environment Research Council and BIRPS' Industrial Associates Program (Amerada Hess Ltd, Amoco, BP Exploration Operating Company Ltd, Chevron (UK) Ltd, Conoco (UK) Ltd, Mobil North Sea Ltd and Shell (UK) Ltd). This paper is Cambridge Earth Sciences contribution number 4176.
References AGSO NORTH WEST SHELF STUDY GROUP. 1994. Deep reflections on the North West shelf: changing perceptions of basin formation. Proceedings Western Australian Basins Symposium, Perth, 63-76. BOWlN, C., PURDY, G. M., JOHNSON, C., SHOR, G., LAWYER, L., HARTONO,H. M. S. & JEZEK, P. 1980. Arc-continent collision in the Banda Sea region. AAPG Bulletin, 64, 868-915. BREEN, N. A., StaYER, E. A. & ROOF, S. 1989. The Wetar backthrust belt, eastern Indonesia: the effects of accretion against an irregularly shaped arc. Tectonics, 8, 85-98. CARDWELL,R. K. & ISACKS,B. L. 1978. Geometry of the subducted lithosphere beneath the Banda Sea in eastern Indonesia from seismicity and fault plane solutions. Journal of Geophysical Research, 87, 2825-2838. CHARLTON, T. R. 1986. A plate tectonic model of the eastern Indonesian collision zone. Nature, 319, 394-396. , DE SMET, M. E. M., SAMODRA,H. 8z KAYE, S. J. 1991. The stratigraphic and structural evolution of the Tanimbar Islands, Eastern Indonesia. Journal of SE Asian Earth Sciences, 6, 343-358. CROSS, T. A. & PILGER, R. H. 1982. Controls of subduction geometry, location of magmatic arcs, and
tectonics of arc and back-arc regions. Geological Society of America Bulletin, 93, 545-562. DEMETS, C., GORDON, R. G., ARGUS, D. E & STEIN, S. 1990. Current plate motions. Geophysical Journal International, 101, 425--478. DROPKIN, M. J., HARRIS, R. A. & ZEITLER, P. K. 1993. An Oligocene forearc crustal flake exposed in a contemporary arc-continent collision: Timor, Indonesia. Geological Society of America Annual Meeting, Abstracts with Programs, 25, 6, A--482. ETHERIDGE, M. • O'BRIEN, G. W. 1994. Structural and tectonic evolution of the Western Australian margin basin system. Australian Petroleum Exploration Association Journal, 34, 906-908. GENRICH, J., BOCK, Y., MCCAFFREY, R., CALAIS, E., STEVENS, C. E T A L . 1994. Kinematics of the eastern Indonesian island arc estimated by global positioning system measurements. EOS abstracts, 75, 162. GRADY, A. E. & BERRY, R. E 1977. Some PaleozoicMesozoic stratigraphic-structural relationships in east Timor and their significance in the tectonics of Timor. Journal of the Geological Society Australia, 24, 203-214. HAMILTON, W. 1979. Tectonics of the Indonesian region. US Geological Survey Professional Paper, 1078.
LOCAL INDENTOR TECTONICS IN THE BANDA ARC 1988. Plate tectonics and island arcs. Geological Society of America Bulletin, 100, 1503-1527. HARRIS, R. A. 1991. Temporal distribution of strain in the active Banda orogen: a reconciliation of rival hypotheses. Journal of SE Asian Earth Sciences, 6, 373-386. HILTON, D. R., HOOGEWERFF,J. A., VAN BERGEN, M. J. & HAMMERSCHMIDT, K. 1992. Mapping magma sources in the west Sunda-Banda arcs, Indonesia: Constraints from helium isotopes. Geochimica et Cosmochimica Acta, 56, 851-859. JACOBSON,R. S., SHOR, G., KIECKHEFER,R. M. & PURDY, G. M. 1979. Seismic refraction and reflection studies in the Timor-Aru Trough system and Australian continental shelf. In: WAT~NS, J. S., MONTADERT, L. & DICKERSON, P. W. (eds) Geological and Geophysical Investigations of Continental Margins. AAPG Memoir, 29, 209-222. JOHNSTON, C. R. & BOWlN, C. O. 1981. Crustal reactions resulting from the mid-Pliocene to recent continentisland arc collision in the Timor region. Bureau of Mineral Resources Journal of Australian Geology & Geophysics, 6, 223-243. JONGSMA, D., WOODSIDE, J. M., HUSON, W., SUPARKA,S. & KADARISMAN,D. 1989. Geophysics and tentative late Cenozoic seismic stratigraphy of the Banda Arc-Australian continent collision zone along three transects. Netherlands Journal of Sea Research, 24, 205-229. KARIG, D. E., BARBER, A. J., CHARLTON, Z. R., KLEMPERER, S. & HUSSONG, D. M. 1987. Nature and distribution of deformation across the Banda Arc-Australian collision zone. Geological Society of America Bulletin, 98, 18-32. LAPOU1LLE,A., HARYONO,H., LARUE, M., PRAMUMIJOYO, S. & LARDY, M. 1985. Age and origin of the seafloor of the Banda Sea (eastern Indonesia). Oceanologica Acta, 8, 379-389. MATrHEWS, D. H. 1986. Seismic reflections from the lower crust around Britain. In: DAWSON, J. B., CARSWELL, D. A., HALL, J. & WEDEPOHL,K. H. (eds) The Nature of the Lower Continental Crust. Geological Society, London, Special Publications, 24, 11-21. MCBRIDE, J. H. & KARIG, D. E. 1987. Crustal structure of the outer Banda arc: new free-air gravity evidence. Tectonophysics, 140, 265-273. MCCAFFREY, R. 1988. Active tectonics of the eastern Sunda and Banda Arcs. Journal of Geophysical Research, 93, 15163-15182. 1992. Oblique plate convergence, slip vectors, and forearc deformation. Journal of Geophysical Research, 97, 8905-8915. 1996. Slip partitioning at convergent plate boundaries of SE Asia. This volume.
--,
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MOLNAR, P., ROECKER, S. W. & JOYOD1WIRYO, Y. S. 1985. Microearthquake seismicity and fault plane solutions related to arc-continent collision in the eastern Sunda Arc, Indonesia. Journal of Geophysical Research, 90, 4511--4528. MILSOM, J. & AUDLEY-CHARLES, M. G. 1985. Postisostatic readjustment in the southern Banda Arc. In: COWARD, M. P. & RIES, A. C. (eds) Collision tectonics. Geological Society, London, Special Publications, 19, 353-364. O'BRIEN, G. W., ETHERIDGE, M. A., WILCOX, J. B., MORSE, M., SYMMONDS, P., NORMAN, C. & NEEDHAM, D. J. 1993. The structural architecture of the Timor Sea, northwestern Australia: implications for basin development and hydrocarbon exploration. Australian Petroleum Exploration Association Journal, 33, 258-278. PIGRAM, C. J., DAVIES, P. J., FEARY, D. A. & SYMONDS, P. A. 1989. Tectonic controls on carbonate platform evolution in southern Papua New Guinea: passive margin to foreland basin. Geology, 17, 199-202. RICHARDSON, A. N. 1993. Lithospheric structure and dynamics of the Banda Arc, eastern Indonesia. PhD thesis, University of London. SCHLt2rrER, H. U. & FRITSCH, J. 1985. Geology and tectonics of the Banda Arc between Tanimbar Island and Aru Island (Indonesia) - results of R/V Sonne Cruise SO-16. Geologisches Jahrbuch Reihe E, 30, 3-41. SENGOR, A. M. C. 1987. Tectonics of the Tethysides: orogenic collage development in a collisional setting. Annual Reviews of Earth & Planetary Science, 15, 213-244. SILVER, E. A., REED, D., MCCAFFREY, R. & JOYODIRWmYO, Y. 1983. Back arc thrusting in the eastern Sunda arc, Indonesia: a consequence of arc-continent collision. Journal of Geophysical Research, 88, 7429-7448. VEEVERS, J. J. 1988. Morphotectonics of Australia's northwestern margin - a review. In: PURCELL,P. G. & PURCELL, R. R. (eds) The North West Shelf, Australia. Proceedings of Petroleum Exploration Society Australia Symposium, Perth, 1988, 19-27. WOODSIDE, J. M., JONGSMA,D., THOMMERET,M., STRANG VAN HEES, G. & PtYNTODEWO 1989. Gravity and magnetic field measurements in the eastern Banda Sea. Netherlands Journal of Sea Research, 24, 185-203. YEATES, A. N., BRADSHAW,M. T., DICKINS,J. M., BRAKEL, A. T., EXON, N. E Er AL. 1987. The Westralian Superbasin: an Australian link with Tethys. In: MCKENZIE, K. G. (ed.) Shallow Tethys 2. Proceedings of Intenational Symposium on Shallow Tethys 2, Wagga Wagga, 199-213.
Detailed processing of seismic reflection data from the frontal part of the Timor trough accretionary wedge, eastern Indonesia B. D. H U G H E S 1'3, K. B A X T E R TM, R. A. C L A R K 1 & D. B. S N Y D E R 2
1 Department of Earth Sciences, The University of Leeds, Leeds LS2 9JT, UK 2 BIRPS, Bullard Laboratories, Madingley Rise, Cambridge, CB30EZ, UK 3 Present address: ABG EXPLORATION, 12 Howard Way, Cromwell Business Park, Newport Pagnell, Bucks MK16 9QR, UK 4 Present address: CSIRO (Division of Exploration & Mining), Floreat Park Laboratories, Wembley, Perth, WA, Australia 6014 Abstract: The DAMAR deep seismic line across the Banda arc in Indonesia provides a valuable insight into the Late Cenozoic collision between the northward-moving Australian plate and the Southeast Asian plate. Previous geophysical investigations across the accretionary wedge have only imaged relatively shallow structures due to the complex structure of the wedge which remains poorly resolved following conventional processing techniques. The design of a detailed processing sequence and its application to a 35 km section of the DAMAR line to include the Australian Northwest shelf, the Timor trough and the frontal part of the accretionary wedge, has allowed an improvement in resolution of structures both within and below the frontal part of the wedge. The Australian margin to the south of the trough contains a number of normal faults which show inversion features. The reprocessing of the accretionary wedge has allowed an improved interpretation of the internal structure in the frontal part of the collision zone and this consists of a number of high angle thrust structures. This has shown that shortening in the frontal part of the accretionary wedge has developed coherent thrust slices imbricated from the subducting Australian margin, rather than an incoherent melange.
The Timor region in eastern Indonesia is often quoted as a present-day fold-and-thrust belt in its earliest stages, and is one of the youngest collisional belts presently active. The study of the Timor region not only provides an understanding of the active collision between the Australian plate and Southeast Asia, but allows a view of the early stages of mountain belt evolution. The development of the region may be divided into two main phases:
of these data to assess the deformation styles, both within the accretionary wedge and on the Australian shelf immediately adjacent to the trough. Accepted models that assume that deformation has recently ceased at the Timor trough are also evaluated in the light of this work. Similar processing techniques to those described may be applied to similar datasets from other areas to increase the resolution of structures both within and below areas of complex geological structure.
• break-up of Gondwanaland and the development of the Australian passive margin; • development of subduction north of Australia with eventual collision of the Australian margin and development of the collision zone presently exposed on Timor. In 1992 the British Institutions Reflection Profiling Syndicate (BIRPS) and the Indonesian Marine Geological Institute commissioned two deep seismic lines across the Timor-Tanimbar trough to the east of Timor, the BANDA survey (Fig. 1). This paper discusses the development of a processing sequence to improve the resolution obtained by conventional processing on part of the DAMAR line (shotpoints 600-950), and the interpretation
Evolution of the Timor trough region One of the most extensive features in Indonesia is the linear, northerly dipping subduction zone delineated by the Java Trench-Timor trough which marks the collision of the northerly moving IndoAustralian plate with the Southeast Asian plate. During the break-up of Gondwanaland the Australian margin progressively rifted southwards with continental break-up on the Northwest Shelf by the Oxfordian (160 Ma) (Pigram & Panggabean 1984). Following rifting, the drift phase of the margin during the Cretaceous and Tertiary was characterized by the deposition of dominantly carbonate sediments. During the Neogene the
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolution o.fSoutheastAsia,
75
76
B.D. HUGHES ET AL.
0 I~
o ~ ~'~
TIMOR
0
I
I
Banda Sea
GunungApi
km 10,01 ' 000/ 4 /L) ° /
DAMAIq~
0
o
Kisar
I
South
o 5 m r n a -I T r o u b a d o u r Well N-°1%
Australian Northwest Shelf 10 ° 125 °
126 °
127 °
East
128 °
129 °
130 °
Fig. 1. Part of the Banda arc in eastern Indonesia showing the location of the BIRPS TIMOR and DAMAR deep seismic lines. The barbed line indicates the trace of the Timor trough, volcanoes are shown as circles (black where active) and values on bathymetric contours are in metres. The section detailed in this paper is shown by the thickened line, located in the immediate vicinity of the trough axis on the DAMAR line, shotpoints 600 to 950.
northerly drift of the Indo-Australian plate produced the development of oceanic subduction to the north of Australia and ultimately the collision of the leading edge of the Australian plate with the Banda arc subduction complex. This resulted in the development of a fold-and-thrust belt exposed on a number of islands, the largest of which is Timor. Subduction is presently active along the Java Trench with decreased activity on the Timor trough. Recent geodetic measurements by McCaffrey (1996) suggest that the Timor collision zone has recently locked on to the northward-moving Australian plate with cessation of subduction at the Timor trough.
Marine geophysical studies in the Timor trough Numerous previous workers have studied the Australian margin and the Timor trough region using marine geophysical techniques (e.g. Bowin et al. 1980; Karig et al. 1987; Charlton, 1988; Masson et al. 1991). Cross-sections across the
Timor trough have previously been imaged using seismic reflection data, and a number of common features have been observed (Fig. 2): • an outer slope increasing in gradient towards the trough axis, underlain by Australian shelf and slope strata (A & B, Fig. 2). Normal faults are present, mainly dipping northwards; • a trough axis with a flat-topped wedge of trough fill sediments (F, Fig. 2) representing sediments trapped between the outer slope and the deformation front; • a deformation front marking the southerly extent of the Timor fold-and-thrust belt; • an accretionary wedge containing individual thrusts and folds representing deformation of the subducted Australian margin. The complex seismic response of accretionary wedges causes the geologically important structures within and beneath them to remain poorly imaged after conventional 2D processing, as seen in many previously published sections. An ideal processing sequence, always data-specific, should
SEISMIC REFLECTION DATA FROM THE TIMOR TROUGH
77
(a)
TW'T (s)
(b) Inner slope Trough fill < >< ><
Outer slope ,,-
>
Deformation front
4
6 TWT
(s)
--"~
4
6
- -/ '
'
'
Fig. 2. Seismic reflection profile (Gulf line AU-38F) across the Timor trough and outer slope south of Timor showing the major morphological features. Adapted from Karig et al. (1987).
deliver optimum signal-to-noise ratio (SNR) while representing subsurface geometries faithfully and with optimum vertical and lateral resolution. These objectives are sometimes compromised against practical limits such as computer resources, but this study has developed a more detailed processing sequence which (compared with conventional processing) has resulted in a significant improvement in SNR, resolution, and hence geological interpretation.
Data processing Seismic reflection data processing techniques are well described in many texts (e.g. Yilmaz 1987). However, 'deep' reflection surveys (i.e. deep relative to hydrocarbon exploration, for which reflection methods are most developed) encounter particular difficulties: data from the Timor trough are quite typical in this respect. They show strong multiple activity, all reflectors are relatively deep, and numerous very strong diffractions are visible.
Migration is an essential process in such heavily faulted sections; with marked variations in reflector dip across the section, choice of suitable migration velocities was critical, but, as is typical of deep reflection surveys, velocities were difficult to measure accurately because of the very limited amount of moveout on the reflections. The data were provided by the BIRPS group already preprocessed and sorted into CMP gathers. The considerable water depth (2-4 s twt) meant that water-bottom multiples in fact only appeared deeper in the section than the zones of interest here: their removal was straightforward and multiple suppression was carried out as part of standard preprocessing as described by Snyder et. al. (1996). Conventional processing was then completed, to ascertain the overall geometry of the accretionary wedge. Velocity analyses were made at 100 CMP (2.5 km) intervals across the section. Given the simple structures south of the Timor trough, this was felt acceptable for that part of the line, but further north, velocities were difficult to determine
78
B.D. HUGHES ET AL.
reliably. The stacked section so produced was migrated using a finite-difference time migration, to allow lateral variations in velocity to be accommodated. Even so, internal structure of the accretionary wedge and the subducting Australian shelf beneath it were poorly resolved and a more detailed processing strategy was imperative for this part of the line. Shotpoints 600-950, spanning the toe of the wedge, are shown conventionally processed in Fig. 3. Key elements of the improved processing sequence (Fig. 4), which yielded the section in Fig. 5 (which shows the same shotpoint range a~Fig. 3), are now discussed briefly. Detailed processing
South of shotpoint 748, no revisions to velocity analyses were thought necessary. As the section becomes structurally more complex northwards, within the accretionary wedge, selection of velocities becomes more important. Instead of regular spacing, velocity analyses were located at the apices of all prominent diffractions, resulting in identification of more accurate velocities. The average separation between velocity analyses thus reduced to 1.6 km; still larger than would have been preferred, but the effect of the strong diffractions was to make additional velocity analyses elsewhere of little value. To refine velocity estimates further, bias from dip effects (inherent in any moveout-based velocities for dipping reflectors) was reduced by an iterative sequence of velocity analysis, DMO (dip moveout), and repeat velocity analysis. Common-offset FK-based DMO (Hale 1984) was used; because of limitations on computer storage,
600
I
700 I
|
DMO was applied separately to near, intermediate, and far offsets and the full dataset then reconstituted. Revised velocities were some 5-10% different from the original in and below the accretionary wedge but effectively unchanged for the shallow-dipping Australian shelf to the south. Successful migration of the resultant stacked section was critically dependent of the use of accurate, true, medium velocity throughout. Below the seabed, optimum migration velocities were determined empirically as percentages of stacking velocities, but that of the water column is well known and was not allowed to vary. In the complex parts of the line, test panels from faulted areas at either end of the shelf slope and from within the accretionary wedge were used. Optimum percentages (typically 80-90%) were chosen and applied to a finite-difference time migration of the full line (shown in Fig. 5). For structurally simple parts of the line, optimum percentages could be seen to correlate clearly with dip (see Hughes 1994 for a full discussion). A depth conversion was done postmigration, using geologically-constrained interval velocities.
Interpretation of the DAMAR line An interpretation of the DAMAR seismic l i n e involved an iteration between geological observations and the processing sequence. Structures identified were examined and additional processing was applied if required to redefine the velocity field allowing an improvement in resolution and interpretation. An interpretation of the reprocessed section is shown in Fig. 5b.
800 I
0 |,,,
900
I
I
SP
10 km I
TWT (s)
-5
Fig. 3. Example of part of the DAMAR line (shotpoints 600-950) processed using a conventional processing sequence. Note the poor lateral and vertical resolution which is attributed to the low signal-to-noise ratio resulting from conventional processing. Compare this with the same section in Fig. 5, reprocessed to give better definition, which has allowed an improvement in the geological interpretation.
SEISMIC REFLECTION DATA FROM THE TIMOR TROUGH
79
PREPROCESSING Carried out by BIRPSgroup independently. Incorporated multiple suppressionand sortingto CMP gathers
I "
VELOCITY ANALYSIS Incorporates analysis of diffractions ......
re-sort to CMP~athers
[ NMO CORRECTION, CMP STACK
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POST-STACK FILTERING AGC, bandpass filters, etc.
1 MIGRATION VELOCITY TESTING On several separate test panels along line
1 l
TIME MIGRATION Constant velocity in water column; variable percentage of stacking velocities sub-seafloor.
I DEPTH CONVERSION
[
Fig. 4. Outline of the processing sequence used to produce the section in Fig. 5. Emphasis is placed on determination of accurate and appropriate velocities. Key elements only are shown: background information on processes may be found in standard texts such as Yilrnaz (1987), and data-specific details are given in Hughes (1994).
The Australian margin slope The Australian margin slope (shotpoints 600-780) dips gradually north into the Timor trough at an angle of approximately 3.5 ° . Reflectors on the adjacent TIMOR line have been tied to the Troubador No 1 Well (see Hughes 1994), located at its southern end, and have been compared and verified with seismic data from the Timor gap to the west of the BANDA lines (see Baxter 1993). Reflectors on DAMAR have been directly correlated with those on the TIMOR section, thus making it possible to estimate that they relate stratigraphically to Mid-Late Tertiary (A), Mid-Base Tertiary (B), Mid-Late Jurassic break-
up unconformity (C) and Top Permian? (D) ages. The upper sediments represent the ongoing post-rift subsidence of the Australian passive margin and are Late Cretaceous to Miocene pelagic mudstones and shelf carbonates (Karig et al. 1987). These are laterally consistent across the section with no prominent angular erosional surfaces. Normal faulting on the DAMAR line is apparent in two main areas (shotpoints 608-635 and shotpoints 740-778) and these faults appear to bound a small graben structure. Some of these faults dip to the south, rather than north towards the trough as commonly described by previous workers. The faults show growth (indicated by thickening of sediments in the hanging wall) during Cenozoic
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SEISMIC REFLECTION DATA FROM THE TIMOR TROUGH sediment deposition and in places reach the sea floor. Inversion structures are apparent on the southernmost, north-dipping, fault (shotpoint 610) with a compressional anticline developed on reflector B. Testing this structure using different migration velocities has shown that this is not an undermigration effect (Hughes 1994). Some thickening of the sediments just below this reflector on the fault can be seen with steep up-thrown sediments immediately overlying low-angle dipping strata, possibly indicating a small thrust, although this is not well imaged. Some complicated zones are apparent on the section and are thought to correlate to normal faults (e.g. in the vicinity of shotpoint 747). The excessive displacement gradient generated by interpreting single faults suggests that this area may contain additional fault terraces, increasing the appearance of drag on the fault zones. This faulting also creates the topographic bulge in the sea floor at shotpoint 760. At shotpoint 635, a south-dipping fault with no apparent inversion similarly shows thickening of sediments in the hanging wall and steepening of reflectors immediately adjacent to the fault. This is also difficult to interpret as a single structure and probably involves terraces. It should be noted that the presence of inversion across this region and potential strike-slip activity may also give out-ofplane-of-section complications which are not considered here.
The trough axis The axis of the trough, marking the surface transition between the shelf slope and the accretionary wedge, is characterized by a change in the regional topographic dip from north to south. However, a significant thickness of trough sediment, as seen on the adjacent TIMOR line and in sections published by previous workers (e.g. Fig. 2) is absent on the DAMAR line. Karig et al. (1987) also noted a change in the geometry of the trough fill along axis with ranges of 0-10 km in width and 0 to >1 km deep. They proposed that this is due to the wedge thrust front being laterally discontinuous and that in some areas the trough fill may have been accreted into the toe of the wedge. Both the section in Fig. 2 and the adjacent TIMOR line show similar features of overthrusting of the trough sediments by the wedge front. From the reprocessing beneath the frontal part of the accretionary wedge, a pre-existing thickness of sediment in the trough does not appear to have been overridden by the wedge (as seen on the TIMOR line), unless it has been fully accreted to the wedge by its recent advancement. The top of the downgoing Australian plate lies at a depth of
81
3.8 km on the TIMOR line beneath a trough sediment fill of c. 630 m, and at a depth of c. 3.2 km at seabed on the DAMAR line at shotpoint 780. Therefore, the greater seabed depth of the TIMOR line may have produced a preferential site for trough sedimentation in comparison with the lack of sediment fill present on the DAMAR section. It may be possible that the inversion of the normal fault beneath the trough axis on the DAMAR line at shotpoint 760 has also contributed to the reduced bathymetry in the trough, although this is speculative.
The accretionary wedge The accretionary wedge is characterized by a change in the surface topography as the Timor collision zone overrides the Australian margin. The sea floor above the wedge shows a relatively smooth profile, with a lack of penetration of thrusts at the wedge front and only minor penetration further back in the wedge. This suggests that the frontal part of the wedge is inactive, with minor out-of-sequence thicknening further back producing internal thickening. This may be due either to the cessation of subduction at the Timor trough in this region, or that the frontal part of the wedge is inactive as internal thickening attempts to return the wedge to a critical state and allow propagation of the wedge front. The internal structure of the wedge is seismically complex. Picking structural and lithological reflectors is obviously difficult in such a complex zone which is likely to contain a high amount of diffracted energy which may not have been migrated back to a source reflector. The use of migration velocity analysis during this study has, however, led to the conclusion that a number of dipping events within the wedge are real reflectors and not diffraction effects. These reflectors are shallow in dip at the toe of the wedge, increasing in dip southwards and are most likely to be thrust faults developed during forward propagation of the thrust front, developing a back-steepening of the thrusts. These show typical thrust geometries with rollover anticlines in the hanging wall above a lowangled detachment. Correlation of reflectors from the Australian margin is difficult in such a highly deformed zone. A strong seismic reflector is defined by a high acoustic impedance contrast at the contact between two lithological units. The lithological units occupying the intervals between the strong reflectors A-D are considered to be homogeneous throughout as indicated by their relatively transparent internal seismic structure, although some internal layering is apparent. These units are
82
B.D. HUGHES ET AL.
thought to be similar to those observed from the Troubadour 1 well located nearby, and consist of." a Recent-Maastrichtian sequence of calcarenites and marls from seabed to 1.55 km depth; a 550 m thick claystone unit (Turonian-Cenomanian); Jurassic sandstones and claystones (2.1-2.8km depth); and Triassic limestones and claystonesiltstone-sandstone sequence (2.8-3.3 km depth). Therefore, during thrusting, imbrication which structurally juxtaposed two of the main lithologies could be expected to produce a reflector with acoustic impedance and phase similar to a stratigraphic contact. The strong convex-upward nature of many reflectors is thought to correlate to hanging wall geometries within thrust sheets rather than being from faults but it is unclear whether the lithological contacts picked correlate with the original stratigraphic reflectors on the Australian margin or whether these are due to lithological juxtapositions created during faulting.
Imaging of structures beneath the accretionary wedge Previous studies across the Timor trough have not generally imaged the structure of the Australian shelf beneath the accretionary wedge. Improved processing during this study has allowed some structures to be imaged beneath the frontal part of the wedge. The decollement itself is not strongly imaged, although the truncation of stratal reflections within the uniform shelf sediments allows an interpretation of its location. Beneath the wedge in the area of shotpoint 820 there appears to be an upwarping of reflectors C and D. This could be either a processing anomaly or a geological feature. Similar anomalous features are well known during processing and are associated with inaccurate velocity determination of the overlying structure, particularly if this is of a high velocity, resulting in a velocity pull-up. However, the feature at shotpoint 820 appears to be of high amplitude and short wavelength, which is inconsistent with any possible velocity anomaly within the overlying wedge. Therefore, this feature is tentatively interpreted as geological. The upper part appears to be truncated by the wedge decollement with no apparent continuation into the wedge. Thus, this structure must pre-date wedge imbrication of this part of the shelf. The shape of the feature on reflector D is similar to that beneath the zones of structural complexity seen elsewhere on the section (shotpoint 635 and shotpoint 747) and may be associated with similar fault structures, although resolved less clearly due to the overlying wedge.
Conclusions Following the design of a reprocessing sequence, an improvement of SNR and resolution of the DAMAR line has been achieved, with the improvement in data quality allowing an increased confidence in the interpretation. Application of similar, data-specific, processing should significantly improve other datasets from areas of complex geological structure. Recent geodetic measurements reported by McCaffrey (1995) have suggested that subduction at the Timor trough has almost stopped. However, observations by Charlton (1988) that the wedge thrust front in the Timor trough to the south of Timor has not advanced significantly in the last 450 000 years have been interpreted as indicative of episodic propagation of the thrust front during continuing subduction by Masson et al. (1991). From a detailed examination of the frontal part of the accretionary wedge on the DAMAR line, it is difficult to resolve this debate. The lack of recent sediment deposition to the south of the thrust front does not allow any estimation of timing of the most recent advance of the wedge and from Charlton's observations both scenarios are possible. A further indication of possible recent thrust activity within the wedge is the identification of active faults either at the thrust front or within the wedge, affecting the more recent sediments which overlie the wedge. Such faults will produce sediment thickening and sharp topographic variations on the wedge top. The reprocessing of the DAMAR line has allowed an improved interpretation of the internal structure of the accretionary wedge which should allow similar structures to be imaged. However, although some faults appear to outcrop at the seabed with the development of minor breaks in topography (e.g. at shotpoints 785, 810, and 922) and may indicate minor internal thickening, the thrust front and much of the internal part of the wedge appear to be recently inactive with continuous, undeformed sediment drape across the outer slope and the wedge front. The lack of a recent significant sediment thickness in the trough axis is attributed to a lack of deposition, rather than due to recent wedge movement imbricating any pre-existing trough fill. From the improved processing of the DAMAR line it is possible to identify the large-scale internal deformation mechanism within the wedge. This is dominated by high-angle thrusts which have imbricated sediments from the upper part of the subducting Australian passive margin. This indicates that the deformation mechanism within the frontal part of the accretionary wedge consists of the imbrication of coherent blocks rather than by a shearing of the subducting Australian margin sediments into an incoherent melange.
SEISMIC REFLECTION DATA FROM THE TIMOR TROUGH
83
References BAXTER, K. 1993. Quantitative modelling of continent
collision: Application to the Timor region, Eastern Indonesia. PhD thesis, University of Liverpool. BowrN, C., PURDY, G. M., JOHNSTON, C., SHOR, G., LAWVER, L., HARTONO,H. M. S. & JEZEK, P. 1980. Arc-continent collision in the Banda Sea region. AAPG Bulletin, 64, 868-915. CHARLTON, T. R. 1988. Tectonic accretion and erosion in steady-state trenches. Tectonophysics, 149, 233-243. HALE, D. 1984. Dip moveout by Fourier transform. Geophysics, 49, 741-757. HUGHES, B. D. 1994. Reprocessing, modelling, and
interpretation of complex seismic reflection data from accretionary wedges: Application to the Timor Trough, Indonesia. MSc thesis, University of Leeds. KARIG, D. E., BARBER, A. J., CHARLTON, T. R., KLEMPERER, S. & HUSSONG,D. M. 1987. Nature and distribution of deformation across the Banda Arc-
Australian collision zone at Timor. Bulletin of the Geological Society of America, 98, 18-32. MASSON, D. G., MILSOM, J., BARBER, A. J., SIKUMBANG, N. & DWIYAYrO, B. 1991. Recent tectonics around the island of Timor, eastern Indonesia. Marine and Petroleum Geology, 8, 35-49. MCCAFFREY,R. 1996. Slip partitioning at convergent plate boundaries of SE Asia. This volume. PIGRAM, C. J. & PANGGABEAN,H. 1984. Rifting of the northern margin of the Australian continent and the origin of some of the microcontinents in eastern Indonesia. Tectonophysics, 107, 331-353. SNYDER, D., PRASETYO, H., BLUNDELL, D. J., PIGRAM, C. J., BARBER, A. J., RICHARDSON, A. & TJOKOSAPROETRO, S. 1996. Style of crustal deformation across the Banda Arc continent-arc collision zone as observed on deep seismic reflection profiles. Tectonics, in press. YtLMAZ, O. 1987. Seismic data processing. Investigations in Geophysics, No. 2. Society of Exploration Geophysicists.
Extension,
collision and curvature
in the eastern Banda
arc
JOHN MILSOM, STEVE KAYE & SARDJONO
Research School of Geological and Geophysical Sciences, Birkbeck College and University College London, Gower Street, London WC1E 6BT, UK A b s t r a c t : The Kai Islands occupy the region of maximum curvature in the east of the Banda arc, where the Aru trough has been regarded as the surface trace of past subduction and present arccontinent collision. Eocene to Pleistocene sediments on Kai Besar, the easternmost island, have not been deeply buried or imbricated but have experienced large-scale extensional faulting. The associated Bouguer gravity high of more than +200 mGal requires upfaulting of the accretionary complex, the attenuated Australian continental crust on which it rests and the underlying mantle at the western side of the Aru trough. Seismic reflection surveys show the deformation front within the Aru trough SE of the Kai Islands but entirely to its west further north. Instead of continuing NNE to an offset near the coast of New Guinea, the collision trace passes through the narrow and shallow strait between Kai Besar and the other islands, and thus mimics the relatively smooth curve of the Bouguer gravity contours, rather than the discontinuities of the bathymetric troughs. The continuity in deep and shallow structures is strong evidence for the existence of the outer arc as a single geological unit prior to the present phase of arccontinent collision.
The B a n d a arc, in eastern Indonesia, is the site o f the collision b e t w e e n the Australian continental margin, which is m o v i n g north with the IndoAustralian plate, and the B a n d a Sea which lies on the SE Asia plate (Fig. 1). The collision has been
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extensively studied in Timor, where the processes involved are relatively simple and where transfer o f imbricated material from the Australian margin to SE Asia has been well d o c u m e n t e d (cf. B a r b e r 1979; Charlton et al. 1991a). Similar processes
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From Hall, R. & Blundell, D. (eds), 1996, Tectonic Evolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 85-94.
85
86
J. MILSOM ET AL.
operate further to the east, where impressive images of continental crust underthrusting the arc have been provided by multichannel seismic surveys near the islands of Kisar and Damar (Richardson & Blundell 1996) and Tanimbar (Schltiter & Fritsch 1985). The similarity of the seismic sections from these two areas is surprising since the collision trace swings markedly to the north in the Tanimbar region to an orientation almost parallel to the present-day SE AsiaAustralia convergence vector. The problem is further compounded on the northern side of the arc, where the island of Seram has been described as geologically a mirror image of Timor (AudleyCharles et al. 1979). The very similar seismic reflection sections from the Seram Trough (Hamilton 1979: Jongsma et al. 1989a) presumably
record collision between Southeast Asia and the Bird's Head peninsula of New Guinea. The Banda arc thus appears to achieve the impossible by being involved in collision simultaneously, and with the same plate, to the north and to the south. As noted by McCaffrey & Abets (1991), either the Australia-New Guinea plate or the SE Asia plate, and quite possibly both, must be undergoing considerable internal deformation. The nature of the tectonic regime in the eastern part of the Banda arc has been controversial from almost the earliest days of plate tectonics. The Timor, Tanimbar, Am and Seram troughs which separate the islands of the outer Banda arc from the Australian continental margin are generally less than 50 km across and only 1.0-2.5 km deep. The Am trough, east of the Kai Islands, is exceptional,
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87
EXTENSION, COLLISION ~ CURVATURE, E BANDA ARC
being about 100 km across at its widest point and reaching a maximum depth of more than 3.5 km (Fig. 2). Fitch & Hamilton (1974) identified this trough as the subduction trace a view contested by Audley-Charles & Milsom (1974), who cited Bowin et al. (1980) as their authority for an extensional origin for the Aru trough and suggested the 7 km deep Weber Basin west of the Kai Islands as an alternative collision site. Although the reason for the great depth of the Weber Basin is still uncertain (but see McCaffrey 1988), later authors have generally followed Hamilton (1979) in placing a collision trace in the Am trough, often transferring motion from its northern end to the Scram trough by strike-slip motion along the Tarera-Aiduna fault in the extreme southwest of the Bird's Head (Fig. 3a). Geophysical data from the Kai Islands favour a third possibility which was hinted at by SchRiter & Fritsch (1985) in one of their illustrations (Fig. 3b) but was not discussed in,their text.
Geology of the Kai Islands The Kai archipelago (Fig. 1 inset) is one of the major island groups of the outer Banda arc, occupying the region of maximum arc curvature. The onshore geology has been described by Achdan & Turkandi (1982) and by Charlton et al. (1991b). The latter authors divided the group into Eastern, Central and Western Provinces, of which the Eastern Province consisted of the high, rugged
and strongly N-S elongated island of Kai Besar. The Central Province included Kai Kecil and the Tayandu Islands and the Western Province comprised a number of small islands, including Kur and Fadol, close to the edge of the Weber Basin. Kai Besar, which contains the major exposures of non-coralline rocks in the archipelago, consists of heavily block-faulted sediments of the Australian margin belonging to the Eocene, deepwater, Elat Formation, the shallow water OligoMiocene Tamangil and Weduar Formations and the Pliocene Weryahan Formation which consists of shallow-water limestone and marl. Achdan & Turkandi (1982) showed the Pliocene rocks in depositional contact on all older formations but Charlton et al. (1991b) failed to find any evidence of this unconformity. All geological discussions of Kai Besar comment on the strong NNE-SSW faulting, which Charlton et al. (1991b) considered to be extensional, both on the basis of the regional geological setting at the edge of the Am trough and because of the consistent pattern of large eastfacing escarpments (reaching to a maximum of 800 m above sea-level) and gently west-tilted sediments. The faults are offset or terminated at WNW-ESE lineaments which were interpreted as transfer faults. Kai Kecil, which is separated from Kai Besar by Selat Nerong (Nerong Strait), consists mainly of slightly raised coral reef. The occasional outcrops of the Weryahan Formation may occupy the cores of gentle anticlinal upwarps. The other islands of the Central Province are generally similar and are
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Fig. 3. The Kai-Bird's Head triple junction, showing plate boundary solutions suggested by (a) Hamilton (1979) and (b) Schltiter & Fritsch (1985). Bathymetry as Fig. 2 but with values omitted.
88
J. MILSOM ET AL.
notable for the presence of the mud volcanoes which are a feature of accretionary complexes around the entire outer Banda arc (cf. Barber et al. 1986). There is no evidence of imbrication and the carbonate platform is therefore assumed to have developed on top of the accretionary complex after convergence had ceased. High grade metamorphic rocks (silicic schists, gneiss and migmatite) which are exposed on the small islands of the Western Province have been correlated by Charlton et al. (1991b) with the Kobipoto Complex of Seram.
Seismic reflection data Examples of multichannel seismic reflection sections obtained by the research vessel R/v S o n n e along lines which crossed the southern end of the Aru trough were published by Schltiter & Fritsch (1985), and sections obtained by the Dutch Snellius II project across the trough NE of the Kai Islands using a 4-channel system have been published by Jongsma et al. (1989b). BGR-93, the northernmost of the Sonne lines, crossed the Am trough only a little south of Kai Besar but did not extend as far west as the offshore prolongation of the island (Fig. 2). The portion of the section reproduced by Schltiter & Fritsch (1985) shows Australian continental margin sediments dipping beneath unstratified material, identified as melange, at a deformation front in the deepest part of the trough, but ends in the west in a region where water depths still exceed 2500 m and so does. not show any of the structures at the trough's western margin. A significant difference between the section obtained on this line and those ?rom lines further south is the upfaulting of the sediments of the continental margin by about 1 s twt beneath the melange wedge about 15 km west of the deformation front. Schliiter & Fritsch (1985) interpreted the fault as a thrust but the evidence for overstepping of the upper block on the lower is not strong; the fault seems to be high angle but could be either normal or reverse. The southernmost of the Snellius lines (Line A2) ran from east to west across the Aru trough about 30 km north of the northern tip of Kai Besar. In the west it fell just short of the prolongation of the axis of the island but it did reach the East Kai Ridge, a bathymetric high between Kai Besar and the Am trough which rises to within 500 m of the sea surface. Along Line A2, the ridge is in water about 2000 m deep and the floor of the trough is at a depth of about 3300 m (Jongsma et al. 1989b). Underthrusting Australian continental margin was not imaged; this may have been because the seismic system used lacked the penetration of the system mounted on the Sonne or because the trough fill is considerably thicker (approaching 1 km), but the
sediments are virtually horizontal and the section can be interpreted in purely extensional terms. Arcward-dipping and arcward:thickening sediments do occupy the shallower basin between the East Kai Ridge and the Kai Plateau but there is no evidence for convergence there and the boundary between the two appears to be a large normal fault. The Kai region has also been covered by a high quality seismic survey which included data recorded to 8 s twt along lines which passed close to the northern and southern ends of Kai Besar (respectively UT-A and UT-B of Fig. 2). Figure 4 shows line-drawing interpretations of these sections. The interpretation of line UT-B which passed just south of the Kai Islands may be compared in part with Line BGR-93. In the southeast, beneath the shallow waters of the Arafura Sea, sediments of the Australian margin have been subject to normal faulting with slight rotation. Faulting increases into the Am trough and the eastern flank of the trough is underlain by fault blocks, some of which are noticeably tilted. The deepest part of the trough appears to coincide with the outcrop on the sea floor of the deformation front and further west the continental margin sequence can be traced, deepening towards the west, as far as the steep NW wall of the trough. There is a very thin layer of flat-lying sediment above the seafloor trace of the deformation front but little or no sediment cover on the accretionary complex within the trough. The abrupt change of sea-floor slope at the foot of the NW wall of the trough strongly suggests normal faulting, and this interpretation is reinforced by the reappearance of the west-deepening continental margin reflectors within the upthrown block. The throw on the fault appears to amount to about 5 s twt. The seismic character of the material which overlies the dipping reflectors and which forms the southern extension of the Kai Plateau is similar to that of the accretionary wedge in the Aru trough, although a little more high-frequency content has been preserved. The implications are clear; the Aru trough is an extensional feature bounded on both sides by normal faults, with the main basin-controlling fault in the west. The Kai Plateau, where crossed by the line, is part of the accretionary prism and therefore equivalent to southern Timor and perhaps to a part of the inner slope down to the Timor trough, but has been displaced upwards along major normal faults which post-date the main convergent movements. The time available for extensional faulting has been very short; there are still volcanic eruptions in the eastern Banda arc and a considerable level of seismic activity although, as demonstrated by McCaffrey & Abers (1991), fault plane solutions suggest extension in the trough and strike-slip
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rather than thrust motion in the Banda Sea. Although the present Am trough is a young feature, the graben framework may well have been established in the Mesozoic, when the passive margin of northwest Australia was formed by rifting. The line drawing in Fig. 4 of the seismic section obtained just to the north of Kai Besar, shows a very different picture. The East Kai Ridge, at about 2000 m, is underlain by sediments which dip gently westward and are similar in seismic character to the Australian continental margin sediments on the southern line. There are no indications of an accretionary complex beneath the ridge, which is consistent with their absence from the Snellius seismic sections of Jongsma et al. (1989b), which were obtained across the whole width of the trough. However, an apparent accretionary complex can be seen beneath the Kai Plateau, overlying westdipping reflectors which emerge at the sea floor in the northern extension of Selat Nerong, the narrow and shallow seaway between Kai Besar and Kai Kecil. The region between this easternmost part of the Kai Plateau and the East Kai Ridge is dominated by diffraction patterns which were probably generated by closely spaced normal faults. Kai Besar is therefore on the Australian side of the deformation front and underlain by Australian continental crust. The deformation front is east of Kai Besar in the south but west of Kai Besar in the north.
Gravity
data
The very strong gravity gradients associated with the Banda arcs have been known ever since the pioneering marine surveys of Vening Meinesz (1932, 1954). The gradient has since also been defined on land, notably in eastern Timor where Bouguer gravity increases over a distance of 50 km from close to zero along the axis of the island to more than +100 reGal at points on the north coast (Chamalaun et al. 1976; Kaye 1990). Similar gradients between similar limiting values have been reported from Seram (Milsom 1977) and from Tanimbar (Kaye 1990). Apart from its size and steepness, the Banda gravity gradient is also notable for its continuity and smooth curvature in plan view, controlled largely by deep crustal structure and therefore seen most clearly on regional maps of Bouguer gravity (Fig. 5). The pattern appears to be little affected by the presence of island groups, confirming the source of the gradient as deep-seated. If the position of the steepest slope can be taken as a guide, then in the Tanimbar group the main island is part of the same geological zone as southern Timor and equivalents of northern Timor are to be found in the small islands closer to the Weber Basin (Kaye 1990). Land gravity measurements have been made throughout the Kai Islands, with the majority of
90
J. MILSOM E T AL.
Fig. 5. The Bouguer gravity field of the Banda Sea region. Contours, at 25 mGal intervals, after Bowin et al. (1980) with additional information from Woodside et al. (1989) and unpublished data. Stipple indicates regions of negative Bouguer gravity.
stations situated within a few metres of the shoreline which provided a ready, if approximate, height reference (Kaye 1990). The Bouguer gravity map (Fig. 6) has been prepared by combining the onshore data with data from lines UT-A and UT-B. The Bouguer gravity low departs from the bathymetric axis, which trends NNE into the Aru trough and instead curves smoothly into the Seram trough, passing to the west of the main islands of the group. The contour pattern is dominated by a high which peaks at more than +200 mGal at the east coast of northern Kai Besar and by a broader low, with a minimum value of less than -30 mGal, over the western islands of the Central Province (Fig. 6). From the low there is a moderately strong gradient eastwards across Kai Kecil and a steeper one westwards across Kur and Fadol towards the Weber Basin. This pattern suggests that the Tayandu Islands belong to the same geological zone as southern Timor and the basinal areas of northern Seram. The main island of the Central Province, Kai Kecil, is then gravitationally analogous to the Timor trough or even to its outer (Australian) flank, although there is no deep marine basin between it and the other islands of the group, either to the east or the west. The strong gravity high which is associated with Kai Besar (Eastern Province) has no parallel elsewhere in the outer Banda
arc or on the Australian shelf, and is exceptional, in global terms, for any area underlain by continental crust. Marine gravity work has shown that the high extends, with diminishing intensity, almost to the coast of New Guinea (Woodside et al. 1989). The gravity profile interpreted in Fig. 7 was extracted from the data shown in Fig. 6 along a line oriented approximately ESE which crosses the centre of the Bouguer maximum on Kai Besar. This is the area in which gravity interpretation has most to offer. A profile based on seismic line UT-B, which passed south of Kai Besar, could have been based on actual gravity observations for a greater distance but would have been less interesting since the interpretation would have closely resembled that already published by Schltiter & Fritsch (1985). On the other hand, complications would have arisen in modelling a profile based on line UT-A because it would run parallel to, rather than across, the trend of the Bouguer contours immediately north of Kai Besar. The profile selected has its origin at the boundary between the West and Central provinces (i.e. between the Tayandu group and Kur and Fadol), close to the start of the Bouguer gravity rise towards the Weber Basin. About 50 km east of the origin, the line passes close to the western end of
EXTENSION, COLLISION ~ CURVATURE,E BANDA ARC
91
13~2°E 1j~tk 180
IN
6°S 0 I
50 Km I
Contour interval 10 reGal I
Fig. 6. Bouguer gravity of the Kai Islands. Onshore data from Kaye (1990). Offshore data from profiles provided by Union Texas (Kai) Inc., with the permission of Pertamina.
UT-A, but diverges thereafter since the seismic line, necessarily, passes to the north of Kai Besar. Despite this, the seismic section provides an important constraint on the shallow structure used in gravity modelling. Deep structures are less well controlled but a satisfactory fit with observation is difficult to obtain without supposing that only the relatively thin outer margin of the Australian continental crust has been subducted and that further crustal thinning occurred during the formation of the Ayu Trough. The gravity low around the Tayandu Islands and the high over Kai Besar require lateral changes in density both at shallow depths, to account for the steepness of the gradients, and in deep structure, to explain the large differences in absolute levels. The peak value of +200 mGal on Kai Besar is typical of intra-oceanic arcs and oceanic islands but can be achieved in an area underlain by continental crust by raising the Moho to an extent which implies both that the crust is thin and that the Aru trough western boundary faults cut down to and displace the Moho. This interpretation is consistent with the seismic reflection data. The gravity data also provide direct support for the existence of an accretionary complex beneath the Central Province. The gravity low requires both crustal thickening, which is provided by the doubling up of crust implied b y subduction, and low density material, either young stratified sedi-
ments or melange, near the surface. There are no indications of significant thicknesses of young undeformed sediments on seismic sections across the Central Province but Charlton et al. (1989b) noted the presence of mud volcanoes, which are characteristic of melange terranes. The model therefore incorporates an accretionary wedge; its density and thickness can be varied widely and still satisfy the gravitational constraints but the essential features of the model are preserved through these changes. The models are most easily brought into agreement with the observed field if a density of 2.2 Mg m-3 or less is used for the main part of the wedge. It seems fair to use the term melange for what is obviously a very disordered assemblage, but geological mapping in the possibly equivalent terrane of southern Timor has shown that large blocks of sediment have retained their original internal structure (cf. Barber 1979). The model includes a number of layers of sediment on the downgoing plate, the youngest of which are incorporated into the melange at the deformation front while the older are preserved to considerable depths beneath the wedge. These features are introduced to maintain compatibility with the seismic sections and are not dictated by the gravitational constraints. Gravity modelling can say little about the degree to which sediments are subducted or are incorporated with little deformation within the melange, and the densities assigned
J. MILSOMET AL.
92 200 f' Bouguer gravity (reGal)
I (Reductiondensity 2.2 Mg,m~) J°°t
~
~
~
Calculated ~ - .
ot, W N W . ~
()bscrvcd
• •
ESE
5~
_ : ..."...,.,.,.o.:...e_ s~lsm°~"ni[,
0
50
100
Kilomctrcs 150
Australian continental
200
crust
250
Fig. 7. Gravity profile and model cross-section across northern Kai Besar. The upper part of the accretionary complex (above the dotted line) has been assigned a density contrast of-0.5 Mg m-3. with standard crust, corresponding to a real density of 2.17 Mg m-3, the lower part a density contrast of -0.3 Mg m-3. Tertiary sediment layers have density contrasts of -0.47, -0.42 and -0.3 Mg m-3 with standard crust. Arc crust, continental crust and Mesozoic sediments have been assigned standard density.
to the sediments are not critical to the success or failure of the model. Discussion There are three possible routes by which the deformation front defined by seismic reflection surveys south of Timor might link with its mirror image defined by Jongsma et al. (1989a) in the Seram trough. The seismic sections of Schltiter & Fritsch (1985) which imaged the deformation front in the eastern parts of the southern Am trough eliminated the western route through the Weber Basin as a possibility and could also be considered as support for the eastern route along the Am trough to a transfer at the Tarera-Aiduna fault. The absence of the front from the seismic sections presented by Jongsma et al. (1989b) is not strong evidence against this latter possibility since the weak source used lacked deep penetration in the thick sediments. However, the UT-A seismic section (Fig. 4) is interpreted as showing a westdipping thrust which separates stratified sediments on Kai Besar from an accretionary wedge in the Kai Central Province. This interpretation is supported by the fact that the Eocene to Pliocene sediments on Kai Besar have been tilted but not imbricated and
show no signs of deep burial. The faulting they have suffered has been interpreted as extensional by Charlton et al. (1991b) and this seems to be in accord with the seismic data. It is almost impossible to reconcile such an interpretation with the existence of a true deformation front beneath the sediments in the Aru trough since fault displacement of the order of l0 km followed by considerable erosion would be needed to expose this front on the upthrown block, and these processes would have had to have taken place in the short period since the subduction zone locked. The trace of past subduction must therefore pass through Selat Nerong and then curve into the Seram trough. The fault locations in Fig. 8, which illustrate this solution, are inevitably schematic, being constrained by seismic data along the seismic lines of Fig. 4, but elsewhere being drawn on the basis of bathymetric features which are themselves not well controlled. Almost certainly the true pattern is more complex, but such complexity would not affect the conclusion that the deformation front broadly parallels the curvature of the Bouguer gravity contours and does not follow the bathymetric trends along the Am trough The cause o f the extreme curvature of the Banda arc is one of the major unsolved problems of SE
EXTENSION, COLLISION •
93
CURVATURE, E BANDA ARC
5°S
J k.~
/
0
c,.ar
-
100 K m
...
1 °E~"
133°E
Fig. 8. The Kai-Tanimbar region, showing suggested interaction of extensional faulting with earlier thrust faulting. Strong black lines show thrust faults of the deformation front, narrower lines mark possible locations of faults of the Aru trough extensional system, as suggested by bathymetry. Because of the absence of major geological differences between northern and southern Kai Besar (cf. Charlton et al, 1991b), the narrow isthmus which links the two is attributed to transfer faulting associated with the extensional system, rather than with the passage of the thrust from the Aru trough to Selat Nerong.
Asian geology (cf. Linthout e t al. 1991). Evaluation of the processes involved has been hampered by the lack of a universally accepted model of arc evolution. There are three broad possibilities, one of which is that the curvature has been a feature of the arc since its inception. This has found little favour (i) because of the problems involved in incorporating a tightly curved arc into reconstructions of the area during the Neogene, (ii) because of the difficulty of transporting some Australian continental fragments to their present positions within the arc, (iii) because of the evidence for some 74 ° counter-clockwise rotation of Seram (Haile 1981) and (iv) because of the
evidence for continuing N - S compression and E - W extension in the Banda Sea (McCaffrey & Abers 1991). An alternative possibility is that the arc was assembled relatively recently from two or more elements with quite different previous histories. Such a solution is attractive since it poses few mechanical difficulties and places few constraints on Tertiary reconstructions, but it has always encountered an obstacle in the evidence for continuity in deep structures provided by regional gravity maps. If it could be shown that the subduction trace coincided with the Aru trough and was offset along the Tarera-Aiduna fault (Fig. 3a),
94
J. MILSOM ETAL.
this objection might be discounte&since the offset w o u l d imply a major discontinuity in the upper, as well as the lower, plate. The seismic reflection evidence that curvature o f the deformation front m a t c h e s the curvature o f the B o u g u e r gravity contours, and the detailed gravity interpretation w h i c h c o n f i r m s this c o n c l u s i o n , nullify this argument. Whatever the geometrical difficulties involved in bending not merely a crustal sliver but
an associated subducted slab, it seems that this is what has happened in the Banda arc. We thank Union Texas (Kai) Inc. for providing access to their unpublished seismic sections and marine gravity data and permitting reproduction of our line drawing interpretations of the seismic data. Sardjono publishes with the permission of the Director, Geological Research and Development Centre, Bandung.
References ACHDAN, A. & TURKANDI,T. 1982. Preliminary geologic map of the Kai (Tayandu and Tual) quadrangles, .Maluku, Indonesia (Scale 1:250,000). Geological Research and Development Centre, Bandung. AUDLEY-CHARLES, M. G. & MILSOM, J. 1974. Comment on 'Plate convergence, transcurrent faults and internal deformation adjacent to south east Asia' by T. J. Fitch. Journal of Geophysical Research, 79, 4980-4981. , CARTER, D. J., BARBER, A. J., NORWCK, M. S. & TJOKROSAPOETRO, S. 1979. Reinterpretation of the geology of Seram: implications for the Banda Arcs and northern Australia. Journal of the Geological Society, London, 136, 547-568. BARBER, A. J. 1979. Structural interpretation of the island of Timor. SEAPEX Proceedings, 4, 9-21. , TJOKROSAPOETRO,S. & CHARLTON,T. R. 1986. Mud volcanoes, shale diapirs, wrench faults and melanges in accretionary complexes, eastern Indonesia. AAPG Bulletin, 70, 1729-1741. BOWIN, C., PURDY, G. M., JOHNSTON, C., SHOR, G., LAWVER, L. HARTONO, H. M. S. & JEZEK, E 1980. Arc-continent collision in the Banda Sea region. AAPG Bulletin, 64, 868-915. CHAMALAUN, E H., LOCKWOOD, K. & WHITE, A. 1976. The Bouguer gravity field and crustal structure of eastern Timor. Tectonophysics, 30, 241-259. CHARLTON,T. R., BARBER, A. J. & BARKHAM,S. T. 1991a. The structural evolution of the Timor collision complex, eastern Indonesia. Journal of Structural Geology, 13, 489-500. , KAYE, S. J., SAMODRA,H. & SARDJONO. 1991b. The geology of the Kai Islands: implications for the evolution of the Aru Trough and the Weber Basin. Marine and Petroleum Geology, 8, 62-69. FITCH, T. J. & HAMILTON,W. 1974. Reply to comments on Audley-Charles, M. G. and Milsom, J. on 'Plate convergence, transcurrent faults and internal deformation adjacent to south east Asia' by T. J. Fitch. Journal of Geophysical Research, 79, 4982-4985. HALLE, N. S. 1981. Palaeomagnetic evidence and the geotectonic history and palaeogeography of Eastern Indonesia. In: BARBER, A. J. & WIRVUSUJONO, S. (eds) The Geology and Tectonics of Eastern Indonesia. Geological Research and Development Centre, Bandung, Special Publication 2, 81-87.
HAMILTON,W. 1979. Tectonics of the Indonesian region. US Geological Survey Professional Paper, 1078. JONGSMA, D., WOODSIDE,J. M., HUSON, W., SUPARKA,S. KADARISMAN, D. 1989a. Geophysics and tentative Late Cenozoic seismic stratigraphy of the Banda Arc-Australian continent collision along three traverses. Netherlands Journal of Sea Research, 24, 205-229. --, HUSON, W., WOODSIDE, J. M., SUPARKA, S., SUMANTRI, T. & BARBER, A. J. 1989b. Bathymetry and geophysics of the Snellius II triple junction and tentative seismic stratigraphy and neotectonics of the northern Aru Trough. Netherlands Journal of Sea Research, 24, 231-250. KAYE, S. K. 1990. The structure of eastern b~donesia: an approach via gravity and other geophysical methods. PhD thesis, University of London. LINTHOUT, K., HELMERS, H. & ANDRIESSEN,E A. M. 1991. Dextral strike-slip in central Seram and 34.5 Ma Rb/Sr ages in pre-Triassic metamorphics related to early Pliocene counterclockwise rotation of the Buru-Seram microplate. Journal of SE Asian Earth Sciences, 6, 335-342. M¢CAFFREY, R. 1988. Active tectonics of the eastern Sunda and Banda arcs. Journal of Geophysical Research, 93, 15163-15182. -& ABERS, G. A. 1991. Orogeny in arc-continent collision: the Banda Arc and western New Guinea. Geology, 19, 563-566. MILSOM, J. 1977. Preliminary gravity map of Seram, eastern Indonesia. Geology, 5, 641-643. RICHARDSON, A. N. & BLUNDELL,D. J. 1996. Continental collision in the Banda arc. This volume. SCHLt3TER, H. U. & FR~TSCH, J. 1985. Geology and tectonics of the Banda arc between Tanimbar Island and Aru Island (Indonesia). Geologisches Jahrbuch, E30, 3-41. VENING MEINESZ, F. A. 1932. Gravity expeditions at sea, Vol I. Netherlands Geological Commission, Delft. -1954. Indonesian archipelago-a geophysical study. Geological Society of America Bulletin, 65, 143-164. WOODS1DE,J. M., JONGSMA,D., THOMMERET,M., STRANG VAN HEES, G. L. & PUNTEDEWO. 1989. Gravity and magnetic measurements in the eastern Banda Sea. Netherlands Journal of Sea Research, 24, 185-203.
Pre-Cretaceous evolution of SE Asian terranes I. M E T C A L F E
Department of Geology & Geophysics, University of New England, Armidale NSW 2351, Australia Abstract: During the last decade, a wide range of geological and geophysical data has led to the recognition of various continental terranes in SE Asia which, on tectonostratigraphic, palaeobiogeographic and palaeomagnetic grounds are 'suspect' or allochthonous in nature. Some of the recognized terranes may be composite and there is some disagreement with regard to the number of terranes and their boundaries. The continental terranes are bounded by sutures (representing former oceans), by narrow mobile belts or major fault zones. Comparative studies of the stratigraphy, palaeontology, and palaeomagnetism of the various pre-Cretaceous continental terranes of East and SE Asia suggest that they were all derived directly or indirectly from Gondwanaland. Asian continental terranes that are placed on the India-Australian margin of Gondwanaland in the Early Palaeozoic include Tarim (here regarded to include the Kunlun and Ala Shan terranes), Qaidam, Indochina/East Malaya (which includes the Qamdo-Simao block of western China), North & South China, Sibumasu, Qiangtang, Lhasa, Kurosegawa, NW and SE Hainan, West Burma and the Woyla terranes. The evolution of Gondwanaland and Tethys during the Palaeozoic and Mesozoic involved the rifting of continental slivers/fragments from northern Gondwanaland, and the northwards drift and amalgamation/accretion of these to form proto-East and SE Asia. Three continental slivers were rifted from the northern margin of Gondwanaland in the Early to Late Devonian (North China, South China, Indochina/East Malaya/Qamdo-Simao, Qaidam and Tarim terranes); Early-Middle Permian (the Cimmerian continent including the Sibumasu and Qiangtang terranes and possibly NW and SE Hainan); and Late Triassic to Late Jurassic (Lhasa, West Burma and Woyla terranes). The northwards drift of these terranes was accompanied by the opening and closing of three successive oceans, the Palaeo-Tethys, Meso-Tethys and Ceno-Tethys. Assembly of Gondwanaland-derived Asian terranes began with the amalgamation of South China and Indochina/East Malaya along the Song Ma/Song Da zone during the Late Devonian/Early Carboniferous to form 'Cathaysialand'. Palaeomagnetic, climatic and bitgeographic data indicate that Cathaysialand and North China were located within the Palaeo-Tethys at low northern/equatorial latitudes during the Late Carboniferous and Permian. Palaeomagnetically determined palaeolatitudes are consistent with the development of tropical Cathaysian floras on these terranes. The Tarim, Kunlun, Qaidam and Ala Shan terranes accreted to Kazakstan/Siberia in the Permian. A major episode of rifting occurred on the northern margin of Gondwanaland in the Late Carboniferous-Early Permian and the Cimmerian continent separated in the late Early Permian resulting in the opening of the Meso-Tethys. Suturing of Sibumasu and Qiangtang to Cathaysialand occurred in the Late Permian-Triassic, closing a major branch of the PalaeoTethys. South and North China amalgamated and then accreted to Laurasia by Late Triassic-Early Jurassic times. The highly disrupted Kurosegawa terrane of Japan, possibly derived from Australian Gondwanaland, accreted to Japanese Eurasia, also in the Late Jurassic. The Lhasa, West Burma and Woyla terranes rifted from NW Australian Gondwanaland in the Late Triassic to Late Jurassic and drifted northwards during the Jurassic and Early Cretaceous as the Ceno-Tethys opened and the Meso-Tethys was destroyed by subduction beneath Eurasia. Accretion of these terranes to proto-SE Asia occurred in the Cretaceous. The SW Borneo and Semitau terranes were derived from the South China/Indochina margin by the opening of a marginal basin in the Cretaceous which was subsequently destroyed by southwards subduction during the rifting of the Reed Bank-Dangerous Grounds terrane from South China when the South China Sea opened. The NW and SE Hainan terranes, which formed part of Early Palaeozoic and possibly Late Palaeozoic Gondwanaland, reached their current positions, relative to South China, sometime in the Jurassic-Cretaceous.
The pre-Cretaceous evolution o f SE Asia has received m u c h attention during the last decade and has been the principal focus of two recent I G C P projects, IGCP 224 (Pre-Jurassic Evolution o f East Asia) and I G C P 321 ( G o n d w a n a Dispersion and
Asian Accretion). The evolution of East and SE Asian terranes, in the framework of the evolution of Gondwanaland, Laurasia, Pangaea and Tethys, has important scientific implications for both regional and global earth sciences. Detailed information on
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolution of SoutheastAsia, Geological Society Special Publication No. 106, pp. 97-122.
97
98
I. METCALFE
the movements (particularly cross-latitudinal) of continental blocks and the changing spatial distribution of seas and oceans provides a basis for investigating globally important problems. Past ocean currents and climatic patterns have been very important in determining biogeographical distributions of both marine and land organisms on the Earth and a knowledge of changing continent/ocean configurations helps us model these past climates and current patterns and leads to a greater understanding of the observed changes in both the distributions of organisms, and sedimentation patterns in the evolving seas and oceans. This paper forms a contribution to the lUGS Global Sedimentary Geology Program's Project Pangaea which is primarily concerned with modelling global climate during the existence of Pangaea. Of importance to this modelling is the continent/ocean configuration, and more specific to East and SE Asian terranes, was Tethys an empty open gap into Panthalassa or did it contain continental fragments, and if so what positions did these occupy relative to the rest of Pangaea? Biologists, botanists and zoologists are becoming increasingly interested in SE Asian terrane movements, particularly in the Mesozoic and Cenozoic, because the movements of continental fragments may have provided land 'rafts' or chains of islands that influenced land plant and animal evolution and dispersal in the region. Another rationale for detailed studies of terrane evolution in the Asian region is the increasing economic importance of these regional studies in understanding metallogenesis and the development and evolution of hydrocarbon and coal-bearing sedimentary basins in the region. It is now well established that continental SE Asia (together with large parts of eastern Asia) comprises a complex assembly of continental terranes (Figs 1 and 2) which are entirely allochthonous to central and northern Asia (Siberia and Kazakhstan). All the SE Asian terranes are interpreted to have been derived directly or indirectly from Gondwanaland and their Palaeozoic and Mesozoic history involved the rifting of terranes from the northern margin of Gondwanaland, their northwards drift and amalgamation/accretion to form proto-SE Asia (Seng0r 1979, 1984; Ridd 1980; Mitchell 1981; Stauffer 1983; AudleyCharles 1983, 1984, 1988, 1991; Metcalfe 1986, 1988, 1990, 1993, 1994a, c ; Klimetz 1987; Seng/3r et al. 1988; Burrett et al. 1990; Nie et al. 1990). The northwards translation of terranes involved the opening and closing of successive Tethys oceans. Previous workers have recognized a two Tethys ocean system comprising Palaeo-Tethys ('Old Tethys') and Neo-Tethys or 'New Tethys' (e.g. Seng6r 1979, 1984, 1989). The rift-driftsuturing (amalgamation/accretion) history of the
SE Asian terranes is here considered within a framework of a three Tethys ocean system following Metcalfe (1994a, c). It is proposed that three continental slivers rifted from the northern margin of Gondwanaland during the Palaeozoic to Mesozoic, opening and closing three successive Tethys oceans, the Palaeo-Tethys, Meso-Tethys and Ceno-Tethys. The Palaeo-Tethys is broadly equivalent to the Palaeo-Tethys of previous workers, although timings of the closure of the different branches of Palaeo-Tethys differ in detail. The Meso-Tethys and Ceno-Tethys are approximately equivalent in age to what has previously been referred to as 'Neo-Tethys' but are here regarded as successive oceans developed by the Triassic-Jurassic rifting of a third continental sliver from northeast Gondwanaland.
SE Asian pre-Cretaceous terranes and their origins The pre-Cretaceous continental terranes of East and SE Asia are shown in Figs. 1 and 2. Some of these terranes may be composite, but where the composite nature is not well established or is controversial, they are treated as single units. The Cretaceous and Cenozoic evolution of the small terranes distributed around the South China Sea and in eastern Indonesia (Fig. 2), including West Irian Jaya, Buru-Seram, Buton, Banggai-Sula, Obi-Bacan, North Palawan, Spratley IslandsDangerous Ground, Reed Bank, Luconia, Macclesfield Bank, Paracel Islands, KelabitLongbowan, Mangkalihat, Paternoster, West Sulawesi, East Sulawesi and Sumba has been discussed by Metcalfe (1990) and will not be repeated here. The pre-Cretaceous terranes discussed below include those which now constitute mainland SE Asia. Some discussion of other East Asian terranes which have a bearing upon the evolution of SE Asia is also included. South China
The South China block is regarded as composite by some authors (e.g. Hsu et al. 1988, 1989, 1990). Hsu et al. (1988; 1989) recognized component Yangtze and Huanan Blocks separated by the 'Xiangganzhe Mesozoic suture'. This interpretation has been challenged by Rogers (1989) and Rowley et al. (1989), and geochronological data from Anhui and Jiangxi Provinces suggest that the suture recognized by Hsu is in fact of Proterozoic age (Chen, J. et al. 1991). Hsu et al. (1990) and Chen et al. (1993) recognize four component blocks, the Yangtze, Xianggui, Cathaysia and Dongnanya blocks separated by three melange zones (Fig. 3).
SE ASIAN TERRANE EVOLUTION
99
TARIM NORTH CHINA
SOUTH CHINA
HT
WOYLA" TERRANES
KT SUTURES Q SongMa Q Aibi-Xingxing Q Xiliao-He G Kunlun G Qinling-Dabie (~) Jinshajiang Q Lancangjiang O Banggong O IndusYarlungZangbo Q Nan-Uttaradit G Raub-Bentong ShahBoundary Woyla Meratus G Boyan G Changning-Menglian (~ Ailaoshan
Fig. 1. Distribution of principal continental terranes and sutures of East and SE Asia. EM, East Malaya; WB, West Burma; SWB, South West Borneo; S, Semitau Terrane; HT, Hainan Island terranes; L, Lhasa Terrane; QT, Qiangtang Terrane; QS, Qamdo-Simao Terrane; SI= Simao Terrane, SG, Songpan Ganzi accretionary complex; KL, Kunlun Terrane; QD, Qaidam Terrane; AL, Ala Shan Terrane; KT, Kurosegawa Terrane.
Palaeomagnetic data suggesting a separation of the Yangtze and Huanan blocks by a latitude difference of 8.3 ° in the Triassic (Chen et al. 1993) have been used to support the pre-Triassic composite nature of South China. However, the shallow inclinations reported from the Yangtze and 'Huanan' blocks (-11.8 and -13.0 respectively) are not significantly different (even w i t h small o~95s), and do not confirm a Triassic separation of these regions. The occurrence of identical endemic Devonian fish faunas from both the Yangtze region and southeast South China, which define a South China Province (Young 1993) also indicate a unified South China block at that time. South China is here regarded as a single tectonic unit since the Palaeozoic following the arguments of Rogers (1989), Rowley et al. (1989) and Chen, H. et al. (1991). Its boundaries with North China and the Indochina block are the
Qin Lin-Dabei suture, and Ailaoshan and Song Ma Sutures, respectively. Lower Palaeozoic shallow marine faunas of South China have affinities with those of northeast Gondwanaland and belong to the Asia-Australian and Austral realms in the Cambrian and Ordovician respectively (Yang, J. 1994; Li 1994). These faunal affinities, together with stratigraphic comparisons suggest that South China had its origin on the Himalaya-Iran region of the Gondwanaland margin (Burrett et al. 1990; Nie et al. 1990; Nie 1991, 1994). The Lower Palaeozoic placement of South China adjacent to the Himalaya-Iran region is consistent with the palaeomagnetic evidence which places South China in mid southern palaeolatitudes in the Ordovician (Lin et al. 1985; Burrett et al. 1990; Fang et al. 1990; Metcalfe 1990). Hoffman (1991) also placed the East and SE Asian blocks
100
i. METCALFE
PHILIPPINE
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SEA
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"
X X X X Suture
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~
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o
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Inferred Terrane Boundary J OceanIc Crust
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~ 1
Extended Continental Crust Continental Blocks and Fragments IOOE
Fig. 2. Distribution of continental blocks and fragments (terranes) and principle sutures of SE Asia (modified after Metcalfe 1990). 1. South China; 2. Indochina; 3. Sibumasu; 4. East Malaya; 5. West Burma; 6. SW Borneo; 7. Semitau; 8. Sikuleh; 9. Natal; 10. West Irian Jaya; 11. Buru-Seram; 12. Buton; 13. Bangai-Sula; 14. Obi-Bacan; 15. North Palawan; 16. Spratley Islands-Dangerous Ground; 17. Reed Bank; 18. Luconia; 19. Macclesfield Bank; 20. Paracel Islands; 21. Kelabit-Longbowan; 22. Mangkalihat; 23. Paternoster; 24. West Sulawesi; 25. East Sulawesi; 26. Sumba; 27. Banda Allochthon; 28. Qiongzhong and Yaxian terranes of Hainan; 29. Simao terrane.
es -~ ss.j s
-'--.
a~'~'~.
,, st" "",,,, _ S
adjacent to the NW margin of Australia in his Neoproterozoic supercontinent reconstruction. Recent comparisons of the Proterozoic basement of South China with Australia and Laurentia led Li et al. (1994) to propose an alternative model whereby South China is placed between Eastern Australia and Laurentia in the Late Proterozoic, forming part of Rodinia. Such a position would be at variance with an Early Palaeozoic position adjacent to Himalayan India, and Proterozoic palaeomagnetic data are required to test this proposition. The Upper Sinian and Palaeozoic tectonostratigraphy of South China is also not comparable with Eastern Australia. A northern Gondwanaland margin origin for South China is favoured here.
J
.,,-
...-......
[
./i
. ~ _ . . _ ~ Island
i
i
Fig. 3. Component blocks and melange zones of South China proposed by Chen et al. (1993).
Sibumasu
The Sibumasu block (Metcalfe 1984, 1988) is bounded to the west by the Shah Boundary Fault
SE A S I A N T E R R A N E E V O L U T I O N
and the Andaman Sea basin and to the southwest by the Woyla suture in Sumatra. Its eastern boundary is formed by the Raub-Bentong suture in Peninsular Malaysia, and the Changning-Menglian and Lancangjian Sutures in western Yunnan. The eastern boundary of Sibumasu in North Thailand and Burma is still not clear. The northwards extension of the Raub-Bentong suture has been regarded by many authors to be the Nan-Uttaradit suture in Thailand. This suture appears to bend northeastwards to be truncated by the Red River-Ailaoshan zone. In western Yunnan, the Changning Menglian suture zone (Wu & Zhang 1987; Liu Benpei et al. 1991; Wu Genyao 1993; Wu Haoruo et al. 1995) which separates the Baoshan/Tenchong blocks (disrupted parts of Sibumasu) from the Qamdo-Simao terrane (Fig. 4) appears to continue southwards into Burma and then possibly into the Chiang Rai region of north Thailand. Deep-marine ribbon-bedded cherts (Fang Chert) in the Chiang Rai region have been dated by radiolarians as Devonian to middle Permian (Sashida et al. 1993). These are associated with basic volcanics which may be subduction-related (Barr et al. 1990). Some authors have suggested that the Sibumasu terrane is composite and have recognized a western 'Mergui Platelet' (Cameron et al. 1980), 'Phuket terrane' (Cooper et al. 1989) or 'Mergui Group nappe' (Mitchell 1992) characterised by the presence of Carboniferous-Permian diamictites. There seems however to be little basis for separating the diamictite-bearing 'terrane' from Sibumasu and the implied thrust contact between the Mergui Group and equivalents and other Palaeozoic strata of Sibumasu has not been demonstrated. In fact, in northwest peninsular Malaysia, a stratigraphic contact can be unequivocally demonstrated (Hutchison 1993). However, new geochronological data from western Thailand (Ahrendt et al. 1993, 1994; Hansen et al. 1994) suggest that the high grade metamorphic rocks previously regarded as Precambrian basement in NW Thailand, have depositional ages of less than 600 Ma. New K-Ar, Rb-Sr and U-Pb ages and structural observations also indicate very young (Tertiary) cooling ages for the metamorphic 'core complex' but older (Mesozoic) ages for deformational ages in overlying low grade metamorphic rocks. This suggests the possibility of eastwards directed thin-skinned tectonics in western Thailand during the Tertiary. If this is the case, the geometry and boundaries of the Sibumasu terrane may have been disrupted and the Palaeozoic and Mesozoic strata may well have been detached from their original basement. Distinctive Gondwanaland faunas (Cambrian to Lower Permian) with NW Australian affinities on Sibumasu (Archbold et al. 1982; Burrett & Stait
|01
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Fig. 4. Principal blocks and suture zones of western Yunnan (after Wu et al. 1995).
1985; Metcalfe 1986, 1988, 1990, 1991, 1993; Burrett et al. 1990; Shi & Waterhouse 1991; Racey et al. 1994; Archbold & Shi 1995) strongly suggest a NW Australian origin for the Sibumasu terrane. This is supported by the presence of Upper Carboniferous-Lower Permian glacial-marine diamictites (Stauffer & Mantajit 1981; Stauffer & Lee 1989, Jin 1994a, b; Wopfner 1994), Lower Permian cool-water faunas and 8180 cool-water indicators (Ingavat & Douglass 1981; Waterhouse 1982; Rao 1988; Fang & Yang 1991) which indicate proximity to the Late Palaeozoic Gondwanaland glaciated region. A Glossopteris flora has also recently been reported south of Baoshan in western Yunnan (Wang & Tan 1994) and typical Lower Permian Gondwanaland spores, including P r i m u s p o l l e n i t e s levius and Schauringipollenites maximus, have been recovered from near Tenchong, western Yunnan (Yang, W. 1994). The palynoflora reported from the Tenchong block (Sibumasu) is also remarkably similar to palynofloras described from the Collie basin of Western Australia (Yang, W. 1994). Gross stratigraphical comparisons between Sibumasu and NW Australia, especially with the Canning Basin (Metcalfe 1992, fig. 3), also show similarities consistent with Sibumasu having been positioned
102
I. METCALFE
40 Sibumasu Block (Ref. at 18N, 95E)
30, LU a 20. E3 }-- 10.
}<
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:::
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--0-
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--dr
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Tertiary
Fig. 5. Palaeolatitude versus time plots for the Sibumasu Block (after Van der Voo 1993).
outboard of NW Australian Gondwanaland in the Palaeozoic. In addition, Palaeozoic palaeomagnetic data indicate palaeolatitudes (Fig. 5) consistent with a position off NW Australian Gondwanaland in the Devonian, Carboniferous and Early Permian (Bunopas 1982; Bunopas et al. 1989; Wu et al. 1989; Metcalfe 1990, 1994a, b; Huang & Opdyke 1991; Van der Voo 1993).
I n d o c h i n a - E a s t Malaya block This block is bounded to the northeast by the Song Ma and Ailaoshan suture zones and to the west by the Changning-Menglian suture in Yunnan, the Nan-Uttaradit suture in Thailand and the RaubBentong suture in Malaysia. The block probably extends northwards as the Qamdo-Simao or Simao block (Wu 1993; C h e n & Xie 1994) which is regarded as a separate, possibly South Chinaderived, tectonic unit by Wu et al. (1995). The Upper Palaeozoic and Mesozoic faunas and floras are Cathaysian/Tethyan types, have affinities to those of South and North China, and show no relationship to Gondwanaland (Metcalfe 1986, 1988). Lower Palaeozoic faunas are poorly known, and their affinities have yet to be clearly demonstrated. The Precambrian core of the block (Kontum Massif) comprises granulite facies rocks exposed in Vietnam, and it has been suggested that this may have originally formed part of the Gondwanaland granulite belt (Katz 1993). It seems likely that Indochina (including the East Malaya and Simao terranes) had its origin on the northern
Gondwanaland margin but this must remain largely a speculation until we have constraining palaeobiogeographical and palaeomagnetic data.
West Burma block Formerly known as the 'Mount Victoria Land' Block (Mitchell 1989), this terrane is here referred to as the West Burma Block to avoid confusion with Victoria Land in Antarctica. Mitchell (1993) has reinterpreted Western Burma (together with the Woyla Group of Sumatra and the Meratus ophiolite of Borneo as a lower Cretaceous mafic arc, produced by SW-directed subduction, which was then thrust on to the Asian margin in the late Early Cretaceous. Following Metcalfe (1990), the author prefers to regard Western Burma as a probable continental fragment now largely obscured by younger sediments and volcanic rocks. This block has a pre-Mesozoic schist basement overlain by Triassic turbidites and Cretaceous ammonite-bearing shales and limestones in the Indoburman Ranges and by an upper Mesozoic-Cenozoic arc association in the Central Lowlands of Burma. There is as yet no direct evidence for the origin of this block but it sutured to Sibumasu in Cretaceous times. The West Burma Block is considered (Metcalfe 1990) a good candidate for part of the continental sliver that provided a source for sediments derived from the northwest in Timor during the Triassic, and which must have rifted from Gondwanaland in the Late Jurassic. This interpretation however remains speculative.
SE ASIAN TERRANE EVOLUTION
S W B o r n e o a n d S e m i t a u blocks
The origins of these two blocks, separated by the Boyan suture, are poorly constrained. Upper Carboniferous and Lower Permian fusulinid faunas and other Lower Permian microfossils are similar to those of South and North China and do not have affinities with those of Malaya or Thailand (Vachard 1990). Triassic and Jurassic floras and faunas have affinities with South China, Indochina, Japan and the Philippines (Hayami 1984; Kimura 1984; Kobayashi & Tamura 1984). There does not appear to be any Gondwanaland connections in the faunas and floras of these two blocks. A South China/Indochina origin seems likely as proposed by Rammlmair (1993) but reconciliation between substantial progressive counter-clockwise rotation of SW Borneo during the Cenozoic and Cretaceous and younger equatorial palaeolatitudes with the various tectonic models is required. H a i n a n I s l a n d terranes
Hainan Island appears to represent at least two tectonostratigraphic terranes but the nature and boundaries of these are still controversial. Yu (1989) and Yang et al. (1989) recognise two small terranes, the northern Quiongzhong and southern Yaxian terranes bounded by the east-west Jiushuo-Lingshui Fault. Chen et al. (1993) however, interpreted the Island as a Mesozoic orogenic belt created by the collision of the Huannan (SE China) and Dongnanya (SE Asia) blocks. Gt~rtir & Sengrr (1992) interpreted the island as originally forming a part of the Sibumasu terrane, being separated from other Sibumasu components in the Mesozoic by strike-slip faulting. The Lower Palaeozoic faunas of the island indicate links with northeast Gondwanaland (Sun 1963; Lin & Jago 1993) and the presence of possible glacialmarine diamictites and Permian faunas with Gondwanaland affinities suggests a northeast Gondwanaland origin (Yang et al. 1989; Yu 1989). Recent field investigations in Hainan (Metcalfe et al. 1994) have confirmed the probable presence of two terranes on Hainan, but the boundary of these is interpreted to be the SW-NE-oriented Baisha Fault and not the east-west Jiushio-Lingshui Fault. The Late Palaeozoic Gondwanaland origin of these terranes remains problematic. Woyla terranes
Several small continental fragments, including the Sikuleh and Natal blocks, now located along the southwest margin of Sumatra were accreted to 'Sundaland' in the Cretaceous and were called the Woyla terranes by Cameron et al. (1980) and Pulunggono & Cameron (1984). Stratigraphic
103
studies of part of the Woyla terranes in the Padang area (Yancey & Alif 1977) reveal similarities with the stratigraphy of the Exmouth Plateau of the NW Australian shelf (Grriir & Seng6r 1992) and limited palaeomagnetic data from the Sikuleh block (Haile 1979) suggest palaeolatitudes of 26°S for the Late Triassic and 10°S in the Late Mesozoic which is consistent with a NW Australian origin of these terranes. K u r o s e g a w a terrane
The Japanese Islands are composed of a rifted Asian basement and a huge accretionary complex, produced by long-lived westwards subduction of the Pacific and Philippine Sea plates beneath Asia, over which the volcanic arc has migrated eastwards. A number of accreted terranes has been recognized within the accretionary complex including oceanic (seamount) and continental allochthonous terranes. The Kurosegawa con= tinental terrane is a highly attenuated and disrupted composite terrane of predominantly Palaeozoic age now sandwiched between two Mesozoic terranes in the Chichibu Belt of SW Japan. It comprises a Lower Palaeozoic basement of igneous and high-grade metamorphic rocks, Silurian-Devonian volcaniclastic sediments with carbonate blocks, serpentinite, a Upper Palaeozoic-Mesozoic chaotic complex and well-bedded Upper PalaeozoicLower Mesozoic cover sediments (Aitchison et al. 1991). Lower and Middle Palaeozoic faunas and floras have affinities with Australia and suggest a north Australian Gondwanaland margin origin for this exotic terrane (Yoshikura et al. 1990; Aitchison 1993). Studies of chromium spinels from serpentinites of the terrane (Hisada et al. 1994) suggest that the serpentinites were emplaced in a forearc setting during the Devonian. The precise location of the terrane in Silurian-Devonian times is at best speculative but a position adjacent to eastern South China, on the Gondwanaland margin, has been suggested (Saito 1992; Hisada 1994). The precise site of original attachment to Gondwanaland and the drift history of this terrane, however, remain largely unknown.
Rifting and separation of Asian terranes from Gondwanaland It is here proposed that three episodes of rifting occurred on the northern margin of Gondwanaland in the Devonian, Carboniferous to Early Permian and Late Triassic-Late Jurassic. Three continental slivers separated from Gondwanaland in the Late Devonian, late Early Permian and Late Triassic to Late Jurassic times (Fig. 6).
104
I. METCALFE in Australia (Fig. 7) proposed by Chen et al. (1993). Nie (1994) has also suggested that the conspicuous Devonian unconformity of South China, and subsequent Devonian to Triassic passive margin sequences along the southern margin of South China record a Devonian rifting episode and continental separation. In addition, a widespread Late Devonian-early Carboniferous unconformity is recognized on most of the Asian blocks and may represent the breakup unconformity related to continental separation. Carboniferous to Early P e r m i a n rifting
Fig. 6. Schematic diagram showing the three continental slivers/collages of terranes, rifted from Gondwanaland and translated northwards by the opening and closing of three successive oceans, the PalaeoTethys, Meso-Tethys and Ceno-Tethys.
D e v o n i a n rifting
Various Chinese continental blocks (North and South China, Tarim, Indochina, Qaidam) that are postulated to have been located along or close to the northern margin of Gondwanaland in the Early Palaeozoic, outboard of Sibumasu and the Tibetan blocks, were by Carboniferous-Permian times located equatorially, did not have Gondwanalandrelated faunas and floras, and could not have formed part of Gondwanaland. Close proximity or actual attachment of these blocks to Gondwanaland in the Devonian is suggested by the distribution of Devonian vertebrate faunas (Long & Burrett 1989; Young 1990, 1993; Ritchie et al. 1992). A Late Devonian separation of these blocks from Gondwanaland is therefore suggested. Late Devonian clockwise rotation of these blocks away from northern Gondwanaland would be consistent with the contemporaneous counter-clockwise rotation of Gondwanaland about an Euler pole located
There is now extensive evidence of extension and rifting along the northern Gondwanaland margin in the Carboniferous to Early Permian. Evidence of Early Permian rifting in Northwest Australia has long been known (Falvey & Mutter 1981; Powell 1976; Bird 1987). Recent deep seismic studies on the Australian NW shelf have confirmed this and suggest that in the mid-Carboniferous to Early Permian a major episode of extension and crustal thinning took place on the margin of NW Australian Gondwanaland, including the present Australian NW shelf, resulting in the development of the Westralian Superbasin (Colwell et al. 1994). Crustal thinning of the Australian NW shelf at this time was most pronounced in the lower crust and was concentrated northwest of the NW shelf Megashear. Separation of the Sibumasu block (as part of the Cimmerian continent) occurred in the late Early Permian. Similar rifting events have been recorded from Northern Pakistan and Afghanistan (Pogue et al. 1992; Boulin 1988), the North Indian margin (Baud 1994), the Malagassy Rift (Wopfner 1994), Oman (Pillevuit 1993) and Iran (St0cklin 1974). Late Triassic to Late Jurassic rifting
Both Permian and Triassic ages have been previously proposed for the separation of the Lhasa block from Gondwanaland. A Permian separation has been advocated, either as a part of the Cimmerian continent (All~gre et al. 1984; Metcalfe 1988, 1990) or as a 'Mega-Lhasa' block which included Iran and Afghanistan (Baud et al. 1993). Evidence for Permian rifting on the North Indian margin and in Tibet (Baud 1994) is here regarded as being related to the separation of the Cimmerian continental strip which included Iran, Afghanistan and the Qiangtang block of Tibet, but not the Lhasa block. A Triassic rifting and separation of the Lhasa block from Gondwanaland has been proposed by several authors including Chang et al. (1986) and Dewey et al. (1988). Recent sedimentological and stratigraphical studies in the Tibetan Himalayas and
SE ASIAN TERRANE EVOLUTION
105
Fig. 7. Reconstructions showing the rapid counter-clockwise rotation of Gondwanaland in the Late Devonian to Early Carboniferous (after Chen et al. 1993).
Nepal (Liu 1992; Liu & Einsele 1994; Ogg & von Rad 1994; von Rad et al. 1994) have documented the Triassic rifting and Late Triassic (Norian) separation of the Lhasa Block from northern Gondwanaland. This Late Triassic episode of rifting is also recognized along the NW shelf of Australia (Colwell et al. 1994) where it continued into the Late Jurassic resulting in the separation of Western Burma and the Woyla terranes (Metcalfe 1990, 1994a, b; G6rtir & Seng6r 1992).
SE Asian Pre-Cenozoic sutures The formation of present-day SE Asia involved the progressive suturing of terranes to each other during Late Palaeozoic to Cenozoic times and their subsequent disruption, principally caused by the collision of India with Eurasia. The ages of sutures in eastern and SE Asia become younger to the south and southeast. Reconstruction of the history of amalgamation and accretion in SE Asia requires constraints on the ages of the various sutures. Constraining data include: ophiolite obduction
ages; melange ages; ages of 'stitching' plutons; ages of collisional or post-collisional plutons; ages of volcanic arcs; major changes in arc chemistry or isotope chemistry; convergence of apparent polar wander paths (APWPs); loops or disruptions in APWPs; ages of blanketing strata; palaeobiogeography; biostratigraphy (e.g. ages of oceanic sediments now located in the suture); stratigraphy/sedimentology (e.g. provenance stitching); structural geology (eg. age of collisionrelated thrusting). It is rarely possible to acquire all types of constraining data for individual sutures. The various SE Asian sutures are shown in Figs 1 and 2. The region formed by nucleation around the South China block and the Palaeozoic and Mesozoic sutures and constraints on their ages are discussed below from oldest to youngest. Song M a and Song Da sutures
The Indochina and South China blocks are separated by a NW-SE-oriented mobile belt in North Vietnam. Two sutures are currently recognized
106
I. METCALFE
within this belt, the Song Ma and Song Da sutures of Palaeozoic and Mesozoic age, respectively. The age of the collision and amalgamation of Indochina and South China is still controversial with some authors favouring an early amalgamation along the Song Ma suture in the Late Devonian-Early Carboniferous (Gatinsky & Hutchison 1987; Hutchison 1989a, b) and others favouring a Late Triassic amalgamation along the Song Da suture (SengOr et al. 1988). Proponents of an early suturing have proposed that the Early Carboniferous event represents the collision between Indochina and South China along the Song Ma suture to form the 'East Asia Continent' or 'Cathaysialand'. Alternatively, those advocating a Mesozoic suturing regard this event as the collision between an arc fragment and an already accreted arc along a SW-dipping subduction zone and consider the final welding of Indochina to South China as having taken place in the Late Triassic along the Song Da suture. The large-scale folding and thrusting and nappe formation which took place in the Early to middle Carboniferous suggest continent-continent collision rather than an arc-continent collision and recognition of the Song Da Permo-Triassic sequences as probably representing rift basin sequences (Tran 1979; Hutchison 1989a, b), supports the amalgamation of Indochina and South China as occurring in the Early Carboniferous. In addition, middle Carboniferous shallow marine carbonates are reported to blanket the Song Ma suture in North Vietnam and palaeobiogeographical data show that pre-middle Carboniferous faunas on each side of the Song Ma zone are distinctly different whilst the middle Carboniferous faunas are essentially similar (Tran 1992). Newly discovered Carboniferous floras on the Indochina block in Northeast Thailand also indicate continental connection between Indochina and South China in the Carboniferous (Laveine et al. 1994). It is also interesting to note that Middle Devonian fishes recently recorded from central Vietnam (Indochina block) by Janvier et al. (1994) do not include typical South China block forms (including the faunas from North Vietnam). This supports an Early Carboniferous amalgamation of the Indochina and South China blocks. The Song Ma suture is here regarded as the probable zone of welding of the South China and Indochina blocks in the Early Carboniferous. The Song Da zone appears to represent a zone of Permo-Triassic rift basins which closed in the Late Triassic and which was reactivated as a major strike-slip shear zone in the Late Mesozoic to Cenozoic. Much more work is required to unravel the very complex geology and tectonic history of North Vietnam/Laos. Identification of the continuation of the Ailaoshan suture of Yunnan (see below)
in Vietnam or Laos, if this is contiguous with the Song Ma suture and hence diachronous in nature, is a major task for the future.
Ailaoshan suture
The Ailaoshan suture zone (Figs 1 and 4) is a narrow NW-SE orientated belt of ophiolitic rocks and oceanic sediments which forms the boundary between the Simao (Indochina) and South China blocks. The ophiolitic rocks of the zone comprise peridotite, gabbro-diabase and basalt. The peridotite includes weakly depleted plagioclase lherzolite and spinel harzburgite with Sm/Nd ratios of mid-ocean ridge type (Zhang et al. 1984; Wang & Zhong 1991). The ophiolitic rocks are associated with deep-marine sedimentary rocks including ribbon-bedded cherts that have yielded some Lower Carboniferous and Lower Permian radiolarians (Wang & Zhong 1991). Upper Triassic rocks (Norian limestones and Rhaetian sandstones) appear to blanket the suture in this region. The suture trends southeast into North Vietnam but its extension there has not been demonstrated. If the suture is Mesozoic in age it may well correlate with the Song Da suture. If it is Palaeozoic in age then it probably correlates with the Song Ma suture. Another possibility is that it is Mesozoic but correlates with the Song Ma suture, closure of the Palaeo-Tethys being diachronous, earlier in the east and younger in the west. It has also been suggested that the Ailaoshan ophiolitic zone possibly comprises two belts, one of which correlates with the Song Ma suture of Vietnam and one that correlates with the Nan-Uttaradit suture in Thailand (Zhang et al. 1984). Further constraints are required on the nature and age of the Ailaoshan suture in order to resolve these various possibilities.
Jinshajiang suture
This suture, which probably correlates with the Ailaoshan suture to the south (Figs 1 and 4), defines the northeastern limit of the Qamdo-Simao block and forms the boundary between this block and the Kunlun terrane and the Songpan Ganzi accretionary complex terrane. The suture has well developed ophiolites, and melange comprises Devonian, Carboniferous and Permian exotic blocks in a Triassic matrix. The ophiolites are regarded as Upper Permian to Lower Triassic and the age of the suture regarded as Late Triassic by Dewey et al. (1988). An older age for this suture has however been suggested recently by Chen& Xie (1994) and Bian (1991) who recognize Upper Permian to Jurassic sediments unconformably overlying Lower Permian ophiolites in the Hoh Xil Range.
SE ASIAN TERRANE EVOLUTION Lancangjiang suture
This suture forms the boundary between the Qamdo-Simao and Qiangtang terranes and is here regarded as representing a segment of the main Palaeo-Tethys ocean. The suture zone rocks include Devonian and Carboniferous turbiditic 'flysch', ultramafics, ocean-floor basic extrusives of Permian age and Carboniferous-Permian melange. These suture zone rocks are also blanketed by Middle Triassic continental clastics ( C h e n & Xie 1994). Carboniferous-Permian island arc rocks are developed along the west side of the suture (Fan & Zhang 1994) and Upper Triassic collisional granitoids are associated with the suture (Wang & Tan 1994). In Tibet and Qinghai, the suture runs along the Lancangjian Thrust and is covered by huge thicknesses of Jurassic sediments in the Tanggula Range. Scattered ultramafic rocks occur in the Muta region of Tibet and in Qinghai (Chen & Xie 1994). The suture appears to be similar in character and age to the Changning-Menglian suture of Western Yunnan and the Raub-Bentong suture in Peninsular Malaysia here considered to represent the main Palaeo-Tethyan ocean. Banggong suture
This suture forms the boundary between the Tibetan Lhasa and Qiangtang terranes. Well developed ophiolites have been recorded which are associated with radiolarian cherts and the age of the ophiolites and their obduction in Tibet is well constrained as Upper Jurassic (Girardeau et al. 1984; Dewey et al. 1988). The suture is blanketed in Tibet by Cretaceous and Palaeogene rocks and structural data indicate that collision took place along this suture around the Jurassic-Cretaceous boundary (Dewey et al. 1988; Fan & Zhang 1994). The extension of this suture in Yunnan is not well preserved due to shearing but some Triassic 'flysch' and ultramafic rocks are found in the Luxi area and blanketing strata are of Middle Jurassic age indicating an earlier closure of the suture in this region. Nan- Uttaradit suture
The Nan-Uttaradit suture zone in Thailand (Figs 1 and 2) has long been regarded as representing the Palaeo-Tethys destroyed by continent-continent collision between Sibumasu and Indochina. The suture has also been generally regarded as being contiguous with the Raub-Bentong suture in Peninsular Malaysia (see below), and more recently, the Changning-Shuangjiang (ChangningMenglian of this paper) suture of SW China (Barr & MacDonald 1987). The precise age of suturing
107
is still controversial with ages of Lower Carboniferous (Helmke 1985b), Permian (Helmke & Kraikhong 1982; Helmke 1983, 1985a; Helmke & Lindenberg 1983), Early Triassic (Cooper et al. 1989; Mitchell 1989; Metcalfe 1990) and Late Triassic (Ridd 1980; Mitchell 1981; Seng6r 1984) being proposed. This wide range of ages reflected paucity and poor quality of constraining data. The suture includes a belt of pre-Permian ophiolitic mafic and ultramafic rocks (Type II ophiolitic associations) with associated blueschists distributed northeast of Uttaradit (Barr et al. 1985; Barr & Macdonald 1987). Hada et al. (1994) have shown that the suture in southeast Thailand (Sra Kaeo-Chanthaburi) comprises a western chertclastic belt and an eastern serpentinite melange belt. The western chert-clastic sequence appears to form a stack of imbricate thrust slices and is dated as Middle Triassic by radiolarians. The serpentinite melange belt includes a wide variety of rock packages including rocks of oceanic, island arc and continental affinities of various ages. Mafic and ultramafic blocks in the melange comprise oceanisland basalts, backarc basin basalts and andesites, island-arc basalts and andesites and suprasubduction cumulates generated in Carboniferous to Permo-Triassic times (Panjasawatwong & Yaowanoiyothin 1993). Limestone blocks in the melange range from upper Lower Permian to middle Permian and a granitic lens has yielded a zircon U-Pb age of 486 _ 5 Ma (Hada et al. 1994). Structural studies of Carboniferous to Permian meta-greywacke and phyllite, which enclose the mafic-ultramafic sequence, in the Sirikit Dam area (Singharajwarapan & Berry 1993) indicate that these represent an accretionary complex. These rocks are associated with Permo-Triassic volcanic and volcaniclastic rocks of dacitic and rhyolitic composition and a relatively unmetamorphosed Lower Triassic sandstone-shale turbidite sequence. Structures indicate east-directed accretionary thrusting and hence westward directed subduction. The suture zone rocks are overlain unconformably by Jurassic redbeds and post-Triassic basaltic lavas which disconformably overly the suture are intraplate continental basalts (Panjasawatwong & Yaowanoiyothin 1993). A Permo-Triassic age for the suture is therefore indicated. Changning-Menglian
suture
The Changning-Menglian suture in western Yunnan (Fig. 4) is a narrow north-south orientated zone of dismembered ophiolites and associated deep-marine sedimentary rocks that are interpreted as representing a segment of the main PalaeoTethys in East Asia (Huang et al. 1984; Zhang et al. 1985; Wu & Zhang 1987; Liu etal. 1991; Wu 1993;
108
I. METCALFE
I " =0
II
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Bathyal sediments Shallow-water clastics Stratigraphic break
Fig. 8. Devonian to Jurassic stratigraphy of the Changning-Menglian suture zone belt (after Liu et al. 1991)
Fang et al. 1994; Wu et al. 1995). The suture can be traced from Menglian northwards through Laochong, Tongchangia to Changning and ophiolitic melange includes blocks of harzburgite, cumulate websterite, gabbro, basalt, limestone and chert in a mud-silt grade matrix. Associated basalts are of mid-ocean ridge and ocean-island types (Wu et al. 1995) and remnants of limestone capped seamounts have been identified in the zone (Liu et al. 1991). Oceanic ribbon-bedded chert-shale sequences have yielded graptolites, conodonts and radiolarians indicating ages ranging from Lower Devonian to Middle Triassic (Qin et al. 1980; Duan et al. 1982; Wu & Zhang 1987; Wu & Li 1989; Liu et al. 1991; see Fig. 8). Limestone blocks and lenses dominantly found within the basalt sequence of the suture and interpreted as seamount caps,
have yielded fusulinids indicative of Lower Carboniferous to Upper Permian ages (Duan et al. 1982; Wu et al. 1995). The suture is truncated to the north of Changning by the Chongshan metamorphic belt and almost certainly continues northwards as the Lancangjiang suture to the west of Deqin (Fig. 4). The southern extension of the Changning-Menglian suture is problematic. The suture can be traced to the China-Myanmar border but its extension into Myanmar has not been documented. It may be offset by major left lateral strike-slip faults then to continue down into Laos along the Dien Bien Phu line of mafic and ultramafic rocks in Laos and hence to connect with the Nan-Uttaradit suture in Thailand and the RaubBentong suture in Peninsular Malaysia. Alternatively, it has been proposed (Huang et al. 1984) that the Changning-Menglian suture extends southwards as the Chiang Mai volcanic belt in northern Thailand which had been interpreted as the remnants of a subduction-related volcanic arc by Bunopas & Vella (1978). The volcanics of this zone have, however, been shown to have formed in an extensional continental setting, possibly a backarc setting related to westwards subduction along the Nan-Uttaradit zone to the east (Macdonald & Barr 1978; Barr et al. 1990). Interestingly, mafic rocks in the Chiang Rai area may possibly have formed in a subduction environment and these are associated with a chert-clastic sequence known as the 'Fang Chert' which has yielded Lower Devonian graptolites, Carboniferous conodonts and radiolarian faunas of Lower Devonian, Carboniferous and Lower to middle Permian ages. Could this be another small disrupted segment of the main Palaeo-Tethyan suture? Extension of the Changning-Menglian suture to the south, rather than connecting it with the Nan-Uttaradit zone which was considered to terminate against the Ailaoshan suture, led Wu et al. (1995) to propose a 'Simao Terrane' of Cathaysian affinity, bounded by the ChangningMenglian suture to the northwest, the Ailaoshan suture to the northeast and the Uttaradit-Nan suture to the southeast (Fig. 9). The similarity in both nature and age of the Nan-Uttaradit and Changning-Menglian sutures lead me to favour their correlation and to here regard the 'Simao Terrane' to be disrupted part or parts of the Qamdo-Simao block which is regarded as an extension of the Indochina terrane. Raub-Bentong
suture
The Raub-Bentong suture of Peninsular Malaysia forms the boundary between the Sibumasu and Indochina/East Malaya terrane and represents the Palaeo-Tethys. The suture zone is exposed as a
109
SE ASIAN TERRANE EVOLUTION
30N ,' a .~ •
,'
j
Pi
*
"/ " '
CHINA
t ?
t
OKunming
D O
fi "~ Pe*, t" ".-I
BURMA ,' : :
,,
te,
Chiang
AND ...
- ....
"LADS .,...'.-
~
..'
,
.~CAMBODIA!
110E I
Fig. 9. Map showing the Simao terrane proposed by Wu et al. (1995) narrow north-south-trending zone extending from Thailand through Raub and Bentong to the east of Malacca. Its extension to the south and southeast is controversial (Fig. 10) but a southwards (disrupted)
extension into Sumatra as suggested by Tjia (1989) and Hutchison (1993) is here favoured. The suture is exposed as a c. 13 km wide zone in Peninsular Malaysia and comprises melange, oceanic sediments (including ribbon-bedded cherts), schist, and discontinuous, narrow, elongate bodies of serpentinized mafic-ultramafic rocks that have been interpreted as representing ophiolite (Tjia 1987, 1989; Hutchison 1989b). The melange has a mud/silt matrix and contains a variety of clasts including ribbon-bedded cherts, limestones, volcanic and volcaniclastic rocks. Basic subduction-related volcanic clasts (which occur commonly in the Nan-Uttaradit suture further north) have not however been reported from the Raub-Bentong suture. The precise age of suturing is still poorly constrained but a latest Permianearliest Triassic suturing age is here favoured. Harbury et al. (1990) presented structural evidence which seems to preclude a Late Triassic collision. Ages of limestone clasts in the melange include Lower and Upper Permian (Metcalfe 1989) which indicate a latest Permian or Triassic age for the melange matrix. The Main Range 'collisional' 'S' Type granites of peninsular Malaysia range from Upper Triassic (230 __+9 Ma) to lowermost Jurassic (207_+ 14 Ma) in age, with a peak of around 210Ma (Liew & Page 1985; Darbyshire 1988). Ages of oceanic deep-marine bedded cherts within the suture zone range from Upper Devonian to Upper Permian (Metcalfe 1992; Spiller & Metcalfe 1993, 1995; Metcalfe & Spiller 1994). These data support the existence o f Palaeo-Tethys between Sibumasu and Indochina/East Malaya from Late Devonian to Late Permian times and suggest collision of Sibumasu with Indochina in the latest Permian or Triassic.
Qinling-Dabie
Fig. 10. Map showing the southwards extension of the Raub-Bentong suture as proposed by various authors.
suture
This suture forms the boundary between the North and South China blocks. Estimates of the age of this suture, based on geological data, range from the Middle Palaeozoic (Zhang et al. 1984; Mattauer et al. 1985, 1991; Wang 1991) to the Permo-Triassic (Wang et al. 1982; Klimetz 1983; Hsu et al. 1987; Seng/Jr 1987; Yin & Nie 1993; Nie & Rowley 1994). Palaeomagnetic data (Lin & Fuller 1990) suggest that the amalgamation of North and South China along the Qin Ling suture had occurred by the Late Triassic-Early Jurassic. These data comprise convergence of APWPs, rapid rotation of the North China block in the Late Triassic-Early Jurassic, convergence of palaeolatitudes and a Jurassic-Cretaceous loop (stagnation point) in the APWPs. The data also seem to be consistent with stratigraphic data and ages of collision-related
110
~. METCALFE
plutons in the Qinling foldbelt (Hsu et al. 1987). Enkin et al. (1992) utilizing palaeomagnetic data, proposed that the suturing could be as late as Jurassic, a view challenged by Nie & Rowley (1994) who cogently argue for a Permo-Triassic collision of South and North China. A PermoTriassic suturing age is here favoured with closure of the suture commencing in the Late Permian in the east, and progressing westwards with development of the Tan-Lu transform fault, into the Middle Triassic.
Shan boundary
Woyla suture
This suture comprises Cretaceous ophiolites and accretionary complex material along the southwest margin of Sumatra and marks the zone of accretion and collision of the small Woyla terranes with Sibumasu in the Late Cretaceous (Wajzer et al. 1991).
Meratus
suture
This suture, also known as the Sagaing or Mandalay suture, forms the boundary between the West Burma and Sibumasu terranes. An Early Cretaceous age for the suture is indicated by Cretaceous thrusts in the backarc belt (Mitchell 1990) and the Upper Cretaceous age of the Western Belt tin-bearing granites.
suture
This suture, located in southeast Borneo, is characterized by subduction melange and ophiolite of middle Cretaceous age (Hamilton 1979) which is overlain by Eocene strata. The ophiolite was obducted in Cenomanian times during an arccontinent collision (Sikumbang 1986).
Boyan suture
The Boyan suture forms the boundary between the small Semitau terrane and the South West Borneo
50N 40N 30N 20N 1ON
10s 2os 3os 4os 50S 1 -~
21-1
15@
16 ©
3 X
17 O
4A
5+
18 ~ )
6I
19 [ ]
7®
20 [ ]
8 ©
9,&
21 O
10 •
22 X
11 ~
12'~
23 o.*o 24 +
13~
14~
25'~
26 •
Fig. 11. Palaeolatitude plots for the Tarim, North China, South China, Indochina, East Malaya and Sibumasu terranes from Devonian/Carboniferousto Tertiary. Sources of data are 1. Lin (1987a, b); 2. Lin et al. (1985); 3. McElhinney et al. (1981); 4. Opdyke et al. (1986); 5. Sasajima & Maenaka (1987); 6. Chan et al. (1984); 7. Achache et al. (1983); 8. Maranate & Vella (1986); 9. Achache & Courtillot (1985); 10. Bunopas et al. (1989); 11. McElhinney et al. (1974); 12. E. A. Schmidtke & M. Fuller (pers. comm. 1994); 13. Haile & Khoo (1980); 14. Haile et al. (1983); 15. Bunopas (1982); 16. Sasajima et al. (1978); 17. Halle (1979); 18. Lin (1987b); 19. Zhao & Coe (1987); 20. Cheng et al. (1988); 21. Bai et al. (1987); 22. Li et al. (1988); 23. McFadden et al. (1988); 24. Zhai et al. (1988); 25. Huang & Opdyke (1991); 26. Enkin et al. (1992). Modified from Metcalfe (1993, 1994c).
SE ASIAN TERRANE EVOLUTION block. The suture zone rocks are Upper Cretaceous melange (Williams et al. 1986; Williams & Harahap 1987) produced during short-lived SWdirected subduction which ceased after collision of the Semitau terrane and then recommenced behind this accreted terrane to produce the Lubok Antu melange and the Sarawak accretionary complex.
111
Terrane evolution Available data suggest that all the east and SE Asian allochthonous terranes formed part of, or were close to, the northern margin of G o n d w a n a l a n d in the Early Palaeozoic. The continental blocks that are placed adjacent to the
PALAEO-PAClFIC LAURENTIA _ SIBERIA
AUSTRALIA CAMBRO-ORDOVICIAN ,~ (TREMADOC)
ANTARCTICA \,
) |!!i!!!!!!~!!i
INDIA t
30S
30N
LATE D E V O N I A N
1
Fig. 12. Reconstructions of eastern Gondwanaland for the Cambro-Ordovician and Late Devonian, showing the postulated positions of the East and SE Asian terranes. NC, North China; SC, South China; T, Tarim; I, Indochina; Q, Qaidam; WC, Western Cimmerian Continent; Qi, Qiangtang; L, Lhasa; S, Sibumasu; WB, West Burma; GI, Greater India; SA, South America. Present day outlines are for reference only.
112
I. METCALFE
Greater India-NW Australian Gondwanaland margin in the Early Palaeozoic include North & South China, Indochina/East Malaya (which includes the Qamdo-Simao block of western China), Tarim (here regarded to include the Kunlun and Ala Shan blocks), Qaidam, Sibumasu, Qiangtang, Lhasa, NW & SE Hainan, West Burma, Kurosegawa and Woyla terranes. Palaeomagnetic and biogeographic evidence indicates that the North and South China, Tarim, Indochina/East Malaya and Qaidam terranes were in equatorial or low northern palaeolatitudes by Carboniferous times (Fig. 11) and their faunas had no affinities
with those of Gondwanaland. These terranes must therefore be placed outboard of Sibumasu, Qiangtang and Lhasa on the Gondwanaland margin in the Early Palaeozoic and must have separated from Gondwanaland in the Devonian (Fig. 12). The separation of these terranes is interpreted to have occurred in the Late Devonian when mainland Gondwanaland rotated rapidly counter-clockwise about an Euler pole in Australia (Chen et al. 1993), and the Palaeo-Tethys opened between the terranes and northern Gondwanaland (Fig. 7). By Early Carboniferous times, South China and Indochina had amalgamated along the Song Ma
EARLY CARBONIFEROUS (VISI~AN)
EARLY PERMIAN
~ ~
LATE PERMIAN
~ 7, • ~
~ ~0
LATE TRIASSIC
wBL
Fig. 13. Palaeogeographicreconstructions of the Tethyanregion for Early Carboniferous,Early Permian, Late Permian and Late Triassic. Present day outlines are for reference only. Symbols as for Fig. 12.
3o
SE ASIANTERRANEEVOLUTION suture to form 'Cathaysialand'. However, a narrow ocean between the Qamdo-Simao portion of the Indochina terrane and South China (now represented by the Ailaoshan suture) persisted until Triassic times (Fig. 13). Palaeomagnetic data indicate that South China was equatorial during the Early Carboniferous (Lin et al. 1985; Lin 1987a; see Fig. 11). No palaeomagnetic data are available for the Carboniferous of Indochina but Middle and Late Carboniferous and younger faunas and floras have strong affinities with South China. Palaeomagnetic and palaeobiogeographic data for the North China block suggest that it was not attached to either South China or Siberia in the Carboniferous and that it lay in low northern palaeolatitudes (Lin 1987a; Lin & Fuller 1990; Nie 1991; Fig. 11). In the Mid-Late Carboniferous, Gondwanaland rotated clockwise, collided with Laurentia to form Pangaea, and eastern Gondwanaland moved from low to high southem palaeolatitudes. The predicted palaeolatitude for Sibumasu, if placed adjacent to NW Australia, would be about 40°S in the Late Carboniferous and recent palaeomagnetic results from the Yunnan part of Sibumasu give a palaeolatitude of 43°S for the Late Carboniferous (Figs 5 and 11) which is consistent with the proposed reconstruction (Fig. 13). Palaeomagnetic data for the Tarim block indicate that it moved from southern to northern palaeolatitudes in the Devonian and Carboniferous (Fig. 11) and that since the Permian, observed latitudes concur with those predicted from Eurasia indicating that it had already sutured to Eurasia (Nie & Rowley 1994; Fig. 14). Data to constrain the palaeoposition of the Qaidam terrane are largely lacking for this time. A major rifting phase occurred on the margin of NE Gondwanaland in the Early Permian (Powell 1976; Falvey & Mutter 1981; Bird 1987; AudleyCharles 1988; Pillevuit et al. 1993; Pogue et al. 1992; Baud 1994; Wopfner 1994) indicating that a substantial continental fragment or fragments separated from Gondwanaland at that time. Audley-Charles (1988) suggested that these terranes were Iran, North Tibet (Qiangtang) and Indochina but later (Audley-Charles 1991) supported the view presented by the present author (Metcalfe 1986, 1988, 1990, 1991, 1992, 1993, 1994a, b) that the rifting terranes were in fact Sibumasu, and Qiangtang (along with other terranes constituting the Cimmerian continent of SengOr). Palaeomagnetic data indicate that Sibumasu travelled rapidly from southern to northern palaeolatitudes in the Permo-Triassic (Figs 5 and 11) supporting this view. Post-Middle Permian faunas of Sibumasu are Tethyan in aspect and show affinities with Indochina and South China and do not contain any Gondwanaland elements (Metcalfe
113
80
TARIM
70
Predicted
from
-
60 50 40 30
~ t t ObSerVed
20
1 i 1
10 0
Irving & Irving (1982) Van der Voo (1990) Besse & Courtillot (1991) Enkin et al. (1992)
---
-10 ~. -20
I
I
I
I
I
I
I
I
I
R
N
E
K2
K1
J3
J2
J1
T3
I
I
T2 T1
I
P2
Fig. 14. Palaeolatitude versus time plots for centre of the Tarim block. Solid black circles are calculated from measured data on the Tarim block as reported by Enkin et al. (1992). Other plots are the palaeolatitudes predicted from the Eurasian reference poles of Irving & Irving (1982), Van der Voo (1990) and Besse & Courtillot (1991), from Nie & Rowley (1994).
1988). Recent work on Permian brachiopod palaeobiogeography (Shi & Archbold 1993, 1994a, b; Shi et al. 1995) has shown that terranes of Sibumasu and other Cimmerian continent components formed part of a Gondwanaland province in the early Early Permian, but by the late Early Permian there was a separate Cimmerian province indicating separation of Cimmeria from Gondwanaland. Upper Permian faunas belong to the equatorial Cathaysian province. The rifting and separation of the Cimmerian continent from Gondwanaland was effected by opening of the Meso-Tethys and northwards destruction of PalaeoTethys beneath Eurasia and the North and South China and Indochina blocks. Continental palaeomagnetic data show that Cathaysialand, characterized by its palaeoequatorial Cathaysian flora, remained in low northern palaeolatitudes during the Permian (Fig. 11). Sibumasu sutured to the Qamdo/ Simao/Indochina/East Malaya portion of Cathaysialand along the Changning-Menglian, Nan-Uttaradit and Raub-Bentong sutures in Permo-Triassic times. The precise age of suturing is still controversial and may well have been diachronous but a latest Permian-Triassic suturing age is here favoured (see above). Thus, by Late Triassic times, Sibumasu had amalgamated with Cathaysialand and this amalgamated superterrane was located in low northern latitudes (Fig. 13). Initial contact between South and North China probably also occurred in the late Permian with
114
i. METCALFE
collision occurring first in the east with development of the Tan-Lu transform fault and final suturing along the Qinling-Dabei suture occurring by Late Triassic times (Nie & Rowley 1994). The presence of coesite- and diamond-bearing ultrahigh pressure metamorphic rocks in the Dabei and Sulu regions (Wang et al. 1992; Okay e t al. 1993; Maruyama et al. 1994) which were probably exhumed during the Triassic (Nie et al. 1995)
indicate that the Qinling-Dabei orogenic system produced vast quantities of sediment during the Triassic to the Songpan Ganzi basin to the west (Zhou & Graham 1993; Nie e t al. 1995). It is not yet known if the small Gondwanaland terranes of Hainan Island, which yield Lower Palaeozoic trilobites and other invertebrates endemic to Australia and Gondwanaland and which have Lower Permian diamictites interpreted by
UINE~.
60~.
I
iili ! ~ :i~:~• PACIFIC OCEAN
c. s ~. . . . '. . . . .:
I
-60
{
c) :
:;::::::
: i
..'~. -
:
L ATE
C R E T A C E O U S . . .'" ...r,.
Fig. 15. Palaeogeographic reconstructions for Eastern Tethys in (a) Late Jurassic, (b) Early Cretaceous and (c) Late Cretaceous times. SG, Songpan Gangzi accretionary complex; SWB, South West Borneo; SE, Semitau; Si, Sikuleh; N, Natal; M, Mangkalihat; WS, West Sulawesi; Ba, Banda Allochthon; ES, East Sulawesi; O, Obi-Bacan; Ba-Su, Bangai-Sula; Bu, Buton; B-S, Buru-Seram; WIJ, West Irian Jaya; Sm, Sumba; PA, Philippine Arc. M numbers represent Indian Ocean magnetic anomalies. Other terrane symbols as in Figs 12 and 13. Modified from Metcalfe (1990) and partly after Smith et al. (1981), AudleyCharles (1988) and Audley-Charles et al. (1988). Present day outlines are for reference only.
SE ASIAN TERRANE EVOLUTION some workers as glacial-marine, were accreted to South China in the Palaeozoic or Mesozoic. The Late Triassic to Late Jurassic saw renewed rifting on the northeast margin of Gondwanaland and initial rifting of the third continental sliver from G o n d w a n a l a n d opening the Ceno-Tethys. The Lhasa terrane separated in the Late Triassic, followed by the West Burma block and Woyla terranes in the Late Jurassic. The Lhasa block moved rapidly northwards in the Late Jurassic to Early Cretaceous, separated from the West Burma block and Woyla terranes by a transform fault (Fig. 15). The Lhasa block sutured to Eurasia in the Early Cretaceous and by Late Cretaceous times, the
115
West Burma block and the Woyla terranes had accreted to proto-SE Asia (Fig. 15). Continued work on the geological evolution of East and SE Asia is currently supported by an Australian Research Council Grant which is gratefully acknowledged. I also thank the Department of Geology & Geophysics, University of New England and the Department of Geology, Australian National University for facilities provided. Thanks go to M. G. Audley-Charles and R. W. Murphy for constructive reviews of the manuscript. This paper forms a contribution to IGCP Project 321 and GSGP Project Pangaea. It was written whilst a Visiting Fellow in the Department of Geology, Australian National University, Canberra.
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Cenozoic SE Asia: reconstructing its aggregation and reorganization GORDON
PACKHAM
Department of Geology and Geophysics, University of Sydney, NSW, Australia 2006 Abstract: Reconstructing SE Asia in the Cenozoic is dependent on being able to model three major tectonic events and their consequences. They are the collision of India with Eurasia, the rotational history of the Philippine Sea plate and the ongoing collision of Australia with eastern Indonesia. Models of the Eocene India-Eurasia collision imply extrusion along major strike-slip faults or crustal thickening and block rotation. Kinematic modelling of the present day tectonics of central Asia indicates rotations and strike-slip faulting and an eastward displacement of crust. Prior to middle Miocene time tectonic activity was closely associated with the collision zone and extrusion of Indochina was supposed to have occurred. Analysis of the volume of the topography formed by the collision indicates eastward displacement and the loss of some crust by expulsion or into the mantle. Palaeomagnetic measurements have found northward displacement of Tibet and strong clockwise rotation east of the eastern syntaxis but unequivocal evidence of southward displacement is rare. Large Palaeogene to early Neogene displacements on strike-slip faults have been documented and related with confidence to the rotations that formed at least the Oligocene South China Sea floor. Early Oligocene docking of the Indoburman Flysch belt south of the eastern syntaxis is difficult to explain in extrusion models. Whether the rotations that have been documented are a response by internal deformation or block rotation is unresolved, as is the amount of extrusion of SE Asia to the south. In the eastern region, rotation and associated northward motion of the Philippine Sea plate as it developed puts constraints on possible models because of a maintained close proximity of its southern margin to the northern edge of the Australian craton. Assembling of continental fragments now in the Banda Sea and derived from eastern New Guinea appears to have occurred by Late Cretaceous time. Formation of the Banda arc may have isolated the Banda Sea from the Australia plate then collision of the Australian margin possibly rotated the Banda Sea counter-clockwise. Although westward movement of the Philippine Sea plate relative to Australia commenced in the early Miocene, convergent leftlateral shear was not possible until the late Miocene thus limiting westward tectonic transport of terranes and the transmission of the shear into the region west of the Banda Sea.
This paper reviews a n u m b e r of the tectonic arguments which have been used to arrive at SE Asian reconstructions. Ideally, reconstructions should be based on the rigorous application of kinematic parameters. While major plate motions provide the large scale k i n e m a t i c f r a m e w o r k for deriving reconstructions, simple rigid plate solutions have limitations because of the uncertainties of the locations of plate boundaries, non-rigid deformation and the progressive reordering of elements within the complex. Two major but contrasted C e n o z o i c collision events have f a s h i o n e d the region. Namely, the collision between the cratonic masses of India and Eurasia Asia in the west and the collision o f arcs and small continental blocks derived from Australia together with the Australian craton in the east and southeast. Accordingly, this review falls into two parts, one dealing with the India-Asia collision and the formation of the South China Sea where the emphasis is biased strongly towards Palaeogene tectonic events and the second with the P h i l i p p i n e - B a n d a - N e w Guinea region
where the focus of interest is on the Neogene. As well as reconstructions covering the whole region, other studies have focused on specific problem areas. The four major regional subsets are: (a) the India-Eurasia collision and its possible displacem e n t of the constituent blocks of Sundaland; (b) development of the South China Sea; (c) motion of the Philippine Sea plate and the Philippine Islands; and (d) the d e v e l o p m e n t of the Banda Sea region and northern New Guinea and arc collisions with the Australian craton.
The India-Asia collision and western SE Asia While there is general agreement that the collision has indented the margin of Eurasia and that strain has been absorbed in substantial crustal thickening, there is controversy about the amount and distribution of the remaining strain and its effect on SE Asian tectonics. In their well k n o w n extrusion
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolution of SoutheastAsia, Geological Society Special Publication No. 106, pp. 123-152.
123
124
G. PACKHAM model, Tapponnier and his co-workers argued that during the collision, continental blocks including Sundaland were extruded possibly some 1000 km southeast from the path of the Indian craton (e.g. Peltzer & Tapponnier 1988). In contrast Dewey et al. (1989) considered that residual strain was confined to extension of the Tibetan Plateau east of the eastern Himalayan syntaxis, accompanied by oroclinal bending and block rotation in SE Asia and west of the western Himalayan syntaxis (Fig. 1). Two of the major ingredients of kinematic tectonic solutions have been (a) estimations of rates of slip on active faults and convergence in zones of shortening, and (b) reconstructions, based on forward modelling of the collision of the Indian craton with Eurasia. This discussion reviews the present day tectonics of Asia and then goes on to examine kinematic models, the sequence of tectonic events, the contribution of palaeomagnetic studies, the implications of South China Sea spreading and some geological relationships along the western margin of SE Asia.
Fig. 1. Rotation model of the India-Asia collision from Dewey et al. (1989). Zones of sinistral transpression (oblique lines), zone of dextral shear (vertical lines), maximum area of extrusion of Tibet (horizontal lines), subduction accretion since 45 Ma (dotted) and motion of India with respect to Eurasia in cm a-~ (arrows).
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CENOZOIC SE ASIA RECONSTRUCTIONS observations, compiled earthquake data and the India-Eurasia convergence rate from the De Mets et al. (1990) NUVEL-1 model. In the model, two stable blocks (Tibet and Tarim) are rotating relative to stable Siberia about poles indicated on Fig. 2. Motion between the blocks is taken up along faults and zones of active shortening although there are large uncertainties in slip rates. The model transfers the estimated 40 mm a-1 of northeastward movement of Tibet into clockwise rotation and extrusion of crustal blocks along curved left-lateral faults. After allowing for shortening at the Longmen Thrust, the South China Block is rotating clockwise and moving southeast at 10-15 m m a -1 with respect to Siberia. Tibet, rather than being a simple rigid rotating block as in the Avouac & Tapponnier (1993) model, is according to Molnar & Lyon-Caen (1989) extending east-west. They calculated from earthquake data that through strike-slip and normal faulting, northern Tibet is shortening north-south at roughly 5 mm a-1 and extending east-west at roughly 1 0 m m a -1. In south Tibet and the Himalayas north-south graben formation results in east-west extension of 18 _ 9 mm a-1. The eastwest extension in southern Tibet and the Himalayas is attributable to radial thrusting on the Himalaya front and in northern Tibet to gravitational collapse. Using radiometric age data, Harrison et al. (1992) suggested that the graben could have originated between 8 Ma and 4 Ma. Molnar and Lyon-Caen (1989) concluded that eastern Tibet has an eastward component of movement with respect to the Tarim Basin and India of around 30-40 mm a-1 but declined to estimate the effect on South China because they could not evaluate the shortening rate in Longmen Shan. Avouac & Tapponnier (1993) estimated that 50% of the motion between India and Siberia is absorbed by the extrusion of Tibet to the northeast (at 40 mm a-1 at the eastern end). Molnar and LyonCaen (1989) estimated it at about a third in their analysis. Neither study took Altai into account. Dewey et al. (1989) approached the problem by adding the estimated present-day shortening rates along slip lines for Eurasia-India convergence and concluded that the residual available for extrusion, which they ascribe to gravitational collapse of the thickened Tibet crust along conjugate strike-slip faults, was not significant. Updating their estimates for the central Himalayas to Tienshan transect, the total India-Siberia convergence is 45 mm a-l (NUVEL-1) less the sum of 13 mm a-1 in Tienshan, 6 mm a-1 in west Kunlun, 5 mm a-1 in central Tibet and 18 mm a -1 in the Himalayas; the remainder is 3 mm a-t. A similar transect from the eastern syntaxis to Altai starting with 55 mm a-1 from NUVEL-1 and deducting the sum of 5 mm a-1 for
125
central Tibet, 3 mm a-1 in Nan Shan, 11 mm a -1 for the Altyn Shan fault and 18 m m a -1 for the Himalayas, leaves a residual of 18 mm a-1. In their topographic analysis Le Pichon et al. (1992) demonstrated that Tienshan and Altai (northeast of Tien Shan), together form an eastward increasing topographic mass. It is therefore likely that the Altai convergence rate is greater than the 13 mm a-1 estimated for Tienshan. Assuming a value of 15 mm a-1 would again give a residual of 3 mm a -1. Although the estimates, apart from NUVEL-1, have large uncertainties this methodology does not provide compelling evidence for extrusion.
D u r a t i o n o f the p r e s e n t t e c t o n i c r e g i m e
The structures presently active may have originated at various times from the early Miocene onwards. The onset of the present tectonic regime in Tienshan has been broadly established as early to middle Miocene (16 +22/-9 Ma, i.e. 7-38 Ma) by Avouac et al. (1993). The post-lower Mesozoic displacement on the Altyn Tagh Fault was estimated as 500 km by Peltzer & Tapponnier (1988). In contrast, Dewey et al. (1989) consider that the long major strike-slip faults such as the Altyn Tagh, Kun Lun and Tang Ting (Xan Shui He), all have displacements of 150 km. Depending on the estimated displacement, the Altyn Tagh Fault the fault is between 5 and 17 Ma old using an estimated slip rate of 30 mm a-1 and ignoring the uncertainty. The Red River Fault (Fig. 6) on the boundary of the South China and Indochina Blocks has been a right-lateral fault since 4.7 Ma but was left-lateral until 17-19 Ma (Leloup et al. 1993). Although not at present active, Quaternary rightlateral slip rate was estimated by Allen et al. (1984) at 2-5 mm a-1. The Pliocene to Recent fightlateral slip on the Red River Fault in the Avouac & Tapponnier model is about 50-75 km, or as little as 10-25 km using the slip rate of Allen et al., Longmen Shan and Qilian Shan presumably absorbing any excess strain. Between 5 and 1719 Ma any residual extrusion of the South China block would have also rotated Indochina because the Red River Fault was inactive.
Kinematic models for the IndiaAsia collision The position of the Indian Craton from the onset of collision, has been forward modelled from plate tectonics by amongst others, Dewey et al. (1989) and Le Pichon et al. (1992). The size of greater India prior to the collision is a matter of conjecture,
126
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Fig. 3. Simplified tectonic map of western Southeast Asia and adjacent regions of central Asia modified from Tapponnier et al. (1986) and Dewey et al. (1988) and unpublished sources, with Jurassic and Cretaceous palaeomagnetic deviations from expected palaeolatitudes and declinations. Data as in Fig. 8. Results from Jurassic rocks are circled. Heavy lines are sutures, large arrows are early to middle Cenozoic fault slip and small arrows are late Cenozoic fault slip.
CENOZOIC SE ASIA RECONSTRUCTIONS
but for a given size and the time of collision, the location of the boundary of Asia can be determined within fairly small limits.
127
limestones are the youngest preserved Himalayan marine sediments (Searle et al. 1987). Harrison et al. (1992), Tapponnier et al. (1986) and Peltzer & Tapponnier (1988) favoured a collision date of 50 Ma.
Time of collision
Besse & Courtillot (1991) pointed out that the convergence rate between India and Eurasia drops sharply between anomalies A22 and A21 (50.347.8 Ma on the scale of Harland et al. 1990). An Indian Ocean spreading ridge reorganization, shown on Fig. 4 as the change from Phase 2 to Phase 3 followed between A20 and A18 (43.941.1 Ma). Both Dewey et al. and Le Pichon date the collision at 45 Ma (early middle Eocene). Subduction-related volcanism north of the suture ends at about 50 Ma and related plutons range from 94 to 41 Ma (Dewey et al. 1988). Middle Eocene
Fig. 4. Sea floor spreading phases in the eastern Indian ocean. Phase 1 is from Late Jurassic to mid-Cretaceous, Phase 2 is from mid Cretaceous to middle Eocene (anomaly 20) and Phase 3 is middle Eocene to the present. Note the location of the Exmouth Plateau (EP) and the Cape Range Fracture Zone (CR) which may have been the northern limit of greater India before rifting off Australia and Antarctica.
Greater India
On the basis of the fit of India against Australia and Antarctica, and the shedding of continental clastics north onto the Exmouth Plateau, Powell et al. (1988) suggested that continental crust of greater India extended north of the West Australian margin to the Cape Range Fracture Zone (Fig. 5). Spreading patterns in the Perth Abyssal Plain and the Bay of Bengal indicate a common breakup date of Valanginian (Powell et al. 1988; Ramana et al. 1994). This proposed extension of greater India 1300 km, north of the reconstructed location of the eastern syntaxis, was used by Le Pichon et a/.(1992) in their model which is discussed below. A smaller greater India has been proposed Dewey et al. (1989), Harrison et al. (1992) and Treloar & Coward (1991). Dewey et al. argue that apart from erosion loss after collision the remainder of the crust is stored in the Himalayas. On this basis, Le Pichon et al. (1992) estimate a minimum size for greater India as 600 km north of the suture, whereas Dewey et al. (1989) give a range from 450-1000 km. In the Harrison et al. and Treloar & Coward models, collision commenced near the western syntaxis leaving a 700 km gap at the eastern syntaxis which closed in the early Oligocene over 10 Ma after the collision onset. Le Pichon et al. (1992) assumed simultaneous collision and located the collision boundary 1000 1200 km south of the Indus-Zangbo suture (Fig. 4). Dewey et al. (1989) and Tapponnier et al. (1986) also assumed simultaneous collision, placing the Asia boundary fifteen degrees further south at its eastern end. The fit of pre-rift India against Antarctica makes it unlikely that it extends east of 95°E. Its southeastern margin runs from east of the Shillong Hills under the eastern Bengal Basin to the foot of the slope parallel to the northeast trending coastline. Some recent authors (e.g. Zhao & Morgan 1985; Peltzer & Tapponnier 1988; Le Pichon et al. 1992) show an eastward prolongation of the craton including the Cretaceous to Palaeogene flysch deposits of the Indo-Burman Ranges on the Indian plate. This is not compatible with the pre-rift reconstruction or the geology of the Bengal Basin where a southeast deepening continent margin is identifiable from the Early Cretaceous onwards (Lindsay et al. 1991). It is argued later that the flysch was part of the Asian margin.
128
G. PACKHAM I 60*E
'
70° ,,...,'1
i
t
80*
90"
II-- " - ~
~
IO0"E
I
I
4
--10*N
-0"
i
1
I
\~
I
Fig. 5. Estimated locations of the Asian margin at the time of the Indian collision from Tapponnier et al. (1986), Dewey et al. (1989) and Le Pichon et aL (1992).
Topographic analysis
Le Pichon et al. (1992) attempted to assess whether significant extrusion occurred by estimating the area of the crust lost during shortening, assuming conservation of continental crust, and comparing it with the volume of the topography that can be attributed to the collision after adding an estimate of the volume of eroded sediment. The model uses the larger greater India indicated above, assumes a simultaneous collision along the craton edge, and extended further north than the Avouac & Tapponnier (1993) model to include Altai. Assuming an average pre-collision elevation of 500 m, Le Pichon et al. found there was a uniform loss of mass west of the east syntaxis and an excess east of it (Fig. 6). The excess accounted for one third to half of the loss to the west. The loss of non-thickened crust can be accounted for by some combination of lateral extrusion and loss of lower crust into the mantle, possibly by eclogitization. The loss would be reduced if the crust was thinner i.e. was at sea-level or below it. For a minimal greater India in which there was 700 km of oceanic crust north of the eastern syntaxis at the time of collision, the loss of continental crust is reduced by about 20%. The transfer of mass to the east of 103°E still remains, but the deficit is reduced to between one third and one eighth, or even less if the crust was thinner. The minimum estimate of Le Pichon et al. (1992) is not very different from the result obtained by the kinematic approach of Dewey et al. (1989). They summed the strain throughout the collision event, along slip circle traverses through the western syntaxis, central Himalayas and the eastern syntaxis, and concluded that the crustal volumes
nearly balanced. No extrusion was required beyond the thickening of the Tibetan Plateau east of the eastem syntaxis. The crust lost from the north of the Indian Craton was assumed by Dewey et aL to have been thin like that of the Exmouth Plateau off Western Australia. The essential differences between the models are the thickness, volume and fate of the greater India crust. Le Pichon et al. assumed that the upper crust was stored in the Himalayas and partly eroded off, but the lower crust was transferred to the mantle or emplaced north of the Indus suture. If all the crust has been transferred under the plateau, half of its elevation would be due to the transfer and the remainder to local shortening. The mechanism Le Pichon et al. favour is injection of Indian crust into the Tibetan lower crust as a low viscosity fluid, as proposed by Zhao & Morgan (1985). Le Pichon et aL rejected the thin crust hypothesis of Dewey et al. because there is no evidence of Eocene and Oligocene deep water sediments overlying the Indian crust fed into the system. A hint of thin (or thinned) crust comes from the Upper Jurassic to Lower Cretaceous section of the Tethyan Himalaya cover north of the Main Central Thrust in Nepal. A clastic deltaic to shelf clastic succession deposited on northward palaeoslope is overlain by post-breakup pelagic slope carbonates (Gibling et al. 1994). 3000-
2000-
ItVS y n t ~
o
IOO0
/o.
' Longitude 80"
;o.
100"
Fig. 6. Estimates of equivalent linear shortening between India and Eurasia, determined by Le Pichon et al. (1992), from the volume of the topography and corrected for erosion assuming conservation of continental crust. Average block elevations are plotted for sea-level and 500 m. The upper curve with filled triangles represents the linear shortening from kinematics for a maximum greater India, the lower curve (added here) with open triangles is for a minimum greater India. Note the eastward displacement of shortening determined from topography.
CENOZOIC SE ASIA RECONSTRUCTIONS
Phases of the collision and their expression in SE Asia Rotation or extrusion of the part of SE Asia west of the Red River Fault has been identified as a major driving mechanism for sea floor spreading in the South China Sea (e.g. Tapponnier et al. (1986) and Briais et al. (1993) between 32 Ma and 16 Ma. This event, resulting from India-Eurasia collision, divides the Cenozoic SE Asian tectonic history into three major phases, namely middle Eocene to early Oligocene, early Oligocene to middle Miocene, and middle Miocene to Recent. M i d d l e E o c e n e to E a r l y O l i g o c e n e
Dewey et al. (1989) proposed that between 45 and 30 Ma, convergence was taken up by the commencement of stacking of northern Himalayan thrust sheets and thickening of the Tibet crust to 70 km elevating it to 3 km by 30 Ma, along with progressive indentation of the Asian margin and bending the older sutures (Fig. 1). Because the Palaeogene and older strata in the Tibetan plateau are strongly deformed and Neogene sediments are essentially flat lying, Dewey et al. (1988) dated the major deformation of the plateau as pre-Miocene and estimated that the shortening was about 50%. Harrison et al. (1992) accepted the degree of deformation of some of the Palaeogene sediments but questioned its generalization over the plateau. They estimated shortening at 150 km (< 20%). Using radiometric data to determine cooling rates in the Quxu pluton near Lhasa, Harrison et al. (1992) suggested thickening, uplift and unroofing of the pluton commenced in the early Miocene (21-18 Ma). Later work by Copeland et al. (1995) related unroofing of granite bodies successively towards the north to uplift on the Gangdese thrust to the south commencing at 27 Ma. This does not conflict with the argument of Dewey et al. (1988) that Palaeogene crustal shortening elevated the Tibetan Plateau. A review by Mercier et al. (1987) of the climatological evidence for the date of plateau uplift from palaeobotanical studies reveals conflicting interpretations. Some authors consider that steady uplift occurred in northern Tibet from the end of the Cretaceous, and in the south (Gangdese Belt) from the middle Eocene, and suggest the Himalayan uplift was later. Other authors consider that the uplift was predominantly Plio-Pleistocene. In their well known plasticine model experiments, Tapponnier and his co-workers (Tapponnier et al. 1982; Peltzer & Tapponnier 1988), partitioned extrusion and crustal thickening equally throughout the collision and conserved all crust. The indentor representing the Indian craton deformed a striped
129
plasticine representing Asia, producing a remarkably good facsimile of the observed fault patterns. In their tectonic evolutionary scheme for Asia, Tapponnier et al. (1982, 1986) and Peltzer & Tapponnier (1988) identified two major phases of extrusion. The first phase between middle Eocene and middle Miocene, had extruded the region south of the Red River Fault a total of 800-1000 km in two stages. Extrusion in the second phase from middle Miocene to Recent employs a model similar to that of Avouac & Tapponnier (1993) for the present tectonic regime, in which extrusion occurred north of the Red River Fault, as discussed above. Tapponnier et al. (1986) suggest thickening of the Shan Plateau and the Lhasa Block may have taken place before the formation of the South China Sea. But in Peltzer & Tapponnier (1988), the strain produced by convergence was apparently partitioned between thickening and extrusion along the Wang Chao and Three Pagodas Faults (Fig. 3). The end of movement on the Wang Chao fault at about 30 Ma has been supported by radiometric data from Maluski et al. (quoted in Huchon et al. 1994). Peltzer & Tapponnier (1988) estimated displacements on both the Wang Chao and the Three Pagodas Faults at 300 km by matching the displacement of the Uttaraditt suture and upper Cretaceous magma belts respectively. The magmatic belts are not as clearly defined on geological maps as portrayed by Peltzer & Tapponnier (1988) and the displacements on the Three Pagodas Fault could well be substantially less than their estimates and may even be right-lateral if the displaced extension of the Uttaraditt suture on the Gulf of Thailand is aligned with the projected BentongRaub suture (Fig. 3). Harrison et al. (1992) explained the delay in movement on the Red River Fault until early Oligocene (35 Ma) by assuming a smaller greater India than Le Pichon et al. (1992) and that collision commenced at the western syntaxis at about 50 Ma leaving a gap of 700 km of oceanic crust at the eastern syntaxis. Movement on the Red River Fault started after the early Oligocene closure of the gap and collision in eastern Tibet. In this scheme, the duration of displacement on the Wang Chao Fault would have been short. E a r l y O l i g o c e n e to E a r l y M i o c e n e
In the second phase, according to Dewey et aL (1989), after 30 Ma when the Tibetan Plateau had achieved considerably buoyancy, India began to indent Asia, deflecting the Asian sutures and commencing block rotation (Fig. 1). Wrenching began along conjugate faults bounding the elevated area and deformation spread north with diminishing
130
G. PACKHAM
intensity, into Tienshan and south into the Himalayas elevating them along the Main Central Thrust and thrusts in the Lesser Himalayas. Uplift of the Gangdese belt in the early Miocene resulted from underthrusting of the northern Himalayas. In their model Peltzer & Tapponnier (1988) proposed that movement on the Red River Fault generated the South China Sea as the southern part of Sundaland moved to the southeast. They estimated the displacement on the fault as 500 km, by matching the extrapolated Jinsha and Uttaraditt Sutures (Fig. 3). The magnitude and direction of displacement on the Red River Fault is supported by Lacassin et al. (1993) who estimated the leftlateral strain in mylonitic gneisses as 330 _+60 km and radiometric dating by Leloup et al. (1993) has found that in the Diancang Shan region in Yunnan, left-lateral ductile shear terminated between 17 and 19 Ma, preceded by 7 km of unroofing at 23 and 19 Ma, due to large scale oblique shear. In the Harrison et al. (1992) variant on the Tapponnier model the main phase of movement on Red River Fault was from 35 to 20 Ma. Half to two thirds of the convergence was taken up by extrusion. The end of the phase is based on evidence of unroofing of the Quxu Pluton near Lhasa, suggesting that significant uplift of the Tibetan Plateau occurred at 18 to 21 Ma rather than after 17 Ma as suggested by Tapponnier et al. (1986). The unroofing is apparently related to uplift associated with movement on the Gangdese Thrust that commenced at 27 Ma (Copeland et al. 1995). Briais et al. (1993) made a kinematic study of the spreading history of the South China Sea and the displacement on the Red River Fault. Spreading commenced at 32 Ma with a ridge jump at 26 Ma and a change in ridge trend from east-west to northeast-southwest at 24 Ma with spreading ending at about 15.5 Ma soon after the cessation of movement on the Red River Fault at 17-19 Ma as discussed above (Fig. 7). Using poles of opening they determined for the South China Sea and assuming that the Indochina Block was attached to the drifting margin Briais et al. (1993) determined the displacement on the Red River Fault as 555 km for a point near the Uttaraditt suture; a good correlation with the estimate of Peltzer & Tapponnier. In the reconstructions of Briais et al. convergence increased across the fault inland and there was extension in the lower Red River region, correlating well with the study of Leloup et aL (1993). The implication is that the Indochina Block and the drifting South China Sea margin formed a single small plate and that spreading was driven by motion of the Indochina Block. This point will be returned to later. A stage pole determined by Huchon et al. (1994) describing the early stage of opening was located at 13°N and 97°E in the
Fig, 7. Interpretation of South China Sea structure modified from Tapponnier et al. (1986) showing: oceanic crust (cross hatched) and shallow areas of continental crust (dotted). LP, Luconia Province; MAB, Macclesfield Bank; NCB, Nam Conson Basin; PA, Palawan Block; PS, Paracel Shoals; RB, Reed Bank; SCAR, Scarborough Seamounts; WNB, West Natuna Basin; YB, Yinggehai Basin.
northeast Andaman Sea. At 30 Ma it would have been located east of the eastern syntaxis but well south of the Indus suture. The closeness of the pole to the Indochina Block suggests that rotation was more important than translation. Although the commencement of South China Sea floor spreading at 32 Ma fits well with the initiation of movement on the Red River Fault, pre-drift rift basins containing Eocene sediments occur on both margins of the South China Sea (Salvidar-Sali et aL 1982; P. Chen et al. 1993). In the case of the Pearl River basin on the south China margin and Reed Bank on the drifted margin, there are both Eocene and Palaeocene sediments (Taylor & Hayes 1980; Yu 1990). That is, South China Sea margin extension commenced earlier than the India-Asia collision and the supposed time of initiation of the Red River Fault.
Early Miocene
to R e c e n t
The uncertainty in dating the initial elevation of the Tibet Plateau was discussed above. Rather than occurring by crustal shortening from late Eocene to
CENOZOIC SE ASIA RECONSTRUCTIONS mid-Oligocene time as proposed by Dewey et al. (1989), Tapponnier et al. (1986) and Harrison et al. (1992), consider that the bulk of the uplift took place after movement on the Red River Fault. This requires transfer of lower crust from the India plate across the Indus suture because the Neogene sediments of the plateau are relatively undeformed. As described previously, the present tectonic regime commenced in middle to late Miocene time. During it some of the convergence was transferred to eastward movement of Tibet relative to the Himalayas and in part to South China as indicated by the early Pliocene commencement of rightlateral movement on the Red River Fault. Elevation of the Tibet Plateau to its present height was approximately coincident. Harrison et al. (1992) suggested it occurred between 8 Ma and 5 Ma and Dewey et al. at about 5 Ma. England & Houseman (1988) explained this last uplift as the result of delamination of the mantle lithosphere beneath the plateau.
Palaeomagnetic data: central Asia and mainland SE Asia The objective of palaeomagnetic studies in central and SE Asia has been to determine the amount of northward movement of sites north of the Himalayas and whether blocks in SE Asia have been rotated and moved south due to extrusion. The palaeomagnetic data have been recently summarized by Y. Chen et al. (1993) and Huang & Opdyke (1993) using different methodologies. Rotations and change in palaeolatitude have been determined by comparing measurements with the Eurasian polar wander path compiled by Besse & Courtillot (1991). The uncertainties taken into account are statistical errors arising from the measurements within and between sites, structural corrections, estimates of rock age, date of magnetiztion and uncertainty in the polar wander path location. Fortunately, the Eurasia polar wander path assembled by Besse & Courtillot (1991) indicates very little movement in the pole location in the Cretaceous between 130 and 80 Ma. Studies have been biased towards Cretaceous sediments but unfortunately many of them are in non-marine successions and hence dating is imprecise. Huang & Opdyke (1993) compared the results from separate sites with the pole for that magnetization age. The Y. Chen et al. (1993) compilation compared the averages of individual studies on Cretaceous rocks for each block with an average Cretaceous pole. Although the results from the two methods are in general comparable Y. Chen et al. lost fine detail. The dataset for Middle Jurassic to Upper Cretaceous sites from Huang &
131
Opdyke (1993) are replotted according to longitude in Fig. 8 and individual sites in the eastern part of the region have been plotted geographically on Fig. 3. With one exception, the Pamir measurements west of the western syntaxis show counterclockwise rotation and possible but minor southward displacement. Tamir poles indicate significant northward motion with greater displacement to the west. The Tarim block data have large errors, Huang & Opdyke summarize it as 1380 _+790 km but suggest several hundred kilometres without significant rotation is more likely. The Tibet data (Lhasa Block) suggest about 1500 km of northward displacement with counter-clockwise rotation increasing to the east. East of the east syntaxis, the Markam sites (eastern Qiangtang Block) are rotated clockwise and unlike Tibet may be displaced southward but the error bars overlap zero displacement. Sites on the northern Shan-Thai Block, southwest of the Red River Fault and west of the Uttaraditt suture, indicate clockwise rotation apart from two locations close to the Red River Fault which are rotated slightly counter clockwise (Figs 3 and 8). Five Shan-Thai sites indicate possible northward displacement. The three Jurassic sites (22, 23 and 25) apparently indicate a strong northward displacement. These may not be significant because the wander path of Besse & Courtillot (1991) has a strong shift in the palaeopole corresponding to a palaeolatitude change at these sites from 49°N to 20°N between 170 Ma and 130 Ma (Oxfordian to Valanginian) after being relatively stationary from 210 Ma (Late Triassic). The fourth site (R on Fig. 8) with possible northward shift is from the Jurassic~Cretaceous Kalaw Red beds in eastern Myanmar on the Shan-Thai Block (Richter et al. 1993). For ages of magnetization between 130 and 80 Ma this sampling locality yields a palaeolatitude displacement between 2.6 and 6.0 degrees northward within a _+6 ° error bar. If there is a northward displacement it is only small. A possible small northward displacement has also been measured at Site 24. Possible southward displacements were found at Cretaceous sites 17, 18 and 19 are (Fig. 8) but the errors make them inconclusive. Yang & Besse (1993) report a palaeolatitude of 23.1 +_ 1.8°N for the medium temperature magnetic component in sediments from the Khok Kruat Formation (Albian-Aptian) in the Khorat Basin on the Indochina Block. After including the uncertainty of the palaeopole location Huang & Opdyke (1993) quote the palaeolatitude change as -8.0 _+5.3 ° northward with a clockwise rotation of 14.4 _+6.1 °. No fold test was possible at the site because of low dips but older formations in the same sequence do give a positive fold test. The
132
G. PACKHAM
Indochina 2O Moved North
°i 80
21,
60
dochina
Pami
01
/--I
/'~ ~
-20
-40
14~,Jl/ Markam
(I,j-/~r 28
I
II
~/
8
r
t
70
'
'
'
'
I
80
. . . .
Ill'
90~
1'
'
'
!
100
'
'
'
,
I
110
. . . .
I
120
. . . .
i
130
Fig. 8. Plots of deviations of palaeomagnetic measurements from the expected palaeolatitude and declination from the equivalent aged Eurasian pole from Besse & Courtillot (1991) plotted against latitude. Data and point numbers are from Huang & Opdyke (1993) except for the point 'R' which is from Richter et al. (1993). The inner bar is the uncertainty of determination from the individual dataset and the outer bar includes the uncertainty of pole determination.
Cretaceous results south of the Red River Fault from the western side of the Shan-Thai Block suggest a small northward m o v e m e n t and a correspondingly small southward m o v e m e n t on the east. Declinations are generally rotated clockwise and
they are deflected around 70 ° clockwise from the Lhasa Block measurements (Figs 3 and 8). At face value, these results could indicate rotation of the Shan-Thai Block about a pole located within it. The Khorat Plateau rotated further
CENOZOIC SE ASIA RECONSTRUCTIONS
133
south could be part of the same domain. Variability in disturbance of declinations suggest that the Shan-Thai Block may not have behaved as a single entity but was internally deformed by displacement along the many faults through the region. Although the rotation can only be dated as later than Early Cretaceous, the event may be of middle to late Tertiary age, since upper Cenozoic sites in central and western Thailand are also disturbed and are mostly but not universally rotated clockwise (McCabe et al. 1993; Richter et al. 1993). Sites of similar age from the Indochina Block in eastern Thailand (Khorat) and Vietnam are not significantly rotated. The South China results indicate possible southward displacement in the eastern part, but in the west site 26 in the Sichuan Basin may have been displaced to the north (6 _+6°). The magnetization age is indicated by Huang & Opdyke as uncertain. Sites 27, 28 and 33, between the Xian Shui He and the Red River Faults, are in the central Hunnan Basin are close to their expected palaeolatitude. Site 33 in the central Yunnan Basin near the Red River Fault is, like the Markam sites, rotated significantly clockwise. In summary, there is significant northward translation of Tibet, counter-clockwise rotation in Pamir, counter-clockwise rotation of eastern Tibet, clockwise rotation of sites east of the eastern syntaxis but disturbed close to the syntaxis and near the Red River Fault. One good Indochina Cretaceous dataset indicates probable southward motion, the remaining Cretaceous results spread from west of the Dien Bien Phu Fault in the Shan-Thai Block are ambiguous but could indicate clockwise block rotation with little change of latitude.
it, was moving at a higher velocity obliquely away from the Indochina block, rapidly decreasing the length of the contact between them. Thus after 26 Ma, spreading was not driven by motion of the Indochina Block. Further, spreading ended about 15.5 Ma, later than the cessation of movement on the Red River Fault (17-19 Ma according to Leloup et al. 1993). To invert the argument, using South China Sea spreading to determine fault movement would substantially overestimate the fault displacement after 26 Ma. Of the 560 km of Red River Fault displacement calculated by Briais et al. (1993), only the 240 km which occurred between the commencement of spreading and 26 Ma can be attributed to driving South China Sea spreading. While fault displacement may have continued, spreading in the South China Sea does not provide a means of determining it. The amount of strike-slip movement may have been small in the last phase of movement on the fault from 23-19 Ma when oblique shear and unroofing occurred. The fault could have been active before 32 Ma driving some of the rifting which preceded South China Sea spreading, although as noted earier this started in Palaeocene time before the commencement of the collision of India. Extrusion of Indochina on the Red River Fault fits well with the Oligocene phase of spreading in the South China Sea, but is difficult to reconcile with published basin histories in the region. In the Nam Con Son Basin (Fig. 7) on the Indochina Block syn-depositional extension occurred (Matthews & Todd 1993) while in the West Natuna Basin to the south there was post extension subsidence (Ginger et al. 1993).
The driving mechanism for South China Sea spreading
The western margin of Sundaland
In the discussion of early Oligocene to early Miocene events, it was pointed out that South China Sea spreading had been used to estimate displacement on the Red River Fault on the assumption that the spreading was driven by extrusion of Indochina on the Red River Fault. During the first stage of opening from 32 to 26 Ma, spreading ridges were short and the Indochina block had a long contact with the drifting side of the basin, extending almost to Reed Bank (Fig. 7). After a ridge jump between anomaly 7 and 6b at 26-24 Ma, spreading began to propagate rapidly to the south. The ridge trend changed from 5-10°E of N to about 60°E of N. The southwestward extension stabilized at about anomaly 6 at 20 Ma. As the spreading pattern changed, the drifting block, consisting of an elongated strip of thinned continental crust with oceanic crust outboard from
The geology of the western margin of Sundaland provides some constraints on models of India-Asia collision which have been largely ignored. Some aspects are examined here. The Java-Sumatra
margin
In the Sumatra-Java margin, there is a Late Cretaceous to middle Eocene hiatus in volcanic activity. Possible explanations are that, prior to the middle Eocene a north facing arc lay to the south and collided with polarity reversal in the middle Eocene or that there was little or no subduction on the Sumatra-Java margin during the hiatus. The latter is supported by the work of Wajzer et al. (1991), who have interpreted the Upper Mesozoic Woyla Group in the Natal area of western Sumatra as a west-facing accretionary prism which has later been disrupted by strike-slip faulting.
134
G. PACKHAM
Packham (1990) suggested that between latest Cretaceous and middle Eocene time, sea floor spreading in the Indian Ocean (Phase 2 in Fig. 4), extended east only as far as its present preserved limit off southern Sumatra, 1300 km east of the Ninetyeast Ridge. East of it lay the older sea floor of the Australia plate, which at that time was moving slowly away from Antarctica (Royer & Sandwell 1989). With middle Eocene reorganization of sea floor spreading, between anomalies 20 and 18 (44-41 Ma), subduction under the Sunda margin commenced or increased from a low rate. Radiometric ages of an andesite and a quartz diorite from northern central Myanmar of 50 and 53 Ma respectively (Mitchell 1993) suggest that subduction-related volcanism occurred there during the Sumatra volcanic hiatus and that the IndiaAustralia plate boundary intersected the Asia margin between Sumatra and Myanmar. The Myanmar dates also fall in the time range of the Linzizong Volcanics of the Lhasa Block (Dewey et al. 1988) and presumably represent the eastern end of the Paleogene volcanic arc. Using the India-Asia reconstruction parameters of Le Pichon et al. (1992), the extrapolated middle Eocene position of the India-Australia plate boundary would intersect the central Sumatra coast 500 km west of its present position (Fig. 9). It is in a similar position in the 45 Ma reconstruction of Peltzer & Tapponnier (1988). For the Sumatra margin to be essentially passive until then requires the India-Australia plate boundary to have been west of it, i.e. Sumatra must have been about 500 km further east relative to South China at the time collision commenced. The distance may be reduced by about 100 km, subtracting movement of western Sumatra on the Burma plate since the inception of the Sumatra Fault.
M y a n m a r a n d the I n d i a - A s i a collision The tectonic position of the Indoburman flysch belt, the time of its docking and the distribution of post-docking strain all have important implications for reconstructions of western SE Asia. As indicated in the discussion of greater India the eastern boundary of the Indian craton runs from south of the Shillong Hills to the Bay of Bengal (Fig. 3). The Indoburman flysch belt has been accreted onto the craton margin after collision. Palaeogeographically, the flysch is an Asian margin accretionary wedge and the proximal Central Basin of Myanmar is a shelf deposit. The flysch is a thick sequence of Senonian to middle Eocene feldspathic turbidites (Bender 1983), which for the most part predate the collision of India with Asia. Their distribution down the Arakan coast to the west of the oceanic
EURASIA
35 \ ,= 4 5 × _ _ X - -..~( ''~
INOIA " ~ PLATE
oo
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ......... ,5
20os 6O"E
PLATE 80"
I 100*E
Fig. 9. Reconstruction of India and western SE Asia for 45 Ma from Peltzer & Tapponnier (1988). To it have been added the proposed India-Australia plate boundary prior to Phase 2 of Indian Ocean Spreading (Anomaly 20-44 Ma), and an X in Assam at the eastern syntaxis and another on the Eurasia plate at the corresponding point at the northern end of the Indoburman flysch belt on the Eurasia plate from the same set of reconstructions for 45, 35 and 15 Ma. Docking of the flysch belt at Assam is thought to have occurred in the Oligocene between 30 and 36 Ma.
crust of the Bengal Basin precludes their derivation from the Indian margin. West of the flysch the Central Basin of Myanmar contains a shelf to nonmarine Albian-Aptian to Pleistocene sedimentary succession. In the Naga Hills near the eastern syntaxis (Fig. 3), the Upper Cretaceous-Eocene flysch is thrust over Palaeocene to upper Eocene platform cover (Mitchell 1993) and both are overlain by the post-docking Oligocene to ?lower Miocene, Barail Formation (Roy 1986). The Barail Formation progrades south from fluvio-deltaic to marine. The stratigraphic relations indicate an early Oligocene docking of the flysch terrane on the India plate, probably within 10 Ma of the India-Asia collision, and movement north on the craton since that time. On a broader scale, the structural units of western Myanmar appear to be an eastward continuation of the Asian margin wrapped around the eastern syntaxis in agreement with palaeomagnetic declination rotations (Fig. 3). The uplift between the Central Basin and the flysch belt corresponds to the Indus suture as indicated by Dewey et al. (1988). At its northeastern extremity in Myanmar, the suture appears to end against a branch of the Sagiang Fault. It is apparently
CENOZOIC SE ASIA RECONSTRUCTIONS displaced 450 km to the south to a belt of ultramarie rocks 100 km north of Mandalay, thence it trends north-northeast, curving north-northwest around the northern end of the Central Basin where it is separated from the Indus suture by the Lohit Thrust. The presence of Triassic flysch in Myanmar on the western margin of the Central Basin links it with the southern Lhasa Block-Indus suture as does the Upper Cretaceous flysch. The basement rocks of Myanmar from the Shan Scarps westward form part of the Cretaceous to Eocene magmatic belt which Mitchell (1993) compares with the Gangdese plutons. From docking at the beginning of Oligocene time, say 36 Ma, until the commencement of Andaman Sea spreading at c. 13 Ma (Curray 1989), the northward transport of the Indoburman flysch belt on the Indian craton relative to fixed Asia would be 1500 km, using the kinematics of Le Pichon et al. (1992). If the Shan-Thai Block was extruded the relative displacement would be that much greater. Identification of such a dislocation is very speculative. Mitchell (1993) suggested 1100 km of right-lateral displacement of the Cretaceous outer magmatic belt exposed in highs within the Central Myanmar basin, relative to the Shan-Thai Block prior to Andaman Sea spreading. Assuming coupling to the India plate this would take 17 Ma. The fault would have to be located between the Central Basin highs and the Sagiang Fault. This would leave the remaining 400 km to be taken up west of the Cretaceous magmatic belt in the Indoburman Ranges, the Andaman arc and pre-rift extension prior to Andaman spreading. A later time of docking would reduce the residual by 64 km per Ma. The position of the northeast comer of the flysch belt has been plotted on the 45 Ma reconstruction of Peltzer & Tapponnier (1988) on Fig. 9 together with its 35 and 15 Ma locations from their other reconstructions. At 35 Ma, about the time of docking according to the geological evidence cited above, their reconstruction shows it about 2000 km to the north. At 15 Ma it is still around 900 km north of the corresponding docking point in the Naga Hills. At 30 Ma when movement ended on the Wang Chao Fault, relative to stationary Eurasia the eastern syntaxis was at 1 I°N and the northern edge of greater India as far north as 22 ° corresponding to the present day location of the western end of the fault. Andaman Sea spreading commenced at about 13 Ma (Curray 1989). Most of the 460 km of extension has been taken up onshore along the westem margin of the Burma plate by right-lateral slip on the Sagiang Fault on the western margin of the Shan Plateau (Fig. 3). The northward movement of the India plate relative to stable Asia over the last 13 Ma was 790 km,
135
leaving a residual of 330 km between the India and Burma plates which cannot be accounted for easily. Summarizing the foregoing, the Indoburman flysch belt was a forearc on the Asian margin at the time of its collision. After docking early in the Oligocene, the Indoburman flysch belt and possibly central Myanmar, were wrapped around the syntaxis and transported north, possibly 2300 km on the Indian Craton along the western SE Asian margin. Attenuation of the crust in the Andaman Sea region in early Miocene time, was followed by sea floor spreading in the Andaman Sea from the middle Miocene onwards accompanied by displacement on the Sagiang Fault. Some of the northward strain could have been translated into rotation of the Shan-Thai and Indochina Blocks and westward transport of Sumatra and possibly adjacent parts of SE Asia. Movement on the Wang Chao and Red River Faults was taking place as the flysch belt and central Myanmar were being transported north. Extrusion models would require the Shan-Thai and Indochina blocks and the flysch belt to be extruded from north or west of the eastern syntaxis in Eocene time, prior to collision at the eastern syntaxis. Alternatively, they could have been squeezed out behind the SE Asian margin after the northern end of the Myanmar flysch belt had docked in southern Assam early in Oligocene time.
Western SE Asia: conclusions Analysis of the present seismicity documents an eastward motion of Tibet and sets the stage for the interpretation of the earlier history. The model proposed by Avouac & Tapponnier (1993) gives a residual southeastward movement of South China of 10-15 mm a-1. At this rate during the last 4.7 Ma of fight-lateral displacement on the Red River Fault, a displacement of only 50-75 km is indicated. At a lower rate based on field studies by Allen et al. (1984) it would be 10-24 km. These latter are compatible with the Dewey et al. (1989) suggestion that most of the eastward movement has been absorbed in the Longmen Shan and that the strike-slip displacements are a fraction of those suggested by Tapponnier et al. (1986). Any pre4.7 Ma extrusion of South China would have moved Indochina as well. Transition to the present tectonic framework is likely to have occurred in late early to middle Miocene time after left-lateral movement on the Red River Fault ceased between 17 and 19 Ma (Leloup et al. 1993). Forward modelling of the India-Eurasia collision from its onset at about 45 Ma produces different results depending on the estimated volume of crust fed into the Himalayan system. Even conservative models result in some extrusion of crust into eastern Tibet. Dewey et al. (1989)
136
G. PACKHAM
assumed conservation of continental crust and a greater India with thin continental crust north of the present craton, and concluded that extrusion is limited to the eastward extension of the Tibetan Plateau east of the eastern syntaxis. With a larger and thicker greater India, Le Pichon et al. (1992) found that there was a deficit of topography between the syntaxes and an excess to the east, supporting the notion of eastward transport, but between a half and one third of lost crust remains unaccounted for. This can be explained by partitioning the loss between extrusion and transfer of lower crust to the mantle by eclogitization. If a smaller greater India is assumed, the deficit reduces to one eighth to one third and even less if the crust was thinner, a result similar to that of Dewey et al. Ahead of the advancing Indian Craton, Dewey et al. (1988, 1989) suggested crustal thickening elevated the Tibetan Plateau to 3 km between 45 and 30 Ma. Palaeogene deformation was suggested because Neogene sediments on the Tibet Plateau are relatively undeformed. India then began to indent Asia, deflecting the old sutures and rotating adjacent blocks as the Tibet crust became more buoyant. After 30 Ma deformation spread north of Tibet and south into the Himalayas. Elevation of the southern margin of the plateau discussed by Harrison et al. (1992) is related to thrusting on the Ghagdese Thrust which commenced at 27 Ma (Copeland et al. 1995) and is independent of dating the elevation of the whole plateau. If plateau elevation did not occur until around 20-25 Ma, then it is not possible to explain how over 20 Ma of shortening was distributed without invoking extrusion. In their models Harrison et al. (1993) and Peltzer & Tapponnier (1988) partitioned strain from convergence between crustal thickening and extrusion. They proposed that Indochina was extruded along the Red River Fault before the uplift of plateau. The latter authors proposed an Eocene to early Oligocene extrusion of the region south of the Wang Chao Fault. Left-lateral displacements of the order of hundreds of kilometres have been documented for the Wang Chao and Red River Faults and radiometric data support an early end of movement on the Wang Chao Fault. Harrison et al. (1992) suggested that uplift of the plateau commenced between 18-21 Ma. at the time of oblique shear on the Red River Fault. Neogene plateau elevation must be explained by some form of crustal underplating by Indian Craton crust across the Indus suture because Neogene sediments of the plateau are mildly deformed. Elevation was accompanied by movement on the Main Central Thrust in the Himalayas and the commencement of deformation to the north of Tibet, passing into the present tectonic regime.
Relating spreading in the South China Sea to determining displacement on the Red River Fault by Briais et al. (1993), although elegant, may be applicable only to the pre-Miocene fault displacement since after that time, the southeastern drifting side of the South China Sea was moving away from Indochina and cannot have been driven by its motion. Only about half of their estimated displacement of 520 km can be demonstrated this way, although some additional slip may have occurred in the pre-spreading rift phase on the China margin. The rift phase enigmatically had a Palaeocene commencement, prior to the India-Asia collision. Palaeomagnetic measurements have been quoted in support of extrusion. A review of the data here reveals that there is good evidence for counterclockwise rotation in the Pamir and northward translation of the Lhasa Block with some counterclockwise rotation. Stronger clockwise rotation predominates in the Indochina and Shan-Thai Blocks but regional changes of latitude consistent with extrusion have not been established. Southward motion of the Khorat Plateau is the only result in the east which falls outside the error bars. The data are compatible with clockwise rotation of the Shah-Thai block around a pole within the block. The dispersion of declination deviations could well be a response to complex deformation such as thrusting on the western margin of the Khorat Basin, and possible displacements on the many faults which cross the region rather than the simple extrusion of stable blocks. Evidence for the location of some entities on the western margin of Sundaland conflicts with extrusion reconstructions by Peltzer & Tapponnier (1988). A Late Cretaceous to middle Eocene hiatus in volcanic activity in Sumatra has been interpreted here to indicate that its middle Eocene position was around 500 km further east adjacent to the slow moving Australia plate (Fig. 9). For the Indoburman flysch belt and the Central Basin of Myanmar to dock at eastern Assam in early Oligocene time, they also had to be substantially east and south of the position reconstructed by Peitzer & Tapponnier, unless they were extruded with other parts of Sundaland prior to collision at the eastern syntaxis. If the docking predated Indochina extrusion, the extruded block was extruded from northeast of the Central Basin inboard from it. After docking, the flysch belt and most of central Myanmar have been transported possibly 2300 km north on the margin of the India plate and rotated clockwise. There are corresponding counter-clockwise rotations in the Lhasa Block. A major dislocation may exist beneath the western part of the Central Basin. Continued movement resulted in the middle Miocene formation of the Burma plate which is separated from the
CENOZOIC SE ASIA RECONSTRUCTIONS remainder of SE Asia by Andaman Sea spreading, the Sumatra and Sagiang Fault. Some of the earlier northward strain may have contributed to the rotation of the Shan-Thai and Indochina Blocks and suggested westward movement of Sumatra. The India-Asia Collision has been responsible for clockwise rotation of parts or all the Shan-Thai and Indochina Blocks, left-lateral slip on the Red River and probably for generation of at least the Oligocene part of the South China Sea floor. After accretion of the flysch belt onto the Indian craton it was carried north while displacement was taking place on the Wang Chao and Red River Faults. In the absence of definitive evidence of southward movement it is probable that, if there was extrusion, it was on a smaller scale than advocated by Tapponnier and his co-workers. While the present day tectonics of Asia are indicative of the largescale processes resulting from the India-Eurasia collision, determination of the time of elevation of the Tibet Plateau is crucial to resolving the kinematics of contending general models. If, in the first 20 Ma after the collision, most of the shortening was not taken up by thickening the crust of the plateau then extrusion must have occurred.
137
10 Ma
Rangin et al. (1990) t
,l
._.._I--I-
....... t
(a)
The Philippine Sea plate and AustraliaSE Asia collisions A number of regional reconstructions have treated the eastern region in some detail. Some of the diversity in interpretation is illustrated in the Miocene to basal Pliocene reconstructions from Rangin e t al. (1990), Daly e t al. (1991) and Packham (1990) in Fig. 10. The earlier regional study of the western Pacific by Jolivet e t al. (1989) has been largely incorporated in Rangin et al. (1990). The kinematics of the model of Rangin e t al. are illustrated in Fig. 1l. Reconstructions by Hall (1996) have made a major advance in the understanding of the regional tectonics. In the following discussion, the history of rotation of the Philippine Sea plate and the Philippine arc and the problem of the Celebes Sea is reviewed to provide background for the consideration of the origin and dispersal of the microcontinental blocks of Northern New Guinea and the Banda Sea, the impact of Philippine Sea plate rotation on Northern New Guinea and the Banda Sea, and the transmission of Pacific plate motion to Northern New Guinea and the Banda Sea.
(b) -
~
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The Philippine Sea plate and the Philippine archipelago Since 5 Ma the Philippine Sea plate has been rotating clockwise relative to Eurasia about a pole
(c) Fig. 10. Reconstructions of southeastern SE Asia: (a) for 10 Ma from Rangin et al. (1990); (b) for late Miocene time from Daly et aI. (1991); and (c) for 5 Ma from Packham (1990).
138
G. PACKHAM
Fig. 11. Kinematic diagram showing trajectories of the various blocks of Southeast Asia digitized and plotted in the reconstructions of Rangin et al. (1990). Plates are Australia (Aus), Caroline (Ca), Eurasia (Eu) and Philippine Sea (Ph).
located north of Japan at 48°N and 157°E at 5.45°/Ma (Seno et al. 1993). The Philippine archipelago straddles the convergent boundary between the Eurasia and Philippine Sea plates (Fig. 12). The left-lateral Philippine Fault which is no older than late early Pliocene (Aurelio et al. 1991), takes up the motion between the essentially anchored western flank of the islands and the oceanic plate to the east. The youthful Philippine Trench is a product of changing arc polarity. Palaeomagnetic data from islands east of the Philippines on the Philippine Sea plate and Ocean Drilling cores have a coherent pattern indicating a large scale clockwise rotation of the whole Philippine Sea plate (Hall et al. 1995b). Because the Philippine Trench is young, it is generally accepted that much of the archipelago was developed on the Philippine Sea plate rather the Eurasian margin. In his reconstructions Hall (1996) has separated Luzon and some of the islands to its south from the southern part of the Philippines and derived them from an extension of northern Borneo, assembling the island arc chain in early Miocene time by transferring Luzon to the Philippine Sea plate. This interpretation is based on the palaeomagnetic results of Fuller et al. (1991) who found a large counter-clockwise rotation in Luzon throughout most of the Tertiary with little change of latitude.
Hall et al. (1995a) estimated that the Philippine Sea plate has rotated c. 90 ° clockwise since 50 Ma. The pre-Pliocene rotation is in two stages. A rotation of 50 ° between 40 and 50 Ma about a pole located at 10°N and 150°E, followed by an interval with no significant rotation until 25 Ma when rotation recommenced, amounting to 34 ° about a pole at 15°N and 160°E (Fig. 13). These findings put severe constraints on the reconstruction of north New Guinea and the Banda Sea. The Miocene rotation may have been initiated by the collision of the northern edge of the Australian craton with the Philippine Sea plate. The general location of the Philippines has been plotted on the Fig. 13 on the assumption that they were on the Philippine Sea plate. The eastern part of the Philippine islands is substantially comprised of rocks built up by island arc processes from the Cretaceous onwards, on oceanic basement dating back to Late Jurassic (e.g. Geary et al. 1988). Continental basement of at least Jurassic age is present in the west around Mindoro and western Panay. It is an extension of the North Palawan Block on the South China Sea drifted margin. Pubellier et al. (1991) have identified a terrane in western Mindanao, including Zamboanga and the Daguma Range (Fig. 12), which may also be continental. The continental blocks, part of the Asia margin, were incorporated in the Philippines
CENOZOIC SE ASIA RECONSTRUCTIONS
Fig. 12. Major tectonic elements and sea floor anomalies in the Philippine-Banda Sea region modified from Hall et al. (1995b). Terranes of continental or suspected continental origin are west of the line in the Philippine Islands which includes Mindoro, west Panay and west Mindanao.
139
as the Philippine Sea plate rotated clockwise. The essentially transform boundary brought the south to southwest facing Philippine arc northward to collide obliquely with the Asian margin. A middle to late Miocene collision between the Palawan Block on the southern flank of the South China Sea has been interpreted differently by Holloway (1982), Sarewitz & Karig (1986), Rangin (1991) and Bird et al. (1993). Transpression and strike-slip continued from latest Miocene time to the present (Bird et al. 1993). The eastern and western terranes of Mindanao were juxtaposed before early Pliocene time by strike-slip movement (Pubellier et al. 1991). Middle to upper Miocene volcanic rocks in western Mindanao have been interpreted by Pubellier et aL (1991) as an east-facing arc on the western side of the Molucca Sea. The arc may have continued south through the Sangihe arc to similar age volcanic rocks in eastern SW Sulawesi (Coffield et al. 1993). Rangin et al. (1990) and Packham (1990) both show this as subduction of the Philippine Sea plate under the Sangihe arc. Daly et al. (1991) indicate subduction of Celebes Sea crust from the west (Fig. 11). An alternative and perhaps a better model, as discussed later, is that before the Banda arc was initiated, the Molucca Sea was part of a northward prolongation of the Australia plate as illustrated by Daly et aI. (1991) in their early Miocene reconstruction.
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Fig. 13. Schematic reconstructions of relationships in the Northern Australia region based on Philippine Sea plate reconstructions from Hall et al. (1995b) for 40, 25 and 15 Ma. Locations are: MLA, the Melanesian arc; PA,a possible Palaeogene Australia-Pacific arc between the Philippine Sea and Australia plates; MRA, the Maramuni arc; BA, the Banda arc; PI, the location of the Philippine Islands; and CB, the Banda Sea Continental Blocks.
140
G. PACKHAM
The Celebes Sea The Celebes Sea and the West Philippine Basin have overlapping ages (Fig. 12). Both contain middle Eocene oceanic crust with similar spreading rates. Weissel (1980) mapped magnetic anomalies 20 to 18 (46-52 Ma) which parallel the Sulu ridge and young to the south, into the North Sulawesi Trench. Anomaly trends differ because of the clockwise rotation of the Philippine Sea plate and possible counter-clockwise rotation of the Celebes Sea (Weissel 1980; Shibuya et al. 1989). The Celebes Sea could be part of a larger domain including southern Borneo and parts of the Malay Peninsular where Fuller et al. (1991) have also documented counter-clockwise rotations. The presence of contradictory results from Borneo and the dating of the rotation make interpretation controversial. Hall (1996) argues for the rotation of Borneo on geological and palaeomagnetic grounds. Silver & Rangin (1991) concluded that the Celebes Sea was either a marginal basin formed behind an arc or is a trapped fragment of a larger entity, including the Molucca Sea now separated from it by the narrow Sangihe arc. Similar sea floor ages are suggested by the lower middle Eocene cherts and limestones associated with ophiolites embedded in what is possibly a lower Miocene forearc terrane described by Moore et al. (1980) from Talaud Island in the Molucca Sea. Assuming the arc is not allochthonous the Celebes Sea has a backarc relationship to Palaeogene volcanic rocks in the north arm of Sulawesi (Ratman 1976) and the eastern part of the southwest arm (Sukamto 1982) where they have been broadly dated as lower Eocene to upper Oligocene. Hall (1996) has linked the Celebes and Molucca Seas to the West Philippine Basin to form a single basin behind a North Sulawesi-Philippine arc. Marginal basin formation was probably enhanced at anomaly 20 time when Australia increased its velocity away from Antarctica from c. 1 0 m m a-1 prior to anomaly 20, to about 30 mm a-I between anomalies 18 and 20 (Royer & Sandwell 1989).
Continental blocks of the Banda Sea and North New Guinea One obvious feature of the terrane distribution in northern New Guinea and the Banda Sea is that oceanic and island arc terranes are to the north of the continental terranes. Intervening between them are the West New Guinea Composite Block and the East Papua Composite Terrane. The Finisterre Terrane continues offshore into the uncollided Melanesian arc. East-west tectonic transport has resulted in duplication of the arc so that the Manus
segment overlaps the New Britain segment (Fig. 14). Three categories of continental blocks are found north of the Australian craton. (1) In the southern Banda arc from Timor to Tanimbar, parts of the Australian craton have been incorporated into the upper plate during collision. (2) North of the Australia plate boundary, in the Banda Sea region are rifted blocks including the basement of Sumba, Buton, Banggai-Sula, Buru, Seram, the Bird's Head, Obi and southern Bacan and submarine ridges south of Buru and Seram. This group could also include the metamorphic belt in the north, central and southeast arms of Sulawesi, and possibly some of the higher tectonic units in Timor. (3) Composite terranes comprising a mixture of continental, arc and oceanic terranes have been accreted onto New Guinea to form the West New Guinea Composite Block and the East New Guinea Composite Terrane. Apart from Sumba and central Sulawesi which are problematical the remaining blocks have an Australian derivation, but there is disagreement about their source, time of detachment from Australia and the means of their tectonic transport.
S u m b a a n d the central S u l a w e s i m e t a m o r p h i c belt
Sumba has a basement of Upper Cretaceous turbidites overlain unconformably by gently dipping Palaeogene shallow water sediments and volcanic rocks (Burollet & Salle 1981), and resembles the stratigraphy of the adjacent Asian margin in southwest Sulawesi and offshore east Java as described by Hasan (1991) and Bransden & Matthews (1992). Sumba is separated from them by the Flores Basin and was probably incorporated in the Banda forearc in later Palaeogene time. Van der Werff et al. (1994) and Rangin et al. (1990) also consider that Sumba was rifted off SE Asia and is a separate block from Timor. Another problematical unit, not usually regarded as one of the drifted blocks, is the metamorphic belt of central Sulawesi which contains blueschists and is overthrust on the east by the East Sulawesi ophiolite belt. A lower Cretaceous metamorphic date shows that these former continental margin deposits are older than the Upper Cretaceous turbidites of southwestern Sulawesi which overlie an ophiolite basement. Two phases of EoceneOligocene metamorphism have been identified in the blueschists (Parkinson 1991). The metamorphic belt extends down the west side of the southeast arm of the island where Helmers et al. (1990) have
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SE ASIA PLATE
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Fig. 14. Plate tectonic framework of the New Guinea region modified from Struckmeyer et al. (1993) showing relative longitude of the former locations of the western edge of the Philippine Sea plate as reconstructed by Hall et al. (1995b).
~"s
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142
G. PACKHAM
obtained an upper Oligocene metamorphic date. Palaeogene volcanic rocks on the eastern side of the southwest ann of the island reported by Ratman (1976) and faulting events recorded in the late Eocene and mid-Oligocene in the platform carbonate Tonasa Limestone to the west of the volcanic arc are compatible with a convergent boundary and a mid-Oligocene collision. Helmers et al. (1990) distinguish the event from the later collision of the Banggai-Sula Block in northern east Sulawesi which Davies (1990) dates as latest Miocene. It can also be distinguished from the mid-Miocene collision of Buton-Tukang Besi with southeast Sulawesi (Davidson 1991). Hall (1996) envisages transfer of the metamorphic belt and the ophiolite from the India plate and to the Eurasia plate at 25 Ma. The eastern and central Sulawesi terrane has general similarity to the West New Guinea Composite Block, which contains amongst other things non-metamorphosed, continental slope sediments, low to medium grade metamorphic rocks of continental affinity, blueschists and ophiolites (Struckmeyer et al. 1993). The reconstruction of Daly et al. (1991) shows eastern and central Sulawesi forming in the oceanic domain, south of the Philippines and colliding with western Sulawesi in the late Miocene (Fig. 10). Although explaining the late Miocene tectonic episode in western Sulawesi, it is incompatible with the continental margin origin and tectonic history of the metamorphic belt.
Microcontinental blocks o f the Banda Sea and the Bird's H e a d
Regionally, the northern Australian craton basement can be divided into two types. In Papua New Guinea there are deformed Palaeozoic sediments and Palaeozoic to Triassic acid intrusive rocks. West, in Irian Jaya and on the northern Australian margin, undeformed Palaeozoic sediments overlie Precambrian basement. The microcontinental blocks of the northern Banda Sea and the Bird's Head characteristically have metamorphic pre-Mesozoic basements and are of the eastern type. Above basement is a Triassic clastic or carbonate shallow marine succession, overlain by deeper water Jurassic to Palaeogene sediments, passing into Cretaceous to lower Palaeogene bathyal to pelagic deposits. The preserved upper Mesozoic to Neogene successions are remarkably continuous, indicating little tectonic disturbance. The stratigraphy of the blocks has been illustrated by Audley-Charles (1988). A lower Jurassic (Pliensbachian-Sinemurian) breakup unconformity in the west Papuan Basin indicates that Indian Ocean sea floor extended east
60N
30N
30S
60S-
120E
180E
240E
Fig. 15. Relative motions of the Australia, Pacific and Eurasia plates in the early Palaeogene (64-56 Ma) from Gordon & Jurdy 0988).
at least to eastern New Guinea at that time (Pigram & Symonds 1991). Because of the divergence between Australia and Pacific motions prior to 43 Ma (Fig. 15) determined by Gordon & Jurdy (1988), it is unlikely that Pacific Ocean spreading would have extended far west of Papua New Guinea. Consequently the Australia plate boundary probably ran west of north to the Asian margin or the southern boundary of the spreading systems of the Celebes and Molucca Seas and West Philippine Basin. Since Phase 2 Indian Ocean sea floor spreading (Late Cretaceous to middle Eocene) may have extended east to western Sumatra (Fig. 4) Jurassic to mid-Cretaceous crust of the Australia plate may have extended from western New Guinea to western Sumatra from the Late Cretaceous to the middle Eocene. Palaeomagnetic studies on the microcontinents suggest that they were separated from the Australian craton by Late Cretaceous time. Ali & Hall (1995) concluded that palaeomagnetic results from an Upper Cretaceous marl (86_ 3 Ma) at a site on Sula indicated a formation palaeolatitude of 19 _+6°S, well to the north of Australia. Wensinck et al. (1989) obtained similar results from palaeomagnetic measurements on Santonian and Maastrichtian sediments from Misool. The palaeolatitudes obtained were 18.5 _ 3.2 ° and 21.5 _ 3.6 ° respectively. Reconstructions by Struckmeyer et al. (1993) place the Late Cretaceous Papuan margin at a palaeolatitude of 30°S, roughly 10°S of the microcontinents at that time. The north-south
CENOZOIC SE ASIA RECONSTRUCTIONS
relationship to Australia has been substantially restored by shortening in the Banda subduction zone. The Sula site and the Santonian Misool sites both give counter-clockwise rotations of around 40 ° but the Maastrichtian site on Misool is not rotated. Ali & Hall (1995) also found strong counter-clockwise rotations in Upper Cretaceous, upper Oligocene and middle Miocene volcanic rocks from north Obi. They relate the rotations to left-lateral shear on the left-lateral Strong Fault Zone since as the sites are adjacent to, or between splinters of the fault. There is no clear evidence of latitudinal separation of the Kemum Terrane of the Bird's Head from the Australian margin (Giddings et al. 1993). By comparing palaeomagnetic results from the Kemum Terrane with the Australian polar wander path they suggested that the terrane, originally located in eastern New Guinea, was rotated 55 ° counter-clockwise between Early Jurassic and middle Eocene time, then in the late Eocene and early Miocene interval moved westward to near its present location. Finally there was 10° of postmiddle Miocene counter-clockwise rotation. Unfortuiaately Cretaceous sediments did not yield poles because of strong overprinting. Their data are open to an alternative interpretation. The westward movement of the terrane to the Bird's Head with a decreased of palaeolatitude and counter-clockwise rotation could have taken place between the Early Jurassic and middle Eocene time followed by a 20 ° late Eocene and early Miocene clockwise rotation. Because the microcontinental blocks and Bird's Head terranes were were not involved in Oligocene tectonism of the New Guinea orogen, they were probably located to the west of the cratonic part of Irian Jaya by early Oligocene time as indicated on Fig. 13. From the foregoing discussion of palaeomagnetic data, Jurassic rifting may have removed them from eastern New Guinea to reach the indicated location by Late Cretaceous time. Hall (1995), Struckmeyer et aL (1993) and Rangin et al. (1990) all use a generally similar location for the blocks in their reconstructions. Using the palaeomagnetic interpretation of Giddings et al. (1993), Struckmeyer et al. (1993) moved the blocks westward by a complex Late Cretaceous to Palaeocene spreading system, followed by southwest movement on a major leftlateral southwest strike-slip fault. It is difficult to relate these events to known plate boundaries and vectors. In their reconstruction Daly et al. (1991) rotated Seram and Buru counter-clockwise to their present position from a location on the northwest Australian margin adjacent to Timor extending north to the Bird's Head of New Guinea. Banggai-Sula derived from the Birdfs Head is
143
located immediately to the north (Fig. 10). In their tectonic evolution, Banggai-Sula is transported west in the late Miocene time. Southward directed deformation in the Bird's Head and left-lateral strike-slip on the Tarera Fault are responsible for rotation of the Burn and Seram, arriving at their present orientation as Pliocene underthrusting occurred in the Seram Trough. This scheme avoids the problem of moving the blocks west from the Papua New Guinea margin but their pre-Triassic metamorphic basement (Kemp & Mogg 1992) indicates affinities with the eastern Australian tectonic province. The location does not conform to the Cretaceous palaeolatitude data but the sense of rotation is in agreement with the measurements. Packham (1990) assumed that initial rifting of the blocks took place in the Mesozoic but that they remained close to the New Guinea margin as marginal plateaus away from clastic deposition. About the beginning of Miocene time, they were transferred to the Philippine Sea plate and started to move west on the northern margin of the developing New Guinea Orogen (Fig. 10). The relative continuity of deposition non-clastic deposition makes this process problematical and in the case of Buton an origin in northern New Guinea in the vicinity of the Bird's Head is assumed. This would involve an earlier episode of westward movement. Again the palaeolatitude determinations do not fit.
Models for North New Guinea and the Banda Sea Although the tectonics of the New Guinea-Banda are driven principally by the west-northwest convergence of the Pacific plate and the northwardmoving Australia plate, the Pacific plate and the Australia plate are not in direct contact in the region. They are separated by the southern end of the Philippine Sea and Caroline plates. The Caroline plate (Fig. 14) was formed in Oligocene time by north-south spreading when, in the Hall et al. (1995b) model, the Philippine Sea plate was stationary. The Caroline plate has a slow counterclockwise motion relative to the Pacific plate about a pole located about 5 ° north of it (Weissel & Anderson 1978). It has been rotating away from the Philippine Sea plate since about 12 Ma from near the apex of the southward opening Ayu Trough and is being slowly subducted under the Philippine Sea plate in the Palau Trench (Fig. 11). Early Miocene volcanism (20 Ma) reported from Palau by Haston et aL (1988) suggests that it may have been part of the Pacific plate before 12 Ma. Caroline plate motion is transmitted westward into the seismically active New Guinea Highland
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region (Abers & McCaffrey 1988) and thence westward into the Banda Sea. In the south Banda region the strain is principally absorbed by thrusting into the Seram Trough but some of it is transmitted westward along the Sorong Fault Zone into Sulawesi and eventually to the North Sulawesi Trench (Silver et al. 1983). West of the Bird's Head, the northward motion of the Australia plate is absorbed in the Tanimbar-Timor trough, backarc thrusting north of Wetar and north-south shortening of the Banda Sea (McCaffrey 1988). The present tectonic configuration, dominated by strong left-lateral shear has led to models in which terranes were transported westward under its influence. Hamilton (1979), Charlton (1986) and Packham (1990) considered that the blocks were driven west from northern New Guinea along with the development of the Sorong Fault. As discussed above models which propose that the microcontinental blocks of the Banda Sea extended west from the Bird's Head before the collision of arc elements with north New Guinea are more probable. Collision tectonics has dominated the explanations for the formation of the New Guinea Orogen (Pigram & Davies 1987; Struckmeyer et al. 1993) but recently extensional tectonics for the Late Palaeogene phase has been invoked by Hill et al. (1993). The Philippine Sea p l a t e a n d N o r t h N e w G u i n e a in the P a l a e o g e n e
In the Melanesian arc middle Eocene volcanic rocks resting on oceanic basement mark the onset of volcanism. The identified western limit of the arc is the Finisterre Terrane of northeast New Guinea (Fig. 14). Distinct from it to the west is the Torricelli Terrane, in which Cretaceous to Oligocene volcanic activity is recorded (Fig. 14). A similar late Cretaceous commencement of arc volcanism is also found nearby in Halmahera (Hall et al. 1991; Ali & Hall 1995) and further afield in the Philippines. Similar volcanic rocks may be preserved in the Arfak, Waigeo, Tosem, Biak/Superiori and Gautier Terranes now located on and adjacent to north New Guinea (Pigram & Davies 1987; Charlton et al. 1991; Struckmeyer et al. 1993). This volcanic assemblage, referred to here informally as the northern arc, may have formed a south-facing arc on the margin of the young Philippine Sea plate as reconstructed by Hall et aI. (1995b) and Hall (1996). The development of the arc commenced before the Eocene rotational phase of the plate and continued through the late Eocene and Oligocene non-rotational phase. The collision of the West New Guinea Composite Block with the New Guinea margin in mid-Oligocene time as Australia moved north has
been regarded as the first stage of the development of the North New Guinea Orogen (Pigram & Davies 1987, Struckmeyer et al. 1993). In their models it had previously been rifted off and collided over a north-dipping subduction zone. In view of the proximity of the southern boundary of the Philippine Sea plate to the northern New Guinea margin at the time, as depicted by Hall et al. (1995b), an alternative explanation is that emplacement of the West New Guinea Composite Block was the result of a collision between part of the Philippine Sea plate and the New Guinea margin. The Torricelli Terrane and the other volcanic terranes to the west listed above may have originated from the margin of the Philippine plate, some of them from the gap between the present Philippine Islands and Halmahera, being left behind during Miocene clockwise rotation of the Philippine Sea plate. Hill et al. (1993), taking a very different approach, interpreted the West New Guinea Composite Block as part of the continental margin in which extension has exposed mid-crustal rocks as core complexes. The Sepik and Ramu Basins to the north of the block were initiated as half-graben in late Oligocene time. The model requires continuous slow southward late Eocene to late Oligocene subduction under the New Guinea margin with some compressional deformation. Most of the convergence was taken up to the north, away from the New Guinea margin. This explains the development of the composite block before the late Oligocene docking of the arc terranes commenced of which the first was the Torricelli Terrane. M i o c e n e B a n d a Sea tectonics
As a consequence of Miocene clockwise rotation of the Philippine Sea plate, Ali & Hall (1995) have determined that at its contact with the northern New Guinea, the plate boundary was a left-lateral strikeslip fault. The northward component of the Philippine Sea plate vector approximately balances the northward Australia plate vector leaving a resultant left-lateral movement of the Philippine Sea plate initiating westward movement of blocks on the Sorong Fault. Until Philippine plate motion changed at the end of the Miocene, strike-slip relationship could not have persisted far west. Westwards along the Sorong Fault, the distance from the closely located Euler Pole of rotation of the Philippine Sea plate increases hence the velocity vector increases and has a more northerly azimuth while the Australia plate vector is nearly constant. The slip direction would swing towards the north. In a simple model of the Philippine Sea plate in
CENOZOIC SE ASIA RECONSTRUCTIONS which the boundary swung to the north, west of Halmahera as envisaged by Rangin et aL (1990) and Daly et al. (1991) the Molucca and Banda Sea floors were on the Australia plate. They could be ocean crust (Fig. 13) trapped when the Banda arc formed by eastward extension of the Sunda arc. The Banda Sea at least has heat flow levels and oceanic sea floor depths appropriate for a late Mesozoic age of the Molucca Sea as discussed above. A rough minimum estimate of the age of the Banda arc can be obtained from the 1000 km length of the subducted slab in the vicinity of Timor (McCaffrey 1989). Using a convergence rate calculated from 7 9 m m a -1 calculated from NUVEL-1, the age is over 12 Ma. Lavas on Wetar range from 7.7 Ma to 3 Ma but granodiorite and diorite intrusive rocks on the island are 12 Ma old (Abbott & Chamalaun 1981). The maximum age is possibly 15 Ma. Simple restoration of the 1000 km of convergence of the Banda arc on the west and 200 km on the east would place the microcontinental blocks in a line running west-northwest from the Bird's Head at the time of the onset of subduction (Fig. 13). In the simple model proposed above, collision of the Buton-Tukang Besi block in the middle Miocene could have triggered the westward extension of the Sunda arc, initiating the Banda arc creating a transitory Banda-Molucca plate. As subduction on its eastern extension was limited by the Australian margin, the crust of the Banda Sea was rotated counter-clockwise about a pole located near the western end of the Timor Trough, isolating the Molucca Sea. With two subduction boundaries and probably a transform southern boundary, the Molucca plate became a triangle of oceanic crust larger than at present, able to adjust to changes in convergence of the surrounding plates. Banda Sea rotation provides an alternative explanation for counter-clockwise palaeomagnetic rotations observed on the microcontinental blocks. Seram may have been further disturbed by thrusting into the Seram Trough. Banda Sea rotation was accompanied by subduction under eastern Sulawesi, so that Sula collided with the Sulawesi ophiolite belt in latest Miocene time, later than Buton. A difficulty with this rotation model is that the pre-rotation location of the microcontinental blocks (Fig. 13) overlaps the Molucca Sea crust shown to the west of Halmahera in the 25 Ma and 15 Ma reconstructions of Hall et al. (1995b). If the Philippine Sea and Australia plates had been a degree or so further apart or a slightly different shape, the overlap would be removed. Alternatively westward motion may have redistributed the blocks to some degree. A further difficulty is that it does not explain the upper Miocene volcanic rocks and granites in central West Sulawesi.
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Some of the wide diversity in interpretation of the Banda Sea region is illustrated in Fig. 10. Rangin et al. (1990) reconstructed the Banda arc as an east-west arc, formed in the middle Miocene, attached at the eastern end to western New Guinea at the northern margin of the craton. The subduction zone was south of a rifted continental block which was incorporated in Timor. The western end of the arc was joined onto the eastern Sunda arc in the Pliocene as Australia moved north. In their reconstructions Daly et al. (1991) subducted the old crust Banda crust under eastern Sulawesi as it moved westward, driven indirectly by Pacific plate motion, to collide with western Sulawesi in late Miocene time. In their model the Banda arc was apparently formed after the collision. The change of direction of Philippine Sea plate motion to close to west at 5 Ma (Seno et al. 1993) introduced a new tectonic regime which will be discussed in the next section.
Caroline~Pacific plate motion and eastern Indonesia
The collision of the Ontong Java Plateau on the Pacific plate with the Melanesian arc in the Solomon Islands (Packham 1973; Vedder 1986) late in early Miocene time was an event remote from eastern Indonesia but its ultimate consequences extend east to the Banda Sea. The collision reversed the polarity of the Melanesian arc, transferring it from the Australia plate to the Pacific plate which transported it westward to collide with northern New Guinea in late Miocene time (Pigram & Davies 1987). With the western end of the arc now identified as the Finisterre Terrane fixed, westward transport ruptured the arc duplicating it for about 1000 km between New Britain and Marius (Fig. 14). This amount of overlap required early late Miocene rupturing to have occurred. Although the commencement of terrane docking has not been dated precisely, deformed upper Miocene sediments in the Ramu Basin are overlain by a thick lower Pliocene to Pleistocene successor basin deposit (Hill et al. 1993). The progressive exposure of northern New Guinea to convergence between the Caroline and Australia plates is illustrated in Fig. 14. At 15 Ma, the eastern edge of the Philippine Sea plate was near the future site of docking of the Finisterre Terrane. Before the Finisterre Terrain docked there was no significant coupling between the Philippine Sea and Australia plates except in the vicinity of the boundary (the Sorong Fault). As already mentioned, the northward component of the motion of the Philippine Sea plate and the Australia plate approximately balanced, leaving a residual left-
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lateral strike-slip. The motion of the Caroline (or Pacific) plate could not be transferred to the Australia plate until docking of the Finisterre Terrane commenced (at about 7 Ma) because the Melanesian arc was south-facing. From about 7 Ma combined crustal shortening and lateral translation extended f r o m the docked part of the Finisterre Terrain to the eastern boundary of the Philippine Sea plate. As the Philippine Sea plate moved west, the interface increased in length from around 600-800 km as the terrane docked to its present length of about 1200 km by 3.8 Ma when northern segment of the arc started moving west with the Caroline plate and extension commenced in the Manus Basin, isolating the South Bismarck plate from the Solomon plate (Taylor 1979). By 4.0 _+ 0.5 Ma, thrusting and uplift had exposed sediments in the Muller Anticline in the western part of the Papuan Thrust Belt (Hill 1991). Hill (1991) estimated the amount of shortening in the Papuan fold belt at c. 100 km. From the topography, Abers & McCaffrey (1988) suggested it was 6 1 k m east of 140°E and 44kin in the western highlands. These latter figures do not take account of loss from erosion. The convergence rate across the Australia-Caroline plate Boundary at 140°E and 2.5°S, is 97 mm a -1 towards 21°S of W (calculated from NUVEL-1 and the CarolinePacific pole of Weissel & Anderson 1978). This is equivalent to north-south and east-west components of 47 and 85 mm a-1 respectively. The amount of north-south shortening in the east is about 325 km. In a Finisterre-west Papuan Fold Belt transect there is probably c. 200 km of shortening across the fold belt and thrusting of the Finisterre Terrain onto the margin identified by Pegler et al. (1995). This accounts for about 4 Ma of shortening. But much of the thrusting in the eastern part had been completed by 4 Ma possibly indicating a start at 6-7 Ma. This leaves over 100 km of shortening unaccounted for. In eastern Irian Jaya shortening has been only possible for a shorter time, probably less than 5 Ma, suggesting a possible 240 km of shortening. As expected, the thrust belt narrows westwards and the highlands are of smaller mass. Even if the Abers & McCaffrey estimate of 44 km of convergence is doubled allowing for erosion, it still leaves about 150 km to be accounted for. The excess can probably be accommodated in underthrusting into the North New Guinea Trench and its former extension in the east. Seismic activity in the New Guinea orogen, indicates that much of the strain is taken up in the west by left-lateral strike-slip, especially near the southern margin and while in the east, thrusting is more common (Abers & McCaffrey 1988). In order to account for the thrust direction in the Seram
Trough, McCaffrey (1988) concluded that the Bird's Head is decoupled from the Australia plate and is moving west or southwest with respect to it, probably along the east-west left-lateral Tarera Fault, south of the Bird's Head. The western end of the fault forms a triple junction between the Bird's Head, Banda Sea and the Australia plate where it meets the Seram and Tanimbar Troughs. The left-lateral component of convergence between the Caroline and Australia plate distributed across the orogen, is c. 600 km in the east, if strain was effectively initiated at about 7 Ma and possibly 400-450 km in the west if commencement was c. 5 Ma. Some of the strain has been absorbed in the eastern Bird's Head in the Lengguru Thrust Belt where orogenic sedimentation commenced in the Pliocene (post-N18, c. 5 Ma) and thrusting continued till at least 2 Ma (Moffat et al. 1991). The remainder of the Bird's Head has behaved as a single entity throughout the Neogene, bounded on the north and south by the Sorong and Tarera left-lateral strike-slip faults. This is kinematically consistent with a post-middle Miocene 10° counter-clockwise rotation of Kemum Terrane identified by Giddings et al. (1993). Much of the strain was probably taken up by westward or southwestward movement of the Bird's Head into the Seram Trough where the seismic zone beneath Seram extends to 300 km (McCaffrey 1988). On stratigraphic evidence from Seram deformation commenced at 5 Ma and continued throughout Pleistocene time (Kemp & Mogg 1992). Possibly 150km of the westward movement of the Bird's Head is expressed in leftlateral displacement of the thrust front on the Tarera Fault. The displacement could have been achieved in less than 2 Ma if the strain was concentrated on the fault. Apart from thrusting in Seram associated with a strike-slip component on the Seram Trough, only an insignificant amount to the convergence southwest of the trough has been transmitted to the southern part of the Banda Sea. Shortening of over 400 km, commencing at c. 5 Ma, can be accounted for in the undeahrust slab beneath Seram plus the shortening in the Lenggum Thrust Belt to which could be added a contribution from oblique subduction into the Noah New Guinea Trench. North of the Seram Trough, shortening has been transmitted to the west into Sulawesi between the Seram Trough and within strands of the Sorong Fault, absorption can be identified in the thrusting in the Banggai-Sula block and East Sulawesi between 5.2 and 3.8 Ma (Davies 1990) and an estimated 250 km of left-lateral slip on the Matano-Palu Faults Sulawesi (Silver et al. 1983). Finally, it should be noted that large-scale tectonic transport of terranes from eastern New
CENOZOIC SE ASIA RECONSTRUCTIONS Guinea to the Banda Sea region could not have been driven by the Caroline plate interacting with New Guinea because the amount of convergence was too small. Further, the Buton-Tukang Besi Block had already collided with eastern Sulawesi and the Sula Block was close to doing so when interaction commenced close to the end of Miocene time.
Eastern SE Asia: conclusions The history of rotation of the Philippine Sea plate has been recently refined by Hall et al. (1995b). Its initial southem latitude location places constraints on reconstruction models of the Banda Sea and the development of the New Guinea Orogen. Two earlier phases of clockwise rotation have been identified prior to the present one. Rotation about the present pole, located northeast of Japan results in westward motion at the southern end of the plate. The rotations about the earlier poles located to the east of the Philippine islands total 90 ° . The first was between 50 and 40 Ma and the second between 25 and 5Ma. The second phase is the more important in regional reconstructions. The Miocene rotational phase has been responsible for the middle to late Miocene collision of the Philippine chain with the Asian Margin in Mindoro and western Mindanao. The colliding arc, with the possible exception of Luzon, developed on the margin of the Philippine Sea plate before Miocene rotation phase. Hall (1996) suggests Luzon developed on the margin of the Eurasia plate and was transferred to the Philippine Sea plate early in Miocene time. The crust of the Celebes and Molucca Sea may have been formed as an eastward Eocene extension of the West Philippine Sea spreading system, north of an arc system linking Sulawesi and the Philippines. There is inconclusive evidence that a domain including the Celebes Sea, Borneo and the Malay Peninsular underwent subsequent counterclockwise rotation. Although most, if not all of the continental rocks in the Timor-Tanimbar orogen have been sliced from the Australian northwest shelf margin during its Pliocene collision with the Banda arc, the basement of Sumba straddling the Banda forearc has probably been rifted off the adjacent Asian margin in the Palaeogene time. The nearby composite block of the central Sulawesi metamorphic belt and the East Sulawesi ophiolite may have originated on the Australia plate and been emplaced by an Oligocene collision with West Sulawesi. Microcontinental blocks of the Banda Sea and the Bird's Head have an intruded and metamorphosed PermoTriassic basement which suggests an origin in eastern New Guinea.
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Late Cretaceous palaeolatitude differences of around 10 ° between New Guinea and Sula, Seram and Misool and an Early Jurassic breakup unconformity in the West Papuan Basin indicate the blocks were probably rifted off at the time of breakup. The breakup and the Mesozoic ocean crust off western Australia suggest that Phase 2 Indian Ocean crust of the Australia plate probably extended from eastern New Guinea to the Asian margin and west to Sumatra, and the microcontinental blocks were contained in it. The lack of tectonic disturbance in the upper Palaeogene to lower Neogene stratigraphy on the blocks indicates they were assembled from the Bird's Head westward before that time away from the developing New Guinea Orogen. The close proximity of the Philippine Sea plate to north New Guinea as reconstructed by Hall et al. (1995b) makes it geometrically improbable that the blocks were transported to the southwest on the Pacific plate after 43 Ma as proposed by Struckmeyer et aL (1993). The first tectonism in the New Guinea Orogen occurred in mid-Oligocene time when the Philippine Sea plate was stationary and well after middle Eocene increase in northward velocity of the Australia plate. According to Pigram & Davies (1987) the West New Guinea Composite Block comprising variously deformed Mesozoic continental margin sediments, granitic intrusions, metamorphic rocks, extrusive rocks and ophiolite masses collided with the New Guinea continental margin. Alternatively it could have been a collision between the New Guinea continental margin and the edge of the Philippine Sea plate. A third possibility suggested by Hill et aL (1993) is that the composite block is an extensional core complex structure developed during Oligocene to early Miocene time in rocks already located on the New Guinea margin. The Torricelli Terrane, north of the composite block, is part of what is interpreted here as the Northwest arc developed on the south facing edge of the Philippine Sea plate. The arc which includes the Philippines, Halmahera and the various terranes of north New Guinea from the Torricelli Terrane west has a volcanic record extending back to Cretaceous time. It appears that docking the Torricelli Terrane on the north New Guinea margin was a late Oligocene event. It may have been detached from the Philippine Sea plate when Miocene clockwise rotation commenced and leftlateral movement on the Sorong Fault was initiated. The present author's preferred pre-15 Ma model is for the Philippine Sea plate boundary to curve northwest along the arc along the strike-slip direction against the Australia plate extended north to include the Molucca and Banda Seas prior to the
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formation of the Banda arc. The 1000 km long subduction zone under the west Banda arc indicates it was formed at c. 15 Ma presumably as an eastward extension of the East Sunda arc, transferring the Molucca and Banda Seas to the Eurasia plate from the Australia plate. Subduction may have been initiated by the collision of Buton with southeast Sulawesi. It would have reduced the latitude differences between the microcontinents and Australia. At about 12 Ma the Molucca plate became a separate entity when eastward subduction under Halmahera started. Australian continental crust first impacted at the east end of the Banda arc and has been subducted only 400 km or so. Differential subduction would have rotated the Banda Sea counter-clockwise, colliding BanggaiSula with western Sulawesi. This model does not explain the late Miocene volcanic and plutonic event in West Sulawesi. The remote collision of the Ontong Java Plateau with the north facing Melanesian arc north of the Solomon Islands set in train events that spread west to Sulawesi. The Melanesian arc has a much shorter history than the western arc having commenced forming on oceanic crust in middle Eocene time. Polarity reversal of the Melanesian arc transferred it to the Pacific plate. Its eastern end eventually collided with northeastern New Guinea to be emplaced as the Finisterre Terrane possibly c. 7 Ma. With the elimination of the subduction zone(s) between them, the convergence between the Caroline and Australia plates was transmitted across the New Guinea margin for the first time. At 7 Ma the eastern edge of the Philippine Sea plate was at about 142°E and c. 600--800 km of the margin was exposed to shortening. This distance progressively increased to c. 1200 km at c. 4 Ma when the Philippine Sea plate boundary was at 136°E. Combined strike-slip and spreading then commenced in the Manus Basin carrying the Manus arc segment west on the Caroline plate. Folding and strike-slip faulting in the New Guinea Orogen spread from east to west. Assuming that convergence commenced at c. 7 Ma and 5 Ma in the east and west respectively the shortening components are c. 330 and 240 km respectively. This can be accommodated in thrusting under the Finisterre Terrane, thrusting in the orogen and underthrusting into the North New Guinea Trench. The strike-slip component in the west is about 430 km. Some of this was taken up in the Lengguru Thrust Belt and possibly by oblique subduction into the North New Guinea Trench and within the orogen. The remainder must have moved the Bird's Head underthrusting it into the Seram Trough and moving it west on the Tarera Fault. McCaffrey (1988) concluded that Bird's Head is at present moving west or southwest with respect to
Australia. North of the Seram Trough there may be partial coupling with the Philippine Sea plate, transmitting strain westward to the thrusts between Banggai-Sula and East Sulawesi and to the PaluMatano fault system where Silver et al. (1983) have suggested there is 250 km of left-lateral movement. More important than explaining the distribution of the strain is the observation that the total amount is insufficient to transport blocks from eastern New Guinea to the Banda Sea region.
Concluding remarks The way in which the strain identified in the neotectonics of central Asia is being distributed in eastern Asia is still uncertain and hence its applicability to the rather different Palaeogene to Early Neogene tectonic regime is yet to be resolved. Kinematics of models partitioning Palaeogene shortening between crustal thickening of Tibet and block extrusion will not be determined until the time of elevation of the Tibet Plateau is confidently known, although substantial displacements have been documented on the Red River and Wang Chat Faults. The style of regional deformation indicated by the dispersal of palaeomagnetic results suggests that simple block rotation does not apply in the northwest Indochina block but it could well be an important process further east of the syntaxis. The regional geology north and east of the of the eastern syntaxis has not yet revealed the link between the Shan-Thai block and the terranes of eastern Tibet and the continuity of the eastern Himalayas to northern Myanmar except in broad terms. The related late Palaeogene palaeogeographical problem of the Indoburman flysch belt could be resolved by a better definition of the time of its docking south of the eastern syntaxis as well as an assessment of its former location on the Asian margin. Although the two sections of this paper have been treated separately, they are interdependent in that reconstruction of the eastern part of SE Asia requires knowledge of the location of the eastern Asian margin from late Oligocene to early Miocene time. One of the key pieces in the jigsaw is Borneo which has been alluded to only in passing. The two major unresolved problems are firstly, the relationship between the South China Sea Cenozoic spreading and deformation of the Rajang flysch belt, and secondly, the implications of possible counter-clockwise rotation of a domain including the island and adjacent regions and it relationship to Sundaland basin histories. While significant advances in the elucidation of the history of the rotation of the Philippine Sea plate have been made, its position relative to Asia is not well constrained as evidenced by the recent
CENOZOIC SE ASIA RECONSTRUCTIONS revision o f the location of the present Euler pole of relative motion by SenD et al. (1993). Allied to the relationship o f the Philippines to the Philippine Sea plate and the Asian margin is the absence of a detailed tectonic model for the Philippine arc from the Cretaceous onwards. A lack of highly detailed stratigraphic and palaeomagnetic studies in the microcontinental blocks, volcanic and oceanic terranes in the Banda Sea and along the northern margin of N e w Guinea hampers a more complete interpretation of the convergence and collision
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history, as do the resolution o f the origin o f the West N e w G u i n e a C o m p o s i t e Block, the central Sulawesi Metamorphic Belt and the age of the volcanic rocks of the Banda Sea and the Sangihe arc. I thank the University of London SE Asian Research Group for sponsoring my attendance at the 1994 conference on the Tectonic Evolution of SE Asia and R. Hall in particular for his assistance and the provision of unpublished manuscripts. Petroconsultants (Australia) Pty. Ltd. have funded the preparation of the text figures.
References ABBOTT, M. J. & CHAMALAUN, F. H. 1981. Geochronology of some Banda Arc Volcanics. In: BARBER, A. J. & WIRYOSUJONO, S. (eds) The Geology and Tectonics of Eastern Indonesia. Geological Research and Development Centre, Bandung, Special Publication, 2, 253-268. ABERS, G. & MCCAFFREY,R. 1988. Active deformation in the New Guinea fold and thrust belt: seismological evidence for strike-slip faulting and basement involved thrusting. Journal of Geophysical Research, 93, 13 332-13 354. ALl, J. R. & HALL, R. 1995. Evolution of the boundary between the Philippine Sea plate: palaeomagnetic evidence from eastern Indonesia. Tectonophysics, 251, 251-275. ALLEN, C. R., GILLESPIE,A. R., HAN YUAN, SLED,K. E., Zhuang Buchum & Zhu Chengnan. 1984. Red River and associated faults, Yunnan Province, China. Quaternary geology, slip rates and seismic hazard. Bulletin of the Geological Society of America, 95, 686-700. AUDLEY-CHARLES,M. G. 1988. Evolution of the southern margin of Tethys (North Australian region) from Early Permian to Late Cretaceous. In: AUDLEYCHARLES, M. G. & HALLAM, A. (eds) Gondwana and Tethys. Geological Society, London, Special Publication, 37, 79-100. AURELIO,M. A., BARRIER,E., RANGIN,C. & MULLER,C. 1991. The Philippine Fault in the Late Cenozoic tectonic evolution of the Bondoc-MasbateN. Leyte area, central Philippines. Journal of Southeast Asian Earth Sciences, 6, 221-238. AVOUAC, J. P. & TAPPO~IER, P. 1993. Kinematic model of active deformation in central Asia. Geophysical Research Letters, 20, 858-898. - - , BAI, M., YoY, H. & WANG, G. 1993. Active thrusting and folding along the northern Tien Shun and the Late Cenozoic rotation of Tarim relative to Dzunggaria and Kazakhstan. Journal of Geophysical Research, 98, 6715-6744. BENDER, F. 1983. Geology of Burma. Gebuder Borntraeger, Bedim BESSE, J. & COURTILLOT,V. 1991. Revised and synthetic apparent polar wander paths of the African, Eurasian, North American and Indian plates, and time polar wander since 200Ma. Journal of Geophysical Research, 96, 4029-4050. BIRD, P. R., QUINTON,N. A., BEESON, M. N. & BRISTOW, C. 1993. Mindoro: a rifted microcontinent in
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Reconstructing Cenozoic SE Asia ROBERT
HALL
SE Asia Research Group, Department of Geology, Royal Holloway, University of London, Egham TW20 OEX, UK Abstract: Reconstructions of SE Asia at 5 Ma intervals for the past 50 Ma are presented. They are constrained by new data from the Philippine Sea plate, which forms the eastern boundary of the region, by recent interpretations of the South China Sea and Eurasian continental margin, forming the western boundary, and by the known motions of the Indian-Australian plate to the south. An attempt is made to satisfy geological and palaeomagnetic data from the region. The implications of these reconstructions for the Tertiary evolution of SE Asia are discussed in the light of other new data from the region. There are two regionally important periods of change during the past 50 Ma. Both appear to be the expression of arc-continent collision and resulted in major changes in the configuration of the region and in the character of plate boundaries. At c. 25 Ma the collision of the Australian continent with the Philippine Sea plate arc caused major effects which propagated westwards through the region. At c. 5 Ma collision of the Philippine arc and the Eurasian continental margin occurred in Taiwan. This appears to be a key to the recent tectonics of the region. Principal features of the model include the following interpretations. Middle Tertiary counter-clockwise rotation of Borneo closed a large proto-South China Sea and led to the development and destruction of marginal basins north of the Celebes Sea. The rotation implies that much of the north Borneo margin was not a subduction, but a strike-slip boundary for most of this period. It also suggests that the central West Philippine Sea, the Celebes Sea and the Makassar Strait formed part of a single marginal basin which opened between late Eocene and mid Oligocene, and narrowed westwards like the present South China Sea. Luzon is suggested to have formed in an arc on the north side of the Celebes Sea-West Philippine Basin, whereas most of the other Philippine islands probably formed part of an arc at the southern edge of the Philippine Sea Plate before the Early Miocene. Arc-continent collision in the early Miocene caused plate boundaries to change and initiated the clockwise rotation of the Philippine Sea plate. Since then the Philippine fragments have moved in a very narrow zone, mainly as part of the Philippine Sea plate, with significant strike-slip motion of fragments at the plate margin. Most subduction under the Philippines was oblique, mainly at the western edge, and north of Mindanao. The Molucca Sea was a very wide area which formed part of the Philippine Sea plate before c. 15 Ma and originated as trapped Indian ocean lithosphere. It has been eliminated by subduction on its east and west sides. The present-day double subduction system never extended north of the present Molucca Sea into the Philippines. The Sulawesi ophiolite has an Indian ocean origin and was emplaced on the west Sulawesi continental margin at the end of the Oligocene. The major change in plate boundaries at the beginning of the Miocene following arc-continent collision of the Australian margin with the Philippine Sea plate arc caused initiation of the Sorong Fault system and led to westward movement of continental fragments which were accreted to Sulawesi during the late Neogene. The Sula platform and Tukang Besi platform formed part of a single large microcontinent with the Bird's Head before c. 15 Ma. They moved to their present positions after slicing of fragments from this microcontinent at different times and each was attached to the Philippine Sea plate for a few million years before collision. Most of the Banda Sea is interpreted to have an extensional origin and to have opened during the late Neogene. The reconstructions imply that there has been little convergence at the north Australian margin in Irian Jaya since the early Miocene and most convergence has occurred during the last c. 5 Ma. Movement of Philippine Sea arc fragments within the northern New Guinea margin along strikeslip zones probably accounts for the terrane character of this orogenic belt.
Plate tectonic reconstructions o f SE Asia have some obvious practical value for the region in helping to understand the d e v e l o p m e n t o f sedim e n t a r y basins, the history o f volcanic arc activity, and similar processes w h i c h are linked through tectonics to the distribution o f natural resources. Reconstructions are also a necessary precursor to
From Hall, R. & Blundell, D.
understanding m o r e fundamental processes that have acted. W h a t are the important controls on the tectonic d e v e l o p m e n t o f the region (e.g. the role o f indentor tectonics), what are the critical events (e.g. different types o f collision event), and what is the nature o f deformation (e.g. rigid plate versus distributed d e f o r m a t i o n ) ? H o w far can plate
(eds), 1996, TectonicEvolutionof SoutheastAsia, Geological Society Special Publication No. 106, pp. 153-184.
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tectonics go in describing the development of the region and the behaviour of crust and lithosphere (e.g. what is the role of strike-slip faulting versus contraction in the development of young orogenic belts)? Similar, more general ~uestions have relevance to collision processes and to the understanding of ancient orogenic belts for which parts of SE Asia have often been used as analogues. During the Cenozoic the region which now forms SE Asia was bounded to the north and west by a Eurasian plate, and to the south by the IndianAustralian plate. The motions of these plates are reasonably well known and their positions provide limits to the zone within which the SE Asian collage of microplates and sub-plate fragments can be moved when attempting plate reconstructions. The boundaries provided by these plate motions have been used as limits in previous reconstructions of Cenozoic SE Asia (e.g. Rangin et al. 1990; Daly et al. 1991; Lee & Lawver 1994). There have been considerable differences in dealing with the eastern boundary of the region. At present, the eastern edge of SE Asia is bounded by the Philippine Sea plate but the motion of this plate has been difficult to link to the global plate circuit because it is surrounded by subduction zones. Its Tertiary movement history has proved controversial, as illustrated by previous reconstructions, and there are major uncertainties in the position of the eastern edge of the region. Rangin et al. (1990) accepted evidence for clockwise rotation of the plate whereas Daly et al. (1991) and Lee & Lawver (1994) did not. Hall et al. (1995b) used palaeomagnetic data from east Indonesia to estimate Tertiary poles of rotation for the Philippine Sea plate and made a new reconstruction of this plate based on these poles, incorporating the effects of marginal basin opening within the plate. Discontinuous clockwise rotation for the entire plate during the last 50 Ma leads to palaeolatitude predictions which closely fit all palaeomagnetic data and also satisfies constraints on rotation inferred from magnetic anomaly skewness and seamount magnetization studies from the Philippine Sea plate. This model has been used to define the eastern margin of SE Asia as the basis for reconstructing the region using the ATLAS computer program (Cambridge Paleomap Services 1993) for the last 50 Ma. Figurel shows the present geography of the region and identifies the principal tectonic elements used in the reconstructions. Approximately sixty fragments (the number changes with age) have been used in reconstructing the region. Mercator projections showing reconstructions of the area bounded by latitudes 20°S and 30°N, and longitudes 90°E and 160°E, are presented for 5, 10, 15, 20, 25, 30, 35, 40, 45 and 50 Ma as Figs 2-11. Details of rotation poles, and fragments used, are
listed in Tables 1 and 2 which are available as Supplementary Publication No. SUP18101 (5 pp) from the Society Library or the British Library Document Supply Centre, Boston Spa, Wetherby, W Yorks LS23 7BQ, UK. The reconstruction is also available as an animation, on floppy discs, which can be run on either a Windows-based PC or a Mac with adequate hard disc space. Contact the author for details.
Methods ATLAS model
The ATLAS model uses a palaeomagnetic reference frame with Africa as the reference fragment with its movement defined relative to magnetic north. Movements of other major plates relative to Africa are based on Cande & Leslie (1986), Cochran (1981), Fisher & Sclater (1983), Klitgord & Schouten (1986), Le Pichon & Francheteau (1978), McKenzie et al. (1970), Royer & Sandwell (1989), Sclater et al. (1981), S~goufin & Patriat (1980) and Srivastava & Tapscott (1986). In these reconstructions South China is used as a reference and this is fixed to Eurasia for the period 50-0 Ma. There has been little Cenozoic motion of Eurasia whichever reference frame is used (e.g. Livermore et al. 1984; Gordon & Jurdy 1986; Besse & Courtillot 1991; Van der Voo 1993) and therefore it remains in a similar position to the present-day in all the reconstructions. In the ATLAS model there are small movements of Eurasia owing to the plate circuit used, particularly in the last 5 Ma, and therefore there are minor differences compared to reconstructions which use a fixed Eurasia (Lee &Lawver 1994; Rangin et al. 1990).
Palaeomagnetic
data
The model attempts to include the important constraints imposed by palaeomagnetism. Palaeomagnetic data can help to put some limits on interpretation of geological data since in principle they provide indications of palaeolatitudes and rotations. Interpretation is not always simple, and in SE Asia it is particularly difficult to reach unambiguous solutions. Van der Voo (1993) discusses in detail the use and problems of using palaeomagnetic results, and provides a particularly clear summary of problems in SE Asia. Besides the obvious drawbacks of collecting data in predominantly tropical, remote and often harsh terrain, there are additional problems such as error limits, remagnetization, and equatorial ambiguities. Not least amongst these are the difficulties of deciding
INDIAN
Malay Basins
::,:: '
PLATE
Gulf of Thailand
;
............
Java Sea
....
....
Palawan
Borneo
' • Bm.t~
SUNDA SHELF
Natuna Basins
.........
......
~r"cles~e~d
So~ China
EURASIA
~ "
•
,~'.'
.~B,,,~_.:._..-.c--/
~/w
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Basin
SOUth
Sula
. . Besi Platform ,~-B~da,~
Sulawesi
Taiwan
f
I
'
1
/
mJ
CAROLINE PLATE _
PHILIPPINE SEA PLATE
PamceVela Basin
-
MADE
RECONSTRUCTIONS
margins during the Tertiary. Principal marine magnetic anomalies are shown schematically for the Indian ocean, Pacific ocean, South China Sea, Philippine Sea and Caroline Sea. Circled Z identifies Zamboanga peninsula of Mindanao. Double lines represent active spreading centres. Narrow lines represent principal bathymetric features of the Philippine Sea plate and the Caroline Ridge; the margins of the oceanic parts of the South China Sea, the Celebes Sea and the Andaman Sea; and deeper parts of the Sulu Sea and the Makassar Strait. Complexities in the Bismarck-Solomon Sea regions are not shown.
Fig. 1. Simplified present-day tectonic configuration of SE Asia. Shaded areas represent mainly ophiolitic, arc and other accreted material added to Eurasian and Australian
Indochtna
..... ~%1:
k/I
156
R. HALL
which interpretations of palaeomagnetic results should be accepted. One major problem with palaeomagnetic data, often not emphasized by the palaeomagnetists, is determining the motion history of a region. This is particularly important in areas where, for example, there are results from Mesozoic or older rocks, but few or no results from Tertiary rocks, such as the Malay peninsula and Thailand. Similarities in declinations of similar age rocks may indicate a common regional rotation history but in some cases similar declinations may be the results of different Tertiary motion histories for separate blocks (cf. Celebes Sea counter-clockwise rotation and age, v. Borneo counter-clockwise rotation and age, which are discussed below). Sea floor magnetic data from the South China Sea, for the Celebes Sea and the West Philippine Sea central basin also provide limits, although in all cases there are uncertainties in anomaly ages and correlation. The distinction between local and regional rotations is far from clear in SE Asia, particularly because good palaeomagnetic data are sparsely scattered in time and space, and modelling such as that attempted here shows clearly the need for long term, systematic and integrated palaeomagneticgeological studies of the region. It is possible to avoid many difficulties with palaeomagnetic data by explaining apparently anomalous or controversial evidence of movements as a consequence of local tectonics. This is often sensible, and Surmont et al. (1994) provide an excellent SE Asian example of how previously inferred large regional rotations are better interpreted as very local consequences of tectonics. However, at the same time they show that smaller systematic regional rotations can be recognized, and we must accept that much essential evidence for movements, for palaeogeographic reconstructions, and certainly for rotation about vertical axes, can only be acquired from palaeomagnetism and therefore to ignore all palaeomagnetic data is a position of despair. The present author has tried to distinguish regional scale movements from local movements but in several cases, principally Sundaland-Borneo and the Philippines, a choice has had to be made between different views on the basis of inadequate data. In these cases decisions were based on regional geological arguments, recognizing that other solutions are possible. But once a critical decision has been made, for example in this model that of accepting a large counter-clockwise rotation of Borneo, the reconstructions become both a development, and a partial test, of the decision; can reasonable reconstructions then be made that are consistent with the geological data set for the whole region? Thus, the reconstructions presented here
should be seen as a way of distinguishing local and regionally important datasets, identifying targets for future work, and a possible model of the region which provides a different, albeit sometimes controversial, interpretation of the development of SE Asia.
Principles and tests The fragments were left at current size in all reconstructions in order that they remain recognizable. This is broadly satisfactory for Neogene reconstructions, except for some areas of volcanic arcs. The present shape of fragments has been maintained although this may contribute to overlap of blocks if significant new crust has been created, which must be the case in the Sunda, Banda and the Philippine arcs. Before the Neogene many of the fragments may have had quite different sizes and shapes or simply may not have existed. This is true for many areas in which the extent or age of the basements is uncertain, for example, in parts of Philippines, Sangihe arc, Sulu arc, Java and Banda arc. In some cases, this has been incorporated by omitting the fragment before a certain time. For instance, most of the Bonin forearc did not exist at c. 50 Ma and probably grew during a period of rapid volcanism in the Eocene (Stem & Bloomer 1992; Taylor 1992). Thus, some fragments may not appear on all reconstructions between Figs 2 and 11. In moving fragments Occam's razor has been used in an attempt to find the simplest possible motion histories. Fragments were first attached, or partially coupled to, a major plate with known motion; most minor fragments can be linked in this way to a major plate (Australia, Eurasia, Philippine Sea). If more complex movements were required, experiments were made with transferring fragments from one major plate to another. During the reconstruction process possible solutions were tested by asking (1) are the palaeomagnetic data satisfied? (2) are the geological data satisfied? and (3) is there overlap of fragments during movements? As discussed below, both (1) and (2) require subjective interpretations and judgements; readers will inevitably have their own views of the value of these. The most important sources are cited and the reader may refer to regional compilations and reviews such as Hamilton (1979); Hutchison (1989) and Van der Voo (1993) for overviews of geology and palaeomagnetism. Below are summarized the main features of the data used and the reconstructions made for principal sub-areas within SE Asia. The implications of the judgements and interpretations made for the development of the region are then discussed.
RECONSTRUCTING CENOZOIC SE ASIA
Regions considered Indochina and South China Sea
In the reconstructions South China is fixed to stable Eurasia. The Indochina block south of the Red River fault and north of the Malay peninsula has been moved using the model of Briais et al. (1993). They suggest c. 550 km of movement on the Red River fault system with left-lateral motion between 32-15 Ma and some dextral movement since the late Miocene. Their average small circle pole (5.3°N, 66.3°E) results in significant overlap of Indochina and South China and this model therefore uses a small circle rotation pole (20.9°S, 61.5°E) further from the Red River fault, as Briais et al. (1993) suggest should be the case, which results in a smaller overlap but still provides the history of contraction and extension that they summarize. The movements on the fault estimated by them yield an approximately linear agedisplacement relationship and the model assumes regular displacement in the interval 15-32 Ma. The area between the Indochina coast and the present north Borneo coast is underlain by continental crust and the north Borneo margin has been fixed to Indochina to provide an indication of the northern edge of the proto-South China Sea created by removing the extrusion of the Indochina block. Taylor & Hayes (1980, 1983) identified ocean floor ~magnetic anomalies and used these to interpret the opening history of the South China Sea. This interpretation has been modified by Briais et al. (1993) and their model of opening, using their calculated poles of rotation, has been used in the reconstructions without change except for reassigning anomaly ages to the Harland et al. (1990) timescale. Palawan and Mindoro have been moved with Reed Bank in the reconstructions. The islands of Palawan and Mindoro are considered to include continental crust of south China origin and the bathymetric contour marking the north side of the Cagayan ridge is assumed to mark the southern limit of continental crust. The northwest part of the Sulu Sea is thus considered to be underlain by continental crust (Hinz et al. 1991).
Borneo
The basement of Borneo of western and interior Borneo consists of Palaeozoic and Mesozoic igneous, sedimentary and metamorphic rocks and this area behaved more or less as a craton during the middle and late Tertiary. To the north are younger additions to this continental core which have been interpreted as subduction accretionary complexes (Hamilton 1979) although this view is not universally accepted. In this work north and
157
south Borneo have been separated at the Lupar line, which is a zone of Mesozoic ophiolites and south Borneo has been treated as a single rigid fragment back to 50 Ma. West Sulawesi separated from east Borneo in the Tertiary, resulting in opening of the Makassar Strait and the development of large sedimentary basins in east Kalimantan. West Sulawesi has a long Tertiary history of igneous activity and the present eastern margin of the Sundaland block appears to have been an active margin for much of the Tertiary. There are two principal large-scale tectonic views of Borneo: one advocates a large counterclockwise rotation of the island, the second argues for no rotation of Borneo. Palaeomagnetic results are reported by Haile et al. (1977), Haile (1979), Schmidtke et al. (1990), Wahyono & Sunata (1987) and Lumadyo et al. (1993). These results are reviewed by Fuller et al. (1991) and Lee & Lawver (1994); the former favour a counter-clockwise rotation of the island and the latter favour no rotation. It is clear that the existing palaeomagnetic data are inadequate to reach a conclusion and those who reject the rotation of Borneo (Lumadyo et al. 1993; Lee & Lawver 1994; Rangin et al. 1990) emphasize the problems with the data. However, although not all the evidence points in the same direction there are also regional geological arguments that favour rotation and this model accepts a counter-clockwise rotation of southern Borneo. The major obstacle to incorporating the rotation in a regional tectonic model is determining the position of the rotation pole. The chosen pole is close to the northwest corner of Borneo (I°N, ll0°E). This allows Borneo to remain part of a Sunda block while permitting the rotational movement to be absorbed within the north Borneo accretionary complexes by closing a proto-South China Sea. It implies some extension between Borneo and the Malay peninsula and allows the southern boundary of Sundaland to rotate northwards. Because the pole is so close to the northwest corner of Borneo it requires no major deformation of the Sunda shelf to the northwest, although minor deformation would be expected. The earliest inversion event in the West Natuna Basin (Ginger et al. 1993) is Early Miocene, which is consistent with the timing chosen for the rotation (see below). The movement requires counter-clockwise motion of west Sulawesi with little latitude change, for which there is some evidence. It fails to account for the similarity of the Thailand and peninsula Malaysia counter-clockwise rotations reported by McElhinny et al. (1974) and Schmidtke et al. (1990). However, poles further from Borneo which could account for these data result in a much larger proto-South China Sea and also require large latitude changes which are not seen in the palaeomagnetic data.
!
\
e
~
~ '
Inner B*a'ndaarc
tw
"/
Sula platform
Okinawa trough
,pr°pagaresi
p.,,DIII~ ~dllb~l~4 j" Ambon
"
:: j}
trench
/
Convergence begins befween Caroline plate •~ andNew Guinea
PLATE ~\
CAROLINE .~
Clockwise rotation of Philippine Sea Plate
20°S
10°S
L~ O0
RECONSTRUCTING CENOZOIC SE ASIA Data from SW Sulawesi indicate that the SW arm was close to its present latitude in the late Jurassic (Haile 1978) and late Palaeogene (Sasajima et al. 1980) but has since rotated counterclockwise by about 45 ° . Directions recorded by late Miocene volcanic rocks of the SW arm are indistinguishable from those of the present field. The palaeomagnetic results from SW Sulawesi are very similar to those from Cretaceous rocks of the Malay Peninsula (McElhinny et al. 1974) and in Borneo (Fuller et al. 1991) there are similar counter-clockwise rotations since the late Mesozoic and before the late Miocene. The limited evidence from west Sulawesi and Borneo suggests that counter-clockwise rotation occurred before the late Miocene and sometime during the late Palaeogene to middle Miocene. This model therefore uses a rotation of 45 ° between 20 and 10 Ma. Thai-Malay
Peninsula
There are a number of faults at the southern boundary of Indochina on which there is likely to have been Tertiary movement although currently the movement histories of these faults are not well known. The Indochina block is separated from Thailand and peninsula Malaysia on a line representing the Three Pagodas and Wang Chao faults. This is undoubtedly an oversimplification and movement of the Malay blocks in the reconstructions cannot account fully for the formation of the sedimentary basins of the western part of the South China Sea. The movement history of the ThaiMalay region is difficult to determine. Offshore, in the western South China Sea, are several large sedimentary basins with complex trans-tensional histories (e.g. Ngah et al. 1996; Tjia & Liew 1996). The land area largely lacks Tertiary rocks and palaeomagnetic results do not provide a clear
159
picture. Post-Cretaceous clockwise rotations are recorded in Thailand and northern Malaysia (Schmidtke et al. 1990; Fuller et al. 1991) whereas counter-clockwise rotations (McElhinny et al. 1974; Halle et al. 1983; Schmidtke et al. 1990; Fuller et al. 1991) are reported from Tertiary and older rocks further south. Therefore a north Malaya block has been separated from a south Malaya block at the Khlong Marui fault. There is no evidence for a major suture separating the Malay peninsula from west Borneo and the counterclockwise rotations recorded are similar from both regions. However, moving both the Malaya blocks with south Borneo results in major overlap of the peninsula and Indochina. Hutchison (1989) observed, based on the compilation of Haile & Briden (1982), that although the declinations recorded in rocks from peninsula Malaysia and Borneo are similar, there are large differences in inclinations; these differences are also seen in more recent results (Fuller et al. 1991). Thus, it is possible that the counter-clockwise rotations of Borneo and Malaysia are of different ages. The model rotates the south Malaya block counterclockwise about the same pole as that used for south Borneo but reduces its counter-clockwise rotation to 15 ° . In contrast, clockwise rotations could be explained by extrusion of Indochina. The north Malaya block is rotated clockwise relative to south China using a pole close to the fragment. This keeps the Indochina and north and south Malaya blocks close together and implies late Tertiary trans-tensional faulting in the zone of the Three Pagodas fault where the Indochina and north Malaya fragments overlap before 20 Ma, Rotations are assumed to have occurred between 20 and 10 Ma in order to maintain the separation of the peninsula and south Borneo and to maintain the northern and southern parts of the peninsula as a
Fig. 2. 5 Ma reconstruction of SE Asia. At this stage the Philippine Sea plate was rotating clockwise about a pole close to its north edge (48°N, 157°E; outside the figure) and the Luzon arc collided with the Eurasian margin in Taiwan. On this and the following figures Eurasian blocks and blocks forming part of Sundaland, with areas accreted to its continental core before the early Tertiary, are shown in yellow. The Sunda Shelf and its extensions are shaded in pale yellow. The present areas of the Indian and Pacific plates are coloured blue. Blocks of Australian continentalorigin are shown in red. Areas shaded in pink are shallow and deep parts of the Australian continental margin. Submarine parts of Sula, Buton-Tukang Besi, and Bird's Head-related fragments are also shaded with pink. Areas shown in green are mainly volcanic arc, ophiolite and accreted material of the Ryukyu islands, the Philippines, north Moluccas, north Borneo, Sulawesi and northern New Guinea. The volcanic islands of the inner Banda arc are shown in orange. Areas within the Philippine Sea plate filled with magenta are remnant arcs. Thin black lines are used to show principal marine magnetic anomalies of the Indian ocean, Pacific ocean, South China Sea, Philippine Sea and Caroline Sea. Thin light blue lines represent marine bathymetry outlining at different stages the present limits of the Philippine Sea plate, Caroline Ridge, Caroline Sea, Sulu Sea, Andaman Sea, margins of the Makassar Strait, and the Java-Sunda, the Izu-Bonin-Mariana-Yap-Palau, Negros and Manila trenches. Complexities in the Bismarck-Solomon regions are not shown. Red lines with short paired arrows represent active spreading centres. Half arrows represent strike-slip motion. Thick black and red lines represent major faults or fragment sutures used in the reconstructions. Long arrows indicate motion directions of major plates. Circular arrows represent rotations.
160
R. HALL
broadly continuous fragment. Some extension is also predicted by the model from about 32 Ma as a result of Indochina extrusion. S u m a t r a a n d the A n d a m a n
Sea
North Sumatra is fixed to the south Malaya block for all the reconstructions. Because of the rotation of south Malaya discussed above, the Sumatran margin before 20 Ma would have been closer to N-S and sub-parallel to the motion vector for the Indian plate. In this configuration it is possible that the partitioning of convergence into an orthogonal subduction component and a parallel strike-slip component would not occur. South Sumatra is fixed to north Sumatra before 15 Ma. As the counterclockwise rotation of the Malay peninsula, Sumatra and Java proceeded, the angle between the Sumatran margin and the Indian plate motion vector would have become less oblique leading to formation of the dextral strike-slip system. Dextral motion along the Sumatran Fault zone is incorporated between 15 and 0 Ma. Rotating north Sumatra with the Malay peninsula can also account for extension in the Andaman Sea. The Andaman region has been included in the reconstructions but the model is probably over-simplified. The bathymetry of the Andaman Sea is complex (Curray et al. 1979) and very simplified bathymetric contours on the east and west sides of the sea are used as markers, fixed to north and south Sumatra respectively. This suggests that before c. 10 Ma there was a small amount of orthogonal extension. After c. 10 Ma extension was greater but highly oblique. This is broadly consistent with the age of the oldest oceanic crust in the Andaman Sea (c. 11 Ma) and the recent pattern of opening (Curray et al. 1979). Java Java is included largely for completion and has been rotated counter-clockwise by 30 ° between 20 and 10 Ma. This is a compromise between the rotations chosen for Sumatra and south Borneo. There is no evidence for great extension of the Java Sea during this period (e.g. Bishop 1980; Van der Weerd & Armin 1992), but if Java is rotated rigidly with south Borneo there is too much overlap of Java and Sumatra. The difference in the amounts of rotation for south Borneo and Java would permit some extension, and probable strikeslip faulting, of the east Java Sea between 2010 Ma. Because the pole of rotation is so close to the Borneo, Java and the north Sumatra blocks there is no significant change in their relative positions, although there is some overlap of Java and Sumatra. This could be accounted for by
assuming arc-parallel extension since both Java and Sumatra are likely to have been smaller than their present-day outlines, as indicated by the extensional histories of Neogene sedimentary basins of Sumatra, and the volume of Neogene arc volcanic rocks. The counter-clockwise rotation gives an assessment of the likely pre-middle Miocene orientation of the subduction zone south of Sumatra and Java which is closer to NW-SE than present-day. The rotation also has implications for the tectonic history of Java (e.g. oblique subduction could have caused strike-slip motion) and for the timing of volcanism. The model predicts important changes in volcanic and tectonic history beginning at c. 20 Ma for both Java and Sumatra. The subduction boundary south of Java must have changed eastwards to a more complex link into the Pacific. Sulawesi
Sulawesi consists of four principal tectonic belts: the west Sulawesi volcano-plutonic arc, the central Sulawesi metamorphic belt, the east Sulawesi ophiolite belt, and the continental fragments of Banggai-Sula, Tukang Besi and Buton. This configuration has been widely interpreted in terms of collision between the eastern micro-continental fragments and the western volcanic arc (e.g. Audley-Charles et al. 1972; Katili 1978; Hamilton 1979; Silver et al. 1983b) resulting in ophiolite emplacement and metamorphism. However, more recent work shows that this apparent simplicity is partly a reflection of incomplete knowledge of the region. There is evidence of several episodes of subduction beneath the west arm of Sulawesi since at least the late Cretaceous (Hamilton 1979) and this formed part of the east Sunda margin since the early Tertiary. Collision has played a significant part in the Tertiary development of the island but the micro-continental fragments arrived later than initially thought (e.g. Davies 1990; Parkinson 1991; Smith & Silver 1991). There has been little palaeomagnetic work on Sulawesi. The earliest results by Haile (1978) from the SW and the SE arms indicated that these arms originated in different regions during the late Jurassic-early Cretaceous (Audley-Charles et al. 1972; Katili 1978). Data from SW Sulawesi indicate that it was close to its present latitude in the late Jurassic (Haile 1978) and late Palaeogene (Sasajima et al. 1980) but rotated clockwise by c. 45 ° between the late Paleogene and late Miocene (Mubroto 1988). These results from SW Sulawesi are very similar to those from Cretaceous rocks of the Malay Peninsula (McElhinny et al. 1974) and Borneo (Fuller et al. 1991). Sasajima et al. (1980) reported Eocene-early Miocene clockwise rotation
1UO °1-
}
INDIAN PLATE
(3
Bomeo rotation complete
\
l l U~1-
,
~
~
f
OUlU
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150°E
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CAROLINE
rotation ~ , of Philippine Sea Plate <
MoluccaSea doublesubduction f established ~ 1 "- ,e, Ayu trough .,~.~%,,...=._ spreading -_ N ~ t - . - -
,. Suluarc activity,,
• Sea,,
i
-~~~ Clodo~ise
20°S J
Fig. 3. Reconstruction of the region at l0 Ma. Between 5-25 Ma the Philippine Sea plate rotated about a pole at 15°N, I60°E. Counter-clockwise rotation of Borneo and related rotations of Sundaland were complete.
9U ~1-
iJ
\
rotationcomplete:
\
Subduction at Manila
PACIFIC PLATE
162
R. HALL
of the east part of the north arm whereas Otofuji et al. (1981) suggested no significant latitude change but clockwise rotation of more than 90 ° by the north arm between the Eocene-early Miocene. There are problems with interpretation of the Otofuji etal. (1981) data (J. C. Briden, pers. comm. 1994), and Surmont et al. (1994) show that 20-25 ° clockwise rotation of the whole north arm has occurred since the Miocene but that larger rotations are related to local shear zones. These rotations are consistent with the late Neogene tectonic history of Sulawesi proposed by Silver et al. (1983a) who reconstruct the island by removing the movement on the Palu, Matano, Tolo, Lawanopo and Kolondale faults. This paper follows their reconstructions and assumes, as they suggest, that this deformation occurred between 5 and 0 Ma. Palaeomagnetic work shows that lavas of the east Sulawesi ophiolite have a clear southern hemisphere origin (Mubroto et al. 1994) and formed at a latitude of 17 _+4°S. This is in marked contrast to similar Cretaceous and Tertiary rocks in the Halmahera islands where palaeomagnetic results from ophiolitic and associated rocks indicate sub-equatorial latitudes of formation (Hall et al. 1995a; Ali & Hall 1995). Palaeolatitudes of Sulawesi rocks are north of Cretaceous and the early Tertiary palaeolatitudes for the northern Australian margin but similar to Cretaceous palaeolatitudes for Sula (Ali & Hall 1995) and Misool (Wensinck et al. 1989). The age and origin of the east Sulawesi ophiolite is uncertain. Simandjuntak (1986, 1992) interprets the ophiolite as formed at an early Cretaceous spreading centre. K-Ar dates reported by Simandjuntak (1986) include 93--48 Ma gabbros and 54-38 Ma basalts. K-Ar ages on lavas by Mubroto et al. (1994) range from 79-16 Ma. Dating and geochemical studies by Girardeau et al. (1995) suggest a 44 Ma age and a backarc origin for part of the ophiolite suggesting that the ophiolite could be composite. Since the ophiolite and west arm were juxtaposed by the early Miocene (Parkinson 1991), the ophiolite is fixed to west Sulawesi from 25-0 Ma and before 25 Ma moved with the Indian plate. Makassar Strait
Geological similarities of east Borneo and west Sulawesi suggest that they have moved apart since the middle Palaeogene (Hamilton 1979) although the timing is not well constrained. The Makassar Strait is thought to be underlain by attenuated continental crust (Dtirbaum & Hinz 1982) and stretching occurred between early Palaeogene and Early Miocene (Situmorang 1982). The Makassar Strait was closed by fitting bathymetric contours on the west and east sides of the north and south
Makassar basins although this is only an approximation due to the thick Neogene sediments of the Mahakam delta, and young deformation in west Sulawesi (Bergman et al. 1996). The best fit is achieved using a pole NE of the north end of the strait at 6°N, 128°E requiring a rotation of 6 °. Because the rotation of Borneo and the Philippine Sea plate results in an alignment of the West Philippine central basin, the Celebes Sea and the Makassar Strait, the author suggests that these basins opened as part of a single basin, probably by spreading which propagated westwards from the West Philippine central basin, as discussed further below. The period of extension is assumed to be the same (44-34 Ma) which is consistent with the stratigraphic interpretation of Situmorang (1982, 1987).
Philippine Sea plate
The Philippine Sea plate provides an eastern boundary for the reconstructions. At present the plate is rotating clockwise about a pole near its northern edge (Seno et al. 1993). However, the Philippine Sea plate is now surrounded by subduction zones which separate it from the oceanic ridge system and consequently its earlier motion with respect to other major plates is difficult to determine. Subduction at the Philippine Trench is young (Cardwell et aI. 1980) and hence much of the Philippines must have been attached to the plate before the late Neogene. At the southern edge of the plate the Indonesian islands of the north Moluccas still form part of the plate. Reconstruction of the Philippine Sea plate and estimation of its past position are discussed by Hall et al. (1995b) with details of the data, the blocks, and the rotation poles used in reconstructing opening of the marginal basins. For the period between 5 and 0 Ma the present Eurasia-Philippine Sea plate pole (Seno et al. 1993) has been used. The rotation poles for the plate used for the period 50-5 Ma are based on palaeomagnetic data (Hall et al. 1995a) collected from the east Indonesian islands of the HalmaheraWaigeo region which contain a good Mesozoic and Tertiary stratigraphic record and indicate a long arc history. These palaeomagnetic data indicate that the Philippine Sea plate has rotated clockwise in a discontinuous manner since the early Tertiary with c. 35 ° clockwise rotation between 25 and 5 Ma, no rotation between 40 and 25 Ma and c. 50 ° clockwise rotation between 50 and 40Ma. Reconstructions based on these results, and including opening of marginal basins within the plate, show that other magnetic data from the plate are consistent with this rotation model (Hall et al. 1995b).
15Ma
\
rotation ~
!
i i
,~
Tukang
activel,
~ ~
k
<-4.
OCIr~a~ti~ise# /
©
PACIFIC PLATE
150°E
Terranesmovingin north
NewGuineastrike-slipsystem
MoluccaSea subductionbegins beneathHalmaheraL & ~ " •~ AyuTrough ~..~ ~ ~ - ~ ~ ~ ~ .~_~ %,. spreadingbegins --~
Bird'sHead "~" q microcontinent in 'baconslicer'. ~ . • ~
/ .~--x
Cagayanfidge collisionw#h north Sulu Palawan (
f
Spreadingends \ in Shikoku "~ basin
20°S
20°N
Fig. 4. Reconstruction of the region at 15 Ma. Rotation of Borneo and parts of Malaya, Sumatra and Java were underway. Strike-slip motion at the southern boundary of the Philippine Sea plate fragmented the Bird's Head microcontinent and moved blocks west in the plate boundary zone. Similar motions were occurring in the north Philippines. The backarc Sulu Sea began to close after collision of the Cagayan ridge with the Palawan margin,
INDIAN
Sumatran fault active
~, Malaya
Clockwise rotation of north
MiddleMiocene
7 EuRAsI,
o~
164
R. HALL
Celebes Sea
ODP Leg 124 results indicate that the Celebes Sea formed in the middle Eocene, probably not far from its present latitude (Silver & Rangin 1991). The palaeomagnetic inclinations indicate palaeolatitudes similar to the present-day latitude; no errors are quoted by Shibuya et al. (1991) but the data allow movement of up to 19 ° according to Silver & Rangin (1991) implying relatively large errors. The palaeomagnetic results of Shibuya et al. (1991) also indicate a counter-clockwise rotation of c. 60 ° between 42 and 20 Ma. The reconstructions in this paper imply a connection between the Celebes Sea and the West Philippine central basin. The suggestion that these two basins were linked was discussed by Silver & Rangin (1991) who argued against it without excluding it, based on apparent inconsistencies of spreading history, rates and palaeomagnetism between the two basins. In fact, there is no major difference between spreading rates estimated from spacing between anomalies 18 and 20 in the Celebes Sea (Weissel 1980) and the West Philippine central basin (Hilde & Lee 1984), and the small difference is consistent with a basin narrowing westwards. The stratigraphy and sedimentology of the two basins are similar (Nichols & Hall 1995) based on drilling by DSDP Legs 31, 59 and ODP Leg 124. The reconstructions also show that the apparently different rotation histories could lead to a sub-parallel alignment of their magnetic anomalies at the time of basin formation. In this model the Celebes Sea basin is suggested to have originally formed an extension of the West Philippine central basin which opened between 44 and 34 Ma, based on the ages of anomalies identified by Hilde & Lee (1984). The model therefore opens the basin and includes a 45 ° counterclockwise rotation between 44 and 34 Ma implying that the rotation occurred during opening. The magnetic anomalies identified by Weissel (1980) indicate the spreading centre was south of the present southern edge of the basin suggesting that part of the ocean has been subducted at the north Sulawesi trench since the late middle Miocene (Rangin & Silver 1991). The model assumes symmetrical spreading and eliminates the southern half of the ocean between 10 and 0 Ma at the north Sulawesi trench. The subduction is interpreted to have resulted largely from rotation of the north arm of Sulawesi (Surmont et al. 1994).
Sulu Sea-Cagayan
ridge
The Sulu Sea is a marginal basin thought to have opened as a backarc basin during the early Miocene (Holloway 1982; Hinz et al. 1991; Rangin & Silver 1991) south of the Cagayan ridge, although the
northern part of the Sulu Sea is underlain by continental crust as noted above. The Cagayan ridge is interpreted as a volcanic arc active for a short period in the early Miocene which collided with the south China margin at the end of the early Miocene (Rangin & Silver 1991). Part of this arc may also be present in Mindoro and Tablas (Marchadier & Rangin 1990). Rangin & Silver (1991) offer two scenarios for the history of this region. Their scenario A has been modelled by southward subduction of a proto-South China Sea beneath the Cagayan ridge between 20 and 15 Ma forming the Sulu Sea. Collision of the Cagayan ridge with Palawan at 15 Ma then resulted in development of a new subduction zone and southward subduction of part of the Sulu Sea beneath the Sulu arc between 15 and 10 Ma.
Philippines
With the exception of Palawan, Mindoro, Zamboanga and nearby parts of the west Philippines, the Philippine archipelago is composed of largely ophiolitic and arc rocks of Cretaceous and Tertiary age. The present Philippine fault is young, < 5 Ma, (Aurelio et al. 1991 ; Quebral et al. 1994) but there is evidence of older strike-slip faulting in the northern Philippines (e.g. Rutland 1968; Karig 1983; Karig et al. 1986; Stephan et al. 1986) possibly dating from the early Miocene. There is widespread evidence of volcanic activity throughout the Neogene implying subduction. West Mindanao east of Zamboanga is omitted before 5 Ma since almost all of this block consists of very young arc material, although it may include some basement of Eurasian continental affinities (Ranneft et al. 1960; Pubellier et al. 1991). It is likely that central Mindanao, although shown on most of the reconstructions, was also much smaller. The Philippines are widely considered to have formed part of an arc system at the edge of the Philippine Sea plate before the Pliocene (e.g. Rangin et al. 1985, 1991; Rangin 1991). South of Luzon palaeomagnetic results indicate clockwise rotations consistent with movement as part of the Philippine Sea plate. However, the Philippines are still palaeomagnetically insufficiently known to attempt a detailed plate tectonic model, even assuming this were possible, since so many fragments would be required. However, the general plate tectonic evolution is known (Rangin et al. 1990) and indicates that most of the Philippines have moved from the south and have collided with the Eurasian margin during the Neogene, although the relative importance of collision and strike-slip tectonics is uncertain. The tectonic evolution of this complex region has been simplified by localising
RECONSTRUCTING CENOZOIC SE ASIA all strike-slip movement on the present Philippine Fault and by moving the Philippines south of Luzon with the Philippine Sea plate. On the whole this avoids overlap of fragments, except between 105 Ma, and provides some limits on the large-scale tectonic setting of the region. The motions for the Philippines south of Luzon in the model are consistent with the palaeomagnetic data which predict clockwise rotations and northward movement (McCabe & Cole 1989; Fuller et al. 1991). The history of Luzon is more controversial. McCabe & Cole (1989), Fuller et al. (1991) and Van der Voo (1993) review the different interpretations of the palaeomagnetic data. This model accepts the Fuller et al. (1991) preferred interpretation of a largely counter-clockwise rotation history with a small latitudinal change for most of the Tertiary. They interpret relatively young clockwise rotations to indicate late Neogene movement with the Philippine Sea plate. Fuller et al. (1983) suggest that the palaeomagnetic results from Luzon indicate tectonic models should involve counter-clockwise rotation of Luzon since mid-Miocene, no important northward motion (_+ 500 km) since then, northward motion of the Zambales complex from equatorial latitudes with counter-clockwise motion since the Eocene, and no significant rotation in the Plio-Pleistocene. If Luzon is moved with the Philippine Sea plate and the rest of the Philippines for the whole of the period 50-0 Ma, most of these conditions are not met and there are major overlaps of Luzon, Sulawesi and the Celebes Sea. However, with the exception of the early counter-clockwise motion of the Zambales complex (which could be incorporated in a more detailed model) all of these constraints are satisfied if Luzon is positioned on the north side of the Celebes Sea before the Neogene. For the Neogene, Luzon was moved using the Philippine Sea plate rotation poles (Hall et al. 1995b) but at slightly lower rates, which implies a partial coupling to the plate, between 200 Ma, and assumed 40 ° counter-clockwise rotation between 25-20 Ma. The position of Luzon in the reconstructions is different from that normally assumed (e.g. Rangin et al. 1990) but can also satisfy many of the geological data, as explained below.
Halmahera
Reconstructions incorporating the rotation of the Philippine Sea plate and removing the effects of subduction at the Philippine Trench can be tested in part against present-day observations in the Molucca Sea region. The Philippine arcs terminate in the south in the Molucca Sea collision zone where the Halmahera and Sangihe arcs are actively
165
converging. West of Halmahera the Molucca Sea plate has an inverted U-shaped configuration and is dipping east under Halmahera and west under the Sangihe arc. Regional seismicity indicates that there are c. 200-300 km of subducted lithosphere beneath Halmahera. On the other side of the Molucca Sea, the Benioff zone associated with the west-dipping slab can be identified to a depth of c. 600 km beneath the Celebes Sea. In central Halmahera a fold-thrust belt forms the boundary between the ophiolitic eastern basement and arc volcanic western basement. Balanced crosssections indicate at least 60 km east-west shortening between east and west Halmahera in the fold-thrust belt and within the east arms (Hall & Nichols 1990). The age of thrusting is between 3 and 1 Ma. This movement is incorporated in the model and probably reflects intra-plate deformation associated with the change in motion of the plate. The western arms of Halmahera are covered by late Neogene to Recent volcanic .products related to eastward subduction of the Molucca Sea Plate. The volcanic arc was initiated at c. 12 Ma and subduction probably began at c. 15 Ma. An older phase of arc volcanic activity commenced in the late Eocene and terminated in the early Miocene. Stratigraphic similarities and the relative position of the Halmahera islands and the east Philippines suggest that they formed part of the same arc system before 5 Ma. The Halmahera islands were moved with the southern Philippine Sea plate in all the reconstructions.
Bird's Head
The Neogene movement of the Bird's Head has been estimated from constraints imposed by reconstruction of the Molucca Sea. Progressively restoring the oceanic crust subducted at the Sangihe and Halmahera Trenches requires a wide ocean and the Bird's Head needs to be moved further south than the northern Australian margin in order that the Bird's Head is south of this ocean. McCaffrey (1996) suggests the Bird's Head is currently moving south relative to Australia and this is incorporated by a small movement in the past 0.5 Ma. Before this time the movement of the Bird's Head has been modelled by assuming leftlateral strike-slip motion along a boundary parallel to the Aru Basin edge between 0.5 and 2 Ma. A small counter-clockwise rotation of the Bird's Head has been incorporated between 4 and 8 Ma based on palaeomagnetic results of Giddings et aI. (1993). A further small strike-slip motion is incorporated between 8 and 12 Ma, again constrained by reconstruction of the Molucca Sea. Before 12 Ma the Bird's Head is fixed to Australia.
20 Ma
"
basins
C"
crust thrust 'h Sulawesi
S~lu Sea opens
F~al spreading of South China Sea
Bird's Head microconfinent dismemberedby Sorong fault splays . . . .
.__~
Cagayan ridge separates f r o m /
ProtoSouth China Sea
/
/I
o
L~'---....~..
Molucca Sea forms part of PhilippineSea Plate
/.~~y/
.,
Spreadingin Shikoku \ basin
I
? ?
Sorong Fault systeminitiated
cA~o,,.~\
lO.
20°N__
20°S
10°S
-~\ 1
y, lSre''r'e
of Philippine Sea Plate
PACIFIC PLATE
I
Fig. 5. Reconstruction of the region at 20 Ma. Rotation of Borneo and parts of Malaya, Sumatra and Java began. Subduction of the Proto-South China Sea caused arc splitting in the Sulu arc and the separation of the active arc of the Cagayan ridge. South China Sea opening propagated SW into the Sunda shelf.
INDIAN PLATE
Early Miocene
E ~ PLATE
I
RECONSTRUCTING CENOZOIC SE ASIA
Sula The Sula platform is a fragment of continental crust widely considered to have been transported west by the Sorong Fault. Its origin is uncertain. Many authors consider it to be a piece of New Guinea that was detached from western Irian Jaya in late Cenozoic time (e.g. Visser & Hermes 1962; Audley-Charles et al. 1972; Hamilton 1979; Silver & Smith 1983). In contrast, Pigram et al. (1985) have proposed that it originated c. 1000 km further east, and was detached to form an independent micro-continent from central Papua New Guinea before the early Cretaceous. All these suggestions are based on stratigraphic features discussed by Pigram et al. (1985). The Sula platform is now attached to east Sulawesi and has a thrust contact with the east Sulawesi ophiolite. It was originally suggested that the collision of the west Sulawesi island arc and the Sula platform resulted in ophiolite emplacement in the east arm (Kiandig 1956; Hamilton 1979; Silver et al. 1983b). However, recent evidence indicates that this suggestion is incorrect. The ophiolite was obducted westward onto west Sulawesi at the end of the Oligocene (Parkinson 1991), whereas thrusting of the ophiolite onto the western edge of the Sula platform occurred in the latest Miocene (Davies 1990) indicating collision of the Sula platform with east Sulawesi must have occurred at c. 5 Ma. Sula is therefore moved with east Sulawesi from 0--5 Ma. Hamilton (1979) shows the Sula platform as a fragment moving along the north side of the Sorong Fault, implying it was attached to the Molucca Sea or Philippine Sea plates. By moving the Sula fragment with the Philippine Sea Plate before 5 Ma it fits closely to the Bird's Head at c. 1011 Ma. Charlton (1996) shows that the Tomori and Salawati basins, both of which are sharply truncated, would have formed the northern and southern parts of a single large basin if the Sula platform were moved back to this position. This need not preclude a Mesozoic separation of a Bird's Head microcontinent from further east on the north Australian margin.
B u t o n - T u k a n g Besi The Tukang Besi platform is a micro-continental fragment that collided with Sulawesi during the Miocene, although the timing of the collision is interpreted differently by different authors. Davidson (1991) suggested Buton to be a microcontinental fragment which collided in the early Miocene with SE Sulawesi, before the Tukang Besi platform. Here, both have been treated as parts of a single block, following Smith & Silver (1991), who consider Buton as part of the Tukang Besi platform.
167
This difference may be explicable if both formed part of an extended Birdfs Head microcontinent, as discussed later. Smith & Silver (1991) argue that collision of the Tukang Besi platform with Sulawesi was complete before the late Middle Miocene, since ophiolitic conglomerates and sandstones of this age (N13-N14) overlie deformed basement rocks. They interpret Lower Miocene conglomerates (N7-N9) that lack ophiolitic debris to indicate uplift associated with initial detachment of the platform from the northern New Guinea margin. If this interpretation is correct, initial uplift associated with detachment from New Guinea occurred between c. 15 and 17 Ma and collision of Tukang Besi with Sulawesi must have been complete by c. 11 Ma. To model this history Tukang Besi was fixed to west and central Sulawesi between 0 and 11 Ma. By attaching the platform to the southern edge of the Philippine Sea plate before that time Tukang Besi returns to a position, west of, and adjacent to the Sula fragment and the Bird's Head by 14 Ma. To obtain the best fit requires rotation of the Tukang Besi block, and this is incorporated by a clockwise rotation of 40 ° . The author therefore interprets the sequence of events to be: at c. 15 Ma a strand of Sorong fault propagates west, south of Tukang Besi; by 14 Ma Tukang Besi is fully attached to Philippine Sea plate; at 11 MaTukang Besi collides with Sulawesi, locking this strand of the Sorong fault and requiring a development of a new fault strand which caused the detachment of Sula. It is interesting to note that the development of Molucca Sea subduction beneath Halmahera begins at c. 13-15 Ma, indicated by K-Ar ages of volcanic rocks and reset ages (Baker & Malaihollo 1996), as well as biostratigraphic ages. Thus, a locking of subduction at the west side of the Molucca Sea, requiring initiation of a new subduction system on its east side, is temporally linked to development of the Sorong fault splay. Seram and Buru Hamilton (1979) suggested there is only shallow seismicity associated with the Seram Trough whereas Cardwell & Isacks (1978) argued that a deep slab was bent round the entire Banda arc. McCaffrey (1989) attempted to reconcile these differences on the basis of an increased number of better located seismic events, and concluded that there are two slabs, one subducted at the Timor trough, and a second subducted at the Seram trough. The Seram slab extends to no more than 300 km whereas the Indian ocean slab is continuous to over 600 km indicating that they record different histories of subduction. The bend in the Indian ocean slab implies that Seram has moved
168
R. HALL
eastwards while Australia has moved north. During the late Neogene the Banda arc has migrated east since the volcanoes become younger and the length of the subducted slab decreases eastwards. This configuration was modelled by moving Seram northwards to its present position relative to the Bird's Head between 4 and 0 Ma and moving Seram eastwards relative to the Bird's Head from 12-4 Ma. This is consistent with, although simplified from, the present tectonic configuration in the Banda Sea inferred by McCaffrey (1989). Buru is currently situated between strands of the Strong Fault system and Hamilton (1979) suggests it may have moved westward from New Guinea. The present author has allowed a small amount of westward movement between 4 and 0 Ma which fits Seram, Buru and the Bird's Head closer together but this is merely a guess.
Caroline plate The Ayu Trough opened during the middle and late Miocene (Weissel & Anderson 1978) and spreading may be continuing at the present day. There has been no subduction at the Caroline-Philippine Sea plate boundary and little convergence at the Caroline-Pacific boundary c. 25 Ma. The model uses the Caroline-Philippine Sea plate rotation pole and rate of Sent et al. (1993) for the period 5-0 Ma and then moves the Caroline plate with the east side of Ayu Trough before this. The Caroline plate is omitted from the reconstructions before 25 Ma.
Implications Timing of major changes The animated reconstructions show clearly that, during the interval 50-0 Ma, although there are important changes in movements and locally there are significant manifestations of these changes, there are two truly regionally important periods of change. Both of these appear to be the expression of arc-continent collision and resulted in major changes in the configuration of the region and in the character of plate boundaries. At c. 25 Ma the collision of the Australian continent with the Philippine Sea plate arc in New Guinea (Fig. 6) caused major effects which propagated westwards through the region. The Philippine Sea plate began to rotate clockwise requiring development of new subduction systems at its western edge. This led to the assembly of the Philippines archipelago, initiated new arc systems from north Sulawesi through to the Philippines, and led to the growth and partial destruction of marginal basins such as the Sulu Sea. The continued rotation of the Philippine Sea plate ultimately resulted in elimina-
tion of the Molucca Sea plate and accretion of fragments from the northern Australian margin into the SE Asian margin, notably in Sulawesi. At c. 5 Ma collision of the Philippine arc and the Eurasian continental margin occurred in Taiwan (Fig. 2). This appears the key to the recent tectonics of the region. Once again, the Philippine Sea plate motion changed, and new subduction systems were initiated, such as those currently active on the east and west sides of the Philippines. Most deformation now seems to be concentrated in the region between the Banda Sea and Taiwan. Summarized below are the major implications of the reconstructions for different parts of SE Asia.
Borneo Accepting rotation of Borneo has some important consequences for the reconstruction of the region, although the amount of rotation could be reduced to about 30 ° without seriously affecting the model. It means that there was initially a very wide protoSouth China Sea (Fig. 1 l) which began to close from c. 44 Ma (Fig. 10). Alternative reconstructions (e.g. Rangin et al. 1990; Lee & Lawyer 1994) which do not incorporate this counter-clockwise rotation, appear to lack the space required for the opening and closing of ocean basins such as that north of the Cagayan ridge and the Sulu Sea. Although the closure of the proto-South China Sea may have been partly driven by extrusion of the Indochina block the reconstructions suggest it was achieved by subduction either at the southeast side of the ocean or within the ocean (Figs 8 and 9). Once subduction was established slab-pull force would have caused extension of the IndochinaSouth China margin leading to formation of oceanic crust and extension of Eurasian continental crust (Fig. 7). Thus, extension in this region may not have been driven by strike-slip-related extrusion. In fact, the model suggests that subduction may have been underway before South China Sea opening began. Opening of the Celebes Sea required northward motion of Luzon because of its position north of the spreading centre, thus implying a subduction zone on the south side of the proto-South China Sea (Fig. 9). The Oligocene reconstructions are thus very similar to those of Taylor & Hayes (1983) except that south Borneo is rotated further. Many of the differences between this model and that of Rangin et al. (1990), which otherwise have many resemblances, result from the new constraints imposed by reconstruction of the Philippine Sea plate. If Borneo is not rotated there are problems in accounting for the evidence of continental crust in west Sulawesi in the early Miocene (see below; Bergman et al. 1996). The extended Bird's Head
110OE
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20°N
Fig, 6. Reconstruction of the region at 25 Ma. Collision of the Australian continental margin in New Guinea, and the Bird's Head microcontinental block in Sulawesi, with the arc from north Sulawesi to Halmahera caused major reorganisation of plate boundaries. The active ridge jumped south in the South China Sea.
!
25 Ma L=eO,gocene
EURASIAN PLATE
I
/
~
1100°E
INDIAN PLATE
/
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extruded
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~
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SulawesiwestArm
Subduction decreases westwards
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~ /Pala N°wa~
Openingof outh ChinaSea northof Macclesfield Bank
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Openingof Parece Vela basinbegins
PACIFIC PLATE
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20°S
10°S_
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10°N_.
20°N_
Fig. 7. Reconstruction of the region at 30 Ma. South China Sea opening was driven by pull of the subducted Proto-South China Sea slab, and possibly by extrusion caused as India indented Eurasia. Arc splitting began in the eastern margin of the Philippine Sea plate with spreading in the Parece Vela basin.
/
|
movement on Red R/vet fau/t
Sin
Mid Oligocene
30 Ma'
I
¸--,,3 0
RECONSTRUCTING CENOZOIC SE ASIA
microcontinent would have been much further north than shown in the reconstructions and hence would have occupied the site of the Molucca Sea. For most of the Oligocene and early Miocene (Figs 6 and 7) the Sundaland margin in Borneo would have been orientated NW-SE and would have been a strike-slip dominated region and therefore the sedimentary basins of northwest Borneo should record an important component of this strike-slip history. This configuration also suggests that much of the sediment now found in north Borneo may have been fed southwards across the Sunda shelf from Indochina. This resolves a problem of the sediments now found in north Borneo which could not have a south Bomeo provenance because of their volume and because most of south Borneo was below sea-level during much of this period. The timing of subduction suggested by the model is similar to that deduced from the north Borneo stratigraphic record by oil company geologists (e.g. T a n & Lamy 1990) and the present author suggests that their 'Deep Regional Unconformity' is one manifestation of Australia-Philippine Sea plate collision and consequent reorganization of plate boundaries. Subduction became increasingly important in NE Borneo between c. 20 and 10 Ma (Figs 3-5) and the proto-South China Sea was entirely eliminated before 10 Ma. For Malay blocks west of Borneo, the reconstructions (Figs 4 and 5) can be regarded only as an approximation. The model suggests greater extension than that recorded in the Malay basins, and the timing of the rotations in the model needs to be improved. The similarities in rotations recorded through this part of Sundaland may be no more than coincidental, but if Bomeo and Java have rotated, there must have been some movement of Sumatra and the Malay peninsula, otherwise there is major overlap of fragments. The implications of rotations of blocks for the wider region are a problem which the palaeomagnetists often neglect, and therefore the model has some value, even if wrong, in attempting to face up to these implications, which in turn emphasize the need for more data to identify timing and to separate local and regional rotations. There can be no doubt that at present the palaeomagnetic data from Sundaland do not on their own convincingly demonstrate regional rotations. Origin o f the Celebes Sea
If palaeomagnetic evidence for the rotation of Borneo is accepted, there is a strong case to be made that the central West Philippine Sea, the Celebes Sea and the Makassar Strait formed part of a single marginal basin (Figs 8 and 9) which
171
opened between late Eocene and mid Oligocene, and widened eastwards like the present South China Sea. This interpretation has been incorporated in the model and the palaeomagnetic data from the Celebes Sea and Philippine Sea can i~e reconciled in such a model. The author follows Hamilton (1979) and many others in interpreting the Makassar Strait as an extended area of crust, probably without oceanic crust, although this need not preclude the Neogene convergent tectonics suggested by Bergman et al. (1996) for its west Sulawesi margin. The reconstructions are least convincing for the period between 44 and 40 Ma when the Philippine Sea plate was rotating rapidly clockwise and the Celebes Sea was opening with counter-clockwise rotation. However, this difficulty may have more to do with the inadequacy of the data on which the timings of rotations are based. For the Philippine Sea plate all we can say is that there was rapid rotation between 50 and 40 Ma (Hall et al. 1995b) and more data from Palaeogene rocks are needed to determine when and at what rate the rotation occurred. It may well have been fully complete by 45 Ma in which case there would be no difficulty with smooth opening of a basin narrowing west. For the Celebes Sea the position of the basin is consistent with a backarc setting related to northward subduction of Indian ocean lithosphere beneath west and north Sulawesi. Further east this setting appears less convincing because of the very large distance between the basin axis and the subduction zone (Figs 8 and 9). There were at least three major basins which opened in SE Asia with a similar east-widening geometry: a Mesozoic proto-South China Sea, the Eocene--Oligocene Philippine-Celebes basin and the Oligo-Miocene South China Sea. The South China Sea opening has been interpreted as linked to India indentation (Tapponnier et al. 1982) and as suggested here may be related to subduction, but this explanation seems unlikely for the PhilippineCelebes basin. Perhaps these three basins reflect some other lithospheric mechanism related to the long-term subduction of lithosphere east and south of SE Asia. Sulawesi collisions
The apparently simple tectonic configuration in Sulawesi of arc-ophiolite-continent is not the result of a single arc-continent collision (e.g. Silver et al. 1983b) but is a consequence of multiple collision events. The east Sulawesi ophiolite has an Indian ocean origin (Mubroto et al. 1994). Emplacement of the ophiolite on the west Sulawesi continental margin occurred at the end of the Oligocene (Parkinson 1991) and was followed by a change in plate boundaries at the beginning of the
i 100°E
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~, i
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Sea
/
i
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l
140°E
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~ f,,-
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Sulawesi arc
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~,,1~
Extensiondriven bypullof subducted proto-SCSslab
• EURASIAN PLATE
i
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9
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10°S
oo_
10 ° N
20°N_
Fig. 8. Reconstruction of the region at 35 Ma. Extension in the Sunda shelf and Eurasian continental margin was driven by pull of the subducted Proto-South China Sea slab. Active spreading of the Celebes Sea-West Philippine Sea basin ended at 34 Ma.
I 90°E ~
/
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35 Ma
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120°EL ~1111~,.--,.,,.,~
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Fig. 9. Reconstruction of the region at 40 Ma. Subduction of the Proto-South China Sea began as a trench became active north of Zamboanga-Luzon, caused by rapid opening of the Celebes Sea-West Philippine Sea. Between 40-50 Ma the Philippine Sea plate rotated clockwise about a pole at 10°N, 150°E.
90°E
-
Eocene
t.,o
174
R. HALL
Miocene (Fig. 6). There is isotopic evidence from geochemistry of igneous rocks for very old continental crust beneath west Sulawesi (Coffield et al. 1993; Priadi et al. 1993; Bergman et al. 1996), probably of Australian origin, because of the extreme isotopic compositions. The ages of the oldest igneous rocks reported (Bergman et al. 1996) imply early Miocene underthrusting of Australian lithosphere. Since the early Miocene there have been at least two further collisions, in SW and east Sulawesi, as fragments of continental crust have been sliced from the Bird's Head microcontinent and transported west for brief periods on the Philippine Sea plate or the Molucca Sea plate which was partially coupled to the Philippine Sea plate. Bird's Head microcontinent
The reconstructions suggest that the Sula platform and Tukang Besi platform formed part of a single large microcontinent with the Bird's Head at c. 15 Ma (Fig. 4). The model shows that movement of small continental fragments to their present positions can be explained easily if they were sliced from this microcontinent at different times and each moved with the Philippine Sea plate for a few million years before collision (Figs 2-5). Thus, the driving force for these motions was the Philippine Sea plate. Temporary locking of the strike-slip system at the southern edge of the Philippine Sea plate required development of new splays of the fault which resulted in the transfer of continental fragments to the Philippine Sea plate. As each fragment docked at the western end of the fault system in Sulawesi, a new splay developed and a new fragment began to move. The cartoons of Hamilton (1979) for the Neogene development of the region are remarkably similar to the predictions of the model although the timing is somewhat different, mainly because of new information from east Indonesia. This model has been described informally as a 'bacon slicer' and this does seem a very appropriate description of the mechanics of the process. The present author differs in one important respect from many interpretations of this region in suggesting that these fragments, although colliders, are not the major causes of contractional deformation associated with their docking. They are merely passive participants riding on the Philippine Sea plate whose major role is to lock a fault strand on their arrival at the Sulawesi margin. The reconstructions of Tukang Besi and Sula with the Bird's Head do not account for the evidence for Australian crust beneath west Sulawesi by the late Early Miocene (Bergman et al. 1996). The extent of this continental crust was estimated and used to outline an additional frag-
ment (Figs 5-9). If this is assumed to be beneath west Sulawesi by 17 Ma but is moved with the Philippine Sea plate from 21-17 Ma, it fits back to the western edge of the Bird's Head microcontinent (Fig. 6). It is suggested here that this fragment records the earliest effect of the 'bacon slicer' and was the first block detached from the Bird's Head microcontinent. It separated as a result of the development of the Sorong Fault system at the southem edge of the Philippine Sea plate and, like the other fragments, moved with the Philippine Sea plate for about 5 Ma. On the basis of the 25 Ma reconstruction, it can be speculated that the Bird's Head microcontinent was a single block at the end of the Oligocene formed by separation from Australia during the Mesozoic, to which had been added at least part of the east Sulawesi ophiolite. The reconstructions do not answer the question of the ultimate origin of the Bird's Head microcontinent, but they do require that the microcontinent has been moving with about the same motion as, and has remained in a similar position relative to, Australia for the past 25Ma. Palaeomagnetic results suggest that the microcontinent was at least 10°N of the north Australian margin in the late Cretaceous (Wensinck et al. 1989; Ali & Hall 1995) but its early Tertiary position is not well constrained in the model. The pre-Neogene geology of the region suggests there may have been several important events in the development of the Banda Sea region (e.g. ophiolite emplacement and metamorphism described by Sopaheluwakan (1990) from Timor), which could record relative movement between Australia and the microcontinent in passage to their 25 Ma positions. Molucca Sea
The model suggests that the Molucca Sea was a very wide area formed by trapping of Indian ocean lithosphere between the north Australian margin and the Philippine Sea plate arc when collision occurred at 25 Ma (Fig. 6). Thus, its age should be pre-Tertiary. It was eliminated by subduction on its east and west sides. Subduction probably began soon after the 25 Ma change in motion of the Philippine Sea plate on the west side of the Molucca Sea in the north Sulawesi-Sangihe arc, consistent with ages of arc volcanic rocks in north Sulawesi (Dow 1976; Effendi 1976; Apandi 1977). The Molucca Sea formed part of the Philippine Sea plate up to c. 15 Ma (Figs 4 and 5) when eastdipping subduction began beneath Halmahera (Baker & Malaihollo 1996). Most of the subduction occurred on the west side so the Molucca Sea has remained partly coupled to the Philippine Sea plate. The double subduction system of the Molucca Sea
RECONSTRUCTING CENOZOIC SE ASIA probably never extended north of its present northern edge into the Philippines. However, there was a oceanic area which was effectively continuous with, and north of, the Molucca Sea which was formerly the central part of the Celebes SeaWest Philippine central basin. In the reconstructions this northern part of the ocean is eliminated between 25 and 12 Ma and the Panay sector of the Philippine arc arrives at the western subduction margin at about 12 Ma. Slightly earlier (15 Ma) the Cagayan arc and Luzon arrive at the Palawan margin (Fig. 4) resulting in collision in Mindoro (Rangin et al. 1985). It may be one or both of these events in the Philippines which caused initiation of the east-dipping subduction system beneath Halmahera and the concomitant development of splays of the Sorong Fault system at the southern edge of the Molucca Sea. The reconstructions of the Philippines are too imperfect to be confident of the relationships between cause and effect but the timing of collision and age of the ophiolites in Mindoro (Rangin et al. 1985) and events in Panay (Rangin et al. 1991) are consistent with the model. Philippines
The Philippines are difficult to reconstruct for a number of reasons. Our understanding of Philippines geology and evolution is still insufficient, although considerable advances have been made in recent years as a result of investigations in the region (see for example, Sarewitz & Karig 1986; Stephan et al. 1986; Rangin 1991; and references therein). Reconstructions using the outlines of present-day fragments can be only approximate since much new crust has been added by arc processes. In order to describe the Philippines more precisely many more fragments need to be used and this is currently beyond the capacity of the ATLAS program. However, it seems that rigid plate tectonics may be an inadequate tool to describe the evolution of the area. At present this is illustrated most clearly at the south end of the archipelago where plate boundaries are ill-defined probably because there is a significant amount of within-plate deformation (Rangin et al. 1996). Karig et al. (1986) have drawn attention to the way in which at present only a small part of the Philippines is completely coupled to the Philippine Sea plate, and how in the past it is likely that coupling was never complete across the entire Philippine system because of strike-slip faulting. Because of the situation of all the Philippine fragments at the edge of plates it is probably unwise to rely too heavily on the evidence of arc volcanism to infer arc continuity. At present it is clear that some volcanism is related to subduction on the west side of the Philippines, some may be related to sub-
175
duction at the Philippine Trench, and some may not be directly related to subduction. Switching of subduction from one side of the Philippines to the other has almost certainly occurred in the past (e.g. Schweller etal. 1983; Karig e t a l . 1986). Some arcs may also be eliminated completely during collision as one arc overrides another, a process that is underway in the Molucca Sea. However, the reconstructions do provide some useful limits on what is feasible. Most of the Philippine islands probably formed part of an arc at the southern edge of the Philippine Sea Plate before the early Miocene '(Figs 7-10) as shown earlier in the reconstructions of Rangin et al. (1990). The choice here for the position of Luzon (Figs 6-10) shows that the geological evidence for subduction can be satisfied in ways other than by including all the Philippines in this arc. Since the early Miocene (Figs 2-6) the Philippine fragments have moved in a very narrow zone, mainly as part of the Philippine Sea plate, within which there appears to have been a substantial component of strike-slip motion (Sarewitz & Karig 1986). Most subduction under the Philippines was oblique, mainly at the western edge, and north of Mindanao. The Philippines are an ephemeral feature. They will probably end up as a composite arc terrane smeared onto the Eurasian continental margin which will be impossible to unravel. The great depths of young sedimentary basins in close juxtaposition to areas of high emergent topography suggest that lithospheric processes operating in this region are not well described by current plate tectonic concepts, and in some ways the Philippines are reminiscent of Pre-Cambrian greenstone belts. There seems no evidence that the Philippines are the result of a collision of two opposed arcs, progressively zipping up southwards towards the present-day Molucca Sea as shown in some reconstructions (e.g. Lewis et al. 1982; Rammlmair 1993). In particular, there is no evidence that the west-facing Halmahera arc extended north of the present north edge of the Molucca Sea into Mindanao (Quebral et al. 1995). Banda Sea
The age and origin of the Banda Sea have long been the subject of dispute. Several workers have suggested it is relatively old, possibly Mesozoic, with the Banda Sea representing trapped oceanic crust (Katili 1975; Bowin et al. 1980; Lapouille et al. 1986; Lee & McCabe 1986) whereas others have preferred a much younger, late Tertiary, age (Carter et al. 1976; Hamilton 1979; R6hault et al. 1995). Silver et al. (1985) suggested that the North Banda Sea may include crust of Pacific origin and identified the Banda ridges as continental
I 100°E
I110°E
North Sulawesi
-, ~ ~
Zambales~ ophioliteformation
ProtoSouth China Sea
1120°E
INDIAN-AUSTRALIAN PLATE
EURASIAN PLATE
?
.=
i130°E
©
PLATE
PHILIPPINE SEA .
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~,~1~ 4~mahera
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PACIFIC PLATE
\
Fig. 10. Reconstruction of the region at 45 Ma. The North New Guinea-Pacific spreading centre was subducted causing forearc magmatism and massive extension of the NE margin of the Philippine Sea plate.
90°E
A
• Middle Eocene
45 Ma
k
o~
RECONSTRUCTING CENOZOIC SE ASIA fragments based on dredging, a conclusion supported by more recent dredging (Villeneuve et al. 1994). Since some of the magnetic lineations of supposed Mesozoic age cross the Banda ridges the inferences based on magnetic anomaly ages may be wrong. Many of the lineations are parallel to the structural fabric of these continental areas (Silver et al. 1985; Rthault et aI. 1991). Despite this, there still appears to be a widespread acceptance that the Banda Sea is floored by old oceanic crust (e.g. Hartono 1990). This is not a conclusion supported by the reconstructions in this paper. In these the Banda volcanic arc is fixed to west Sulawesi during the late Neogene and the arc-trench gap is assumed to have maintained its present width. Timor and the islands of the outer Banda arc are fixed to Australia for the same period. The Birds' Head microcontinent, including the island of Seram, has been moved, as described above, to satisfy the constraints imposed by reconstruction of the Molucca Sea and the distribution of the deep subducted slabs inferred from seismicity. The reconstructions that result show that before c. 10 Ma the gap between Seram and Timor (Fig. 4) was filled by oceanic crust, now subducted beneath the Banda arc, which could indeed have been of Indian ocean origin and Mesozoic age. Timor arrived at the trench at c. 4-3 Ma consistent with geological data from Timor (e.g. Carter et al. 1976; Audley-Charles 1986; Harris 1991). However, before c. 10 Ma the relative distance between Timor and Seram is maintained (Figs 4-6) suggesting there was no subduction in the eastern Banda Sea area. The reconstructions suggest subduction began at c. 10 Ma (Fig. 3), resulting in the eastward propagation of the volcanic arc consistent with the very young age of the volcanoes in the inner Banda arc (Abbot & Chamalaun 1981; McCaffrey 1989). The north Banda basin extended as Seram moved east and the arc propagated east. The eastward movement of Seram is consistent with tectonic inferences from present seismicity (McCaffrey 1989) and geological observations on Seram (Linthout et al. 1991). The arc propagated east to the longitude of Seram at c. 5 Ma (Fig. 2) which is close to the age of the well known ambonites of this area. The reconstructions also show the south Banda basin extending rapidly between 5 and 0 Ma. Therefore, both the north and south Banda Sea can be interpreted as having an extensional origin and to have opened during the late Neogene. These interpretations are consistent with the age of young volcanics dredged in the Banda Sea (R6hault et al. 1995). The great depth of the south Banda Sea is still a problem, although it is known that Parsons & Sclater's (1977) age-depth relationships often do not hold in small ocean basins. Van Gool et al.
177
(1987) recorded high heat flows in several NW Banda Sea basins and suggested that they are not isostatically and thermally compensated. Caroline plate and New Guinea
The consequences of fixing the Caroline plate to the east side of Ayu Trough for Caroline-Pacific boundary motion are that there has been predominantly left-lateral motion since 25 Ma, minor oblique convergence between 15 and 5 Ma and minor oblique extension between 5 and 0 Ma (Figs 2-6). The timing of convergence and extension are partly dependent on the model since the period of 15-5 Ma was chosen as the interval of the main opening of the Ayu Trough, based on Weissel & Anderson (1978). However, these conclusions are consistent with the evidence of young oblique extension in the Sorol Trough (Weissel & Anderson 1978). The northern New Guinea arc terranes have been omitted from the reconstructions for simplicity and because there are insufficient data to reconstruct them adequately. However, the reconstructions do predict that there would have been relatively little convergence between the northern Australian margin and the Caroline plate during the Neogene. Most of the convergence that is required should have occurred in the past 5 Ma (Fig. 2) and would have been oblique. At the present day it appears that much of the convergence is distributed (McCaffrey & Abers 1991; Puntodewo et al. 1994) implying that only a proportion need be absorbed by subduction. This is consistent with the shallow seismicity associated with the New Guinea trench and lack of active volcanoes (Hamilton 1979; Cooper & Taylor 1987), and the absence of Neogene volcanicity in Irian Jaya and the Bird's Head (Pieters et al. 1983; Dow & Sukamto 1984). The reconstructions require young (5-0 Ma) underthrusting on the sector of the New Guinea trench east of the Bird's Head as interpreted by Hamilton (1979) and Cooper & Taylor (1987) but not north of the Bird's Head, consistent with observations of Milsom et al. (1992). The apparent complexity of the northern New Guinea margin (cf. Pigram & Davies 1987) appears to be due less to multiple collisions than to strike-slip faulting and fragmentation of the arc which collided with the Australian margin in the early Miocene. No reconstruction has been attempted here of the SW Pacific north and east of New Guinea, and the reader is referred to Yan & Kroenke (1993).
Final comments The reconstructions presented here differ significantly from earlier attempts to describe the
~
'\
1100°E
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~
EURASIAN PLATE
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Zamboanga
~ 1120°E
Southand ast Sulawesi
INDIAN-AUSTRALIAN PLATE
• West Sulawesi
ProtoSouth China Sea
North ..~ Palawan/
Mindom
Taiwan
1130°E
Fig. 11. Reconstruction of the region at 50 Ma. The early rotation of the Philippine Sea plate began.
90°E
'~/
End Early Eocene
50 Ma
SouthChina
I
1140°E
NORTH NEW GUINEA PLATE
t
1150°E
20°S
o
...j oo
RECONSTRUCTING CENOZOIC SE ASIA development of SE Asia. Daly et "al. (1991) and Lee & L a w v e r (1994) neglect the rotation of the Philippine Sea plate and this accounts for major differences in our interpretations of the eastern parts of the region. Rangin et al. (1990) modelled clockwise rotation of the Philippine Sea plate and their reconstructions are quite similar to those here for the Philippines region but the models differ on the position of the Philippine Sea plate, the rotation of Borneo and the position of Luzon. As noted earlier, these differences are partly a consequence of different interpretations of inadequate data. Thus, even if these new reconstructions are rejected they do at least serve to draw attention to the need for many more good quality palaeomagnetic data from the region, in addition to g e o l o g i c a l data, particularly on the timing, chemistry and character of volcanic activity. Biogeographical data could also be especially useful in testing the different interpretations of the region and there is a need for a joint approach from botanists, zoologists and geologists. If the large frame is used it also becomes clear how much one is limited by the area, by the extent of possible oceanic crust, etc.; these limitations are much less obvious in local reconstructions where it is easier to move problems outside the area of immediate interest. The reconstructions suggest that the indentation of Eurasia by India has played a much less important role in the development of SE Asia than often assumed. They suggest that collision of the Australian continent has driven major rotations, and that the movement of smaller plates, such as the Philippine Sea plate and the Borneo microplate, is the result of the northward movement of Australia. Events which are thus 'caused' by movements of such plates may therefore be the consequence of more fundamental regional movements. Thus, caution is necessary in correlating events of similar age and interpreting their causes. The difficulties of reconstructing the Philippines are partly due to our still inadequate knowledge of this region but may
179
also reflect our limited understanding of arc processes, particularly at the lithospheric scale. The intra-oceanic history of the region leaves a poor record; arcs are ephemeral features on the geological timescale and may disappear completely, a process currently underway in the Molucca Sea. Probably the most difficult feature to incorporate is the role of strike-slip faulting. Like Karig et al. (1986) and Rangin et al. (1990) the present author considers that strike-slip faulting has played a major part in the development of the Philippines and the reconstructions confirm that there is limited space for the convergent motions often inferred from the arc volcanic record. These reconstructions also point to the importance of strike-slip faulting in other parts of the region such as Borneo and New Guinea, and suggest a need for re-examination of data and interpretations based on purely convergent collision models. I am particularly grateful to Simon Baker who has provided me with invaluable support, not only with the work involved in digitizing the fragment outlines used in the reconstructions, but with discussion and ideas at many stages. I also thank the numerous people with whom I have discussed the geology of SE Asia over many years who have contributed criticism, ideas and comments, including D. A. Agustiyanto, J. R. Ali, C. D. Anderson, M. G. Audley-Charles, P. D. Ballantyne, S. J. Baker, A. J. Barber, J. C. Briden, T. R. Charlton, E. Forde, R. Garrard, N. Haile, A. S. Hakim, S. Hidayat, T. W. C. Hilde, M. Fuller, Kusnama, J. A. E Malaihollo, R. McCabe, J. S. Milsom, A. H. G. Mitchell, S. J. Moss, R. Murphy, G. J. Nichols, C. D. Parkinson, M. Pubellier, R. Quebral, C. Rangin, H. Rickard, S. J. Roberts, N. Sikumbang, T. O. Simandjuntak, M. J. de Smet, B. Taylor and S. L. Tobing. Financial support has been provided at various stages by NERC award GR3/7149, grants from the Royal Society, the University of London SE Asia Research Group, and the University of London Central Research Fund. Work in Indonesia was facilitated by GRDC, Bandung and Directors including H. M. S. Hartono, M. Untung, R. Sukamto and I. Bahar. I thank R. J. Bailey, Gordon Packham and Claude Rangin for reviews and many helpful comments on the manuscript.
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Contrasting tectonic styles in the Neogene orogenic belts of Indonesia T. O. S I M A N D J U N T A K 1 & A. J. B A R B E R 2
1 Geological Research and Development Centre, Jalan Diponegoro 57, Bandung 40122, Indonesia 2 Southeast Asia Research Group, Department of Geology, Royal Holloway, University of London, Egham TW20 OEX, UK Abstract: The recent compilation of a new tectonic map of Indonesia as part of the Geotectonic Map Project of East Asia has prompted a reassessment of the contrasted tectonic styles represented by currently developing orogenic belts. These orogenic styles provide a range of models illustrating the diversity and complexity of tectonic processes which may provide the key to the interpretation of other orogenic belts elsewhere in the world. The following distinctive types of orogenies have been recognized within Indonesia. 1. Sunda Orogeny in Java and Nusa Tenggara: involving subduction of oceanic crust with normal convergence, producing an orogenic belt of Andean type with trench, accretionary complex, forearc basin and Quaternary magmatic arc with active volcanoes built on the margin of the Sundaland continent. 2. Barisan Orogeny in Sumatra: with strongly oblique convergence and major strike slip transcurrent fault movement within the magmatic arc, along which a segment of continental crust is being displaced northwards along the western margin of Sundaland. 3. Talaud Orogeny in the northern Molucca Sea: convergence of the Sangihe and Halmahera oceanic magmatic arcs as the Molucca Sea Plate subsides beneath them. 4. Sulawesi Orogeny in eastern Sulawesi: collision of microcontinental blocks with subduction systems along the eastern margin of Sundaland. 5. Banda Orogeny in the southern Banda Arc between Sumba and Tanimbar: collision of the northern margin of the Australian continent with the subduction system along the southern segment of the Banda Arc. 6. Melanesian Orogeny in Irian Jaya and Papua New Guinea: a more advanced stage in the collision of the northern margin of the Australian continent with a magmatic arc on the Philippine Sea plate, commencing in the early Miocene with a partial reversal of polarity and the subduction of the Caroline plate beneath the collision zone. The present phase of orogenic activity in most of these occurrences commenced in midMiocene times and orogenic processes are still in progress.
Since the commencement of the first five year National Development Plan of Indonesia (Pelita I) in 1970 the geology of the whole country has been systematically and intensively investigated both by government institutions and by the private sector. These investigations have included the systematic geological mapping of the Indonesia archipelago, initially by the Geological Survey of Indonesia and later by the Geological Research and Development Centre (GRDC), partly in collaboration with the United States Geological Survey (USGS), the British Geological Survey (BGS), the Australian Bureau of Mineral Resources (BMR) (now the Australian Geological Survey Organisation: AGSO) and the Japanese International Cooperation Agency (JICA). At the same time geological and geophysical research has been continually in progress in many parts of Indonesia in collaboration with geoscientists from the University of London, UK; University of California, Santa Cruz, USA; Institute of Geophysics, Hawaii, USA;
INSU, France; the Snellius Programme, Netherlands; and JICA, Japan. Geological and geophysical investigations have also been carried out by the Indonesian government institutions and the private sector, including both domestic and foreign companies, in the exploration for energy and mineral resources. The result of these investigations has been an immense increase in the knowledge and understanding of the geological and tectonic development of the archipelago. The present paper is a review of this current understanding of the tectonic development of the Indonesian archipelago through Neogene times to the present day. The review arose out of the responsibilities of one author (TOS) in the compilation of the new Geotectonic Map of Indonesia as a member of the Working Group on the Geotectonic Map of East Asia organized by the Commission for the Geological Map of the World (CGMW) and the Committee for Coordination of Joint Prospecting for Mineral Resources in Asian
FromHall, R. & Blundell, D. (eds), 1996, TectonicEvolutionof SoutheastAsia, Geological Society Special Publication No. 106, pp. 185-201.
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T.O. SIMANDJUNTAK t~ A. J. BARBER
Offshore Areas (CCOP) as part of the NW Quadrant Circum-Pacific Map Project. Tectonic
setting
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The present physiographic and tectonic configuration of Indonesia is considered to have developed since late Neogene times due to the interaction of three of the Earth's major lithospheric plates: the NNW-moving (c. 10 cm a-]) Philippine Sea plate; the NNE-moving (c. 8 c m a -1) Indo-Australian plate and the stationary, or slowly SE-moving (c. 0.4 cm a-1) Eurasian plate (Minster & Jordan 1978). The Indonesian archipelago therefore represents an immensely complicated triple junction, involving a complex pattern of small marginal ocean basins and microcontinental blocks bounded by subduction zones, extensional margins and major transcurrent faults. On the basis of geological and geophysical characteristics five regions of crust of different origins can be distinguished in the Indonesian region (Fig. 1). (1) Southeastern promontory of the Eurasian plate forming the Sundaland continental craton in Sumatra, West Java and Western Kalimantan; (2) Philippine Sea oceanic plate in the northeast; (3) Australian continental craton extending into the Indonesian region in Irian Jaya and the Arafura and Sahul
Platforms; (4) Indian oceanic plate in the southwest; (5) a transition zone, marking the zone of current plate interaction with active seismicity and volcanism extending through western Sumatra, eastern Java and the Banda arcs to northern Irian Jaya and through Sulawesi and the Moluccas to Mindanao in the Philippines in the central part of the region. In this zone subduction is still active, with the development of thrust, transcurrent and extensional faults. This region is also characterized by allochthonous cohtinental microplates with Tertiary and Mesozoic sediments overlying Palaeozoic basement, juxtaposed against Cretaceous and Tertiary terranes to form collision complexes.
Tectonic development of Indonesia P r e - N e o g e n e tectonics Western Indonesia. A concentric accretionary model for the tectonic evolution of western Indonesia, with the addition of successive orogenic belts to a continental core in Sumatra and Kalimantan from the Palaeozoic through the Mesozoic and Tertiary to the present day, was proposed by Katili (1973) and Gage & Wing (1980). Basaltic to andesitic volcanic rocks and granites dated at 276-298 Ma, and volcaniclastic
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NEOGENE OROGENIC BELTS OF INDONESIA sediments associated with limestones containing a Permian fauna in northern and central Sumatra, have been interpreted as representing a magmatic arc with associated forearc or backarc regions (Katili 1973; Silitonga & Kastowo 1975; Rosidi & Santoso 1976; Cameron et al. 1980; Simandjuntak et al. 1981). A similar assemblage of rocks was described from West Kalimantan (Gage & Wing 1980; Pieters & Supriatna 1990; GRDC 1992), from Sarawak (Tate 1991) and from the Main Range of peninsular Malaysia (Hutchison 1973; Gage & Wing 1980). Pupilli (1973) and Gage & Wing (1980) suggested that these Permian magmatic arc systems were related to an Asia-ward subduction system in Sumatra and an Indian Ocean-ward subduction system in Kalimantan. A similar opposed double subduction system is proposed by Hutchison (1973) and Pupilli (1973) for the Triassic, with the Malaysian-Indonesian Tin Belt representing the magmatic belt related to subduction from the Asian directed subduction system, while the opposing system is represented by the Serian Volcanics in Sarawak. Cretaceous accretion complexes related to the Asian-ward subduction system can be traced through the melange wedges of this age in North and West Sumatra and Bengkulu, in Ciletuh and Karangsambung in Java, the Meratus Mountains of SE Kalimantan and in Bantimala and Wasapondo in South and central Sulawesi (Asikin 1974; Simandjuntak 1980; Sukamto 1986; Wajzer et al. 1991; GRDC 1995). Cretaceous magmatic arcs related to this system are represented by granites, basaltic-andesitic volcanics and volcaniclastics at Gumai and Garba Mountains in South Sumatra, the Meratus Mountains of SE Kalimantan and in SW Sulawesi (Musper 1937; Katili 1973; de Coster 1974; Sukamto 1975; Sikumbang & Heryanto 1986; GRDC 1992). These rocks together with associated sediments may represent forearc, trench slope, volcanic arc and backarc assemblages. The opposing Indian Ocean-ward directed system is represented by volcanic rocks and associated granites in the Anambas, Tambelan and Natuna islands in the South China Sea (Katili 1973; Pupilli 1973), the Schwaner Mountains of West Kalimantan and the Kuching-Sibu zone in Sarawak (Haile 1972). Palaeogene subduction systems can be traced by melanges through the outer arc islands of Nias, Pagai and Sipora, offshore Sumatra and in S. Java (Katili 1973; Karig et al. 1978; Hamilton 1979) and by volcanics and intrusives, the 'Older Andesites' of van Bemmelen (1949) associated with volcaniclastic, volcanic and hemipelagic sediments in Sumatra, Java and W Sulawesi (Katili 1973; de Coster 1974; Djuri & Sudjatmiko 1975; Sukamto 1975; Koesoemadinata et al. 1978; Cameron et al.
187
1980; Sukamto & Simandjuntak 1983), extending into N. Kalimantan, Sabah and Sarawak (Hutchison 1988; Pieters & Supriatna 1990; Tongkul 1991). Palaeogene volcaniclastic, siliciclastic and carbonate sediments on the eastern flanks of the Barisan Mountains in Sumatra and in S. Java, SE Kalimantan and South Sulawesi, followed by shallow marine to terrestrial deposits with economic coals and hydrocarbon source rocks, may represent interarc and backarc sequences related to the Palaeogene subduction systems of western Indonesia (de Coster 1974; Koesoemadinata et al. 1978; Cameron et al. 1980; GRDC 1995). I n d o n e s i a . In the Neogene a number of microcontinents, e.g. Buton, Banggai-Sula, collided with subduction systems along the eastern margin of Sundaland to form the present tectonic framework of eastern Indonesia. The pre-Neogene history of eastern Indonesia relates therefore to the origin and earlier history of these microcontinents before these collisions took place. All the microcontinents are considered originally to have formed part of the northern margin of the Australian continent and to have separated during Mesozoic or Palaeogene time. The history of break up and the subsequent development of these microcontinents is represented by sedimentary sequences on the northern margin of Australia, in northern Irian Jaya, the Arafura and Sahul platforms and within the microcontinents themselves (Dow 1977; Pieters et al. 1983; Pigram & Panggabean 1984; Simandjuntak 1986; Dow et al. 1988; Garrard et al. 1988). Thermal doming, volcanism, rifting and subsidence to form sedimentary basins commenced during the Palaeozoic on the northern margin of the Australian continent, which at that time formed part of Gondwanaland (Veevers 1984), and intensified during the Permian and Triassic (cf. Bird & Cook 1991 from Timor). The exact timing of separation of the microcontinents from Australia is still very much in dispute. Pigram & Panggabean (1984), based on the model of breakup sequences proposed by Falvey & Mutter (1981), suggested that the separation of microcontinents from central Papua New Guinea occurred in the early Jurassic between 141 ° and 145°E longitude. Rifting with the deposition of red beds in Irian Jaya and Papua New Guinea was followed by the deposition of a passive margin sequence which continued into the early Tertiary. It was earlier suggested that the microcontinents were detached from Irian Jaya in the mid-Tertiary and displaced westwards by movements along the Sorong Fault and/or its subsidiary traces (Visser & Hermes 1962; Krause 1965; Katili 1971; Gribi 1973; Hamilton 1979; Silver & Smith 1983). Eastern
188
T.O. SIMANDJUNTAK8z A. J. BARBER
However, the Jurassic passive margin sequence preserved in the microcontinents is overlain paraunconformably by Upper Cretaceous pelagic calcilutites; the Early to mid-Cretaceous was a period of non-deposition in the microcontinental blocks (Pigram et al. 1985; Simandjuntak 1986; Garrard et al. 1988). This depositional hiatus, followed by the abrupt deepening of the margins of the microcontinents in the Late Cretaceous, has been used to argue that the microcontinents were already detached and displaced from the northern margin of Australia by this time (Simandjuntak 1986, 1994; R6hault et al. 1991).
contribution to the development of five orogenic types in Indonesia (Fig. 2). Sunda orogeny
The Late Neogene Sunda orogeny affected the segment of the Indonesian arc between West Java and the islands of Nusa Tenggara as far east as Flores (Fig. 3). In this segment of the arc convergence between the Indian Ocean and SE Asian plates is normal to the subduction trace in the Java Trench with a rate of c. 7 cm a-1. The subduction system comprises an accretionary complex composed of offscraped Indian Ocean floor materials in the Java forearc ridge, a forearc basin developed on extended continental crust and containing late Palaeogene to Recent sediments. The volcanic arc which forms the backbone of Java and forms the islands to the east is constructed on continental crust, in West Java, on Mesozoic accretionary complexes in Central and East Java and on oceanic crust in Sumbawa and Flores. To the north of the arc in Java and in the Java Sea Tertiary backarc basins have developed on continental crust in the Sunda Shelf and on oceanic crust north of Bali and Flores. The basins on the Sunda Shelf formed in the late Palaeogene as rift basins in a terrestrial environment and subsided in the Neogene to be transgressed and covered by marine sediments. In the Late Neogene this system was affected by compression associated with the Sunda orogeny.
Neogene orogenies in Indonesia The present physiography of the Indonesian archipelago can be attributed directly to Neogene orogenic events. These events included plate convergence with subduction beneath the margins of Sundaland to produce a Cordilleran type of orogeny, a unique arc-arc collision in the Moluccas, collision of arcs and microcontinents and major continental blocks with the construction of mountain belts, transpression and transtension along major transcurrent belts, the construction of foreland fold and thrust belts, back-arc thrusting and reversal of subduction polarity. Examples of all of these types of orogenic event can be found among the islands of the Indonesian archipelago. These events will be described in terms of their
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NEOGENE OROGENIC BELTS OF INDONESIA
189
Java) and Pekalongan (Central Java) show that segments of the backarc thrusts are currently active (Kertapati et al. 1972). The causes of the Late Neogene Sunda orogeny, with a change from an extensional to a compressional regime across the subduction system, are not clear. Compression may develop when smooth subduction of the downgoing oceanic plate is interrupted by topographic irregularities on the sea floor. This explanation could apply in the East Java-Bali segment of the subduction system where the Roo Rise is beginning to enter the subduction trench (Fig. 3), but does not explain compression elsewhere in the arc. It may be that the compression is localized along the northern margin of the volcanic arc due to magmatic intrusion and the consequent uplift of the arc.
In N. Java Mio-Pliocene turbidites are deformed into tight, locally isoclinal folds, readily observed in the field and on aerial photographs, while in S. Java-Nusa Tenggara older volcanic sequences are folded, faulted and uplifted to form mountains more than 3500 m above sea level. This phase of folding is associated in time with the intrusion of acid plutons, the uplift of the volcanic arc, the development of a major thrust system in which the volcanic arc is overthrust towards the Java Sea to the north, and the subsidence of the backarc basins, with the deposition of fine siliciclastic sediments, marls and carbonates of Plio-Pleistocene age. In N. Java the trace of a major backthrust, the Barabis-Kendeng Thrust (Simandjuntak 1992) can be traced from the Sunda Strait eastwards across Java and through the Bali Basin into the Flores Thrust, north of Flores (Prasetyo 1988). The thrust may continue eastwards as the Wetar Thrust to the north of Timor. The Barabis-Kendeng Thrust has been imaged in seismic reflection profiles in the northern part of West Java (Supryanto & Ibrahim 1993) and offshore north of Flores (Hamilton 1979; Prasetyo 1988), whilst the Bouguer gravity anomaly pattern in the northern part of East Java indicates the location of the Kendeng Thrust. In Central Java the thrust is cut and disrupted by the Cimandari and Citandui Faults which have wrench components of movement (Dardji et al. 1994). Earthquakes recorded at Majalengka, Brebes (West
Barisan orogeny
The Late Neogene Barisan orogeny affected the segment of the Indonesian arc occupied by the island of Sumatra (Fig. 4). Convergence in this segment of the arc between the Indian Ocean and Southeast Asian plates at a rate of c. 7 cm a-1, is currently markedly oblique (50-65°). The subduction system includes an accretionary complex, exposed in the offshore islands such as Nias (Moore & Karig 1980), and a forearc basin. The volcanic arc is constructed on continental crust and
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Fig. 3. Sunda orogeny in Java and Nusa Tenggara. Line of section illustrated in Fig. 9 is indicated.
190
T . O . SIMANDJUNTAK •
(Koesoemadinata et a/.1978; Koning & Aulia 1985; Wang et al. 1989; Situmorang et al. 1991). Continuation of movements along the fault led to the opening of the Andaman Sea (Curray et al. 1979; Harding 1983). Commencement of the Barisan orogeny was marked by the uplift of the Barisan Mountains and arc volcanism, signalled by the influx of volcaniclastic sediments and regressive sequences in the Sumatran back-arc basin in the mid-Miocene. Uplift was accompanied by intrusions in the volcanic arc and transpressive movements along the Barisan Fault System. Fault movement resulted in the imbrication, duplexing and stacking of crustal slices to form a large-scale positive flower
the volcanoes sit on an uplifted pre-Tertiary basement terrane which forms the Barisan Mountains running the whole length of Sumatra. Both the basement and the volcanic arc are dissected by the Barisan dextral transcurrent fault system. Extensive Tertiary backarc basins lie to the northeast, behind the arc. Sumatra forms part of the Sundaland continental craton. In the Palaeogene the region was affected by extension and subsidence, resulting in the formation of rift basins. Some of these basins, such as the coal-bearing Ombilin Basin of West Sumatra, lie within the Barisan Fault System and have been interpreted as pull-apart basins formed by transtensional movements along the fault
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main map). Line of section illustrated in Fig. 9 is indicated.
NEOGENE OROGENIC BELTS OF INDONESIA
structure, leading to the erosion of the cover and exposure of the basement rocks which now rise to nearly 4000 m above sea level. Uplift of the Barisan Mountains was accompanied by subsidence of the forearc and back-arc basins. Plio-Pleistocene transpressive movements along the Barisan Fault System are considered to have induced fold structures in the sediments of the back-arc basins on axes which trend at an angle of 20 ° to the main fault (de Coster 1974; Hamilton 1979). The folds are commonly associated with small-scale wrench faults and may be related to large-scale wrench faults in the underlying basement (Tiltman 1990). Pleistocene movements along the fault system have, at least locally been transtensional with the opening of pull-apart basins, often holding lakes, such as Laut Tawar, Toba, Singkarak, Diatas, Dibawah, Kerinci (Fig. 4, inset), Ranau or a Recent sedimentary fill as in the Semangko Valley. Recent earthquakes resulting in the cessation of sulphuric hot springs and the disappearance of ponds at Tarutung (North Sumatra), and at Padang Panjang in Central Sumatra, indicate dextral transpressional reactivation along the northern segment of the Barisan Fault System. In Lampung Province, S. Sumatra NW-SE extensional cracks formed during an earthquake near Liwa in early 1994, indicating sinistral transtensional movement along this segment of the fault. The Neogene Barisan orogeny is attributed to variations in the rate of subduction of the Indian Ocean plate and the response of SE Asia to the continuing collision of India with the southern margin of Asia and the adjustment of crustal blocks by movements along major transcurrent faults (Tapponnier et al. 1982). The obliquity of subduction is considered to be responsible for the development of the Barisan Fault and the detachment of the Sumatran forearc to form a 'sliver plate', which is partially coupled to the northward movement of the Indian Ocean plate (Fitch 1972; Curray 1989; McCaffrey 1991). Movements along the Barisan Fault are considered to be responsible for the uplift of the Barisan Mountains and the transtensional and transpressional effects seen along the fault trace. Results of break-out analysis of sediments from boreholes in the Sumatran backarc basins show that the effects of oblique subduction are also being transmitted across the Barisan Fault into the backarc (Mount & Suppe 1992). Talaud o r o g e n y
The Neogene Talaud orogeny of the N. Moluccas provides the only example of an active arc-arc collision in the world. The Molucca Sea is bounded
191
to the west by the Sangihe volcanic arc and to the east by the Halmahera Arc (Fig. 5). Seismicity shows that the Sangihe Arc is underlain by a westdipping Benioff zone extending to a depth of 700 km, while an east-dipping Benioff zone underlies the Halmahera arc to a depth of 200 km (Silver & Moore 1978; Hamilton 1979; Sukamto et al. 1981). The shallow Molucca Sea between the two arcs is considered to be underlain by the collided accretionary complexes of the two opposing arcs uplifted to form the central TalaudMayu Ridge, the underlying Molucca Sea plate having subsided back into the mantle between them. Hamilton (1979), on the basis of seismic reflection profiles, suggested that the volcanic aprons from the forearcs of the Sangihe and Halmahera arcs had been thrust backwards across their corresponding arcs as the result of the collision. Recent Seabeam and seismic reflection data from the northern part of the Molucca Sea indicate that the floor of the sea consists entirely of the Sangihe forearc which has been thrust eastwards over the Halmahera forearc (Rangin et al. 1996). To the south the collision complex is truncated by a major strand of the Sorong Fault System. The Sangihe subduction system may swing round to the west and continue as the Batui Thrust in the East Arm of Sulawesi. Frequent earthquakes show that all these structures are currently active (McCaffrey et al. 1983; Simandjuntak 1989; Kertapati et al. 1972). Sulawesi orogeny
Sulawesi is made up of three structural units: preCretaceous accretionary material in the west upon which later developed a Neogene volcanic arc; Central Sulawesi and part of the SE Arm are composed of metamorphic rocks; the E. Arm and the remainder of the SE Arm are made up of a major ophiolite complex. The metamorphic rocks in Central Sulawesi and in the SE Arm include materials of both continental and oceanic derivation. These rocks are locally affected by high pressure metamorphism forming blueschists, developed in an east dipping Paleogene midoceanic subduction zone. Subduction resulted in the obduction of the East Sulawesi Ophiolite westwards across the metamorphic belt, accompanied by the extrusion of ophiolitic melange (Parkinson 1991). Neogene orogeny in Sulawesi was initiated by the collision of the two microcontinental blocks of Buton-Tukangbesi and Banggai-Sula with the eastern part of the island (Fig. 6). These two microcontinental blocks, having separated from the northern continental margin of Australia, possibly from the region of central New Guinea as has been
192
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previously discussed, were carried westwards along the Sorong transcurrent fault zone by movements of the Philippine Sea plate (Ali & Hall 1995) and collided with the eastern margin of the ophiolite complex. The collision caused the obduction of the ophiolite onto the microcontinental blocks and the shortening and thickening of the ophiolite by
imbrication. The leading edges of the ButonTukangbesi and Banggai-Sula microcontinents were thrust beneath the ophiolite, uplifting the tightly folded, faulted and imbricated ophiolite and its pelagic cover to heights of more than 3000 m above sea-level (Smith 1983; Simandjuntak 1986; Garrard et al. 1988; Fortuin et al. 1990; Davidson
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1991). Also as a result of the collision the metamorphic belt of Central Sulawesi was thrust westwards over West Sulawesi and uplifted to form mountain ranges of nearly 3000 m. Overthrusting resulted in the formation of a foreland fold and thrust belt in Tertiary sediments, the Majene Fold Belt, which continues to develop westwards to the present day, affecting Recent sediments in the Makassar Strait (Coffield et al. 1993; Bergman et al. 1996).
Associated with the collision, or following shortly after, was the development of the NNWSSE trending Palu-Koro sinistral transcurrent fault, along which eastern Sulawesi has been displaced northwards with respect to western Sulawesi. Simandjuntak (1993a) has discussed the effects of transtensional movements along this fault and related displacement of the island of Sumba to its present anomalous position in the Banda forearc. More recent transtensional movements during the
194
T.O. SIMANDJUNTAK t~ A. J. BARBER
Quaternary, continuing to the present time, are responsible for opening pull-apart basins, such as those of Lakes Poso, Matano and Towuti, as well as the Palu depression. Recent earthquakes along the Palu-Koro and related faults show that the system is currently active. Banda orogeny
The Neogene Banda orogeny is due to a continentarc collision where the northern margin of the Australian continent, moving NNE at c. 7 cm a-1 is colliding with the subduction system along the southern side of the Banda Arcs (Fig. 7). The deformation front, marking the southern limit of the collision complex, lies along the 2 km deep Timor trough. Seismic reflection profiling across the trough shows Australian passive margin sediments passing northwards down beneath the trough where, at the deformation front, the upper parts of the sequence have been uplifted on thrust surfaces to form fold ridges in an accretionary complex (Karig et al. 1987). The structure of the collision zone is exposed in
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the island of Timor (Barber et al. 1977; Charlton et al. 1991; Harris 1991). The island is composed of sediments of Permian to Pliocene age, of Australian affinity which have been folded, thrust and imbricated. The Australian sediments have been thrust beneath an ophiolite slab with an underlying metamorphic sole, representing the trace of the Benioff zone prior to collision, but now uplifted to heights of more than 3000 m above sea-level in Timor (Sopaheluwakan 1990). Most of this uplift occurred in the Neogene, as Miocene, Pliocene and Pleistocene forearc sediments resting on the complex show a history of deposition shallowing upwards from bathyal facies through shallow water deposits to Pleistocene coral reefs (de Smet et al. 1990). These facies changes indicate two periods of rapid uplift, one 2 Ma and the other c. 100 000 years ago, representing different stages in the collision. The collision between the Australian margin and the Banda Arc subduction system appears to have commenced in the region that is now East Timor, and the collision is at its most advanced stage in this segment of the arc. Here the forearc between
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NEOGENE OROGENIC BELTS OF INDONESIA the volcanic arc and the deformation front has been reduced to a width of less than 100 km, compared with over 400 km for the forearc east of Sumba where collision has commenced only recently. Continued northward movement of the Australian continent has been accommodated by the uplift of the collision complex in Timor and lateral extension along conjugate faults (Charlton et al. 1991). Compression of the forearc has virtually eliminated the forearc basin which is only 40 km wide to the north of East Timor. Furthermore, shallow seismic activity is absent beneath East Timor suggesting that the downgoing plate and the collision complex have become locked in this segment of the arc. Volcanic activity in the islands of Alor and Wetar to the north of East Timor ceased about 3 Ma (Abbott & Chamalaun 1981) and the volcanic arc now rests on a southwarddipping thrust plane, the Wetar Thrust. The development of a small accretionary complex to the north of the thrust (Breen et al. 1989) suggests that the whole of the forearc and the volcanic arc are being carried northwards with the movement of Australia, and are overthrusting the Banda Sea floor to the north. These may be the first indications of reversal of the polarity of the subduction system, with the Banda Sea floor passing down southwards beneath the northern margin of Australia. Recent deep-seismic reflection profiles to depths of up to 100 km, a short distance to the east of East Tirnor, show reflections representing thrust surfaces dipping both northwards and southwards beneath
the collision complex which has been uplifted as a wedge by the convergence of the Australian and the Banda Sea lithosphere (Snyder et al. 1996). Melanesian orogeny
The Neogene Melanesian orogeny in Irian Jaya and Papua New Guinea is considered to be the result of oblique convergence of the Australian and Philippine Sea (and Caroline) plates (Dow & Sukamto 1984). In the late Palaeogene or very early Neogene, the northern promontory of the Australian Continent collided with an oceanic island arc constructed on the southern margin of the Philippine Sea plate (Hall et al. 1995). The remnants of this arc are now distributed through the northern part of New Guinea (Pigram & Davies 1987), disrupted by transcurrent fault movements to form discrete terranes (Fig. 8). The Central Ranges of New Guinea are composed of a major ophiolite complex, here interpreted as the oceanic lithosphere which underlay the oceanic island arc, subsequently uplifted as the result of the collision with Australia. To the south of the ophiolite the Australian passive margin sediments are deformed by thin-skinned thrust tectonics into a foreland fold and thrust belt, with folding, imbrication and duplexing. At deeper levels seismicity indicates that the underlying Australian continental crust is also involved in the thrusting (Abers & McCaffrey 1988). The whole collision belt is being thrust southwards over the northern
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196
T. O. SIMANDJUNTAK & A. J. BARBER
margin of the Australian continent. The southern margin of the overthrust belt is marked by the Asmat Thrust (Fig. 8). The progress of the Banda and Melanesian orogenies in eastern Indonesia has been controlled by irrregularities in the northern margin of the Australian continent (Charlton 1986). Collision c o m m e n c e d in New Guinea. Using the k n o w n rate of northward m o v e m e n t of the Australian continent, Simandjuntak (1993b) postulated that at the time of the original collision, the subduction zone to the north of New Guinea was continuous with the subduction zone to the south of the Banda Arcs. Continued northward m o v e m e n t of Australia since the collision has displaced the collision zone northwards. Simundjuntak (1993b) also suggested that the western m a r g i n of the N e w G u i n e a promontory is marked by the Waipona Fault which transects the Asmat Thrust. He speculated that the northward m o v e m e n t of New Guinea relative to the Banda Arcs is responsible for the northward curvature of the Banda Arcs near Tanimbar and for the curvature of the Lengguru Fold Belt of the Bird's Head of Irian Jaya. Transcurrent m o v e m e n t along this fault is transpressive near Tanimbar and transtensional in the Aru Trough Following the collision, continued northward m o v e m e n t of Australia has thrust the Palaeogene arc, now attached to the northern margin o f Australia, over the Caroline plate to the north in a m u c h quoted example of polarity reversal (Dewey & Bird 1970; Johnson & Jaques 1980; Cooper & Taylor 1987). Recent geophysical studies in the Manokwari Trough immediately to the north of Irian Jaya, however, show that subduction is no longer in progress there (Milsom et al. 1992). Extensive outcrops of andesitic volcanic rocks of
mid- to late Miocene age throughout northern New Guinea indicate that southward subduction of the Philippine Sea plate beneath Australia occurred at that time. Quaternary and Recent volcanism is largely restricted to Papua New Guinea, and the potassic nature of the volcanic rocks suggests that they are related to extension rather than to subduction (Dow 1977). The northern part of New Guinea is transected by a major E - W strike-slip fault system, the New Guinea Megashear, which continues westwards into the Sorong Fault System of the Moluccas, already mentioned. Some of the records of shallow earthquake activity, especially concentrated in central Irian Jaya, may be related to continued m o v e m e n t on this fault system. Most records, however, show a thrust sense of movement related to compression in the M o m b e r a m a Thrust Belt (Abers & McCaffrey 1988). These thrust movements may be the effects of transpression which may also be responsible for the uplift of the Central Ranges to a height of nearly 6000 m. Transtensional movements in other parts of the fault system may be responsible for the opening up of rifts such as the Markham Valley in Papua New Guinea (Hill & Gleadow 1989). Transcurrent fault movements in northern New Guinea are attributed to the c o n t i n u e d westward m o v e m e n t of the C a r o l i n e / P h i l i p p i n e Sea plate at a rate of 12.5 cm a -l relative to the Australian plate.
Conclusions The whole range of plate tectonic and orogenic processes of subduction, accretion, construction of volcanic arcs, back-arc thrusting, strike-slip faulting, formation of sliver plates, arc-arc,
Fig. 9. Comparative interpretative cross-sections across the Neogene Orogenic Belts of Indonesia, all at the same scale. (a) Sunda orogeny. Convergence between Indian Ocean and Eurasian plates is normal to the subduction trench. The system consists of an accretionary complex, forearc basin, volcanic arc and backarc basin. Compression across the system has resulted in backarc thrusting. (b) Barisan orogeny. Oblique convergence of the Indian Ocean and Eurasian plates. The system is modified by the development of a 'sliver plate' where the forearc is driven northwards along the Barisan strike-slip fault by the movement of the Indian Ocean plate. (c) Talaud orogeny. The Molucca Sea plate is subsiding between the colliding Sangihe and Halmahera forearcs. The Sangihe forearc has been thrust over the Halmahera forearc to form the Talaud Ridge. Subduction of the Sulawesi Sea plate has commenced only recently. (d) Sulawesi orogeny. The Banggai-Sula microcontinent has collided with the ophiolite in the East Arm of Sulawesi. Repercussions of this collision include strike-slip movements along the Palu-Koro Fault and overthrusting in the Makassar Strait. (e) Banda orogeny. The Australian craton has been subducted ~.eneath the accretionary and collision complex in the Timor Ridge. The ridge is composed of accreted Australian continental margin sediments and earlier collided microcontinents. Subduction has ceased in this section of the collision zone, but the collision complex is being driven over the volcanic arc, which is in turn being driven over the Banda Sea plate to the north, in the earliest stages of subduction reversal. (f) Melanesian orogeny. The Australian Craton has been subducted beneath a Palaeogene volcanic arc with the development of a foreland fold and thrust belt from its overlying sediments. Ophiolite and arc volcanics in the central Ranges represent the uplifted hanging wall of the subduction zone. The system has reversed its polarity with the subduction of the Caroline Sea plate but the system is truncated by movement along the Sorong strike-slip fault zone due to the westward movement of the Caroline Sea plate. Data are drawn from references quoted in the text of the paper.
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micro-continental-arc and continental-arc collision, foreland fold and thrust belt formation, obduction and the uplift o f mountain belts m a y be f o u n d currently in progress in the I n d o n e s i a n archipelago. Our present understanding o f the structure o f the N e o g e n e - R e c e n t orogenic belts o f Indonesia is illustrated in a series o f cross-sections in Fig. 9. These present-day e x a m p l e s o f orogenic processes p r o v i d e models for the interpretation o f older orogenic belts such as the A l p i n e - H i m a l a y a n system, the H e r c y n i d e s and the Caledonides.
The authors are indebted to the Director of the Geological Research and Development Centre, Bandung, for permission to publish this paper. H. Faeni and Sudarto of GRDC, and Steve Moss, Simon Baker and Andy McCarthy of the University of London assisted in the preparation of the illustrations. Robert Hall and Derek Blundell invited TOS to attend the meeting on the 'Tectonic Evolution of Southeast Asia' organized at the Geological Society, and arranged sponsorship. Robert Hall, Tim Charlton and Steve Moss made many suggestions for the improvement of the paper.
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1986. Sedimentology and tectonics of the collision complex in the East Arm of Sulawesi, Indonesia. PhD Thesis, University of London. 1989. Tunjaman ganda melahirkan jalur gunung api muda di Minahasa. Suara Pembaruan, 16 June 1989. 1992. Neogene tectonic development of the Indonesian Archipelago. In: IGCP 246 Conference. IGCP-IAGI-GRDC, Bandung. - 1993a. Tectonic origin of Sumba Island. Journal of Geology and Mineral Resources, 22, 10-20. 1993b. Neogene tectonics and orogenesis of Indonesia. Journal of Geology and Mineral Resources, 20, 2-32. 1994. Tectonic development of the Indonesian Archipelago and its beating on the occurrence of hydrocarbons. AAPG Bulletin, 78, 1162. • SURONO & BUHITRISNA, T. 1981. Geological Map of West Central and East Kalimantan Area. Scale 1:1,000,000. Geological Research and Development Centre, Bandung (Open File). SITUMORANG,B., GUNTUR,A., YULIHANTO,B., HIMAWAN, R. & GAMAL JACOB, H. 1991. Structural development of the Ombilin Basin, West Sumatra. Proceedings of the Indonesian Petroleum Association, 20, 1-16. SMITH, R. E. 1983. Sedimentology and tectonics of a Miocene collision complex and overlying Late Neogene orogenic clastic strata, Buton Island, Eastern Indonesia. PhD Thesis, University of California, Santa Cruz. SNYDER, D. B., PRASETYO,H., BLUNDELL,D. J., FhGRAM, C. J., BARBER, A. J., ET AL. 1995. Style of crustal deformation across the Banda Arc continent-arc collision zone as observed on deep seismic reflection profiles. Tectonics, in press. SOPAHELUWAKAN, J 1990. Ophiolite obduction in the Mutis Complex, Timor Eastern Indonesia: an example of inverted isobaric medium/high pressure metamorphism. PhD Thesis, Free University Press, Amsterdam. SUKAMTO, R. 1975. Geological Map of Indonesia Sheet VIII, Ujung Pandang. Scale 1:1,000,000. Geological Survey of Indonesia, Bandung. 1986. Tectonik Sulawesi Selatan dengan acuan khusus, ciri-ciri himpunan batuan daerah Bantimala. Doctoral Thesis, ITB, Bandung. - (~ SIMANDJUNTAK,T. O. 1983. Tectonic relationship between geological provinces of Western Sulawesi, Eastern Sulawesi and Banggai-Sula in the light of sedimentological aspects. Bulletin Geological Research and Development Centre, 7, 1-12. , APANDI, Z., SUPRIATNA, S (~ YASIN, A. 1981. The geology and tectonics of Halmahera Island and surrounding areas. In: BARBER, A. J. & WIRYOSUJONO, S. (eds) The Geology and Tectonics of Eastern Indonesia. Geological Research and Development Centre, Bandung, 2, 349-362. SUPRYANTO (~ IBRAHIM, A. T. M. 1993. Pengembangan kemampuan penambangan lepas pantai, masa depan Indonesia. Proceedings of the Indonesian Association of Geologists, 2, 1162-1174. --
NEOGENE OROGENIC BELTS OF INDONESIA TAPPONNIER, P., PEWTZER,G., LE DAIN, A. Y., ARM1JO,R. t~£ COBBOLD, P. 1982. Propagating extrusion tectonics in Asia: new insights from simple experiments with plasticine. Geology, 10, 611-616. TATE, R. B. 1991. Cross-border correlation of geological formations in Sarawak and Kalimantan. Bulletin of the Geological Society of Malaysia, 28, 63-95. TILTMAN, C. J. 1990. A structural model for North Sumatra. In: Lemigas Scientific Contribution on Petroleum Science and Technology. Special Issue, 24-44. TONGKUL, F. 1991. Tectonic evolution of Sabah, Malaysia. Journal of Southeast Asian Earth Sciences, 6, 395-405. VAN BEMMELEN,R. W. 1949. The Geology of Indonesia. Government Printing Office, The Hague VEEVERS, J. J. 1984. The Phanerozoic Earth History of Australia. Oxford University Press.
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VISSER, W. A. & HERMES,J. J. 1962. Geological results of the exploration for oil in Netherlands New Guinea. Verhandelingen Koninklijk Nederlands Geologisch en Mijnbouwkundig Genootschap, Geologische Serie, 20, 1-265. WAJZER, M. R., BARBER, A. J., HIDAYAT,S. & SUHARSONO 1991. Accretion, collision and strike-slip faulting: the Woyla Group as a key to the tectonic evolution of North Sumatra. Journal of Southeast Asian Earth Sciences, 6, 447--463. WANG, J. E, BUDIJANTO, E M., JOHNSON, M. I. & SIRINGORIN60, R. S. A. 1989. Neogene seismic sequence and structural styles in 'B' and Peusangan Blocks, North Sumatra Basin, Indonesia. Proceedings of the Indonesian Petroleum Association, 18, 339-361.
Palaeomagnetism of the Sibumasu and Indochina blocks: implications for the extrusion tectonic model BRYAN RICHTER
& MICHAEL
FULLER
Department of Geological Sciences, University of California, Santa Barbara CA 93106, USA. email:
[email protected] Abstract: The Jurassic-Cretaceous Kalaw redbeds of Myanmar yield a prefolding Late
Cretaceous to Early Palaeogene magnetization which records c. 25-30 ° of clockwise (CW) rotation relative to the South China block. This corresponds to 10-15 ° CW relative to the Lower Cretaceous Khorat Plateau VGP. The data also show 5 ° of northward transport relative to the 100 Ma South China VGP or 12° relative to the Khorat Lower Cretaceous VGP. Similar CW rotations are measured in remagnetized Palaeozoic carbonates in Peninsular Thailand and Langkawi Island, Malaysia. These block motions most likely took place between the Late Cretaceous and the Late Oligocene. These and other recently published data have several implications for the extrusion tectonic model: (i) Sundaland has only rotated 25-30 ° CW relative to South China during the Tertiary; (ii) southeastward translation is only 300-500 kin; and (iii) Sundaland is composed of smaller sub-blocks, some of which have moved northward. This is interpreted to indicate that deformation of the Sibumasu block is dominated by the oblique subduction of the Indian Ocean Plate while deformation of the Indochina block is dominated by extrusion, in turn driven by convergence between the Indian Craton and Eurasia.
SE Asia has been located at the intersection of the Eurasian, Indian, Australian, Pacific and Philippine Sea plates throughout much of the Cenozoic (e.g. Hamilton 1979). These plate interactions culminated in the Palaeogene to present collision of India with Tibet and the Neogene to present collision of Australia with Sulawesi, Java and Borneo. The boundary conditions set up by the relative motions between these plates have had a profound impact upon the Cenozoic motions of the terranes and micro-plates which comprise SE Asia. Convergence between the Indo-Australia and Eurasian plates continues today and ultimately the terranes of SE Asia will form a broad orogenic belt between these two plates. It has become clear over the last decade that SE Asia has moved with respect to Eurasia during the Cenozoic. Field data which document these motions have been difficult to obtain and often more difficult to interpret. Thus, much of what is postulated about Cenozoic motions is derived from laboratory experiments (e.g. Tapponnier et al. 1986) suggesting that much of the deformation associated with the India-Eurasia collision has been accommodated by rigid plate-wide clockwise (CW) rotation and southeastward translation of the Indochina and Sibumasu blocks out of the collision zone. The available Mesozoic and Cenozoic palaeomagnetic data from the Indochina and Sibumasu blocks qualitatively support the predictions of these
laboratory models. However, ambiguities regarding the tectonic significance of these palaeomagnetic data remain. Many of the tectonic conclusions are based upon several key sampling localities or well preserved chronostratigraphic successions and very little is known about the intervening regions or time periods. With regard to the actual palaeomagnetic analyses, it has proved difficult to discriminate between primary and secondary magnetizations. A review of the literature on the Khorat Plateau, for example, shows the same basic dataset interpreted first as a primary magnetization, reinterpreted as a secondary magnetization, then finally reinterpreted as a primary magnetization (Achache et al. 1983; Chen & Courtillot 1989; Yang & Besse 1993). It is imperative, therefore, to reevaluate existing datasets as new data become available. Despite these difficulties, palaeomagnetism remains one of the few direct records of ancient plate motions available to us. Towards this end we review the available palaeomagnetic data from Sundaland and we present new palaeomagnetic data collected from the Kalaw Redbeds of the Shan Plateau of Myanmar and from Palaeozoic limestones located in Central and Peninsular Thailand. Moreover, we evaluate the tectonic significance of these palaeomagnetic directions. For example, are they caused by local or plate-wide rotations? Are they controlled by structural deformation? Finally, we conclude by evaluating existing tectonic models and propose modifications which make the models
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 203-224.
203
204
B. RICHTER • M. FULLER
more consistent with the measured rotations and translations.
Regional tectonic framework The data presented in this paper have been collected in Thailand and Myanmar from the Tertiary geological 'plate' known as Sundaland (Fig. 1). Sundaland is composed of several Palaeozoic and Lower Mesozoic terranes (Fig. 2). The origin of these blocks and their suturing ages have, among others, been studied by Metcalfe (1988, 1990). Hutchison (1989) provides an excellent summary of additional studies. The new data presented in this study were collected from the elongate N-S trending Sibumasu block (Shan-Thai). In our discussion we compare these data to published data from the Indochina block (Fig. 2). Sibumasu block
The Sibumasu block consists of portions of Myanmar, Thailand, northwest Malaysia, and Sumatra (Fig. 2). It is bounded to the west by the dextral Sagaing Fault, the Andaman Sea and the dextral Sumatran Fault Zone; to the east by the Bentong Raub line, the extension of the BentongRaub line into the Gulf of Thailand, and finally by the Nan-Uttaradit Ophiolite line in northern
30°N
20°N
10 ° N
Thailand; and to the north by the Red River Fault and the Himalayan Syntaxis (Metcalfe 1988). It may have originally been continuous with the Qiangtang or Lhasa blocks but intense deformation makes it difficult to confirm this. There is some agreement that the long axis of the Sibumasu block was originally orientated approximately E-W and was attached to the northern margin of Gondwanaland during the Early Palaeozoic (e.g. Hutchison 1989). It rifted from Gondwanaland in the Middle or Late Palaeozoic and began travelling northward. Collision timing along the Bentong-Raub line in Peninsular Malaysia is widely thought to be marked by the Late Triassic S-type granites of the Main Range Province (e.g. Cobbing et al. 1986). In contrast, Helmcke (1986), Barr & Macdonald (1991), and Barr et al. (1990) have shown that the proposed suture zones in Northern Thailand (Nan River Belt, Phetchabun Fold belt) have a more complicated collisional history which, although it includes the Late Triassic deformation seen elsewhere, began in the Upper or even Middle Palaeozoic. These contrasting collisional ages can be explained in several ways. (a) The Nan Suture may not correlate directly with the Bentong Raub suture and the Sibumasu block may have been broken into northern and southern halves during the Late Palaeozoic; (b) there may have been post-collision lateral translations along these suture zones which have obscured the original collisional relationships; (c) the northern part of the Sibumasu block may have collided with northern Indochina in the Late Palaeozoic but the collision did not go to completion, forming a partially amalgamated superterrane with a small trapped oceanic basin which finally closed in the Late Triassic; or (d) one or more marginal 'back-arc' basins may have formed and then been consumed prior to the final collisional episode in the Late Triassic. The last two hypotheses appear the most likely. Fortunately, almost all researchers agree that the major blocks which comprise the core of SE Asia had assembled and become sutured to South China (Eurasia) by the Late Triassic-Early Jurassic (Hutchison 1989 and references within). Indochina block
90 *
100 °
110 °
120 °
Fig. 1. Regional geography of Southeast Asia. Light circles show location of sites discussed in this study and heavy circles indicate the sites of Yang & Besse (1993).
The Indochina block is bounded to the north by the Red River Fault (Song-Ma and Song-Da suture zone) and to the east by the South China Sea marginal basin (Fig. 2). The southern boundary is poorly documented and it is not clear if the Indochina block has always been continuous with the Borneo block, which has similar Mesozoic igneous sequences, or if there is an E-W trending Late Palaeozoic suture zone near the Natuna and
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Diamictite ~ Carbonate~ Marine ~Continental ]~E-] Chert ~ Congl. [-~T] Volcani- ~ Evaporite rTFTT]Stratigraphic (GlacioShale, Redbeds clastics Break Marine?) Mudstone (S/S +Cong.) Fig. 2. Tectonostratigraphicterrane summaryfor SoutheastAsia showingPalaeozoicterrane boundaries and regional stratigraphiccorrelations.Tertiarystrata in peninsular Thailand and the Shan Plateau of Myanmar are only locallypreserved in small Oligo-Mioceneintermontanebasins. Diagrammodified from Metcalfe(1988).
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206
B. RICHTER & M. FULLER
Mekong basins. The western boundary has traditionally been interpreted to be the NanUttaradit suture zone. This suture is thought to extend southward, beneath the Central Thai Basin, into the Gulf of Thailand and ultimately to the Late Triassic Bentong-Raub line in Malaysia (Metcalfe 1988; Hutchison 1989). In contrast, Barr & Macdonald (1991) conclude that the Nan suture actually marks a Permian suture between the Indochina block and the recently identified Sukhothai terrane. They place the Late Triassic suture zone farther west, along the eastern margin of the Northern Thai granite province. Unfortunately, the Late Palaeogene to Neogene Central Thai basin and the Pattani trough almost completely cover the suture zone from 16°N to 7°N and it is not yet possible to correlate definitively sutures along the length of the Indochina and Sibumasu blocks. Precambrian basement is exposed in the Kontum Massif of Vietnam and presumably forms the basement of the entire Indochina block. It may have originated on Gondwanaland, possibly along the eastern margin of Australia, but must have rifted away by the Early Carboniferous (Metcalfe 1988). Moderately to strongly deformed Palaeozoic sediments are widely distributed but the block is dominated by thick, relatively undeformed Mesozoic continental sediments of the postIndosinian Khorat Basin. Most palaeomagnetic studies in the Khorat Plateau are concentrated in this Mesozoic section.
Previous palaeomagnetic studies Bunopas (1982) examined the palaeomagnetism of a wide variety of rock types in Palaeozoic and Mesozoic strata throughout both the Indochina and Sibumasu terranes using orientated hand samples. His reconnaissance study formed the foundation for additional studies on Neogene basalts in Northern and Central Thailand (Barr et al. 1976; Barr & MacDonald 1979; McCabe et al. 1988); upon Upper Palaeogene to Neogene intermontane basins throughout Thailand (Richter et al. 1993); and upon Upper Palaeozoic and Mesozoic continental strata from the Khorat Plateau (Barr et al. 1978; Achache et al. 1983; Achache & Courtillot 1985; Maranate & Vella 1986; Chen & Courtillot 1989; Yang & Besse 1993). Upper Palaeozoic and Mesozoic o f the Khorat Plateau As noted above, the majority of palaeomagnetic data has been collected from Upper Palaeozoic and Mesozoic sediments of the Khorat Plateau. Many
of these early studies failed to recognize the potential significance of secondary magnetizations and their data are only presented in tectonically corrected coordinates. Yang & Besse (1993) further note that many of the demagnetization procedures may not have been sufficient for discriminating between primary and secondary magnetizations. Despite these concerns, the consistency between these studies is remarkable. Mean values of c. 3040 ° of CW deflection with positive inclinations of 300-40 ° are found in almost all of the Khorat studies. Furthermore, structural dips are low and corrected and uncorrected directions commonly overlap within their respective 0~95 error estimates. The primary concern with these earlier data is distinguishing between primary and secondary magnetizations and then assigning an age to that magnetization. Yang & Besse (1993) present new data from Upper Permian limestones, the Upper Triassic Huai Hin Lat formation, the Lower Jurassic Nam-Phong formation, the Upper Jurassic Sao Khua Formation, and the Lower Cretaceous Khok Kruat Formation. These data are of high enough resolution to allow an approximate chronology of magnetizations and rotations to be reconstructed. Their results are reviewed below and summarized in Fig. 3 and Table 2. The Permian Limestone samples yield a high coercivity CW deflected component which fails a fold test at the 95% level (Fig. 3). Synfolding analysis indicates that kappa is at a maximum at 30% unfolding and thus the magnetization is best interpreted as a synfolding remagnetization. At this locality, Upper Triassic continental sediments unconformably overlie steeply dipping, strongly deformed Permian units. Thus, the Late Triassic Indosinian orogeny is a strong candidate for the folding and initial remagnetization. The Upper Triassic Huai Hin Lat formation also gives a CW deflected direction. Based upon the interpretation of a positive fold test in the overlying Lower Jurassic Nam-Phong Fm they conclude that this is a primary magnetization and they present a tectonically corrected Late Triassic VGP at 52.1 °N, 169.8°E (o~95=7.3) (Table 2). This VGP is indistinguishable from the remagnetized Permian VGP. The Lower Jurassic Nam-Phong formation also yields a CW deflected magnetization. This is found in both positive and reversed polarities and passes a fold test at the 95% level (Table 2).The tectonically corrected VGP for this unit is located at 54.4°N, 175.6°E ((x95 =4.9) and is statistically indistinguishable from the Permian and Late Triassic VGPs. However, Yang & Besse (1993) caution that the folding may have occurred anytime between the mid-late Yanshanian orogeny and the Late Tertiary.
PALAEOMAGNETISM OF THE SIBUMASU BLOCK N
N
90 °
Ju and KI Redbeds Khorat Plateau, Thailand
207
The Aptian-Albian Khok Kruat Formation yields medium and high coercivity components which are quite close to each other (I, D = 37.2 °, 26.0 ° and 35.3 ° , 29.8 ° respectively). Bedding corrections are all less than 12° and the fold test is inconclusive. Despite this, Yang & Besse (1993) conclude that this is a primary Lower Cretaceous VGP. As above, we urge caution in assuming at Early Cretaceous age. The small ~95' the inconclusive fold test, and the lack of reversed polarities leave open the possibility of a younger remagnetization event.
Mesozoic of peninsular Thailand and peninsular Malaysia
N
90°
+ In Situ ++ + + +++++1-90 ° Permian Carbonates Khorat Plateau, Thailand
Fig. 3. Lower hemisphere equal area projections of site mean directions measured in the Khorat Plateau by Yang & Besse (1993). Solid symbols denote positive polarities while open symbols denote negative polarities. Ellipses are the 95% confidence interval about the mean (alpha95) but these have been omitted from the Ju + K1 plot for clarity. See Table 1 for exact values and text for discussion.
We do not follow them in assigning an Early Jurassic age to this pole because the age of the folding is so poorly understood. The interpretation of a positive fold test is reasonable, but without good age constraints on the age of folding we can only say that this is a pre-folding VGP with a maximum age of Early Jurassic. The Upper Jurassic Sao Khua Formation yields a high coercivity CW deflected direction (Table 2 and Fig. 3). This unit is poorly exposed and dips are very low. As such, fold tests are inconclusive. Polarity may be a useful argument in that one would expect to see an even distribution between normal and reversed polarities if the magnetization truly formed in the Late Jurassic. The lack of a conclusive fold test and the preponderance of normal polarities leaves open the possibility that this magnetization was acquired during the Cretaceous long normal period.
The southernmost extent of the CW rotations described above is presently not clear and it is useful to review the available palaeomagnetic data from peninsular Malaysia. The principal work from peninsular Malaysia is by McElhinny et al. (1974). They detected two distinct populations of palaeomagnetic data: (1) the Pengerang rhyolites, Singapore gabbro and dykes, Bentong Group, Singa Formation and Sempah Fm. displayed inclinations of 20-40 ° with CW rotated declinations of 20-33°; and (2) the Kuantan-Massai dykes and the Segamat basalts displayed inclinations of 30-40 ° with counter-clockwise (CCW) rotated declinations of 40 °. Haile & Khoo (1980) detected 20-30 ° of CCW rotation in the Maran, Teka and Kluang redbeds of Malaysia while Schmidtke et al. (1990) confirmed the CCW rotated declinations in mafic dikes at Kuantan and mafic flows at Segamat. McElhinny et al. (1974) argued that either the poles or the field directions of three of their four CW rotated sites passed a fold test. They estimated the age of folding to be Late Triassic and thus their CW rotated magnetization was assigned a preLate Triassic age. In the context of new data and statistical analyses, however, it is not clear that the CW rotation is a pre-folding magnetization and even if it is, it could be as young as Late Cretaceous. There are additional unpublished analyses conducted at the UCSB palaeomagnetism laboratory which bear on the review of Malaysian data. Schmidtke (pers. comm. 1991) measured CCW deflections of similar magnitude (30-45 ° ) in samples collected from the poorly dated Singapore and Lanchang dikes, the Jurassic-Cretaceous Raub redbeds, and the Permo-Triassic Kodiang Limestone. The Bukit Kemaman, Bukit Temiang, Lanchang and Tanjung Penyabong dykes display between 6 ° and 23 ° of CCW declination deflection while sites from the Mesozoic Raub and
9
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7.9
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150.2
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-
-
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-
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8.6
6.0
1.6
-
-
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-
Corr. k
-
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-32.7 -25.8 37.9 -7.5
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15.9 -6.4
8.5
11.7 16.1 4.3 4.3 7.6 17.1 0 10.9 5.2 7.2 7.9 5.5 12.9
Corr. A ° Lat.
E171.2 ° E174.5 ° E173.8 ° E173.0 °
N49.2 ° E172.4 °
N56.7 ° E181.3 ° N54.1 ° E 1 8 2 . 8 ° N55.4 ° E182.1 °
° ° ° °
E122.9 ° E138.5 ° E12.4 ° E27.4 °
N60.5 ° E164.3 °
N 5 2 . 6 ° E207.3 ° N 6 2 . 4 ° E156.3 ° N59.6 ° E187.3 °
N13.4 ° N52.9 ° N58.8 ° N67.7 °
N46.7 ° E 1 9 8 . 4 °
N53.1 ° E 1 8 0 . 6 °
N20.2 N42.7 N37.1 N33.4
N24.8 ° E172.0 ° N49.9 ° E137 °
N40.4 ° E354.9 ° N69.5 ° E164.3 °
N46.4 ° E190.6 °
Corr. VGP
N46.3 ° E 1 6 7 . 2 ° N41.8 ° E161.9 °
N47.3 ° E 3 5 7 . 4 ° N58.7 ° E 1 9 3 . 8 °
N50.6 ° E197.0 °
in situ VGP
values a n d italics indicate the p r e f e r r e d direction b a s e d u p o n interpretation o f fold tests.
n o r t h w a r d t r a n s l a t i o n s as p o s i t i v e a n d s o u t h w a r d t r a n s l a t i o n s as n e g a t i v e ; in s i t u , g e o g r a p h i c a l c o o r d i n a t e s ; C o r r . , s t r u c t u r a l l y c o r r e c t e d c o o r d i n a t e s . B o l d t y p e i n d i c a t e s m e a n
I, i n c l i n a t i o n (°); D, D e c l i n a t i o n (°); ¢~95, t h e 9 5 % c o n f i d e n c e i n t e r v a l a b o u t t h e m e a n d i r e c t i o n ; k, t h e p r e c i s i o n p a r a m e t e r k a p p a ; A L a t , p r e s e n t l a t i t u d e m i n u s p a l a e o l a t i t u d e , w i t h
Limestone VGP mean
33.5 36.1
19.2 17.8
14.9
60.8
Silurian Setul Limestone, Langkawi Island, Malaysia SM-88-62 N6.2 ° E99.8 ° 9 SM-88-63 N6.2 ° E99.8 ° 5 SM88-62, 63 N6.2 ° E99.8 ° 14
14.1 10.3 11.5
75.2 50.5 56.7
Permo-Triassic Lampang Limestone, Central Thailand T91-8 N18°15.75 ' E99°36.92 ' 4 40.3 T91-9 N18°15.75 ' E99°36.92 ' 4 40.7 T91-10 N18°15.75" E99°36.92 ' 7 40.7 T 9 1 - 8 , 9, 10 N18°15.75 ' E99°36.92 ' 15 41.0
3.1 11.4 8.8 18.9
43.2 48.3 27.1 37.2
Limestone, Peninsular Thailand N7°55 ' E99°55 ' 7 N10.95 ° E99.28 ° 9 N9.9 ° E99.1 ° 7 N7°36.43 ' E100°2.04 ' 6
14.9 24.1
9.2 11.9 17.8 8.5 11.1 12.4 17.1 11.9 11.5 8.0 7.1 10.3 8.3 19.1
in situ tx95
38.0 46.3 37.6 22.6
Permian Ratburi T90-10 T90-27 T90-45 T91-39
317.3 32.1
in situ D
Late Palaeozoic Plateau Limestone, Shan Plateau, Myanmar B 9 1 - 1 2 + 14 N20°48.89" E96°44.039 ' 6 17.6 B91-13 N20°40.89 ' E96°44.039 ' 3 26.5
N in situ (samples) I
58.5 24.1 27.2 42.9 38.3 352.9 40.1 54.3 40.4 41.7 48.0 44.8 341.1 39.3
Location
Juro-Cretaceous Kalaw redbeds of Shan Plateau (Shan-Thai) B91-8 N20°48.37" E96°26.98 ' 7 -10.2 B91-10 N20°47.98" E96°27.14 ' 5 13.8 B91-11 N20°47.98 ' E96°27.14" 5 21.9 B91-17 N20°38" E96°34 ' 8 22.8 B91-18 N20°38.95 ' E96°33.02" 6 36.6 B91-19 N20°42.24 ' E96°38.52 ' 7 77.0 B91-20 N20°45 ' E96°32" 8 -0.2 B92-54 N20°47.06 ' E96°27.83 ' 4 -22.1 B92-55 N20°45.40 ' E96°28.78 ' 5 27.9 B92-57 N20°41.75 ' E96°30.16 ' 7 14.3 B92-58 N20°42.24 ' E96°38.52 ' 5 -34.0 B92-60 N20°38.27 ' E96°33.43 ' 6 21,6 B92-81 N20°42.31 ' E96°30.55 ' 7 70.7 Mean N20°42 ' E96°33 ' 80 18.6
Site
T a b l e 1. S u m m a r y o f the n e w p a l e o m a g n e t i c d a t a p r e s e n t e d in this p a p e r oo
to
Formation
N in situ (samples) I
.
. 11.9
ct95
D
. 24.1
in situ
in situ
. 42.0
k
in situ
in situ
13.8
A° Lat.
pre-Khorat Group pre-Khorat Group
<) Devonian 0 Devonian
F o r m a t is the s a m e as Table 1.
OD+OP+OTr OD+OP+OTr
Khorat Group Khorat Group
0 Permian 0 Permian
$ Upper Permian $ Upper Permian
3 3
27 25
9 9
102 13
Palaeozoic of Khorat Plateau (Indochina Block)
73 88
56 61
Huai Hin Lat F m Nam-Phong F m Composite Sao Khua F m Khok Kruat Fm
$ $ $ $ $
Upper Triassic Lower Triassic Tru + J1 Upper Jurassic Lower Cretaceous
69 64
0 J + Tr 0 J + Tr
38.2 38.3
46.2 -39.4
33.4 37.9
39.0 39.4
31.7 25.4 117 38.1 37.2
36.2 37.2
29.7 32.3
39.5 183.6
25.5 27.6
24.8 33.0
36.9 39.0 27.9 27.1 26.0
40.8 32.3
3.1 8.1
11.3 12.5
14.4 15,0
12.6 6.0
13.3 15.3 38.2 3.1 2.5
15.9 7.0
22.0 25.0
120.0 99.0
42.0 68.0
17.6 75.4
33.8 14.0 9.8 247.4 388.5
19.0 120.0
-4.5 -4.5
-10.5 -5.3
-1.2 -4.3
-5.4 -5.7
-0.5 3.3 18.8 -4.8 -4.6
-3.1 -3.8
Mesozoic of Khorat Plateau (Indochina Block) 0 Chen & Courtillot (1989) $ Yang & Besse (1993)
Mesozoic of Western and Peninsular Thailand (Shan-Thai) Mesozoic Barr et al. (1978) 19 . Mesozoic Barretal. (1978) 5 13.8
Age
Table 2. Summary of published palaeomagnetic data
40.2 '~ 31.6
-12.1 8.0
31.4 31.7
31.8 39.4
44.4 40.1 41.8 37.3 40.5
50.0 46.9
38.8 32.8
Corr. Inc.
44.4 43.9
62.0 192.6
60.1 54.7
51.4 33.0
39.5 37.2 38.1 26.6 28.1
43.6 36.4
35.6 34.2
Corr. Dec.
4.3 17.1
11.3 31.6
15.4 18.5
12.6 6.0
8.3 6.6 4.7 2.6 2.4
14.1 9.3
12.0 9.6
Corr. c~95
11.0 6.0
120.0 16.0
36.0 45.0
17.6 75.4
85.0 71.6 77.5 339.2 398.5
23.0 70.0
31.7 11.8
Corr. k
-5.9 -0.1
23.1 21.0
0.0 -0.2
-0.6 -5.7
-9.4 -6.2 -7.4 -4.3 -7.0
-13.8 -11.1
-4.3 -0.3
Corr. A ° Lat.
N61.6 ° E179.1 ° N59.2 ° E179.3 °
N52.1 ° E169.8 ° N83.7 ° E134.9 °
N65.7 ° E186.2 ° N63.6 ° E179.3 °
N66.0 ° E174.9 ° N48.5 ° E176.15 °
N54.7 ° E185.5 ° N52.2 ° E191.4 ° N53.2 ° E189.2 ° not given not given
N51.3 ° E181.9 ° N59.3 ° E180.8 °
-
VGP
in situ
N48.0 ° E177.4 ° N48.1 ° E186.1 °
N24.5 ° E208.2 ° N65.6 ° E251.3 °
N32.8 ° E183.4 ° N37.9 ° E184.2 °
N58.45 ° E176.15 °
N52.1 ° E169.8 ° N54.4 ° E175.6 ° N53.6 ° E173.3 ° N64.8 ° E175.6 ° N62.7 ° E173.3 °
N48.1 ° E165.6 ° N54.8 ° E168.1 °
N56.3 ° E!76.6 ° N57.5° E184.0 °
Corr. VGP
tO
©
~Z
E ©
Z
> © E >
210
B. RICHTER & M. FULLER
Tembeling redbeds reveal a broad spectrum of directions between the CW and CCW rotated end members identified by McElhinny et al. (1974). These data have never been published because it has been difficult to evaluate the age of the magnetization and the influence of structural deformation and present field chemical weathering upon the magnetic signal. These unpublished data are presently being reevaluated. To summarize, the domain of CW rotations observed in Mesozoic rocks of the Shan and Khorat Plateaus only extends as far south as Langkawi Island, Malaysia. South of this the Mesozoic section shows a wide spectrum of CCW and CW rotated directions. The CW end members are very similar to the Shan/Khorat directions while the CCW end members are very similar to directions found in Borneo (Schmidtke et al. 1990). Thus, it appears that peninsular Malaysia is a transition zone between a CW rotating Indochina-Sibumasu block and a CCW rotating Borneo block. Cenozoic of Thailand
McCabe et al. (1988, 1993), building upon earlier work by Barr et al. (1976) and Barr & Macdonald (1979), detected small CW rotations in Neogene basalts from northern and western Thailand but detected no rotation of Neogene basalts located on the Khorat Plateau. These basalts are part of a tholeiitic to alkalic suite of Late Cenozoic basalts that are exposed throughout the Indochina block (Barr & Macdonald 1981). They conclude that volcanic activity began by at least 12 Ma and that there is no systematic relationship between the age and geographical distribution of volcanism. McCabe et al. (1988) initially interpreted the data as a Neogene CW rotation of western Thailand relative to both Eurasia and the Khorat Plateau while McCabe et al. (1993) reinterpreted the data as local block rotations driven by intracontinental deformation. Richter et al. (1993) presented reconnaissance data from several Neogene intermontane basins in Thailand. The Krabi and Nong Ya Pong basins of peninsular Thailand show small CW declinations comparable to those measured in Neogene basalts. In contrast, the reconnaissance data from the Mae Moh basin of north-central Thailand shows predominantly small CCW declinations. Although it is possible that a 15 ° rigid body CW rotation of peninsular Thailand took place during the Middle to Late Neogene it is very unlikely that a 20 ° CCW rotation of north-central Thailand took place concurrently. Thus, local rotations are a strong possibility. In contrast to McCabe et al. (1993), however, we conclude that the dispersion in Neogene palaeo-
magnetic data does not demonstrate that chaotic Neogene local rotations have obscured all Mesozoic motions. There is demonstrable consistency in Mesozoic data collected over a large region while all Neogene data have been collected from small discontinuous exposures which, in many cases, are structurally bounded. Thus, it appears that regional block motions are preserved in the Khorat and Shan Plateaus while data collected from the fold belts which bound these plateaus and from the Central Thai basin are probably complicated by Neogene strike-slip faulting.
Geology of palaeomagnetic sampling sites A complete review of the regional stratigraphy of the Sibumasu and Indochina blocks is beyond the scope of this paper but regional stratigraphic sections are presented in Fig. 2. The reader is referred to Hutchison (1989) for regional summaries; to Bunopas (1982), Snansieng et al. (1983) and Nutalaya et al. (1983) for Thailand; and Chibber (1934) and Bender (1983) for Burma for a more detailed stratigraphic discussion. M e s o z o i c continental s e d i m e n t s o f Shan Plateau, M y a n m a r
The Kalaw redbeds are an extensive sequence of continental sediments which are exposed along the western margin of the Shan Plateau east of the Pan Laung and Shan Boundary Faults (Fig. 4). They lie near the northern end of a linear belt of Upper Mesozoic continental deposits which include the Raub and Tembling redbeds of peninsular Malaysia and the Chumpon and Kanchanaburi redbeds of Thailand (Metcalfe 1988; Bender 1983). Mitchell (1992) indicates that they are coeval with the lower portion of the Khorat Plateau redbed sequence. Diagnostic fossils within the Kalaw sequence itself are lacking and the age is poorly constrained (Win Swe, pers. comm. 1992). The redbeds unconformably overlie the Lower-Middle Jurassic Loi-An (Pan Laung) Series and are unconformably overlain by Neogene to Quaternary non-marine sediments (Bender 1983). Most authors agree that the sequence ranges from Middle or Upper Jurassic to Cretaceous (Bender 1983; Wolfart et al. 1984; Myanmar Oil and Gas Enterprise, unpublished geological map) but it must be emphasized that geological mapping and dating within the Shan Plateau are still at a reconnaissance stage. The Kalaw sequence is moderately to strongly folded about sub-horizontal NNW-SSE trending axes. Folding is commonly thought to have taken place during the Middle to Upper Cretaceous
PALAEOMAGNETISM OF THE SIBUMASU BLOCK
95°E
21ON
--
15'
30'
Mt. Popa Chauk ~ ~ tO
-
_
45'
96°E
15' 20ON
_
_
_
_ _
Meiktilla
~ha
Taunggyi
Q Kalaw
Exolanation
"~ ~k
Q
QuaternaryCover
~/\
~
NeogeneBasalt
"~/~
~
Cretaceousto Lower Paleogene Intrusives
Pz
l/ ~,
Pz
Pz
Pt
)lnle
Jurassic-Cretaceous Kalaw Redbeds 45'
97OE
45'
. _ _ _ _.. , , o
a
_
30'
/ I \
45' 30'
15'
211
PaleozoicCarbonates Mogok Series Gneiss, Marble, Schist~
30'
\
Chaung Magyi Group
fJ
15'
Faults (air photo lineaments) / Roads
19ON
0
20
40
60
80
100 km
Fig. 4. Geological sketch map of the western edge of the Shan Plateau at the latitude of the Kalaw redbeds (modified from Bender 1983). Samples were collected at c. 400 m (0.25 mile) intervals along the Thazi-Kalaw road (dashed line). The dextral Sagaing fault is just off the western boundary of the map. (Wolfartet al. 1984; Win Swe, pers. comm. 1992) but Palaeogene sediments are absent at Kalaw and the age of the folding cannot be directly bracketed by the age of undeformed overlying sediments. Yang & Besse (1993) similarly caution that the folding of Upper Jurassic-Lower Cretaceous units in the Khorat Plateau may have occurred anytime between the mid-late Yanshanian orogeny and the Late Tertiary. There are several pieces of data which point towards a post-Cretaceous regional uplift and folding event. First, MacDonald et al. (1993) show that the Doi Inthanon metamorphic core complex in Northwest Thailand was formed and then uplifted between the Late Cretaceous (72 _ 1 Ma) and the Latest Oligocene epochs (25 _ 4 Ma). Second, Rangin (pers. comm. 1992) reports that Mesozoic continental sediments of the Shan Plateau are thrust over Eocene sediments of the Central Basin of Thailand. Third, thrust-cored anticlines of the
Chauk Oil Field in the forearc basin of Myanmar first formed during the Oligocene, as did the E-W trending cross basin highs which separate the forearc and back-arc basins into smaller sub-basins (Bender 1983). Fourth, a significant Oligocene unconformity is present throughout northern Myanmar (Bender 1983). Finally, the intermontane basins of Thailand as well as the Pattani trough began subsiding rapidly in the Late Oligocene (Polachan et al. 1991). Thus, there are several regional geological events which point to a Tertiary uplift and folding event.
Mesozoic and Palaeozoic carbonates of Thailand Upper Palaeozoic and Lower Mesozoic limestones were sampled throughout peninsular Malaysia, Thailand and Myanmar (Figs 1 and 2). These
212
B. RICHTER & M. FULLER
include the Ordovician to Silurian Setul Limestone exposed on Langkawi Island (Jones 1978), the Upper Palaeozoic Plateau Limestone of the Shan region of Myanmar (Bender 1983), the Lower to Middle Permian Ratburi Limestone exposed throughout peninsular Thailand (Nutalaya et al. 1983; Baird & Bosence 1993), and the Triassic Lampang Group of central Thailand (Nutalaya et al. 1983). The Palaeozoic units represent subtropical to tropical shallow marine shelf carbonates which dominated the Sibumasu block prior to the Indosinian orogeny while the more limited occurrences of Lower Mesozoic rocks record localized shallow marine deposition (Hutchison 1989). These limestones have almost certainly undergone the Late Mesozoic to Palaeogene deformation seen in the Kalaw redbeds but may also have experienced an older Late Palaeozoic or Late Triassic deformation (Jones 1978). Baird & Bosence (1993) conclude that the Ratburi Limestone in northern peninsular Thailand has undergone at least three episodes of diagenetic alteration. They argue that the high temperature isotopic signatures which characterize the final diagenetic event is likely the result of heating caused by the intrusion of Cretaceous granites. Silicification, tensional calcite veins and karstification characterize the Palaeogene evolution of the unit. Thus, the folding recorded in these carbonates could have taken place in either the Late Triassic-Early Jurassic Indosinian orogeny or subsequently during Late Mesozoic-Palaeogene subduction.
Palaeomagnetic sampling and analyses Sample locations are summarized in Table 1. Between six and ten individually orientated cores were drilled at each site using a portable gasoline powered diamond coring drill. These were orientated with a Pomeroy orientating platform and a Brunton compass and no correction for secular variation was required. None of the samples required the use of a sun compass. If exposure permitted, additional sites were collected several tens of metres up or down section. Each site collected during the 1991 and 1992 field seasons was located with a hand held Magellan GPS Navl000 Pro Global Positioning System unit and locations are accurate to _+30 m. Sites collected during the 1990 field season were located with local base maps and location errors can be as large as +_300 m.
Laboratory analyses Palaeomagnetic analyses were conducted at the University of California, Santa Barbara using either a 2-G Enterprises 3-axis cryogenic magnetometer
or a Molyneux Molspin spinner magnetometer. Three methods of demagnetization were employed depending upon the magnetic behaviour of individual samples: (a) alternating field de magnetization (AFD), (b) continuous thermal demagnetization (CTD) (Dunn & Fuller 1984), and (c) stepwise thermal demagnetization.
Statistical procedures Orthogonal vector plots (after Zijderveld 1967) were constructed for each sample in order to characterize its demagnetization behaviour. Principal component analysis was conducted using computer software written by S. L Gillette (© 1985) which utilizes the least squares regression technique of Kirschvink (1980). In addition, remagnetization circles were fitted to samples displaying two component behaviour using the Gillette software and directions were calculated after the method of Halls (1978). In all cases, site mean directions, confidence intervals (t~95), and precision parameters (k), were calculated after the method of Fisher (1953). Folded strata were analyzed using the methods of McElhinny (1964) and McFadden (1990).
New palaeomagnetic data Kalaw redbeds, Myanmar Palaeomagnetic samples were collected from 14 sites located along the Meiktila-Kalaw highway (Fig. 4). The traverse crosses several parallel folds and bedding attitudes vary considerably. Thus, it was possible to conduct a detailed fold test. Samples were collected primarily from medium to fine grained red sandstones which vary from weakly friable to well cemented. Hematite cements are present in all samples while the strongly cemented samples also contain as much as 30- 40% carbonate cement. In the latter case, hematite dust rims completely encircle the detrital quartz grains and appear to predate carbonate cementation. Conglomerates within the sequence were also sampled in the hope of obtaining conglomerate tests but the clasts are dominantly composed of very weakly magnetized limestones which exhibited variable behaviour during demagnetization. Orthogonal vector plots from demagnetization experiments are presented in Fig. 5. The magnetic behaviour of the medium to fine grained units within the Kalaw redbeds is excellent and in almost all cases, independent of grain size or degree of cementation, samples display single component behaviour above 150-250 ° C. The low temperature component is less rotated than the stable endpoint
PALAEOMAGNETISM OF THE SIBUMASUBLOCK
Thermal Demag B92-58-4C
W Up S [~L%I~36dC, , , ~\'%. i \'%458oc I \- 77oc
Thermal Demag B92-57-4B
AF Demag B92-57-4B
N
EDn] I i ~ ) 9 5 " _ [C~24°C
S
213
W Up
WUp :
:
:
:
:
N
19 0 0 Oe,, 1300 Oe
s I
-
: : 4 °C
~! "K"
:
:
N
oc
-,%
E Dn
/
...-.'1 K "~50 Oe
I.O'~,~ ~ , ~ 00e
0n] 1.o . \
",.2.90 oC
1900Oe |
500oC
Fig. 5. Characteristic orthogonal demagnetizationplots for samples collected from the Kalaw Redbeds, Myanmar. Closed symbols represent declination data plotted in the horizontal plane while open symbols represent the projection of inclination data upon the vertical N-S plane. Inset plots show the normalized intensity decay throughout a demagnetization experiment. Demagnetization steps are in degrees celsius or Oersteds and initial intensities are in Gcm3/g. Magnetic behaviour is excellent and after a soft present field component is removed the magnetization decays univectorially to the origin. Samples which did not display this behaviour were not incorporated into the site mean values. and most likely reflects recent chemical weathering in the present field direction. The remaining magnetization is extremely stable to 550°C above which intensity begins to drop off. In all samples a stable direction remains until temperatures of 640°C at which point magnetic behavior becomes erratic. These intensities have not been corrected for measurement at high temperature but the intensity drop at 580°C indicates that a fraction of the magnetic signal resided in magnetite. Thus, a combination of AF and thermal demagnetization was employed to determine qualitatively the relative importance of magnetite and hematite carriers. Sample B92-57-4b (Fig. 5) was first subjected to AF demagnetization in peak fields of 200 mT (2000 Oe) and was subsequently thermally demagnetized to 630°C. Approximately 60% of original intensity was still present after treatment in 200 mT fields, indicating that the majority of the signal resides in a mineral which is very resistant to AF demagnetization. In order to eliminate the possibility of an AF resistant pyrrhotite component, samples from sites B92-54, 55, 57, 58, 60 and 81 were thermally demagnetized in stepwise fashion to 400°C and then AF demagnetized in peak fields of 200 mT. The behaviour of these samples indicates that goethite or pyrrhotite, which are very resistant to AF demagnetization but not to thermal demagnetization, are not present. Hematite, therefore, is the primary magnetic carrier with a small portion of the signal residing in magnetite. There is no change in direction when the samples are AF or thermally demagnetized and thus both magnetite and hematite are interpreted to carry the same magnetization.
Thirteen of the fourteen sites show CW declinations of c. 40 ° with shallow positive inclinations (Fig. 6 and Table 1). The in situ mean inclination and declination are 18.6 ° and 39.3 ° respectively while the structurally corrected values are 23.4 ° and 44.7 °. This gives a corrected VGP at 46.4°N 190.6°E. After application of individual bedding corrections the o~95 decreases from 18.0 to 6.9 while kappa increases from 5.8 to 34.0. Statistical analysis of the significance of this fold test indicates that it is positive at the 99% level using the test of McElhinny (1964) and is positive at the 95% level using the SCOS1 procedure of McFadden (1990). In order to see if the Kalaw redbeds have a magnetic fabric which might have affected the remanence direction, we measured the anisotropy of magnetic susceptibility (AMS) for 16 representative samples. These data show a weakly anisotropic magnetic fabric with a strong clustering of maximum directions parallel to the regional fold axis and a girdle of minimum and intermediate directions perpendicular to the maximum direction (Fig. 7). The remanence direction is located approximately 80-90 ° away from the maximum anisotropy and does not appear to be controlled by it. All of the available data, therefore, indicate that the Kalaw redbeds possess a pre-folding magnetization which has not been altered by subsequent deformation.
Palaeozoic and Mesozoic carbonates A variety of Palaeozoic and Lower Mesozoic limestones also display c. 40 ° of CW deflection.
214
B. RICHTER & M. FULLER N
Kalaw R Shan P l a t e a u , Myanmar /
/
+ +
~
+
+ ~ ~ \ + ~ ~ ~ + + Corrected / ' + + + +++++Jr--~'-~r / Cooroinales y+-
90 °
270°--~-++++++++'~++ +++++++~90 °
Fig. 6. Positive fold test at the 99% level in the Jurassic-Cretaceous Kalaw redbeds of the Shan Plateau of Myanmar. The in situ and structurally corrected directions are 18.6°, 39.3° and 23.4 °, 44.7 °, respectively. Solid symbols denote positive polarities while open symbols denote negative polarities. Ellipses are the 95% confidence interval about the mean (alpha95). See Table 1 for individual site mean directions and fold test statistics.
Orthogonal vector plots for the Ratburi Limestones are presented in Fig. 8 and directional data are presented in Fig. 9 and Table 1. These samples display three types of demagnetization behaviour: (a) univectorial decay to the origin, (b) a strong low temperature overprint followed by univectorial decay of a high temperature component, and (c) multicomponent behaviour in which a high temperature signal cannot be isolated. In most cases the intensity of the signal is swamped by instrument noise before the Curie point is reached.
Nonh
270*
"
90*
AMSMinimum I+
~//
,, ,,MS,,,a=um {;
•
Axis 180 °
Fig. 7. Anisotropy of magnetic susceptibility (AMS) data for the Kalaw redbeds. See Fig. 3 for an explanation of symbols. The magnetic fabric is weakly developed with the maximum susceptibility clustered around the average fold axis for the region. However, the remanence direction does not appear to be controlled by it (see Fig. 6).
All three types of behaviour could be found at the same site in samples collected less than 1 m apart. This can be partially attributed to variable mineralogy (magnetite v. pyrrhotite), variable amounts of veining and recrystallization, and extremely weak initial magnetizations. Samples displaying multicomponent magnetizations have not been used in site mean calculations but there is still substantial dispersion in the mean directions. These sites are distributed between latitudes 6°N to 20°N (Fig. 9) and show a CW direction throughout much of the peninsular Thailand. Fold tests in the Setul Limestone of Langkawi Island, Malaysia, and the Lampang Limestone of north-central Thailand are negative (Table 1 and Fig. 10) and indicate that the magnetism was acquired after the youngest episode of folding (Fig. 11). Although the errors are large it is clear that the in situ poles cluster in a single region while the corrected poles are dispersed. As in the prior two single fold tests, this regional attitude test on the VGP'S indicates that the magnetization was acquired after folding.
Age of magnetizations The magnetization measured in the Palaeozoic and Mesozoic limestones is a secondary chemical remanence which is carried by pyrrhotite and low coercivity magnetite. There are two tectonic events which could have driven this remagnetization: the late Triassic Indosinian orogeny, and a combination of Late Cretaceous magmatic activity and the Late Cretaceous to Palaeogene? uplift event discussed above. The lack of a Mesozoic age progression of magnetizations makes it difficult to discriminate between these two possibilities and
PALAEOMAGNETISM OF THE SIBUMASU BLOCK
215
Thermal Demag T90-45-2 S
W Up : : '~78°C
:
:
:
N
- - ~ 85°C '~,
121 °C
Dn
C
Thermal Demag T90-27-6 W Up
t
o.ok
E Dn
500°C
T90-27-9Thermal Demag w up . . . .
-.
,. i ,
/f
_
~%
tu
23°C .~ o . - ~ - . -
( 10
.~ -'.~ 7 ~
282o(~ / .~ E Dn
500°C Fig. 8. Orthogonal demagnetization plots for continuous thermal demagnetization experiments conducted on samples from the Ratburi Limestones. These samples display three types of demagnetization behaviour: (a) Univectorial decay to the origin; (b) a strong low temperature overprint followed by univectorial decay of a high temperature component; and (c) multicomponent behaviour in which a high temperature signal can not be isolated. Samples displaying Type (a) or Type (b) behaviour were used to calculate site mean values. See Figs 3 and 5 for an explanation of symbols.
Fig. 9. In situ palaeomagnetic directions of Palaeozoic and Mesozoic limestones from Thailand and Malaysia. These show CW declination deflections similar to that seen in the Kalaw redbeds. These directions are not disturbed by the NE-SW trending Ranong strike-slip fault zone (RFZ). This indicates that at least some fault blocks in peninsular Thailand have not rotated with respect to each other. However, these data cannot distinguish between a plate wide rotation or a systematic 'domino' style rotation.
the errors overlap all of the VGPs reported from the Mesozoic of the Khorat Plateau. Thus, we can only confidently say that the pole measured in these limestones is no older than L a t e Triassic and probably no younger than Late Cretaceous. In view of the fact that the mean VGP coincides with the Late Triassic pole (Fig. 11) calculated for the Khorat Plateau by Yang & Besse (1993), a Late Triassic remagnetization is more likely.
216
B. RICHTER & M. FULLER
N N
90°
(a)
N
N +
90° +
Corrected/
+~~+ + ++ +7
90°
(b) Fig. 10. Negative fold tests in (a) Silurian Setul Limestones, Langkawi Island, Malaysia; and (b) the Early Mesozoic Lampang Limestone, North Central Thailand. See Table 1 for numerical data and Fig. 3 for an explanation of symbols.
Conversely, the age of the magnetization found in the Kalaw redbeds is clearly pre-folding. Thus the magnetization could be a primary depositional/ diagenetic magnetization which is as old as the stratigraphic age (Late Jurassic to Cretaceous) or it could be a much younger pre-folding chemical magnetization which is tied to the Late Cretaceous to Palaeogene uplift event described elsewhere in this paper. In order to refine the age of the Kalaw pole we have prepared tectonic reconstructions of South China, Indochina, the Shan Plateau, and the Lhasa and Qiangtang blocks at various Mesozoic time slices relative to the GAD and examined the reconstructed position of the Shah Plateau (Fig. 12). The reconstructions were prepared on an Evans and Sutherland workstation courtesy of G. Ramsayer and I. Norton of Exxon Exploration Company. These were made by directly restoring the palaeolatitude using the mean VGP position and then by empirically rotating the terrane along lines of palaeolatitude until space problems were minimized and 300--400 km of sinistral offset was present along the Red River fault (Table 3). A small range of solutions is valid as long as the 95% confidence ellipse for the VGP encloses the GAD axis. The positions of the South China and North China blocks are taken largely from Enkin et al. (1992) but have been modified by Ian Norton (pers. comm. 1994). The Lhasa and Qiangtang blocks are essentially held fixed to South China but compressional deformation across these blocks has been restored by using plate circuits to reconstruct the position of India at the time of collision and
r-----s ~______~: :COn-6dti~d: : i
/
"
i
:i:
J
180 °
Fig. 11. Virtual geomagnetic poles for the Ratburi, Triassic and Setul limestones of Thailand and Malaysia. This plot indicates a negative regional fold test but the errors are large. The corrected Khorat Tu, Ju, and K1 poles have been reproduced from Yang & Besse (1993). The mean in situ value for this study (open circle) coincides with the Khorat Tu pole. This suggests a Late Triassic remagnetization age for the carbonates measured during this study.
217
PALAEOMAGNETISM OF THE SIBUMASU BLOCK Table 3. Summary of the rotation poles, relative to the GAD, used to reconstruct the Shan and Khorat Plateaus against the reconstructed position of South China block (see Fig. 12)
Time slice (Ma)
Shan Plateau (Shan-Thai Block) Lat.
60 80 120 140 160 180
-2.0 -4.1 -13.1 -18.8 -30.7 -29.2
Long. Angle 99.6 100.5 95.6 93.1 86.9 88.3
43.5 43.8 44.7 46.0 50.6 50.4
Khorat Plateau (Indochina Block) Lat.
Long.
Angle
-17.4 -26.7 -35.4 -40.4 -51.9 -51.6
83.6 80.9 78.8 75.4 70.0 69.3
29.3 31.3 34.2 36.8 45.2 46.0
then by retrodeforming Lhasa and Qiangtang out to the reconstructed northern edge of India. This analysis shows that the Jurassic and Early Cretaceous reconstructions all have serious over-
Restored Khorat Plateau JI-Ke Pole Restored
Shan Plateau
laps or underlaps which are inconsistent with the geological data. The Late Cretaceous and Late Palaeocene reconstructions show the smallest space problems and also show the correct amount of displacement along the Red River Fault. This analysis suggests that the magnetization measured in the Kalaw redbeds is most likely to be Late Cretaceous to Palaeocene? in age.
Discussion of palaeomagnetic data Regional rigid block rotations v. chaotic local rotations As d i s c u s s e d above, U p p e r P a l a e o g e n e and Neogene palaeomagnetic data show a wide variety of CW, unrotated, and occasionally C C W rotated directions. McCabe et al. (1993) argue that the apparently chaotic N e o g e n e declinations in Thailand record local block rotations and that older terrane motions have been obscured. We present an alternative hypothesis. The Neogene rotations
Shan J-K pole I if held fixed to I Indochina _1 "t v~t~
x'--J I 00o
~ . ~ 120 ° E
120 Ma / Early Cretaceous
60 ° N 120 ° E
~60
Ma Late 1I Paleocene |
Fig. 12. Plate reconstructions of South China (Eurasia) relative to the geocentric axial dipole (GAD) at various time slices. For each time slice the VGPs for the Khorat and Shan Plateaus have been restored to the dipole axis and the respective terranes then restored along lines of palaeolatitude until space problems were minimized. The white Shan Plateau represents its position if held fixed to Indochina and then rotated CW relative to South China. The filled Shan Plateau is the position obtained by restoring the Shan VGP directly to the GAD. The white lines are the coastlines of South China and Indochina. Sinistral displacement along the Red River fault can be estimated by comparing these coastlines. See Table 3 for rotation poles and the text for additional discussion.
218
B. RICHTER & M. FULLER
appear to be limited to extensional and transtensional zones in the Central Thai province where strike-slip faults are well documented (Polachan et al.1991). The Central Thai province is bounded on either side by moderately to highly deformed fold belts and then by the relatively undeformed Shan and Khorat plateaus. Although faults have been mapped along the margins of these plateaus, the interiors are remarkably undeformed (Bender 1983; Hutchison 1989). Thus, it appears that these large crustal blocks have only deformed along their margins. Data collected from the marginal fold belts or the transtensional zones between these blocks probably record local rotations driven by transtension and strike-slip motion but data collected from the interior of these blocks probably record large-scale Late Mesozoic and Tertiary terrane motions. Lamb's (1994) discussion of the behaviour of the brittle crust in wide plate boundary zones provides a theoretical basis for this hypothesis. He proposes a simple two layer model of the lithosphere in which the brittle upper crust sits upon ductile lower crust (Fig. 13). The brittle crust consists of rigid blocks of varying sizes which can only deform by rotating and translating. Thus, the fine scale velocity field will be discontinuous with abrupt velocity changes across faults. When viewed at a large scale, these velocity variations are averaged out and match the velocity field of the underlying ductilely deforming layer. Thus, an overall CW rotation and southeastward translation in the lower crust could be recorded in the upper crust as a mixture of rotations. Sundaland is almost certainly broken into smaller crustal blocks than
have previously been recognized but there is ample evidence which indicates that terrane motions can still be measured.
Rotations The VGPs for the Khorat and Shan Plateaus and the APW path for Eurasia-South China (Enkin et al. 1992) are shown in Fig. 14. The mean Kalaw VGP is rotated 47.6 ° CW relative to the GAD, 30.4 ° CW relative to the Upper Cretaceous (100 Ma) Eurasia-South China reference pole, and 15.4 ° CW relative to the Khorat Lower Cretaceous VGP presented by Yang & Besse (1993). A similar CW rotation can be calculated relative to other Cretaceous or Palaeogene segments of the EurasiaSouth China APW path. Thus, we conclude that the Shan Plateau VGP is rotated c. 25-30 ° relative to South China and c. 10-15 ° relative to the Khorat Plateau (Indochina Block). Revisions in the ages or error estimates of the Shan and Khorat VGPs are unlikely to alter this basic conclusion. The palaeomagnetic data do not uniquely indicate when this rotation took place but the geological evidence indicates that it occurred during the Late Cretaceous and Palaeogene.
Latitude shifts Palaeolatitudes have been calculated for all of the data presented in Tables 1 and 2. These data show a clear pattern with the Khorat Plateau travelling slightly southward while the Shan Plateau has travelled northward by 8.5 ° +/- 3 ° relative to the
Rotating and
Fine scale
Large scale
translating
velocity field
velocity field
blocks
/.,,-J.,,,,..,,
I"'-" IP t
J'
"1-
•
1.
,,;.".,.,
""
""
,,
"
,,,'//// ,r.......Koo°,°°o..,,o.
f; :.:r Fig. 13. Velocity field model presented by Lamb (1994) in an attempt to explain brittle deformation in wide plate boundary zones. This model for continental deformation may be able to explain the variety of palaeomagnetic rotations observed in SE Asia. See text for discussion.
PALAEOMAGNETISM OF THE SIBUMASUBLOCK
.ur
219
J,
rat~ ~Khora(Tu(corr.)~l "-~Xo,.2_ ~/
9/
180*
Fig. 14. Composite VGP-APW plot which compares the motions of the South China block with the VGPs for the Indochina (Khorat Plateau), and Sibumasu (Shan Plateau) blocks. The APW path for South China is reproduced from Enkin et al. (1992) and the Khorat VGP positions are reproduced from Yang & Besse (1993). Despite the ambiguity in the age of the Shan Plateau pole (K1 to Tpal?) it is clearly rotated CW relative to the equivalent range of Khorat Plateau and South China positions. Likewise, it is clearly displaced northward. The exact amount of rotation and northward translation is dependent upon the inferred age of the pole.
GAD over the same time period. This corresponds to c. 12° northward motion relative to the Khorat Plateau Lower Cretaceous VGP and c. 5 ° relative to the 100 Ma Eurasia-South China VGE AMS measurements (Fig. 7) show that a strong bedding plane fabric is not present and thus shallow inclinations do not seem to be related to a depositional alignment of platy hematite. The fold axis is almost horizontal and thus shallow inclinations cannot be attributed to an overlooked plunge correction. These observations indicate that the small inclinations do not result from a problem with the palaeomagnetic signal. However, there are some potential structural complications. The Kalaw redbeds are exposed along the very western edge of the Shan Plateau within a zone of faulting that is bounded to the west by the N-S trending Sagaing-Shan Boundary-Pan Laung Fault system and bounded to the east by unnamed NW-SE trending faults (Fig. 4). It is possible that this triangular sliver is partially detached from most of the Shan Plateau. This leaves open the alternative hypotheses that a separate 'Kalaw block' has been tilted or has been translated northward relative to the Shan Plateau. We have collected extensively from Silurian, Devonian and Permian carbonates exposed near Taunggyi (Fig. 4), well away from the Pan Laung Fault. The inclinations from these samples should indicate if there is a separate Kalaw block which has moved northward relative to the rest of the Shah Plateau. Figure 12 shows the positions of the Shah Plateau, relative to the Khorat Plateau, assuming end member models of no relative northward motion and the total palaeomagnetically determined northward motion. As a cautionary note, the
error estimates on the palaeomagnetic data do not take into account external geological sources of error such as poor age control, limited outcrop, the uncertainty in recognizing remagnetizations and other factors. Thus the real errors are likely to be larger than shown. A more realistic reconstruction would restore the Shan Plateau to a position between the end members shown here. There is little doubt that oblique convergence has driven the Burma block almost 500 km northward during the Neogene (Curray et al. 1979). Furthermore, the Indian Ocean plate has been obliquely subducting beneath the western margin of Sundaland since the Late Cretaceous. Depending upon the timing of the CW rotation of Sundaland, this oblique subduction could have driven northward translation of all or part of the Sibumasu block during the Eocene and Oligocene epochs. What is presently unresolved is the size of these blocks and how displacement is distributed. Are the blocks just small slivers within the western edge of the Mergui shelf and Shan Plateau? Are they larger blocks bounded by faults such as the Three Pagodas and Wang Chao (Mae Ping) Faults? Is the entire 'rigid' Sibumasu block moving? Is displacement centralized on a master fault, similar to the present Sagaing fault, or is it distributed as small displacements along many faults? Additional research is needed to choose between these possibilities.
Review of tectonic models Extrusion tectonics
The extrusion tectonic model has evolved from the initial slip-line field theory experiments of Molnar
220
B. RICHTER & M. FULLER
& Tapponnier (1975) and Tapponnier & Molnar (1976) to scaled down mechanical experiments with plasticine (Tapponnier et al. 1982, 1986; see also Jolivet et al. 1990). Peltzer & Tapponnier (1988) added significantly to earlier studies in that they examined the impact of distance between indentor and the margin of the block as well as the impact of vertical and horizontal inhomogeneities upon extrusion. The type, location and timing of structures which developed during the plasticine extrusion experiments are remarkably similar to those mapped in SE Asia. These observations led Tapponnier et al. (1986) to conclude that the Eocene collision and subsequent penetration of India into Eurasia was accommodated primarily by more than 1000 km of eastward expulsion and c. 20 ° of CW rotation of Sundaland along the Red River fault during the Oligocene and Early Miocene. Beginning in the Middle Miocene the deformation front shifted northward and the South China block was likewise extruded eastward and rotated 20 ° CW between the Red River and Altyn Tagh faults. Thus, Sundaland is thought to have undergone a rigid body CW rotation of 40 ° since the Late Palaeogene (Fig. 15). A more detailed review of the extrusion model is presented in Richter (1995).
Time 2
Time 3
(a)
(b)
D e x t r a l m e g a s h e a r tectonics The dextral megashear model was elegantly summarized by England & Molnar (1990) who stated that 'the image of lateral transport on (east-west trending) faults, known also as continental escape, extrusion, or expulsion, is an illusion, and that instead the left-lateral slip on east-striking planes in eastern Tibet is a manifestation of north-striking right-lateral shear'. As this dextral shear evolves, the sinistral faults and the N W - S E trending blocks which they bound must rotate clockwise around local axes (Fig. 15). An important consequence is internal deformation which allows E - W shortening and N - S lengthening. Furthermore, Dewey et al. (1989) conclude that the India-Eurasia collision has been accommodated primarily by lithospheric thickening of Eurasia in a widening east-west trending zone which is bounded on the west by a sinistral megashear extending from Makran to Baikal and to the east by a dextral megashear which extends from Sumatra to the Tanlu Fault system. The tectonic evolution of eastern Tibet, Myanmar and Indochina would, therefore, be dominated by northsouth striking dextral shear and associated CW rotations around local vertical axes with little to no southeastward extrusion.
Extension in Central Thai Basins? ~
(c) Fig. 15. Schematic comparison of (a) the extrusion and (b) megashear model at the beginning, middle and end of motion. Compare this with (e) which attempts to integrate internal deformation into the extrusion model. In this model, Sundaland consists of large fault bounded blocks which rotate CW as a consequence of extrusion. But, as extrusion progresses, the blocks closest to the collision zone are displaced farther northward along N and NW-SE trending strike-slip faults (e.g. the Sagaing, Uttaradit or Three Pagodas faults). As motion continues, antithetic strike-slip faults will likely form and local zones of extension and compression will be created. Blocks caught within sinistral zones may rotate CCW while those caught in dextral zones may rotate CW. As the westernmost block (Shan Plateau) moves northward it may rotate CW, driving rifting in the Central Thai basins.
PALAEOMAGNETISM OF THE SIBUMASU BLOCK Polachan et al. (1991) have proposed a tectonic model for the evolution of the Tertiary basins of Thailand which is based upon extrapolation of the dextral simple shear strain ellipsoid presented by Wilcox et al. (1973) to the regional scale. This is based upon the systematic geometrical relationship between the north-south trending Tertiary basins and the conjugate NW-SE and NE-SW trending strike-slip faults of Thailand. The shapes and spatial distribution of these basins suggest that they formed as lazy-S and lazy-Z basins in direct response to strike-slip faulting. Within this framework the Red River, Mae Ping, Three Pagodas and Sumatra faults, which all trend NW-SE, represent the master dextral faults while the Northern Thailand, Uttaradit, Ranong and Klong Marui faults, which trend NE-SE, represent the conjugate sinistral faults. A genetic relationship between strike-slip faulting and basin formation is also supported by more detailed studies conducted by Remus et al. (1993) who have documented wrench features within the Phetchuban graben. The common theme of all three models is that southeastward extrusion is minimal, CW rotations are driven by dextral shear across plate boundaries, and Sundaland is broken into smaller sub blocks which have rotated in sympathy but which are separated by strike-slip faults. However, a simple dextral megashear model cannot explain northward transport of blocks along the western boundary of Sundaland.
Preferred tectonic model The extrusion model is an elegant way to link many of the geological features of Sundaland but data collected over the last decade indicate that the model must be refined (see Richter 1995). In summary, the new model must: (1) allow for crustal thickening and shortening within the collision zone (e.g. Dewey et al. 1989; Le Pichon et al. 1992; Houseman & England 1993); (2) recognize the importance of dextral shear along the western margin which may drive internal deformation within the extruding block (e.g. Dewey et al. 1989; Polachan et al. 1991; Richter et al. 1993); (3) reconcile palaeomagnetic data which show 20-30 ° of CW rotation of Indochina and Sibumasu relative to South China, rather than the predicted 40-50 ° (Yang & Besse 1993 and this study); (4) reconcile estimates of fault slip on the Red River fault which are far smaller (300-400 km) than the predicted thousands of kilometres (Tapponnier et al. 1990; Scharer et al. 1990; Lacassin et al. 1993; Leloup et a1.1993); (5) integrate basin analysis data which indicate that most or all of the fault motion on the Red River Fault is accommodated by transtension in the
221
Yinggehai basin and is not transmitted into the South China Sea (Qiming & Quanxing 1991; I. O. Norton, pers. comm. 1994); (6) explain Cretaceous and Palaeocene basin rifting ages along the South China shelf which predate extrusion (e.g. Chen & Pei 1993; G. Grabowski, pers. comm. 1994); (7) explain palaeomagnetic data which show CCW rotations in Borneo, Palawan, Sulawesi and the Celebes Sea (Schmitdke et al.1990; Fuller et al. 1991); and (8) account for interaction with the Australian and Philippine Sea plates along the southeastern margin of Sundaland (Richter et al. 1992; Hall et al. 1995). An alternative hypothesis is that Sundaland is composed of smaller sub-blocks which have rotated CW together but which have slid past each other (Fig. 15). Deformation of the Sibumasu block is dominated by the oblique subduction of the Indian Ocean Plate, resulting in CW rotations and regional dextral displacements along N-S and NW-SE trending faults. At this stage, we have not attempted a detailed structural analysis and it is expected that motions between smaller sub-blocks could be both dextral and sinistral. The model proposed by Lamb (1994) provides a good analogy (Fig. 15). The Indochina block, however, is too far behind the plate boundary to feel this motion and it deforms in response to extrusion, which is driven by convergence between the Indian Craton and Eurasia. These two modes of deformation meet in the Central Thai Basin, and could drive extension and uplift of the Doi Inthanon metamorphic core complex (Macdonald et al. 1993), the formation of intermontane basins, and the formation of the Central Thai basin and the Gulf of Thailand (Polachan et al. 1991). Finally, Sundaland is bounded to the southeast by the Australian plate and the Philippine Sea Plate (PSP). We hypothesize that the extruding block cannot simply override these plate boundaries and is partially or completely coupled to them. Thus, extrusion works with northward subduction of Australian oceanic crust and northward motion and CW rotation of the PSP to drive the CCW rotation of Borneo.
Conclusions (1) The Jurassic-Cretaceous Kalaw redbeds of the Shan Plateau of Myanmar yield a mean prefolding magnetization which is rotated CW 47.6 ° relative to the GAD. The age of this magnetization is interpreted to be Late Cretaceous or Early Palaeogene. The exact amount of rotation relative to South China varies with the inferred age but is approximately 25-30 ° CW over the range of possible ages. This rotation most likely took place between the Late Cretaceous and Early Miocene.
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B. RICHTER & M. FULLER
(2) The mean Kalaw VGP is rotated 15.4 ° CW relative to the Khorat Lower Cretaceous VGE When both palaeomagnetic and geological errors are considered, this is consistent with a small CW rotation between the Khorat and Shan Plateaus. Such a rotation may be responsible for extension in the Central Thai basin and uplift and unroofing of the Doi Inthanon metamorphic complex in Northern Thailand. This rotation most likely began in the Middle or Late Oligocene. (3) Neogene and Upper Palaeogene palaeomagnetic data from Thailand show a variety of CW, CCW and unrotated declinations. We hypothesize that Sundaland is composed of large rigid blocks (the Shan and Khorat Plateaus, peninsular Thailand) which are separated by extensional and transtensional zones (Central Thai Basin, Pattani trough, Intermontane basins). The young scattered rotations appear to be limited to these extensional and transtensional zones where a strike-slip component of motion could easily drive local rotations. Within the deformation model proposed by Lamb (1994) the rotations recorded in the intermontane basins and Neogene basalts may record the fine scale velocity field while the rotations in the Shan and Khorat plateaus may record the large-scale velocity field. (4) The palaeolatitude data from the Kalaw redbeds show 8 ° of northward transport relative to the GAD, 12° relative to the Khorat Lower Cretaceous VGP, and 5 ° relative to the 100 Ma South China VGE The exact amount of northward motion varies with the inferred age of the respective VGPs. Part of this latitudinal motion can be explained by the extrusion model but plate reconstructions show that some or all of it must be accommodated by overall dextral motion between the Indochina and Sibumasu blocks or possibly within the western margin of the Sibumasu block. (5) CW rotations have been measured in Palaeozoic carbonates in peninsular Thailand and northern peninsular Malaysia. These were most likely remagnetized during the Late Triassic Indosinian orogeny but may have also been remagnetized during the Cretaceous. These data establish that a domain of CW rotation extends at least as far south as Langkawi Island, Malaysia.
South of this the pattern becomes more complex and mixed CW and CCW declinations are found in the same units. This zone of mixed CW and CCW grades southeastward into Borneo where the dominant sense of rotation is CCW. (6) The palaeomagnetic and geological data from Sundaland suggest several modifications to the extrusion tectonic model. Northern Sundaland has only rotated 25-30 ° CW with respect to South China during the Tertiary. The total southeastward translation of the Indochina block was only 300500 km. The extruding block is composed of smaller rigid blocks (the Shan and Khorat Plateaus, peninsular Thailand) which are separated by extensional and transtensional zones (Central Thai Basin, Pattani trough, Intermontane basins) which have accommodated internal deformation. As such, a regional CW 'domino' rotation is preserved in the large blocks while a variety of mixed rotations is found in the transtensional zones. Deformation of the Sibumasu block resulted from oblique subduction of the Indian Ocean Plate, whereas deformation of the Indochina block was dominated by extrusion driven by convergence between the Indian Craton and Eurasia. The extruding block could not simply override plate boundaries located to the south and east and was partially or completely coupled to them. Thus, extrusion worked with northward subduction of Australian oceanic crust and northward motion and CW rotation of the PSP to drive the CCW rotation of Borneo. This study would have been impossible were it not for the assistance of numerous colleagues throughout Thailand, Myanmar, and Malaysia. We would like specifically to thank S. Bunopas and V. Thitipawarn (DMR, Bangkok); U. T. Myint (MOGE); J. Murphy, T. Acomb, and N. Timble (Amoco Myanmar); C. Hutchison (University of Malaya, Kuala Lumpur); N. Halle (Petronas); A. Rb. Samsudin (University Kebangsan Malaysia); I. Norton, D. Maughn, G. Ramseyer, M. Fitzgerald, D. Leary and G. Grabowski (Exxon). Sample collection was assisted by C. Anderson and S. Cisowski. B. Dunn assisted in data analyses and interpretation. The study was funded by the National Science Foundation as well as by contributions by an industrial consortium consisting of Amoco, Chevron, Exxon and Unocal. Finally, we thank J. Briden and R. Hall for critical reviews which greatly enhanced the quality of the manuscript.
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PALAEOMAGNETISM OF THE SIBUMASU BLOCK Late Cenozoic basalts of Southeast Asia. Geological Society of America Bulletin, 92, 1069-1142. & -1991. Towards a Late Palaeozoic-Early Mesozoic Tectonic Model for Thailand. Journal of Thai Geosciences, 1, 11-22. & Haile, N. S. 1978. Reconnaissance Paleomagnetic Measurements on Triassic and Jurassic Sedimentary rocks from Thailand. Geological Society of Malaysia Bulletin, 10, 53-62. & REYNOLDS,P. 1976. Paleomagnetism and Age of the Lampang Basalt (Northern Thailand) and the Age of the Underlying Pebble Tools. Journal of the Geological Society of Thailand, 2, 1-10. ~, TANTISUKRIT, C., YAOWANOIYOTHIN, W. & MACDONALD, A. 1990. Petrology and Tectonic implications of Upper Palaeozoic volcanic rocks of the Chiang Mai belt, northern Thailand. Journal of Southeast Asian Earth Science, 4, 37-47. BENDER, E 1983. Geology of Burma - Beitrage zur Regionalen Geologie der Erde. Gebruder Borntraeger, Berlin. BUNOPAS, S. 1982. Paleogeographic history of Western Thailand and adjacent parts of Southeast Asia a plate tectonics interpretation. Geological Survey, Paper 5, Department of Mineral Resources, Thailand. CHEN, S. & PEI, C. 1993. Geology and geochemistry of source rocks of the Eastern Pearl River Mouth Basin, South China Sea. Journal of Southeast Asian Earth Sciences, 8, 1-4, 393-406. CHEN, Y. & COURTILLOT, V. 1989. Widespread Cenozoic(?) remagnetization in Thailand and its implications for the India-Asia collision. Earth and Planetary Science Letters, 93, 113-122. CHIBBER, H. 1934. The Geology of Burma. MacMillan, London. fOBBING, E., MALLICK,I., PITFIELD, P. & TEOH, L. 1986. The Granites of the Southeast Asian Tin Belt. Journal of the Geological Society of London, 143, 537-550. CURRA¥, J. R. et al. 1979. Tectonics of the Andaman Sea and Burma. In: WATK~S, J., MOrCrADERT,L., & DICKERSON,R W.(eds) Geological and Geophysical Investigations of Continental Margins. AAPG Memoir 29, 189-198. DEWEY, J., CANDE, S. & PrrMAN, W. 1989. Tectonic Evolution of the India/Eurasia Collision Zone. Eclogae Geologica Helvitica, 82, 717-734. DUNS, R. & FULLER,M. 1984. Thermal demagnetization with measurement at high temperature (abs). LOS, Transactions of the American Geophysical Union, 65, 863. ENGLAND,P. & MOLNAR,P. 1990. Right lateral shear and rotation as the explanation for strike-slip faulting in eastern Tibet. Nature, 344, 140-142. ENKIN, R., YANG, ZHENYU,CHEN, Y. & COURTILLOT,V. 1992, Paleomagnetic Constraints on the Geodynamic History of the Major Blocks of China From the Permian to the Present. Journal of Geophysical Research, 97, 13 953-13 989. FISHER, R. A. 1953. Dispersion on a sphere. Proceedings Royal Society of London, 217, 295-305. FULLER,M., HASTON,R., LIN, J., RiCHTER,B., SCHMIDTKE, E. & ALMASCO,J. 1991. Tertiary Paleomagnetism -
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Timing of the Shan-Thai-lndochina collision: new evidence from the Pak Lay Foldbelt of the Lao PDR R O B E R T B. STOKES 1, 4, PAUL F. LOVATT S M I T H 2 & KO S O U M P H O N P H A K D Y 3
1 Surbiton Geological Services, 14 Dennan Road, Surbiton KT6 7RY, UK e Monument Resources (Overseas) Limited, 80 Petty France, London SW1H 9EX, UK 3 Department of Geology and Mines, Vientiane, Lao PDR 4 School of Geological Sciences, Kingston University, Penrhyn Road, Kingston upon Thames KT1 2EE, UK Abstract: The Pak Lay Foldbelt in the northwest of the Lao PDR is a product of the Indosinian orogeny, resulting from the collision of Shan-Thai and Indochina along a suture now marked by the Nan-Uttaradit ophiolite zone. Current hypotheses place the timing of this collision in either the Permian or the Triassic. Recent field work and subsequent laboratory analyses suggest that, in the area to the south of Pak Lay: pre-collision sediments are as young as Middle to Upper Jurassic; arc-related volcanism continued from the Triassic into the Late Jurassic; all these sediments and volcanic rocks form an imbricate zone; and the Cretaceous Khorat Group rests with marked unconformity above the imbricate wedge. From this we conclude that the Shan-ThaiIndochina suturing occurred in the Late Jurassic.
This paper presents a reinterpretation of the stratigraphy, structure and igneous activity in the southern part of Sainabouli Province in the northwest of the Lao PDR. It results from fieldwork, sampling and subsequent analyses undertaken by Monument Resources (Overseas) Limited and the Shlapak Development Company in the hydrocarbon exploration of their Contract Area. A total of six weeks of fieldwork was carried out during the months of November and December in 1992 and 1993 making a reconnaissance geological survey of the Contract Area from the Muang Kenthao region in the south to the Sainabouli Basin in the north. The area of the present study lies to the west of the Nam Khong (Mekong) River and to the south of Muang Pak Lay.
Current interpretations
Tectonic setting Workman (1972) described the Pak Lay region as the exposed part of a NNE-trending Indosinian linear foldbelt developed along the western margin of the Indosinia (Annamia) cratonic block. The folding was attributed to an early phase (approximately Late Triassic) of the Pacific Orogeny (Workman & Page 1967). In plate tectonic reconstructions, continental SE Asia has been shown to be a composite of allochthonous terranes which accreted to one another during the Palaeozoic and Mesozoic (Metcalfe 1988). There is general agreement that
they had all sutured to each other by the Late Triassic. Of these major terranes, only the ShanThai Block (also called Sibumasu) and the Indochina Block (also called Indosinia) are directly related to the Pak Lay Foldbelt. According to Cooper et al. (1989), NE-dipping subduction was initiated in the Permian beneath Indochina in addition to a possible southwest-dipping subduction beneath Shan-Thai. The two plates collided in the Early Triassic creating (inter alia) the Pak Lay-Petchabun Foldbelt. Extensional collapse of the Indosinian orogen during the Triassic allowed continental extensional basins to develop in northeast Thailand and adjacent parts of Lao, and then thermal subsidence, during the Jurassic and Cretaceous, allowed the red-bed Khorat Megasequence to be deposited over much of northeast Thailand and the Lao PDR. The suture between the Shan-Thai and Indochina blocks is generally accepted as being marked by the Nan-Uttaradit ophiolitic zone (Hutchison 1989, Barr & Macdonald 1991). This narrow zone extends northeast from Uttaradit along the Nan River (Fig. 1). It is believed to extend through the Lao PDR, past Luang Prabang, and into Vietnam as the Lay Chau Fault. Its location within Lao is uncertain.
Geological history of the Pak Lay Foldbelt The first geological interpretation of the Pak Lay area (Fig. 2) was produced from fieldwork by Bourret (1925) who summarized the stratigraphy,
FromHall, R. & Blundell, D. (eds), 1996, TectonicEvolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 225-232.
225
226
R . B . STOKES ET AL.
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o
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n
- ~ Himalayan Folding
Fig. 1. Regional geological summary of the Monument/SDC Contract Area of the Lao PDR showing the area covered by this study.
PAK LAY FOLDBELT, LAO PDR
,, 18o15,N .......................... ........
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Granite Triassic Phalat Formation in East
• (~) Locality mentioned in text
20km I
Fig. 2. Simplified map of the geology of the Pak Lay Foldbelt including locations mentioned in the text.
228
R.B. STOKES ET
elucidated from superposition, as follows from base to top: (1) uncrushed crystalline basement; (2) terrain ancien: pre-Uralian shales and sandstones, Uralian limestones and cherts, very highly folded on N-S trends; (3) andesitic volcanic rocks: no trace of crushing in the border region where they are unconformable on older strata; (4) terrain rouge: gently dipping in the border mountain region; (5) gr6s sup6fieurs: horizontal or hardly dipping, except on the Nam Lay. Bourret's reasoning for this stratigraphic history can be summarized as follows. (i) The age of the granites is Permian or Early Triassic because they are post-tectonic and therefore younger than the intense folding which affected the region in preTriassic times. The granites pre-date the Mid Triassic 'terrain rouge' even though no clasts of granite could be found in the red-beds. (ii) The oldest sediments (the 'terrain ancien') are highly folded, often vertical, sandy shales and fine sandstones with calcareous shales, cut by numerous quartz veins. In the southern part of the province they occur only in the deepest parts of the valleys, because the higher parts are all formed by 'roches vertes'. The strike is always N-S and dips are usually very steep and sometimes vertical. In the area to the east of the Mekong (and to the west of Chiang Khan) these sediments are not metamorphosed, but elsewhere they are. Bourret dates these sediments as pre-Uralian and probably mainly Devonian. (iii) A mass of andesitic rocks forms an elevated plateau, between the Nam Sang
AL.
and Pak Leng, which is cut by deep W-E fiver valleys on the steep eastern scarp. According to Bourret (p. 86) 'we can observe everywhere in these valleys the superposition of the roches vertes on the 'ancient' shales'. Bourret found no evidence of bedding in the andesites. On the western margins of the plateau, however, stratified tufts showed moderate dips. (iv) The andesites are younger than the Uralian because they have locally 'digested' fusulinid limestones, and they are older than Triassic because pebbles of the andesites are included in breccias and conglomerates of the 'terrain rouge'. They can only be, therefore, either Permian or Lower Triassic. Bourret states that they are horizontally bedded in general. Workman & Page (1967; repeated in Page & Workman 1968) carried out a photogeological study following a field traverse of the area and established a stratigraphy which was very similar to that of Bourret. Their summary is reproduced here with ages taken from their text. (1) Deposition of Palaeozoic sediments (Upper Carboniferous to Permian argillites; thin beds of shale and siltstone, with occasional fine-grained sandstones. The shales are generally black or dark grey, often studded with crystals of pyrite). (2) Cimmerian 1 folding (approximately Upper Triassic). (3) Intrusion of granitic rocks. (4) Triassic volcanic episode. (5) Deposition of Khorat Group (Triassic to Jurassic). (6) Gentle epeirogenic folding and large-scale faulting. The end of the older Cimmerian 1 folding was
Ban Nam Pou Nong Phuk Heo Ban Kok Ke I , Nam Hoi [ ] I
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LEGEND Gres superieurs Jurassic Terrain rouge Triassic volcanics Triassic Granite intrusion Palaeozoic sediments
Fig. 3. Cross-section interpretation of the Pak Lay Foldbelt taken from Bourret (1925) and modified from information in Fontaine & Workman (1978).
J
PAK LAY FOLDBELT, LAO PDR marked by post-tectonic granodiorite intrusion, after which erosion and planation took place before the Triassic volcanic episode and the deposition of the Khorat group. In his review of 1972 (repeated in Fontaine & Workman 1978), Workman added (inter alia) the following points to the geology of the Pak Lay area. (1) The intrusive rocks, mostly with compositions in the range of granodiorite to tonalite, were emplaced in an early phase of the Indosinian orogeny, probably in the Early Triassic. (2) The thick volcanic formation in the Triassic at the base of the continental Mesozoic series conformably underlies the gently dipping red beds of the Upper Triassic to Jurassic. The group consists of andesite and trachyte lava flows interbedded with siliceous and feldspathic crystal-tufts and agglomerates. These are approximately Middle Triassic in age. The currently-accepted history of this foldbelt is that presented by Fontaine & Workman (1978) which can be deduced from the cross section in Fig. 3 and summarized as follows: (1) deposition of Palaeozoic sediments; (2) Indosinian orogeny (Early or Middle Triassic) resulting in isoclinal folding of the Upper Palaeozoic formations along N-S axes; (3) intrusion of granitic rocks (probably Early Triassic); (4) period of erosion and planation; (5) Triassic volcanic episode (approximately Middle Triassic); (6) deposition of the continental Khorat Group (Late Triassic to Jurassic) conformably overlying the thick volcanic formation; (7) gentle epeirogenic folding and large-scale faulting. This view of the history of the foldbelt is largely responsible for the acceptance of a Triassic age for the Indosinian orogeny and the related collision of Shan-Thai to Indochina.
New evidence from the Pak Lay Foldbelt
229
based essentially on foraminifera, come from the localities numbered 1-4 on Fig. 2. Their ages and lithologies are as follows: (1) Upper Murgabian to Lower Midian fossiliferous packstone; (2) Midian (or possibly higher i.e. Dzhulfian) fossiliferous boundstone; (3). Midian to Dzhulfian fossiliferous packstone and boundstone at Ban Buamlao; (4) yielded a wagenophyllid coral of probable Murgabian age. The remainder of the sedimentary rock samples are almost universally barren of fossils, but one exception is critical to our dating of the clastic sequence. Thinly-bedded, grey to dark grey claystones at Locality 5 have yielded a lean, moderately well preserved, terrestrially derived palynomorph assemblage most characteristic of a Middle to Upper Jurassic age. Taxa present include Classopollis spp., Calliatisporites dampierri, Cerebropollenites mesozoicus, Uvaesporites sp. and P. elatioides, along with some rare, reworked striate bisaccate forms of Permian to Triassic character.
Radiometric ages of the volcanic rocks Volcanic rocks have long been known from Permian and Triassic sequences in the Pak Lay and Loei-Petchabun foldbelts. Our data show that this vulcanicity continued into the Late Jurassic with the youngest age being obtained from volcanic rocks in the far west of the area. The field relationships of the Bajocian basalt from Locality 8 are unknown, but the Kimmeridgian alkali basalt from Locality 9 is part of a volcanoclastic sequence which dips 50°E with a strike of 175 °, conforming to the regional structure seen in the sedimentary sequences, and thus suggesting that it is extrusive. Details of the dates obtained are summarized in Table 1.
Biostratigraphy of the sediments Our samples of the 'Uralian' limestones in the Pak Lay region gave only Permian ages within the range Murghabian to Dorashamian. These ages,
Radiometric ages of the granites The Triassic age of the widespread granites in the Pak Lay and Loei Foldbelts is confirmed by our
Table 1. Ages of volcanic and plutonic igneous rocksfrom the Pak Lay Foldbelt determinedfrom whole-rock K-Ar analyses (localities indicatedon Fig. 2) Locality number 6 7 8 9 10 11
Lithology volcaniclastic lithic wacke crystal tuff basalt alkali basalt granite granite
Radiometric date ( M a )
Chronostratigraphic Age
210.4 _ 5.2 226.7 _+5.4 167.2 +_5.2 152.4 _+6.3 213.8 + 8.9 117.0 _+3.0
Late Triassic (Norian) Late Triassic (Carnian) Middle Jurassic (Bajocian) Late Jurassic (Kimmeridgian) Late Triassic (Norian) Mid-Cretaceous (Aptian)
230
R.B. STOKES ET AL.
dating of the Ban Pakthoun granite (Locality 10) which intrudes the (?)Upper Permian Phalat Formation in the east of the area. The dating of the Ban Samxong granite (Locality 11), previously regarded as Triassic, was made to test the Jurassic age for part (Locality 5) of the stratified sequence which it intrudes. The mid-Cretaceous age of the granite supports the biostratigraphy of the argillites. Details of the dates obtained are given in Table 1. Structure Relationship of the volcanic rocks to the Indosinian unconformity. The present authors have been unable to find any evidence in support of Bourret's conclusions, followed by all subsequent workers, that the volcanic rocks are unconformable above the Palaeozoic sediments, that they are conformable beneath the Khorat Group, and that they post-date the major episode of folding. On the contrary, both theoretical considerations and field evidence lead to the conclusion that the volcanic activity preceded the Indosinian orogenic event. The widespread basaltic to andesitic volcanic rocks have been interpreted, in their southern extension within the Petchabun belt in east central Thailand, to have formed above an east-dipping subduction zone (Bunopas & Vella 1983). This volcanism would, therefore, have pre-dated the collision event. Bourret's assertion that the volcanic rocks occupy the high ground and the sediments the low ground (and, therefore, that the volcanic rocks are unconformable above the folded sediments) is
demonstrably untrue. For example, the road from Ban Kengsao (on the Mekong) to Ban Bouamlao (in the Nam Gnang valley) cuts through the major NNE-trending ridge to the southwest of Pak Lay. The crest of this ridge exposes a thick sequence of claystones and fine to medium grained tufts. All the rocks dip steeply to the east. Passing westwards down from the ridge to the broad alluvial valley of the Nam Gnang, volcanic rocks predominate in the scattered outcrops. Imbrication in the pre-Khorat sequence. Bourret's assertion that the structure of this region is simple seems false. Mapping the distribution of the volcanic rocks is particularly difficult, as Whittle (1970) pointed out. With few exceptions, dips in the pre-Khorat rocks are relatively steep. The intimate association of the volcanic rocks, claystones and Permian limestones, and their apparent conformity suggest that they have been strongly tectonized and show either isoclinal folding or that the Pak Lay Foldbelt is a zone of structural imbrication which has juxtaposed sediments and volcanic rocks of varying ages. On the ground it was found that there were numerous unpredictable changes from argillites to volcanic rocks and back along our traverses. Where bedding is seen within the volcanic rocks, it is steeply dipping and of the same order of magnitude and orientation as is seen in nearby sediments. At Locality 12, volcanic tufts strike 036 ° and dip 75 ° to the SE; oil-stained siltstones and claystones roughly 1 km to the SSE at Locality 13 strike at 014 ° and dip at 76 ° to the
NW
SE
LEGEND
Imbricate zone of Late Palaeozoic to Late Jurassic sediments and Triassic to Late Jurassic volcanics
Granite and its MetamorphicAureole Khorat Group (Cretaceous) Late Jurassic Sediments Triassic to Late Jurassic Volcanics .
.
.
.
.
.
Mid Permian Limestones
Fig. 4. Diagrammatic cross-section showing the revised interpretation of the Pak Lay Foldbelt.
PAK LAY FOLDBELT, LAO PDR ESE. Due to a lack of dating of most exposures, imbricate thrusts can only be demonstrated where Permian limestones appear to overlie Triassic or Jurassic volcanic rocks in the northwest of the study area. Pre-Khorat
Group
(lndosinian)
unconformity.
There is clearly a major angular unconformity between the Upper Jurassic argillites and volcanic rocks, and the overlying Cretaceous Khorat Group. At Locality 5, the former dip 50°E with a strike of 008 °. The Khorat Group shows a consistent regional strike (140°-160 °) and gentle dip (c. 20°SW) in the area of Locality 14.
History of the Pak Lay Foldbelt Whilst individually none of the evidence above is unequivocal, these obervations together lead to the following conclusions concerning the geological history of the Pak Lay Foldbelt. The western part of the Pak Lay Foldbelt consists of (?)Upper Carboniferous to Upper Jurassic sediments and associated Triassic to Upper Jurassic lavas, agglomerates and tufts. The Triassic to Jurassic volcanic activity is interpreted as arcrelated and to have occurred during a period of subduction which preceded the collision event. The latter can, therefore, be no older than Late Jurassic. The Upper Palaeozoic to Jurassic sequence of the western zone was deformed by a compressional event at the end of the Jurassic which produced an imbricate, accretionary wedge with steeply dipping strata striking NNE. These structures result from the collision of the Shan-Thai and Indochina blocks during the Indosinian orogeny. Following a period of considerable erosion, the Cretaceous Khorat Group was deposited unconformably, overstepping the truncated strata of Carboniferous to Jurassic age. A major granite was intruded later in the mid-Cretaceous, probably associated with the basin inversion in the Khorat region before the deposition of the Upper Cretaceous Phon Hong Group (Lovatt Smith et al. 1996). Tilting to the SW and subsequent erosion of the uplifted Paklay-Petchabun Foldbelt has stripped the Cretaceous from much of the area revealing the structurally complex foldbelt. Our schematic reinterpretation of the line of section given by Bourret (1925) is presented as Fig. 4.
231
Discussion The results presented above relate to the area west of the Nam Khong (Mekong) river. The area to the east of the Mekong consists essentially of lesstightly folded Palaeozoic strata which were intruded by granites in the Late Triassic. The line of separation between these two major tectonostratigraphic terranes is difficult to fix. It was regarded as the Kenthao-Saiapoun fault zone by Workman & Page (1967). We have drawn it somewhat arbitrarily along a number of faults along the eastern margin of the C-J unit on Fig. 2. The relationship of these terranes with each other is obscure. The Late Triassic deformation and plutonic activity in the east represent an earlier event in the history of the western margin of the Indochina Block. The revised dating of the Ban Samxong granite resolves two anomalies noted by earlier workers. Firstly it explains why, contrary to his expectations, Bourret found no pebbles of the granite in the basal conglomerates of the Khorat Group. This is now to be expected, since the granite is of a later age. Secondly, the Cretaceous age explains why Workman & Page (1967) found it to be 'unlike any of the other intrusive rocks of the area .... ', since all the other granites which they examined are of Triassic age. Cooper et al. (1989) suggested that the Khorat Group was deposited during a phase of thermal subsidence following continental collision and extension during the Late Triassic. However, as Racey et al. (1996) point out, since the majority (if not the totality) of the Khorat Group is Cretaceous in age, a different mechanism for basin formation must be invoked. Our results suggest that the continental collision was in the Late Jurassic and that the thermal subsidence model is still acceptable. We thank the Directors of Monument Resources (Overseas) Limited, the Shlapak Development Company and the Department of Geology and Mines (Vientiane) for permission to publish this work. We are particularly pleased to thank the numerous villagers who gave help and hospitality during the field work. The paper has benefited from reviews by Charlie Bristow (Birkbeck College, London) and Ian Metcalfe (University of New England). All micropalaeontology, palynology and radiometric (K-As) datings were carried out by the Geochem Group, Chester.
References BARR, S. M. & MACDONALD,A. S. 1991. Toward a late Palaeozoic-early Mesozoic tectonic model for Thailand. Journal of Thai Geosciences, 1,
11-22. BOURRET, R. 1925. Etudes geologiques dans la region
de Pak-Lay (Moyen Laos). Bulletin du Service GEologique de l'Indochine, 24. BUNOPAS, S. & VELLA, P. 1983. Tectonic and geologic evolution of Thailand. In: Ntra'ALAYA, P. (ed.) Proceedings of a workshop on stratigraphic
232
R.B. STOKES ET AL.
correlation of Thailand and Malaysia, Bangkok. 1, 307-322. COOPER, M. A., HERBERT, R. & HILL, G. S. 1989. The Structural Evolution of Triassic Intermontane Basins in Northeastern Thailand. In: THANASUTmPITAK, T. & OUNCHANUM, P. (eds) Proceedings of the International Symposium on Intermontane Basins: Geology & Resources, Chiang Mai. 231-242. FONTAINE, H. & WORKMAN, D. R. 1978. Review of the Geology and Mineral Resources of Kampuchea, Laos and Vietnam. In: NUTALAVA, P. (ed.) Proceedings of the Third Regional Conference on Geology and Mineral Resources of Southeast Asia, Bangkok November 14-18, 1978. Asian Institute of Technology, Bangkok, 539-603. HUTCHISON, C. S. 1989. Geological Evolution of SouthEast Asia. Oxford University Press. LOVATT SMITH, P. E, STOKES, R. B., BRISTOW, C. & CARTER, A. 1996. Mid-Cretaceous inversion in the northern Khorat Plateau of Lao PDR and Thailand. This volume. METCALFE, I. 1988. Origin and assembly of south-east Asian continental terranes. In: AUDLEY-CHARLES, M. G. & HALLAM, A. (eds) Gondwana and Tethys.
Geological Society, London, Special Publication, 37, 101-118. PAGE, B. G. N. & WORKMAN,D. R. 1968. Geological and Geochemical Investigations in the Mekong Valley, between Vientiane and Sayaboury and at Ban Houei Sai. Institute of Geological Sciences, Overseas Division, Report, 9. RACEY, A., LOVE, M. A., CANHAM, A. C., GOODALL, J. G. S., POLACHAN, S. & JONES, P. D. 1996. Stratigraphy and reservoir potential of the Mesozoic Khorat Group North Eastern Thailand: Part 1, stratigraphy and sedimentary evolution. Journal of Petroleum Geology, in press. WHITTLE,G. 1970. Pak Lay Sheet. In: Notes on 1:250 000 Photogeological Reconnaissance Map of Laos. Institute of Geological Sciences, Overseas Division. WORKMAN, D. R. 1972. Geology of Laos, Cambodia, South Vietnam and the eastern part of Thailand a review. Institute of Geological Sciences, Overseas Division, Report, 19. -& PAGE, B. G. N. 1967. Interim Report on the regional geology and geochemistry of the Sanakham-Pak Lay area of western Laos. Institute of Geological Sciences, Overseas Division, Report, 8.
Mid-Cretaceous inversion in the Northern Khorat Plateau of Lao PDR and Thailand PAUL F. LOVATT SMITH l, R O B E R T B. STOKES 2, C H A R L I E B R I S T O W 3 & ANDREW CARTER 3
1 25 Tredegar Road, London E3 2EH, UK 2 Surbiton Geological Services, 14 Dennan Road, Surbiton, Surrey KT6 TRY UK 3 Department of Geology, Birkbeck College, University of London, Malet Street, London WC1E 7HX, UK
Abstract: Evidence is presented from the study area for the occurrence of a regional compressive tectonic event in the mid-Cretaceous(Aptian-Cenomanian). This is tentatively attributed to the effects of a distant continent-continent collision to the west. Pre-existing structural trends were reactivated parallel to palaeo-continental sutures to the northeast (Song Ma and Song Da) and west (Nan). The event interrupted the latest Jurassic to earliest Palaeocene subsidence and continental sedimentationof the Khorat Plateau Basin. The primary effects are shown by seismic data to be regional tilting, compressive folding, reverse faulting and inversion of the basin subsidence pattern. The data do not support the current view that such structuring was initiated during the Tertiary. However they do show that there was some reactivation of mid-Cretaceous structures during regional uplift in the Tertiary. This revision of timing of structuring suggests the hydrocarbon potential of the area may be greater than previously anticipated.
The study area comprises the MonumenffShlapak Contract Area in the Lao PDR and the western margins of the Khorat Plateau in Thailand (Fig. 1). The data presented in this paper results from Monument's hydrocarbon exploration programme in Lao PDR, and from sedimentological studies of the Khorat Group in Thailand. Geological fieldwork in the Lao PDR, totalling 19 weeks, was undertaken by PLS, RBS and others during the dry seasons of November-December 1991, January, March-April and November-December 1992, and November-December 1993. This resulted in the production of 30 geological maps at a scale of 1:100,000. Samples were analysed in the laboratory for petrography, geochemistry, biostratigraphy and isotopic dating. Geophysical surveys in the Vientiane Basin were conducted in April 1992 and March-May 1993 and resulted in 4 2 0 k m of seismic, 2146 km of airborne gravity, some 500 km of surface gravity and 6450 km of aeromagnetic data. Fission track analysis on samples from the Khorat Group cropping out in the Khorat monocline of Thailand was carried out by CB and AC.
Regional setting The Lao PDR study area is situated on the northern and western margins of the Khorat Plateau Basin. This is a large area of cropping out Mesozoic (mainly Cretaceous) continental deposits covering the northeast of Thailand, parts of Lao PDR and
Cambodia (Fig. 1). The basin system was once more extensive but has been reduced in area and thickness by significant regional Tertiary erosion. Present-day sub-basins of the Khorat Plateau Basin are defined by the distribution of Upper Cretaceous sediments. The Vientiane and Pakxan topographic plains are the Lao extensions of the Upper Cretaceous Sakhon Nakhon Basin. Around the northern margin of this basin the underlying Khorat Group (uppermost Jurassic to Aptian) is exposed in a pattern of eroded anticlines and outlying synclines (Fig. 2). The Phetchabun Foldbelt separates this eastern area from the NakhonThai/Sayabouri Basin where Mesozoic sediments belonging to both the Upper Cretaceous Phon Hong Group and the Khorat Group are also preserved. The similarity of the Cretaceous succession of this basin to that further to the east indicates that both areas can be considered as part of the same basin system. The surrounding massifs consist mainly of Precambrian to Upper Palaeozoic rocks, though some Upper Palaeozoic to Upper Jurassic sediments and volcanics are exposed in the Phetchabun Foldbelt. Two structural trends are present in the study area. An approximately north-south trend predominates in the west. In the Nakhon-Thai/ Sayabouri Basin, Cretaceous sediments are gently folded and faulted on this trend. Underlying sediments in this area and in the Phetchabun Foldbelt have been more severely tectonized by east-west
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolutionof SoutheastAsia, Geological Society Special Publication No. 106, pp. 233-247.
233
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compression. Along the east side of the foldbelt Mesozoic sediments reappear in the Khorat Monocline, which defines the westem margin of the Khorat Plateau Basin. Within the central and
eastern part of this Basin a northwest--southeast structural trend becomes more dominant, parallel to the Annamitic Foldbelt. This trend is characterized by large wavelength anticlines which have topo-
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graphic expression. These include the Nam Leuk, Khammouan and Phu Phan Uplifts (Figs 1 and 2). The structures are often bounded by deep-seated, high angle reverse faults with large throws, often of more than 1 km.
Stratigraphy The stratigraphy of the Vientiane Basin is summarized in Fig. 3 and is based on that proposed by Ha Luong Tin & Do The Que (1991), being here modified and further subdivided in order to facilitate correlation with the better known sequences in Thailand. The maximum thicknesses indicated in Fig. 3 are based mainly on geophysical data supplemented by outcrop measurements. The chronostratigraphic section in Fig. 4 is based on an east-west seismic line across the Vientiane Plain. The Permian is present as marine platform and marginal facies, comprising limestones and clastic sediments. The Triassic crops out only in the west of the study area where it is present as extrusive volcanics and volcaniclastics. To the east its facies is assumed to be fluvio-lacustrine, similar to its equivalents in Thailand. The Khorat Group comprises mainly fluvial sediments. The Lao Khorat Group stratigraphy substantially follows that of Thailand, recently updated by Racey et al. (1994), except that the lowest unit, the Upper Norian to Rhaetian Phulekphey Formation (Thai Nam Phong Formation equivalent), has been assigned to the
pre-Khorat section. This is because a long hiatus between this formation and the overlying Nam Set Formation is indicated by the absence of LowerMiddle Jurassic sediments. The Phon Hong Group stratigraphic nomenclature is based on the results of potash exploration carded out between 1983 and 1986 by Vietnamese geologists, quoted by Ha Luong Tin & Do The Que (1991). In the Vientiane area the Thangon Formation reaches a maximum thickness of 700 m and consists of three evaporitic sedimentary cycles. A basal conglomerate has been noted in one locality in the west of the Vientiane Basin (Howlett 1993). The basal evaporite cycle contains a halite bed which reaches a maximum thickness of 340 m in boreholes. In the southeast part of the Vientiane Plain this salt has been mobilized into pillows and small diapirs. The evidence for an Albian to Cenomanian age for the Thangon (Maha Sarakham equivalent) Formation evaporites is based on biostratigraphic data from palynological analysis of cores taken in the Sakhon Nakhon Basin of Thailand (Sattayarak et aI. 1991). This formation was identified by Utha-Aroon (1991) as being deposited in a continental setting. The Saysomboun Formation is up to 150 m thick in the Vientiane area. It consists of predominantly red-brown fluvial claystones grading upwards into siltstones and fine sandstones. The claystones are dated by palynology as Upper Cretaceous both in the Vientiane by Ha Luong Tin & Do The Que
236
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(1991) and in the Savannakhet Basin of Central Laos (Huynh Xuan Dang and Boutheung Phengthavongsa 1989). In the Pakxan Plain the upper part of the Saysomboun Formation is preserved and contains massive beds of red-brown, cross-bedded sandstones. One of these sandstone units is over 180 m in thickness and forms a scarped ridge (Sayphou Khout) in the centre of the
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The present authors prefer to place it in the Upper Cretaceous, based on the palynological evidence cited above. Neogene and Quaternary alluvial sediments occur locally, infilling abandoned river channels. In boreholes they reach thicknesses of up to 80 m. No older Cenozoic sediments are found in the Mesozoic basinal areas.
Current interpretations of postEarly Cretaceous inversion history There are currently two main interpretations of the post-Early Cretaceous structural history of the Khorat Plateau Basin, relating to the timing of inversion with respect to the deposition of the Upper Cretaceous Phon Hong Group. Cooper et al. (1989) interpreted the Khorat megasequences as being deposited in a basin formed by thermal subsidence during the Jurassic and Cretaceous, which was inverted prior to the deposition of the Maha Sarakham Formation evaporites. This inversion created a rimmed basin(s) within which the evaporites were deposited. The inversion was stated to be caused by the early stages of collision between India and Eurasia in the region of central Burma (Myanmar),
stress being fed to the Phetchabun Foldbelt via the Mae Ping strike-slip fault zone. They inferred a Late Cretaceous to Early Tertiary age for this inversion. Most other authors interpret the inversion as beginning after the deposition of the Phon Hong Group, during the Tertiary. Mouret et al. (1993) date the main inversion as beginning at c. 65 Ma (earliest Palaeocene). This is based on fission track data from outcrops in Thailand of the Phra Wihan Formation indicating that the maximum palaeotemperature, and therefore the onset of uplift, occurred at that time. These and other data are interpreted to demonstrate uplift and erosion of 3000 m for surface outcrops of Khorat Group sediments in the Phu Phra area of Thailand, which is on the northem margin of the Phu Phan Uplift. A similar age for the onset of inversion is also supported by Kozar et al. (1992).
Evidence of a two phase inversion history While our fission track data are in close agreement with those of Mouret et al. (1993) indicating that substantial uplift and erosion began at 60 Ma (midPalaeocene), other data show that this was the second phase of structuring to have affected the
238
P.F. LOVATT SMITH E T A L .
Khorat Group. The initial event occurred prior to the deposition of the Phon Hong Group in the midCretaceous and was responsible for regional tilting, compressive folding, reverse faulting and inversion of the basin subsidence pattern. The Tertiary event reactivated some of these trends but its main effect was to produce regional uplift and substantial erosion. This two phase model is consistent with the evidence published from Thailand mentioned above.~ The evidence in the study areas for this interpretation is listed below.
Unconformity at the base of the Phon Hong Group The Upper Cretaceous sedimentary cover has been more or less completely eroded in many areas of the Khorat Plateau due to Tertiary uplift, obscuring the evidence of mid-Cretaceous structuring. However an unconformity at the base of the Thangon (Maha Sarakham) Formation has previously been noted from seismic data in Thailand (Sattayarak et al. 1991). Fortunately, in the Vientiane area of Lao PDR. the preservation of a relatively thick section of Phon Hong Group sediments means that the character of the unconformity is readily discernible. The unconformity has been noted from shallow borehole data at the margin of the Vientiane Plain of Lao PDR by Ha Luong Tin & Do The Que (1991). A conglomerate containing clasts of siltstone, interpreted to be derived from the Khorat Group, has also been identified at the base of the Phon Hong Group at outcrop in the west of the Vientiane Basin (Howlett 1993). Seismic data from the Vientiane area show that this unconformity represents a more significant tectonic event than was previously realized. Four seismic lines (Figs 5-8) demonstrate this. Figure 5 shows the aspect of the Phon Hong Group towards the northern limit of its distribution. Horizontal Thangon Formation reflectors onlap uneroded Khorat Group both in the west (onto the Khorat Monocline) and in the east (at the base of the Nam Leuk Uplift). Although the shallowest reflectors may be confused by recent weathering effects it is apparent from these data that most of the Nam Leuk Uplift and Khorat Monocline structuring in this area occurred during the midCretaceous. The horizontal dip of the Phon Hong Group reflectors shows that subsequent Tertiary reactivation of these structures has been relatively minor. Figure 6 is a detail from a north-south line which intersects the line of Fig. 5. It demonstrates erosional truncation during the mid-Cretaceous of the upper Khorat Group reflectors on a reverse
fault-bounded horst. There is clear onlap by horizontal Thangon Formation reflectors over this feature. The reverse faults moved during or before the deposition of the Lower Salt Member of the Thangon Formation evaporites (AlbianCenomanian), since only the highest part of the Early Cretaceous horst has been eroded. The large reverse fault-bounded anticline seen on Fig. 7 is along a surface trend from the unfaulted Nam Leuk Uplift anticline seen in Fig. 5. The relief on both structures is of a similar order of between 1000 and 1400 m. The bulk of the reverse fault movement is therefore inferred to have occurred during the mid-Cretaceous event. It is interpreted that the movement occurred both before and during the deposition of the Thangon Formation, giving rise to a fault scarp and localized development of a syn-fault talus fan, against which the Thangon Formation evaporites were deposited. Some Tertiary reactivation is evident on this fault, shown by dipping Phon Hong Group reflectors on the upthrown side. Finally, Fig. 8 is a detail from the western margin of the Upper Cretaceous evaporite basin. Again, the shallowest horizontal reflectors in the Phon Hong Group may be confused by recent weathering effects but westward onlap at the base of the Phon Hong Group is strongly indicated. Together with the onlap seen along trend on Fig. 5 this indicates that the regional tilting of the Khorat Monocline was initiated in the mid-Cretaceous.
Sediment thickness and the distribution of halokinetic structures Figure 9 illustrates three seismic lines from the Vientiane Plain which demonstrate that an inverse thickness relationship exists in this area between the present day thicknesses of the Khorat Group and Phon Hong Group. The data show the presence of a complete section of Khorat Group sediments, thickening towards the northeast from 1900 m on line 1 to 4000 m on line 3. This demonstrates that during the Early Cretaceous there was an increase in the rate of subsidence and deposition from southeast to northwest. The subsidence pattern during the Late Cretaceous can be interpreted by examining the distribution of halokinetic structures in the Thangon Formation. The salt pillows and diapirs are assumed to have been generated by increasing overburden pressure of Upper Cretaceous sediments up to the time of maximum burial during the earliest Palaeocene. Line 1 shows that the most intense salt movement has occurred in the southeast, indicating that this was the area of thickest Upper Cretaceous palaeo-overburden. However
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P.F. LOVATT SMITH ET AL.
seismic lines 2 and 3 show that towards the northwest no salt movement has occurred, indicating a thinner Upper Cretaceous palaeo-overburden. The opposing thickness trends between Khorat and Phon Hong Groups are shown by this data to be subsidence-related and demonstrate a midCretaceous inversion of the subsidence pattern. It should be noted that in the Vientiane area gentle salt pillows are currently present at very shallow depths (Fig. 8), supporting fission track data which show that a substantial amount of Tertiary uplift and erosion occurred after maximum burial in the earliest Palaeocene. In the area of the Vientiane Plain the authors have calculated maximum Tertiary erosion of c. 1000 m from the character of the halokinetic structuring.
Abrupt facies change At the level of the mid-Cretaceous unconformity there is an abrupt change in facies from the Aptian Ban Thalat Formation of fluvial claystones, siltstones and sandstones to the Albian-Cenomanian Thangon Formation of restricted, continental evaporites. This facies change occurs throughout the Khorat Plateau Basin. It is difficult to envisage any other way that such a facies change could have been brought about other than by the effects of tectonism. These evaporites were suggested by Cooper et al. (1989) to have been deposited in restricted basins created by the topographic expression of syntectonic uplifts. Such potential drainage barriers are exemplified by the marginal uplifts visible on Figs 5-8. Three successively thinner evaporite cycles are present in the Phon Hong Group indicating three progressively more minor tectonic episodes.
these source rocks occurred during the Late Cretaceous burial. Thereafter only relatively minor quantities of hydrocarbons are likely to have been generated. Since trap formation nearly always pre-dates hydrocarbon generation these two timing constraints impose a mid-Cretaceous age of structural development for such gas-bearing traps, further supporting the evidence for mid-Cretaceous tectonism.
Lack of structural dip in Phon Hong Group At outcrop, the Phon Hong Group rarely dips at more than a few degrees, whereas the Khorat Group shows steeper dips and is even vertical or overturned adjacent to major faults. Although this could be explained by erosion of steeply-dipping Phon Hong Group sediments during the Tertiary, it is interpreted by us as supporting evidence for a mid-Cretaceous event.
Cretaceous granite intrusions K-Ar dating on granite intrusions from various parts of the Monument Contract Area gives ages of 78.3_+ 1.7 Ma (Campanian, Ban Boung Koang, No. 3 on Fig. 1), 94.9 + 2.1 Ma (Cenomanian, Keng Daeng, No. 1 on Fig. 1), l 1 7 + 3 . 0 M a (Aptian, Muang Kenthao, No. 2 on Fig. 1), and 137.2 +_3.0 Ma (Valanginian, Ban Phontiou, No. 4 on Fig. 1). Although no detailed geochemical analysis has been carried out, these granites are inferred to indicate crustal melting following the Indosinian continent-continent collision. The timing of their intrusion may be coincident with interruptions in the post-collision Cretaceous thermal subsidence of the Khorat Plateau Basin including, at c. 95 Ma, a mid-Cretaceous event.
Presence of trapped hydrocarbons The presence of thermally-derived gas in reservoirs within closed structural highs has been successfully demonstrated by many exploration wells in the Khorat Plateau of Thailand. Published seismic data over these structures show that they are defined by internally conformable, folded Khorat Group sediments (Kozar et al. 1992). This indicates that the structuring episode which formed these traps post-dates the Early Cretaceous. It can be assumed that the source rock for the gas lies in the pre-Cretaceous section since organicallyrich sediments within the Khorat and Phon Hong Groups are volumetrically insignificant. It is generally agreed that the most likely source rocks are within the Permian and Triassic section. The authors' burial history modelling indicates that the period of peak generation of hydrocarbons from
Fission track data from Thailand Data were obtained from seven outcrops of the Phra Wihan and Khok Kruat Formations on the Khorat Monocline in Thailand southwest of Khon Kaen. The data show a spread of ages all of which are younger than the depositional age (c. 145-112 Ma). This indicates that the fission tracks have experienced some annealing. Despite variations in mean age from 84 + 8 to 40 + 5 Ma, statistical modelling indicates that cooling began prior to 60 Ma and that the rock has experienced temperatures of around l l0°C. The model shows that the Cretaceous thermal history is poorly constrained because it has been overprinted by later heating. The dominant thermal event was at 60Ma, but there is no evidence to suggest that an episode of uplift did not occur in the mid-Cretaceous.
245
CRETACEOUS INVERSION IN THE KHORAT PLATEAU
Regional tectonic model The pre-Cretaceous tectonic history of the Indochina region is still not well understood. However there is agreement that a number of continental fragments including the Indochina Plate, on whose northern margin the Khorat Plateau Basin is situated, rifted from the northern margin of Gondwana during the Palaeozoic. These fragments drifted northwards, eventually accreting to the Eurasian margin during a series of continentcontinent collisions (Mouret 1994; Metcalfe 1996) of which the India-Eurasia collision is the latest. The timing of the collision of the Indochina Plate with its two neighbours, the South China Plate and the Shan Thai Plate still requires more research. Recent data from the Pak Lay Foldbelt show that tectonism occurred during the Late Jurassic (Stokes et al. 1996), indicating that the Shan-ThaiIndochina collision may have occurred later than previously thought. However it is evident that at the onset of Late Jurassic-Cretaceous basin subsidence in the Khorat Plateau and surrounding areas both of these sutures had already closed (Fig. 10). The Khorat Plateau and other continental sedimentary basins in the region (Fig. 1) may have formed as foreland basins as a result of flexural subsidence in front of the orogenic belts created by these sutures. Amongst the supporting evidence for this theory is that the depocentre of the Lower Cretaceous sedimentary basin in the Khorat Plateau runs parallel to the Song Ma-Song Da suture and Annamitic Foldbelt trend (N. Sattayarak, pers. comm. 1994) and that palaeocurrent data in the Khorat Group show a source from the Annamitic Foldbelt (Howlett 1993; Mouret et al. 1993). Also, there is no evidence of major syn-depositional faulting in the Khorat Plateau Basin, indicating that the stretching (13) factor was low. The onset of structuring and inversion of the Khorat Group as described in this paper can no longer be attributed to the collision of the India Plate with central Burma, as was proposed by Cooper et al. (1989), since this collision began during the Eocene (Daly et al. 1991). An alternative tectonic model must therefore be invoked. In the absence of a more convincing alternative, we suggest that mid-Cretaceous tectonism is best explained by a continental collision to the west, such as that of Western Burma with Shan-Thai which is stated by Metcalfe (1996) to have occurred in the Cretaceous. The constraining forces created by such a collision would have placed the entire region in compression, leading to the structural effects already described. The Nan suture runs parallel to the north-south Phetchabun Foldbelt trend, and the Song Ma-Song Da suture runs parallel to the northwest-southeast inversion trend
•
s c.,,A /
J
i
0
1000 km
Fig. 10. Pre-Cretaceous suture zones in SE Asia (after Cooper et al. 1989).
described above (Fig. 10). Both of these trends were compressionally reactivated during the mid-Cretaceous event in the study area. Subsidence resumed following this hiatus, initially in new evaporitic basins created by the inversion, and continued until the earliest Palaeocene. From this time onwards the Indochina area was subjected to a transtensional stress regime. Rifting occurred during the Oligo-Miocene both to the east (South China Sea) and west (Thailand Central Plain and Gulf of Thailand) of the Khorat Plateau (Daly et al. 1991), however there is no evidence of rifting in the Khorat Plateau itself. The evidence already cited indicates that the study area underwent some reactivation of mid-Cretaceous trends but that the main effect was to produce regional uplift and substantial erosion of much of the Upper Cretaceous sedimentary cover.
Implications for petroleum exploration Burial history models which assume a Tertiary age for the timing of post-Early Cretaceous inversion find it difficult to explain why, if trap formation post-dated the main period of hydrocarbon generation, most, if not all such inversion structures which have been tested by drilling have proven to contain gas. As discussed above the presence of hydrocarbons in structural traps defined at the base
246
P.F. LOVATT SMITH ET AL.
of the Khorat Group is much better explained by mid-Cretaceous timing of structural development. Tertiary reactivation of mid-Cretaceous structures has nevertheless enhanced the volume of some traps, which might explain why some structures are underfilled with gas (e.g. the Nam Phong gas field). The implication is that the petroleum exploration potential of the area is not only confirmed but may be greater than previously anticipated.
We thank Monument Resources Overseas Limited and Shlapak Development Company for permission to publish as well as colleagues in Monument for their help and advice. We also thank our numerous Lao and Thai colleagues, particularly in the Department of Geology and Mines, Ministry of Industry, Vientiane and the Mineral Fuels Division, Department of Mineral Resources, Bangkok, for their help, and the many Lao villagers for their hospitality during the fieldwork.
References COOPER, M. A., HERBERT, R. & HILL, G. S. 1989. The structural evolution of Triassic intermontane basins in northeastern Thailand. In" THANASUTHIPITAK,T. (ed.) Proceedings of International Symposium on Intermontane Basins: Geology and Resources, Chiang Mai. 231-242. DALY, M. C., COOPER,M. A., WILSON, ]., SMITH, D. G., HOOPER, B. G. D. 1991. Cenozoic plate tectonics and basin evolution in Indonesia. Marine and Petroleum Geology, 8, 1-21. HA LUONG TIN & DO THE QUE. 1991. Determination of salt-bearing stratum depth and its structure in Vientiane region by geophysical data. In: TRAN VAN TRI (ed.) Proceedings Second Conference on Geology of lndochina, Hanoi. 2, 32-38. HOWLETT, P. 1993. Sedimentology of the Khorat Group. MSc Thesis, Birkbeck College, University of London. HUYNH XUAN DANG & BOUTHEUNG PHENGTHAVONGSA. 1989. Rock salt of the south of Laos. In: Geology of Kampuchea, Laos and Vietnam. Dept of Mines and Geology, Hanoi, 1, 28-30. KOZAR, M. G., CRANDALL, G. E, & HALL, S. F. 1992. Integrated structural and stratigraphic study of the Khorat basin, Rat Buri Limestone (Permian) Thailand. In: PIANCHAROEN,C. (ed.) Proceedings National Conference on Geologic Resources of Thailand: Potential for Future Development. Bangkok, 692-736. METCALFE,I. 1996. Pre-Cretaceous evolution of SE Asian terranes. This volume. MOURET, C. 1994. Geological history of northeastern Thailand since the Carboniferous. Relations with Indochina and Carboniferous to Early Cenozoic
evolution model. In: Proceedings International Symposium on Stratigraphic Correlation of SE Asia, Bangkok. 132-158. --, HEGGEMANN,H., GOUADAIN,J. & KRISADASIMA,S. 1993. Geological history of the siliciclastic Mesozoic strata of the Khorat Group in the Phu Phan Range area, Northeastern Thailand. In: THANASUTIPITAK,T. (ed.) Proceedings International Symposium on Biostratigraphy of Mainland Southeast Asia: Facies and Palaeontology, Chiang Mai, 23-49. RACEY, A., GOODALL,J. G. S., LOVE, M. A., POLACHAY,S. & JONES, P. D. 1994. New age data for the Mesozoic Khorat Group of northeast Thailand. In: Proceedings International Symposium on Stratigraphic Correlation of SE Asia, Bangkok. 245-252. SATTAVARAK, N. 1983. Review of the continental Mesozoic stratigraphy of Thailand. In: NUTALYA,P. (ed.) Proceedings of Workshop on Stratigraphic Correlation of Thailand and Malaysia, Bangkok. 1, 127-140. --, POLACHAN, S. t~ CHARUSIRISAWAD, R. 1991. Cretaceous rocksalt in the northeastern part of Thailand. In: GEOSEA VII Abstracts. Bangkok, 36. STOKES, R. B., LOVATTSMITH,P. F. & SOUMPHONPHAKDY, K. 1996. Timing of the Shan-Thai-Indochina collision: new evidence from the Pak Lay Foldbelt of the Lao PDR. This volume. UTHA-AROON, C. 1993, Continental origin of the Maha Sarakham evaporites, northeastern Thailand. Journal of Southeast Asian Earth Sciences, 8, 193-207.
The 'Rajang accretionary prism' and 'Lupar Line' problem of Borneo CHARLES
S. H U T C H I S O N
c/o Department of Geology, University of Malaya, 59100 Kuala Lumpur, Malaysia Abstract: The Rajang Group, composed of the Belaga and Lupar Formations of Sarawak and the Embaluh Group and Selangkai Formation (in part) in Kalimantan, had a turbidite sedimentation history from Early Cretaceous to Late Eocene. These rocks generally young northwards. Inliers within the eastern Miri Zone have been mapped as the Kelalan and Mulu Formations. The Rajang Group was compressed into a steeply dipping quartz-veined phyllite-quartzite complex by the Sarawak orogeny and unconformably overlain locally in the north by the Upper Eocene continental to neritic Tatau Formation, extensively in the south by Middle to Upper Eocene basal continental sequences of the Ketungau, Mandai and Melawi basins, and widely in the north by the Upper Oligocene coastal to marine Nyalau Formation. In Sabah and East Kalimantan, the Upper Cretaceous to Upper Eocene Mentarang, Sapulut, Trusmadi, and possibly the East Crocker Formations, also belong to the Rajang Group. The West Crocker Formation demonstrates rapid facies changes into the more shaly Temburong Formation, and was deposited as sandy turbidites throughout the Oligocene. To the south their equivalents are the nearshore Kelabit and Long Bawang Formations. The West Crocker Formation was folded and uplifted in several Miocene pulses, resulting in regional unconformities and igneous events at Mount Kinabalu. The West Crocker Formation has not been metamorphosed, and dips are shallower than in the Belaga and Trusmadi Formations. Its provenance probably was from the uplifted Upper Cretaceous to Eocene Lurah and Kelalan formations of NE Kalimantan and East Sarawak. It is therefore proposed that the West Crocker and Temburong formations be excluded from the Rajang Group. Middle to Late Miocene Crocker Formation uplift and deformation, herein called the Sabah orogeny, was synchronous with spectacular basin inversion throughout the South China Sea and in the Meratus Mountains of Kalimantan. Uplift ceased in the Late Miocene and undeformed post-inversion formations unconformably overlie inverted folded structures. The Rajang Group flysch-belt may be interpreted as a north-facing accretionary prism. The Schwaner Mountains represent a subducfion-related Lower to Upper Cretaceous volcanoplutonic arc. Scattered Eocene volcanism, Miocene Sintang intrusives and Pliocene Metalung volcanics in Central Kalimantan and Sarawak post-date subduction. The Rajang Basin (Danau Sea) rapidly narrowed and by Eocene time subduction was transformed to collision as the Rajang Group was compressed between the Schwaner Mountain Zone and the Luconia-Balingian-Miri block. The Ketungau Basin is in sharp contact with the Rajang Group along the bounding Lupar Fault, which can be traced northwards into the East Natuna region. Palaeocurrents show that the Upper Eocene basal sandstones have a provenance in the metamorphosed Sibu Zone. The Melawi and Mandai basins of Kalimantan also unconformably overlie the flysch-belt, The basins are not forearc and were formed after transformation of the accretionary prism to a landmass formed of a collisional orogenic complex.
The models of tectonic evolution of the South China Sea region conflict with the d o c u m e n t e d geology of Borneo and especially with the Rajang Group. These models have then been applied to analyze the tectonic evolution of Borneo. The m o d e l of Taylor & Hayes (1980, 1983) wrongly shows active subduction of Mesozoic oceanic lithosphere continuously through the Palaeogene, b e c o m i n g extinct by the Middle Miocene when the Luconia Shoals were interpreted to have collided with the Northwest Borneo Trough. The model o f Holloway (1981) is not significantly different. The mistake can be traced back to Hamilton (1979), w h o described extensive tracts o f B o r n e o as
'm61ange', interpreted as s y n o n y m o u s with subduction-related accretionary wedges. The m o d e l of Ru & Pigott (1986) ignores the West Baram Line and wrongly extends the Northwest Borneo (Palawan) Trough through the Luconia and Balingian provinces (their fig. 1), and has active Late Eocene, southeastwards subduction. The model o f James (1984) perpetuated this mistake by not closing in the Late Eocene the Rajang Basin, or Danau Sea of Haile (1994). These models wrongly concur that subduction continued until the Early Miocene. The mistake of continuing the Northwest Borneo (Palawan) Trough along the southern margin o f the Luconia Province, ignoring
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolutionof SoutheastAsia, Geological Society Special Publication No. 106, pp. 247-261.
247
248
C.S. HUTCHISON
the West Baram Line, is also made by Tongkul (1990). All these models are flawed because they also ignore the peripheral, yet equally important region. Thus, Ru & Pigott (1986) and Williams et al. (1988) accept the palaeomagnetic evidence that West Borneo has rotated anti-clockwise (Schmidtke et al. 1990), but they ignore the evidence for clockwise rotation of Indochina (Achache et al. 1983). Williams et al. (1988) bring peninsular Malaysia and Sumatra together only in the Early Tertiary; an impossibility because they share the same Palaeozoic and Mesozoic stratigraphy and structure without displacement (Hutchison 1993). The Mid Oligocene to Early Miocene N-S extensional opening of the east sub-basin of the South China Sea, first detailed by Taylor & Hayes (1980), has been widely accepted and forms the basis of most subsequent models of the region, including contiguous Borneo. Using the data of Taylor & Hayes (1980, 1983) and Chen (1987), Briais et al. (1993) have produced a refined scenario of South China Sea spreading history, composed of three episodes: NW-SE extension from 32-30 Ma, followed by N-S extension from 30-26 Ma, then by NW-SE extension from 2316 Ma. The structural trends for the latest, Early Miocene, episode have been resolved in the Scarborough Seamounts region by Pautot et al. (1986) and Briais et al. (1989) to be 50" (magnetic anomalies) and 140° (transform faults). The choice of magnetic anomaly designations for the main eastern basin was influenced by the only direct geological constraint of volcanics dredged from the Scarborough Seamounts, radiometrically dated 10-15 Ma (Hrkinian et al. 1989). The volcanics are thought to have been extruded along the still weak relict spreading axis. Briais et al. (1993) admit that their interpretation is not unique. Some of their illustrated anomaly designations do not inspire confidence. Their scenario should prevail until the ODP programme can include the South China Sea Basin. The practice of designating anomalies without direct geological constraint has led to serious misinterpretations in the nearby Sulu Sea (Lee & McCabe 1986; Hutchison 1992a). More carefully constructed evolutionary scenarios of the Borneo margin of the South China Sea were painted by Tan & Lamy (1990) and Hazebroek & Tan (1993), which benefited from comprehensive petroleum company data. However their inferred subduction history from Late Cretaceous through Late Eocene, using the Lupar and Mersing lines, and from Late Eocene through Middle Miocene, using the Northwest Borneo (NW Sabah) trough, is not matched by identified subduction-related post Palaeocene volcanic arcs in Borneo.
The Rajang Group of Sarawak The Rajang Group consists of the Belaga and Lupar formations, which are isoclinally folded, with dips usually between 80 and 90° (Fig. 1). It extends across the border into Kalimantan as the Embaluh Group (Tare 1991). This monotonous flysch group is of interbedded sandstone and mudstone, is locally metamorphosed to lower greenschist facies phyllite and slate, and commonly veined by quartz (Kirk 1957; Liechti et al. 1960; Wolfenden 1960). The outcrop width of the group perpendicular to the E-W strike from the Lupar Line to the Tatau Horst is 200 km. Thickness cannot be determined and has been guessed to be in the neighbourhood of 15 km (Haile 1974), but the basis for his estimate is unknown. The Rajang Group constitutes the Sibu Zone of Sarawak (Fig. 1), outlined along the south by the Lupar Line ophiolite and along the north by the Bukit Mersing Line ophiolite (Hutchison 1989), but it also crops out north of this in the Tatau Horst and as various inliers within the Miri Zone, as the Kelalan and Mulu Formations. The Lupar Line ophiolite occurs as blocks in a 20-25 km wide mrlange zone, known as the Lubok Antu Mrlange. Along its SW side, the mrlange is in sharp contact with the basal Silantek Formation beds of the Ketungau Basin along the steep linear Lupar Fault. The Rajang Group has been interpreted as a deep marine fan deposit. Most outcrops have thin fine to medium grained sand layers which may be described as distal turbidite, but some outcrops show thicker sandstones. Most are characterized by prominent sole marks. Adjacent to the Lubok Antu Mrlange, the 5 km wide outcrop of the Lupar Formation is predominantly a turbidite sequence similar to the contiguous Layar Member of the Belaga Formation (Liechti et al. 1960; Tan 1979). It was mapped separately because it incorporates large bodies of ophiolitic rocks. Microfossils indicate an Upper Cretaceous age of Santonian to Maastrichtian. The Lupar Formation thus represents the oldest part of the Rajang Group of Sarawak (Table 1). From the south, adjacent to the Lupar Formation, to the Tatau Horst in the north, the Belaga Formation has been subdivided into the following members: Layar, Kapit, Pelagus, Metah and Bawang. The fossils indicate a progressive younging northwards from Upper Cretaceous Layar to Upper Eocene Bawang. The steep dips and the systematic northwards younging and ophiolite and mrlange association have led to the plausible interpretation of a northwards facing accretionary prism, related to Late Cretaceous (85 Ma) to Late Eocene (45 Ma) southwards subduction beneath the Kuching Zone of Sarawak of the Rajang Basin, named the Danau Sea by Haile (1994).
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Fig. 1. Sketch geological map of Sarawak, Sabah and contiguous Ka]imantan (Borneo) to emphasize the Rajang Group. The eastern Mid Zone contains several inliers of Kelalan and Mulu Formations, interpreted to be equivalent to the Rajang Group. The Sibu Zone contains several large Pliocene volcanic massifs. Within the Embaluh Group outcrop they are known as the Metalung Volcanics. Based on Tan (1982), Lim (1985), Pieters et al. (1987) and Pieters & Supriatna (1989).
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Table 1. Chronology of events discussed in the text Schwaner Post-subduction Rajang Group Rajang Group N. W. Sabah South China Sea Mountains Volcanism deposition deposition Crocker Ranges volcano& plutonism Sarawak& Sabah & plutonic arc W. Kalimantan E. Kalimantan 5 ..Plioeene ....................... Metalung Voles.................................................................................................... Post-inversion strata ................. 10 upper Mr. Kinabalu Shallow Regional 15 middle Deep Regional ~ SABAH OROGENYu" 20 Miocene Sintang 25 .... lower ............................. intrusives .... Deposition of the Melawi ........................................................ Deposition of the .................... 30 Oligocene Ketungau and Mandai Kelabit Fin. [W. Crocker Fm ] South China Sea 35 ..................................................................... basins ..................................... [ including the ] .............. basins .......................... 40 MiHler Tatau Fnt Batu Gadlng [ Temburong Fm l 45 Eocene ~ S A R A W A K OROGENY ~ ~ y ~ . 50 Nyaan, Piyubung /lk /l~ [ E. Crocker Fro. ]
Ma
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Palaeocurrents in the Lupar Formation turbiditic sandstones indicate a major current direction towards 50-60 °, with a subordinate maximum towards 80-90 ° (Tan 1979). The provenance of the earliest part of the Rajang Group turbiditic sandstones was from the Kuching Zone, lying to the SW and W. Most of this zone formed an integral part of the extensive Late Cretaceous landmass known as Sundaland, upon which continental Eocene formations were widely deposited (Hutchison 1992b). The fluvial system, which brought sediments from the interior of Sundaland, must have reached the Danau Sea in the neighbourhood of what is now the Lupar Line. The geological record favours a narrow continental shelf as shown by outcrops of the Jurassic-Cretaceous Bau Limestone rapidly giving way to turbiditic Pedawan Formation of the Kuching Zone (Tang 1992), so it may be inferred that there was a rapid descent from land in the Kuching Zone to deep marine conditions even before the Lupar Line was reached. The lower, Upper Cretaceous, part of the continental Kayan Sandstone (Tan 1986) may be the on-land equivalent of the deep marine Lupar Formation and Layar Member of the Belaga Formation (Fig. 1). However, Tate (1991) and R. J. Morley (pers. comm. 1993) have expressed doubt about the older age palynological determinations by Mtiller (in Tan 1986) of the Kayan Sandstone and regard it as Upper Eocene.
Relationship to younger formations
One of the most spectacular Malaysian angular unconformities is exposed on the east end of the Tatau Horst (Wolfenden 1960; Halle & Ho 1991). The gently dipping basal conglomerate of the Tatau Formation rests on steeply dipping and complexly folded Upper Eocene Bawang Member turbidites of the Belaga Formation. The unconformity is palaeontologically dated as Upper Eocene. The Tatau Formation is an Upper Eocene to Oligocene sequence of conglomerate, sandstone and shale. It is continental to fluvio-marine, becoming neritic upwards, and includes the sub-aerial Arip Volcanics. From this informative outcrop, it may be inferred that the Danau Sea was eliminated and deformation, metamorphism,-uplift and peneplanation of the Belaga Formation to form a landmass was completed before the end of the Eocene. The author proposes to name this the Sarawak orogeny (Table 1). This information invalidates the tectonic model of James (1984), reproduced by Hutchison (1989). An equally spectacular unconformity occurs at the Bukit Besungai area of Batu Gading (Fig. 1) in the Middle Baram region (Adams & Haak 1962). Highly fossiliferous Upper Eocene limestone overlies with spectacular angular unconformity an Upper Cretaceous steeply dipping sequence of turbiditic Kelalan Formation, the eastern equivalent
RAJANG GROUP AND LUPAR LINE, BORNEO of the Belaga Formation (Haile 1962). This quarried location along the upper reaches of the Baram River (Fig. 1) is of identical significance to the Tatau Horst locality. The Tatau Formation is overlain by a thin (550 m) shale-sandstone sequence known as the Buan Formation (Wolfenden 1960). However, the major transgression over the landmass was accomplished by the Upper Oligocene to Lower Miocene Nyalau Formation, estimated to be about 5 km thick. It rests conformably on the Buan Formation and with marked unconformity on the Belaga Formation. The Nyalau Formation forms many outliers over the main Belaga Formation outcrops of the Sibu Zone (the largest at Merit Pila is shown on Fig. 1) and the unconformity is documented by Kirk (1957) and Wolfenden (1960). This is a very sandy formation in the west around Bintulu, becoming more muddy eastwards, where it is known as the Setap Shale. Detailed analysis by Shell indicates that a western landmass, known as the Penian High (Fig. 1), progressively gave way eastwards to a coastal plain rich in coal beds, then an inner neritic continental shelf. The outer shelf was characterized by a narrow NNW-SSE trending carbonate platform (Subis Limestone), seen at Niah. The edge of the shelf was marked by a continental slope, known as the West Baram Line, east of which bathyal conditions prevailed in a region which became the Baram Delta in the Late Miocene.
Southwards extension into Kalimantan An understanding of the Rajang Group is impossible without knowledge of its southwards extension, perpendicular to strike, into Kalimantan. The Lupar Formation and the Layar and Kapit Members of the Belaga Formation are known in northern Kalimantan as the Upper Cretaceous to Eocene Embaluh Group (Fig. 1; Table 1), which is not sub-divided (Tate 1991). This flysch sequence is strongly deformed. Bedding is in places overturned, and two fold phases have been identified. East of the Kapuas lakes, the main fold axis orientation is horizontal, trending 280 ° . The folding is asymmetrical, with southerly dips predominant (Williams et al. 1988). However, overall stratigraphic younging is towards the north, as in Sarawak. South-dipping thrust faults have been identified within the Embaluh Group. Southern outcrops along the Upper Mahakam River are reported to be unmetamorphosed (S. J. Moss pers. comm. 1995). The Embaluh Group is succeeded southwards in the Sintang District (Williams & Heryanto 1986) by the Lower Cretaceous (Valanginian) to Upper
251
Cretaceous (Turonian) Selangkai Formation, which youngs northwards and was first described by Zeijlmans van Emmichoven (1939). It has been restudied by Williams & Heryanto (1986), who described it as predominantly of calcareous mudstone with intercalations of pebbly and bouldery mudstone, graded sandstone and rare limestone and conglomerate. However, the formation is lithologically variable and some stratigraphic sections are dominated by turbiditic sandstone or other mass flow deposits. The structural and stratigraphic complexity has not been resolved, but Zeijlmans van Emmichoven (1939) suggested a formation thickness in excess of 3 km. The Selangkai Formation occupies a 45 km wide belt in which the main fold axes and outcrop trend WNW, with some cross folding and some strongly cleaved and thrust faulted zones. The lower part of the formation is in fault contact with the Boyan Mrlange, where it is tightly folded and cleaved. The dominant calcareous mudstone is a dark grey strongly sheared rock. It commonly contains, large blocks of fossiliferous limestone. Williams & Heryanto (1986) interpreted the sandstone beds, conglomerate and calcareous mudstone containing dispersed boulders as mass flow deposits. The sandstone beds have prominent sole marks and are turbiditic. The .-general appearance of the Selangkai Formation is that of a deep basin submarine fan into which blocks of limestone have been transported from a more stable shelf. Abundant volcanic detritus indicates a nearby contemporaneous volcanic arc, probably in the Schwaner Mountains (Williams & Heryanto 1986). The Upper Cretaceous (Turonian) Ingar Formation is in fault contact with the Selangkai Formation and both are folded and unconformably overlain by the Dangkan Sandstone, which forms the lower (Upper Eocene?) sequence of the Melawi Basin (Williams & Heryanto 1986). Unlike the Selangkai Formation, the Ingar Formation is predominantly muddy with an absence of turbiditic sandstone. It seems reasonable to conclude that the Danau Sea deep water fill is of Valanginian age in the south (Selangkai Formation), younging northwards through the Embaluh Group (= Lupar Formation and lower Belaga Formation), and eventually reaching its youngest (Late Eocene) age near the Mersing Line at the Tatau Horst. Nevertheless, the picture of a general northwards younging may be a gross oversimplification of this poorly known terrain. The Embaluh Group of the Upper Mahakam and Boh rivers of Kalimantan has yielded Middle Eocene planktonic foraminifera (Carter & Yeats 1973). The Selangkai Formation is similar both in its partial turbiditic character and age to the Pedawan Formation, which lies west across the Lupar Fault in the Kuching Zone.
252
C.S. HUTCHISON
The Lupar Fault appears from the Sarawak viewpoint to represent the most southerly termination of turbiditic rocks. However, the flysch belt extends southwards into the Sintang region of Kalimantan. The Lupar Line has therefore been given undue importance. It is the prominent fault margin between the Upper Eocene Silantek Formation of the Ketungan Basin and the Sibu Zone flysch belt. Indeed, within the Kuching Zone of Sarawak, there are abundant Lower to Upper Cretaceous turbidites known as the Pedawan Formation lying W and SW of the Lupar Line (Ting 1992). Ophiolitic m61ange zones are not confined to the Lupar Line. They also occur along the Bukit Mersing Line, and along the Semitau Ridge, known as the Boyan M61ange, although clasts in the latter are predominantly of quartz-rich sedimentary and metamorphic rocks (Williams et al. 1988). All of these m61ange belts lie close to and parallel to major faults which bound superimposed Tertiary basins. The Semitau Ridge margins are characterized by north dipping thrusts post-dated by Upper Oligocene sedimentary rocks.
the convexity would favour subduction from the south. The ophiolitic rocks of the Meratus Mountains have been dated as Lower Aptian (116 Ma) by Sikumbang (1986) and are associated with calcareous mudstone, sandstone and limestone. These ophiolites would seem to be ideal candidates to be paired to the Early Cretaceous volcanoplutonic arc of the Schwaner Mountains (Fig. 1). However, Sikumbang (1986) considered that there was no subduction relationship between them, and he concluded that the Meratus Mountains contain the record of a compressed marginal basin and paired volcanic arc. The Meratus ophiolite formed part of the Late Cretaceous landmass upon which diamond-bearing serpentinite conglomerate (the Pamali Breccia) was fluvially deposited (Bergman et al. 1987). However, the Meratus did not uplift to form the present day mountains until the Late Miocene (Hutchison 1992b). Before the uplift, the now separate Barito, Asem-Asem and Pasir basins were once one and the same. The Schwaner Mountains
Search for the paired volcano-plutonic arc Most authors (e.g. Hamilton 1979; James 1984) presume that the Upper Cretaceous through Upper Eocene Rajang Group of Sarawak (now modified to begin in the Lower Cretaceous by including Kalimantan) represents a northwards-facing accretionary prism, but there has been no clear identification of an appropriately positioned contemporaneous subduction-related volcano-plutonic arc. Gower (1990) concluded that the data from Borneo 'support the postulate of southeasterlydirected subduction of the South China Sea beneath Borneo during the early Tertiary.' At the same time he admitted that 'there are no convincing reports of early Tertiary magmatism, coeval with the postulated subduction.' The following discussion may clarify the situation of the appropriately distanced terrain lying 200-250 km to the south.
Convexity o f the arcs
The island of Borneo is dominated by a framework of arcuate structures, all convex towards the south and southwest (Fig. 1). This feature was emphasized by the earlier Dutch workers (Van Bemmelen 1949). Hamilton (1979) noted the importance of the Schwaner Mountains volcano-plutonic arc thus 'Presumably these igneous rocks are paired to the Cretaceous m61ange complexes that occur in both northwestern and southeastern Borneo'. He favoured subduction from the north, even though
The rigid 'core' of Borneo is formed by the Schwaner Mountains. Their northern boundary forms the southern margin of the Tertiary Melawi Basin (Fig. 1), and extends inland along an E-ESE trend, which cuts the equator east of Pontianak. The Schwaner Mountains extend eastwards more than 4 ° of longitude to disappear beneath the Cenozoic NW Kutei Basin, and they extend over 2 ° of latitude southwards to disappear beneath the Quaternary sediments of the coastal plains, which are underlain by the Billiton and Barito basins of SE Kalimantan. The region was divided by Van Bemmelen (1949) by an approximately E-W line along latitude l°S into the 'Schwaner Zone' and a 'southern region'. However, subsequent mapping (Amiruddin & Trail 1989; Pieters & Supriatna 1989) has shown that both zones form part of the Schwaner batholith, of which a major part is the Sepauk tonalite, with the Sukadana granite of the Ketapang batholith occurring in the SW corner. The Schwaner Zone, east of Pontianak, is mostly of 1-type (subduction-related) calc-alkaline plutonic rocks ranging from granodiorite to gabbro, dominated by the Sepauk tonalite, associated with a variety of metamorphic rocks mapped by Pieters & Supriatna (1989) as Palaeozoic, probably Carboniferous to Permian, though their age is indefinite. The 'southern region', lying east of Ketapang, is composed of Jurassic-Cretaceous flysch and contact metamorphosed limestone (the Ketapang Complex of Van Bemmelen 1949), and volcanic rocks (the Matan Complex of Van
RAJANG GROUP AND LUPAR LINE, BORNEO
Bemmelen), intruded by granitic batholiths (Pieters & Supriatna 1989). The Bunga basalt and Kerabai volcanics, formerly included in the 'Matan Complex', range from basalt, through andesite, to dacite and rhyolite, and represent a mature volcanic arc, which Tate (1991) paired with the Upper Cretaceous part of the Rajang Group, representing southwards subduction. The plutonic complex of the Schwaner Zone (Sepauk tonalite) has been dated by the whole-rock K-Ar method by Williams et al. (1988) and Bladon et al. (1989) at 104 (Albian) to 123 Ma (late Valanginian). Earlier, McDougall (in Haile et al. 1977), using biotite and hornblende mineral separates, determined K-At ages ranging from 75 (Campanian) to l l 5 M a (Barremian). The two datasets overlap and extend over the Lower and Upper Cretaceous. Williams et aI. (1988) report only 4 K-Ar dates from the potassic Sukadana granite of the 'southern region', lying within the range 86-91 Ma. However McDougall (in Haile et al. 1977) reported 10 mineral separate K-Ar dates from this province, eight of which lie within the range 78-86 Ma, but two granites gave considerably older ages of 127 and 153 Ma. Therefore this province contains important Late Cretaceous plutonic activity, dominated by alkaline granite, but there also occur some Lower Cretaceous to Upper Jurassic granitoids. The volcanic rocks of the 'southern region' were considered too weathered by Haile et al. (1977) for accurate dating. However, Pieters & Supriatna (1989) reported Upper Cretaceous K-Ar ages of 65-75 Ma (Maastrichtian-Campanian). In summary, most of the plutonic rocks of the Schwaner Mountains may be paired to the Selangkai Formation, which contains abundant volcanic detritus. Volcanism and high-level plutonism continued at least into the Maastrichtian, when the Embaluh Group, Lupar Formation and Layar Member of the Belaga Formation were being deposited. The Cretaceous part of the Rajang Group may therefore be reasonably interpreted as a northwards-facing accretionary prism, paired to the Cretaceous Schwaner Mountains volcano-plutonic arc (Table 1 and Fig. 2).
N o r t h o f the S c h w a n e r M o u n t a i n s
A narrow E - W trending zone through the Upper Kapuas region of central Kalimantan, with sporadic occurrences of Early Eocene acid volcanic and pyroclastic rocks (Fig. 1), of restricted 48-50 Ma age, has been interpreted by several authors to post-date subduction and be related to the early rift stages of formation of the Ketungau, Mandai, Melawi and West Kutei basins (Tate 1991; Doutch
253
1992; Van de Weerd & Armin 1992). They can be demonstrated to post-date subduction, because in several localities the Eocene volcanics unconformably overlie either the Boyan M61ange or the Embaluh Group. The Piyabung Volcanics, of gently dipping subaerial lithic and vitric tuff and agglomerate up to 200 m thick, extend 40 km E-W from 112 to 112 ° 30' within the Semitau Ridge at 0 ° 18'N (Pieters et al. 1987). A K-Ar whole rock age of 49.9 was obtained (Bladon et al. 1989). The Nyaan Volcanics, of 200-900 m of acid to intermediate tuff, agglomerate, ignimbrite and dacite, occur much farther east in the Mahakam area of the West Kutei Basin, at 114 ° 50% and 0 ° 50'N. They gave a K-Ar date of 48.6 Ma (Tate 1991) and 50 ___2.5 Ma (Pieters et al. 1987). In the Kelian district of the Upper Kutei, Van Leeuwen et al. (1990) describe Upper Eocene rhyolitic volcanic and pyroclastic rocks in a region dominated by Miocene volcanism. The Eocene igneous rocks are exclusively silicic, and have not been demonstrated to be subduction-related. In the same general region, Wain & Berod (1989) show Upper Cretaceous to Eocene volcanic rocks on their maps, but do not describe them. Near Singkawang, far to the west near the west coast at l°N, the Bawang Dacite gave a K-At date of 51.3 Ma. It is part of the Serantak Volcanics, which are less than 300 m thick. A much more extensive body of intermediate to basic volcanics lies entirely within the Mandai Basin in the Mtiller Mountains at 113 ° 20'E, 0 ° 30'N, K-Ar dated 40.9 Ma (Pieters et al. 1987). These intermediate to basic tufts and flows, many hundreds of metres thick, are interbedded with the basal sandstones along the Mandai River. The volcanic rocks were first documented by Molengraaff (1902). Zeijlmans van Emmichoven (1939) confirmed the existence of volcanic rocks, predominantly of basic-intermediate to basic composition in the region of the Mandai Palaeogene basin east of the Lake District (Kapuas), equivalent to the Mtiller Mountains of Molengraaff (1902). On the north side of the Rajang Group, the acidic Arip Volcanics and associated Piring Granodiorite of the Tatau Formation have not been dated but post-date the metamorphism of the Rajang Group. They are comparable to volcanics described above from the southern side of the orogenic flysch belt. It may therefore be concluded that there is no Eocene subduction-related arc to the north of the Schwaner Mountains, and that the predominantly silicic and pyroclastic eruptions are an integral part of the basin extensional process. It is therefore doubtful if the term accretionary prism should be applied to the whole of the Rajang Group, for only its Cretaceous part can be paired to a volcanic arc,
254
c . s . HUTCHISON
within the Schwaner Mountains. The PalaeoceneEocene part should not be classified as an accretionary prism, for subduction-related volcanoplutonic activity had ceased, and the situation was one of collision (Fig. 2). A province of abundant small epizonal stocks, sills and dykes extends from the Kuching Zone throughout the Ketungau Basin, with maximum development within the eastern Ketungau Basin (Williams & Heryanto 1986). They have been named the Sintang Intrusives (Williams & Harahap 1987). They range from pyroxene andesite to microgranite and are I-type. K-Ar dates from Sintang range from 30--23 Ma in the south and 18-16 Ma in the north. The stocks preferentially intrude the basinal strata, and have metamorphosed the Silantek Formation coals to anthracite (Tan 1979). Volcanic rocks overlie the Belaga Formation in the Usun Apau, Hose and other massifs (Kirk 1968). They are equated with the Metalung Volcanics, which overlie the Embaluh Group of Kalimantan (Doutch 1992). Although poorly studied, they appear to be Pliocene for they overlie outliers of the Nyalau Formation. In Kalimantan, they have been dated by the K-Ar method within the range 1.6-8.2 Ma (Bladon et al. 1989). They have been extrapolated much farther east, where they intrude the Mentarang Formation (BRGM 1982), and have yielded similar dates. However K-Ar ages as old as 25 Ma may not be from equivalents of the Metalung volcanics.
Ketungau Basin In Sarawak, the basal formation of the Ketungau Basin is known as the Silantek Formation. Only the lower part, which outcrops adjacent to the Lupar Line, can be dated by fossil content, but the age range is generally considered to be Upper Eocene to Miocene. The Upper Eocene Basal Sandstone Member outcrops on the Marup Ridge and is steeply dipping (Tan 1979). The strata flatten out southwards away from the Lupar Fault. The average down-dip palaeocurrent direction of the lower member is 228 ° from ripple marks and 194 ° from cross bedding (Tan 1979) indicating that the provenance was from the northeast. This is consistent with the petrology of the conglomerate and sandstone, which contain clasts of quartzite, schist, meta-greywacke and mafic volcanics, indicating provenance from uplifted and eroding Lupar Formation and Layar Member of the Belaga Formation. The base of the lower member is nearshore to shallow marine, but the Silantek Formation rapidly becomes continental upwards and has red-beds in the upper part, followed by the thick flat lying continental cross bedded Plateau
Sandstone. The Sibu Zone was therefore in the process of inverting to form a landmass during the early stages of Silantek Formation deposition. The steep dips of the lower Silantek Formation adjacent to the Lupar Line imply that the Rajang Group orogenic belt continued to uplift after deposition of the basal Silantek Formation, resulting in the boundary becoming the Lupar Fault. The rapid elimination of marine conditions from the Silantek Formation indicates continuing inversion of the Rajang Group flysch-belt. Accordingly it is wrong to refer to the Silantek Formation (Ketungau Basin) as forearc (Williams et al. 1988), since the Rajang basin had been eliminated and the Sibu Zone transformed from an accretionary prism into an orogenic belt welding together the Schwaner Mountains Zone of Sundaland onto the Balingian-Luconia Province (Miri Zone) (Fig. 2). The Rajang basin, or Danau Sea, was not related to the younger South China Sea and should not be referred to as the Proto South China Sea (Haile 1994). The Lupar Fault is the bounding margin of the Ketungau Basin, which developed by extensional tectonics affecting the Rajang Group orogenic belt. Studies to the north in the East Natuna area have shown that a similar situation persists, where Tertiary basins are unconformably superimposed on phyllitic basement (White & Wing 1978), which can be equated to the Rajang Group. The basin margins likewise commonly form prominent faults, similar to the Lupar Fault, but the trend has been bent into a N-S alignment northwards from the coast of Sarawak. During their early development, the Ketungau and Melawi basins were one and the same (Doutch 1992). They have subsequently become separated by uplift of the Semitau Ridge.
Rajang Group of Sabah and East Kalimantan The Rajang Group of Sabah, as presently defined, is of much longer depositional duration (Table 1) and its stratigraphy and structure have not yet been satisfactorily resolved. However, it can be subdivided into a 'Lower Part', whose sedimentation history extends from Late Cretaceous to Late Eocene, and an 'Upper Part', extending from Oligocene to Early Miocene (Lim 1985). It is the Lower Part only which extends into Kalimantan and Sarawak. The Embaluh Group appears to continue east of 115° as the Upper Cretaceous to Eocene Mentarang Formation, extensively exposed and of distal turbidite character, strongly folded and displaying slaty cleavage (BRGM 1982). Northwards along
255
RAJANG GROUP AND LUPAR LINE, BORNEO
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256
C.S. HUTCHISON
strike in SW Sabah, it has been mapped as the Sapulut Formation (Collenette 1965). No slaty cleavage was mentioned by Collenette, who described the mudstones as 'fissile and indurated', although previous workers (see Reinhard & Wenk 1951) referred to the Sapulut, as well as the Trusmadi Formation, as a "slate formation'. The farthest extension into Sabah is known as the Trusmadi Formation, well exposed in accessible outcrops from Ranau (near Mount Kinabalu) along the main road towards Telupid. The predominantly Palaeocene-Eocene Trusmadi Formation is a tightly folded, generally steeply-dipping, phyllitic distal turbidite, characterized by quartz veining, and identical in appearance to large parts of the Lupar and Belaga Formations of Sarawak, but in general more shaly. The upper parts of the Mentarang and Sapulut Formations show indications of shallowing of the sea, and limestones and jasper conglomerates are recorded in both (Hutchison 1989). In Kalimantan, the Mentarang Formation is unconformably overlain by the Upper Eocene Sebuku Formation basal conglomerate, followed by reefal limestone (BRGM 1982). Turbiditic sedimentation was therefore eliminated by tectonic uplift by Late Eocene, a situation identical to that in Sarawak (Table 1). A folding and metamorphic event clearly must separate the Trusmadi from the unmetamorphosed and less deformed Crocker Formation. This event is named here the Sarawak orogeny. However, only fault contacts have been observed between the two formations (Collenette 1965; Jacobson 1970). The very sandy Crocker Formation turbidite, containing several extremely thick sandstones, is usually considered as Palaeocene to Lower Miocene (Lim 1985). The upper western part has been mapped as the turbiditic West Crocker and Temburong Formations by Wilson (1964). The latter is very fossiliferous and has an age of Oligocene to Lower Miocene. The 'East Crocker Formation' is less well known and its reported Palaeocene and Eocene age is poorly constrained palaeontologically. Structural relations between the "West' and 'East' Crocker have not been resolved. The Sarawak equivalent of the Oligocene to Lower Miocene West Crocker and Temburong Formations is the Setap Shale. The unconformity at the top of the Trusmadi and Mentarang Formations indicates that the West Crocker and Temburong Formations should not be included as part of the Rajang Group of Sarawak. Palaeocurrent analysis on sole marks of the West Crocker and Temburong formations (Stauffer 1968) indicate northwards directed currents. The provenance is most likely to be the Lurah and Long Bawang formations, which were uplifted before the end of the Eocene; an interesting heavy mineral
provenance study remains to be carried out. These shallow marine formations are considered the time equivalent of the deep marine Mentarang Formation (BRGM 1982). If the Mentarang, Sapulut and Trusmadi Formation turbidites represent an accretionary prism, then the Late Cretaceous to Eocene paired volcanic arc, which cannot be seen in Sabah, needs to be sought in the northern arm of Sulawesi, which was contiguous with eastern Sabah before the Late Eocene opening of the Celebes Sea (Hutchison 1989). However, the volcanic arc activity of north Sulawesi began in the Early Miocene. Older rocks have been interpreted as ophiolite, with preTertiary metamorphic and intrusive rocks (Carlile et al. 1990). The Upper Cretaceous to Eocene Tinombo Formation in the north arm of Sulawesi contains only minor calc-alkaline and tholeiitic suites (Kavalieris et al. 1992). An outstanding problem remains, and that is the relationship between the shallow marine Upper Cretaceous to Eocene Lurah Formation (BRGM 1982) and the West Crocker-Temburong Formation. The areal distribution of formations indicates that while sediments of the West Crocker Formation were being transported from the south, there lay a landmass predominantly made of the Lurah Formation in that direction. Just across the border in Sarawak, the Upper Cretaceous to Eocene Kelalan Formation (Fig. 1) has been mapped by Haile (1962). He correlated it with the Belaga Formation, and outcrops at Batu Gading are of turbidite identical to the Belaga Formation. In the region south along strike from the West Crocker and Temburong Formations, Haile (1962) mapped the shallow marine coal-bearing Kelabit Formation, which has yielded good Oligocene to Lower Miocene fossils, and it is the proximal along-strike equivalent of the deeper water West Crocker and Temburong Formation turbidites. The Kelabit Formation is probably equivalent to the Long Bawang Formation, mapped in contiguous Kalimantan in a region probably underlain by evaporites, for all groundwater wells produce brine (BRGM 1982).
Uplift o f the Crocker Formation Deformation resulting in spectacular episodic uplift of the Crocker Formation is well documented in the offshore oil fields by several prominent unconformity surfaces (Levell 1987; Tan & Lamy 1990; Hazebroek & Tan 1993). The first is the Deep Regional Unconformity (15 Ma, Middle Miocene) and the latest is the Shallow Regional Unconformity (9 Ma, Upper Miocene). The tectonic front, or northwestwards limit of tectonic deformation and uplift, progressively moved ocean-wards
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Fig. 3. (a) The main structural elements of northwest Sabah (after Tan & Lamy 1990; Rice-Oxley 1991). (b) Late Miocene palaeo-environments for stages IV E/F (after Rice-Oxley 1991), showing also the positions of identified slump scars along the Late Miocene continental slope (shelf edge), from Levell & Kusumajaya (1985) and Hutchison (1995). (c) Sea-ward limits of the important regional unconformities, which successively migrated away from the tectonic front of the uplifting West Crocker Formation fold-thrust belt (based on Levell 1987). Palaeo-environments: LCE lower coastal plain; C, coastal; HIN, holomarine inner neritic; HMN, holomarine middle neritic; HON, holomarine outer neritic; BAT, bathyal. Unconformities: DRU, deep regional unconformity; LIU, lower intermediate; U1U, upper intermediate; SRU, shallow regional unconformity. Oil fields: 1. Samarang; 2. Glayzer; 3. Erb West; 4. Ketam; 5. Tembungo; 6. St Joseph; 7. SW Emerald; 8. Furious; 9. Barton.
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(Fig. 3). The unconformities extended quite close to the continental slope (tectonic front), so that the fluvial system brought sands to the edge of the very narrow continental shelf to spill over the slope into the bathyal area as submarine slumps, such as the one described by Hutchison (1995), and to feed turbidite fans. The persistence of outer neritic to bathyal marine conditions, without unconformities, west and northwest of the tectonic front has been documented by Rice-Oxley (1991). The Late Miocene tectonic front of the West CrockerTemburong orogen approximately coincides with the Jerudong Line, Morris Fault, and Transition zone between the Inboard and Outboard belts of Shell geologists (Fig. 3). The Mid to Late Miocene inversion of the West Crocker-Temburong basin to form the Crocker Ranges of Sabah has never been given a name, but the term Sabah Orogeny seems appropriate. Its cause remains obscure. The spectacular Late Miocene granitoid forming the core of Mount Kinabalu (4101 m) is a product of this orogeny, with whole rock K-Ar dates commonly as old as 10 Ma (Jacobson 1970).
The Sabah orogeny regionally This orogenic event was widespread throughout the South China Sea, but only in the Zhu Jiang Kou (Pearl River Mouth) Basin has it been given a name: Dongsha Movement ( S u e t al. 1989). However, this is not a good regional name, since the inversion there is not spectacular. The nearby Song Hong-Yingge Hai Basin contains very impressive Middle to Late Miocene inversion structures, well displayed in regional seismic records, but they have not been published. The best published account of spectacular basin inversion is by Ginger et al. (1993) in the West Natuna area, where the Anambas Graben became the Boundary High during Middle to Late Miocene inversion. The eroded anticlines are unconformably overlain by fiat lying latest Miocene-Pliocene Muda Formation. In the contiguous Penyu and Malay basins, inversion structures have not yet been analyzed and described because of a lack of seismic resolution at depth. However, the Upper Miocene unconformity has been well documented
in the southern end of the Malay Basin (Armitage & Viotti 1977; Nik Ramli 1988). Even though the Middle to Late Miocene inversion structures may not have been identified and described, the Upper Miocene unconformity is widely documented throughout Southeast Asia. All of the South China Sea basins rapidly returned to post-inversion subsidence after erosion of the inversion structures, so that the basins remain today beneath sea-level. Much farther south, the Late Miocene inversion is well displayed in the Meratus Mountains, which formed the basement to the Eocene through Miocene sequence, then rose up only in the Late Miocene to form a ridge which subdivided a formerly much more extensive basin (Hutchison 1992b). The post-inversion unconformity is also seen in Central Sumatra (Hutchison 1993). Both in the Meratus and Central Sumatra the uplifted basins have not subsequently subsided beneath sea-level.
Summary Although the interior of Borneo remains poorly known, the following broad evolutionary scenario may now be summarized. The turbiditic Rajang Group of Sarawak may be usefully extended into northern Kalimantan and central Sabah. It may be interpreted as an accretionary prism, compressed and uplifted between the Schwaner Mountains volcano-plutonic arc and the Luconia-Balingian-Miri microcontinent of Sarawak by the Upper Eocene Sarawak orogeny, to become an integral part of the extensive Upper Eocene Sundaland landmass upon which the Ketungau and other continental basins were developed. The Oligocene to Lower Miocene turbiditic West Crocker and Temburong Formations of Sabah should be excluded from the Rajang Group. Their nearshore equivalent has been mapped to the south as the Kelabit and Long Bawang Formations, suggesting provenance from eroding Rajang Group outcrops of the Sarawak Orogen. The West Crocker and Temburong Formations were deformed and uplifted by the Middle to Upper Miocene Sabah orogeny.
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Tectonic and Geologic Evolution of Southeast Asian Seas and Islands. American Geophysical Union,
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Geophysical Monograph, 23, Washington, 89-104. & -1983. Origin and history of the South China Sea Basin. In: HAYES, D. E. (ed.) The Tectonic and
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the British Territories in Borneo, Kuching, Memoir, 11. ZEIJLMANSVANEMMICHOVEN,C. P. A. 1939. De Geologie van het centrale en oostelijke deel van de Westerafdeeling van Borneo. Jaarboek van het Mijnwezen in Nederlandsch-lndie Verlhandelingen, 7-186.
Origin and tectonic significance of the metamorphic rocks associated with the Darvel Bay Ophiolite, Sabah, Malaysia S H A R I F F A. K. O M A N G 1 & A. J. B A R B E R 2
1 Department of Earth Sciences, Faculty of Science and Natural Resources, Universiti Kebangsaan Malaysia, Sabah Campus, Locked Bag No 62, 88996 Kota Kinabalu, Sabah, Malaysia 2 SE Asia Research Group, Department of Geology, Royal Holloway, University of London, Egham TW20 OEX, UK Abstract: Banded hornblende gneiss, foliated amphibolite, hornblende, chlorite and siliceous
schist form lenses in an 8 km wide belt within the Darvel Bay Ophiolite Complex. Foliation in the belt is generally steep to vertical, striking parallel to the trend of the belt and lineations are sub-horizontal. Mineral and geochemical studies show that the metamorphic rocks represent banded and isotropic gabbros, plagiogranites, doleritic and basaltic dykes, basaltic volcanics and cherts formed at a spreadingridge in a supra-subductionzone environment, which were deformed at high temperatures but low pressures along a transform fault. Incorporation of supracrustal cherts indicates that the transform extended for hundreds of kilometres between spreading centres. Garnet pyroxenites and amphibolites found as clasts in Miocene volcanic agglomerates formed at high pressures, and temperatures are interpreted as derived from a metamorphic sole underlying the complex, formed during subduction of ocean crust and the emplacement of the ophiolite complex on Sabah.
Ophiolite outcrops are distributed throughout the eastern part of Sabah, East Malaysia, from Banggi Island in the north to Ranau and Telupid and the Lahad Datu area in the south (Fig.l, inset). The Darvel Bay Ophiolite Complex in the south is the most extensive outcrop, extending 100 km westwards from Darvel Bay. The greater part of the complex consists of peridotite, largely serpentinized, but it also includes cumulate pyroxenites, layered and massive gabbros and diorites, metamorphosed to varying degrees. The complex is also cut by dolerite dykes, although these are never so abundant to be termed a sheeted dyke complex. These rock types are closely associated with outcrops of the Chert-Spilite Formation, composed of pillow basalt, banded ribbon chert, turbiditic sandstones, mainly volcaniclastic but with some rare quartz sandstones, and a few occurrences of massive limestone. The ophiolite complex has been well described in the publications of the Geological Survey of Malaysia (Reinhard & Wenk 1955; Fitch 1955; Dhonau & Hutchison 1966; Koopmans 1967). The complex has been interpreted as a segment of ocean floor, either of a Proto-South China Sea (Holloway 1981; Rangin et al. 1990) or of the Celebes Sea (Hutchison 1988). A wide range of K-Ar age dates has been obtained from the rocks of the ophiolite complex from 210Ma (Leong 1971) to 137 Ma
(Rangin et al. 1990). Cherts from the Chert-Spilite Formation have yielded radiolaria of Lower Cretaceous age (Leong 1977; Rangin et al. 1990; Aitchison 1994). Massive limestones associated with the Chert-Spilite Formation contain Cretaceous foraminifera (Leong 1974). The ChertSpilite Formation is interpreted as representing ocean floor sediments which were deposited on top of the ophiolite (Hutchison 1975), and carbonate cappings to seamounts. Since the underlying oceanic crust is unlikely to be much older than the oldest overlying sediments, the Early Jurassic K-Ar ages are regarded as spurious (Hutchison 1988). Fragments of ophiolitic rocks are found as clasts in Eocene sediments (Newton-Smith 1967; Rangin et al. 1990), suggesting that the ophiolite complex had been obducted onto Sabah either in the latest Cretaceous or earliest Palaeogene. In the early Miocene the ophiolite complex formed the basement to a volcanic arc, possibly related to continued subduction of the Proto-South China Sea. Mitchell et al. (1986) and Rangin (1989) have suggested that, as a result of continued compression, the complex was backthrust over the Celebes Sea floor to the south. The ophiolite complex is surrounded by chaotic melange deposits (Fig. 1) which contain fragments of all the rock units represented in the complex and the Chert-Spilite Formation, as well as
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolution of SoutheastAsia, Geological Society Special Publication No. 106, pp. 263-279.
263
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Fig. 1. Geological map of the Darvel Bay Ophiolite Complex near Lahad Datu showing location of samples used in this study. Inset map shows distribution of ophiolite complexes in Sabah. B, Banggi Island; R, Ranau; T, Telupid; DB, Darvel Bay.
y o u n g e r sediments and volcanic rocks. Clennell (1991) considered that the melanges were f o r m e d by p r o c e s s e s o f s e d i m e n t a r y s l u m p i n g and diapirism, coincident with the collision o f microcontinental blocks with the north Sabah margin and the extension o f the arc basement related to the d e v e l o p m e n t o f the Sulu Sea in late early and early mid M i o c e n e times. The ophiolite c o m p l e x and the melanges are overlain u n c o n f o r m a b l y by Late M i o c e n e to Pliocene sediments o f the 'circular basins' and in the Quaternary again f o r m e d the basement to a volcanic arc which extends from M i n d a n a o in the Philippines, through the Sulu archipelago to the Dent and S e m p o r n a peninsulas o f Sabah.
Metamorphic
rocks
A l t h o u g h the rocks f o r m i n g the Darvel B a y Ophiolite Complex are variably m e t a m o r p h o s e d throughout, d y n a m i c a l l y m e t a m o r p h o s e d rocks are concentrated in an E - W belt c. 8 k m wide extending westwards from Lahad Datu on the north side o f Darvel Bay (Fig.l) which can be traced for c. 40 k m to the west. Metamorphic rocks are well exposed in coastal sections around the shores o f Darvel Bay and particularly around the small islands within the bay, w h e r e they have been described previously by D h o n a u & H u t c h i s o n (1966) and Hutchison & D h o n a u (1969, 1971) f r o m the north side o f the bay, and by Koopmans (1967)
DARVEL BAY OPHIOLITE, SABAH
from the south. Inland exposures are poor, apart from road-cuts, quarries and scarce river sections. The metamorphic rocks have been described as metagabbros, amphibolites, hornblende gneisses and schists. Reinhard & Wenk (1955) described the dynamically metamorphosed rocks as 'Crystalline Schists' and Koopmans (1967) identified them as the 'Crystalline Basement Complex', with the implication that they were the oldest rock unit in Sabah. However, K-Ar dating reported by Dhonau & Hutchison (1966) showed that these metamorphic rocks are of Cretaceous age, the same age as the Darvel Bay Ophiolite Complex, and form an intrinsic part of the complex The structure of the dynamically metamorphosed belt has been fully described by Dhonau & Hutchison (1966). The metamorphic rocks are typically banded, schistose and lineated. Through most of the belt the banding and schistosity are vertical or steeply dipping and strike E-W, parallel to the general trend of the belt, swinging round to a more NW-SE trend in the islands of Darvel Bay. The lineation is sub-horizontal, lying in the plane of the schistosity and plunges at low angles either to E or W. From their mapping of the structure in the metamorphic rocks, Dhonau & Hutchison (1966) identified two large-scale open postmetamorphic monoclinal flexures on E-W axes affecting the foliation in the islands of Pulau Saddle, Pulau Bohayan and Pulau Silumpat. This paper gives an account of the field relations, petrography, mineral chemistry, geochemistry and K-At isotopic dating of the metamorphic rocks associated with the Darvel Bay Ophiolite Complex and discusses their origins and significance in the tectonic evolution of Sabah. A separate occurrence of metamorphic rock is found among Miocene volcanics at Tungku, 50 km to the east of Lahad Datu (Reinhard & Wenk 1955). Garnet amphibolite from this locality was also included in the present study.
Banded hornblende gneiss The dominant metamorphic rock type in the metamorphic belt within the Darvel Bay Ophiolite Complex is a banded gneiss, the 'Silumpat Gneiss' of Dhonau & Hutchison (1966) and Hutchison & Dhonau (1971), with alternating bands of white feldspathic and black or dark greenish ferromagnesian bands, well exposed on the north shore of Darvel Bay, west of Lahad Datu and in road cuts along the Silam Road. The bands range in thickness from tens of centimetres to millimetres. The banding is paralleled by a schistosity, with the alignment of feldspathic and ferromagnesian aggregates which may also be elongated to form a rodding type of lineation. The intensity of the
265
schistosity increases in localized shear zones. The banding is commonly folded into tight intrafolial isoclinal folds which may show the closed eyed outcrop pattern characteristic of sheath folds. Where folds are present the schistosity is parallel to the axial planes of the folds. In the more intensely deformed shear zones aligned hornblende and feldspar crystals define a mineral lineation, lying in the plane of the schistosity.
Amphibolite Amphibolites are commonly found associated with the banded gneisses in the field. These may form extensive outcrops, as on the northern side of Sakar Island or as smaller bodies which may show crosscutting, intrusive relationships to the hornblende gneisses. The grain size of these amphibolitic rocks is variable. Coarse grained amphibolites show the characteristic distribution of white feldspathic and black or green ferr
