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Geologica l Structures and Moving Plates t'
A.G. PARK, SSe. PhD Read er i n Geo log y U nive rsity 01 Keele
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Blackie Glasgow an d Lon don Pub li shed in the USA by Chapman and Hall New York
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Black ie Ilt Son Lid Bish o pbr igg s. Glasgow G64 2NZ 7 Lei cester Place, Lond oo W C2H 7BP Distributed in the USA by Chapman and Hall in associa tion w ith M el huen. I.,c 29 w es t 35th St, New Yori, NY 1000 1- 2291
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1988 Btackie Ilt 50rl lid Fir51 published 1988
All rights reserved No part o f this publiQIr;on m ay'" reproduced. stored in a re lfieval sys tem , or ' ra nsmirtttd. in any for m o r by "n r me"ns. electronic. mechanical. recording or o therwise. withou r prior permi$$ion of the Publishe rs British Libr ary Cataloguing in Publicat ion Dat a
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Par k, R.G. Geological su uclures and moving plates. 1. Prospecting 2. Plate tec to nics I, Til le 622' , 15 TN2 70 ISBN ()'2 16-92249-6 ISBN o-216-92250-X Pbk
libra ry of Cong ress Calaloging-in.f'ubliution Dat a Park, R.G. IR. Grahaiml Geological st rUClures and mo ving platn. Bibliography : p . Incl ude s index. 1. Geology, St rUChHa J. I. Titl e.
OE601.PJ 45 1987 ISBN 04 12-0 162 14
2, Plate tectcmcs.
551.1'36
87-6390
ISBN 04 12-01631-1 (pbk.) Filmset by Best-set "rv oesener lid Pri nted in Great Britai., by Tho m so n Lit ho LId., East Kilbr ide, Scotland
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Preface The great classical tectcnicians, such as Suess, Argand and Wegener, att em pt ed (0 und ersta nd . without the benefi t of th e plate tecto nic theory. the workings of the Ea rt h engine as a who le, and the part that deformation played in that who le. In my student days, I deri ved great pleasure and benefit from De Sitter's textbook on str uctura l geology where the study of geo logical struct ures a nd maj or Earth structure received more or less equal rreatment. Since then, until relatively recently, there has been a tendency for str uctu ral geology to become
more parochial an d inward-looking, despite the enormous ad vances in und erstand ing that th e pla te tectonic revolutio n has brough t ab o ut. I have long felt the need, th erefo re, for a bo ok tha t wo uld give students a tectonic overview in which geo logical structures an d deformation could be seen in their context as byproducts of the plate tectonic system. This boo k att empts to integrate struct ural geology and plate tectonics (often taught quite separately) by dealing wit h the theo retical background knowledge necessa ry to understand plate movements an d plate interactions. Thus the mechanical properties o f plates, sources o f stress and stress dist ribution in thelithosphere, and the causes of pla te mot ion , are examined first, followed by a d iscuss ion o f the kinematic aspects of relative-plate movements and interact io ns. The seco nd part o f the book deals with so me modern case st udies - examples where presentday structures ca n be related wit h some degree of confidence to plate movements, such as the Central Asian co llisio n zo ne, th e Lesser Antilles subduction zo ne in the Ca ribbea n, an d the Rhine- Ruhr rift system. T hese are discussed in terms of four main types of plate tecto nic regime:
divergent, convergent, str ike-slip, and intraplate. In the th ird section o f t he book, example s of class ical orogenic belts, o f bot h Phanerozoic a nd Precam brian age, a re discussed a nd interpret ed in the light of the principles established in the earlier cha pters. Thus the Alps are discussed in terms o f Africa n - Euro pea n plate interactions, and the Cordillera n orogenic belt in terms o f Mesozo ic subd uction and subsequent str ike-slip co llage tectonics. A more speculative approac h is necessary in the Precam brian examples, where the d iffering tecto nic styles of, for example, the mid-Proterozoic Gre nville P rovince an d the Archa ean greensto ne belt terrains may reflect genuine d ifferences in lit hosphere behavio ur. T he boo k is ai med at readers who a re already familia r with th e basic principles and nom enclature of geotectonics and structural geology, who understand plate tectonic theor y a nd its su pporting evide nce, an d who are familiar with its cent ral role in modern geology. In orde r to keep the book to a reaso nable length, I have deliberately concent rated o n the role o f geological st ructure in plate tecto nics and resisted the temp tat ion to include more than passing reference to ot her releva nt to pics, such as petro genesis. Finally, I am indebted, firstly, to two anonymous reviewers who read the first draft of the manu script and made a number o f helpful suggestions for its improvement; secon dly, to the many a ut ho rs who have allowed me to reprod uce dia gra ms from their publi shed work (these are ind ividually acknowledged in the figure captions); and thirdly, to my wife, friends and co lleagues who have bo rne wit h me thro ugh the tra uma of the writing o f t his work . RG P
To those structural and tectonic geologists who, throu gh their stim ulating lectures and papers, ha ve nourished my interest in geotet:tonics over the y ears, and prov ided the motivation 10 write this book
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Cont ents Introduction The rl ~le -I(:l;l tmic ' rn ol ullOn' The rok of the hlhol.phcrc
~
,,
I'l.ate motion
2
Tbc lm f'Ol13ncc o f plate ooun.J...ri c s Gcol ogical slruct urcs Q ro go;: nic or mobile bel l' in lhe JCOlog lC al r asl Tbe prohk m o f .he Prceamorian
3 3 4
•
2 The lilhosphert': so me importa nt
proper ucs
5
2. ' Lil~ rc . ~l hcnos~l(' ilnd mo:"''''phcl c 2.2 Some sho rl-lc rm mcchanin l prcpcnes
s
Of l hc " l ~ rc
Summ ary 2. 3 Thcrmal sUIK'1 UfC o f Il'te Iilho..phc rc Sum ma ry 24 M.tnl lc con veclion : lhe" role o f Ihe" hl hO$phc rc Modclh ng ron veCllo n Summa ry 2 ' SourCC$of,u~ in Ihc h 'hoophcre Pla te boundary forces l .oad i n g ~ ~
No n·re nc:,.,ab lc 'He M socrces AmplifICation o f ~. reu Summary 2.6 11IC
3
Plate m o vement a n d p lat e h ound a ri e s
Diverge nt (ext('nsion aIJ It'(( nnic regimes 73 4 , I T )pcs 01 <: lIl; n~(ln a l rCI:,mc 4,2 Ou:lIn ndgc ~ l ttl;m d 43 Conllnellt"l, i!lS The Afr~Ar ..hial\ nfl , )'\ tem The: Keny.. Of Eir.lcrn Rlfl The G ulf of Suez Or igin or tbe n f. S~ lcm Thc Rhine Rirt 4' Elllcnwna l rI OVlOC~ at cu nvcrgenl houndilna The B.1$in·a nd ·R an gc pnwlnCC
,
RL'<.Um.:
B..cs -..rc SPlt....:!'"g N~ ins
4. '
III
5
IS
C on verge n t tec to nic r e gimes Geomc lry o f t rcnch \ y, .eTm Mor phol og y a nd str uct ure of ~I ..nd UC\ 5.: i\ mil:it y and t he" mechamsm o f
22 24
24
:.uhdUCl l()fl
" '"ae
Stru ct ure of acc re tiona ry
27
52 Some
29
Act i_e co lll,i. ," beus G ro"s st ruc une o f collisio n Ilclb Fl•• ke rccuoeics and onducnon Thr ust he lls Inde n t..tion A m;,thematica l mode l o f a coi llsin n l one Th e Hima b y;" a nd Ce ntral A,ia T he Ce nt ra l Asia n collage Deep snuc t urc o f t he H ima layas and Tibet Str uct ur e o f t he Wc sle rn Hima la ya 5.5 So ut heast Asia A n mco mple tc co llage Timor
JI 37 37 41 41
"
47 47
47
4'
6
"" 54
"
St r i ke -s l i p a n d o bllq ue-s tt p r e g imes
0. 1 Ch ar a ctc ns uc s of st rike -slip regimes Ca uses of geo me trica l co mp le xit y 0.2 Drsptaccd or e xot ic te rranes 0 .) The SOl n An dre as rauuac oc Sc ismici ty Sire )) and hca t Displacem e n t gc o mclry St ruc tu re o f Ihe SOIn l" ~b na d ill r;ct Structure of thc Bil! Mc nd regio n Ocenmc tr a nsfo rm f.. u lt ~ The Charl ie G ,lIn.. a nd Gloria It ect ure zoncs Fasl-,liWlng fracl u rc zoees o n the f. aM Pacific r id ge
5~
Migfil io n o f p la te bou ndarjes Stahle a nd ulll>l ahlc mple j un Clions 3.2 T he influc nce o f pla te geo me try o n t he k ifMOm atic patt e rn A hwlu.c plale mol ion 13 T he d fccls o f rela live plal e moTio n. t p1alc bounda ri" Mo ve me nts across OIl ddor mahlc boundary Summa r}'
subd uctioe zones
5.3 Cclhcon
J2 J2
3.' Kine ma tic behaviour o f p lates
;l(1 ' ~
com f'1c 1~
11IC Pe r u Irc no.:h be l....c c n 1"· an d 14"5 The Barhados ridge lUmpkx T he M al rdn CO ml' le . 1ll<' Aegea n ar c
29 JI
now
51>
,..
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70 71
72
v
K7
'" 91
" " '3 W,
99 It"
IUS
5 I SuM uc, illn
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S II UCIUICS i1sSQl;Iillcd w ,lh ( "<:",, io n,, 1
n:glmcs
'01
II
73 73
112 11 2 113 11 3 11 3 11 4 J22 J22
125 133 ' JI
, 31>
119
'40 ''I '44 ' 47
147 ISO
IS' 154 ' 54 IS' IS.
10 ' 166
'00
167 171 I7S
177 177 In
17. '"
I" I'" ' 87
GlOlOGICA I. S I I1 UCTU Ill'$ AND MOVING " I.Al k 'i
7
Intraplate tectonic
rc~imes
7 . 1 1)[...... "nd rh.. ,a(1eri'>l lOo
IXX
S,-l The C;lk dnn i.." uru~en ie hell o f lhe Norl h Al l"nI 'e ro:.:,.. " I{c gi" n..I "':lImS TC~I"nic ' ul"'I,\lI..... .., " f the B, ili.Ji 1 ~1c.." The NW f"leI,lAd (l' m.,' I) ~t rud ,,'e: " f the M. '; ne th rLlSI b\..11 111c N" n h..- rn lI il!h1;>,""" le llanc: hUlle 2)
or inl r" l'l ..lc
''''
' l UM1 u rc
D,·'.' Tmin.lhu" .. r'... .,.·nl \\:n tc;al
.....,,,,' mc nh in ['bh: in l..,ri..."
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72 The R tr>....;a n pl"d.. rm : .. IY!"'-":Il tnlf,,('b I..rq :ion'?
,., ,~,
7.J InU" rbh: ......in.. 11lc P",.\ h.l ..in
Tbe (j ' ;UUriol n I " ghla n.h. le" anc It ''nc: 3) Tbc MKlt m,J Val k y ( l nnc 4 ) n l,' S.. " th'''ln Uf'l..noh ( Ztllle 5)
'''' l<,l:'i
llJl: Mio.i liF-"''' O"..;n 11M:' T~,Utk n i M in ( higm "r IOlf;lp l;,lc "',,,in, 7. 4 Ell:Imrlc~ o f :ad'vc ma rine t>
1 h,: SO Kllh ..... 11..'" the So l....a}' sul u re ( lOnc~
'''' '. 7
" -It)
11lc x an,J in"vi ,m C" k:..... n"J~ " I" Ie:1''Clun ic inlc rpr " l ,ll itm "rtne North ,\ 11,,1111<,: ( ·"k:.kll1i.k s
So: a a nd lhc At lan l ic ....HlllrlCnlal m ;lrf:1lI
nr lhc US A Tbc Nort h Sea b asin TIle A tI" JlI;C con tin enta l ma'!:in (If the nor the rn USA 7.5 Inlr.lpl;llc up lifl'
Th e Fen ntlso,: nm h an up lifl
Th e ( 'u lu rad u PlaIC;IU O rit:in " f i01wpl a lc up lift.
8
Phaueroz nic oro geni c bells : som e e xam ples
~"
21. 1
""
211l\
<;,2
»7 210 2111
R(g i<.ln;.ol lcC1" nicc...nlc~1
21Z
l l l)lo ry of pi'll ': movcme nrs
214 2 15 221
Thc.' Cnulillc:nm ..... danc.l l h ru~1 he ll tn lhe IClTil nc:, R J The Uc fcy nia n u rugen ic he la .lfWc ,...·rn EulUpc and NUr1h Ameria. The Allc ghc nian belt The WOo l Eu rope a n !oC\.1 '" The SW Dr i l i~h b les Pla tc -tcc tcmc in tc rp rcranon of lhe A lla nlic H ercyn ian region
Orogeny in the Pre cambrian 9, 1
Mc-dih.:rrilnca fI
S, (".anad i:an !'o«l,)r The we, l,·f n .:ul l:ogc te lOC of , u"P"!
9
20(, 207
R. l The A lpmc " ,..g,'nil: beltof tile w cacrn
S l ruel ur.. 1It am c ....m ~ uh hc A lp< 8. 2 Tht.. C" n:Jillcl .. n .... .,genic hell u ( Nm l h Americ:..
"" ,'"....
\1 .3
'J.4
222 22(. 2;N
m
231 2](,
24 1
'69
'''' '" '8' '"
nr
111c Lew.";an Ctllllpk:_ \1 _5 The AIl'h ,'can : '" <J ifkrenl kintl o r "ruge ny" The Nor lll A lla nl ioc CU IO" G rcr n'h mc he ll' "r the S upc:flur Pruv inc c:
Refer en ces Indt"'\
269
Precambria n eh ru ncJlogy PI,.L! c move me nts in the Pre cam bnan 27(1 Pretc nvcuc pl,ll,- Illtwem ,'n ls 27J LOl l,' Prnl e lUlt".' Pa n-A rn ean lw,' It272 l',t1I·Al ric''' 1 h i' l
ia Thc Mi,j· I' ru l,·"..... i c G renville Svccon..r..·et:i,Lft 'Y"le m The G rcn "i l!.: P",,,irK·C Th c Sycc~ >rt,or" cgi;m hdl f>I"l c -le,"1" llIc m tcrprO:la l ltlft 01 lho: G r(' nvilk-S" ' 'C''Il<,r.. c~".n '~' l,·m Ea rly Pr..I,·w" »c 1....· 11, lhot.- N ,orlll A II;IIl1ic rCt:" ,n The l,.;ohrol<Jtor he ll Tht: l<.'.. · ;"'i " n · N a~,-,ugl<"l ",h ,,,, '~ e nl : Ihe " ~ (fn N"'p-lUgl<1...i.Ji:m
,.,,.. 30 '
]<J2
sn
3 13 327
1 Introduction The them e of this book is t he re lationship bet ween geo logical struct ures and plat e teeto nic theory. 711t, plate-tectonic 'revolution'
le is no w un iver sally ac kno wledged tha t t he plate tecto nic theo ry has bro ught abo ut a revolution in our percepti on of geology in almos t a ll its branches. In th e case of st ructura l geology , the a pplications of the pla te tecto nic model are par ticularly o bvio us. a nd ha ve affected in a funda men ta l wa y o u r interpret at io n bo th of geologica l structures in the na rro w se nse, and of o rogeni c be lts. T he two ke y disciplines o f struct ural geo logy a nd plait' leetonics (o r gf:ou! cum ;cs) a re usuall y ta ug ht se parately. and are o ften dealt with in diff erent textboo ks. Yet t he g re at ex pa nsion in research publica tio ns de a ling with th e a pplicat ions of plate tecto nic t he o ry to oroge nic be lts. a nd to the interpret a tio n 'I f geologica l st ruc ture s, deman ds an in tegrated a pproac h which it is th e aim of th is book to pro vide . Classical geologicalt heorists we re fascinat ed by o roge nic belt s a nd ot he r majo r ea rt h st ructures, a nd spec u lated wide ly o n their o rigin . J;ISiigGi fres e nts an int e resting ove rview of the evolution of classical ideas o n o roge nes is in th e ope ni ng cha pte r of 'Orogeny' by Miyashir o et (/1. (1982). This wo rk is a ve ry re ada ble account of the WilY in which the co nce pt of o roge ny has bee n tra nsfo rmed by pla te tecto nic theo ry. Th e weakness in pre -1970 theo ry la y in th e a bse nce of a ge nera lly acce pte d tec to nic mode l t hat could sa tisfac to rily expla in nOI o nly geological structures , but a lso th e dist ributio n a nd va riatio n bo th o f igneo us an d met a morphic activity, and of sedime nta ry fac ies. II is in t he successful linkage bet wee n previo usly unrel ata ble phe no me na th a i t he st re ngt h a nd success o f th e plate tectoni c mod el lie . T he co nce pt o f p lates ar ose fro m the o bser vation t hat la rge a re as o f th e crust have appa rently suffe red very lill ie la te ra l disto rtio n
a lth ough the y ha ve tra vel led sev era l tho usand kilome tres. if the e vidence of con tinental d rift is acce pted . T he det a iled a nd acc ura te ' jigsa w' tit of th e o pposing coas tlines o f Am e rica and Africa fo r exa mple . ..fte r ... ........ Jc'" of d rift , testif ies to th is lac k of distort ion. In th e oce ans a lso . a re fou nd reg ular linea r magne t ic stripes a nd fau lts th a t have ma inta ined thei r sha pe fo r le ns o f millions of yea rs . T his evide nce re in fo rces the co nclusions re ached by studyi ng th e d istri bu tio n o f tectoni c mo vements tha i there are large slab Ie a rea s (co ntinen tal c ra to ns a nd dee p oce a n bas ins) tha t s uffe r lill ie inte rn al de fo rma tio n and ex hibit o nly slo w vert ical mo vem ents , while at the sa me time mov ing la tera lly as co he re nt units a t ra tes 10 to toO limes faste r. T he recogn it ion o f the lit hosphe re pla te as the fundamental kinematic unit no w unde rlies t he stud y o f a ll su rface te cto nic prec esses. , ~ .",it jill"" Jut
...........
II
nd '
I
,
'4'!.......olJ.... a nd th is subdi visio n has re placed the o ld dichot omy be twee n oroge nic and allorogenic re gion s o r acti vity .
m
U 0.
T he vert ica l ex te nt of a plat e is defined by t he base o f the ~ .. th e stro ng o ute r laye r o f the Ea rth lhat res ts o n th e und e rlying we ak e r _ _ ws;JLli4P. TIle lithosp her e includes both t he c rust a nd pa rt of th e uppe r man t le , a nd has a n average thick ness of a ro und IOO km (sec Figure 2. 1). Th e co nce pt of t he plate is th er e fo re bo und up with the pro pe rt ies a nd be haviou r o f th e lith osphe re a nd asth e nosphe re. discussed in Cha pte r 2.
.
.
.
o (see 2.2 ). T he most use ful d istinctio n be tween th e lithosphe re a nd th e unde rlying asthe nosphe re is in terms o f the highe r viscos ity of th e for me r. Since th e visco sity is co ntro lled by the te rn-
2
GEOL OGI CA L ST RUcrU RES AN D MOVIN G PLATI::S
assuming a like ly value for the magni tude of the availab le stress so urces (fo rces) on it , it is possible using numerical mod ellin g tech niqu es to predict the st rengt h of that piece o f lithosphe re and the time tha t will elapse before failure . In this way, we can ultimately estima te the st ress cond itions acco mpanyi ng the formatio n of geo logical struct ures , hath at plate bo unda ries and also wit hin plates , at the time whe n new plate bou ndaries are init iated .
Fp'nU' ; in o the r word s,
• and 2.5). Th e study of the nature and influence of forces or stresses on a bod y is
~see
known as
t
' . (e.g.
plateau up lifts and ocean ridges). A numbe r o f me tho ds are available fo r the dete rmina tion of stress within th e lithosp here , part icularly in the upper part of the crust, where stress can be dire ctly measured (see 2.6). Th ese meth ods yield a bewi lde ring vari ety of estimates, bot h of th e act ual magnit ude of stress carried by the lithosphere , and of t he stre ngth o f the litho-
sphere. The orientation of stress on the other hand . usually hear s a simple relationship 10 the more o bvious loc al stress so urces . Th e dist ribution of measured st ress can o nly be understood by co nsidering th e lo ng-term strength of the lithosphere , th at is, its stre ngth over periods of the orde r o f ten s of Ma (see 2.7).
( the prin ciple of stress amplification.). Thi s exp lains why the magnitudes of the availa ble st ress so urces are so much s malle r th an act ual mea sured values near the surface . Know ing the temperatu re st ructure o f a given piece o f lithosphere , and
Geological st ruct ures are co ntro lled bot h by . - - (o r force) and by Th e .; aic study of the co nditio ns under which deforma tion takes place mu st be com plemen ted by th e study of th e relative mot ions of the vario us plates o r blocks conce rned. T his type of ap proach is termed br" 7 .#;. For al1 practical pu rposes, we can regard relati ve plate motio n as ta king place at constant velocity. T he accelera tions and decele ratio ns that occ ur take pla ce o ver such lon g periods o f time th at the forces ge ne rated are much too small to be significant. Plat e movements at co nstant velocity do not of them sel ves create forces, o r prod uce deform at io n. The forces associated with re lative moti on a re created by resistan ces across planar bo undaries o f relative moti on or visco us dr ag be tween two o ppose d mo ving bloc ks . In this way the kine matic and dy namic app roaches to defo rma tio n are linked . If it is accepted that plates can be regarded as stro ng rigid shells, th eir relative moti on across the surface o f the Ea rt h ca n be described in term s of the simple ru les o f motio n on a sphere . A ny relative mot ion betwee n two plates o n t he surface of a sphe re becomes an angular rotation abo ut an axis thro ugh the centre of the Ea rth , wh ich intersects t he Earth's surface at two poin ts ca lled the poles of rotation. for that movem en t. Th e direction o f movem ent at the surface of the Earth is parallel to a set of small circles abo ut th e axis o f rotat ion . As rea lized ori ginalis classic pape r on ly by ' any such fault s displacing the
INTROO UCI'IO N
boundary betwee n two plates must he pa rallel to these small circles ; th at is, parallel to the direction of relative mo lion betwee n the two plates.
P ies (II"') Iii n:tIi I: ; , F 'tt ,r '_ ue. T hey analysed the p""" .llI'_ direction of motio n from 5 !
and . . . .bo rde ring th e Pacific plate , and sho wed that they gave . .......on• ..................u·. ..(see 3.1). T his d irection is parallel to the small-circle arcs obtained from the or ientatio n o f the San Andreas fault. ....
'i ' ,it• • Relative plate velocities can be fou nd by analysing the magnetic str atigrap hy, so that for any plate pair sharing a spreading ridge , the movement vecto r . can be fo und (see Figure 3.2). a II 1 ,::d e wch' n l ' .. , • • Ii ,.... II 1'. .....1:2 l . .ti,U .S 6:b ?5 0' I b e P'h?5 a . ,..... C8Z\?h• •k:ubatah 6 4iRJi4iian I In th is way the relative motions of ali six major plates were dete rsee Figure mined by , 3,1. The principles gove rning plate kinem atic behaviour are d iscussed in 3. 1.
The obvious link between seismicity and present-day tecton ic activity suggested that the seismic zones must represent the bou ndaries of the stable blocks, and that each block or plate could be defined by a continuo us be lt of seismic activity (Isacks et al., 19(8 ). Since the seismic activity represe nts fault mo vements with high strain rates, each plate must be in a state of relative motio n with respect to each of
~
its neighbours, Ul~"::::::::::::::: are recognized : (i) :" ! ..~ where adjo ining plat es are mo ving ap art and new plate is being created at ocean ridges; (ii) iil!fj ad. .. f :ria where adj oining plates f
3
are mo ving toget her and plate mate rial is be ing destroyed by subductio n at oce an trenches ; ami (iii) _ ,; 7 'rsFllFhlM ' wher e adjoi ning plates are mo ving later ally past each o the r with a ho rizontal strike-slip sense o f d isplacement along tra nsfo rm fault s. The sense of di sp l ace m e n~ these bou ndaries can be ded uced from fi rst ·~ n studies of individual earthq uakes that in gene ral co nfi rm the re lative move ments inferred fro m other evide nce such as palaeomagnetism and magnetic stratigra phy. Fo llowing th ese prin ciples, a networ k of bo unda ries may be d rawn divid ing the prese nt ~ Eart h's surface into" (see Figure 3. 1): . " , "' i ~ , "" .__ . and . toget her with a number of smaller plates associate d main ly with de structive bo undaries , especially aroun d the margins of the Pacific Ocean . Continenta l m U Qim nwy or ma y pm co rrespond with c hlte bo undaries, T hose that do , such as the weste rn margin o f the America n contine nts, are ter med ' those that lie within pla tes a re termed j
'w_
T he position o f the plate bo undaries de te rmines the location of the mor e significant tectonic activity, a nd the type of bounda ry controls the natu re of th e tecto nic pro cesses ope rat ing ther e , Th e three fold division into constructive , destru ctive, and conse rvative plate bou nda ries is reflected in the o rganization o f this book. Structures of cur re ntly active plate boundar ies are co nsidered in terms o f th ree funda menta l types of tecton ic regime : divergent regimes relating to co nstructive bou nd aries (Chapte r 4) ; con vergent regimes relat ing to de structive bound aries (Cha pter 5); and strike -slip and oblique regimes relatin g to conservative bo unda ries o r to those with a compon e nt o f strike-para llel motio n (C hapte r 6), Intraplate regimes are d iscussed in Cha pter 7. Geological structures It is assumed that the reader has an adeq uate
wor king know led ge of structura l geology. A
4
GEOL OG ICAL STRUC r URES A."JD MOVING I'LAHS
familiar ity with the va rio us types of geological str ucture an d their o rigin is implicit in the description of struc tures in Chap ters 4-9 . In considering the relatio nship be tween geo logical str uctures and plate mo vem ents. certain aspects of struct ura l geol ogy are o bviou sly mo re relev ant th an othe rs. Orientation of majo r structures (fo lds a nd faults), intensity of deforma tio n, magn itude a nd or ien tatio n of the hulk st rain axes, an d st ruct ural sym me tr y a ll convey import ant info rmation abo ut the way in which the cr ust respo nds to re lative plate move me nts at pla te bo und a r ies. In assess ing t he region al tectonic significance of geological st ruct ure the refo re , the orientation o f folds and fau lts is critica l. and also particula rly the or ienta tio n of t he tran spo rt d irection , since th is will be re lated to the kinematic convergen ce (or dive rgence) direction. In high-st rain zo nes, this d irection will be close to the maximum principal stra in axis. In te rms of st rain , we are co nce rne d essentially with bulk prope rties and bulk geo met ry, and with how these re late to the large-scale kinematic pattern . Fo r this purpose , a gcometr ical o verview o r ove r-simp lification of the large-scale structural pa tt ern is more useful tha n a co nside ration of the structura l detail. at o utcrop sca le fo r e xample. The same pri nciple applies to strain rates - a very impor tant co nt rol on deformation . Bulk st rain rates relating to mo untain be lts o r la rge zo nes are cited, and can be compared wit h theo retical strain rates derived from th e mech anical behaviou r of t he lithosphere , but str ain rates o n sma ller sca les arc gene rally no t discussed . Symme try is anot he r impo rtant aspect of the la rge-scale str uct ure. The verge nce directi on . o r facing d irection , of thrusts. ov er folds and othe r asymmetric st ruct ures is of fundam ental significance in understandin g the way in which a piece o f crust has been defo rmed . So me structural geo log ists wo uld ar gue th at movements on low-an gle faults and she ar zo nes are the dom inant mechan ism in the bulk defor mation of t he crust. Such st ructures impart an obv ious asymme t ry to th e str uct ural pat tern . Tectonic processes dri ven by simple shear are
favoured in co nvergen t regi mes because o f the basic asymmet ry inherent in the subduction process. A similar basic asy mme t ry characterizes ,III strike -slip regimes . Rece nt studies o f exte nsiona l reg imes indic at e that qu asisymmetrical fa ult-bloc k ar range ments a t high crustal leve ls, by det achmen t at lowe r level s o n low-ang le deco llement planes. a re pa rt o f an asymmetrical system ove ra ll. Orogenic or mobile helix in/he geological pas /
Tectonic effects o f great interest to the structural geo logist are produced in act ive convergent regimes (see Chapte r 5). It is those that provide worki ng models that ca n he used to interpret the or oge nic belts. the co nve rgen t regimes of the geological past. C hap te r 8 e xa mines a selection of such be lts o f Phane rozoic age , com mencing with Mesozo ic-Cenozoic exam ples where the plate-tecto nic se tt ing is reaso nabl y well constrained . By showing how vario us work ers have interprete d structure in terms of plate tectoni c processes, the pr inciples underlying the relationship betwee n plate motion and st ruct ure may be illustrated . The pro hlem of {he Precambrian
Examples of Precambrian mobi le belts are discussed in C hapte r 9. Despite the fact th at t he Precambrian occ upies approximate ly eightninths of geo log ical time . remarkably lill ie is known abo ut how t he plate tectonic process o pera ted during th at period , o r indeed whet he r it ope rated at all in the ea rlier part of Earth histor y. T he examples discussed s how sufficien t simila rity to Pha nerozoic syste ms from the Early Prot erozoic o nwards to sugges t that t he plate tecton ic mod el is probably a pplicable, albei t in modified form , for the last 2500 Ma . T here is as yet no gene ral co nse nsus co nce rning the type of plate regime in the Arc haean, and even t hose indicat io ns that we have arc biased towards t he later part of t he Archaea n. lill ie o r no th ing is known abo ut large-scale structure prior to abou t 3000 Ma ue.
2 The lithosphere: some important properties The base of the lithosphere is usually defined o n the basis (If a relati vely rapid change in se ismic wave ve locity (o f bot h P and S waves) , and specifically of a fall in the rate o f increase of Vr an d V•• which takes place at de pth s of around l OO-1 50 km within the upper mant le . T his cha nge in ve locity is related to ch anges in de nsity and rheo logy, which in turn arc re lat ed to the geottverm - the profile of temperat ure vari ation downwards throu gh the crust and upper mantle . A ltho ugh there may be petrographic differe nces be twee n t he lit hosph eric a nd ust hcnospheric ma nt le , these arc no t the mai n factor in diffe rentiating the IwO layers, which a re dis ting uishe d most co nve nie ntly in te rms of their viscosity , The viscosity of the a S l he nos~ here is usua lly es timated to be aro und 102 1 _ 102 po ise in co nt rast wit h that in the lower part of the lithosph ere which proba bly varies from 102 .1 po ise upwa rds (see MeKenzie . 1':167). It is this ra pid decrease in viscosit y tha t ena bles the solid mate ria l of the asthe nosphere to flow at a gcologicatty significa nt rate . ca rry ing the more viscou s and therefo re stronge r lithosph e re above it. T he relative plasticity of the asthe nosp here is d ue mainly to the e ffect of e levated te mpe rature on the rheology of the material , which is governed by a flow law in which the st ra in rate is bo th te mperature- a nd st ress-de pe ndent (see 2.2 ). A significant fract ion of ast henosphe re mat eri al is believed to be co mposed of mel t
2. 1 Lit hosphere, asthenosphere and mesosphere
In his mod el of co ntinental dr ift , We gener ( IY29) o rigina lly visual ized pieces of continc ntal crust moving acr oss a pla stic ocea nic crust. However this id ea was abandoned many YC
ago whe n it was realize d t hat ocea nic rocks could not behave in a suffi cientl y ductile manner near the surface . When the plate tectonic model was bei ng dev eloped in th e 196()s, it was realized t hai t he moving plate s included ocea nic as well as co nt inen tal crust. The oceanic crust is only about 7 krn th ick and cou ld not re main und istorted whe n subjected 10 the horizont al st resses associated with pla te
tectonic proc esses. Plates must he there fore he conside ra bly thicker than the crust and include part of the uppe r ma nt le as well. The lithosphere is defi ned un seis molo gical crite ria ;:IS the strong o uter layer o f the Ea rt h: it can equ ally be regar ded as the coo l surface layer of the Ea rt h's convective system. T his convective syste m is d iscussed fur ther be low (see 2.4) but it is importa nt at the outset 10 point out that the lithosph ere can no t he COIIsidered in isola tion ; th e bo undary be twee n it and the und erlying asthenosphere is tran sitional. and is continua lly changing; and there is a consta nt interactio n be twee n the lithosphe re and the other par ts of the co nvective syste m (Figure 2. 1) . Oe Ur uc t ion
C onSlru~tion
}"iRurt 2. 1 A ~l hcnosphere -li thO"rhn .. - mc sos phc rc inrcra c1;on . Th e h lh""p her~ g;l i n~ ma le r;,,1 from the ;1~lh ~n
5
6
GEOLOGiC A L ST RUCTU RES AN O MOVING PLATES
(perhaps as much as 10% ) although th is may decrease away from sites of upwelling convection curre nts. Th e asthenosphe re is co ntinuo usly fed by up rising mater ial fro m the underlying m esosp here. Mesosphere is transfor med to asthenosphere simply by a change in rheol ogy -
t he
same piece of man tle mat erial may commence in the mesosphere. he changed into asthenosphe re at a site of convective upwelling, with a decrease in viscosity and increase in now rate. lt may subseq ue ntly be tra nsfor med again into lithosphe re by a reve rsal of this process - by cooling and a conseq ue nt rise in viscosity. In addition. of course, there a rc material changes: partial melting ca uses magmas to leave the system and move up into the lithosphere , volatiles may enter and leave the system, etc., so that the composition of the asthenosphere presumably varies significa ntly at the sites of upwelling, but is proba bly much more unifo rm be neat h stable plate interiors. T he lithosphere grows at the expe nse of the asthenosphere mainly at ocea n ridges. Here new lithosphere is generated by the cooling of asthc nospheric mat erial as it is carried laterally away from the hotter ridge axes. Again the syste m is very complex in detail, but can be simplifie d in te rms of a model of continuous accretio n of lithosphere alo ng an essentially ther mal bounda ry in the flan king regions of an ocean ridge (Figure 2. 1). As mate rial is carried across this boundary, it changes from asthenosphere to lithosphere. T hus altho ugh new material is be ing con tinuously emplaced below the ridge, and is thereafte r being transpo rted laterally away from the ridge axis, the actual boundary to the lithosphere is statio nary with respect to the upwe lling the rmal source, as long as the thermal conditio ns are unchanged. Material points within the lithosphere move although the lithosphere bounda ries themselves may remain fixed. Away from the ridge , the cooling of ocea nic lithosphere proceeds more slowly, although differences can be detected in no rmal ocean basin lithosphere thickness that are related to
its age (see 2.3). The older a piece of ocea nic lithosphere , the coo ler it will be , and the deeper will be the the rmal bounda ry which defines its base. It has bee n estimated that ocean-basin lithosphe re varies in thickness from arou nd SOkm at the ridge crest to about ISOkm in the oldest parts furthest from the crest. There is thus a simple relationship between thickness a nd age that can be determined fairly accu rately for most parts of the ocean using the methods of magnetic stratigraphy. The oldest and cooles t parts of the oceanic lithosphere are either attached to continents and co ntinue to migrate later ally with them, or form subducting slabs which desce nd through the asthenosphere to merge indistinguishably with the mesosphere. These slabs the n form the cool downward-flo wing limbs of convective 'cells' . They sink because they are cooler and therefore more dense than the surro unding mat erial. An oceanic lithosphere plate may thus exte nd from a ridge crest to a trench and therefro m down to the base of the asthe nosphere (Figure 2.1). As it descends. it becomes warme r and will lose mate rial to the asthenosphere by pa rtial melting. Al its base. it may disappear. o r brea k into sectio ns, but the material of which it is made will generally descend, carried down by the coo l return limb of the convective circulation. but indistinguishable seismically from t he adjacent mesospheric material. It thus appea rs thai the lithosphere plates, because of their strength and coottnulty, display a patte rn of movement which is pa rt of a mo re fundament al mantl e circulatio n to be discussed in 2.4. 2.2 Some short-term mechanical propert ies of the lithosphere The formation of geological structures is ultimately dependen t o n the mechan ical properties of the material in which they are formed . It is essen tial therefore to discuss the mechanical properties of the lithosphere in o rder to discover how. and under what conditions, geo logical deformation is produ ced . The important
I
I
I
I
rue LITHOSPHERE: SOMe IMPORTANT PROPERTIES
mecha nical properties which cont rol defo rmation are elastici ty. viscosity, fract ure streng th and yield st re ngth. Th ese properties var y with rock compos ition , depth and tempe rature . The infor ma tio n pr ovid ed by labor ator y experiments on rock ma terials can provide estimates for these mechanical par ame ters unde r a limited ra nge o f co nd itio ns. Howe ver , a more useful method o f stu dying the mechan ica l behaviour o f th e lithosphere as a whole is to make simplifying assumptions ab o ut bulk properties, and to use ma the mat ical mod els to determine how th ese propert ies inte ract and vary in changing physical co nditio ns (see 2.7). The main so urce of inform atio n abo ut the short-term mecha nica l pro perties o f lithosphere plates co mes fro m indirect geo physical methods, part icularly the study o f seismic waves. The velocity of se ismic waves provides information abo ut the elastic prope rties of a plate (elasticity a nd rigidity) a nd also , of course, its effec tive thickness, as explained above. Th e values of elasticity o r rigidity derived from se ismic wave velocities define the elastic prope rties of t ht:: lithosphere ove r very short time period s (0 . 1s to 1 h). Howe ver the lithosphere strength ca lculated in this way (often called the ' instantaneous st rength') is very much grea te r th an its stre ngth when subjected to forces for periods o f te ns of Ma . Useful informa tio n may also be gained from the study of lateral va riatio ns in bot h P- and S~ wave velocity. Figur e 2.2A is a map o f t he North American co ntine nt showing the variation in mean P,,-wave veloc ity (1' waves propagated in the up permost ma ntle ). Figure 2.28 shows in addition the variation in mean crustal velocity togethe r with co ntours of crustal thickness. T here is clea rly a crude re lationship betwee n these ; most of the Pvwave velocity variatio n in the crust app ea rs to be due to the crustal th ick ness variat ions. Latera l differences in t he lithosphere . however , are more evide nt whe n the velocity o f t he P" waves is studied (Figure 2. 28). Th e map shows a decrease in P" veloci ty from abo ut 8.2 km/s o n the North American crato n to below 7.8 km/s
7
in regions o f current or recent volcan ism a long the cen tra l Cord illera n orogeni c belt (fo r example in the Basin-an d-Range province and the Cascades volcan ic arc) . T hese are regions of warm er lithosphere with higher surface heat flow an d stee per geotherma l grad ients (see 2.7) which act to reduce Pcwave velocities in the mantl e part of the lithosphere . Very similar effects are see n in othe r curr ently o r recently active regions. For example the velocity o f P" waves beneath t he Japanese arc is 7.57 .7 km/s co mpared with 8 .0 -8. 1 km/s fo r t hose below the adjoining Pacific ocean . In continen tal rift zones and also at ocean ridges , the re is a co rres po nding reduct ion of P,,-wave ve locity co mpa red with nearby stab le regio ns. T hus the stu dy of t he regional va riat ion o f P,,wave velocity reveals zo nes of anoma lo usly weak , warm lithosphe re which must pe rsist for long per iods of time and are thus relevant to the lo nger-te rm st rength prope rties of the lithosphere as well as 10 its instantaneous stre ngth. SrI waves are S waves propaga ted in the uppe rmost man tle . Like Pn waves, thei r ve lo city is affected by cha nge in elastic pro perti es. U nlike P" waves though , th e S" waves do not pe ne trate the low-velocit y zone du e to their sho rt wave le ngth. The efficiency of tra nsmissio n of th ese waves is shown o n a wo rld-wide sca le in Figure 2.3 . It is clear that stab le plate interiors are zones of efficie nt transmission. whereas active tectonic zo nes (island arcs, ocean ridges etc .) corres po nd to zo nes o f inefficien t tran smission. A not he r geophysical method of studying lateral variatio ns in lithosp here stre ngth is t hrough the study of the dispersion of surface waves ( Rayleigh an d Love waves) . Dispersion is a mea sur e of the spread in am plitude and wavele ngth with in a wave tra in , an d occurs because of the variation in elastic modul us and density with distan ce from the so urce . T he depth of pe netra tio n of a surface wave is directly de pende nt on its wavelen gth . Thus if the velocities are ca lculated as a funct io n of wave lengt h. the rigidit y of the mater ial can be
8
GEOLOGIC A L ST RUCT U RES AI\'D MO VI NG PLAT ES
, .a
A
E stlm a' ed Pn ( k m/u c )
e,
y . foc U ~
1.9
8.0
B
"on ",u '.'
.... .",
>• .• , ../...
' .2 h , I . . .
n
"'9011,
< ..... o.~ .,. 1 < S. ' h,I..,
.... . . ,yo, .1
"'OO" r
< ' .2 8 oW• • •
<:"" to•• or
...."''''''''. .,". COM ••• •
0
••••,.t .".b .
,-
... ' . .. . 1 ' 0 , .
te , .
....
~o< " "
•• I..
r
1...1
on " do or
olt, < e k .., . ..
FiRllre 2.2 (A) Estimated P, seis mic wave veloc ity fo r uppe r man tic in the USA. Afl<:TWyllie ( 1 ~71 ), fro m Herrin ( 1969). (8) Var iatio ns in cr ustal thickness, mea n crusta l velocity, and uppe r me nuc Icl\>cll y (see A) in the USA . After Wyllie (197 1), from Paklser and Zie tz ( 1%5) .
9
I Hi: LITHOSPHERE: SOME IMPORTANT PROPH n ES
E' fo e 'e nl transm.ssion Inelt, cie n l lransmi ss, on
l./ H~u r~
2.-' Regions of <)(Iicicnl and inefficienl propagation o f
S~
seismic waves in lhc uppe r mantle (sec
l e~t).
Ar,el
Molnar and Oliver (1%'J ).
estimated as a functio n of dept h. Th is method depends on the simu ltaneo us study of body waves (e.g. P waves) a nd surface waves, and reveals zones of anomalo usly weak man tle below currently active volcanic region s co rresponding to those revea led by P,; wave anal ysis. Seismic wave attenuation is the red uction of amplitude with dista nce and time due to e nergy loss. and is measured by th e qu ant ity Q. The amount o f att enuatio n Q is re lated to the strength o f the ma te rial thro ugh which the wave is pro pagated . T he ' tow-velo city zo ne' (asthenosphere) is charac te rized by low values of Q. Further evidence as to the presen ce of anomalo us zones co mes fro m the study o f electncal conductivity in the Ea rt h. Rocks arc weak co nducto rs o f ele ctric curre nts. It is possible by study ing rapid variatio ns in t he Ea rt h's magnetic field to isolate effec ts caused by electric curren ts in th e Ea rth 's upper at mo-
sphere which are due to ch anges in the fl ow o f radiation and charged pa rticles fro m the Su n. Th e resulting magnet ic field induces e lectric cur ren ts within the Ea rth. which in turn ca use rapid ly cha nging modificatio ns to the magnetic field. By isola ting the seconda ry va riation , the stre ngth o f the e lectric curre nts , and he nce the elec t rical co nductivity of t he Ear th , ca n be est ima ted . Ano malies in electrical co nd uctivity we re foun d to be assoc iated with island arcs and co ntine ntal rift zones (see e .g . Herman ce . 19R2). Th ese anoma lies are zo nes of poor co nd uctivity t hat co rrelate with regio ns of unusually warm lithosphe re . / Thus seve ral inde pende nt geophysical met hods indicate t hat active tecto nic zo nes (volcan ic arcs, ocean ridges an d co ntinental rifts) d isplay ano ma lous physica l properties that co rres pond to those exhibited by the asthenosp here . We may co nclude tha t such
10
GE O LOGICA L SlRUC1 URES AND MOVING P LATES
zones possess abno rmally low instantaneou s stre ngth and can be regarded as zones of anomalou sly thin and wea k lit hosphe re . Sum mary For time pe riod s in the ran ge seco nds to hou rs, t he lithosphere gener ally beha ves ;IS a stro ng e lastic a nd rigid body wit h effec tively infin ite viscosit y. Active tecto nic zones exhibit ano malous physical prop e rties indicat ed by low P" wave ve locity, grea ter surface-wave d ispe rsion, lo w Q (high deg ree of atte nuat ion) , inefficient prop aga tion of S, waves, and low electrical co nd uctivity . Th e st rength of the lithosp heric mantle in these anomalous zo nes is in many cases (c.g . Japan) co mparable with that in t he ast henosphere. implying tha t the ' insta nta neo us' t hickness of the lit hosphere is co nside rably red uced . In simple terms, plates are th inne r and wea ke r in active tecton ic zones. As we shall now see , this is related to t heir thermal st ructure .
2.3 Th ermal structure of the lithosphere Th e ave rage total rate o f hea t loss through the Ea rth's sur face is about 2.4 x Ityu cal/year. T his represents an enormous loss o f ene rgy, seve ra l orde rs of magnitude greater than the to tal loss associa ted with earthq uake or vo lcanic activity. Mean values of heat fl ow per unit surface area for t he differen t co ntinents and oceans arc shown in T able 2.1 . It is clear that the average continenta l and oceanic hea t fl ow is esse ntially the same . It ap pear s that more than 9970 o f the Ea rth 's su rface has a ' no rmal' heat fl ow of around 1.5 HFU ( = 6OmW m- 2) , with ,10 0 rnalo us zo nes o f ve ry much higher heat now. Note that 1 HF U = l llcal cm - 2s- 1 = 40mW 01 - 2. T hese localized zones ca n o nly be explain ed by the tra nsfe r of mate rial bringing thermal energy from deepe r so urces and co rrespon d mostly to areas of curre nt o r recent volcanic activity, such as island arcs, major conti nenta l rift zo nes and oce an ridges . T he measurement of heat fl ow o n land is
TaMe 2. 1 region s.
Mean hC;ll ftow fo r cc minc rna l ~ n d ocea nic
Regio n
N
q
S.D .
255
1.49 1.65 1.43 1.44 1.71
0.54
Von Herzrn Ulld Lt <'. J%Y A Ucontinent s All oce ans A lt;llllic Indian
Pacific A rct ic Mediterranea n se as Ma rginal scus
2329 4<~
J31 12.' 2
29
7J 2/'1'
J.23 1.33 2. 13
1.14 1.07
109 1.24 0 .33
o.es 0.6)
G irdlt,.1 % 7 Africa Ja pa n A ustr alia Eu rope
NiAmcrica
I' )R
'0
"
44
1.20 2.2 1 1.76 l. 9 1 1.26
0.21 2.73 0.62 1.70 U.57
N '" numhcr of oI>scrvalio"s ; q : ar;l hmel;c mn ll nl heal flo w ill /
much mo re difficult than at sea because of va rio us s urface e ffects, which arc minimized by the blank eting effe ct o f the overlying water in the oce ans. II is necessary to d rill 10 depths of about 300 m or mo re o n land to produ ce accurate rcsults. In general . estimates o f hea t flow in the ocea ns arc accurate to within IOn;." but continental estima tes a rc very variable in their accuracy. Regio nal estimates of mean heat fl ow based o n mo re than 10 observations a rc believed to he accur ate 10 within 0.2 HFU . Loca l depar tur es from these mean values of mo re than th is amount arc co nside red to be significant. Table 2.2 shows selected hea t fl ow values for the major types of tecto nic pro vince . from which it may be see n that low mean heat fl ow values cha racteri ze the Precambri an shields and tha t high mean heat flows are associated with Ce nozo ic volca nic areas. High heal flows are also associated with ocea n ridges and island a rcs. Pro fi les of heat fl ow values across the mid-At lantic ridge , and the Kur ile and Jap an islan d arcs (F igure 2 .4) show very marked but relatively narrow an omalies with a rather rap id
THE ll TlWSP IiER E: SOME IMPORTANT PRO I'ERTIES
Co ntinen ta l heat flow d ata d ivided ;010 s hield, intermediate (You nger Proterozoic to Pha ne rozoic' cr ust) and thermally act ive regions . After Kusznir a nd PMI<.
Table 2.1
(1984).
He at flow Hf U lIl W m - ~
Regio n A Shll'/ d
Supe rior Pfov i nee ~ Wl~! AUS tU It;1 1 WeS! A frica ( N igc r) ~ South lod ia 2
]4
± 8
1I.,sS
39 ± ,s
0.91\
20 ± "
0.50
49 ± K
1.23
±
1.03
B . JI1It'fJnt'diu lt' Easte rn US A ~ England a nd Wal es~
'"
57 ± 17 59 ± 23
1.43 1.48
Cenlta l Europe (Bohe mian m assi f)~ Nor1hern C hina ' Mean Yo unge r Prote rozoic ' Mean Pa laeozoic '
73± 75 ± 15 SO ± S 62 ± 20
1.R.l
Mea n A rch aea n + olde r P rll1{' rnzoi e~
41
"
1.,s9
US 1.55
C Thermatiy active 101 ± 35 (73 ± 20) Baikal rif1' 97 ± 22 (SE flank - Lo we r Patac o zoic p (55 ± 10) East Af rican rif1' IUS ± SI (ft ~ n ks)-' (52 ± 17) Basin-and-Runge Provinc e! 92 ± :n (E . lbnk-Colo r;ldo c.la lca u)4 (611) Rhmc g tahen '
{f1a n l<.s-R hc l1 i~ h massifl"
Il~ 'l 1\,_
2.N!
(1 .83) 2.'0 (Uil) 2 .(,3
(UU) 2.30 (I. SO)
data from 1'" llack and Ch41'man (19n ) ' , V,tn,d l" ""d
P"lbd (19!1U11, Mnrgan ( 19112'. 1910' , in I'w,~' )_
change with in a few 100k m to norma l regional values. Sctate r a nd Fra nche tca u (1970) co m pa red the profile of ave rage measured heat flo w values across the Ea st Pacific ridge with a theore tical pro file e xpec ted fro m a 75 km-thick cooting lithosph er e model (Fi gure 2.5A ). T he close corres po nde nce was held to suppo rt the ocean-floo r sprea ding model of th e growth o f lithosphe re at t he r idge by add ition o f ne w hot ma nt le mat er ial. As the new ocea n floo r spreads awa y fro m the r idge axis. it coo ls from the su rface downwards a nd gradually t hicken s (see Figure 2.58 ) . T he dec rease o f hea t flow with distilnee from the r idge axis is the refo re a direct conse que nce o f a coo ling a nd thicke ning lithosphe re plat e . This re la tio nship ma y be checked by plo tti ng the va ria tio n o f hea t flow
11
with age of the ocea n floor ( Figure 2.5 C). Th e hea t flow decreases steeply un til a n age of a bout 50 Ma , afte r which it bec o mes more or less cons ta nt. T he pro files acro ss the volcan ic a rcs s how a noma lously low values a t the trenches as well as high values ove r the a rcs. T he dist ribu tion o f t hese zones o f high and low heat flow ar e s ho wn for the no rth a nd west Pacifi c region in Figure 2.6A . T he relat ionship with tre nches a nd vo lcan ic isla nd a rcs is ve ry clea r. Figure 2.6H illustra tes in pro file the depressio n o f iso ther ms associa ted with the su bducting slab . It ca n he see n from Ta ble 2.2 t ha t t he re is a te nd e ncy fo r the hea l flow to increase with decrease in age of oroge ny for Pha ne rozo ic a nd Preca mbrian o rogen ic belts. but t he diffe re nces a re sma ll in rela t ion to the e rro r. T his decr ease of continent al hea t flow with age was in vestigated in mo re deta il by Vito re lJo a nd POlI,lCk (19RO). Th e y showed ( Figu re 2.7) tha t the decrease with te cto nic age (i.e . age since t he last major tectonothe rmal eve nt) co uld be a ppro xima tely filled to an e xpone ntial c urve re prese nti ng the gradual decay of a tr a nsie nt the rmal pe rt urba tio n together wit h a co mpo ne nt o f heat loss due to gradu ai erosio n of radi oge nic crust.
Summary Th e re is no significa nt d iffer ence bet ween mea n co ntinental an d mea n ocea nic heat flow . La rge hea t flow a no malies occur ove r very nar row zo nes mak ing up less t ha n 1% of the Ea rt h's surface a rea . Zones of high hea t flow com prise the active tectonic reg io ns where magm a has bee n introduce d into high levels in the lit hospher e - t he active vo lca nic a rcs. co ntine nta l rifts a nd ocea n ridges. Low hea t flow zones ar e associa te d wit h t he tre nches . Bo th ocean ic a nd co ntine nta l cru st sho w a progressive decrease in heat flo w with incr ea se in the rma l age . T hese va riation s in hea t Row reflect variati on s in geot her mal gradie nt tha t have impo rta nt implicat io ns fo r lithosphe re stre ngth. as we sha ll see in 2.7.
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12
13
T H E UT HOS I'H EII:I,: SO ME I MI'ORTAt'JT PROPERTI ES
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t'iJ:ure 2,5 (A ) Heat flo w profile from the Easl Pacific rid ge crest across the North Pacific. Mean obse rved value s (clo,;cd circles) arc cumpatcd with a theore tica l pr()liIe ror " coo ling Ii l h()~phc re 'S km Ihid O pen circle s represen t mean va lues increased hy 15% to allow for ,I possible hi;,s nca r Ihe ridge. From Uyeda ( 197X). (11) Schematic diagram showing male rial flow lines (d ashed) and isother ms (so lid) al an oce a n ridge . T he solidus tempe rature 1~ is the isot her m rep resenting the boundary betwee n the lithosphe re an d the ast he nosp he re . Af ter Uyeda (1 9711). (C) Vnn.uic n of ho.';\1 flow with ago.' of ocean crust. showing cooling with incr easing age , After Sclutc r ( 1972 ),
14
G EOLOG ICAL SlKUCl'U ll.I"S A ND MOV1N(j I' LAI I::S
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til.: Wc~l Pucinc sho wing dec rease d hea l nnw a t tren ches I
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,.nd incrc,[""d heal "ow 'Il volcanic ;H~ and hack-ar c region s. From lJYCUil (1971'). (8) Schem atic pro file across oj sot-duc uon zone shuw ing ...lc prcsscd isothe rms a lo ng the su lxluCl i ng sla b . A ft cr W yllie ( 1971) ,
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t"il:u rc 2.7 Decre ase (It co nt inent;,1 heal fl"w with tec toni c age of c rust. T he up per c urve (visua lly HUed ) is interpret ed ;IS ma de up o f thre e co mpo ne nts: I is radio genic heal [rom th e cruse II is heal fru m a tra uaicntt hc rma l per turha tion associated wilh a tecronorh c rmat cvc ru, a nd I II is back ground heat llow from a deeper source. Dou hlc oa rs rc pre se nt the standa rd e rror, s ingle ba rs the sl
ra e
U THOSI'HIC Kl.: : SOM I'. IMf-ORT-ANT I'ROt' !:.Kfl f.5
2,.1 ~I a ll tle convectio n: t he r ole of t he lithosph er e The Enn h is a hea t e ngine, a nd the ult imat e source of almost all the e ne rgy requi red in geological processes is Ihermal in origin: ~ris tng out of the tra nsfe r of heat (part ly o riginal , partly radioge nic ) from the inter ior of [he Earth 10 it" exterio r. A rt hur Holmes ( lY2lJ) proposed ther mal convection c urre nts as the oriving mecha nism fo r co ntine nta l dr ift , bu t the sugges tio n was o nly se riously conside red by most geologists t hirt y yea rs la te r. whe n enough evidence h;IIJ accumu lated to convince the m that continental dnft a nd oce a n-fluor spread ing were viable hypo theses and that co nvec tion was the onl y adeq uate ex planatio n for these rn~c~~~ . . It is now ge ne rally accepted thai the solid material of the Earth is capab le of flow give n a long enough time scale. The rat e of flow is governed hy the effeclive viscosity of the mat e rial. which is st rongly te mp era ture-depe ndent. An estimate of the mean viscosity of the oute r shell of the Ea rt h ca n be det e rmined from t he isostatic respon se of t he lit hosphere . F~r e ~ · ample . the up ward move me nt of Sca~d m'l vla in response 10 t he re mova l of th e Pleistoce ne ia::sheCican he acc ur atel y measured by o bse rvations on raised be aches. whic h s how a maximum elevatio n o f c.3OO m in 1I l()() yea rs. This is e xtremely rapid in geo togicat terms an d 21 indicates a mea n viscosi ty of a bout 10 po ise . This mean value rep resents a ran ge in viscosity thai m ust de crease downwa rds wit h increase in tem perature . Flow in the upp e r ma ntic has , until recently, been assumed to he gove rned by du cti le cree p in either dry or wet o livine. Flo w la ws ba sed on dislocation c ree p have bee n p rov ided by Kohtsted r and G oet ze ( 1974 ) and G oe tze (1m) for dr y o livine . a nd by Post ( 1977) fo r wet dunit e , based on labora tory da ta . These now laws a re o f the for m ~ = A e xp BITols -
1
where t is str ain ra te . T te m pe rature a nd
0
15
si ress. A and B ar e constants ami arc st ro ngly te m pe rat ure-de pe ncJc nt. 11 was o riginally tho ught that o livine was the durnina nt mineral throu ghu ul t he upper man lie , a nd .....as replaced by a minera l with similar co mpos ition hut with a spi nel st ruct ure (i.c . a denser ph ase) in the lowe r mantle . How ever . pe n o logists have mo re rece nt ly suggeste d a mor e complex compositio nal structure . A nd e rso n a nd Bass ( 11)86) summa rize the e vidence in fa vour of a het erogen eo us up pe r ma ntic domi naled in the lowe r pa rr by clinopyroxene a nd ga rne t. the base o f which ma y be composed la rgel y o f subd ucted ma ter i.al, a nd sugges t tha i olivine is domina nt o n l ~ I ~ th ~ uppe r part o f the uppe r ma ntle . OilVille IS now conside red 10 he unsta ble below a bo ut 400 km. Below 650km the re is a majo r ch a nge in physical pro perties which is e x pl a i ne~ as ~h e result o f a cha nge to de nse pe rovskuc -fi kc mine ral . Th e flow la ws fo r these ma te rials a rc not know n . A lt hou gh it has been sugges ted that the lower man tic ma y nor he ca pab le of convecnve flow. the bala nce of opinio n no w is in favou r of con vection in bot h upper a nd lo we r man tle h ut proba bly not d irectly linked in a la rge mantle-wide cell. . Th e ground ru les fo r rhe pla te tectonic theory ha ving bee n laid down in 1968 , the ke y role of convect ion in t his new t heo ry was analyse d in a cla ssic pape r by Mcken zie (1969) e ntitled 'Speculations o n the co nseque nces a nd ca uses of pla te moti o ns' . McK enzie po inted OUI t hat lar ge-scale co nvect ion throu ghou t at least the uppe r ma ntle , as o rigina lly e nvis aged by Holmes. provided the on ly adequa te mec hani sm for pla te movem en ts. VI SCO US forces we re required 10 couple the pla tes 10 the moving ma ntle be low. a nd vertical mo ve me nts of both hot a nd cold materia l mu st also be involved to co mple te the co nvec tive ci rcula lio n. Plat e mo vem ents a re thu s se e n as pa rt o f a la rge-scale convec tive process t ha t is in a se nse bo th a co nseq ue nce and a ca use of the mo ti on. Th is co ncept was a t va ria nce with a suggestion by Elsasse r (1967) that the motio n o f the
16
G EO LOGICAL STRUcr U I<ES AND MOVIN G PLA TES
pl ates was governed by the pull o f the cold
sinking slabs in subduction zones. Althoug h the ' slab-pull' force is an importa nt cle me nt in lithosphere dynam ics, it can eas ily be shown tha t it is only one of severa l forces o f co mpa rab le magn itu de acting o n the lit hosp he re , an d thai much of the ocea nic lithosphe re is und e r co m press io n ra the r than ten sion as req uired by t he Elsasse r model (see 2.5, 2.6). T he implicat ion of Mcke nzie's ar gum e nt is tha t such fo rces a re seco nda ry, t hat no single fo rce provides t he d riving mecha nism fo r plate tectonics. a nd that all suc h fo rces a rise from the co nvect ive flow sys te m itsel f. The early models of Holmes (1929) and Ru ncorn ( 1962) ( Figure 2.8) implied a link betwee n ocean ridges (or mo re ge ne rally. const ructive plate boun da ries) and rising currents o n the one hand . and between subductio n zo nes (o r destructive boundaries) and falling cu rre nts on the other. It beca me apparent, however, th at such a link could o nly be tra nsitor y, since distances between ridges and tre nches are continuously changing o n a geo logical time-sca le. an d tha t all plate bo unda ries migrate with respect to a fixed man tle referen ce fram e . It was co nsidered by Runcorn and othe rs
f igure 2.8 Th e Runcorn ( 1%2 ) mode l of ma ntic- co nvection . Fro m R unCOfIJ ( 1962).
that the position s of majo r convec tive upf t ows and downttows in the ma ntle should be refl ected in distortions of t he geoid. The geoid (Figure 2.9) is the sea -leve l equi potential surface of the gravity field of the Ea rth. and this surface defines the depar tures from rad ial symme try of the d ist ribut ion o f mass wit hin the Earth. 11 is measur ed by ext reme ly accurate satellite obse rvat io ns. The geo id su rface reveals departu res fro m a simp le sphero id of flattening which are of seve ra l orders of magnitude . T he largest type of anomaly has a halfwavele ngth of about 4000 km, and is comparab le in dime nsions with the major plates. It produ ces elevations and dep ressio ns o f the o rder of 50- toO m o n the geo id surface . Rath e r sma ller-scale disto rtio ns with hal fwavele ngths in the ran ge HX)()- 1500k m co rrespo nd to t he majo r ocea nic ' swe lls' like those o f Hawaii an d Bermuda . These a re acco mpanied by positive geoi d anomalies of 6- 8 m. A third scale of ano maly can be detected with a ha lf-wave le ngth o f IUO- 250 km. T he long-wavele ngth co mpone nts of the Ea rth' s gravity fie ld are conside red to result mainly from density co ntras ts associate d with mant le convec tion. T hus a mass excess is associated with a co ld dense downward current (giving a geo id swell), a nd a mass deficiency with a war m , less den se upward curre nt, giving a geo id depression. However , t hese effe cts are largely o ffset by the dynam ically main tained defo rma tion of the surface to pography. (e.g. ridges an d tre nches) that produ ces effects o n the geo id that are o pposite in sign but co mparable in magnitude . In o ther wor ds. the existe nce o f ge ne ral isostatic co mpensation in these majo r to pogr aph ic structures redu ces the net geo id an omaly to around zero , parti cularly in the case of the ridges. Th e lon g-wavelen gth anomalies probabl y rela te to swells an d de press ions of perhaps
Fjgur e 2.9 The shape of Ihe geoid. Dilfe re nt views (a - n of the geoid shape ( 5C(: tex t). Co ntours a l 20 m intervals show de pa rtu res of the Smi thso nia n Sta ndar d Earth II geoid from a sphe roid of f1a u.:ninS 1f29g.25. Positiv e erevanon is shaded. Fro m Gough ( t977).
IHI:;
I.Il'HO~I'HER E :
SOME IMPOIUAN T PIWPLRflt:S
17
18
GEOLOGICA L STlWCTU RES AN D Mo v I NG PLATE S
several k m magni tude on the core -man tle
interface. tha t wo uld be associa ted wit h rising and falling convec tive co lumns. Anomalies with a deep so urce shou ld pr imarily re flect the den sity imbala nce ; thus uptlows in the lowe r mantle p rod uce nega tive and down llows positive anom alies. T he opposite appe ars 10 he true of the shallow d isturba nces rooted in the ast henosphere . whe re the ano malies reflect the de for med shape of the lithosphe re ; fo r example t he Hawaii swell is associated with a positive anomaly . Figure 2. 10 co mpares the Large-wavele ngth a nomalies with the present plate bo undary networ k. A ltho ugh ther e is a poo r cor relatio n in genera l. and a total lack of correlat ion
be twe en ridges and negative ano ma lies, the major subd uctio n zo nes a re situated in regions o f posit ive ano ma ly. McKenzie ( 1969) presu med that ridges must e xist over bo th falling and rising curre nts. This was ex plai ned beca use the creatio n o f lithosp he re at a ridge requires a la rge vo lume o f ho t mant le ma terial that,
beca use it rises morc o r less adiaba tically, co nvec ts little hea t except within the lithosphe re, an d so produces litt le d istor tio n of the isot herms within the sub-lit hosphe ric mant le. In subduction zo nes ho we ver. the op posi te is the case . No change in tem perature structure withi n the lithos phere occ urs until it cc mmen ces its descen t into the ma ntle . This motion d istorts the isothe rms by seve ral hund red km throughout the upper mant le (see Figur e 2.68 ). T hus the sub-lithos phe ric ma ntle loses materia l be neat h ridges but loses heat bene at h island arcs. Th e o pposite is true for the lithosp he re , as ex plained in 2.1. Mckenzie co ncluded tha t t he cold des ccndin g slabs prod uce large hor izon tal a nd ve rtical temper ature gradients a nd sho uld therefo re gove rn the position of the descending limb of any co nvection ce ll. If the subd uction zone moves, t he sinking curre nt sho uld move with it. Coo l oceanic lithospher e is able to sink beca use it is den ser tha n ast bc nosphc ric rnan-
•
(
t
f Figurt 2.10 Co mours at 10 m intervals of the height of the non-hydros tanc geoid com pared with the plate bou nd ary nelwork. Tr enches, hatched lines; ridges . dou ble lines; faults , single lines. Major trenches all occur in positive areas. Afte r McKen zie ( 1969).
t r I
rHE LlT HOS PH E KE: SO ME IMPO Kr ANT PKOI'ERIIES
tic, desp ite the nega tive buoy ancy o f the oceanic crus t , which is pro ba bly overcome by the transfo rmation o f gabbro to ecl ogite as It sinks. Con tinental lithosp he re , on the other hand, has a muc h lowe r mea n de nsity tha n oceanic lithosp here , since abo ut 20-30% o f it is composed of crus tal ma te rial, compared with only abo ut 5% in t he case o f ocea nic lithosphe re . Con t inenta l lithosphe re will therefore res ist subd ucti on , a nd con tinen ts, once formed , will be very di fficult 10 destroy. Dewey an d Bird ( 1970) showed how continent -island a rc a nd co ntinen t -con tinent collisions arc t he inevita ble conseq ue nce o f con tinued subd uct ion, and how subd uct io n must cease when co llision takes place . T he reafter , for continued convergence to occur, the site of subduction must move away fro m the collisio n zone in o rde r t hat further su hd uctio n of oceanic materia l ma y take place . T he pa tte rn of mantle convect ion must therefore depe nd to a large exten t o n the position of the co ntinents and must move wit h the conti nen ts , since t he latter contro l the position o f the subd uction zones. Howe ve r. convect ive cha nges ma y lag behind the cont inen t move me nts becau se o f the time take n fo r the old co ld slab below the collision zo ne to wa rm up . The lust maj or reo rga nizat io n o f t he convective patte rn was prob ab ly assoc iate d with the continen tal ama lgam at ion that for med Pa ngaea. The supe rconti ne nt appea rs to have bee n almost surro unde d by subd uctio n zo nes. T he Pacific Ocea n o f t hat t ime for med a bo ut half the surface a rea of the Earth (Fi gu re 3.5), a nd probably poss esse d ,I simp le sy mme trical ridge system that ma y reflect t he co nte mpo ra ry rising convec tive limbs. Since that time . the Pacific ridge sys te m has moved no rt hwa rds a nd been overridd en by con tinen tal plate . pa rticularly on its eastern side . Geo me tric co nsequences of pla te movem e nts ha ve thus te nded 10 move ridges a way from t heir longe r-lived ma ntle roots. Howe ve r. the link be tween fa lling cur re nts an d subd uctio n zo nes may ha ve beenlonger-lasti ng . an d ma y pa rt ly ex plai n the relationship be twee n geoid a noma lies an d trenches see n on Figure 2. 10.
19
G ou gh (1977) has co mpared the shape of the long-wavel e ngth an oma ly ( Figure 2.9) to the two spi ral strips covering a tenn is ball. T he high . or positive ano maly, has on e lobe covering the wes te rn Paci fi c with its t re nch system, an d including most of Aust ralia , an d is joi ned across the Ar ctic a nd no rthern A tlan tic regio n to a second lob e cove ring the so uthe rn A tlan tic a nd so uthe rn Ind ian ocea ns, ex ten d ing west wa rds ove r the A ndes. T he low, o r negat ive a no ma ly, has o ne lobe cent red sou t h of India an d cove ring m uch o f Asia and the Ind ia n O cean , which is jo ined ac ross the A nta rct ic to a second lobe cove ring t he eastern Pacific, North Ame rica , the Bra zilian shield and the weste rn A tlantic. T hus the low st rips surro und the high lobes an d vice ve rsa . Gough a rgues that this pa tte rn ma y ind icat e a glo bal singlecell convective system whe re the up curren ts pro ba bly unde rlie the low geoi d strip (a ltho ugh the o pposite co uld be the case) . Who le-ma ntle co nvec t io n ce lls were env isaged by earl ier worke rs (c.g. Hol mes, 1929; R uncom . 19(2) a nd a re still advoca ted by so me (e .g. Ke nyo n a nd T urcotte, 1983). Howeve r, ma ny investiga to rs now bel ie ve t hat the c ha nge in che mical a nd physical prope rt ies a t 650 km dept h, which prod uces t he seism ic d iscontinuity se pa rat ing upper fro m lowe r ma ntle , re prese nts a ba rr ier to comp lete circu lat ion . a nd tha t separate convect ive pa tte rns ma y he inde pe nde nt o r on ly pa rtially coupled (see e .g. Le Picho n an d Huchon . 1984) . It is poss ib le tha t the rising currents a re cont rolled by the te m pe rat ure dis tributio n at the base o f the man tle . but arc unco upled fro m the lit hosp he re at the surface , whe reas the fa lling cu rre nts arc mo re strongly in fluen ced by lit hosp he re move me nts. Figure 2. 11 sho ws diagram matically ho w a poss ible ma ntle con vcctivc sys te m might link with plate mo veme nts. T he positio n o f ho t columns in the ma nt le is go verned lar gel y by the sites of upwell ing liq uid ou ter core mate rial. An im po rta nt sou rce of informa tion on the E ar th's convective syste m is provide d by t he so-called ' ho t spots' or man tle plumes (Wilson, 1965; Mo rga n , 1972 ). T hese a re long-lived
20
GEO LOGI CAL STR UCTUR ES AND MO VI NG PLATES LITHOSPHERE eM ,
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so urces of vo lca nic activity which arc 's tution ar y' . o r at le ast mo ve much more slowly with respect ( 0 a fixed man tic referen ce fram e , than the plates. Co nseq ue ntly t hey lea ve a linea r 'w ick' in t he form of a chain o f volcanoes on the surface of t he moving oce anic pla tes. A goo d ex ample is the Hawaii- Empero r chain in the Pacific O cean (Figure 3. 10). Iceland is an o the r exam ple , actua lly situated on a ridge , so that there is a symme trical arran geme nt of linear ch ains running to wards t he present position fro m each side . Morgan used the ho t-spot frame o f refere nce to det ermine the ' a bso lute' mot io ns of the plat es ( Figure 3. 12). T he oceanic hot spots are associated both with topographic swells (wit h amplit udes of ahout 1 km) and also wit h positive geoid ano ma lies ( with amplitude s of around 10m) see Par son s et al. ( 1983) . T he to pograp hic swells are ca used by wa rme r and less den se lithosph ere similar to that o f th e oce an ridges . Both swells and anomal ies are e lo ngated in the direct ion of plate mo vement. It is be lieved that the size and spacing of t he ho t spots indicates the presence of a convective ci rcu latio n pattern which is much sma ller in scale (l.e . with wavel ength of 2000- 3000 km) t ha n the majo r circulation respo nsible for the pia te movemen ts, and co mparable in size to the thickness o f the up pe r mantle.
,I
~'; gu ~ 2.11 Car t'''lll rCl'rcsc ntiog I'0ssihk mantle c onvccuvc II" ", patte rn s. A simple whole. Illalil ic pnnc m (A 1 dege ne ra tes into pilrl ly. linkc:
An even smalle r-scale convec tio n is suggested by the panem of short-wavelength (200500 km) undulatio ns of t he geoid obse rved in the cent ral Pacific ocea n (F igu re 2. 12; Haxby and Weissel, 1983). T hese featu res are elongated paral lel with the ' a bso lute ' motion of th e plates, like the hot -spo t chains , hut arc vo lcanically quiet. Thi s pat tern is int er preted as the resu lt o f small-scale instabilities in coo ling o f the plate as it moves away from the ridge , and is thought to be assoc iated wit h now patte rns with in the asth enosp here .
~
C
Modelling convection
Th ere have been many attempts is simulate mantle co nvection by mean s of mod els of bo th expe rime ntal a nd mat hematical type . Both kinds of mod el a re highly artificial and give only a rough guide to the way co nvect io n might ope rate . A stimulating acco unt of the ph ysics of the be haviour of the Ea rt h is prov ided by E lder (1976) in his book , The Bo wels of the Earth. T his acco unt se rves as a useful illustr ation of bot h t he possi bilities an d limitation s of mod ell ing the Earth 's internal be haviour . B Elder shows that thermal co nductio n j S i~U1 totally inadeq ua te to e xplain the heat flow and y ~'I tempe rature distributio n in the uppe rmos t cp ll parts of the Eart h. and that convec tive flow is 'll~ :; esse ntial to transpo rt the heaT and energy r rh.
A
B t"lItUIl' 2.12 (A) Gr avity 'IO nm~l y penc rn n"cr pan uf the Pacific ptate , obse rved as Ct'nl\lUf~ un the sea surface , measured b)''''' tclliIC. ( B) Huthyr ucmc co nto urs lhe sa me regio n as (A ). plnll ell a~ rcxidnaldcpth (Ihe d iffe re nce hc [weco the .k:p!hallrihulahk to elk,ling oce a nic plal e "noJ Ihe oh"c rwu .kl'lh). Bot h nl'1Jharc "mout hed In cl imin,lh: fluctuations with u.dc nglhs ,,'",ncr than 5/.. 1km. Low s on h..111 mal'" ar,' dUlled. Slightly dung'l l.: pusi .;vc gC\li,j unom ahc s t:orrcl alc with l"f"'!:lal'hic highs. The mups a rc oncn tcd s uch Ihill lhc mot ion "f [he plalCis [('win lls the lcft , in the d hecuon o f elongation ...r lilt- elliptical anom"lin . A lle r MI.-Ke nzie ( t9R.' ).
or
21
22
GEO LOGICA L Sn W CTU lI: 1:.S AN D MO VING PLATES
required fo r surface processes. Fur therm or e , t he re ma r ka ble similarity o ve r ge o logica l time o f suites o f basic igneous rocks indica tes a wellmixed (well-sti rred') source, which again indi-
ca tes convection. Elder uses the pr inciples o f fl uid dynamics to tre a t the Ea rth as an e xa mple of th e rm a l turbule nce in a medium with va riable visco sity. T he fundame ntal d riving mecha nism for co nvectio n is a ho rizo ntal te mp e ra ture g rad ient which prov ide s cha nges in den sit y. T hese in tu rn generate buo yancy for ces which init ia te and maintain motio n. In an unstable syste m above a critica l Rayleigh number, any small pe rtur bation in th e te m pe ra ture field will become amplified and generate mo tion . T he Ra yleig h nu mbe r ( Ra ) is a me asure o f the e ffective ness o f th e bu o yan cy fo rces ac ting aga inst th e co mb ined re sist a nce to mo tion o f visco sity a nd th e d iffusio n o f te mp e rature varia tio ns by t he r ma l co nd uc tivity. For a bod y o f give n sha pe an d bo unda ry te mp e rature dist ribut ion . fhe Rayleigh n um ber is g ive n by :
whe re y is the coefficient of cu bica l e xpa nsio n. S is the loc al ac celera tio n d ue to g ra vity . b. T t he t em perat ur e d iffe ren ce . h the wid th of t he bo d y (rad ius of t he Eart h). k the th e rma l d iffusivit y an d v the kine mat ic viscosity. E lder assum es t he follow ing va lues fo r these pa rarneters- y « IOK . g = IO m s - 2• b. T:::: 25IK}K . h 6370 km . k =: 1O- 6 m 2 s - l . a nd v 6.46 x 1016 m l s -l c m2s -l . giving a Ra yleigh nu mbe r o f lOy. Co nside ra ble unce rta in ly a ttac he s to t he va lue o f v, whic h ma y lie in th e r ange 10 1 ~ _ 1O " lm 2 s - 1 fo r th e mant le . Ho we ve r, the re leva nt va lue s of t he kinema tic visco sity lie in th e upper pa rt s o f the ma ntle a nd a re k now n mo re acc ura te ly. Va lues fo r y, b. T a nd k are pro bably accurate within a factor o f 2. Ho weve r. it is clear that th e Ra yle igh number mu st be la rge . Fo r bod ies with Ra ... lO5, vigoro us non -ste ad y co nvectio n is indi ca ted , domi nated by chao tic e dd ying mo tio n wit h e dd ies o f a wide ran ge o f size. T his mo del is q uite d iffe rent fro m t he re gu la r ce ll model o f Ru ncorn (1959)
=
=
bu t sim ila r to th e kind of mod e l infe rre d by McK e nzie a nd ot he rs from the re latio ns hip betwee n pl at e bounda ries and geoid anomalies. El de r's model pred icts a the rm al sub -laye r (co rre spo nd ing to th e lith osph e re ) wh ich is a co o led an d high ly visco us buffe r be twee n t he we ll-m ixed inte rior a nd th e co nstan t-te mperatu re surface . T he thi ckness of this la ye r is a function of th e Ra yle igh n um be r. Vigo ro us in te rio r flo w te nds to th in the su b-laye r. which mu st pe riod ically ove rturn for convect ion to be ma int aine d . The beh aviou r of the sub -laye r is sim ulate d using a po t of ho t o il. broader th a n its depth , insulat ed o n its base and sides. a nd cooled fro m abo ve . T he co oled sub-laye r g row s to a maximum thick ness de ter mined by t he va lue o f Ra , then ove rturn s. Befo re it docs so . it fo rms a se ries of ed d ies o f th e sa me d ime nsion s as the dept h of t he sub-laye r (Fi gure 2.13A). These e ddi es prod uce lar ge d istortions o f the ini tiall y ve rtica lly stra t ified te mpe ra ture fiel d . Each edd y en tra ins coo l fluid fro m t he sur face a nd br ings hot mat er ial close r to the sur fac e . Eve n tually, blo bs o f cool fluid fa ll out o f the sub-laye r into the inte rio r a nd in itia te d isruptio n o f the su b-laye r. T his process , which is e ffective ly ran dom , simulates t he behavio ur of th e lith osphe re as it is a tt acked by rid ges and plume s , a nd eve ntua lly by su bd uctio n. A further usefu l an a log ue o f Ea rt h behav io ur is prov ided by e xpe rime nts simula ting th e e ffec ts o f a contine ntal slab on th e co nvective syste m (Figur e 2. 138 ). Elde r finds that an asym me trica l edd y is ge ne rate d by a co nu nc nta l edge ; this pro vides a la te ral force t hat co uld act to propel the co ntine nt (d . the subdu ction suction forc e, 2.5 ) . There is a stro ng upwell ing imme d ia te ly beh ind t he le ad ing ed ge that sim ula tes volca nic a rc produc tion. A ny th ermal pe rtu rbat ion be lo w th e co ntine nta l sheet would te nd to pro pel th e shee t late ra lly, unless it we re ce nt ra lly situate d.
Summary
c ~
t s
t.
ti b n h b
P
r
P
•
r:
'4
(
-
W
~
T he st ud y of the geo id a no ma ly pa ttern sug- ge sts that the ma ntle co nta ins a t le ast th ree sca les of co nvect ive circulatio n. The first , or
II~ E Ll IH OSI'I~ ER E :
SOME I MPORTAN T l' IWf>[Rfi ES
large-scale circula tio n is co mparable wi th the dimensio n" o f the plates a nd r eturns materi al from trenches to ridge s . T ilt' se cond . of in te rmediate sca le, is co mparabl e to the depth o f Inc uppe r mant le in sca le , an d co nt ro ls th e ho t spots. T he third has a smal l SC;11C co mpar a blc to the widt h o f the a sth enosphe re , In terms of plntc kin e matics . ma n t le con vection docs nOI appear to p rov ide a direct link between ustbc no sp hc rc no w a nd pla te mov e rncm . Any lar ge plat e would be e xpe c ted to have differe nt d irec tio ns o f asth e nosph e re Ilo w beneath it. T his o bse rvat io n is of c ritica l rmportance in e valu at ing t he mech an ism s of plate move ment and the force s actin g o n th eplates. as we sha ll now sec (2 .5 ) .
The co ncl usio ns th at can be d raw n fro m Eld er 's mo del be a r an in teresting re se mblance 10 tho se o f McK e nzie , a nd ca n be s umm a rize d as follo ws .
[i]
Descen ding coo l currents (s ubd uction zo nes ) ex e rt a majo r co ntro l on the co nve ctive syste m ( ii) Con tinent s ge ne ra te thei r o wn sys te m of sm a ll-sca le ed dies wh ic h ge ne ra te la te ra l force s a nd vo lcani c ac tivity ( iii) Co nt ine n ta l plates d o no t ride passive ly o n th e ba ck o f a ho rizo nta l ma nt le flow (iv) T he re is no d irec t link be twee n as t he nosp he re Ho w a nd plate mo ve men t (pla te s a re no t d rive n by 'm a ntic d rag ' )
A
() 0
23
t"il:u rc 2. D (A) Laboratory simulunon (using hUI nil cootc u fro m ;,ho ve) o f n>nvcclio n in the uppe r man tle neil( " s l" liomuy cu nlinl'lll ,,1 nI,u gin. T he «kerch shu w, the dirc cuo n of fluid mol ion Fro", Elde r ( 1<)76). U11 S~cl r h of co nvective ll,)W paucr n for an i' ol ..1cd migr" ling (co nl ' Ol'nt"l) s hn' l with nnuchcd. tr," lo ng o CCdnic c ru' l, hilsed Oil ;, nu me ric..l simulation . Fw ru Elde r ( 1976),
00
I
w E
24
GEOLOGICA L STRUCTU KES AN D MOVI NG I' L ATE S
(v) Plate move ment is not d irect ly co ntro lled by upwelling curre nts (i.c . plates arc not prima rily dr iven by ' ridge push') . 2.5
Sources or stress in the lithosphere
BC( ilUSe o f its co mpar ative st re ngt h, the lit hosphe re can suppo rt s ubsta ntial de viat o nc st resses . Estimates of maximum stress differe nces produced by surface top ogra phy yield values of 2{K)-300 M Pa ( = 2- 3 kiloba rs; I MPa = 10 bars) in the uppe r cr ust (Birch, 1964: Jeffreys, 1( 70). However , such large stress d iffe rences can no t be maintained in the lowe r part of t he li thosphere or in the asrbcno-
sphe re because o f visco us cr ee p . Kuszni r a nd Pa rk ( 19R4) es timate t ha t ave rage stress levels within the present co nt ine ntal plates probably lie within t he range - 25 to +25 MP a dis tr ib uted ove r t he w ho le t hickness o f t he lit hosphere. Intr a pla te exte nsio nal tectonic e ffects in the form o f gra be n , rifts and sed ime ntary basins are widespread in the maj or conunental plates, b ut co mpressional intra plat e effects a ppear in co ntras t to be rela tivel y uncommon . Th e defo rma tio n o f oce anic plat es seems to be neithe r per vasive nor exte nsive , being ge nerally co nfined to brittle deform at ion in the upper crust associated with insignifican t str ains, o r 10 lithosphe re flexure associ ated with sedime nt o r se amo unt loadin g (see Bod ine et ai. , 19K1). T he mag nitude o f the stresses invo lved within lithosph ere plat es must clea rly be large enou gh to promot e frequ ent t ho ugh localized exte nsio nal failure in co ntine ntal lithosphere , but too sma ll for co mpress io nal failure e xcep t unde r exceptio nally favourab le circu msta nces. T he so urces o f stress in the lit hosph ere have been investigated by Forsyth and Uyeda ( 1975), Turcotte and O xbur gh ( 1976). Richardson et at. (1976) and Bott and Kusznir ( Il.JK4) (see T able 2.3). Stress systems affect ing t he lit hosphe re ca n be co nve niently d ivided into renewable and non-renewa ble type s (Ba tt. 19R2). Renewable stresses are those that pe rsist as a res ult of the co ntinued presen ce or rea pp lication o f the ca usa tive forces, even alth ou gh the stra in ene rgy is progressive ly
dissipated . The two mai n exam ples are stresses ar ising from plat e boun da ry forces and fro m isosta t ica lly co mpe nsated sur face load s. No nrenewable stresses arc those tha t lire dissipated by re lease o f the str ain en e rgy initia lly present. Bending st resses, membrane st resses and thermal stresses are exa mp les o f this type . T he value o f stress lit a ny po int in the lithosphe re results fro m t he supe rimpos it ion of se veral diffe re nt stresses and is affected by local variations in mechanica l prope rt ies. It is con venien t to treat the lit hosphere as a single un it o f varying du ctility in which the a pplied fo rces a re averaged o ver the whole lithosp her e tbick ncss. Th is simplificatio n may o nly be a pp lied to intraplate lit hosph ere . A t plate bo unda ries. o r where majo r int e rnal deform ation is takin g place . it will no longer be a pplicab le.
1
r
s s
" L
I.
I.
Plate boundary forces As inert ial forces arc negligible , the dr iving and res ist ive forces acting o n a moving plate must be in yna mic equilibrium rc st ress: distriliution within plates dcpcnas critically u ~n whether the plates are dri ve n by force s o n thei r edges or by the underlying mant le drug. The various types of dri ving and resist ive forces that may act on a plat e have bee n SUIll marized by Fo rsyt h and U yed a (11)75) and Botr and Kusznir (1984). T hese are as follows (T ahle 2.3). (i) T he slah-pull '[orce ( Figure 2. 14A ) acts o n a su bducting plate an d results fro m the negative buo yan cy o f the coo ler, den ser lithosphere of the sinking sla b. T his force is po tentially Ih e largest o f the plat e bou ndar y forces, hut is partl y cou nteracted by resistan ces prod uced by s ink ing, dow nbending and co llisio n. Bo tt and Kusznir est imate a mag nit ude of 0- 50 MPa for th is force . (ii ) T he .l"ulJel ucrion merion fo ret', origina lly recognized by Elsasse r ( 197 1) and named the ' trench suction fo rce' , is cause d by the effect of subdu ction on t he ove rlying plate . and is estimated to be around 20 MPa in magnitude . Both the slab-p ull and subd uct ion-suction forces will prod uce tensiona l stresses in ad jace nt litho-
1.1
11
M
Sf Sl
d. Sl
(J
fr; C{
T 01 n. m fa B. as sn A
THE UT HOSP H I'; RE: SO MI'. IMPORTANT
25
P ROI' E RT1 ~S
T~hle
2..' Summary of the principal mec h,lI1 i, m, of , lress ge ne ra lio n in the lithosphe re . wilh their mo re importa nt propert ies amIes tima ted magnitudes. From Bo n and K.., znir ( 191\4 ). Stre sses
Re newah ll' '"
Ap p, o xlmale le vel of , Ire" d iffe renc e
non -ren ewa ble s tre sses
( '''Illprcssinn or te nsion
Re ne wable
T en sion (norm.llly)
n. so MP"
Ren ewa hle
Ridg.: push (plate b oundar y for ce) M;tnl!c con vccno n or astheno spher e dr ag
Mcchanem
Suhdul1itln sla h pull (plale houndary for ce )
effcct-,
(? )'
Y,',
Yes
T cnsi(l[l (no rmally)
ll- JIlM Pa ( ?) '
y"
Yes
Renewable
Compression
20 ~.'\O M P a
Yes
Yes
Re newable
Bo lh
1- :;U MP,1
Yo.
Pro bably nOI
Renewable
BOlh (mai nly
MI'a • •
No
(-<,,'lilly per hap s
5H MPa • •
Yo.
Locally yes
Bnr h
U p 10 :;IMIMP"
No
No 0)
h UI
co ntin ually ge ne rated
BOlh
Up to IIM MIM Pa
No
No (?)
Non-renew able
BOlh
U p to 5lMIMP"
No
No ('!)
Non -re newable
BOlh
Up 10 IOOM P"
No
No (? )
[pluto hnundary for ce )
Luhosphercloudin g
{lIne,' mpcns"lcd)
Re ne wa ble
Lilho,phe...: hcndin g (dlle In IlI"ds)
Non-re ne wa ble N on-ren ew able
•
_anahle in . pace and r,me ocu ""c " r _arml,,,n in 2km cle_alion.
'Ill~hl y · · ~nr
hOlh (m ainly te nsion ]
Lhb ospbcrc be ndin g (due io subduct ion)
Membrane effects
l~
te nsio n]
Lilhosphere load ing (compensa ted)
Thermal effects (C\loling and su b(joclin~ lilhosph e re )
St res ses
signilic" ol in tect onics
Suhd ud inn lrene h ~e !ln n
suhJeel 10 amplification
rC';M a ncc~ , ~ lI hd uchon
sphere, provided (hat the resistance fo rces are suffic iently lo w. Th ese resistances arc highl y dependent o n the length and velo city of t he subducting slab. (iii) Th e ridge-push Lore (Figure 2.148) acts at ocea n ridges, he lping 10 fo rce the plates apa rt and causing lateral compression within the adja ce nt ocea n prates. This is a buoya ncy force arising fro m the mass of hotter , less dense mate rial makin g up the ridge, and is calculated to be 20- 30M Pa in magnitude. [iv ] Th e mantle drag f orce. is the force acting on the base of a movi ng plate . Because of the re latively low viscosity of t he asthenosphere , th is force is co nsidered to be small compared to the plate bou nda ry fo rces, According to Schubert et af. ( 197H) . the shear
_d ocily, lcnglh of ~I " h, elC
stresses p rod uced by a major convect ive cel l of the same dim ensions as the plate wou ld prod uce a maximum st ress of ab o ut 40 MPa in the lit hosp here . However , the evi de nce d iscussed in 2.4 suggests that it is much mo re likely that sma lle r co nvective cells underlie t he large lit hosph ere plates, a nd tha t these will exe rt shear forces in va rying directions, whose effects will probably large ly cancel out. It would appear that pla tes are dr ive n mo re by edge for ces th an by man tle dr ag. (v) Resistance forces are prod uced at ocean ridges and at transform faults. Th ese 'lppear to be small co mpared with the resistance at converge nt plate boundaries. It is possible to calc ulate theo retical st ress
26
GE O l.OGI CAL STRUCTU RlS AN D MOVING PLATES
.. .. .
A. Tr enc h
~
~
..
~
. · · .' . . . .. . . . . . . . . .. . . ·· . . . . . . . . . .. . . . . . . ·. . . . . . . . . . . . .. .. .. . . . .
B. Rid ge
· ·· . . .. .. . .. . . . . .. . ... .. , .. . .. . . . . . .. .. . . . . . ··· . . . . . . .
.
C. Plat eau uplit t Fi!:ur e 2.14 O rigin of th e main plat e d riving Ior ccs "Cling on the lithosphe re , dcnvcd from dc n~;l ~ imbalnnccs and gravilalio ll<.l l ioad ins_ 8 a nd C a rc ex amples o r to pographi c loads iw sl" lically compe nsated hy II lower-density ",. thcn ospbceic roo t. Th in a rrows , ho w gravitano nat and isos tatic rorccs , an d thic k a rro ws show ller ivcd fo rces acting parallel to the P!;' l":s . Litho , phe re . hl.lnk; usthcn osphe re a nd mC$Oliphcrc . dOlled ; F.... s ubducnon-s uc no n force ; f'f" sl"h- pull Iorce ; F. r • rid ge-push Iorcc ; ,..,... . pl" h:;lu u plift for ce (sec T able 2.3) .
d istributions fo r ce rta in simplified plate seenaries. Ba tt and Kusznir co nside r t hree ( Figure 2. 15). (a) A plate wit h ocean ridges on bo th sides should be in co mpressio n throu gho ut. An ex ample is the prese nt Af rican plate with ridges on three sides. (b ) A p late with an ocean ridge o n one side and a subduct ion zo ne o n the othe r might be expec te d to sho w a grad ation fro m co mp ress ion near t he ridge to tension nca r the subduction zo ne. altho ugh t he effect
of resistan ce might offset the te nsiona l fo rce . An exa mple is the Pacific plutc , with a ridge o n its sout heast side and tre nches O il its weste rn an d nor thern sides ; anot her is the So uth American plate with it ridge o n its ca st side an d a subduction zone on its west. (c) A plate with subduction zones on both sides might be expected to be in te nsio n th ro ughout. A lthough there is no present-day example of t his, the early Mesozoic Pan gae a is pre sumed to ha ve fo rmed a large con tine ntal plate o f t his kind , with subd uctio n zones aro und the ma rgins. Tensional stresses genera ted by these subduction zones, in conjunction with co ntine ntul hot -spo t activity. may have caused the brea k-up o f Pan gaea. It fo llows that widespread ext ensional effec ts may be con fined to circumsta nces like those of case (c). and would be rep laced as soo n as splitt ing and rifting co mmenced by grada tio nal stress syste ms of type (b). /, Loca l modificatio ns of the stress system wit hin plates may be caused by irreg ula rit ies of plat e boun dary shape . by variatio ns in physical prop e rties . by lines of weakness. a nd by geome trical inco mpatibilit ies of shap e as plat es evolve , all of which Illay cause st ress co ncentr atio ns that tend to localize defo rmation if it occurs. L VlIdillg .\f r('.HCS
Local stress fields arc prod uced when t he lithosphe re is loade d hy surface topog rap hy (e .g. mo untain ran ges) or hy lateral density va riatio ns. Small to pographic load s. less than abou t 50 km wide . produce insignifican t stresses. Loads greate r tha n 50 km tha t Me no l isosta tically co mpe nsated will pro duc e ben ding stresses (see below) . However. significant stresses are prod uced where a large topographic load is isostatica lly com pensated by lower-de nsity material at depth. for exa mple the thicke ned continen tal crust below a mou ntain ran ge . o r an ocean ridge. which is co mpensa ted by less dense asthe nosphe ric ma ntle. T he co mbined effec ts of t he su rface load a nd the upthrust of the low-density compensating material pro-
THE LlnI OSPHERE : SOME IMPORTANT PROPERTIES
Oceanic au8l
L.itt109phefe in C
Oce an ridge
27
;:;>"~:'7· ·'." :···:··'· ." ':'· ·." :'· ""':''''~<::: Cootinemal aust
Utt'Iosphere
,a l
Tension (?l
Tension (1)
-- - - -
-
-
-
-
- - - -
Corr1>ression
'liansion_ - - - - - - - - - ~ Ioo
. :::.,':, ' ,.: ... .,.': ::::Bb tel
duce ho rizo ntal deviato nc tension in the regio n betwee n the lo ad an d the co mpe nsatory mass (Figure 2. 14C) . Such a stress system will cause a piece o f con tinenta l crust in reg ions o f mountain ranges o r plateau uplift to be in tensio n relative to adj oi ning regio ns, with stress d ifferences of the orde r of 50 MPa. C rust in the adjoining regio ns wo uld suffer corres ponding compre ssio n. T he same effect will occur at passive con ti ne ntal ma rgins.
Non-renewable stress sources Several diffe re nt so urces of stress exist that theore tically can pro duce large stress d iffercnces in the crust, giving initial strains of 1% or more . Howeve r, becau se they arc non -
Figu rt 2. 15 E xam ples of simple str ess systems in lithosphe ric plates ge ne rated by pla te bou nda ry forces. (0 ) R idge -push terce developed ;11 r idges on opposite sidL"S of a con tine nta l plate, ca using the whole plate to be in co mpression; a n exam ple is the p rese nt A frican plusc . (b) Rid ge-p ush fo rce on n ne stdc o f a pla te and subd ucnon suctio n force o n lhe other, causing a srress sysle m grad ing from compression at the rillgc to te nsion , posvibly , a t the tre nch ; a n exa mple is the pres ent Sou th A me rican plate. (el Rid ge push force on one side of a pla te a nd slab pull on the ethe r, ca using a gradation a l strcss syste m as in (b) ; a possible example is Ca rbo niferous basi n fo rma tiun in Bnla in. (,I) Subd ucricn s uction lin bot h side s of a contine ntal pla te , produc ing te nsio n t hr oughou t: an poss ible exa mple is Panilaea immedia tely p rior 10 its b rea k-up . F"" ridge -pus h; F"" slab-pull; F.u • subducnon suc tion; R md • ma n tic drag. Fro m eo« a nd Kusznir ( 191l4 ).
ren ewable , they will be subs tantia lly relieved by creep and brill le failure over geo logically rathe r sho rt pe riods o f time . Such sources will never prod uce large stresse s o ver the whole th ickness of the lithosp here , since they will be partly relieved before they are able to reach their theoret ical value. Exa mples of t his kind of stress are bend ing st resses. membr ane stresses and thermal stresses. Be nd ing stresses arise from flexure of the lithosphe re resul ting fro m unco mpen sated loads and downbend ing at subd uctio n zon es. Horizontal compression occurs on the co ncave side of the ben d . and te nsion o n th e co nvex side . Because the lithosphere shows visco-e lastic behaviou r over geo logical time periods, t he init i al~ sses calculated using an elastic model
--.
28
G EOLOG ICAL STRUCTU RES A N D MOVING PL,AU:S
(et . walcott. 1970) of around 500 MPa for a load of about 500 km width , would ra pidly decay t~ about half that value in the up per part o f the lithosp here and totall y disappear in the lower part , if the visco-elastic behavio ur of the lithosphe re is. ta ken into account (Kusznir and Karn er. 1985). Th is problem is considered 411 ~rea le r length in 2.7. Because the stress system IS non -renewa ble , it is nOI compa rable with Ihe plare bou nda ry fo rces in its lon g-ter m effects. M~m.h ran~ stresses are caused by changes in the radius of curvature of a plate as it migra tes from the pole to the eq uator o r vice versa (Turcotte , 1974a ). However , in a very similar way to bending stresses , mem bra ne stresses will neve r reach their theo retical maximum (calculated at aro und IOOMPa oy Turcou e ) beca use of continuo us visco us rela xation in Ihe long time pe riod take n for the plate 10 move 10 its new positio n. A third common so urce o f no n-renewable stress is ca used by te mperature cha nges with in the lithospher e that give rise 10 thermal stresses. These are most impo rtant in the ocea nic lithosphere as it cools afte r fo rmat ton 411 a ridge, and again as it beats up after subduction. Tensile stresses para llel to the ridge axis o f up to 400 MPa are calculated by Tu rcott e (1974b) and both comp ressive and rensite stresses o f the o rder o f 600 MPa in su bducting lithosph er e are suggested by Ho use a nd Jacob ( 1982). Like the o ther non -renewable stresses , however , it appears unlikely that they play an important role in tectonic defo rmation. although they may have a signifi cant local weake ning e ffect on the lithosph ere . Other sou rces of stress include changes in volume associated with phase transitio ns in the mantle , eros ion (surface unloading) , which is locally impo rta nt in joint form al ion , tida l de fo rmatio n, which yie lds negligible stresses. a ~ lateral v~ r ~at io ns in stre ngth ca used by majo r compos itional bo unda ries (e .g. mar gins o f magma chambers o r of sed imentary basins). None o f these is a possible sou rce o f maj or
tectonic deformatio n.
•
A mplificatiQI/ 0/ stress
The stresses gener ated by the majo r plate bou nda ry and klitd ing forces have bee n calculated as applying to tbe whole thickness o f the hthosphere . However , because the lithos phere beha ves as a visco-elastic body. stress will decay in rbe lower. warmer, a nd less viscous pa rt, a nd become concentrated in the upper pa rt wher e clastic/brittle behaviour is more important. In the case o f renewable stresses. since the fo rce is constantly app lied , and retains the same value , the e ffective stress can be do ubled if the effective thickness o f the lithosphere is halved . Kusznir and Bolt ( 1977) and Kusznlr and Par k ( 19R4) have deve loped a model which investigates mathematically how a visco-elastic lithosphere with downwa rd var iati on in viscosity responds to a co nstant applied fo rce. They show that. depending o n the tem peratu re structure of the lithosp her e and on the size of the applied fo rce . there is an a mplification of the stress in the upper pa rt of the lithosphere of abou t x 2 for coo l continen tal shield regio ns after I Ma. a nd abo ut xg for very warm lithosphere . like tha t o f the Basin-a nd-Range province o f the USA. after only 1000 years . With larger initial stresses o r a longer time of application, the lithospher e fails complete ly. when the strength of the strongest part is overcome. and large strains can de velop . Th is principle o f stress amplification is critically impo rta nt in explaining how the relat ively small forces available from renewable plate bo undary so urces (giving stresses in the ra nge 1O- 30MPa when applied over the who le thickness of the co ntinental lithosphere) a rc nevertheless able to ove rcome the known strength o f rocks in the middle and upper crust, which is in the range 100-400 MPa . Summary
The sources of stress in the lithospher e that a re likely to be o f tectonic significance arc rene wable buoyancy fo rces arising from de nsity
TH E
Ll TH OS P H E ~E :
SOME IMPORTANT PROPERTIES
contrasts acting at plate boundaries. T hese buoyancy forces pro vide th e principal dynamic ope rati ng me ch an ism of the p late sys tem. T hey arc o pposed by various resistance for ces du e 10 subduction and co llisio n, and alo ng tra nsform fau lts. Ma nt le dr ag forces may assist or oppose them . Buoyan cy for ces also arise fro m isostatically co mpensated loads on the lithosphe re . Co ntine nts, platea u uplif ts ami mountain ran ges all ge ner ate their own interna l tensional st resses and produce co rre spo ndi ng compressional stres ses in the ad join ing lithosphe re. These fo rces co mbine to p roduce ave rage stresses in the lit hosp here that are pro bab ly in the range - 25 to +25 MPa . Howeve r, because of the visco-ela stic prope rties of the lit hosphere, these ra the r small st resses become amplified ove r pe riods of 10"_1011 years to levels tha t are capable of loca lly ov erco ming the strength of the lithosphere and causing geologically sign ificant strains . T he ex te nt to which this happens dep ends on th e loca l strength of the lithos phe re . which is in turn controlled by the geothermal gradi ent. Thi s subject is explored in mor e de tail in 2.7 . Many ot he r sources o f stress ex ist that arc not considered to be of tecto nic significance . Examples o f these are no n-re newa ble forces. which give theoretically large va lue s, such as thermal, be nding and memb ran e stresses. However, the st resses produ ced by these effects will be, in geologica l te rms, rapid ly dissipated by viscous cre ep and , being no n-re ne wable , will not produce important lon g-term fo rces. 2.6 The determinati on of stress in the lithosphere There are num ber of ways of o btain ing estimates o f the present state of stres s in the lithosphere . So me melhod s provide measu remen ts of both the magn itude and the or ie nta tion of the principal str esses; othe rs measure orientation o r magnitude on ly. Direct measure me nt of the magnitude and orienta tion of the princip al stress axes (0 1)
29
in the up pe rmost cru st (in -situ srress ) is carried out by several different me thods. Some em ploy a strain Kauge , wh ich recor ds small elastic st rains prod uced within rock across a cavit y. Measurem ent s are made eithe r at the surface o r at de pth , in mines or boreho les. O vercoring with a large-d iamete r dr ill bit is employed to relieve the stress aro und the st rain gaug e. Flatjaek measurem ents arc also widel y used . The flatjack is a t hin hyd raulicpre ssure cell in which the pressure is increased un til it cance ls the s train disp lace ments created by cutt ing a hole or slot in the roc k. Both overcoring an d flatja ck techni qu es employ the principle of stress relief. A n alternative type of method is the hydrofracture techniqu e in which hydraul ic fracturing is induced arti ficially in the rock, in a bore hole sec tion .
0 2, 0))
Str ess magnitude Ncar-surface measurements o f in-situ stress give widel y va rying ma gnitudes (T ab le 2.4). Th e use of the hydr ofracturc techniq ue ap pears 10 have given mo re co nsiste nt resu lts tha n the st ress-re lief met hods. Available hydr ofra cture dat a suggest st resses of seve ral hund red bars (tens of MPa ) to dep ths of several km (Haimson , 11J77; McGarr and Ga y, 1978). McGarr (1980) sho ws, o n the basis o f determinatio ns from North A merica , southe rn Africa and Au str alia , that, o n average . ma ximu m she ar stress increa ses linearly with de pt h at a rate of 38 M Pa/km in so ft rock s and 6.6M Pa/km in ha rd rock s. Brace and Kohlstedt (1980) poin t o ut that the limits of upper lithosphere stress can be de te rmine d on the assum ptio ns (i) that rock s are fract ured . and that friction on the fractures co ntrols the stress at shallow depths, an d (ii) t hat upper crustal st ren gth is based o n the strength of quart z. AI a given depth , depe nding o n the temperat ure grad ient , stren gth beco mes dependent on the cree p properties of mine rals , particularly quartz, o livine and feldspa r (see 2. 7). Fo r dr y rock s. the maximum strength in
30
GEO LOGICA L ST RUCTURES AND MOVING PLATES
Tablt 2.4
Som e str ain-relie f in-situ stress measure me nts in No rt h A me rica. A lIef Sb;tr :",d Syh-,; ( 1973), Data source s:
J, Hoo ker ami Jo hnso n ( 19M ) ; 2, Hoo ke r and Joh nso n (1967) ; J . Se llar s (1%9 ); 4, Obcn ( 1%2); !i. Moru 7.i ( 196R) ; fl, Eisbacher and Bid enstci n ( 197 J) ; 7. Agarwal [ unpubt.}. Location
Q,
0.,
Depth ,
( ba rs)
(ba rs)
if > SO m
Barre. Vermont Proctor , v ermom Tewksbury. Massachuse tts
"'
W. Chelmsford. M as....acn uscu s Nyack. New Y or k 51.Peters, Pennsylvania
145
Rapidan. Virginia Mr. Airy. No rth Ca ro lina U thoni a, G eo rgia
~1
R1
,. 12
"' '" 102 11 1
Litho nia. Georgia Douglasville , G eorg ia Ca rthage . Missou ri G ran ite ville , MisM'lur i Teoy. Ok laho ma M..rhle F..Us. T e ra s Niagar" Falls. New Yo rk Harbcnon, O h io Sudb ury , Ont a rio Elliol l.
440 510 210
Mo rgamow n. Pen nsylva nia
370 370 510
" ""
T T
22 2.6 I. '
35
1.,
5 23
2.4 2.' 1.2 2.1 1.5
" R1
"
T rend of <1 , Reference Rock type
N. 14" E. N . ~W E .
N. 2° E.
N.1 4°E.
N.
flOE .
N . l\7" E.
N. H"E. N. 4'J" E.
I. ' 23
N . M~ W .
53
I.,
"
2.1 l.5
N. Xol " w. N . :W W. N. SS" C N . 90" W. ENE .
lUI
(~
- 0.7'
"
N . 07" E .
N. 2"E.
230 - 440' 180 200
RSll
1.,
300- 400
' 2
E.
'".0
7fXl
"
N E.
= 0.85 On, fo r 3 < On < 200 MPa , = 60 ± JO + 0.6 0" fo r on > 200 MPa
where T and On a re the shearing and norma l stresses respective ly at which friction is o vercome on a fracture. Brace and Kohlsted t find , using mainly hydrofracture stress measurements, that Byerlee's law appears to pred ict horizontal stresses satisfacto rily down to about 4 km. In dry rocks the maximum stre ngth is considered 10 be 850 MPa in com pression and 300MPa in extensio n. The influence of pore fluid pressure is impo rtant, and the values of stress obtained assum ing that the po re- fluid pressure is hydrosta tic, are redu ced to 600 MPa for compression and 200 MPa fo r extension. These results app ly dow n to a level of about 25 km. below which a tempe raturedependent flow law will become increasingly important. Th e calculatio n of stre ngth distnbutio n with depth . and the influence o f the thermal grad ient on it, are dea lt with in 2.7.
,~
700
1 1 1 1 2 1 1 1 1. 7 1, 7 I, 7 1 1 1 1
N. 4° W . N. ZO W.
1.7
64 19
35 217 73 73 l SI
the crust is given by a '"linear friction law, Byerlee's law (Bye rlee . 1968) which gives: and
",la,
' 0 13.0
N. 27' E .
Dolomite: Pa ragn eiss
Granitc Diabase NUTtle Diabase
Granite G ranite G neiss G ne iss Limeston e G ra nite G ra nite
Granite
J
Dolom ite Limesto ne
0 0 1. 7
Sand sto ne Sa nds ton e Sa ndstone
•5
NE.
Gr an ite
Diabase
A nother app roac h in estimat ing stress magnitudes in the upper lithosphe re is th rough labo rator y measurements of common rock stre ngths (see Ta ble 2.5). The se can give us minimum estimates of the strength of fractu red uppe r crust. Many regions exhibit uppercrustal te nsile fractur ing but no evide nce of co mpressio nal failure. in which case the ava ilTa hle 2.5 U niaxial com pressive str ength. Cu. fo r a n u~ bc r of roc ks. Mult iply by 100 to co nvc nrc MPa , Rock
COl (k b ars)
G ra nite , w e » ... rly
2.29
Ou anz ue , Chesbirc
4 .(J()
Dia base , Fr ederick
4.K7
Mar b le , Te nnessee G ra n ite , Ch arcoa l
1.52 1.73
Sha le , w itwate rsran d Gran ite Aplite (Chert)
5.87
O ua n ane , Witwat e rsrand Do lerite. Ka rroo Ma rble , Wom hc ya n Sa nds to ne , Go sro rd Lime stone , So len hofen
1.72
Re fe re nce: Brace ( l964b) Walsh ( l %5c ) Coo k ( 1965 ) Coo k ri a l. ( IlX..(,)
2.00 3.3 1
o.n
0.37 2.24
Wiebolst'l a l. ( 19M)
31
TH E LITHO SP H ERE: SO ME IMPORTANT PROI' ERTIES
able stress levels are bracke ted by the values of com pressive and tens ile stre ngth of the approp riate materia ls. These ligures may be contrasted with the strength estimates ca lculated from vario us topograp hic loads (mountains, sea mounts, erc.) supported by bo th co ntine ntal and ocea nic crust, which yield stre ngth estimates of aro und 100MPa (Hea rd, 1976 ; McN utt , 19RO) . It is clea r that the values o f stress magnitude ob tained in these various ways give us no indicatio n of the rea l long-term stre ngth of the lithosp here as a who le , as explained in 2.4.
Stress m agnitude in the manlie
Estimates of stress magnitude at various depths in the lithospheri c mantle and below have been made by studying sub-grain size and
ec
dislocat ion density in samples o f mantle peridotit e brought to the surface as xeno liths in lavas, kimherlites , etc. Recrystallizatio n at high temperatures and stresses takes place by grain-bounda ry migratio n, and by sub-gr ain rot ation at low temperatures, or lo w stresses . o r bot h (Sellars, 1978; G uillopc and Po irier, 1979). Since both processes are highly stressdependen t, analysis o f o livine gra in size may be used to estimate stress magnitude . Plots of grain size against depth o f or igin show a prominent grain-size discontinuity at a critical depth that can be interp reted as the leve l o f change-over from the sub-grain rota tion mechanism to grain-bo undary migration. T he system can be calibra ted in terms o f stress using the piezo meter (stress meier ) deve loped by Ross et al, ( 19HO). co nsisting o f a n experimentally derived grain-size/stress relatio nship. Th e results of th is method are applied to
I ,
•
po
-+--
:;;
'"
r.
ec
ra
I
- ;f
~
--
- ,..
--
:,/,
sc I
ec
§ --
--
{So.I!hom
-d 1
'-
ec
ax
f-
':: ~E."""'z."..
rc
(9otin
Mrial l
'"
+,=-q::. =:JF4= -tI-lIt
ec
, 0
,
0
A
e
0
eo
eo
so
eo
so
eo
B
F'ilu~ 2.16 Stressestimates for infra-crato nic mantle (southe rn Africa) (A ). and for a continental exte nsion zone (Basinand-Range Province. USA) (8 ). based on the olivine grain-size piezo meter. Estimates fo r shallow samples (above the da~1\ed lirte) arc based on the SGR piezometer (Mercie r, t9RU) and those for the deep samples on the piezome ter of Ross t'l
al. 1I9llO). Fro m Mercie r ( 191l0).
32
GeO LOGICAL STI/.UCTUll:f.S AND MOVING I'LATES
vanous tectonic provinces by Me rcier ( 1980) in the fo rm of stress- depth curves ( Figur e 2. 16). T hese may be co mpared with the theo retica l stress- de pth curves give n by Kusznlr and Pa rk (see 2.7) . T he re ar e significan t diffe rences between the stress - de pth profiles for the relatively coo l co ntine ntal shield lit hosphere . o f southern Africa ( Figure 2.16A) and t he wa rme r Basin -and -Ra nge lithosphere (Figure 2. 168 ) - compare Figure 2.26. T he st ress estimates for the infra-crat oni c mantle below so ut hern A frica vary from 15 MPa a t BO km de pth to 5 MPa a t 140 km . the n decre ase ver y slowly with de pth to 4 MPa at 240 kru, indi ca ting a lithospher e base a t aro und 140km . T he lo w stress in the asthen osphe re is a useful co nfirmation o f the minor ro le of the ma ntle d rag force in plate dynam ics. T he stress estimates for the mantle below t he Basin-and-R ange province re veal a much thinner lithosphe re with a base around 56 krn, a co mp arab le va lue o f asthen osphere stress, but a much highe r va lue of stress in the lithos phe re - up to 70 MPa.
• Sum mary: stress magni tude Es timates for the uppe rmost crust vary widely but ar e o f litt le significance for bulk lithosphe re st rengt h. The st ronges t part o f the lithosp here fo r average geo the rmal grad ients lies at mid-crustal levels ( aro und 20 -25 km) and dep ends on the shea r strength of qu art z. Fo r roc ks with hyd rosta tic fluid pressures. the maximum st rengt h would be around 600 MPa in compress io n and 300 MPa in ex tensio n. Intrapla te st ress levels mu st co mmo nly be bracketed by these values, but will rarely excee d the high er. Below the zo ne o f ma ximum stre ngth. t he strengt h depends on te mpera ture-d ependent flow laws for minerals such as quar tz, feldspa r and o livine, a nd stress leve ls decrease with depth to le vels of 4- 5 MPa in the s ub-lithospheric mantle . The long-term strength of the lit hosphere is clearly much lower than these stress esti ma tes suggest , as indicated by estimates o f l OOMPa fro m the effects of to pograp hic load ing. A
rea listic mod el of lithosphere stre ngth must ta ke account o f stress , de pth , tem per ature grudie nt and t ime . a nd a model of this type de velo pe d by Kusznir a nd Park is discussed in 2.7 . S tress orientati on
Mea sur em ent s o f stress o rientation are mo re di rectly relevant to tectonic anal ysis than are magnitud e estimat es since they can be mor e easily rela ted bo th to visible st ructures and to the kinematic pattern . The most widely used method for o btaining stress orientation data is by analysing earthquake focal mechanisms. A n individua l fau lt-plane so lutio n gives a choice o f two possible vecto rs. o ne of which lies in the fault plane . an d t he ot he r pe rpen dicula r to it. Whe re the o rientation of the fault pla ne is known . the refor e , a unique determin etion of the slip vecto r is possible. Some areas s ho w rema rkable consiste ncy o f slip vectors ove r a large area, and the orientation of slip vecto rs around the va rious plate bou ndarie s was one of th e impo rta nt criteria used by McKenzie and Parke r ( 1967) an d by Isack s et at , ( 1968) to demonstr ate the rigid be haviour of plates (see 3. 1). The calculat ion of the orientation o f the prin cipa l stress tensors requires more information than th e slip vectors . If the shea r plan e respo nsible for t he earthquake was initiated by the s tress syste m be ing invest igated . and not by so me previou s eve nt, the stress or ientation can be ca lculated ap proximately by assuming a value for t he angle be twee n the stress o rie ntation and t he she ar plane . O ver la rge a reas . the meth od may give satisfacto ry and co nsiste nt resu lts. Fo r example. the co mpressive stress distri but ion in the region o f the Ja panese arc gives results which a re gene rally co nsiste nt with a mode l where the horizon tal co mpo ne nt of pr incipal stress acts parallel to the direction of relat ive plate motion (Figure 2. 17). Where fau lt planes with varying o rientation exist. however . the method gives a regiona lly va rying st ress orientatio n. which seems unl ikel y. Ge phart and Fo rsyth (1985) have discussed
IH E LITHOSP H E RE; SO ME IMI' ORTANT PMO I' J::: RTl ES
•
,
,
t
' ~rP ~ "'"
EU
.
". ~
PA
:
•
uu' ~
L.
PH /
' 3 ~ "e
3 ~ ' ..
" O'~
,
..~ ~ .(
I~O ' E
t·i ~u rt 2. t 7 Dircenons uf ma xim um honzon tnl Clll11preS,j,m;!1strcvs fu r intc rmcd ra te and <J eep e unh q ua kcs in the Japanese are system. Toothed lines. tr en ch e s; T- T, ImnsfllTm Iault ; fA , Pa cifi c plat e : PH , Philipp ine p l
zu.
this problem in relation to the focal mechanism pattern of recen t ea rthquake s in New England . T hey show that. if the eart hquakes arc assumed to result from movem ent s on a set of variably o riented pla nes of weakn ess. a mean o rienta tio n fo r the principa l stress axes can be found . T hey use a method deve loped by Angelier (1979) for de termining principa l stress directions fro m sticke nside o rie ntatio ns on a set of fault s urfaces o f wide ly varying orientatio n. Gep hart and Fo rsyth show tha t their data (Ire consiste nt with a single regio nal stresso rientatio n with EN E-WSW to NE -SW maximum co mpressive stress. and ncar -vertical minimum st ress. which is co nsisten t with estimates from othe r so urces . A met hod o f d etermining in-situ st ress o rie nlations from borehole wall fract ures (b o r('h o/~ breakolds) is descr ibed by Go ugh and Bell (1982). Brea kouts were fi rst ide ntified in oil wells in weste rn Ca nada by Cox ( 1970) as borehole inte rvals wit h e longate cross-sec-
33
tio ns, who se 10 l1g axes were aligned region ally in a NW -SE direc tion. Gough and Bell explain t he break outs as zone s of spelling along shear fract ures . develo ped beca use of the increa sed st ress d ifference caused by the boreho le. T he bre ako uts are arranged in pa irs at 1800 to each other and de fine a n azimut h par allel to the smaller horizontal principal stress. T hey ge nera lly le ngthen the diameter by 8- 10%. T he au thor s discuss results Irom three se parate area s; the Rangely o ilfield. Co lorado . t he east Texas basin , an d the Norman Wells area in No rt hwest Te rrito ries. Ca nada . At the Ra ngely oilfi eld . breakout stres s o rientat ions show good agree ment with direct measurem ents made by induced hydra ulic fracturing . and with eart hqua ke focal mechani sm solu tio ns. Dula ( 198 1) co mpa res t he in-situ stress de terminat ions in t he Ra ngely basin with stress o rie ntations determined from fabric eleme nts (e.g . q uartz de fo rmatio n lamellae and microfracture o rientatio n] o f Lar amide age and co ncludes that the presen t W NW- ESE or ien tatio n o f 01 may have been co nstant since late Laramide times. A large numbe r of b reakout d ata in the ea st Texas basin co llated by Brown et al, (1980) show a consistent patt ern (Figure 2. 18) interp reted by Go ugh and Bell as the result o f a NE -SW maximum horizont al stress . A t No rman We lls. breako ut data fro m two wells show good agreem ent with fracture o rienta tio n and indicate a NE -SW maximum hor izontal co mpressive st ress also . Thi s directio n see ms to be unifo rm over a large part o f the No rt h Ame rican crato n (see Figure 2.20). Stress mea su rements using this tech nique have also been mad e in the oceanic crust at holes dr illed in the Deep-Sea Drilling Program . New ma rk et al . ( 1984) describe two examples in the East Pacific. Th e fi rst is o n the Nazca plate west of the Peru - C hile t rench (Figure 2. 19A ) and indicates a maximum ho rizontal principa l stress orie nted NE - SW. T he min imu m ho rizontal stress is nea rly parallel to the pre dic ted tensio nal stre ss ca used by the slab pu ll force at the t rench. Th e second e xample is fro m the Pacific plate to the west o f the last example an d IBOO km west of the Ea st Pacifi c
34
Gt-.O LOGICA L ST KUcr UII.ES AND MOVI :-O G PLAT ES
Fil:urr 2, 18 Mea n o ne nta tio ns of borehole b rc,lk,oUIS, in sand-
stones of the Schuler Iormano n of E. Texas, in rctaricn 10 the local ex tensio nal faul t pa tte rn . The break-o ur unentaticns indica te a unifor m N E-SW ma ximum bo nzonta ! com press ive stress. After
Gough and Bell (1982).
ridge (Figure 2.198 ). Here the maximum horizontal compressive stress indicated by the breakout data has an azimuth of 1200 (NW SE) whic h agrees well wit h the focal-plan e solutions of local tra nsform faults. T hese are or iented WNW-ESE with a strike-slip se nse, par allel to the relative plate move me nt vecto r. Thi s resu lt is int erpreted as co nfirmation tha t t he stress syste m in an ocea nic p late near the ridge is do minated by the co mpressive ridgepush force. Regional pattern ofstress orientation
The most co mplete regiona l data co verage is of North America . Zo back and Zoback ( 1980) present a gene ralized stress map (Figure 2.20) based on over 200 measureme nts. Several distinct stress prov inces can be distinguished which can readily be correlat ed with the wellknown tecto nic prov inces. T he most important of these are as follows. (i) Mid-continent o r stable continental interior: NE - SW compression. (ii) Atlantic coastal province (Appalachian
orogen ic belt): NW- SE to WNW- ESE compression. (iii) Gulf coast province: exte nsio n perpendicular to co ntinental margin. (iv) Cordillera n o rogenic pro vince : this is a co mplex zo ne co ntaining several subprovinces, but the most co nsistent stress patte rn is the E- W extensio n see n in the active tectonic areas of the Basin-andRange . Rio G rande rift and no rthern Rocky Mou ntains. The transition between the stress provinces in the west is relatively sharp (less than about 75 km typically) but more gradual in the east , part icularly between the mid-continen tal and Atlan tic coastal provinces. Within each of these provinces, the stress o rientation is uniform to within abou t ± 15°. T he co rres pondence be tween the dat a derived from the relatively shallow borehole measureme nts and those from earthquake focal mechan ism solutions (from depths in the runge 5- J5 km) suggests that this unifo rm stress field is representative of the whole of the strong upper crust.
35
TH E LITHOS PH ERE: SOME IMPORTANT PRO PERTIES
F; g ur~ 2.19 (A) Stress o rie nt ation determined lrom borehole hn:;lk<Jul~ nca r silt: 504, be tween th e Cocos ridge ;lO d lh~' Pcru -c Ctulc Hench. The longer Mrow s rcptcscut the maximum , and lhe shor le r Ihe min imum , prin <.:i pal horizontal 'tre~,. Fro m Ne wmark ,'I ul. (1<)1l4) . (8) Stress "Tlenl alion delerminl'd Irorn breakou t drrcclio llS at hole 5<)7c, we~1 " f the Erst Pacific ridg e . Heavy aHOW., indicate ma ~i mum, a nd ligh t short arrows min imu m . principal hunzouta! st ress (tirecl iolls. Llghl I"ng arro"" imJk,ll e Ihe , Iire\'· til,n o [ relative Pacific - Nazca plate moti on . Afte r Newmark CI (,I. ( J'J1l4).
A
...
r1 6 0"
.,.
Hawaii
00'
Me xi c o
10'
0'
Mi o c en e
30'
B
W
0\
ce -
"'0'
.,.
.,.
' 0'
00'
".
' J.
,,'
' 0'
, 0'
o m o
5o ()
>-
r
C/l
.....;
,,'
JJ'
;<:
c
Q
c
;<:
f] ,0'
Jo· '00
l OO
)?O
·,, ~ ( S
~."'O "( T I "S
,,'
.,.
>z o
:s: o sz o
"
r >-;
m
C/l
'20 '
II!!-
lOS-
100·
se-
'0'
.,.
' 0'
".
Figure 2.20 Generalized stress map of the USA . The least principal stress is always horizontal, but may be either extens iona l (o utward pointin g) or compressional (inward-point ing). Stress provinces. bound ed by heavy lines.
TH E U THOSPH ERI'. : SOME IMPORTANT PROPi: RTlf. S
T he stress-field changes at so me bou nda ries merel y involve a swa pping o f stress axes. Fo r example , in the change fro m mid-continenta l through Co rdille ran to San A nd rea s pro vinces, 0 \ changes on ly slightly in o rientati on altho ugh it varies from compress io nal to exte nsio nal. Th is co mple x st ress field can be bro ken down into three simple clemen ts: (i) a weste rn or Pacific supcrpro vince re lated to Pacific plate-boundary processes; (ii) a ce nt ral sta ble intraplate field co ntro lled by bo th Pacific and Atlantic plate bou ndary forces; and (iii) t he marginal fields of the A tlantic and Gu lf co ast provinces , which probabl y repr esent local modifications of the intrapl ate field The extensional G ulf Co ast field is attributed 10 the effect of the passive contine ntal ma rgin (see 2.5). T he co mpressio nal Atlantic pro vince field may possibly reflect an o riginal NW -S E ridgepush force during the earlier phas es of Atlantic opening. It is con veni ent 10 co nside r int rapl ate stress fie lds and pla te bo undar y stress fields , separately.
•
Intraplate stress The mid-conti nent pro vince discussed ab o ve. and its exte nsion into Canada, ma y be regarded as a typical co ntine ntal intra plat e region. Th is was shown o riginall y by Sba r and Sykes (1973) who demonst rated the co nsisten t NE-SW to E- W max imum st ress ori entatio n pauem using data derived from over cor ing. hydrofraeture and focal-rn echanisrn met hod s. 'lhe stress magnitudes obtained from the insitu measurem ents show a wide ra nge , from 10 to 600 bars ( 1- 60 MPa) , but the reg ion ove rall is one of high horizo ntal co mpressive stress . A similar rema rkable regularity o f al o rientation is de mon stra ted by Froid cvaux et al . (1980) in a study of eight sites in F rance using the fl atjack met hod . Th e NW - SE 01 orient ation obtained by them compar es closely with the results for the Rhine - Ruhr rift system (see Figure 4 .168). Richardson a af. ( 1976) and Solomo n et at. (1980) present a compilation of wor ld-wide
37
intraplate stress or ientation data o btained pr imarily from earthq uake foca l-me chani sm a nalysis ( Figure 2.21A ) . Th e most reliable results co me fro m Eu ro pe and Nort h Am e rica , as we have see n. In these areas the foca l-pla ne solu tions ca n be checked by in-situ measurement s. T he So uth A me rican co ntine nt shows a similar o rientalion 10 North A me rica. No te also the N -S to NNE-SSW compression in India and t he E - W co mp ressio n in sout he rn A frica . Most ocea nic lithosphe re o lde r than abo ut 20 Ma also appear s to be in horizonta l co mpression. T he au thors comp are this stress distri but ion with a num ber of theo retica l models making differen t assum ptio ns abou t the vario us dri ving fo rces available (see 2.5 , Table 2.3). Th e most success ful fi t is obtai ned using a model (F igure 2.2 18) which employs equa l ridgepush an d slab- pull driving forces , ignor es the subduction-suction force , and has a very low value for the drag force res isting plate mo tion . Al thou gh the mea suremen ts o f intrapla te st ress arc not nearly good enough for a co nvincing analysis, the evide nce so far sugges ts th at intrapla te stress can be explained to a fi rst app roximation by the simp le inte ractio n o f th ese two ma in driving fo rces , slab-pull and ridge-push .
Stress at rime boundaries Oli ver et 01. (1973) investigated the stress o rientat ion in sinking slabs for fou rte en different subduction zones, using earthquak e focal mechan ism data . Th ey fo und thai in the grea t major ity of cases there was a major co mpone nt of either down-dip co mpress ion or down -dip exte nsion, pa rallel to the inclinat ion of the slab (Figure 2.23). Moreo ver the exten sio nal so lutio ns were co nfined to high-leve l ea rthquak es in slabs where eithe r t he re were no deep earthq uakes o r there was a gap between the upper and lower eart hquak e zo nes . Compressio nal so lutio ns were associated eithe r with the lower part s o f the slab , o r with the whole slab, where se ismicity was co ntinuous. T his distri but ion was exp lained by
38
B
GEOl.OGl CAL STRUcr URES AND MOVING PLATES
"
Oli ver et al , on the basis of the relative strengths o f the slab-pull fo rce , which exerts a down-dip te nsion on the slab, and an opposing resistance force. The resistance produces a co mpressional stress which becomes lar ge r as the slab sinks dee per into the mesosphen c mantle with its increased strength (Figure 2.238 ). This work W:l S very infl ue ntial in persuad ing geologists that plate motion could not be simply explained either by slab-pull o r by ridge-push, but was co ntrolled by the interaction of several primary forces (see 2.5) . The o rientat ion of the maximum horizontal stress across subduction zones generally shows a consistent parallelism with the convergence
FIgure 2.2 1 (A) Wo rld distr thution uf pri ncipal horizont al dcvimonc stresses inferred fru m mi,j-plate ea rthq uake mccbanisms. 'Fnang jes re present th rust f;lU lt mechanisms, square s normal fauIL . and cu cles strik e-s lip. Ar ro ws de note the ho nzc nta l projection of the inlcrred stress axes. From Richa rds on 1'1 ul. ( 197fl), with permissio n. (8 ) Principal horizonral dcviatoric st resses in the lithosph e re fo r a mode l o f plate driving Iorces (sec text] : Axe s without arrows indica te co mpre ssi on and those with a rrows . te nsio n. Rela tive magnitude ind ica ted hy le ngth of axis. Fro m Solo mon ('I al . ( I'JXU ), with permission
direction . Th is regular patte rn is seen for example in both the Aleut ian arc and the Japanese arc (Figure 2. 17) (Nakamura and Uyeda . 1980). However the compressive stress fi eld is oft en confi ned to the volcanic arc itself, and is replaced by an extensiona l stress fi eld in the back -arc regio n. Nak amu ra a nd Uyeda suggest that whe re there is no active ope ning o r spreading of the back-arc region (e .g. in the Peru- Chile. Japan and Kurile zones) the compressive stress associated with the subduction zone may be transmitt ed directly to the interior of the ove rriding plate . Whe re back-arc exten sion is o ccurr ing, however , either the 01 stress weakens and swaps over to become 03 without
T HE. l.ln l OSI' H EIl.I::: SOME. IMI'OR rANT PR01'ERTIE S
, - u-- ,,-c"'"', --- - --- - ,
39
00 "
,"
:»-/?kt> '\ .....
'{
.
"
•
•
..
I
"
..
00 '
Jo'igur e 2.22 Extensional and strik c-slip focal mechan ism solutions tor ca rl hquakes in the A frican rifl syslc nl . Aftcr Fairhead and G ird ler (1':17 1).
any cha nge in directio n, or the re may be II grad ual swing in d irect io n o f at , as see n in the Aleutian ar c. T he o rienta tion o f the maxim um ho rizon tal stress at co llisio na l boundaries is ge ne ra lly perpe ndicu la r [Q th e bo undary o r in th e di rection of rela tive plate mo tion . T he focal-plan e solutions to sha llow e a rthq uakes bo t h in the nor the rn pa rt of th e India n pla te an din Ce ntr a l Asia no rth of the India- Asia co llision sutu re show a co nsiste nt NN E - SSW o rie nta tion (Pigure 2.2I A ). The stress o rien tation a t constructive boundaries (i.e . ocea n ridges and maj or co nt ine nta l rift zo nes) is we ll esta blished fro m e a rthq ua ke foca l-mech anism stud ies . T hese s how pre dominant ly no rma l fa ult mo vem ent wit h a subho rizo ntal e xte nsio na l stress parallel to th e direct io n of rela tive pla te mo tion . as dem on-
stra red by Sykes (1967) fro m mo re tha n 50 focal -pl ane so lut ions for the world rift system. Fa irhead a nd Girdle r ( 1971) show bo th ex te nsio nal a nd strike-sl ip so lutions fo r t he A frica n rift syste m (Fig ure 2.22). T he ex te nsio nal stresses a re a pproxima te ly par a lle l to the re lative pla te mov ement vecto rs: NE - SW in t he Re d Sea and NW - SE in the A fr ica n rifts. T he lo we r pa rt of the Re d Sea shows predomina ntly st rike-s lip sol ut io ns.
Conservative boundaries. Tra nsform faults a re associated with e a rthquakes with a strikeslip se nse of movemen t , o ppos ite to the direclion o f a ppa re nt displace me nt (Tuzo Wilso n . 1963). Sykes ( 1967) sho ws a good e xampl e fro m the G ulf of Ca liforn ia (Figure 6. to). Th e o rie nta tio n o f th e principa l stress axes ca nno t be sim ply derived fro m t he slip vectors , how -
40
GEO LOG ICA L ST RUcrU KES A t'o!D MOVlf'JG PLAT ES
o s:
c c
•
•
ca
Z
• - '"
•c
OC
100
•
•
0 0
o·0
200
:; 400
0
a
C) 500 I600 I700
L
0 0 0
;,
""'t,
E
o
S o
'i'
N
,
~
c
0 0
e
c
m
i'"
300 ~
o
o
I
,•
u
"0
o
C
~
. ~,. • ~
:
L
•
•
•
0
~
0
0
~
~
0
~
~
f?9 ~
8: La
~D~ D
GJ
A B
A
. ~• ••
L -_
_....
_ _ _ ~~~ reng th
I
.
c
0
.~ ,-~~== ~~ . LJ I ~---" o
~~,
••
o c
0 0
c
c
_
Inc reasin g streng th
-- --High--strength -- --- - --- - --- - - - - - - - - B Figure 2.23 (A ) Down-dip stress ploued as a funct io n of dept h fo r fou rteen subduction w iles. Filled circles. do wn-dip extension; open circles. compression. Cn lSSCs rep resent ocher oriemauons. Small symbols represent tess accurate
determinatio ns. Th e enclose d rectang ular area s rep resen t the a pprox imate detrib ution or eart hquakes in the sl " h~ . ( 8 ) D iagram sho wing poss ible explanation or the distribution of mess types in (A l. Sla bs which just pe netrate the asth eno sphere milf he characte rized o nly by down -dip exte nsion (I ); as the slahl; penetrate (utlh el into the higher-strength mesosphere, they arc subjected (0 ccmn ression in their lower pHI (2) and eventually thro ughout (3) . Break -up o f the slab is envisaged in (4). From Uyeda ( 1978) . alter lsaeks and Molnar ( I%'J).
ever. since a uniqu e solutio n ca nnot be pro vided from a set of para llel fault s. Examina tion of the stress orientation o n intraplate sea fl oo r (see e. g. Figure 2. 19A , B) suggests that (11 is
ge nerally oblique to active transform fracture zone s in accorda nce with classical fault th eo ry. A similar ar rangement can be dem onst rated in contine ntal strike-slip zones which form plate
T H~ LlIH O S I> H ~ H : SO M ~
boun da r ies. Fo r instance . in the Sa n A nd reas fa ult zo ne . the a rrange men t of fa ults and o f indi vidual slip vecro rs is very complex in de tail, hu t the overa ll stress field appear s to be mo re uni fo rm. with a N E -SW 0, o rientati on. o bliqu e to the t rend of the Sa n And reas fa ult. as sho wn in Figu re 2.211 {Zo bac k and Zo beck, 19R1J) .
Summary The ra ther sparse datu available fo r intrapl at e stress fields sho w co nside rable regula rity. Stress o rien ta tions ca n he explained to a first appro xima tio n hy the co mhi ned effects of ne ighbouring plate boun dary forces. The important forces appea r to he a sym me trical ridge-p ush fo rce a nd a symmet rical ' tre nch pull' for ce co m bi ning the effec ts o f sla b pull. su bd uctio n suction. and the va rious o pposi ng resista nces. Stress data fo r the plat e bo unda ries themse lves a re a bunda nt a nd sho w a consis te nt pa ue m o f e xtension al stress p urullcl to d ivergent motio n, com pressiona l stress pa ra llel to conve rge nt mot io n . and obliq ue stress fields across tr a nsfo rm fau lts. Ext ensio na l stress fields a t dive rge nt bou nda rie s a re restrict ed 10 the region of th e rift zo ne . a nd a re ra pid ly replaced by co m pressive intra pla te stresses o n eithe r side . Co mpressive st ress fie lds il l co nvergen t hou nd a ries Me replaced by exte nsional stress fie lds in ma ny hack-arc regio ns o n the upper pla te . hut no rma lly co nt inue int o the typical co m pressive intrapla te st ress field o n the ocean ic subduct ing plate . Stress d istribu tio ns in subduct ing sla bs co nfi rm the pictu re o btained by world-w ide st ress modelling. o f a com hi ned sla b-pull/ ridge- push driving mecha nism for plat e mo tio n , wit h exte nsion al st resses confined to t he up pe r parts of slabs o nly in the ea rly stages of subd uctio n. The effect of the mantle d rag fo rce aprea rs to be minimal.
2.7 The tong-term st rengt h of th e lith osphere The stre ngth of the lithosp he re co ntrols both the initia tion a nd subseq ue nt evolutio n of majo r zo nes of deform at ion . T he res po nse of a
lMPORTA!"T
PR O PEJl.TI~S
41
piece o f lithos phe re to a n a ppl ied tect o nic fo rce is depende nt o n the vertic al d ist ribution of both duct ile und britt le stre ng th. which in turn is co ntrolled by the va rying r heo logy wi th de pth shown by lithosphere mat e rial. Whereas brittle st reng th is co ntrolled prima rily by litho stati c pressure a nd increases with de pth, du ctile stre ngth is co ntrolle d by te mperat ure a nd decrea ses with depth beca use of the geothe rmal gradie nt. In tectonically sta ble lit hosp he re subjected to an a pplied fo rce . an upper regio n of brittle defo rm at ion a nd a lowe r region o f du ctile deformat ion will be se pa ra ted by a stro ng co mpe tent clas tic region (Figure 2.24A) . If we ca n assume that the various la yers of the lithosphe re a re welded toge t he r. th e stre ngth o f this elastic region cont rols the bulk strength o f the who le lithosp he re a nd o nly very sm all st rains ca n occu r initially. If the a pplied fo rce is increased . or me rely with the passage of tim e unde r a co nstan t applied fo rce . the region of bntt!e deformat io n will exte nd dow nwa rds. and the regio n of du ct ile deform ation will e xle nd upwa rds. eve ntuall y redu cing the co mpe te ", elast ic core to zero . Whe n this ha ppe ns. a rapid inc rease in stra in ac ross t he who le thickness o f the lit hosphe re ca n ta ke p lace , produ cin g geo logica lly significa nt le ve ls o f de formation ( Figure 2.24 8) . T his process has been te rmed whole-lithosph ere [ailure ( Kusznir , 19H2). T his process will be accel era ted if the geothe rm al gradie nt becom es steepe r. beca use of the temperature con tro l ove r du ctile stre ngth . T he vert ica l distribution of stress whic h results from a n applied force is co ntro lled by the va riation of both britt le a nd d uct ile st re ngth with de pt h. This va riation in strength is the refore c ritically impo rta nt in defining the va lue o f the force which m ust be a pplied to t he lithosphe re in o rde r 10 produce significan t de fo rma t ion. E vidence fo r the va ria tion of stre ngth with de pt h is pro vided by the stress-dep th curves of Mer cie r (l 9RO) discu ssed earlie r (see Figure 2. l h). Kuszni r a nd Par k (.1982. 1984a ,b ) use a ma them aticnt mod el to calculat e this stress d ist ribut ion ass uming Ma xwell visco-elastic
42
GEOLOGICA L STRu c r U RES AND MOVIN G PLA TES
p ro perties for the lithosphe re . Det ails of the mode l are give n in Figure 2. 24. Importan t assumpt ions are tha i the total horizontal force ari sing fro m the initial applied force is co nse rved , an d that the lithosp here unde rgoe s a uni form st rain with depth. Because of the siliceo us nature of most
upper-crusta l rocks. and the relativel y low st rength of qu artz , it is assum ed tha t defor mation in the upper crust is co ntrolled by the behaviou r of this mine ral (see White . 1976). Th e rheol ogy of the lowe r crust is un certain , but it appea rs likel y thai, in view of the proba ble importan ce of basic mate rial, defe r-
TIME OR STRESS OR HEA T Fl O W
A
)
,, , ' -
,
, I
,
,
SRlnLE I DUCTI LE T RA N S ITI O N
tw
c
DUCTILE
ASTHENOSPHERE
• STRESS
-
--
-
Fjgure 2. 24 (A ) Th e regio ns of brill le, d uctile and elastic be haviou r in the lithosph ere shown diag ra mmatjcatly. Given a large eno ugh applied str ess, th e clastic core will reduce to zero with time as the brit tle an d duct ile deformation spreads downwards and up ward s respect ively. WLF (Whole-lithos phe re failure ) occ urs when the elast ic region di'\.a ppears_ Increasing heat no w has the same effect as increasing stress . (8) Schematic re prese ntatio n of lithosphere respo nse to an appli ed ho rizo nt al stress. ( I ) Initial ela stic respo nse causes uniform distr ibut ion of strain and stress with depth . (2) Ductile cre ep in the lo we r lithosp here causes st ress deca y there , and results in st ress amplification in the uppe r lithosphere , sufficient to ca use fractur e in the up permost parts. (3) Further stress amplification results in stres s levels in the stro ng upper part of th e lithosphere sufficient to ca use com plete failure an d conseq ue ntial la rge strains. From Kusznir and Park
(1984).
T H E L1T HOS PlI f, ll.E: SOMI; IMPORT ANT PRO PERTI ES
43
MOlhpmOllcul m ode! fOT lilhwl'here di'jormOfion Lithos phe re of initia l th ic kness L is su hJccl~(1 10 an initia l ap plie d ho nz on tal stress , II", in the A dire ction. Pla ne slra in (U1 = 0) is assu med in the pe rpe nd icula r horiz ont ul drrccnon and the rcsullin!,: vc o i\,,,1 stress " , is assume d lu be l ew , Co nserva tion o f horiz on ta l forc e and the acsu mpuons. tha t lhe vanous laye rs of the lithosphe re
f'
iI, d z
de, O "nd "' U
J,
"
where o, is borizonta l s tress, r, is lotal hm izu nta l s lr;,in a nd z is d e pth . T hese eq ua lions , toge the r with the constitutive c quauons Ior a viscoelast ic ma terial , allow the f" llo wlng mtc gral e q ua tion to he Io rrn ulatc d , gi~' i ns (l~ as a fu nct io n o f ti,nc " nu dc plh : 0"
dl ' -
=
±J fI': I
dz +
<1','
"
wher e k "- I
~ ,,! .lndl
11,(2
~
v ) - 0 .( I - 2v)
"'I
where E is Yo u ng's mudulu s, v Poisson's ratio a nd II apparent viscos,ty.
A simila r equ atio n c os ts for 0, . Fracture h
Dislocauon cree p:
•
'"
i ~o. 7 x III e xp ( -- :'-T - ~-tl) ( II I
t
-
"
0.,) ,
lo r (a , - 0.1) < 2 kba r Durn Law :
"
t
..
X
10'""'XI' ( - 55550 ( I _ (a, 1\ - all) ' ) T 5 s ,
for (a, - 0 ) ) > 2 kha r whe re (a, - all is in kb ar ( I kha r = 1110 MPa ). The d ucti le d efor ma tion o f q uart z wilhin the cr ust is assum ed to correspo nd to tha t of dislocati on cree p. Creep rate s in qcanz are co ntro lled stro ngly by the a mo un t o f wate r. Th e co nt ine n ta l up pe r c rust is assum ed to de form acco rd ing 10 a WCI q uartz rheology with 50% quartz . T he lowe r crus t is assu med 10 defor m acco rding to a dr y q uartz rheolo gy wilh HI'x' q uar tz , or a pla gioclase rheo logy wilh 411'Y" o r .~ O 'Y" plagio clilsc . C reep rilles for wei and d ry quar tz arc ha, cd o n the expe nme nta l ","o rk o f Koc h et al. ( 19110). Wei quartz: t """
4" e xp (-193 - -T-32)( ..'"
0 , -
01 ) 2 '
00
S- I
Dry qua rtz:
c
where (0 , -
a,l
is in kba r.
s, , x IW, exp ( -- 21\7XX) -T(a , - 01) "- S ,
44
GEO LOG IC A L ST RU crU KES A N D MOVI i'-G r LII H .$
marion is co ntro lled by the ductile beh av iour of plagioclase , which de forms mo re readily than pyroxene or ol ivine . Flow curves plott ing strain-rate against temperature for various appropriate minera ls and rocks are compa red on Figure 2 .25. Th e role of water is critical; wei quartz is mu ch weaker than dry, and whereas wet quartz may he assum ed 10 co nt rol uppe r-cru sta l rheology, we might expect a mid-crusta l region of essentially gra nito id co mposition to he cont rolled hy d ry quartz deformation . Ku szn ir an d Par k investiga te two cru sta l st rength models . on e with a 50'Yu wet qua rtz rheo logy overlying a 50% plagiocla se rheology. and the o ther a three -laye r mod el comprising a 50% we t q uar tz laye r ove rlying a 50% dry qua rt z laye r ov erl ying a 40'X, plagio clase layer. The rheology o f the up per ma nt le is assumed to he cont ro lled hy t he behaviou r o f dry olivin e .
T he critical tempe ratu res req uired to ge nerate significant stra in rates for these differen t rheo logies a rc reac hed at depths tha t a re depe ndent o n the geo thermal gradie nt. Different geothe rmal grad ien ts may be cburacrcr ized by the sur face heat flow , q. Figure 2.26A shows calculated st ress-dl.'pth (o r strengt hdepth] pro fi les at various times afte r the initial appl icat ion of a ten sile force of JOl 2 N/m (i.e . eq uivalent to a fo rce of 1.5 x 1017 N ap plied ove r the who le thickness o f the litho sphe re) . Thi s force is applied to co nt inental lithosphere with a sur face heat now q = 6OmWm - 2 (cor respo nding to the continental ave rage - see T ab le 2 . 1). As tun c prog resses, duc tile cree p in the lowe r lithosphe re rl'sull'i in dissipation of stress there an d its transfer upwards to the coo ler no n-d uctile up per lithosphere , where il becom es amphlicd 10 a level sufticien r to ge nerate brittle fail ure in the topmost levels of
T EMPERA T URE,
•
200
4 00
roc 800
1000
120 0
//
,
w
10 12
w
>-
-c a:,
14
z
-c 16 a:
>'D
o a
18
-'
20 22 t"ij1,ur e 2.25 Vanaricn in log struin ra te with tc r npc ruru re for n ll11 nlhcr ,.1 minera ls an d rocks imponunt in ductil e lithosph c rc def orma tio n. T he cu rves a rc de rived fro m expcnmc mat da tu Iromthc full< ming s...urcc s: quartz ( xoc f d al. , 1')110 ): anorthosit e (Shelton and T ullis. 1911 1): d iopsid c and websterite (A .'': Lll hunem. 197Xj. FWIl> Ku szrur and Park ( 19X7)
THE L1T HOS I' H ERE:
A
se ve IMPO RTANT
a,(MPa)
.,
0 . ',,",PaJ
o
"
45
PROPERHES
.,
a .IMP.}
eo
, - --
--"
UPO~' CtuS!
We t o....., ll
Lo",e, CtVSI
.:
10 ' y'
'--_ _---.J reo
'"
B
,
'00
0.""'. ,"
•
'"
o
0.""" ,,, eo
reo
' 00 f• • lO"Nlm
.·ii:ur~ 2.26 (A) Stress plotted agains t de pth et various limes afte r the application of a tens ile tecto nic fo rce of IU ' 2 Ntm to conuucrnat li thosphere with a surface heal flow of 61) rn w m - :. Note the development of low-st ress (low-s trength) regions above composuionat (and the re fo re rheological] boundunes. (8) Suess ploncd 'tgainSI depth for ,t range of
geothermal gra dients, co rres pon ding 10 sur face heal flows of 45 mOO mW m - !, ill I Ma after the apphcanon of the same tensile force
46
GEO LOGIC.... L STRUCl"l JIU3S AND MOVING !'LAreS
the lithosph e re . T wo major stress -de pth discon tin uit ies a re ap pa re nt a t th e changes in rheol ogy bet ween upper and lower crust. and
betwee n c rust and man tic . Th ese ar e lr nporIan ! wea k zon es whi ch wo uld assume critica l tecto nic importan ce if large strains were 10 occ ur. T he effect of differ e nt te mpera ture gra dients o n the st ress-de pth rel a tio nship is illustra te d in Figur e 2.268, fo r the sa me init ial ten sile force ap plied fo r a pe riod o f I Ma. Ve ry litt le uppc rlithosphe re stress increase is evide nt in the coolest lithosph e re mod el (co rrespond ing to a verage co ntine ntal shie ld wit h q == 45 m W m- 2 ) . Howe ver . as the geo the rm al grad ient steepens, th e regio n of duc tile de formatio n e xte nds progressivel y up wa rds , co nccn tru nn g th e st ress in th e uppe r lithosp he re . For he at flow leve ls of SO a nd 90 mW m - 2 • co rres po nding to the hottest regio ns of the co ntine nta l litho sphe re (c .g. the Basin-a nd-Ra nge province of the western US A) the stress is en tirely confined ttl th e crust , and the st rength d iscon tinu ity a t the m id-cr usta l rheo logy cha nge is st ill ev ident. At 90 m Wm - 2 , wholelith osphe re fail ure is a bo ut to la ke place as the
c/"=
el as tic co re has been red uce d to zero (sec Figure 2.24A). T hese results may be co nvenient ly summa rized by e mp loying the concept o f critica l st ress, define d as that level o f stress req uired to produ ce whole- lith osphe re failure within 1 Ma , a nd which wo uld the refor e be e xpected to produce geo log ica lly sign ificant de format ion . Figure 2.27 shows critical st ress plo tte d aga inst he a t flow fo r exte nsional a nd com press ive stress co mpa red with the theo ret ica l st ress levels ca lcula ted fo r va rio us stress so urces (see T a ble 2.3 ). Figure 2.27A shows the cr itical st ress/ hea t flow curve for exte nsio n, to ge th e r with t heo ret ical maximu m stress le vels prod uced by platea u up lift (50 M Pa ) a nd subd uctio n suct ion (30 M Pa), the two mos t im po rta nt so urces o f lith osph e re exte nsion a l stress (see 2.5). A th eo ret ica l ma ximum ne t e xte nsional stress of ROMl' a cou ld he produced by co mbin ing th e two e ffects. Howe ver , as disc ussed earlier. a mo re re a listic ma ximum net stress is co nside red to be abo ut ± 25 M Pa . Suc h a stress inte rsect s the critica l st ress curve a t a he a t flow va lue o f a bo ut 70 mw m -2. defining a re gio n of po te nt ial defo rma tio n.
,,, reo
'"
R.oge push (rna .)
• ~
"":::! ""i
g
5
20
ao
P'Ob2bllt "'..._ M I sl ' U S
I I
10
P'_ ble
ma•. net
I I I I
"
I 5
a
c
as
50
I
s
EXTENSION
,
75
A
10 0
t 25
'.5 0
. , . ....
sl'lt$)
I I I I
COMPRESSION I I
I 0
HEAT FL OW (0) mW
25
50
75
10 0
125
150
-a
B
figurt 2.27 Curves
H l l:: Ll ftlOSrH I::I~E: SO ME IMl'ORlAN T PROr'ERnl::S
These result s demo nstra te tha t . fo r re a so nable esti mates o f th e likely a vai lab le e xte nsio nal fo rce , sign ifican t e xtensiona l deforma rio n ma y be expec ted in lithosp he re wit h heat -flo w levels fo und for exam ple in typ ica l re gion s o f Pa lae o zo ic o rogenic c rus t (see Ta ble 2.2). Th is confo rms with the Obse rved occ u rren ce of intra plat e exte nsional zo nes in suc h regio ns.
47
typi cal ocea n-b asi n bear Ho w val ues in th e ra nge 40 - 50m W m - 2 (sim ilar to t he con tinen ta l shield va lues ) . it ca n be seen fro m Fig u re 2.2711 tha t a n unre a listicall y high val ue o f c rit ical stress wou ld be re q uired to p ro d uce de fo rma tion. No likel y so u rce o f st ress e xists, t he refore. tha t will produce significant co mp re ssio na l intraplate deformatio n - ind ee d the re is no e vidence of such de forma tion in the ocea n ba sins at p resen t.
Comp ressiona l strength: Beca use rocks a re mu ch st ronger und e r co mp re ssio n th a n und e r te nsio n , t he co mpressive st re ngth of the litho s phe re as a who le is co nside rably g re ater than its ex te ns io nal stre ng th . Figu re 2.27 B s ho ws tha t for a he a t flo w q of HO mW m - 2 . suffic ien t to p romote sign ificant ex tens ion wit h a st ress of a ro u nd 20 M Pa . a stress of o ve r 40 M Pa would be req ui red to produce sig nifica n t co mp ressio na l d eform at io n - a va lue t ha t is o u tside the ra nge of norrnul com pressive stress so u rces . The p roba ble ma ximum a va ila ble ne t stress of 25 M Pa re q uires a heat flo w o f o ver 90 m\\' m- 2 10 pr o du ce sign ifican t deforma tio n. Suc h a high he at flo w is curre nt ly found on ly in th e honest re gio ns . which a rc curre ntly un dergo ing e x te ns ion. rather tha n co mp ressio n. T he mod e l there fo re pred icts t ha t major co mp ressio na l de fo rma tio n shou ld not normall y o ccu r with in plates a nd we might e xpec t it 10 be re str ict ed to conve rge n t plat e boundaries where la rge co llision resistance fo rces can occ ur. That intraplate co mpressio nal d e forma tion is uncom mon is wid e ly acknowledge d . The fe w cases where it see ms 10 occu r need to be exa mined clo se ly. Pe rhaps the litho spher e il> unusually we ak in suc h zones . Oceanic lithosp he re
Althou gh ocea nic lithosph e re is muc h thinne r than contine nt a l. its co mposit ion re nd e rs it significa ntly stro nger . since its rheo logy is contro lled almost e nt irely by th e d eforma t io n of olivine. T he crit ica l stress c u rve fo r typica l ocean- bas in litho sph ere the ref ore cor res po nds to that fo r an a ll-o livine litho sp he re . W ith
Evolution uf.wrenglh du ring lithosphere extension and com pression : crustal thickness Be ca use of the significa nt di ffe ren ce s in rheolo gy between the q uu rtzo -feldspathic ma te rial o f the c r us t and the oli vine -rich ma te rial of the m a nt le . the b ulk strength o f the lit ho sph e re de pe nds ve ry muc h on the relative p ropo rtio ns o f these two di fferen t ma te ria ls . an d thus o n crusta l thickness . Figu re 2.2XA sho ws st ress d e pth p ro files. calcula te d fro m th e Ku szni r -. Pa rk lit hos phere defo rma tion model . fo r lith o sp he re wit h cr usta l th ick ne sse s o f 2tJ. 35 an d 60 km . T hese sho w ve ry sig mficnr ud iffe ren ce s . T he thin-crust mod el is st ronge r o ve ra ll a nd sho ws no stre ng th d isco n tinu ities . Th e thickc rus t mo del sho ws mo re th an twice t he st ress level . bu t the stress is co nce ntra te d in the up pe r crust. T hese res ults are fo r a n ave rage geothe rmal gradien t a nd a force of to N/m a fte r 1 Ma . Fo r high e r he a t flows . t he thi nc rust mo d e l is 3- 4 ti me s st ro nge r tha n average cr ust. T he a bov e re la tio nship assum e s tha t the geot he rm al gradien t is co ns tan t d urin g t he a pplicat ion of the force. If. how e ve r . t he c rusta l t hick ness cha nges as a resul t o f deformat io n. fo r exa m ple by exte nsio na l t hinning . th e pr oce ss itse lf changes the the r ma l st ruc tu re of the lit ho sp he re . T his sit uat io n has been inves tiga ted in a modified ver sio n of the lith o sp he re mod e l de scribed a bo ve ( Kusz nir an d Pa rk . 1987 ) which co nside rs the cv ol ution o f litho sphere st re ng th du ring exte ns ion. ta king accou nt o f the c ha ngi ng tempe rature st ructu re. T he p ro ce ss of lit ho sp her e ex tensio n re su lts
A 0
O~ MPa
0
o ...... ec
O.MP OI
ec
'0
0
eo
"
WO
0
eo
.............. .
. .'".
, '"
...... -
..... .. ....
-
o
r
•• 0 w
..........
'0
60
. .
O usr.1 1'hIcknns 2O<m
3!:okm
-. . -- - .. . 6O>m
Q_60 m Wm "
A
.. ! •• • <
F. tO
,
"
0
~~
.
c;
10
.,
N / ...
"
· · · o·•
-
" o
o
rso
'00 1'(1 . 1 lI ow
( Q! .....
20
Thermal Ag. 10M • ••••••. . •
,2s
-" ,.
5 0 "1.
•• &
__
,
exte nsio nal strcnglh (the cri lkal fo rce req uired to gene rate WLF in I Mal in " cru~ l a l Ihickncwoul tIow plo t. Schematic trajectories indicate last, slow and inte rmediate lithosp he re ex te nsio n rat es. The evofunon of cr u~ I ;Ll stre ngth ;n compression molY be visual ized by ex te nding lhe inte rmed iate U lcnsion rate u pwar ds 'iencal mode l; from KUSlllir and Par k
or
.... ..... .... ....
. ....
...... o ..,....,::.
fi,.u" 2.18 (A) Sire» ploued against <Jcplh for mtltJd~ wilh c l us lal Ihicknc$.."C'S of 2(1, 3~ and flOkm ,,1 1 M. af ler .hc ap p lica liu o of a lc nf>ile force or 10 ' 1 Nlm 10 lilhu!Orhcr c w u h he al flow, q '" 60mW m ! . Numeri c...l modc:l ~ fro m Kusznir and Par k (I~K7) . (H) Conluu ~ lithosphe re
or
m'2
B
c
..
. --- --- -
,
.
,
.._.._
,
t -IO'"
( 1987).
48
49
IHl L11HOS I'I! EIlE : SOME IMI'OIl IA Nl" I'KOPfX ll lS
in two o pposi ng effec ts: [ i ] a steepening o f the
georherm b rough t a bo u t by bri nging the hotte r asthen osp he re ne ar e r to the su rface . wh ich we a ken s t he lith osph ere ; and (ii) a thin ning o f the cr ust whic h, as we hav e se e n . will act 10 strengt he n t he lit ho sp he re . T h us the lithosp here ma y s ho w ei the r a ne t weak e nin g o r a ne t stren gt hening d uring extension dependi ng o n wh ich effect do mina te s . If th e e xtensio n rakes p lace slow ly. the geot be rm ma y ha ve t ime to rc -cq uitib ratc . that is, th e base o f the litho sph e re W I ll mo ve do wn war d s to co m pensate fur the thin ning effect as the e xtra hea l is lo vt, Slow rates of ex te ns ion wilt th e re fo re re sult in a ne t strcngthe ning (ts trai n ha rde n ing ' ) of the lithos phe re because th e crus ta l th inn ing effect is dom inan t. Ra pid ex tensio n o n the ot her ha nd will lead to ne t we a ke n ing , si nce th e temperatu re rise wi lt mo re th an ba la nce the effect o f crus ta l th inn ing. Fig ure 2.2H B SIl\l WS co nto urs o f litho sp he re stre ngth in a c rustal thickness / he a t no w plo t. T raje cto ries o f cha nging crustul thick ne ss at co nst a nt hea t flo w clea rly lend to an incre ase in stre ngt h. T rajec to rie s whic h sho w a la rge change in he a t flo w (co rres po nding to a fasl extensio n rute) p ro d uce a decre ase in strength. A n int e rme dia te rat e o f exte nsio n wo u ld Cau se no or ve ry litt le ne t change . Figur e 2.2RC sho ws the q ua nti ta tive re su lts of the lit ho sp he re st re ng th mo del mo d ified to take acco u nt o f c hangi ng te m pe rat u re structure . T he resu lts a re sho wn in the form o f a plot o f litho sp he re st re ng th (cr itica l fo rce ) against beta va lue ({3 ) fo r ex tensiona l strain rates of 10- 14 a nd 10.- 1\ - I , and ' the rmal ages' of 10 a nd 50 Ma . Not e th at {3. th e lit ho sp he re stre tching factor ( Mc Ke nz ie, 1\}7&1) is e qu ivale nt to the streng m » ( 1 + e ) . whe re e is th e extension , in th e terminolo gy used in structurul ge o logy. Thu s a val ue o f fJ :=: 2 co rrespond s to a d oubl ing of the origina l widt h and a hal ving o f the o rigi na l th ick ne ss o f th e lit ho sphere segme n t in q ues tion . T he th erma l age is define d as the time since the last majo r tecto notherma l e ve nt (oroge ny ). The ev o lut io n o f e xte ns io nal stren gth is stro ngly de pe nde n t o n the initia l therm al
state . T he fas te r str ai n rat e in the wa rmer litho sph e re p rod uce s appro xima te ly co ns ta nt stre ngth. wh ereas the slowe r ra te cau ses ra pid strum harde ning after {3 :=: 1.5 . F ast stra in ra te s ca nno t be ini tia ted in the coole r lithos p he re mo d el (the rma l age o f 50 M,1 ) beca use of th e u nre ali sti call y h igh in itial strengt h re q uired . Th e mod el t here fore p red icts . firstl y, that fast e xte ns ion ra tes (~ 1O-1 4 S -I ) a re on ly possibl e for hot . ther mally yo u ng litho sphe re thu t will p ro d uce lo ca lly in te nse e xten siona l defo rmurion. with str ain softeni ng. le ad ing 10 la rge {3 va lues a nd ult im atel y, if the fo rce pe rsists , to t he co mp le te rift ing of the co nt ine nt al cr us t and the fo rmatio n of a n ocean . Seco ndly, s lower exten sion ra te s ( $ IO - l ~S -I ) wi ll p ro duce stra in h a rd en ing a nd ge ne ra te a fin ite fJ va lue o f a ro u nd 1.5 . A s e ach section of lith osphere ha rde ns . the lo cus o f inte nse d efo rm ation would be e xpect ed to sp re ad late rally to invo lve a m uch wider re gio n of e xte ns iona l de fo rma t ion (see Figu re 2.29). This c ritica l f1 value of 1.5 is in re ma rka ble agre e me nt with the es timated {3 va lues fro m a wide ra nge of intr a-co ntin e ntal exte nsio nal b asi ns (Table 2.6) wh ic h sho w a n average {3 value o f 1.4 - 1.5 .
Evo lutio n ofslrenglh in compressive deform atio n T he pro gr e ssive increa se in c rust a l th ick ne ss whi ch re su lts fro m co m pr essive d eformat io n the o re tically p rod uce s th e re verse situ a tio n to ·L . llIe 2.6 Es tima te d vdlues o f CX ICllsinn in vMious con line nl., ! h dSlns . Fro m Ku sznrr and PMk ( 19H7) (u; lIi1 frum
G D. Karner ].
p Nor th SC
I 55- I.Y 1,2-1.3 1.1-1.3
P,m no n;dn Bils in
I ,H- 2.7
Acgc, m
14
Vie nna e Ol sin P'l fis Basin
1,0- 1.11
W~·"M·!{ B;) ~; ll
e..
w o rcest e r si n Bass Basin Gippsland l3<1 sin
U 1 1- 1.25
1.2 1.25-1 .5
"
50
GEOL OGIC A L STRUcrUR E.<; AND MOVING PLAT ES
,.,
~
...
A
e g 10 -"
..,.., ...,
I
....
..-.[i~ ~ l j l:+1 I I I I
B
F I $! S t. a ln Rale
•
•
_-----.ln~_ la r g e jl • n Ig h
Q
S lo w Str ain Ra , a
•
DC
sm all ft
.
C
- lo w
D
Q
TH E U1HOSI' H ERt:: : SOM E IM PORTA NT PII.OPEII.T1ES
that JUSI describ ed for ext en siona l dcform ano n . That is , fast strain ra tes prod uce stra in softening, whereas slo w strain rates produ ce struin hardening. However , fast st rain ra tes arc proh ibited by the high initial for ce req uired (see Figure 2.28) and realistic strain rat es will produce stra in harde ning a fter the crust has thicken ed beyo nd abou t 50 km, tha t is , after a finite amount o f sho rte ning (pe rhap s 50'Y... ) has taken place. Compressive deforma tion of t he lithosp he re is therefore a se lf-limiting p rocess, in co ntrast to extensio nal defor mation .
Rheological control ofdetachment horizons We have seen that th e stre ngth d ist ribution wi th depth de pends in de tail on the position of major rheo logical ch anges in the lithosphere , and particularl y in the crust. Kusznir an d Park consider two crusta l mod els, bo th o f which have appare nt co unter pa rts in natu re . Th e first (sec Figure 2 .26) is a two-layer crust with a granod iorite rheolo gy, co nt rolled by wei qua rtz deforma tion , ove rlying a gab hroic rheo logy controlled by plagioclase defo rma tio n. Th is type of crust may co rrespo nd to ma ny parts of the stable co ntine ntal lithosp he re with a well developed Conrad se ismic disconti nuit y at c. 17 kill depth . T he seco nd is a three- layer crus t wit h a middle-crustal layer of broadly granitic composition co ntrolle d by dr y-qua rtz de formation (Figur e 2.30). T his type of cru st ma y correspo nd to that o f the no rthe rn Scot lan d Caledonian terrain as sho wn o n the L1SPB seismic refractio n line ( Ba mfo rd, 1979). Othe r types of crust undoubtedl y exist with a mu ch more complex laye red st ructu re and seve ral significan t rheo logical changes. Major rheo logical changes of th is t ype form
51
zo nes of low du ctile stre ngth, as sho wn in Figure 2.30, which wou ld be e xpec ted to pro vide detachm ent hori zo ns du ring major lithos phere dc formano n. T he imp orta nce of th e various zo nes of pote ntial low st reng th depe nds very much on the temperature gradie nt. Figu re 2.30A shows that for a threelayer cru st in extension, all three zones are well developed on ly for inte rmediate hea t-flow reg imes (q - 70 mW m- 2 ) . AI higher hea t flows , o nly the up pe r zo nes arc significant. Thus we might expect thut a major detach ment horizon would be most like ly to develo p :11 mid - to uppe r-crustal levels for wa rm lithosph e re like that of the Basin-and -R an ge province . Blundell et al. ( 1985) show t hat in the UI RPS ' M OlS I" de ep -re flectio n line across northern Scot land (Br ewer et at., 1984) the majo r Mesozoic extensiona l fau lts flatten out a rou nd the 20 krn-dce p horizon correspo nding to the base of the middle-c rusta ! laye r ident ified by Bamford ( 1979) from the U SPtl profi le ( Figure 2.30ll ). III this case therefore, we migh t co nclude that the geother mal grad ien t was not unu sually Slee p and that hea t-flow leve ls perhap s co rresponded to those o f present-day Hercynian orogenic region s in Europe, wit h a thermal age o f c.200 Ma. It is inter est ing that the e xte nsional fau lts pen et rate much deepe r in t he Precambrian shield regio n west of the C aledonian fron t where the lithosphe re would have bee n much coo ler during the early Mesozo ic exte nsio n.
Summary The rheo logy and stre ngth of the lithosphere ca n only he understoo d in a precise quantitative way by mean s of mat hematica l modell ing.
Fij(urt 1.19 Ca rtoo n erostal models sho wing Ihe differen t styles of eXle n, illn;11Jdl,r l1la t;on expected with fast an d slow rates of lithospher e exte nsion. A t fast rate s (e .g. [U" '~) srrain so ltc ning might be expected 10 localize u~ lo rma l ~on ncar the orieinal sire 01 WL F, cau sing prn grcs.,ive narrowin g and mtcnssficanon of the al'lIve de formatio n, leading to high P values and el'enlua lly ' " n usl;.l se par atio n . Al slo w strain ra tes (c .g. 10- ':') loc al. st rain harden ing mighl be e XflCcte~ 10 I ranS ~e r ddurm
A 0
or
Ox MPa 40
0
80
WO
~. .
~o ~ r.>Q
Ox MPa 40
80
. . .. 1 .. . .. . . . . . . . .
O x MPa 40
0
~
. .
0
80
~. . . .
..
O x MPa 40
VI
N
80
~ . . . . . . . . . . . .. . .
I~ • • • • • · · ' · · · · · '
An
------- --.
__ ,- - - - - - Moho-
- - -- -- - -----
- - - - --- - - - - -o
01
AO
C'I'l
0 r 0
E
o
.:t:
o)-
J:
I-
fb0
r
vO
::j '"c
Q c
~O
'"
r
l
Q=50 mWm- 2
60 mWm- 2
l
70 mWm- 2
~
C'I'l Vl )-
z
80 mWrr.- 2
0
:s:: <
0
1°° ~
B ,
Z
o 20 -------1 k rn
OIF
"'"
~
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I
T H F. LITH OSPHERE: SO ME IMPORTA:" T PROPERTI ES
T here ex ists a critical and co mplex inte rp lay of ap plied force , therma l sta te, strengt h and strai n-rate tha t de termi nes how de formation is initialed and ho w it proceeds. T he res ults of the particular mode lling exe rcise just described give precise es timates of the strength of the lithosphere under differ ent t he rma l regimes. Th ese estima tes are in close ag ree men t with obse rvatio ns of tecto nic behavio ur and hea l Ro w at t he pr ese nt lim e , and with theo retical es timates (see 2.5) of the likely fo rce levels avai la ble to defo rm the lithosphe re . A modificat io n o f the model allo ws the evo lutio n of lit hosp he re str ength during ex tens ion and compressio n to be exam ined . T he model predicts that fast initial st rain rat es e ~ 1O- 14 S- 1 and wa rm lithosphe re are necessar y 10 pr oduce large exte nsio ns, and eve ntually oceans; thai strain rates of c. IO - 15 S- 1 will produce broad zo nes o f ex te nsiona l deformatio n with a finite limi tin g f3 va lue of a bout t .5 (correspon di ng clo se ly with o bse rva tions); an d finally
53
that initial strain rates o f Io- I(, s - I o r slower will never succeed in ac hieving any sign ificant level of e xte nsion. In the case of compress ive deformation . much higher level s of a pplied force are re quired to Initiate defo rmati on than would no rma lly be e xpec ted away from plate bo unda ries. and realist ic str ain rat es are predi cted to ca use st rain har de ning afte r a finite amo unt of sho rtening , possibly 30 -50%). D uring ex ten sion a l o r co mp ressio nal deform ation . major rh eological bounda ries in the lithosphere (espe cially at mid -cru sta l leve ls) fo rm important zo nes of low du ct ile str ength. T hese zones may prov ide det achm ent ho rizons during the defo rmation . Wh ich detach menI zon e is selec ted de pe nds o n t he tempe rature gradie nt; higher detachment levels are favou red in warme r lithosph er e with a you ng the rma l age . w hereas lower levels are favoured in cooler lithosp he re with an old er t he rmal age.
•
~'i~u r~ 2.30 (A ) Stress plotted againsl dep th for 11 three-la yer crusta ! mode l with a ra nge of sur face heat flows. sho wing the development of low-str en gth zones in th e middl e and lower cruvt. The st ress-depth distributions correspond to I Mil after the a ppliealion of a ten sile fo rce of 1 (J I ~N m ' . Th e lo w-strength regio ns re present the probable sites of detachm en t ho rizo ns. wO . wet q uartz ; DO. t.lry quartz. An . unonbosite ; 0 1. ol ivine . Nu mericalmo del; fro m Kuszuir and Park (19&7). (8 ) Mo t.le! o f the MOI ST profile across no rth Scotla nd. afte r Blundell t r QI. ( 19K5 ). showing the th ree main crustal layer s recognized in the L1 SPB profile hy Ila mfo rt.l 1 ). I . lower crust (7 km s ' I. "basic gra nulite }; 2. middle cru st (6.4 km 5- 1• inte rmediate, granuluc -facics Lcwisian ); 3, ul'pcr crust (6. IS km s - ' . amp hiho lite-fae lcs Caled onian meta morphics}. Th e uppe rmost (o nnrnilmented) layer co rres po nds with unmet arnorphoscd sedim en ts ho rn Tcrridonien to Mesozoic in age . Ff ', R annan 'f"ull' ; OI F, Out er Isles Iault. Note that most of the majo r lo w-angle exte nsional fll ulls sole o ut alo ng th e to p o f laye r I at a bou t 211 km depth . whi\·h ther efor e represe nts 11 ke y deta chm ent ho rizo n d uring po stCaledonian ex tension. Th e Iaults in Ihe W'; SI, how ever. includin g a branch of th-e O uter Isles Iault, appe ar to detac h along the Moh o. Fro m Kusznir and Park ( 19S71.
3 Plate movement and plate boundaries 3. 1 Kinema tic beh a viou r of plates
linear (i.e . tangential) ve locit y vect ors, whic h can be co nside ra ble over la rge a reas. T he first ste p ther efore in a na lysing the structures a t a ny active or rece nt pla te bo unda ry is to dete rm ine th is rela tive ve loci ty vec to r. It is co nve nie nt in ge ne ral te rms to distinguish th ree main types of rela tive mot io n: divergent, convergent a nd strike-s lip , a nd these types correspond in t urn to t he ma in types of tectonic regime a rising respectively from co nst ructive, destructive a nd con se rvative plate boundar ies. However we must recog nize tha t the rel ative move men t vecto r may mak e a ny an gle with the plat e bounda ry and tha t, in ge neral terms, most vectors will be ob lique. Th us, in practice , a dive rgen t or a co nvergent boundary may a lso ex hibit a co mpo ne nt of strike-slip motion whic h will im pose a simple-shea r st ress ac ross t he bo undary. Be ca use new plat e is crea ted at d ive rgen t boundari es, a nd old plate destro yed at convergent bounda ries , obliq ue relative motion ca n ea sily be accommoda ted. Howe ver , since plate is conse rved a t transfo rm fau lts , the move me nt there should theoret ically be pure str ike-s lip with no ob lique compon e nt. Th is co nd itio n is viola ted whe n a c ha nge in rela tive pla te mo tio n occurs, in which case o ld tra nsfo rm fault s may suffe r e xte nsional or co mpressio nal move me nts. Good examp les o f suc h c hanges in pla te motion have bee n stud ied in t he Indian a nd easte r n Pacific oceans (se e Figures 3.6. 3.8).
A majo r implicatio n of the pla te tec ton ic model is that cru sta l deformation is ultimately co nt ro lled by the relative movemen ts o f the lithosphe re pla tes. We have seen thai the pla tes ha ve co nside rable la te ra l stre ngth a nd suffer litt le late ral d isto rtio n over lime pe riod s o f le ns o r e ven hundreds of Ma, and that they can tr a nsmi t ho rizo nta l stresses th ro ugh long dist ances in a regu la r fas hion . We obse rve that large re lative move ments are co nfined to piaIe bo undaries, where crustal deformation is t herefore concentrated. Th e d eterminu tion , bot h qualita tively a nd q ua ntitative ly, of t he rela tive mo tio n be twee n two adjoining plates a t t he ir co mmo n bo unda ry is therefore essen tial in und e rstand ing the de format io n that ta kes place the re . The co nve rse is equa lly important : a knowledge of the deforma tion al structure a t a plate bou ndary ca n provide usef ul informatio n a bo ut re lat ive plate mo vem ents, especially in pre-Mesozoic time when e vide nce from oceun-tloor stratigra phy, so vita l in interpre ting more rece nt pla te tec ton ic histo ry, is lacking . T he pri nciples gove rn ing t he merion of plat es ove r a sphe rical surface we re first ex plained by Mc Kenzie and Pa rke r ( 1967), Mo rga n (1968) . Le Pic ho n (1968 ) and lsac ks er al , ( 1%8) . By usin g t he ori e ntat ion of t ransform fa ults. slip vec to rs fro m earthquake foca lme chanism so lutions and sprea di ng ra tes o n rid ges, rel a tive mov e me nt vec to rs ma y be fo und for the five majo r plates in re lat ion to a n ar bitra rily sta tio na ry Anta rct ic plate ( Figure 3. 1). The magnitude of these velocity vectors varies fro m 2 to 17 ern/ye a r. Using known re lat ive mo veme nt vectors, the direct io n a nd a mount of relati ve mo veme nt at a ny plat e bou ndar y ca n be fo und by co nstructin g a vecto r tria ngle (Fi gure 3.2) . Althou gh the d irection a nd speed of angula r ro ta tio n is uniform, as measured o n a sphe rica l surface. ma p proj ectio n ind icat es differe nces in the
Mi!:rarion of plate boundaries O ne of the co nseq ue nces of re lat ive plat e mo tion is that plate boundaries th em selves ma y migrat e in rel ation to o ne a no ther. T he prese nt plate bo undar y network as shown in Figure 3. 1 is a tran sie nt one, and ma ny of th e bounda ries shown a re mo ving at a de te rmina ble ra te . Th e simples t exa mp le which de mo nstrat es thi s is rhe bou nda ry o f t he 54
55
PL AT E Mo v t:M EN T A 1'< O PLA TE OOU/'oi OA RIE S 16 0 '
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Figure J .2 De termina tio n or the re lativ e ve locity vecto rs or th ree pl.. lc:!> meetin g at a Irir le junl:tion . (A ) Th ree plat es A , B and C a rc hou nd ed by rid ges. The velocuy o f R re tat ive 10 A (V /l<") is pa ra llel to tra nsfo rm fa ults o n the A lB Ixluooltry , "n J Inc m"s"il ooc ca n be d Clc rminc d from 1m, sp reilding ra te " nl! is re p rese me d by OP in thc vecto r diag ra m . Simila rly VOIO is re p rese nted by PQ . The u nknow n VC'ClOI VNI"" is represent ed by the line QO . (B) Th is Iriple june lio n involves a tr ansfor m fau lt . II , idse an d a Irc nett . VII<" ca n be J e te rm ined in the 'Same wlY u in (A), and the d irec lio n o f the CIS vector is give n by lhe oric nla lion o f the Ira nsform Iauh . VA l (" ca n be dc:tc, mined if the o rie nlation is known (by the Ori1: nlal ion o f tfansform Iau hs CUlli ng 11K' lre nch ) o r if the ma gnilude o f V o " can be de te rmined .
56
GEOLOG ICAL STRUCTURES AN D MOVING PLAT ES
Anta rctic plate. Th e movemen t vectors o f Figure 3.1 were co nstructed assuming a stationa ry Antarctic pla te . Ho wever t his pla te is completely surro unded by r idges 'o ff set by tra nsfo r m faults . and new plat e mat e ria l is bei ng cre a ted at each. It follo ws th ai the Antarctic plate is gro wing in size and that. with refere nce 10 a fixed Antarctic co ntine nt, all th e co nstructive bou ndaries surro unding t he Anta rctic plat e must be mo ving outwards. A simi la r argume nt ma y be a pp lied to the Am e rican and Eur asian plates. Since both these pla tes are grow ing by the addition o f oceanic material along t he mid-Atl antic ridge . eit he r the destructive west Pacific bo und a ry must migra te e astwa rds. o r th e destruc tive/ transfo rm c as t Pacific bounda ry must mig ra te westw a rds , o r both . In o the r wo rds. the Paci fic plate must be shrink ing . a nd the destr uctiv e bo unda ries o n its NW and SE sides ar e a pproach ing each ot he r. Stahle and un stable triple junctions
It was recognized e a rly in the evolutio n of p la te te cto nic theory by McK enzie a nd Morgan (1 969) th a t the re must be poi nts o n the Ea rth's su rface wher e th ree pla tes me e t. Such poi nts we re te r me d triple jU1lctions . Th e y di vide d triple junctio ns into ty pes accord ing to th e natu re of th e bounda ries invo lved . T hus if R sym bolizes ridge . T tre nch , a nd F t ra nsfo rm fa ul t. a n RTF junctio n is o ne invo lving rhe meeting o f all th ree ty pes of bo unda ry. Simila rly we may have RRR , T IT , 'IT R junctio ns. a nd so o n . Mc Ke nzie a nd Mo rga n (eco~o i ze 16 poss ible types o f triple junctio n and discuss t he sta bility o f e ac h. A j unc tion is sta ble if it mai nta ins its shape t hro ug h ti me (disrega rd ing its absol ute moti o n). Some e xa mpl es a re immedi at e ly o bvio us: an RRR j unctio n will a lwa ys be stable . A good exam ple of a n RRR "junct Io n IS whe re t he Galapa cos rj~ge meets t he E . Pacific rid e west of Ce ntra l America ( Figure .8A ). se pa ra ting t he srna cos p a e in the no rth east fro m t he Pa cific plate in the west a nd th e Nazca plat e in the so ut h. It is cle ar th at co ntinued spre ad ing o n all thre e ridges will no t affect the bas ic geo me try of the triple junct io n. In co ntras t , XFF a nd RRF
junctions will a lways he unsta b!! . Th e sli' b jlj! ~ th e rem ,lining tv pes IS depe nde nt o n the geo me t ry. T he kinematic behaviou r a nd sta bility of a give n trip le ju nctio n ma y be det e rmined by d ra wing the ap prop riate vecto r tria ngle (se e Figure 3.2). T he sum of the rela tive ve loci ties o f the three plat es m ust be zc nU l.e . V illA + " V CIII + V A l(' = 0, where V HiA is t he velocity o f B rela tive to A , e rc. ) provided that the plates a re rig id. T he le ngths 0 1'. PQ. an d QO o f t he vecto r tr ian gle represent th e velo c ities V RIA , Vo n an d V A l e respectivel y. If we kno w V Ql4 gnd Von , V A I C ca n he dl;lerm jned. No w co nside r tile mo vem ent of a po int o n a boundar y. If we ta ke t he RR R case (Figu re 3.3A) , a point P o n the ridge axis AB will mo ve with respect to A and is represented by th e m id po int of A ll in th e velocity tria ngle . Simila rly po ints Q a nd R o n ridge axes BC a nd CA will be re prese nte d by the mid -point s o f BC a nd CA respectively. T he ve locity of a ll po ints P o n the A lB ridge axis IS re prese nt e d by t he da sh e d line ah paralle l to the AIIl rid ge a xis , that of a ll po ints Q hy he paralle l to th e BIC ridge axis , and that o f all poin ts R by en par a lle l to the CIA ridge axis. T he condition fo r stability is tha t ab . b e and ca meet at a point. Th is po in t re presen ts the velocity o f the t riple junctio n, which fo r co nve nience we sha ll assu me is stat io nary. In th e R R R case, a lthough the geometry is alw ays stable . the tr iple junct ion will migrate if t he sprea ding ra tes o n the ridges a re no t equal (co mpa re Figu res 3.3A a nd B ). Co nside r now th e TIT e xa mple give n by Mc k enzie a nd Mor gan (Fig ure 3 .3C). He re , plate A is stat io na ry, plate B is subd uct ing be low pla tes A a nd C. a nd pla te C is subd ucting be lo w pla te A . T he rela tive mo tion vectors a re sho wn , a nd a re o f co urse diff er ent fo r e ac h bo unda ry. Figur e 3.3C(2) shows the pos itio n of plates B and C at so me la te r tim e , as if th e y we re allowed to proce ed hor izon ta lly inste ad o f be ing subd ucte d . The trench BC has migrate d up the A lB bo undary beca use it is co ns uming plate B a nd no t plat e C; tha t is , th e ma rgin of plate C with B is fixed to plat e C a nd must migr a te with it. On e co nseq ue nce of thi s is the o bv io us geo me tric cha nge of th e t riple
of
57
PLAT " MOV e M ENT AND PL ATE BOUN D ARIES
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Ftgure 3.3 Stable and unst able triple Ju nClio ns. ( A . B) RR R ju nclio ns. The ~e loeilY or a point /' on the ridge a xis A B moves with rcspcc t m A "I h" lr th e Vc!'II;;ly " r pl.uc fJ. 11 is the re fore re pfl:~<'nteu hy the mid-pointof All . Similarly . Q anU R represe ntthe vctocuics or poi nts on ridge axe s He and AC rcspc c nvclv
58
GEOLOG ICAL ST RUcrU II.E$ AND MQVl f'J (j PL ATES
ju nctio n itse lf; a nothe r is th e relunve mo veme nt vector a t a po int such as .r, originally o n the A lB bo unda ry. T he mo tio n pe rcei ved o n pla te A u nde rgoes a n abrupt c ha nge as th e Be ho undary mo ves past the point x , T his cha nge is di achro no us a nd pro gressive ly reaches a ll po ints o n th e A ll bo und ar y give n e nough time . It is e as y no w 10 visua lize the condi tion for sta bility of thi s type of trip le junct io n (sec Figure 3.3 D ,£::). Since poi nts on the AB bo und a ry do no t mo ve re lat ive to A , line ab is drawn throu gh A in th e velocity tr ian gle . Line lie is d rawn thro ugh A for the same reaso n. and lin e be must be drawn thro ugh C. T herefore ab obe a nd co will o nly mee t if be is pa ra llel to A C ( Figure 3.3 D), tha t is, if t he CA mo ve me nt vec to r is pa ralle l 10 the DC bo undary. In that case , th e bou ndary will nOI m igrat e a nd t he tr iple j unctio n will be sta ble . No te t ha t th e new confi g ura tio n of the triple junctio n in Figure 3.3C(2) is ac tua lly stab le (sec Figure 3.3E) a ltho ugh it is migrating with re ference to its origina l position . In Figure 3.4 a n ex a m ple give n by Mc Ke nzie a nd Pa rker ( 1967) "fro m the N E Pacif ic is sho wn . T he presen t pla te geo mc try is shown in Figure 3.4A (4) . Plat e vel oc ities a re show n wit h refere nce to a st a tio nar y Am e rica n plate . The re a re two triple junct io ns: an FIT in the no rth (Fi gure 3.4B) an d an R IT in the sou th (Fi gure 3.4C) . Bot h j unc t ion s involve three plates , A me rica n , Pacif ic a nd Fa rallo n (th e Fa ra llo n plate is now d ivided into se pa ra te pa rts a nd par tly inco rporated into o the r pla tes) . Th e evolution of t he plate sys te m is sho wn in Figu re 3.4A( 1- 4). Since the Pacific pla te is moving nort hwestwards a nd the FuralIo n pla te nort he ast , a ny ma te rial poi nt on th e ridge (such as th e inte rse ctio n with t he Murra y tra nsform ) is mo ving appro xima tely no rt hwa rds. A s t he easte rn ridg e - t ran sfo rm junctio n t ra ve ls no rth , it e nco unters t he trenc h o n th e no rtheast s ide of the Fa rallon pla te to fo rm a triple ju nction . Im medi a te ly th e Pacific pla te co mes int o co ntac t wit h the A merican plat e , the rela tive mo ve me nt vecto r beco mes no rthwest , cha nging th e boundary to a tra nsfo rm fa ult. T hus the so uthe rn RT F tri ple ju nction is fo rme d . Since th e ridge ax is is st ill mo ving
north , plate s F a nd P togeth e r with the ridge itsel f arc bei ng co ns umed by th e t re nch. Ho wever , the pos ition o f the tri ple junc tio n is sta tion a ry with respe ct to t he A me rican pla te. As mor e of the Pac ific plate is co nsumed , the transfo rm fa ult le ngthe ns and the no n hero FIT triple junction m igrates alo ng t he mar gin o f th e A merica n plat e , progressivel y co nverting tren ch to tra nsform . Thus bo t h triple ju nctio ns posse ss sta hle geo me try but ar e mo ving relative to eac h o thcr. If the San And re as fa ult we re no t para lle l to th e Pacific! Am e rican mo veme nt vecto r, th e ju nctio ns wo uld not be stable (se e Fig ure 3.4B) . A very co mmo n type o f inte rsect io n occu rs whe re a fau lt mce t'i j! rjdl:.c , initia lly fo rming a n R RF juncllon .M cK e nzie a nd Morga n show th at such a triple junctio n is a lways un sta ble and evolves in to a n R,FF type . A good e xa mple occurs in the ce nt ra l A tlantIC , at the jun ct io n o f thc A me rica n, Eur asia n a nd Africa n plates , whe re the m id-Atla nt ic ridge mee ts t he Azo res G ibra lta r fra cture zone (Figur e 3.1). A nothe r e xa mple is t he junction o f t he C hile fract ure zo ne wit h th e East Pacific ridge , a t th e SW co rne r of the Naz ca pla te (Figure 3.8A ). A t hird is th e yo ung tr ip le ju nctio n fo rmed a t th e mee ting of the Carlsberg ridge a nd the Owen fractu re zone in the NW Ind ian O ce a n, du e to th e breakaway o f the A rabia n plat e (Figure 3.( 8). This ty pe of ju nc tio n is sta ble if th e two fau lts lie on the same s ma ll circl e (i .c . a re ef fective ly a single fa ult) .IS is the case in these e xa mp les. Th e evolution o f both the Pacific a nd t he Indian ocea ns th ro ugh Meso zoi c- Tert ia ry time pro vides usef ul illust rat io ns of th e kinema tic evolutio n of pla te structure . In broad te r ms , th e evolut ion o f world-wide pla te structure since th e e a rly Mesozoic is brought a bo ut by mea ns of t he bre a k-up of Pa nga ea thro ugh the opening of the A tla ntic a nd Indi a n oce ans, a nd th e co nseq ue nti al shrinking o f the Pa cific (Figure 3.5). Th e open ing of th e At la ntic invo lved a re lative ly simple seq ue nce of moveme nts that result ed in a ne t co nve rge nce of the Am er ica n a nd Eurasia n plat es a cro ss the Pacific. T he initia l stage too k place d uring the
59
"'lATE MOVEMENT AND PLAn :; UO UNO AIHf-S
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4 ~'ig u rc 3.4 Imuunon nnd cvolunon of the mplc junction, of rhe Sa il Andrea f" ult ( A, 1- 41, four s'agcs in the evolution o f the pla te boundary system in th e reg ion or the San An dr eas fau lt ill ~3 Ma. 30 Ma. In Ma OP, and lhe present , assu ming a sraucnary A me rican pla te , a fte r A twat e r ( 1970). Whe n the Pacific plat e mee ts the Ame rica n pl" tc (2), two triple juoctio ns arc fo rmed . joi ned t>y the San A ndrc; ls tra nsform (;lull, wh ic h gr;ulu"lly le ng the ns
Jurassic. betwee n 200 and 17UMa IW. with the opening of t he Ce ntral A tlantic (Figure 3.5A . B). Th is mo veme nt was bounded on both sides by major transfor m faults , bet ween Newfound -
la nd an d Gibraltar in t he north , and be twee n the Bahamas a nd G uinea in the so uth. T hus while So uth Am e rica and A frica remained unified to the sout h , and No rt h A merica and
60
GEOLOGICA L ST RUCTU IH, S AN D MOVING PLATES
I'L A n " M O V EM I'.N r AS D PL A I t:: II 0 U S [) A It 1l,S
Euro pe to che no rth . there was a crockwisc rota ria n of the lane r ilway fro m Gondwa nala nd thai re sult e d in a sinistra l dis pla cem en t thro ug h the Me dite rra nean a nd a co nve rge nce across T et hys . The second stage to o k place ma inly during the Cret ace o us . betwe e n 170 and 65 Mit. whe n the So ut h A tla nt ic open e d abou t a rot atio n axis in the Ce nt ra l At la nt ic, in ad dit ion to co ntinued spre ad ing the re . The se mo vem ents hroughl about a furthe r ro ta tio n of North Ame rica - Eura sia a nd a co nseq ue ntial co nve rgence ac ross T e th ys ( Figure 3.5 e) . By the m idCretaceo us . ;1 ne w rift a p pe a red th ro ug h the L abrado r Se a a nd across the Arctic O ce a n , splitting La ura sia into two se pa ra te plat e s (Figure 3.5D) . In the e a rly Ce no zo ic (65 M•• UI'). this spre a d ing axis was re placed hy t he presentl y act ive ridge t h ro ug h Icela nd . se par ating Gree nla nd fro m no rt hwe st Europe ( Figure 3.5£). T his third stage wa s a cco mpa nied by th e complete clos ure of the T e t hys oce a n . The evolution o f th e Ind ia n O ce a n is mo re complex ( Figur e 3.6 ). A mode l for t he pla te tectonic ev ol utio n of t he regio n wa s put fo rward hy Mck e nzie a nd Scluter ( 197 1). T hey suggest that the e a rlie st mo veme nt s loo k place along a rift se pa ra ting th e Ind ian fro m the African co nt ine nts ( Figure 3.5 e) . Ho we ver , the magnetic reco rd o f t his mo veme nt has bee n largely o blite rat ed . A r a ro und 90Ma ag o , in the tate Cre tace o us . the mo ve me nt of Ind ia changed d ramaticall y. a nd a no rt hwards mo ve ment comme nce d d ue to a fa st-sp re a di ng rid ge between the Indi a n a nd A ust ra lia -A nt a rct ica continents ( the n joi ned ) ( Figure 3.5 /)) . This movem e nt was accom mo da ted to th e north by su bd uc tio n of T e thys ocea n plate along the so uthe rn ma rgin o f E urasia. M uch o f this no rthwa rd move me nt wa s con strai ned
tlll,u n 3.5 Palaeomagn etic reconsl ruct io n of lhe con l illC nt~ illuslraling the b re ak-u p o f Pa ngaea in Mcstlzuk C(1I01oi( time. Lam bert eq ual-area S· po lar stctcogra phic prlljn 1111fl!; 'II : (A ) 2m Ma ur ( hll e T rias..s ic); ( H) 1411 MOl IIf (!.Jle Jur",,"sic): ( C) . IIIO Ma liP ( m;J ·Crel ;I~'lUS ) ; (l» . W M~ liP (l'al a< 'OCcnc) ; ( E ). 211 Ma liP (c a rly M iuccn.: ); iloo (Fl. P"",,-·m. Frum Sm ilh ,ill tJ Bride n (1 'J77)
61
be tween two majo r N - $ tra nsfo rm faul ts . the Chagos a nd N inet y-E ast rid ge s. At about 45 Ma ago , the spreadi ng rid ge e xte nd e d eastwa rds . ca us ing the se pa ration o f A ust ra lia fro m An tarctica ( Fig ure 3.5 E) . T he re cord o f t his phase of mo vem e nt , wh ich la ste d un til a bou t 35 M a ago , ca n he se e n in the NW Ind ia n O ce an , IS a set of mag ne tic stripes an d inac tive t ra ns fo rm fault s tha t are o blique 10 the p re se nt ly a cti ve rid ge a nd tra ns fo rm syste m ( Figure 3.6) . T he third major cha nge in kinem at ic pa ttern was rel at e d to a cha nge in spre a d ing dir ect io n of ab ou t 45°, accompa nie d by a fu rt he r e xte ns io n a t th e N W e nd o f t he o ld ridge into the G ul f o f Ade n . T his marke d kine mat ic cha nge appears to he d iac hro no us . co m me ncing Mo und 35 MOl lll' a t an o ma ly 12 in t he ce nt ra l urea an d p ro gressing no rt hwe st wa rd s un til ubo ut IOM a Il l' whe n the Gul f of Ad e n o pe ning co mmenced. T he ch a nge in dir ectio n is re flecte d in the ocean fl oor in a d isco rdance th a t re prese nts a ve rt ica l ' unconformity' with ne w stripes c utting: o ld stri pes and o ld tran sfor ms ( Figure 3.( 8 ). l t ha s bee n suggested that the kine ma tic cha nge is re la te d to th e co llision be tween the Ind ia n a nd E ura sian con tine nts . T he e ffec t of t his co llision ma y have bee n to bring ubout a c ha nge in th e re la tive mov em e nt vec tor be twe e n the Ind ia n a nd Eurasia n plat es . T he evo lution o f the Pacific O cea n can be reco nst ructed hy e xam ining t he Me so zo ic m agne tic stripe pa tte rn. La rso n an d Pitma n ( 1972 ) pre se nt an inre rp rcuuion o f the pla te struc ture within the Pacific in e a rly Cret ac eo us t ime (Figure J .7A ). A t th a t lime, the rid ge sys te m a p pea rs to ha ve bee n much more sy mme t rica lly a rranged . with two RRR tr iple junctio ns se para ting fo ur oce a nic pla tes : the Kula pla te in th e no rth . the Fa ra llon plate in th e north eus t . the Pacific plate in the so ut hwes t and th e Phoeni x pla te in t he so uthea st. S ubductio n zo nes fo rme d a n a lmosl co ntinuo us rim a ro und t he Pacific O ce a n of th a t time (the ' pro to Pa cific ' . we mighl ca ll it) . Accordin g to Pitm a n a nd Hayes ( 196R). th er e ha s been a co ntin uo us no rth wa rds mo vem ent of the no rt hern pa rts o f this p rot o-Pac ific Ocea n since th e e a rly
62
GEOLOGICA L S'I IWCTURES
A ~D
MOVING PLAT ES
I'i gurl' 3.6 ( A ) Pre ....-nr-duy I'lJ~i li"I1' of the G on d wanaland co nt ine nts. with si'''l''lilictJ di, I, ihUlIllll of occ.ur rid ges. ma gnetic smpc I"' llcrns ,,,,d In,",r.' rm ra ulis. MA R, mid-A tlantic rid ge ; AA R , A tl'Lnli!:-A nlarcti<; nd gc ; SW lR . SW Indi an O cea n ridge; Sl; IR , SE Indian O cean mlge ; ClR , central l mhan Ocean ridge . (H) Sin' plifi.:d mar the we stern Indian O cean reg io n ~htlwil1g. the dl\CUUJ ,lI\(C III magne lic anomaly puucr ns and !rim ,form d irec tions OI l am)JIl;lly S (.'3 Ma lIP) ca used hy a cha n~c in the Ind;.tn - A nlilfClic pl.uc vec tor. Af ter La ughton 1'/
"r
•
I
AI,;can plate I
.;: :.:
I
f--t--(-;f-;,~~'J--"" f-t''---
, '<§
- -;>(
,"'.." . . "
Antarct ic plat e ~ta t;onary) /."
.~,
Mesozo ic. With spreadi ng occurring on all fi ve ridges. the northe rn RRR junction would migrate nort h relative ( 0 the southe rn. When subd uction ceased alo ng the Antarctic margin. no rth wa rd movement o f the so uthe rn RRR jun ction would lake place relative to a fi xed An tar ctica . causing the whole proto- Pacific syste m to migrate nor thwards. The Kula plate was ra pidly subdueted throughout Cre taceous time aro und the no rth and no rthwest Pacifi c rim, according to the Pitman and Hayes reconstruction. They show four stages in this process in the Alaskan region (Figure 3.78 ). By ea rly Palaeocene time (li(l Ma ago ). the Kula piare
PI.ATE
M OV EM ~ NT
AND PLATE IJOl;NOAIl IES
63
Kula Plate
-
" ~.
Pacific Plate
".
A
"
B Alask a
a Lale Oelaceous (about 75 million YNfI, ~l
C Ea.ly Paleoce ne (about 60 mill ion
years~ )
b l4le Cretaceous (aboul 70 million years 8QO )
d hrly Pliocene (about 6 mill ion yea'1i ago )
Figure 3.7 (A) Arrange me nl of pla tes in Ihe PacifIC O cean OIl around I IOMa UP (early C retaceo us lime) . Note the two RRR trtple [unctions and the projected Farilllo n- Kula ridge (see B ). T he Cen tral Atlantic has com me nced ope ning. After Larson and Pnman ( 1972). (1J) Schernanc .tiag.ams showing fo ur stages in the cvolc uo n of the mag netic snipe patte rn in the NE Pacific Oce an ; I, Pacific pla tc ; II, Kula plale ; II I. Ame rican plate . IV , Farullo n plalC. Fro m Uyeda ( l'n ll ) . afte r Pumnn and Hayes ( 11)(>t\ ).
GEO LOGICA L STRUCTURES AN D
had almo st e ntire ly d isappeared , and the A leutian subduct io n zo ne was co nsuming pro gressively old e r ma teri al belon ging to the Pacific plate. It is di fficult to int e rpre t Figure 3.7 8(4) by itself, but it beco mes clear once the stages of e vo lution a re fo llowed throug h. The ridge be tween the Pacific a nd Farallon plat es ha s migrated northeastwards . a nd is about to be co nsumed by the A leutia n tre nch . T he ot he r impo rtant e ffec t o n the Pacific region was the progressive westwa rds e ncr oachm ent of the Amer ican plat e bro ught a bo ut by the op ening of the At lant ic. Co ney (1973) has calcul at ed th a t the western mar gin o f No rt h A me rica has moved abo ut 3700 Km westwa rds from the ea rly Mesozoic un til the for mation of the San A ndrea s fault a bo ut 10 Ma ago . Almost all o f the Farallon plate has been co nsu med by this mov e ment, along the weste rn A mer ica n subd uction zo ne . Th e later stages of th is pro cess a re discussed above (see Figu re 3.4). Two maj or c ha nges in movem e nt direc tio n can be t raced in the magnetic a no ma ly pa tte rn of the Paci fic floor. Th e first . ar ou nd the beg inning o f the Cr et aceous, is ma rked by it disco nti nuit y in tra nsfor m direction s in the Ha waii regio n (Fi gure 3.10). At 45 Ma BP, the East Pacific rid ge extended westwards to co nnect with the Ind ian O ce an , by splitting Austra lia from A nta rc tica , but no co rrespo ndi ng cha nge can he see n in the Paci fic floor a t thi s time . Th e seco nd major c ha nge occurred 10 Ma ago whe n thc plate bound ar y syste m in the east-ce ntral Pacific W,IS complet e ly reo rga nized ( Herron , 1972). Two new plates we re formed . the Cocos pla te west o f Cen tr al Am eri ca , a nd the Nasca plate west of the Pc ru C hile tr ench ( Figure 3.8A ). A new section of the Ea st Pacific ridge was formed be twee n latitude 4SOS and the Gulf o f Ca liforni a, brea king through the old ridge a t the G alapa gos triple jun ct ion (Figu re 3.8B ). As in the Ind ian O cean , the c ha nge in rel ative mo vemen t vec to rs is clea rly marked by an abru pt cha nge in the orie nta tio n o f the t ra nsfor m faul ts, which run a pprox imate ly E - W until ano maly 5. wher e they a rc re placed by a
~lOVIN G
l'lATES
NW -SE se t. T he heavily frag me nted h ut still active Chile ridge represen ts the so uthwes t bounda ry of the old Farallon pla te . of which relics oc cu r in bot h the Co cos and Nasca pla tes. Th e reaso n for this cha nge ca nnot be esta blishe d with ce rtai nty. bu t seve ra l factors may have contrib ute d . T hc c rea t io n of the new ridge re presents a cha nge in re la tive mo vemen: direction between the Pacific an d Am e rica n plat es and corresponds in time with (i) a de crease in sp reading ra te in the At la ntic, (ii) the subd ucti on of the no rthe rn part o f the Eas t Pacific ridge be low North A merica (a nd the co nseq ue ntial initia tion o f the Sa n Andreas fault). and (iii) the chan ge in plate st ructure in the Ind ian O cea n already di scu ssed. A maj or facto r which ma y have in flue nced wo rld-wide pla te struct ure is.nf co urse , the collisio n of Ind ia with Asia. alt ho ugh the ex act time o f this e vent is unc ert ain (see 5.4) . Th e st ruct ure of the western Pacific. which was the site of destr uct io n o f Pacific pla te throughout Mesozo ic and Tertiary time . IS dominated hy effec ts cr ea ted hy the northwards advance o f Au str alia (o n the Ind ia n plate) a nd by t he creation of nume rou s bac ka rc spreadi ng basins (see 4.4) . The structure of the co mple x Indo nesia n region is discussed in detai l in 5.5 . 3.2 T he iutluence of plat e geometry on the kinem atic pa tter n In the preced ing sect io n, we hav e assumed that the rela tive pla te mo vem e nt vect ors wer e co nsta nt. a nd that thc y, toge ther with the sha pe of the plate bounda ry netwo rk , contro lled the subseq ue nt geom e t ry of the ne two rk. Pla te tecto nic theory , to a first approximation , assu mes a co nsta nt ne two rk geo me t ry. However , unde r cert ain circu msta nces, plat e bo undari es may he deformed as a resu lt of plate move me nts, thus violating the princip le o f ' rigid ' pla tes. Such de fo rmation is la rgely confined to des tructive boundaries, and pa rt icula rly affects continental lit hosphe re which , as. we have seen, is softe r a nd weaker tha n ocea nic.
~
Ff;:/:::
r
PACI FIC
.................
<,
.............
..........
.........
. r i d ge
{-NA S:A
<, ................ <,
........ ................ ................
t··
. Ga / a p a g os •
•
•
t
0
I
0
AMERICA
,
.
, -
-
... ,
-
~_. - -=."", . -. _-
'"
r
}>
...........
~
tT. 3:
0
<
(T:
:.:.~·O·OS
..........
3: rr.
;
}>
z
.5;'
~
0
a.'"
t
~ .~ -~_1::
J1
l~ - " -? n(t-.. ""---
........
jJi1Gtl ,-= II ,.
........ ....... _
...' _.
'"
at
_• •
'.
~f~lj
0
r
}>
-i
(T:
to
0
C Z
0
}>
~
A
rn
I:::: :.::::: :\--l-
(f)
N%'~i~&'i:1
' 4 0·
6 S0s
Figur e 3.8 Change in plate structure in [he East-Central Pacific Ocean. (A ) Present plate boundary network. The ea rthquake zone marking the destruc tive western boundary of the American plate is stippled. (B) Discord ance in magnetic stripe and transfor m fault patt ern at anomaly 6 (a pprox. 10 Ma sp) . Pre-anomaly 6 ridge segments are dotted , active ridge segments in black. New ocea n crust since anomaly 6 is stippled. After Herron (1972).
0\ V1
: :"~"'"el"~ I~~"i-hUS ''''::::1',;:th:_,:;~n:~lcad;ng sug~~;dest ~e~~~",defom~:t:'~:UI:~n; oec~n~a;ds, e~;nl "~:~e bound~ry ~,~:gon o(~f:~re GEOLOG ICAL ST'U
"
crURES AND MOVI N
.
G PLATES
.
cd that subduction
' . S such ridges wa hie relief and by
of
at rbe cus pate sha pe
Vogt (1973) could, in of "land arcs to the by dest s, ructive 3Ihe 9A) . He em be P f asersnuc n g ocean
locally " ' : : : :
the gre,
the inte rac tion 0 . g plate (FIgure . floor 0 f the subduc tin .
whe n driven by 3,98), the rnov
....'...'..~ ..
v ~J /) -7 '
..
t'igu~e
3.9
ridges (A ~r Aseismic the NW Paclhc
and island arcs M iyashiro tI at. O cean. A fter bcrnutir diagram
( 1982). (8 ) S~ k-a rc spreading showing how. ~c a t the cusps of may be inhibite resence of ;In
TIOOC'
'---=- ...
the ar~ by ~he /rom Mi.y a~tll ro aseismic ( ndg " I pe rrmss.on.
el a [.
) Mo"'oal
"0
1982).
WII I ...~
will be eon-
PLATE Mo v EM ENT A N\) I' L A l l; HOUNDA IU t:S
strai ned at the positio ns o f the ridge inte rsections. T he rate of piatc co nverge nce will he at u minimum there . but will increase to a ma ximum betw een the inters ection s . produ cing an arcuate pattern . Good exa mple s of this arc the Emperor sea mount chain at the inte rsection o f the Ale ut ian and Ku rile arcs, and the MarcusNecker a nd Caroline ridges at each en d of the Mari anas a rc ( Figu re 3.9A ). Such deformation is consisten t with rigid-plate theory bec ause it results fro m a progressive change in the position of down-bending and does not involve active lateral d istor tion at the surface. De form ation o f con tine ntal plat es ap pear s 10 result mainly from coll ision , and may involve major ch an ges in geo met ry bo th of the plate bo undar y and of the plate inte rio r. T he best example o f such defo rma tion at t he presen t day occurs in the Cent ral As ian regio n described in 5.4. T he de forma tio n result s from the collision of the co ntine nta l part of the Indian plate with the so uthe rn mar gin o f the (continental) E urasian plate . Once the intervening ocea nic plat e had been co nsumed , furthe r co nver ge nce wou ld ha ve bee n inhibi ted by the buoyancy o f tfie conti nental part of the Indian plate . T he processes of subd uctio n an d collision are consider ed in det ail in C hapter 5. It is importan t to recogn ize th at co llisio n is t he most e ffective way of alte ring plate kinem atic patterns, oft en in a quire dra matic and world wide fash ion . Envisage the co llision o f two opposing co ntinental margins, both typica lly irregular in shape , and ob liq ue to each ot he r and to the co nve rgen ce d irection . A t the first point of contact betwee n the two o pposing margins, resistance to co nverge nce will be introduced which may 'lei eithe r to change the converge nce vector , or to defo rm the bo undary geome try , or both. Th e tee Ionic effects of wedge-shape d prot rusions of one plat e as it mee ts another at a co llisio n bo und ary are d iscussed in the ' indentation' model of Tapp o nnier and Molnar (1976) and applied to the Ind ia Asia co llision. Th e mode l is o f gen era l application and involves a protrusion or "inde nter" which causes local stress co nce ntrations in the
67
indent cd plate suffic ient to ove rcome its st rengt h and to prod uce widespread d istortions. 'A hsol ute' phI/(' motion
Th e me thods of analysing plate mot ion developed by McKenzie and Parker (1967) give vecto rs for relative mot io n o nly, and Figure 3. 1 is bLlSCU o n t he assumption of a statio na ry Antarctic plate . A meth od for dete rmining ' absotu te' p late mo tion was suggested by wilson ( 1% 5). Wilson noted that , at a number of locations sca ttered ove r the Earth 's surfac e . volcanic activity ap pea rs to have been co ncentrat cd over long periods of time . Wilson ca lled t hese areas ' hot spot s' and identi fi ed severa l. includ ing Hawaii and Icela nd . He showed th at t he motion o f an oceanic plate ove r o ne o f these hot spots wou ld result in a linea r cha in o f volcan ic islan ds becom ing progressively olde r from tile currentl y active volca nic centr e . Figure 3. 10 shows the Hawaii- E mpe ror chain o f volcanic islands and sea-mo unts in the Pacifi c interpreted acco rding to the Wilson mod e l. The ages of the vulcan icity range fro m ] 0 Ma at the distal end o f the chain, adjace nt to t he Aleut ian trench , to the presen t hot-spot locat ion in the Hawaii islands at the so uthern e nd. T he bend in the rkjge is inte rpreted as a cha nge in pla te velocity vector. occ urr ing at c.35 Ma UP (sec above) . Wilson also ide ntified late ral chains o n eithe r side of the mid-Atlan tic ridge , such as the T ristan da Cunha - Walvis ridge o ff SW A frica ( Figure 3. 11). In this cas e , he sho wed that the prese nt ridge axis is offset from t he hot-spot site by ab ou t 400- 500k m, and suggested that the ridge or iginally lay o ver the hot spot, but had been moved westward s over the last 25 Ma as a result of a change in the pole o f ro tation for the Am eric a -Africa separation. Morgan (1972) de veloped Wilson's ideas furt her and reco nstructed a se t of 'abso lute' plate move ment vector s with refe rence to t he hot-spo t frame o f reference ( Figure 3. 12). He fou nd that the relat ive movement betwee n the hot spots has been ver y much less th an that
68
G EOL OGICAL STRU CTU RES AN D MOVING PLA f ES , ~O
, ~O
o
,~ O
~o
Pacific Ocean
A
~o
'0 0
B
L
~
-
~~
~
_
Progressive ly Older 0 A
I'ib:U fe 3. 10 ( A) Lo c anon o f the E mpe ro r and Hawa iia n volcani c rstaud a nd sc.r-ruou ru c hains in the nor the rn Pacific O cea n. (8) M ode! to illustrat e tne formation of rtc volca nic c hums Ily I1lOv e "'cn ! o f the occ ani c lith osph e re (lve r a ' 1i ~..:,J' hot.xpot. After W i lson ( 1%3) .
" . ST.HE LENA
" ,' A F R I C A ."
,.;. ". OI S COV ER Y S EA MO U NT .,pO C H A I N
\i.~fj;~:-"f;.,
o
' O OO~ m
MID- O C E AN RIDGE
Figu re 3. 11 Map of lhe sou thern Atla ntic Oce an s howing vo lca nic ce ntr es (T ri.s t;m. G ough cIC.) offse l fro m the pr esen t position of the mid-o cean ridge . Acco rd ing to Wilso n (1973) th e line throu gh the sou thwes t en ds o f me volcanic cha ins s hows the position of the ridge 25 Ma ago . since which lime the ridge has migrated westwa rds. A fter Wilso n (1973).
69
PL ATE MOVEM ENT A N D PLAT! , BOU N D A RI ES
so'
,"'.
,"'.
iso-
~.
00 '
~.
o'
oc-
sc'
~.
... oo'
~.
c-
"'.
...
00 '
~.
~.
,ao'
.se-
' 00 '
isc'
,ac-
so'
00 '
"'.
, o'
,
co-
, 00 '
Figure 3. 12 Plal~ movemen t vectors rcl.ulvc III a lixcu hot-spot lramc " r re ference . Lengths or arr Owx a rc pro po rt ional [0 plalc vclocuics. From U ycJ .1 ( 197X) . alter M
between the plate bou nda ries over the last 200Ma or so. A lth ough it is li kely that the ho i spots also migrate wit h lime re lative to a notional fixed man tle frame . th is migra tion is probably an order o f magnitude slower th an thai of the pla tes . Since the ha l spo ts are formed by the upward migration o f hot mantl e materi a l. th ey must be relat ed to the mantl e con vect ive sys te m. We ha ve see n (2.4) thai the convec tive circulatio n in the man tle must be comp lex , and co nstantly changing beca use of the co nstra ints imposed o n the surface mobility of the plat es by t hei r st rength and geome tr y. T he lateral movement of ridges away fro m their likel y locus of crea tion is a good illustra tion of thi s. It is thou ght that the hot s po ts re present the sites o f ascent of dee p mant le ' plumes' that form the rising co lumns of a convecti ve circulat ion pattern , se pa ra te fro m
but linked with . the plat e movemen t circulatio n (see Figure 2. [ I). A n int erestin g feature of the 'a bso lute' plate movem ent vect or patte rn is the much grea te r vel ocities of the who lly or mainly oce a nic plat es, such as the Pacific , Coco s an d Nasca plat es, co mp ared with those plat es carr ying large continental masses such as the Eurasia n. A me rica n and Ant arct ic plates. It might be tho ught that the presen ce of la rge pieces of continental lithosph ere acts as a resista nce to motio n. Howeve r, the fast-movin g Indi an plate argue s against thi s notion. It is clear from Figure 3. 13 t ha t the significa nt fact or affecting the velocity of a given pla te is the len gth of attached subduction zo ne, indicating that the subd uctio n process (i.e . slab pu ll) is a major kine mati c co ntro l, as well as an impo rtant dynami c co ntro l. as show n in 2.5.
70
G EO L CXifCA I. STIWCrURES ANO MOVING PLAT ES
30
o
10
Velocity (cent imeters per year ) Figure 3.13 Pro por tio n of pl"l.:: circumference ("10 ) connected to subductin g slahs v. 'absolu te' veloci ty of pl,llcs. Note Iha l :.11 rho prates with lo ng subd uctio n zone s have velocities great er etw n 5 ern/year . Fro m Forsyth and Uyeda ( 1975)
•
3. 3 Th e effects of relative plate motion at plate boundaries It is clear fro m Figures 3. 1 and 3. 12 that the
movement vecto r across plate boundaries is ge ne ra lly o blique, a nd tha r o nly in the case of t ra nsfo rm faults is the re lative mo tion const ra ined in a pa rticular d irect ion (para llel to the boundar y). T he impor tan ce o f oblique rela tive movemen ts in orogenic belts was ori ginally highlighted by Harland ( 1971), who introduced the te rms ' transpression' and ' transte nsio n' to de scribe tecto nic regi mes ex hibiting ele me nts of both strike-s lip moti on and eit her co nvergence (co mpression) o r d ivergence (exte nsio n) respectively. In the interve ning years, these ideas have been co mpara tively neglected by structural geo logists, who ha ve been a pplying them extensively to oroge nic be lts only qui te recently. Assuming initia lly that we are dea ling with movem ent s that are wholly in th e hori zontal plane ( Le. tan gent ial to the Ea rth 's surface ),
there are eight possible catego ries of relative motion
PL AT E
Sinistr al
.\1 0V [ M E ~ T A~O
De x tral
Normal
A. Mo ve m en t s in t h e no rt zor uet p la n e
strt ke -sn o M o v emen t Dip-Slip '-:::-c",,'7:"':"'-:,",,':7:'~r":-:-----1 Mo vemen l rSin istr al De x lral None
, "" <, . "" <:
Thr ust
~
Normal
~ "' , ~ , "'"
None
.
>--:
~
~
x. Away f ro m observe r
• • To wards observer
8 Mo vem ent s in t hr e e dimen sions (v er tical pr ot fle ) on inc lined b o u n d ar ie s. tll:urt 3. 14 C llcgmi,;;, or movemen t ,Ier"" ptatc h"u n ' da rie s: (A) . in the hm iZlInl,,1 pla nc ; (8). in three dimcn ,iun.' (verlk"l profile). l n ~ " l h c.e,c . the bcuvy ;lITOW, m:,rk the movement veClor, :Illd th,;; tight arrows the cornponcuts of motion "Inng und ano" Ihc houmJMY·
add itio n to the categories o f d iverge nt mot ion listed above applied 10 a vert ica l bo undary. A t co nservat ive bo unda ries . sinist ra l o r de xtr a l strike-slip mot ion a pplies e ither to ve rtical or incline d planes. M o vements across
(l
deformable boundary
Th e above a na lysis ignores o ne o f the most importa nt asp e cts of plate boundar ies , wh ich is that the y d o no t re pr ese nt a d iscre te plane , bu t a volume of def o rm a ble mat e ria l. O ro ge nic belts a re, to a lar ge ex tent, a n ex pression of th is de fo rmatio n. We the ref o re ha ve to ta ke into accou nt rel a tive movements across the bounda ry as we ll as a long it. T his is partic ularly obvious in th e case o f construc tive boundaries, wher e new p late ma te ria l is crea ted to accommodat e th e div er gen t mo ve me nt. It ix the re fore nece ssar y to co nsider the pla te bound a ry
PLAT E
BOU~OARl t:S
71
as a de formable shee t ra the r tha n :J plane , in o rder to det e rm ine t he rela tio nshi p be tween plate movcmcurs and de fo rmatio n. T o each of t he categories of relative mo vem en t liste d in Figur e 3.14, must be ad ded a co mpo ne nt of e ithe r co nve rgen t o r dive rgent mo ve men t across the sheet , resulting in ei th e r compression o r ex te nsion of t he sheet in t hat d ire ctio n. T hese move me nts
• 72
GEOLOGICA L STIl UCru RES AN O MOVI NG PLATES
,
de red , we shall see how far these pri nciples hav e been put into pr actice .
... ,{!;p~ .--(' / / ,
Summ ary
..
/ ,'
7
,::>;":J'
v
A
c FiJ;:.ur., J . 15 (A) rran spr csskm, and I R). u a nstc nsion ( plan views ) produced oy a mo vem ent VCClur (hCHVy arrows) oblique 10 a plate Otlundary rcprc:.cnlcd hy ~ dcl orm ahk ~hcCI Th e " Shl ;,rmws represe nt the COIllpon cn ts of move me nt across an d alullg the bou ndary . (Cl A nuc c -dlm c nsion al diug rum illuxrruting trans pres sion . In
each
C
the deformed shape is stippled.
•
usefu lness . In Chapte rs 8 a nd 9, whe re individu al examples of or ogen ic belts arc co nsi-
In a pplying plat e tecton ic theor y to the study o f geo logical st ructures in orogenic belts, the prima ry objec t is 10 rela te rela tive plute mov eme nts a t pia rc bounda ries to stra in with in the be lt . Since obliqu e move me nts across plat e bo und a ries a re th e no r m rat he r th an th e e xce pt ion. obl ique co nverge nce a nd d ive rge nce , bo th in th e horizontut plane an d in three dimen sions. must be co nsidered . Eight ca te go r ies o f relat ive mo vem en t across a pla te bo unda ry ar c recog nize d . By co nside ring the plate boundar y as a defo rma ble sheet ra th e r than a discre te plane. the furt he r possibility a rise s o f co mpressio n or e xte nsion taki ng pla ce no rma l to the wa lls of the shee t, ca using vo lume c ha nges of the kind e xp resse d in oroge nic belt s by crustal thicke ning. a nd a t co nstr uctive bo unda ries by the crea tio n of new lit hosph e re . A t a typic a l plat e bo undary, oblique-sl ip e xte nsio nal or co mpressi o nal move me nt s are expressed in terms of tra nste nsio na l or tre ns pression al str a ins wit hin th e rnate riul adjacen t 10 the bo unda ry.
• 4 Divergent (extensional) tectonic regimes ru re o n the Earth' s s urface. su rpassing e ven th e g reat mountain ra nges in sca le. A typ ica l sectio n o f ridge is a bout IOOO- 2000km wide and 2- 3 km high . The m id-A tlantic ridge occupies about one-third of the surface a re a of th e A tlan tic Ocean (Figure 4.1A ). Th e tecto nica lly act ive ce ntral rift is marked by a zo ne of co ncentra ted earthq ua ke ac tivity. The wide to pogra phic swe ll with ce ntral rift is a lso cha rac te rist ic of th e contine nta l rift zones ( Figu re 4 .2A ) wh ich displa y approxi m ately th e sa me d ime nsion s. Acco rd ing to Me nard a nd Sm ith (1966) the ridge syste m as a whole ma kes up 32 .7 % of the surface a rea of th e ocea ns, o r 23.2% o f tha t o f th e Ea rt h. The geophysical struct ure o f ridges is now known in co nsiderable de ta il. The very large excess to pog ra phic ma ss of the ridge is a lmost exactly compe nsat ed by a mass deficiency
The main so urces of ex te nsio nal stress in the lithosphere arise fro m the de nsity imbala nces produced by ocea n ridges. con tinenta l ma rgins and pla tea u upl ifts , a nd hy th e fo rces a rising from subd uction (see 2.5) . 1) laI C3 11 upl ifts
in·
A. Plate boundary regimes
(i) (ii)
Ocean rid ges Con tinenta l rifts o f type I (pla te boun daryl (iii) Co ntine nt a l ex te nsio na l pro vinces at conve rge nt bo und a ries (lv) Ba ck-arc e xte nsio na l provinces (v) Strike-slip ex te nsiona l p ro vinces
B. Intraplate regimes (vi) Co nt ine nta l rift s of type II (intr apla te ) (vii) Int rapla te ex te nsio nal bas ins.
In this cha pte r. we s hall d iscuss e xa m ples of types ( i) -( iv) . T ype (v) will he di scussed in Chapter 6. a nd intra pla te re gim es in C ha pte r 7.
Figu~ " . 1 The mid·Atl ant ic ridge . showi ng a nlfal rifl and d istr ibution o f t'a rt l!.qual.c epiantrcs . After Hee re n ( 1962).
Th e wo rld- wide ne t wo rk o f ocea n rid ges co nstitut es the most sig nifica nt topog raphic fe a-
73
• 74
GEOLOGICAL STRUCTURES AND MOVING PLATES
A
Seismic Bell
"',
2.000~--
2,500
....
...",..".,
r' -r:
.Mal....
J
E!elQion
l~~
1 I
Con9o
Northern Rhodesio
"
AIonQ 30° N 101.
~
I I
I
Lake TOllQOnyika I
:
Lake Nyc...
I
.
• Indian Ocean
"~-s-:& 100
200
AIoRQ 14· 5 kit
400
500
600
Noutico' miles
mgal
300
300 -Observed 200
- - - Calculated
L-
-'-
---''--
/j/.m
--'
B
caused by a volume of less dense mantle material below the ridge. Figure 4.28 shows profiles combining gravity information with seismic refraction data that indicate anornalously low seismic velocities in the region occupied by the low-density material (see 2.2). The most satisfactory explanation of this anomalous structure is that the asthenosphere effectively rises much closer to the surface, and that the lithosphere thickens away from the ridge crest as it cools (see 2.1). The presence of
200
,
Figure 4.2 (A) Topographic protile across the mid-Atlantic ridge, compared with profiles across the E. African rift system, Note in cach case the central rift situated on a broad topographic swell. x25 vertical exaggeration. From Heezen (1%2). (8) Possible density model of thc crust and upper mantle structure beneath the midAtlantic ridge, showing densities in g/cru' assigncd to each layer. From [jolt (1971). after Talwani et al. (1%5).
material with the properties of asthenospheric mantle has been confirmed by other geophysical evidence: low electrical conductivity, high attenuation of seismic waves, high surface wave dispersion, and inefficient propagation of Sn waves, which correlate with the zone of high heat flow along the ridges (see 2.3). The active tectonic zones occupy a narrow central rift valley about 100 km wide, in which the earthquake activity and vulcanicity are concentrated. Rather low heat flows have been
DIVERGENT (EXTENSIONAL) TECTONIC REGIMES
75
measured from areas adjacent to the high heatdisplacements characterize the flanking fault scarps, which dip at about 60° inwards, forming flow zone along the ridge axis. This pattern is thought to be due to convective circulation of the walls of the rift. These features indicate fluids, which gives rise to intense hydrothermal extension normal to the rift axis but oblique to activity. the plate divergence vector. Detailed information about ocean-ridge The vertical displacement on the extensional morphology and its relationship to volcanic fissures of the axial zone is always less than 1 m and tectonic activity was obtained in the but, in the marginal tectonic provinces, the FAMOUS project (Heirtzler and van Andel, throws increase to between 1 and 3 m, with the 1977) which employed manned submersible formation of tilted blocks. On the inwarddives along a section of the mid-Atlantic ridge facing walls of the rift, major vertical scarps occur, with throws of 5- HXJ rn, on planes west of the Azores islands, between latitudes 36°30' and 37°N. The manned dive programme dipping around 60°, bounding fault blocks tilted away from the rift at 5-7°. was supplemented by a number of techniques including seismic refraction, side-scan sonar, The horizontal extension measured across surface sampling and deep drilling. This enthe inner rift on these faults and fissures amounted to 5.7'X, on the western side and 8% ablcd a detailed picture to be obtained of the morphology of the ridge and its tectonic conon the eastern. The difference is proportional struction (Ballard and van Andel, 1977). to the difference in spreading rate. New rift ~e ridge in this ~~<:tor is. topographically axis positions appear to occur along the lower Jomplex arlQi~-~fset.b.Y_s..~ve~aU~;icturc~~~e~~ • slopes of the volcanic edifices rather than (transform faults). Bathymetric profiles show through the middle. Consequently, short tranranges of rift mountains with peaks at a depth sient transform faults are required to accommodatc the spreading. The width of this central of 1300 m on each side of the rift, separated by 30-32 km of valley ~oor which slopes gently zone of offset rifts is about I km. The central towards the rift axis. The axis is offset towards axial ridge contains volcanic material with the western margin of the rift. The asymmetry dates of 20000 to 35000 years, but the marof the central rift in relation to the margins of ginal ridges contain material 100000-160000 the first magnetic anomaly suggests asymmetric years old (Figure 4.3A). Thus the inner rift spreading of 0.7- 1.0 ern/year on the west side contains the youngest volcanic rock and is interpreted as the result of axial volcanic and 1.2- 1.4 em/year on the cast side of the rift. The fracture zones are located in deep Uactivity in the form of small piles or cones of shaped valleys up to 3 km deep and 10 krn lava 5-7 m high and 10-15 m wide draped by wide. Seismically active scarps of the order of radiating lava tubes. This vulcanicity is accorn100m high mark the tectonically active portion panied by pure extension, in contrast to the of the fracture zone, and sheared rocks were faulting in the rift walls, which results from dredged up to 10 krn on either side of the shear. It is suggested that, shortly after formasouthern fracture zone. tion , the volcanic edifice in the axial zone The rift floor contains an inner rift valley collapses vertically along the boundaries of the (Figure 4.3) with a floor 1-3 km in width, central ridge, whereas in the outer portions of bordered by sloping terraces. The centre of this the valley, older volcanic blocks are uplifted to inner rift valley floor consists of a line of form the walls of the inner rift (Figure 4.3C). elongated hills up to 1 km wide and 100- 2(XJ m Since the distance from the boundary wall to high, lying at a depth of about 2500m. The the axis is correlated with spreading rate, it morphology of the rift valley was found to seems likely that the uplift of the walls is result mainly from volcanic activity modified related to the thickening of the lithosphere as it moves laterally away from the locus of new by tectonic effects. Vertical tension fractures occur on the inner rift floor and dip-slip volcanic activity and cools. The crustal thick-
76
GEOLOG ICA L STII: UC I U HES
ness in the inne r rift valley is only about 3 km , based on ther mal mod els (Sleep , 1975). A bo ut 1O- 20'Y" o f this thick ness is made up of pillo w basalts. ove rlying ocea nic crust o f laye r 2 type (pres umably she e te d d ykes) , which in (u rn o ve rlies a ma gma chambe r th a t ;IPPC;' fS [0 extend t he whole width of the inner rift .
A~ D ~ O Y I N (j
T he ho rizon ta l ten sional stress e xe rted across the whole width o f the ridge , d uc 10 its de ns;Iy cont ras t with Ihe adjoining ocea n floo r , is therefor e mm smu tcd across u very th in , wea k. br ill Ie laye r in the cent ra! rift zo ne . This must cause repealed extensional failure , enah ling the spreading process \0 co ntinue.
3~'*
-
OM"
•
J> I.ATES
",'
,,'
q ""opo ME TERS
-
I
•
Egu", 4.3 (A) Distributio n of fau lts [ hachured lines) and volcanic hodies (~ h ade d) io me inner rifl m oe o f lhe midA tlantic ridge at ;lOoN. The da rker s hading indicates the younges t volcanism; d\lb . vents a nd cres t tines of volca noes: thic k ar ro ws, dip o r fault bloc ks: thin c urved a rru ws. vulca nic I lcw lobes. From Balla rd "od V;lI1 A ndd (1977)
77
DIVERGENT (EXTENSIONAL) TECTONIC REGIMES
B W.WAll 300 m
~
E WAll W MARG. HIGH
~ II ~ ~
00
500m
ft
CENTRAL HIGH EASTERN WESTERN TROUGH TROUGH,:.<.--.........---7
..
~
J\
RIFT
E MARG HIGH
• FLOW DIRECTION A VENT
Figure 4.3 (8) Diagrammatic profile of the inner rift zone showing structural subdivisions. (C) Diagrammatic crustal crosssection of the inner rill valley, showing magma chamber in which lateral magmatic differentiation and cumulate dcposilion are taking place. Later volcanism on the flanks draws on differentiated magma. and crustal thickness increases towards the main boundary faults. Band C from Ballard and Van Andel (1()77)
The type of topography described in the area appears to be characteristic of ridges with rather slow spreading rates. Ridges with moderate to high spreading rates (e.g. the East Pacific ridge between 2loN and 200S) are characterized by central ridges with narrow, shallow axial depressions (Figure 4.4). Francheteau and Ballard (19R3) describe the results of a detailed survey of three small sections of the East Pacific ridge at 2loN, l3°N and 200S respectively, and compare these with the FAMOUS results from the Atlantic. They show that at moderate spreading rates (e.g. at 2loN) there is a shallow axial depression comparable in width to the rift valley on the Atlantic ridge (3-5 km) and with similar volcanic edifices 1050 m in height. The fault-controlled walls of this rift average 100 m in height. The axial trough is situated on a central swell up to 7 km in width. However the highest relief occurs on the uplifted blocks immediately flanking the axial trough. At faster spreading rates (e.g. BON) there is only a very narrow axial graben, 200 m in width and 50 m deep, situated on the FAMOUS
central swell. At the fastest spreading rates, the graben is absent. and there is only a prominent axial ridge. ~ km wide and 200 m high, situated on the central swell. This swell is still about 7 km wide, hut is now 500 m high. These features characterize the East Pacific ridge at 20 oS, and are superimposed on a general ridge topography that is much smoother and flatter than that of the mid-Atlantic ridge. In this section of the East Pacific ridge, the spreading rate is ahout 17 ern/year. The ridge is almost aseismic and displays no signs of major faulting on the axial ridge. which appears to he of volcanic construction. Fluid lavas forming sheet flows are more prominent than pillow lavas in the axial region, and would tend to cover the fissures as soon as they opened up. This would account for their relative scarcity. Hydrothermal activity is evident from the existence of numerous vents and. locally, massive sulphide deposits along the axial zone. Faults and fissures are comparatively uncommon. Francheteau and Ballard propose a model for an ideal accreting segment of ocean ridge
78 ~
~
i::
:;; C,
G EO LOGICA L STRUCr Uln:S AND MOVI:"G PLATES
MAR 36°N
<0" ' 00 Il OO
160 0 2000 0
to
: '5
,
20
25
,"oo~PR 26000
~
It ~
c,
,"OOF
2~
5
: 7.5
ao
00
21°N
10
, ' --t:i ~ PR 13°N
2800 ~ 0
2.5
5
,:
, 7,5
''
'0
"OO~ : : EPR 20°5 3000 • • , ,
3200
400 3 0
,
I
25
,
5
7,5
10
'~ 5
D IS TANCE (km ) FiJ::ur", 4 .4 Comparisu n of 1<)[1<'I;'''PI'I ,,,, prulilc ac ro ss Itll: mid -A flnnuc rilli!'- a[ .1I,"N wilh p m fik s " e"", 1I H<~ c M:Clions o f the [;h l Pacific ridge c h.u.rc tc rrzcd h y dilfcrcru sp rea ding rarc s , OI l 1 1~N «(' n n ycar) , L~o N (10 .2 c rnlyl" ar ) '111.1 20"S (If> em! year) . The pm; lion of the illl1cr ,if! zone in the mid-Atlan tic ridge is projected omo the ot her profiles for comp
(Figure 4.5) . The segment is boun ded by active transform faults which are associated with to pographic depressions. They suggest that each segment is unde rlain by a separate magma reservoi r that lenses o ur d ue to cooling at the transform -bounded ends of the segment. The magma chambe r will thus be thicker and the overlying crustal ' lid' th inne r. in the central pan of the segment, resulting in iso sta tic uplift and to the observed to pograp hic highs. These ho lle r regio ns where the lid is thinnest will prod uce surface- fed fluid lavas rat he r than pillow lavas. which will occur distally. around the fl anks of the highs. Lichtman and Eisscn (19S3) suggest that e ruptive phases migrate in a probably random fashion along the ridge crest. T he initial, highly active ph ase is charac te rized by magmatic inflation. increase in instant aneo us spread ing rate , and eru ptio n o f sheet -flow lavas, and lasts
for abo ut 10 years. Then , as the pu lse of replenishment moves to ano the r poin t on the ridge axis. tbe previous site is characteri zed by pillow-flow e rup tions and decreasing hyd rotherma l activity. The explanation for the d ifferences in to pegraphy bet wee n fast and slow-spreading ridges appears to lie in the diffe ring effects o f broad and narrow axial magma chambers. Slee p and Rosendahl (1979) constructed numerical fl uid dynamical models for fast and slow-spreading ridge sections, and produ ce calculated pro files which closely match the observed topography. T hey conclude that ridges C<1I1 be classifi ed into slow-spreading (half-sp read ing rates less than 4 cm/year) with axial rifts abo ut I km deep ; fast-spreading (half-spreading rates grea te r than 3 em/yea r) with axial ridges severa l hundred met res high; and hot -spot ridges with spreading half-rates less than 4 em/year . pro-
79
D!VE II.GENT ( EX f F.NSIONAI. ) T EOON IC REG IMI;S
I AI
-
l SI LARCE fRANSFONM FAULl
-c'J-'
FISSURES ANO rsocIS
~ I
.--
I, I
III
SHE£! Fl OwS Pill OWS
-
~
III
- l ONE 1 VOLCANICS
[)
SMAll TfUIVSrOR.=':"='~"'~'Iii=!!i!...-'==;'::
DlCi[A Sl IN HYDtOTHlIlMA/ AWVITY, NCUArl IN DEPTH
,
<- LARCE TRANSFORM FAUlT
TOPOGRAPHIC HICH;
-
EN ECHELON f l!UPTlV[ FISSURES
Figure 4.5 M,,<Jd 1m an i,k alil cu accreting sc!:mclll uf '>CCJll ridge . , h"wing IUl'ugrJ l'luc . v" kan;c, tectonic anJ h)'dmtilc, m;,1 v"r i,lli<>n unJ ing 1, ;on,Iorm [,lllih (1\ ). ",,<.1 Ih..: prcd.ctcd ;u r•m gc mcn t of _mall scale U;llI,fwl1l [,' ulh and c n-cchckm c ru puv c flssurcs. Fm m Fr" nchc h:,' u and lJdlla roJ (I'nn )
nounccd axial rid ges and higher general e leva tion . In the kis t-s pread ing ridges . th e tcpogr uph y is co nsid e red to he th e co nsequen ce of isostati ca lly co m pe nsat ed ther ma l expansion d ue to a la rge ma gma cha mbe r. In the slo w-spreading ridges. the rifts result from viscous he ad loss in t he upwe lling mat erial ascendi ng thro ugh a na rr o w cond uit. The ho tspo t type of rid ge is associated with a thick c rust o ve rlying a thick m agm a cha mbe r a nd produces abnorma lly e levated topography, as see n fo r e xa m ple in the R eykjan es ridg e nea r Icel a nd . a nd in th e Azo res sec tio n o f th e mid A tlantic rid ge , wh ere ax ial pe ak s rea ch nearl y to se a-leve l. Th e rel ati o nship be tween rift ing and spre ading o r d iverge nce d irect io n is no t as simple as mig ht a t first be ass ume d. Theo ret icall y. e xten sion ma y occur ac ross a plane ma kin g an an gle with th e plat e d ive rge nce. or spre ad ing direc tion (see Figure 3. 14). H owe ve r . in practice . the a xia l rift fissures a ppe a r to be ge ne ra te d by pure ex te nsio n , wit h no co m po ne nt of la te ra l shea r. Moreove r. th e norma l fa ults in the a xia l re gion a re also ge ne ra ted by pure d ip-slip
mo tion . Th e ' nor ma l' pauem of a spre ad ing ridge will ther efore he a crenellat e o ne (se e Figure 4.6A ) with rift sec tio ns o ffse t by s ho rt t ra nsforms similar to th e structure fo und in th e I' A ~ O U S ar ea. O nce fo rme d . these tra nsforms will be of co nstant len gth if sp re ad ing is uni form on both sides . Ho we ve r t here is e vidence that ridges can e ithe r 's t ra ig hte n' o r form mor e exaggera te d cr e nella tion s with time , by asy mme tric spreading o n e ithe r side of th e axis, balanced by alt e rna tion s o f faster a nd slowe r spre ading ac ross th e tra nsfo rm s bo und ing e ac h sect ion ( Figure 4.6A ). Th is process is believed by Me na rd ( 19R4) to accou nt fo r th e c renellated C re taceous a no ma ly pa tte rn in th e Pacific . which is sa nd wiched be twee n e a rlie r a nd la te r st ra ight sec tio ns. If me am ou nt o f spre ading a lo ng ;1 rift va ries , a di ffe re nt type o f ste pped patt ern results. cbaructer izcd by wedge-sha pe d seg ments ( Figure 4.6 8 ). Me nard sh o ws how a ste pped ridge ma y e vo lve from a st raig ht o ne by alterna tion s of fast and slo w sectio ns ac ross the bo undary tran sfo rm s , combined with va ria tions be twe e n fast a nd slo w with in the segm en ts . Th is process e xpla ins
80
Ge OLOGICA L STRUcrU RI:':S AND Mo v I NG I' LAT ii S
A Slll
2
F .A S
has been subje cted to de ta ile d exa mina tio n. Icel a nd lies on the site o f (Wilson. 1963; Mo rgan. 1971) , bel
r
F ,AST
"' III (c. f oMa ee) T he cu rren tly a ctive rift run s thro ugh the middl e o f th e isla nd in a com plex patt ern
,
h ~"
U
r ,4ST
I
,
B
llik.t
ai~
( Figure 4.711 ). T he act ive r ift co incides with a lo ne of high hc ur flow a nd act ive highte m pe ra t ure ste a m fields. E;II.:h ne w sec tion o f rift a ppe a rs to have fo rm ed first as a fa ultbo unde d to pogra ph ic dep ressio n in which accum ulat e d thick se q ue nces (If volcaniclas tic se di me nts prior to the first fiss ure e ruptio ns. T he rift zo ne cha nges ori e nta tio n so uth o f t he ce ntra l tran sfo rm from N - $ (0 NE - SW. and agai n so ut h o f the so ut he rn tra nsfo rm to ENE WSW. Accordi ng to • this pattern e vo lved fro m a muc h simple r pat te rn ab o ut 4 .5 Ma ago ( Figure 4.7A ) whe n a sing le
",..""'~...,JoiI"i!!l"'lOl1~~il!;oll nI iI t he j unc-
• Fi~u r r 4 .6 Mod els illu- tranng lh..: cr Cll110n uf crcnCll;tICd a nd ste p ped ridge patterns hy asymmcmc spee..din g. (A) Evolut ion o r a c renelate ridge from a straight one by crCOI lio n an d le ngthe ning o f trans form la u l l ~ !>Cr ar at;ng seg ments of laste r a no slo we r !'f'fca ding. (II) Evo lution " f a s tc ppo..'
Fro m M el1ard ( IQIW)
ho w a change in plate diverge nce direc tio n ca n be acco mmoda te d . For e xa mple , th e postto Ma ste pped pa ttern o f the C a rlsber g ridge in the NW India n O cea n ( Figur e 3.6) is .. res po nse 10 a cha nge in relative mo ve me nt vecto r between the A frican an d Intitan plates tha t re sulte d in a discorda nce between the o ld a nd new se ts of magnetic stripes a nd tra nsfo r ms. an<
T his a rea is the only pa rt of th e cu rren tly active ocean ridge sys te m tha t is ex posed o n land a nd
non with the present ce nt ralt ra nsfor m. whe reupo n it cha nged di rec tio n to NE - SW. Spreading rat es a t th is time we re lo w. aho ut'" "'!R"• d ivergen t mo tion .
T he a xial rift o n the islan d is a 70 km -wide zo ne o f f lood bas al ts yo unge r than 0 .7 Ma . and has bee n stud ied in det a il hy Sae mundsson in th e no rth ern pa rt o f the island ( Fig ure 4.8 ). Th e curre ntly act ive hel t co nt a ins a series of fissu re swarms paralle l 10 the ge ne ra l rift orie nta tio n . togethe r with N - $ no rm a l faults a nd a number of cald e ras a nd ce ntr a l volcanoes. Th e o lde r basa lts form ing the fla nks of the rift a re ge nt ly tilt ed to wa rds the rift axis a t an gles o f 5- Itl". inc re asin g to steeper d ips (20-35°) nea r th e rift ma rgins. Fu rth e r west , the d ip changes to around 25°W ind ica ting a broad fle xure a long th e rift flan ks. Within th e s hari ENE- WSW sec tio n. o n the
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A de tailed a nalysis of the Irac t urcs in this so uthe rn area (Jeffe ris a nd
81
Dl V[ RG ENI (EX T ENS IONA l. ) TECTON I(" REGIMES
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Fi~urr 4.7 Ptare tectonic intcrp rcunion " f the Icel a nd flft ( Ao ) Reconstruction of the rifl of 45 M" Ul' before the e astwards , hifl Ilf the ce nt re! 'lq; m..:: n1. Ne \\ e,,~lem rift wn ..::~ Me hq :1I1I1il1£ to <J e\lcl
80
G J;:O LOG ICA L ST RUCTU RES AI' O
A
a
B
Fi ~u r ~ 4.6 Mmlch illll, U slo ", ;It \)rrnSiIC e nds o f e llch se g.uc nt , cleat ing wed geshape d segments; ridges e ve n tually become or th<>gu n,,1 III the transfo rm Iaults and 10 the new spf"ading directio n, Fro m Menard ( 11)"4 )
ho w a change in pla te dive rgence d irection can OC accommodated . For example. the post10 Ma stepped pat tern of the C arlsberg ridge in th e NW India n Ocean (Figure 3.0) is a response to a change in relative mo vem ent vec tor bet wee n the Af rican and Indian plates that result ed in a discor dan ce between the o ld and new se ts o f magnetic stripes and transforms.
M O V I N(~ I' LA T~S
has bee n subj ec ted to deta iled examination . Icelan d lies o n the site of "'!.... (Wilso n. 1963; Mor gan . 1971) a..... h_ _ _ COllI' mn fill (c.16M a liP) _fir. The cur rently active rift runs through the middl e o f the island in a co mplex patter n di I Wi (Fi gure 4."7 8 ). T he active rift co incides with a zo ne of high heat now an d active highte mperature stea m fields . Each new sec tion of rift appears to have fo rmed first as a fault" ho unded topographic de pression ill which accum ulated thick sequences of volca niclastic sed ime nts prior to the first fissu re eruptions. Th e rift zone changes or ient atio n sou t h of the ce nt ral t ransfor m fro m N- S to NE - SW , and again so uth o f the sout he rn tr ansfo rm to E NE WSW . Acco rdin g to . , this pa tte rn evolved from a much simpler patt ern abo ut 4 .5Ma ago (F igure 4.7A ) when a single "",u.di · lb _., until the ju nctio n with the present cent ral t ransform . whereupon it changed direction to NE -SW. Sp read ing rates at this time were low,
"'iM
1....AItO
T he axial rift on the islan d is a 7U km -wide zo ne of floo d basalts yo unger than U.7 Ma . an d has bee n stud ied in detail by Sacm undsson in the northern part o f the island ( Figure 4 .8) . T he curre ntly active belt contains a series of fiss ure swarms parallel to the ge ne ral rift orienta tion , together with N- S no rmal faults a nti a numbe r o f cald eras an d central volcanoes . T he older basalts formin g the flanks of the rift are ge ntly tilted towards the rift axis at an gles of 5 - 1U", increa sing io steeper dips (20- 35°) ncar the rift ma rgins. Furthe r west , the dip changes to aro und 2Y W ind icating a b road fl e xure along the rift flanks. Wit hin the shor t E NE- WSW sec tion. o n the
rise i"lmic:h,• • b.h:Mi'td"o
..........1-T his area is the on ly par t of the currently active oce an ridge syste m that is exposed on land and
oitw iPl'J'dPW",- A det ailed analysis of the fr ac-
tures in th is so uthe rn area ( Je ffe ris and
81
[)[ VE HC/ON T (EX T ENS IONAL) ', ECT O N I C HEGIM f.S
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t'il:ure 4.7 Plure tectonic in l<,;r pr<,;ta tio Jl of the lcctuml rif!. (A) Reeon~lrucliun "r the rif( of 4 5 M" UP befo re the e ast wards shifl of the cc m rul segme nt. Ne'" c u-ac ru rift zones arc I>eginning to de velop. mark ed hy llexlIr,,1 I ll lUg hs ""he r..: thid scdimcuts ar~' accumulatin g. (8) Present p.uu-m . afte r the sh if! was comple ted. T hl' new rifts were inill
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Figure 4.11 Structural map of NE Iceland showing position s of fissures . fissur e eruptions a nd central volcanoes within the act ive rift zo ne and the flexure o f the olde r basalts in the lIank ing regio ns. From Sucmundsso n ( 197<1)
D1VEflGENl (E Xl~.N SIONAL) I I.C I ON IC IlH ilM ES
Vo ight , I\)HI ) indica tes l WO ma in orien tatio ns . one NN E -SSW pa ra lle l to the IOC,1 ! tren d of dykes an d fa u lts, an d the ot her a b road E - W sys te m ra nging from 07(f 10 1:'100 in tre nd T hc NN E-SS W syste m is inte rpre ted as extensio nal fractures (i ndicat ing about (l.·n:, ex tension ) a rising from th e stress field respo nsib le for the nor ma l fa ulting a nd d y ke Injectio n . T he E- W set o ccu rs nc ar t he edges of the active zon e a nd indicates a reo rie n ta t ion of the active st ress fie ld , IS th e cr us t moves away fro m racrures a ru intC I"f'~l(,,(~ the acti ve zo nc. S: s the u.~ . u h o f hcrmcetnsn stresses u uc u. (JOling a mi indica t ex te nsion 0 1 the o rd r I"
'N Fuca l m ech anism so lutiolit';of ea rt hqua kes in this N E -SW zo ne g ive un ifo rm hor izo n ta l lea st ~l re.'''i .. xes o rie nted NW_S i . Since the pla te bound ar y here h as an E N E - WSW s tr ike , these e xte nsion al zo nes ar e e n-ec helon zo nes of norma l ex ten sion dc f ning an obliq ue s pread ing ridge on th e rcgiona! scale . North and south o f this section , th e sp re adi ng d irec tion coinc ides with the di rection of extensi o n in the normal way .
4._ Con tinen ta l rir s The clas sificatio n a nd o rigin o f co nti ne nta l rifts, has bee n a subject of gre at debate a nd co ntro ve rsy for m;m y d ecades . A n um be r of symposia ha ve b een d e vot ed to the topic in recent years (c. g. Ne u mann and Ra mbe rg , 1978; l ilies , lIJRI ; Pubnaso n. 19X2; a nd Mo rgan and Bake r , IIJH3). Some we ll-kn o wn rift systems, for exam p le the fri can- Red Se " .a.Gulf c t den systcm. I h ~ Hhine Ruhr SYS(CI I Gslo graben . h Haik ul ft , an d t N.io G rande ri f h ave bee n st ud ied in great det ail. A p rima ry classifica tio n of cu rre ntly ac tive rifts wo uld a tte m pt to s ub d ivide the m into usu ucnv pial bou those repr ese nting danes , and t . " Ihal a intraplat Ho we ve r this subd ivision is not as clcn rcut as it seems. since many a ppare ntly in traplate rifts h ave been inter p rete d as resu lt ing d ire ct ly o r indirectly fro m pl a te bo u nda ry p ro ce sse s , a nd some are d irect ly con nec te d with plate boun -
dar ics a ltho ugh t he y do 11 01 themselves fo rm plate boundaries . Certain of t he la tter ty pe o f con tinenta l rif ts prov ide goo d evi d e nce as to the nat u re o f the p ro ce sse s of in it ia tio n of construc tive bo u ndar ies in co nrinentn l litho sphe re . iee (i ulf I d e rift ·s loJ I ~l l l a.:li ":tlnslH ~ t i\/cltr ans forlll pial u nuary wh ich Iin kli with th e Indian O ce a ;JKI: YlllClll cl:UU plov ide;:s H m odern a nalogue -.,1 h lit sys te m wluct es uhed in th b rea k'. 'u p uf l)ilO!!aca. T h is rift sys te m is d iscusse d in d ct ui l hclnw. Exa m ina tion of the pa ssi ve co ntinenta l ma rg ins of t he A tlan tic a nd Ind ia n Oceans prov ides su ppo rting evi de nce for th is process . Equally clea rly, m,my ap pa ren tl y in tr a pla te rifts ha ve existed ove r lo ng pe riod s o f geolog ica l lime without o pe ning to fo rm ocea ns, T he Bai ka l a nd Africa n rifts arc e xamples o f such structures tha t a re cu rren tly aelive . 1e BailLiI rift tJri!iina ll'l ! in ('rl' I,Ken us IIIn \: . ( Logatc hcv et IIf. • liJ7x a nd Ihe E ilst Afric a n n fls n the 1illl:cn (Ba ke r /'1 al., 1( 72). P reS("n1ly ill" nih.li... ift !!ouch ell th ' r.~ lilian O slo gra ben lid th e- Nort h Se ; b asin eibvio usly ne ve r mad e the tran sit ion to constructive bo unda ries , ;11thn ugh appreciable e xte ns ion to ok place . T he .fuj/~tJ arm; )f rift tr iple j u nctio ns a rc au importa n t class of rifl lirst reco g nized by tiurk a nd @ ( llJ73), w ho sugges ted tha t enn rin ntal lin"n t oo rlac h_ the joining tUltelhe;: 0 p ai rs { rifts from eujacen triple J U lkOl lUi lS l O l in ~n l i n uuuli bu t irreguta . I - l r ~ l i\/ bn un d a r,- ( Figur e 4 .9). The rifts tha i we re no t d e vel oped in the ev e ntu a l sp reading be came ' fai led a rm ' g rabe n, o r IIltu'ugell. (Shatsky. 1\)55). ~ e h rifts . a ltho ug h structu ra lly a nd genet ica lly li nked to the pl ate bou nd a ry . a rc no t p art o f it. A geed e xa m p le 01 u ;lluIIH.'t' ge n i th e Benue trough in w est lri C3 1 Fig ure 4. 11: Burke an d Dewe y, IlJ73) . The Nige r triple ju ncti o n deve lope d in th e C re taceous pr io r to the opening of th e So ut h Atla nti c . Th e ot he r two r ift s d e ve lop ed into the A tla n tic Ocea n whil e th e Bc nue t rou gh beca me a fa iled arm . St ruc tu ra lly. a rift is essen tially an el o ngat e do w nfaultcd b loc k or gra ben . Ho we ve r , majo r
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84
GE01.0G ICAL STlW CrU IU,S AND \l OVING 1' 1.1\ 1lOS
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rifts co mpr ise eu mplc .·l cm~ u e ten. ion." {Hulk. . and exhibil 'ismi eotceni.. a nd di m ClIlar ylc aturc tha t a rc dir ect ly re lated 10 the prima ry ex tension a l st ruc ture. Ho rizon tal e xte ns io n is th e ref o re fun dame nta l to all rifts . a lt ho ugh in ma ny cas es the am o unt o f e xte nsion ma y be q uite sma ll {arou nd IO'X, fo r t he Rhi ne a nd Bai ka l rifts fo r exa mple ). AU ritt. t:Xhibi an om aluu s cr usta e and 1JPPC ni ~n,l l!l plOfliCtl. usually inte rpret ed as the re sult of cr ustal and/or lithos phe re thinning a nd e xte nsio n , o r asthcnosph cr ic d ia pirism. Th e geophysica l c baructcrisrics ar c ve ry sim ila r to
those o f oc e a n rid ges and are explai ned in te rms of a re gio n of low- de nsity ma ntle mat e ria l, whic h has been te rmed the iJln , co rre la ting with high heat How . T his pillow unde rlies t he r ift an d o fte n su pports a n uplifted pla te a u o r do me in the nan king regio ns. Associa te d vulca nici ty is highly va riable bo th in ty pe a nd amoun t . So me rifts e xhibit ve ry lill ie vulcan icity while in o t her s vo lcan ic roc ks a rc a bunda nt. Ma~mati:.lU is tj'picull Pt· mudal wuh ooth a lkali basalt h olit • nl!
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In ma ny rifts . the vo lcan ic ro cks
DIV HUiEKI ( L:X r EN SION A L ) I ECl Ol" lC Io:LGI MLS '" IS; 19b ' . t,; Ot W ;ly trulII t he-rilt i: The re is usuall y a com p lex se q ue nce o f e vents inv olving sc vc rul episo d e s o f fault ing and vulca nism . Many riflS m e asso eialcJ wit h IHo"J JOIn ,,1 uplift s as sugges ted hy the Bu rk e a nd Dewe y model , h ut the re a rc arg u men ts as to wh ether these ar c ,I con se que nce or ,I cause of the rifling , o r whe the r both ar c as pec ts of a more Iund ar ncnta l J n vlllg me cha n ism . A ccord ing to Bur k • ,.IUd D ewey ( 1"J7.'ij. It£b;ureini tialc o 0CI0 a ro ult of m an uc pl umes u tliapi r.t. wh ich produce bo th th e d om es and the vulca nicity . In lhis model , th e rifts arc a co nse q ue nce of t he initial m a nt le upwelling. Suc h r ifts ha ve be e n termed ·IIl'lIItlc"m;tiv'llec.l by . mdic tl( 19H2) . Howeve r m,11l Y rills do nOI lit this mo d e l. A n alternativ e lI10J C of origin is w here the ri ft IS prod uced ent irely , IS a result of lithospher e extension. The con di tio ns neces sary for complete c xtcn sionu! fai lu re o f th e litho sp he re were d isc usse d in 2.7 . lt appe ;1fs nuu such conditio ns C01l1 re ad ily be me t . panicufu rl y a t times of UIl U.Sl 1011 plate contig umuons wh en continental in traplat e ex tensiona l stress would he ill a m axi mum . Su ch a pe riod mu st hav e preceded the b rea k-u p of l'un gacu . whe n that supercontine nt was surro unded h y subduct ion zones . thu s m ax imizi ng the po tenti a! effec t o f the subducti on -suction for ce (sec 2.5 ). E xtra mantle heat source s ma y no t have been ne cessary for failure to occ ur , a lthough fail u re would pre ferentiall y o ccu r in wa rmer , yo u nger orogenic crust rathe r t ha n in th e co o le r sh iel d are as. Rif ts o riginati ng in this way arc termed , · thus p hC t C-al: l i v ll l c d '~hy Cond ie ( 1\)1';2 ). Clear ly such rifts . o nce formed, will produce lithosphere thinning and volcanism a nd rmg fu • . )111 .. l\:
ev olv e in to o ceans if the stress co nd itio ns were app rop riat e (sec 2.7 ). T hese IWO fu ndamentall y diffe rent types of rift may he distin guished most e asily in the ir initia l stages . Plumc-gcucrarcd rifts ought to co m m e nce w it h doming , a nd sho u ld e xhib it abundant vulcani cit y from the earlies t stage s . Str ess-gen erat ed rift s , on the o ther hand , shou ld comm en ce w it h grabe n a nd basi ns o f scdirncntarion . and de velo p vulcan icit y a t a later stage ( Figu re 4. 10). Some authors have used the terms uno . to d e sc ribe the mannc-gc nc rare d and lithosphe rege nerated type s respectively (Scngor a nd Bu rke , 197H). H o we ve r , this terminology be gs the q ue stio n o f wh ich mechanism is m o re ' ac tive ', an d is the refore con fusi ng . F urt he rmore , the classificat io n cut s across th e more w id ely used subdivision between c u rre nt ly act ive and c urren tly inactive l1.r ' fossil' rifts t ha i have be en activ e in the pa st. ~) me acti ve rifts fall e as ily into one_0!:Jlthe r_ of thcsc cat.:gmic s . T he I ~ a s t A fr ican rift sys te m for exam ple , with its ahundnn t vu lcanicit y . a lt ho ugh e xhib iting extensional stress loca lly , e x i ~ l s within a con ti nenta l plate in a sta te of gene ra l compression (sec Figur e 2.2 1). At first s igh t. therefore . this rift is a good ca ndid ate fo r the mantl e-generated ca te gory. Ho wc ve r , " ..; we s ha ll se c bel o w. the E ast African rif1 IS connected viii the Afar triple junct io n with th e Red Sea -Gu lf o f Aden con struct ive plate bo unda ry . a nd th e whole netw or k need s 10 he co nsid ered in ilS e ntir ety . Moreo ve r , t he sta le of st ress al the time of initi a tio n of th e rift syst em may be ver y diff er e nt 10 t he p re se nt o ne. Mantle -ac tivated rifting requires either sub -
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li thospheric thin nin g (rcr osio n') or the emplaceme nt of an astbc nosphc ric d iapir within the lithospher e, preferabl y spreading alon g the base of th e crust for maxim um ef fect ( Figur e 4. lOh ) . It has been argued that th e diu pir
mech anism
IS
more efficient an d th at the
th inn ing mechanism involves too long a time-
scale (Tu rcotte and Em crman , 1983; Marc schal, 19R3). Neu gebauer (1983) has sum ma rized th e stages in the dev e lopment of a rift generated by suc h a mec ha nism. Th e initiation of the rift requ ire s a sma ll pe rtu rba tion in the inve rted de nsity inte rface at the base o f the litho sphe re . T his per tu rba tion cau ses a sur face uplift w ith a wavelength o f around 500 km for reason able valu es of cr ustal a nd astbeno sphc ric viscosit ies. When th e amplit ude of the pe rt urbat ion s rea ches a critica l level , th e o ve rlying layers a re defo rmed a nd the c rus t is thinned . At this stagc , th e evolutio n o f th e rift becomes more rapid . T he diupi r, no w fo rme d , rises at its maxim um rat e (c.5 km / Ma ) a nd is acco mpa nied by mo re rap id a nd exte nsive crusta l thinni ng a nd hig h-le vel rifting. Volc anic ac tivity ma y no w co mme nce . Eve ntua lly the diupir loses its . buoya ncy d ue to coo ling, when the supply of ne w low -d e nsity m at erial dimi nish es. A t thi s stage t he influence o f the ma ss exc ess abo ve the dia pir , ca used by the volume of so lidified volc a nics , pre vail s, ca using ba sin subside nce a nd a ces sation of vulcanism. The Baikal and hine rift ha ve bee eoellicJereU.lililil active exam ples of th lithmphe regenerated I YfJC'I The y ar e int ra plate , exhibi t o nly min or vu lca nic ity and arc assoc iated in the ir e a rly stag es with sed ime nt ary basins ( Loga rcbcv et at., 1978 ; lilies , 1978) . Th e state of stress with in the Eurasian plate at th e tim e of initiat ion of thes e rifts ca n on ly he guesse d at. but it is not unre aso na ble to envisa ge a sta te of ge ne ra l E - W ex te nsio n that woul d be re placed in the we"! by co mpress io n when th e No rt h At lant ic ridge bro ke th ro ugh between Br itai n a nd Gree nla nd in t he e a rly Ge noeoie. These rifts the re fo re co uld possib ly be asc ribe d to stress ge ne ra tion, since the y we re initiat ed in late Mesozo ic ( Ba ika l) to ea rly Ce no zoi c time ( Rhine ). H o we ver se ve ra l
a ut ho rs a rgue for sp ecia l kine ma t ic facto rs in th e initia tio n o f th ese rifts."Ur -onsidc rs tha t . 1 onh u . m pr~iu re sul ting fro m the A lpin e co llision W
54 ) T wo e xam ples of c urre nt ly activ e intra -con tinen ta l rift syste ms will no w be e xamine d in more deta il: the Rhine an d Afr o-Ara bian rift syste ms. T hes e e xhibit con trast ing prope rt ies in so me respects but nei ther ca n be co nvincin gly classified into e ithe r ma ntle-ge nera ted o r
D IVl:!l (; I'N I (E X H. NSION,\ L ) IIOCro NK" 1l:~;GIME S
md.Jut uilldi. uplift- tha i h ave bee n lllaJor eem res of alk.dinc vulcmirily s ince the Minta -ecu EJhiupi_. n. • an d I>;'trhl fl re spcc lively , Northwe st o f Da rfur a rc tw o other ce ntres, at Ti bcsu and Hogga r. that do not a ppe a r to he dir ectl y lin ked wi th the ma in nft ne twork. A ccor d ing to Fai rhead (1976 ) , t he Eth iopian and Kenya dcma l upl ifts a rc th e focal point s fur the vo lca n ism of the rift syst e m . It IS in these a rea s th at t he lit ho sp he re has und ergone the g rea tes t amount of th in nill g. G eoph ysica l ev ide nce suggests th at with III th e dom al up lifts , the cr ust a wa y fro III th e rift s is of norma! th ickne ss hut is u nd erl a in by ho t . low -de nsity man tle ma te rial with an ornalou sly low se ismic ve loci ties wi th in (he upper
lithosphere- gen erated typ e s . In this , , IS in oth er respe cts , they ca n probably be re garded :JS fairly typ ical.
'*
T he we ll-known rifts of East Africa arc pa rt of a much larger re gion a l sys tem tha t e xte nds across Cent ral Africa to th e west to link u p with the A tlantic O cea n on one side, and e mb races (he Red Sea -G u lf of Aden plat e boundary on th e o the r ( F igur e 4.11). T o the south . th e two ma in brunches 0 1 me EaS! African sys te m join an d co n tin ue southward s to mee t th e In d ia n O cean at Bcira . in Moza mbiquc . Associate d with these rift s ale thr ee
,
,
Fil:ur c 4.t l M"in d ""' "nl s "f I n" A fm A r"h,an r i ft ~y~le m . liT, Bellu" 1«lugh. NI C Ng""" n<.k re : A G , All u G ahl" , W I~ , wc!'> le rn
E. A lrtc.m . EIC eas te rn Eo
"
X7
Arrie-m ; G A.
G ull " f A<J ~" . RS , R~'d S<:" ;md GS , G ulf <>1 SU"l rills. f' l '1ll<J 1'2 arc the f'" k s "r ''I t,cIUH1 for the A rJh lJIl INlIbi"l1 '1I1 <J Ar " bi'luIS"ln,!i· ia n pl.uc mov ements rcxpccuvcty . Alter Girdtcr ;md D,,, r'lcn H ( t972)
•
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88
G I:O LOGlCAL ST RL:cn'; RfS AN I) MO VING I'I.i\ l ES
50 kill o f the mant ic (Gass et a!., 1978 ) , indica ling tha t as the nospheric mate rial has bee n e m place d
abou t I OOOkm in length " long i ts maj or axis par allel 10 the rift . It had a maximum heig ht of 1400 m d uring the l ate Pliocene to m id-
Plci-aoccn c . T he maj o r faults of th e Ke nya rift zone (Figure 4 . 12A ) defi ne a co mplex branch ing graben str uctu re wit h an ove rall N - S tre nd . al thou gh individual faults a nd g ra be n segme nts ge ne ra lly strike N NW- SSE or NNE-SSW . T he well-de fined cen tral gra be n (the 'G regory rift') rr.rve rses the e llip tica l uplift and , al its nor the rn an d so uthe rn e nds, is re place d h y less
The Kenya OT E U.I//: '-" Rift The st ruc tu re of this rift is su mmarized by Baker and w ohlc nberg (197 1). T he r ift exle nd s fro m T a nz a nia, whe re it joins the Weste rn Rift . to the Re d Sea ,H the A fa r triple j unction in Ethi op ia . It c rosses the Ke nya d om a l u plift. which is e llip tica l in pla n a nd
ETHIOPIA
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B Fi~u r~ 4 .1 ~ ( II ) Frn: ·,I IJ .lIld Il""gua !lr " vil y ; " h'lI1>ll ~ 1,,', ' lilc~ ;,( h,." I II( (;,~IC rn E. A fr ic"n n i l in K,'n y" . l"gClhc r wllh an IlIlc rl' rl' I"I IV,· m"dd " l\ i,r~l ng l h,' gruvuy \1' <.ic' Il, ily iUll1l l'oi ,'ld y IInde'f lying thl' ritt . From l).IIr ao ,11 ctal, ( J')7.~)
well-defined broad de pression s ma r ked hy splay.:£:mlt. The 111<1111 grab en is ClO - 70 km wide and 7511 km long, und is bounded hy norma l faults a rranged in a n e n-cc bcio u IM UCH Be tween the e nds of udjuc cut en -echelon fault s. sloping ram ps de scend (rom the mar ginal plateau x to the rilt 110m. Major fault escarpments a rc preserved . runging up 10 2OtIO m ill tM:;~ht . II '" eslllua ic u trom Ih ~ hi,,'kncl'o , u' the ri tt till th at th e to tul tlUUWli un th e m:l.inrll fault!'iO rnR. N.'- _ km . T he 110m of the graben is cut by abundant young minor faul ts subparallel to the gra be n wa lls. Th e ea rliest rift -related structure is a mid Cenozoic (pre -Miocene) monoclina l fle xure along the western rift tlank . D ur ing the Mio cene, e xte nsive vulcanicity occu rred . and the
first ex te nsive rift fau lts deve loped In the e a rly Plioce ne . Voluminous vulcani cit y. of prcdominarcl y alkaline affinity. c har acterized the period from the Plioce ne to the prese nt day. accompani ed by periodic fau lt move ments. At lea st 5 km or c rusta l extensio n is required to ex pla in the displacem e nts on the visib le fault s. and th is figur e should perhaps he doubled to tak e acco unt o f the co ncealed fau lts bcnc.uh the graben floor. Th e a mount of e xte nsio n IS too grea t to he e xplai ned as the result of crustal dom ing alone . Th rift dO!lcly folloW!!: in c -axis 0 1'3 re ~i() nil " ncgauv iluu~uer Cl lloma l ~ ( Figure 4 .128) interpre ted ot:). a ,I:d U -thin Inhos ph cr T he gra vity data also sugges t the prese nce of a narrow shallow cru stal body of dense mat erial
or
90
GEOL OGIC A L STRUCTU RES A N O MovI NG I' LATES
thought to re prese nt a wedge-shaped basic intr usive mass don ut to k ill wid e a t its to p ,
reaching to
J 500
m below sea -level. Focal
me chanism so lu tio ns of e a rthqua kes a lo ng the r ift (sec Figure 2.22) indicate mainly W N W ESE to N W -SE ext e nsion . If we assume IO k m o f crus ta l ext ension ac ross the Ke nya rift since the Mioce ne , this re prese nts a su ai n-nue of about 1O - 1.~/s . Fur ther north , 111 Ethiopia . the rift sys te m wid e ns and the a ppare nt rate o f e xte nsio n is 3 S r nm/yca r. co rres po nd ing to a stra in-ra te of about J x JO - 14/s across the 25 km-wide zon e (Tr yggvason , 19H2). T his differe nce suggests an i ncreasing rut e o f widen i ng to wards the
triple junction. The prog ressivel y more a lka line tren d of the vulc ani cit y fro m Ethio p ia 10 Kc nya sugge st s tha i tec tonic wideni ng ha s
p rog re sse d so u th wa rds ( Mohr , 19H2). T he East African ri ft is linked across Centra l Africa to two other doma l upl ifts, a t Darfur and A damaouu . h y means o f two rifts: the N W - SE Abu Gabru rift and th e NE -SW : Ngaoundcrc rift (Fi gu re 4. 11: Browne an d Fai rhc ad, 19H3) . The latte r pa ra llel s and is lin ked with the Bc n uc trough , wh ich , accord ing to Bur ke an d Dew e y (1973) , fo rms t he failed arm of a t rip le junction with the p ro to A tlantic ri ft. T he Ngaounde re and Abu Gabra riffs arc subsid ing se d ime nt- filled trou gh s which have be en active s ince the Cretaceous. T he sou th-
we st e nd o f th e Ng uo un dc rc rift crosses base ment ov e r the Adamaoua uplift , wh ere th e ba se me n t struc tu re I S see n to be. a dextral sh e ar zo ne o f rim-African age ( De Almeida a nd B lac k , 1967 ) . Thi s zo ne ca n be. trace d in to Brazil as the Pcm umb uca line a me n t. T h is Preca m b ria n st ructure was rea ctivat e d in the Lo we r C re taceous du ring the initial sepa ra tion o f Sout h A me rica fro m Africa . Depositi on o cc u rred u ntil SOMa ago wh e n it wa s te rmi nat e d by compres sive foldin g. T he line a me nt was re -a ctivated agai n du ring the Cenozo ic, whe n the basemen t uplifts we re fo rm ed with thei r associa ted vulcanic ity . Faults pa ra llel to the N gaoundc re rift show dex tral move ments wit h a to tal d isplacemen t of 40 km . The N W - SE Abu G a b ra rift consis ts of a t Su km-widc trough filled with C re tace o us and Cenozoic sed ime n t... , loca lly mo re t ha n 4500 m thi ck . F au lting is pa rallel 10 th e trend of th e rift , wh ich is co nsid ered to he p u rely c xrc nSiOlMI T he Cenozoi c tectonic movements on th e Ce ntra l Af rica n rift sys te m
Fil:urc 4. 1.' Ar r"n geln cn l o f rift ~ ,mJ volc ani c <.Jo rnes In w est and Ce ntra l Alriea . T he "no w indic ate s the infer red drrccnon of o pe n ing of the Abu G ah ra rif! and rd atcd <.J e xlral shear ,do ng. the Ngaoundcrc rift. Aft er Browne '"10.1 Fnirh cnd ( I 'I~J) _
•
,,'
9]
D IVE IlG EN I (EX n:NSION flL) ( f C"I O NI C IlEGI M ES
T h C ulf 0 ~ Icn ( Fi gure 4.14) i . lm rw~ til oce an! nl!\\ , and co mme nced openin g 10Ma ago in th hu e·M iocen8 ( La ugh ton a ol., 1 ~ 70 ) . Ab out 260 krn of co ntine ntal sepa ration has ta ken place at an average rate o f about 2c m/yea r about a pole situated in NE Africa (Figure 4. 11). Thi s rift re prese nts an extension of the Carlsberg ridge In the NW Ind ian Occun discussed in 3. 1. T he Red Sea rift is also r too rcd by ocean ic crust (G ird ler, 1(69) and the magne tic uno maly patt er n ind icat es an ave rage divergence rate of 2 cm/ycar for t he last 3- 4 Ma. Both the G ulf of Aden :JI1 d the Red Sea have undergone sub side nce and volca nism since the Cretaceo us.
Angclier ( 1985) pro vides a d etailed st ruct ural analysis of the G ulf of Suez seg me nt of the Red Sea rift syste m ( Figure 4. 11). T his area lies at
the no rthw est en d o f the rift be tween the Mediterranean Sea in the north and the G ulf of Aqa ba -Dead Sea (tra nsfor m) rift in the so ut h. Th e mai n structures trend NW - SE par allel to the ori entation of the rift , and to the ax is of the rn.un Red Sea rift . The flanks o f the rift co nsist of blocks tilted at 5-35° away from the rift axis, and bou nde d by large no rmal faults ma rk ing the margins o f the main gra be n ( Figure 4 .15). T here is no evide nce of the rift prior to the Oligocene . Nor is there (lily ev ide nce o f pre-rift doming in the plate aux bord er ing rhe rift . Immediately pre-rift str at a o f Eocene age are widespr ead III the axial zo ne . Faull movements and limn ed tilting ,Ippea r to have tak en place throu gho ut the Miocen e and co ntinue to the present . Analysis of the late Cen ozoic fault geo met ry indic ates a main set of fau lts with a mean or ie ntat ion of 135J4()0 with minor se ts at mmo. 120° an d 155°. Fa ult disptaccm cm s arc predominantl y norma l dip -slip WIth a small dextr al compon ent. Frac -
ARA BIA N PL A TE
I
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, 50°'
t·igurt 4.14 Ma gnetic a no maly patter n ill the G ulf o r Ade n sho wing offsets alo ng tr a nsform Iaulrs pa f
92
G EO l.OGICA I. Slll.UCrU RlS AND MOVING l'I .r\ f f.S
t
f,
f,
f,
Fi li;urc 4. 15
c
Structur e of the G u l f 01 S U(l lifl. Ulo.;k ui"l;r" m, 11 - /1 show Ih e c ~ol u l " lI1 o t ;' 'Y ';[ CIll or lilled h h,eks
aCC"OlO m " u ;lting to gr;,u u
lures appear 10 have deve lop ed perpend icular to bedding by pure ext ens ion an d [ 0 have bee n Totaled duri ng block tilring , whe n {he di p-slip moveme nts loo k place (Figure 4. 15). T he fau lt geo me try ind icates an e xte nsio nal hor izontal stress o rie nte d a t 045°. T he amount of e xton-
sion is esti mated ,II 20-30% . Estimates based o n the a mount of subs idence (d. Mcken zie , 197R) yield highe r va lues o f 45- 50'Yo ove r the e ntire width of the 80 kill rift secti on . It is likely tha t the fault reconstruction method unde restima tes the e xte nsio n.
[)! \ 'FlH. I r-. I ( I.Xlf. NS 10 NA I ) IrC IO:-IIC REGI M f.S ' \ , II/
Th e Af'ro-A r.rhinn nft system consists o f
93
In th e a rgu men t as to wh et he r the rift sys te m is lit ho sph e re - or stress -generated . il is impo rtout to o bse rve t hat th e ba<;is o f the e xte nsio na l ne two rk wa s in exi st en ce in the Cretaceo us. be fo re th e maj or volcanic cen tres we re fo rm e d U ndo ub te d ly in Ea st Africa th e rift is ge ne tic a lly re late d to , a nd pos t-dates. the vo lca nic do me. T h is rift milY t hus he sa id to be tit bo-phc re-gc ncr ated. Ho we ver in the co nte xt of th e ne twor k as a wh o le , th e location o f the dom ul up lifts is likel y to he controlled by the ear lier , pr oba bly stress-gene rated . zone s o f fa ilu re a nd wea kn ess .
J '
fWI
ir
The R hine rift sys te m ( F igu re 4 . ]()) is o ne o f the be st -kno w n a nd inte nsive ly studied e xarnplcs o f cu rre n tly Clive ext en 10n <1 1 rift . A s tudy by !llit.."S a nd G re inc (] 97H) sum ma rizes the tecto nic evo lut io n of the Rhine gra be n an d rw .llc !II il h II ' de vel opment o f h Al flint' "lruJ!l' n . Te cton ic act ivity in the R hin e sys te m "ppe ar s to ha ve commence d in t he Uti e 'a~lu hum ' I M a • g , wh ich coi nci de d with th e in it ia tio n o f co mp rcssiouul dcf or rnution in t he A lps . Riflin ~ n mh;u h",iJ cm.'e: 1 u .... ..~ JUI-in101o Iii Eocen !nO Oligocene us th ~ Ipj n
'um rrc~,i\ll lal dcrurm aIJUI~onlin ue d-,
rc nchin g its cl ima x at the Eoccn e - Dligoccne bou nda ry . In t he mid- M iocene 10 e a rly Plio cene , e xte nsion a l ac tivity app ears to ha ve decrea sed as the A lpi ne co nve rge nce slo we d down . to be replaced b y re gio na l epei roge nic u p lift. In id -PIi n
l'I.A I ES
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D1V[ KG ENI ( EXH ; NS IO NA l. ) -11::( T O N I C K I::GI M ES
uon (Figu re 4 .l oH ) ind ica tes a ge ne ra l NW SE -L1irecte d 0 1 both in t he Alps a nd across the Rhine rift. T h is di rection is ob liq ue to th e rift , which tre nd s ar OJ 5° in its sou the rn sec to r. an d is dea rly res po nsib le fo r th e current si nis tra l mov em en ts.
The fau lt pa ttern is co m p lex ( Figu re 4. 16A1. Most of the ac uvc fa ult s in the g ra ben st ri ke abou t 170"'. A rel a tivel y si m ple o rthogo na l se t o f NW -SE a nd NE -S W no rma l fau lts o u tside the rift cha nge to a co mple x patt ern o f b ran ching fault s , cu rv ing fro m NE - SW , pa rall e l 10 the rift , into a N -S tren d , ind icat ing a sinistral s hea r com ponen t. Dip-slip d isplaceme n ts occu r o n bo th syn t hetic and anti thetic fault sets. indica ti ng ex te nsion in a n EN E- wsw direction co ns iste nt with th e trend of (J I ' At runny localities, for me r d ip-slip fa ult s p ar a lle l to the grabe n have be e n o ve rp ri nt ed by ho rizorual suckcnsid c st ri
95
crus t (' .24 km) with in th e graben . T he d ee p struc tu re . toge th er with gr avity an d he a t-flow data , arc inte rp reted as a n ex p ressio n o f a mantle diapir whi ch fi rst pr oduced th e e ar ly volca nic p hase of t he syste m, beginning about IOO Ma lIP, a nd was foll ow e d by t he te cto n ic effec ts assoc ia ted with the extension . Kusznir and Park ( 19H4) ha ve d isc ussed t he Initia tion of t he R hine grubc n in terms o f t he exte nsional strengt h of t he lit ho sphere d u ring the initi ation o f the rift. Using t he ir mat he mat ica t mo del , a nd the heat Ilow of 73 m W m - 2 me asu red for the rift think s (se e Table 2.2) the y calcula te th a t a n c xrcnsio na t stress 0 1" c. IOMPa wo u ld he req uir ed 10 in itiate th e rifting . The origin o f the e xte ns ion is lik e ly to be relat ed 10 t he ge neral la te C rctacco us -. Palaeoc ene e xte ns ion precedin g the break -up of La ur asia (d. Bolt and Kus z nir . 1984) a t a lim e wh e n th e su percon tine nt was sub jecte d to extensional subduct ion-suct io n fo rce s o n al l sides (sec F igure 2. 15) . Th e effect of th e N -S Al pine co llisio n is sugges ted hy Wics and G reiner as a n impo rtant fact o r in the initia tio n of t he rift T h is e ve n t , ho weve r , is mu ch la ter (in Eoce ne-Oligoce ne tim e) tha n t he initi at io n o f the c rusta l e xte ns ion , but ma y hav e contr ibut cd 10 the in iu ano n o f the ac tual rift
featu re . . '( h.' n.~ lU lI al pru,·in cl,'s OIl ("ullvc rgt'n
htmnda.-jC'S
It ma y see m at first paradox ica l th at e xte nsio nal reg imes shou ld be associa ted with co nve rge nt p lat e bou nd ar ies . J lo wc vc r we h a ve seen (sec 2.5) that the effect o f the su bd uct io n process. unde r ce rtain circ ums tances, is to e xert a te nsio nal st ress on the pla le s o n eit h e r side o f the su b duct io n zo ne , as first suggeste d by bts (-IV7" . E xte nsio na l tecto n ic reg imes fo rmed in thi s Wil y arc found on bo th co n ti ne n tal and o ce a n ic c rust o n the u pper s lab of t he su bd uc t ion zon e . II has been suggeste d (see 5 . 1) th at th e a ng le o f s ub duction is im po rta nt in dete rmining the sta te of stress in the up pe r sla b, an d tha t sha llow-d ip pi ng slabs a re assoc ia ted with co m-
G EOLOGICAL STlWCTUR ES AJ'.;{)
pressional . an d Sl ee p with ex te nsional . stress. II is im po rta nt [0 remem be r tha t the upper slabs of su bd uc tion zones a rc th e sites of en ha nce d he at no w assoc iate d with the p rodu ctio n o f volcanic a rcs, an d th,tl much sma ller e xtonsiona ! stresses will be re q uired to p ro d uce litho sp here fail u re tn the up pe r slab u nd e r th ese con ditio ns T hu s, althou gh the c x tc nsien a! st ress gene rated by the subductio n p rocess is a pplied eq ua lly to bo th plat es , on ly the the rmally wea ke ned u p pe r p lat e will fa il T he UlOl>t int e nsivel y tudic e xa mr l nf" conl ill"ntal extensional-tectoni re gime rel ated ('f c uvc rgem p ial bo unda i Indlll lhtedl th Uasin.oa nd -Kan!! rovinc C1 ' lh . l ~'liS wh ich we sha ll now e xa mine In so me
det ail It'
fin.HI/ -mll
- Ufli l
'c' uovince
In the so uth , th is e xtensio nal tect onic province (Figu re s 4.17, 4. 18) lies immed ia tel y cast o f th e S a n A nd rea s fa ul t, wh ich mar ks the A meri can p la te bou nd a ry. In nor th er n Cali fo rnia a nd O re go n , th e province-ties c ast o f the Casc ades vol can ic arc. T he Ca scad es cha in is re lated 10 the subduction zone marki ng the bo unda ry be tween th e J ua n d e Fu ca a nd A merica n p lat e s in the north . The prov ince is abo u t 1000k m wide a t its ma ximu m in the no rt h , b u t narro ws so ut hw a rds as it bends aro und the m ar gin of the Colorado plat eau 10 lin k with the Ri o G ran de rif t. T he p ro vince is char acte r ized by t10llual hltll lin!!! . ismicilf \ h i~h helll lIow (c. 9OmWm - 2 (sec 2.3) a nd II high rcg ion a P'
elcvau on e T he mode rn extens iona l str uc tu re is s u mma rizcd by Zoqba ck, r t a ( JIJM l . lt co nsists o f a line a r topograph y of elo nga te ra nges se pa ra ted by basins filled with Cenozoic an d Q u ate rnary te rre stria l sedi me nts. The ra nge b loc ks a rc sp aced abou t 25-35 km apa rt from c res t to crest, with in te rve ning basi ns lO-20k m in wid th. T hus horsts a nd g ra ben a rc o f a p pr o xima te ly eq ua l dim en sion s. T his b loc k struc tu re is co ntro lled by no rm al fa ult ing in res ponse to a stress field with a E~ minlOlll
horizontal stress us .nmcateds 'J earthquak
novu-o
"' LATIO S
tutin n - nd · n~i ",...., (Zoback and ZOb'ICk, 19RO) . A lignm e nt o f volc a nic feeder d ykes and de taile d fa ult slip d ata indica te that t he pre se nt s tress sys te m was a lso cb.uactcnstic of the la te Cenozoic ( Figure 4. 178 ) . T his stress field a pp ea rs to be fai rly consta n t thro ugh o ut the no rt he rn a nd so u the rn sec tors of the provi nce , an d co ntin ues into the Rio G rande rift. Zoback et ul, (198 1) suggest th a t this mode rn extensio na l struc tu re d eve lo ped ea rlier (c.]J M a Ill' ) in the sou th tha n in the north (L IO Mu 111') . A co mpa rison of the stress orie ntation data with the fau lt a nd topog ra phic tre nds sho ws th a t the latter arc a pproximately pe rpe nd icula r 10 the dir ect io n of exte ns io n in the no rt h , bu t oblique in the sou th . Eaton ( 19KO) not e s th at the sout her n pa rt o f t he p ro vince ind icates lo w levels of seism icity compared wit h the northe rn, and suggests that the former sec tor is no w la rgel y q u iesce n t struc tu ra lly . It is likel y the re fo re tha t th e o rie n ta tio n of t he stress fiel d ha s cha nge d since the s truct ure of the sou thern sector W :'IS initia ted . Sei smi cit y is co nce ntrated in broad be lts 100- f50 km wide alon g the margins of th e no rthern secto r, de lim itin g a relati vel y ase ismi c re gio n in the ce n tra l part or th e G re a t Basin . Seism ic activity is concentrate d in th e d e p th ra nge 5 - 16 km . M uch d iscu ssio n has ccnlux.l un h p_mbh:n of il<' hi. bigh-lc vet .uo fm.. l Iaul ex tensio n i acconunod a te d der1h It now seems clear that , alt hough a fe w fa u lts ex hib it listr ic ge o me t ry (i.c . a rc co nca ve upwa rds) a t sha llo w dept hs of 4 -5 kr n, most stee p faul ts wh ose geomet ry ha s bee n invest igate d a t de p th co n tinue to di p un iform ly. The pre va le nce of tilted blocks and the regio na l co nsis te ncy o f lilt (S te wa rt , 1980 ) suggests that 1he xtensio n hall be n a ccornpbsh ed m a inly~' ~It Ijlt~ 10(; · mcchamsrn in th upper e rul'i4 4ie tachin~ on m aj o r1 n W'= angll.: d lsp c nrs'a dc:pltu. o~ 1 km nd hus pa £il l i n~ he britt pper-crust a t exte nsion Irom mor du ul e fo rma tio n 1n th e towe recrus (We rnic ke, 19X1 ) . T he amou nt of e xte nsio n of t he mod e rn phase of d e for mation is estima ted by Zo back et
Ioca mechnms
U h:&iUf CJIlCUb
r •
,e e
,
,-
,
,
•• • •• • u
-e
• • 2
<
•• c •
.-
•
,
j
98
G EOLOGICA L ST RUCTURES AND MOVI :-lG PLATES
A
,.
o N.SA ·« \ -\-7' L 4.5
/x;;r-.
. ~ ' . '. ~. '
,,
CP
./
. R.GH
,
-, -,
,,
,
Fi~u re ~ .Ill ( II) "'1
al . (198 1) as 17- 23% from faul t a nalysis. Much lar ger estim ates . up to 100 - 300% ( H am ilton . 1975; Wernick e . 1981). a re a ttr ibuted by Zoback et al , to the e ffects o f the ea rlie r phase of e xte nsion (sec be lo w) . The prese nt crustal str ucture a nd tect onic patt e rn o f the Basin-a nd -Ra nge pro vince ca nno t be con sid er ed in iso lation . as the r th product s ~ It tectoni bistor ' lending back 10 M esm~ni and la te Pi:lI~lJA)i ' tim.; D uji ng the ea rly Mesozo ic. a subductio n zon e e xte nde d alon g t he who le coastal be lt of west e rn Am erica with a volcanic arc to the east re prese nted by t he Cordi llera M esozote hal h olilb bdl (Figure 4.17A ) . At hou t HO MII RP magmatism cease an d w replaced .o 0011100 prcssiullal dd unn.auon 0 I ra niide a nd
Sevie o rogeni
n ate
re taceces lu early
Cenozoi tim (c.80- 40 Ma 8P ) . T he foldt hrust be lt res ult ing fro m this co mpress io nal
ph ase affected a broad region up 10 1500 km wide . mar ket! by a se ries o f baseme nt uplifts resti ng on th rust fa ults. T hese st ructures result ed from N E - SW to E NE- WSW co mpression . . his: p atter n f cornpressioua deforma t" ln has been .attribul d by Goney ( 1'113 ) 10 th e ffects.ct.low-eng je !'IubdUCfinn Rlnng th~ ,plal buuuJilry a nd nearly perpendicular-eo-i The compressiona l regime e nde d bet wee n 40 and 50 Ma ago , proba bly as a resu lt o f the reorga niza tion in Pacific p lat e movem e nt pa tterns discusse d ea rlier (3. 1). It was replaced by a ph ase of ca lc-a lkali ne volcani sm tha t lasted un til the co llision of the Faratlon -Pacitic ridge changed the western bounda ry of the Am erican plate from a sub ductio n zo ne to a t rans for m fau lt. As we sa w ea rlier (Figure 3.4A) . th is cha nge was diachronous. a nd the ridge - fa ult - tre nch tripl e junc tio n ma rking the no rthern e nd of the San A ndr ea s fa ult migra-
D IVrlH,EN I ( t:XTLNS IONA L) T ECTOi':I C Il.EGL'.1ES
ted no n hwc st wa rd s fro m a bo ut 25 Ma ago unt il the present da y. The migrati on W
99
e xte ns ion .'
e hanf! JI 0 «ecuon illustr.. JHB lUa y ehlll.-u to (he nUfl hwanJ rmgrau on o f the so u thern end n t th 06UlKIuetio _on.. prcoa UI C 0 h
-t"'"
ri ~ur
ICmimna l SI
,
G iven the p re se nt crustal thickness o f a bo ut 30 km , a ne t e xte nsio n of around 80 - 100% ma y be calculated if we assume tha t rhc crus t att ain e d a th ick ness o f 45 - 50 krn (co mp a ra ble 10 rhat o f the Colorado plate au) as a resu lt of th e Meso zoic co mpressio na l p hase ,
I? e.wnu!
;n
H
100
G EOLO GIC AL ST RU C r U RE$ ANO MOV ING I' LArE S
CONFUSION
RANGE
HOUSE
RANGE
o 5
'0 E
•
15f-=,--.""••• "" c'"c,.... ",..."".... _OI,
20
.
01'''' 0' "
25
. '
I
,.
P,
p
.
D
D. _
..
' f' SO
30
c" Cc
0" ,
V P. No. _
-, Y
I
_
s".. ·••
_-
c_ ... '._<0' <0_ ... .
t" . ....
00
',
,lY _ , •••
".. . "."i
400
600
80 0
Figure 4. 19 Structu ral uuc rprctunon o f COCO R P <.leep seis mic relk el,on linc acn», pan or the U",i n' iln
re placed by the curre nt ex tensiona l phase with a al ori entation of WNW - ESE betw een c.20IOM a agu . E xte nsio n was acco mplished by thc form ation o f tilted blo cks a nd by rot ation on listric faults de tachi ng on low-a ngle normal faults! shear zone s. The clockw ise ro tat ion of extensional stra in axes has bee n att ributed [ 0 the no rt hwa rd migr atio n of the end of t he subd uctio n zon e as it is replaced by the nort hward s-ext end ing San And reas fa ult .
A series of ucean i hasim (often termed ' marginal basins' ) are sit uated
spread ing' 10 exp lain thes e marg inal basins is d ue to Karig ( 1971 ). In Kangs model (show n in Figur e 4.23A) , new spreading aXIs is devcloped abov e the de scending slab becau se of the diapiric rise of ho t mantic ma terial relea sed by the su bd uction process. A n alter native way of viewing this process is ,IS a secondary conve ctive cell of the kind predicted by the ther mal mode l o f Elder (see Figure
,I
2. 13),
A study of the magnetic a no ma ly puucrn of the basins o f the west Pacific (Figure 4.22) shows that they ar e all relative ly you ng and short-lived in compa riso n with t he Pacific plate itself. T he oldest basins in th is regio n commenced spreadi ng ab out 60 Ma UP and the inactive basi ns gene rally had a life o f o nly abou t lOMa . Among the basins that arc still active arc the onin lIOUl(R., the-Mar iana trough . ..m d the Lau tro ugh/H avre basi n. The Japa Parece Vel' and -South 1-:ij' basins are examples o f inactive systems (F igure 4.2213) . W iss I t IlJXt1 s hows that the co mp lex magnetic str ipe patte rns of the se basin s cannot be
101
I)I VFIl " FNl ( F.XTU" SIONA L) ·1EC f'ON IC IlE GIM ES S ~ VI~R
.0._ .0,. ..' --. . ."
D~S ~Rr
CA N YON
~
RANGE
.....
,
'0
'0 --..-. = . 1000
'0
1200
14 0 0
160 0
10 krn
Se ,. ....
~ _...
]
~----_
650 "..
o •• 0
•
''''--1 '-:.....-.------------/
..../
I
.
fi gllrt 4.Ul Ca rtoo n sec tio n Illustrating a possible interp retatio n or the for mation of tbc Basin-and-R ange province. SAF, San And reas fallll; SN , Sierra Nevada : BR , Basin-and-Runge province ; WF. w asatch Front: CP, Co lo rado plateau ; S R Southe rn Rock y moun tain s: EFR , East fmn l ulthc Rocky mountains; NFD . Newfo undla nd ; CE , connnental edge ; MAR , mid-Atlan tic ridge; PM. zo ne of part ial melt ing. From Gllugh ( 19l>4)
102
GEOLOGICAL STRUCTURES AND MOVING PLATES
,
~v
'f..~
BOWERS BASIN OKHOTSK BASIN
PARECE VELA BASIN MARIANA TROUGH
Figure 4.21 Distribution of marginal or back-arc basins in the north and west Pacific Ocean. Subduction mnes indicated by toothed lines. After KariB (1914).
interpreted by a simple back-arc extension mechanism. Only some of the most recent spreading patterns can be simply related to the present arc geometry (Figure 4.22A). Most of the basins in this region show patterns of magnetic lineations that are repeated across active or extinct spreading ridges, although the pattern is often complicated by the superimposition of an active system on an inactive system with a quite different trend. For example, NNE-SSW spreading in the West Philippine basin changes to E- W spreading in the Parece-Vela basin, across the KyushuPalau ridge (Figure 4.22-B). In several basins, the spreading axis is offset towards the volcanic arc. This is particularly evident in the Tonga-
Kermadec and Marianas arcs (Fipre 4.228) and may be because the ftank region of the volcanic arc is the hottest and weakest part of the basin. The magnetic anomaly patterns in general indicate that back-arc spreading rates are similar to those. on the main ocean ridges, although the duration of the spreading episodes is much shorter. It would appear that tectonic conditions favourable for the generation of back-arc basins are either relaxed relatively quickly. or are easily interrupted, for instance by buoyant ocean ftoor material on the descending slab arriving at the trench. The South Fiji and Lau basins appear to have formed as a result of the evolution of
DIVERGENT (EXTENSIONAL) TECfONIC REGIMES
103
A
. ,.
.
/-7 ,
..
..20 ......
"22/ aD
"24/ ..25"
FIIU~
4.12 (A) Spreading directions in marginal basins in the western Pacific Ocean. (B) Magnetic stripe patterns, aseismic ridges (bachured) and spreading axes (heavy lines with arrows) in part of the western Pacific ocean. Dashed lines indicate inactive, continuous lines active, spreading axes. Based on Weissel(1981).
HJ.l
GEOL OGICA L STRUCTU RES A N D M OVI NG PlATES B
GR I\Vl f ATlONA L
FORCE MARGIN AL
~
8"'S IN
~,,, m m;4?>
$ '"
· 1~ I TRENC H I r.
I SLAB ROl l -e "'CK
Figun 4.23 MOIlelS 10 explain marg inal o r back-a rc basins. (A ) T he Karig model : ' " ,,('I.-arc scre ading' . A lte r Uyeda ( 197N) . ( 8) T he 'trench roll-hack' model.
thre e , rathe r tha n two , plate s, although the det ails are not ver y clear. Although .pHU1~ I the back·a, basinlli in explained by the hack-ar the Pacific: can spreading mech anism of Knit ( Figure 4.23A ), it has been suggested by several au thors (see Chase , 1978a,b ; Molnar and Atwate r, 1978) tha t back-arc basin formatio n may be re lated mor e plausib ly to globa l plate motions. In this model, back-arc ext ens ion occurs when the resultant of the velocities of th e ove rrid ing plate and the t rench ' ro ll-back' motion has a compo nent d irected away from the t rench , whe n viewed in a hot- spo t frame of referen ce . Trroch m Y-back (Figure 4 .238) was defined by Dswe" (1980) as the gradu al seaward migration o f a trench caused by the gravitational pull of the descendin g slab. Th is motion is grea ter for old, co ld ocea n plate t han fo r younger , warmer ocea n plate , which accou nts fo r its impo rta nce in the weste rn Pacific, where trenches are subd uct ing o ld Mesozoic ocean crust . When t he co mponen t o f mot io n is d irected from the tre nch o utwards, towards the ocea n , tension al failure will occur in the over-riding plate along the volcan ic axis. T he spreading direction will be governed, accord ing to this model, by the directio n of the resultant velocity vector , and spreading occurs passive ly just as in the ocea n ridges. Acco rd ing to the world
map o f 'abso lute' p late motion s (C hase , 1978 ; Minster and Jo rdan, 1978) severa l of the basin s discussed above (e .g. the Mar ian as and Lau Havre basins) are o pe ning in acco rdan ce with the p red ictio ns of t his model. Hynes and Mati (1985) , in a study of the To nga- Kermadec and Mar ian as a rcs, ha ve argued that the roll-hack mec ha nism may not be viab le beca use it ca nnot account fo r the variability in back-arc spread ing a lo ng 1I specific margin , a nd in parti cu lar ca nnot explain its recen t initiation . Th ey suggest that a mo re att ractive explanatio n for tre nch migration is a cha nge in pr ofile of the descend ing slab , due to changes in the relati ve rat es of moti on of the co nve rging plates - a mechani sm proposed by Furlo ng et al. (1982) o n the basis of thermal mod elling. Th e mechanism proposes th at an increase in velocity of co mpar at ively o ld , coo l, and stro ng oceanic lithosphe re will produce a decrease in slab inclination at shallow dept hs. However, due to the st reng th of t he slab , t he line o f co mmencemen t o f downbe nding migrates oceanwa rds , pulling the t rench away from the upper plate . In other respect s, however , Hynes and Mott confirm th at the global p late mo tion model is applica ble to these two arcs in t hat t hey both ind icate absolute trench motion towards the Pacifi c.
105
OIV ERGEN I ( EXTE NSION A L) n crON IC REGIM ES
A
"
, "
Fil:urc 4.24 Me tho ds o f acco mmo da ting e xte nsion . (A ) The wed ge su bsrd c ncc mod e l fo r graben furrnano n . rrorn
,
8rilll. lay ..
,
... •
,
n oll and Muhcn ( 1':l113) , ( 8) Block rotation above a low-. a ngle e xte nsio na l fault-shea r zo ne . A lte r We rnick e ( 1981)
, 00.0<:""
B
I
..
~, I f . . '
~~~'-"-'\W~---L.2~Z!J
r
KM
• 4.5 Str uct ures assoc iate d with exte nsiona l regimes
Classical views on faulting (e.g. Ande rson , 1951) and on the form ation o f graben a nd rifts (Veiling Mei nesa, 1950) envisaged thai ext ension was acco mmodated primaril y by dip -slip movem en ts on sleep fault s. or by filling of extensional fi ssur es by magma . However t he amount of ex te nsion ac hie ved by displace me nI on a stee p no rmal fault is limited by the depth to which a crustal block can sink , and begs the question of how the displacem ent is accommodated at depth - see fo r ex ample the keystone or wedge model for rifting ( Figure 4.24A ) proposed by Batt ( 1976). It is clea r that some addit ional mechanism is req uired to produce extensio ns la rge r tha n a few km. Th e importa nce of altern ati ve fault mecha nisms in achieving large exte nsio ns has o nly been discovered relatively recently , mainl y by field workers in the Basin-and-Ra nge prov ince (e .g. Davis et al ., 1981; Wernicke . 198 1) and by the
analysis of seismic records obtained in petroleum exp lorat ion of marin e basins, for ex ample the Nort h Sea (e .g. G ibbs, 1984) . Wernicke a nd Burchfie l ( 1982) present a geo met ric and kinematic a nalysis of extensio nal fault systems which highlights three pr incipal met hods of ac hieving large hori zo ntal disp lacemen ts. These are: (i) the use of low-angle normal faults or deta chme nt hor izons ; (ii) the rotation o f fault blocks , and (iii) the use of curved or 'listric' fault plan es. T he demon stration by Werni cke ( 198 1) that large disp lacements co uld be accommodated by a combination of rotated imbricate blocks det aching o n a lowangle fau lt o r du ctile shear zo ne (F igure 4.248) was o f great importa nce in the study of exte nsio nal regimes. If fault blocks are allowed to rotate (Figu re 4 .25A) , th e am oun t of displacement is cumulative , ea ch new fault displacem ent addin g an increment of exte nsion without necessaril y causing an y add itional de pr ession of t he surface or add itional crustal thinning. T he listr ic
106
GEO LOGICA L S flHJCTU Kf.S ANO MO VING PLAT ES
A
_ _ 'C·_I~~"O~"~'=~rr-_
..,
·""""1
-
o-
,
L-_----',
j
B -- - - -=--=-=-=~-
-
H anging Wa" Bloc k
1 e
1I. ' rlc F I " lt Dela chmen!
•
c
H':~,'+~?,n~'" Rollo • • •
Ha ng,"g Wa ll Synclint
Fif::u rt 4.2.5 Gco mcl ry (If e xte ns ton. (A) Block ro ta tion abo ve a detachme nt . Triangular gap s A arc lefl . ( 8 ) Limic Iauh with hangingwall ro ll-ove r a nticline . Areas A and B are equal. The adjus tment in h.mgi ngwaU shape implies in ternal stra in. (C) Fla t/ ra mp geometr y o f fau ll p roduce s geome trically necessa ry folding in h afl~i ngwal L (A ). ( R) a nd (C) f rom Gih~ ( 19M ) Fla l
fault (Figure 4.258) achieves a rotation in the hanging wall me rely by its d isplacem en t , and is accompan ied by an acco mmodation anti cline termed a 'rollo ver'. Th e role of t he low-a ngle det achment fault is critical in both st ruct ures . Th is detachmen t horizon will gene rally possess a ramp -flat geome try (Figure 4.25C) similar to
t hat fou nd in co mpressional thru st be lts (Dahlst ro m, 1970 ; Boyer and Ellio tt , 1982) . Both t hese mechani sms req uire accompan ying du etile deformation to alleviate space problems. These pro blems can be minimize d by the introduction of an titheti c listric fau lts, producing listr ic ' fans' , which have the effect o f thinni ng
DIVEKG ENT ( t: XI' EN.sl0N A L) n CION IC II.EGIME S
a
107
A SN UO'" Cou " ' . ' Fa"
,
,
b
• c
d
Figu~
4.U. More complex extensio nal geometry .
A (Q,b) Thin ning and exte nsio n in the hangingwall by
antithetic fault systems. (c,d) Horsetail or lisln C (an producin g a series of ride rs on the sole lault , formed by the migration o f the sole r;oult into the foo twall. Fro m Gibbs ( 19R4)
108
GcO LOG ICAl. STRUCT U Rl:S AN D MOVJ!'JG PLATES
B
a
flat
/
/
/
•.
, b
E I IItn ~ Du~.:.:.""';;::"
•
__.....
_
c Cetm_ High " ....1"1 W._ Counl... F""
,
\
S t>ortcut Faull
, 7;·7·'-~"~----_J.-~
2
2~
4.16 B(a- c) Evolution o( ,10 extension al d up lex with associa ted list Fi ll.U",
ric fan. eoemer fan and ele vated cent ral high. From G itms ( 1985)
the ro llover in the hangingwall. Th e addition of syru hetic fault sets in the footwall prod uces complex graben structures (Figure 4.26A. B). Further complications are produced if the basal detachment has a ram p-flat geometry, in which case t he detachment may migrate into t he footwall 10 produce extensional duplexes analog-
ous to those found in thrust zones (Figure 4.268) .
Gibbs (1984) presents an interpretation of the central graben of the North Sea basin (see Figure 7.10) based on both seismic and well control. His section illustrates well how the conventional model of a symmetrical 'key-
109
DIVl::IWl; f'jT (EXTt NSI ONAL) Tt CTONIC IttGlMES
stone' grabe n has bee n replaced by an asymmetrical structure where the key clement is a low-angle exte nsio nal fault. Although the structure as a who le as see n at the surface retains an overall symmetry, individual elements in the structure are asymmetric al . typically fo rming v -shaped ' half-grabe n' with an incl ined fault o n one side , and inclined rotated bedd ing on the oth e r. An impo rtant conseq uence o f the low-angle detachme nt is that displa cemen ts may be tran sferred consid erable dista nces laterally. Extensiona l structures at the surface may be accommodated by d uctile displacemen ts in the lower crust , bu t the precise form which these take cannot usually be discovered . Lower crustal exte nsio n could be accommodated largely by pure shea r ex tensio nal thinnin g, or alternat ively by simple-s hear d isplacements o n low-angle shear zo nes, or by a co mbinatio n of both mecha nisms. The importance o f midcrustal shear zones in the defo rma tion of highgrade Preca mbrian com plexes is stressed in 9.4. Deep seismic profitin g across the Mesozoi c basins o ff the coast o f no rthern Scotland (the MOI ST line), co mbined 'with dat a o n deepcrusta l str ucture obtained fro m the o n-land deep seismic profile L1SP B (Bamford et al., 1977) ind icate tha t the fault d isplacement s forming the basins (mostly of half-graben type) are tra nsfe rred along major mid-crustal detachme nt hori zons using a netwo rk of preexisting faults and she ar zones used as thru sts during the Ca ledo nian orogeny (see Figure 2.30). It appea rs likely that in this case the exte nsiona l displaceme nts cut through the whole thickness of the crust. These results have important implications for cont inenta l separatio n and the structure of passive continenta l ma rgins. Gibb s (1984) illustrat es a typical pa ssive continen tal margin structure ba sed o n a sectio n in the Bay of Biscay (af te r De Charpal et at ., 1978) which demon strates the importance of a basal det achmen t horizo n. T he det achmen t effectively tran sfers the lowe r crustal displacement away from the continent towards the ocean . Th is
mechan ism allows broad zo nes of upper cr ustal thinnin g to take place by exte nsio nal faulting as shown above, leavin g the lowe r crust unaffected. The exte nsio nal structures o f passive cont inental margins are thus likely to be allochtho nous, and the basa l de tachme nt plane will transfer the displace me nts to the o riginal site of the rift, whe re the who le lithosphere was brok en through (see Figure 2.29) . Exte nsional fault syste ms frequ ently contain many stee p faults with strike-slip d isplacements. These faults appea r to be integral to the displa cement pattern and are termed transfer faul ts by G ibbs. The y play the same role as oceanic transform faults in tran sferr ing d isplacem ents from o ne dip- slip move ment plane 10 ano ther, bUI differ in that all the movement planes norm ally deta ch on the so le fault, ra ther than at the base of the lith osphere. Tra nsfer faults may separate imbri cate fault-fold pack ages that a re quite d istinct a nd uncorrelateable across the transfer faults (Figure 4.27). B
A
c
c . ~-_ .:
.' C ~
C'
Figure 4.27 Geometry of transfe r faults. (A) Plan o f a simple transfer faull connecting two normal Iaults . ( 8 ) plan of two transfer faults separating three zones with different arrangements of norm al listric faults , all detaching on the same basal plane. (C) Sectio ns AA ' . 88' and CC' across (8). After Gillhs ( 19M)
110
GEOLOG ICAL ST RUCrUR I::S AND MOV ING PIA n ' s
-- --- - - - ~ ~;;..=-=.:c.=..=-~= Foot wa ll
,, T he problem of how listric faults e volve is centra l in the underst and ing of the extensional faulting mechanism (Figure 4.28). This proble m is addressed by Jackson and McKe nzie (1983) in the light o f ob servations in a zone of active seismic faulting in G reece , the Aegea n and weste rn Turkey. Seismological observations o f t he ea rthq uake source are used to
f igurl' 4.2tI Dia~ra l11ma lic prolilc illustrating elevation or thl' f<xl lwa ll. a nd the development of ,mlithclio.: fault s an d of a second-generation synthetic f:lull in the ha ngingwall o r an exrcnsio nal listric fa uIL system . From Jackso n and M cKenzie (1 911J)
det e rmine th e fa ult orie nta t ion a nd slip vec to r
at the origin of fault failure. typically at a depth of 8- 12 km. T he grea t majorit y of large no rm al -faul t ea rthquakes in this region have d ips of 40- 5(r at their foci, and the d ips appear to corres pond with o bse rved surface d ips in at least two cases. Jackson a nd McKenzie therefo re suggest that fault s norm ally pro pagate to . ·i\::ure 4.29 (.a -d) Mood sho wing the evo lut ion of an exte nsiona l [aull syste m with incre' lsing amou nts of extension (fJ .. 1.t15 - I.'J) . f'.lIOrmal f'lU ll obhq uc to beddi ng (rlr5t-ordcr fa ulls) ; T. te nsion al fract ures; TI-'. seco nd- an d third·...rder faults, a pprox ima tel y pe rpe ndicu lar to bedding . 8 .. ave ra ge lill of blocks in each cuse. From A ogelicr and Co lletta (19B3)
DIVE I(Gf.N T ( EXT ENSION A L ) T EcrO,"l ]C I(EG IM ES
the surface as planes, and th at the listric geometry is deve loped in a se mi-ductile zo ne below the depth of fault initia tion. t hat is, between 10 and 15 km depth . lncrcusing cx tcnsion in the britt le layer is prod uced by rotat ion of the initially plana r fau lts as sho wn in Figure 4.25A . T he rot ation produ ces a gradual increase in d ip o f th e initial fl at surface o r bedding, acco mpa nied by the elevatio n of the hangingwalls and the depression of the foo twalls (Figure 4.28) . These ve rtical movem ent s are the isostatic respo nse to any dip-slip fault displaceme nt and may be clea rly see n for recent fault movements in the Aegean, because sea -leve l is a co nve nient datum . It is geo met rically nece ssa ry for a co ncaveupwards curved fau lt 10 form in o rde r that voids do not occur as displa cem e nt ta kes place . Even if the fault in t he ductile layer is initially plana r a nd meets the brittle fau lt at an an gle , the result of movem ent o n the fault syste m will cause accomm oda tion stra ins and seco nda ry faulting in the regio n of t he change in dip. T his process will eve ntua lly result in a listric geo metry by abrasion of tne hangingwall angle .
111
Accor ding to Ja ckso n an d McKenzie , antithetic faults arc nuclea ted at th is zo ne of st ram co nce nt rat io n whe re the ab rupt change in dip occurs. After the initial fault set has been ro tated to much lo we r angles of dip, new sets o f faults may fo rm at steeper angles. As the ea rlier faults are cut by t he new faults. the fo rmer lock , an d move ment is transferred to t he younge r se t. Angelier and Co lletta ( 1983) dem onstrate t his principle in a stu dy of the evo lutio n of exte nsional fault geo metry . co mpa ring the G ulf o f Suez (with 10- 50% extension, see Figure 4.15), the western Gul f of Califo rnia (50-100%) an d the southe rn Basin-and -Range province (up to 200% ). T hey cons ide r tha t at sma ll ex te nsions (10- 20% ), blocks are gen tly tilted up to 10" be tween parallel , widely-spaced fault s dipping at 60-65° ( Figure 4.29a,b) . Many close ly-sp aced ve rtical tension fractures are developed appr oxima tely perpe ndicular to bedding during the e arly stages o f extension, and these are used for t he later d ip-slip displacements when they have bee n rot ated into a suitable att itude (Figure 4.29c,d ).
5 Convergent tectonic regimes 5.1 Subduction
Th e co ncept of a spec ial type of orogeny related 10 the subductio n process is im plied by the plate te ctonic model, a nd was first clea rly e xplained by Dewey and Bird (1970) in the ir cla ssic pa pe r 'M o untain belts a nd the new globa l tecto nics' , They demonstrat ed that t he process of subd uction a t a dest ruct ive plate boundary inevitab ly pr oduces a characte ristic asso ciation of rocks and struct ure s tha t. in pre plat e tecto nic termino logy. would ha ve been regarded as a type of o rogeni c hell . T hese ideas we re fo resha dowed 10 so me ex ten t by R.S. Dietz (1963) who proposed an ' actualisric' model to explain the fo rmat ion of ' eugeosynclines', which were so familiar in t he lite rature on oroge nic belts. In his model , an accretionary sedimentary wedge is formed o n the contine nta l slope, on isostatica lly depressed ocea nic crust , and subseq ue ntly defor med and accre ted I ~ the contine ntal margin . At about the same time , the essen tial link was made betwee n trench formation and s ubd uction (see Hess , 1962) that led d irectl y to the ocea n-floo r spreading hypothesis and to the plate tecton ic theory. The two essential fea tures of the subd uctio n zo ne are the volcanic or magmati c arc an d the tren ch (Figure 5.1). The tre nch typically co ntains a th ick prism of sed iment s overlying the
...
TRENCH
volca nic roc ks of the oce anic crust. T hese sed iments arc usuall undcfor med on the'"OUier and':ffoor oC th.l:__trenc .,-6u C 1?eco m..f - deformed at the foo l of the inner Irench w Jl... Thi s inner "'IrCnch region , characterized by high pressures and low geothermal gradients (see 2 .3) , was suggested by Takeuchi and Uyed a ( 1965) as the site of for mation of the blueschist metamor phic be lts characte ristic of the ci rcum -Pa cific region . Th ese bel ts form a paired set with the high- tempera ture , lowpressure me tamo rphic belt s fou nd on the inner side of islan d arcs and mo untain belt s. The lat te r co rrespo nd to the zon e of high gco the rmal gra dient associated with the volcan ic a rcs, o n the dow n-dip side of the subd uction zo ne . T he p rese ntly-exposed, dissected orogenic belts o f the circum-Pa cific reg ion , with their paired met amorphic belt s. t hus represen t the uplifted prod ucts o f o riginally active arc-trenc h sys te ms. Dewey and Bird visualized the seque nce of events in the evolutio n o f a subd uctio n zo ne as follo ws.~he~s u bducti ng ocea nic plat e.begins to desce nd . Iorming a trench I. th rusting occurs a t ils inner-wa ll - form ing---; _~eries of -thrust. wedges of oce an-floor. mater ial such aschen.. argillite, carbo nates and even basic and ultra. basic igneo us materi al. The th rust wedg ing 'p rocess results in a tecton ic e leva tion on the inner side of the tren ch 10 form a ridge . Th is
nan
CO NTINEN TAL MARGIN
ARC-T RENC H
0' FLANK OF REMN ANT ARC
UTHO SPHERE
"i!:ure 5.1 Sche matic pro file across an island-arc subd uction zo ne , showing the main tecto nic featu res. Afte r Windle y ( 1977).
112
CONVERG ENT TECTO NIC REGIMES
process may be accompanied by gravity sliding of par t of this ma terial back i n to the tre nch .
Geometry of t r~nch systems The present tren ch network is s how n in Figure 3.1. T renches occur o n the ocean side of volca nic island arcs and active co ntinen tal margins. Some tren ch es ar e remarka bly co nlinuou s ove r great d istances. The Peru -Chile trench is 4500 km lo ng and the To nga tr en ch about 700 krn. Typical wid ths _ ~l ~~ 2 ~ 1!!!£. WOkm a~_Q..J.! s he twee n 2 and 3 .k m .bl:low ~ The ocean.has in.Jl oo r or u p---.!~U I km be low , sea-Ievell although up to 2 km of sedime ntary trenc h fill milY be Rrese nt; The positio n of trenc hes bo rderi ng act ive co nt inen ta l margi ns is o bvio usly det ermined by the locat io n and shape o f the margin . Tren ch es bo rde ring island chains, howeve r, are typ ica lly arcuate . T he explana tio n for th is curva ture (discusse d in 3.2) is tho ugh t to be d ue to hack -ar c sprea di ng of ocean crust which is inh ibited at t he cus ps of the arcs by ase ismic ridges or ot he r obstr uctions (see Figure 3.9) . •
Morphology and ~'rrucrure of island arcs A typ ical gravi ty ano maly profile is show n in Figure 5.2 toget her with a crusta l str ucture profile base d on co mb ined grav ity and seismic refra ction data. T re nches arc o bvio usly not in isosta tic eq uilibri um , and re present a mass deficie ncy which is largel y expla ined by the depth of water in the tr ench. The cr usta l thickn ess does no t ap pe ar to va ry across the trench . It may be conclude d, the refore , th at the grav ity anom aly ma y be sa tisfacto rily explained by a downward force du e to the subduction "precess depressing the crust ; t hat is. the tre nch topo grap hy is dy na mically mai ntained . The variable sed iment infill (some tren ches have very littl e sedi me nt, o the rs are virtua lly full) ap pe ars to have little influen ce on the amount o f crustal depressio n and de pends purel y o n sedime nt supply. The negati ve grav ity anoma ly assoc iat ed with the t renc h is flanked by positive ana-
1I3
matie s: a large pos itive ano maly follows the vo lcanic arc, a nd a smaller broa d anomaly occ urs on the oce anwa rd side of the t rench . The latte r anoma ly has been exp lained by Wa ils and Ta lwa ni (1974) as the result o f upward ben ding of the ocean ic plat e as il a pproaches the trench , an d is a co nseq uence of the late ral strength and co nti nuity of the plate . T he positive anoma ly associated with the volcanic arc is explained by the mass excess re presented by the re lative ly dense vo lcanic rocks of the arc. T he lower crustal layer is grea tly increased in thickness below the arc (F igu re 5.2 ). which possesses a cr ustal structu re similar. 10 that o f t he co ntine nts. The elevated top ogra phy of the vo lcanic pile is thus o nly pa rtly co mpensated isostati cally.
Seismicity and the m echanism of subduction Th e seismicity assoc iat ed with sub duc tio n zo nes is one of t hei r mos t ch aracteristic fea tu res . T he di ppi ng zone of ea rt hqua ke foci wide ly kno wn as th e Benioff zon e constitutes one of the most impo rtant pieces o f evide nce for the hypothesis of subduct io n o f oceanic lit hosphere . We have already d iscusse d the seismic evide nce fo r stress o rientation and d istribu tion in s ubd ucting s labs (see 2. 6). The tempor al and spatial d ist rib ut io n of eart hquakes in subductio n zones was investigat ed by Ma gi ( 1973) who demon str ates, firstly, a pro gression in tim e of earthqua kes from sha llower 10 deepe r levels o n t he slab , an d seco nd ly, that maj or sha llow ea rthq ua kes are always preced ed by a mar ked increase in deep seismic act ivity. He suggests that stra in is accumula ted gradu ally ne ar the s urface by co ntinued co nve rgence . and that a large eart hqua ke occurs whe n the accum ulat ed st rain rea ches a critical va lue. However, be fore t he main shock occ urs, the region expe rie nces nume rou s sma lle r shoc ks that ind icat e a slight move ment along a restricted secto r o f th e slab (Figure 5.3 ). Thi s move ment is transmitted progressivel y dow n t he sla b at a rate of about 50 kmlyea r and te rminates at t he lower e nd of the slab in a large shoc k. The s udde n down wa rds mo ve-
J 14
GEOL.OGICA L ST RUCTU RES A N D MOVI NG PlAT ES
mgal
200 ' 00
- 100
•
•
- 200
Free air anomaly (sea) Bouguer anom aly (land ) Computed gravity a nomaly
- 200
- 400
Nares
Outer ridge
Puerto Rico trenc h
ON basin
Puerto
• • ~i~~ ~ ,
Ve nezue lan ba Si: - S
2Q
ted 40 Unconsolida sedime nt D,p'h
o
(.m)
s veroctrv den sity
2.1 km/ s
2.0 g/cm'
Consolt at Rt
seiinilml 3.8 2.4
U~..SJust
Lower cr ust
Ma ntl e
5.5 2.7
7 .0 3.0
8.2 3 .4
'-"'" '""
IZ.2J
Figur f 5.2 G ravity-anomaly and crustal-structure profile across a typical island arc (Puerto Rice ). From UOII ( 1971) . ener T alwa ni et Ill . ( 1959) .
ment at the end of the slab rapidly propagates upwards to trigger the large shallow ea rthquake at the top of the slab. The large shallow ea rthquakes may have either compressional or extensional focal-plane solutions (see Figure 2.23) and both may be explained by the above mechanism (Figure 5.3). Compressional solutions are explained by underthrust faulting (Figure 5.3A) . Compressional solutions are explained by underthr ust faultin g (Figure. 5.3A ) and extensional solutions by normal faulting (Figure 5.38 ). Both types of fault have the same 'effect of releasing the upper portion of the slab, previously 'stuck' at the
trench, and allowing it to move downwa rds. A comparison of the seismicity of a large number of subduction zones led Ruff and Kanamori (1983) 10 suggest that the extent of 'seismic coupling' is an importa nt control on earthquake magnitude. The degree of seismic coupling is controlled by the number and size of 'asperities' on the slab, that is, of stro ng regions that resist the motion of the slab and have to be broken through before motion can be continued. Earthquake magnitude also appears to correlate both with age of subducted lithosphere and with conve rgence rate (Figure 5.4). The largest eart hquakes occur in
115
CONVERGENT TECTOI':(C RlOG1 MES
Conlinental Lirholllhe ,e
Oc eanic li lhosp/l ete
f'e ne ll
\,
Under thrust Fault
Normal Fault
G, ea t 5 110110_
G' ea l 5 110110_ Eo,llIqlla ke
~r1 l1 qlla ke
A
Figure 5.J Schematic diagram illustr ating the mechanism for shallow and deep earthquakes in subduction zones . (A) illustrates the underth rust fault (U F) case . and ( 8) the normal faul! (NF) case for the production of great shallow eanh quakcs. Hatched areas deno te thc faulted segment. From Magi ( 197J)
zones where young. warm lithosphere is subducting at fast convergence rates. Ruff and Kanamor i note that most of the seismic slip on subduction zones occurs above a dep th of about 40 km and that this depth correspo nds to a sharp bend in some slabs. They suggest that below this level, seismic activity is relat ed to
the basatr-eclogite phase change which produces superplastic deformation within the slab. The various factors affecting the geo metry of subduction zones (Figure 5.5) are analysed by Cross and Pilger (1982). They recognize fo ur interdependent factors: ate of relative plate convergence: velocity of absolute uppe r-
12 .---,.-- =--,.--,.-~,.-_,.-_ ,.---,
o
,1'8 E
u
!! II.
6 4
2
140
120
BKl
60
Age, my.
40
20
0
Figure 5.4 The correla tion between ear1l\quake magnitude . plate oonvergence rate. and lithosphe re age for 21 subductio n zones. T he characteristic magnitude values are shown for each zone in a conve rgence rate v. age plot. Regression lines of constant magnitude are shown. The broken line encloses the suhductioo zones whe re back-arc spreading is thought tu occur (in the lower lefl pa rt o f the diagram) . The diagram shows that larger ea rthquakes are associated Ilo'ilb furer convergence and younger age. From Ruff and Kanamo ri ( 1983)
116
GEOLOG ICAL SI RUCTU RES AND MOVIN( j 1'l.A'IES
CO NVERG ENC E A A re F A ST
I
ABSO LUTE MO TIO N OF UP PEA P LATE
150-600
T
l<,
I
m
FAST
. .. ... . . ..
~
.. .
SlO W T km
600 -1000
~.m~ --.
~?@V SLOW
....
~
T km _ •
-~
". "
SUBDUCTION OF ASE ISMIC
RIDGE S
14-- > 600 _
~~
AGE OF OCEANIC LIT HOSPHERE
,...!9 0 · ' 5~ (>50 MYlr 11m
OLD
YOUNG «50 MY) T
t'igurc 5.5
Principal con tro ls on the
geometr y of subductio n zones. Fuur contro ls arc illust ra ted : conve rge nce rate , abso lute motio n o f uppcr plate, subd uelion (If ase ismic ridges , and age of oceanic lithosphere, each of whi ch influences stlO
dip. and hence th e size of The arc-trench gap . After Cr oss and Pilger (1982).
plat e motion toward s the tre nch ; iii ~ ge of the
ocea nic lithosphere of the su ucting plate; a nd IV) presence or absence of intra plate 'o bstacles' such as sea-mounts or ocea nic plateaux. The effect of each of these factors was examined in examples where the effects of the other factors could be minimized. t"ra and Pilge onclude that large t are associated with a low subduction angle (a relationship proposed earlier by Luyendyke, 1970), depressed isotherms, a nd a large arctrench gap (150- 600 km). An important consequence of the low subduction angle is the increased le ngth of the inclined Benioff zone. A contemporary example is the b1exiuoR subduction zone, whe re the Gx:ok platc is destroyed below the JcaD late. Slow convergence rates, in contrast, are associated with steep slab dips and sho rt arc-trench gaps. Absolute motion of the uppe r plate is an important factor independe nt of the conver-
gence rate, since the trench is fixed to the upper plate and must move with it relative to ascending or descending mantle now regimes. Thus rapid upper-plate mot ion may override the tre nch and reduce the infl uence of gravitational sinking on the slab. f ast absolute upperplate motion towards the trench also produces a low subduction angle a nd a large arc- trench gap. The position of the a rc is liable to change, and a new arc may develop 6OO-1000km inland from the old. Again the Mexican subduction zone may be used as an exa mple. Slow or retrograde absolute motion of the upper plate has the opposite effect of steepe ning the slab and causing a seaward migration of the trench. The Central America n subduction zone, where the Cocos plate descends beneath the Caribbean plate, is an example. In co n t ra ~t to the subduction zone further nort h, where the North American plate is rapidly overriding the tre nch, the Caribbean plate has a small component of motion away from the subdue-
I
~UM10 ~ r-t-tf/l~ f!-,,{7 0 CONVERGEN T TECTO NI C REGIM ES
tic n zo ne an d the arc- t rench gap is much smalle r.
A ..roq clalion . bel wc;' p age-,o f..,s.ubduclcd he ' l.1.sla b.l~ng l b as.ncred by Fa rrar and Lowe ( ' 978). The related corre latio n be twee n age an d slab inclinat ion is a
~c.:J i l h os
conseq uence of the incre asing thickness, aver age density and decreasing sea-ffoo r elevation produced by t he grad ual cooling o f ocean basin
lithosphere as it mo ves awa y fro m its site of (ormation 'II a ridge (sec 2.3) . Since most of the increase in thickness is prod uced in the mantle rat her than in th e crust, the mean density o f the lithosph er e increa ses , thus reducing bu oya ncy. increasing the slab dip, and hence poten tia lly decrea sing the arc-trench gap. Howeve r thi s ef fect is opposed by a tende ncy for the zo ne o f magma gene rat ion 10 occur at higher leve ls in yo unge r. warmer slabs, thu s de crea sing the a rc- tren ch gap . A
test o f this relat ionship IS provided by the subduction o f the Nazca plat e alo ng t he Pe ru C hile tren ch ( Figu re 5.6) . C ross and Pilger (1982 ) dem ons tr ate t hat t he sout hwa rds de crease in age of Nazca p late being subd ucted along this trench co rre lates wit h a change to s hallower and less well-defined seis micity, and with a decrease in the a rc- tre nch ga p. 1h Pillrofe of eDn.iu:isJ&.e:., ~Qun L. ~ aim.. o r illisanic p at 'au o n the subdu cting plate a ll involve reg ion s of red uced mean den sity w hich increase the re lative buo yan cy of t he slab an d conseq ue nt ly ~~~Dg.I.~ subductio n. Ve ry low a ngles of subduction are co mmo n, and the vo lcanic arc may be complet e ly ex tinguished. ,t{ur and Ben-A yr atUIJD ~1 9831. n ol e several exa mples of volc an ic ga ps 10 sub ductio n zones wh ich t hey relate to ob liq ue co nsumption of aseismic ridgcs. Examples cited arc t he Coco s, Nazca and Ju an
+------ -~,,~ ---+ '5· '. t .
'.
, ..
~ ••••1. •
i "j ••
--
~f ...- RidI' o
=
.
SOUlll AMERICA
-::..~
~ ~ 9 MY
,m 9-2& MY I;;;l
2&-36 MY
• AC1M VOlCANOS
+-- - - - - - - ----+60' 55'
1115'
117
' ;iJ;: lIrr 5 .6 Rel atio nship bet wee n the width of the urc-Ircnch ga p and the age of subductcd lilh\>s phcr e altlr1g the sourhcn Per u - Chile trench. Note lhal lhe width of me gap decreases southwards wilh decre asing age of oce an ic crust of the Nazca pl.nc . Fro m Cross and Pilge r (19112)
118
GEOLOGICAL ST RUCl"U RES
Fe rna ndez ridges o n the Cocos and Nazca plates (F igure 5. 7). T he effec t is most strongly mark ed in the case of the Nazca ridge which , becau se o f its ma rked obl iquity 10 t he d irection of subd uctio n. has swept acro ss a long sec tor of the subd uctio n zon e . T his has caused a large gap in t he vulca nici ty in no rt h and central Peru , t hat coi ncides with a very low subduction angle « tOO) ( Barazangi and lsacks. 1976). Two add itional effects were noted by C ross a nd Pilger. Accre tio n of sed iment in trenches ten ds ( 0 flatten the slah inclination at shallow depths, beca use the weight of t he accretiona ry p rism depresses the plate pr ior to subduction (Ka rig a aJ., 1976). Secondly , long-conti nued subd uctio n may t hicken the upper plate beca use of (he cumulative effects of accretion and depression of (he isotherms (James, 1972). tat~ n: 5S
il} (he uppe r plate and
Figure 5.7 Map sho wing [he relat io nship bet ween the positions of aseismic ridges o n the Nazca and Cocos pla tes and gaps in the volca nic arc. No te the wide gap associa ted with the Oblique Nazca ridge , di scussed in the text . From Nur and Ben -Avraha m (1983)
'"" o'
NAZCA PLATE
\ \ ec
'\
\""
\"" ZO'
=:::::::>
•
--- , -
..
ACTIV E VOLCANO
-
S[lS IiIlC I'\,I,N( CM' P' " lit ...
':.0
_ T J I E"'CH
110-
100"
'""
} ec
•
PLA T ES
sp nse!luently affecLl hc strucltlrss...p.uWuced. there . Mode rate 10 stee p subduction is asso d ated with extensi o nal stre §§, in the upper plate , and shallow subductio n with co mpressio n. These relati on ships are a co nsequence of t he relative size of the co nt ributions o f: (I) thermally-indu ced isostat ic up lift of the upper plates, co upled with the subd uctio n-suction force (prod ucing tension); an d (ii) the shear stress produced along the plate co ntac t (p ro. du cing com pression) . ~ la c:.s.;ue..noI~ mally in co.mp~ ion.. (see 2.6 ). Low-an gle subduction increases the area of mechanical co upling between the two pla tes, and allows shear st resses with a large component of hor izontal compress ion to be transmitted into the upper plate. Ste ep subd uction mini mizes th is effec t and maximizes the effect s of the extensio nal fo rces. As d iscussed earlier (2.5) th e stal e of stress
D UUoUUU_i,ljor acto descri ed above aU. co ntro l t
A~O M O V r ~ G
"" ..,'
CONvfRGfNl TECTON IC REGIM ES
across subduc tion zone s is affected mainly by the opposing effects of negative buoyancy generated by the subducti ng slab (pr oducing extension) and the comp ressive res istive forces opposing the conve rgent mot ion. As So lomo n and Sleep (I 980) po inted ou t, the general state of intraplate stress suggests that these two effects are broad ly in balance, with some subduction zones exhibiting net extension and others net compressio n. Bayly (1982) has poi nted o ut , fo llowing Scholz and Page ( 1970) that the curvature of arcuate tre nches , togeth er with the curvature of the Eart h's surface, must force a subducting slab into a mo re co nstricted space at depth, It is suggested that th is constriction is taken up by buckling below a depth o f about 100 km involving a range o f dips from 3SO to 55° in a slab initially dipping at 45° (Figure 5.8). There is some evidenc e tha t plate geometry as indicated by seismic foci distribution is compatible with this model . Structure of accretionary cpm plexes
A combination of se ismic refl ection, deep-sea drilling and side-sc an so nar pro grammes in a
11 9
number o f active subd uction zones has led to a significant increase in unde rstanding of the structu res and processes of defo rmat ion affecting accretionary sed imenta ry wedges at subduc tion zones. We sha ll exa mine in detail results from stud ies in the Peru trench in the West Pacifi c, the Barbados ridge in the Caribbean, the Makran com plex in the A rabian Sea and the He llenic trench in the Med iterr ane an . These examples cover the main types o f active subduction zone currently recog nized. Th ese stud ies have suggested that ~ruCl u r . I( a ctive accrctwnaI n s:ms...;(c a nalogues {. o nland fQl d- I ~..I.~~L. be lJ.S . The main processes a nd structures that have bee n inferred fo r accretionar y comp lexes are shown schematically in Figure 5.9 (see Scho ll et al. , 1980). The front or leadin g edge of the prism is domi nated by fron ta l accretion where by of/· scraping of the sed imentary cove r (F igur e 5.90) is achieved by a synthetic imbr icate thrust co mplex a bove a basal deco llement plane that underth rusts the sedimentary prism. T his basal plane and othe r majo r thrusts are thought to act as condu its for dewat er ing of the buried sediments. Furth er down the deco llement plane , underpla ting or 'subcrc uon' may ta ke
A
B
--I
ld .,! ... .,
rigu ~ 5,8 Schema tic mode ls to illustrate the buckling of a subducted slab du e to the geome tric sho rtening effec t brought loout by curvatu re o f the Ea rth's sur face (A) , and by the arcuate outcrop of the zo nes (8). After Bayly ( 1982).
120
GEO LOGIC A L ST RUCTU RES A r" D MQV l r"G PLAT ES
-..
~ ~"
.... --
~ -~
/
~=---=='==::: ;r .:-:-:.
fa>\""':-:
. -
"•
w
"w> o
20 0
I'il;ure 5.9 Struct ures and process es in an idealized accretio nary prism . (a) Zo ne o f frontal accre tion by imbrica te thrusting whe re the u pper pari uf the incuming sed imenta ry shee t is off5f: ruped; (" ) decollem en t be nea th whidl lhc dee pe r pan of the inco ming sec tion is unde rt hrust ; the dtcollcmClll pla ne and fault a t I t ) ma y se rve as dt'I'II
(n
place by the forma tion of thrust duplexes. which have the effect of thicke ning and raising the accretionary complex (Watk ins et al. , 1981) . More distal portio ns of the complex may be characte rized by strongly deformed, steeply dipping mater ial exhibiting tight folding and cleavage . Near-s urface parts of the complex may show gravity spreading and slumping of the sediment pile. Th e process of accre tion at a subduction zone may thus be visualized as the progressive upward stacking of the sedimentary pile by the successive emplace me nt of new thrust wedges at the base, causing ove rall thickening and uplift of the prism. Event ually this process produces a topographic ridge or trench - slope
break , which is separated from the volcanic arc or continenta l margin by an undeformed fo rearc bas in .
The sedimenta ry cover on the ocea nic plate is not all accreted on to the upper plate. Hilde (1983) has highlighted the importance of graben in preserving pockets of sediment which are then subducted along with the oceanic crust (Figure 5.10) , a mechanism first suggested by lsacks et al , (1968). Ocea nic plates are typically scarred by nor mal faults that are caused by flexural extension as the slab bends down into the trench . Accord ing to Hilde's model, the grabe n may be em pty on app roaching the tre nch but become filled tectonically with materia l scraped from the upper slab. Anoth er
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122
GEOLOGI CA L ST RUcrU RE.S AND MO VI NG f' LATES
tivcty small sedi men t volume in trench complexes com pared with the calculated accretion rates (Gillu ly, 1971 ). T he seco nd is the crustal isot opic signat ure of ce rta in mant le-de rived mag mas (see Mcken zie . t9R3). $.2 Some active subduction zones
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t' iR\lrt 5.10 (8) Mode l illus tra ting the process of sedi men t subd uction by tecto nic d isplace ment of sediment into grabe n. which preserve the sediment be neath the ove rriding plate. From WIde ( 1983)
mechanism for remov ing sed ime nta ry materi al fro m the inner tre nch slope is suggested by Cande and Leslie ( 1986) in a study of the south C hile t rench. T hey point to the large base ment re lief associate d with th e transfor m fault s, in which large pocket s of sediment can be preserved . and also to the erosive effect of major fault scarps jf they move ob liquel y across the base of the upper plate . Th e importance of these mechanisms is thai they provide answers to two problems. On e is the apparent discrepancy between the rela -
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w arsi et al. ( 1983) repor t the results of a study, com bini ng GLOR IA side-sca n sonogra phy and se ismic re fl ection data , on the Per u tre nch in the regio n of its intersection with the Mendano fracture zo ne (F igure 5. I IA). T his section of tre nch is interesti ng in tha t no obvious accretion o f sediments seems to occur o n the inner tren ch wall, and it has bee n supposed that sedime nt subd uction is do mina nt an d that the up per plate is being tectoni cally ero ded (Ku lm et al., 19RI; Hussong and Wipperma n, 198 1). T he Nazca plate in this regio n ex hibits a NNW - SSE sea- floor spre ading fabri c (F igure 5. 11B ) con sisting of fau lt block s formed at the sprea d ing axis. T he bloc ks a re generally tilted , and are bou nde d by normal faults with sca rps facing in bo th directions. Ind ividual blocks d isp lay tOO- 200 m relief and are draped by 100 -1 50 m o f pelagic sed iments. So me bloc ks ca n be traced for ove r IQOk m along st rike. T he Menda no frac ture zo ne co nsists o f a se ries of pa rallel ridges and troughs with up to 1 km relief. It widens from less th an 50 km across in the west to abo ut 100 km near the trench. Several sea- floor volcanoes were found , abo ut 10 km in diamete r and rising to over 700 m from the sea-floor. At abo ut 100 km west of the tre nch , the Nazca plat e starts to be nd do wnwards tow ards the trench . In this zo ne , the spreading fabric is ove rp rinted by a set of new faults sub-parallel to the trench axis and to the sp reading fabr ic in the nor th , but o blique so uth of 12°$. T he new faults displace the pelagic sed iments with maximum ve rtical throw s of about 200 m in so me cases , and have a spacing of 3-5 km . Th e fau lts form graben, some of which can be tra ced for mo re than IOOkm .
123
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The tr ench axis is turbidite-fi lled, with a flat l100r at a maxim um de pth of 6300 m. The turbidites mask the structures on the subducting plate in the north, wher e they are folded against the base of the inne r slope . Th e
tectonic defor mation front appears as a single co ntinuo us feat ure . It is believed that this tectonic accretion is tempora ry and that the sediments will eventua lly be emplaced in the graben by slumping and will be subducted.
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CON VHlG t;NT T ECTONI C IlE(OlMES
This scdimc r uury till thins southwards and terminate s at abo ut 11 5"5 . Reflection profiles across a large partl y-subductcd graben showed two stages in t he progressive fill ing of the graben by slumped sedime nts. the section nearest the t renc h ax is being the more co mplctcty fille d .
The Barbados ridg e co mp lex
This region , described by w est brook ( 1982) . has been inte rpreted as an e xa mple of a mat ure fore-arc co mplex. It p roba bly re presents an intermediat e stage bet ween the young, deepwater . accretio nary prisms associated with well-defined tre nches. such as the western America n exa mple alread y described . and the various uplifted . fossil co mplexes o f Mesozoic/ early Cenozoic age forming inactive arc systerns a round the Pacific rim a nd in Indo nesia . TIle Barbados ridge is a N - S eleva led structure lying 150 km cast o f the active volcanic arc of t he Lesse r An tilles (Figures 5.12, 5.13). Th is a rc ma rks the site o f the Lesser Antilles subdu ction zo ne. where the Ameri cas plate passes be nea th the Ca ribbean plate at a rate of abo ut 2c m/ year. T he arc is bou nde d o n both sides by major t ransfo rm faul ts ( Figure 6.12..1 ). Wes t of the volcanic arc lies the Gre nada back-a rc bas in. T he island of Barbados is the highest part o f the ridge , which extends for ove r 500 km. Th e ridge is an accre tionary co mplex mad e up of sedimentary roc ks lying in a linear trou gh in the volca nic base ment. The axis o f this tro ugh reaches a depth of 20 kill be neath Ba rbad os. a nd co rres ponds to an isostatic grav ity low . T he tro ugh is interpre ted as the site of the original trench marking th e line of s ubductio n. T he easte rn part o f the ridge is mark ed by a positive isostatic gra vity ano maly co rrespond ing to the outer tren ch slope of other complexes. Th e deformat ion fro nt lies at the eastern margin o f the co mplex. on the lower continenta l slope ( Figure 5. 128 ). T he co mplex beco mes increasingly deformed westwards . and the island of Barbados ex hibits stro ngly folded and faulted Eoce ne fl ysch ove rlain by
125
Upper Eocene 10 Miocene pe lagic sed ime nts. West o f t he ridge . and betwe e n It and the volca nic arc. lies the T obago trough. co ntaining about 3 km of undc for mcd sed ime nts. The style of deformat ion varies alon g the defo rmation front. In the so ut h. ge ntly asymmetric cast -facing folds with am plitudes of 500 m and wavele ngth s of R- 9 km ride on th rusts which dip westwar ds at 2lr . T hese th rusts are interp ret ed as listric faults de tac hing on a major decolle men t plane . paralle l to basem ent . that can be traced at least 30 km westwards beneath t he co mple x. Unde formed bedd ed sedim ents lie be low th is majo r de colle ment. In the north of the co mplex. the deform atio n is so inte nse and chaotic that no obv ious struc ture ca n be disce rned from the seismic pro files. T his change co rrespo nds to a nort hward s increase in slo pe gradie nt and to a decrease 10 width a nd height of the com plex. Th ese changes may relate to a nor thwards decrease in sed iment supply. A series of ste ps in the ridge topography ap pear to be caused by the inte rsectio n of ridges in rhc ocea nic baseme nt , These arc o bliq ue to the line of subduction a nd wou ld p roduce pe nding and latera l swee ping of sediments as desc ribed abo ve for the Pe ru t rench. Th e str ucture o f th e southe rn part o f t he ce nt ral Barbados ridge a nd Barbados trough has been investigated by mea ns of d etailed SlO A8EAM-sonar bat hyme tric mapping a nd a high-reso lut ion seis mic survey ( Biju-Duval et al. , 1982). T he seismic profiles ( Figure 5. 17) show clearly thc asy mme t ric folds an d reve rse faults o f the frontal ove rthrust region (F igure 5.14A ). To the west , broad. km-widc synclines a re evide nt . with syntec ton ic infilling in their upper parts . sepa rated by narrow asymme tric an ticlines associated with stee p reve rse fault s dippin g both cas t and west ( Figure 5. 148 ). O n the west side of th e Bar bad os ridge , the sedime nts arc defor med into ge ntle west-facing asy mmetric folds associa ted in places wit h westwards-directed reve rse fault s (F igure 5. 14C) . Th e ridge co mplex t hus displays a degree of ove rall st ructura l symme try. In t he nort h. th rusts ha ve been proved in
126
GEO LOGICAL SfRUCI'URES AND MOVIl" G PLATES
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GEO LOGIC AL STRUcrU RES AN D MOVING I' LAr ES
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GEOLOGiCA L STRU crU IlES AN D MOVIN G PLA TES ATI,.AMn C
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131
CONVER GE NT TECfOr-J1C REGI M ES
drill cores in the DSDP d rilling p roject leg 78A (Moo re et 01. , 1982). Nea r the postulated basal deco llement plane (which was not penetrated by the drill ing o wing to technical pro blems), drilling revea led zones of intense deformation with fract ured mudstone passing dow nwards into inte nsely fo liated 'scaly' mudstone revealing slickensides , a nd ultimate ly to a tectonic breccia . Abnorma lly high fl uid pressure s were measured . ro ughly eq uivalent 10 lithostatic pressure. The se high ffui d p ressures undouhredly facilitated the underthrusting process as o riginally envisaged by Hubbe rt and Rubey (1959). Co rrelatio n o f d rillco re sections with seismic profiles enabled Moore et al , to reco nstru ct the stratigraphy of the accre tionary wedge (Figure 5. 14D ) . T he offscrapcd sequence cons ists of Miocen e and younge r ocea nic deposits. Th e laye red sequence below consists of U pper Cretaceous to Lo wer Mioce ne pelagic clays, resting o n oceanic basement, which are heing underth rust below the younger deposits. T he Lesser Ant illes subduction zone has been in existe nce since the early Eoce ne (about
50 Ma IW), much lon ger than most ot her active subduction zo nes. It there fo re prov ides us with a use ful mode l with which to com pare supposed fossil examples in o rogenic belt s. Du ring th is time , the position of the trenc h migrated eastwards relative to th e South Ame rican continent and to the mid-Atla ntic ridge , owing to spreading within the Ca ribbean . An evolutionary model of the subductio n zone (Figure 5. 15) demonstrates how the tre nch has bee n first filled, then o bliterated by the building of an O ligocene accre tiona ry ridge (the pre sent Bar bados ridge ), ca using the Upper Miocene to Recent accretiona ry com plex to migrat e eastwards to its present positio n.
The Makran com plex
The accretiona ry comp lex of the Makran (see White , 1982; Platt et al. , 1985) lies along the cont inenta l margin of Ira n a nd Pak istan o n the north side of the G ulf of Oman (Figures 5.16, 5. 18). The complex is fo rmed by the no rthwards subductio n of the ocean ic part of the Arabia n plate beneath the Eu rasia n plate . The
J."iKu rr 5. 16 Location of the Makran accret ionary pns m in tbe Gu lf of Oman . Note tha t the accretionary prism is situaled at the: subduction zone marking the: boundary o r rhe Arabi an and Eurasian plates [inset) , and is truncate d on hs e:astern side by the Murray transform fault and its comine ntal contin uation. Stars mar k volcanic centres along the active: volcanic arc; black areas are ophiolite o utcrops ; thin lines on land are faults; ticked lines mar k boundaries of major de pressions. Aft er While (1982)
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subduction zone is terminated o n its eas tern side by the Murray tra nsform fault that separates the A rabian and Indian plates (see Figure 3.6) and continues no rthwards on the co ntinent 'IS the O rnach Nat -Chama n fault system (see Figure 5.30). The subduction zone ends in the west at the Stra its o f Ho rmuz, whe re the Arabian and Eurasian continents a re in contact. Sed iment 6-7 km thick covers ocea nic crust in the G ulf o f Oman , which is thou ght to be be tween 70 and 120Ma o ld. The active volcanic a rc consists o f a chain of Cenozoic volcanoes situ ated 400- 600 km north of the coast (F igure 5.16). The re is no top ographic trench, and the accre tionar y co mplex is unusually broad . about 300 km in width . mo re than half of which lies o nsho re. Seismic reflection profiles across the o ffshore part of the complex sho w a linear patte rn o f ridges with some intervening tro ughs (White . 19M2) . Fold ing appea rs to ta ke place initially at the southe rnmost o r fro nta l part of the prism which see ms to have migrated so uthwards at it rate of IUkm/Ma. The se fro ntal folds are then incorporated into the accretio nary complex by uplift along a basal thrust. Little subsequent deform at ion appears to have occurred in th is sector of the complex. Howeve r, 70 km (Q the north of the prese nt fro nt, a further uplift occurs which rises e ventually abo ve sea level IOHm nort h of the fro nt to form the o nsho re Makran . Her e a th ick, faulted fly sch seq uence is exposed . exte nding about 200 km inland to the no rth. The onshore str ucture , summarized in Figure 5. 17, is described by Platt et 01. ( 1985 ) . A co nco rdant seq uence of ma rine sed iments
133
co mmences with Oligoce ne to mid -Miocene abyss al plai n deposits. fo llowed by Upp er Miocene slope deposits. and by a late Miocene to Pliocene shallow-water shelf sequence . indicating rapid shoa ling o f the sed ime ntary prism in the mid-Miocen e . T here is appa rently no fi eld evide nce for accre tiona ry structure prior to the depositio n o f the slope and shelf sediments . Nor is there evide nce for the progressive growth of structures d uring deposition, althou gh the growth of gentle folds might be undetectable owing to the effects of the la ter de fo rma tion , which caused 25 -30% shortening. T his main deform ation occu rred afte r the earl y Pliocene (4 MOl f1p) at a time when the accr eti onary fro nt prob ab ly lay 70 - 100 km so uth of the presen t sho re line , and has resulted in a se ries of E- W to ENE - WSW , asymmetric. S-facing folds and associated reverse faults (Figure 5.17B ) . The uplift of the onshore Makra n and the acco mpan ying defo rmat ion are tho ught to have been accomplished by underplating at depth (see Figure 5. 17B). T he au thors sugges t that th is process may have o perated by the form atio n of a p rogressively widening duplex at a ramp in the basal thrust. Such a structu re would cause tilting of the uppe r part of the seq uence , leading to shoa ling and possibly to syn-sed imentary defo rmat ion , but not to major fo ldin g o r faulting. Plait et al. a tte mp t to apply mass balance ca lculatio ns on the accretiona ry process by co mparing the likely quanti ty o f sediment input with the estimated volum e of the prism. T he y show that a significant proportion of the sedime ntary sequence must have been und er -
Fillurt 5.t7 Structure of the onshore Makran complex . (A) Simplified 5u uetural map of the coasrat Makran. showing major rotos and reverse fa ults. Note laleral facies change between Ta tar and Parkini rormancns. After PIau d al. (I 9RS). (B ) (a ) Section across Ihc Kulan,h syncline (see line on Figure A) showing the retau onship between structure and Iacies . Dashed lines represent lime plan es, (b) Interpr etalion of la) during the Pliocene . suggesting Ihal the shelf and slope sequences may have been dcpo sucd on an undisturbed abyssal plain sequence tha t was being uplifled along a majo r dku llemenl surface. Note lhe suggested duple. seructure causing co ntemporaneous uplift of the non hern limb of the Kulanch synd ine. I() Seetit'n across the faulted sequence easr of sccnon (A ) . illustra ting Ihe senes o f northward-you nging sequences bounded by reverse Iaults characterizing the southern limb of the Kulaneh syncline. Pg. Panjgur; Bg. Branguli; Pk . Parkini formauons: aHOWS represenl younging directions . (B ) and (C) fm m Piau ~I al . ( 1985 )
134
GEOLOGICAL ST RUCTURES AND MOVI NG PLATES
pla ted or subd uctcd , enough fo r a layer arou nd
6 km th ick above Ihe ocea nic crust (sec Figure 5 .178). The A egean arc
The co mplex Aeg ea n region (Fi gure 5. 18) has been inten sive ly studied o ver the last deca de . The basic plate bou ndary netw o rk and re lative plate mo tions were es tab lished by Mckenzie ( 1972), and s ubseque ntly refined in a comprehensive review of the neotectonic patt ern of
the region using earthquake data , La ndsat photographs and se ismic refraction recor ds (Mc kenzie. 1979b) . Seve ral distinct tectonic units are appare nt. T he H ellenic french, alo ng which the Af rican plate is descen ding be low E urasia, lies immediately sou thwes t and south o f the Hellenic arc, wh ich is a no n-volca nic isla nd a rc exte nding from mai nland Greece west of the Pelopcnnesos to C re te and Rhodes.
30
Black Sea
. 0
On the sout h side of the tre nch is the bro ad Mediterra nean ridge o n the sea-floo r betwee n G reece and No rth Africa. No rth of C rete lies the Cretan Sea basin , T he active volcanic arc runs from the eas tern Pelopon nesos t hrou gh t he southe rn Aegean Sea to the coas t of Anato lia. A large tectonica lly active region north of t he volcanic arc co nsists of the A egean Sea basin and the ad jo ining land masses of main land G reec e in the west and An atolia in t he east. T his E urasian hinterl and is divided into two sepa rate pa rts by a major fault syste m con necting the north en d of t he subd uction zone with the Nort h Anatolia n st rike-s lip fault runn ing along th e so uth side o f the Black Sea. Th e region south of this fault is recognized as a sepa rate small plate , the Anat o lian plate , which is moving westwards as a result of the N-S co nvergence of the A rab ian and E urasian plat es. The Hellenic trench syste m is desc ribe d by
'0
30
AFRICAN PlATE
Figure 5. 18 Map showin g lhe loca tion of the Aegea n arc in the gene ral tectonic selling of the Easte rn Mediterra nean region . Note the deformed so uther n edg e of the Euras ian plate to the nort h. and the African and Arabian plates to the south . The large arrows de note the movement vectors of the sout hern plates in relation to each othe r and to the Eurasian plate. assumed to be stati onary . Movem ent direct ions of the Anatolian and Aegean blocks rela tive 10 the main Eurasian plate are shown hy smaller arr ows. After Merci er ( t98 1) .
CON VERGENT TECTON I( REGIMES
Huchon et al . ( 1982) who repor t the resu lts of a
oa red by large nor mal fau lts with well-de fi ned slicke nsides. Hu chon et al , bel ieve therefor e that the inne r wall falls within the e xten sional tec ton ic province 10 the nor th of the subduction zone. In contrast, the ou ter t renc h slope and trench l100 r arc characterize d by obvio us compressional featu res in the form o f fo lds , small th rusts. and con jugate st rike-s lip faults. T his zo ne therefore be lo ngs to the co mp ressio nal domain indica ted by the ea rthquake focal mechan ism solutio ns. Le Pichon et al, ( 19l:l1) report the results o f se veral SEAO (,;AM so na r traver ses across t he Mediterranean ridge. It had previous ly bee n suggested by Ryan et ol , (1970) that most o f the sedimentary co ver o n the Mediter ran ea n floor is bein g tecto nically thickened along this ridge , rather than being accreted at th e tre nch. T he survey by Le Pichon et at. sho wed the presen ce of an extensive fold syste m affecting the 34 km o f uppe rmost Miocen e (Messinian) to Oua tcm ary sedimentary cover. A set o f co njugale st rike-slip faults cuts the folded sed iment s, The co mpressional tecto nic regime , which occupies the no rthe rn half of the 250 kmwide ridge , extends as far as the trench axis as described above . Th e Mediterran ean ridge is thus interpret ed as an accretio nar y structure , similar to the Barbad os ridge already desc ribed, whe re an upper layer o f soft sedime nts is be ing deformed abo ve a deco lleme nt horizon . It is sugges ted (Le Pichon et al., 1982) that a thick layer of evaporites may play an import an t role in t he deco upling of the deeper su bducti ng layers from the shallow shorte ning layers,
so na r survey o f the det ailed mor phology and struc ture of par ts o f the tre nch system . T hree main !'\W- SE seg me nts. the Matapan, Go rrys and Po seidon tre nches, are separated by NE - SW line ar trou ghs, the southe rn o f which of fset s the tren ch sinistrally (Figure 5 . 1 ~) . T he trench sh allows so utheastwards from ove r 5 km 10 a round 3 km de pth . At its eastern end , the Poseidon trench bends sharp ly into the t\: E-SW Pliny trench , T he Srrabo tre nch lies furt her to the so ut heast, parallel to th e Pliny tre nch but no t d irectly connected with it. Th e Strabo tre nch is re latively shallow and poo rly defi ned . Bo th these southeaste rn trenches have litt le sed ime ntary cover. The present co nve rge nce vector across the tre nch is abou t pe rpendicular to the main tren ch but makes an ang le of about 35° with t he two sou theastern tre nches ( Figure 5.19). T he result s o f the deta iled survey by Hucho n aal. revealed a set of ridges an d tro ughs on the outer slo pe of the Mat qpan t renc h, gene rally parallel to the trench axis. These arc inter preted as folds. Within th e Pliny t rench, howeve r, the structure is quite dif fere nt. A series o f e n echelon troughs about lO km long and 2 km wide occur with in the main tren ch, oriented abou t 15° anticloc kwise o f the ax is. These arc inte rpreted as sinist ra l strike -slip fault segments, which are co nsisten t wit h the oblique co nvergence vecto r. The inne r wall of the Matapan t rench was examined directly by sub mersible , It see ms to be re latively inactive tec tonically a nd is domiSEABEA .\l
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135
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Figur e 5, 19 Sche matic map of lhe Hel lenic trench system, showing the five main segment s in rclanon 10 the co nvergence di recti on be tween Ihe Afri can and Aeg ea n plates : see also inSCI veelor Iriangle rela ting 10 African (AI') , Aegean (A e) and E urasian (Eu) retanv e plait' motion . A(le r Huchon tl ai, (1982).
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Figu re 5.2 1 Teetonl Cimc rprc tauo n of lhe Aegean Se4 ( A) Tec tonic summary. based o n infor mallo n fro m Figure 5.211 a nd adjoi lling a reas. sho wing inferred c ~ t e n~io n d trectic ns o n land (heavy arrows) and se ~ a reas (light a rro ws ). No te me s wing in tre nd (bro ke n hnes ) fro m NW - Sl: in thc westto NE - SW in the ca vt. A fte r A ngel ic! (I (1/. ( 1982). (8 ) Com pute d res to ra tion 10 its o riginal sha pe ( I) of Ihc present a rea (2); the melho d uses est imat es of the CXlc nsiOIl facto r, P. which var ies in the range 1. 1-1.9. resto ring to eq ual sq ua res. Fro m An gelic! <'I ul . (191'2 )
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138
GEOL OGICAL STHUCTU Il.ES A ND MOVIN G PLA f ES
f\tc K enzic ( 197Xa ) showed that rapid extensio n is taking place in the Aeg ea n Sea and adjoining mai nla nd areas , and thai the crust ben e at h the Aegean Sea is o nly about 30 km t hic k, compared with SOk m in the main lan d a reas . Foca l mechan ism so lutio ns of eart hq ua kes in the regio n of the Hell enic tren ch indica te thrust mot ion with a mea n s lip vector o rien ted at 2 110 o n a shall ow pla ne dipping NE . T hus th e co nve rge nce d irect ion across the Helle nic tre nch appea rs 10 be constrained by southwestwards motion o f the A egean region
perpendi cular 10 the central sec tor of the tr en ch ( Figure 5. 18) . In co ntr ast , the focal mecha nism solutions fo r shallow earthquakes in th e Aegean Se a region indicate a preponde rance of norm al di p-slip motion , but with a st rike-slip co mpo ne nt in so me cases . On mainla nd Greece . ear thq uakes with simila r foca lplane so lutio ns a re asso ciated with su rface normal fa ulting. Th e strike of the dip-stip motion ap pears to vary from NW - SE to E - W . Mc Ke nzie sugges ts an ov erall NNE- SSW extension . In-situ str ess de te rminations using the ovcrcori ng tech nique - (2.6) ind icat e a predo minan tly N - 5 hori zon tal exten sion over the who le area of mainland Greece and the Aegean basin (Paqui n et 01.• 1982). A n a na lysis of the geom etry of recent fault pattern s in the Hellenic arc and adjoining Creta n Sea basin is reported by Angel ier et al, (1982), using dat a from land 'surveys. satellite pho togra phs and SEA BEAM surveys. Th e a ut hors find that mo st of the late Miocene to R ece nt faul ts of the so uthe rn Hellenic arc (principa lly Crete ) a re pure normal fau lts which str ike either pa rallel o r o blique to the trend o f the arc (F igu re 5 .20) . T he Cretan Sea basin is also dominated by nor mal fault s. some o f wh ich can be traced o nshore . Most fau lts here are pa ra llel to the trend of the arc, E- W in the ce nt re, swinging round to NW-SE in the west and to NE- SW in th e east. These result s co nfirm the exte nsio nal nature of the back-a rc regio n (F igure 5.2I A ). It is of interest to obse rve th at this extensional region includes the pr esent vo lcanic are , un like the oceanic examples discussed earlie r (sec 4. 3) . Ange lie r
et
al, note that if t he ar c is restored to its presumed pre-stretchi ng positio n . t he o rientatio n of the fau lts be comes much mor e uniformly E - W (Figure 5.2 18) . The upl ift of the arc might at first a ppea r to be inco mpatible with an ex tens io nal regime . The a uthors suggest. however , that the lower 3-6 km o f sed imenta ry co ve r, which is initially subducted at the tren ch , becom es und erpla ted onto the uppe r plate below the Helle nic are, thus ca using the up lift. T he Aegean basin the refore appe ars to form a se para te micro-plat e , represen ting the upper slab of the subductio n zo ne , which is movin g rather rap idly so uthwest over the African pla te crea ting a zone o f exte nsio n, pa rticularl y in its no rthern pan ( Figure 5.18). Th is ba sin rc prcse ms a specialtype of ba ck -ar c sp read ing ba sin formed on co nti nental crust. but is in some respect s an alogous 10 the oc ea nic spre ading bas ins d iscussed in 4.4 . Lc Pichon and A ngel ier (1979) sugges t thai this movem e nt is a resul t o f the withdrawal southwards of the subd uct ing slab (co mpare the trench ro llback mechani sm discus sed in 4.3). T his mecha nism is tested using a finite eleme nt model by De Bremaec ker {'J at. (1982) and shown to give a much better match to the ob served stress field tha n two alte rnat ive mod e ls: the Ar abian inden ter mod el a nd the grav ity-spreading model based o n the clcvaticn differen ce between Aege a and the Med iterranean floor .
5.3 Collision Th e co llision of two pieces of co ntine ntal crust is an inevitable con seque nce of the co ntinued subd uction of oceanic lithosph ere . T he mu ch greater buoyancy of co ntine nta l co mpa red wit h oceanic crust mak es the forme r d ifficult if no t impossible to subd uct. The rel at ion ship between subd uction, collision and orogeny in the new p lat e tecton ic t heory was clearly illust rated by Dewey and Bird (1970). They recognize two type s of co llision , continent island arc and co nti ne nt -cont ine nt , and demo nst rate that the collisiona l oroge nic belt s
CO ~ V E R(; E N T
so for med di ffer fund a me ntally fro m the asymmet ric subd uctio n orogenic belt s of islan d a rc or continent-ma rgi n type already described . Dewe y a nd Gird Illustrate the simplest such situa tio n (Figure 5.22): the co llisio n o f a passive co ntine ntal ma rgin with a n active or subd uct ing co ntine ntal ma rgin . Clea rly subductio n mus t precede collision . so that the effects of the subduction 'o roge ny' must be incorpo ra ted int o the su bsequent co llisio nal orogen ic belt. Th e pol ari ty or asy mmetry o f the subd uctio n structure con trols. al least initially. the colf..ional st ructure. T his pr incipie is illustra ted in Figure 5.22, whe re the left dippin g und erth rust struc ture of the subd uelion zon e co ntinues as rightwa rd-di rected o ver thrustin g a fte r coll ision. va rious alte rna tive and more com ple x sce na rios a re possible of course: bo th op posing margins ma y possess
trench
deep -se e
continental $hell
tlgure 5.22 Diagrammatic seque nee of st,lge~ in lhe u ausro rma non o f a subductio n lo ne to a continental ooUision zo ne by the approac h uf two contine nts and the closure: or the inte rvening ocean . Afte r D ewey and Bird
( 910).
139
TECTON I C Ri'GI MES
sub d uctio n zon es for e xample , a nd co mplex collisio nal belts may be form ed by mult iple accre tio n o f islan d arcs or s mall co nt ine nta l fr agm ents. T he wide Cen tral Asia n o rogenic belt has been cre a te d in th is wa y, as we shall see . Conv er gen ce con tinu es aft er initial con tact o f the opposing pieces of bu oya nt cr ust. The e xte nt o f this co ntin ued converge nce is the single most importan t factor in the crea tion of a n oroge nic o r moun tai n bell. since continent al co nverge nce leads to crusta! thicke ning and con seq ue ntly to isostati c uplift.
,m'e "u/h:mm
mn
Recent mount ain belt s a tt ribu ta ble to contine nta l co llision a rc fou nd mainly alon g a comp lex zon e co nn ecting the .A.lP:i._ the imal-j'~ a nd J.otJone£ia- Thi s zo ne is the result o f the l~L.Oi · Genomi0 co nvergence of u..ralioi<W\o'illl-A fr ica . A rabia . I ntl i a ~ n d A u s... ~ ral i
140
GeOLOGICA L $T II.UC I U RE$ ANn MOV ING Pl.ATE S
obliquely ac ross this re gion co nnecti ng the no rthe rn an d southe rn branches. T he co mplexi ty o f t his ar rangement W <-l S a ttr ibuted by De we y a af. ( 1973) to th e move me nts a nd inte racti ons of va rious m icr opl at es th at existed in the Med ite rran e an region during the co nve rgence bet wee n E uro pe an d A frica . These mo vem en ts we re con trolled in pa n by major cha nges in th e co nverge nce vecto r bet ween the
ma in plat es. Since t he Med iterranean Sea has no t co mpletel y closed . the full e ffects of co ntinent a l collisio n have yet to be expe rienced . T he Alp s a nd th e Pyren ee s a re e xa mples o f the co llisio n of rel at ively sma ll pia les ( Iberia n a nd Adri atic) with Europ e . Th e cl ima x o f th e com pressio nal movement s th ai produced these c hai ns occurred in Ol igoce ne and Miocen e times , a nd the be lts e xhib it o nly res idua l tecto nic ac t ivity . Th e A lps ar e descr ibed in C hapter R as a n exam ple o f a Phanerozoi c orogeni c belt . Fur t he r ea st, the Zagros mountain s ma rk the site of co llision be tween the E uras ia n and Ar abian pla tes ( Figure 5. 18). Dewe y a nd Bird noted t hat the Za gros cr ush zon e mar ks th e suture bel ween th~ two plates . A Mesozoi c seq ue nce o f oph iolit e . chert, Ilysch and me la nge ma rks the s ite o f a subd uct io n zo ne alo ng th e so uthe rn margin o f th e Ira nia n pla te a u. T he th ick Phane ro zoi c ca rbo na tesha le cove r o f th e Arabi a n shie ld to th e so uth , which is underthr usting th e Ir a nia n plat eau , is deforme d in a se ries of asymm et ric folds facing sout hwes t , de ve lo ped abo ve a ba sa l decoll eme nt hori zon in late Precam br ia n to Ca mbria n sa lt deposits. Thi s zo ne passes la te ra lly into the Mak ran subd uct io n zo ne a lready descr ibed . A str ike -slip zone co nnects t he east e rn e nd of the Mak ran complex to th e major co llisio n comple x of Cen tra l A sia , produ ced by th e co llisio n with Indi a . This zone (d escribed he lo w) co nsists , in addi tion to the main Hi ma la ya n ra nge, a num ber of re lat ed morpho-te cto nic units inclu ding th e Tibetan plat e a u and th e Tle n Sha n a nd A ltai ra nges fa r to th e no rt h. Th e e aste rn end of the co llisio n zo ne connects via a lar gel y st rike-slip bounda ry with th e Indo nesia n subd uction zone . Her e the oce a nic
pa rt o f th e Ind ia n plate is being dest royed in a n arcuat e zon e from th e Andama n island s alo ng the Sunda arc as far as T imor. T he easte rn co ntinuati on of this he ll, fro m Ti mo r 10 New Gui nea, is affecte d by the coll ision o f the A ustra lian co nti nen t al crust. Thi s are a ex hihits a rel at ivel y juven ile phase of co ntinen tal coll ision . a nd the Ban da arc is an ex a mple o f a co nt ine nt- isla nd ar c co llisio n o ro gen y (see 5.5 ). ,r) .. r/l ·tfir
CO
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It" I
A ctive a nd rece nt mou nta in bel ts a re cha ractc rized by th ickened crust (to be twee n x 1.5 and x 2 no rma l t hickness ). T hus thic knesses of .n re co mmo n , a nd ove r ' 1
CO N V ll~G t NT
141
TE CTONI C RI'.GI ME.S
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Upper mant le
Fi gur~ 5.23 Bou gue r and isos tauc gravity anu mal y and crusta l stru ctur e profil..:s across the w este rn Alp s. Pcwave se ismic velocities arc given in krn s "'. Not e that the mountain range IS isosuuicatly compensated hUI 'hal a posiuv c a noma ly is associated Wil li the d ense lvrc.. pcndotuc . inte rpre ted a~ u pth rust mantle m;lIcri,11 (sec sccnon x. f ) . From BOll (1'J7l)
con taining yo ung granite batholiths such as the Andes. Flak e tectoni cs and obduction It was shown by O xburgh ( 1972). based on studies in the Eas te rn Alps . that co llisio n may involve detachme nt and overr id ing of pari of the crust o n to the opposing co ntinent while the remainder o f the crust and lithosphe re descend ed below it (Fig ure 5.24) . T his process was termed fla ke tectonics. In the Eastern Alps, a pre-Mesozoic metamo rphic basemen I of the Europea n plate has bee n overridden from the south by an allochtho nous thrust sheet of pre-Up per Palaeozoic crys ta lline base-
rnc nt derived fro m the so ut hern plate . Be-
tween the two base ment shee rs lies a highly deform ed pelagic and vo lcanic seque nce derived fro m the intervening ocea nic area . now largely subducted. Oxburgh suggests thai the initiation of the flaking p rocess is d ue to the buoyancy and to pograp hic ele vat io n o f the opposing contine ntal margin, and that a lowangle crustal split propagated back into the adjoi ning plate along a convenient zone of weak ness. He po ints out that the separatio n of the uppe r third o f the continenta l crust would redu ce the buoyancy of the subd ucted crust 10 one-half its o riginal value. T his would facilita te co ntinued subduction and allow converge nce to proceed . The existe nce o f mid-crustal de-
142
GEO LOGICA L ST RUCTUR ES A ND MOVI N G PLAT ES
A
B
T rench
j
y
--- .
.... ... . ..... ... .. .. . .... ...... ...
'
c y Fi p;ur~ 5.24 Th e flake tectonic mech anism . (A) Ca rtoon sho wing the overth rusti ng uf the up per pa r! of the cr ust (ro m contine nt C o ver thai o f contine nt A , and thc und e rlmu Sling uf the lowe r pari o f C following tile suhd uctc d oceanic crust {blac k}. Th e unit 8 is mar ine sedimen ta ry co ve r fro m the regiu n be twee n the twu co nt inent s. A fter Ox bu rgh ( 19n ). ( B . C) Ca rtoons s ho wing the Icrmat ion o r a crusta l flake by the detach -
\
.;; -
-
-
'- -
- - 1/
. . . .... . . . .. . . . .. . . . . . . . -
menl of a leading pa rt of the continent at X whe re the co llision al s tresse s will be
co nce nt rat ed . Fro m O xbu rgh ( 1972)
tachmenI ho rizon s d iscusse d in 2.7 (see Figure 2 .29) would ass ist t his pr ocess . A similar interpretatio n is applied to t he Himalayas (sec below) . Wh ere detachment of the who le crust takes place , the process has bee n termed A mpferer subduction o r A- subdu ction , to distinguish it fro m subduction of th e whole lithosph ere (8subduction) . Th e basal crustal weak zone (see Figure 2.29) is a part icu lar ly favourable site for detachment , and exp lains the occurre nce of very high-pr essu re meta morphic rocks within orogenic belts. II was realized by Cole ma n (1971) and
.... Dewey and Bird (1971) that the ophiolite co mplexes of o rogenic bells co uld rep resee frag me nts of oceanic crust emp laced on 10 co ntinental crust by a process which was termed obduction. T heir p resen ce co uld therefore be used as a valuable indicato r of a suture representing a forme r subductio n zo ne. This idea is now ge nerally accepted . A n initial prob lem with the obdu ctio n process was why dense ocea nic crust sho uld some times be detached and thrust over less dense co ntinental crust rather tha n be subd ucte d . Dewey and Bird (1971) illustrate thr ee poss ible ways iB which op hiolite obductio n co uld occ ur (Figure
143
CONV£RGENl TECTON IC REGI M ES
A
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Hl: urt 5.25 Pc ssibfc me cha nisms of obd uctio n. lA. B) Obd uct ion re sulting fro m the back-th ru stin g o f o cean ic litho sphe re o f the leading pla te on to the con tinenta l part o ( the lead ing pla te. (C) Upli ft uf rhe upper ocea nic plate in an intr a -oce anic subd uction zone resu lts eventua lly in its e mplace me n t o n lhe app roaching co ntine nt. (0 ) O bd uction o mc th e uppe r pla te o f ma rginal basin Iu hosphcrc belonging to the up per plate . Fro m Dew e y a nd Bird (197 1)
5.25): (i) compressional deformation of the desce nding slab, as found in the Medite rranean ridge . involving thrust wedges of oceanic hasement ; (ii) ove n hrusting of an oceanic uppe r plate du ring subduction; and (iii) backthrusting of ocea nic crust in the upper plate on to the continent. O phiolite sequences form linear helts extending up to several hundreds of km along
strike but never exceed 15 km in thickness. A typical cross-section exhibits the four main oceanic crustal layers: pelagic sediments, pillow lavas. sheeted dykes and laye red gabbros , overlying ultrabasic mantle-type mater ial, but there are significant differences be tween many ophiolites and the standard ocea n ridge crustal section. For example. typical ophiolites exhibit a basal crustal laye r which is much reduced in
144
GEOLOG ICAL STRUCTUIlES AND MOV ING PLAT ES
t hickness. an d there is co nside rable var iation among o phio li tes in th e development o f the
shee ted dy ke layer - in so me , it is co mpletely a bsent. It has bee n sugges ted that . fo r these and other reason s, many ophio lites re prese nt ano malou socean lit hosphere prod uced in back-
a rc sp reading basins rat her t ha n in true ocean s (Miyash iro , 1973). Sp ray ( 1IJH3 ) d raws 1I1Ic n lion to t he sig nifica nce o f the basa l tectono metamo rphic zone or 'sal e' fou nd in many
ophiolite co mplexes. T hese highly de fo rmed zones acq uired their fabr ic at high te mper alures, up to gran ulite facies in some cases, a nd we re forme d , acco rding to Spr ay . while the ocea nic lithos phere was still hot, an d within 5 Ma of their init ial magm atic crystallizatio n. This suggests that the init ial decoup ling o f the o phiolite too k place , at or ncar its origin at a sp read ing cen tre , along t he lithosphere ast he nos phere boundar y. T his bo und ary wou ld be situ ated at a dept h of only abou t 25 km in litho sphere less t han 5 Ma old . T he possibi lity the refor e a rises t ha t the det achmen t alo ng which an o phiolite eve ntu ally beco mes o bd ucted was created as a result of tec tonic activity at the spread ing cen tre . perhap s unrelated (0 the co nve rge nt mo vements which ca used the o bd uctio n. Thrust belts It is already appare nt that the geo met ry of co llisio n zo nes favour s the initiation and deve lopment of thrust belts within the co ntine nta l cru st. We have seen that such belt s are fundame ntal to (he accretio n process in subd uction zones, and it is to be ex pected that th ese zones sho uld to some extent co ntrol subse q ue nt s ho rtening of the crust du ring the co llisio n p rocess. T hrust belt s fall naturally into t wo classes of po larity: synthetic belt s d ipping tow ards the co ntin ent . parallel to the initia l subductio n zo ne . and antithetic be lts. dipping in th e op posite d irectio n. typically fo und at the o uter margin of an o roge nic belt . se pa rating it fro m the undeformed stable crato n or fo re/and. Such belts are termed [o reland thrust be lts , Complexit ies occur if the flake or
A-subd uctio n process supe rimposes antithet ic thrusting on syn the tic, o r where collision takes place he twee n co ntine nta l margins wit h th rust belts o f opposed po larit y. T hese pro blem s were first clearly stated hy Roede r ( 1973) in an anal ysis of the geo me tric relat ion ships bet ween thrusting and pla te mo ve me nts. Th e bas ic po lari ty of t hrust-driven co llisio nal shorte ning is det ermin ed by the pre-existing subd uct io n zo ne . Cont inued co nverge nce after the initial co nti ne nta l co ntact is e nsured by the fact th at the negative buoya ncy fo rce provided by the sinking sla b is still acting on the unde rth rust plate , as lo ng as it remains at(ached . Most co llidin g sla bs will have sect ions alo ng st rike that are still subducti ng. in all the major co llision oro ge nies d iscussed he re , subdu ct ion o f ocea nic lithosph e re co ntinues along part o f t he destruc tive boun dar y. For examp le, the India n plate is still partly dr ive n by slabpull in Indon esia . the A rab ian plate at the Makran , and the African plate at the Hel lenic tre nch . Moreo ver the ridge-pu sh force co ntinues to ope rate as be fore . T hese fo rces arc co unte ract ed by a co llisional resist ance force (see 2.5) which must increase with the ex te nt o f crusta l o ve rla p a nd th ickening . If we take the Hima layan co llision as an example , this process o f crustal convergen ce may last for up to about 40 Ma. T he way in which the process see ms to o pera te , by underth rusting of co ntinen tal crust , force s the o roge n to deform internall y in an asymmet ric manner. T hrust be lts may be d ivide d int o thinskinned or thick -sk inned (Figure 5.2 6) depending o n whethe r the ba sal o r sa le th rust sha llows at de pth or stee pe ns downwards to meet the base of t he crust. Rece nt geo met ric and kinematic mod els of thrust be lts have been deri ved mai nly fro m wo rk in the thi n-ski nned Rock y Mo unt ains be lt ( Bally et al ., 1966; Price . 1981), in we ll-bed ded sedi me ntary roc ks, and ap plied to the Mo ine thrust zo ne (Elliott and Joh nson, 1980; McClay and Coward . 1981). the Scandinavian C aledo nides ( Hossack . 1978) , the A ppalac hians ( Hatche r, 198 1; Brewer a al., 1981) , an d elsewhere . Useful summaries of the geome try and
145
CQ:,,/ V!.'. I(GENT I"C ro NI C II.EGIME S
"
- ..... '
~lgur~ 5.26 Pro files illus lnlli ng thin-skm ncd (upper lWO) a nd thsck-skin ned (lower lwo ) thrust tectonics. All scc uon s a rc true scale . Fro m So per a nd Ba rb... r (19S2) . willi pe rmission , a fter Ha tche r ( 19SI). Price (19RI) . Hsu (1979 ) a nd Shackleton (1981). respe ctively.
mechanism o f th rust zones are provided by Dahlstrom (1970) , Royer and Elliott (1982) and Butle r (1982). In essence. sho rte ning is achieved by a process of thickenin g by crustal overlap. whereby o lde r. or structurally lower. material is slacked upon younger , o r stru cturally higher materia l. Th e stacking IS
achieve d by tra nsfer along thrusts which ha ve a staircase tra jectory of alternating flats and ramps . T he geo metry is similar to thai for exte nsio nal faulti ng. described in 4.4 (see Figures 4.25, 4.26). Supe rimpositio n o f hangingwa ll ramps upon footwall flats produces geo metrically necessary folds in the hanging-
146
G EOLOGICAL STRUCTURES AN D MOV ING PLAT ES
wall . a nd the la ter al mo vem en t o f a thrust sheet fro m flat to ramp to rlat pro duces continuously migrat ing zones o f i nterna l strain in the mo ving sheet. W hen move ment of the
first thrust hccomes d ifficult , du e to increasing resista nce, a new thrust pro pa gates fo rwa rds. co nnec ts upwa rds with t he old . and tra nsfers the no w inactive upper thrust pas.s ively forwards in ' piggybac k' ma nner. A se ries of imbricate thrust wedges (h orses) forme d in this way forms a duplex structure. The duplex has an active floor thrust and a n inactive roof thr ust. Stacked duplexes may for m to prod uce nappe co mplexes such as tho se of all the major thr ust belts. Excellent examples may be seen in cross-sections o f the Himalayas ( Figure 5.3 6), the Rocky Mo unta ins ( Figure 8 .1 1) and the Mo ine thrust zo ne of the C aledo nides ( Figure 8.23). Th e ab ove syste m achieves the objective of sho rte ning t he cove r in an o roge n , but avoid s the proble m of how t he ba se ment is sho rtened, and how the dis place ments a re tra nsferred thro ugh the lo we r crust an d ma ntle lithosphe re . Thi s pro blem is add ressed by Cowa rd (1983) who poin ts out that the ev idence from the in ne r par ts of orogeni c belts s uch as the Alps and the Himalayas indicates the importance of steep thr usts o r shear zo nes which tra nsfer de e p cr ustal rock s to the surface (F igure 5.35). Matt auer (1986) s ugges ts thai sho rte ning in the Himalayas has been ac hieved by sub-ho rizo ntal displace men ts alo ng maj o r decolleme nt ho rizo ns at ( i) t he baseme nt cover co ntact , (ii) t he mid-cr usta l sei smic disco nt inuity. and ( iii) the base of the crust. These d isplaceme nts are t ransferr ed upwards along stee p ramps co nnecting the maj or detachments. The style of deforma tio n varies co nside rab ly with cr ustal level. T ypica l thin -skinned fa ult fold mo rphology associated wit h cataclastic deformatio n processes in discrete zones gives way downward s to more pervasive plastic deformation with the deve lopment of slaty cleav age , and to wide zo nes of du ctil e deformation at high metam orphic grade s. In the Himal aya n mod el (Figur e 5.3 5), t he th in-
skinned t hrusting is a hig h-level ou te r ex press ion of disp lace me nts of an esse ntially thicks kinned nature invo lving the whole crust. Since she ar zo nes widen with de pth due to rise in am bient tempe ratu re (sec c. g. Lockett and Kuszni r. 1982) , dis place ments with in midd le an d lower crustal rocks are d ist ributed th ro ugh wide zo nes o f ductile defo rmation , which may am algam a te to invo lve mo st of the lo we r cr ust. The o rigin of such wide be lts of deform ati on , when they arc fo und in old o rogeni c be lts, may no t be obv ious. In an ideal shear zone , a thrust d is placement is tr ansformed into a zo ne of simple she ar. However , in rea l shear zone s a compo nen t of shorte ning or exte nsio n across the zon e result s in the supe rimposi t io n o f a pure she ar co mpo ne nt to the simple shear st rain of the ideal zo ne (see Figure 3.15). T he pu re shea r compo nen t will becom e increasingly importan t with de pth due 10 the combined effects of gravitational load and el evat ed tem pera ture , en hanci ng t he du ctilit y o f the rocks ( Figure 5.2 7). Unle ss the stra in patt erns of highly deformed metam orph ic belts can be geomet rically re lated to high-level displa cem e nts, as is possible to so me ex te nt in ce rt ain young mo untain be lts. the ir origin may not be obvi-
•
t
I
I
"igu rt 5.21 Cartoo n to illustrate the variati on in slyle of deformation down ward s in the crust , from disp lacement do minated at upper levels to hulk stra in-domin ated at lower levels. From Co ward ( 1911J)
CON VERGE NT TlCTO N1C REG IM ES
ous. This di fficulty has led to great debat e and cont rover sy among str uctural geologists who have stud ied old o rogen ic belts. It is in fact diffic ult to es tablish the exte nt to which thru stdrive n co llisio n sho rtening IS fundament al 10 the deformat ion of oroge nic belts, or whethe r other mecha nisms me eq ually important. Ind entation
The co ncept of indenta tion was developed by Molna r and Tappo nnier in their study of the Ce ntra l Asia n collisio n zone of India and Eu rasia (Molna r and Tnppo nnicr , 1975; Tapponnier and Mo lnar, 1976 , 1977; Tapponnicr et at., 1982) , T hey ob se rved that the active tecto nic areas of Central Asia indicated by current se ismicity formed a number of d iscrete zones affect ing a regio n up to 4000 km wide , northeast o f the Himalayan fro nt (Figure 5.28), wherea s India , in co ntrast, is re latively unaffect ed . They sho wed o n the basis o f magnetic stra tigraphy and palaeo magne tic evidence tha t Ind ia must have moved at least 2000 km into Asia satcc the time of initial contact. II is clea r, however, fro m the Asian crusta l struc ture tha t J (K)() km of crustal sho rtening has not occ urre d. Tappon nier and Molnar ther efo re suggest that Ind ia has acted as a ' rigid' indent er dri ven into the more ' plastic' Asian co ntinent (Figure 5.29A ) which has reacted by a co mbinatio n of thrust and strikeslip displacements. T he ability o f Asia to shor ten by later al displacem ent is infl uenced by the ho unda ry co nd itio ns of the Asian plate . In the east , the presen ce of a cont inuous subductio n zo ne was he ld to a llow later al ' extrusion' of Asia n con tinenta l lithosphere ove r the oceanic Pacific plate . To the west , continuous co ntine ntal lithosphe re exte nds to Europe and the At lantic with no co mparable possibility of extrusio n. Onl y to the so uthwes t is so me latera l move ment possible , wher e the westwards-d irecte d wedge of Afghanistan can move towards oceanic 'space' in the Arabian Sea and ultim atel y the Medit erran ea n. Th e indenta tio n process commences at the prot rusions o f India that a rc presumed to be
147
the fi rst po ints o f contact. T hese act to conccnuutc the stress and initiate failu re . T he au tho rs co nside r that two wedges o f Asian crust, the Indo-China block and the China block, have escaped to the sou theast as a result o f the no rthwa rd prog ress of the inde nter (Figure 5.29A). O f the 25CKl- 3500km o f co nver ge nce estimat ed by Molnar a nd Tapp o nnicr between NE India and Asia , bet ween HX)(l and 25()(lk m is considere d to be achieve d by strikeslip moveme nts. Tappon nicr et af . ( 1982) illustrat e the applicatio ns of 'ex trusio n tecton ics' to the defo rmatio n of Ce ntral Asia by means of indentatio n experiments using plasticine (Figure 5.29 8 ). T he indentat io n principle has been applied to other o rogen ic belts. For exam ple T homas (1983) shows how the irregular ma rgin of the App alachia n-Ouachita o roge nic belt of eastern North America co uld be ex plained in ter ms o f a se ries of recesses and salients of the orogenic fron t. These ar e explained as the result of respe ctively stronger and weaker secto rs of the or iginal co ntinenta l margin, co rrespo nding per haps to basement domes or rift depressions. A mathematical model of a collision zon e
England and McKenzie (1981) not e the limitation s imposed by the two-dimensio nal nature of the inde ntation model , and report the results of numeri cal experimen ts which ta ke account of vert ical as well as horizontal strain in a block of mat eria l subjected to a co nstant rate of sho rte ning. T hey assume that variatio n of the hor izont al compone nt o f velocity with dep th is negligible , and thai the gradients of crustal thickness variation are sma ll. T hese assumptions imply that the strai n-rate o f the lithosphere is gove rned by the strength of its strongest part (see 2.7) and th at the effects of heterogeneous b rittle fault deformation in the uppermost layer s can be ignored. The y ob tain the mos t rea listic results using visco us ma terial with a non-Newtoni an po wer -law rheology. Their model pred icts that , for a wide ran ge of rheological parameter s, th ickenin g o f the con-
148
GEO LOGIC A L SI KU(.'lU K£S A NI) MO VI N G I' L A IES
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•lgll'-" 5 .28 Sc he matic s ummary ma p o f the tectonic pnncrn of easte rn A sia. Hea vy lines, maj o r faults o r pla te bo und a ries ; ope n-too thed hnes, active subd uction zone s; close d toot he d lilies . ma jo r intr aco ntinentalth rusts ; lar ge open arrows, major block movement di rections retanve to the main Eurasian pnue: small black arrows, recent extension; nu mbe rs re prese nt phases of e xte nsional move ment consider ed to he rela ted Itl lhe co n tine nta l converge nce; ( I) 50 - 17Ma liP; (2) 170 Ma liP to prese nt; (3) active and projecte d fut ure extension. From Tn ppc nnier 1."1 el. (1985)
tinental crust occurs over areas with dimensio ns at least as large as those of the indenting conti ne nt. To give crustal thickne sses approximating to th ose of the Himalayas after 32 Ma ,
a powe r-law rheo logy is requi red where the stress term is raised to about n = 3 (sec 2.7). The crustal thickness in front o f the indenter is limited by the strength of the lithosphere ,
149
CONV!iRG ENl' TECTO NIC REG IM ES
,
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FigUf t' S.29 Th e indenta tion mode l. (A ) C artoo n represent ation o f the results of an inde n ta tio n ex pe rime nt o n a bloc k of plasticine . Successive s tages (u -c) rep re se nt the progrcsstve mo ve me nt o( the inde nte r (grey block) ir uo the d ucutc plasticin e block . The Ia utt patte rn p ro d uced is compa ra ble with th at of th e Ccm ral Asian collision zone . ( 8 ) T wo, s tage mode l s howing ill mo re detail how the seque nce of fa ult move me nts uccom -
modatcs IU lhe indenta tion. Note the me thod of late ral ext r usion of the two bloc ks BI and B1 . identified with SE A sia a nd S. C h llla respectively. Th e re ucrs I, K a nd T ide nt ify inte rsection po inls on the block that change the ir pos ition d u ring . he expe rime nt. (A). ( 8 ) fro m T ap po nnic r ~I al. ( 191l5)
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150
GEO LOGI CA L STR UClU RES AN D MOVIN G PLATES
and as the maximum perm issible thic kness is app roac hed , lateral stretching occ urs in this region . Th is is an interesting ana logue o f the st ruct ure north o f the Hima layas (see Figu re 5.30). An impo rtant result of the model is tha t realistic resu lts arc obtained where t he fo rces arising from the crustal thickness con tr asts arc as important in determining stra in as those a rising from the or iginal bo und ar y co nd itio ns. T o mai ntain cr usta l th ickness co ntrast s of about 30 km. similar to those of th e Himalayas , t he lithosph ere is required to sustain shear stresses of abou t 30 MPa at strain rates of abo ut 1O -1 ~/s . Th ese stress es and strain rates a rc co nsisten t with estimates of available stresses from plate boundary force s (see 2 .5) an d with defor mat io n rat es fo r mod ern mou ntain belt s. 5.4 T he Himalayas and Central Asia The rece nt explosion o f interest in th e Ce ntral Asian co llision zone is due largely to the wo rk of Mo lnar and Tappo nnier , discu ssed abo ve . In thr ee influen tial papers, th ese author s exam ine the pattern of recent tecto nic activity in the regio n and att empt to explai n it by a series of moveme nts related to plate co llision (Molnar and Tapponnier, 1975; Tappo nnier and Mol nar, 1976. 1977). Current tecton ic activity as ind icated by seismic and recen t morphotcct onic data cove rs an enorm ous region extending over 3000 km northea st of the Him alayas (Fig ure 5.28) . Th is activity is co ncentrated in a num be r of active belts o f deforma tion tha t are separa ted by co mparatively stable blocks. T he pri ncipal tecto nic un its arc indicated in Figure 5.30. T he Him alay an fold-t hrust belt is bounded on bot h sides by major strike-slip be lts - the O uetta Chaman fau lt system in the west , and the Sitt ang zone in Burma in the ea st. These belts define the margins of a large piece of con tinental lith osphe re which , acco rd ing to Mol nar and T appo nnie r, has d riven in a NNE d irection into the As ian crust. Th e plate bo unda ry lies alo ng the Indus- Z angbo (Tsan gpo ) suture wh ich lies on the no rt h side of the Himalayas.
Th is suture connects throu gh complex str ikeslip zo nes of defo rmatio n wit h the Owen fractu re zone dividing the Indian plat e fro m the Arabia n plate in the west (see Figure 3.6), a nd with the Andam an trench , at the no rthern end of the Indonesian subd uction zone . in the cast. T he so uthern limit of the Hi malayan foldth rust belt is the main Hi malayan bo undary . or fro ntal, th rust which lies about 300 krn south of the suture with the Ind ian plate . North of the Himalayan belt are seve ral othe r major fold-t hrust belt s. no tab ly the Parn ir, T ien Shan . A lta i and Nan Sha n ranges, separated by st able blocks such as the T ibetan plateau and the Ta rim basin . Foc al mechanism d ata fro m all these belts yield mostly N- S th rust so lutions. The other majo r component of rece nt tecto nic activity is st rike-slip faulti ng. A number of major strike-slip faults exte nding for dista nces o f the order o f lOOQkm accou nt for much of the recent seismic activity. North of the Hi malayas these form a conjugate set wit h NW -SE dextral and NE - SW sinistr al displa cements. In the sout heast. bot h sinistral and de xtral faults appear to be bent into a mo re N- S or ienta tion. Similarly, in t he west , t he E-W dex tral Herat fault meets the N- S to NE - SW Oueue - C haman lineament defining t he Afgha nistan wedg e, which is moving sou thwest in relation to India . Th ese moveme nts are exp lained by T ap ponnier and Mo lna r as lateral extrusio ns resu lting from nor th wards indentation of India into A sia (sec Figure 5.29). T he th ird main element in the recent tectoni c pattern is exte nsio na l. The NE- SW Baikal rift system lies at the no rthern margin of the active tecto nic zo ne, and t he Sha nsi grabe n system at the eastern margin (Figure 5.28). Stud ies of the active faulting o f Ti bct (Molna r and Tapponnie r, 1978; Ni and York , 1978) reveal ed that the most recent faults arc N- S nor mal fault s (Figure 5.30). Foca l mechan ism so lut ions of earthquakes in Cen tral Tibet yield approximately E- W slip vectors . These result s indicate th at the T ibetan plateau has been subjected to E- W extens ion since t he late Cenozoic. Bo th sets of aut hors explain the
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Figure 5.30 T ectonic summary map o f the Cent ral A sian col lision zone, showing the main fault patte rn and the inferred relati ve movement di rections o f the In dian, T ibet an and Tari m blocks. From Ni and Y ork ( 1978), with per mission .
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152
GF.OLOGKAL ST RUCTURES AN D MOVING PI.AT ES
extension as a seco nda ry result of the N-S co nverge nce o f Ind ia and Asia. Ni a nd Yo rk suggest an eas terly spreading mechanism rcs ulling fro m a wedging effect in the west . due to conve rge nce o f the E -W th rusts in the so uth and the NE -S W Alty n Tagh st rike-slip fault in the nort h. Molna r and 'l'a ppo nnier attr ibute the extension 10 E -
w la teral
flo w of lo wer-
crustal material in response to crustal s ho rte ning. It is inte resting to not e t hai la ter al stre tching o f t his type is pred ict ed by the mathematical model of E ngland and M cK enzie
referred to ea rlier. Th e magnetic st rat igrap hy record in the
Indian Ocea n (sec 3.6) allows accur ate rcccn st ruct ions
(0
be made of t he co nverge nce o f
Ind ia and Eurasia since the late Cre taceous (Figure 5.31). AI around 38 Ma RP. at t he Eoce ne - O ligoce ne bou nda ry, t he rat e and d irection of co nve rge nce cha nged abrup tly .
Prior to 38 Ma 01'. the converge nce rate was bet ween IU and tx cm/ycar in a N NE directio n. After 30 Ma Ill'. the rate slowed to about 5cm/year in a nort hwa rds direction. It is likely , but unp ro ved , tha t this change re lates 10 the initial contact bet ween the two con tinen tal masses , which would pro bably have bee n in the no rt hwestern ' ho rn' of the Ind ian co nnncnt . in northe rn Pak istan . Since th at time . ap pro xima tely 150(J km of co nvergence betwee n the two con ti nents has ta ke n place . Stratigraphic evidence from the IndusZangbo suture l o ne ( Mitch ell. 1984) ind icates that early Cretaceo us ocea n-floo r sediments and ophiolites were subjected 10 th rusting in lat e Lower Cr etaceou s times indicating the presence of a subduction zone dipping to the no rt h . T he Ncfacing str uctures relat ing to the final collisio n of India with Asia arc of Eocene age . T he manner in which nus converge nce has bee n accommodated has been the subject of much debate . It had been tho ug ht for man y years t hat Ind ia had underthrust As ia and thus effec tively do ubled the crusta l thickness beneat h Ti be t (see e .g. Ho lmes, 1978. Figure 29. 4). Mo lnar and T ap po nnie r suggest tha t pr ob ab ly only 5{X) -}OOO km of horizo ntal shor te ning can be tak en up in the fo ld-th rust be lts and thilt the rem a inder has been accommodated by E- W exte nsion using displa cements on the major mike-slip faults We shall exa mine str uctu ral ev ide nce be aring on t his question later. The Central Asian collage
90" pcsuic ns of India al various times h o m 71 Ma B P , reco nstructed from ocea nic magnet ic anomaly data. Th e northern boundary of Indi a, and the position of Asia are ilrbitrarily fixed for times before the prese nt. Note the an ticloc kwise ro tation of India fro m 71 to IOMa HP . Af ter Molnar and Tappcnnier ( 1975). J"i(:un : 5.31 Successive
Many of the act ive movemen t zo nes of Figure 2.8 a re re-ac tivated tecton ic be lls of mueh o lder derivation . Central and Ea stern Asia is a tecton ic co llage or co mposite co ntinent for med by the accretion of separate blocks at vario us times (Figur e 5.32). T he main block s are the Sibe rian co ntine nt in the north . the North China or Sino-Ko rean block . t he So uth China block , an d the So uthe ast Asia block in the east, the Ta rim and Ti bet blocks in Ce ntral Asia, and the Kazakhstan and Afghan istan
153
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sho wing the main r ctucom agneucafty defined block s. After Mcfilhinn y 1'1 al . (19RI) .
blocks in the west. T he pa laeomagne tic evidence for t he se par ate e xiste nce o f these blocks in pre-Mesozoic times is discu ssed by McElh inny et al. (198 1), who s uggest that t he three easte rn blocks we re situa ted near the equator in Permian times. and were successively accret ed to th e Siberia n block du ring the Mesozoi c. Acco rd ing to Lin et af. (1985) . the Nort h C hina block join ed Sibe ria in the lat e Permian and the Sou th C hina block in the earl y Jurassic. fro m a posit io n along the nor t heast marg in o f Gondwa na la nd . Mit chell ( 198 1) discusses the co mplex area extending fro m T ibet int o SE Asia , and recog nizes a Ce ntral T ibet / Indo-China bloc k tho ught 10 hav e accre ted to Asia in the late T riassic. The sout he rn T ibe t blo ck is co nsi-
dered to be cont inu ous wit h a N - S block extendi ng thro ugh easte rn Burma , cast o f the Sittang sutu re , and to have collided wit h Indo C hina in the late T riassic also . Howe ver. the Sou t h and Cen tra l T ibe t blocks do nOI a ppea r to have joined un til t he ear ly C retace ous. Th e geo logy of the Ban gan g Nujang su tu re se pa rating the se two T ibetan blocks is d isc ussed by Allegre et af. (1984). Th ey dem on strate tha t highly defo rmed midd le to Uppe r J urassic sed ime nts are ove rt hr ust by op hiolit es and o verlain u nco nfor mabl y by Uppe r Cretaceo us volcanic roc ks. The a ut hor s suggest that an island-arc s ubduct ion zo ne dipping to t he south was terminat ed in U pper J urassic to Lo we r Cre taceo us times by co llision between t he two blocks. Mo lnar and C he n (1978) po int out that c
154
GEQl.OG I(,\L ST IW Cl U R£S AND MOVING PLATES
a pal aeomagnetic reconstruction places Cen tra l T ibet a t a bo ut la ti tud e gON in th e la te Cretaceous, and that abou t 3000 km of subseq uen t co nve rgence m ust have taken place befo re il reached its pre sent position. This impl ies that Ti be t may no t have been acc reted 10 Sibe ria until later t ha n the ot her accre tion
eve nts just referred 10 . T he precise history or acc um ulatio n of the co llage is still subject to co nside rab le uncerta inty. T he s utu res sepa rating these bloc ks a re zones o f subduction a nd coll ision w hich re prese nt major st ruc tura l weakn esses in the Asia n co ntinent. T hese zo nes of wea kness have been re-acti va ted during the Eoce ne to Re cen t conve rge nce, and exp lain the patte rn a nd e xte nt o f rece nt tecto nic activity .
Deep structure of the Him alayas and Tibet A co mbi na tio n o f grav ity a nd dee p seis mic p rofiles (Misbra. }\)82) ind ica tes a cr ustal thickness of 58 km bel ow the Him ala yas. 71 krn u nde r th e Karak orum, a t rhe nor thwest e nd o f t he H imalayan ra nge , an d 55 km be low the Pamir range (Figurc 5 .33A). A mid-c rustal re flect or a bo ut 14 - 15 km ab ove the Moh o a ppears to ex te nd acr oss the whole cen tral sec tio n , a nd is inte rpre ted as the basal deco lleme nt se pa rat ing the As ian plate fro m un der th rust Indian pla te . H im ct at, ( 19R4) re po rt a ste p ill the Moho a few ten s ofkm nor th o f Mt. Everes t be twee n the 70 km-deep T ibet Moho a nd t he 55 km-dee p H imal aya n Moh o . A bo ve the Moh o is a not he r pro mine nt reflecto r which is int e rprete d as a probab le crust- ma ntle int e rface at 35 km de pt h , reinforcing the unde rthrus t mode l. Howe ve r, t he arr a nge ment was no t thou ght to suppo rt a simple doubling o f the c rus t , but ra the r a sepa rate dccouplin g and thrust ing of the upp e r and lowe r c rusta l la ye rs. A llegre et al . (1984 ) re po rt result s from a se ismic ref raction study across the Indus Tsang Po (za ngbo) sutu re nea r Lhasa , tha t reve al a co m plex Moh o topogr aphy involving se veral ste ps (Figure 5.338) . These a re in te rpre ted as the sites o f o ve rt hrust ma ntle wedges, direc te d so ut hwa rds. The T ibe tan plateau has an a ve rage eleva-
no n of about 5 km and a n a ve rage crust al thick ness of about 70 km . T he main pe riod of uplif t a ppea rs to be pos t- Mioce ne (Guo, 1980). A study o f P, a nd S; seis mic wa ve prope rties ben eat h the H imalayas an d T ibet . repo rted by Barazangi an d Ni (1982) ind icat es ef ficient pro paga tion of SIt waves in the uppe rmost ma ntle be nea th t he T ibe tan platea u, toge t her wit h rel a tively high veloci ties of both PIt a nd S~ waves benea th most of the T ibet a n plat eau , s imila r to t hose fou nd be low stable Preca mbria n s hield regions (sec 2.2). These result s we re held to bc con sisten t with a mode l in which shie ld-like Ind ia n co nt ine ntal lithosphere undc n hrusrs Tibe t a t a sha llow ang le (about 15°) as o riginally sugges ted by Argund (1924) . An alt er nat ive mod el , in which hot , wea k c rust and uppe r ma ntl e is being shorte ned be nea th T ibe t ( D e we y and Bu r ke , 1973) is not support ed by th ese results. Recen t st ruc tura l wo rk in the Weste rn Him ala yas . wh ich we shall now d iscuss , sugges ts thai crust al struc ture is m uch mo re complex in de ta il tha n either of these mode ls en visages .
Structure of /he Western Him alaya A cr ustal profi le ac ross t he Ka ra korum ra nge in No rt h Pakistan is desc ribed by Cowa rd et al. (1982). T he sec t ion ex te nds fro m the Indus Za ngbo sut ure in the no rt h to the main Himala ya n bound ar y t hrust in the so uth (F igure 5.34) a nd crosse s a maj or s hear zone te rmed the Main Mantle Thrust ( MMT). On its sou th sid e is a 10- 20 km-wide inte nsel y defo rmed zo ne of blue-schists an d a mp hibolitcs. T hese co mprise inte rfo lded ba se ment a nd cov e r rocks belo nging to the Indi a n plate , a nd ar c ov e rthrust by highly deform ed a nd meta morphosed roc ks of the Ko his ta n seque nce. Th is seque nce co mme nces with the basicunrab asic Chiles co mplex , more than 8 km thick a nd 300 km lo ng, which is ove rlain by pillow lavas a nd gre ywac kes, intr uded by gab bro s, dio rites a nd ton alites . T hese rock s are deformcd un der gran ulite- facies co nd itions a nd are interpre ted as a s lice o f (he lower crust upth rust along the Main Ma ntle thru st. T he ea rly high -grade fabrics and assoc iated
A
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tl ll.u", S.33 (A) Gravity and deep seismic structure of a crusrat section across the western Himalayan , Parnir and Alai ranges. No te ( I) that the main high ranges of the Karakorum and Pamirs appear to be largely compensated isosta ucahy , and (2) the base or the Cru~l at abo ut 70 km beneath rhe Karakorum. Data west o r Karak ul are fro m Pakistan sources and east of this line from the USSR . Aner Mishra ( 1982). (8) Deep-se ismic refraction profile across the Tsang Po (Za ngbo) sutu re near Lhasa, showing the ap pare ntly stepped nature of the Moho . Th e lower diagra m is an interpretative cart oon indicating a possible crustal structure. Mantle, bache red ; ocea nic crust , black; MBT, main bou ndary thrust ; MCT, ma in central thrus t; KeT, Kangmar thrust: ITS , Indus- Tsang Po sutu re ; ENS, Bangong- Nujiang suture; CT4-6, un-named thr usts within the Lhasa block . After Allcgrl: et al . (1984) .
155
GE01.OG ICAI. ST KUCTU KES A N D MOVlr"IG PU.TES
156
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157
CON V[)tG ENl' 1 E(TONI C REGt M ES
folds ar c re fold ed by a major sy ncline (t he Jaglot sy ncl ine ) t hat appea rs to Invo lve the who le uppe r crust o f K ohistan . Mo st of th e Kohistan se que nce is st e eply di p ping a nd th e la ter folds a rc tight with stee p ax ia l pla ne s. Howe ver . so ut h o f the MMT t he st ructu re is domi nat ed by ge ntle sou t h-d ip p ing th rus t we d ges ( Figu re 5.34 /J). The stee pe n ing o f the MMT and the Kobistan se q ue nc e may he du e 10 pa ssive back -tilting p rod uced b y move men ts o n the yo u nge r th rusts as th e y mo ve u p ramps to the so u th. o r po ssib ly to northwa rd s bac kthrust ing o n a so ut hw a rd s-di pping thr ust.
NN.
Figure 5 .35 sho ws a model (C o wa rd a nd BUl le r . IY85) o f th e posr-coltisional rcct o nic ev ol ut ion o f th e Ka rako ru m re gion . No te firstly t ha t the d eep e a rt hqua ke s beneat h T ibet a rc att ri b u te d to a no rtb-di rccre d b ack -t hr ust invo lving the whol e Indi an litho sp here . a nd sec o nd ly th at the isostatic re spon se o f o ve r-t h ick e n ing o f the lith osphe re result s in th e up lift of the reg io n betwee n th e Pam ir ran ge a nd Ko his ta n. A bala nced sectio n fro m the M MT 10 t he undcfo rmed Ind ia n plat e (Cowa rd and Bu tle r , 1985) le ad s to a sho rte n ing esti ma te of 64 % ( Figu re 5.36) .
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I'igu rf 5.35 Int er pr etative cartoo n profiles i ll u~I ';ll i n g the post-cothsion tectonic evolut ion oltbc nonhero margin of the Indian plate . Randomdashes, maude rocks of Kchistan complex : stipple , Indian crust in Iootwalt to H imalayan sole thrust; large arrows. predic ted vert ical movements ar ising fro m lithosphe re load ing: MM T. main mantle thrusr ; MBT, main 1ll.lUndil ry thrust : L VZ . base or I ndian plat e lit hosphere (hypot hetical) , From Co ward and Butler ( 19li5)
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Figure 5.36 Simplified balanced (a) and restored (b ) sect ions across the Pakist an Him ala yas , from the main mantle thru st out crop to the fore land . Sect ion is con structed to minimize necessar y displacem ent s . From Coward and Butler ( 1985)
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T he H ima laya n range thu s represents a fore la nd thrust belt , cu rrently active at its so ut he rn limi t in the Salt Range whe re the basal th rust is still moving , result ing from co llision man y hundred s o f km to the north . The site of t h e most rece nt col lision has ye t 10 be esta blished ; it may lie alo ng t he no rthe rn ma rgin o f Ihe T ari m bas in, along the line of the l' umir -.T ien Sha n ra nges much further north tha n o riginally thought. U ntil m uc h more geo log ical fiel d wo rk is undert aken in these re mote region s, this question may not be lin ally reso lved . The whole process o f c rustal thicke ning a nd sho rte ning invol ved in thi s colli sion o roge ny appe a rs to ha ve tak e n a bout 40 M Ol to rea ch its prese nt sta te, a nd is not yet co mplete. A n instruct ive com pariso n ma y be made with the Caledonia n o roge ny in Britain (see 8.4 ) whe re the lat e Ca ledo nia n forela nd th rus t bell o f NW Scotland is linked with closu re along a sut ure 300km to t he sout h , ac ross an intervening co llage of block s with a much ea rlie r o rogenic history.
5.5 Southeast Asia A ll incomplete collage Southeast Asia ma y be ta ken as an exam ple of a co llisio n o roge nic be lt at an ea r ly stage in its de velopme nt . It is inst ructive to specula te on the e xtreme complexity of the accre t ionary terran e that wou ld result from co mplete cont inental co llisio n of t his regio n with closure of all th e ocea nic basins. T his notio na l acn etio nary te rr a ne ma y be usefull y co mpared with Ce ntral As ia , or indeed with older o roge nic belt s, as a wa rning aga inst ove r-simplistic reconst ructio ns ! The prese nt tect on ic frame work of the regio n is sum ma rize d in Figure 5.37, a nd represe nts the co mplex interact ion of three mai n plat es: the Indi an plat e to the so uth with Aust ralia n co nt ine ntal c rust in its easte rn half; t he Southe as t Asia n pa rt of the Eurasian plate to t he northwest , a nd t he Pacific plat e to the northeast. Subd uctio n o f oceanic Ind ian pla te
159
is taking place at the J ava tr e nch below the Su nda ar c. Th is s ub duct io n zone ex le nds south of Suma tra an d Ja va eastw a rds to the edge of t he T imo r Sea where ocean ic crust of the India n plate gives way to co nti ne ntal Aus tralian crust. The c urre nt ly act ive volca nic arc ext e nds fro m western Su mat ra thro ugh Ja va an d the smaller isla nds 10 th e east. Th e A ust ralian co ntine ntal cr ust ext e nds nort hwards to incl ude New G uinea (Ir ian). No rth of thi s plate lies a very co mplica te d region consisting of co m para tively yo ung bac ka rc spreading bas ins a nd isla nd a rcs which lie be tween the mai n Pacific plate to the east and the As ian co nt ine nta l ma rgin in the west. The Neoge ne vo lca nic a rc runs through the island of Sulawesi, east of Bor neo , a nd joi ns the act ive a rc in t he so uthe rn Phi lippi ne islan ds. Cha rlto n ( 1986) exp la ins so me of the co mplexit y of t he prese nt patt e rn by postulating moveme nts along a series of N E -SW sinis tral strike-slip fa ults that resu lt from t he geo metrica l arrangeme nt o f co ntine nta l a nd oceanic plate d ur ing the initial collisio n (Figure 5.38). Due to the small a rea o f initia l con tinental ove rla p , and the greater ease of no rthwa rd tr a vel over the oceani c Pacific plat e , fragments of the no rth wes t corne r of Australia arc progressivel y sliced off , a nd a tta che d to As ia. T he position of Ne w G uinea , about 1500k m to th e northeast of th e present Ind ian /A sia n plate bounda ry at t he Su nda a re, is a conseq uence o f thi s cumulative st rike-s lip displacem e nt. AI present, co nt ine nt - isla nd arc co llisio n is tak ing place alo ng the so uthe rn s ide of the Ba nd a A rc on T imor and the ad jacent isla nds. C ha rlto n believes tha t t he initia l co llisio n, the products of which ar e now to be fou nd in eas te rn Sulawesi , look place pri or to midMioce ne times when the major Indi a n pla tc rcorientation refe rred to ea rlier look place . T he present converge nce vector be tween t he IndoA ust ralian and Eurasian pla tes is 020" a nd t ha t betwee n the Asian a nd Pac ific plat es is 1100 (see Figure 3. I). Prio r to the mid -Mioce ne rearrangement , the Indo-A ustral ia n plate was tr avell ing a pp rox ima te ly no rt hwards relative to
160
G EOLOGIC A l. STlWC TU RES Ar>lO MOV ING PLA T ES
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Eurasia (see Figure 3.5e- E). However , a complicating factor is the so utheastwards movement of t he Indo-China block resulting fro m
the India-Asia collision (sec Figure 5.28). This move ment must have caused a n eastw ards shift in the Pacifid Asian boundary relative to the Indian plat e , so t hat the co nve rgence dir ectio n
be tween the Aus tralian part o f the IndoAustralian plate and SE Asia is ac tually NNW. Figure 5.39 shows a reconstruction of the relative positions o f Australia and SE Asia in late Cretace ous 10 late Pliocene times which may be compared with the simplifi ed model of Figure 5.38.
CO N V E ~ G E NT
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TECfONIC REGIMES
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Figure 5.38 Seq uenc e or ':ilU' IlHl mill's ilIusu"l;og the tecton ic evolution of the eastern Indo nesian reg ion. T he three plates arc th e E urasian plate (A . !:llan\;;) , the Pacific pl" le (8. UMk slipplc ) and the Indo-A ust ralian prate (c. oc ea nic, light st ipple ; co nline ,u "l. crosses t. Subducticn Niles . too thed lines. Loca l sea -floo r spreadi ng associ at ed with tra nstensiona l zones is indica ted oy .bshc () o rna ment. T he dotted lines in (b) arc pore e ua t co mpleme ntary strike-slip faults Ihal do nOI beco me active. T he effect ot rhc culhsion is 10 develo p a ser ies of t ransfo rm fa u l l~ which trans fer pieces o f the nOrlhwCSlern cor ner of the AU ~lr a h "n conuoc m to the sout heaste rn po rtion of the Eurasian plate. from Ch arlton ( 19l«'i) . with permission.
Tim or
The island of T imor form s pari of the co llision zo ne between the Austra lian continen tal shelf and the Banda island arc. Accordi ng to Chartton's model , the co llision oroge ny of T imo r is the result of a seco nda ry co llision bel ween a piece of the nor thwest co rner of A ustralia alread y welde d to Asia, and a more southwesterl y po rtio n of Au stralia (see Figure 5.38). A n impo rtant feature of the Banda arc is that the Pe rmian and Mesozoic rocks of the para -a utochtho nou s units d isplay Australian a ffi nities , whereas the over lying alloc hthon ous th rust shee ts conta in strata of the same age ra nge but with tro pical facies comparable with the co ntemporary rocks of the Sunda arc and
Borneo . These allochthonous th rust sheets are directed sout hwards , away fro m the Banda Sea and towards the Austra lian continent. The nearest count e rpa rts to these allocht honous units lie on the no rth side o f the Banda Sea , in Sulawesi (Figur e 5.37). The form er continuity of these unit s has there fore been disrupted by the opening o f the Banda Sea . T he formation of th is oceanic spread ing basin may relate in part to the regio nal southeastwards exte nsion o f the Indo-Chi na con tinental margin already d iscussed , and partly to back-arc exte nsion relating to the Banda subduction zone . A geological cross-section of li mo r (Figure 5.40A ) shows the basic structural framework. A basal low-angle sa le thrust (Tl) carries the allochthon across the para-au tochtho nous Permian
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Figure 5.39 Palaeogeographic reco nstructio ns of the easte rn Indonesian region during the late Cretaceo us (I) , mid-Miocene (2) and late Pliocene (3) periods. Present-day outlines of land areas are for refere nce only. The allochtho nous elements of the Banda arc arc shown in hatched orna ment in (1). Back-arc spreadi ng areas are stippled. WS, West Sulawesi; ES, East Sulawesi; CNG , centra l New Guinea ; SNG, south New Guinea; NNG , nort h New Guinea ; SER , Seram. Note that the present north orientation of the Austral ian continent has different or ientations in the thr ee diagrams. After Carte r et al. (1976).
CO NVE RGENT T ECTO NIC
N
!I/ofOR
Tl HOR
B ANO A
163
~F.G I MF. S
5
f R OUG H
W f f AR S f RAIT
Sf A
... . B
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IT]
Teclon,c !lo~ e 01 lOteOtc n o ~ pe
WelO'
Soul" Bando Sea
S' , a ,' WS T,mOt
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o
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conhn enf
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200
ws Wela, Su'u, e wT We ra , T h ' u ~1
' 00
ctu R~,on 01 Ouat e''''''1 isoslOt,c u~ li "
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Figur e S.40 (A) D iagrammatic section across Ti mo r , sho wing the seq ue nce of allochthono us shee ts carried Oil thr usts TJ- T5 o ver lying the pat a-autochrhon o us A ustralian assemblages. U neve n ver tical str ipes , oceanic crust : V-symbols, volcan ic-arc assem blage ; black , At apu pu she et of amphibo lite and se rpe ntine; closed triangles, O cussi shee t of Per mianJurassic sediments and volcanics; eve n vertica l stripes, Lolotai meta mo rp hic complex; small regula r dot s, Kc bano- Hiomar sheet of bat hyal sedimen ts. Note post -emplace ment de posits of primary Bo bo na ro otistostrom e , and rewo rked Bo bc narc sedime nts . From Caner (I at, ( 197(1) ( 8) Tecto nic car too n illustrating an inte rp retat io n of the geo physical and geo logical struct ure of Ti mo r. Not e the postulate d rupture of the Au stralian continen tal lithosphe re alo ng a thr ust with a surfac e out cro p in the werar stra it; thi s inte rpre tation differs significantly fro m that of ( A ). Eart hq uak e foci represe nted by o pe n circles. From Milso m and Audley-Cha rjes f 19M ), with perm issio n.
GEO LOGICA L Sn W CfU RES AND 1>lOVll" G PLATl:S
164 Allt
S t ,a l iq ,a phic a nd st ruc t ura l ev e ot s in Timo r
BP
o ",
Te ct onic event s
Upli f t o f Pl e is to ce ne co r er re e ls a nd a llu via l f,
2 ",
De po s itio n o f co r er r ee fs ond a ll uv iu m d ur; nq
2 ",
- - - - - L oc ol o nq u lor u nconf or mi l y - - - - -
I
Ple i s to cene ( N . 2 2 - 23 )
Wid upr e a d subae rio l eros ion Genlle f o l d i n Q ot V i que q ue t urb id it es De po sit io n of Vi qu e que tu r b i d i t u
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t N ce te
Ma rl Fo rmot ion - Lo t e Pli o c e ne - Earl y
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s ters tc ceee N. 2 1-22 ) w ;lh sll bcr e, iol eros ion in North e r n Ti mo,
Deposi ti on of Sa bael! t jme stcne Formol ion ( Lo" Pl io cene N. 2 1) in s ho llo w wo l e '
~
•, ••>
• > ..... :3 Mo
, ",
Deposi tion Of u p per po r i of Bo lu P ul in Ltm estce e i n deep woler ( Lo te Pl io cene N. 2 1)
- - ---Loc OI onq u lo t unc onfor m ity - - - - -
(Cessa tio n 0 1 volcani c ocl iv ily in Al au r a and We lo r 0 1 inner Ban da Ar c 1 Em placem ent of Ih ruS! ,he.ls wilh o y. r lyinq Bobona ro Sc aly Cloy OIi, lo slr o m, an d lowe r pa rI o f Botu P u l ih li mes l o ne in Ihe Mid P lioc ene ( N. 20l
'" 4 Mo
De posi l io n of lowe r par I o f Bal u Pulih li mestone o n Ih , Bo bo na ro Sc aly Cl o y in I he Early Pliocene ( N. 18 - t9) Eros ion of po ro · o u i oc h i ho n FoldinG. fau lTinQ and tccc t imbr ical ion 0 1 para - a ulochlon
'" 5 Mo
DepOsi lion 01 you nqesl mem be r 01 paraau t och l hon o us Austr ali a n co nl ine nt a l marq in tec te s (Ea rly P lioce ne N. 18)
'" 7 lola
Se dimenlation in Ih, o lio ch i hono u' el'men ts c ho nqe , from ,ho llo w we re r c e erc e Limeslo ne (Ea r ly Miocene N. 8 ) 10 deep woler Miomo ll u l u lh ( Lo l. Mio ce ne N. 17 1
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CO!" Vl:KGEN I r ECT ON"lC II.I'.GI M ES
10 Cretaceous seq ue nce . Above TI is t he Kalbano thrust shee t ( 1'2). which is an imb ncatc slack or du plex co ntain ing ea rly C retace ous sedi me nts . Th is thr ust shee r is ove rlain by th ree highe r t hrust shee ts wit h o phio litic material o n the top most and most norther ly. Th e th rust seque nce is unco nform ably ove rlain by late Plioce ne to late Pleistoce ne turb idites. Th e alloc ht honous units revea l a different de for matio n histo ry from that of the unde rlying para-a utoc hthono us Aust ralian units. T he re is evide nce in t he allochthon for defe rrnat ion and igneo us act ivity be fore e mplacement o n the A ust ralian margin. T he you ngest unit in the allochth o n is of late Miocene age . T he para-autochtho no us seq uence reflects the deep-water contine ntal slope an d rise en viro nment of the nort hern A ustra lian co ntinental shelf. Th e yo unges t unit in t his sequence is ea rly Pliocene in age . Th e re is no ev ide nce for any deformation in these rocks pr io r 10 this date. The de formation accompanying the emplacement o f the a llochtho n the refo re took place in mid-P liocene "t imes. which is ta ken as the dat e o f collisio n. Afte r the co llisio nal deformatio n, the T imo r region experienced vertical uplift of at least 3 km in the last 3 Ma. T his uplift is well document ed from the shallowing and erosio n of mar ine seq ue nces, and
165
the for ma tion a nd uplift o f co ral reefs a nd all uvial te rraces (Ta ble 5.1). T he gravity fi eld in the Banda ;HC regio n is discussed by Milsom and Audlcy -Ch artcs ( 19H6). A pro mine nt negative Bo ugue r anomaly ex te nds through T imor and Jarndc na , fo llo wing the arc round to Scram. A sma ller posi tive anomaly fo llows the active volcan ic arc to the no rt h and also exte nds ove r thc Banda Sea . T he no rt hern limit of the negative ano ma ly runs through the island of T imo r. Reca lculating the gravity fi eld assumin g isos tatic co mpe nsation stillleaves a substan tial negat ive ano maly, although the positive anomaly is virtua lly eli minated. T hese results suggest t hat the southern part o f T imor is depressed below its isostatic level, whe reas the nort h coa st , which has been subjec ted to rapid recent uplift. is now near eq uilibrium. Figure 5 408 shows an inte rpretat ive model profile of the area . The negative a noma ly is att ributed to the subducted oceanic slab which must still exist below t he inne r arc from seismic ev iden ce . Co mplications arise fro nt ho rizo ntal short ening. fi rstly o n t he south-directed th rus ts o n Tim o r, and secondly by a postu lated no rth -directed thrust in the straits be tween T imor and the volcanic arc . T he rise o f northe rn T imor is attri buted to isostatic adj ustment resulting from its detachme nt fro m t he sinking slab.
Table 5.1 Summ ary of tate Cen ozoic stratig ruphic and tectonic h i~ l o r~ of T imor. Fro m Milsom and A udlc y.-Charlcs ( 1985)
6 Strike-slip and o bliq ue -sli p reqrrnes
6 . 1 C ha ra cte r istics of sir ike-slip reglmes
Major strike-slip move me nt never takes place along a single faul t plane , but is distribut ed thro ugh a zone . In the case of the San An d rea s fault system. this zo ne is about 100 km in width . So me of the grea t ocean ic fracture zones arc over 50k m wide . A simple mod el of a strike-s lip boundar y consists of two pla tes slidi ng past each ot he r, with comple te censer vation o f plate area , and no con verge nce or di vergence across the bo unda ry. T his mod el must be re placed by a model involving a bo und ary zo ne of finite wid th within which co mplex tecto nic effects tak e place . The general kinematic relat ionships were described in 3.3, whe re the significance of oblique relat ive movements across plat e bo undaries was stressed . It was co ncluded that, in gen eral, plat e movements at bo unda ries were tm p§Ilrcss igl1"a1 or " ap sts nsjRQjl l, with compon ent s of compressio n or extension acros s t he boun dary. and t hat the boun da ry s ho uld be co nside red as a deformable shes t rath er than a plane . T he impor tance of the continental strike-s lip regime was highlighte d in an influential pap er by Read jQ!' ( 1980). He pointed o ut that a strike-s lip tectonic regime created a special type of o rogenic belt characte rized by ju lkp§, seismic activitX_i'Qd de fQrmatjo p, by jwoo rtapl differs ntia! yertisal w p¥f we pts by ~ and xaried sedimenta tion , but by co mparat ively feeb le magma tic and metamo rph ic act ivity. Major strike-s lip zones are co mmon o n the co ntine nts . and il is freq ue ntly unclear whethe r o r no t these are plate bo undaries . For exam ple, the maj or stri ke-slip zones resulti ng from the India - Asia co llisio n (sec Figure 5.28) are most ly the result of in terna l defo rmat io n of the E urasia n plate , and it is not pract ica ble to use the rigid plate mod el in this area . Nevert heless, individual fault zo nes may co nstitute major be lts of strike-slip deform atio n sim ilar in their e ffects to the plat e bo undary type . Some may pe net ra te the who le th ickness of the
In the early days of the plate tectonic theory , atte ntion was concen trated on the tectonic effects of dest ructive and CO Pslr u£t j\'s p ! a",~ bslllpdar jcs. Ho we ve r. the de tailed study o f oceanic transfor m faul ts and of maj or contine ntal tr ansform faul ts, pa rtic ular ly the Sa n
A nd reas fault syste m , ha s led to the recognilio n that stri ke-slip or tra nsfo rm regi mes arc also o f fundamental tecton ic import ance.
If we exam ine again the piatc bo undary ne twork in Figu re 3. 1, we Sec that co nser vative or st rike-s lip bo undaries make up a significant proport ion of the Io taI boundar y len gth . In addit io n 10 t he la rge nu mbe r o f minor tr a nsfo rm offse ts of the ocean ri dges. there are lo ng sect ions of bo undary mad e up e nt irely of tr a nsform faul t. Th e most stri king examp le forms the northeastern bo undary of the Pacific plate . He re the San Andreas fault jo ins the end o f the East Pacific ridge in the G ulf of C aliforn ia to the J uan de Fuca ridge west of Washington and O regon in the nort hweste rn USA . No rt h of this sho rt sectio n o f ridge . anot he r ma jor strike -slip fault syste m co nnecting the Chugach- Fairweathcr O uee n Charlott e islands fa ults exte nds off the co ast of Western Canada and Alaska . A nothe r maj o r co nt ine ntal strike-s lip bo undary is the Alpine fau lt of New Zealand , which connects subductio n zo nes to the no rth an d so uth marking the bo undary between the Pacific and IndoA ust ralian plate s. There are a num ber of major oce anic transform fa ults. A mo ng the more impo rtant arc the Owen frac tu re zone be twee n the A rabian and Ind ian plat es (sec Figure 3.68 ), the Azores fra cture zo ne co nnecting the midAt lant ic ridge and the Mediterranean subductio n zo ne. and the major fracture zo ne connect ing the Sco tia arc with the C hile t rench. The mo rph ology and str ucture of oceanic t ransform fau lts is discussed below (6.4) . c
166
StRI KE-SUP At'lD OBLIQ UE-S U P
...
A
~ EG IMES
167
S YN THE TIC STR IK E 'S LI P
NOR MAL
F" Ul TS
AN T ITHET IC
STRI KE -SLIP FAUl T
.... B
• >
J.'igurf 6. 1 (A ) Diagramm a tic re prese nta tio n of the str uctural pane m produced by a de xtral simple -shear couple, afte r Ha rd ing (1974 ), a nd Reading (1980) , (8) Diagrams sho wing the or ic muuo n patter ns of faulls and fold axes d ur ing dextral simple shear (middle diagr am), • unde r transpressio n [ top diagra m) and transtensio n [bonc m diag ram ). C, co mpress ion axis; E. e xtensio n axis: N , no rmal Iau hs ; T, thrust fa ults; R. R', Riedel shear s or srnk c-shp fa ults : V, ve ins, dykes o r extensio n fracture s; F. fold axes. Not e that transpression res ults in cloc kwise rota tion of co mpression and exte nsion axes. and tra nSle nsion in anncl oc kwise rotation o f stress axes. Th e opposite would o r course hold for sin istra l shear. Fro m San derso n and Marchini (1984), wilh pe rmission .
lithosphe re . while o thers may detach on lowangle decollement planes with in or at the base o f the crust. Studies of deeply eroded Precambrian orogenic belts de monstrate the importa nce of
major st rike-slip shear zones at deepe r crustal levels. For example the Precambria n of South G ree nland ex hibits several major orogenic
belts th at re presen t middle- and lower-crustal d uctile co unterparts o f the high-level strike-slip fault zo ne (see Figures 9,17. 9. 18).
Causes ofgeometricalcomplexity If we assume the strike-slip boundary to be a defor mab le shee t. the bulk strain can be
168
GEOLOG ICAL s n WCTU K J;.S AND M OVI NG PLATE S
B
Figu re 6 .2 Loc"l ctl'nr ' <·Ssi<>I1
la u ll oveda p
considered as a comb ination of pure and
simple shear. The pure shear compo nent ar ises fro m the exte nsional or co mpress io nal co mponent ac ross the shee t. and the simple shea r compo nent from the stri ke-slip displacement (see fi gure 3. 14). Tr anstension and t ranspres-
sio n involve a cha nge in sur face area hUI not necessarily In volume. Compression across the shee t at cons ta nt volume will be co mpensate d by extension , most pro ba bly in the vertica l dim ension . leading to crustal thickening and uplift. Exten sion across the sheet may be co mpen sat ed by vertical shortening lead ing to crustal thinning and depr ession . IllS ''f"nmef [Lk
and
:fS::y;.r~~:t ~q ps~nession u anSlcnsion (" r ' X . derson a n Marchlnj .• ( 1984) and their effects are summa rized in Figure 6.18. The importance o f volume cha nges sho uld not be ove rlook ed : extensional moveme nts. particula rly in the oceanic lithosphere , will normally be accompa nied by emplace ment of new mantle material, thus add ing to lithosphe re volum e . and volumetr ica lly less importan t cha nges also acco mpany met amorphic effects in both co mpression and extensio n. T he effect of simple shear strain is summa rized in Figure 6.1A . In a bloc k of roc k defor ming heterogeneou sly. the d irecti ons of extensio n and compress ion tire give n by the o rientation of t he bulk simple-shea r st rain ellipsoid . Vario us types of struct ure ma y form .
and different comb inations o f str ucture will be appro priate in d ifferen t mat e rials. Fold axes will parallel the long axis o f the stra in e llipse . Co njuga te sets of st rike-slip fault s may for m. one §)'p!hc ljc. ~ k i91! ./ Wp ll am'k w ilb !be ma in strike-d ip di rr£!j Q ~ . and the o ther antithet ic. ma king a large angle with this dir ection . Inclined faults will have normal dip-slip compon ent s par allel to the long ax is of the ellipse. an d thru st co mpone nts parallel to the shor t ax is. T hus whe re the se nse of movement o f thc major strike-slip displacem en t is unk nown , it can be dedu ced from the relation ship of an y of these subsidiary str uctures to t he bo und aries of the deformation zo ne. Othe r geo me trical effe cts arise o ut of the natur e of the fau lt movements. On ly o ne sma ll sec to r of a faul! is active at an y given time . and t he d isplacem ent must the refor e be tak en up elsew he re by hete rogeneo us st rain. This fault termination effect is illust rated in Figure 6 .2A. Local zo nes o f compression o r extensio n are prod uced at the ends of displaced seg me nts. Simultaneous movem ents o n en-echelon fau lts also produce local zo nes o f compressio n o r ex te nsio n (Figure 6.28) . The most importan t geometrica l effects are produ ced by cha nges in di rection of strike-slip fau lts (Figure 6.3A ). As the two opposed blocks move past each o the r, local zo nes o f co nverge nce or diverg en ce occur, which produce co mpressio nal and extensional effec ts
ST RIKl:-S Ll P AN O O RLlQU l:-S Ll P Rl:G IMES
respectively. In a co mplex fault netwo rk. this process will lead to alterna te zo nes o f raised and dep ressed fau lt b locks (F igure 6.3 8) . These effects arc ana logous to t he geo me trica l effects created by the ramp-flat geometry in dip-slip fault displacemen ts (see e .g. Figu re 4.26). Th e combination o f fo lds and fau lts prod uced by these local zo nes o f co mp ress io n and exte nsio n have been termed flo wn .\"ITIl'fllT by Harding and Lowell ( 197IJ) . Pos itive
es
169
ttower s are uplifted zones with a co mp ress ional co mpo ne nt acro ss the strike-s lip belt . and negat ive fl ower s ar e dep ressed zon es with an e xtens io nal co mpo ne nt ( Figur e 6.4). Strike-slip duplex structures may be creat ed in an analogous manner to thr ust and exte nsional d up lexes ( Figure 6.5). Pieces from one side o f the ma in fault may be sliced o ff and transferred to the other side as t he active fault tak es a new course . O n a large scale. this
Fi~u re 6.3 Th e ef fect o f c hange s in Iault oricmauon . CA) Diagr a ms sho wing th e ge ne ration of trunsp rcssive (u ppe r d iagra m) and Hanste nsion a l (lo wer ) reg imes wilhin a region of offsel in an otherw ise pure s trike -shp zon e . Tr a nsp ression a l and transte ns jon a! reg ions a re s tip ple d; fold axes, ~ing le lines, and e xte nsion al fissures. dou ble line s. Bo u nd a ries of tre nsp ressio nal a nd Ira OS I('n sion al re gio ns a re shown as Iauhs [ toot hed lines ) 10 indic at e aeoo mmod a lion of the strain discontinuity. F rom Sa nderson a nd Ma rc hini ( 19M ). (8) D i" gra ms illuslrn ling the Iormarion 01 ra ise d and de pressed blocks hy coovcrge ncc a nd dive rgenc e alo ng curved fault ~cgmen lS. du ring strike -sllr mo tion . Fro m Read ing ( 19IllJ)
B a Be/ore movemenr
170
GEOLOGICA L S TRUC tURES AND MOVlf'lG I' LAH S
POSITIVE flOWER STRUCTURE
.... .
. . .":.,
','
t'j g\l r~
6.4 Pos itive a nd negative flowu produc ed oy co nve rge nce a nd d ive rge nce respectively in smkc-shp motion. Dot a nd CfOSl; symbols within circles indica te o ut-of-page and into-pag e compoue nrs of mot io n. rcspect ;vdy. Al ter a n unpublished diagram of N. w oodcock . Jlme/ II Tes
O u l 0 1 p a gll
l ruc p ag e
NEGA TIVE FLOWER STRUCTlFIE
B
, -+
2
--)
- ---
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2
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II 3
3
tlRure (l.S Diagrams illustrat ing the format ion of strike-slip d uplex structure in transpression (A) and transtension (8). Not e tha t the str uct ures a re ana lo gous mo rphol ogicall y 10 compressio nal aoo ex tension al d ip-slip fau lt duplexes . respect ivel y.
5 TR II\ E-5lJP AN D 013 U QC E-SlJ l' RE G IM ES
proc ess may produce fa r-tr ave lled bloc ks tha t are alloch tho no us or exotic in re lat ion to th e bloc k with which th e y a rc no w associated . Lar ge di sp laced bloc ks o f this typ e
Sedim entation in strike-slip zo nes Despit e th e fact th a t the mai n disp laceme nts in strike -slip fau lt zo nes a rc hori zon ta l , the most obvious dis place me nts lo ca lly a rc us ua lly d ipslip (sec Figu re s 6 . 1-6.3). It is the ve rt ica l move ment s o f fa ul t block s re lati ve to e ach other which pr o d uce t he most impo r tan t strat igraphic e ffec ts o f a strike -slip zo ne . T here are obvio us di ffe rences be twee n tra nste nsio n and tra ns pr essio n in th is re spe ct . since tra nstensio n le ad s pre d o m inantl y to depre ssio n a nd transp re ssion to up lift . T hus tr e nsprcssion a! zones will be e ro de d a nd th e de rive d sed ime nts will be ca rrie d o utside t he st rike- slip zo ne , possibly to d istan t a reas. Fo r e xa mple t he majo r trunsp re ssive A lpin e fau lt zo ne of Ne w Zealan d is no t a n a re a of major se d ime nta tio n . In tran ste nsio na l zo nes o n th e o the r ha nd , majo r se d ime nta ry basins fo rm , which o n lan d are no rma lly with in re ach o f a bundant se d iment su pply. Transte nsiona l basins a re typi cal ly lacu stri ne , and bo rde red hy alluvia l fa ns. An e xce lle nt example of such a zo ne is th e De ad Se a r ift . which fo rms pa rt of the wes tern mar gin o f th e Arabia n plate . Fre un d et 01. (1968) d iscu ss th e evidence for a maj or sin istral d ispla ce me nt along th is zo ne , a nd re lat e it to th e sc dimentary reco rd . Se ve ra l rho mb -sha ped gra be n occur in the zo ne (F igure 6.6A , 8) , so me o f which co nta in la ke s , of which th e Dead Se a a nd th e Se a o f G a lilee a re well-k nown e xa mples . These grabe n are pull-a part fea tures fo rmed in regio ns of e xte ns io na l fa ult ove rlap (sec Figur e 6.28). T he to ta l sinistral mo ve me nt is lto km as me asure d by th e d isplace me nt o f va rio us igneous a nd sed ime nta ry marke rs of Pre cambrian to lat e C re ta ceo us
17l
age . Qu e nnel! ( 1959) a tt ributes 67 km o f th is mo vem e n t to the early Miocen e. a fte r which a n inact ive pe riod allowed the n ft to lill u p with re d be ds until rive rs fro m the ca st we re abl e to no w a cross th e rift to wa rds the Mc ditc rr nnca n. Figur e 6.6C dem on strat e s the evidence fo r sinistral d isplace me nt of the se Mioce ne river s by 43 km in the Plioce ne to Pleis toce ne pe riod . Some mo ve me nts ar e ve ry rece nt. Zak a nd Fre und (1966) dem on stra te d dis plac em e nts of 150 m in t he Lisa n marl of th e Jo rda n valle y. Th is fo rm ation has bee n dat ed a t 23 000 ye a r li P by the rad ioca rbo n me thod A well -doc ument ed e xa mple of a Pa lae o zo ic strike -slip zone is the Midland Va lley o f Sco tla nd in t he De vo nia n pe riod . descri bed by Bluc k (1980 ) - sec 8.4 . T he best ac tive ex a mp le is the in ten s ively studied Sa n A nd re as fault zone which we s hall exam ine in 6 .3.
6.2 Displa ced or e xotic terranes T he co nce pt of displaced or exotic terranes a rose from o bserva tio ns in th e No rt h A mc rica n Cord ille ran ben . over 70% of which is re ga rde d as a co llage o f susp ect ferm fles of pro ba bly a lloch t hon o us o rigin (W ilso n, 1968; Mo nge r et 01., 1972; Jo ne s et 01. , 1972). Altho ugh d isplaced te rran e s milY of co urse be fou nd a t an y co nve rge nt plate bo unda ry , a nd a rc pro m inent fo r e xa mp le in t he o rt hog o nal co nve rge nt regime of the Cent ra l Asia n co llisio n zo ne , t he y a re pa rt ic ularl y associ a ted wit h o bliq ue co nve rge nce o r str ike-slip re gimes , an d te nd to acc um ula te a t geo me t rica lly favo ura ble lo ca tion s a lo ng suc h bo unda ries . T he d ist ribution and nature o f more tha n 50 sus pe ct te rr an e s in we ste rn N . A merica (sec Figu re 8.9) is sum ma rized by Co ney et 01. ( 1980 ) who la y do wn ce rta in pr inciples in the ir reco gni tio n. A terran e e xhibits int erna l ho moge ne ity and cont in uity of stra tigra phy , a nd of tecto nic style a nd seq ue nce , a nd is d ist ing uishabl e from adjo ining te rra nes by d isco ntin uit ies of str ucture o r strat igra phy t ha i ca nno t be ex plained on th e ba sis of no rma l fac ies o r te cto nic cha nge s. Most terrane bounda ries se pa ra te to tall y d istinct roc k se q ue nces and/o r
172
GEOLOGICAL S'IIWCru Kl;;S AND MOVING PL AT l:::S
Ma ri
o
A""
km
30
!600m JERUSALEM
Nahal se -e s
1500 10 0 0 m
500
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Fig ure 6.6 (A , B) Rhom tHh ~ pcd gra hc: n formed in en-e c helon scgrnen ts. resulting from strike-slip mo veme nt on the Dead Sea trans fo rm (;Iull zo ne . NOll; difference in scale bet wee n (A ) and (8) (0 Displac eme nt o r Miocene river sp te rn across the Dea d Sea fault zon e . (A ) - ( e) »ncr Fre und ('I at, (19M ),
faunas. and many contain palaeo magne tic rcco rds thai differ strongly from those of the sta ble craton o r of adjace nt te rranes. A suspec t te rra ne may be proved 10 he allochtho nous o r exotic if its faunal o r pala eomagneti c signat ure pro ves it to o riginate a co nside rable d istan ce from its prese nt position. Most of the suspect terran es of N. Am erica appear to ha ve co llided and accret ed to the craton margin during Mesozoic and early Ce nozo ic time . Man y s how ev ide nce o f an or igin far distant from t hei r present pos itio n, and ma y also have undergo ne tr anslatio ns of hundred s of km afte r co llision. Palaeom agnet ic ev ide nce a lso indicates significant rotat ion s abo ut the ve rt ical in man y cases.
Dur ing muc h of the 120 Ma period during which the te rrane disp lacem en t process occur red . the co ntine ntal margin was a subduction zo ne , so that displacement and acc ret ion look place by a process of oblique co nve rgence . co mbining und ert hrusting a nd st rike-s lip movemen ts. It appears that the str ike-s lip componen t was dextra l t hroughout. and the terran es see m to have o riginated far to the so uth of thei r present position , in so me cases possibly on the othe r side of the Pacific Ocean . Silver and Smith (1983) discuss the western Pacific Ocean as an ac tive exa mple of the terrane displace me nt process. As is clear from Figur e 6 .7. obl ique co nverge nce is ta king p lace
173
S I Kl n .-SLl I' AND OHLl QU E- SLlP KEGJM €S
bet ween the mai n Paci fic plate ,H1d Austrutia. T he a utho rs po lilt ou t t hat thi s monon ap pea rs to ha ve s liced off pieces of ocea nic plate au and island a re , together with Irugmcnts of eo nunc mal A ustr al ia , a nd ea rned the m northwards. Lar ge ophioluc m,J SSlCS have been em p lace d in New G uinea ,H1d New Cutedonia as a resu lt 0 1 thi s o bliq ue conve rge nce . Th e bo un dar y be twee n the Indo -A ustralia n and Eur asian platc-, is alxu the sce ne of oblique
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conv er gen ce (sec Figu re 5.37,5.31::), the effects of which have alr ea dy been discussed . Major strike -slip d isplace me nts along NE -SW fa ults have t ran sfe rred po rtion s of the Au stralian conn nc rual pl ate to Ihc Eurasian plate . creaung a co llage zo ne on the sourhcus tc rn ma rgin of rhc Eurasia n plat e . La rge rot ations ca n be pro ved in this regio n. For e xam ple H a ile ( 1978 ) has sho wn fro m palaeo magne tic c vidon ee tha t Sc ra m . in the no rt he rn arm of the
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174
GEO LOGICA L ST RUCTU RfS A N D MO VI NG PLArES
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Ba nda a re, has under gon e a large clockw ise rotation due to thc ope ni ng o f the B anda Sea. O nce accre tion is complete , thc re aso n fo r this rotati on will no lo nge r be o bvious. The lar ge rotat ion s not ed in the terran e collage of the North A merican Co rd illera ma y ha ve o riginated in th e sa me way. T he obliq ue co nverge nce be tween the oceanic part of t he Indo-A ust ra lia n plat e a nd So uthe ast A sia has res ulte d in la rge dex tra l strike-slip d isplace me nts a long t he con tinen ta l mar gin in Bur ma . T ha iland a nd wes te rn Indonesia (Fig ure 5.28) . In Suma tra , the A sian plate ha s respo nded to the o bliq ue subd uctio n by fo rm ing a dext ral strike-s lip fau lt that trave rses the isla nd , a nd d ispl aces th e le a ding edge of th e vo lca nic arc no rt hwa rds in re la tio n to the re ma inde r of th e o verriding pla te . In th is case, th e co mbina tio n o f stee p st rike-s lip faul ting and subd uctio n ha s clearly been favo ure d , rathe r th an str a igh tfo rward obliq ue subd uction , as a me an s of ac hiev ing th e o bliq ue conve rge nce (Bec k, 1993). Bec k di scusses th e possible condit ions co ntro lling th e mechanism of obli q ue co nve rge nce, ' a nd co ncl udes tha t three fact o rs favour the combine d strike-s lip! subductio n me chan ism: (i) a la rge a ngle of conver ge nce (i .e. th e co nve rge nce di re ct io n makes a sma ll an gle with t he trench) ; ( ii) a shallo w a ngle of sub d uct ion; an d (iii) th e therma l ' softe ning' e ffec t of the volcan ic a rc. 6.3 The Sa n And r eas fa ult zone The Sa n And reas fa ult syste m is pro ba bly the most widel y kno wn a nd inten sivel y studied in the world . A tt entio n has been focused pa rticula rly by t wo ma jo r earthq uak es; the San Fra ncisco ea rthq uake of 1906 wit h a ma gn itude
175
of 8.25 ( R ichte r sca le ), a nd th e San Fe rna ndo ea rthquake of 197 1 wit h a magn itude o f 6.6 . T he devasta tio n ca use d by the 1906 e a rthq ua ke , an d the po te ntia l for a major ca tastro phe if a shock o f com pa ra ble magn itude affects one of th e mo re hea vily po pula ted pa rts o f Cali forn ia , have st imulated a ma jor prog ram me o f study in th is fa ult 'lo ne. T he ki ne ma tic histo ry of the reg io n is summa rized in 3. 1. Before a bo ut 38 Ma BP, the regio n was a subd uct io n zo ne with o blique co nve rgence tak ing pla ce betwe en th e oce a nic Fa ra llon pla te a nd the ov er ridi ng No rth A meri can co ntine nt (se e Figure 3.4A ). A s we ha ve see n, o bliq ue co nverge nce d uring the Mesozoic a nd ea rly Ce no zo ic W
~l gu ~ 6.8 Simplified map of the principal fa ults and ut hcr su uctural cleme nts in lhe San A ndreas fault zo ne of Califo rnia and no rth er n M exico . CM. Cape Mendocino ; SC. She ller Cove ; PA . Poinl Arena; GV, Gr ear Valley; S A , San Francisco Bay; SN , Sierra Nevada; SJ, San JUlIO Bau tista bend ; SB . Salinian block ; SN, Santa Maria basin; BB, Big Bend (o f the San Andreas Iault}; SB, Ventu ra basin ; MB . Mojave bloc k; CI. Channel Islands: T R. Tra nsverse Ranges; LA . Los An geles basin; G B. San Go rgione be nd: CO . so uth Ca lifornia offshore borderland; ET, eastern Transver se Ra nges; PR o Peninsular Ranges; ST . Salton trough ; BP. Baja Ca lifornia pen insula: Gc, Gulf or California ; BO . Baja Californ ia offshor e borderland; SO . Sonor a . Numbe red fauhs: 1, San And reas; 2. Mendocino fracture zone ; 3, Oregon subd uction zone ; I I . Big Pine; 12, While Wolf-Kern; 13. Garlock; 15. San Gahrie l; 19. Elsinore ; 2 1, San Jacinto ; for names of other numbered faults , see source . From Crowell (1m)
176
G EOLOGIC AL $T RUCTU RI'.$ AN D MOVING PLAT ES
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Joigu rt: 6.9 Distr ibution of earthq uakes o f mag nitude:5 or greater in the period 1934-69 in sout hern Ca liforn ia, in rela tio n to the princi pal faults . Dots indicate magn itudes of 5.0-5.\}; circles. magn itudes of 6 o r grea ter . From A nderson ( 197 1)
separated by the San Andreas fault zone . The growth o f the zone from the initial point of co ntact no rthw ards is docume nted by the northward progression of the end ing of activity on the Neoge ne volcanic a rc (see Figure 4.18).
The fault zone is about 1200 km lon g and l00 km wide , and consists o f a complex netwo rk of faults (Figure 6.8 ; see And erson , 1971). Most o f these trend NW-SE subpara llel to the main Sa n An dreas fault (see for
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exa mple the San Ja cinto and E lsinore f..tults}. but m
The reg ion is ve ry acuvc seis mically : 7JOO earthqu akes we re reco rde d with a magnit ude of 4 or greater in so ut he rn California in rhc period 1934-69. Th e loca tio n of the lar ger recent eart hq ua kes is shown in Figure 6.Y. Most are un connected with the San Andreas fault itse lf, hut arc associ ated with par allel strike-s lip fa ults such as the San Jacin to and Imperi al faults. and with t he White Wolf fuult , which int ersects the Sa n An d reas fault at 90" in the region u f the ' Big Ben d'. Several of such faults with t re nds ma king a large angle wit h the main fault show o verthrust dis placements. Fo r example. th e la rge earthq ua kes in Kern County and San Ferna nd o were assoc iated with ove rt hrust movem ents. Ma ny o f the faul ts of Figu re 6 .8 ap pe ar to be curre nt ly inactive . Stress and hem fl o w
In-situ stress det e rmina tions in the vicinit y of the San A ndr eas fault using the hydra ulic fract ure techni q ue are re po rted by Z oback et al. ( 1980) . T hey invest iga ted the va riatio n of stress wit h dist ance from the fa ult and also with dep th . In shallow we lls loca ted alon g profiles across two sect io ns of the fault whe re slow creep is taking plac e . t he di rectio n of maximum horizonta l compressive stress was found to he N- S. approx imate ly 45° to the (rend o f the fault . The shear st ress was found to increa se with distance fro m the fault to a value of about 5 MPa 34 km from the fau lt . At 4 km fro m the fa ult , the she ar st ress increa ses from about 2.5 MPa at 150- 300 m depth to a bout
177
XMPa at 750- 850 m de pt h. Thi s increase suggests thm the mean st ress at seis mogcmc depths ( lCl - 15km) mus t he several te ns o f MP
Dis placem ent geometry The rate of disp laceme nt alon g the transfor m bounda ry can be calcul ated fro m the ma gnetic sea-floor strat igrap hy. This varies from 1.3cm/yea r in the early Mioce ne 10 5.5cml year since th e Pliocen e ( A twate r and Mol nar, 1(73). Seismic techniq ues yield a val ue of arou nd 4 em/ye ar for recent movements, and geodetic measurement s give an es timate o f 57c m/year ( Anderson , 197 1). Accord ing to Crowe ll (1979) the to tal dis pl ace me nt alon g t he fault syste m is around l OOOkm . Howeve r geo logic al evide nce ind icates on ly ab out 300 km of dex tral displaceme nt on the San A ndreas fa ult itself since Ihe mid-Miocene (C rowell, 1979). and t he rem ainder of the movemen t is pro ba bly d istrib uted a mo ng many s malle r fa ults. A significant pr opo rt ion may be taken up by a major fa ult alon g th e con tine ntal ma rgin offsho re . According to Hein (1973) . for
178
GEO l.OGIC A L. STRUCTU RES A N D MOVING Pl.A TES
example . a subm ari ne fan lying on th e ocea n 11001 has been displ aced from i ts source by a
dista nce o f 300-550 km . Th e to tal observed dex tra l displa cement on the ma in Sa n A ndrea s fa ult is 600 km as measu red by the offset of the Sierras base me nt. T hus th e fa ult is interp ret ed as having a two-s tage histo ry o f mo vem ent : the fi rst , o f late C retaceous 10 Palaeoc ene age , prod uced
abou t 300 km of displacemen t; the seco nd is the curren t phase co mmencing in the Miocen e. The early mo ve me nt is te nta tive ly linked with a possible transform boundary bet ween the Kula a nd Nor th A me rican pla tes a nd e nded when the Kula - Far atlo n ridge swe pt no rthwa rds up th e Ame rican co ntin en tal margin (see Figure 3.7A ). T hese ea rly moveme nts were assoc iated with the ma jor terran e displaceme nt process referred to ea rlier. Th e st raight sec tio n" of the ma in San A ndreas fault appear to be prec isely parallel
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with sma ll circles about the pole of rotatio n for the Pacific- A me rican plate motio n, as det er mined by Minster et al. ( 1974). Slip alon g these straight sectio ns is exhibited in the form of slow ' creep' and by man y sma ll-magnitude earthqu akes . The curved sectio ns, in co ntrast , ap pear to be locked , and such sec tions arc obvio us sites for large ea rt hquake s such as those a lrea dy men tioned . T he most prominent curved sec tion is known as the Big B end region (see below ), where activity is prese ntly co ncentrated on the White Wolf and o ther faults (see Figures 6.9, 6. 12). In othe r place s, bends appenr to be in the process o f being by-passed. Fo r exam ple, at the San Gorgione Bend ( Figure 6.8) , major move ment appears now to be taken up a long the San Jacinto fault , thus ' stra ightening our ' the main fau lt line , transferr ing the majo r movemen t to the west and iso lating an inactive slice on the eastern side of the new fau lt line . At the so utheaste rn end of the fau lt zone , the San Jacinto and Elsinor e faults end at the ma rgins of pull-apa rt basins. The se fo rm part o f a system o f side-stepping spreading cent res within the Gulf o f California , which has o pened by a process of oblique rifting (Figure 6.10) . T he pull-apart basins are bounded by no rmal fau lts with tre nds ob lique o r pe rpendicular to the m ike-slip faults (see Figure 6.28). T his proce ss has resulted in the no rt hwestwar ds movement of the Baja Califo rnia penins ula by a co mbinat ion of E- W sprea ding and NW-S E st rike-slip faulting. Rhom bo id depressions are also form ing in the o ffshore regions o f Ca liforn ia by the same mechanism .
PAC I F IC OC£ A N
..', Hcure 6.10 Co mbinat io n o( sp rea ding ridges and transform Iaults in the Gulf of Califor nia by which nort hwest wards displacement of the Baja California pcninsulil hils been achieved . Not e the patte rn of ear thq uake c picent rcs , man y of which are clearly associa ted with strike -slip move men ts. After Isacks i f al . ( 1%8 ).
St ruct ure a/ the Santa Maria district
In no rthe rn and ce ntral Califo rnia, the San And reas fault lies relatively close to the continental margin , both o n and o ffshore , for about 450 km from its nort hern end until it cuts inland towards th e Big Bend region ( Figure 6 .8). Th e Coast Ranges, which lie o n its weste rn side , contain a number of e longate Neogene basins thaI t rend slight ly o bliq ue to , and ant iclockwise o f, the line of the San
STKua :-S LlI' ANI) OIl LlQUf.;-SLlI' REGIMES
179
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riJ:U~!' 6. 11 Simplified structura l map and secnon of the Sa rna M~r; a o il dismct , Ca lifo rnia . Note lhe oh hque relations hip !>cl ween fold a xes (conv entional symhols) and str ike -slip Iaults (heavy IlOes ) chaructc r jsuc ur Slr ike.s lip rc ~Im es . Qa . alluvium ; Qt! . non-marine Ouale rna ry: Trl. Pliocen e matine ; Till . Mio cen e marr ne; Mioce ne or Oligoce ne no nmann e : Cretaceou s mar ine ; f . Franciscan depONh. Numbers re fe r 10 oi l field s [ see so urce re fe re nce ); o il fields arc Slipplcd o n the ma p and she wn in black in the sccuon . After Blake iN al .
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And reas fault. Th ese basi ns are se para ted by parallel elongate uplifts. T he structu re o f these basins. which is related to moveme nts o n the fault zo ne , is o f interest becau se of th e o il fields occupying man y of t he anticli nal a reas. T he Santa Ma ria d istrict lies at the southeast end of the Coas t R anges, northwest of Santa Barba ra . The structu re is describ ed by Blake et al.
(1978) and summarized in Figure 6.11. T he Neoge ne San ta Mar ia basin contains a marine sequence, up to 4S00m t hick, of mainly
has bee n shorte ned in a NNW -SSE direct io n an d exte nded in a WNW - ESE di rection due to progressive simple shear . T his mod el applies ge nerally 10 t he San A ndr eas fau lt zone , which exhibits a complex arrangeme nt of up lifted blocks und ergoing co mpression and depressed bas ins unde rgoing exte nsion, who se sed ime ntary infill is be ing folde d as a result of s ho rtening in a di rection an ticlockwise of t he stri keslip o rientation . in acco rdance with the simpleshear mod el.
Pliocene - Pleistocene age, lying unconformably o n F ranciscan base me nt . Th e sedime ntary seque nce was folded and loca lly thrust in late Quate rnary lime , formi ng broad , open , pe riclinal folds with WNW-ESE axe s. T he oil is contai ned in fra ctured shale of Up pe r Miocene age and occ urs mainly in anticlinal traps (Figure 6 .11 , secti o n) . Th e oblique relationship between t he fo lds and bo th the San Andreas fa ult and the pa rallel fault to the nor the ast, is co nsistent wit h the geomet ric model o f Figure 6.1 for a dextral m ike-slip regime , and suggests that the whole sed ime nta ry co ver o f the Coast Ran ges bloc k
Structu re of the Big Bend Region. T he co mplex regio n of the intersection of the Sa n A nd re as and Garlock fa ults (Figu re 6. 12) is discussed in detail by Boh an non and Howell (1982 ). They poi nt o ut the geome tric incom pat ibility of simultaneous moveme nt o n the dex tral Sa n A ndreas fault and the sinist ral Garlock and Big Pine faults which Inte rsect it. The Garlock fault has a displaceme nt of 60 km , much of which is probab ly late Ce nozo ic to Quaternary in age , and it was active historica lly. Th e Big Pine fault dis place ment is
180
GEOLOGICA l. STRUCTU RES A ND MOVI NG PLATE S
COAST RANGES
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6.12 Bloc k mooet ro illustrat e the rela tio nship betw een the vario us raul! bloc ks in the region o f the "Big Bend' . Note the ne cessa ry upth rust structures alo ng the Ga rlock la ult (d. Figu re 6. 13). F ro m Anderson ( 1971)
probably mos tly of Pliocene 10 Pleistocene age . The a ut hors suggest thai the San A ndr ea s fault pro ba bly o rigina ted as a straight linear featu re , but was deformed by extension in the Basinand-Range Province to the no rt h (see 4 .3). Figu re 6 . 13 sho ws how t he st ructure might have evo lved. Continuous sinistral movement o n t he Ga rlock faul t produced a bend in the San Andreas fa ult , since neither fau lt co uld cut the o the r wh ile both were act ive . Th e e ffect of the be nd was to prod uce a local compress ional region in the nort heast ern quadrant , where E- W ov erth rusts a re found . Th e northwardmo ving western bloc k may either have moved laterally west ward s or deformed inte rnally as it slid past t he bend . A similar bu t less intense zo ne o f co mpressio n exists in the southwestern
qu adrant where the sinistral Big Pine fault meets the San And reas fa ult. These sinistra l faul ts a re uncon nected but ha ve moved closer toge the r as a result of the dextr al displacem ent o n the main fau lt. T he bend ing of t he San A ndreas fault would ca use internal de formation within the Moj ave bloc k in the southwestern qu adrant , which wou ld suffe r N-S sho rte ning an d E-W ex te nsio n (compare Figur e 6.1 3A ,D ). T his deformation is a conseq ue nce o f th e cha nge in angle of t he bou ndaries of the bloc k . This mod el is co mpati ble with the late Cenozo ic deformat ion pattern in the Mojave desert. If the whole th ickn ess of the crust were involv ed in t hese move men ts, it is difficult to visua lize how the excess volume re presen ted
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by the triangle of overlap (shown with stipp led ornament in Figure 6.I3 D) could be accommodated . The problem is ove rcome if. as seems like ly. the faults det ach o n a lo w-angle decolle ment surface at depth . o r are o the rwise transfe rred at mid-crustal leve ls to a b roade r zone of du ctile deformatio n. 6.4 Oceenlc tra nsfor m raul ts
Transform fault s make up a significant proportion of the oce anic plate bounda ry networ k (see Figu re 3. 1). Th ey we re originally defi ned and their significance exp lained by Wilson ( 1965). and their o rientations were used in the ea rly atte mpt by Mck e nzie and Par ker ( 1969) 10 define and describe the mechan ism o f plate mot ion . Ocean ic transfor m faults may be divided into majo r plate bo unda ry faults and the re lat ively mino r, of ten transient offset fault s associated with the spread ing process at ridges. Th e latter are d iscussed in 4.2. Some major transfo rm
t'iKU", 6.0 Modd 10 illustrate the su ueluroll evolution of the int crscelio n of the: Garlnd, (GF) and San A ndr ea s (SA F) fa ulls (A) in itial ~i lU ~ t ion wuh a unifo rm NNW SSE o ric ma uo n of the SAF. (8) Ea rty \ Iip o n the G F is acco mpanied by bendin g
faults ca n be di rectly att rib uted to irregularities in the shape of the initial conti nental rifl syste m (see Figure 4.9). As the passive continental margins spread apart to form an oce an. the transform faults pro pagate co ntinuo usly inwards 10 form permanent boundaries to the sp reading segme nts. The majo r fractu re zones o f the eq uatoria l Central At lantic (c .g. the St. Pau l. Romanchc and Chain zones) ori ginated in this way. Other transfo rm faults o riginate as a geo metrical respon se to kinematic changes, as illustrated in Figure 4.5. The mo rphology and general structure of the major oceanic fracture zones arc described by Menar d and Chase ( 1970). Most possess a prom ine nt scarp , ofte n with an assoc iated ridge and trough . although any of these featu res may be abse nt. The re is oflen conside rab le topographic vari ation along the length o f a given fracture zone . T he width varies from a few km to ove r 50 km in the case o f the largest zone s. Linear gra vity and magnetic a nomalies are o ften associa te d with the zones. Th e or igin o f
182
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n gurt 6.1" (A . 8) Simplified ma p showing th", ridgea nd-valley topog raph y of the Vema (A.l and Rom anch e (8 ) tran sform fault zones ;n the Cen tral A tlanti c Ocean.
Note lhal areas of maximum heigh! ..ccur opposite the ends of the spreading .ut's. across the tra nsfor m "'i1 UCY5. The a rrow in ( B) points to part of the ridge Ihal ....as a t sea le vel abo ut S Ma ~, . (e) Diagra ms illustr ating th e th ree main lypeSof base menl morpho logy profile ( M- N ) ac ross tr ansfo rm Iracture zones . ( A)-(C) fro m Iln nall i (1918)
these anomalies has caused some debate; they have bee n att ributed both to magmat ic intrusion and to hydrotherm al alteration, and both
processes probably contribute. The morphology of fracture zones is discussed in detail by Bcn aui (1978) who au ributes the topog raphic variation primarily to the effects of diffe rential cooling and subsidence. Figure 6.14C shows the different types of topog raphic profile found at fracture zones. The trough itself is considered to be a thermal contraction effect related to the shape of the magma chamber at spread ing axes (see Figure 4.5). Type A would be the expected normal profile if this were the only control on the topo graphy. The elevated side is closer to the spreading axis and has therefore cooled and subsided less than the other. However type A fracture zones are comparably rare, and only
M=':'f
found in certain Pacific zones. Profil es of type B or C, or of an intermediate type, are typical of most fracture zones. In the Atlantic Ocean in particular, major topographic ridges are associated with the great transform faults marking the sinistra l offset of the mid-Atlantic ridge between the Central and South At lantic. T he Vema fracture zone, which offsets the mid-Atlantic ridge at l l' N, shows a topographic profile approaching type 8 (Figure 6.14A). A prominent ridge occurs on the south side of the valley marking the transform fault zone. T he ridge rises to a height of over 5 km from the adjacent ocean floo r, and is presently about 600 m below sea-level. There is even evidence of recent eme rgence. The Romanche fracture zone (Figure 6.148 ) appea rs to approach type C in topography, since major transverse ridges border the seismically active
S flHKE -SLlI' AND OII UQUE -SUr REGIMES
transfo rm va lley , a nd e xte nd fro m the ridge Intersection to t he A frica n and South Ame rican contine ntal ma rgins . Simila r topographic pro files with on e o r mo re ridges ha ve been observed in t he Owen fract ure wile in the India n Ocea n , in the A lula fract ure zone in the Gulf of A de n , a nd in t he Ch a rlie Gibbs fract ure zone in the At lan t ic (see below ). Bon atti notes t hat la rge fractu re zo nes are generally characte rized by ridges , ru nni ng para llel to the ma in tra nsfo rm fault-zone valley , whose su m mits may reac h to 1 km o r mor e above the e xpected level fo r 'norm a l' ocean ic c rust of th at age. The nature of t he rocks fo rmin g these r idges (mostly sc r pe ntinizcd pe r idotit e ) indica tes th a t they a rc mostly for me d by the uplift o f norm a l ocea nic lithosphere ma ter ia l, ra the r than by magm atic intr usio n. T he fractu re zones a ppea r to be affect e d by int ense vertica l mo tion re q uiring subside nce ra tes m uch higher than e xpecte d from no rmal ocea n cr ust. Bc narti a rgues th at these vert ica l mo tion s ca nnot he expla ined o n the bas is of sta nd a rd oce a n-spre ad ing theory . and tha t so me add it ional mech anism is re-
*
quired . Th e zone of max imum e levat io n of the tran sve rse ridges occ urs ncar the spreadi ng ay:<; on the opposite side of th e fra cture zo ne valley. suggesting th at th e rid ge m ight he d ue 10 Ihe th e rm al anom aly ge ne ra ted by the sprea ding ax is. It has bee n ca lculate d tha t tempera ture cha nges o f l OO"C e xte nd ing to 20 km from th e fra ct ure zone a nd to a de pth of 70 km ma y be produced ( Louden and For syt h , 1976) . Howeve r. this e ffect wo uld o nly produce a n uplift of a few hund red m at most ,
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whereas topograp hic relief of se ve ra l km needs to be ex plai ned . Bon ut ti concludes that up lifts of t his ma gnitu de can on ly be ca used by co mpress io na l fo rces. and c ites severa l lines o f evidence fo r the ex iste nce o f bot h co mpress io n an d ext en sio n ac ross fractu re zones , as o rigina lly suggested by De wey (1975 ). (i) Focal mec hanism solu tio ns of ocean ic int ra pla te e a rt hquakes ind icate a state o f ge ne ral hori zonta l co mpressio n (see 2.6). In par ticular , two eart hqua kes in th e re gion o f the Cha irn a nd C ha rco t fracture zones in th e Cent ral Atla nt ic e xhibit com p ressio na l thrust solut ions with 01 pe rpendicula r to th e str ike of the fa ult (Sy kes and Sba r, 1974) . (ii) Se ismic re fl e ctio n profiles ac ross sed ime nt- filled fractu re va lleys in se ve ral zo nes sho w foldi ng a nd other co mpressional fe atu re s. (i ii ) Mylonitized rocks dredged fro m man y of th e lar ge frac ture zo ne ridges suggest t hat the ir uplift was te cto nic. Exte nsio n is suggested in se ve ra l zo nes by the g rabe n - ho rst mo rph ol ogy. ho th in a gross sense a nd with in the sed ime nt fill, and by the occu rre nce of min or volcan ism. Th e re ar c se vera l possible causes of hori zon ta l co mpress io na l a nd e xte nsio na l forces. As po inted out in 3.3. rela t ive pla te mot ion ve ctors ar e norma lly o blique to pla te bo undaries . Eve n altho ugh a tr a nsfo rm boundar y no rm a lly o riginates para lle l 10 t he re lati ve plate mo ve me nt vector , changes in this vect or occur d uring th e cou rse of the plate mot io n, re sulting in a compone nt o f co nve rge nce o r div erge nce across the bou nda ry. O n a more loca l sca le, tbe prese nce of sinuo us o r e n-ec he lon fau lts within the fra cture
.
_ .-- ~
...Figure 6.15 Dia gram matic plan illust ratmg th e e jjects of differential horizonta l ther mal co ntraction o f oceanic lithosphe re along a tra nsfo rm fracture lone. Maxim um compression occurs a t the e nds of ridge segments , a nd e xte nsion occurs between these . From Bcn au i (1978)
y
184
GEOLOG ICA L S l'RU CTU RES AND MOVING PLAT ES
zon e will produce zon es of co mpress io n or extension as descr ibed ea rlier (see Figu re 6.2 , 6.3). T he rma l co nt ract ion o f the cooling plate us it moves a way from the sp readi ng a xis is a n im porta nt source o f minor e xte nsional stress , as shown in 2.5. Co llette (1974) has cs nm a rcd that a ho rizontal cont raction of 0.3 km wou ld res ult fro m the cooling th rou gh 20{f'C o f a 150k m segme nt of axial ridge . As show n in Figure 6.15. a zo ne o f maximum com pression m ust ex ist across a fract ure zo ne at the en d of each sp rea ding se cto r, an d a zo ne of ma ximum exte nsio n be twee n the o ffse t sp read ing sectors . Of these so urces of stress . cha nges in rela tive plate mo tion arc proba bly th e most effect ive in prod ucing lar ge stresses across fracture zo nes , a nd arc the most obvious reason for up lift o n on ly one s ide of zones suc h as the Ve ma a nd O we n fract ure zo nes .
The Charlie Gibbs and Glo ria fracture zo nes Sea rle ( 1979. 1986) describes the results of a sid e-sc a n so na r study . using the CLORI A sys te m , of seve ral A tla ntic fract ure zo nes . He notes a marked asymmet ry of fracture va lleys on the shor t-offset sec tions of the mid-A tla ntic ridge , wit h a steep sca rp on the older side . facing the younge r lith osphe re , a nd a ge ntle slope o n the
younger side. Seismic re fl ect ion and gravity profiles ac ross the Kur cha to v frac ture zone ( Figure 6. 16) indica te tha t t he asy mmet ry of the to pogra phy is a reflect ion o f the c rustal str uct ure . Th e asy m me try is a ttrib uted to differe ntia l uplift o f the o riginal ridge crest co mpa red with the transfor m valley floo r. Th e topograp hic effect is prese rved o n t he walls of the transfor m valley as the ocean floo r cools a nd moves away fro m the sprea ding axis. T he co mbina tion of diffe rent ial uplift and simple shear produ ces a set of obliqu e normal
fault scarps as shown in Figure 6.17. These have been found in a number o f fract ure zones, a nd a re pa rticula r ly clearl y demo nst ra ted in the Charlie Gibhs fracture Wil e , which displaces t he mid-A tlan tic ridge a round 5(fN. T wo sepa ra te fa ults we re fo un d . a t 52°(}6'N a nd at 52°36'N. se pa ra ted by a sho rt N-S spreading secto r: T he so uthe rn fract ure valley ( Figure 6.18) is almost se dime nt-free . a nd disp la ys the nature o f the baseme nt struct ure clearly. T he valley is V-sha ped . about 2 km dee p . a nd 15- 20 km ac ross at the top , which lies a t a round 2{XKl m depth . A steeper inner region can be recognized , which pro bably co rres po nds to the 'act ive transfo rm do mai n' recognized by Fra nc he tea u et al , (1976). T his in ner valle y is 5- 10 km wide a nd lies bel ow
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ti Rurt 6. 16 (A) Seismic reflection pro fite across the Kurch atov fractur e zone at 29"W showing the asymmetric natu re of tbe valley. Heavy line is basement . fine: tine: is sedime nt surface. From Se:arle (t979). with pe rmission. ( 8 ) Free-air gravity (A ). Not e that the asymmetry in the surface to pograp hy ex presses a ano maly profile and crustal mod el alon g the line dee p-seated asymme try in crust..1structu re . Th e figures in the cr ustal model are specific gravities of the layers used for the model . The crosses in the gra vity profile are the com pute d values . Aft er Searle (1979).
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S n W(E-SL lI' AND OHUQ UE-S UI' REG1MI:S
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H~ur" 6. 17 Diagram ;IIU~lr;llin g n method of for rnauon of oblique scarp' ill lra 'hlor m frac tu re zones , Lighl lines represe nt sche ma tic co ntou rs; hC3vy lines are Ia uhs; large MruW~ sho w regio nul d,ps; ~.., ~ U arro ws loc al stress fields. At A . le n~ion g..shes and norm al faults fo rm in ne wly c rea ted hrbo sph cr c ncar , pre.,uIOg ,, ~ is -transfo r m fa ull mtcrsccuo n-; Uptih from median valle y floo r 10 cresurl m"unl,lIn, tK:CU r.. al B , T hus SC
.lgu ~ 6. 18 Side -scan so na r ph o tograp h musaic , and tccic mc imc tpr e unic n. 0 1 rne Cha rlie Gib bs tra nsfo rm fra cture zone . Inferr ed ma jo r smke -slip Ia ult , heavy hnM; o thcr fau lts. light lines , with tid s indicating inferred do wnthro w; o ther lineamen ts, dashed . Fro m Searle ( 1979 )
3 km depth . Th e walls a re mod era tely stee p, with slopes of up to 30°, a nd conta in many E- \V sca rps attributed to no rmal faulti ng . T he N-S mid-Atlantic-ridge fault st ruct ures curve
sinistrally into t he top of the t ransfo rm valley but do not e xtend into the inne r 'ac tive' portio n. T he bottom o f t he valley exhibits lo ng str aight E - W scarps thai are thought 10
186
GEOLOGICA L ST RUCru RES AND M OVI NG PLATES
represe nt the activ e st rike -slip fault traces. A linea r E -W baseme nt rid ge , a few km across, borde rs th e inactiv e port ion of the nort hern tra nsform. Major oblique sca rps, trc ndm g NW -S E at 45° to the spread ing direc tio n , occ ur o n inactive frac ture zo ne walls. especia lly on the so ut h side o f the no rth ern tra nsform valley . T hese a re about 15-20km lon g and spaced abo ut 15-20km ap art. Sea rle att rib utes these to obliq ue norm al fa ults for med unde r a de xtral simple-shea r regim e . Anothe r fract ure zon e described in SOme de tail by Sea rle is th e G loria fractu re zo ne which runs f rom t he mid-A tlantic ridge ncar t he Azores to Gibraltar , and fo r ms {he boundary between the E urasian and A frican plat es
(see Figure 3. 1). According 10 McK enzie ( 1972), this sec tio n of the pla te bou ndary is ex te nsional at the A zores e nd . bU I bec o mes compressional through the Strai ts o f G ibr alta r, with a pure st rike-slip sec tio n betwee n . Along th is central sectio n, se ismic reflec tion pro files indicated the prese nce of a scarp. 100- 500 m high , o n the sout h side of the valle y mark ing the active fault . T his valley is v -shapcd a nd between 5
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Fi,::urf 6. 19 (A ) Simplified sketch of teoomc hneameru r cuc rn . inferred from scnographs, of an area of
the East Pacific ridge between )0 and 50S. The three main fracture zones each consist of several smallerscale fractu re zones . Note the bend ing of the oceanfloor lineament fabric into the Iracture zones. 'me heavy line is the inle rred plat e bo undary. (B ) Summary mode l illustrating the proposed tecto nic pattern al II ridge - tra nsfo rm inters ection for a fast'~prcad i ng ridge. Full ar TOWS indicate infe rred deviuror jc stress duecucn; hal( arrows. shea r stress . ( A) and ( 8 ) fro m Sea rle ( 19113)
$ I Il IKE - S U I' AN D OIJLlQ UE- SLJ I' REG IME S
truncated by it. A broa d ridge on the north side of the valley . be tween 23° a nd 24°W , ca mcs the older structu res on it. A less prominent and narrow er ridge occ urs o n the so ut h side betwee n 200 a nd 22°W. Th ese detailed observat io ns on fracture zones confirm the trans form fa ult model described by Bonani ( 197H) . Tra nsve rse ridges carry ing base me nt s tr uctures a re clear ly the prod uct o f diffe rent ial uplift rat he r than intru sion. An impo rta nt re fi nemen t to the Bcna u i model is sugges te d by the o blique sca rps which imply the o pe rat ion o f a narrow zone o f simple shear. abo ut IO km wide , within whic h e xten sional normal faults form a t 4SO to the spread ing d irection . wit h long transfor m offse ts (proba bly > 20 km ). a true strike -slip fa ult develop s in a na rr ow valley in the ce nt re of this zone. Fa.l'I-sllj l/Jill g !flluufe zones ou the Easl
Pacific ridge Searle ( I WO ) describes 11 rat he r di ffe re nt morp ho-tecton ic pa tte rn in the multip le offse t zone o f the East Pacific Ridge be twee n 3° an d 50s using the G l.O RIA side sca n son ar syste m (Figure 6. 19). Nine se pa ra te t ra nsfo rm fau lts were ide ntified in t hree grou ps corresponding to the Ouebrado a nd Gofar fracture zo nes a nd to the previ ou sly uniden tified D isco ver y Iraclure w ile. The spacings be twe en the individu al transfo rm fau lts ran ge fro m 5 to Io krn . a nd the offsets ra nge fro m 24 to 93 km. As in the Atlantic fracture zo nes. the individ ual t ra nsform faults occu py narrow valleys a few km in width . but in the case o f the closely space d
IR7
zo nes , the valleys me rge into a single broad va lley. Several fault sca rps occ ur in the va lle ys. Th e active fa ult is not co nfined to the bottom o f the valley. bUI occ upies various positio ns o n the flanks or e ve n the la p of the slope . Th e per vas ive spreading fa bric fou nd o n the Pacific ocean floor is modifi ed near the tr a nsfo rm fau lts, begin ning abo ut 4-10 km from the fau lt with a ge nt le curve . T his brings the fab ric to a tren d of a bo ut 5SO from the nor mal di rec tion within 2- 3 km of the tra nsform fau lt. In some case s the fabri c becomes near ly asympt oti c to the fau lt. T he s igmo idal nat ure o f t he curvature , a nd the degree of obliquity, con t ras ts wit h t he A tlant ic e xa mples described above . T he di rection of cu rvature is similar to t hat o f the A tla ntic fract ure zones a nd is in t he op pos ite se nse 10 the st rike-slip displace me nt (Figure 6_19). Sea rle ex plains th is curva ture hy a gradual cha nge in o rien tat ion within a na rrow zo ne of simp le s hea r alo ng the tra nsfo rm fa ult . Linea r feat ures formed at tow an gles to the t ran sform fault arc inte rpre ted as Riedel shea rs (sy nt he t ic s hears with the same se nse o f str ikeslip displacem e nt as the main fa ul t - sec Figure 6.1 ). Th e pa tter n o f mu ltiple . close ly-spaced tra nsfo rm faults is t ho ught by Sea rle to be typica l of fast-spreadi ng ridge o ffsets. H e also suggests tha t t his pa rt icula r zo ne ma y ha ve develo ped in res po nse to a sma ll ( Hf ) clockwise cha nge in spre ad ing direction . whic h wou ld e ncou rage the deve lopme nt o f t ra nsfo rm fau lts with a small e xte nsio na l com po ne nt. T he ne t e ffect would be to produce a n ove rall clock wise cha nge in dive rge nce di rectio n.
7 Int ra plat e t ectonic regimes
7. 1 Types an d cnaracrcrtsttcs or intrapl at e st ruc ture
Cen tra l Asia (5.4) . Lithosphe re loadi ng a nd the resulting crusta l thic kenin g will prod uce isostatic up lift in the reg ion o f th e load , a nd fle xu ra l de press ion in the regio n beyond the load , prod uein g [ o r<'ltmd basins , Th ese ca nnot he co nside re d as ge nuinely int rapla te phe no me na. Ho we ver , many st ructu res situ.ucd fur fro m plate bo unda ries ma y have no o bvious co nnec tio n wit h bou ndary structu re ... . Fo r e xample t he Ba ikal rift . north of the H ima la ya n pla te bo unda ry (sec Figure S.2g) a nd the Ce nozoic co m press io na l fold be lt o f so ut hern E ngla nd , situa te d 700 km nort h o f the A lpine fro nt , arc bo th e xam ples of e nigma tic structures that ap pe ar to be intr a plate . but fo r wh ich a co nnec tio n with plate bo unda ry teeIon ic ef fects ma y he postula ted . Processes lha t ar c unre lat e d o r only indirect ly relat e d to plat e mo vem e nts include isos tatic respo nses to cha nges in th e de nsity str ucture of th e lithosphe re or asth e nosp he re , such as int ra pla te hot spots , a nd un lo ad ing du e to the re mo va l of icc shee ts. A speci al type of int ra pla te basin is forme d a t passive co ntine nta l ma rgins. Such basins typicall y show a lo wer fa ulte d sec tion co rresponding to a n e arly rift ing stage in the evo lutio n of the basin. A t this stage , th e basin is the produ ct of a d ivergent plat e bounda ry regime (se c 4.3 ). Ho we ve r. d ur ing the la ter sta ges of its e vo lut io n, th e basi n bec o mes part of the sta ble plat e interior. as the bo unda ry mo ves awa y fro m its initial po sitio n with co ntinued se a-floo r sprea ding . Th e A tlantic coa sta l basin s provide exce lle nt exa mples o f such structures. Co ntin ued depressio n in t hese basins is prod uced by cooling and sinking o f th e oce a nic lithosphe re a ided by th e g ra vitat io na l load of th e se dime nt pile . Vert ical mo vem en ts in the oce a nic lithosphe re a re re lat ive ly simply ex pla ine d o n the basis of the cooling mod e l desc ribed in 2.3. O ce a n floor is progressive ly de pressed with increasing age a nd distance fro m t he spre ad ing
Accord ing to classica l plate te cto nic the or y, pla tes a re ess e ntially stable inte rna lly. a nd te cto nic eff ects a rc co nce ntra ted at th e ir bou nda ries. Ho we ve r it has always been recognized tha t thi s was only true to a first a pproxima tio n , a nd tha t a ll regio ns of the Ea rt h's su rface e xpe rie nce tecto nic effects 10 so me degr ee . T he most co mmo n type of in traplate (w it hin plat e ) tecto nic act ivity is undo ubt ed ly vert ica l mo veme nts. Acc ura te geode tic measure me nt s invo lving precise levell ing a nd o the r tcch ruqu es have shown that most part s of th e crust a re und e rgo ing slow uplift or depressio n . Th e fre q uency a nd distribut io n o f intra pla te e a rt hq uak es is a lso an indica tio n of wid esp read tect o nic act ivity, a lbe it to ,I not abl y lesse r deg re e th a n a t plate bo unda r ies. T he vertica l mo vements , how eve r , ar c ty pica lly at lea st a n o rder of mag nitud e slowe r than th e mo ve me nts associ a ted with plat e bo und ary activity . La tera l va riat io ns in these int ra pla te ve rt ica l mo veme nts cre a te a syste m of basins an d interve ning uplifts th at is th e c ha rac te ristic st ruct ure of all co ntinental intra plat e re gio ns. Th e o rigin of th ese vert ica l int rapl at e mov eme nts is still the cause of co nside ra ble unce rta inty an d debate . It is co nve nie nt to di vide the po ssib le modes of o rigin in to th ose rel ating to horizont al pia te mo ve men ts an d those t hat a re no t d irectl y re la ted to such mo ve men Is. Mo veme nts in t he fo rme r ca tego ry ta ke place as a result of ho rizo nt al d isto rt io ns of the plat e int e rio r, e ither by e xte nsio n o r by sho rteni ng, a nd thus violate t he principle of the la te rally ' rigid' plate . Th e wide ly used mode l of the e xte nsio na l basin developed o rigina lly by MeKen zie (1978 ) is depende nt o n a n initia l assum pt io n of lateral intrapla te e xte nsion . Plate bou ndary processes can prod uce ve rtical mo ve me nts at co nside ra ble d ista nces fro m the bo unda ry. as we have see n in th e case o f 188
I NT RA /' I.AT E T ECT ONIC REGI M ES
189
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centre . Depart ures from this gen e ral rule occ ur in areas nf hot-spo t ac tivity. where the oce an crust becom es mo re bunyan I due to the tbe rmal effec ts o n the den sity structure. Similar variatio ns associat ed with fracture zones were discussed in 6.4 , Cratonic basins vary wide ly in mo rph ology and st ructure . Man y appe ar to be underlain by rift system s. in which case it is possib le to use the lit hosphere stre tching model to exp lain their ge neratio n. Th e No rth Sea is an excellent example o f this type of basin. and is described in de tail below . Determ ination of recen t vertical movem ents in ploseinterio rs
The most wide ly used method of gai ning accura te info rma tion conce rning present -da y movement s is by the geode tic tech niqu e o f repeated precise levelling and trian gulation . Info rmatio n abou t ra the r longer-term movements ca n also be obtained from mea su ring o ld lake and sea sho re levels. T he distribu tion of
t'i~u r(>
7. 1 Recent vcrucat movcrn c uts in ofthe USA . Nul<: p
recent ve rtical movements in the stab le Arnc rican co ntine ntal plate is shown in Figure 7. 1. Note that the rates of movem ent arc low - in the ran ge 0 - 5 nun/yea r - an o rder o f magnitud e smaller than those associa ted with hor izo ntal plate movemen t. Se vera l depressed region s are associa ted with subside nce alo ng t he pass ive co ntinental margin . particular ly in the Gulf of Mexico. Eq ual ly prominent is the uplifted region o f the Great Lak es. attributed 10 post-glacial rebound . Recent movements in the Russian platfor m (Figure 7.4) d isp lay a simila r pattern , T he ave rage rates ca lculated fo r recent ve rtical movem en ts a re much higher (b y more than x 10) than those calculated from the study of sed ime ntary basins measured over millio ns o f yea rs. As we shall see, these ave rage o nly about O.2m m/year. T his suggests that crato nic mo vements are e pisod ic or oscillatory ove r periods o f the ord er o f H~ years o r mo re , Perhaps periods of relati vely rapid dep ression o r uplift ar e separated by much longe r periods o f co mparative quiescence ,
190
T he re
GEOLOGICAL S"IIWCl'U RES A N D MO \l I 'CG I'L A I lOS IS
so me e vide nce po intin g to a co r-
re lat ion be t wee n pr esent mo vement and topogr aphy . 1\ rcas CU Trent Iy u ndergo i ng dep ression arc already low -l yin g,
uplift arc alr ea d y elev a te d. con tir m ing th a t the curre nt move ments a rc pnn o f a longe r-te rm process.
7.2 Th e Ru..sia n platfo rm : a typical intraplate regio n'! The stable inter ior o f t he Europ ean part of the Eurasian piatc co nsists of th e Fe nnosca ndia n or Balti c shield and the Russian ptatjorm ( Figure 7 .2). T his la rge re gio n ha s existe d as a cra to n since m id- Pro terozoic tim es a nd is surrounded by orogenic hells o f Phanerozoic age: the Ca ledonia n belt in the north wes t. t he He rcyn ia n belt s of wes te rn E urope a nd th e Ura ls o n the so ut hwes te rn an d eas te rn sides respectivel y, a nd the Al pi ne bel l in th e so uth . In a re vie w o f t he Ru ssia n platfo rm by Ale inikov et ot. ( 19HO), e leve n m ajor sedi mentary basins a nd six uplifts a re recogniz ed . In t he ce ntra l part of t h ~ pla tfor m . depositio n
of se dime nts co mme nced in late Prot e ro zoic nm c. a ro und 14 ()O M ,l ago , o n high ly defo rme d a nd meta mor phose d early Prot erozoi c base rncru. In th e nonncustcr n pa rt of t he pla tform ho we ve r, de posi tion did no t co mme nce un til t he Ordo vician . Th e aut hors s ubdivide th e st ruct u res fo und with in the platfo rm into thre e ma in types: (i) la rge circular de pression s (sy nec tises v o r up lifts (antectises v cause d by flexure of the Ea rt h's cr ust; ( ii) lar ge elongate gr abe n (tluili cogens) d ue mainly to faulring; and (ii i) c mb aymc nrs at the ma rgins of the platfo rm associat ed with subs idi ng bas ins of plate bo undary type . M OSI of the la rge structures ar c a t le ast 1000 km across a nd represen t crus ta l fle xures with amplitudes of around 1500 m an ave rage . A ulacoge ns a rc th e do m ina nt type o f basin : of the e leve n major bas ins recogn ized , se ven ma y be clas sified as a ulacoge ns. Th e formation o f a ulac oge ns was virtually confi ned to two pe riod s. th e la te Prote rozoic a nd the midPalaeozoic ( Fig ure 7.3A, lJ). Most a ula coge ns a ppe
".
~'iRU rl' 7.2 Tec tonic se tting of the Russian platform. The platform lind Bailie
toge ther comprise a Phanerozoic craton surrounded by orog enic belts. L_L1JJL1J_Li::I:1::::t :r:::lL1J:: :>---'::d.::':=LJ shield
IN n UU' LA I E I ECTO:-.K IO-.GI ' I FS
gens ur e replaced by ea rly Pataco zcic basin v (compa re Figure 7. 3A ,8 ). For exampl e , t he mid- Russian aulacoge n is t he s ite o f the m uch larger Mosco w sy neclise d uring t he latest Preca mb rian anti e ar ly Pa lae o zo ic. By O rdovician and S iluri an ti me s , o nly two maj or ba sins existe d within the platform : the Balti c syncerise , whic h represe nts 0) . Th e other ma jo r basi ns to nned d uring th is pe r io d were the D neipc r - Do nc tz syneclise . formed on th e site o f t he ea rlier a ula co gen . a much re duced Mosco w basin , and the new Ul'yanovskc-S a ra tov de pre ssio n for me d o n the site of th e Vol ga - U ral an teclise . Several im po rta nt co nclusio ns ca n be drawn fro m th e an a lysis o f th e se intraplate str uctures . A lmost all the major basins o rigina ted as aul aco gen s (i.e . rifts in ou r pre viou s te rmino logy) th at form ed in o nly IwO main per iods . Man y pa rts o f th e pla tfo rm a ppear to ha ve alte rna te d be twe e n ac tive basin subs ide nce a nd e ithe r neutra l o r upli ft beha viou r. Suc h a re as ex hibi t t he pr operl y of in version (se e BclousSOY, 1962 ) whe re a part o f th e crus t re ver se s a lo ng-co nt inu e d d ire ction o f ve rt ical mo ve men t . Inve rsio n is cha rac te rist ic o f intraplate re gio ns as we ll as of o roge nic belts a nd severa l good exam ples ma y be noted in Figure 7.3. T h us parts of t he Upper Pa laeozoic Vo ronezhU kraine u pli ft fo rm ed la te Pre cambri an o r ea rly Pala eozoic basins , a nd o the r pa rts fo rme d a M eso zoi c bas in. O ve ra ll move me nt of th e plat for m was
I t) 1
dm \ nwardv. in con t ras t to th e ncigh bounng Baltic shield. which was ge ne ra lly u plift e d during the sa me per iod . T he major basins existe d Fo r pe riod s rangm g from 160 to n OMa wuh
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Figure 7.3 Four stages in the tectonic evolution of thc Russian platfor m, showing the locatio n of the major intraplate structures: (A ) Upper (Late) Proterozoic; (B ) Early Palaeozoic; (e) Mid-Late Palaeozoic; (D) Mesozoic and Cenozoic. I, Baltic shield; 2, Ukrainian anteclise; 3, Timan ridge; 4, Voronezh anteclise; 5, Volga-U ral anteclise: 0, Pre-Caspian syneclise; 7. Pechora syneclise ; 8, Moscovian syneclise; 9, Baltic syneclise; 10, Pachelma aulacogen ; 11, Dneipe r-D onetsk aulacoge n; 12, Pechora-Kolvin aulacogen; 13, Vyatka aulacogen; 14, Mid-Russian aulacogen; 15, Kaltasa aulacogen ; 1o, Sernovods k-Abd ulino aulacogen; 17, Ul'yanovsk-Saratov dep ression ; 18, Ors hansk syneclise; 19, Dneiper-Do netsk syneclise. From Aleinikov et al . (1980), with permission.
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194
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and in the large de pressio n extendi ng sou theast of Moscow towards the Caspian Sea . T his pa ttern indica tes th at the major phil form structures of the Mesozo ic ( Figure 7.3D ) a TC still active. but tha i the T,(t CS of movement are arou nd two orders o f magnit ude greater than those measured ove r periods o f JOO - lOOO Ma. The Russ ian pla tform is a particular ly wellstudied examp le o f a maj or int raplate region.
Other co ntine nts, howe ver , display a similar pattern of to ng-co ntin ued eq uidimensional or non-linea r basins separa ted by broad uplifts. Th is pattern may be see n on tectonic maps of all the co ntine nts and is particularly clearly d isplayed o n t he tectonic maps of Nort h A merica and A frica . 7.3 Inlrapl ale basins We have seen fro m o ur study of the Russian platform th at major tecto nic basins can be recognized t he re, charac terized by co ntinued dep ression o ver pe riod s of l OO-I OOO Ma . These bas ins are large features with dimen sio ns a pproxi mately 500 - 1000 km across, and represe nt depressio ns in the basement of , in some case s, ove r 5 km de pth . T hey are ther e-
t'i ~u r " 7.4 Patte rn of rece nt ve rt ical movemcr ns in the Russia n plutfcrur. Contours in mmrycar Based on n;: .lcvd ling data. The sup pled arca i ~ under go ing ~u h~i de ll ce . From Vita-Finzi ( 1'J,s6) . alte r Mal1skov" (1% 7).
fore ma jo r cr ustal structures and form . with the associated uplift s, the most im po rtan t and wide spread type o f int raplate regime . O ver a long lime-scal e . the ve rt ical mo vements respo nsible for t hese st ructures co nstitute a significant de part ure from intrap late sta bility. We sha ll now loo k at se ver al classical and well-st ud ied examples o f intra plate sedimentary ba sins fro m d ifferent cont inents: the Paris bas in in Weste rn E urope , the Michigan basin in the north ern USA , and t he T aoudeni bas in in West Africa . T hese arc exam ples of former mar ine ba sins that are now ina ct ive o r which for m sha llow co ntine ntal depressions. In a later sec tio n (7.4 ) we sha ll exami ne in more de tai l t he No rth Sea bas in and t he At lan tic co ntinent al margin of the northern USA , which are examples of currently active marine basins whose str uct ur e is comparat ively well know n be cause of oil and gas exp loration. The Paris basin This basi n . descr ibed by Megn ie n and Po merol (1980), about 600 km in diamet er, is sit uated in no rth ern France , and is one of the be st -de fined basin struct ures in E urope . It rests on Pa laeo-
195
IN T II.APLA TE T l::(.TO N LC II.H' IM ES
zoic base men t to the west (Massif Armo rica in) , south (Massif Ce ntral) a nd e as t (A rde nnes massi f) , b UI e xte nds to tile north int o th e English C ha nnel . a nd no rt hea stwa rds to war ds the North Sea . T he basin proba bly ori gina ted d urin g U ppe r T riassic lime . hut ex iste d as a well-defined basi n str uc ture onl y d uring the J urassic and Cre ta ce o us pe riods. D ur ing th is lime , a to tal maximum thickn ess of 2\jOIl m of sediments acc um ulated . Ho we ve r. t he a re a o f ma ximu m subsidence migrated southwards for a distance o f 60 km during this pe rio d . so th at the tot a l thick ne ss a t an yone pa lm a long this line is rat her less than th is . T he authors d em onst rat e , fro m a stud y of the va riat io n in cu mula tive thickness with time , a ra the r ste ady ra te o f accum ulation o f sed ime nts with an ave rage rat e o f 2 1.5 m/M a (0.2 m m/yea r). The a ut ho rs attrib ut e the growt h of the basin o ve r th is period to g rad ua l flexing o f the c rust ind uced by th e sed ime nt load ing , bU I do no t co mme nt o n t he init ial mo de of ori gin of the basin .
The Michigan basin This basin has been in ten sivel y stud ied by mea ns o f nume ro us bo re holes , and g ravity a nd mag ne t ic sur veys. Th e basin was fo rmed in mid -Ordov icia n time a nd lasted at le ast until the late Carbo nife ro us. T he basa l mid O rdovicia n sedi me nts res t un co nformably o n Pre ca mb rian basem en t. Structural con to urs o n the baseme nt surface ( Figure 7.5) ind ica te a n unusua lly regu lar, a lmost pe rfe ctly circula r, shape with a grad ua l incre ase in de pth to wa rds a we ll-defined ce ntra l point. Most Pa lae ozoic unit s thicke n tow a rds the ce ntre of th e basi n , a nd facies va ria tio ns indic a te co nsiste ntly deeper-wa ter co nd itio ns th e re . Subsid en ce therefore ha s co ntinued thro ughou t most o f the life time o f the basin. The total thi ckness in the ce ntre is about 3 km , fro m wh ich a n ave rage ra te of subside nce of 24 m1Ma (0 .24 mm /ye a r) is o bta ined - very similar to tha t of th e Pa ris basin. A sma ll positive free -a ir gra vity a no ma ly over the basin is a ttribute d by w alcot t (1970)
,
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Fil;ua 7. 5 Th o: ~ h ~ p<.: of the Mic higan bas m , shown by su uel ur a l co n to u rs (m n x 10-' ) on the Precam brian basem e n t C ircled tria ngle s ind icate bo reho les. N Oh~ grad ual lind steepeni ng d ip or the base men t surface to wa rds th e ce ntre of the bas in. From Sleep ( 1900). wilh pe rmission .
to re gio nal subside nce ca use d by load ing. The re se e ms to be no ge ne ra l agreem ent , howe ve r, ove r the o rigin o f the structure .
The Taoudeni basin This basin, describe d by Bronne r et at. (1980), is o ne of the most prominent st ruct ura l features of the African cra to n. II lies in w est A fr ica , m ainly in Ma urita nia and Ma li, in the western Saha ra region (Figu re 7.6) a nd is ab out 1300 km across , with an a rea of 2 x 1O"'km2 . The sed ime nta ry thick ness varies fro m IOO(J1500 m . The basin res ts o n A rchae an a nd Ea rly Proterozoic ba seme nt whic h form s sh ie ld ar eas to th e north and so uth . It is bounded o n th e west by th e Pal ae ozoic Maurita nide belt , a nd
196
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I Nrll.Al' LAT E T Ec r ON IC
la te Pre cambrian to ea rly Cambr ian Pharu side belt . The se dime nta ry fi ll ran ges in age fro m mid Prot e ro zo ic ( l IO()- !O()O Ma) to Ca rbo nife ro us . The ce nt ra l pa ri of th e basin is co ve red by a thin veneer o f Meso zo ic and Ce nozo ic sc dimc nt s. T he se q ue nce with in t he basin is s ubd ivided into fou r supe rgro ups se par ated by unco nform ities or di sco nfo rm ities. T hese sho w quite differe nt subside nce ra tes, va ryin g fro m 4 m/M a fo r supe rg ro up I {late Precambria n) to !6 m/ Ma for supe rg ro up 4 ( De vonia n) . T he mean subside nce ra te for the who le pe riod of activity is a bo ut S m/ Ma (= (I.(IS mm/yea r) . T he ma in basin co nt a ins two sm a lle r bas ins with depths o f a bo ut 2S00 m. a nd a curved linear trough with a maxi mum dc pth o f 5ekm . T his tro ugh appe a rs to ha ve e xisted as a majo r rift zo ne in th e late Pre ca mbri an . a nd docs no t e xte nd in to the Pha ne ro zoi c, since the seco nd supe rg ro up rests un confo rma bly on th e 10 the east by th e
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Figur~ 1.1 Sche malic model illustrating lhe rcctomc crre"'C1 of sedimeni iood ing al a co ntinental mar gin. In (u) a 200 km-widc Iransilio n be twee n continental and oc eanic crUSI ts assumed; (b ) sbo ws ca leu1aled ru ull of loa ding, using the A iry mode l. assum ing a sedime nt den sity of 24S0 kglm·'. and mantle density o r 33()() kglm~ ; (cl sho ws calculated result using a ftcxu rlli lollding model . assuming a lIcll.u ra l rigidily o r 2 x IIPNm, and den sities as in (/I) . Note that the inward edge o r the load in (c) mignllcs inwards over lhe co nlinen!. Fro m BOll ( 1980)
II.I;:Gl~ ES
J97
rift- fil l sed ime nts of su pe rg ro up 1, onla pping the basemen t
O rigin of intraplate basins T he o r igin of se di ment ary ba sins is discussed by Ba lly ( 1980. 19H2), who cla ssifi es ba sins into 19 types base d o n the ir tecton ic loca tio n. H is funda me nta l divisio n is be tween basins associ ated with 'm cgasut u res' (i.e. pla te bounda ries ) a nd th ose loca ted o n ' rigid' lithosphere (i.e. in tra plate) . Of the intr a pla te basins. he reco gnizes the follo w ing catego ries:
(1) Rela ted 10 format ion of oce anic cr ust (oceanic basins) (a ) Rifts (b) O ce anic tra nsform fa ult-associ a te d basins (c) Ocean ic a byssa l plains (d ) Atlantic-type pa ssive ma rgins (sh elf. slope and rise ) which s tra dd le con rinenra l a nd oce a nic crus t [ i) O verlying e a rlie r rift syste ms (ii) O verlying e a rlier tra nsfo rm syste ms (iii) O ve rlying e a rlie r back-a rc basi ns (2) Loc ate d o n pre-Mesozoic con tinenta llithosphe re (cratonic basins) (a) Loca ted o n e a rlie r rifte d graben (b) Loca ted o n forme r back-a rc basins. O ce a nic basins of type s 1(3) a nd I (b) ha ve a lre ad y bee n discussed (see 4.2 a nd 6.4) . Basins of type I (c) ar e co ntrolled by the age an d cooling-re lated depressio n of oce a nic lithosphe re as it mo ves away fro m spre ad ing ridges. Such basins are impo rtan t a t p rese nt in te rm s of thei r a re a l e xte nt. since (he y co nstitute most
198
G IOOLOGI C,\ L STRUClUR E:S AN I) MOVING PLATES
o f the ocean floo r . but are not of such great
geologicut Inte rest because they arc not prese rve d in the pre-Mesozoi c geologica! record . Passive-m a rgin bas ins . in co nt ras t. a re of great impo rt ance because of the large volumes of ter rigenou s sedimen t thai accumulate in th em. a nd because they be come welded to the con tinen tal crust du r ing collisional o roge nesis. Ball y distingu ishes three type s acco rd ing 10 whe the r the bas ins we re initialed over rifts. t ra nsfo rm faults . or bac k-ar c spreadi ng-basi ns.
Ty pes d (i) and d(iii) arc extensional in origin. whereas Iype d( ii) is strike-slip. However. in eac h case the initial de pression results from the crea tion of ocea nic lit hosph er e tha t becom es isostat icall y de presse d in rel a tion to surrou ndmg a reas. Co nt ine ntal o r cr a to nic basins of intraplate type are di vided into two catego ries: those loca ted on ea rlie r rifts. an d tho se loca ted o n fo rme r ba ck-a rc ba sins. In bo th cases. the origin o f the basi ns is rega rded as e xtension al, ca using crustal t hinning a nd de pression o f the surface. T he im plica tion (If Ba lly's classificati on is tha t the ori gin of intr a plate basins is rel ate d 10 a n inhe rite d crustal structure . that is, o ne resulting from a forme r pe riod of tectonic activity thai ca used the su rface de pressio n. T here is. howev e r. co nside ra ble co nt roversy over the o rigin of the de pressions req uired to c rea te intra pla te basin s, a nd the es ta blishme nt of a ge ne rally agreed model has been ha mpe red by the lack of e vide nce as to the dee p str uct ure of ma ny basins. A number of propo sed mec ha nisms a re cited by Bally: ( i) sedime nt load ing; (ii) isostat ic response to coo ling a nd de nsity increase ; (iii ) lithosph ere str etc hing a nd thinning ; ( iv) e mplace me nt o f de nse . ma ntle-deri ved . igneous material ; (v) de nsity increa se o f lowe r-crust al rock s due to ga bbro-eclogite phase cha nge ; (vi) com ple te basitlcation o r oceanization o f the co ntine nta l crust by t he supply of ult ramafic ma terial fro m the ma ntl e ; a nd (vii) creep of duct ile middle- to lower-crust al ma te rial to wards the ocea n at a passive margin . These mechanism s a re by no mea ns mutual-
Iy excl usive. a nd all are pro ba bly o perative 10 some exte nt . with the e xce ptio n of ( vi). Co mple rc ocea niza tion . proposed by Belo ussov ( 1968) . is implaus ible since a mixtu re of c rustal a nd ma ntic den siti es will still be lighte r t ha n ma ntle ma te r ial a nd will be unabl e to sin k into it (see A rty ushkov et al.• 1 9 ~O) . Howe ve r, pa rt ial ccca nization of t he crust is ess en tially the process e nvisaged in mecha nisms (iv) and (v). T here ap pea rs to be widespre ad ag reeme nt that most basi ns result fro m a n isost at ic tesponse 10 a po rtion of lithospher e that has become , for so me reason . mo re de nse than its su rro undi ngs. T he incr ease in de nsity ma y a rise in se veral dif feren t wa ys: in the ocea ns, an d in co ntine ntal rift zon es. it results fro m straig htforwa rd coo ling of a n initiall y heat ed region ; else whe re . it is a conseq uence of t he e mplace me nt of den se man t le -deri ved ma ter ial within the crust. T he main a rea o f disagreeme nt ap pears 10 be be twee n those who rega rd lith osphe re stre tchi ng an d rifti ng as the main, or indeed the o nly. met hod for genera ting this crusta l t ra nsfo rma tion , a nd those who bel ieve that such a proc ess proceeds inde pe nden t ly of t he no rmal plate tec to nic processes, a nd witho ut significa nt lith osph eri c st re tching . T he mechan ism which is now widel y re ferred to as the Mc Kenzie st re tc hing mod el ( MeKe nzie . 1( 78) ass umes tha t basin fo rma tio n is the res ult of a n ' insta nta neo us' lithosphere exte nsio n. T his proce ss thins the lithosph ere . producing a surface de pression a nd a co rresponding upwa rds bu lge in the base of the lit hospher e . The thinned litho sp he re ge ne ra tes a the rm al an om al y. since the wa rm ast he nosphe re is now close r to the surface in the stre tc hed por tio n. Howe ver. as the initial basin fills with sed ime nt. the the rma l a no ma ly gradually deca ys. a nd the base of t he litho spher e re t urns to its origina l le vel. This slo w cooling results in grad ual isosta tic s ubside nce ove r a pe riod o f a bout 6OMa . The initi al McKen zie mod el e nvisaged instantaneous stre tching for simp licity. Howe ve r. Ja rvis a nd McKe nzie (1980) ha ve sho wn tha t fo r most basins t he insta ntaneo us model
I NT RA f' I .Al'I'. I ECroN IC REGIMES
predicts the subsidence geo metry accura tely. prov ided that the d urat io n o f the initial stretching is tess tha n 601[3 Mu, whe re the stretc hing factor [3 is less than 2 . and 601[ 1- ( ll[lll if [3 ~ 2. T hus fo r a value o f [J = 1.5 co rrespo nd ing to the mean value for 11 basins (see Table 2.6) the durat ion o f initial stre tching should be less than 27 Ma for the model to ap ply. The model ap pea rs 10 accoun t successfull y fo r the stratigraph y and subsidence histo ry in the Pan no nia n basin of Hungar y and the No rth Sell basin (Ch rist ie und Sclatc r, 19HO) . Subsidence du e to sed ime nt loa din g is the simple grav itatio nal response to impos ing an exira mass on the sur face (Dietz. 1963 ; w alCo li , 1972) and requires an initial depression
1<J9
fo r sufficie nt sed imen t 10 accurnu lutc . Bot! (1980) , in an ada ptatio n of the flex ure model deve loped by Walco tt ( 1972), sho ws that . in a 200 km wide tran sitio n zo ne at a passive co ntinent al margin ( Figure 7Jl) , a th ick pile of sedime nt at a ma jor delta , such as t hat or the Nige r. will produce a down warpin g ex ten din g for about ISO km be yond the area of the initial load . Th e sed ime nt load ing mechanism can produce a sedim entary pile of twice the initial water dep th for sed ime nt wit h a me an lknsi ly of 250 kgfm~, and of nea rly th ree times the initial water depth for sed ime nt with a mean de nsity of 255 kgfm). Thick she lf successio ns formed in wa te r with a de pth o f less than ab o ut 200 m canno t for m by this mechan ism. but to tal
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o
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Figure 7.8 Simplified maps illu' lrilling two stages: (A ) E.u ly Jurassic. ( 8 ) Mid . dle Jurassic. in the tect onic evolutio n of the North Sea basin. Open stipple, land areas; erose seipple , major rin s with bo u nd a ry faults indicated by heavy tines with lick on downthrown side. Th e dense dashed are a in (8 ) represe nts the uplift comprising the Ce ntral Nort h Sea dome . the Shetland platform and adjoininil iand areas of northern Br uain . ' V ' marks areas or co ntempo raneous volcanicity. G, Gr eenla nd . RD. Rockall Ban k; FP. Fa rce s Plateau; UP. J1e~ritkiln plat (o rm: sr, Shet lands platfoe rn; FS. Fe encscandia; IR . Irish massif; W . w elsa massif: cr, G ramp ian - Pen nine ma~f ; A M. Armorican massif; L B, London 8rabant massif; lB . lberian massif; VB. Vinde lician -Bohemian massif. After Zieg ler (1982) .
200
G EOLOG ICAL STRUCT URES AN D MOVIN G PLAT ES
thick nesses of a bout 14km co uld fo rm nea r the ba se o f the contine nta l slo pe if the initia l depth is g rea ter than this. T he sedi me nt loading mechanism pro vides a me a ns o f e xpla ining the co ntinue d e vo lutio n of basins lon g aft e r th eir initial causa l mecha nism s have cea sed to opera te . Th e process thus e xpla ins th e! ve ry lo ng life (ra nging fro m 1007no Ma ) of the major inactive basins di scussed e a rlier, despite the re la tively sho r t lime periods required by the init ial e xte nsio nal o r th erma l cooli ng eve nts (of th e orde r o f tens of Ma on ly) . It is significa nt tha t man y o f the major bas ins disc ussed above ap pea r to be situated over e a rly rifts. It appea rs like ly therefore tha t most , if not a ll. majo r basins that a re ge nui nel y int ra plate ( i.e . e xcludi ng passive-m ar gin types) o rigina te d hy a p rocess of exten sional .rilu ng t ha t a llowed dense mat er ial to be e mplace d locall y within the c rust. or at th e base o f thin ned crust . giving rise to isos ta tic de pressio n. The amount of e xte nsio n need not he la rge : a nar ro w zone of se vere local ized th inning with acco mpanying basificaticn . o r a wide zon e of less in ten se thinning, ma y ea ch prod uce la rge basins du e to the su bse q ue nt isostatic res ponses. a lthough th e detai led geometry will d iffer (see Figure 2.29). Basins o riginat ing by cr usta l flexure req uire a n initial lo ad such as thai prov ide d by a th rust shee t. Bas ins of this type a re the refore no t ge nuinely intraplate in o rigin. but a re asso ciated with destruc tive pl ate boundaries. The y differ fro m ex te nsiona l basins in not ex hibit ing c rustal thi nn ing, and ca n ther efore be distinguished fro m the m by geophys ical means.
7.4 Exam ples of ac tive marine basins: the North Sea a nd the Atlantic continental margin of (he US A
The Nor th Sea basin This la rge ma r ine basin is situa ted o n co ntinental crust of th e northwest Eurasia n plate
be tween Brita in a nd Scand ina via , ope ning into the At lantic O cean in the no rth . It is loze ngeshaped. me asu ring 1000 km a long its lon ger side in a N- S direct ion . a nd a bo ut SOOkm ac ross from eas t to west. With the e xceptio n of a narro w st rip along th e coast of No rwa y. the basin is eve rywhe re less than 200m in dept h . It recei ves terrigen o us sedi ment from a numbe r of major rivers in eas te rn Britain , the Nethe rlands. West Ge rm any and southern Norway. T he histor y of the basin is summarized by Ziegle r ( 19H2) . T he ba sin pro ba bly o riginated in th e Devon ian peri od , IS o ne o f a number of continental ex te nsiona l r ift bas ins developed in the region of the Caledonian orogenic be lt in Brita in a nd Norway (see 8.3). D uring the Pe rm ian , two se pa ra te ba sins ex isted in th e no rthern and so uthe rn No rth Se a respectively. sepa ra te d by th e mid -North Sea high . T he prese nt Nort h Sea basin was initia ted in the T riassic pe riod as pa rt of an extensive rift system connec ting with the m ajor ArcticAt lan tic rift. T he two major rifts in th e North Sea a re t he Viking an d Ce nt ra l gra be n. which lie mid wa y betwee n Norway a nd Shetla nd. and between No rway and mainla nd Bri tain respcctively (Figure 7 .~) . Th ese two rifts mee t in a tri ple junctio n with a g raben th at extends westwa rds into th e Mora y Fir th . In the ea rly J urassic. a rise in sea-level produce d a wide spread ma rine tran sgr essio n across th e No rt h Sea bas in linkin g th e A rct ic a nd Tethyan Oceans via t he rift systems of Western E urope . S ubsidence a ppears to have continued th rou ghout th e e arly Jurassic , but was inte r rupte d in mid -Ju rassic time s by the c re a tio n of a lar ge rift ed dom e. 2- 3 km high , in t he ce n tra l No rth Sea (Fig ure 7.88) . A large vo lca nic ce nt re was establishe d in the area of th e tri ple junction and severa l sma lle r ce nt res alo ng the Viking G raben a nd to the east. T his upl ift ph ase was associate d with th e majo r regional kinematic cha nge th at resulted in the open ing of the Ce nt ral A tlant ic (see Fig ure 3.58 ). By lat e Jurassic time , th e dome had subsided , and de e p- wa ter co nd itio ns had become re-established througho ut th e bas in . Con-
201
INTRAPLATE TEc rO NK REGI MES
A 3 :s~ VERTICA L [XAGG£ RAT'Of\l
6
,
VIKING GRABEN -'
.. o nllTIAIl'I'
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.
f W.AEOCEN[
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200
100
250km
B SHETLA NQ ISL ANQS
VIKIN G
GRABEN
NORWAY
".
"
eo
so
., 0
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PAE - AI'T SEDUIlENTS
o
I'O$T -A'F'T SEDIMENTS
Figur, 7_9 (A ) Simplificd st ructural cross-sections. (8). seismic-refraction crustal pro f dc, ac ross the Viking G rabe n. Figure> in ( 8) arc mean P -I'I;lVC sci~m ic: velocities. Fro m Zieglcr ( 1'J1l2)
20 2
G t,:O I.OG ICAI. STll.UCIU II.l'S AN !)
tinued e xte ns io n took place by the westwards move me nt of Britain in rel at ion 10 bo th Scand inavia and main lan d Europe . The ex te nsion appears 10 have con tinue d int o the ea rly C retaceou s. wit h re ne wed mo vements o n fa ults in the ma in rift zo nes producing sea-nom re lief of up to 1 km. In the lat e C retaceous. riftin g in NW Euro pe appe a rs to ha ve beco me co nce ntn ucd 1I10 ng the abo rtive Labrado r 5('3 spre ad ing axis he twee n G ree nland a nd Nort h Am eri ca . Al thou gh subs ide nce continue d in the No rt h Se a basin. th er e is no e vide nce for significa nt co ntinued ex te nsion. During th is pe riod . up to 2 km of Uppe r C re taceous cha lks a nd ma r ls infillcJ the topograpntc depressio ns of the Viking and Ce ntral graben (Fi gu re 7.9A.C). Further subside nce of the basin look pla ce during the Cenozoic. whe n a ma ximum Ihk kness of 3.5 km of sediments was deposited in the ce ntral a re as of th e basin . The re gio n of ma ximum Ce nozoic se dime ntat io n pro bably co rrespo nds to the zune o f ma ximum crus ta l thinning produced during th e ea rlier exte nsional phase (Dona to und T ull y. 19N1). Gra ..uy profil es indic
w
M o v t~ (j
Pl .... res
grabe n. A typical cr usta l profi le across the riil ( Figu re 7 .4 1l ) indicat e s no rma l c r usta l t hic k-
ncsscs o f 30- 35 km benea th Norway and S het la nd . decre asing 10 a bo ut 20 km in the Viking grabe n whe re ther e is a n K- llI krn sedime nt fill. Th e uppe r mant le d isplays nor ma l Pcwa ve velocitie s throu ghout . Sclat c r and C hristie (l9XO) propose that th e Meso zoi c crus ta l thinning was acco mp lished by an e xte nsion of f3 = I.H- 2.0 IXO- 11M''''' ). B each ( 19X6) describes a deep seismic rcfl cclio n profile across the Viking grabe n fr om no rt h of the She unnds to Ber ge n (Figure 1. 11). Th e profile illustrates the upper c rusta l st ructure clea rly a nd shows th e asym me tric na ture of th e rif! structure. Jl OW known to he cha racte n suc nf ma ny exten sio nal provinces (sec 4.4) . Th e Viking gra ben is see n as an asymme tric Jrtll/ -gm ht'll forme d by till ed fau lt bloc ks of Jurassic a nd olde r stra ta, with a con siste nt wester ly tilt . The Jurassic bloc k-fuuned "Iruetur f" is buried by Hat.l yin[:. C re taceous sedi menls that had co mple te ly huried the highes t bloc k hy end -Cre taceo us time (Figure 7.9A ). T he bloc ks a rc bounded by eas twa rdsdipping fau lts. with a weak listri c geome try. IhOiI appear 10 det ach on a broad zo ne of lo wer crusta l reflectors inter pret ed by Beach as a mujor , low-an gle shea r zone (ct. Figur e 2.30)
E .... • IO.U
100....' 0 ' .........
F"'l"" 7. 10 Compolill: in te rpreta lm: acrud ural profile Kros5 the Cc nlr.ll Gr;al1otn of 1M St'IU lhcm No rt h and wett co mr cl . Ve rticil kale i. lwo-wa y lime . From G ibbl ( t985) . wilh fot'rmiu.iorJ.
sei~mic
~a.
based on
203
l!'oil RAI'L,' I ~ 1I,C"lO :'llC REGIMI'.5
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t1IC urr 1. 11 Inl <: Tp fc l"'lion " I UCI;P M: i~lIl ic IcflCl1i,m prufll c X fOM Ih.c Viki0E- 1 ';1"-:0 . 11k:: ~hat.lc d zone In IA) is inte rpre ted a$;l major cXl cn~i...nal s bcur m oC' J"K<.ing from fhe middl e cruSl in the W('SI, into lhe m"nl le in the C'lst . Easl.di Pflillg IIOrm :11 r:au ll~ de laching u n Ih il l~lnc a rc ~ho .....n dnl\ed . Not e t he majo r Cfu'lt :tI-lICa !c ro lkwer ant id inc on the ea\ lc rn ~itlc o f the l>CCl iu n. Tbe M.'\:l iu n ~hOW!i iI l und ;Imo:ollll a~ymm.:u)' t h;lI cu n l ras h "'nih the in tc rp ll'ta lit.'n (If Figure 7.'J. Th e !ol:hcm atic mod e ! (U ) ~uggcsll; h" ..... t he Iilhl""f'hc n: mllY have acc<mlmod,IICd t he n ll'o, itm . FNlm " e ach (19116), wuh f'Crm i", iun.
Thi s detachment appea rs to risc westw ards to meet th e lOp of the base me nt at the western side of the gra be n. T he e levated G utlfa ks bloc k. in the centre o f th e gra be n. is sho wn 10 be a till ed fa ult bloc k like the o the rs, but situated over the area of greatest exte nsion. and bounded by the faull wu h the lar gest throw , The easte rn side of the graben is sho wn
10 he siructurally q uite diffe rent. A ve ry large .
crustal-scale. roll-o ver anticli ne (see Figure 4.25) acco mmodates the slip alo ng a dee p de tachme nt tha t is be lieved to lie within the uppe r mantle at a depth of abou t 40 km 15- 20 km be lo w the ba se of th e cr ust. As see n ear lier (Figure 7.98) . the crus t thi ns to 20 - 25 km over tbe graben region . ln teresungly,
204
G EO I.O ,",O \' I:'II G "L\H S
th e ur e a of maximum th in ning is d isplaced eastwa rds of the grabe n axiv. which \ uppnr l\ th e asy mmet ric e xte nsio n mod e l proposed hy Wernicke (19H5) . Thi s model (see Figur e 4 .24IJ ) ex plai ns la rge extension s by block faul tin g a t h igh le vels . detach ing o n a lo wa ngle finn;11 phase . while no t responsible fo r t he initiation of the basin. was a n impo rta nt cl e ment in ih subseq ue nt de velopment . 111is e xte nsional ph..sc , last ing approximat el y t"{lMa . invol ved a major c rusta l e xte nsio n o f a ro und 1.5 (SO'Yo ). calc uluted by com pa ring the pre- and post -Mesozoi c crusta l thicknesses . Tota l extension . assumi ng an original c rustal thickness of 31 km . is as mu ch as 1.8 ( Wood a nd Ba rton , 19tH) . Th e diff e re nce may be explained by a n earlier pha se o f exte nsio n fo r which nu de tai led e vide nce is a vail able. The J urassic ex ten sio na l phase requ ires a strain rat e of about 2 x IO - It> a nd co rresponds In the slower type of ex ten sio nal riftin g . prod ucing a wide zo ne of ea tenslona l deforma tion (see 2.7, Figure 2.29). Sctate r a nd Christie (JlJ80) view the cvulu·
lion o f the ba sin in Cr e taceou s a nd Ce nozo ic limes ;1<, the conseq ue nce of isost a tic dep ressio n. thut result ed Irom cooli ng III the lithosphe re ali the the rm al .ultllllal)' produced hy the initia l stre tching disappe ared .
The A tkuu ic conunental tnurgin of the northern USA
Th e l..·UI1I111ent'll shelf an d stope horde ring the Allanl ie en asl of the nort he rn USA is an ac tive exa m ple o f Ihe passive -margin type o f int raplat e ba sin. The fl'giun ha s been intensive ly stud ied using borehole , sei..mic reflectio n. a nd gruvit y datu . A mod el profile across the basin (Figure 7.12 ) i!'> described hy Saw yer ('1 al. ( llJX2) . T he profile crosses the Hahimorc trou gh . Incaled cast o f the New Je rse y coastline . whe re up 10 IXkm o f sedime n t has been d eposit ed since the lat e Triassic. T he pe riod of ma ximum subsidcoce oc curred during the Jurassic . when the initial e xte nsion in the Ce ntral Atlantic took place . Th e J u ras..ic sedi me nts re st on :I companuivcly thin wed ge of T riassic red bed s. volca n ics and eva porites. which them selves rest direcuy on the co nn ncntal basemen t. Abo ve the Ju rassic stra ta. a much thinn er sequ e nce of C re taceous sed ime nts is dra ped over the edge of t he ccnn nen tal she lf without showing ma rked th ick ness va ria tion. Th e ba sin is nu t reco gnizable as a separate struct ure in the Ce nozoic sedi me nta ry seq ue nce, which is clearly separa te d by the edge nr the co ntinen ta l she lf int o she lf a nd ocean-basi n asse mbtages. Sawyer et al. use th is exa mple to test the McK e nzie st re tching mod e! o f bas in subsidence. The y as...ume tha t the ma in e xte nsiona l event commenced al about 200 M il fir with the fo rma tion of the la te T riassic rift sedime ntatio n. a nd ceased a bout 175 Ma 8P with t he for mati o n of the first oce a nic c rust . Furt he r ex te nsio n would be ta ke n up by oce a n-floor sprea d ing. Theo reti cal subsidence: c urves ba sed on the Mckenzie mod el show the a mount of subside nce a t a give n lime aft er Iniuation for
205
I NlIl A Pl.AU .. n .cruNIC REGl MI'S
COST B- 2
COST B-3
WELL
w{LL
\ =:; I
'0
'0
'0
'0 V(- s )(
200
' 00
DISTANCE If\l Kill
tl J:ur., 7. 11 lurc rprc tanvc pruhk· at·n ....' Ih., ....' "li"'·nl "l m'II(:IllU r Ihc .:uwcm l JS A. In lhe '':j;'''" or the Hal1 iru" re uough . .. r N,'w kr ",') The I', ,,iliun , ,,1 1""01 well, . ("OST 112 ami 1\.\ . ,J,,,,'u, .,.'t.I in Ih.: 10:11• •".: , 11" .. n , fhHl1 S"",,>er ,./ ,II
(a~l
( 1'JI1.2)
different values o f Ihe stret ching factor p. Subside nce is initi'llly linear {i.e . for the first 25 Ma) during the contine ntal stretching phase . and is followed by ther mal subsidence be ginning at 175 Ma and co ntinuing to t he pn.-scnt . . Th e model pred icts that t he init ial subsidence is la rger , a bout 40% o f the total . whereas the subseq ue nt thermal subsidence. acco unting for the remaining 110% . lasts for ove r lOn Ma . Act ual subside nce curves (Figure 7. 13A) were calc ulated from sed iment thicknesses in two wells (CO ST 1\2 and 8 3), after mak ing corr ect ions for sedime nt compac tion . isos tatic responses to sed iment loading. pala eo-depth of wat er . and e ustat ic sea-leve l changes . The curve for the COST 83 well shows ,I rea son able lit to the model exten sion curves ove r the latter part of t he time ran ge fo r a stre tching factor of be twee n 5 and infin ity. However. since the se lWO we lls are situa ted over the edge of the ocea nic crust. they canno t be used to give an acc urate es tima te of co ntine ntal ext ension. since the extension at the oce an margin is
effec tivel y infinite . A be tte r guide to the co ntine nta l ex tensio n in the basin is provid ed by two wells situated in a t raverse furt he r north where the hasin is ra ther shallower (abo ut II km). The curve for the mor e weste rly well (CO ST G I) on the nort her n prorik I Figur e 7. 138) co rres po nds to a st retching meter of be tween 1.66 and 2.5. and the mo re easte rly 10 a factor of betwee n 2.5 and infinity. These res ults suggest t hat the Mck e nzie mod el gives a reasonable a pproximatio n to the subside nce history of a pa ssive margin basi n. al least over the greater pari of the cooling stage . Moreover. s uch a basin wou ld he expec ted 10 show vary ing su bsid en ce rates co rrespo nding 10 model ext en sio ns ra nging fro m a minimum (comt ner ual} valu e to infinity over the ocea nic pan o f the ba sin. Pas...we-ma rgin bas ins are a product of teeIonic processes relat ed 10 diverge nt plat e boundaries (see Cha pte r 4). However. since (heir effects a re retained within the lithosphe re lon g afte r the plat e bo unda ry has migrated
2O/i
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away fro m them . the y ca n properly be regarded as intrap late structures.
7.S Inll'1lpblllt uplifts Intr aplate uplifts are as important as basins in tecto nic histo ry o f 1M plate inte riors. 11Icy oct"U p~' a similar surface a rea . a nd ~r long I~
ClII''fft ....111
dro ~ frum fbi:
..,..mflt'd
McK.cntJC'
~
".t..n
C'''k~
mndo.-t. PkK A .. ~ hum the lbtl of the f'OST R ~ ..-ell I FlJUfC' 7 .12) lhe ......F.n 0{ lhe t>oX'.ni(""", , da u hi bctvo«n 1~Ifl.'1inll a ll''''' fOf U I(nUon "f hct~" ~ ~ i"flnily. Pl." B i. dc m~d from __IICOST G 1. Iotl ll.ted ' 5('1 lo rn NE 01 lB . n n cnnl ino,:nl a l Cl'\l 1It. Not t t hall hi:' ,J. lill heu,' lit • k......... C'lI lt'lll>kJn n l I.M -2..5. lbc CUI"\.'" ~C' CIl k u la lcd ..~ \ llm,"s .. lIe..ur i.1 ("hu lk r1l1l c mudd . J\<,j d C'U Of ha n; ref e r III paliK'C_lll(' t dc fllh n limilln. 111M.! li n : r Xlr nffi S:I.... \Tr t'f a/. ( I 'JSl j
n.c
I""
pe riods o f lime mu st approx imately balance depressio ns vo lumetrica lly in o rder 10 maintain their sedi men t supply. Uphfts are more diffi cu ll 10 ~l ucJy than basins because , in man y cases. the Mr.l1ig.rapbic record is eithe r inromplete o r lolally missing. The major Preca mbri an shid J regio ns arc exposed beca use they are ur lirlS. bu t the de tailed histo ry of their
l N IR APL A l E If;.n tlf',I C
uplif t h a ~ no t , in ge ne ral. been recor ded . Th ose region s o f the Northern He misphe re cove red by the O uare mary icc shee rs show high recen t upl ift rates tha t ha ve bee n amibu tcd to po st -gtaciu l isostat ic response to the rem oval of the load . H owever , these sa me a reas ha ve acted as uplifts over much lon ger pe riods , a nd the O uarcma ry movements mer ely accen tuate a lon g-cont in ued tectonic trend . Ma ny co ntine ntal up lifts a re associated with rift zones , an d ar c tho ught to be ge nc nca tly rel a ted to them (sec 4.3) . A mong uphtt s of lhi.; type a rc the Ea st African a nd Ethiopian pla tea ux. the R heni sh shield (sec Figure 4. 16A) and the Voronez h-Ukraine up lifl (If the Ru ssian platform ( Figure 7.Jq . Th e origin o f such struct ures is gene rally presum ed to he thermal in view o f th ei r association with vulcan icity and . in the rece nt examples. with high heat 110w _ O the r upli fts ar c anrlbutcd to present plat e-boundary processes (c .g. the T ibe tan pla te au - sec 5.4 ) or arc associated with past plat e bounda ries. The DeCCiUl plateau , fur example , mark s the site of Cre tac eous const ructlvc bo unda ry. Ma ny up lifts . howe ve r , ar c situat ed fur from plat e bo und a ries o r rifl zo nes and th eir origin is nOI
rece nt deformation . These uplift s involve vert ica l movemen ts ra nging up 10 100 llun/yea r over pe riods o f the o rde r of 10000 yca rs [Tabl e 7. 1). T hc forme rly glacia te d area s of N. A me rica . Fennoscand ia a nd Sco tlan d yield uplift rates ve ry similar to t hose of the prese nt day glaciated regio ns of G reenla nd a nd Fran . . Josef Land . T he patt ern of uplirt of Fe nnoscandia ohrained fro m ra ised pos t-glacia l sho relin es is shown in Figure 7. 14. T he co ntou rs s how a n ov al uplift. I XiK l k m lon g in :1 N E - SW dircclio n, an d about II KKlkm ucross. T he cent ral portion has heen elevat ed over 250 III since NUIII ftC co rrespo nding to an a ve ra ge up lift nne of :!X mm/yea r. A very sim ilar recen t upli ft pancm has been de te rmine d hy precise rclevelling since IH35. Th e ma xim um uplifl o f t) mm/ye a r occurs in the G ulf o f Bothnia . The uplift corr espo nds with n ma rked , ncgalive , free-air gravity a nomaly that has bee n inte rpre ted as the res idua l mass de ficien cy produced hy the re mo val of t he icc shee t. T he e xisten ce of zones of active sei srnicit y. fogcthc r with evidence of recent Iault r novcrncms , com plica tes the tec ton ic int erpre tat io n o f the uplift . and suggests that
,I
so obvious. W e shall nnw conside r two e xa mples of
active uplift s, t he Fe nno scandian up lifl a nd the Co lo rado plateau .
17,t' Colorado Plateau
This st ruct ure, sum ma rize d by McG cl chin et al , ( 1982) is freq uently ci ted as an e xam ple uf curre ntly active up lift . It lies imm edia tely ea<;1 of the Basin-and -R ange province of the .... es te rn USA desc ribed in 4.4 (sec Figure 4. 17A) . T he
The Fe nnoscandian uplift
Th e ' pos t-glaci al' uplifts of Lauren tia (northe rn N. A merica) a nd Fen noscand ia (Fi nla nd and Sca ndi na via ] a rc familiar exa mples of T."
7. 1
Upl irl (in m mlycill r) in curnnlty.1Id r«:en1lf glacia lcd arc.... f ro m ViII -finn ( 19tltJ), l iter ""..acn l icc u UR'nlirk
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208
o I
G EOLOG ICA L STR UCTU RES AN D MOVING PUTES
km
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n gU" '7.14
M.p 1howinl OOflIUUI'5. in m , o r
uplifl ul Ihe O.IIic: ara lino;c 6lIOO ec. From Vit. ·finti ( 19Xb1. aller Zeunef (I~) .
plateau has a mean eleva tio n of 1&X)m and is about 700 km across. Uedeformed Mesozoic strata are exposed a t the surface. The crust be nea th the Co lorado plateau is un usually thick (4S km) an d ex hibits shieldlike propert ies from su rface- wave behaviour. Th e upper mantle . in co ntrast, is chara cterized by an anomalo usly low P" wave ve locity of 7.8 kmls. Heat now measur em ents ind icat e a surface flux of about 70 mWm - 2:. Th e plateau lies within the sa me regio n o f Ce nozoic igneous activit y as the Basin-end-Range province. bur has not suffered the e xte nsio nal effect s experie nced th ere and in the Rio G rande rift 10 the
east. The plateau is interp reted as a re mnant of a large uplifted regio n of th ickened crust in the western US A produced in th e late Cre taceous Laramide o roge ny. Th e bulk of the present uplift of l.S- 2km . however . occurre d in (he Miocene . betw een about 18- 10 Ma HP, giving
an average up lift rate of about 200 mlMa d uring th e period o f extension in the neighbouring Basin-and-R ange provi nce. Th er e is no general consensus regarding the uplift mechanism for the Co lo rado plat eau . bUI the most likely cause appears 10 be the isosralac response to Ihe Mioce ne regional thermal eve nt. Th is eve nt may have produced a region of anomalously warm mantle beneath the plateau crus t which, un like th e adj oi ning regions, was too stro ng o r not warm eno ugh to prod uce the exte nsio nal failure seen there .
Origin 01intraplate up/ills
or (he many mechanisms proposed to explain up lifts. we shall conside r the two most plausible: isostat ic and neural. Isostatic uplift may be the respo nse 10 a cha nge in either lithosphere mass. or mean lithos phere density. Th us an uplift may be the response 10 a redu c-
INTRAP Uo TE TECTONIC IlI::GIMES
tion in density caused by loca l heat ing of the lithosphe re . for exa mple above a mantle plume . This mechanism is th e sa me as that proposed for oce anic ridges and plateaux , and is the rever se o f the thermal process responsible for basin form ation . Uplifts produced in this way exhibit high heat flow, are associated with vulcanicity. and are likely (0 be related to rifts. Examples of such structures are the East African and Rhenish upl ifts refe rred to ea rlier. Plateaux o n the ocea n floor have a similar origin. Th e removal of mat er ial by erosion from an uplift will also p rod uce an isostatic response. The second importan t mechanism is lithosphere flexure . Because of the short-term mechanical rigidity of the lithos phere. downward be nding tc for m , a basin in one place is inevitably accompan ied by an upward bending around the mar gins of the dep resssion. In o ther words. the litho sphe re is tilted towards the
209
area of the down-bend . The uplift will be compensated by the flow of asthenosphcric mate rial into the bulge at the base of the lithosphere crea ted by the up lift. The processes of uplift and depression are complementa ry, and probably balance out . broadly. over long pe riods of time . Bo th processes must be esse ntially self-limiting in o rde r to conserve crustal thicknes.... Erosion of uplifts results in crustal thinning, and ove rcompensation by dense asthenosph ere will produce a cou nterbalancing downwa rd force . The reverse is true o f basins: over-thickening of the crust prod uces a red uction in ove rall lithospher e density. and results in a co unterbalancing upward s force . Th is may explain why the intraplate tecton ic patt ern ove r time per iods of the order o f hund reds o f Ma is cha racterized by periodic inversions o f the kind seen in the Russian platfor m.
8 Phanerozoic orogenic belts: some examples We have established in the foregoing chapters a set of principles and methods, both on theore tical grounds and by looking at 'act uatistic' or active exa mples, linking geological structure with plate movements and plate behaviour. In this and in the following chapter, we shall ap ply these principles and methods in order to understand the structure of certa in orogenic belts. Obvio usly. very few belts can be discussed here, a nd much of the essential detail must be omitted for reasons of space. However , it is ho ped , by discussing a few selected well-studied exa mples, to give a gene ral impression of how the various tectonic patt erns expressed in these belts have been linked with plate tectonic models. Plate tectonic interpretation has been acbieved with varying degrees of success: in general, the interpretation of young orogenic belts formed during the Mesozoic- Cenozoic period is much more tightly constrained than in the case of Palaeozoic and , more especia lly, Precambria n examples. The reason for this difference lies in the co mparative certainty with which the successive positions of the various plates and plate fragments can be tracked , by ocean-floor magne tic stratigraphy and other means, since the break-up of Pangaea abo ut 200 Ma 81'. Before this lime, the oceanic record is missing, and we have to rely on much less accurate palaeomagnetic restorations in plotting the former relative positions of the continental fragmen ts. We shall look at four examples: the Alpine orogenic be]t in the Western Med iterranean, the Cordilleran Mesozoic- Cenozoic bell in western North America , the Hercynian system of Western Europe and its North American counterpart , the Atleghenian , and finally the Caledonian belt of the North Atlantic region . These four examples provide a complete range in tectonic environment : they e ncompass divergent , convergent and strike-slip regimes, subduction, and collision, of both continentarc and continent-continent type. Most oro-
genic belts will find anal ogues somewhe re in these examples. 8.1 The Alpine orogenic be lt or the Western Mroiterranean An active destructive plate boundary extends from the Gibraltar fracture zone in the At lantic Ocean (see Figure 3.1) to the weste rn boundary of the Pacific plate in eastern Indonesia. Th is boundary includes subduction zones such as the Aegean are, the Makr an and the Sunda arc (see 5.2) and the major collision zones between Eurasia in the north and Arabia and India to the south. The Western Medlterranean sector of this active boundary lies mostly below the Med iterranean Sea , continuing the line of the Azores-G ibraltar fracture zone to Sicily. Here a short subd uction arc links it to the Eastern Mediterranean trench system (Figure 8.1). This active boundary lies within a wide zone of Mesozoic- Cenozoic activity comprising several sepa rate but relat ed belts. South of the suture lies the Atlas be lt stretching from Morocco to Tunisia. Across the Straits of Gibraltar is the Betic Cordillera, along the south coast of Spain, which is separated by a large stable craton, making up most of the Iberian peninsula, from the Pyrenean belt between Spain and France. The latter belt meets the main Alpine belt east of the Gulf of Lions. The Alps are joi ned to the south by the Apennine belt, running along the Italian peninsula, to the east by the Dinaride chain of Yugoslavia, and 10 the north east by the Carpathian chain. The explanation for this complex pattern lies in the plate tectonic history of the region. which, according to Dewey et al. (1973) and subsequent workers, has involved separate but interre lated movements of a number of microplates or minor continental fragments in addition to the main Europea n and Afr ican plates. The microplates include Iberia, the Carnics, Moesia, Apulia and Rhodope (Figure 8.2).
210
211
PIt ANERO ZOIC OROGEN IC RU TS: SO ME EX.... MPLES
·....... -. :;: :: ::: : ::. ·· , ... ·... . ,
"
"
--
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"'igu ~ 8. 1 Map ~howing lhe h,:cton ic scllinll of Inc Alpine orog" nic bell in Inc Mcd ilcnanean rcgion . Microphu C5 o r major cros!al 1'1101:"1(1 within the hell are ~n in bold k llen. Tbe main pblC5 bordering 1he bdl (nl lcd u rn.lffiC nl) arc the Eurasian , Affican .nd Ar abian p1111C5. Tbe p1ale bouooary (lJlO!>l l ~ dnIl'\M."t ive) bl:IWtt n 1 ~ iI _hown n a hea¥) too thed line . Arro ws indicate direct ions of movemcnt rehrlive to lhe- Eurasian plate. No1e the main lCdon o r the AI.... chain, discussed in the tcxt ; FR , FrcflCh ; SW. Swiu; 100 E. Er..tem . Afler Wiook y ( 1977) aoo Dewey r'I al . (1973).
8
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AMERICA
....
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=
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.•
TETHYAN Pl.ATE
;..
Fliun 8.2 (A) La te Trill~sit palaeogeographic and plate tectonic: teroJl5tf\lction or the A lpine orogenic belt system. From Windley (1m). sner Dewey et al. ( 1973). (8) Scncmalic map . howinll timing (in MOl II'). direction and amount of Mc:sow ic-Ccnowic motion of plates and bled. relative 10 NW Europe . From Dewey ( 1982)
212
GEO L.OGICAL.
~'RUcrUR £S
Th e Ca rnic a nd Apulian frag me nts ama lgamated to fo rm the present-day Ad riatic micro-
plate (Figure 8.1) that forms the southern hinterland of the A lps. Regional tectonic context
A useful summa ry of the tectonic evo lutio n is co ntained in Wind ley (lm). MesozoicCe nozo ic histo ry of t he Western Medite rran ea n has bee n dominated by a major plate reconstruction involving the opening of the Atlantic Ocean to the west a nd the clos ure of the Tethys
Ocea n (see Figure 3.5). In the late Tri assic pe riod, Western E urope formed a landmass co ntai ning con tine nta l red-bed basins bordered by a wide ca rbonate platfor m along the nor thern side o f the Tethys Ocean (F igure 8.2A ). A similar carbona te plat form formed the north-
ern and easte rn Ranks of the African-Arabian contine nt. New ocea nic crust was fa nned in extensional basins now reeognizable in several abd ucted ophiolite complexes. The Triassic carbonate platforms began to disintegrate in the ea rly Jura ssic, when widespread extensiona l faulting a nd rifting we re experienced, associated with volcanic activity (Figure 8.3A) . Subsidence led to deeper -water, muddy and pelagic facies throughout the region. The next major change marked the comme ncement of convergence and subduction (Figure 8.38). The forme r existence of trenches is often deduced from the widesp read appearance of the so-called 'flysch' facies in the Cretaceous and Eocene pe riods. Th is sedimentary assemblage consists of marine shales with intercalated coarse clastic material, typically turbiditic, and associated with olistostromes or massive slump deposits. Assemblages of this type are characteristic of modern trenches but also of extensio nal basins such as those of the present-day Western Mediterra nea n, and of major submarine delta fans. They have therefore been regarded traditionally as synorogenic depos its created as part of the orogenic process. In pla te tecton ic term s, they represent both the erosional prod ucts of the uplifted zones of convergent boundaries (e.g . volcanic
AN D MOVING Pl.ATES
arcs) and also the associated deposit ional environments such as tre nches, and fore- and back-arc basins. The main fl ysch deposits, which are of Cretaceous age, are found in the external Zone of the Alps, extending eastwards into the Carpathians. These de posits have long been regarded as marking the Alpine 'foredeep basin' receiving the eros ional products of the uplifted mountain range to the south and east (d. Argand, 1916). Th is foredeep basin has been interpreted in plate tectonic reconstructions of the region as a trench bordering an uplifted arc 10 the south. Howeve r. Hsu (1972) has pointed out that fl ysch sedimentation in the eas tern Alps continued for abou t 50 Ma , and that in a normal subduction zone , such material would have been re moved much more quickly. The explanation he suggests is tha t the relative motion of the tre nch was largely strikeslip. This is in agreement with the relative plate movement vector , as we shall see . Associated with this stage in the Alpine orogeny was the development of blue-schist metamorphism in the oceanic material, interpreted as the effect of subduction; the appearance of acid to intermedia te vulcanicity indicating the presence of volcanic arcs; and the development of thrusting and obduction marking the closure of small ocea nic basins. Continued convergence led ultimately to continent-continent collision in several parts of t he Alpine belt, and part icularly in the main Alpine chain itself, whe re the Adriatic continental plate came into contact with the European continent. This eve nt led to progressive movement of thrust sheets and to general uplift. The process took place during Eocene to Oligocene time, and was accompanied by high-grade metamorphism. The climax of regional oro ge nic uplift in the Alps occurred in the Miocene period (Figure 8.3C) . During the final stages of convergence and continenta l collision, an importa nt change look place on the foreland, where largely contin ental clastic deposits were formed in foredeep basins situated along the craton margin. Nonmarine clastic sequences of the type formed in
PH .... NEKOZ.O IC OROG EN IC BELTS : SOME EXAMPLES
213
A NOItTH ""EIt IC"
APU LIA
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1ft, BP
the later stages of orogenesis are termed 'm olasse' in classical geosynclinal or orogenic theory. These deposits contain a high proportion of coarse, clastic material, along with intercalated coals and evaporites. Molasse
tlgure 8.3 Pro posed plate bo undary schem es for ea rly Jurassic (It) . laic C retaceo us ( 8) and tate Ce nozoic (C) times for the Al pine system. from Windley ( 19n ). aller Dewey t lal. ( 1973) .
deposition in the Alpine belt overlapped the period of ftyschdepo sition in genera l, although within the Alps proper, molasse deposition -replaced ftysc h during the Oligocene period and continued into the Pliocene.
214
GEOLOGI CA L STRUcrU RES At-lD MOVING r LATES
History of plate movements
The record of plate movemen ts within the Western Med ite rra nean region was fi rst docu me nted by De wey et al. (1973) whose account of the plate tecton ic evolution of the area forms the basis for most subsequent models. The lat e Triassic 10 early Jurassic riflin g phase was expresse d not o nly along the line of subseq uent ocea n o pening of the Central Atlantic, but also along the boundaries of several 'mi cropl ates' : the Carnics and Moesia atta ched to E urope , and Apulia, Rhod ope a nd the Turkish pla te a ltached to Africa (Figure 8.3A) . T hese con tinental blocks subsequently moved eastwards a nd north eastwards respectively, crea ting new ocean basins behind them . The commenceme nt of ope ning in the Central Atlantic is generally da ted at 180Ma ae in the ea rly Jurassic. Th is date corresponds with the collapse of the carbonate platforms noted above. As a result of this op ening, the African plate moved in a n ESE to E dire ction parallel to the Azore s-G ibraila r fracture zo ne. A t the same time , furt her opening of th e new ocea n basins was accompa nied by subd uction of the main Teth yan ocea nic plat e along the Black Sea -Caucasus line (Figure 8.3A ). Th is phase of movement continued until abo ut 80 Ma UP in late Cretaceo us time , when an important change in relat ive plate vectors took place . The cha nge resulted from the ope ning of the North Atla ntic Ocea n west of Spain, and e xtending to the Davis Strait be tween Greenland and N. America , togeth er with the linked ope ning of the Bay of Biscay and the conseq uentia l rotation of Iberia : Because this part of the A tlantic was opening faster than the Ce ntral Atlant ic, the overall movement vecto r of Africa reversed to become NW to W relative to Europe (Figure 8.28) . Th e rotation of the Iberian pe ninsula duri ng this phase has been det ermined palacomagnetically (Van der Voo, 1969). Durin g this second phase of essentially sinistral relative movement be tween Africa and Euro pe , conver gent motion too k place in the Alps, marked by Ilysch sed imentation in the Piemont trough .
As can be seen from Figure 8.28 , the convergence vector at this time is appro ximatel y perpendicu lar 10 the N c-Setre ndi ng Western Alps, bUI almost parallel to the E- W-trending Ea stern Alps. At about 52 Ma DP another importan t cha nge in relative motion occ urred . caused by the opening of the main N.Atlantic- A rctic Ocean between G reenland and Scandinavia. This cha nge was marke d by a NW-d irected movement of Africa towards Eur ope. During this third phase, from Uppe r Eoce ne to Lower Ol igocene time , the main deforma tio n and metamorphism of the Alps took place , although flysch sedi menta tion continued in the Helveu c zone to the north. No te that the convergence vector is now oblique to both the Easte rn and We stern segments of the A lpine chain, but with a stro ng converge nt compo nent in each case . T hus movement is dextral transpressive in the Eastern Alps bu t sinistral transpressive along the N- S to NW-SE French A lps, and only truly converge nt along the sho rt NE -SW segment of the Central or Swiss Alps. The movement uplifted the ea rlier fold belt , and crea ted the grea t gravity-sliding nappes of the Pennine and Ultrahelveti c syste ms (see Figure 8.5). Th is stage also corresponds to the commence ment of molasse deposition in the fored eeps to the north and in the Po plain to the south. By O ligocene times, all Mesozoic ocea n . crust in the Western Medit errane an had disappeared . The present oceanic are as result from the opening of new marginal basins created by the antidockwise rotation of Italy (a nd the Dinarides) and of Sardinia-Corsrco away from the Iberian peninsula in the Miocene . Again these movements are reliably det ermined from palaeomagneti c data (Zi jderveld et aJ., 1970a,b) . The final change in convergence di rect ion took place about 10Ma UP during the Miocene (Figure 8.28). From then until the presen t, Africa has been moving northward s relative to Europe. This change is reflected in the intraplate structures such as the Rhi ne - R uhr rift system (see 4.3). During this phase , the main
PHANERO ZO IC OROGEN IC BELTS; SO ME EXAMPLES
defo rma tion occurred in the H elvetic zo ne and
Italy. T he latter belt is truncated o n its ea stern side by the unde formed deposits of the Hunga rian plain. The re is considera ble variation alo ng the
in the So uthern A lps, o n the margins of t he Africa n pla te . vand finally, in the e arly Plio-
cene, in the J ura.
str ike of the Al ps o f which a usef ul summa ry is provided by Debel mas et al . (1983) in th e form
Structural f rame work ofthe A lps
of four characte ristic pro fi les across the belt. We shall concentrate on the interesting centr al
The A lps proper (as distinct from the A lpides o r A lpine chains, which are much more widely distr ibuted) com prise th ree main secto rs: the French, Swiss, and Eas tern , or Austr ian , A lps (Figure 8. 1). The Fren ch (or French-Italian) Alps exte nd from the Medite rranean coast ne ar Nice , to the Swiss border in an arc tha t varies in tre nd from NW-SE in the south to NE- $W in the nort h. The change in trend is rat her sudde n and occurs in the regio n of Gre noble. The Swiss A lps continue in a NE $W d irection but gradually bend into the E - W trend of the Eastern Alps in Austria and N.
f Ol DED JURA
region whe re the NW- SE French secto r be nds around into the NE-SW Swiss secto r (Figure s 8.4 , 8.5). The re arc eleven main tectonic zo nes, on ly some of which can be recognized in any give n profil e . (1) the no rthern foreland consists of undefo rmed Mesozoic- Ce nozo ic cover on a Hercynian crystalline basement ; in the central Franco-Swiss secto r, this zone is known as the plateau Jura. (2) The fold ed Jura zone , see n only in this central secto r. consists of re latively simple fold-thrust cover resting o n a shallow basal detachment. The synclines
figure 8.4 M3p showing the main tectonic sub divisions o r the Swiss and nort he rn Fre nch Alps. The zone numbers lire refe rred to in the texr . After Ramsay (1963).
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EXTERNAL ZONES
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EXlernal Zones
Pia ~ont FaCIes
Bria~~meis FaCIes
Opoolltes
Molasse
intrusions
~····
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Figure 8.5 Representative structural profiles across the northern French (upper diagram) and Swiss (lower) Alps (see lines on Figure 8.4). showing the outcrop of the major structural units and tectonic zones. SB. Sub-Brianconnais zone ; P, Piernont zone; W. Wildhorn nappe; M. Morcles napp e ; DJ. Diablercts nappe. After Debelmas et al. (1983) .
PI-l AN ERO ZO IC O ROGENIC BEI.TS : SOME EXAMPLES
of Mesozo ic folded cover con tain weak lydefo rmed Miocene molasse depo sits. The amount of deform ation increases so utheastwards. (3) The Molasse srougn is a flexural forela nd basin developed du ring the O ligocene 10 Pliocene pe riod in respo nse to thr ust loadin g to the so utheast. The molasse is undeform ed in the cent ral and western part s of the basin bu t is involved in th rusting o n its so uthea st side . Th e sole thrust passes beneath the und efo rmed molasse to link with the next zone . (4) The Dauphinais zone (o r sub-A lpine chains) contains the most highly deformed of the external zones of the Alps. In this zone , Hercynian basement and a th ick platfo rm Mesozoic cover has bee n involved in major thru st shee ts which are para -autochtho nous in th at they have travelled on ly a sho rt distance from thei r origin. Gravity sliding in the Ti nee nappes of the Alpes Ma ritimes north of Nice (Figure 8.6A ) is described by Graham (1981). He attrib utes the 26 km o f shortening see n in the Triassic cover to gliding on weak decolle ment planes in Triassic eva porite deposits. The gravity gliding is attributed to uplift of the A rgente ra basement massif to the no rth (Figure 8 .6 B) . This zone is replaced along-strike in Switze rland by the Heiv etic nappes. These consist o f basement blocks of the Aiguilles Rou ge and Mont Blanc massifs to get her with the ir paraautochthonous Mesozoic cover . In a study of the Helvetic nappes, Ramsay (1981) and Ramsay et al, (1983) integ rate the major and minor structure and fabrics developed in the p rogressive strain histo ry o f the na ppes. He shows that the folding and intern al strain are related to movement along sub-horizontal shea r zones that ar e the deeper-seated equivalent of thrusts. Figure 8.7 is a pro file across the Morcles, D iablere t and Wildhom nappes (see Figure 8.5) , which consist o f detached Mesozoic cove r . The profile illustrat es the gene ral form and stratigraphy o f the Heivet ic nappes . By studying the strain histor y o f the variou s parts of the nappe complex, Ramsay shows that Ihe ea rliest strains result fro m NNW elo ngatio n arising from sub-ve rtical sho rte ning. These early stra ins are o nly shown in the
217
uppermost nappes which have experienced a lo nger deform ation history than the lower. Moreover , the 'ligher nappes were displaced under condit ions of lowe r co nfining restra int tha n the lowe r and sho w more variable strain patt erns. Late r strains are due to mark ed extension pa rallel to the fold axes. Th ese observat ions are consistent with ea rly duc tile tr anslations along lo w-angle shear zones that steepen downwards into the intense ly defor med shea r zones seen in the basem ent. The next thr ee zones co nstitute the internal Alpine zones. (5) Th e Embru nais- Ebaye nappes o f the Valais zone occur in the so uthern French secto r, where they are thru st over the rocks of the sub-Alpine chains. Th ey conta in allochthono us material deri ved fro m the interna l Pennine nappe zones , and comp rise a lower unit of Mesozoic cover slices and an upper unit of Cre taceo us flysch . The lowe r unit continues no rth ward s as the suo -Brianconnais zone. (6) Th e Brianconnals zon e consists of numerous superimposed units exhibiting a fan arra nge ment, with more weste rly structu res verging west and mo re easterly ver ging eas t (Figure 8.5). The stratigraphic seque nce is characterized by very thick T riassic she lf deposits o n a Permo-Ca rboniferous basement, o verlain by a very thin J urassic and Cre taceous cover, with many stratigraphic gaps, and is interpreted as a pelagic gea nticlinal zone . Deformatio n is inte nse, and shows two main phases; an earlier, characterized by nor thwest wards thrusting, and a late r, related to so utheastwards back-thru sting o r retroch arriage (see e .g. Platt a nd Lister , 1985). The Brianco nnais zone in the Swiss Alps is rep resented by the Sa int Bernard Nappe. Th e later back-thrust ing phase is dated by H unziker (l986) fro m mica cooling ages a nd apatite fission-tr ack ages, a nd is att ribu ted to the N - S Miocen e collision move ment. (7) The Piemont, o r Schistes L ustres zone , is the most easterly of the intern al o r Pennine nappe zones. It contains a number of comple xly defo rmed nappes co ntaining a stratigra phic seq uence that changes fro m west to east. The externa l unit s possess a th ick Triassic
218
GEOLOGICAL STRUCTU RES AN D MOVING PLATES
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B shelf sequence on a Carboniferous base ment, similar to that of the Brtanconnais zone . However . the internal units contain typical o phio litic ocea n-crust assemblages with Jur assic to mid-Cretaceous pelagic sediments . These nappes show a similar cha nge in vergence across the zone to the Brianconnais nappes . It is this
8.6 (A) Representative downplunge: structural p rofile across the note nappe complex. (8) Sequence or diagrammatic profiles illustrat ing a gravity sliding model for the Io rmaticn of the Tinee nappes. (A) , (8) from Gra ham
(1981)
zone thai contains the evide nce fo r Upper Cretaceous obduction linked with high-pressure met amorphism. In the northern French sector, this zone is reduced to se veral klippen resting o n the Brianconnais nappes (Figure 8.5). In the eastern part of the zone lie the Lanzo peridotites. interpre ted as the top most par t of
PHANEROZ OIC O ROGENIC BELTS: SO ME EXAMPLES
Coml>!"""
219
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1'1gure 8.7 Co mpos ite structural profile across the western Helvetic Alps, showing the alloc hthonous Morcles and Wild horn nappe s ove rlying the autochthon of the Dauphincis l one (see Figure s 8.4 , 8.5). Fro m Ramsay (1981)
the uppe r mantle, bounded on its south side by African crystalline basemen t of the Ivrea zone. In Switzerland , the lateral equ ivalents of the Piemont nappes are found in the complex Monte Rosa nappe, with its associated ophiolites. The Pre-Alps of Switzerland represent a large klippe of Premont -zone material resting on the Molasse basin, at least 50k m from the nearest Pennine rocks, having travelled across the intervening Helve ric zone. (8) In the Swiss Alps and Eastern Alps of Austria, the Penni ne nappes are over lain by the next zone , the Austro-A lpine nappes . These contain crystalline basement of the Adriatic plate with its Triassic to Jurassic cove r. T he Dent Blanche nappe in the central Swiss Alps forms an Austro-Alpine klippe resting on the Piemont nappes. These nappes root in the Sesia Lanzo zone on the SE side of the Piemcn t zone . (9) The So uthern Alps zo ne consists of a simple south-verging fold-thrust bell that is separated from the zones to the north by the Insub ric a nd Tonale faults. This zone is on ly recognized in the easte rn Alpine secto r. Th e crystalline basement of the south-
ern Alps zone is known as the Ivrea zone. In the southern sector , the Po basin (zone 10) with its thick molasse deposits conceals the southern Alpine margin . Zo ne 11 is the undeformed Adriatic plate or African fo reland . It is concealed by the Po basin in the south, but is represented by the Ivrea zone in the central sec tor.
In summary, then , the three main tectonic units of the central or Swiss Alps are the Helvetic, Pennine a nd Austro-Alpine nappe assemblages. The Austro-A lpine sheets a rc the topmost unit, a nd represent the relatively thin basement and cover from the African (Adriatic) plate, which have been 'fla ked' off the top of that plate as first suggested by Oxburgh 1972 (see Figure 5.24). The Pennine nappes, with their ophiolitic sequence, represent the thinned contine ntal margin and oceanic crust or the subducted margin of the Europea n plate. The Helvetic nappes represent the platConn sedimentary cove r from the Europea n plate, stripped off and transported back towards the foreland. 11 The metamorph ic history of the Alps reflects
220
GEO LOGICAl. STRUCTURES AND MOVING PLATES
the above changes in tecto nic en vironment. NW-SE convergent phase in Up per Eoce neThe ear ly high-pressure. low-te mper ature Lowe r Oligocene limes. In the southern me tamo rphism , giving rise to blue-schists and French Alps. Merle and. Bron. (l 9R4} demoneclog ites, is associa ted ..with subd uction and - -ctrate -thar the Par paillon nappe, a Pen nine obduction d uring the early stages o f convernappe thai has ove rridde n the exte rnal zo ne, gencc in the late Cretaceous. The later, exhibits an earlier mo vement to the no rthhigher-temperature, lo wer -pressure phase was west , followed by a southeastwards mo veme nt superimposed o n the former to give gree nschistattributed to gravity sliding away from the facies co nd itions throughou t the intern al zones uplifted bell to the northeast. coinciding with the peak of tecton ic act ivity. Butle r et 01. (1986) present a balanced crustal-scale sectio n across the central (FrancoA ttempts are being made in many pa rts of the A lps to relate individual movemen t and Swiss) secto r of the Alps by restoring the Frontal Pennine Thrust , which ma rks the stra in histories of the nappes to an o verall kine matic pattern that is compatible with the boun da ry bet ween the exte rnal and internal plate tecto nic model o utlined earlier. This can zones (Figure 8.8). This section has bee n be achieved less easily in the intern al zones resto red parallel to the main WNW-directed than in the exte rnal, owing 10 the mo re ductile convergence d irection in the exte rna l thrust belt (i.e ., that of the Oligocene mo veme nts). deform ation and more complex strain history Shorte ning estimates from individual of the former. Howeve r, in se veral areas, northwestwards tra nspo rt di rections (see e .g. balanced sections demo nstrate a minimum of 140 km displacement along the Frontal Penn ine Butler, 1983) ap pear to co rrelate with the main
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figu re 8.8 Balanced IIfW restore d sections iIluslrating lhe dee p struct ure or the: nor thwestern ellie-mal Alpine lhru st belt. Motassc. open circle orna ment; otber eete rnet rover sedimenu. slipp lcd ; EBM . exte rnal Belledon ne massir; FIT. rto nlal Penninc thru st; UHT . Ulrra belveuc lhrust ; AA T. A ustro-A lpine Ihrust (suture ). from Butle r ~I Qf. ( 1986)
PHANEROZOIC OROGE NIC BELTS: SOME EXAMPLES
Thrust (Figu re 8.8) . This imp lies an eq uivalent widt h o f footw all in the form o f a mid- to lo we r crustal wedge projecting be neat h the internal zone o f the Alps . However . the geomet ry of this wed ge is dep end ent upo n the preconve rge nt crustal geo metry: in pa rticular, the extent o f an y base me nt thinning . The au tho rs assume an o riginal crustal thickness o f 25 km , corresponding to t he thick ness on the undeformed Fre nch crato n. The pro fi le inco rpo rat es se ismic re fraction a nd gravity data ( Perrier and vialon . 1980) defining the depressio n o f the Moho to a depth of c.Sn km below t he southe rn part of t he interna l zo ne (sec Figure 5.26). Howev e r. the Moh o has been displace d upward s and northwar ds along a thrust regarded by Perrie r and Vlalo n as the A lpine so le thrust. Th e displace ment o f the high-density Ivrea body to a relatively high crustal level (see gravity profile in Figure 5.23 ) is attrib uted by Butler es al. to the Fron ta l Pe nnine thrust. Th ey reg ard this th rust as lying within the midd le crust for most of t he widt h of the internal zo nes . since no deep crustal rock s are exposed exce pt at t he eastern ma rgin. T hus both the ex te rna l and intern al zo nes are att ributed . according to this model , to thi n-ski nned thru st movements. The Aust ro-A lpine thru st, which t ransports the European- Africa n suture over the Europea n plate . restores to a po sitio n benea t h t he Po valley. No precise estima te is available for the shortening across the Pe nnine nappes , which are difficult to restore beca use of their comple x three-dime nsion al strain. According to T rumpy ( 1973), during the main converge nt phase be twee n late Eocene and early Oli go cene time. at least 300 km o f crustal sho rte ning took place acro ss the whole belt in a pe riod of 5- 6 Ma , giving an average deforma tion rate o f 5-6 cm/yea r. 8.2 The Cord illeran oroge nic helt of Nor th America
At its simp les t, th e Co rd illera n o rogenic be lt o f North America may be d ivided into two major tectonic zones. Th e eastern zone comprises a
221
Mesozo ic to earl y Ce nozoic foreland thrust be lt produce d by co nve rge nt defo rmat ion of t he co ntinen tal ma rgin of th e A merican p late . In the so uthern part o f this zone , a later Ce nozo ic exte nsional regime has been superimpose d , giving risc to t he Basin-and-Range p rov ince desc ribed in 4.3 . Th e rocks an d history o f this zo ne can be rel ated 10 each o the r and to th e stable co ntine ntal interior in a co he re nt and sensib le manne r, a nd will be described fi rst. Th e weste rn zo ne is a co mple x tecto nic collage of suspect te rranes (see 6.2) , man y of which are dem o nst rably alloc hthon ous. and whose relation sh ips with t he co ntine ntal margin to t he east are eit he r spec ulative o r no n-existen t. This western zo ne was co nside red to re presen t the 'eugeosyndinar belt. paired with the ' mio geosynclina!' eastern belt in the o lde r lite ratu re . be fore the importa nce of strike-slip displace ments and exot ic terranes was d isco vered . Th e str ucture o f t he o roge nic belt is summarized in Figure 8.9 . Th e easte rn margin of the be lt is re presented by the t hrust fro nt. which possesses a sinuous course from the A laskaYuk on bo rde r to the G ulf o f Mexico , defi ning a belt that va ries in widt h from 600 km to nearl y 1400 km . Th e autochthono us par t o f the fore land thrust be lt is defined by the prov ed limits of the N . American craton ic basem ent. The Nort h Slope terran e is co nside red to be prob ab ly autochthonous, but all the te rranes west of a line from the weste rn bo undary o f the No rt h Slope te rra ne , southwards along the weste rn cra tonic margin, are suspect. Seve ra l a re invo lved in the thrust be lt (e. g. the Easte rn assemblage o f Brit ish Co lumbia) . Man y of the terranes in the western belt have unde rgone lar ge strike -slip displace ments relative to Nort h A merica. The so uthwestern part of the orogenic belt is traversed by the Sa n Andreas fault zo ne , along whic h de xtra l strike-slip mo tio n is presently taking place. Th is active zone is described in 6.3. Act ive subd uctio n is also tak ing place at the western mar gin of the o rogenic belt alo ng the British Co lumbia - Washingto nO regon sector, an d is respo nsible fo r the Cascades volca nic arc (see Figure 4. 18).
222
GEOLOGICAt. STRUCTURES AND Mo v ING p!.A u .s
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Figu ~ 8.9 Tectonic summary map or the Nor th Ame rican Co rdilleran oroge nic be ll . showing lhe division between the western collage zone o r suspect and displaced terranes, and Ihe eastern paramccruboncus fo ld-thrust belt underlain by Nonn American enll onie basement. Sec: socrce for a c:ontplele list o f the terranes, and thei r descn rnions. NS, Nonh Slope and E, Eastern assemblage are terranes referred to in Ihe text . The SOUl he m pa n of the map should be referred to Figure 4.17(A) for grealer detail. From Coney ~ ai, (1980)
The Cordilleran fo reland thrust belt in 1M S . CAnadian sector
The eastern part of the Cordille ra n orogenic belt of N. America consists of an east-directed set of thrust shee ts defin ing a belt up to 300 km wide, developed on Precambrian basement of
the N. Ame rican craton. A detailed description of the southern Ca nadian Rocky Mountains sector (Figure 8.10) is provided by Price (1985). The sedimentary cover consists of an easte rly-thinning wedge, divisible into ' preorogenic' and 'synorogenic' domains. The pre-orogenic cover consists of 'miogeoclinal' continental slope 10 continental shelf deposits ranging in age from Late Prote rozoic 10 midJu rassic in the east, and in the west, of a 'eugeoclinat' assemblage of basic to acid '101eanic rocks and immature clastic ff yscb deposits, The synoroge nic cover consists of clastic deposits of late Jurassic to Palaeogene age, accumulated in a foredeep Or foreland basin. The axis of this basin appears to have migrated north -eastwards during this period as the thrust-thickened load moved towards the craton. The Late Proterozoic sequence comprises two quite different units. The Belt- Purcell Supergroup consists of up to 11 km of marine clastic sediments followed by carbonat es deposited at a rifted continental margin. The overlying Windermere Supergroup consists of up to 9 km of coarse, immature sandstone s and conglomera tes, interbedded with shales and carbonates, lying unconformably on the BeltPurcell rocks, and partly der ived from them. The basin in which these beds accumulated appears 10 have been bounded by a major extensional fault, which subsequently reve rsed its displacement direction during the thrusting. The Lower Palaeozoic strata consist of a thin sequence, up to c.l km thick, of shelf carbonates and shales that thicken westwards to about 5 km along a carbonate bank margin. West of this line. the re is a facies change in most units to dark mudstones with inte rbedded basic volcanic rocks and coarse clastic deposits. This facies change is interp reted as evidence of a back-arc or marginal basin west of the continental shelf, with a volcanic arc on its western side. The Upper Palaeozoic strata are unconformable on the Lower . In the basin domai n, the Lower Carbonife rous deposits are similar to those of the Lower Palaeozoic. a nd rest on
PHANERozorc OROGENI C BELTS : SOME E.XAMPlES
them with angular discordance. In the plat form seq uence, Upper Devonian carbonates overstep their Lower Palaeozoic equivalents. This distinction between shelf and miogeoclinal slope environments is maintained in the Mesozoic seque nces. 100 m of Ju rassic marine shales o n the platform are the lateral equivalent of a 1 km-thick seque nce of Triassic [0 J urassic shallow-marine deposits, consisting of assorted sandstones , carbonates and evaporites. The basin asse mblage of equivalent age is 10km thick a nd contains a high proportion of volcanic a nd volcanogenic deposits of mainly andesitic de rivation. Deformed mid- to late Jurassic granitic pluto ns are associated with these rocks. which are also cut by younger. Cretaceo us granites. The changes in depositional environment recognized in these facies variations mark the boundaries of major tectonic provinces within the Rocky Mountains belt. Important changes in thickness and mechanical properties of the cover have con trolled the nature of the foldthrust defo rmation. Consequently. abrupt changes in tectonic patt ern take place both across and along strike, due to these lateral changes in facies and thickness. The belt is interpre ted as a typical. thinskinned , foreland thrust belt, where many of the ideas incorpo rated into modern thrusttectonic theory we re develope d (see e.g. Dahlstro m, 1970). The structure is dominated by thrust faults that a re primarily west-dipping and eastwards-verging (Figures 8.10, 8. 11). Flexural-slip folds are developed in association with the thrusts. many of which die out in the cores of anticlines. Thrusts in the eastern part of the belt commonly detach on weak horizons such as the Upp er Cretaceous Alberta Gro up, the Jurassic marine shales of the Fernie Group, a nd a t the base of the thick Upper Permian Paliser limestone. The thru sts in the eastern part of the belt , from the thrust front to the Rocky Mountain Trench, appea r to have developed in piggyback sequence, by eastward displacement of the cover over the autochthonous platform (Figure 8.11). Preca mbrian baseme nt with
223
similar geophysical prope rties can be traced at least as far as the eastern part of the Purcell anticlinorium (Figure 8.11) without any disruption of its distinctive NE-SW magnetic fabr ic. In the western part of the belt, the Protero zoic strata of the Bell-Purcell and Winderme re Supergrcups show variable penetrative strain and metamorphic grade, both of which increase progressively westwards. Accurate section balancing is therefore confined to the eastern section. A major change in str uctural level, amount ing to about 20 km o f stratigraphic succession. takes place across the Kootenay A re, where Belt- Purcell strata in the core of the Purcell anticlinorium are adjacent to T riassic- Jura ssic basin deposits to the west. The Kootenay Arc is therefore interpreted as a major west-facing monocline marking the western edge of the North Amer ican craton. T his structure corresponds to a change in crustal thickness from c.50-55 km at the Purcell a nticlinorium to 30- 40 km beneath the interior part of the Cordillera, and also to a corresponding increase in the negative Bouguer gravity anomaly. The size of this anomaly can be explained by the calculated increase in crustal thickness of IOkm. However, in orde r to balance the thin-skinned crustal shorte ning in the east, the 40km -thick crust of the autochthonous platform is required to extend westwards beneath the Purcell anticlinorium to the edge of the Kootenay Arc (Figure 8.11, 8.12) . Th is crustal structure confirms the view, based on the study of the sedi menta ry facies , that the thick miogeoclinal strata of the Proterozoic and Lower Palaeozoic sequences accumulated on thinned continental crust at the contine ntal margin. The minimum tota l shorte ning achieved by the thin-skinned part of the belt is estimated to be J70 km. Deformation in the thrust belt a ppears to have spanned a period of almost 100Ma from late Jurassic to Palaeoce ne time. Uplift and erosion of shelf assemblages is first recorded in Upper Ju rassic deposits, and early reverse! transfer faults in the Purcell anticlinorium are cut by early to mid-Cretaceous batholiths.
224
G EOLOGICA L STlWcr UII.ES AN D MOVI NG PLATES
225
PHANlRO ZOIC OROGENIC BELTS : SO ME EXAMPLES f _ _' _
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" gure 8.10 Geological map of the furd anJ Ih ruSI· fo ld he ll or Ih e so uthern CanaJi an Rock y Mountains. in tbe cent ral sector of the N. Americ an Cordille ran o rogen ic he ll. For names of key faults an d batbofhhs distin guishe d by teue rs, sec source. Fro m Price ( l'Jlll )
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FigUR 8.11 SW- NE struc tural profile aeross upper line on Figure R.IO. Faults idcn lified on Ihe sec tion arc : Pu. Purce ll; Ba, Bou r&C,)u; Sm. Sulphur Mountain; Ru, Rundle ; t".lnglismaldie; LD. Lac des Arcs; M,. McCo nne ll; HI. Burnt Ti mber ; 08. O ld Bald y; 8 z . Br azeau. Not e diftc renl ornament in Koot enay arc (nJlw ) and in foredeep elastics (blank) Ior clamy. Fro m Pnce (198 1)
226
G EO LOGICAL STRU(.iURES AND MOVING PLATES
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0(-:.:.:.:.:J. FiRU~ 8. 12 Diagrammatic sections 10 illustra te an iOIC' rprcli1lion o f the evoruuce of the Purcell an tid inori um (see Figure 8.11) (4) Restored original sectio n; (b) prese nt sectiofl drawn to eliminate effects o f erosion pre- and post- lhrusling: (c) sehemanc representation showing d isplace ment (A 10 A' etc.). m, manue : c. continenta l Crusl;p. Bell- Purcell assemblage; .... windermere lS."Cmblage;Lp . lower Palaeozoic assemblages; up -m . Upper Palaeozoic and T riassic-J unlssic asse mblages . From Price ( 1981)
Th e late Cre taceo us gra nite plutons of the Kootenay Arc a re pos t-tecto nic in relation to the deformation fabric. Furth er east , the McConnell and Lewis thrusts post-date Upper Cretaceous slope de posits but pre-date late Eocene -early Oligocen e foredeep-basin molasse. Price estimates thai a t teast 100 km of horizontal displacement occurre d across the thrust belt during thi s pe riod o f less than 3O Ma, corresponding to a rate of 3 kmlMa. o r 3mmlyear. T he Purcell anticlinorium is interpreted as the geo metric co nsequence of lifting the thick sed imenta ry prism . ori ginally deposited o n the atte nuated crust o f the co ntinental margin, o n to the main part o f the cra to n (Figure 8.12). Crusta l conve rge nce of possibly 200 km o ver the whole width of the be lt is thus acco mmoda-
led 10 a large exte nt by ove rlapping o f the already thinn ed co ntinen tal crus t along the continenta l margin . and is viewed by Price as an exa mple of intraplate co nve rgence . involving the destru ction of a marginal basin sit uated behind (east o f) the main eastward-dipping Co rdillera n subd uctio n zone . The western collage zone ofsuspect terran es
Th e concept o f displa ced or suspec t terranes (see 6.2) was developed in th is regio n (see Wilson , 1968; Mon ger et al., 1972; Jon es et d ., 1972). In Figure 8.9 the distributio n o f more th an fifty sus pect terranes ide ntified by Co ney et at. (J 98O) is shown . Th e prin ciples govern ing their recognition are discussed in 6.2. Adjoining terranes may be d istin guished by discon-
PHAN EROZOIC OROGENIC BELTS: SOME EXAMPLES
tinuities of structure or stratigraphy across their boundaries, tha t cannot be explained on the basis of nor mal facies or tectonic changes. Many terranes contain palaeomagnetic records that differ stro ngly from those of the stable craton, or of adjoining terra nes. Te rranes are regarded as allochthonous or exotic if their faunal or palaeomagnetic signatures indicate that the y originated a considerable dista nce from their present position relative to the craton. Many terr anes show evidence of an origin far to the south of their present latitude, and many also have undergone translations of hundreds of km after collision. Palaeomagnetic evidence also indicates significa nt rotations about the ve rtical in many cases (e.g. the large terra ne in Oregon. labelled S in Figure 8.9). The histor y of the western zone can be pieced together by comparing the stratigraphy of the autochth onous a nd parautochtbonous foreland seque nces with those in the suspect terra nes. As we have seen, the western boundary of North America was a passive continental margin throughout late Precambrian and ea rly Palaeozoic time. during which a broad miogeoclinal terrace developed. Apart from a brief period of convergence and collision in the mid-Palaeozoic, this situation continued into the late Palaeozoic. In late Triassic to mid-Jurassic time, however. a subduction zone became established which eventually consumed the Palaeozoic proto-Pacific ocea n. All the Palaeozoic te rranes now found outside the Palaeozoic passive continental margin must therefore be suspect, and must have accreted to that margin during Mesosozic--Ce nozoic time. You nger terranes outside that margin must also be suspect, although their allochthonous natur e may be more difficult to prove unless they include Palaeozoic basement. Most of the suspect terranes listed by Coney et at. cont ain sedimentary and volcanic sequences of oceanic affi nity, and rocks olde r than mid-Palaeozoic are rare . A few contain pieces of oceanic crust (e .g. the Cache Creek terr ane of Western Ca nada . and the Klamath Mount ains terrane of Californ ia - see Figure 8.9). The Cache Cree k terrane contains Per-
227
mian Te thyan faunas quite distinct from those found in adjoining blocks. Other terr anes represent fragments of island arcs of late Palaeozoic to J urassic age. The large Stikine terrane of Western Canada (Figure 8.9) contains a Lowe r Carbo niferous to Perm ian volcanic sequence overlain by Upper Triassic to mid-J urassic volcanogenic strata . This terr ane has no continental basement. O ther terranes represen t volcanic arcs fonn ed on older basement sliced from a distant continen tal margin. Several terranes ca n be shown to have amalgamated before their final accretion to the North Ame rican craton . For example Jones et at. (1977) de monstrate that Wrangellia collided with the Alexande r te rrane before final accretion to Western Ca nada and Alaska. These terranes contain diffe rent Palaeozoic baseme nt rocks originating far to the south, but display similar Upper Jura ssic to Cre taceous sequences and evidence of volcanic arc activity. The combined terrane accre ted to the continental margin in mid-Cretaceous times. Since its accretion, furthe r fragmenta tion has occurred, and the terr ane now extends in several detached pieces ove r 2000 km from Oregon to Alaska. The process of strike-slip terrane accretion appears to have extend ed over a period of at least 120 Ma from mid-J urassic to ea rly Ce nozoic time. Dur ing most of this period, the continental margin was a subduction zone, so that accretion took place by a process of oblique convergence combining underthrusting with strike-slip moveme nts. The former presence of subduction zones is at tested by the belts of highly deformed chert , ophiolite and greywacke sequences, metamorphosed in blueschist facies, such as the Franciscan comp lex of California. The strike-slip compo nent appears to have been dextral throughout , so that the accreting material seems to have originated consistently to the south of its final resting place. Many of the fragments of volcanic arcs may be totally foreign to North or even South America, and may have travelled from the far side of the
228
GEOLOG ICAl. STRUCTU RES AND MOVING PLATES
Pacific Ocean. Erns t (1984) provides a q ua ntitative analysis o f the process . By assu ming symme trica l sp rea d ing at the East Pacific ridge , a figu re o f about IOQ(Xl k m of western overriding o f Pacific ocean plate is derived . To this E - W co nve rgence sho uld be added se veral thousand km of northward drift of t he Pacific plate .
of their struc tural, metamo rphic and igneous cha rac teristics. The Hercyno-typc , of which the Wes t E uropean Variscides are the type example. were diffe rentiated fro m the Alpinotype by (i) large volumes of gra nito id pluton. (ii) regional low-pressure , high-tem perature metamorph ism , and (ii i) poorly-developed fo ld-thrust tectoni c sho rte ning. However these c haracteristics do nOI apply to the who le Hercyni an be lt. In No rth A mer ica the Her8.3 T he Her cynian orogenic belts of Wester n cynian o roge ny is represented by a linear foldEurope a nd Nort h America thru st be lt co nta ining Barrovian meta mo rphic An o rogeni c belt of Hercynian age. often rocks and few gra nites. Mo reo ve r linear foldtermed the Variscan belt. occupies most of thru st be lts ex ist also in t he ma rginal zo nes of Wes tern E urope south of a line thro ugh the the E uro pean Hercynioes . in SW E ngla nd and sou t hern Bri tish Isles and northern Ge rmany, S. Wales. and in the Ca nta bria n- Asturian and west of the Tornquist line mar king the chain . for exa mple ( Figures 8.17, 8. 19). edge of the R ussian platform (Fig ures 8. 13, The regional co ntex t o f the Hercynian 8 .16). O n the eastern side of the Russian be lts is summarized in Figu re 8.13. Following platform , the Urals be lt formed during the the Caledo nian orogeny, the co ntine nts of sa me pe riod . In Nort h Am erica, the equiLaurentia a nd Baltica had become we lded valale nt oroge ny is termed the Alleghenian, to gether as far south as the nort hern Appala and in Nort h Africa , the Maurilanian.- - .- " chia ns '- To the so uth iay the proto-T eth ys Th e o roge ny spa ns mid-Devonia n to ea rly Ocea n , with Gondwanaland on its so uthe rn Perm ian time . and immediat el y follows the side . At the end o f the Hercynian o rogen y, Caledonian oroge ny. In E urope , the HercynA frica had co llided with La ure ntia to fo rm the ian belt is oblique to the earlier Ca ledonian All eghen ian sec to r of the Hercynides. Many belt , but in Nort h America , the two he lls a re autho rs have pointed o ut the importan ce o f pa rallel . and partl y supe rimposed , and are de xtra l shea r with in the E urope an He rcynldcs d ifficult to d istinguish from each othe r in man y (e .g . A rthaud and Ma lle , 19n ). A ge ne rally no rt hwest wa rds move me nt o f Africa in relaareas. Useful ge ne ral de scriptions of the be lt are provided by Windley ( 1977), Zi egler ( 1975) tio n to La ur entia -Nor the rn Euro pe explains both co nverge nt mo vement in the Alleghenian and Weber (1984) . The preferred name fo r the Euro pean oroge nic be ll is the Variscides (H utsec tor a nd dextral st rike-slip effec ts in Western to n and Sanderson . 1985). but Hercynian is Eu rope . and fo rms the basis of most plate pr obably the more inte rnationa lly accep table tectonic re const ructions. name for the orogeny world-wide . A simple subdivisio n of the Hercynian belt The width of the bel t in E uro pe is abo ut (f igure 8. 13) is made by Dewey and Burke 2000 km. and the structural and stratigraphic ( 1973) . T he o utermos t ZOne is part of the pat tern is difficult to interpret beca use the variHercynian fore land o n which fo rmed basins of o us o utcrops are separated by post-He rcynia n contine ntal deposit s duri ng the De vo nian. cover a nd . in the so uth , by the ove rprinting she lf deposits in the Lo wer Ca rbo niferous. and effec ts of t he Al pine orogeny . The E uropean coal basins in th e Upper C arbo niferous. T his He rcynides, or Variscides , have tr aditi onall y zone is represented in S. Wales and in the been regarde d as a different type o f oroge nic west ern side of the Alleghenian belt . The belt to both the Ca ledo nides and the Alps. mid dle zo ne conta ins bo th ma rine and no nThus Z wart ( 1967) classifies orogenic be lts int o mari ne ea rly Devo nian sed iments, mid Hercyno-t ype and Al pine -type on the basis Devo nia n basic volcanic roc ks, and mainly
PHANElt07-OlC OItOGENIC BELTS : SOME l:XAMPLES
L AURENTIA I SA LTlCA
229
,
•
TETH YS
OCEAN lONES
em
1
BID
2 IT] 3 •
GA....IlES
shales in the early Carbonifero us. Flysch basins, exhib iting the ' Culm' facies , formed in the mid-Carboniferous, and were subjected to northward -direc ted thrust movements. Th e inne r zo ne contains a number o f Prec amb rian basem ent blocks, such as the Bohemi an (Moldanubian) , Ar mo rican, and central Iber ian massifs. Devonia n sedim ent ation in this zone was largely co ntro lled by t he distributio n of the basement bloc ks. Sedimentary sequ e nces are generally thin, and carbo nates are typ ical. In the Lower Carbo nifero us, sedime ntat ion was inte rrupted by tholeiitic vulca nism. The zone is cha racterized by high-temperatu re , lowpressu re regio nal metamorphism. and by abundant gra nitic plutons and loca l migmatites. In t he uppe rmost Ca rbonifero us, a numbe r of intermontane sed imentary basins deve loped , together with pot assic ignirnbritic vulca nism. Three main phases of deformation a re recogni zed within t he period occupi ed by the Her cynian orogeny in the West European be lt, eac h of which ca n be detected over most of the belt. Th ese phases are the Bretonic (c.345 Ma DP) , the Sudetic (c.325Ma) and the A sturic (290-295 Ma ). Th e Bretonic phase is responsible for the wides pread Devonia n-Carboniferou s u nconformity. Acco rding to Ziegler (1975) , significant shorte ning occu rred across th e be lt at that time. T he Sudetic deforma -
Figurt 8.13 Outline map o f the tectonic setting and pr incipal subdivisions of the He rcynian o rogenic bell system of w estem Eur ope and No rth America. Zone s: 1. dis continuous forela nd basins; 2 , externa l zone characterized by Upper Carboni ferous flysch basins and fold·t hrusl belts; 3. internal zone characterized by basement massifs, hightemperature , low-p ressure metamorphism and abunda nt granite plutons. A fter Windley ( 19n) and Dewey and Burke (1973).
tion corres ponds to the main uplift phase o f the inte rio r o f the Hercynian belt , and was associated with t he main episode of granitic emplacement and acid to intermediate vulcanicity. Th e Asturic ph ase , in the uppermost Carbonife ro us, p rod uced the marginal belts o f fold-t hrust de fo rma tion as well as furt her defo rmat ion in the inte rior zone . The A l/eghenitm bell
The Phanerozoic orogeni c syste m of eastern North Ameri ca is d ivided into three separate secto rs: the Nort he rn Appalachians, exte nding from Newfoundland to the Hudson River; the Ce ntra l-Sou thern Appalachians fro m the re to Ce ntra l Al abama ; and the O uachita -Marathon belt (rom no rt hern Mississippi to Texas. Th e Nonhern Appalachians are prima rily Ca ledo nian in age ( Aca d ian and Taconic), but in addition suffered Hercy nian de formatio n in the so uth-easte rn pa rt o f the belt. Th e Ce ntral- Southe rn Appalachian belt is the type area o f the Alle ghe nian orogeny. Th e belt he re is abo ut 2000 km lon g and 500 km across (Figu re 8. 14). It consists of fo ur main zones bo unde d o n the At lanti c side by younger deposits of the coa stal plain. Th e outermost , fo re land , zon e comprises (he Appalach ian and Black Wa rrior basins, which contain unde formed o r wea kly-defo rmed Upper
230
GEO LOG ICA L ST RUCTU RES AN D MOVIN G PLATES
Palaeozoic (mainly Carboniferou s) stra ta . Lower Ca rbo nifero us (Mississipp ian) marine ca rbo nates are ove rlain by Uppe r Carbo nife ro us (Pe nnsylva nian) fluvia l o r de ltaic deposits, wit h an ove rall thickness o f gene rally unde r I km. These st rata a re affected by
fold ing near the so utheast margin of the zone. Th e th ree zones making up the Alleghenian o rogenic be lt are k no wn as the va ltey-ondRidge, th e Blue Ridge a nd the Piedmont provin ces (Figure 8. 15) . Th e Valley-and-Rid ge prov ince contains a thick Palaeozoic successio n without apprecia ble break between Silurian and Devo nian , o r be tween Devo nian and Ca rbo nifero us . Th e facies o f the Carbo nifero us are similar to those of the foreland. Import ant coa l-be aring deposi ts occur in the Upper Carbo nifero us. Thi s p ro....ince has long bee n con side red to be an example of a major thinskinned thru st belt (see e .g. Gw inn, 19(4 ) .
The eastern bo undary of the pro v ince IS marked by a ma jo r fau lt. southeast o f which lies the Blue Ridge pro v ince, co nsisting of an upth rust block o f Precambrian (Or enville ) crystalline basement together with late Precamb rian to early Pa laeozo ic sedimentary cover . Th e Pied mo nt belt consists of meta mo rphic rocks of probably pre-Carbonife rous age , cut by ab undant granite and gab bro intrusions of Carbo nifero us age (330- 260 Ma) , so me of which are st ro ngly deformed and gneissose . Thi s be lt is inte rpre ted as a Ca rboniferous isla nd arc. Th e A lleghenian structu re o f the Ce ntralSou thern A ppalachians is dominated by wes twa rds o ver thrusting towards the fo reland . A major decollement ho rizon within Silurian sa lt deposits forms a rela ti....ely shallow detachme nt surface for thin-skinned thrusting in the Valley-and-Ridge pro v ince . Th e COCO RP deep-
~lgurr 8. 14 Tec romc summary map o f the Ap p.alachian eroge nic ben of North America . No te lhe subdiv ision inlo extem at thru st-fold bells and interna l Pied mont and Slate bells. TIle eas tern end of the Ou achita-c Mararhc n be ll is shown in lhe extreme S\\'. After Coo k i f at. ( 1981).
.
CANAD'
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OUACHITA -MARATHON BELT
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recue
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PHAN EROZ OIC OROGl::NIC BELTS : SOM E EXAMPL ES
seismic profil e across Ge orgia (Cook et al., 198 1) ap pear s to confir m this model (Fig ure 8. 158 ) in resp ect of the Valley-and-Ridge . Blue Rid ge and inne r Pied mo nt be lts. Two alte rn at ive mod els ar e propose d for the ea stern Piedmon t be lt; on e env isages a mid-cru stal det achme nt ex tend ing to the edge of the Coastal Plain , then descen ding to the Moho ; and the ot he r a zone of deep thrusts desce nding to the Moho beneat h the eastern Piedmont , along the King Mo unta in be lt. In both models, the con tinen tal crus t of the easte rn Piedmont and C oastal Plain is shown 10 be substan tially thinne r abo ut 30 km , co mpared with c.45 km in the ma in Appalachian be lt. T hus the maj o r pan o f t he be lt is allochthonous, involving disp lacements of up 10 severa l hund red km. T he age of the defo rmat ion ap pears to span a lo ng pe riod of time . Ea rlier t hrusts have bee n dat ed at c.380 Ma and 356 Ma , but the main A lleghen ian de forma tio n appears to relate to post-met amorphic displacements o f late Carbo nifer cc us to Permian age (270- 240 Ma BP) . T he ma in de fo rmation is gene ra lly am ibuted to co llision with- No rth Africa . T he Ouachita -Ma rathon be lt to the south ( Figure 8.14) is thou gh t to be related to a qui te sepa rate co llision with a differe nt microco ntine nt , wh ich took place in mid-Upper Carbon ife ro ust imes . Both orogenic belts involve seq ue nces of shelf-slope sediments of t he North American plate , toge ther with portions belo nging to the adva ncing Go nd wanala nd plates. A n earlier collision took place in the No rt hern Ap palachian belt (see late r) whe re a co ntinental fragme nt known as A valonla collided with the No rth American craton in mid -Devonian time , giving rise to a n Acadi an orogenic phase the re . Th us both to the north and 10 the south of the main A lleghenian sec to r of the North American Hercynian be ll , co llision with microplates preceded the main Afri ca n-North Ame rica n co llision in endCa rbo nife ro us time .
The WeSl European sector We ber (1984) summarizes the evidence for the nat ure of the p re-Hercynian basement in the
231
West Europea n Var iscides (Fig ure 8.16), and concludes that , ove r most o f the re gion, the basement is no o lder than ab out 700 Ma BI' {i.e . de rived in the Cadomian o roge ny of Late Protero zoic age) . Exception s are the Armorican and Bohem ian massifs. which are found ed on o lder Precam b rian blocks. T he evid en ce fo r the nature of the basem ent co mes mainly from a study o f 81Srfl6Sr initial ratios ind icating that the Hercynian gra nite s are derived from me lts of re lat ivel y yo ung contine nta l crust (Vidal et 01. , 198 1). T he Ca domian orogeny appears to have succeeded a period of ge ne rally oceanic sed ime nta tio n over mo st of the West E uropean regio n . We be r a lso d iscusses the eviden ce re lating to the ex iste nce of the Ca ledonian o roge ny within the Va risca n be ll. A ltho ugh there has bee n no seve re regional de form ation , invo lving significant crusta l shortening, a widesp read suite of gra nite plutons was emp laced in O rdov ician to Silurian times . This Lower Palaeozo ic magm ati sm is broadly coeval with a high-grade metamorphi c eve nt represent ed fo r ex a mple in the granulite-facie s rocks o f the Saxon G ran ulitg eb irge . T he stratigraphic reco rd sugges ts that this high-grade event took place at depth du ring co ntinuo us sedime ntation at the surface , since a complete stra tigrap hic sequence from late Precambrian to Ca rbonifero us occurs within the adj acen t Sa xoth uringia n zo ne . We be r s uggests, following Catstere n et al. (1978), th at bot h the gra nite e mp lace ment and t he subsequent hightempe rature metamo rph ism we re produced by e xtensio nal cr ustal thinning and rifting , enab ling the warme r as the nosphere ma terial 10 rise to high leve ls within the lithosphere (see 4.2) . If these ide as a rc co rrect, the imp lication is that the nature of the Ca led onian 'o rogeny' changes d ra ma tica lly from no rthern to southern E urope , from an essentia lly co nvergent regim e to a d ivergent one . Another importa nt o ro ge nic event that is usually regarded as pre -Hercynia n is a pre Upper Devonian phase of deformation and gra nite emplacement recognized in the basement co mple xes of the Saxot huri ngian zone , t he Bohe mia n massif, and the Massif Central, for example , where me ta morphic rocks with
N
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Figure 8.15 (A) Structural profile across the Southern Appalaehi an orogenic be lt. sho wing the main tecton ic units and structures. Pcb , Precamb rian ; Ccr , Cambrian; S. Silur ian ; D. De vonian ; M, Mississippian ; P, Penn sylvanian .
233
PH A /'II EROZ OI C OROGENIC BELTS: SOMI'. EXA M PLES
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the Variscan be lt system of w estern Europe. From Franke (1985)
234
GEOLOGICAL
sr eucru aes
int rusive granites, yie lding ages in the range 370 - 400 Ma, are overlain by Uppe r Devonian clastic sedime nts. Unlike t he ea rlier rifling phase , t his mid-Devonia n (A cadian) event was assoc iated with significant crus tal shorte ning, invo lving fold ing a nd th rusting o f the metamo rp hic basement, and producing nappes of granulite-facies rock s restin g o n lower-grade materi al. In the Saar 1 borehole, within the Mid-Germa n C rysta lline Rise (Figure 8.17A ), unmetamorphosed mi d-D evonian sedi ments rest o n Lowe r De vonian crys talline basem en t, indicating rapid uplift during Ihis episode . Howeve r other basemen t complexes show much younger K-Ar cooling ages, indicat ing that uplift to high crustal lev els d id no t occur until the Lo wer Car bo nife rous. A lthough t he maj or folding and me tamo rphism of t he baseme nt comp lexes is of ea rly Devonian age , the wea kly me tamorphosed or un met amorp hoscd Upper Palaeozoic cove r was not fo ld ed until the Ca rbonifero us. Web er
AN D M OVI NG PLA1'ES
suggests that co ntinued uplift of the ba sement massifs too k place under co nditions of ge ne ral crustal sho rte ning t hroughout Upper Devo nian and Lowe r Carbonifero us time , documen ted by repea ted influxes of turbiditic fl ysch in the neighbo ur ing basins. such as the Rhemschcs Schieferge birge. These moveme nts too k place alo ng shear zo nes, producing belt s of mylon itic gneisses in the basemen t crystalline complexes. In Figure B.l7A a sectio n across the Rh e nohcrcynikum ( Rhenisches Schieferge birge ) and nor thern Saxothuringian zo nes of nort hern Ger many is sho wn. The profile illustrates consiste ntly Nw -verging overfolds and reverse faults in the greywacke cove r thro ugho ut the Rhenohercynikum. Th ese structu res are inte rpr ete d as a t hin-sk inned th rust co mplex detaching at a shallow dept h. Webe r estima tes that the basement o f the Mid -German high has trave lled up to 100 km to the northwest. In the narro w No rthern Phyllite zo ne, betwee n the Rhen o hercynik um and the Mid -G erman
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Figurt 8. 17 Schematic slructural pron jes IICIOSS the Variscan be ll of w esre m E urope . (A) NW - SE trave rse across the Rh enohercynik um (R henisches Schiefergebirge) to fhe Saxcthuringian zone . From Weber ( l911S)
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Figure 8.17 Schematic structural profiles across the Variscan belt of Western Europe. (8) W- E traverse from western Galicia to the Cantabr ian mounta ins (N. Spain) ; and N-S traverse ~ c ros s the Massif Centr al (France). From Matte and Burg (1981)
tv
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236
GEOLOGICAL STRUCTU RES AND MOVING PLATES
Crys talline Rise. metamorphic temperat ures reached 400 -4S00C, bu t elsewhe re in t he cove r o f the Rhc nober cynlkum . temperatures were typically in the range 200 - 300"C. T he age of the Nw -directed fold-thru st defor mation is late Devoni an to ea rly Carbo niferous ( BreIonic) and the re is ev ide nce of a northward prog ress ion of t he deformation from about 330 Ma in the so ut h to c.300 Ma in the north.
Weber notes that there is no evidence of the developm ent of ocea nic crus t. o r o f its subd uctio n , in the Ger ma n Va riscides . He explains the o roge ny as an initial phase of int racratonic extension a nd rifting in the Lo wer Pal aeozoic. follo wed by intracon tinental crustal shortening by A-subd uctio n, o r intra-c rusta l slicing. o f the kind sugges ted in the Himalayas (see 5.4). Th is co nve rge nt d eformat ion co ntinued into the early Carbo nifero us ( Bre to nic) . He po ints o ut that th e structures o n the so ut he rn side of the West E uropean Vari scides ve rge sout hwa rds (see e.g, pro file across the Massif Central in Figure 8. 17B). giving the belt as a who le a bila teral str uct ural symmet ry . A N-dip ping subduc tion zo ne o n the south side of
the Variscan be lt . along t he no rthe rn margin o f the prot o -Tethys O cean , is co nside red to be a possible explanati on of the str uctural patt ern (see Figure 8.20). However evidence as to the natu re of t he Variscan st ructure of the southern pa rt of the bel t is difficult to assemble ow ing to the effects of the Alpine oroge ny.
The S W British Isles The western exte nsio n of the R he nohe rcynia n zone of north Germany (zo ne 2 of Figure 8. 13) occ urs in SW England an d SW Ireland ( Figure 8 .16A) . T o the no rt h is the foreland zone (zo ne I) o f Dewey and Bur ke ( 1973) rep resented in S . Wales . T he rocks of zon e 2 sho w on ly lowgrade met amorphism (up to gree nschis t facies) and are cut by a maj o r post -tectonic gra nite pluton of probable Permian age . the Co rnubian ba tholith. The sedi me nta ry seq uence invol ved in the de forma tio n co nsists mai nly of Devon ian to late Ca rbo nifero us flysch , gradi ng laterally no rth wards (in the Devoni an) into shallow-marine shelf dep osits a nd co nt inen tal red-beds . Lo wer Palaeozo ic rock s occur in
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237
PHANE ROZOIC OROGEN IC BELTS: SOME EXAMPLES
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c anticlinal fold cores in S. Wales , a long the margins of the belt. However , there are no large nappes carrying Lowe r Palaeozoic or basement rocks such as those in the Alleghenian. Dewey (1982) and Leede r (1982) attribute the formation of the Uppe r Palaeozoic basin of SW Brita in to back-arc crustal exte nsion, related to a subduction zone through southern France, and est imate a stretc hing facto r P of about 2. The structure of the fold-thrust belt of SW Britain is summarized by Coward and Smallwood (1985) . T hey point out that the belt shows many characteristic fe at ures of a thinskinned foreland thrust belt, but that in many
Figurt 8. 18 The external fold-thrust bel! o f t he SW British Isles. (It ) Tecto nic summary map showing the tre nds of the main folds and thrusts in SW Ireland , S. Wales, the Mend ips, and SW England, with the positions or the profiles M - N. P- Q , and R-S o f Figure 8.\9. After Co ward and Smallwood ( 1985). (8) Summary map or the v ariscan bell of W. Euro pe. B , Brussels; L , London: P, Paris: 8M . Bavarian massif; RM , Rhenish massif. (C) Schema tic diagram showing structures associated with postulated ob lique closure : major th rusts, pinned at one later al tip , suffer some rotational displaceme nt and develop extensional strains along their traces. ( B), (C) from Coward and Smallwood ( 1985)
areas the de tails of the thrust geometry a re obscure d by the absen ce of a well-dated layercake stratigraphy. In Figure 8.18 the main elements of the regional stru cture are shown. There are conside rable changes both alongstrike and from north to south. SW Ireland is dominated by rather upright folds which face south in the south (see balanced section by Cooper et ai., 1984, and Figure 8.19) . In S. Wales and the Mendi ps, good layer-cake stratigraphy defines a series of Nvergi ng overfolds and thrusts, and enables balanced sections to be constructed across the belt (see Williams and Chapman , 1985; Hancock et ai., 1983).
238
GEOLOGICAL STRUCH JRES AND MOVING PLATES
A
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figure 8.19 Simplified 5truet \lu l profiles across the Variscan be lt of (he S W Brit ish Isles. (A) S-N section , long the west Pem brc kesnue coast . Vertica l and honzc ntal scales eq ua l. (8) Syno ptic N- S secnc ns: (a) fro m Pem broke 10 SW Cornwall ( P- Q ); (b ) th e Mend ips 10 S. Devoe ( M -N) ; (t) an inte rpret ation of (a) ; and (d) ac ross SW Irela nd ( R-S). From Coward and Smallwood (1985)
Coward and Smallwood show two sections across the south Pemb ro keshire sector th at illustra te the general nature of the de forma tio n (Figure 8. 19). The northern part of the section crosses an imbricate zone develop ed in Uppe r
Ca rboniferous strata. These structures detach on a shallow sole thrust dipping gently south. This zone is bounded on its southern side by the John ston and Ritec thrusts. which defi ne the northern limit of a zone of major folds in
PHAN EROZOIC OROGENI C BELTS : SO ME EXAMPLES
Devonia n and Lower Ca rboniferous cover. Some of the major anticlines expose Lower Palaeozo ic strata in their co res, and Precambrian basem en t is bro ught up along the Jo hnston thrust. Th e structures verge northwards in the northe rn belt but fan throug h the vertical in the southe rn belt to verge south in the souther n part of the sector. The authors suggest a rather deepe r detachmen t level for the structures in the sout he rn belt. T he folds are considere d to have been fo rmed by buckle-shortening and subseque ntly thrust northwards alo ng the Ritec thrust , which is o ut of sequence. Resto red sections indicate abou t 45% shorte ning in the southe rn belt, and 25% in the northern . The structu res in the imbricate northern be lt are considered to have fo rmed in piggyback manncr , but the majo r Johnston a nd Ritcct thrusts appea r to have climbed up, out of seque nce, from a deeper detachment leve l. The autho rs suggest that this lat er movement has folded and uplifted the higher-level detac hme nt , at the base of the Upper Carbonifero us, above the e rosion surface in the southern belt (Figu re 8.19A) . The roc ks in tbcsouth show intern al strain and cleavage indicating compression across the strike of the bell, and so me exte nsion along it. Th e str ucture of Devon and Co rnwall prescnts a more complex geometrical problem . The Culm synclinorium in the no rth exhibits a fan-like arrangemen t of folds, and is separated by a zo ne of nor mal fa ults from a central zone of napp es that face no rthwards (Figure 8. 198). The southern belt co nsists of Nw-directed thrusts, one of which carries the high-grade metamorphic-igneous complex of the Lizard (see Rattey a nd Sande rson , 1984). Cleavage throughout the area is we ll-developed , showing high strains. A majo r high-strain zone, 25k m wide, marks the Tintagcl decoupling zone, for which a displaceme nt of over 20 km is estimated (Shackleton et al., 1982). This zone is considered to mark the de tachment below the C ulm structures, and to cont inue beneat h the southe rn belt . A ltho ugh the primar y di rection of thrust transport is to the no rth, major back-thrusts
239
are recog nized alo ng the sout h side of the Culm synclinorium, and may explain the steep zone affecti ng the south Devon nappes (Figure 8. 198). Th rou ghout south Devon and Corn wall, the exte nsion lineations indicate a NNW tra nspo rt di rect ion, approximately normal to the thrust outcrops. The back-th rusts, however , are E - W, ob liq ue to the ea rlier structu res. Cowa rd and Smallwood suggest that the ob liq uity reflects diffe rential displacement with a dextral sense. They rela te this o bserva tion to the pattern of struct ures ove r the region as a who le (Figure 8. 188) and obse rve that the slightly arcuat e patte rn of the fold tren d in SW Ire land is no rmal to the NNW transport direction , whe reas the eas tern extension of the front al be lt thro ugh S. Wales and the Mendips exhibits an E - W to WNW- ESE trend , o blique to the inferred transport direction. This prompts the autho rs to suggest a bow-shaped displace ment with the maximum displacement in SW Ireland, becoming less to the cast. This variable displaceme nt mod el could explain the exte nsion pa rallel to the fold axes in S. Wales by rotat io n of the thr ust sheet arou nd its lateral tip (Figure 8. 18C). Th e age of the deform atio n appears to vary from south to north . Uplift of south Devo n and Co rnwall occurred in late Devonian to ea rly Ca rbon ifero us times, from K-Ar datin g of slates . However, stra tigra phic evidence points to mid-Carboni ferous movement s on the majo r thrus t shee ts co ntributing flysch sed iments to the Culm basin. Late Ca rboniferous ages are ind icated for the uplift of the Culm synclino rium and the fold-thrus t be lts of S. Wales and the Me ndips. A n Upper Carbo niferous age is also indicated fo r the back-thrusting in south Devon . Coward and Smallwood estimate the total sho rte ning across the belt to be about 150 km within a tot al time of about 90 Ma, giving a rat her slow aver age rate of 0.4 emf yea r. T he lack of angular uncon formities throughout the main Devonian and Carboniferous outc rops of the region suggests submarine deformat ion, without the developm ent of major uplifted la ndmasses. The presen ce of
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PHA NEROZOIC O ROGENIC ll ELTS: SOM" EXAMPLES
Devonian olistostromes a nd the Culm fl ysch deposits, ho wever , suggest submarine tectonic instability rela ted to the migra ting thrust sheers. The structure of the SW British Isles represents a westward co ntin uatio n, the refore, of the outcr thin-skinned Variscan zone, and shows obvious similarities 10 the Rhe noh ercynian zo ne of north Ge rma ny. Th e transport direction there a nd e lsewhe re in Western Europe is also to the northwest , suggesting a continuity throu ghout the Variscides from the Massif Central and Bohem ian Massif northwards. The bend in the Variscan front from Ireland to Ge rmany is a pparently dete rmined by the limits of the o rigina l basin. T his shape, markedly o blique to the transpo rt direction of the nappes , co upled with the prese nce of the Il-'SS easily defo rma ble Armorican Massif, may be respo nsible , acco rding to Coward and Smallwood, for the later al displaceme nt of the main outer compressio nal zo ne from SW Brilain 10 the Rhen ohe rcynian, and suggests a strongly u ansprcssivc belt between (Figure 8.188 ). An importan t implication of the estimated 150km shor te ning across the British sector is that the o rigina l co ntinental crust of the southe rn British Isles must ex te nd beneath
F"JgU~ 8.20 Plate tectonic evolunon o r the AppalachianHercynian system. (A) Early Dinantian tectonic setting: BBl. Brevard-Blue Ridge zone; CC, Cedar Cree k. uplifl; MCO , Ma s..~ir Central ocean; MV, Midland Valley; D B, Oquirrh basin ; PCD , PreCaspian depression; RM, Roberts Moun tain thru st complex; RSG, Rheniscbes Schie· Iergebirgc graben; SU J' , St Lawrence graben; UO, Urals ocean; YJ, Ymer Island defo rmatio n; coarse stipple. oceanic crust; vertical lines. ddormat io n; arrows, movement dn ccnoo s: black. circle , average Laurasia -Goodwana rcrauon pole . (B) Namurian; AB, Anada rko basin; DL. Dimple limestone ; 11., Idaho lineament ; MO. Mara· then ocean; NR , Nema ha ridge ; RC, Rough C reek fault zone; S8 , Sverdrup basin; crosses. are.. or uplift; ot her ornament as (A ). (C) Westphali an (A sturie phase) : C. coal basins of the He rcynian foreland; PB , Pa radox basin; V , mafic volcanism and sill intrusion ; other symbols as for (A ) and (B) . (D) Early Pcrm ian ~ DB , Delaware basin; MNSH . mid-Nort h Sea high; OG, Os lo graben ; RB , Rotliegendes basin; VV, Val Verde basin; light hachured line . limits of Zc:mslein marine transgression; broad arrow, boreal provenance of Zc:ch.Slcin transgression; ot her symbols as for (A) - (C) . (A) - (D) from Dewey ( 1982)
24\
the A rmorican Massif which there fore should be regard ed as allochtho nous. Plate-tectonic interpretatio n of the A tlar uic H ercy nian region
The evolution of this regio n is d iscussed by Dewey ( 1982) , who points out that the best approach to the prob lem of Uppe r Palaeozoic plate movements is to work backwa rds from the relative certainty of the ea rly Permian contine ntal fi t, using palaeom agneticall y der ived movement vector s (Figure 8.20) . This exercise co nfi rms previous views expressed by A rtha ud and Mall e ( 1977) and o the rs that the West European Variscides are con tro lled by an importan t compone nt of dextral strike-slip motion d uring Upper Ca rbo niferous times. Arthaud and Matte had suggested that the Eu ropean Variscides fo rmed a be lt of complex strike-slip movement joi ning the northern end of the A lleghenian co llision suture with the so uthern end of the Uralides suture. Many of the complexities of the West Europea n belt can be att ributed to the effects of a str ike -slip regime: (i) varying and loca lly stro ngly-curved structural trends; (ii) rapid changes in metamorphic grade across major stee p shea r zo nes; (iii) low-angle thrusts emplacing thin flakes of crustal and 10000IIy upper-ma ntle rocks; and {iv) obducted oph iolites generated in small intraco ntinental basins. Th is tecto nic patt e rn co ntrasts marked ly with the linear Alleghenian be lt. att ributed to relatively straightforward co llision normal to its strike. A minimum shortening of 200 km acro ss the be lt gives a minimum strike-slip d isplacement for the West European belt. Accord ing to Dewey's mod el, in the ea rly Lower Carboniferous (D inantian), closure of a mino r ocea n basin prod uced by late Devonian crusta l stretching (see above) led to collision in the Massif Ce ntral, a nd the southwestwards e mplacement of nappes (Figure 8.20A , and Figure 8. 17). Probably at the sa me time, collisional sho rtening occurred in the Pied mont zone of the Alleghen ian. In mid-Carboniferous times. the Sudet ic
242
GEOLOG ICAL STII.UCTU RES AND MOVING I'LA1"ES
ph ase in E urope is a ttrib uted (0 co ntinued Intrac rustal shortening of the flysch basins genera ted no rth of the collisional uplifts of the previous phase ( Figure 8.208). Nort hwa rd migration of thrust stack ing led to flexural crustal depressions o n the margins of the forel and . giving rise to the coa l basins of S. Wales and the Ruhr . Similar effects of the progressive westwa rds migration of the thrusting a re see n in the Allcghenian bell. The main collisiona l deformation and uplift appears to have been comp leted in Westphalian limes in Weste rn E urope, during t he Astu dc phase (Figure 8.20C) In late Ca rboniferous time. and con tinuing into the Perm ian, changes in plate vectors re lat ed 10 the collision arc expressed in th e fo rmati o n of rifts in the No rth At lantic. between G ree nland and Norway . and in the Os lo graben . Mea nwhile. continued collision took place across the Central Appalachians until well into the Permian period (Figure 8.20D) . 8.4 T he Caledonia n orogenic: bell of the North Atla ntic reg;on R~;onal setting
The Ca ledo nides of the British Isles, togeth er with thei r extension no rthwards into Scandi navia and G reen land . and southwards into Nova Scotia and New Brunswick , are probably, after the A lps. the most inte nsively studied and best known of the Phanerozoic o roge nic be lts. Much of the early work on str uct ural and metamorphic geology too k place in the Scott ish Highlands, which has experie nce d pe riod ic invasion s by geo logists. in order to test out new structural or tectonic ideas, since the early mapping was completed . The exte nt of the belt before the opening of the Atla ntic is shown in figure 8.21. Il occupies a coasta l belt in East G reenland and Western Scandinavia, extending northwards to include Spitzbergen and Franz Josef Land in the A rctic Ocean. SOuthward, the belt embraces most of the Brit ish Isles, and extend s to Newfoundland and the Northern Ap palachians
in North Amer ica. To the eas t and southeast. the belt is tru ncated by the younger Variscan belt. Ziegle r ( 1985) summarizes the available evidence from Western and Central Europe . which indicates that a co mplex system of late Caledonian fold bells occupied much of this region. The eas tern margin of the be ll in southe rn Norway crosses beneat h younger cover through Denmark and Poland. The southweste rn branch of the Caledo nian belt is usually known as the Acadian belt . and is cha racterized by a rather later o rogen ic climax (mid- to late Devon ian). Th e fo rma tion of the main Caledo nian belt was completed by the ea rly De vonian . The prese nt width of the main Caledo nian belt, afte r removing the interven ing oceanic crust, is abo ut 1000 km , bu t this ove restimates the Devonian width by probably about 300k m becau se of ea rly Mesozo ic cru stal exte nsion along the Atlantic margins. Th e foundat ion s of the be lt are to be see n in the Precambrian shield regions of No rth Ame rica- Green land to the west, and Fennoscandia to the east. These regio ns are mainly composed of large Archaean and Early Proterozoic crat ons. hut are also crossed by linear Mid- and Late Proterozoic orogenic belts that form an important component of the Ca ledo nian basement. The ea rlier of the se belts is the G renvilleSveconorwegian bell, which ceased to be active about tOOOMa ago. The Grenville sector lies, parallel 10 the Acadian secto r of the Caledonides, along the southeaste rn side of the Canedian shield, and crosses the British Isles to join the Sveconorwegian branch in sou thern Norway a nd southwest Swede n, where it forms a N-S bell along the western side of the f ennoscandian shield. Most of Scotla nd appea rs to be fo rmed from a basement of G renville age, reworked during the Caledonian o roge ny. The later of the Precambrian belts that in8uenced the Caledonian basement is the Tim anides of northern Russia, which form the northeaste rn bou ndary to the fennosca ndian shield, and has a possible counterpa rt in northeast G ree nland. Th is belt remained active until the mid-Cam brian. A belt of similar age in south-
PH .... N EROZ.OIC O KOGEN IC Bf. LTS : SOM I:::
ex.. . MP Lf. S
243
• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •••• • • • • •
?
• • • • • • • • • • • • •
TIME OF CONSOLIDATION
l%~ g Mod ·Comb
ABBREVIATIONS
•
L8M c London- BrabanT M.
• • •
N.A.
Narlh Atmari<:an M.
• • • • • • • • • ••••••• • • • • • • • • • •
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.
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Fll:urt 8. 2\ Locauon and e ~ te nt or the Caledonian orogenie belt o r the North Atlanti c regio n prior to Mesozoic opening of lhe Atlantic ocean . showing its relat ionship 10 the older Precambrian shields and to ot he r Palaeozoic orogenic belts. From Ziegler ( 1985)
ern Brit ain and Western Eu rope is known there as the Cadomi an. Rocks affected by the Cadomian o roge ny form a num be r of isolated cratonic areas within th e younger orogenic belts in th e so ut h of the region. Of these . the Londo n-Brabant massif and the Ava lon platform are importan t in defining the so utheaste rn margin of Cale do nian tect on ic activity
in E ngland a nd Newfound lan d respectively. Th e re lat ive posi tio ns of eac h side o f the Caledonian be lt have cha nged conside ra bly du e to large sinist ra l strike-slip move ments (c. IBOO km according to Dewey a nd Shacklet on , 1986) of mid- Devonia n age . Wh en this movem ent is restor ed , we find that no rthern Newfou ndlan d is co ntiguous to S. Britain , the
244
GEOLOGICAL STRUCTURES AND MO VING PLATES
G reenland craton faces t he Scandinav ian Ca ledonides, and the East Greenland Ca ledonide s face the A rctic Ocean. We shall now discuss the British Isles a nd Scandinavia n sectors of the belt in some de tail , refe rr ing brie fly to (he A ppalach ian secto r as well. Tectonic subdivision of the British Isles T he pr incipal tectonic units of the Caledonides o f Br itain arc shown in Figure 8.22. T hey a re : ( 1) the N W forel and; (2) the Northern H ighlands ; (3) the G ramp ian Highlands ; (4) the Midland Valley ; (5) t he Southern Uplands ; (6)
f
Nort h-W est Fo reland
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Flgurc 8.22 Location of the main tectonic zones of the Caledonia n orogenic bell in the British Isles.
t he Lake Dtstria, (7) the lrish Sea block ; (8 ) the Welsh basin; and (9) the Midlands platform (t he British part of the London- Brabant massif of Figure 8.21). Most of these zo nes, no rt h of the Welsh basin , ca n be traced into Ireland . A fundamenta l distinction has bee n recognized for many years between the northern ' meta mor phic' Ca ledon ides ( Read , 1961) and the southe rn Caledon ides, which ex hibit at most the lowest metamorphic grades and arc charac te rized by slates. Following the original suggestion of Wilson ( 1966) th at a suture t hrough the middle of the British Ca ledo nides represe nted the line of closure of the ' protoAtla ntic' Ocean , Dewey ( 1969, 1971) prese nted the fi rst plate tectonic model for the Ca ledonides of the regio n. Dewey (1971) highlighted the significance o f Read's subd ivision , and termed the northern part (zo nes 2-3) the orthoteaonic bell , interpreted as part of a No rt h American plate , and the sout hern part (zo nes 4 - 8) the paratectonic bell, most o f which was interpreted as part of a so uthern , Europ ean , plate . The suture is now universally regarded as lying between zones 5 a nd 6, through the Solway Firth . Since t he pionee ring work o f Dewe y, many plate tecton ic mod els have been proposed , and the British Isles secto r of the Caledo nian belt is no w regarded as a co llege of terran es, all displaced to a grea ter or lesser exten t from the ir o rigina l positions relative to the North A merica n cra to n. Major strike-slip bo undaries separate zo nes 2 and 3 (t he G reat Glen Fault ), zones 3 and 4 (the Highland Boun dary Fault) and zones 4 and 5{the South ern Upland Fault). Strike-Slip d isplaceme nts are also recognized betwe en zones 6 and 7, a nd zones 7 and 8, and proba bly ex ist along the Solway suture as well. Thus all six intern al zo nes are suspect terranes (see 6.2) . Much of the eviden ce required in any analysis of the tecton ic pattern and history o f this region co mes from the study o f the base men t in t he differe nt zones (Watson and D unning, 1979) . Dewey poin ted o ut the majo r d ifference betwee n the gneissose basement of the or thotecto nic Caledo nides (i.e . the northe rn plate)
PHANEROZOIC OROGENIC BELTS : SOME EXAMPLES
and the low-grade rocks cut by acid plutonic complexes that formed the basement of the southern plate. These differences correspond to importa nt contrasts in geophysical characteristics of the deeper crust. The deep seismic U SPB profile (Bamford et al .. 1977) indicates a layer with high seismic velocities at depths of 6-12 km below zones 2- 4. This layer is absent south of the Solway Firth (Figure 8.26A ). Studies of crustal heat production by Richardson and Oxburgh (1978) suggest that much of the upper contine ntal crust of E ngland and Wales consists of low-grade metamorphic rocks with acid plutons. There are important differences also in the age of the basement in each case. The last major oroge ny to affect the southern basement is the Cadomian, yielding dates ranging from c.800 Ma downwards. In the north , the basement is either Lewisian (c.1700Ma IlP) or Gre nvillian (c.l000 Ma). These differences in the nature of the base ment are compleme nted by differe nces in the age of the cover. The Caledonian orogeny is, by genera l agreement, post-G renvillian in age, and in the north involves sedime ntary cover at least as old as 800Ma (the Torridon Gro up and, possibly, the Grampian Group). In the south, the oldest cove r rocks to be affected are usually Cambrian, or very late Precambrian, in age. We shall describe the general stra tigraphic and structural features of each of the zones in turn , before discussing their possible plate tectonic context.
245
to the 'true' foreland of North Ame ricaGree nland. However, on either side of the Outer Hebrides fa ult zone, the rocks of zone 1 are virtually unaffected by Caledonian deforma tion except near the ma rgin of the Moine thrust zone, where the cover is tilted ge ntly towards the southeast. The baseme nt consists of the Lewisian complex (see 9_5), which comprises late Archaea n crust formed about 2900Ma BP, and reworked during the Laxfordian orogeny about 1700 Ma SP . The rocks consist predominantly of granulite- or upper amphibolite-facies gneisses. The Lewisian basement is overlain by three distinct units of unmeta morphosed pre-Caledonian sedimentary cover, the Stoer Gro up (c. 1000 Ma old), the Sleat and Torridon Gro ups (c.800 Ma old) and the Ca mbro-O rdovician sequence. The Stoer , Slea t and Tor ridon Groups are largely continental, flu viatile redbeds, whereas the Cambro-Ordovician is a thin marine shelf sequence consisting mainly of or thoqua rrzites and carbonates. This forela nd zone is involved in the major Moine thrust belt, in which a number of distinct nappes or thrust sheets can be recognized , usually with complex intern al deformation . However , the typical zone 1 stratigraphy can be recognized throughout this thrust belt except in the uppermost thrust shee t, the Moine nappe . The Moine thrust, which underlies the Moine nappe, marks the western boundary of the Northern Highland zone (zone 2), whose stratigraphic and structural characteristics are quite distinct from those of zone 1.
The NW fo reland [z one I )
This region , often termed the 'Hebridean craton' , is neither a true foreland nor a true craton. It is transected by a major low-angle fault zone (the Outer Hebrides fault) which may have acted as a Ca ledonian thrust. and another major low-angle structure , the Flannan fault zone, is identified by deep seismic refraction at depth, and projects to the surface on the continental shelf west of the Outer Hebrides. It is likely therefore that all the nonhero zones are allochth onous with respect
Structure Of the Moille thrust bell
The structure of this classical forela nd thrust belt is summarized by Elliott a nd Johnson (1980) a nd McClay and Coward (1981). The first mapping and compre hensive description of the belt were carried out by Peach et al, (1907) of the UK Geo logical Survey, and a considerable amount of detailed research has been directed at the belt subsequently. The belt extends along strike for over 190km (Figure 8.23) and is up to 11km in width. A
246
GEOLOGICA L STRUCTU Rl;.S A N D MOVIl" G PI.ATES
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Figuft 8.23 Struc ture of the Moine thrust zo ne of NW Scotland . ( A ) Locat ion map (inse t) and diagrammatic sections across the northern end of the be ll at loch Eriboll. showing slages ( 1-4) in its evolution. Profile (5) shows a late r stage (If the central part o f (4) . Note the propa gation of the thrust ing ncrt bwestwercs, in general, (i.e . piggyback ) but with some: oer-o l-sequeece move ments along the Arnabollr hrusts. MT. Moine: thrust; OHT, Outer Hebrides thrust (fa ult) ; ST, sole Ihrust ; VAT, upper Amabcllthrusr: SBS, Sgurr Beag slide; GGF, Grear Glen Iault; M , Moine complex; L, Lewisi an com plex. Afle r McCay and Coward ( 1981). (B ) Structural profile across the Kinlochewe are a, towards the sout hern end of the Moine thrusl zone , illustraling out-of-sequence movemenl of the Kinlochewe thrust (k) which cuts dow nward s aeross lower rhrc sts, and is interp reted as extensional, due probably 10 grayilalional gliding. Fro m McCay and Coward ( 1981)
PHANEROZOIC OROGENIC BELTS : SOMi: EXAMPLES
series of nappes o r thr ust sheets are recognized , resting o n the basal or sole thrust. The nappes appea r to be lensoid in character. wedging out laterally to be rep laced by o ther nap pes along strike. The uppermost nappe. termed the Moine napp e, resting on the Moine thrust, mar ks the western limit of the Moine Complex a nd is discussed belo w. The nappes are divided into a lower se ries of imbricate shee ts that are esse ntially thru st duplexes of stacked Ca mbro-O rdovician cover. around 300 III thick. Above the imbricate nappes are several large nappes carrying Lcwisian basement toget her with Ca mbro-O rdovician cover. South of Vllapoo l, T orri donian cover also appears in the higher nappes. Th ese upper nappes arc up to 500 m thick and usually deformed inte rna lly by minor thrusts and overfold s. T he main nappes, fro m north to south, are the A rna boll, G lencoul, Ben More. Kinlochewe-Kishorn a nd Tarskavaig nappes. The nappe zone is widest at Assynt, where the Glencoul and overlying Ben MOre nappes result in a n an tiformal bulge of the o verlying Moine thrust, a nd in so utheas t Skye, where the thick Tarskavaig nappe o verlies the Kishorn nappe. Elsewhe re , for example between UlIapool and Klnlochcwe, large nappes are absent at the surface o utcrop. In Figure 8.23A a series of diagrammatic sections across the thrust belt at Loch Eriboll in the north is shown. Th e thrust seq uence here co nsists of lowe r and upper imbricate duplexes carrying o nly Cambro-O rdovician cover, ove rlain by the Arnaboll nappe . Th is nappe is a dou ble duplex structure with internalth rust Slacking of rather thicker slices of Lewisian baseme nt and Ca mbrian cover. The Moine thrust for ms the roof thrust to this complex duple x. The tra nsport direct ion of the thru st sheets is towards WNW and the thru sts appear to have propagated from eas t 10 west . since the uppe r thrusts are defo rmed by the lower. However. in some parts of the belt the higher nappes appear to have unde rgone later re activation (in some cases involving local extension) that resulted in the ir cutt ing aero ss structures in the
247
underlying imbricate nappes. Th e Kinlochewe nappe , for example, locally cuts down the stratigraphic sequence in the underlying nap pe (Figure 8.23B). McClay and Coward suggest that late re-activation took place on some of the uppe r thrusts due to gra vitatio nal sliding. If the Ca rn bro-O rodovician strata are returned to their o riginal hor izontal o rientatio n, the Kinlochewe thrust has an ap preciable dip to wards the fo reland. It is possible that several of these higher roof thrusts, deform ed into hangingwa u amifo rms by mo vements o n underlying thrusts. may have become gravitationally unstable and slid towards the forelan d , cutting down Ihrough the underlying nappes. It is also possible tha t ren ewed movem ents on thrusts at a deeper level caused o ut-ofseq uence movement s near the surface . Displacements on the lower thrusts, cstimated by section balancing. vary from 3.5 10 30 km; however. movemen t on the Moine thrust is much grea te r: a minimum displacement of 40 km has been estimated by various aut hors but the actual displacement may be nearer 100 km (E lliott a nd Johnson , 1980). The da te of the movem ents has been estabfish ed at 430 Ma DP (mid-Silurian ) from the age of intrusive igneous rocks e mplaced d uring the thru st seq uence (Va n Breeme n et at.• 1979). How this relates to the movem ent s in o ther majo r thrusts to the south, and to the collisio nal o rogeny as a whole , will be discussed below.
The Northern Highlands terrane (zone 2) The Northern Highlands exte nds from the Moine thrust in the west to the G rea t G len faull (Figure 8.24) and varies from 30 to 50 km in widt h. The Caledo nian part of this te rrane consists of the Moine Co mple x, toge the r with its interleaved Lewisian basement. The psammites an d petites of the Moine succession represent a thick seque nce of fluviatile o r deltaic sediments that rest unconform ably on the Lewisian basemen t. The Moine Co mplex and its Lewisian basement were highly deformed and metamorphosed in the Gre nville
...'
248
G I:::OLOGlC ..... L ST RUCtU RES AND MO VI N G PLA T ES
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Flgurt 8.2..4 O utline tectonic map o f the foreland and Nonbe m Highlands zones of the Caledonides in north Scotland . Note the two major nappes, Moine and Sgun Beag. making up Ihe Northern Highlands. From wtectesrer ( 1985)
PHANf.RO ZOIC OROGE NIC BELTS: SOME EXAMPLES
o rogeny at c.1oooMa BP, and were late r reworked in the Caledo nian orogeny. The o riginal margin of the Grenvi lle belt probably lay a sho rt distance east of the present Moine thrust o utcrop , and paral lel to it, but has bee n tra nsported westwards abo ve the Moine thrust , so tha t the margins of Grenvill ian and Caledo nian defor mation are no w almost coinc ident. Caledo nian effects within the Moine complex arc difficult to distinguish from the earlier Grenv illian. Pervasive d uctile defor mation has appreciably a ffected the whole terra in, but effects are most intense along several majo r ductile shear zones or slides. The Moine thrust itself is the most westerl y of these. It is a dee pleve l structure, as shown by the development of a wide zone of mylonite (over 600 m thick in places), that possesses a well-developed elongation fab ric in the direction of thrust transport. T he mylonite shee t is cut by later , brittle , highle vel thrusts that have transported the deeper mater ial to the surface. It is considered that the high-leve l brittle thru sts pass downwards into mylonites and ultimate ly to broad ductile shear zones , with increasing depth. T he supe rimpositio n of Moine comp lex ' abo ve Lewisian ap pears to disobey the usual rule that thrusts place olde r rocks above younger. T he expla nation see ms to be that the Lewisian-Moine unconformi ty had been highly deformed in the G renville o rogen y prior to the thrust moveme nts. The G lenelg a rea (Ba rber and May, 1976) provides a link between the complicated structure of the Moine Complex and the rather simple structure of the thru st belt. Immed iatel y cas t of the Moine thrust in this area , the Lewisian baseme nt and Moine cover are interfolded . Howe ver, the degree of de formation in the Lewisian in the western and easte rn parts of the area shows a marked cont rast. In the Lewisian of the weste rn outcrop , the deform ation is similar to that see n in the basement nappes of the thrust zone to the west. In the easte rn part of the area, both Lewisian and Moine rocks are highly deformed. Three phases of deformation ca n be distinguished,
249
associated with high-grade met amo rphism of Grenville age, prio r to the development of the mylonite fabric, which is the first Caledo nian str ucture see n here , and is affected in turn by two furt her phases of Caledonian deformation. These latter three phases are the only deforma tions see n in the cove r west of the Moine thrust. Furthe r to the ea st, the situation becomes even more co mplicated. The grade of Catcdo nian me tamo rph ism increases to amphibolite facies. and complex interfe rence patterns are formed o n both minor and major scales by the superimpositio n of duct ile defo rmatio ns of Grenville and Caledo nian age. In the regio n between Strathcono n and Gle n Affric, To bisch et al. (1970) summarize the results of detailed structura l mapping ove r a region, 450 km2 in ext ent, of inte rfo lded Lewisian and Moinian rocks. The authors recognize seven distinct sets of folds , of which the last four are Caledo nian and the first three probabl y Grenvillian. The second Caledonian phase (termed the Monar folds by the authors) produced the typical 'Caledonoid' NE-SW folds , with gene rally stee p axia l planes, found over wide areas of the Scott ish Highlands. Th e Caledo nian folds affect a majo r shear zone, termed the Sgurr Beag slide (Figure 8.24; Tanner , 1970; Rathbone a al., 1983; Kelley and Powell, 1985) which is a deeper-level counterpart to the Moine thru st zone. Th is slide exte nds fo r the whole length of the Northe rn Highlands terrane (it is known as the Naver slide in Sutherland) and divides the western part of the Moine Complex (the Morar Division ) from the cent ral part (the G lenfinnan Division). The slide consists of a zone of highly defo rmed rocks in which the strain increases ove r a dista nce of abo ut 500 m to a maximum alo ng a nar ro w zone that usually corresponds to a Moine - Lewisian j unctio n. Linea r shape fabrics in the slide zo ne indicate a WNW transport direction similar to that of the Moine thrust. Although now usually stee ply-dipping, the slide is co nsidered to be an originally lowangle displacement plane, eq uivalent to the Moine thrust but at a mid-crustal level.
250
GEO LOG ICAL STRUCTURES AND MOV ING PLATes
Winchester (1985) , by matching amph ibolites on e ithe r side of the slide . estimates a westwards displacement of more than 140 km . Kell ey and Powell ( 1985) co nclude that it m ust
have formed at de pths grea ter than 15 km at around 450 Ma S f . duri ng t he peri od of Caledo nian me tamorphism. T hey suggest that a
period of c.25 Ma may have elapsed betwee n the d uctile movements on this slide and th e mo vem ents alo ng the Moine thru st zone . T he la rge disp lace me nts in both the Mo ine an d Sgurr Beag structures , co upled with th e lac k of a ny deep c rustal roc ks brought up along t he m, a nd wit h the ge ne ral unifo rmity of grade ac ross the nappes, suggested to Co ward ( 1980) a nd Rathbone a ul. (1983) thai the d isplaceme nts in itia lly had a ve ry low angle of inclinat io n, pe rha ps follo wing major horizo ntal zone s of weak ness within the midd le crust fo r lar ge distan ces (see Figure 8.268) . O n t he othe r ha nd , So pe r a nd Ba rber ( 1982) a rgue in favo ur of a ' thic k-skinned ' model for the No rt he rn H igh lan ds (Figure 8.25) , whe re bot h the Mo ine thrust a nd t he up pe r slides steepe n a t de pth then become l ist ric a nd det ach a t the Moh o . This model is simila r to the one suggeste d for the H imalayas (see Figure 5.26). A co m pa rison with the deep seis mic reflection profile (Fi gu re 8.268) s ugges ts that the thr usts in fact detach at a level about 8 km above the Mo ho . Sope r and Barbe r att rib ute most of t he dis place me nt on the Moine thrust to an early ph ase of defo rmat ion (D I-D2) prior to the peak of Ca ledonian me tamorphism at c.450 Ma BP, a nd o nly a bo ut 30 km to the lat e pos tmetamorphic ph ase at c.420 Ma seen in the thrust be lt. A n int e resting geomet rica l result of this mod el is the requ iremen t fo r a n exot ic ' roo f nap pe ' , now co mplete ly re moved by e ros io n, to provide the roof of the c rustal d uplex structure a nd to bu ry the Moine Comple x to a suitable depth during the pe riod of Ca ledonia n me tam orphism. The a uthors po int out that only crusta l-sca le overt hrusti ng produces the degree of cr ustal th icke ning required in majo r orogenic belts. T he sigmo idal nat ure of the thrust/shea r zo ne displace me nt surfaces is a t-
Iribu ted to deep-crusta l ductile sho rte ning. a nd uppe r-c rusta l grav ity sp rea d ing , act ing o n a plane with a n initially e ven dip o f around 3QO. Coward et ai , ( 1983), in a discussio n of Soper a nd Ba rbe r's model, poin t out that the measured displacements on the thin nappes of the thrust zone require a low-a ngle d isplace men t e xte nd ing a t least 60 km bac k from the present Moine thrust outcrop. Thus the majo r ram p must be at least 60 km be hind the thrust fro nt . Soper a nd Bar be r, following Mitc hell (1978), a tt ribute t he Ca ledo nian o roge ny of the No rt hern Highla nds to the effects of a co llisio n , duri ng the pe rio d 470 - 450 Ma DP, o f the North A me rica n con tine nta l margin with a subduction zone an d isla nd a rc syste m lying to t he so ut h. The pos tula ted up per exotic nap pe could be a n obducted ophiolite nappe derived fro m the subd uction zone as a result of this collisio n. Th e la te r ( D4) events at the thru st fro nt (a t 420 Ma ee) are a n rib ute d to t he final continenta l co llisio n along the Solway s uture . The co nce pt of a missing high-le vel o phiolite na ppe is also a feat ure of t he Dewe y an d Shackleton (1984) mod el fo r the pla te tecto nic evolut ion of the who le Caledonia n be lt (sec be low) . T he y po int o ut that the U nst ophioli te complex in She tland ma y be par t of such a nappe a nd tha t ophio lite nappe s a re we ll known in the Appalachian be lt.
The G rampian Highlands terrane (z one 3) T his te rrane (50-75 km in widt h) is se pa rat ed fro m the Northe rn Highlands terra ne by the G rea t G len Fau lt , which has lon g bee n recognized to have a maj or sinis tra l st rike- slip displace me nt, o rigina lly es tima ted a t IOQ km (Kennedy, 1946) a nd more recentl y at 160km ( Winc heste r, 1973) o n the basis of th e appa re nt displacem e nt of me ta morp hic zones. A much smaller Mesozo ic de xtral disp lace ment of 30 km has also bee n recognized ( Hol ga te , 1969 ). Sinist ral displace me nt of the North American co ntine nt in re lation to Euro pe , based on palaeom agne tic da ta , is m uch large r, appro xima te ly 2000 km ( Van der -voo a nd Scctese, 1981) . Much of thi s displace me nt ,
25 1
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however , may be take n up a long other major strike-slip faults. and possibly along a hidden fault near the contine ntal margin. Consequently, the Grampian Highlands ter rane is displaced in relat ion 10 the Nort hern Highlands and, althoug h similarities in strati-
graphy and structure e xist betwee n them, it is difficult to make direct connections. Reconstru ctions involving profiles across the Scottish Highlands as a whole should be lh c refo~ be viewed with caution. With the Winchester fit, at the DI-D2 period, ther e is an overlap of
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Fip ~ S.U (A ) Mndel o f (he crustal structure of nonhero Britain based on the u s ee deep-seismic rd leelion survey profile. Seismic P·wave velocities of (he different lajers are shown in (he ke~ . From Bamford ~t Ill . ( 1978). (8) Possible interpretation of the MOIST de ep-seismic reflect io n pro file across nort bem Scotla nd . Reflecto rs indicated b y solid lines ; faults (dashed) inferred ; D. Devo nian ; PI , Permo-Triassic. Th e depth conversion exaggerates the dep ths of sedimentary basins by a IitCIO, o f 1.5- 2. M T( B) is the preferred position of the Moille thros!. from Iwo possible solutions given ill the source reference. Note that the major faults an: extenssoeat, hut are inferred to be reactivated Caledo nian thrusts. From Brewer and Smythe (1984)
PH A N ERO Z OIC OROGEN IC BELTS : SOME EXA M PL ES
only around 50 km of presently-exposed ground across the fault. Th e majo r difference be tween the Gr ampian and No rthe rn Highlands lies in the nature of the pre-Ca ledon ian cove r. The Gra mpian Highlands is made up of an extremely thick seq uence of late Precambrian to Ca mbrian sedime nts formin g the G ramp ian Group and the overlying Dalradia n Supergroup. The Gra mpian G roup , often te rmed the 'yo ung Moines' consists of a very similar sedimentary association to the Pre-G renville Moine rocks of the North ern Highla nds. A n inlier of rocks corre lated with these older Moine rocks has been ide ntifi ed in the nort heastern G rampian Highlands by Piasecki and Van Breemen (1979) who gave them the name 'Central Highland Division' . Afte r restor ing the postulated movem ent o n the Great Glen fault, these rocks lie alo ng-strike from the Gle nfinnan Division of the Moine co mplex. T hey are separated by a major slide , the G rampian slide, from the G ram pian- Dalradia n cover sequence. The main moveme nt o n this slide is dated at c.750 M a ee in an event widely known as the Morar ian (La mbert , 19(9) . Since this event affected both basement and cover, the deposition of the G rampian Group must predate 750 Ma ae. No majo r unconformities have been ident ified in the thick G rampian-Dalradian seque nce, which implies that movements on the slide were tak ing place at dept h du ring co ntinued sedime nta tion at the surface. Th is in turn suggests that the G rampian slide , in commo n with seve ral of the oth er major displacement zo nes in the Scottish Highlands, originated as an early extensio nal fault and was later reactivated in the Caledon ian orogeny as a co mpressional thrust (d. Sope r and Barber . 1982).
Lewisian basem ent appears at the southwestern end of the G rampian Highlands terrane, in lslay, where it is overlain by Da lradian rocks above the Loch Skerrols thrust, tentatively co rre lated with the Moine thrust. Plant et al, (1984) suggest that the Moine co mplex (G renvillian) basement ends along the NW- SE Cr uacha n line in the south-
253
west of the terrane , allowing Dalradia n sediments to rest directly on Lcwisian baseme nt. Several lines of evidence indicate that the nature of the baseme nt of the G rampian Highlands is the same as that of the Northern Highlands. The L1SPB seismic profil e (F igure R.26A) shows that the IWO lower layers recognized by their different seismic velocities can he traced across the Highland Boundary Faull. An analysis of inherit ed zirco ns fro m Caledonian granites in bo th terranes shows a similar pattern, suggesting a de rivatio n by partial melting of mid-Prot erozoic crust in the age range 1800-1000 Ma. T he G rampian basement is inferred therefo re to be co mposed of Lewisian crust modified and added 10 du ring the Grenville orogeny . A regio nal geo che mical ana lysis of the Caledo nian rocks of zones 1- 3 (Planl et 01. , 1984) brings out a majo r geochemical discontinuity within the G ramp ian te rrane , following the Moine -Dalradian boundary betwee n the G rampian Group and Dalradia n Supergrou p. The To rrodinian , Moine, and G rampian G roup sedime nts indicate de riva tio n from a commo n source domi nat ed by interme diate to acid calc-alkaline rocks of Archaean and Proterozoic age, resembling the present Ketilidian belt of S. G reen land. T he geochemistry of the Da lradia n sedime nts in contrast, point s to significant co ntributio ns from basic to ultrabasic vulcan icity a nd from Ba , Pb and Zn mineralization . T hese geoche mical features confirm previous inter pret ations (e.g. Harr is and Pitcher, 1975) suggesting that the Dalradia n successsion accumulated in a mari ne, intracontine ntal basin formed by stretching and rifting of the conti nental crust. The original margin of the basin, where much thicker Da lradian seq uences formed o n thinned Moine crust, probably ra n o bliquely across the present G rampian terrane in such a way that the main G rampian division o utcrop represents the edge of the old continent , and the Dalradian o utcrop of the SW Highlands and Highland borders represents the basin. The structure of the Grampian Highlands has occasio ned co nside rable debate over the
254
G EO LOGICAl- STKUCTUII:fS AN D MOVING PLATES
yea rs. Th e o riginal interpretation in terms of a se ries of large nappes was put forward by Bailey (1922). This model has been considera bly modi fie d bUI, in its fundame nta l aspe cts , still re mains the basis for modern inte rpretation s. A re gio nal synthesis o f the structure in the SW Highlands is presen ted by Roberts and Tr eagus ( 1977). The majo r nappes in this region (Figure 8.27A) are: (i) the Appin nappe, rest ing o n the Fo rt William slide , which m arks th e co ntact betwee n th e G ra mpian Di vision and the ove rlying D alradian; and (ii) th e Ba/lachu/ish -Tay fUlppe which rests on
the Ballach ulish slide . Th e latt er slide passes southwa rds into the Iltay bounda ry slide that underlies the Tay nap pe to the south. In the northeastern part of the G rampian Highlands terrane , these nappe s are overlain by the Banff nappe which includes a sheet of gncissose basement dated at c.700 Ma Sf (Stua rt et aJ., 1977). Th is nap pe is regarded as allochthon ous, and possibly de rived fro m a southern continental block of Cado mian age. An interesting aspect of the major structure of the region is that the nappes in the no rthwest face to the northwest, where as those in the south-
A
~
majO!" slides
....
fold axial planes lautt lacing d irection
/--.
.i Highland / / boundary / teen
B
'"
02
Figure 8.27 Struct ure o f the G rampian Highlands. (A ) Interp retative model showing the principal majo r structu res of the G rampian Highlands. 01 structures : AS. Appin syncline; KA , Kinlochlevcn anticline. 02 structures: 5 85. Sgurr Beag synfo rm; BLA , Batlachulish antiform; DO. Drurnocbter dome ; SMS, Stob Mhor synform. 0 3 structure; BA. Bohespic antiform. Arte r Thomas (1979). (B) Interpre tative profiles illustrating a possible two-stage model (0 1, D2) for the tectonic evolution of the G rampian Highlands. 1, Abe rfoyle-Ardrishaig anticline; 2, Batlachulish-c Loch Awe syncline; 3, Kinlochleven - Islay anticline; 4, Appin syncline; I, Ben l ui-Stob Bhan fold ; II , Kinlochleven- Ben Chulm fold; squaretoo thed lines. major slides. O rnament indicates different subgroups: from boncm to top : Gra mpian Grou p (blank), and the Locheber , (slay, Easdale , Crinan subgroups, and the Southern Highland G roup (oute rmost). From Rcoe rts and Tre agus (1m )
PHA1'IEROZO IC OKOG l:1'IlC l:lI, LTS : SOME EXAMPLES
east face southeastwards (Figure 8.27A). Th is gave rise to the 'fo untain of nappes' hypothesis that visualized the Dalradian cover being squeezed laterally by converging basement hlocks on either side and escaping sideways by gravitational flow. Robe rts and Treagus sho wed that three deforma tion phases controlled the major structure, and that the D I phase was associated with major isoclinal folds and with a penetrative fi ne-grained fabric. Several major recumbent folds , forme rly thought to be 01 , were shown to be of 0 2 age. T hese folds developed during the metamorphic climax. The later upright 0 3 folds refold the earlie r recumbent structures into locally steep attitudes . T heir model to explain this structure (Figure 8.278 ) attributes the or iginally upright 01 folds to the lateral squeezing effect, and the recumbent 0 2 folds to the consequential gravitatio nal spreading, followed in 0 3 by a further horizontal contraction. The major slides appear to have been initiated in 01 times but have clearly also been re-activated in D2. Harris a al, (1976) suggest that the southeastwards transport of the large Tay nappe was largely accomplished during the 0 2 deformation, when about 6 km of horizontal transport took place. Both the Fort William and Ballachulish slides emplace younger strata on olde r, a nd arc lags rather than thrusts. Like the Gr ampian slide to the north , these slides may be primarily extensional in origin. The age of the main 0 1-02 deformation overlaps the main Caledonian metamorphism in this region. This is the "type area for the 'G rampia n orogeny' (La mbe rt and McKerrow, 1976) which is dated at c.490 Ma BP (Arenig) by a va rie ty of methods. Note that there is a progressive change in the apparent age of initial movement on the main slide zones from 490 Ma in this region to 450 Ma at the Sgurr Beag slide , and 420 Ma in the Moine thrust zone. An alternat ive to the 'fountain-of-nappes' hypothesis is provided by Dewey and Shackleton (I 984), who believe that all the early (0 1-02) structures were NW-facing and that
255
there is no 'root zone' within the present boundary of the terra ne (Figure 8.34). They attribute the NW-directed nappe pile to an obduction oro geny in which the highest nappe is the lost ophiolite nappe of Soper and Barbe r (1982). T he obduction is thought to have taken place by detachment at a ridge-fracture ZOne inter action (Figure 8.34C) producing an overriding young ocea nic plate that progressively abd ucts on 10 continental shelf and ultimately over the continent itself. Th is major nappe is represented in the Belts' Cove and Bay of Islands oph iolite complexes in Newfoundland , at Trondhjeim in central Norway, and steep slices are prese rved along the faulted southern boundary of the terr ane , for example along the Highland Boundary fault. Later SE-facing structures are att ributed to 'retrocharriage' or back-folding, resulting from northward subduction commencing a t c.460 Ma BP. The latt er process resulted in uplift of the Scottish Highlands during the period 460-440 Ma BP . The Southern Uplands accretionary prism, or its lateral equivale nt , is thoug ht 10 represent the southern margin of this subduction zone. The Midland ValLey (z one 4)
Major strike-slip displacements have occurred along both the Highland Boundary and Southe rn Uplands faults that form the north-western and south-eastern margins of the 40 km-widc Midland Valley terrane (F igure 8.22). Sinistra l displacements duri ng the Devonian have been established by Bluck (1980), and de xtral displaceme nts during the Carboniferous are described by Read (1987). Th is zone is therefore a displaced terrane with no direct relationships to the zones to the north and south. The terr ane consists of a hidden baseme nt of probable Grenville type (Watson and Dunnin g, 1979) , similar to that underlying the main part of the Grampian Highlands (Figure 8.26A). The upper layer of metamorp hic Grampian- Dalradian sediment s, however, is missing, and in its place is a thick, gently folded sequence of Ordovician to Permi an sediments and volcanics. Over most
256
G EO LOGICA L STRUCTURES AND MOVING PU,TES
of th e zone, the Caledonian histor y is obscured
by the younger Devo nian and Carbonifero us cove r. T he absence of an unconform ity at the base of the Devonia n in the Silurian inlier of Lesmahagow indicat es that the e nd-Siluria n collisio n did not result in deform ation of the Lower Palaeozoic cover here. In the southweste rn part of the terrane, however a majo r unconfo rmity separates the a bducted oph iolite shee t of Girvan -Ballantrae from the over lying mid-Ord ovician strata. The Upper Devo nian tectonic history of the Midland Valley is discu ssed by Bluck ( 1980) who demo nstrates thai , du ring this period, it formed a strike-slip extensional graben or pullapa rt basin (sec 6.1) bounded by sinistral bou ndary faults o n eac h side . Over 3 km of con tinenta l red beds formed in this basin, fed by rising Caledo nian mou nta in belts o n both sides. Th e Upper Devoni an sed ime nts rest unconform ably on a gently-folded Lower Devonian sequence that accumulated in two basins, sep arated by a volcanic chain. Bluck con siders that the Midland Valley acted as a sinistral strike-slip-co ntro lled grabe n basin during both the Lower and Upper Devoni an , but his detailed acco unt is confined to the
this same sou rce . Each is an asymmetr ic halfgrab en, with a wedge-shaped co nglome rate de posit thinning so utheastwards away from the source . This type of sedimen tar y basin structu re is typical of strike-slip exte nsio nal regions (see 6. 1, and Reading, 1980).
I
Upper . A n ana lysis of the deform ation patte rn in the Lower Devonian rocks indicat es a preferred orie ntation of folds in a E NE - WSW direct ion, 350 clockwise from the o rientat ion of the bo undary fault. Th is is consistent with the relationship predicted for a sinistral strike-s lip region (see Figure 6. 1). The Upp er Devonian (U ppc rOld Red Sandsto ne) seque nce begins with alluvial fan deposits and e nds with coasta l sed iments. Palaeocurren t directio ns are generally eastwards or northeastwards. To explain the d isposition of the coarse conglomera te wedges in the alluvial fan deposit s, Bluck post ulates a se ries of faultbounded basins along the NW margin of the main graben , within which each sequence thickens to the northwest (Figure 8.28). As the basement moves north-eastwar ds with the creation of each new extensional fault, a succession of new basins is formed, filling from
The Sou/hem Uplands (zon e 5 )
This terrane is bounded on its no rthwestern side by the Sout hern Uplands fault, and o n its southeaste rn side by the hidden Solway suture (Figure 8.29). It has suffered strike-slip displacement in relation to the Midland Valley and the terranes to the northwest. Th e extent of displacement is not known but may be considerable, as the source areas for the O rdovician sediments in the terrane can no t be fou nd in grou nd prese ntly adjacen t to it. Moreover the prod ucts of the Ordovician uplift of the G rampian terrane to the north are not foun d in the O rdovician basin to the south (Bluck and Leake , 1986). Th e terrane is approximately 75 km in width, and consists of a thick seque nce of highly defo rmed Ordovician and Silurian strata, resting o n a hidden basement thought by Watson and Du nning ( 1979) to be of G re nville or Lewisian type. A seismic discon tinuity at a depth of 12km beneat h the central part of the terrane is inter preted as a major deco llement , the Ettrick Valley thrust, which carries the folded and thrust cover o ver the weak Moffat Shales horizon (Weir, 1974). A major seismic discontinuit y dipp ing northwestward s at an angle of 15- 250 has also bee n detected in dee p seismic profiles offshore (Beamish and Smythe, 1986) and is equated with the Solway suture . The deep basement of the Southern Up lands thus underlies this suture and presumably belongs to the southern plate . The major component of the sedimentary pile is a thick sequence of greywackes of turb idite o rigin, attributed generally to a trench environment. Associated with the sediments is an older sequence of pelagic sed iment s with occasional basalts.
257
PHANE ROZOIC OROGt'NIC [Hi LTS: SOMI::: EXAMPLES
A
o <
"
. .. . d
eot to w le
• E
•
b
, /
Figu re 8.28 Diagrammatic interpr etation o( Ihe U ppe r Old Red Sand stone (Upper Devon ian) facies distribut ion in the so uth west Midland Valley of Sco lland in terms o f a sinis/ra l strike-slip model . A . B and C rep rese nt the pr ogressive evolution of sedi me nta ry bas ins as ItIc oa!lemen l 10 the Mid land Valley moves nc nhea srward s in relation 10 the bloc k bo unded by lhe Highland Boundary fau ll on the NW side o f the Midland Valley. S uccessive po suion s of the SW boundary fau lt
The structure is dom inated by NE - SWtrending, SE-facing, monoclinal folds separated by tracts of unfolded , stee ply-dipping beds. Mudstones exhibit gene rally steep slaty cleavage. The terrane contains te n or more distinct stratigraphic sequences which differ considerably across-strike over distances of a few km, but which can be followed along-strike over distances of 100km or more. Conside rable re petition of stratigraphic seque nces takes place across the strike, much of which is a ttn-
butable to a series of steep reverse faults. Some of these are relatively minor, but others separate blocks with major stratigraphic differences, whose constituent strata were originally formed large distances apart. Within these major fault-bounded blocks, shown in Figure 8.29A , the overa ll sequence generally youngs towards the nor thwest, while progressively younge r deposits are found towa rds the southeast. Leggett et at. (1979) interpret the Southern Uplands as a Lower Palaeozoic accretionary
G EOLOGI CAL STRUC ru ll. ES AND MOVI NG PLATES
---- - ---::::------ ---
........ [IIJ post -CaledonIan co.....,
--
~ l.-te Celedon;an g,anites [ ] upperslopeba,; n ecc.elio poism • Ballant ophiolites
o
30km
B uppe r slope basin
l,emergent tre nch I lo we< trench ,I..,. s lo pe ,
', bre a k ; Southern Upland : : fault NW
I
,
Coekbumtand
I lf ench
c
Solway fi n h
s,
1 ~ /t 0 l
,') t ,' • Oldest Midland Velley inti" ".
FiguI'"C 8. 29 Str ucture of the Southern Uplands of Scotland. (A ) 'Tecton ic summary mapof the Southe rn Uplands showing the positions of the main Iaults bounding the different blocks. (B) Diagrammatic profile a,r055lhe area 10 iIlustrale the acceeuoe ary prism model. (A) , (B) , ene r Leggett t l al. (1979). (C) Diagram 10 show how the emplacement of successive undert hrust wedges, using the Moffat shales as a detachment honzon, can result in steepe ning a nd back-rotation of the ea rlier-formed IhfUst blocks . After EalC5 (1979).
prism, formed at a subduction zone on the nort hwest side of the Iapetu s Ocean, along the Laurentian continental margin. The fault-bound ed blocks are interpreted as packets of sediment , offscraped from the downgoing plate, a nd stacked up and steepened in the fore-arc region to form an evenw ally e mergent ridge at the trench - slope break (Figure 8.298) . Th is ridge corresponds to the
conjectural 'Cockbumland' long recognized by stratigraphers as a Silurian landmass in the northern part of the te rrane. This tectonic enviro nment is reminiscent of the Barbados ridge complex discussed in 5.2. Accretion is considered to have commenced during Llandeilo time {mid-Ordovician) and continued until the end of the Wenlockian , a period of about 45 Ma.
PHAf'I"'WZOIC O KOG Ef'lIC BELTS: SO ME EXAMI'l. ES
The south side of the Sol W il Y suture (zones 6-8) A majo r intracont inental suture has long been postul ated within the Caledonides of the British Isles beca use of differences between the Camb ro-O rdovician fa unas of Girvan and the NW Highlands on the on e hand, which exhibit No rth Ame rican affinities, and those of Wales o n the o the r, which exhibit Euro pean affinities. The suture is therefore held 10 represent the site of closure of a vanished Lowe r Palaeozo ic ocea n, part of the Iapetu s Ocea n. T he suture can be de tected by majo r differe nces in base me nt characteristics on either side of the Solwa y line , and can be traced through Ireland alo ng the Navao-Shannon fault ( war son and Du nning, 1979). So uth of the suture, four Caledo nian IccIoni c units a rc recognized , the Lake District zon e, the Irish Sea block , the Welsh basin . and the Midlands platform . The Lake Disrricr- fslc of Ma n-Lelnstc r belt is generally rega rded as the site of an early O rdovician volcanic arc o n the northern margin of a southe rn continenta l plate (Figure 8.32) (see e.g. Moseley, 1977). So uth of this zone , the Irish Sea block exposes pre -Cale do nian Cadomian basement in A nglese y, the L1eyn peninsula in N. Wales, and in SE Irelan d. Th is basement is pa rt of the Cadomian o roge nic belt, subjected to deforma tion and metamo rphism prior to the Lower O rdov ician. The age of this o rogeny in Anglese y, and the strat igrap hic relationship between the Irish Sea block and the zones on either side, is still controversial. All three zones arc suspect terranes with possib le strike-slip d isplaceme nts betwee n them. T he Welsh basin has long bee n regar ded as a classical geos yncline , following Jones ( 1938). It conta ins a thick sequence (c. IO km) of Lower Palaeozoic sediments, including a high propo rtion of turbid ites. Volcan ic rocks form an important compo ne nt of the basin, part icularl y in the O rdovician of north and southwest Wales. Deformati on is highly variable, ranging from tight folds associated with highly-strained , cleaved slates to rather gentle flexures with no associat ed cleavage. Rocks from dee per
259
parts of the basin exhibit pre hnite -pumpcllyite metamorphic facies. The Welsh basin has been widely inte rpret ed as a back-arc extensio na l basin, formed on thinned con tinenta l crust , and related to a southw ard-d ipping subd uction l o ne (see Figure 8.32) . Ok ada and Smith ( 1980) point out, howeve r. that a better tectonic analogy fo r the Siluria n is a fore-a rc basin of the type presently forming la ndward of the Nankai trough off SW Japan . A similar basin was described west of the Barbados ridge , in the Lesser Antill es subd uction zone of the Caribbean (sec Figure 5.13). In the mo dern Japanese analogy, the Japan ese islands co rres pond with the Midlands platform, and the Nankai trough subductio n zone is repr esent ed by a postulated trench no rthwest of A nglesey. Ok ada and Smith believe tha t the subd uction zo ne may have migrated southeastwa rds towar ds the Midlands platfo rm. due to tecto nic e rosion, fro m its position in the O rdovician when the North Wales volcanic rocks were for med . The Ir ish Sea block, forming the tre nch-slo pe break in early Silurian times, became emerge nt in the later Silurian, thus iso lating the basin. The main Caledon ian defo rmation of the basin, like that of all the zo nes up to and including the Southern Upla nds. is e nd-Silurian in age and is attributed to collisio n between the Midlands platform and the Lau ren tian continent. Woodcock ( 191)4) has demon strated the importa nce of dextral strike-slip movemen ts along majo r lineam ents running pa rallel to the south-eastern mar gin of the Welsh basin. A long o ne of these featu res , the Pontesford lineament , Woodcock demonstrates patte rns of branching faults in basement inliers and localized belts of comp ressional folds in the cover cha racteristic of a strike-slip regime (Figure 8.30) . Th e Midlands platform is not , as o nce tho ught, the no rthe rn margin of a large Europea n co ntinent in Lower Palaeozoic times, but a co mpa ratively small region of undcformed Lower Palaeozoic strata resting on late Precambrian volcanic a nd sed imentar y rocks thai have bee n affecte d by the Cadom ian orogeny.
~
Post Silurian Co ver
o
Snvtan
L~
~;~~m,,--, t;.c~r
t::/ ",,:j Uocer Orcovlcan
i
R:;:;:;:;:;:;:;:::J Lower Ordovician f:,~~:~'cJ Precambrian & ' • • • ~+
Cambrian
B
A
J
"
< , •• ,
" Figure 8.30 Map sum ma rizing impo rta nt Ca ledon ian structures o f mid-Wales and the Welsh Borders. Folds, con tinuous thick lines; faults , dashed lines . Note : (1) the: major C wm MaWl- Po ntcsfor d - U nley and Q lUrch SUc llon {aull zones; (2) the oonccnlral ion of Iaults , and part icularly folds alo ng a relatively narrow zone between U aodri ndod Wells and a un; (3) occasio nal o blique rela tio nships be tween fol d axes and the majoc faults. A - 8 and C-D arc cress-sections across the central pa ri of the linea ment , showing positive flower structures (sec Figure 6 .4). After w ccdccck ( 1984).
PHANEROZOI C OROGEN IC BELTS: SOME EXA M PL ES
Th e late Pr eca mbrian sequence rests in turn o n an earlier Precam br ian basem ent, of which on ly a few frag me nts a re pr eser ved , bu t whose geophysical characte ristics diffe r considera bly fro m those of the Laure ntia n base men t no rt h of the sut ure (W atson and Dunning , 1979). Similar rocks form the A valon te rra ne th at co nstit utes the sou thwestern margi n of the Appalachian belt fro m Newfo undland to eastern Massachuse tts (F igure 8.21). Ou r knowledge of the natur e of this so uthe rn crato n in Caledon ian times is ve ry limited , but it is clear that, in co ntras t to t he two large co ntine ntal masses of Laurentia and Baltica , the Midlands platform was pa rt of a relatively minor block . T he e nd-Silurian coll ision gave rise to relatively mild de format ion with litt le sho rte ning in the southe rn pa rt of the Briti sh sector, quite unlike the co mplex thrust belts o f Sca nd inavia and NW Scotla nd .
The Scandi navian Caledonides T he relation ship bet ween th e Scandina vian and British Caledonides is shown in Figures 8.21 a nd 8.33. Althoug h the Scand inav ian be lt appears to be a d irect co ntinuatio n a long-strike of the Scott ish Highlan ds, sepa rated by o nly 500 km across t he North Sea , the re a re co nsiderable d iffe rences in str uct ure . T he Scand inavian Caledo nides co nsist of an 1800 km-lo ng belt, with a pr esent outcrop width of up to 300 km, of na ppes directed east wards o n to the Fennosca ndian shield . The polarit y of the be lt is t hus opposed to th at of the NW-directed nap pes of zo nes 1- 3 in th e Bri tish sector . Th e eastern margin of th e belt curves t hro ugh Den mar k towar ds the Po lish Ca ledon ides and there is no d irect connection with the so utheastern part o f the Briti sh Caledo nides. T he alloch thonou s nappes of th e Scand inavian Ca ledonides ( Figure 8.31) conta in stratigraphic successio ns ex tend ing up to t he early Silu rian , and the main Caledonian o roge ny (the Scandian) is dated as mid-Silur ian to Ea rly Devon ian . T he re is evidence also , in certa in districts, of a n early -Caledo nian defo rmat ion ( the Finnmarkianv of Late Ca mbrian to early Ordov ician age (540-490 Ma ee).
261
T he nappe sequence , summarized by Ro berts and Gee (1985) , is divided into fou r se parate comp lexes: the Lowe r, Midd le , Upper and Up permost Allochthons. T he Lo wer Alloch thon is composed mainly of a sequ ence of late Precam brian to early Palaeo zoic sed ime nts t hat have been invo lved in t hin-skinned t hrust defor mat ion in a zone along the enti re eastern margin o f the belt , bo und ed in t he east by a sole thrust. Further west , Preca mbrian crystalline basem ent is also involved in nap pes o f t he Lower A llocht ho n. Th is seq uence is over lain by the Middle A llochthon , which is composed main ly of highly-defo rmed Precamb rian crystalline rocks, together with unfossilifero us, prob ab ly late Precambrian , psammites. T he roc ks of th is unit a re extensively mylon itized , ex hibit grea ter and mor e penet rative deformat io n than the underlying unit , and arc meta morphosed in midd le- to upper-greenschist facies. T he Midd le Allochthon co nta ins a suite of late Precam bri an pre-tectonic basic dykes and an exte nsive syntecto nic mafi c/ult ramafi c to alka line igneo us com plex of late Camb rian to ea rly Ordovician age . Th e Upper Allochthon co nta ins defo rmed volca no-sedimentar y successions derived from a variety of tectonic envi ro nments: island a re, fore -arc and back-arc bas in, and ocea nic. T he nappes containing these seq uences have been transported seve ral hund red km eastwa rds. Two defo rmatio ns can be reco gnized , an early Ordov ician phase that has bee n attributed to the o bductio n of the ophiolite asse mb lages, and a later Siluro- Devonian event. Metamorphism during both phases reached upper amphibolite facies . T his unit may th us co rrespond with the missing op hiolite nap pe of t he Scot tish Highlands. T he Uppermost Allochthon co nsists of a series of nap pes co mprising migmati tic gneisses and high-grade met asedi ments, together with units of lower-grad e suprac rustal rocks. T hese roc ks are int ruded by synorogenic gra nito id and gabbro plutons. Some of the gneisses in t hese nap pes appear to represen t pr e-Caledon ian base ment with the Precambrian supracrus ta l cover.Seve ral lhrusts licescontain ophiolite fragments and associated low-grad e metasedime nts.
262
G EO LOG ICAL STRU(T URf"S AND MOVIN G PL ATES
~
Ba...-u
~
OtMlni an
Ed
bolic Nappe C~
•
Ck eri;
see
~
UJ Cf yalallinll
D Ba ilie
GfOuo
She ets
eo.-
o .....-
Sr-ts
AIIOCtdh:>noo..-
~ =-I::::¥ , .. Thrust
flgnt
\
B
Figurr 8.JI Structu ral summary of the Scand inavian Caledonides. ( A) Map showing principal tectonic units: exter nal un ils com prise the a utoch thonous basemen t and cove r and the ex ternal crys talline nappes ; intern al units comprise (he oceanic and exo tic na ppe co mple xes.
Th e later stages in the evolution of the belt were marked by regional uplift of the western part of the orog enic be lt, le ading to the accumulation of early to mid-Devonian molasse deposits in fault-controlled extensional basins. . Ramsayer at. (1985) est imate that a total shor tening o f c.400 k m may have taken place across th e be lt in northern Norw ay. although
the displaceme nt o n the lowermost nappe diminishes to the nort he ast. In the nort hern section a dear dist inction can be made into two
groups of nappes: an e arlier Finnmark ian nappe complex and a later Scandian nappe complex, e ach containing distinctive sedimentary sequences - late Preca mbria n to Ca mbrian, and Ordovician to Silurian respectively. The Finnmarkian orogeny commenced dur ing the Upper Cambrian in the interior of the bell and progressed toward s the craton, e nding in ea rly O rdovician times. The Scandian orogeny created a new group of nappes that in places overrode the Finnmarkian nappes , at-
263
PHANEROZO IC OROGENI C BELTS: SOME EXAMPL ES
,m ,
o
" ,
c
c
-- - -- - - - - _
Jo>l..,""""
--_
..',
....
l ' a dihon a l
Iron\.
...>--- -
t 'iJ:url:' 8.3 1 Structural summary or the Scandinavian Caledonides. (8) Schematic profiles across the lines marked in (A ). Note thai A -A ' is at a dilfcrcm scale to the others. Vertical scate equals bonzont al. From Hossack and Cooper (19R6) , afte r various sources.
ready dee ply eroded. In addition, Finnmarkian thrusts were locally reactivated . Hossack and Cooper (1986) divide the nappe complex into two zones (Figure 8.31A): an external zone of thrust sheet s that have been emplaced southeastwards onto the Fennoscandian craton, and an intern al zone of exotic nappes with NE-SW stre tching directions, parallel to the strike of the belt. These two sets of nappes must therefore have differe nt emplacement histories. The external zone comprises the Lower, Middle and Upper Allochthons of Roberts and Gee, and the inte rnal zone corresponds to their Uppermost Allochthon. Hossack and Cooper claim that the pree rosion thrust front lay much further to the east than the present ou tcrop, based on its position in the Oslo Graben (Figure 8.31A). T hus the width of the thrust belt in the cover
sheets of the Lower Allochthon is very much wider (up to nearly 3OO km) tha n is apparent at outcrop . The widt h of this zone expands in the north to cover the whole exposed width of the belt (see section AA ' in Figure 8.318 ) . The exposed thrust front , according to Hossack and Cooper, corresponds to the position of a series of frontal ramps where the sale thrust cuts down from the Ca mbrian black shales into the late Precambrian sequence. The crystalline basement nappes of the Middle Allochthon, like the cover of the Lower Allochthon, are thought to have been derived from the Fennoscandian shield. A restored section of a profile in the south through Oslo (section DD ' in Figure 8.318) indicates that the sole thrust ramps down through crystalline basement beneath the Jo tun nappe , about 475 km west of the present
264
GEOL OGICA L STRUcrU RES AND MOVI NG PL AT ES
t hrust fro nt , but ca st of the autochthonous basement outcrop of SW No rway.
The 'oceanic' nappes of the Upper Allocht ho n cove r a wide surface area in ce ntral a nd northern N orwa y (Fi gure 8.31A ) and include the well -known Seve and K oli nappes. Sedi-
men ts within these nap pes contain faunas with Baltic affinities but a higher oceanic nappe contains material with No rth American affinities (Gee, 1975). Th e geochemistry of the volca nic rock s is consiste nt with an ocea n-fl oo r origin (Fu mes et af . , 1(82). These nappes a re there fore con sidered to rep resen t abducted slices of oce anic crust originat ing o n bo th sides of the Iapetus Ocea n. Since the major deformation of the op hiolites of the Upper Altocbt ho n is Finnmarkian in age, the auth or s date t he obduction of the ocea nic mat erial as Lo wer to Midd le Ordovici an. Th e exotic nappes of the internal zone , o r Uppermost A llocht hon , form large ou tcrops alo ng the coastal bell of centra l and no rt hern Norway (Fig ure 8.31A ). They overlie the Lofote n base men t complex, which Hossack and Cooper believe to be alloch thonous and pa rt o f t he Midd le A llocht hon (see Figure 8.318, sec tion 88') . Hossack and Co oper point ou t that t hese internal nappes must have bee n derived either fro m a micro -continent within the Iapet us Ocean separat ing Baltica fro m the Laurentian co ntinen t, or fro m the latter co ntine nt itself , since the y over lie the oce anic mat erial of t he Upper Allocht hon. Th e highly deform ed rocks of these nappes exhibit NE -SW--o ricnted shea th folds and elongation lineat ions indicat ing either emplacemen t parallel to the strike of the orogen, or possibly oblique sou thward emplace ment in a transpr essional regime. Th e main deformation of the Up per mos t, Upper and Midd le Allocht ho ns is regarded as a Finnmarkian eve nt, rath er similar to the G rampian o rogeny in Sco tla nd . The Scand ian or end-Caled onian or ogen y produce d the first deforma tio n in the thrust sheets of the Lo wer Allocht hon, but can also be recognized in the higher nappes, suggesting tha t their e mplacement was in part a Scandian
even t. Some re activatio n of the earlier thrusts is indica ted by a met amo rp hic overpr int of " .420 Ma BP found in certa in of the upper nappes. Th e dat es of the Scandian event arc dia chronous across the oroge n fro m c .450 Ma BP in the central part o f the belt to c.420 Ma at the thr ust fro nt, indicating a movemen t rate of c.2.Bcm/yea r. Hossack and Cooper pro pose a plate tectonic mod e l for the e volutio n of the Scandinav ian Ca ledo nides that explains the Finnma rkian event in terms of the obduction of a slab of oceanic crust co ntain ing an islan d a rc; and the Sca ndian o r e nd-Ca ledo nian eve nt as a collisio n of Laure ntia with Baltica during wh ich t he latt er und erth rust the form er (see Figure 8.348 ). Plate tectonic iruerpreuuia n ofthe Norrh A tlantic Caledon ides
Th e first plate tecton ic inte rp reta tion of this region was made by Dewey (1969) . He describes a model ( Figure 8 .32) invo lving a NW· d ipp ing subdu ction zone be low t he Gram pian Highlands in the British secto r, and two SE· d ipping subductio n zo nes, one below the Irish Sea block in Cambrian to Lowe r Ordovician times, and a later one below t he Lake Dist rict in Upper Ordovician ti mes , with ccntinent-. co ntine nt co llision takin g place du ring the late Silurian. Many subseque nt refinements an d alternatives ha ve bee n suggested , but the tectonic fra mework suggested by Dewey is still the basis of most modern views, T he evo lutio n of the belt may be said to co mme nce with the brea k-up of a late Preca mb ria n co ntinen t that is doc umented by palae omagnetic ev ide nce (P iper, 1985) and by the diversification of fau nas in the ea rly Cambrian . Differen ces in early Ca mb rian shelly faun as between Lau re ntia and Ba ltica are well kno wn. McMenamin (1982) presents ev idence t hat this faun al sepa ration co mmenced in the late Precam brian with ce rta in be nthic ' Ed iaca ran' soft-bod ied faun as in th e period 650 600 Ma ee. Ev ide nce for widespread rifting and intraco ntine nta l ex te nsion prece ding this
PHANEROZO IC OROGEN IC Bf.l.TS : SOME F. XAMP l. ES
265
A
"'........e-.n
,--
c o
E
, i
,...-f
l o... C 0<1"0 •••1 _
Hg urt 8.32 Schernanc sections Illustratin g me Dewey (1969) model for the tectonic evo lution of the British Caledonides . DL , Durncss lirncstoae ; M , Moine complex; D , Dalradian sediments; G V. Girvan ; MN C, Moine thrust ; BV , Baltantra e vokan jcs: M e . Mona complex: ISH • Irish Sea block: PH F, Pomesfo rd Hill (ilull ; CSF, Church Snenon Iauir; IIBS . l llghtand Bo rder Series; SMT, South Mayo trough: Rfl WV. Bcrrowdale volcanics; D F, Dinorwic Iault; SV. Su okesrown volcanics; 8 Z1, 2, 3, successive sul>duction zones. From Dewey (1%9 )
continen tal separatio n is pro vided by the developme nt o f fault -controlled To rrido nian and Grampi an-Dalradian basins in Scotla nd around 800 Ma ago . Th e Iapetus Ocean is widely regarded as having opened initially in the early Ca mbrian (Ande rton, 1980) at a time of dr amatic chang e
in the nature of the Dalradia n basin. Aft er the de position of the Port Askaig tillite , a product of the Ve ndian glaciation of c.640 Ma Bi>, rapid su bsidence took place along growth faults, producing turbidite-filled basins, succeeded by the extrusion of basic volcanics and by further subs ide nce and t urb idite sedimentatio n. By
266
GEO LOG ICA L ST RUCTU RES A ND MOVING PLATES
Low er Ca mbrian times, both fa unal and pal aeomagnet ic evidence indicate an appreciable se para tio n of Laure ntian, Ba ltic and southe rn co ntine ntal masses, with an interve ning Ia petu s O cean . The nature of the so ut hern contine ntal mass is unclea r. Th e Britis h pa rt, termed t he Midlands platform , is usuall y regarded as the lateral eq uivale nt of the Ava lonian te rra ne in the Appal achia ns. T he fa unas of this continental mass show diffe re nces from bot h Laurentia and Baltica, and Avalonia
probably represents a large island or series of islands se parated from Baltica by oceanic cr ust.
In a revised model, Dewey ( 1982) demo nstr ates the importance of obliq ue plat e co nver-
D
.. .
rn:m
CIIJ -~ --....... ,
2
['2ZJ
3
~
4
D:JJ
gence d uring Ordovician and Silurian times , a possibility explored ea rlie r by Phillips et at. ( 1976) . Dewey' s model involves a large dextral strike-slip compo nent of relative mov em ent along the British secto r o f the belt d uri ng t he life of the subd uction zon es. Thi s relative mo tion changed aft e r end-Silurian co llision to a sinistral strike-slip regi me which produced around 1000 km o f re lative moti o n along the Ap pa lach ian- British sect o r of the belt. Dewey and Shack leto n ( 1984) prese nt a furt her revised tectonic mod el ( Figures 8.33 , 8.34) in which the early C aledo nian e ve nt, termed t he Grampian in Sco tland and Irel an d , the Finnmark ian in Sca ndina via, the Hum berian in Newfou nd land , and the Taconic in the
5 6
7
LAURENTIAN FORELAN D
.r NI *loun dll nd
~ AF
.
.. ~
Connema,a
~ OF ;S~
AVllon iln Zonl
ql
F"lgun 8.33 Tect c mc 5ummilry map o f the British and Scandinavia n Caledonides and Nonhero A ppalachi ans, restored to the pre-Mesozo ic fit of Bullard t l al. (1965) : 1, inrraCa mbrian 10 ea rly Ordovician sequence s in Gr amp ian zone ; 2. O rdovician are terranes; 3, oceanic and arc terranes so uth of the suture ; 4, ophiolite co mplexes ; S, O rdovician·SiJurian accretionary prisms; 6. G renville basement within the oroge n ; 7. Avalonian-Cadomian terrane on the south side of the o rogen. BOI , Bay of Island s; LR . Lon g Range ; HB. Hare Bay oph iolite; M , Mings Bighl op hiolite ; B, Ben's Co ve ophiolite ; RF, Reach Ieuk: DF, Dove r fault; CB, Cle..... Bay ophio lite; LN, loch Nafooe y faull zone ; BL, Ballantr ae op hiolite ; PO , Port soy: SUF. Southern Up lands fault; fPS. Iapetus sUlure; U, Unst op hiolit<:; BA , Bergen ares; TR, Trondheim. After Dewey and Shack jeton ( 1984).
267
PHA N EROZ O IC O ROGEN IC HELTS : SO ME EXAMPL ES
,
--~-'"'-L-~ .
.;'jr--_._ _
"'''
""
A LAt"
•
Cetnbo' ian
Ii
o
i! . . " ' ,,. "
B
tl1 _
z"""
Oceen i" cr.-' Conl......... 1 e ,,*
c;:;:'! ._ o
c
~
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•
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F1gur<e 8.34 (A ) Schemat ic N -S structural profile along line indicated in Figure 11.33. Mr, Moine mrcst: SBS, Sgurr Beag slide ; f WS , Fon William slide; BA S, Ballachulish slide; 185 , (hay bound ary slide; G GF, G reet Gle n fault; H8FZ , Highland Boundar y fault zone; SUFZ , Southern Uplands faul! zone. From Dewey and Schackteto n (1984). ( B) Seque ntial cartoo ns 10 illustrat e a tecton ic interpretation o f the Scandinavian Caledonides . From Hossack and Cooper (19116) . (C) App roximately true-scale schematic sections illustrating a model for the evolution of the G rampian o rogeny. An o phiolite is overthrust onto oceanic lithosphere at a fracture zone and thereafter progressively obd ucred onto the contine ntal rise and shelf, prod ucing the 0 1- 2 de fo rmations in the Grampian Highlands (sections a- t-). Section (d) shows shortening and thickening of the sedimentary pile. leading eventually to a reversal of subduction polarity (..). Fro m Dewey and Shackleto n (1984)
Appa lachians of New York and New England, is att ributed to the obd uction of a giant op hiolite nappe, up to 15 km thick, generated at a ridge-transform intersection in the Ordovician Iape tus Ocea n. T his eve nt appears to have been diachronous, taking plaee in preL1a ndeilian times in Britain, before the end of the L1andeilo period in Newfoundla nd, and in mid-Ca radoc time in New York a nd New England . In Newfoundland, progressive dia-
ehronous defor mation ca n be established , commencing at about 495 Ma BP in the southeast, and e nding at 455 Ma BP in the northwest . The obducted ophiolite complexes were formed in early Ord ovician time a nd only a short period elapsed before their obduction. The earlier stages of this process would have been submarine (Figure 8.34C). Emergence would only have taken place whe n the stacked crust reached thicknesses of c.30 km. During
268
GEOLOGICAL ST RUCTUR ES AND MOVING PLATES
the sa me pe riod, a volcanic arc, resting on an oce anic fo undation, lay to the southeast of the abdu cted portion of the ophio lite , and is rep resented in numerous places along the be lt , includ ing th e Mid land Valley terrane of Britain and t he Noi re Dame terrane in Newfoundland . T he co llisio n of th is arc terrane with Laurenti a may have ended the obduction. De wey and Shackleton sugge st that t he co ntinue d co nvergent motion caused firstl y (i) the northwestward s obducti on , then (ii) sou th-eastwa rds subduction , for ming (he volcanic arc , then ( iii) co llisio n of t he aTC with Laurentia , followed by (iv) nor th wcstwa rds subduction in the midOrdovic ian to prod uce the Sout hern Up lands accret io nary pr ism. T he latte r, Llandeilian , e ve nt is held to explain the late 5-facing str uct ures in the G rampian Highlands refe rred to earlier , a nd is cor re lated with the upl ift of that te rrane . A fte r this dia chro nous orogenic phase , during which co nverge nt moti on was obliq ue with a dextr al strike -slip compone nt , the direction o f plat e co nve rgence appea rs to have changed . Evidence for large . sinist ral displacement s
have already been discussed , for exa mp le t he 160km sinist ral d isplace ment of Grampian meta morphic zo nes alon g the G reat Glen Fault. Dewe y and Shacklet on suggest th at a tot al sinistral displace me nt o f 1800 km occurred be tween the Ca radocian and the early Devon ian . Ocea nic closure and co ntinental collision appea r to have been dia chronous. Co llision of Laure ntia and Ba ltica took place during the Silurian period , and was followed by endSiluria n to mid -D evonian co llision of Avalonia with the co nsol idated Lau rentian- Balt ic co ntinen t. Th e au thor s sugges t that it was the latter collisio n event which ind uced the sinistral st rike-slip motion between La ure ntia and Baltica , and between Laurent ia a nd Ava lon ia. Of the 1800 km st rike-slip d ispl acem ent be. tween Lau rentia and Avalonia, the y estimate that 500 km may have been tak en up by co nvergence across the nor th Ge rman - Polish branch of the Caledo nides and th e rem aining 1300 km in the d isp lacem ent of Sca ndi navia re lat ive to No rth A me rica (see Figure 8.33 ).
9 Orog eny in the Preca mbrian As we have see n in the previous chapter , it be-
T llblt 9.1
comes pro gressively more difficult to int erpret
the st ructure of o rogenic belts as we loo k furt he r back in lime , beyo nd the period when accurate continental reco nstructio ns can be made fro m ocean ic palaeomagne tic da ta . Th is d ifficulty is magnified in the Precambrian because of t he inherent inaccuracies of con tine ntal pa laeo magnet ic reco nstructio ns. and is compounded by o ther problems. Th e familiar stratigraphic certa inties co nveyed by accurate palaeo ntolo gical dating in the Phanerozoic are abse nt throughout much o f the Precambria n. T he widely-used radiometric da ting meth ods cann ot ye t give the same p recision in subd ivid ing and corre lating stratigraphic sequences. The na ture of the roc ks is different: much of the Precambrian o utcrop consists of uplifted deep-crustal material. ofte n highly-deformed and meta morp hosed , whose original geometrical relatio nships to adjo ining outcrops and regions cannot be accuratel y reconstructed . Pro bably the most importa nt prob lem, and cer tainly the most stimulating one , in interpreting Precambrian rocks is the uncertainty over the extent to which present-da y processes can be regarded as useful analogues in the Precambrian. Alth ough it is a fundamenta l tenet of geo logy that ' the presen t is the key to the past', we find d ifficu lty in unlocking so me of the o lder doo rs!
C hro no logical subd ivision of Ihe Precamb ri
Eo n
Pro terozoic
Er a
Ca nadian sche me
t.."
Had rynian
Middle
Hehk ian
Ea rly
A phebia n
M, 1000 1600
2500
t.." A rchaea n
2900
Middle
3400 Ea rly 4000 ?
Phanerozo ic. Thus the major subdivisions , Proterozo ic a nd Archaean, have the status of eons , like the Phanerozo ic, and the bounda ry between them is at 2500 Ma BP. The subd ivisions Late , Middle and Ear ly Proter ozoic are in widespread use, altho ugh the time pla nes marking the bound aries be tween the m are not yet gene rally agreed . Tab le 9.1 sho ws the system used in the Ca nad ian shield as an example . Fur ther stratigraphic subd ivisio ns ar c in use in seve ral d iffer ent co untries (sec H arland et al., 1982) but a re no t intern atio nally agreed. Major oroge nies in the Phanerozoic occur at intervals of the o rde r of 200 Ma , and last for period s of the ord er of lOOMa. In the Precambrian , a series of 'o roge nic cycles ' have been recognized (Sutto n, 1963) that a re much lon ger than the Phaner ozoic o rogenie s. The Archaean alone extends at least from 3800 Ma Dr, the age of the oldest da ted co ntine ntal rocks, to 2500 Ma - a pe riod lo nger than the whole of Phane rozoic time . Within this period , it is not generally possible to recognize se parate widespread o roge nies such as the Ca ledo nian o r the Hercynian . Nor can we define o roge nic be lts in the Archaean . A lthough local mobile be lts have bee n recognized , such as the lim popo be lt of southern Africa , bordered by more stable zones, these are not com para ble in scale or in inferred process to the Pha nerozoic orogen s.
Precambrian chronology In this chapter we shalt discuss examples of Precambrian o roge nesis from the fo ur main periods of Precam brian time : the A rchaean , and the Early, Middle, and Late Proterozoic (Table 9. 1). The basis of the stratigraphic subd ivision of the Precamb rian is chrono logical: arbitrary dat es are chose n to re present the bounda ries betwee n the subdivisions, which may corre spo nd to major stratigra phic break s in so me areas but not in others. The size of the time division s is much la rger than in the 269
270
GEOLOGICA L STRUCT URES A ND MOVI NG PLAT ES
9. 1 Plate movemen ts in (he Preca mbria n
The quest ion of what kind of plate tecton ic process ope rated during the Precambrian (a nd particularly during the Archaean) , is one tha t has caused considerable debate among Preca mbrian geo logists since the plate tecto nic
theo ry was established. O pinion has ranged wide ly. Alone ex tre me is the view he ld by Baer (1977) and others tha t ' mod e rn ' plate
processes did not com me nce until the late Precambrian . T his view is bas ed on the argu men t t hat in warme r. thinn er . early Precambria n lit hos phere , eclogi te could not form in
the mafic ocea nic cru st , mak ing the ocea nic lithosphere too buoyan t to subduct. Hea t loss
and tecto nic movements were concentrated in the softer co ntine ntallithosphere . At the ot her extre me is the position taken by Burke et al. (1976) and Tarney and Windley (1977) for example, who believe that the early Preca mbrian plate tecton ic processes were essen tially similar to those ope rating at presen t, and differ o nly in thei r rate , in the size of the plates , and in other relatively mino r respects. Th e key to the problem lies in understand ing Precambrian heat productio n and heat loss. This problem is addressed by Bickle ( 1980) who, fo llowing McKenzie and Weiss (1975), points out that higher heat productio n during the earlier histo ry of the Ea rth would lead to mo re rapid movement of thinne r plates, whose mo tio ns would be governed by smaller-scale convective cells tha n those inferred fo r the prese nt. . Since the thickness of the ocea nic lithosphe re is governed by the supply of heat to the base of the lithosphere , an increase in heat supply will result in a thinn er lithosphere and less e fficient operation of the plate creationsubd uction processes. However this effect may be offset by a faster rate of plate creation, and by the subduction of younger, hott er oceanic plate. Bickle points out that about 45% of the prese nt heat loss throu gh the Ea rth's surface arises from the plate crea tion-subduction process . a nd that if this process ceased , average thermal gradie nts in the continents would be
nea rly doubled . Our knowledge of Archaean ther mal gradie nts is imperfect, but it is clear from pressure - tem perature estimates in Archaea n tower-cr ustal rocks that the rmal gradients in the A rchaean continental crust were not greatly diffe rent from today's values, despite the much higher rate of heat production . Pressure estimates from lower-crustal gran ulite-facies gneisses, suc h as the Archaean Scou rie gne isses of NW Scotland, indicate crustal thicknesses of at least 30km and possibly 50 km, at the time of formation (see e.g. Cartwright a nd Barnicoat, 1987). A marked rise in geothe rmal grad ient would have re~ suited in the wholesale melti ng of the rocks at such dept hs. Bickle concludes that the prolific eruption of vo lca nic rock s appea rs to be the on ly mechanism capable of transpo rting sufficient heat , and that a plate tectonic process is necessa ry for recycling these volcanic rocks. Bickle 's thermal model suggests a sixfold increase in the areal rate of plate productio n in the late Archaea n, compared with the present. A corresponding increase in the overall rate of subduction is requi red to keep the system in balance. Many authors have no ted the high proportio n of to na litic pluton ic intrusions in the Archaean that are interpreted as melting products of subd ucted ocea nic crust, the dee pseated counte rpa rts of volcanic-arc andesites. It is gene rally be lieved by Precambrian geelogists that a large proportio n of the present con tinental crus t was created during the late A rchaean (see e .g. Dewey and Windley, 1981). A de tailed study by Ta ylor et al. ( 1980) of the Pb-isotop e composition of the Arc haea n crato n of So uth G ree nland (see below) indicates two ages of derivat ion for the mantlederived Pb in the igneous rocks: one at aro und 3000-2800 Ma BP and the o ther at c.3700 Ma. The y found no evide nce in regions of younger A rchaean crust of the material with olde r Pb, suggesting that the grea te r part of the craton was fo rmed, ra ther than rewo rked, in the late A rchaean. Rb- Sr and Sm- Nd isotope studies of Ea rly Proter ozoic terrains in Canada indicare that the bulk of the crust there was also
271
OROG EN Y IN THE PRECAMBRIAN
fo rmed in the late Archaean period (McCulloch and wasserburg, 1978). The opin ion of many Precambrian workers seems to be in favour of a convective platetectonic process in the ea rly Precambrian, op erating at a much faste r rate tha n at pre sent , and that this process was respo nsible for the creation of a substantial proportion of the continental crust d uring the approximately 500 Ma pe riod of the late Arc haea n. Th e way in which this plate tectonic process opera led must have been rather different fro m the mode l accepted for the Phanerozoic. One of the most significant features of ea rly Precamb rian mobile belt s is the absence of oph iolites and of low-te mperature , high-pressure metamorph ic rocks , the accept ed indicators of subd uctio n in mobile belts from the La te Proterozo ic o nwards. T his was o ne of the facto rs that led Bae r (1977) to reject subduction in the early Preca mbrian. Baer a rgues that in a thinner , warmer lithosphe re, eclogite cou ld not form, preventing the density inversio n necessary to power the subd uction process from taking place . A possible solution to the A rchaea n subductio n problem is suggested by A rndt (1983) and by Nisbet and Fowler (1983) , who point ou t that the A rchaean oceanic crust may have bee n largely komatiit ic in composition, corre spo nding to that of the ultramafi c ko matiitic basa lts found in Archaea n green-
t U An UII
Proterozoic plate mo vements
Appa rent polar wande r paths have bee n constructed for the Prote rozoic of a ll the majo r Precambrian shield reg ions. T hese arc regarded as evide nce of co ntinental plate movem ent s during this period . The polar wande r curves exhibit a number of ' hairpin' be nds interpreted as major collision orogenies resulting in ab rupt changes in plate mo tion . Irving and McG lynn (1981) discuss the Precambrian palaeom agnetic reco rd from No rth America (Laure ntia) and conclude thai the data are not suffi ciently se nsitive to de tect relative moveme nts within the Lauren tian domain d uring the Archaea n and Early Proterozoic, but that large latitudi nal move me nts relative to the pole occurred with an average rate of 5-6 em/yea r. Three loops and hairpins in the Early to MidProterozoic ap pare nt polar wande r pa th for Lauren tia (Figure 9.l) arc identified with the Hudsonian orogeny at c, 1750 Ma BP t the Keween awan or ea rly Grenville extensional
• "!!."!9"!IP' ..!.!""
o~
fw.:-
sto ne belts. An ocean ic crust of this co mposition, moving over an asthenosphe re that was hotter , and consequently less den se and less viscous than at present, would move late rally more easily. When coo led, such a lithosphe re could be denser than the asthenosphe re and be read ily subdu ctible.
-
-
_
--I
."'"
"'" "'"
-
"'"
Figure 9 . l Plo t showing the vena tio n in magnetic palaeolatitud e with lime for the Lauren tia n shield. No te the ab rupt changes ('hairpins') al 1750, 1150 and 1(0) Ma Dr. Calc ulated for Winnipeg (5O"N, 97"S). Age uncerta inly of individ ua l poin ts are in Ma . Fro m Irving and M~>(j ly nn (19M)
272
GEOLOGICAL STRUCTU RES AND MOVING PLATES
eve nt at c. 1150 Ma BP, and the ma in end G re nville co nve rge nt deformatio n a nd up lift at c.950 Ma 8 f> (see 9.3) . Pipe r ( 1982) believes tha t the polar wand er path s for the var ious co ntinen tal blocks during the Proterozoic are sufficie ntly similar to be interpre ted in te rms o f a Prot erozo ic supe rco ntine nt (Fi gur e 9.12) . Howe ve r, it sho uld be e mp has ized thai co nside rab le rela tive motion may be concea led by th ese paths, an d that there is still so me contro versy ove r this proposa l. Ne ver theless, we ma y co nclude tha t the palaeo magne tic dat a for the Proterozoic sup porl contine ntal pla te mov eme nts similar 10 those of th e Pha ne rozoic . The re is con side rab le controversy a bout t he possible e xiste nce of collisional s utures in Precambrian mob ile belts. We shall discuss so me o f the ev idence in the examples 10 fo llow. On the o ne hand , there is th e belief t hat most Prot e rozoic mobi le belts are intraco ntine ntal with res pec t to a major Proterozoic supe rco ntine nt, and tha t plate boundary processes, if they exist, occur o nly around the margins of the superco ntine nt. O the rs believe that cryptic sutures are present in man y supposed ly intracontine nta l bells but are pa laeo magnet ica lly undet ectable. Th e evide nce hinges at p resen t on the interpretation of magmatic roc ks: wheth er or not they ca n be interpreted as subd uction-de rived . We shall d iscuss exa mp les of intracontinen tal belt s where suc h roc ks are abse nt; in such cases a co llisional model is difficult to substantiate. T he impressio n ga ined is tha t intraplate co ntine ntal mobi lity was mor e widespread d uring the Prot erozoic than d uring the Pha nerozoic, and that this differe nce is to be exp lained by an increase in contine ntal lithosphere strength in late Precam bri an times. In t he ir study of the evo lution of the co ntinen tal crust , Dewey and Windley (1981) inte rpret t he Precambria n record as follows: the Archaea n is a period of rapid crustal growth ach ieved by the amalga mation of vol"canic arcs to form abo ut 85% of the presen t co ntinental mass by 2500 Ma SP . Duri ng th e Early Proterozoic , large crato ns were formed
by th e amalgamatio n of Archaean fragmen ts, and grad ually stabilized, th icken ed and d ifferentia ted . Intern al mo vement belt s resulted from marginal collisio ns. By Late Prot erozoic time , modern accre tionary margins were in existence . 9.2 Late Proterozoi c Pan-Africa n belts Are as of Lat e Proterozo ic to ea rly Palaeozoic tecto no-the rmal act ivity form a lar ge part of the surface are a o f Gondwa nalan d ( Figure 9.2), part icu larly in A frica . Th ese tecto nicall y active zo nes cannot be ter med orogen ic be lts, since in many instances they do not form clearly de fi ned linear features. Nor is the term 'o roge nic' necessarily app licable, since it implies an analogy with Phan erozoic mountain belts whose tectonic o rigins are clearly es ta blished . Many Precambrian geologists prefer the term mobile bell or mobile zone fo r such regions, and we shall follow this practice here . Th e Pan-A frican system of mo bile cr ustal zones co mp rises a reas of tec ton ic, magmatic and metamorphic activity in the age ra nge 1000-450 Ma. These mobile zo nes sepa ra te cratons that were tectonica lly stable d uring th is period . such as the West Af rica n. Co ngo and Kalahari crato ns o f Africa , Guya na Brazil in So uth Ame rica, and Pe ninsular Ind ia . Palaeom agnet ic evidence ( Mcwilliams , 1981) shows that Gondwanaland existed as a supe rcontine nt o nly fro m latest Prec ambri an or early Palaeozoic time un til its break-up in the Mesozoic. Th e formatio n of Gondwanaland is therefore a Pan -Africa n eve nt. Prior to this event, the palaeomagnetic reco rd suggests that two supercontine nts e xisted with markedl y differe nt app aren t polar-wand er pat hs. O ne comprises the Precambrian shield areas of Africa and So uth Ame rica. and may be termed West Go ndwana ; the o ther. East Gond wana , co mprises the Precambrian shie lds of Ind ia , Austral ia and A ntarctica . Th e two supe rco ntinen ts are presumed to have co llid ed along the Pan -African Mozambiqu e belt that extends along th e eas t coast o f Afr ica (figure 9 .2). Other Pan-Afri can mobil e be lts may be inte r-
273
OROG EN Y IN TH E PRI::CA M HRIAN
1, ~'i gu rt
9.2 Tecton ic summary map of
Gondwaoatnnd reconstructed acccrdrng In Smith and Hallam (1970) , showing the extent o r Pan-African mobile regions and the Late Proterozo ic cratons . A , Arabian crat on ; W, West African ; C , Gu yana n: n, Brazilian; S, Sao Francisco ; C, Co ngo; l , Indian; Y , Yilgarn ; N , Kimberley erc.; G . SE A ustralian. M B. Mozambique belt ; D B, Darnaran bell. The area with ' V' ornament is the Hejaz accretionary arc province. The Palaeozoic orogenic belt is shown in ruled o rnament . After McW illiams ( 19KI) .
pre ted as intracrat on ic or int racontinental . such as t hc Dama ra - Zam besi be ll betwee n the Congo and Kalahar i cratons. In th is case there is no pa laeoma gnet ic evide nce for ocean closu re . A th ird type o f Pan -African mo bile zone is rep rese nted by th e large regio n o f tectonotherma l activity co vering much of north ea stern Africa from Morocco to Ethiopia and t he neigh bo ur ing Ar ab ian shie ld. T his zo ne is marginal to the We st Gondwana continent. and co uld be analogou s, to some ext ent. to the Co rdille ran o roge nic belt o f the western Am ericas. Wc shall d iscuss three diffe rent Pa nAfrica n domai ns to illust rat e t hese co nt rasting tecto nic se ttings: the A rabian-Nubian shield , t he Mozambique belt , and the Da mara bell.
Pan-African history of the A rabian-Nubian shield T he Pan-African mobile zo ne of no rt heast Af rica and Arabia is exposed in the ad joining shield area s o f Nubia a nd Saudi A rabi a. A fter closing t he Cenozo ic Red Sea rift , the zo ne is abo ut llOOkm in width , a nd exte nds fro m the nort hern end of the Red Sea to Et hiopia , where it is inte rrup ted b y younger rock s of the African Rift syste m . To the south, it is co ntinuou s with the Mozamb ique belt . Thc tectonic inte rp ret ation of the region is
discu ssed by Ga ss ( 1981) , who points out that tectonothermal activity ex te nde d fro m c. 1200 Ma unlit c.450 Ma Hr . fro m Mid-Pro terozoic to Lower Palaeozoic lime s. This period of mobiliry thus spans a longer time th an the who le of the Phanerozoic , altho ugh it is compar ab le with the life of the Co rd illera n o roge nic be lt of Nort h Am e rica , for example . T he Pan Af rican as o riginally de fined by Ke nnedy ( 1964) was restricted 10 the peri od 650-450 Ma BP. G ass recog nizes t hree maj o r divisio ns termed Lower, Middle and Upper Pan-A frican respectively. although th ere is co nsiderable ove rlap in the radiom etric age ranges o f rocks assigned to t hese divisio ns (Figure 9.38 ). Th e o ldest roc ks of the Lower Pan -A frica n a rc metasedimen tary qu artzites tho ught to represent a passive- ma rgin sedim ent ary wedge fl anking the Archaean Nile craton (Fi gure 9.3A) . No ev ide nce of co ntine nta l basem e nt o lde r than 1200 Ma BP has bee n found within this Pan-A frican zo ne . The quartzites ar e overla in by a thick seq uence (> 12 km) of basalts and basaltic a nde site s with ocean ic affinities , inte rcalated with greywackes. carbon at es and che rts. Th ese supr acrus tal rock s we re invaded by basic plutons dated at c.900 Ma BP. In the Midd le Pan -A frican , the bulk of the rocks of the region were formed ; 50 - 60% o f
274
GEOLOGICA L STII.UCTU RES AND MOVING PLA1'ES
Cairo
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275
OIWG f.N Y I N THE PRECAMBRIAN -
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figure 9.3 ( B) Schema tic represe ntatic n of the age ra nges of magmatic activity in NE Africa and Arabia , divided into Lowe r. Middle :md Uppe r Pan-African an d post-Pan-A frica n periods. NOll' lha l activity is effectively continuous. From Gass ( 1981)
these comprise pluton ic rocks of gabb roic to gran itic composition. These invade supracrustal volcanic rocks of andesitfc to rhyolitic character. Stro mato litic limestones and cherts a rc widespread, but form a relatively minor component by volume of the supracrustal assemblage. The overall picture obtained from a very complex outcrop pattern is of a series of e mergent volcanic arcs depositing material in sha llow subsiding basins, some of which accumulated more than 10km of material. The volcanic rocks appea r 10 have become more siliceous with time. T his supracrustal assemblage was deformed and invaded by a series of syntectonic and post-tectonic dioritic to granodioritic plutons. The latter range in age from c.820 to c_660 Ma. The plutons often form broad linear zones around 50 km wide, aligned in a N-S to NE-SW direction. These zones a re interpreted as the roots of volcanic island arcs. Between the zones lie belts of mainly supracrustal material in which a number of ophiolite masses have been identified (Figure 9.3). Bells of ophiolite fragments are interpreted as pieces of ocea nic lithosphe re preserved along collision sutures (Shackleton , 1986). The relationship of some of these linear ophiolite zones to adjoining dated rocks suggests at least three periods of emplacement, and therefore collision, at c. lOOO Ma, 800 Ma and 600Ma BP respectively. The base of the Upper Pan-Af rican is mark ed by a regional unconformity at the base of a series of unme tamorphosed and undeformed
silicic volcanics and volcaniclastic sediments. The basal units comprise fl uviatile to shallowwater clastic sediments with intercala ted stromatolitic limestones. This supracrustal asse mblage is intruded by calc-alkaline granite s and granod iorites with ages in the range 650610 Ma. The geochemical characteristics of the Uppe r Pan-African igneous rocks are also indicative of a volcanic arc, but the lack of metamorp hism and defor mation , except for block faulting, suggests that this arc was developed on continental crust. T he end of the Pan-African, according to Gass, is signalled by the cessation of calcalkaline magmatism (i.e. of subduction) and the incoming of pera lkaline magmatism comme ncing about 600 Ma BP. The above sequence of events is interp re ted as evide nce of the grad ual and progressive cratonization of northeast Africa by the successive building and subsequent accretion of ocea nic volcanic arcs. Evidence for inte rvening ocea nic material is present in the form of ophiolite zones, which may represent either cryptic sutures Or the remnants of obducted sheets (Shackleton, 1986). According to Shackleto n, in the Eastern Desert of Egypt, a huge shee t of oph iolitic melange overlain by calc-alkaline volcanic rocks has bee n thrust over continental shelf deposits. T he thrust has a gentle regional dip to the southeast, but is refolded so as to form a number of separate outcrops (Figure 9.4) over a distance of 350 km. A NW-SE elongation lineation
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FI&u~ 9.4 Interpretat ive struct ural profiles across the Eas ter n Desert o f Egypl al 2fl"N (u ), and from weo! Haimur in EgypilO Sol Ha med in the Suda n al around 2O"N (b). to show how Ihe ophiolite outcrops may he linked 10 form lhe regional gentl)' di pping sbeeu interpreted as IUIU Il: zones (see Ihe three nor thwestern sheets of Figure 9.3A). From Shackleton (19RS)
OROGE NY IN TH E I'RE CAMB RIAN
indicat es northwestward s t ranslat ion with a minim um displacement of 100 km. T he shee t is con sidered to re present a co llisio nal suture tre nding NE - SW across t he Red Sea and dippi ng so uth-eastwards. Prese nt-day vo lcanic island arcs rar ely exceed 150 km in width with active magmatic cor es less tha n 50 km across. If th e Pan-Africa n a rc system were of similar dimensions, more than ten coul d be acco mmodated within the Nub ian-A rabian belt duri ng the 300 Ma o r so o f subd uctio n-re late d magmatism , represenling nume ro us subd uction zones probably wide ly separate d in time and space . T he prese nce of older co ntinental crust on the e aste rn side o f the Pan-African belt , in Sa ud i Ar ab ia (Stacey and Hedge. 1984) suggcsts that a rc accretio n may have been finally te rminat ed by co ntinent -continent collision, pos sibly rep rese nti ng the northward co ntinuation of the Moza mbique belt now to be descri bed .
The Mo zambique belt Th e Pan-A frican mo bile zone of the ArabianNub ian shie ld is dir ectly along-strike of the Moza mbiq ue belt , which. acco rdi ng 10 Shackleton (1977, 1978). is a d irect co ntinuation of it. However, as Shac kleto n po ints out , the lowgrade met amorphic assemblages of island-arc deri vation in the north are very different from the high-grade metasedim ent s and gran ito id gne isses of th e Mo zambique belt in East A frica . Th is belt co ntains lar ge areas of o lder co ntinen tal basement yield ing A rchaean to Mid-P roterozoic ages a nd variably affected by Pan-A frican deformat ion and metamorphism with ages ranging from c. 950 to 550 Ma . In the cen tral part o f th e Mozam biqu e belt in Kenya and Ta nza nia, o nly Pan-African ages have been obta ined, which suggests that olde r con nn en tal materi al her e may have been thoroughly re wor ked and its geochro nological signat ure obli tera ted .
T he wester n front of th e belt in the E . Af rica n secto r lies alo ng the marg in of the Tanzanian craton , whe re E- W Archaean stru c-
277
lu res are sharply tr uncated along a zone of mylo nites several km wide (Figure 9 .5). East of the mylon ite be lt, th ere is a tra nsition fro m britt le 10 ductile deformatio n co rres ponding to an increa se in metamor phic grade . T he eastern limit of this zo ne of stro ng Pan -African deformatio n and metamorph ism is thought to lie between mainland A frica and Madaga scar, which co nta ins extensive Ar chaean and MidProte rozoi c doma ins. O n th e easte rn side o f Mada gascar , prior to the Mesozoic brea k-up of Gondwanaland , lay the ea rly Precambrian shie ld of Penin sular Ind ia , which exhibits a quite different pa laeo magnetic pola r-wander path fro m Africa and is therefo re co nside red to have collid ed with Africa d uring the Pa nAfrican period of activi ty. T he rock asse mblage in the Mozamb ique bell in the E. African sector consists o f granitoid gneisses with wides prea d and abun dan t intercalatio ns of metasediments. including qua rtzites, marb les and graphitic pe lites. Th e qu artzites are more abundan t in the west, whe reas marbles are co mmone r in the east. Along the wester n fro nt , q uartzites rest unco nfo rmab ly on the A rchaea n base ment of the Tanzan ian crato n. Shackle to n co nsiders that the metasedim enta ry assemb lage as a whole prob ably rep rese nts a con tine nta l shelf sequence with an o rigina l ea stwa rds facies change fro m near -shor e sands to deeper -water limestones, rest ing o n older granitoid basemen t now transform ed to gneiss. II is a fam iliar problem in P reca mbria n resea rch that the detailed stratigraphic data necessary to subst antiate this interpretation ca nnot be o btained. There are thr ee zones within the be lt tha t Shack leton identifies as poss ible ophio lite hor izons, one of which can be traced for more than 150 km. Met avolcani c roc ks associated with one o f these op hiolite bodi es yields a PanA frican age of c.660 Ma . Th e western half of th e E . African sector of the belt is domi nated by E- o r NE -dipp ing planar structures (my lonite fabri cs, foliations and litho logical layeri ng) . Thi s st ruct ura l package is interpreted by Shac kleto n as a n imbri cat e stack of th rust slices of Ar chaean base-
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I
O ROGE NY I N TH E PREC.... MBR I....N
rncn t and subordinate Proterozoic sed ime ntary cove r, with t he o phiolite shee t represe nti ng a high structur al level. If the o phio lite bel ts represen t t he same shee t, ove rlying th e platform cove r, this shee t must root in a suture lying on the easte rn side o f the be lt (F igure 9 .5) . Elongat ion lineatio ns in th is zo ne are co nsiste ntly NW-SE in trend . However, in the ce nt ral pa rt o f the secto r. the linea tions are N- S. Shackleton an d Ries (1984) interpre t th is cha nge in trend as the res ult o f a later st rikeslip relative move me nt betw ee n the two con tine nts after collision. They point in support to t he lack of cor responde nce betw een the E- Wt rend ing A rchae an greensto ne belts of the Tanza nian cra to n, o n the west side of the belt , an d the N- S-trending high-grad e gneisses of t he Mad agasca r A rchaean.
The Dumaran belt in Nam ibia T he Pan-A frica n mob ile belt syste m in SW Afri ca co nsists of two branc hes ( Figure 9 .2): o ne pa rallel to t he coast o n the west side of th e Co ngo and Kala har i cratons, the Gab on-Cape belt , an d the o ther trend ing almost at right angles to it and sepa ra ting the two cratons. T he la tter belt is known as th e Damaran belt and links with the Zambcsi belt further cast. T he struct ure of the sou thwe stern end of the Da ma ran bel l, representing the zo ne of intersect ion of the two Pan -A frican bells, has bee n studied by Cowa rd ( 1981) . Th e Da ma ra n belt is oft en descri bed as an exa mple of an intracrato nic mo bile belt (see e. g. Shackleto n , 1976) . T here is no evide nce of relat ive mo vement be tween the Congo and Kalah ari cratons, within the limits of resol ution of the palaeomagnetic data . T he West Gondwana craton (F igure 9.2) appears to have beh aved as a cohe re nt unit bot h before and afte r the Pan Af rica n eve nt s. However, q uite large re lative moveme nts (of pe rha ps up to 1000km ) co uld not be de tec ted palaeom agnetically, and the evidence leaves open the possibilit y eit he r o f quite large intraco ntine ntal displace men ts, or of the opening and subseque nt closure of a small ocean basin.
279
A map of the Dam ara belt (F igure 9.6A) is described by Hawkesworth et at. (1986). Basement granites o f the Co ngo and Ka lahar i cratons form respectively the north western and southeastern margins o f the belt , which is about 350 km across . Sma ll basem ent inliers occur with in the belt as well. A large proportion of the ou tcro p is made up of metased ime nts divided into two separate sequences. T he olde r seque nce, the Nos lb Group , co mprises coa rse clastic deposits ove rlain by shelf carbonates up to 4 km in th ickness. Po tassic lavas an d assoc iated alkali ne plutons found alo ng the northern mar gin ar e t ho ught to be coe val with this G ro up. T he ove rlying sequen ce con sists of ca rbonates, qu artzites, pelites a nd, locally, graphitic schists tha t reac h a thickne ss of 16 km in the no rt h. Ca rbonates are dom ina nt in the nort hwest, but biot ite-schists rep resenti ng clastic deposits a re more abundant in the so uthwest. T he latt er contain a layer o f mafic vo lcanic rock , t he Match Less belt , tra ced for over 350 km , although only a few m th ick. T he centre of the belt is marked by a zo ne of syntecto nic to post-tecton ic granitoid plutons ra nging fro m diorites 10 gra nites and including highly potassic varieties . The Dam aran sed imen ts were depo sited between 1000 and 750 Ma BP, an d most of the Damaran intr usions were emplaced :n the peri od 570- 450 Ma BP during the later part of the Pan-African timespan. Widespread regional metamorphism occu rred at the same tim e. Acco rding to Coward (1981) , th e major phase of deformation in the belt , which pro duced the NE -SW struct ural t ren d , predates the earliest gra nites dat ed at 560-550 Ma. Later deformation in the ce ntra l zone produced block-faulti ng an d a rching. However , along th e sou thwestern mar gin, defo rma tion associated with SE-directed thrusting co ntinued until abo ut 500 Ma OP . A later suite of pos t-tectonic gra nites was emplaced around 500-450 Ma OP. Geoche mical stud ies of t he me tased ime nts suggest that they co ntai n ve ry litt le new material, but th at t he rocks of the bel t, both sed ime nta ry and igneou s, mainly co nsist of
280
G EOLOGICA L STRUCT U RES A N D MOV ING Pl.AT FS
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Figure 9.6 Str ucture of Ihe Da ma ra bell. (A ) Tecto nic; ske tch ind icating I ~ infe rr ed mo vement duecnoes rela ling 10 the three main phases of de form at io n. N K. Na uklof l nappes; ."ii, Swakopmund; W. Windhoek ; A. area of inter fe rence structures be tween F ilind F2 ; too thed lines , thrust zo nes. From Cow ard ( 198) . wit h pe rmissio n. ( 8) Simpl ified map o f the Da mar a Ribeira be ll, showi ng the diSiribu,ion o f basement. grani tic intrusions arid metasedimentary cover . and the arrangement of met a mo r phic isog rads in the re gio n o f inte nect ion bel"....cc o the Dama ran a nd coastal be lls . F rom Hawkesworth ~I af. ( 1980 ). with permissio n.
rewo rked basemen t ma teria l. A possible exce ptio n is the mafic Matchle ss be lt, which has a mid-ocean-ridge basalt chemistry. However, the gran itoid intrusio ns display intrap late rat her than subd uction-related magma tism. The balance of the geoc hemical evidence is the refo re in favo ur of an intraco ntinent al origi n for the belt . T he structural study by Coward (1981) ide ntifies three main region al moveme nt phases from the structural histo ry (Figure 9.68): the first involved sinist ral converge nce along the nort hern arm of the Gabo n- Ca pe be lt. pro-
ducing overthrusting towards the so utheast along the Sesfonte in thru..s t zo ne. This phase , according to Co ward, is not represented within the Da mara be lt itself. Th e second ph ase produced refolding o n N-S axes in the northern arm , but in the Da mara belt is associated with recumbent west-verging structures with an ENE-WSW elongation lineation . The se structu res are interpreted by Coward as a tow-angle shear zone with a movement sense sub-parallel to the trend of the be lt. The most intense deformation occurred along the central zone, where the synkinematic granitoid int rusions
OROGE NY IN THE PIlECAMBIlIAN
are concentra ted . T hese structures in turn are defo rm ed by more upright folds, tr end ing NE- SW to E NE - WSW, that ve rge no rthwes twards alon g the northe rn margin of the be lt . and so utheas twa rds alo ng the so uthe rn ma rgin. T he latt e r marg in is a major thrust zone car rying stro ngly-defo rmed Dam aran me tase dime nts onto the Kala ha ri craton. Coward inte rprets the s tructura l paucm in term s of an early NW -SE ob lique co nverge nce alo ng the no rthern arm , follo wed by orthogo nal conve rge nce ac ross the northern be lt. coupled wit h diff erential strike-slip movemen ts in the Dam ara n belt alon g a low-angle shea r zon e para llel to th e tr end of the belt . Th e t hird pha se re pr esen ts o rthogona l conve rgence across the Damaran be lt. However , Hawk eswort h er al, ( 1986) cite evidence indicatin g that t he Damaran belt may be olde r than the G abon- Ca pe be lt. The re seems to be no ge neral agree ment co nce rning the plate-tec ton ic inte rpretati on of t hese two belts. It has bee n sugges ted that the Mat ch less ma fic be ll may re present a co llisio na l s ut ure . Howe ver the re is no evide nce for subduction -re la ted magmati sm , and the o the r ev ide nce is suggest ive more of an ex te nsiona l rift environmen t, producing a high-tem pe ratu re belt . Subseq ue nt t ran spre ssionel and co nve rge nt mo vements along this thinned and th erma lly weakened zo ne co uld have been purely intracontinental, reflecting differential movement s bet ween t he neighbouring crato ns. U ncertainties of thi s kind are co mmon in the int erpretation of Precambrian mobi le be lts and illust rat e t hc problems caused by impe rfectio ns in the quality of the evide nce availab le 10 us.
281
the Ea rt h, and Deamley (1966) believed tha t the Grenville mobile be lts he ra lded the sta rt of a third major o rogenic cycle, f unda mentally diffe rent fro m the preceding t wo, that was to ex le nd into Phanerozoic time 10 include all subseque nt orogenies . H is belief was fou nded on the observatio n th at the Grenvi lle mobile be lts were relat ivel y na rrow, regu lar , an d linear, cutting across all pr evio us be lts , and we re more a nalogous to t he Pha nerozoic be lts than to the mo bile zo nes of the preced ing H ud sonia n cycle and of the Archaean. La te r work, and the a pplicati on of plate tecto nic p rinciples , has tended to red uce t he significance of the G renville ' revol utio n' and to emphasize simila rities be tween pre -G renville and post-G renville be lts . Thus there a rc Ea rly Prote rozoic be lts with strong similarities to so me of the Pan-Africa n zones described above . Th e Mid -Prot erozoic mobi le belt system of the North A tlant ic regio n comp rises three bra nches : one is the G renv ille bel t, sensu stricto, along [he south-easte rn margin of the Ca nad ian shie ld; the second is th e Svcco no rwegian bel t o f SW Swe de n and so uthern Norw ay ; and the third is the Ca rotinidian belt of E . Greenland ( Figure 9.7) . After resto ring the Mesozo ic ex te nsio n acros s t he North Atlant ic, these three be lts form a co ntinuo us Y-sha pe d syste m intersecti ng in the region of the British Isles. T he northern o r C arolinidian arm is poorly known a nd is interrupted by the Caled o nian belt . W e sha ll d iscuss o nly the Grenville an d Svecono rwegia n be lts , and the ir possib le interrelationships . Th e Grenv ille Province
9.3 T he Mid-Prot er ozoic GrenvilleSveecnorwegie n system
The e xposed part o f the Nort h American G renville be lt , in C anada a nd in th e Ad irondack Mounta ins of New York State in the D uring th e Middl e Proterozoic period (1800 1000 Ma BP) , an ex te nsive network o f mobi le - no rt he ast USA , is known as th e G renville belts was form ed in bot h Lau rasi a and GondProvince (Fi gur e 9.8) . In C anad a, t his be lt wanala nd . The 'G re nville o rogenic cycle' was exte nds for 1500 km alo ng strike, and is over recognized by Sutt on (1963) as o ne of t he 600km in widt h. The date of t he G renville ma jor s ub divisions in the tecto nic histo ry of 'orogeny' has traditio nally been regard ed as
282
GEOLOGI CAL STRUCTURES A ND MOVING PLATES
fig u~
UPOSE D
£="'1 La
oascUll(o
r:-:--:l
c. 1000 MOl BP, a nd the Gre nville Pro vin ce was affected by a majo r tectono-the rmal even t
a roun d this time . Most K-A r ages ra nge from 1100 M Ol to 8(X)MOl . signify ing slow uplift and
coo ling over a lo ng pe riod. However o lder igneo us and tectonic e ve nts a lso affect the Pro vince: in particular , a region al suite o f intrusive plu tons with da tes ranging fro m c. 1500 to c. 1400 Ma . A general description of the Pro vince and a tecton ic interp retat ion are prov ided by Baer ( 198 1) and Da vidso n ( 1985) . The maj or par t of the Province appears to co nsist of A rchaean and Ea rly Prote rozoic basement th at can he tr aced into the Gre nville be lt from the ad jo ining cra ton . T he distinctive sed ime ntary asse mblage of th e La br ado r belt (see 9.4 ), deformed about 180Q Ma BP in the Hudson ian orogeny, can be recognized o ver a
L:....:..J
9.1
Location of the Mid-P ro -
terozoi c G renville -SVC:COllOrwcgia n o ro-
ge nic belt syste m o f the No rt h Atlanti c regio n (do tted ornamen t). together with the late r non bcm and sou the rn Caledonide: belts.
distance of 400 km from the margin of the G renville belt into t he inte rior. whe re it is progress ive ly deformed and d isrupted by Grenvillian defor matio n ( Roach an d Duffel l, 1974). Thi s de formation has p rod uced ove rt urne d fo lds tre ndi ng sub-pa ra llel to the Gr enville front. A second maj o r phase of deformatio n produced N - $ to NNW- SSE· t rending folds in a wide zone in the inte rior of the be ll . Th e Archaean and Early Prot erozoic basemen t of the Province is intruded by a major suite of plutons co nsisting of anorthosites and related rocks dated at 1500- 1400 Ma . T hese bodies for m a wide be lt that e xtends from t he Hopedale reg ion o f northern La brador to t he Adiro ndack s (Figure 9.8A) . The belt th us cuts oblique ly across th e G re nville Province . extend ing 400 km north o f the G renville fro nt at
Figun: 9.8 (A ~ Simplified tecto nic ma p of lhe G re nville be ll. Note the detnbunon of pre-G renville anon hcsue -mangerue pluton s and of Prot erozoic su pracrustal reek s. Th e Early Pro tero zoic sedime nts of the Labrador be lt (sec Figure 9.15) can be traced lo r a lon g distance inlo the Grenville belt. Th e princip af tccto nic zone s are : I, the Forelan d zone ; II, the G re nville Iro nt zo ne, consi$ting dominantly of basement gnei$.<;CS uplifted in re lat ion to the forelan d ; III , the Ce nlral G neiss zone, co nsisting of high -grad e gne iSSC$ with inte rfoldcd G renville Group met asedim ents, sub-div ided into the O nta rio (A ) and Q uebec (8 ) sectors, which differ mainly in the orie nta tion of the st ruct ures ; IV, the Ce ntra l Mel ascdi menlary zone, which co nta ins the mai n o utcrop of the G ren ville Supe rgrou p ; V, the Ce ntral Granulite terrain, characte rized by gra nulile-Iacie:; gneisses and anorthosites; VA is the Adiro ndack sector ; VI, the Baie Comea u segment , whidt is similar to V bUI in amphibo lite Iacies; V II , the Eas te rn G ren ville zone, which co nta ins simila r roc ks to V I bu t co nta ins in add ition lo w-grade suprac rust al cover similar 10 th ai of the Nain pro vince 10 the eonb. (8 ) Struct ura l sketch sections across th e G re nville belt (N W- SE) to illustrate the gene ral relatio nship between basem en t gneisses (wiggly o rna ment) , A rchaean metasedim ents (g ree nsto ne belt facies) (close stipp le), and Proterozoic supracrustal cove r (dashed o rna ment). Silts and diapirs of the enerthosue-mengerue suite are also shown. Alter Wynne-Edwards ( 1972).
283
O ROG ENY I N TH E PRECAMBRI AN
A
Nova Scotia
. . .. ". D
L2.J 0
AnonlloS
Ma ~;le .
MOtllO", t e p Lutona
P
0 11>0' tect ork:
auart Zo·loklsP. I~ >C
- " ". ~
a_sse, (me.....' )
". B
0
16 0
~m
b~".,
Bouodar y w,th younge.
co"",
c I roc~ .
284
GE O LOG ICAL. ST RUCTURES AND MOVING PLATES
its no rthern end. Th e emplace ment of these plu to ns is genera lly co nsidered to pre-date the deposition o f the sedi me nts of the G ren ville G ro up, altho ugh Bae r be lieves t hat later mo bilizatio n of ce rtain plu to ns occ urred dur ing the G ren villian deforma tio n . The weste rn boundar y of the ano rthos ite belt co incides with the Chibou gam au- Ga tineau lineament . which ma rks a majo r cha nge in characte r of t he basem ent gne isses. BaCT suggests thai a ve rtica l displaceme nt o n this lineament predates the deposition of the Grenville Group sedime nts th at occur on both sides of it. T he metased ime nts co mpr ise a sequence o f carbo na tes. q uartzites and petit es associated with both felsic and ma fic volcanic roc ks. Th e mar bles alo ne are at least l 5 km thick. Da tes of 1300Ma and 1250 Ma on members of the volca nic suite are he ld to give the depositio nal age for the Group. T he nort hwestern margin of the metasediment ou tcrop is mar ked by a line of alkaline pluton s da ted at 1280 Ma, which is interpr eted by Ba er as ev ide nce of a co ntine ntal rift grabe n, in which th e thick sed imentary pile was deposited . Regional exte nsion is also ind ica ted by the fo rm ation o f graben at Seal Lake (a t c. 1300 Ma 81'). along the Kewee nawan rift (at c. l 130 Ma), in the Ga rda r Province ofS . G reenland (a t1245- 1020 Ma) , and by the occurre nce of dyke swa rms pa rallel to the G re nville fron t (a t c. 12OQ Ma). T he greater part o f the G renv ille Prov ince has been affect ed by a high-grad e metamorph ic event yielding uppe r-a mph ibo lite to gran ulitefacies assemb lage s in sed iments and base ment rocks. This event is d ated at c. 1150 Ma ee in the southwes te rn part o f the Province . Bae r no tes the co nce ntratio n of Rb - Sr whole-rock da tes of aro und 1100 Ma in a cent ral zone of st ro ng NE - SW de forma tio n , and suggest s th at the re may be two main G renville tectonic eve nts , o ne at c. l 100 Ma ee, and the second at c. 950 Ma, re late d to the final uplift of the Pro vince . Major th ru st be lts occ ur along several secto rs o f the Gre nville fr ont. In the nor th , in the Seal Lake region , thrustin g is da ted at c.950 Ma ee. In the Cent ra l G ne iss bell (zo ne II of
Figure 9.8) , a series of sinuo us she ar zones separating d istinct gneissose doma ins is de scribed by Da vidso n (1985). A majo r SEdipping shear zone also separates this doma in from the adj oining Central Met ased ime ntary belt (zone III ) . The minor str uctu res indicat e a no rthwestward s sense of movement on these zo nes, tentatively d ate d at c. l l 00 Ma ae. The present crustal thickness of the Pro vince var ies fro m a maximum of 50 km alo ng parts of the front to between 30- 45 km elsew here . Th ese data sugges t that tectonic th ickening by th rusting in t he marginal zone is still preserv ed , whereas the high ly metamor phosed ce ntral zo ne co rrespo nds to an up lifted seg ment of o riginally thickened crus t, now mu ch reduced in thickn ess. Acco rding to Baer and Dav idso n, there is no good evidence in t he exposed Grenvi lle belt for oceanic closure in the form of a bd uct ed ophio lites or crypt ic sut ures; however , Windley (1987) believes that there is ev ide nce fo r a collisio nal sutu re wit hin the Ce nt ral Met asedimen tary be ll. We sha ll co nsider the plat e tecto nic implicat ions of t he Gre nville o rogen y later , after d iscussing t he Svcco norwegian . The S vecono rwegian belt T he Mid-Pro terozoic Sveconorwegia n mobile belt occupies the western half of so uthern Sweden, and form s the Precambrian fore lan d to the Ca ledonides in southern Norway. Mid Pro terozo ic dates in the Prec ambr ian base ment gneisses within the Caledo nides e nab le the belt to be traced northwards to link pres umably with the Carolinides belt of E . G reenla nd (F igure 9.7). This be lt, and t he associated tecto no the rmal events, h as been co rrelated with the G renville belt for many ye ars . Th e maxi mum exposed width of the belt is about 600 km acro ss south Nor way a nd the northern part of the Swed ish secto r, and is divided into two parts by the Permian Oslo G raben . The ma in Sveco norwegia n tecton othermal eve nt (orogeny? ) is ge nerally assigned to the period 1200 - 900 Ma Bp. A general description of th e Sveco norwegia n
OROGEN Y IN T HE PRECAMBRIA N
bel t (Figure 9.9) is provided by Berthelsen ( 1980) . The easte rn boundary of the belt is mar ked by a major shea r zone , term ed the Protogine Zene , se parating the Eastern Segment of the be ll from the Svecokareliun (Ea rly Prot er ozoic) crato n 10 the east. This shear zone ext end s from Skene in the south to the Caledonian fron t, and is marked by a wide bell of inten se defo rma tion, know n in Sweden as the ' centra l Swedi sh schistosity zone'. South of La ke Vattc rn, and in the v armtand- Kop parberg secto r no rth of Lake Vancm . the zone is a low-angle west-dipping thrust that, in the latt er sector , carries c.1670 Ma-old late Svecoka relian granites eastwards on to post-Sveco karelian se dimentary cover o n the eastern cra ton. Berthelse n est imates a minimum d isplacement of 50 km o n the th rust in ViirmlanJ . This th rust cuts an earlier easterly-dippi ng shear zone from Lak e Vanc m northwards.
....
P '01 01l1ne Zone
-,
,/
\
10 0 km
\ \
\
\ 1
285
The rocks o f the easte rn segment consist pre do minantly of gra nito id gne isses apparently der ived mainly from Svecokarelian plutons, and known as the Pregothi an gneisses. Occasional sup racrustal remn an ts beco me more freq uent to wards the west, a nd minor amphibo lite bod ies of vario us ages are common. K- Ar cooling ages record regio nal uplift of the belt bet wee n 1050 and 950 Ma RP. Most of the dcto rma tlon of the Pregoth ian gneisse s in the Easte rn Seg ment is pro bably Sveco karclia n in age (La rso n tt al. , 1986). Th e Eastern Segm ent is separa ted from the Median Segment by another major west-dipping shea r zo ne te rmed the Mylonite Zone . No rth of Lake Vanern, this zone also cuts an earlier NE·dipp ing shea r zone (Figure 9.10) in a similar mann er to the Pro togine zone to the east. Th e rocks of the Median Segment consist of Pregothlan basement gneisses similar to FiR,urt 9. 9 Principa l tec to nic sub-div isions of the ce ntr a l part of thc Sveconorwc gian mobile be lt in SW Swede n a nd S . Norw ay. SVK, Svecokarclia n ( Ea rly Prot e rozoic) c ra to n; £ S , Eas tern segmen t : MS, Med ia n segme nt: O. OSlfold - Marst ra n
286
G EOLOGIC A l- STR UCl"URES AN D MOVI NG Pl,.ATE S
EA STER N
---
I
I
SEG ME N T
LAKE
•
VANE1N
r1
"
l<1gu", 9.10 Simplified lecton;c map of the Ostfold , Median and Easte rn segments or the Svewnorwcgian belt in SE No rway and adjo ining area s of Swede n. Note the paue m o f linear structures (do ts with Oil";) re-o riente d near the majo r lo w-a ngle shea r zon es . The o utcrop of the D al fo rma tion is sho wn with de nse shadi ng. The post-tecton ic Bo hus-dddefj ord granite is also shuwn. Afl cr Be rthelsen ( 191lO).
those of the Eastern Seg ment but more granodio ritic in co mpos itio n , on ave rage. Th ese rocks have yielded an Rb- Sr age o f c.1700 Ma . In the western part of the zone , supracrustal gneisses of the A mM- Kroppc fj .1i. 1I Gro up, thou ght to be of Svecoka retia n age , occur togethe r with a younger seq uence known as the Oa1 G ro up , deposited abo ut l050Ma S P. The Dal G ro up consists of weakly-metamo rph osed sandsto nes, pelites and impure carbo nates, with basic vo lcanic rocks, and rests unconformably on the Pregothian and Amal gneisses (Figure 9. 10). Both the Da l seq uence and the under lying gneisses are involved in stro ng Svecono rwegian defor mation , pa rticu-
larl y alo ng a thi rd majo r west-dipping shea r zo ne known as the Dalsland Boundary Th rust. T his th rust separates the Median from thc Ostf old or Ostfold-Marstrand Segment , and displaces o lder gneissose basement of the Os tfo ld Segment over Dal rocks. T his major displacement see n in the nort h appears to be replaced southwards by a net wor k of shea r zones . one o f which is the prominent Gota A /v Zon~ th at runs through Gothenb urg (Figure
9.9). Th e Ostfold Segme nt is dominated by supracrusta l gneisses kno wn as the Stora Le -. Marstr and formation. These rocks com prise se mipelitic to psammitic metasediments and
287
OROGENY IN TH E PRECAMR RIAN
mafic volcanic rocks, and are cut by a large nu mbe r of intrusive bodi es of vario us ages and compos itio ns. The older of these yield Svecokaretian ages of c. I7DO Ma that provide a minimum age of deposition for the supracrustal seq ue nce. The structural histo ry of the No rwegian pa rt of this complex segment is described by Hageskov (1980) , a nd of the Swedish part by Park et at. ( 1987; see Ta ble 9.2). The magmatic rocks are described by Samuelsso n and Ahiill ( 1985) and A hiill and Daly (1985). The o lder calcalkali ne granitoid plu to ns of groups A and B give Sveco karelian ages, and may be regarded as pa rt of the basemen t to the Sveconorwegian be lt. However , the group C intru sion s dated at 1420-1 220Ma rep resent an impo rtan t postSvecoka relian magmat ic suite, which has e xperie nced o nly Sveconorwegia n tectonothe rmal activity. The suite includes a regio nal mafic dyke swarm that is recognized along the west coast from the Oslo district to western Oru st , including the Koster islands west of
Stro mstad. The suite also includes alkaline auge n granites of ma rke dly d iffere nt chemistry to that of the preceding calc-alka line pluto ns. Three main phases of Sveconorwcgian deformation a re recognized by Park et al., the ea rliest of which is accompan ied by amph ibolite-facies metamo rphism dated a t c. l090 Ma, and the later by mainly gree nschist-facies. The late Sveconorwegian structures arc cut by the post-tectonic pera luminous Bo husIddefjo rd gra nite, da ted at 891 Ma. The Sveconorwegian structures In the Ostfol d-Marstrand belt (Figure 9.11) are related by Park et al. to movements o n seve ral shear zones, including a major lo w-angle she ar zone with an early NE- SW and a late r NW- SE movement direction. The bel t is bounded in the west by ano ther major N- $ shear zone, which is steep , with a sinistral, strike-slip se nse of mo vemen t. Th is zone extends off the west coast of Sweden to co nnect with the Oslo Fjo rd shear zo ne in so uthern Norway (Figure 9.9). The Ostfold-Marstr and
Table 9.2
(~e
Simplified chro nology of the Lysekil-Marstra nd erea
Figure 9.9). From Park ~I al. (19R7). Age ,
Rocks and saructures
Metamorphism
I. Deposition of
Z
<
:;
Sim a Le-Marstrand sediments and emplaceme nt o f conte mporaneous mafic volcanics 2. Emplacement of group A intrusions (mainly graniloid)
'"< 3 o I deform ation §'" ••Emplacement of group B intrusions (ma inly granitoid ) > ~
~
5. 0 2 deformat ion
M,
c. 1700
MI amphibolue-Iacies with (weak?) migrnatiric vei ning
c. 1650 M2 arnpbibohee-Iacics with intense regio nal migmatitic veining
6. Emplaceme nt or group C intrusions (bimodal)
~ 420
,2."0 z;
7. 0 3 deformat ion
:ii
. _ - ------------------- -- ----- - - _ ._------- - --- - - -- -- -- ? - - ? - - ., ----- - ., -------_.
-e
~
8. Da deform ation
~
o z
§ > VI
9. D5 deformation 10. Emplacement of group 0 Bchus gran ite
M3 amphibolne- Iacies
probably lower grade, and locally retrogressive
1090
28R
••
G EO LOGICAL STRUCTU RES AN D MOVING PLATES
/- - ? -
, r
.....
<,
--- ........
~,
, -,
r \ J
.,.
,.,
D Segment is interp reted as a deeper-leve l. more ductile pari of the mobile belt whe re the basement is more intensely reworked by the Svecono rweglan deform ation than in the more easter ly segments. West of the Oslo Fjord shear zone lies the Kongsberg-Bamble segment of south Norway (Figure 9.9), interpreted by Berthelsen as a western eq uivalent to the Os tfold segment. However, the precise relationship be twee n the two segments is obscured by the yo unger rocks of the O slo graben, and by an unknown str ikeslip displacement along the Oslo Fjord shea r zone . The Ko ngsberg-Bamble segment is separated from the Western Subprovince by yet ano ther major shea r zone , the KristiansandBang shea r zone (Hageskov, 1980). Thi s zone d ips to the east beneath the Kongsbe rg-. Bamb le segment. The Western Subp rovince contains large num bers of pluton ic igneous rocks including charnockit es, monzo nites and , anorthosites , in addition to granites and gra nod iorites. Most of these intrusions appear to have been emplaced between 1200 and 850 Ma
fil:,ur t ' .11 lntc rpretanv e structu ral profiles across the L~sekil - Marst rand area (see Figure 9.9). 52 and the e mplacement of the A, B and C gra nites are prc-Svecon c rwegia n in age. The Svcconorwcgian structures 0 3 and D4 are related 10 movemen ts o n major lo w-angle shear zones and slee p tr ansfer wnes. Fro m Park el a/. (19&7 )
BP, alt hou gh o lde r Sveco karelia n ages have also been o bt ained (see review by De maiffe and Michot , 1985) . In western Ro galand , two major Sveconorwegian deformatio ns arc recognized (F alkum and Pede rsen , 1979) dat ed et c. l l00 Ma and c.l 000 Ma respectively. Th ese produce largescale west-verg ing nappes with a N- S tre nd. Magmatic episodes preceded , separated and follo wed these deform atio ns. Th e re gio nal metamorphism is characteri stica lly high-temper at ure granulite- to amphibolite- facies in this wester n sub-pro vince. Th e Telemark supracrusta l sequence in the easte rn pa rt of the sub-province, co nsisting predomi nantly of qu artzites and metavolcanic rock s, probab ly deposited between 1225 and l000Ma BP, may re prese nt the lateral equi valent o f the Da l Fo rm ation of the medi an segment to the east. Despite t he fact that the Svecono rwegian mo bile belt is comparatively well known , the re are majo r d ifficulties in explaining it in terms of plate tectonic models. Th e wide zone of calc-
OROGENY IN T H E PRECAMBRIAN
alka line plutons mak ing up the baseme nt of the ea stern segments is ge nerally considered to re p resen t an e asterly-dipp ing subduction zon e , o r seri es of zo nes (see Ber thel sen , 1980) , which is a westwards con tinuatio n of the Svecokarelian magmatic arc. There is no ev idence of any pre-Svecok arelian baseme nt within t he Sveco no rwegian be ll , which is probably co mposed of a succession o f Sveco karelian volca nic arcs si tuated alo ng the Sveco ka relian contine ntal margin . Th e po st-Sve cokarelian histor y of the region co mmences with extensional rifling associa ted with the emplace ment of dyke swa rms and alkali-granites in t he period 14201220Ma BP in SW Sweden and SE Norwa y. During th is period , the Telemark shelf seq uen ce was de posited in SE Norway and, possi bly later , the Da l deposits were for med in Sweden in an ex te nsional basin. T he ma in Sveconorwegian deforma tion in th e pe riod 1100 -1 000 Ma BP represents, acco rdin g to Berthelsen , a co llision be tween the We stern Subprovince and the easte rn segments. T he structural evidence from SW Swede n sugges ts that th e conve rge nt movements invo lved an ear lier northeastwards movemen t o n a low-angle s hear zone and on a related stee p NE-SW tr an sfer zo ne, followed by later so ut h-eastwa rds or eas twards movem ents on th e zones furt her eas t ( Pa rk et 01., 1987). In Norway, the so utheastwards movement s ap pe ar to be domina nt (Fig ure 9.10).
Plate-tecton ic interpretation of the G renvilleSveconorwegian system T he pala eo magn etic evidence sugges ts that the maj or Early Pro tero zoic shield regions may h ave fo rme d a single supercontinent at the co mme nce me nt of Mid-P rot erozoic time (Piper , 1982) . On Piper's reco nstructio n (Figure 9.12A), the G renville an d Sveconorwegian be lts a re sub-paralle l, and for m th e so ut he rn margin of the supe rco ntinent . An impo rta nt rifti ng phase is reflected in magmatism and in th e form ation of ex tensiona l sediment ary-volcanic basins both in the Grenville Provi nce a nd in the Ostfold -Marst rand seg -
289
menl of the Sveconorwegian belt. T his phase may be re lated to a region al rifting th at resu lted in the bre ak- up of the eastern end of the supercontinent ( Figure 9.12B), and its subseque nt rotation between 1200 and 1000 Ma SP ( Pa tchett el 01., 1978; Stearn and Piper, 1984). Subsequent closure of the interven ing oceanic region wo uld have resulted in collision alo ng the Grenville sector, and conve rgence
290
GEO LOGICAL ST RUCI"URES AND MO VING PLATES
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....g un 9.12 (A ) Palaeomagnetic reconstruct ion of the continents in the Proterozoic (the Proterozoic 'super-contiaent"] based on Piper ( 1976). A US, Australia ; IN , India ; EU. Europe. Note the ~il ion and exte nt o f Earl y Proterozoic mobile belts in Nort h America and Europe (H , Hudson ian; N. Nags., ugloq idian; K. Ketilidian; SV. Svecoka relian) and the position or the Grenville -Sveconorwegian belt alo ng the margin of the supe r-cc nnne nt. (8 . C) Schemali<: mode l based on the palaeom agnetic resto ratio ns (see Stea m a nd Piper. 1984) of North A merica-Europe be fore a nd afte r the I(KX) Ma-cld end-G renville event, interpreted es a collisional o rogeny. Note Ihat the com mencement o f ecratioe of Europe retauve to North America is related 10 Ihe regional extensio nal eve nt al c. l200 Ma, and that early Sveconorwegian movements (and perhaps Grenyillian al<.o) are attributed 10 cclfisicn o f a micropt ate at c.1100 Ma, prior to the main collision event . Note abo Ihe difference in conve rgence direction beth betwee n the (Wit events, and. in the late r event, betwee n Ihe sinistral component along the Sveconorwegia n suture and the dexlral component along the G renyille suture .
Ca nadian shield . O ther important Archaean cratons are the Slave craton in northwest Ca nada, the North Atlantic craton of S. Gre enland , and the Kola craton of northern Finland and the USSR. The mobile zones that surround these cratons are very much wider than the Grenville or Sveconorwegian belts or , for
example, the Caledonide belts of the Phanerozoic, but are not dissimilar in some respects to certain of the Pan-African zones, or to the Cenozoic bell of Central Asia. The Svecokarelian of Scandinavia and the Hudsonian of the Canadian shield both occupy zones abou t 1000km across. These mobile zones differ
OROGEN Y I N TH E
fundamen tally in characte r from o ne part to another, both along and across strike . Gene ralizatio ns made abou t Ea rly Prot erozo ic tectonic regimes, part icularly in contrast with A rchaea n and Mid- Pro terozoic regimes, a re ofte n base d on one o r two par ticular exa mples, whereas the ne twork as a who le should ideally be viewed in its e nti rety. We shall discuss briefly two parts of this comp lex network: the Labr ador be lt, which forms pa n o f the Hudsonian regime of Ca nada; and the Lewisian-Nagssugtoqldian belt of Scotla nd and S. G reenland.
P R ECAM8 l{I A ~
291
9. 14). The southern sector deve loped into an ocean that subsequently closed , whereas the If -shaped no rthern pa n of the syste m formed intra-cont inental rift basins by crus tal thinning and stretch ing. This nor the rn belt was subsequently defo rmed by shorte ning, which rc-
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The Labrador belt This belt fo rms pa rt of the circum-Supe rio r belt of the Canadian shie ld (Figure 9.13) summa rized by Ba raga r and Scoates (1981). They believe that this belt evo lved from an an nular rift surrounding the Superior crato n (Figur e
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Figure 9. 13 Arra nxemenl of Ea rly Proterozoic mob ile be lts and A rchaea n crato ns in No rth Am erica-G reen land . The circum-Superior be ll co nsists o f the Southern province in the south and lhe Ch urchill pro vince in the nort h and no rthe as t Within the latter province are the nar rower sup racrus tal belts of the Belcher islands (88 ). Cape Smilh (e S8 ) and Labrado r (L 8) be lls. Th e Labra do r sector of the Churchill province (see Figure 9.15) is bo unde d on its northeast side by the North Atla ntic craton ( NA C) _ see Figure 9.23.
Figu re 9. 14 Interp retation o f the circu m-Supe rior belt (see Figure 9.13) in terms of an initia l annu lar extensio na l zone created by southward movemen t of a cont inent al pla te (2) Iollowed by co llision and co mpressio n (or tran spression . as appropriate ) as plate movement was reverse d. Th e eve nts relating to the SE side of the cra to n are speculative becau se of ove rp rinting in the Gr en ville belt. Fro m Baragar and Scoates ( 1981)
292
GEOLOGICA L STRUcrU RES AN D MOVING PLATF.5
suited fro m the convergence accompan ying
oce anic closure along the southe rn secto r. A simple open ing and closing model along these lines wou ld sa tisfy the palaeomagneti c da ta,
which preclude any large relat ive displa cements be twee n the Slave. Supe r ior a nd North
Atlantic crato ns. The simple geom etry of the system demands that bot h dive rgent and converge nt mo vements sho uld be ob lique (transtensional then t ran sp ress iona l) along the no rtheaste rn a nd west ern sec tors respectivel y,
hut more orthogonal on the no rthwestern sec to r. T he L abrador belt . form ing the northea ste rn sector of th is syste m , is s imilar in man y respects to the C ape Smith a nd Be lche r belts
o n the no rth-weste rn side of the Supe rior crato n (Figure 9.13). It consists of a weste rn supracrustal zone (Figure 9.15) , often term ed the Labrado r ' geosyncline' or ' trough' , about 100 km across, with a much wider zone of mod ified Archaea n basement rocks o n its nort hea stern side : T he belt is bound ed to the so uthwest by the Supe rior crato n, and to the no rthea st by the East Nain Province (part of the N. Atlantic crato n -see Figure 9. 13). The Lab rador trough has been inten sively stud ied, and is described by Dimrot h (1981). To the sout heast . the belt is truncated by the Grenville belt (see abo ve) and to the northwest it ben ds round into the Cape Smith be ll. The supracrustal rocks of the Lab rador trough consist of a thick seque nce of sed iments a nd volcanic rocks. The earliest deposits rest o n A rchaea n baseme nt of the margins of the supracrustal belt, and co nsist of about l500 m of coarse arkosic red beds. These are overlain by shelf deposits of orthoqua rtzite. dolom ite and iro n-for matio n, reaching a maximum thickness in the west of about 1500 m also. The she lf deposits are par tly e roded and uncon formab ly over lain in the central pa rt of the tro ugh by locally-derived conglomeratic mass-flow deposits a nd greywackes. These a re succeeded by voluminous basaltic and andesitic lavas mo re than 5 km thick. Most of the clastic sed iments appear to have be en derived from the fore land, and the greywackes are compo sitionally dis-
tinct from the typical synoro genic fl ysch of the Alps and other Phanerozoic orogenic belts. The western margin of the belt is defined by a zone of imbricate th rust sheets dipping to the east. Those detach o n a low-angle deco llem ent plane as in the classical thin-skinned thrust model. East of the thrust belt is a zo ne of tight 10 isoclinal fo lds overturned to the west. Fold axes generally trend NNW and are associated with a strong foliation. Seve ral gene ration s of fo lds with associated foliat ions are superimposed on the large-scale overfold s in the more complex eas tern zone. Accordi ng to Dimrot h, the structural patte rn is st rongly influe nced by ea rly syn-sedimentary growt h fau lts defining a series of blocks, some of which have subsequent ly become overt hrust. The overall structure is illustrated in Figure 9. 16. It is clea rly asymmetric with a westerly vergence . The meta morphic grade increases from pumpellyite-prebnite in the west to amphibolite in the east. Th e metamo rphism is believed to be caused by de ep burial unde r a moder ate geothe rmal gradient. Metamorphism was syntectonic in the west but syn- to posttectonic in the eas t. The age of the trough deposits is not accurately known, but must post-date the 2700 Maold Archaea n basement. The main de forma tion and me tamorphism have bee n dated at 1800-1600 Ma. mostly by the K-Ar me thod ; however the data are poorly constrained a nd their significance unclear . The neighbouring Cape Smith volcanic rocks have yielded a n Rb -Sr date of 2300 Ma , which probably represent s the date of formation of the supracrustal sequence in both areas . Dimroth emphasizes that the mafic volcanic seq uence is not an op hiolite complex bu t bears more similarity to plate au basalts of submarine origin. Mo reo ver , none of the units is allochtho nous: the thrusts are not continuous over long distances, and basement inliers can be recognized in the interior of the be ll. The shortening of abou t 100 km estimated from the exposed structure is attributed by Dimroth to A-subdu ction, in which an eastern lithosphere slab is detached from the crust and dips be low
293
OROG EN Y IN THE PRECAMB RIAN
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294
GEOl.OG ICA.L ST RUcrURES AND MOV I NG PLATES
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f<'il:ure 9. 16 Diagrar nrn anc tect onic !"rofile across Ihe Lab rador he ll (see Figure 9.15) 10 illustrate the asymmetric arrangemen t o r SW -ve rging thrusts and folds. Afle r Dim roth ( 19M 1).
the weste rn craton. He thus visualizes the ' crustal sho rte ning be ing transferred to the base of th e crust by the thrusting and then taken up by overlapping of th e man tle lit hosphere . Seve ral quest ions are posed by this interpr etalion ; the ori gin of th e wide belt of reworked basem ent a nd Hud so nian granites in the eas t is not acco unted for . no r doe s the mode l explain th e ea rly stretching phase in which the basin was formed . Th er e seems no reaso n to suppose th at th e origin of this belt is any differ ent from th at o f other intra -con tine ntal belts, such as the Damara belt already described , where ea rly divergen ce a nd crusta l th inning is followed by co nve rgent shorte ning. How the mantle part o f th e lithosph er e acco mmodates to such move ments is as yet speculative . If t he mod el sugges ted by Baragar and Scoates is correc t. we might ex pect the co nvergen t deform atio n in the be lt to be achieved by dextral transpression . The Lewisian -Nagssugtocidian system : the western Nagssugtooidian
Th e Nagssugtoqidian mobile be lt of S. G ree nland mar ks th e northern bou nda ry of the North A tlantic cra to n (Figure 9.17A). On the west coast . the belt is ove r 300 km in width. A major sinistral shea r zone separates it from the Rink ia n mo bile belt , o f probably similar age , to the north, but the relat ionship betwee n the two belts is not yet understood . O n the east
coast. the bell is 240 km acros s and is bou nded to the north by an A rchae an block. A general descrip tion o f the be ll is given by Escher et at. and Brid gwater ( 1976), and in a series of papers ed ited by Ko rstgard (1979). The belt cons ists pre dominant ly of re-worked Archaean basement gneisses with interl ayered and infolded belts of suprac rustal metased iments and me tavolcanic rock s (Fig ure 9. 178). T he sup racrustal rocks are d ivided into an ea rlier Archaean group . consisting of pelltes. impu re marb les. and q ua rtzites. and are associated with amphibolites of probable igneous origin. Younger supracrustal rocks tha t post date the Archaean met amorph ism occur o nly in isolated outcrops. mainly in th e nor thern part of the be lt. Th ese consist of pelitic schists, q uar tzites, ma rbles and a mphibolites. Th e qu artzites preserve current bedd ing, and the metasedimen ts as a who le exhibit a lo wer degree of meta mor phism than the base ment gneisses. O nly a few small gra nitic intrusions occu r, which post-da te the main Nagssugtoqidian deformation. Pegmatites, on the ot her hand , arc ab unda nt. In th e so uthern part of the belt . a region al swa rm of met adole rite dykes , the Kangamiut dykes, intru des the A rchaean crato n; these dykes are prog ressively deform ed and met amorphosed with in the Nagssugtoql dian be ll. T he Nagssug toqidian deformation has p roduced seve ral broad , NE - SW, steep shea r zones sepa rated by areas of wea ker dcform a-
OROG El" Y IN THl:': PRECAMBRIAN
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tion. Figure 9.18 is a map of the Nordre Stre rnfjord shear zone, south of Agte . which is a 16 km-wide zone with a sinistral strike-slip sense of movement. Near the southe rn margin of the belt . between Itivdleq and Holsteinsborg, two main phases of Nagssugtoqidian deformation ean be distinguished : the earlier. termed Nag I by the Gree nland geologists. produces dextral strike-slip movements on the stee p E- W ltivdlcq shear zone. and pre-dates the e mplacement of Kangarmut dykes. T he later N ag 2 phase is a regionally more important and pervasive deformation that results in overthrusting to the southeast. Detailed studies of the structure of this critical marginal region of the belt have been made by Watte rson and his colleagues (eg. see Grocon , 1979). The Nagssugtoqidian metamorp hism is high-
grade throughout the belt; a central granu litefacies zone is bounded to the north and south by regions of amphibolite facies. The Lewisian complex
Afte r removing the effects of the North Atlantic opening. the easte rn Nagssugtoqidian and the Lewisian complex of NW Scotland lie along-strike, and only about 400km apart (Figure 9.22A). Similarities between the two belts have been noted by Myers (1987) . However as the Lewisian complex is probably the most intensively studied piece of Precamb rian crust in the world. we shall concentrate our attention now on that region. A general description of the Lewisian complex is provided by Park and Tarne y (1987). and a structural
296
GEO LOG ICAl , ST RUCTU RES A ND MOVING PLAn..s 6S'
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Figun' 9. 18 Simplified structura l map of the No rdr e Stre mfjord shear zone in the ceeual part o f the be ll (see Figure 9 .178) . Th is sinistral shear zone is defined by various roc k units and structural tren ds that become aligned within a NE- SW bell of intense deformat io n. Black , amphibolire s; ruled omamcm, metasediments: crosses, gra nitic and chamoc kitic intru sio ns; blank areas , gra nitic to tona litie gneisses. Afte r Ole se n t: f af. (1979) .
interpreta tion by Coward and Park (1987). The complex forms a well-exposed strip along the northwest coast of the Scottish mainland and in the islands of the Outer Hebrides (Figure 9.19). The exposed width of the belt is about 260 km but, unlike the eastern Nagssugtoqidian, the margins of the belt are not seen . The bulk of the complex is formed of Archaean tonalitic to granodioritic gneisses that are preserved in a relatively unmodified state in the central mainland region and in seve ral small enclaves elsewhere . The remainder of the Lewisian complex has experienced intense Ea rly Proterozoic deformation and high-grade metamorphism. In addition to the Archaean basement gneisses. the complex includes two narrow belts of Early Proterozoic supracrustal rocks and associated intrusions, at South Harr is in the Outer Hebrides and at Gairloch and Loch Maree on the mainland. The supracrustal assemblage ,
known on the mainland as the Loch Moree Group, consists of mafic volcanic rocks with associated narrow bands of siliceous schist, banded-iron-formation, graphite schist and marble, overla in by a thick seq uence of metagreywackes. These rocks were deposited probably around 2000Ma BP. The other importan t Proterozoic addition to the complex is the wellknown Scourie dyke swarm. These mafic dykes are dated at c.2400Ma ee, but some members of the swarm may be as young as 1900Ma in age. They occupy the same key stratigraphic position in Ea rly Proterozoic chronology as the Kangamiut dykes in Gree nland, separating the ea rlier Inverian deformation from the later Laxfordian phases. As in W. Gree nland, the Early Prote rozoic deformation appears to be related to the development of major steep shear zones, the effect of which, over much of the Lewisian, has been obscured by the younger deformations.
OItOGE~ Y
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t"lgure 9. 19 Loca tion map iIlu ~ lfal in g some impo rtant feat ures o f tbe Lewisian complex of NW Scotland . OH F. Ou ter Hebrides Iaulr; SH IC. SO\l1h Harris igneous co mplex; UZ. Laxford shea f zone; CSZ . Canisp shear zone ; GF, G ruinard 'front'; LMG , Loeh Maree G ro up; C, Carnmore; D. Diabaig; K. Kcomore ; R. Rona . From Park and Tarney ( 1987).
T he major Inverian shea r zones probably occu pied (he whole of the no rthe rn and soothe rn regions of .the mainland Lewisian, and most of the O uter Hebrides. Their initiation is tho ught to be associated with the uplift of the
central Archaean block that is dated at c.2500 Ma by the K- Ar method . The Inverian defor mation took place under am phibo lite-facies conditions, resu lting in extensive retrog ression of Archaean gran ulite-facies assemb lages.
GEOLO GICAL STRUCruKES AN D MOVING PL A.TES Qi. b . ig
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4 flgurt 9.20 Sequence o f eenocn pro files i1tu$lrating the Ea rly Proterozoic tecto nic evolution or the Lewisien complex. (I) The l nve nan event , inte rpreted as resetting from the iniliation o r a network of shear zones: note lhal deep-crustal granulites from A are transferred to A ' . (2) The emplacement or the Scoune dykes and tbe Locn Maree Group ( LMG) in an extensio nal (lra nsle"" ronal) environment. (3) The Ladordian 0 1- D2 events, attr ibuted 10 movement s approximately perpendicular to the line of section o n the shear zone netwcrk .estebhshed in (1). NOle that the relationship betwee n the Outer Hebr ides and mainland pans o r the composite profile are speculative. (4) The Lad ordian 0 3 event, producing upright folds and shear zones by movements oblique 10 the line of Section and with a component o r compression along it. ca, central block (central region o r Figure 9.19). From Park and Tarney (1987)
O ROGEN Y IN TH E PRECA MBRIAN
Minor I nve rian shear zones also cut the ot he rwise unmodified Archa ean centra l block. According to Cowa rd an d Park , the majo r zo nes dip be neath the central block , det aching on a low-an gle shear zo ne t hat underlies the block ( Figure 9 .20 (l». The In vcrian shea r zo nes of the mainlan d exhibit a mainly d ip-slip sense of movem ent but with a sma ll de xtra l st rike-slip co mpo nent. T he Sco urie dyke swarm was emplaced over a wide are a from t he no rth coast to as far south as Barra in the O ute r Hebrides. In areas where the y arc less deformed , the dykes ex hibit ev ide nce of d ilatation al emplaceme nt in a d extr al she ar regime . An extensional regime is also ind icated by th e emplace ment of the su p racr ustal rocks of the Loch Maree G roup (Figur e 9 .20 (2» . Alt hough deposited poss ibly 400 Ma after the ea rliest dated Scourie dykes , it is likel y t hat later me mbers of the dyke swarm may have bee n associ ated with the emplacem ent of t he G roup , and supracrustal rock s at G ai rloc h are cut by dykes with typical Sco urie d yke chemistry. The dyke swarm and t he supr acrus tal bas in may repre sent a longco ntin ued ex tensiona l or tran stensional regime. A lthoug h fou r phases of Laxfor dian (postdy ke) de format io n ca n be recognized over mu ch of t he Le wisia n outcro p , the seq uence ca n be simp lified to two events of majo r regional significance . The ea rlier (0 1/0 2) is associated with amphibo lite-facies metamor- : p hism , and is more pervasive and intense t ha n t he second (03), which results in a refoldin g of t he 0 1-2 fabric under mainly greenschistfacies co ndit ions. The 0 3 phase is respo nsible for t he widespre ad , upright , NW -SE folds wh ich are a promi nen t feature of the Lewisian outcro p pa ttern . Minor pegmatite and granite sheets wit h an age of c.1800 Ma post-da te 01 - 2 and pr e-date 0 3. The latter eve nt is p rovisionally dated at c_ I600 Ma from wides pread resetting of K- A r ages in the range 1600- 1400 Ma . The major 0 1- 2 she ar zones are oriented NW - SE, approx ima te ly paralle l to the In ve ria n st ructures and to the Scourie dykes . They vary in att it ude: steep zo nes occur within
299
the ce nt ral block; inclined zone s at Laxfo rd in the no rth and To rri don in the so uth dip ben eath the central block and detach on a lowangle she ar zo ne that co mes to the sur face in the north and so ut h, acco rding to the mod el of Coward and Park ( Figure 9. 20 (3». Th e whole of the Ou ter Hebrides is interp reted as part of this o riginally gently-inclined zo ne . The she ar zo nes are regar ded as an inter-co nnected netwo rk of displace ment planes separating less deformed or und eformed bloc ks whose rela tive move ments ca n be est ab lished by st udying the sense of movem en t o n the shear zo nes. Th e inclined 0 1 zo nes sho w typically mod erateplunging lineatio ns ind icating both de xtral st rike -slip and no rmal dip-slip compo ne nts, whe rea s the sub-horizo ntal 'flats' d isplay NW SE movemen t directions (Fig ure 9.21). 'In c 0 1- 2 defo rmation pa ttern ove rall is prob ably indicative of a co ntin ued co mpone nt of dextra l st rike -slip move me nt. Dur ing the D3 defor mation, two majo r stee p NW -SE shea r zones we re fo rmed , at La nga vat in So uth Harris, and at Ga irloc h. Both ex hibit a de xtr al st rike-s lip co mpo ne nt of movem e nt and arc associated wit h stro ng compress ional sho rte ning ac ross the be lt, forming the prom inent NW -SE upright F3 folds ( Figure 9.20 (4» . The D3 regime is interpreted as dex t ral transprcssio nal ove rall. These changes in tecton ic regime arc summarized in Figure 9 .22 , which shows ho w the y might be interprete d by ch anges in converge nce direction across t he Le wisia n-Nagssugtoqidian o ett. The first phase ( Invcrian-Nag 1) may relate to ove rall N- S or NNW - SSE co nverge nce . Th is arra nge me nt was o riginally suggested by Watte rson (1978) to expla in the co njugate pattern of E arly Pro tero zoic shear zo nes . It relates the coeva l sinist ral displace ment s o n NE -SW zo nes in W . G ree nland an d the dextral/overt h rust displace ments on the NW-SE zo nes in Scotla nd . Th e ea rly Laxfordia n-Nag 2 regime is marked by a change 10 dextral strike-slip in Scotl and a nd overt hrust in W. G ree nla nd . There is no evide nce e ithe r in th e Lewisian o r in the Nagssugtoqidian complexe s of colli -
300
G EOLOGICAL STRUCfU RES AN D MOVING PLATES
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movements e n the inclined shea r zones al Laxford and be tween Oruinard Bay and Diabajg, comb ined with NW- SE strikeparallel movement s on me shear-zone f lats 10 lhe northeast and southwest. From Coward and Park ( 1987).
sia n sutures or of for mer ocea n basins. Nor is there any indication of calc-alkaline magmatism that might betray the former presence of a subd uctio n zone . The belt appears to consist almost entire ly of pre -exist ing Arc haean baseme nt that has been subjected to essentially int raplate tecton ic movements. These movemen ts have not on ly ca used intense defo rma t ion, but have resulted in considerable crusta l heating and the loca l e mplaceme nt of magmas. It is instructive to compare this intra plate
belt with the coeval Ketilidia n belt to the so uth (in S. Greenland) a nd with the Svecokarelian belt to the east (see Figur e 9.12A). These bells display ab undant calc-alkaline volcanic and pluton ic magmatism, and are widely tho ught to represent an Ea rly Proterozoic destructive co ntine ntal mar gin. It is tempting, following Watterson (1978) to ascribe the intraplate defor matio n of the be lts we have j ust examined to processes occurring at that margin. about lOOO km to the so uth.
30 1
O ROGE N Y IN TH E PRECA MBRIAN
INV, NAG 1
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fit:u ~ 9.21 (JI) Recomtruction the Ea rty Prot trozotc belts of Gr« nlol'n,j a nd Srotland a lte r rc~nl l he: enccu o r the No rth A llani te ope ning . 1be rn.tonoho n is ~d on the rcll'lO\l a l oceanic CTU!>I and oa lhe: 3!o1lU mpl lOn or an a~e rage SO% lh inn ing o r COnl ine ntal et u51 o n lhe eontinemal sk lvcs . Tbe NagMugloqidian-Uwi§.ian hell (blad ) a ppcal5 10 lie between two more 5lablt A rchaean ·ptatC$· 10 the no rt h and sou th (ruled omamcn l) . Th e Kel ilidi an belt Ii« o n tbe south side of the A rchaea n craton o r S. G ree nland . (8) Sequ.eoce o r ca rtoo n diagrams i llu ~t rll i ng an interpretation o r th e kinem atic history of the be lt, based o n a change in movement direc tion of th e northern plate with respe ct 10 the southern . Do minan tly co nve rgent mo vement during the lnvcria n and Nag. I pe riod cha nges to dominantly srrlke-shp in the Lcwisian , but co nvergen t in the western NagMugtoq idian, d uri ng Laxfordian 01 - 2 Nag . 2, and back to do minanlly eoe vergcn t in LAxlo rd ian 0 3 limes in the Lcwisian. From Co ward and Park ( 1987).
or
9. 5 Th e Arc haean: a different kind or orogeny?
Rocks of Archaean age for m a number of stable crato ns within the Proterozoic shield
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regions of all the main contine ntal masses (see e.g. Figure 9.13). In addition, a large proportion of the Proterozoic shields consists of reworked Archaea n crust, as we have seen . Archaean regions are traditionally divided
302
GEO LOGICAL STRUCTURES AND MOVING PLATES
into two q uite differe nt t ypes: th e granitegreenstone terrains and the high-grade gneiss terrains . The granite-gree nsto ne terrains co nsist of greenstone belts surro unded an d cu t by granito id plutons. and meta morphosed typica lly in greenschist o r lower facies. T he high-
grade gneiss terra ins consist predom inantly of granulite- 10 amphibolite-facies gneisses of varying type s bu t inclu ding a high pro port io n of broa dly gran itic co mposition. T he high-grad e gneiss terrains a re the prod uct of tectonoth ermal activity of a similar na ture to thai assoc iated with youn ger Precambrian mobile belts, altho ugh the Arc haea n
terrains exhibit certain special character istics. Th e gra nite-gree nstone terrains ar e unique to th e Archaean : there are no precise analogues in t he younger stra tigraph ic recor d . Ce rta in Archaea n crato ns co nsist entire ly of o ne or ot he r of th ese two types of terrain , while in others the two are found in association . We shall discuss two example s in de ta il: the highgrade gneiss terr ain of t he North Atlantic craton, an d the Superior Province of the Ca nadi an shield . Th e latter is basica lly a gra nite-gree nstone ter rain but is crossed by seve ral belts of high-gra de gne iss, and is bo rdc red by region s of high-grad e gne iss te rrain o n its no rth- western and north-ea stern sides. The North A tlantic craton , Th e A rchaean highgrade gneiss terrain of S. Gree nland and the
adjoi ning part of Labrad or is known as the North Atlanti c crato n (B ridgwater et al. , 1973). Th is craton (F igure 9.23), about 700 km across from north to south, and over 500 km fro m west to east, was a regio n of continuo us mob ility durin g A rchaean times, an d con sists almost solely of gneisses in upper amphibolite to gra nulite facies. Bet ween 80 an d 90% of these gneisses ar e broad ly granit ic in composition, predomina ntly to nalitic to gra nodioritic. Within the gne isses are relatively narr ow ba nds an d inclusions of met asedimentar y gne isses such as quar tzites , pelitic an d semipelitic schists, marbl es and banded-Iro n-format ion , and of met a-igneous amp hibolite s and anorthosites. In the pas t, the or igin o f the gra nito id gneisses has bee n hotl y de bated , and the opinion was widely held that many of the gneisses represented gra nitized sediments of broadly se mipelitic co mposition . However, modern geoc hemical studies have dem onstrated t hat the bulk of the gneisses are defo rmed and metamor phosed calc-alkaline rocks probably of pluto nic origin (see e .g . Weaver and Tarn ey, 1987). The sed imen tar y asse mblage is suggestive of an ep icont ine ntal shelf env iron ment, and co ntras ts markedl y with the greenstone-belt assemb lage . Another importan tco mponent of the terrain is the anorthositeleu cogabbro co mplex described by Windley ( 1973) . T his co mplex co nsists of an essoclaFiKU~ 9.23 Summary tectonic map of the North Atlantic craton, with the surrounding Early Pre rerozoic belts. After Bridgwate r t ' 1;1 /. ( 1976) .
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tion o f anort hosite , teucogabbro , and minor gabbro , and exhibits p rom inen t igneous layering . The assemb lage is expose d over a large a rea (F igure 9.24) due to complex fo lding, but is co nside red to represe nt a single sheet. T he rocks o f the crato n have undergo ne inten se and rep eal ed defo rmat io n and met amo rphism over a pe riod of mar e than 100 Ma .
303
Bridgwater et al, (1976) propo se a seque nce of 15 separate events for the A rchaean of S. G ree nland, summarized in T able 9.3. Th e seq uence may be d ivided into two main cr ustfor ming cycles. T he ear lier cycle (events 1-3, Table 9.3) culminated in the emplacement o f granites at c.3750 Ma UP. The later cycle includes a number of separate intrusive event s,
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304
GEOLOGICAl. STRUcrURES AI'lD MOVING PLATES
T a ble ' .3 Simplified seq uence of Archaea n e ve nts in S. G reen land . A fte r Esc he r ~I al. ( 1976) .
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1. Formation of early cr ust (source for
lsua sediments) 2. De posilion of lsua sed ime nts a nd vo lcan ic rocks 3. Int rusio n of A milsoq gran itic loeb 4. Deformation and me tamorphism 5. E mplacement of Ame ralik ba sic d yke swarm 6. Deposition of Male nc sedim e nts and volcanic roc k
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30002800 30002700 c.2700 c.2bOO
8. Inte nse deformation 9. E mplaceme nt of uhraba sic bod ies and calc-alk aline 8ra nilic shee ts ( Nuk gneisses) 10. Intense deforma liq n 11. E mplace me nt of gra nite s and o the r igneous bodies 12. H igh-grade metamorphi sm 13. De position of Ta rto q G ro up sup racrustal rocks 14. Local ized dcf orrnaticn in shea r zones 15. Em place me nt of K-gran ill."Sa nd regio nal pcgma ntes
of which the most important is the emplaceme nt of the Nuk granitic suite and the accompa nying deformatio n a nd metamo rphism in the period 3040-27()()Ma BP. Th ere is very little evidence as to the history of the region in the intervening pe riod of about 7ooM a. T he lsua supracrustal assemblage of the earlier cycle bea rs some similarity to that of the gree nstone belts described below. 11 consists of a mafic and ultramafic volcanic suite with associated metasediments including carbonates, banded -iran-formation , quartzites and metagreywackes. The supracrustal belt is only about 2 km wide at its maximum , but extends for over 30 km in an arcuate o utcrop. Because of the limited outcrop and the effects of later eve nts, it is not possible to draw definite co nclusions about the tectoni c environment of these very early rocks, except that they indicate , in a general way, a similarity in all essential respects to the much later (c.3300 Ma old) greenstone belts of Africa, Aust ralia and elsewhere.
As pointed out earlie r, the Pb-isotopic evidence indicates that the early crust-forming event is limited to a relatively small area and that the bulk of the co ntinental crust of the craton was add ed d uring the younger cycle. The Malene supracrustal assemblage of this yo unger cycle represents a sequence that is much more typical of the high-grade gneiss terrains in gene ral. It consists of basic metavolcanic rocks, including well-preserved pillow lavas, together with semipelitic to pelitic gne isses, pure and impure marbles, and thin quartzites. T his assemblage is ve ry widely distributed throughout the craton, and has been interpreted by Bridgwate r and Fyfe (1974) as indicating small marine basins ove rlying thin co ntine ntal crust. However , other autho rs (Burke et al ., 1976; Windley and Smith, 1976) view the high-grade terr ains , including the No rth Atlantic craton , as the prod uct of Andean-type active continental ma rgins crea ted by the subductio n of oceanic lithosphe re. T he voluminous calc-alkaline tonalitic magmas in their view are generated by subd uction. It has bee n pointed ou t that the smaller thick ness and lo we r relative density of Archaean oceanic lithosphere may produce much shallowe r angles of subduction (see e.g. Dewey, 1977) that wo uld have important implications for the width of the mobile belt, the pattern of magma emplacement, and the style of deformatio n. A dominan t feature of the North Atlantic craton and of o the r high-grade terrains is a high-str ain structure, produced by very intense defo rmation, which is e xpressed in complex interleaving of basement , supracrustal cover , and various intrusive igneous sheet s 0 0 a regional scale. Th is structure appears to have been initially sub-ho rizo ntal, although subsequently refolded by more upright folds. The defo rmation praducing this structure in S. G reenland embraces eve nts 8 and 9 of Table 9.3_ Figure 9.24 shows the outcro p pattern of the Fiske naesset anorthosite sheet. Although refolded by later structures , it is still traceable over an area of aro und 3600 km, indicating that the high-strain structure is e ffectively horizontal over areas of that size .
305
OROGE NY IN THE rRECAMDRIAN
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Figure 9.25 WNW- ESE structural profiles across lhe eortbwcsr Buksefjorde n erea in the central west -coast A rchaean. The section illustrates the refolding o r the e arly recumbent rold-lhrusl structure (DI- 3) by later upright roros (D4). Ma, Malene amphibonte ; A m, A mitsoq gneiss: N , Nuq gneiss: A a, Arnitsoq augen gneiss: Ms, Malene supracrustal gneisses; A kA , Akilia association; Q . pegmatite and rrucrogranite; orr, 02 lhrusl. From Chadwick and Nutman (1979)
Chadwick and Nutman (1979) describe in det ail the str uctural e volution of the Buksefjorden area in the centra l part of the west coast outcrop of the craton. They divide the structural seque nce into four main events, of which the first is related to the early Amitsoq crustforming cycle. Their 02 event is the fi rst regional high-strain defor matio n (correspo nding to even t 8 of Tab le 9.3) and resulted in the isoclinal folding and thrusting of Amitsoq basement and Malene cover. The 03 structure s are large recumbent nappes, refolding the 0 2 structures and also deforming the Nlik granitic shee ts. 04 prod uced the upright folds that dominate the outcrop patte rn. High-grade metamorphism accompanied the 0 3 deformation and continued after 04 .
No mode rn ana logue has been established for the regional, sub-horizontal, high-stra in structure of the 02-03 type. According to the uniformita rian view expressed by Windley (1981) and others, the high-strain horizontal structures would be found at low levels in a typical modern volcanic arc complex. However , this ana logy has not yet been satisfactorily demonstrated . Park (1981, 1982) discusses a number of possible mechanisms that could explain this type of structure: subduction, gravity spreading, mantle deco up li ngv thinned-crust collision, and tectonic underplating (A -subduction) (Pigure 9.26). Majo r differences in the subduction angle, the thickness and strength of the litho-sphere , and the ease of detachment of
306
G EOLOGICA L STRUCTURES AN D MOVING PLATES
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Figurt' ' .16 Possible mechan isms for u plaining the high-strain stl'\lCtU~5 of, Arch aean high-grade terrain s. (A) Subd uctio n: mor e likely to produce SIU P to mod er atel y inclined high-strain zones at deep levels in the crust . (8) G ravity sp reading associated with solid diap irism: requires sub-horizontal mass flow in source a rea for domes (blank) which would produ ce radial o r conve rgent a>nSlrictional strain patte rns (not observed ). (C) Mantle deco upling: horizo ntal shear zo ne forms al the base of the auSI (see Figure 2.29) d ue to differe ntial movemen ts in either convergent or dive rgent regimes (this mechani sm is similar 10 thai req uired in Ampfere r subd uction) . (D ) Com press ion of thinned au$!: extensional strain in the extensional phase is re-inlorced by th rust-se nse shea r zones during subsequent conve rgence. (E) Tectonic underplating: Ihis process is associated with Ampferer scbducucn, and requires the tec ton ic se para no n or crust from ma ntle lithosphe re. Repeated slid ng o f the crust ju xtaposes supracrustal and dee p-crus tal mererial as fou nd in SW G ree nland . (A) - (D) fro m Park ( 1982) ; (E) from Park and Tamey ( 1987).
continental from ocea nic crust, or from mantle lithosphere, can be expected in the Archaean. Any tectonic model for the Arch aean at present will be highly speculative, and no generally acceptable model yet exists. If we assume that subduction opera ted at a faster rate during the Archaean than at present ,
the 3000-2700 Ma crust-building cycle of the North Atlantic craton must presumably involve the welding together of a whole series of volcanic arcs. The structural pattern should therefore reflect a succession of extensional, tra nslational and collisional eve nts associated with the gradual accumulation and tectono-
OROGION Y I N THE PRECAM8RIAN
magmat ic thickening of the A rchae a n crust. Th e high-strain horizo ntal structures probab ly fo rmed in low-angle shear zones represe nting large horizo ntal translat io ns at deep crustal levels at some stage in this process. Greenstone belts of the Superior Pro vince
Th e Superior Province (Figure 9.27) is one of the best examples of a gra nite-gree nstone terr ain (see e.g. Goo dwin, 1981). It is the lar gest of the A rchaean crato ns, be ing abo ut 450 x 300 km in exte nt , and is se parated from the neighbo uring No rth A tlan tic crato n by the 300 km-wide Lab rador belt (see Figure 9.13) . Th e Pro vince contains two main components: crysta lline gran ito id rocks varying fro m undeformed igneous plutons to highly-defor med gneisses , and the gree nstone belts , which are o utcro ps of supr acrusta l seq uences consisting of volcanic rocks of va rious kinds , predominantly mafi c, togeth er with metasedi ments. The proportion of greenstone belt to ' granite ' varies thro ugho ut the province , being much higher in the south than in the north (Figure 9.27) . High-grade gneisses occur in several narrow be lts, a nd also in the north-western and north-eastern pa rts of the pro vince . Since the gree nsto ne belt outcro ps are usually delimited by the discordant margins of yo unger plutons, the former exte nt , shape and relation ships of the belt s is impossible to reconstruct. It is tho ught that many of the greenstone o utcro ps rep resent the fragmented re lics of much larger gree nsto ne basins. However, some of the large gree nstone belt s, such as the Abitibi be lt, con tain mo re loca lized basins that existed for part of their evo lutionary histo ry. The supracrustal assemblages of the gree nstone be lts co nsist of roughly eq ual propo rtions of sedi ments and volcanic ma te rial deposited around 2700 Ma BP. Ther e is typically a lower sequence dominated by mafic lavas, and an upper do minated by coarse clastic sed iments, especially greywackes. Freq ue ntly, an unconformity sepa rates the two seque nces. In the Abi tibi belt in the so utheast, a number of separate volcan ic piles can be recognized , that
307
grade late rally into sedi ments. The lavas are co mmonly tho leiitic basalts and andesites, but rhyolites , rhyo-dacites and ultramafic types also occur . Of lesser import ance vo lumetrically are sed iment types such as quartzites, ba ndediron-formation , sulphide-rich black shales, and minor carbonates . These sed iments are usua lly fou nd in associatio n with the earlier volcanic seq uences, whereas the coa rse clastic deposits are mo re common in the uppe r part of the successio n. Est imated thicknesses shown in Figure 9.27 range from 20000- 59000 feet (6t 8km). While some of these fi gures may be overestimates, due to unrecognized struc tural repet ition , the re can be no do ubt that the gree nstone sequences are commonly of the orde r of 5-10km in thickness. A lthoug h the original extent of the basins is impossible to estimate , it is thought that , in ge ner al, greenstone sequences we re formed in relatively shallow ma rine basins of probab ly regio nal extent. Through time , vertical move ments thought to be due to differential load ing of the granitic basemen t, caused a restriction of the basins to approximately their present size , and gave rise to the younge r clastic-dom inated sequences (see e.g. Bickle and Eriksson , 1982). The granitoid element of the ter rain consists partly of gneissose basement that pre-dates the forma tion of the gree nsto ne belts, a nd partly of younger post-greensto ne pluton s. In practice, it is not possible at prese nt to determine the o rigin of much of the granite outcrop because of poor geochronological contro l. Ages of c.2900-3000 Ma have been o btained from some of the olde r basement gneisses, in which fragme nts of an o lder series of greenstone belts occur. Overall, the province exhibits an E - W structural trend, expressed in the elongate e lliptical shape of ma ny of the gra nito id are as, by the orientatio n of the gree nsto ne belt s themselves, and a lso by the o rien tation of the high-grade gneiss belts. O n the scale of the individual gree nstone belts , the defonnat ion patte rn is much more variab le . Figure 9.28A is a map showing the gra nite-greenstone relation ships in the Keno ra district , in the south-
ce
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Figure 9.27 Distribution of greenstone belts (ruled ornament) in the Superior province of the Canadian shield . The blank areas are mainly granitoid rocks . The Abitibi belt is the large green stone outcrop in the southeast , crossing from Ontario to Quebec. From Stockwell et at. (1970), with permission .
309
OROGENY IN T HE PRECA MRRIA N
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6km Figure 'J.28 Tectonic patt erns in greenstone belts (A ) Gra nite-greenstone relat ionships in the Kenora area . SW Superio r prov ince. G ranite . dotted ; green stone out crop . blank ; fold axes. dashed . Note the roughly eq uidlrne nsional shape of the gran ites , and how they ex hibit convex margins to the green stone s, which, togeth er with the {old axes, ' wrap around' them. ( B) Simp lified structural map of the Bigston e Lak e greens tone belt , nort hern Manitoba. The ea rly foliation (5 1) wraps arou nd the gra nite margins whereas the late r deformation ap pears to relate to large strike-slip she ar zones (5Z1 - 3). (A) , ( B) {rom Park ( 1982).
310
GEOLOGIC.... L STRUCf UIUiS AN D MOVING PL.ATES
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Figure 9.19 Strut lural moeJd s 10 show the pane rns of Sl ruCUlfe5 associated wilh tbe so lid d iapirism model for greensto ne be lt e volutio n. (.4) Structu res expected nea r the eem ee of th e sutnid ing g ree nsto ne bell. A llitudc: of faults a nd fold axial plan es changes from Sleep to inclined , moving fro m the anna t 's ink' lowilrd!;the margi ns. ( 8) Structures expected nea r (he mu gi"" of a subsid ing gree nstone bell. A highly asymmetric penern of Iojds and thrusts is predK'ted attbe site o r lbe rim syncline produced by marginal depression of the greenstones. (A ), (8) from Gorman et ill . ( 1978).
311
OROGEN Y IN TH E PRECAMBR IAN
weste rn pari of the province. Here , t he gra nite pluton s are weakly deforme d or un deformed , and mo re or less eq uidimensional , a nd their o rigina l rela tio nships to the g reensto ne o ut cro ps is clear. T he rocks of the gree nsto ne se q ue nce a re deformed in such a way that the fold axes and folia tio ns ' wra p a rou nd ' the ma rgins o f the plutons. Th is rel ationship . obscured in ma ny be lls fur lhe r no rt h beca use of mor e int e nse la te A rchaean deformat ion , is typical of greenstone be lls of many o ther gra nite -greenstone te rr ai ns. In a stu d y o f the Bigsron e Lake greenstone be ll in no rth e rn Ma nito ba , Pa rk an d Ermanovies ( 1978) de mo nstr a ted th a i the deforma tio n se q ue nce in the gree nston e belt consiste d of tw o mai n phases. The first is associate d with re gio na l high st ra ins, pen e tr a tive fabrics , an d isoclina l folds which loca lly a re seen to wra p ar o und the ma rgins of the borde ring plutons (Figure 9.288). The second prod uces loca l re fold ing and cre nula tio n schis tos ittes, an d is associa te d wit h m ajor stee p du cti le shear zones that cut across the be lt and affect both gree nsto ne and gra nite ou tcrops. Th e type of struct ural pattern associa ted
with the first deform ation he re , a nd in the Kenora ou tcrops , is most easily ex pla ined by th e d iapir ic behaviou r of a solid granitoid basem e nt loaded by a den se r greensto ne ba sin . T he gra vitationa l e ffec t of th e de nsity inve rsio n is 10 ind uce upward a nd outward flow of the gra nitoid substra tum. T he gree nsto ne materi a l becomes trap ped in synclina l kee ls whose shapes a re controlled by the diapir ma rgins. This process has bee n discussed by Gorman et al , (1978) a nd Schwe rd tne r et at. ( 1979) and mode lled numerica lly by Ma resch a l a nd W e SI (1980) , Figure 9.29 illustra tes th e s tructural patte rns expected in ce ntra l an d ma rgina l ar eas of g ree nstone be lts accordi ng to the Gorman et al. mode l. T he symme trica l uprigh t or m ildlyfann e d Dl structu re of th e B igston e Lake be lt fits profile A of the model. Tight c verfold s and t hrusts found as e a rly stru ctu re s in o th e r gree nstone belt s (see e.g. Coward , 1976 , fig ure 4) fit profile B of the mode l. T he late r stee p d uct ile she a r zones tha t affect the Bigstone La ke bel t a re pan of a regional ser o f such structur es t hat domi nate the la te r A rchaea n deform ation o f the Supe rio r Pro vince . Pa r k (1981 ) shows th at , in th e
~ ~ . .. ..
0-
_ _
· - fil -
.-1'7 -
-:::-:-=
A l.te
1( · 9r~ni tli!S
I
1 11 11 11 '~'~&!~J'l'~~)o11 1 1 1 B Figu,n.' 9.30 Ur uformirarian plate-tectonic mterp re tanon ofbi gh-grade gneiss and greenstone terrains. High.grade terrains an: Interp reted as Andean -type convergent margins and greenstone belts as back-arc basins (A). Collision welds microconnne ms toget her and causes greenstone deformation (8) Contrasl Figure 9.26. After Windley (1977).
3J 2
GEOLOG ICAL SnUCTURES AND MOVING ?LA TES
weste rn part of the pro vince , these shear zones fo rm a co njugate set of NE -SW sinistra l and NW - SE dex tral zones tha t indicate a late Arch aean N-S compressio n across the province . and suggests tha t the erato n was by tha i time stro ng enough to transmit a constant . regional stress. This tectonic regime is very similar to the one described for the Ear ly Prote rozoic shea r zones of the S. GreenlandScotland region (see Figure 9.17A). Bot h sets o f str uctures probably formed aro und the time of the Early Proterozoic-A rchaean bo und ary be twee n 2600 and 2500 Ma BP. The orig in and tectonic significance of the greensto ne basins is sti ll speculative . Figure 9.30 shows the Farney-wi nd ley mod el in which the gree nstone belts are interpreted as backarc extensional basins related to subd uction. In an alte rna tive model suggeste d by Drury ( 1977). the basins are visualized as resulting fro m late ral compression of a thin. wea k lithosp here exerted by a shallow-subd ucting slab . Pa rk ( 1982) interpre ts the basins as intraplate extensional str uct ures ana logous to the initial stages of present-day con tinental rift zones.
None of these explanations seems enti rely satisfactory . The subductio n-collisio n mod el requires a number of unrecognized su tures; th e compressio nal model fails to explain the init ially no n-linear pat te rn of t he gree nstone de fo rmat ion; and the intrap late mod el igno res the bimodal nature o f the volcanic suites. which argues for a volcanic arc analogue . Opinion since these papers were published appears to favour some kind of exte nsional basin model . involving thi nned continen tal crus t in the Supe rior exa mples. but oceanic crust in some of t he o lde r examples. closely associated with volcanic island arcs derived by subduction . Present-day beck-arc basins arc the closest mod ern analogues . but many important d iffere nces are appare nt be twee n such basi ns and the gran ite-gree nstone terr ains. The gran ite-greenstone te rrai ns represent o ne of t he best demonstratio ns in the geo logica l reco rd. that uniformita rian principles and prese nt-day plat e tecto nic analogues may not always be ap pro priate in dea ling with the d istant past.
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Index A-subduction 142, 144, 236, 292, 305 Acadian phase 229, 234 Acadian sector 242 accretion 175 arc 277 Irontal 120 accretionary complexes 119, 120, 131 margins 272 accretionary prism 112, 118, 119, 120, 125. 157, 266, 267 Makran 131 S. Uplands 255, 258, 268 accretionary terrain 159 active transform domain 184 Adirondack Mountains (USA) 281, 282, 283 Aegean Sea 110, III, 134, 136, 138 Afghanistan block 152 Agt~ 295, 296 Aiguilles Rouges 215 Airy anomaly ISS Alabama, Central 229 Alai range ISS Alaska 166. 227 Alberta Group 223 alkali-granites, plutons 279, 284. 287,289 Alleghenian 237 Allochthon Lower (Scandinavian Caledonides) 264 Middle 264 Upper 261, 264 Uppermost 261, 263 Alpes Maritimes 217 Alpine (orogenic) belt 190, 210, 211,212,213,230 Alpine collision 95 Alpine front 188 Alpine - Himalayan system 139 Alpine orogeny 94, 212 Alps 93, 95. 139, 140, 145, 146, 211,212,214,215,217. 219, 220,221,292 Austrian 215 Eastern 211,214,215,219 French 211, 215, 216,.220 Southern 215 Swiss 211.214,215,216,219 Western 214 Ahai range 140, ISO
Andaman Islands 140 Amgl gneisses 286 Amgl-Kropp.:.(jjill .Group 286 Amits;q cycle 305 Ampferer subduction see A-subduction Anatolia 134 Andean-type active continental margins 304 Andean-type convergent margins 311 Andes 141 Anglesey 259 anteclise 190 Ukrainian 191, 193 Volga - Ural 191, 193 Voronezh 191, 193 anticline, rollover 203 Appalachian - Ouachita orogenic belt 147 Appalachians 144, 145, 228. 243, 266 Central 242 Central - Sout hern 229 Northern 229, 242. 266, 267 Apulian microplate 212 Arabia ·139, 210, 274, 275 Arabian - Nubian shield 273. 277 Arabian Sea 119, 147 Arabian shield 140, 273 arc Aegean 134, 210 Aleutian 38, 39, 66, 67 Banda 159. 160. 161, 162. 175 Caribbean Cascades volcanic 96,97,221 Hellenic 134, 136,139" Izu-Bonin 66 Japanese 7. 10. 12, 32, 38 Kootenay 223. 225, 226 Kurile 10, 12, 38, 66, 67 Makran volcanic 131, 210 Marianas 66,67, 102, 103, 104 Peru - Chile 38 Puerto Rico 114 Ryukyu 66 Scotia 100, 166 Sunda 140, 159, 161,210 Tonaa - Kermadec 102, 104 arc - trench gap 116. 117 arcs island/trench systems 100 island 7, 9, 10, 18, 66, 112, 113, 134, 139, 159, 173,227,230, 250. 261, 264, 275, 277
327
island accretion 139 volcanic 9, II, 14, 22, 96, 99, 102,112, 113, 117, 120. 125, 133, 134, 136. 159, 163, 165, 175, 176,212.222,227,259, 268, 270, 272, 275, 289, 305, 306, 312 volcanic island II, 277 Arctic Ocean 61, 200, 242, 244 Asia 140, 147, 152, 159, 188,290 eastern 148 Assynt 247 asthenosphere 5, 9 asthenospheric diapirism 84, 86 asthenospheric mantle 18 Asturic phase 229, 241, 242 Atlantic coastal province
(Appalachian orogenic belt)
34
Atlantic coruinental margin 204 Atlantic ocean, evolution of 58
Atlantic (stress) province 37 Atlas mountains 139, 211 aulacogen 83, 190, 191 Dneiper-Donetsk 191,193 mid-Russian 19~ Australia 139. 140, 159. 160, 161, 162, 173,304 Australia - Irian Craton 160 Austro - Alpine klippe 219 Avalon platform 243 Avalonia 231,268 Avalonian - Cadomian (orogeny) 266 Azores 75, 79, 186 B-subduction 142 lJ·value 53 back arc 14, 112 basin see basin, back-arc extension 136, 161,237 extensional provinces 73, 86 spreading 66, 100, 102, 104, 113. us, 136, 144, 159, 162 back-thrusting 143, 157,217 Baja California 178 balanced sections 220, 237 Ballachulish slide 254, 255, 267 Ballantrae 265, 267 Baltic (Fennoscandian) shield 190, 191, 193, 243 Bahia 228, 261, 264, 266, 267, 268 Baltimore trough 204, 205
.:l28
INDEX
Bamble 285 Banda Sea 161, 163, 165, 175 Barbados 125, 126, 127, 133 Barbados ridge complex 258 Barra 297, 299 Barrovian metamorphic (sequence)
228 basalt - eclogite phase change
115 basin Aegean Sea 134, 136 Anadarko 241 Appalachian 229 back-arc 212,259,261,311 back-arc extensional 64, 197, 198,222, 312 Black Warrior 229 cratonic 189, 198 Cretan Sea 134, 136 Culm 239 Dalradian 265 East Texas 33, 34 extensional 212, 259, 262, 289 fore-arc 120, 212, 259, 261 foredeep 212,222, 226 foreland 188,217, 222, 229 Grenada 126 intracontinental 241 intraplate 188, 194, 197, 198 Japan 100, 102 Lau 102, 103 Lau - Havre 104 marginal 103, 104, 214, 222, 226 Marianas 104 Michigan 194, 195 Molasse 219 North Sea lOS, 108, 194, 199, 200,204 Pannonian (Hungary) 199 Parece - Vela 100, 102, 103 Paris 49, 194 passive-margin 204, 205 Po 219 pull-apart 178, 256 Santa Maria 175 Shikoku 103 South Fiji 100, 102, 103 Taoudeni 194, 195, 196 Tarim ISO, 151 Welsh 244, 259, 266 West Philippine 102, 103 Basin-and-Rallle Province 7, 28, 32, 36, 46, 5 I, 86, 96, 97, 98, 99,100,101,105,111,180, 207,208,221 Bay of Bisay 109, 214 belt Abitibi 307, 308 Acadian 242, 243 Alle.henia.. 210,229,241,242 Appalachian 261 Appenine 210 Atlas 210 Belcher islands 291, 292
Blue Ridge 231 (see Blue Ridge province) Cadomian 243 Caledonian (orogenic) 190, 200, 210, 228, 243, 290 Cape Smith 291, 292 Carolinidian 281 Central Gneiss (Grenville Province) 284 Central Metasedimentary (Grenville Province) 284 Cordillerian (orogenic) 98, 171, 210, 221, 222, 225 circum-Pacific 139 circum-superior 291 Damaran 273, 279, 280, 281, 294 Damara-Zambesi 273 fold-thrust 119, 222, 228, 229, 237 (see also thrust-fold bell) Gabon -Cape 279, 280, 28\ Grenville 249, 282, 290, 292 Grenville - Sveconorwegian 242, 282 Hercynian 210 imbricate (thrust) 239 Ketilidian 253, 300, 301 King Mountain 231 Labrador 282, 283, 291, 292, 293, 294, 307 Lewisian - Nagssugtoqidian 291, 299 Limpopo 269 Matchless 279, 280, 281 Mauritanide 195 Mozambique 272, 273, 277, 278 mylonite 277 (see mylonite zone) Nagssugtoqidian 294, 295 Nagssugtoqidian - Lewisian 295, 301 Northern Appalachian 231 Nubian - Arabian 277 onhotectomc 244 Ouachita - Marathon 229, 230, 231 paratectonic 244 Piedmont 231 Pyrenean 210 Rinkian 294, 295 slate 230 Svecokarelian 300 Sveco- Norwegian 282,284,285, 286, 288, 219, 290 Urals 228 Valley-and-Ridge 231 Variscan 228,231,235,236,237, 238, 242, 243 Zambesi 279 Belt-Purcell SUJ!"I'Broup 222, 223 Betic Cordillera 210 Big Bend 175, 177, 178, 179, 180 Black Sea 134, 214 blocks allochthonous 171 exotic 171
Blue Ridge Province (zone) 230, 233, 241 blue-schist (belts, facies) 220, 227 Bohus granite 285 Bohus-Iddefjord granite 286, 287 Borneo 159, 161 boundaries collisional 39 conservative 3,39, 71 constructive 3, 16, 39, 70, 71 convergent 212 destructive 3, 16, 144 breakouts, borehole 33, 35 Bretonic phase 229, 236 British Isles 228, 242, 243, 244, 259 south-west 236, 237, 238, 241 buckling (of slabs) 119 Buksefjorden 305 Burma ISO, 153, 175 Byerlee's law 30 calc-alkaline magmatism 275, 300, 304 plutons 287, 289, 302 volcanics 274, 276 Caledonian front 285 Caledonides 146, 190, 242, 248, 259, 264, 268, 282, 284 British 261, 265, 266 East Greenland 244 German - Polish 268 Polish 261 Scandinavian 144, 244, 261, 262, 263, 266, 267 Canadian shield 281,282, 290, 291, 302, 308 Cantabrian - Asturian chain 228 Cantabrian MO'lOtains 235 Caribbean 119, lIS, 259 Carnic (microplate) 212 Carpathian (chain) 139,210,2\1, 212 Caspian Sea 194 Cascades volanic arc 7 Caucasus chain 139 Central Asian collage 152 Central Atlantic, .peniag 59 Central block (Lewisiaa) 298 Centntl Highillftd Diviaion 2S3 Central North Sea dome 199 Central Tibet blocks IS3 Chibougamau - GIltineau lineament 284 Chilas complex 154, 156 Chile ridge 64 China block 147 Churchill Province 283, 291 Coast Ranges 177, 171, 179, 180 Coastal plain (Appal1ldlian) 231, 232,233 Cockburnland 258 COCORP deep seismic reflection line, profile 100, 230, 233 data 99
329
INDEX coefficient of cubical expansion 22 collage (tectonic terrane) 154, 159, 175 collision 136, 147, 152, 153, 154, 159, 161,231,256,259,261, 264, 266, 279, 289, 311 Central Asian 149, 150, 171 continent - continent 19, 136, 212, 277 continent- island arc 19, 136, 159 India - Asia 160, 166 thinned-crust 305 collision resistance force 47, 144 collisional (orogenic) belts 136, 140 Columbia River basalts 98 compressional regions 71, 135 shortening 299 structures 168 Conrad seismic discontinuity 51 continental collision 67, 139, 250, 268 (see also collision) continental margin 289 active 113, 139 passive 27, 139 convection 20, 22 cell, currents 6, 18, 19, 25, 270 convective circulation pattern 20 convective flow system 16, 23 convergence 70, 139, 140, 144, 147, 152, 154, 169,281,292,299 continental 147 direction of 4, 160, 214, 220, 289 oblique 71, 171, 172, 173, 175, 227,268, 281, 289 oblique (plate) 266 rate of 114, 115, 116, 152 convergent movement 71,214,289, 301 convergent shortening 294 Cord illeran collage 173 Cordilleran orogenic belt, province 7, 34, 175,273 Cornwall 238, 239 craton 190 African 195 Archaean 301 Brazilian 273 Congo 272, 273, 279 Guyana - Brazil 272 Guyanan 273 (Peninsular) Indian 272, 273 Kalahari 272, 273, 279, 281 Kola 290 Nile 273 N. Atlantic 290,291,292,294, 295, 302, 304, 306, 307 Slave 290, 292 Superior 291, 292
Svecokarelian 285 Tanzanian 277, 278, 279 West African 272, 273 West Gondwana cratonization 275 creep, dislocation 43
Darn Law
43
creep strain rate
48
Crete 134, 137 critical taper 120 Cruachan line (Scottish Caledonides) 253 crust-forming cycles 303 Culm facies 229 Culm synclinorium 238, 239 Dal formation, Group 286, 288, 289 Dalradian Supergroup 253 Darfur 87, 90 Davis Strait 214 Dead Sea 171 decollement 4, 119, 120, 125, 133, 135, 140, 146, 154, 167, 181,217,230,256, 292 deformation collisional 242 compressional 94, 98, 99, 143 convergent 236 dry-quartz fold-thrust
5I 236 plagioclase 5 I
wet-quartz 5I front 123, 126, 129, 133 depression rate of 191 Ul'vanovak-Saratov 191, 193 detachment 4, 141, 142, 144, 146, 177,215,223,230,231,239, 250, 258, 299, 305 detachment horizon 5I, 53, 99, 105, 106, 109, 142 Devon 238, 239 dewatering 119, 120 Diabaig 300 diapir 85 dilatational emplacement 299 Dinaride chain 210, 214 dip-slip movement 105 direction, facing 4 vergence 4 Discovery Chain 68 displacement rate 177, 226 divergence 70, 169, 294 direction 4 oblique 71 rate of 91 divergent motion 71 divergent regime 188 DSDP 125 duplex (thrust) 133, 146, 165, 170, 247,250 strike-slip 169 dyke swarm 284, 289, 294
Scourie 296, 299 earthquake focal mechanism of
32, 37
focal mechanism solutions 33, 34, 37, 39, 54, 83, 90, 96, 97, 135, 136, 150, 183 Kern County 177 magnitudes of 176 San Fernando 177 East Africa 277 East Nain Province 292 East Pacific Rise 123 Eastern assemblage (British Columbia) 221 Eastern Desert (Egypt)' 276 Eastern Segment (Sveconorwegian Province) 285, 286 eclogite 19, 220, 270 electrical conductivity 9, 74 elongation lineations 264, 275, 279, 280 Embrunais - Ebaye Nappes 2\ 5 Emperor seamount chain 66, 67, 68 England, south-west 228, 236, 237 Ethiopia 87, 88, 273 eugeoclinial 222 eugeosyncline 112, 22\ exotic terranes 221· extension 92, 93, 95, 98, 99, 106, 110, III, 136, 138, 150, 167 back-arc 38, 99, 102, 104 crustal 89, 90 intracratonic (see also intraplate extension) 236 oblique-slip 300 extension factor (6) 138, 202, 206 extensional basin 188 duplex lOB fault systems 105, III faults 84, 110 fissures 105 movement 148 provinces, regions 71, 73, 95, 105,221,299 rifting 204, 281, 289 strain 99, 100 structures 94, 168 extrusion tectonics 147 facing direction .255 failed arm (rift) 94 failure, Griffith 43 whole-lithosphere 41, 42, 46 FAMOUS project 75,77,79 fan . counter 107, lOS horsetail 107 listric 106, 107, 108, 110 fault (_ also transform fault) Alpine 166, 171 Altyn Tagh 151, 152 Bil Pine h5, 179, 181 Church Stretton 260, 265 Elsinore 175, 177, 178 Flannan 53, 245 floor 108
.J.JV
Garlock 175, 177, 179, 180, 181 Great Glen 144,246, 247, 250, 252, 253, 254, 267 Herat 150 Highland Boundary 244, 252, 253, 255, 257, 267 Imperial 177 Insubric 219 Najd 274 Navan - Shannon 259 North Anatolian 234 Ornach Nal-Chaman 131 Outer Hebrides (Outer Isles) 53, 245, 297 Pontesford Linley 260 Quetta - Chaman 150, 151 roof 108 San Andreas 178, 180, 181 San Gabriel 175 San Jacinto 175, 177, 178 shortcut 108 sale 107, 109 Southern Uplands 244, 252, 255, 256, 258, 266, 267 Tonale 219 White Wolf 177, 178 fault overlap 168 fault plane, focal-plane solutions 3, 32, 37, 39, 95, 114 faulting underthrust 114, 115 faults antithetic 95, 107, 110, 168, 177 antithetic strike-slip 167 Chugach-Fairweather, Queen Charlotte Islands 166 en-echelon 89, 168, 183 extensional 94, 95, 105, 109, 145, 212 growth 265, 292 listric 96, 105, 106, 109, 133, 202 offset 181 splay 89 synthetic 95, 110, 168 synthetic strike - slip 167 transfer 109, 223 transform 2, 3, 25, 27, 39, 54, 58, 59, 61, 62, 64, 70, 75, 78, 80, 91, 109, 161, 166, 178, 182, 183, 187, 197, 198 (51'1' atso transform fault) White Wolf - Kern 175 fault zones, San Andreas 3, 36, 40, 58,59,64,96,97,98, 100, 101, 166, 175, 176, 177, 179,221 faunal separation 264 Fennoscandia 207 Fennoscandian craton, shield 190, 242,243,261,263 (51'1' Baltic shield) Fernie: Group 223 Finllrnarkian (oroamy) 263, 266 Fiskenaesset 303 fissure eruptions 80, 82 flake tectonics 141, 142
IN!>"X
natjack measurements 29, 37 nat 145, 146 footwall 145 flexural depression 242 flexure model 199 flower structure 169, 260 negative 170 positive 169, 170 flysch 140, 212, 213, 214, 217, 222, 229, 234, 236, 239, 242, 292 fold belt, Southern England 188 force, buoyancy 29 force critical 48, 49 plateau uplift 26 forces mantle drag 25, 27, 29, 32, 37 plate boundary 24,41, 150 resistance 25, 29, 38 ridge-push 25,21>,27, 34, 37, 38, 41 slab-pull 16,24,26,27,33,37, 38,41 subduction - suction 22, 24, 26, 27,37,41,46,85,95,118 trench-pull 41 trench suction 24 fore-arc 258 fore-arc complex 125 foredeep 214 foreland 215 African 219 Baltic 266 Hercynian 228 Laurentian 266 Fort William slide 254, 255, 267 fracture zone (oceanic) 166, 181, 182, 186, 187, 189,267 Alula 183 Azores-<:iiltraltar 58,166,210,214 Chain 181, 183 Charcot 113 Charlie Oi. .s 183, 184, 185, 186 Chile 58,65 Discovery 186, 187 Gibralter 210 Gloria 184, 186 Gofar 186, 187 K~chatov 184 Mendana 122, 123, 124 'fountain of nappes' hypothesis 255 fractures en-echelon 80 extensional 83, 167 tensional I 10, III fraClllrina, hydraulic 33 Francisean complex (California) 227 Franz Josef Land 242 gabbro-eclogite phase change Gairloch 296, 297, 298 Galicia 235
198
Gardar Province (Greenland) 284 geoid 16, 20 geoid anomalies 16, 18, 19,20, 21 geosyncline 259 Timan 191, 192 geotherm 5, 49, 140 geothermal gradient 27, 30, 32, 41, 44,45,46,47,51,112,270 Gibraltar 186 Girvan 258, 259, 265 Glen Affric 249 Glenelg 249 Glenfinnan Division 249, 253 GLORIA side-scan sonography 122, 187 Gondwanaland 61,62, 153,228, 231,272,273,277,281 Gothenburg 285, 286 graben 83, 88, 89, 91, 94, 95, 96, 109, 120, 122, 171, 172, 190, 197, 203, 204, 256 Central 200, 202 formation of 105 half 109, 202, 256 0510 83,241,242,263,284, 288 Shansi 150 Viking 200, 202, 203 graben-horst morphology 183 Grampian Division 254 Group 245, 253 Highlands 244, 253, 254, 264, 266, 267, 268 slide 253, 255 granite - greenstone terrains 302, 307,311 gravitational sliding (51'1' gravity sliding) .raYity spreading 120, 250, 255 gravity anomaly 181, 197 Bousuer 89, 113, 114, 141, ISS, t65, 223 free-air 89, 113, 114, 184, 195, 207 gravity data 204 profile 154, ISS, 202, 221 slidiftg 112,217,218,220,247 sliclina nappes 114 sh. .ping 120 gravity-spreading model 136, 305 Great llasin (USA) 96 Great Lakes (N. America) 189 Great Valley 177, 180 Greece I 10, 134 Greenland 214, 242, 270, 281, 284, 290, 291, 294, 295, 299, 300, 301, 102, 303, 304 Greenland - Scotland resion 312 greenstone belt 279, 302, 304, 307, 301, 309, 310, 311 BilStone Lake 309, 311 Grenoble 215 Grenville front 282, 283, 284 Grenville group 284
331
INDEX Grenville province 281, 282, 284, 289 Grenville Supergroup 283 Grenville - Sveconorwegian system 281 Gruinard Bay 300 Gulf of Aden 61,85,87,91,94 Gulf of Bothnia 207 Gulf of Califbrnia 39, 64, III, 166, 178 Gulf Coast province 34 Gulf of Mexico 189 Gulf of Suez 91, 111 Gullfaks block 203 hairpin bend 271 hanging wall antiform 247 Hawaii 64 Hawaiian ridge 68 Hawaii - Emperor bend 68 chain 20,67 heat now 10, II, 12, 13, 14,20, 42,44,46,47,48,49,51,53, 74,75,80,96, 140, 177,207,208 Hebridean craton 245 Hercynian belt 190, 231 foreland 241 front 236, 237 orogenic belts, regions 51, 228, 229 Hercynides 228 high-grade gneisses 307 high-grade gneiss terrains 302, 304, 311 Himalayan fold-thrust belt 150 front 147 Frontal thrust 151 Himalayas 139, 140, 144, 146, 148, 150, 151, ISS, 159, 236, 250 Pakistan 158 Hoggar 87 Holsteinsbolll 295 Hopedale 282 horse 108, 146 horst 96 hot-spot Hawaii 67 Iceland 67 hot-spot frame of reference 20,-69, 104 hot-spots 20, 26, 67, 78, 79, 80, 84, 188, 189 H unsarian plain 215 Hudson River 229 Hudsonian 190, 291 Hudsonian Cycle 281 Humberian (orogeny) 266 hydraulic fl'llCtare (hydrofracture) technique 29,30,37, 177 hydrothermal activity 75, 77, 78, 79 Iapetus Ocean 258, 159, 264, 265, 266,267 Iberia, Iberian peninsula 210, 214
ice-sheet Pleistocene 15 Scandinavian 15 Iceland 79, 80, 82 Iltay boundary slide 254, 267 imbricate 236, 277 (see also imbricate zone, thrust sheet) indentation model 67, 136, 147, 149, 150 indenter 148 Indian ocean, evolution of 58, 61 Indo - China block 147, 160 Indonesia 139, 144, 161, 162, 175, 210 interference patterns 249 intracontinental shortening 236 extension 264 mobile belts 272 movements 281 intraplate basin 204 compression 24 convergence 226 extension 24, 47, 204 magmatism 280, 312 movements 300 structure 188, 191, 193,206,214 uplifts 206 Inverian 296, 298, 301 inversion 191,209 1reland 241, 259 South-west 236, 237, 238, 239 Irian Jaya 160 Irish Sea block 244, 259, 264, 265 Isle of Man - Leinster (zone) 259 isostatic rebound 207 Isua supracrustal assemblage 304 Italian peninsula 210 Itivdleq 295 Ivrea body 221 J aglot syncline 156, 157 Jamdena 165 Japan sea 103 Java 159, 160 Jura 215 folded 2U plateau 215, 216 Kangamuit dykes, Greenland 294, 295,296 Karakorum 154, ISS, 157 Kazakhstan block 152, 153 Kenora 307, 309, 311 Kenya 17,89, 277 Keweenawan event 271 Kinloehewe 246 Kohistan U6, 158 complex 157 sequence 154, 156, 157 komatiitic oceanic crust 271 Konasberg 285 Konpbera - Bamble segment 285, 288 Koster islands 287
Labrador 302 geosyncline, trough 292 Sea 61, 202 lag 255 Lake District 244, 259, 264 Lake Viinern 285, 286 Lake Viittern 285 Lanzo peridotite 218 Laramide (orogeny) 33, 208 compressional deformation 99 lateral tip 239 Lau trough/Havre basin 100 Laurasia 281 break-up of 95 Laurasia - Gondwana 241 Laurentia 207,228,261,264, 266, 267, 268, 271 Laurentian shield 271 Laurentian - Greenland shield 243 lavas, sheet-flow 78 Laxford 298, 299, 300, 301 Laxfordian (deformation) 296 Lesmahagow 256 Lesser Antilles 125, 126 Lesser Antilles volcanic arc 126 Lewisian complex 245, 295, 297, 298 Lewisian - Nagssugtoqidian system 294,299 Lhasa 154, 155 LlSPB deep-seismic reflection profile 51, 53, 109, 245, 252, 253 listric (geometry) III, 250 lithosphere, compressional strength of 47 • continental extension of 49 (definition) 55 mathematical model of deformation 43 evolution of strength of 47 extension of 49, 51, 85 extension factor (Il) 48, 49 flexure 24, 27, 209 long-term strength of 41 strain hardening of 49 stretching of 198 stretching factor (see lithosphere extension factor) thinning 86 L10yn peninsula 259 loads, topographic 31, 32 Loch Eriboll 246, 247 Loeh Laxford 297 Loch Maree 296, 297 Loeb Maree Group 296, 297, 298,
m
Loch Torridon 297 Lofoten 264Lysekil- Marstrand 287, 288 Madaaascar 277, 279 magnetic anomaly data pattern of 91
152, 181
332 Makran complex 119. 125. 133. 134. 144 Malene supracrustal assemblage. gneisses 304. 305 Mali 195 Manitoba 309. 310 mantle convection 15. 19.69.85, 100 decoupling 305 diapir 95 dr.g (force) 23. 24 plume 19. 67. 85. 99, 209 marginal basin 100 margins active 3 p•• sive 3 M.ssachu.ells 261 massif', Aiguille.-Rouges 216, 217 Ardennes 195 Argentera 217 Armorican 195.229, 233, 241 BeUedonne 216, 220 Bohemian (Moldanubian) 229. 231,241 Central (France) 195,231,235, 236,24\ Centro-Iberian 233 Iberian 229 London-Brabant 199,243. 244 Mt Blanc 215, 216. 217 Rhenish 237 Mauritania 195 McKenzie extensional. stretching model 198, 204, 206 Median seament (Sveconorwegian proviace) 285, 286 Mediterr.nean 119, 136, 140, 147, 210, 211 eaSlern \)4 Mendips 237, 238, 239 m.-phere 6 metamorpbi<: cere complexes 99 microplate 211,214 micropl.les Aeaean 211 Apulia 211 Camics 210, 214 lberi. 2'0 Moesia 210, 214 Rhodope 210, 214 Turkish 211 mid-Qa-man Crystalline Rise 234, 236 mid-Nonh Sea HiSh 200,241 Midland Valley (Scotland) 171, 141, 244, 155, 256, 2H, 258, 167 Midlands platform (Ensland) 244. m, 161, 266 miopoclinal 222,223,227 mioposYftclinal (belt) 221. 293 Mississippi 129 mobile bell 272 iatracontinental 273
INDEX intracratonic 273. 279 Moffat shales 256, 258 Moho 154. 155,202,221,231.250 Moine Complex 247, 249, 250, 253. 265 MOIST deep-seismic reflection profile 51,53, 109 Mojave block 180 molasse 213,214.217,219. 226. 276 Molasse trough 215.216. 217 Moldanubikum 233 Mon. complex 265 Monar folds 249 Morar division 249 Morarian event 253 Mor.y Firth 200 Morocco 210, 273 movements cratonic 189 direction of 3, 237, 280, 284, 287. 299, 301 oblique 70 recent vertical 189, 191, 194 strike-slip 39, \47. 171, 172,241, 266. 281, 287. 295, 299. 101 movement rate 264 Ml Everest 154 Murray transform fault 58. 131, 175 N.gssugtoqidian 296 N.g (Nagssugtoqidian) 2,295 Nain Province N.mibi. 279 N.n Shan range ISO, 151 nappe 217, 262, 263, 264, 278, 288 allochtbonous 261 Appin 254 Arn.boU 247 Ba1acltulish- T.y 254 Banff 254 Ben More 247 Dent- BI.nche 215. 216, 219 Diableret 217,219 exotic roof 250, 25 I Glencoul 247 Jotun 263 Kinlochewe - Kishorn 247 Koli 264 MoiRe 245, 2047, 248 Monte Reas 215, 216, 219 Moreles 217,219 ophiolite 261, 267 P.rpaillon 220 St Bernard 216. 217 Seve 264 SPiff BeaS 248 Tank.v.ig 2047 T.y 254,255 Wildhorn 217,219 n.ppes, Austro-Alpine 219 Embrunais - Ebaye 217 exotic 25I. 262, 263. 264
Helvetic 215.217.219 imbricate 247 Pennine 214.217.219,220.221 Piemont 219 S. Devon 239 Tinee 217,218 Naver slide 248. 249 New Brunswick 242 New Caledonia 173 Newfoundland 229, 242, 243, 255, 261,266.268 New Guinea (Irian) \40, 159. 160. 162. 173 New Hebrides 173 New Jersey 204. 205 New Zealand 166, 171 Nice 215,217 Nordre Strornfjord 295. 296 Norman WeUs 33 Nonh American Cordillera 175 craton 222. 223, 227, 231 North Allantic - Arctic Ocean 214 Nonh China block 152, 153 Nonh Sea 195, 200, 202 Nonh Sea basin 49, 83. 189. 202 Nonh Sea furrow 93 Noth Sea graben 95 North Slope terrane 221 NW foreland zone (Scottish Caledonides) 244,245 Nosib Group 279 Nova Scotia 242 Nubian shield 273 Nilk sr.nitic suite 304, 305 obduction 141, 142, 143, 212. 218. 220,255,261,264,261,268, 275 ol;>lique scarps 186, 187 ~.noor f.bri<: 116 ~ pl.leallx 117 _ ridse 210 OC*. ridse vulc.nicily 75, 16 ~sweU
lIcrmudan 16 .....aii.n 16, 18 oc:eanization 198 off-.pilll 119, 120, 125, 258 olisdloltrome 163, 212, 241 ophiolite 140. 142, 143, 144, 152. 153, 165, 171,211,219,227, 255,261,264,166,267,268, 271,274,275,276,277,278, 292 Ballantr.e 166 Bay of Islands 255, 266 Bell'S Cove 255, 266 Girv.n - BaIl.nlrae 256 UIlSI 250 ophiolites, abducted 241, 250, 284 ophiolile CDmplelles 212 Oregon 166, 227 orogenic bell
333
INDEX Alpine-type 228 Appalachian 230 Ca\edonian '24'2, '244 Hercyno-type 228 Southern Appalachian 232 orogenic cycle 269 orogenic phase, Acadian 231 orogeny Alleghenian 228, 229, 231 Alpine 236 Cadomian 231, 245, 254, 259 Caledonian 159,228,231,245, 249, 264, 269 collision 139, 271 Finnmarkian 261,262,264 Grampian 255, 264, 267 Grenvillian 245,247,253,281 Hercynian 269 Hudsonian 271,282 Laramide 97, 98 Laxfordian 245 Mauritanian 228 Scandian 261, 262, 264 Sevier 97, 98 subduction 139 Orust 287 Oslo 263, 287 Oslofjord 285 Ostfold - Marstrand segment 285, 286,289 Ostfold Segment 286 Outer Hebrides 245, 296, 297, 298, 299 overcoring 29, 37, 94, 136 overthrust, frontal 125 overthrusting 139, 143 Pacific Ocean evolution of S8, 61, 6S palaeomagnetic data, evidence ·147, 171, 2S0, 264, 266, 269, 272, 279, 289, 292 reconstruction 61 Paliser limestone 223 Pamir range ISO, isr, IS4, IS', IS7, 1S9 Pan-African ('orogeny') 272, 273, 273,277,279,281,290 Lower, Middle and Upper 273,27S Pangaea 19,26,27, S8, 61, 83, 8S break-up of 210 parautochthonous (structural units) 217
pBSlive (contiMlltal) margin lOll, 188, 197, 198,199,227,273 Peloponnesos 134 Pembrokeshirc 238 PenDine nappes 214,217,219,220, 221 peralkaline ma.matism 27S Pernambuca lineament 90 Pharuside fold beh 196, 197 phase transitions 28 Philippine islands 1S9
Piedmont province 230 Piemont trough 214 viel-ometet ~sttess metet' 1\ olivine grain-size
31
piggyback (thrust) sequence 146, 223, 239 pillow-flow eruptions 78 plate Adriatic 140, 219 Aegean 135 African 3, 55, S8, 62, 80, 134, 13S, 136, 144, 186,211,214, 215 American, Americas 3, 26, SS, S6, S8, S9, 64, 6S, 69, 96, 125, 126, 175,221 Anatolian 134 Antarctic 3, 54, SS, 56, 62, 67, 69 Arabian 58, 62, 87, 90, 91, 131, 134, 140, 144, ISO, 166, 171, 211 Asian (see also Eurasian) Caribbean 65, 116, 125, 126, 130 Cocos 56, 64, 65, 69, 116, 118 Eurasian 3, 55, 56, 58, 67, 69, 86, 131, 134, 135, 140, 147, 159, 161, 166, 173, 186, 190, 200, 211 European 221 Farallon 58,61.63,64, 175 Iberian 140 Indian 39, 55, 62, 64, 67, 69, 80, 131, 140, 144, 150, 151, 154, 157, 159, 160, 166 Indo - Australian (see also Indian) 3, 159, 160, 161, 166, 173, 175 Juan de Fuca 96 Kula 61,62,63, 178 Nazca 33, S6, 58, 64, 65, 69, 117, 118, 122, 123 N. American 116, 178 Nubian 87, 90 Pacific 3, 26, 33, 55, 56, 58, 59, 61,63,64,69,98, 100, 147, 159, 161, 166, 173, 175,228 Phoenix 61, 63 Somalian 87, 90, 91 S. American 27 Tethyan 214 Turkish 214 plate boundaries collisional 86 conservative 54 constructive 73, 83, 166 convergent 25,86,95, 171 dalruetive 112, 166, 210 diver.ent 73,:w.l oblique movements at 166 plate convergence 19 direction of 268 plate divergence 75, 79, 187 direction of 80 plate evolution, Pacific Ocean 63
plate motions
absolute
20, 67
l<:'\~\\'l<:' \~
plate velocities 3 plateau Colorado 36, 96, 97, 98, 99, 101, 207, 208 Deccan 207 East African 207 Ethiopian 207 Iranian 140 Snake River 98 Tibetan 140, ISO, lSI, 154,207 plateau uplifts 26, 27, 29, 46, 73 plateau, oceanic 209 plates, elasticity and rigidity of 7 Poisson's ratio
43
polar wander curves 271, 272, 277 Pontesford lineament 259 pole of rotation 2, 3 Po plain 214 Po valley 221 Port Askaig tillite 265 post-glacial rebound 189 pre-Alps 21S, 216, 219 Precambrian chronology 269 heat production 270 pre-Caspian depression 241 Pregothian gneisses 285, 286 prehnite - pumpellyite facies 259 pressure, pore-fluid 30 Proterozoic supercontinent 272, 290 proto - Atlantic Ocean 244 proto - Pacific Ocean 227 pull-apart (basins) 171 Purcell anticlinorium 223, 22S, 226 Pyrenees (range) 139,140,211 ramp 133, 145, 146, 250, 263 hangingwall 145 ramp-flat geometry 106, 169 Rangely oilfield 33 Rayleigh number 22 recesses 147 Red Sea 85, 87, 88, 94, 277 regimes convergent 3,4,231 divergent 3, 231 extensional 4 intraplate 73 oblique 3 strike-slip 3,4, 73, 166, 171,241 relative motion convergent 54 diversent S4 strike-slip S4 retrocharriage/back-folding 217, 2SS Reykjanes peninsula 80 Rheinisches Schiefergebirge 234, 241 Rhenish shield 93, 207 Rheno-Hercynikum 233, 234, 236 rheology dry olivine 44
334 dry quartz 44 gabbroic S I grandiorite S I 147 plagioclas. 43, 44power-law 148 quartz 43 wet quartz 44 Rhine Graben 49 riders (on 501. fault) 107 ridge (oceanic) 3, 6, 7, 9, 10, II, 13, 16, 2S, 26, 28, 39, S6, S7, S8, 61, 62,67, 73, 7S, 117, 143,209 Atlantic 77 Atlantic - Antarctic 62 Barbados 119, 12S, 126, 127, 129, 133, 13S, 2S9 Carlsberg S8, 80, 91 Carnegie 118 Caroline 66, 67 central Indian Ocean 62 Cocos 3S, 117 Chagos 61 crenellate 79, 80 East Pacific 11, 13,33, 3S, S6, S8, 64, 6S, 77, 78, 99, 166, 17S, 186, 187,228 Farallon - Kula 63, 178 Galapagos S6 Juan d. Fuca 166, 17S Juan F.rnand.z 117 Kyushu - Palau 66, 102 Marcus - Necker 66, 67 Mediterranean 134, 13S, 143 mid-Atlantic 10, 12, S6, 62, 73, 74, 7S, 76, 78, 79, 101, 12S, 166, 182, 184, 18S, 186 Nazca 117, 118 Ninety-East 61 Reykjan.s 79 SE Indian Ocean 62 SW Indian Ocean 62 stepped 80 Trillan d. Cunha - Walvis 67 Walvis 6lI ridge offset 166 ridle pllSh 24, 144 ridle system, Indian Ocean 83 ridge-transform intersection 267 ridges 18, 26, 56, 62, 69, 70, 178, 181 aseismic 66, 103, 113, 116, 117, 118 fast-spreading 79 slow-spreadina 77 Riedel shears 167, 186, 187 rift 81,82,86,89,91, 9S, 109, 1117, 202, 204,207, 291 Abu- Gabra 87, 90, 94 Adamaoua \l4 Arctic - Atlantic 200 Baikal 83, 84, 86, ISO, 188 continental 181,284,312 Darfur 94 non-Newtonian
INDEX Dead Sea 171 East African 74, 83, 8S, 86, 87, 89, 90, 94 Ethiopian 94 Gulf of Aden 83, 87, 90, 94 Gulf of Aqaba - Dead Sea (transform) 91 Gulf of Suez 87, 92, 94 Iceland 81 Kenya 88, 90, 94 Ngaound.r. 87, 90, 94 proto - Atlantic 90 R.d Sea 39, 83, 87, 87, 90, 94, 273 Rhine 84, 86, 94, 9S Rio Grand. 34, 36, 83, 96, 97, 98,208 rift formation lOS rift pillow 84 rift system African 39, 86, 273 African - Red Sea - Gulf of Aden (Afro-Arabian) 83, 86, 87, 94 Central African 90 Rhine - Ruhr 37, 83, 93, 214 rift vall.y 76, 77 rift vulcanicity 84, 8S, 86, 87, 89, 90,94 rifts, rifting 70, 8S, 190, 191, 198, 200, 214, 242, 264 active 8S aseismic 90 axial 78, 79 continental II, 73, 83, 2S3 extensional 200,212, 289 fail.d arm 83, 84 intrtplat. 83 lithosph.r.·activaled, "Ienerated 8S, 87, 94 mantl.-aaivated 8S, 86 plsUv. 8S Itrcss....nerated 94 Rocky MOUnlaias 34, 36, 101, 144, 145, 146, 123 Canadian 122, 22S
ronever 106 root zone (of nappes) 25S Ruhr 242 Ruuian platform 189, 190, 191, 193, 194,207,209,228 Sahara
195 saJimu 147 Salt rtlll. 158, 159 San Gorlioni t1cnd 17S, 178 Santa Maria 178, 179 Sardinia - Corsica 214 Saudi ~ia (_ ttbo Arabian shi.ld) 273, 277 S8lI0II Grtnullt,.bir,. 231 Scotland 248, 291, m, 301 north-west 246, 270, 295, 297 northern 244, 151, 253 south-west 254
Scottish Highlands 2S I, 2S9 Scourie 297 dykes 298 gneisses 270 Sea of Crete 137 Sea of Galilee 171 SEABEAM sonar 133, 13S, 136, 137 Seal Lake 284 sea-mounts 116, 117 section balancing 223, 247 sediment loading 197, 199, 200 lei5lftic coupling 114 discontinuity 2S6 profiles 12S, 133, 154, ISS, 256 seillllic refieaion data, profil.s 119, 121, 129, UI, 183, 184, 203, 204, 2S0 seismic r.fraction data, prcfiles 7S, 140,201,202,221 seiMlic slip II S seilMic velocities 141 P....ave 201 seillftic wave 7 .tenuation of 9, 74 p. 7, 154 . Sa 7, 74, IS4 Iei_icity 117, 147, 177 Scram 160, 162, 16S, 173 Sevier Desert derachment 99, 101 Sgurr Beag slide 246, 249, 2SS, 267 shallow SUbduction 312 shear zones (see also zones) 4, lOS, 109, 146, IS4, 167,202, 203, 204, 234, 237, 241, 249, 250, 210, 281, 284, 286, 287, 2111, 119, 294, 296, 298, 300, 307, :J09, '11, 312 Canisp 197 Diabe!J JOO
'-rian 1'7, 299 Idwdleq
m
ItriltiansaINI- .... 288 LMford 1'7, JOO Nenlre Strflllfjord 29S, 296 GIIofjord 217, 2IlI s'-" folds 264 S1MIIHd 202 ~hiIW
_iomtal PIwambrian shoot"';nl rate
'2
10, S1
239
Siberia (block) 153, 154 Sicily 210 side-lC8n _ _ 75, 119, 18S Sierra NC\'da 16, 101, 17S, 180 Si. . - "Or- block 152 Sklne 215 Skye 247 slab bucklilll 119 slab dip inclination 37, 104, 116, 117, 118 slab profile 104
INDEX Sia ve province 291 Sleat Group 245 slickenside striations 95 slides 249 Sol Hamed 276 solid diapirism 310, 311 Solway Firth 244, 245, 258 South East Asia 149, 153, 159, 160, 175 South East Asia block 152, 153 South China 149 South China block 152, 153 South Great Plains (province) 36 .South Harris 296, 297, 298 South Harris igneous complex 297 South Tibet block 153 Southern province (Canadian shield) 291 Southern Uplands (Scotland) 244, 256, 257, 258 -r Spitzbergen 242 spreading fabric 122, 124, 181staircase trajectory 145 " Stoer Group 245 q Stora Le Marstrand formation, 286, 287 strain gauge, use in stress determination 29 strain hardening 5 I, 53 strain rate 3, 4, 44, 49, 50, 90, 99, 147, 150,204 strain softening 5 I Straits of Gibraltar 210 Straits of Hormuz 131 Strathconon 249 strength instantaneous 7 uniaxial compressive 30 stress 41 asthenosphere 32 '; bearing 24, 26, 27, 28 compressive 94 critical 46 determination of 29 down-dip 40 extensional 85, 92, 96, 97, l'I8 in-situ 29, 30, 33, 93, 94, 96, 97, 136, 177 intraplate 32, 37,41, 85, 119 loading 26 magnitude of 24,29, 30, 31, 32, 37 membrane 24, 27, 28 non-renewable 24, 25, 27, 28 renewable 24, 25, 28 sources of 24, 25 in subduction zones 38 thermal 24, 27, 28, 29 thermoelastic 83 stress amplification 28, 42 stress concentration 147 stress-depth discontinuities 46
stress-depth relationship 32, 41, 46, 47,48 stress field 83,96, 185 stress orientation 32, 33, 34, 35, 37,40,41,98 stress provinces (USA) 34, 36 stress relief 29 stretching factor B 199, 205, 237 strike-slip, displacement 3, 159, 168, 173, 175,221,244,250, 255, 256, 259, 288 model 257 regime 259 sub-Alpine chains 217 subcretion 119, 120 subduction, angle of 95, 116, 118, 175, 304, 305 subduction (in Precambrian) 271 subduction rate 306 Lesser Antilles 125, 126, 127, 130, 259 Mediterranean 166 Makran 140 Mexican 140 Oregon 175 vulcanicity 118 subsidence, rate of 195, 197, 205 Sudan 276 Sudetic phase 229, 241 Sulawesi 159, 160, 161, 162 Sumatra 159, 175 Sunda craton 160 Superior Province 283, 289, 302, 307, 308, 309 surface unloading 28 surface waves (seismic) 9, 208 dispersion of 7, 74 suspect terranes 221 Sutherland 249 suture 140, 142, 154, 159, 221, 244,261,266, 274, 276, 278, 279, 300, 312 Jlrustro - Alpine 220, 221 Banging - Nujang 153, 155 collisional 241, 272, 275, 277, 281,284 Lapetus 266 Indus 151 Indus - Tsang Po (Zangbo) 150, 152, 154, 155 Siuang 153 Solway 244, 250, 256, 259 Uralides 241 Wetar 163 Sveconorwegian (province, orogeny) 286, 287, 288 syneclise 190 Baltic 191, 193 Dneiper - Donetz 191, 193 Moscovian (Moscow) 191, 193 Pechura 191, 193 Pre-Caspian 191, 193 synorogenic deposits 212
335 Taconic 229, 266 Tanzania 88, 277 Tarim basin 159 Tarim block 152, 153 Taurus mountains 139 tectonic age 14 collage 152, 221 subsidence 206 tectonics, thin-skinned (thrust) 146, 241 Telemark 288, 289 tension fractures 75, 92 terrane J53 allochthonous 227 Alexander 227 Avalon 261, 266 Cache Creek 227 displaced 171,173,222,226,255 exotic 171,175,221,227 Grampian (Highlands) 250, 251, 256 Klamath Mountains 227 Midland Valley (Scotland) 255, 268 Northern Highlands 247,249 Notre Dame (Newfoundland) 268 Stikine 227 suspect 172, 175, 221, 222, 226, 227, 244 terrane accretion, strike-slip 227 terrane displacement 178 Tethys (ocean) 61,94,139,162, 200, 204, 212, 236 proto- 228 Texas 229 Thailand 175 thermal anomaly 183, 198,204 concentration 183 diffusivity 22 gradient (see geothermal gradient) sub-layer 22 subsidence 205 turbulence 22 thick-skinned (thrust) model 250, 251 thrust, Alpine sole 221 thrust Austro - Alpine 220, 221 Arnaboll 246 back 236, 238, 239 blind 238 Dalsland Boundary 285, 286 Ettrick Valley 256 fleer 146,251 (see fold-thrust belts) Frontal Pennine 220, 221 Himalayan Boundary 150, 154 Johnson 238, 239 Kinlochewe 246, 247 Lewis 226 Loch Skerrols 253 Main Boundary (Himalayas) ISS, 157, 158
336 Main Central (Himalayas) 155 Main Mantle (Himalayas) 154, 156, 157, 158 McConnell 226 Moine 246,247,248,251,252, 253, 265, 266, 267 out-of-sequence 239, 247 Outer Hebrides 246 Ritec 238, 239 Roberts Mountain 241 roof 146,247,251 sole, Alpine 221 sole 144, 161, 217, 238, 246, 261, 263 Ultrahelvetic 220 thrust belts 144, 220, 247, 263, 284 antithetic 144 compressional 106 foreland 144, 159,221, 222, 223, 225,245 Moine 244, 245 synthetic 144 thick-skinned 144, 223, 230, 237 thrust complex synthetic 119 thin-skinned 234 thrust zone 108, 145, 281 (see thrust belt) thrust, Moine 144, 146, 246, 250, 255 thrust, Sesfontein 280 thrust displacement 285 thrust duplexes 120 thrust-fold belts 230 thrust front 263 thrust sheet allochthonous 163, 165, 217 imbricate 292 Kalbano 292 Kalbano 165 parantochthonous 163, 165 thrust stacking 145 thrU5ling &ntilhetic 144 piayback 246 synthetic 144 thin·skinned 145, 146,221,261, 292 Tibesti 87 Tibet 140, 150, 152, 153, 154 Tibet block 152, 153 Tien Shan range 140, 150, 151, 159 Timanides 242, 243 Timor 140, 159, 160, 161, 163, 164, 165 Timor Sea 159 Tintagel decoupling zone 239 Tornquist line, zone 204, 208 Tonidon 299 Torridon Group 245 transform fault (St't' also under fault) Dead Sea 172 M\lrray 58, 131, 175
INDEX
Romanche 182 Vema 182 vulcanicity 181 transform valley 183, 184, 185 transport direction 4, 220, 239, 247,249 transpression 70,71,72, 166, 167, 168, 169, 170, 171,214, 241, 264,281,291,292,294, 299 transtension 70, 71, 72, 166, 167, 168, 169, 170, 171,292,298, 299 Transverse ranges (USA) 180 trench (oceanic) 3, 56, 57, 58, 59, 104, 112, 116, 122, 125, 135, 258 (st't' also trough) Andaman 150 Aleutian 64, 102 Chile 166 Eastern Mediterranean 210 Gortys 135 Hellenic 119, 134, 135, 136, 138, 144 Japan 121 Java 159 Kuril 102 Matapan 135 Peru, Peru - Chile 33, 35, 64, 113, 117, 119, 122, 124, 133 Pliny 135 Poseidon 135 Puerto- Rico 114 Rocky Mountain 223, 225 Strabo 135 Tonga 113 Western American 99 trench lcometry 113 trench roll-back 104, U' trencll·llope break 120, 258, 259 trench wall, inner 112 trenches 3, 6, 14, 16, 19. 26, 66, us, 119, 212 triple junctioll 55, 56, 57, 58, 59, 61, B3, 90, 98, 175, 200 Afar B5, 88, 90 Galapqos 64 Mendocino 177 NilCf B3 Trondhjeim 255 (JW also trmc:h) troul" Barbados 126, 133 Benue B3, B7, so Bonin 100 Grenada 127 lau 100 Marianas 100 Nallkai 259 Tobago 126, 127, 133 Tunisia 210 turbidite apron 112 Turkey 110 Ullapool
247
Ultrahelvetic nappes 214 underplating 119,120,133 tectonic (A-subduction) 305 underthrust(ing) 120, 125, 129, 139, 142, 144, 152, 172,227,258,264 uplifts 190, 194, 206, 207, 209, 242 Adamaoua 90 Baltic 208 domal 85, B7, 90, 94 East African 209 epeirogenic 94 Air 94 HOllar 94 Tibesti 94 FennosCandian 207 .intraplate 208 isostatic 208 orosenic 212 plateau post·glacial 207 rates of 207, 208 Rhenish 209 Ukraine 190 Voronezh - Ukraine 191, 207 uPiler·plate motion (at convergent boundary) 116 Uralides, Urals 190 Valley and Ridge province 230, 232, 233 Variscan front 241 Variscides, Variscan zone 190, 228, 241 Varmland - Kopparberg 285 vector
convergence 159 _vement 72, 214, 241 "Ie·movement (.velocity) 39, 64, 67.69, 212 ....live·m.. v....ent (-velocity) 55, 58, 71,10 lip (fault) 12, 39, 41, 54, 110, 136, 150 _ triangle 3,54,56,57, 1,35 .....y P_ve 7, 202, 252 P._ve 7, 20ll lIiIaric:·wavc 5, 7, 8 wiec:iey trianclc 58 V...-n IIaciation 265 .....-e 217,218, 292 WIeeIit)' 5, 15 ...-rent 43 effective 15 1dIIematic 22 viscous relaxation 28 volcanic centre 200 volcanic cIemc 90 volCAnism, cak:...lkaJioe 98, 99 VOlcanoes, central 10, 82 Wadi Haimur 276 Wales 259, 260 South 228, 237, 237, 242
INDEX Wasatch Front 101 Washington 166 waves (see seismic waves) wedge subsidence (theory) 105 West European Variscides 231, 236, 241 West Gondwana continent 273 West Sulawesi 162 Western Alps 141 Western Medineranean 210,212,214 Western sub-province (Sveconorwegian belt) 285, 288, 289 Wetar Strait 163 Windermere Supergroup 222, 223 Wrangellia 227 Young's modulus Yugosla via 210
43
Blue-Ridge 241 Brianconnais 216, 217, 218 Central Gneiss (Grenville province) 282 Central Metasedimentary (Grenville province) 282 central Swedish Schistoslty 285 collage (N. American Cordillera) 173, 222, 226 collision 19, 144, 147,210 continental rift 7, 9, 10, 39 (see also continental rift) continental strike-slip 40 Dauphinois 216,217, 219 en-echelon extension 83 external (of Alps) 216 fault see fault zones Folded Jura 216 Foreland (of Hercynides) 236 fracture
Zagros (range) 134, 140 Zagros crush zone 140 "' zone (see also fault zones; fracture zones; shear zones; subduction zones) Benioff 113,116
'il
, I
)., '
see fracture zone
Grampian 266 Helvetic 214,215,216,219 imbricate 238, 2'8 inner Piedmont 233 Internal (Pennine) 216,217 Ivrea 215, 216, 219 low-velocity 9
337 Moldanubian 237 mylonite 249, 285 (see mylonite) Northern Highlands (Scottish Caledonides) 248 Northern Phyllite (German Variscides) 234 Piedmont (Appalachians) 216, 232, 241 Piernont (Alps) 215,217 Protogine 285 Rhenohercynian 236, 237,241 (see Rhenohercynikum) Saxothuringian 231, 233, 234, 237 seismic 3,113 Sesia-Lanzo 215,216,219 Schistes Lustres 217 Sinang 150, 151 Southern Alpine 216, 219 strike-slip 171 sub-Brianconnais 217 subduction see subduction zones transfer 288, 289 transform fracture 40, 185 transtensional 161 Valaise 216,217
