Geological Structures and Moving Plates

• Geologica l Structures and Moving Plates t' A.G. PARK, SSe. PhD Read er i n Geo log y U nive rsity 01 Keele • Bla...

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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

•

Blackie Glasgow an d Lon don Pub li shed in the USA by Chapman and Hall New York

•

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

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 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 -

-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 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 SCQ

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::

-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--:

~

~

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 ;":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

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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;'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

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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

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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

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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

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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).

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Figure 5.15 Sequence of cart oon model profiles illustrating the: tectonic evolution of the southern pari of the Lesser Antilles subduction zone . '0 ' and 'S re fer 10 possible palaeo-environments for the deposition of the oceanic and Scotland formations, respectively. Th iek sedime ntary inf luxes in Eocene and Neogene limes are inferred to have come main ly from the South American conti nent. From Biju-D uval tl 01 . (1982)

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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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-

•

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"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

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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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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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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

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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: 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---'::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

z

0

;;: 0

....;

rr.

(/)

~

'S t=;(

AULACOGENS MARGINAL BASINS (PERICRATONS)

/ ' BOUNDARV OF THE RUSSIA N PLA TFORM

If

?

INNER BASINS AND ARCH (SINECLVSES AND ANTECLVSES)

A" ..... SUPERIMPOSED VENDIAN

... ... 11 INNER BASIN (SINE CLVS E)

c

D DaTI

Tz-N

-

-

300 Km

300 K m

1

\)~ z > '" r-

>

rn

.-,

rr. -.,

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o z n

:c

r::

o

3::

v;

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.

\Q W

194

G1:::0LOG I( AI. Sl KUCTU KES AN D MOVING I' LA1 ES

, ,,~",,+--
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 indicer ,./ ,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

G H II I I(; ICAI

sr eucruees

AND MO \' ! ' G

o.---~---~----.-----.---,

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f. - C O ST B - 3 F" LEXU R A L

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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'

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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\ z o ~ o < z o .."

r

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MESOZOIC/CENOZOIC COVER

BASEMENT Europeen

"t_N~Ppe

0.~~m~r~ ~a~p~

~

~

SOUTHERN ALPINE ZONES

"I. Rosa massif Matterhorn Dent-BlancllS »» ' - - -

NW

()

>

I

Plemonl Zone

BrlallJonnals Zone

5o r

20 km

,

o

s

m

EXlernal Zones

Pia ~ont FaCIes

Bria~~meis FaCIes

Opoolltes

Molasse

intrusions

~····

~ ~

...... _ _ ..-.... .···· DD

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

-

sw CO(

OE C ROVS

UNIT

,

IH ' _ 1IIE1 / "'-- -- - - - - - -_

,

NE

;~.~~;;;: COL CPOuS LISTRIC NORHAL FAULT

A

1

Fi~Uft

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

u,,;t~

I f J J J Ui'lJOIl"'''

Is' .

H · t · 1· I 'i l(~u/I«JU' Io'a /O" " in .., ,, lSI.

lllllllllllJ llu "'o.m Is'.

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

.-..,w·. .'

w.w

e

.'

se -,

ESE

.nj - D c.._.

" 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 )

RO C KY "' NT ICL I N O A I U ~

PURCELL

I(OO;rE N...Y ...AC

J,lOU N T ", IN

ow

, -s

.,,1---

• ROCK Y MOU N T ...rNS

FOOT HIL LS

S...

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S"'S EMENT

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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

•,

,

•m.se !

3. ,

see I

"URCEU .. "Tlel/NOII/tVM

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b. DUP-M

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

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em

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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).

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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

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233

PH A /'II EROZ OI C OROGENIC BELTS: SOMI'. EXA M PLES

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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

A

N

r front (probably 1101 the lip} to hiqh le vel imbricates .

S. Pe mbrc ke s hire

s 50 km arcuate trend problbly more ~ m ovemenl in .....

"

",?/;; -=~""""""'" wesl

~~

b

.. lo ld llcing d irection """-thru $t CORNWALl. X Inlilorm »r I ~'r' X sy nfo rm

237

PHANE ROZOIC OROGEN IC BELTS: SOME EXAMPLES

HERCYNIAN FRON T

........8 /

..;

8RITTANY SHEAR ZONES

~ RHE NO

/

,

...

?.. )

:'

~. , ..~~~

-, >

\ ?/

~ ') P'-. '-...... '-•.""" ~ ( ~ICARTIA~ --...... ~,

\

-, ;'CA OQM IA N 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 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

100 I

li m it of Precambrian

Outcrops v v v C&lcalkali ne Volcanics =-=-.: Marble - .. Ophiolite Jlll!m Po ssibl e Suture Zone 1/// Generafized Str ike

Fau ll

v- v .....

25"N

W Umn o f Calcalkali ne

Volc.nicslafter V.ill 98J )

f

f ....\ .

, ••

I I

"·E

Figun: 9.3 (A ) O utline tectonic map of NE.Africa and the related part of Arabia, allowing for the: CS timill cd displacement on Ihc: Najd Iault. Note: Ihc: positions of postulated suture zones. From Sllacklc:lon (1986)

275

OIWG f.N Y I N THE PRECAMBRIAN -

-

lo_ Po"... h. .'o

I

I I

E'

A r Cha • • "

~

lIanlle

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mod ar at a

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l ol d

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L. .le,' rigu~ ' .:U Block diagram illustratin g the effects o f obtique-slip ' extensional

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

A

L AX 1.2. NAG 2

(5

or

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

?

B

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) .

lEW ISIA, N

C;l

£3 Pro : o : .lIl Ar~n Q"110ll

[9 _ _

'111

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EliJ OI',null, ' 1 _

O ROG l:NY IN THE r Rl:CAM8R IAf'I

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,

ea-

!

,

o

5

".

F,·'··..) ~ _

,

10~ m

w....1to

c:J G~-' _ ""

I ,·...

,~

___

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Fe....

F"lillre 9.24 Simplified geological map o f the Flskenaesset region, in the central pari o f the Archaean craton on the west coast o f G reenland. From Bridgwarer ~I Qt. (1976)

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) .

M. ?

375"

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

..

7. Emplacement of stratiform gabbro-

enonhc sncs

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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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-

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