Geology 1982 Geology-1982-Tapponnier-611-6 Tapponnier 611 6

8/18/2019 Geology 1982 Geology-1982-Tapponnier-611-6 Tapponnier 611 6

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Propagat ing extrus ion tectonics in Asia: New ins ights

f rom s imple exper iments wi th p las t ic ine

P. Tapponnier, G. Peltzer, A. Y. Le Dain, R. Armijo

Labor ato ire de Physique et Méc aniq ue des Ma tériax Terres tres, Inst i tu í de Phys ique du Globe, Univer s i té P. e t M. Curie ,

4 Place Jussieu, 75230 Paris , France

P. Cobbo ld

Cen t re Arm or ica in d E tude St ruc tu ra le des Soc les , Un ive rs ité de Rennes , 35042 Rennes , F rance

A B S T R A C T

Pl a ne i nde nta t i o n e x pe r i me nts o n uni l a te r a l l y c o nf i ne d b l o c ks o f p l a s t i c i ne he l p us to

unde r s ta nd f i n i te i n tr a c o nt i ne nta l de fo r ma t i o n a nd the e v o l ut i o n o f s tr i ke -s l i p fa u l t i ng i n

eastern Asia . Several large left- la tera l str ike-s l ip faul ts may have been activated

suc c e ss i v e l y , e s se nt i a l l y o ne a t a t i me . T he e x pe r i m e nts sug g e s t tha t the pe n e tr a t i o n o f

Indi a i n to As i a ha s r o ta te d =2 5 ° ) a nd e x tr ude d =8 0 0 km ) Indo c hi na to the so uthe a s t

a long the then left- la tera l Red River faul t in the f irst 20 to 30 m.y . o f the co l l i s ion. This

pr o c e ss c a n a c c o unt fo r the o pe ni ng o f the So uth Chi na Se a be fo r e l a te Mi o c e ne t i me .

E x tr us i o n te c to n i c s the n mi g r a te d no r th , a c t i v a t i ng the Al ty n T a g h fa u l t a s a s e c o nd

ma jo r l e f t - l a te r a l fa u l t a nd mo v i ng so uthe r n Chi na hundr e ds o f k i l o me tr e s to the e a s t . As

th i s o c c ur r e d , Ind o c hi na ke pt r o ta t i ng c l o c kwi se a s mu c h a s 4 0 ° ) , but the se nse o f

mo t i o n r e v e r se d o n the Re d Ri v e r a nd o the r s tr i ke -s l i p fa u l t s i n the so uth . Ope ni ng o f the

Me r g ui ba s i n a nd And a m a n S e a up to the pr e se nt ) a l so a ppe a r s to be a s i mpl e k i ne m a t i c

c o ns e que nc e o f the e x tr us i o n . Re c e nt r i f t s i n no r the a s te r n C hi na a nd Yu nna n m a y be

c o ns i de r e d i nc i p i e nt a na l o g s o f the So uth Chi na a nd An da m a n Se a s . Othe r T e r t i a ry

te c to n i c fe a tur e s suc h a s the se d i me nta r y ba s i ns o f the Gul f o f T ha i l a nd ma y be e x p l a i ne d

a s c o l l i s i o na l e f fe c t s , i f o ne use s o ur e x pe r i me nts a s a g u i de . T he e x pe r i me nts a l so sug g e s t

that a major left- la tera l str ike-s l ip faul t and r i ft system wi l l propagate across the Tien

Sha n, Mo ng o l i a , a nd B a i ka l to the Se a o f Okho tsk .

I N T R O D U C T I O N

I n t ense and w idesp r ead se i sm ic i t y i n cen -

t r a l and ea s t e r n As i a can be i n t e r p r e t ed a s

a d i r ec t consequence o f t he co l l i s i on

b e t w e e n I n d i a a n d A s i a ( M o l n a r a n d T a p -

ponn i e r , 1975 ) . A l a r ge pa r t o f t he active

deformation o f t he As i a n con t i ne n t can be

acc oun t ed f o r by g i an t s t r i ke - s l i p f au l t s

t h a t g u i d e t h e i n s t a n t a n e o u s t e c t o n i c s

and a l l ow the ex t r u s ion o f t he As i an c r u s t

a n d l i t h o s p h e r e s i d e w a y s , m o s t l y t o t h e

e a s t , in f r o n t o f i m p i n g i n g I n d i a ( T a p p o n -

nier an d M ol na r , 1977, 1979) (Fig . 1). Th e

bas i c con cep t i s t ha t a r i g id d i e ( I nd i a )

i n d e n t s a p l a s t i c b o d y ( A s i a ) . P l a n e -

s t r a in p l a s t i c i t y t heo r y ach i eves som e

deg r ee o f success i n p r ed i c t i ng t he i n s t an -

t aneous k inem at i c s and s t r e ss f i e l d f o r d i f -

f e r e n t i n d e n t e r g e o m e t r i e s a n d b o u n d a r y

c o n d i t i o n s t h o u g h t t o b e g r o s s l y a n a l o g o u s

to t hos e o f t he I nd i a - Eu r as i a co l l i s i on

( T a p p o n n i e r a n d M o l n a r , 1 9 7 6) . H o w e v e r ,

t h i s ap p r oa ch i s lim i t ed by it s s im p l i c i t y

a n d b y d r a s t i c a s s u m p t i o n s a b o u t t h e

r h e o l o g y o f t h e d e f o r m a b l e m a t e r i a l .

M o r e o v e r , e a r t h q u a k e s i n A s i a r e f l e c t o n l y

t h e f i n a l i n s t a n t i n a c o m p l e x d e f o r m a t i o n

p r ocess t ha t has been i n p r og r ess f o r a t

l ea s t 40 m .y . S ince t he i n i t i a l su tu r i ng o f

t he two con t i nen t s , I nd i a , excep t i n t he

n o r t h , h a s r e m a i n e d r i g id ( M o l n a r a n d

Ta pp on n ie r , 1981 ) and has pen e t r a t ed a t

l ea s t 2 ,000 km in to As i a ( Molna r and Tap -

pon n i e r , 1975 ) . I t i s t he r e f o r e im p or t an t t o

u n d e r s t a n d t h e

evolution in time

o f i n t r a -

c o n t i n e n t a l d e f o r m a t i o n i n A s i a , n o t o n l y

f o r l oca l r easo ns bu t a l so because i t i s t he

b e s t e x a m p l e w e h a v e f o r u n d e r s t a n d i n g

t h e m a n y c o n t i n e n t a l c o l l is i o n s t h a t h a v e

c o m m a n d e d t h e f o r m a t i o n of o r o g e n i c

bel ts in the past 2 b .y .

I n o r d e r t o s im u la t e such a co m p lex evo -

l u t i o n , o n e c a n m a k e n u m e r i c a l m o d e l s

( f o r exa m p le , Da ign i e r e s and o the r s , 1978 )

o r , u s i n g v i s u a l m a r k e r s , s t u d y e x p e r i m e n -

t a l l y t h e d e f o r m a t i o n o f a n a l o g m a t e r i a l s

such a s p l a s t i c i ne , pa r a f f i n , o r c lay ( C ob -

bo ld , 1975 ; Da ign i e r e s , 1975 ). In bo th

cases , m os t l y p l ane - s t r a in o r p l ane - s t r e ss

s i t ua t i ons a r e eas i l y access ib l e , and i t has

been d i f f i cu l t t o accoun t f o r t he e f f ec t o f

b o d y f o r c e s . T h i s r e n d e r s u n c e r t a i n a n y

q u a n t i t a t i v e c o m p a r i s o n w i t h t h e d e f o r m a -

t i on o f t he l i t hosphe r e wher e c r u s t a l t h i ck -

en ing and t h inn ing , wh ich a r e r e l a t ed t o

g r av i t y in an e ssen t i a l way , can be im por -

t a n t . N e v e r t h e l e s s , b o t h a p p r o a c h e s p r o -

v i d e k e y s t r u c t u r a l a n d k i n e m a t i c

i n f o r m a t i o n . W e h a v e c h o s e n t o e x p l o r e

e x p e r i m e n t a l l y t h e I n d i a - E u r a s i a c o l l i s i o n

f o r two geo lo g i c r easo ns : ( 1 ) t o s t ud y t he

g r o w t h , m e c h a n i s m , a n d i n t e r a c t i o n o f

faults as we l l a s t he i r i n f l uence on t he d i s -

p l ac em e n t f i e l d , and ( 2 ) t o ana lyze large

finite displacements and deformations.

W e

have bu i l t a s im p le bu t ve r sa t i l e i nde n t a -

t i o n m a c h i n e w i t h w h i c h d i f f e r e n t m a t e r i a l s

c a n be d e f o r m e d u n d e r d i f f e r e n t b o u n d a r y

cond i t i ons ( P e l t ze r and o the r s , 1982 ) . We

pr esen t he r e t he f i r s t r e su l t s o f som e expe r -

im en t s and d i scuss new in s igh t s t hey p r o -

v ide i n to t he Te r t i a r y t ec ton i c s o f As i a .

G E O L O G Y , v . 1 0, p . 6 1 1 - 6 1 6 , D e c e m b e r 1 98 2


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Figure 1 . Schemat ic map of Cenozoic ex trus ion tec tonics and la rge fau l ts in eas tern As ia . Heavy l ines = major fau l ts or p la te boundaries ; th in

l ines = less important fau l ts . Open barbs indica te subduct ion; so l id barbs indica te in tracont inenta l thrusts . Whi te a rrows represent qua l i ta t ive

major b lock mot ions wi th respe ct to Siberia rota t ions are not repres ented) . B lack arrow s indica te d i rec t ion of ex trus ion-re la ted e x tens ion.

Num ber s re fer to ex trus ion ph ases: 1 =5 0 to 20 m.y . B .P. ; 2 = 20 to 0 m.y . B .P. ; 3 = most rec ent an d future . Arrow s on fau l ts in west ern Ma la y

Gul f o f Tha i land , and southw estern Chin a Sea earl ies t ex trus ion phase ) do not cor resp ond to pres ent-d ay m ot ions .

6 2

G E O L O G Y , D e c em b e r


3/6

E X P E R I M E N T A L S E T U P A N D

D E F O R M A B L E M A T E R I A L

A r e c t a n g u l a r b l o c k o f d e f o r m a b l e m a t e -

r ia l i s p laced in a t ranspa ren t p la s t ic (Pe rs -

pex ) box 11 cm h igh and 30 cm s quare .

P lane ho r izon ta l s t ra in i s ma in ta ined by

two Perspex p la te s , wh ich bound the lower

and upper su r faces o f the b lock and a re

lub r ica ted wi th ta lc powder . T h is a ssembly

rests in a larger one, which keeps the

t e m p e r a t u r e c o n s t a n t ( 2 5 ± 1 ° C ) . T h e

def o rm ab l e b lock i tse l f i s com pose d o f ve r -

t ica l l aye rs , 5 mm th ick , o f hom oge neo us

p las t ic ine we lded toge the r wi th t r ich lo roe -

thane . By us ing a l te rna t ing ye l low and

v io le t l aye rs o f p la s t ic ine (Cobbo ld , 1975) ,

o n e c a n m a k e a c c u r a t e m e a s u r e m e n t s o f

fau l t d i sp lacemen ts and duc t i le s t ra ins and

tes t the ve r t ica l inva r iance o f the de fo rma-

t i o n . S t a n d a r d H a r b u t t p l a s t i c i n e e x h i b i t s

l i t t l e s t ra in ha rden ing and c lose ly fo l lows

the Von Mises y ie ld ing c r i te r ion . I t s

s teady-s ta te rheo logy can be desc r ibed by a

power- f low law o f the type t = Ka

n

where

n -7.5 at 25 °C (M cC lay , 1976). Th e yield

stress is ab ou t 10

5

Pa fo r s t ra in ra te s on

the order of 10

7

s ' (Dai gnie res, 1975).

T he in den te r (5 cm wide ) can be

adva nced a t a con s tan t ra te (2 .5 cm /h he re )

in a g iven d i rec t ion by means o f a sc rew-

j a c k c o u p l e d t o a s t e p p i n g m o t o r . F o r s i m -

p l e g e o m e t r i c a n d k i n e m a t i c c o m p a r i s o n s

wi th the co l l i s ion be tween Ind ia and A s ia

(== 5 cm /y r of conv erg enc e alo ng a coll i-

s ion f ro n t o f = 2 ,000 km) , the sca le fac to rs

( r a t i o s b e t w e e n h o m o l o g o u s q u a n t i t i e s i n

m o d e l a n d n a t u r a l p r o t o t y p e ) a r e

= 2.5 x 10 * for length, K

x

- 5.7 x 10

12

fo r t ime , and K

s

- K

s

• K,'* = 4.4

x

| 0

3

for

ve loc i t ie s . I f g rav i ta t io na l and ine r t ia l fo r -

ces were neg l ig ib le wi th re spec t to su r face

fo rces , we wou ld be ju s t i f ied in choos ing

two o f these sca le fac to rs independen t ly o f

o n e a n o t h e r .

E F F E C T S O F T H E F R E E L A T E R A L

B O U N D A R Y C O N D I T I O N I N

P L A N E S T R A I N

T h e a s y m m e t r y o f c o l l i si o n a l d e f o r m a -

tion s in Asia (Fig. 1) sugge sts that c ont i-

nen ta l l i thosphere in wes te rn E uras ia o f fe rs

more re s i s tance to la te ra l mo t ions than do

subduc t ion zones a long the Pac i f ic and

I n d o n e s i a n m a r g i n s ( M o l n a r a n d T a p p o n -

n ie r , 1975 ; T ap po nn i e r and M olna r , 1977,

1979). The firs t goal of our s tudy was thus

to te s t th i s hypo thes is in the s i tua t ion o f a

f ree la te ra l bou nda ry . T h e re su l t s o f th i s

ex per im ent (D , E, F in Figs. 2 and 3) are

com pare d wi th those o f a symm etr ica l

exp eri m ent (A, B, C in Figs. 2 and 3)

wher e the p la s t ic ine b lock i s con f in ed by

ver t ica l Pe rspex p la te s on bo th s ides . In the

asym met r ica l exp e r ime n ts , the f ree s ide lie s

init ia lly 5 cm aw ay fro m the r ight t ip of the

inden te r . F igu re 2 shows th ree success ive

s tages o f de fo rmat ion obse rved in the p la s -

t ic ine fo r inc reas ing amoun ts o f pene t ra -

t ion o f the inden te r . In bo th expe r im en ts ,

the inden te r d i sp lacemen ts a t each s tage

were abou t (1 .2 cm (A and D) , 3 .5 cm (B

and E ) , and 6 .3 cm (C and F ) . D efo rm a-

t i o n s a r e c o n c e n t r a t e d a l o n g n a r r o w s h e a r

zones o r fau l t s , wh ich appea r rap id ly ,

approx ima te ly pa ra l le l to p lanes o f max i -

mu m shea r s t re ss ( in s ta n tane ous s l ip

l ines ) . T he geometry , k inemat ic s , and evo-

lu t ion o f these fau l t s have a p ro found

in f luence on the d isp lacemen t f ie ld (F igs . 2 ,

3 ) . As found in numer ica l so lu t ions (Da ig -

nieres and others , 1978), the influence of

the f ree la te ra l bound ary chang es the

d e f o r m a t i o n p a t t e r n i n a d r a s t i c m a n n e r .

In the bilaterally confined e x p e r i m e n t s ,

t h e d e f o r m a t i o n f l u c t u a t e s i n a p a t t e r n t h a t

keeps an ove ra l l symmetry , and r igh t -

la te ra l and le f t - la te ra l fau l t s p lay ba la nc ing

ro les . T he t r iangu la r reg ion in f ron t o f the

inden te r soon f reezes ( dead t r iang le )

and becom es we lded to and mo ves a lo ng

wi th the inden te r . Hence , du r ing mos t o f

the exper imen t , the la rges t d i sp lacemen ts

occur a long the s ides o f th i s t r iang le (F and

F \ F ig . 3, B, C) . As the inden te r pene t ra te s

fa r the r in to the p la s t ic ine , new fau l t s ,

a l te rna te ly le f t - la te ra l and r igh t - la te r a l ,

success ive ly fo rm nea r the apex o f the t r i -

ang le (F ig . 2 ) ; cumula t ive o f f se ts a long

them se ldom exceed abo u t 10 mm (F ig . 3 ) .

T he fau l t s d ie ou t a few cen t ime t res away

f rom the inden te r .

In the unilaterally confined e x p e r i m e n t s ,

d e f o r m a t i o n q u i c k l y b e c o m e s a s y m m e t r i -

ca l ; p red ic tab ly , fau l t s tha t a l low d isp lace -

men t s towar d the f ree s ide take the lead ing

par t . In pa r t icu la r , the le f t - la te ra l fau l t th a t

o r ig ina te s a t the le f t t ip o f the inden te r (F )

in F ig . 3) g rows an d cu rves ou t to jo in one

of the inhe ren t d i scon t inu i t ie s o f the mode l

(an in te r face be tween two laye rs o f p la s t i -

c ine ) , a long wh ich i t p ropaga tes to the f ree

s u r f a c e . T h e r e a f t e r , it b e c o m e s p r e d o m i -

nan t and gu ides the ex t rus ion and ro ta t ion

of a b lock o f p la s t ic ine whose s ize depends

on the inden te r wid th and on the d is tance

to the f ree s ide . Ma x im um of fse ts a long

this fault can reac h 25 to 35 m m (ove r

t w i c e t h e m a x i m u m o f f s e t s o b s e r v e d a l o n g

fau l t s in the symmetr ica l expe r im en t , F

and F ' excep ted ) . T he ex t rude d b lo ck

ro ta te s abou t 25° c lockwise a lmos t r ig id ly .

As in the symmetr ica l expe r imen t , inc reas -

ing pene t ra t ion o f the inden te r causes the

d e f o r m a t i o n t o m i g r a t e f a r t h e r i n t o t h e

p las t ic ine and to repea t i t se l f in some way

(E in Figs. 2 an d 3); a seco nd ext ru sio n

phase deve lops a long a second ma jo r fau l

(F2) wh ich a l lows la rge d isp lacemen ts to

the r igh t and c lockwise ro ta t ion o f a

second b lock , ana logous in s ize to the f i r s

one (E in F igs . 2 and 3 ) . Fau l t s ac t ive du r

ing the f i r s t ex t rus ion phase e ssen t ia l ly

s top , and so me (no t ab ly the le f t segm en t o

F | ) a r e d e f o r m e d d u r i n g t h e s e c o n d p h a s e

T he ro ta t ion o f the f i r s t b lock con t inues t

a ma x i mu m of 40° (F in F igs . 2 and 3 ) .

I n u n i l a t e r a l l y c o n f i n e d e x p e r i m e n t s ,

the re ex is t s no s teady-s ta te dead t r iang le

(D , E, F in Figs. 2 and 3). Th us, t he left

ex t rem i t ie s o f F | an d F2 mig ra te and bend

progress ive ly aga ins t the f ron t edge o f the

i n d e n t e r . N u m e r o u s g a p s , a k i n t o p u l l -

apa r t s a long s t r ike -s l ip fau l t s in the ea r th ,

open a long the le f t - la te ra l fau l t s to the

right (E, F in Figs. 2 and 3). Th e larg est

ones fo rm a long F ] and F2 , nea r the f ree

edge . F i te rm ina t es in to a pa r t icu la r ly

s p e c t a c u l a r w e d g e - s h a p e d g a p E an d F in

F ig . 3) who se open i ng re su l t s f rom t he

ro ta t ion o f the f i r s t ex t r ude d b lock . T he

k i n e m a t i c s o f t h e e x p e r i m e n t r e q u i r e t h e

p r o g r e s s i v e w i d e n i n g o f a n o t h e r w e d g e -

shaped gap be tween the r igh t s ide o f the

i n d e n t e r a n d t h e e x t r u d e d p l a s t i c i n e

E ,

F

in Figs. 2 an d 3).

T E R T I A R Y T E C T O N I C E V O L U T I O N

O F C E N T R A L A N D E A S T E R N A S I A

I n t e r p r e t a t i o n s o f n a t u r a l t e c t o n i c s i t u a

t ions wi th the he lp o f mode l expe r imen ts

mus t be don e wi th g rea t cau t ion . P lan e

h o r i z o n t a l s t r a i n e x p e r i m e n t s f o c u s a t t e n -

t ion on s t r ike -s l ip fau l t ing to the exc lus ion

of equa l ly imp or t an t tec ton ic p rocess es

s u c h a s t h r u s t f a u l t i n g ( T a p p o n n i e r a n d

M oln a r , 1976) . T he in f lue nce o f an iso -

t r o p i c l a y e r i n g o n t h e d e f o r m a t i o n o f

p la s t ic ine i s no t easy to a ssess . More

im por tan t , i t is s t i ll d i f f icu l t to sca le m a te -

r ia l p rope r t ie s , pa r t ly because o f ou r igno

a n c e o f t h e l o n g - t e r m a v e r a g e m e c h a n i c a l

behav io r o f the con t inen ta l c rus t and

l i thosphere . I f one mere ly looks a t geomet

r ic and k in em at ic a spec ts , howeve r , the

r e s e m b l a n c e b e t w e e n F i g u r e 1 a n d F i g u r e

3F (Pe l tze r and o the rs , 1982) wa r ra n ts

a t t e n t i o n .

F i r s t , whereas le f t - la te ra l o f f se t s a long

the la rge s t r ike -s l ip fau l t s o f cen t ra l Ch ina

m a y r e a c h s e v e r a l h u n d r e d k i l o m e t r e s

( T a p p o n n i e r a n d M o l n a r , 1 9 77 ), r ig h t -

la te ra l o f f se t s a lon g the sma l le r s t r ike -s l ip

fau l t s o f the T ien Shan and Al ta i do no t

e x c e e d t e n s o f k i l o m e t r e s ( T a p p o n n i e r a n d

Mo lna r , 1979) (F ig . 1 ) . Ord e rs o f mag n i -

tude o f o f f se t s on co r re spo nd i ng fau l t s in

G E O L O G Y , D e c e m b e r 1 98 2 6


4/6

t h e u n i l a t e r a l l y c o n f i n e d e x p e r i m e n t s a r e

c o m p a r a b l e ( 1 0 m m i n t h e e x p e r i m e n t a r e

equiva len t to 400 km in A s ia) (E , F in F ig .

3 ). S e c o n d , t h e g a p s t h a t f o r m a l o n g t h e

lef t - l a t e r a l f au l t s in the un i l a t e r a l ly con-

F ined exp er im ent r e f l ec t an ex tens i ona l

c h a r a c t e r o f t h e d e f o r m a t i o n , a b s e n t i n t h e

b i l a t e r a l l y c o n f i n e d e x p e r i m e n t , a n d t h u s

es s en t i a l ly a r e r e l a t ed to the ex i s t ence of

the f r ee bou nd ary (E , F in F igs . 2 and 3).

A l though the i r s hapes a r e in f luenced by ' : he

layer ing of the p las t i c ine , the gaps a r e

a n a l o g o u s t o th e r i ft s a n d e x t e n s i o n a l b a -

s i n s o f n o r t h e a s t C h i n a ( S h a n s i ) , M o n g o l i a ,

a n d S i b e r i a ( B a i k a l ) , w h o s e f o r m a t i o n , i n

th i s par t o f nor thern A s ia only can be

v iew ed as a d i r ec t cons equence of the s mal l

r e s i s t a n c e t o e a s t w a r d e x t r u s i o n o p p o s e d

b y s u b d u c t i o n z o n e s a l o n g t h e P a c i f i c m a r -

g i n o f A s i a ( T a p p o n n i e r a n d M o l n a r , 1 9 77 ,

1 9 7 9 ) . T h e e x p e r i m e n t s , t h e r e f o r e , s u p p o r t

the idea of a d ras t i c as ymmet ry in the geo-

d y n a m i c b o u n d a r y c o n d i t i o n s i n A s i a .

M o r e i n t e r e s t i n g a r e n e w i n f e r e n c e s t h a t

c a n b e d r a w n f r o m t h e e x p e r i m e n t s . One

l e f t - l a t e r a l s t r ike- s l ip f au l t , t he A l tyn

T a g h - G a n s u - W e i H e f a u l t , is p r e d o m i n a n t

in the r ecen t deformat ion of A s ia (F ig . 1 ) .

I t i s a t l eas t 2 , 500 km long (Ta pp on nie r

and M oln ar , 1977), and i t s Ter t i a ry of f s e t

ma y r each 500 to 700 km in the w es t ,

w h e r e t h e P e r m i a n - T r i a s s i c m a r g i n o f

L a u r a s i a a p p e a r s t o h a v e b e e n s h e a r e d a n d

d i s p la c e d b y t h a t a m o u n t ( A c a d e m i a

S i n i c a , 1 9 7 1 ; T a p p o n n i e r a n d o t h e r s , 1 9 81 ).

Thi s can be accounted for i f one iden t i f i es

the f au l t w i th one of the major f au l t s (F | o r

F2) em an at i ng f rom the l e f t t i p o f the

indenter (E , F in F ig . 3 ) . The c lockw is e

b e n d i n g a n d f l a t t e n i n g o f t h e w e s t e r n

e x t r e m i t y o f t h e A l t y n T a g h f a u l t a g a i n s t

t h e K a r a k o r u m r a n g e s o u t h o f K h o t a n a n d

Y a r k a n d ( K , Y i n F ig . 1 ) ( A c a d e m i a S i n i c a ,

1 9 71 ) c a n b e c o m p a r e d t o t h e d e f o r m a t i o n

of the l e f t ex t r e mi t i es o f F | o r F2 aga ins t

the f ron t edge of the indenter in the

exper iment (E , F in F ig . 3 ) .

T h e m o s t i m p o r t a n t i m p l i c a t i o n s o f

t h e s e e x p e r i m e n t s c o n c e r n t h e e v o l u t i o n

a n d n o r t h w a r d m i g r a t i o n o f l a r g e - s c a l e

s t r i k e - sl i p f a u l t i n g i n A s i a . S e a f l o o r -

s p r e a d i n g r e c o n s t r u c t i o n s a l l o w 2 , 5 0 0 t o

3 , 5 0 0 k m o f c o n v e r g e n c e b e t w e e n north-

eastern India

and A s ia s ince 40 to 50

m . y . a g o ( M o l n a r a n d T a p p o n n i e r , 1 9 75 ).

G iven the uncer t a in t i es in the da te o f co l l i -

s ion , and a l l ow in g for 1 , 000 to 1 , 500 km of

s h o r t e n i n g t h r o u g h t h i c k e n i n g of t h e c o n -

t i n e n t a l c r u s t , t h e a m o u n t o f c o n v e r g e n c e

t o b e a c c o u n t e d f o r b y e x t r u s i o n a l o n g

s t r ike- s l ip f au l t s w ould thus be be tw een

1 , 000 an d 2 , 500 km. I f i t has r eac hed 2 , 000

Figu re 2 . Th ree successive sta ges of two ind entat io n exper im ents on plast ic ine. A, B, C: bi lat-

era l ly confined ; D, E , F: uni latera l ly confin ed free s ide on r ight) . Inde nter dis place men ts for A

and D = 1 .2 cm; for B and E = 3 .5 cm; for C an d F = 6 .3 cm . Note na rrow shear zo nes or faul ts ,

and op en gap s on r ight s ide in E and F after Pel tzer and others, 1982 ) .

k m , c o m p a r i s o n o f p r e s e n t - d a y A s i a n t e c -

t o n i c s w i t h a s t a g e i n t e r m e d i a t e b e t w e e n E

a n d F in th e u n i la t e r a l l y c o n f i n e d e x p e r i -

me nt (F igs . 2 , 3 ) w ould be ap pro pr i a t e , and

t h e p r o p a g a t i n g p a t t e r n o f d e f o r m a t i o n

w h i c h c a u s e s su c c e s si v e e x t r u s i o n a n d r o t a -

t ion of p las t i c ine b locks in the un i l a t e r a l ly

c o n f i n e d e x p e r i m e n t m i g h t h a v e h a d a n

equiva len t in A s ia . We s ugges t iden t i fy ing

F2 in the exper iment w i th the Altyn Tagh

fault and F | w i th the Red River fault (F ig .

I ; E, F in Fig. 3) . The Tert iary tectonics of

eas te rn A s ia w ould thus r e f l ec t the s ucces -

s ion in t ime of tw o major phas es o f con t i -

n e n t a l e x t r u s i o n .

In the f i rs t 20 to 30 m.y. of the c ol l is ion ,

t h e n o r t h w a r d d r i v e o f c o n t i n e n t a l I n d i a

i n t o A s i a w o u l d h a v e c a u s e d t h e e x t r u s i o n

(800 to 1 , 000 km ) to the s ou the as t and

c l o c k w i s e r o t a t i o n ( - 2 5 ° ) o f m o s t o f I n d o -

ch ina and the Sunda s he l f (F ig . I ) . Huge

left-lateral movemen ts along the Red Rive

fault ( a r e a c t i v a t e d T r i a s s i c s u t u r e a l o n g

s ome of i t s l eng th) w ould have l ed to the

6 I 4 G E O L O G Y . D e c e m b e r 1 98


5/6

C F

Figure 3 . Hand drawings of faul ts observed in exper iments of Figure 2 , w i th cumulat ive offsets

in mi l l imetres smal l num bers) after Pel tzer and others, 1982 ) . In uni latera l ly confine d expe r i -

men t, two majo r faul ts F1 and F2) gui de success ive extrusion of two blocks. In stage F, block s

1 a nd 2 c a n be compar e d to I ndoch ina a nd s outher n Ch ina , and open gaps 1 ,1 +2 , and 2 to

S outh Ch i na S ea , Andama n S e a , and nor theas te r n Ch ina , r e spec t i ve ly .

opening of the South China Sea f r o m a n

in i t ia l r i f t b ranch ing o f f the ex t remi ty o f

the fau l t , much as the Shans i r i f t does now

o f f t h e A l t y n T a g h - G a n s u - W e i H e f a u l t .

T h e S o u t h C h i n a S e a w o u l d b e a n a l o g o u s

to the la rge gap a t the r igh t ex t remi ty o f F j

in the exper imen t . T he an iso t ropy o f the

p las t ic ine de te rmines the o r ien ta t ion o f th i s

gap , bu t i t s wedge shape i s a consequence

of the ro ta t ion o f the b lock ex t ruded a long

F | . T h u s , o n e m i g h t p r e d i c t m o r e o p e n i n g

in the eas te rn pa r t o f the Sou th Ch ina Sea .

Ava i la b le da t a on the Sou th Ch ina Sea

(T ay lo r and H ayes , 1980; Ho l low ay , 1981)

suppor t th i s in te rp re ta t ion . T he onse t o f

T er t ia ry ex tens ion i s imprec ise ly da ted , bu t

mag ne t ic anom al ie s can be iden t i f ied on ly

be tween 32 and 17 m.y . B.P . T here has

b e e n m o r e s e a f l o o r s p r e a d i n g i n t h e n o r t h -

eas te rn pa r t o f the sea , whose wedge

shape (F ig . I ) may ind ica te tha t the sp read -

i n g a x i s h a s p r o p a g a t e d w e s t w a r d ( C o u r t i l -

lo t , 1980) . T he exper imen ts a l so sugges t

t h a t cessation of spreading in the So u th

Ch ina Sea in the ea r ly Miocen e i s a conse -

q u e n c e o f n o r t h w a r d m i g r a t i o n o f c o n t i -

nen ta l ex t r us io n . As the Red R ive r fau l t ,

s lowly ove r taken by Ind ia , cou ld no longer

e f f ic ien t ly p lay i t s ex t rud ing ro le , l e f t -

la te ra l mo t ion on i t wou ld have come to a

ha l t . T he Al tyn T a gh fau l t wou ld have

taken the lead ing ro le a s the new le f t -

l a t e r a l g i a n t r e q u i r e d f o r c o n t i n e n t a l e x t r u -

sion to proc eed . Thu s, 10 to 20 m.y. ago , a

second phas e o f ex t ru s ion wou ld have

b e g u n , m o v i n g T i b e t a n d s o u t h e r n C h i n a

seve ra l hundred k i lometres eas tward (F ig .

I ) , a s Ind ich in a kep t ro t a t in g c lockwise (a s

m u c h a s = 4 0 ° ) . A p p a r e n t l y , t h e A l t y n

T a g h - G a n s u - W e i H e f a u l t i s s t il l p r o p a -

ga t ing eas tward , inch ing c lose r to the

P a c i f i c s u b d u c t i o n z o n e . C r u s t a l e x t e n s i o n

nea r i t s eas te r n ex t rem i ty (Shans i Ri f t ,

No r th C h ina p la ins , Bo Ha i gu l f ) may g ive

b i r th to an ocean bas in comparab le to the

S o u t h C h i n a S e a .

T he k inemat ic s o f the un i la te ra l ly con-

f ined expe r ime n t he lp exp la in o the r la rge -

sca le T e r t ia ry tec ton ic fea tu res in sou theas t

As ia . We sugges t tha t the M ergu i bas in ,

A n d a m a n S e a , a n d l o w l a n d s o f B u r m a

(F ig . I ) a re mere ly a consequ ence o f the

c o l l i s i o n - d r i v e n e x t r u s i o n a n d c o r r e s p o n d

t o t h e w e d g e - s h a p e d g a p t h a t keeps open-

ing a long the r igh t s ide o f the inden te r until

the end of the experiment (Figs . 2 , 3) . Th e

A n d a m a n - M e r g u i b a s i n , w i d e r i n t h e

s o u t h , n a r r o w s p r o g r e s s i v e l y n o r t h w a r d

in to the Burmese lowlands , where huge

t h i c k n e s s e s o f T e r t i a r y s e d i m e n t s p r o b a b l y

c o v e r o c e a n i c c r u s t ( M i t c h e l l a n d M c K e r -

row, 1975) . E x te ns ion m ay have s ta r ted in

the Ol igocene in the sou th (Mergu i bas in )

(H am il to n , 1979), and Mio cene ocean f loo r

i s w e l l d o c u m e n t e d i n t h e A n d a m a n S e a

(C urra y and o the rs , 1978) . In con t ra s t to

t h e S o u t h C h i n a S e a , active extension

toge the r wi th r igh t - la te ra l s t r ike -s l ip

mot ion a long the Saga ing fau l t i s still in

progress

i n t h e A n d a m a n S e a .

Complex i t ie s o f the tec ton ics o f Burma ,

T h a i l a n d , a n d Y u n n a n m a y a l s o b e

a c c o u n t e d f o r in t h i s p r o p a g a t i n g e x t r u s i o n

scena r io . Offs e ts obse rved the re on L an d-

sa t images and in the f ie ld on la rge con ju -

ga te s t r ike -s l ip fau l t s a re no t cons is ten t

w i t h t h e p r e s e n t o r i e n t a t i o n ( a p p r o x i -

m a t e l y n o r t h - s o u t h ) o f t h e m a x i m u m

h o r i z o n t a l s t r e s s i n f e r r e d f r o m f a u l t - p l a n e

so lu t io ns o f ea r thq uak es (A. Y. L e Da i n

and o the rs , in p rep . ) . T h is pa radox can be

so lved i f Indoch ina and Malays ia a re

ro ta ted back (an t ic lockwise ) abou t 40° ; the

ea r ly geometry and k inemat ic s o f the

G E O L O G Y . D e c e m b e r I 9 82 6 I 5


6/6

s t r i k e - sl i p f a u l t s o f s o u t h e r n Y u n n a n a n d

e a s t e r n B u r m a t h e n b e c o m e c o m p a t i b l e

w i t h e x t r u s i o n a l o n g t h e R e d R i v e r f a u l t ,

w h i l e t h o s e o f f a u l t s i n n o r t h e a s t e r n a n d

p e n i n s u l a r T h a i l a n d s u g g e s t a n e a r l i e r ,

l i m i t e d , e x t ru s i o n pha se (F i g . 1 ). A t t he

ons e t o f c o l l i s i on , i n a f a i l e d a t t e m pt t o

s e p a r a t e t h e s o u t h e r n p a r t o f t h e S u n d a

s h e l f f r o m m a i n l a n d I n d o c h i n a , t h i s p h a s e

m a y h a v e s h a p e d t h e M a l a y P e n i n s u l a , a n d

c r e a t e d , a s p u l l - a p a r t s a n d r i f t s , t h e T e r -

t i a ry ba s i n s o f t he Gul f o f T h a i l a nd a n d : he

s o u t h w e s t e r n C h i n a S e a ( H a m i l t o n , 1 9 7 9 ).

A l t h o u g h r e v e r s a l o f t h e s e n s e o f m o t i o n

o n e a r l y l e f t - l a t e r a l f a u l t s a p p e a r s t o h a v e

oc c ur r e d i n sou t he a s t As i a , i t ha s no t be e n

d e t e c t e d s o f a r i n o u r e x p e r i m e n t s . T h e

m o s t o u t s t a n d i n g i l l u s t r a t i o n is t h e R e d

R i v e r f a u l t , n o w c l e a r l y r i g h t - l a t e r a l ( T a p -

p o n n i e r - a n d M o l n a r , 1 9 7 7 ) . P e r h a p s t h i s i s

b e c a u s e o f t h e m o r e c o m p l e x b o u n d a r y

c ond i t i ons i n As i a ; i nc i p i e n t c o l l i s i on wi t h

A u s t r a l i a m a y n o w r es i s t s o u t h e a s t w a r d

m o t i o n o f I n d o n e s i a , o r I n d i a a n d I n d o -

c h i n a m a y n o w b e w e l d e d b y m o r e e x t e n -

s i v e c o l l i s io n a l o n g t h e ( n d o - B u r m a n

r a n g e s . I n a n y e v e n t , r i g h t - l a t e r a l f a u l t i n g

on t he R e d R i ve r f a u l t a nd r i f t i ng i n Yu n-

n a n c a n b e v i e w e d a s c o n s e q u e n c e s o f t h e

p r e s e n t e x t r u s i o n p h a s e . M o r e o v e r , t h e

Y u n n a n r i f t s m a y c o r r e s p o n d t o a n i n c i p -

i e nt a n a l o g o f t h e A n d a m a n - M e r g u i b a s i n

(Fig. 1) .

E v e n t u a l l y , c o n t i n e n t a l e x t r u s i o n c o u l d

m i g r a t e f a r t h e r n o r t h . I f i d e n t i f i e d w i th t h e

n o r t h e r n P a m i r s , t h e n o r t h w e s t e r n t i p o f

t he Ind i a n i nd e n t e r a l r e a d y li e s a f e w

h u n d r e d k i l o m e t r e s n o r t h o f t h e w e s t e r n

e x t re m i t y o f t he Al t yn T a g h fa u l t (F i g . I ) .

C o m p a r i s o n s w i t h a d v a n c e d s t a g e s o f t h e

u n i l a t e r a l l y c o n f i n e d e x p e r i m e n t s u g g e s t

t h a t if I n d i a c o n t i n u e s it s n o r t h w a r d j o u r -

n e y i n t o t h e A s i a n c o n t i n e n t , a t h i r d m a j o r

l e f t - l a t e ra l s t r i ke - s l i p f a u l t sys t e m sho u l d

p r o p a g a t e t o t h e S ea o f O k h o t s k , c o n n e c t -

i n g f a u l t s in t h e T i e n S h a n , M o n g o l i a , a n d

Ba ika l (Fi g. 1) .

G E O D Y N M I C I M P L I C T I O N S

I t i s i m p o r t a n t t h a t t h e e x p e r i m e n t u s e d

he re a s a gu i de be s i m p l e . Phy s i c a l a nd

g e o m e t r i c f a c t o r s t h a t m a k e i t s u c c e s s f u l

a r e p l a n e - s t r a i n i n d e n t a t i o n n e a r a f r e e

l a t e r a l b o u n d a r y , t h e d e v e l o p m e n t o f f a u l t s

i n d e f o r m i n g p l a s t ic i n e , a n d t h e a n i s o t r o p i c

l a y e r i n g t h a t h a s c o u n t e r p a r t s i n A s i a i n

t h e f o r m o f i n i t ia l l y a p p r o x i m a t e l y

s o u t h e a s t - t r e n d i n g P a l e o z o i c a n d M e s o z o i c

s u t u r e s . I f g r o s s l y c o r r e c t , t h e i n t e r p r e t a -

t i on we p re se n t i n f l a t e s t he e f fe c t s o f

Ind i a ' s c o l l i s i on wi t h As i a by a f a c t o r o f

p e r h a p s t w o , a n d i t s g e o d y n a m i c i m p l i c a -

t i o n s s h o u l d n o t b e u n d e r e s t i m a t e d .

N o t o n l y c a n t h e

boundary forces

i nvo l ve d i n

continen tal collision

a c t i v a t e

g r e a t o v e r t h r u s t s a n d l a r g e s t r i k e - s l i p ' f a u l tS

t h a t d e v e l o p r i f t s n e a r t h e i r d i s ta l e x t r e m i -

t i e s , bu t t he y c a n g i ve b i r t h , t h rough t he

u l t i m a t e e vo l u t i o n o f t he se r i f t s , t o fu l l y

g r o w n

marginal ocean basins

s o m e t i m e s

a c t i v e f o r a s h o r t t i m e . O n e w o n d e r s h o w

s l a b pu l l a nd r i dge push c a n be t he m a i n

s o u r c e s o f s u c h l o n g - l a s t i n g f o r c e s .

U n i f o r m i t a r i a n t h e o r y s t a t e s t h a t t h e

pre se n t is t he ke y t o t he pa s t . Ye t , e x t r a p-

o l a t i o n o f p r e s e n t - d a y m o t i o n t o t h e p a s t

m u s t b e d o n e w i t h c a u t i o n s i n c e

large

strike-slip faults

suc h a s t he R e d R i ve r

f a u l t , c a n

change sense

d u r i n g c o n t i n e n t a l

c o l l i s i o n . O n t h e o t h e r h a n d , b e c a u s e w e

m a y be l ook i ng , i n As i a , a t suc c e s s i ve

s t a g e s o f a n e x t r u s i o n p r o c e s s t h a t r e p e a t s

i t se l f in t ime,

the past

m a y be use d a s a

key

to the present a n d f u t u r e : t h e o p e n i n g o f

t h e A n d a m a n a n d S o u t h C h i n a S e a s

s t r o n g l y s u p p o r t s t h e i d e a t h a t t h e S h a n s i ,

B a i k a l , a n d Y u n n a n r i f t s a r e p u r e c o l li -

s i o n a l e f f e c t s , a n d m a y t h e m s e l v e s e v o l v e

i n t o oc e a n b a s i ns if Ind i a doe s no t c om e t o

a h a l t . O u r e x p e r i m e n t s s u g g e s t t h a t t h i s

c a n o c c u r e v e n

without any driving action

by the asthenosphere immediately

underneath.

Faulting

m a y b e t h e

dominant mode of

deformation of the continental lithosphere.

I n m a n y r e s p e c t s ,

fault propagation recon-

ciles intracontinental deformation and

plate tectonics.

Af t e r a d i f fus e s t a ge o f

s e m i c o n t i n u o u s d e f o r m a t i o n , w h e r e t h e

l i t h o s p h e r e o f t h e c o ll i d e d c o n t i n e n t l o o k s

f o r a n e a s y e s c a p e , o n e m a j o r s t r i k e - s l i p

a n d r i f t s y s t e m , u s i n g al l c o n v e n i e n t l i t h o -

s p h e r i c w e a k n e s s e s i t e n c o u n t e r s , p r o p a -

g a t e s t o a s t e a d y - s t a t e f r e e p l a t e

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t o n i c s , a n d s t r a t i g r a p h y b u t a l s o p l a t e

k i n e m a t i c s s h o u l d h e l p u s t e s t q u a n t i t a -

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Manuscr ip t received May 18 , 1982

Revised manuscr ip t received Ju ly 27 , 1982

Manuscr ip t accepted Augus t 2 , 1982

616

P R I N T E D I N U S A

GEOLOGY. December 1982

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