15
Bulletin of the Seismological Society of America Vol. 84 No. 4 pp. 1097-11 11 August 1994 Crustal Thickness Variation in the Andean Foreland Argentina from Converted Waves by Marc Regnier, Jer-Ming Chiu, Robert Smalley, Jr., Bryan L. Isacks, and Mario Araujo bstract Local network three-component digital data from the San Juan area, Argentina, provide the first seismological images of the deep crustal structure in the Andean foreland above a horizontal segment of the subducted Nazca plate. We have identified S-to-P seismic phases converted on the Moho by analysis of seismograms formed by taking the product of the radial and vertical com- ponents (R * Z) from intermediate-depth earthquakes in the Benioff zone directly beneath the network. Under the Sierras Pampeanas, the Moho is estimated to be at a depth of about 52 km. Beneath the eastern Precordillera, the Moho deepens to 55/57 km and further west, beneath the central Precordillera, to 60 km. We therefore estimate a 5 ° to 10 ° westward dip to the Moho under the Andean foreland. In both the Sierras Pampeanas and Precordillera provinces, the thickness of the ductile lower crust is comparable to that for a stable con- tinental crust, while the brittle upper crust, implied from the spatial distribution of the seismicity, shows an important thickness increase. This rheological be- havior is interpreted to be a consequence of the flattening of the dip of the subducted plate. The east-west deepening of the seismicity, the Moho, and the depth to basement under the Precordillera all suggest that the Andean foreland lithosphere is underthrusting beneath the Andes. The associated shortening is estimated to be about 15 across the Sierras Pampeanas. It is estimated to be at least 38 across the Precordillera for the part of the crust beneath the d6collement, while it is up to 70 for the part above the drcollement. Introduction Recently, several articles have dealt with kinematic evolution of the Andean orogen (Isacks, 1988; Allmen- dinger et al. 1990; Sheffels, 1990), and they have shown that models involving a large amount of crustal short- ening could satisfactorily explain the main morphotec- tonic features of the Andes. In an attempt to quantify this amount of shortening across the entire Andean oro- genic belt at 30 ° S latitude, AUmendinger et al. (1990) proposed a family of models that can account for the uplift of the high Cordillera but differ in the manner that they link upper and lower crustal deformation. A key parameter of these models to further constrain the mode of deform ation in the lower crust is the crustal thickness. Indeed, knowing the upper crustal structure and the depth extent of the brittle deformation, the determination of the Moho depth should allow us to better estimate the amount of ductile thickening in the lower crust. To date, very few measures of crustal thickness have been obtained in South America, and especially in the Andes. To provide such important information, a portable seismic network (PANDA, Chiu et al. 1991) was operated in the prov- ince of San Juan, Argentina, (Figs. 1 and 2) from Au- gust 1987 to May 1988. The network was located above a subhorizontal subducting segment of the Nazca plate. The numerous intermediate-depth earthquakes from the Wadati-Benioff zone (Fig. 3) recorded by the whole network provide an opportunity to study the crustal structure of the Andean foreland and Precordillera thrust belt through sampling by upgoing ray paths. In this area, the foreland is a distinctive morphotectonic province characterized by a series of mountains and basins, the Sierras Pampeanas. Although the Sierras Pampeanas are not part of the morphological Andes, they are an integral part of the Andean deformation, and shortening in that province is important to the overall estimation of crustal shortening. The Precordillera is the easternmost belt of the Andes and its relationship to the Sierras Pampeanas is not well understood, especially at depth where there are very few structural constraints. The amount of crus- tal thickening, however, can be estimated from the crus- tal thickness beneath seismic stations, as determined from travel-time analysis of Moho converted waves from un- 1097

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Bulletin of the Seismological Society of America, Vol. 84, No. 4, pp. 1097-1111, August 1994

C r u s t a l T h i c k n e s s V a r i a t i o n i n t h e A n d e a n F o r e l a n d , A r g e n t i n a ,

f r o m C o n v e r t e d W a v e s

b y M a r c R e g n i e r , J e r - M i n g C h i u , R o b e r t S m a l l e y , J r . , B r y a n L . I s a c ks , a n d M a r i o A r a u j o

Abstract L o c a l n e t w o r k t h r e e - c o m p o n e n t d i g i t a l d a t a f r o m t h e S a n J u a n a r e a ,

A r g e n t i n a , p r o v i d e t h e f ir s t s e i s m o l o g i c a l i m a g e s o f t h e d e e p c r u s ta l s t r u c t u re

i n th e A n d e a n f o r e l a n d a b o v e a h o r i z o n ta l s e g m e n t o f t h e s u b d u c t e d N a z c a p l a te .

W e h a v e i d e n t i f i e d S - t o - P s e i s m i c p h a s e s c o n v e r t e d o n t h e M o h o b y a n a l y s i s

o f s e i s m o g r a m s f o r m e d b y t a k i n g t h e p r o d u c t o f t he r a d ia l a n d v e r t i ca l c o m -

p o n e n t s ( R * Z ) f r o m i n t e r m e d i a t e - d e p t h e a r t h q u a k e s i n th e B e n i o f f z o n e d i r e c tl y

b e n e a t h t h e n e t w o r k . U n d e r t h e S i e rr a s P a m p e a n a s , t h e M o h o i s e s t i m a t e d t o

b e a t a d e p t h o f a b o u t 5 2 k m . B e n e a t h t h e e a s te r n P r e c o r d i ll e r a , t h e M o h o

d e e p e n s t o 5 5 / 5 7 k m a n d f u r th e r w e s t , b e n e a t h t h e c e n t ra l P r e c o r d il l er a , t o 6 0

k m . W e t h e r e f o r e e s t i m a t e a 5 ° t o 1 0 ° w e s t w a r d d i p t o t h e M o h o u n d e r t h e

A n d e a n f o r e la n d . I n b o th t h e S ie r r as P a m p e a n a s a n d P r e c o r d i ll e r a p r o v i n c e s ,

t h e t h i c k n e s s o f t h e d u c t i l e l o w e r c r u s t i s c o m p a r a b l e t o th a t f o r a s t a b l e c o n -

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

o f t h e s e i s m i c i t y , s h o w s a n i m p o r t a n t t h i c k n e s s i n c r e a s e . T h i s r h e o l o g i c a l b e -

h a v i o r i s in t e r p r e t e d t o b e a c o n s e q u e n c e o f t h e f la t t e n i n g o f t h e d i p o f t h e

s u b d u c t e d p l a te . T h e e a s t - w e s t d e e p e n i n g o f t h e s e is m i c i t y , t h e M o h o , a n d th e

d e p t h t o b a s e m e n t u n d e r t h e P r e c o r d i l l e r a a ll s u g g e s t th a t t h e A n d e a n f o r e l a n d

l i t h o s p h e r e i s u n d e r t h r u s t i n g b e n e a t h t h e A n d e s . T h e a s s o c i a t e d s h o r t e n i n g i s

e s t i m a t e d t o b e a b o u t 1 5 % a c r o s s t h e S i e r ra s P a m p e a n a s . I t i s e s t i m a t e d t o b e

a t le a s t 3 8 % a c r o s s t h e P r e c o r d i l l e r a f o r t h e p a r t o f t h e c r u s t b e n e a t h t h e

d 6 c o l l e m e n t , w h i l e i t i s u p t o 7 0 % f o r t h e p a r t a b o v e t h e d r c o l l e m e n t .

I n t r o d u c t i o n

Recen t l y , severa l a r t i c l es have dea l t w i t h k i nemat i c

evo l u t i on o f t he Andean o rogen ( I sacks , 1988 ; Al l men-

dinger e t a l . , 1990; Sheffels , 1990) , and they have shown

t ha t model s i nvo l v i ng a l a rge amou n t o f c rus t a l sho r t-

en i ng cou l d sa t i s f ac t o r i l y exp l a i n t he mai n morpho t ec-

t on i c f ea t u res o f t he Andes . In an a t t empt t o quan t i fy

t h is am oun t o f sho r t en i ng across t he en t ir e And ean o ro -

gen i c be l t a t 30 ° S l a t i t ude , AU mend i nger e t a l . (1990)

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

up l i f t o f the h i gh Cord i l l e r a bu t d i f f e r i n the m anner t ha t

t hey l i nk upper and l ower c rus t a l defo rmat i on . A key

paramet er o f t hese model s t o fu r t her cons t r a i n t he mode

of defo rm at i on i n t he l owe r c rus t is t he c rus t a l t h ickness .

Indeed, knowing the upper crustal s t ructure and the depth

ex t en t o f t he b r i tt l e defo rmat i on , t he de t e rmi na t i on o f t he

Moho depth should al low us to bet ter est imate the amount

o f duc t i l e t h i cken i ng i n t he l ower c rus t . To da t e , very

few me asu res o f c rus t a l th i ckness have b een ob t a i ned i n

Sou t h Amer i ca , and espec i a l l y i n t he Andes . To p rov i de

such i mpor t an t i n fo rmat i on , a po r t ab l e se i smi c ne t work

(PANDA, Chiu e t a l . , 1991) was opera t ed i n t he p rov -

i nce o f San Juan , Argen t i na , (F i gs . 1 and 2 ) from Au-

gus t 1987 t o May 1988 . The ne t work was l oca t ed above

a subhor i zon t a l subduct i ng segm en t o f t he Nazca p l a t e .

The numerous i n t e rmed i a t e -dep t h ear t hquakes f rom t he

W a d a t i - B e n i o f f z o n e ( F i g . 3 ) r e c o r d e d b y t h e w h o l e

ne t work p rov i de an oppor t un i t y t o s t udy t he c rus t a l

s t ruc tu re o f the A ndean fo re l and and P reco rd i l le r a t h rus t

be l t th rough sampl i ng by up go i ng r ay pa t hs . In t h i s a r ea ,

t he fo re l and i s a d i s t i nc t i ve morpho t ec t on i c p rov i nce

charac t e r i zed by a se r i es o f moun t a i ns and bas i ns , t he

S i er r as Pampeanas . Al t hough t he S i e r r as Pampeanas a r e

no t par t o f t he morpho l og i ca l An des , t hey a re an i n t egra l

par t o f t he Andean defo rmat i on , and sho r t en ing i n t ha t

p rov i nce i s i mpor t an t t o t he overa l l es t i mat i on o f c rus t a l

sho r t en i ng . The Preco rd i l l e r a i s t he eas t e rnmos t be l t o f

t he Andes and i t s r e l a t i onsh i p t o t he S i e r r as Pampeanas

i s no t wel l under s t ood , espec i a l l y a t dep t h where t here

are very f ew s t ruc t u ra l cons t r a in t s . T he am oun t o f c rus -

t a l t h i cken ing , how ever , ca n be es t i mat ed f rom t he c rus -

tal thickness beneath seismic stat ions, as determined from

t rave l - t i me ana l ys i s o f Moho conver t ed waves f rom un-

1097

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1098 M. Regnier , J . -M. Chiu, R. Sma l ley, J r . , B . L . I sacks , and M. Araujo

d e r l y i n g i n t e r m e d i a t e - d e p t h e v e n t s ( F i g . 3 ) . S i g n i f i c a n t

a m o u n t s o f c r u s ta l t h i c k e n i n g c a n t h e n b e r e l a te d t o

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

a n d t h e P r e c o r d i l l e r a .

R e g i o n a l S e t t i n g

T h e S i e r r as P a m p e a n a s , a r e g io n o f P r e c a m b r i a n

m e t a m o r p h i c b a s e m e n t b l o c k u p l i f t s ( C a m i n o s e t a l . ,

1 9 8 2 ) , i s th e e a s t e r n m o s t p r o v i n c e o f th e A n d e a n o r o g e n

b e t w e e n 2 8 ° a n d 3 3 ° S ( F i g . 1 ) . T h e s e r a n g e s a r e e a s t

o f a n d a d j a c e n t to t h e P r e c o r d il l e ra , a n a r r o w f o M a n d

2 0

S

t h ru s t b e l t w h i c h e n d s e a s t w a r d t h e p h y s i o g r a p h i c A n d e s

( F i g . 1 ) . S uch t h i ck- sk i nned d e f or m a t i on eas t o f t he th i n -

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

o v e r l y i n g s u b h o r i z o n t al s u b d u c t i o n ( D i c k e n s o n a n d S n y -

d e r , 1 9 7 8 ) . B o t h p r o v i n c e s h a v e a b i m o d a l d e p t h d is t ri -

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

e v e n t s o f t h e s u b h o r i z o n ta l W a d a t i - B e n i o f f z o n e o f t h e

s u b d u c t e d N a z c a p l a t e a n d t h e c r u s t a l e v e n t s l o c a t e d i n

t h e u p p e r p l a t e ( F ig . 3 ) . T h e S i e r r a s P a m p e a n a s m o u n -

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

w h o s e s e i sm i c a c t iv i t y e x t e n d s d o w n t o a m a x i m u m d e p t h

o f 3 0 k m ( K a d i n s k y - C a d e a n d R e i l in g e r , 1 9 8 5 ; S m a l l e y

a n d Is a c k s , 1 9 9 0 ; R e g n i e r e t a l . , 1 9 9 2 ) . I n t h e P r e c o r -

d i l le r a , t he s e i smi c i t y i s conf i ned t o t he baseme nt a t dep t h

b e t w e e n 5 a n d 3 5 k m i n a p a t t e r n o f s e g m e n t e d f a u l t s

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

( S m a l l e y e t a l . , 1 9 9 3 ) . O n g o i n g d e f o r m a t i o n s s ta r t e d 1 0

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

t o a f f e c t t h e e a s t e r n P r e c o r d i l l e r a 2 . 3 m . y . a g o ( J o h n s -

s o n e t a l . , 1 9 8 4 ; W h i t n e y e t a l . , 1984; Bas t i a s e t a l . ,1 9 8 4 ) . T h r u s t i n g i n t h e S i e r r a s P a m p e a n a s i s e s t i m a t e d

t o b e y o u n g e r t h an 1 0 m . y . a n d c o i n c i d e n t w i th t h e s h u t-

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

( K a y e t a l . , 1 9 8 8 ) .

D a t a a n d M e t h o d

2 5

3 0

7 5 W 7 0 6 5

F i gu r e 1 . M ap of wes t er n S ou t h Amer i ca i l-lustrat ing upper plate tectonic features, volcanica r c , and con t our s o f t he W ada t i - Ben i of f zone(WBZ). Along-s t r ike segmentat ion of the upperplate tectonic provinces correla tes wi th the a long-strike segmentation of the WBZ (Jordan e t a l . ,

1983). The distribution of Neog ene volcanos (opencircles) (Isacks, 1988) shows an active ma gm aticarc over the s teep WBZ segment and the absenceof an act ive arc over the f la t segment .

C r us t a l and uppe r mant l e ve l oc i t y s tr uc t u r es have be en

s u c c e s s f u l l y i n v e s t i g a t e d u s i n g c o n v e r t e d w a v e s a t i n -

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

1 9 7 0 ; C h i u e t a l . , 1 9 8 6 ; L i e t a l . , 1 9 9 2 ) a n d t e l e s e i s m i c

e p i c e n t r a l d i st a n c e s ( B a t h a n d S t e f a n s s o n , 1 9 6 6 ; J o r d a n

a n d F r a z e r , 1 9 7 5 ; L a n g s t o n , 1 9 7 9 ) . I n p a r t i c u l a r , c o n -v e r t e d p h a s e s a r e e x t r e m e l y u s e f u l to d e t e r m i n e t h e v e -

l o c i ty a n d g e o m e t r y o f d e e p s t ru c t u re s t h a t c a n n o t b e

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

f i le s . A t s h o r t e p i c e n t r a l d i s t a n c e , b e c a u s e o f th e l a r g e

S - w a v e a m p l i t u d e , t h e m o s t p r o n o u n c e d c o n v e r t e d p h a se

i s o f t e n t h e S - t o - P p h a s e ( S p ) , a n d b o t h i t s r e l a t i v e t ra v e l

t i m e s T ( S - S p ) a n d a m p l i t u d e s c a n b e u s e d t o c o n s t r a i n

t h e v e l o c i t y m o d e l w h e r e t h e c o n v e r s i o n s o c c u r re d . I n

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

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

W a d a t i - B e n i o f f z o n e a n d r e c o r d ed b y a l o ca l n e tw o r k

o f t h r e e - c o m p o n e n t s e i s m i c s ta t io n s . T h e n e t w o r k w a s

o p e r a t e d i n t h e S a n J u a n a r e a f r o m S e p t e m b e r 1 9 8 7 t o

M a y 1 9 88 a n d c o v e r e d a n ar e a o f r o u g h l y 1 5 0 k m n o r t h -

s o u th b y 1 0 0 k m e a s t - w e s t ( F i g . 2 ). T h e n e t w o r k c o n -

s i s te d o f 2 6 s i t e s , t h r e e o f w h i c h ( C F A , L F L , a n d L L A )

c o n t a i n e d s m a l l - a p e r t u r e ( s e v e r a l m e t e r s t o 1 k i n ) m u l t i -

s t a t io n a r r a y s ( s i x to 1 1 t h r e e - c o m p o n e n t s t a ti o n s ) . O u r

s t u d y u s e s a s u b s e t o f a b o u t 3 0 0 A q u a l i t y e v e n t s ( e v e n t s

w i t h w e l l - r e c o r d e d a r ri v a l s th r o u g h t h e w h o l e n e t w o r k )

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

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

d e s c r i b e d i n R e g n i e r e t a l . ( 1 9 9 2 ) . I n i t ia l l o c a t i o n s w e r e

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Crustal Thickness Variation in the Andean Foreland, Argentina, from Converted Waves 1 0 9 9

- 6 9 ° - 6 8 ° - 6 7 °

-31 °

- 3 2 °

-31 °

- 3 2 "

- 6 9 ° - 6 8 ° - 6 7 °

F i g u r e 2 . M a p o f S a n J u a n re g i o n . M o u n t a i n r a n g e s a r e s h a d e d l i gh t f o r t h e

sed imenta ry rocks o f the th in - sk inned Precord i l l e ra and da rk fo r the basement

b lock up l i ft s o f the S ie r ras Pam peanas . In te rm ontane va l l eys o f Carbon i fe rous toQua te rna ry sed iments a re shown in whi te . The eas te rn Precord i l l e ra i s the cha inof mounta ins eas t o f the Matagusa nos V a l ley . The se i smic ne tw ork s ta t ions a re

s h o w n b y o p e n t r i a n gl e s f o r t h e t h r e e -c o m p o n e n t h i g h - g a i n / l o w - g a i n s t a t io n s a n dso l id tr i ang les fo r the smal l -aper tu re a r rays comp osed o f mul t ip le th ree -com po-n e n t h i g h - g a i n / l o w - g a i n o r h i g h - g a i n / h i g h - g a i n s t a ti o n s. I n t e r m e d i at e - d e p th e p i -

cen te r s (JHD oca t ions ) used in th i s s tudy a re a l so shown.

E

c' -

Q

"( 3

W E

0 0 2 5 0 3 0 0 k m

50

1 0 0

1 5 0

5 0 1 O 0 1 5 0 2 0 0, & ,A , m l k , t , ~ , , L , ~ a W ~ t , . , & •

' 'o ~ % o o ' %. ~ s ~ O oo° o ~ : : ~ % ~ o ~ ' ° ~

o o

o o

o o o "~° oo o 0 o

o

F i g u r e 3 . S e l e c t e d s h a l lo w - a n d i n t er m e d i a t e- d e p t h e a r th q u a k e s f r o m t h e S a n

J u a n r e g i o n . T h e e a s t - w e s t c r o s s s e c t i o n s h o w s s e i s m i c i t y h a s b i m o d a l d e p t hd is t r ibu t ion (c rus ta l and WBZ) . Crus ta l se i sm ic i ty is a ssoc ia ted wi th the Pampe anrange S ie r ra P ie de Pa lo and the eas te rn s ide o f the Precord il l e ra . P ro jec t ion o fthe loca t ions o f the PANDA s ta t ions a re ind ica ted by the so l id t r i ang les .

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1100 M. Regnier , J . -M. Chiu, R. Smal ley, J r . , B . L . I sacks , and M. Araujo

ob t a i ned us i ng HYP OI NVE RS E ( Kl e i n , 1978) and a t h r ee -

l a y e r v e l o c it y m o d e l ( T a b l e 1 ) m o d i f i e d f r o m t h e f o u r -

l a y e r m o d e l o f B o l l in g e r a n d L a n g e r ( 1 9 8 8 ) . B o t h P a n d

S a r r iv a l s w e r e u s e d f o r l o c a t in g t h e e a r t h q u a k e s w i t h a

Vp/Vs o f 1 . 7 5 , w h i c h w a s o b t a i n e d f r o m a l e a s t - s q u a r e s

f i t o f a l i n e o n W a d a t i d i a g r a m s ( V l a s i t y , 1 9 8 8 ) . A v -

e r a g e r m s r e s i d ua l s o f a p p r o x i m a t e l y 0 . 3 s e c w e r e f o u n d

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

i t. F i n a l l o c a t i o n s w e r e o b t a i n e d u s i n g j o i n t h y p o c e n t r a l

d e t e r m i n a t i o n , JH D , ( F r o h l i c h , 1 9 7 9 ) , w h i c h r e d u c e d th e

a v e r a g e r m s r e s i d u a l o f t h e d a t a se t to 0 . 2 s e c . F i n a l l y ,

o v e r 9 0 % o f t h e l o c a t io n s h a v e e s t im a t e s o f e p i c e n tr a l

a n d d e p t h e r ro r s o f 1 k m a n d 2 k m , r e s p e c t iv e l y .

F i g u r e 4 s h o w s a n e x a m p l e o f a n i n t e r m e d i a te - d e p t h

e a r t h q u a k e r e c o r d e d a t o n e s t a t io n o f t h e n e t w o r k . T h e

d a t a s u g g e s ts a r ri v a ls o f e n e r g y o n t h e Z c o m p o n e n t b e -

t w e e n t h e P a n d S a r r i v a l s b u t i t i s n o t p o s s i b l e t o u n -

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

i n t h e r a w d a t a , a s i t i s m i x e d w i t h s c a t t e r e d e n e r g y . T o

e n h a n c e b o d y - w a v e p h a s e s t h a t a re c o h e r e n t o n t w o o ft h e t h r e e d i r e c t i o n s Z , R , a n d T , t h e p r o d u c t s R * Z a n d

T * Z a r e c o m p u t e d . I m p o r t a n t p r o p e r t i e s o f t h e R Z p r o d -

u c t , p o i n t e d o u t b y J a c o b a n d B o o t h ( 1 9 7 7 ) , a r e th a t w a v e s

w i t h e l l i p t i c a l p a r t i c l e m o t i o n a r e a t t e n u a t e d a n d c o h e r -

e n t r e c t il i n e a r m o t i o n i s e n h a n c e d . I n a d d i t i o n , P a n d S

w a v e t y p e s h a v e o p p o s i t e s i g n s ; f o r P t h e R Z p r o d u c t i s

p o s i t i v e , w h i l e f o r S t h e R Z , p r o d u c t i s n e g a t i v e . B e -

c a u s e o f t h e h i g h - f r e q u e n c y c o n t e n t o f th e b o d y w a v e s ,

t h e p r o d u c t s w e r e c o m p u t e d w i t h th e r a w d a t a . L o w - p a s s

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

c a s e s , i n a lo s s o f e f f i c i e n c y o f th e p r o d u c t o p e r a t i o n .

S i n c e w e d i d n o t u s e a m p l i tu d e s , t h e c o m p u t e d p r o d u c t s

h a v e t h e n b e e n l o w - p a s s f i l t e re d b e l o w 5 H z t o b e m o r ee a s i l y i n t e r p r e t e d . O n t h e R Z p r o d u c t ( F i g . 4 ) , i t i s n o w

p o s s i b l e t o i d e n t i f y c l e a r P - t y p e a r r i v a l s b e t w e e n t h e P

and S , whi ch we i n t e r p r e t a s S-to-P (Sp) c o n v e r t e d w a v e s

a t d i s c o n t i n u i t i e s b e n e a t h t h e n e t w o r k . T h e f i r s t l a r g e

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

S-to-P c o n v e r s i o n f r o m t h e M o h o . T h e c o d a o f th i s ar -

r i v al , m a i n l y c o m p o s e d o f s u c c e s s iv e Sp p h a s e s , i n d i -

c a t e s t h a t t h e c r u s t i s h i g h l y l a y e r e d a n d / o r f r a c t u r e d .

A s w e a r e d e a l in g w i t h t h r e e - c o m p o n e n t d a ta , w e a l so

T a b l e 1

Crusta l V eloci ty Models*

Three Layersover Half-SpaceModel

Depth (km) Velocity (krn/see)

0 .0 5 . 8 8

1 0 . 0 6 . 2 0

3 2 . 0 7 . 3 0

4 5 . 0 8 . 1 0

* T h r e e - l a y e r m o d e l u s e d i n S a n J u a n a r e a ; th e t o p l a y e r

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

( B o l l i n g e r a n d L a n g e r , 1 9 8 8) a n d t h e b o t t o m t w o l a y e r s

f r o m r e g i o n a l e a r t h q u a k e s t u d i e s ( V o l p o n i , 1 9 6 8) .

c o m p u t e t h e T Z p r o d u c t . T h e t w o p r o d u c t s s h o w t h a t

b o d y - w a v e e n e r g y , i n b o t h t h e d i r e c t P a n d s e c o n d a r y

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

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

c o m p l e x 3 D v e l o c i t y st r u c tu r e ( L a n g s t o n , 1 9 7 7 ) . H o w -

e v e r , w e d i d n o t o b s e r v e s e c o n d a r y a r r i v a l s s y s t e m a t i -

c a l ly a t a n y s t a ti o n . N o d e p e n d a n c e o f t h e p r e s e n c e o f

th e Sp p h a s e w i t h d i s t a n c e o r b a c k a z i m u t h c o u l d b e e s -

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

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

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

s i d e r i n g th e s m a l l s e i s m i c w a v e l e n g t h u s e d i n t h i s s tu d y ,

b e t w e e n 0 . 5 a n d 1 k i n , i t is n o t s u rp r i s i n g t o o b s e r v e

i m p o r t a n t v a r i a t i o n i n t h e Sp a m p l i t u d e .

S i e r r a P i e d e P a l o

T h e r e c o r d s e c t i o n o f F i g u r e 5 p r e s e n t s t h e R * Z

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

n e t w o r k l o c a t e d o n t h e S i e r r a P i e d e P a l o . T o r e m o v et h e s o u r c e d e p t h e f f e c t w h e n s e a r c h in g f o r t h e Sp p h a s e ,

t r a c e s a r e l i n e d u p o n t h e S a r r i v a l s . T h e y a r e a l s o c u t

b e f o r e t h e S a r r i v a l b e c a u s e t h e R Z p r o d u c t o f t h a t p h a s e

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

s i g n a l w h e n d i s p l a y e d . O n e c a n s e e l a r g e a r r iv a l s o f e n -

e r g y b e t w e e n t h e P a n d S p h a s e . F r o m i t s p o l a r i t y a n d

appar en t phase ve l oc i t y ac r os s t he a r r ay , w e i n t e r p r e t t h i s

b o d y w a v e t o b e t h e S-to-P c o n v e r t e d w a v e f r o m t h e

M o h o . T h e o r e t i c a l a r ri v a l t i m e s c u r v e s f o r S a n d M o h o

S-to-P c o n v e r s i o n a r e a l s o s h o w n . T h e y a r e c a l c u l a t e d

u s i n g t h e s a m e v e l o c i t y m o d e l ( B o l l i n g e r a n d L a n g e r ,

1 9 8 8 ) u s e d f o r t h e l o c a t io n s b u t w i t h a 5 0 - k i n - d e e p M o h o

i n s t e a d o f t h e i n i ti a l 4 5 - k m d e p t h . T h e v e l o c i t i e s o f t h em o d e l h a v e n o t b e e n c h a n g e d .

F o r e a c h s t a t io n , a l e a s t - sq u a r e s o l u t i o n o f th e d e p t h

o f a h o r iz o n t a l M o h o w a s e s t i m a t e d , m i n i m i z i n g t h e r e s-

i d u e s o f t h e r e l a t i v e S-Sp t i m e f o r a v a r y i n g M o h o d e p t h .

T h e S-Sp r e l a ti v e t im e s w e r e m o d e l e d u s i n g t h e v e l o c i t y

m o d e l o f B o l l in g e r a n d L a n g e r ( 1 9 8 8 ) a n d a l l o w i n g o n l y

t h e M o h o d e p t h t o v a r y . T h e t o p l a y e rs o f t h e B o l l in g e r

a n d L a n g e r v e l o c i t y m o d e l w e r e d e r i v e d f r o m s h a l lo w

r e f r a c t i o n s u r v e y s i n t h e S a n J u a n a r e a ( Y P F , u n p u b -

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

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

p e t e n t s e d i m e n t a r y r o c k . W e t h e r e f o r e c o n s id e r th a t w e

h a v e a g o o d c o n t r o l o n t h e u p p e r c r u s t a l s t r u c t u r e b e -

n e a t h t h e s t a t i o n s a n d t h a t i t w o u l d n o t b e a p p r o p r i a t e to

c o n s t ra i n s o m e o f t h e u p p e r c r u s t p a r a m e t e r s f u r t h e r w i t h

o u r d a t a. T h e m a j o r s o u r c e o f e rr o r c o m e s f r o m p i c k i n g

th e Sp p h a s e . E v e n o n t h e p r o d u c t s , i t c a n b e s l i g h t l y

e m e r g e n t a n d w e e s t im a t e t h e u n c e r t a in t y i n p i c k i n g t h e

a r r i v a l t i m e o f t h e Sp p h a s e t o b e 0 . 2 s e c . T h e u n c e r -

t a i n t y f r o m e a r t h q u a k e l o c a t i o n i s o n e o r d e r l e ss a n d w i l l

n o t s e r i o u s ly a f f e c t o u r M o h o d e p t h d e t e r m i n a t i o n s i n ce

th e S-Sp t i m e d o e s n o t v a r y s i g n i f i ca n t l y o v e r t h e r a n g e

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

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Crustal Thickness Variat ion in the Andean Foreland, A rgent ina, f rom Converted W aves 1101

A n o t h e r p o s s ib l e s o u r c e o f e r r o r o n t h e M o h o d e p t h d e -

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

o f r a y p a t h s u s e d i n t h i s s t u d y . F i g u r e 6 r e p r e s e n t s a

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

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

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

m o d e l . F i r s t , o n e c a n s e e t h a t a t a g i v e n d i s t a n c e t h e P s

a n d S p p h a s e d o n o t c o n v e r t a t t h e s a m e p l a c e o n t h e

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

c o n v e r s i o n p o i n t o f t h e S p p h a s e w i t h d i s t a n c e , w h i l e

t h e c o n v e r s i o n p o i n t o f t h e P s r e m a i n s p r e t t y m u c h b e -

l ow t he s t a t i on ( see i nse t i n F i g . 6 ) . T h i s l a rge va r i a t i on

s p r e ad s t h e s a m p l i n g o f t h e M o h o o v e r a w i d e a r e a a n d

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

p e n d e n t v a r i a t i o n i n M o h o d e p t h . S i n c e w e a r e u s i n g

shor t -pe r i od da t a , i t i s a l so l i ke l y t ha t a "mi n i mum no i se"

i s p r e s e n t in t h e M o h o d e p t h m e a s u r e m e n t s d u e t o s h o r t

w a v e l e n g t h t o p o g r a p h i c v a r i a ti o n s o f t h e M o h o . I n o r d e r

t o r e d u c e t h e e f f e c t o f d is t a n c e d e p e n d e n t v a r i a t io n o f

t h e M o h o d e p t h , w e h a v e o n l y u s e d e v e n t s w i t h e p ic e n -t r a l d i s t a n c e t o e a c h i n d i v i d u a l s t a t i o n l e s s t h a n 5 0 k i n .

F i g u r e 7 d i s p l a y s a n e x a m p l e o f r m s c u r v e a s a f u n c t i o n

o f M o h o d e p t h f o r t h e st a ti o n i 1 5. A s e x p e c t e d , f o r a

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

p e n d e n c e o n M o h o d e p t h . F o r t h e P i e d e P a lo s u b n e t -

w o r k , t h e r m s a r e g e n e r a l l y i n f e r i o r t o 0 . 3 s e c w i t h a n

a v e r a g e o f 0 . 2 s e c , r o u g h l y c o r r e s p o n d i n g t o a n e r r o r o f

- --2 km on t he M o ho dep t h de t e rmi na t i on . F i gure 8 shows

a m a p w i t h M o h o d e p t h s d e t e r m i n e d a t a s i n g l e s t a t i o n .

T h e d e p t h b e n e a t h P i e d e P a l o r a n g e s f r o m 4 8 t o 5 5 k i n .

A s y s t e m a ti c i n c r e a s e i n M o h o d e p t h i s o b s e r v a b l e f r o m

s o u t h to n o r t h w i t h a n a v e r a g e M o h o d e p t h o f 5 0 k m

and 53 k m i n t he so u t he rn and nor t he rn pa r t s o f t he s ie r r a ,

r e s p e c t i v e l y . T h i s r e s u l t is i n a g r e e m e n t w i t h t h e t e c t o n i c

m o d e l p r o p o s e d b y R e g n i e r e t a l . ( 1 9 9 2 ) o f a r a n g e d i -

v i d e d i n t w o b l o c k s .

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

l o o k e d a t S p p h a s e s f r o m i n t e r m e d i a te - d e p t h e a r t h q u a k e s

l o c a t e d b e n e a t h t h e S i e r r a d e l a V a l l e F e r t il a n d r e c o r d e d

a t t h e w e s t e r n s t a t i o n s o f th e n e t w o r k . F i g u r e 9 s h o w s

a r e c o r d s e c t i o n f o r P r e c o r d i l l e r a s t a t i o n s o f R Z p r o d -

u c t s . F o r e a c h t r a c e , b a c k a z i m u t h i s r o u g h l y e a s t w a r d .

T h e d i s t a n c e i s l a r g e e n o u g h s o t h e S - t o - P c o n v e r s i o n

p o i n t l i e s b e n e a t h t h e S i e r r a P i e d e P a l o ( s e e F i g . 6 ) .

T h e o r e t i c a l a r r iv a l t i m e s f o r S a n d S - t o - P c o n v e r s i o n s

f r o m a 5 0 - k i n - d e e p M o h o a r e a l s o s h o w n . T h e o n s e t o fc o h e r e n t a r r iv a l o f e n e r g y b e t w e e n t h e P a n d S l in e u p

w e l l a l o n g t h e S - t o - P c u r v e . T h i s r e s u l t a g r e e s w i t h t h e

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

u s i n g u n d e r n e a t h i n t e rm e d i a t e - d e p t h e a r t h q u a k e s .

T h e a v e r a g e c r u s ta l t h ic k n e s s o f 5 2 k m f o u n d b e -

n e a t h t h e S i e r r a P i e d e P a l o i s g r e a t e r t h a n t h e a v e r a g e

A = 1 6 .6 0 ° b a z = - 1 . 2 8 ° H = 1 0 4 . k m

2.5 5 7.5 10 12.5 15 17.5 20 22.5 25 27.5 30

i0 3

0

t (sec)

r* z

(o.-5. hz)

t* z

Fig ur e 4. Exam ple of an intermediate-depth ear thquake recorded at one sta-t ion on the S ier ra P ie de Palo. Th e three-comp onent data are normal ized together .There are suggest ions on the raw data of ar rivals o f energy between the P andS arrivals. T he Sp phase i s c lear ly revealed by the R Z product . Both productsare low-pass fi l tered for easier interpretation. Ene rgy in the TZ prod uct indicatesa co mp lex 3D veloci ty s tructure .

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1102

E

5

15

25

35

45

55

65

75

M . R e g n i e r , J . - M . C h i u , R . S m a l l e y , J r . , B . L . I s a ck s , a n d M . A r a u j o

8 5 ~ ~ _ L ~ _ _ I \

0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0

t ( s e c )

F i g u r e 5 . M u l t i p l e s t a t i o n s / m a n y e a r th q u a ke s RZ p r o d u c t r e c o r d s e c t i o n th a t

c l e a r l y sh o w s c o h e r e n t a r r i v a l o f e n e r g y b e t w e e n P a n d S p h a s e s a c r o s s t h e P i ed e P a l o s u b n e t w o r k ( t r a c e s a r e c u t a n d l i n e d u p o n t h e S a r r i v a l s ) . T h e o r e t i c a l

a r r i v a l t i m e s f o r S a n d S-to-P c o n v e r s i o n s f r o m a 5 0 - k i n - d e e p M o h o a r e a l s o

s h o w n .

0

-2 0

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0

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d e l t a ( k m )

-20 0 20 40 60 80 100 120 140 160

,=8 .10 p . . . . . . . . . . . . S

Vs = 4.63

Figure 6. G e o m e t r y o f r a y p a t h s u s e d i n t h i s s t u d y . N o t e t h a t t h e S-to-P c o n -

v e r s i o n p o i n t v a r i e s s t r o n g l y w i t h e p i c e n t r a l d i s t a n c e ( s e e i n s e t ) .

~ 6 0

1 2 0

8o ~

4 0 x :

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Crustal Thickness Variation in the Andean Foreland, Argentina, from Converted Waves 1103

4 0 - k m c r u s t a l th i c k n e s s e s t i m a t e d f o r t h e B r a z i l i a n c r a -

t on (Assumpcao and S uarez , 1988) and i nd i ca t e s t he c rus t

h a s u n d e r g o n e a s ig n i f i c a n t a m o u n t o f c ru s t a l th i c k e n -

i n g , a b o u t 1 2 k i n , w h i c h i s a 3 0 % i n c r e a s e . I f w e a s s u m e

t h e b r i t t l e a c t i v i t y d o m a i n i s 3 0 - k m t h i c k , a s i m a g e d b y

t h e c r u s t a l s e i s m i c i t y b e n e a t h P i e d e P a l o ( R e g n i e r e t a l . ,

1 9 9 2 ) , w e c a n t h e n e s t i m a t e t h e d u c t i l e l o w e r c r u s t t o

b e 2 2 - k i n t h i c k , t h e d i f fe r e n c e b e t w e e n t h e M o h o d e p t h

a n d t h e u p p e r c r u s t a l t h i c k n e s s . A c c o r d i n g t o C o n d i e

( 1 9 8 9 ) , t h is v a l u e f o r t h e l o w e r c r u s ta l t h i c k n e s s i s v e r y

c l o s e t o t h a t f o r a s h i e ld o r p l a t f o r m a r e a ( 1 9 t o 2 1 k i n ) .

T h i s w o u l d i n d i c a t e t h a t m o s t c r u s t a l t h i c k e n i n g o c c u r s

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

t h e d u c t i le d o m a i n o f t h e c ru s t . A s p o i n t e d o u t b y S i b s o n

( 1 9 8 2 ) , t h e c u t o f f d e p t h f o r m i c r o s e i s m i c i t y , i d e n t i f ie d

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

g e o t h e r m . H e r e , a m o d e r a t e t o lo w h e a t f l o w , d u e t o t h e

d o u b l i n g o f l it h o s p h e r e a n d t h e a b s e n c e o f v o l c a n i c a r e

( J o r d a n e t a l . , 1 9 8 3 ) , a s s o c i a t e d w i t h a n i m p o r t a n t s t ra i n

r a t e , can ex p l a i n t he unusu a l l y l a rge t h i ckness o f t he b r i t tl eu p p e r c r u s t .

P r e c o r d i l l e r a

I n o r d e r to s u r v e y t h e M o h o u n d e r t h e w h o l e n e t -

w o r k , w e h a v e a l s o l o o k e d f o r S p c o n v e r t e d w a v e s a t

t he s t a t i ons l oca t ed on t he P record i l l e r a . F i gure 10 shows

a r e c o r d s e c t i o n o f th e R Z p r o d u c t f o r m u l t i p l e e v e n t s a t

t h e s t a ti o n s o f t h e e a s t e r n P r e c o r d i l l e r a ( F ig s . 2 a n d 8 ) .

t(S)-t(Sp) le as t square s inversion

2 .0

~2

\

\

\

/

] I I )

35 45 55

moho depth (km)

/J

/

J

/

65

1. 5

!_ j 1 .o

t

~ 0.5

riIi 0 .0

75

Fig ure 7. Least -square invers ion resul ts to es-t imate the Moho depth f rom the re la t ive ar r ivaltimes T(S)-T(Sp) observed at one s ta tion. T he rmsresiduals are plotted against Moho depth for theBollinger and Lang er (1988) crustal velocity model.Source distances are less than 50 kin.

I t c lea r l y shows a r r iva l s o f ene rgy be t wee n P and S phases .

L a r g e v a r i a t i o n s , i n b a c k a z i m u t h t o g e t h e r w i th l a t er a l

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

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

a ll w a v e f o n n o f th e S- to - P e n e r g y a n d f o r t h e s c a t t e r o f

o n s e t o f e n e r g y a r o u n d t h e c o m p u t e d t r a v e l- t im e c u r v e .

B e c a u s e t h e e n e r g y i s s p l i t b e t w e e n t h e r a d i a l a n d t a n -

g e n t i a l c o m p o n e n t ( F i g . 4 ) f o r o f f - a z i m u t h a r r iv a l s d u e

t o n o n p l a n a r o r d i p p i n g i n t e r f a c e s , t h e R Z p r o d u c t i s n o t

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

and shows i mpor t an t ampl i t ude va r i a t i ons . A 55-km-deep

M o h o v e l o c i t y m o d e l h a s b e e n u s e d t o c o m p u t e t h e c u rv e s

s h o w n o n F i g u r e 1 0 . F u r t h e r w e s t , t h e d a t a r e c o r d e d a t

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

c e n t r a l P r e c o r d i l l e r a ( F i g s. 2 a n d 8 ) e x h i b i t v e r y c l e a r

S p p h a s e s o n t h e v e r t i c a l c o m p o n e n t a f t e r l o w - p a s s f i l -

t e r i n g b e l o w 3 H z . F i g u r e 1 1 s h o w s a m u l t i p l e e v e n t s

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

s t a t io n s ( i 1 2 , o 1 1 , a n d o 1 0 ) . A c l e a r p h a s e i s o b s e r v e d

t h r o u g h o u t t h e s e c t i o n a t a b o u t t h e t i m e o f t h e c o m p u t e dS- to - P c o n v e r t e d p h a s e fo r a 5 9 - k m - d e e p M o h o . T r a c e s

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

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

o r d s e c t i o n s . A t v e r y c l o s e d i s t a n c e s , t h e S- Sp r e l a t i v e

t r a v e l t im e i s q u i te i n s e n s i t i v e t o e a r t h q u a k e d e p t h , w h i l e

th e S -P t i m e , i . e . , t h e l e n g t h o f p l o t t e d s i g n a l , s h o w s

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

T h e M o h o d e p t h f o u n d b e n e a t h s t a t i o n o 0 9 l o c a t e d b e -

t w e e n t h e S i e r r a P ie d e P a l o a n d t h e e a s t e r n P r e c o r d i l l e r a

i s 52 km (F i g . 8 ) . T h i s i s s i mi l a r o r s l i gh t l y l a rge r t han

t h e a v e r a g e 5 0 - k in M o h o d e p t h f o u n d b e n e a t h t h e s o u th -

e r n b l o c k o f t h e S i e r r a P i e d e P a l o , n o r t h e a s t o f s t a t io n

o 0 9 . T h a t w o u l d i n d i c a t e t h a t t h e M o h o d o e s n o t s h a l -l o w , a t l e a st o n t h e w e s t s i d e o f t h e S i e r r a P i e d e P a l o ,

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

t h e S i e r r a P i e d e P a l o t o w a r d t h e P r e c o r d i l l e r a . F r o m

b o t h t h e r e c o r d s e c t i o n s ( F ig s . 5 , 1 0 , a n d 1 1 ) a n d t h e

M o h o d e p t h s d i s p l a y e d o n F i g . 8 , t h e r e i s a s y s t e m a t i c

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

a 3 - t o 5 - k m d i f f e r e n c e i n t o ta l c r u s ta l t h i c k n e s s b e t w e e n

t h e S i e r r a P i e d e P a l o a n d t h e e a s t e r n P r e c o r d i l l e r a , a n d

t h e r e i s a ls o a t h i c k n e s s i n c r e a s e b e t w e e n t h e e a s t e r n a n d

t h e c e n t r a l P r e c o r d i l l e r a . T h e d i s t a n c e b e t w e e n t h e e a s t -

e rn and cen t r a l P r ecord i l l e r a i s abou t 25 km , whi ch y i e l ds

a n 1 1 ° w e s t w a r d s l o p i n g M o h o f o r a 5 - k i n d e p t h d i f f e r -

e n c e b e t w e e n t h e m . U s i n g t h is s lo p e e s t i m a t e , th e M o h o

w o u l d b e a t a 7 0- 1~ n d e p t h b e n e a t h t h e f r o n t a l C o r d i l le r a

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

F i g u r e 1 2 sh o w s t h e e f f e c t o f d i p o n S- Sp r e l a t i ve

t i m e . T h e t w o c u r v e s a r e f o r a f l a t ( s o l i d l i n e ) a n d a

d i p p i n g M o h o ( d a s h e d l i n e ) w i t h a 5 5 - k i n M o h o d e p t h

b e n e a t h t h e s t a ti o n ( s e e i n s e rt ) . T h e t i m e d i f f e r e n c e s b e -

t w e e n t h e t w o c u r v e s a t d i s t a n c e s g r e a t e r th a n 1 2 0 k m

a r e b i g g e r t h a n 1 s e c . , w h i c h i s r e s o l v a b l e i n t h e d a t a i f

s u c h d i s t a n c e d e p e n d a n t v a r i a t i o n o f t h e S- Sp t i m e e x -

i s t s . T o c h e c k w h e t h e r t h e r e i s a r e g i o n a l t r e n d o r n o t ,

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110 4 M. Regn ier , J . -M. Chiu, R. Smal ley, J r . , B . L . I sacks , and M. Araujo

w e h a v e m a p p e d S-Sp r e s i d u a l t i m e s a t a l l s t a t i o n s o n

t h e P r e c o r d i l l e r a o n a s i n g l e p o l a r p l o t f o r a h o r i z o n t a l

c r u s t 5 5 - o r 6 0 - k m t h i c k d e p e n d i n g o n s t a t i o n s l o c a t i o n

( F i g . 1 3 ) . T h e s i z e o f th e s y m b o l s i s p r o p o r t i o n a l t o th e

r e s i d u a l . T h e r e s i d u a l s f o r e v e n t s b e n e a t h t h e s t a t i o n s ( A

< 4 0 k i n ) a re v e r y s m a l l , b o t h p o s i t i v e a n d n e g a t i v e ,

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

t o th e c r u s ta l m o d e l . B e y o n d 4 0 k m , t h e re s i d u a l s p a t -

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

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

t i m e d i f f e r e n c e s o f t h e d a s h e d c u r v e ( d i p p in g M o h o ) m i -

n u s t h e s o l i d c u r v e ( f la t M o h o ) f r o m F i g u r e 1 2 . T h e

w e s t w a r d i n c r e a s e o f t h e p o s i t i v e r e si d u a l s i n d i c a t e s a

w e s t w a r d i n c r e a s e o f t h e M o h o d e p t h . A 5 ° t o 1 0 ° wes t -

w a r d s l o p e f o r th e M o h o b e n e a t h t h e P r e c o r d i ll e r a c o u l d

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

D i s c u s s i o n

O n e i m p o r t a n t r e s u l t o f t h i s s t u d y is t h a t th e p r o d u c to f o r th o g o n a l c o m p o n e n t s t o e n h a n c e b o d y - w a v e s i g n a l

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

r e f l e c t e d / c o n v e r t e d w a v e s a r e w e a k a n d h i d d e n b y s c a t -

t e r e d e n e r g y . S y n d e r et al. ( 1 9 9 0 ) , i n p r o c e s s i n g d e e p

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

c o n c l u s i o n t h a t t h e P a m p e a n a s b a s e m e n t h a s l o w s e is m i c

r e f l e c t a n c e . N o f e a t u r e i n t h e l o w e r c r u s t c o u l d b e i d e n -

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

t h e a b s e n c e o f o b s e r v a b l e P-to-S c o n v e r t e d w a v e s f r o m

t h e l o w e r c r u s t i n o u r d a t a . I n d e e d , P - w a v e i n c i d e n c e

o n t h e M o h o f r o m s h o r t - d i s t a n c e i n t e r m e d i a t e - d e p t h

e a r t h q u a k e s i s g e n e r a l l y s t e e p ( F i g . 6 ) f o r a Ps r a y p a t h ,

a n d t h i s p h a s e w i l l h a v e s i g n i f i c a n t a m p l i t u d e o n l y i f a

h i g h e n o u g h v e l o c i t y c o n t r a s t is p r e s e n t . S i n c e w e s e l -

d o m f o u n d S - w a v e p h a s e s i n t h e P c o d a o f t h e r a d ia l

c o m p o n e n t o r l a r g e n e g a t i v e p u l s e s o n t h e RZ p r o d u c t

( F ig . 4 ) w i t h i n t h e e x p e c t e d t i m e r a n g e f o r Ps p h a s e s ,

t h e l o w e r c r u s t a p p e a r s t o h a v e l o w v e l o c i t y c o n t r a s t.

T h e v e r y h ig h a m p l i tu d e o f t h e S w a v e c a n , h o w e v e r ,

g e n e r a t e o b s e r v a b l e Sp p h a s e s ( F i g . 4 ) . T h e s e q u a l i t a t iv e

i n t e r p r e t a t i o n s o n t h e l o w e r c r u s t v e l o c i t i e s a g r e e w i t h

t h e v e l o c i t y m o d e l u s e d i n th i s s t u d y ( V o l p o n i , 1 9 6 8 ;

B o l l i n g e r a n d L a n g e r , 1 9 8 8 ) , w h i c h c o m p r i s e s a v e r y

h i g h l o w e r c r u s t v e l o c i t y (V p = 7 . 3 k m / s e c ) . F o r s u c h

a c r u s t a l v e l o c i t y m o d e l , a l o w v e l o c i t y c o n t r a s t a c r o s st h e M o h o d i s c o n t i n u i t y i s l i k e l y . W e d i d n o t m o d e l t h e

a m p l i t u d e o f t h e S-to-P c o n v e r t e d a r r i v a l s f o r s e v e r a l

r e a s o n s . F i r s t , b o d y - w a v e a m p l i t u d e s a r e s t ro n g l y d e -

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

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

- 6 9 ° - 6 8 ° - 6 7 °

- 3 1 °

. 3 2 ° ~ o 0 6

52

- - 3 1 °

- _ 3 2 °

i 0 5 0 k m L

. 6 9 ° - 6 8 ° - 6 7 °

Fig ure 8. Map of San Juan region showing the Moh o depth determined ateach individual s ta t ion. There i s a sys temat ic increase of the Moho depth f romsouth to nor th across the S ier ra P ie de Palo. Th ere i s a l so a regional eas t -we stincrease of the Moho depth.

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Crustal Thickness Variat ion in the Andean Foreland , Argent ina, f rom Converted Wa ves 1105

( R e t a , 1 9 9 2 ) , m a k i n g a m p l i tu d e m o d e l i n g h a z a r d o u s .

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

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

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

i n g . I t i s i n t e r e s t i n g t o n o t e t h e f r e q u e n c y c o n t e n t v a r i -

a t io n o f t h e M o h o S p p h a s e a s o n e m o v e s f r o m s t a t i o n s

i n t h e e a s t e r n t o t h e c e n t r a l P r e c o r d i l l e r a , a s i t i s p r o b -

a b l y r e l a t e d t o v a r i a ti o n o f t h e t h i c k n e s s o f t h e M o h o

t r a n s i t i o n z o n e . T h e l o w - f r e q u e n c y S p p h a s e f r o m b e -

n e a t h t h e c e n t r a l P r e c o r d i l l e r a (F i g . 1 1 ) w o u l d i n d i c a t e

a t h i n n e r M o h o t r a n s i t i o n z o n e t h a n t h a t f o u n d u n d e r t h e

e a s t e r n P r e c o r d i l l e r a a n d S i e r r a P i e d e P a l o . A l t e r n a -

t i v e l y , i t c a n a l s o i n d i c a t e a l a r g e r v e l o c i t y j u m p a c r o s s

t h e M o h o .

O u r m o d e l o f c r u s ta l t h i c k n e s s v a r i a t i o n is s u m -

m a r i z e d i n a r e g i o n a l c a r t o o n ( F i g . 1 4 ) i n c o r p o r a t i n g r e -

s u i ts o f t h i s s t u d y w i t h l a t e s t t e c t o n i c m o d e l f o r t h e A n -

d e a n f o r e l a n d ( R e g n i e r e t a l . , 1992; S mal l ey e t a l . , 1993) .

T h e M o h o d e p t h d e t e r m i n a t io n h a s b e e n a c h i e v e d s o l e l y

w i t h tr a v e l - ti m e m o d e l i n g . T h e v e l o c i t y m o d e l w e u s e dw a s d e r i v e d b y V o l p o n i ( 1 9 6 8 ) f o r t h e l o w e r c r u s t f r o m

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

b y B o l l i n g e r a n d L a n g e r ( 1 9 8 8 ) f o r t h e u p p e r c r u s t f r o m

s h a l l o w r e f r a c t i o n p r o f i l e s . I t is v a l i d f o r t h e S i e r r a P i e

d e P a l o a r e a . O u r M o h o s t u d y i m p l ic i t ly m a k e s t h e a s-

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

t h e S i e r r a P i e d e P a l o t o t h e P r e c o r d i l l e r a s i n c e d e t a i l e d

v e l o c i t y m o d e l s f o r b o t h a r e a s a r e u n a v a i l a b l e . B e c a u s e

t h e r e i s a t r a d e - o f f b e t w e e n c r u s t a l t h i c k n e s s e s a n d v e -

l o c i ti e s i n c r u s t a l s t ru c t u r e d e t e r m i n a t i o n , t h e i n c r e a s e o f

th e S - S p t i m e f r o m e a s t t o w e s t c o u l d b e a l s o e x p l a i n e d

b y a n e a s t - w e s t d e c r e a s e o f th e m e a n c r u st a l v e l o c it y

i n s t e a d o f a n e a s t - w e s t i n c r e a s e o f th e c r u s t a l t h i c k n e s s .

H o w e v e r , i t i s i n t e r e s t i n g t o n o t e t h a t t h e c r u s t a l s e i s -

m i c i t y is a ls o d e e p e n i n g w e s t w a r d , w i t h m a x i m u m d e p t h

r a n g i n g f r o m 3 0 k m b e n e a t h t h e S i e r ra P i e d e P a l o ( R e g -

n i e r e t a l . , 1 9 9 2 ) t o b e t w e e n 3 5 a n d 4 0 k m b e n e / l t h t h e

e a s t e r n P r e c o r d i l l e r a ( S m a l l e y e t a l . , 1 9 9 3 ) . B o t h m a x -

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

J H D l o c a t i o n s , w h i c h c o r r e c t f o r l a t er a l c r u s t a l s t r u c tu r e

v a r i a t i o n s ( P u j o l e t a l . , 1 9 9 1 ) . B e t w e e n t h e S i e r r a P i e

d e P a l o a n d t h e e a s t e r n P r e c o r d i l l e r a , t h e i n c r e a s e o f t h e

M o h o d e p t h i s e q u i v a l e n t to t h e d e e p e n i n g o f t h e s e is -

m i c i t y , c a . 5 k m , i n d i c a t i n g t h a t b o t h t h e M o h o a n d t h e

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

s l o p e . T h i s s i m i l a r b e h a v i o r o f b o t h b o u n d a r i e s s t r o n g l ysugges t s an eas t - we s t c r us t a l th i ckness va r i a t ion a s a ma j or

c o m p o n e n t o f t he S - S p t i m e s i n c r e a s e ; t h e a m o u n t o f l a t-

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

d a ta . A n i m p o r t a n t c o n s e q u e n c e o f o u r m o d e l ( F i g . 1 4 )

i s t ha t , a l ong t he s ame eas t - wes t p r o f i l e , t he duc t i l e l ower

c r u s t s h o w s n o t h i c k n e s s v a r i a t i o n a n d r e m a i n s 2 1 - k m

1 2 0 _ _

130 I

140

E'150 __

.1.-.,

"t 3

160 _

170 _

180 __

190 J

0 5

\

\

\

10 15 20 25 30

t (sec)

35 40 45 50

F i g u r e 9. Record sect ion a t Precordi l lera s ta t ions of R Z product f rom inter -mediate-depth earthquakes beneath the Sierras Pampeanas province. Delta is largeenough so the S- to-P convers ion points lay beneath the S ier ra P ie de Palo (F ig.6) . The 50-km Moho depth es t imated f rom this record sect ion i s in agreementwi th the Moh o depth determinat ions a t the s ingle s ta t ion on S ier ra P ie de Palo.

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110 6 M. Regnier , J . -M. Chiu, R. Sma l ley, J r . , B . L . I sacks , and M. Araujo

t h i c k , s l o p i n g 5 ° to 1 0° w e s t w a r d . T h e c r u s t a l t h i c k e n i n g

o c c u r s t h r o u g h a n i n c r e a s e o f t h e b r i tt l e u p p e r c r u s t b e -

n e a t h b o t h t h e P a m p e a n f o r e l a n d a n d t h e P r e c o r d i l l e r a .

A l t h o u g h t h e t w o a r e a s h a v e d i f f e r e n t s e i s m i c i t y p a t -

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

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

e v e n t ( 1 9 4 4 f o r t h e P r e c o r d i l l e r a a n d 1 9 7 7 f o r t h e S i e r r a

P i e d e P a l o ) , t h e y p r o b a b l y h a v e c o m p a r a b l e c r u s t a l

t h e o l o g y . T h e d e e p e n i n g o f b o t h t h e d u c t il e l o w e r c r u s t

a n d t h e s e i s m i e i t y s u g g e s t s t h a t t h e P a m p e a n c r u s t i s u n -

d e r t h ru s t in g b e n e a t h t h e P r e c o r d il l e ra . T h e e a s t - w e s t i n-

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

m o d e l . T h e d e p t h t o t h e P r e c a m b r i a n b a s e m e n t i s v a r i -

a b l e i n t h e f o r e l a n d b a s i n w e s t o f th e S i e r r a P i e d e P a l o

( T u l u m v a l l e y , F ig . 2 ) , b u t n o d e e p e r t h a n 5 k m ( B o l -

l i n g e r a n d L a n g e r , 1 9 8 8 ) . W e s t w a r d , t h e d e p t h t o t h e

d 6 c o l l e m e n t t h i c k en s f r o m 9 k m b e n e a t h t h e e a s t e rn P r e -

c o r d i l l e r a t o a b o u t 1 5 k m b e n e a t h t h e c e n t r a l P r e c o r d i l -

l e r a ( A l l m e n d i n g e r e t a l . , 1990) . T h e i nc r ease o f t he dep t h

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

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

d e r t h r u s t s b e n e a t h t h e P r e c o r d i l l e r a n t h i n - s k i n n e d b e l t .

A s p o i n t e d o u t b y A l l m e n d i n g e r e t a l . ( 1 9 9 0 ) , c r u s -

t a l s c a le b a l a n c e d s e c t i o n s a r e v e r y p o w e r f u l i n th i s s e g -

m e n t o f t h e A n d e s f o r t w o m a j o r r e a s o n s : ( 1 ) th e a b s e n c e

o f i m p o r t a n t m a g m a t i c a d d i t io n t o t h e a n d e a n c r u s t d u r -

i n g t h e l a st - 9 m . y . ( K a y e t a l . , 1988) and ( 2 ) t he

boundar y cond i t i ons , bo t h hor i zon t a l l y and ve r t i ca l l y , s e t

u p b y t h e s u b d u c t e d p l a t e , w h i c h s e v e r e l y l im i t th e d o -

m a i n o f d e f o r m a t i o n o f t h e S o u t h A m e r i c a n l i t h o sp h e r e

a b o v e t h e N a z c a p l a t e . F i n a l l y , t h e g e o m e t r y o f t h e p la t e

c o n v e r g e n c e a n d t h e to p o g r a p h y o f t h e A n d e s d o n o t s h o w

e v i d e n c e o f i n d e n ta t i o n a n d o f l a rg e l a t e ra l m o t i o n o f

l i t h o s p h e r i c b l o c k s ( I s a c k s , 1 9 8 8 ) , w h i c h a l l o w s u s t o

10

20

30

40

50E

J

-~ 60

70

80

90

100 L--~

0 5 10 15 20 25 30 35 40

t (see)

F i gu r e 10 . M ul t ip l e even t s R Z product reco rd sect ion a t s ta t ions of the eas ternPrecordi l lera (o15, o12 , LLA, i l l , i09, i07, i08, and 007) . A m ajor ar r ival ofenergy i s observed between the P and S phases throughout the sect ion a t aboutthe t ime of the computed S - t o - P conver t ed phase f o r a 55- km - deep M oho . As i nFigure 5, traces are cut at the S arrival .

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Crustal Thickness Variation in the Andean Foreland, Argentina, from Converted Waves 1 1 0 7

a s s u m e a 1 D s h o r t e n i n g a c r o s s t h e o r o g e n . A l t h o u g h i t

i s v e r y l i k e l y t h a t a 3 D d i s t r i b u t io n o f d e f o r m a t i o n o c -

c u r s o f t e n l o c a l ly , t h e 1 D s h o r t e n in g m o d e l i s p r o b a b l y

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

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

e n i n g a c r o s s th e o r o g e n w h e n c o m p u t e d f r o m t h i c k e n i n g

i n f e r r e d i n a s i n g l e c r o s s s e c t i o n o f t h e A n d e s o n l y .

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

o f t h e S i e r r a s P a m p e a n a s p r o v i n c e f o r t h e l a s t 5 t o 1 0

m . y . , d u r i n g w h i c h c o m p r e s s i v e f o r c e s c a u s e d t h ic k -

s k i n n e d d e f o r m a t i o n ( C a m i n o s , 1 9 7 9) . T h e a c t u a l e a s t -

w e s t w i d t h o f t h e S i e r ra s P a m p e a n a s p r o v i n c e i s a b o u t

5 ° ( J o r d a n a n d A l l m e n d i n g e r , 1 9 8 6 ) , w h i c h i s 4 81 k m

a t t h e l a t i t u d e 3 0 ° s o u th . W e e s t i m a t e d i n t h i s s t u d y t h e

c r u s t a l t h i c k n e s s t o b e 5 2 k m a t t h e w e s t e r n m a r g i n o f

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

4 0 k m . T h e i n i ti a l w i d t h w a s t h e n 6 2 5 k m t o m a t c h t h e

s a m e a r e a a f t e r d e f o r m a t i o n . T h e t o t a l s h o rt e n i n g i s 1 4 1

k m o r 2 9 % . T h i s v a l u e m u s t b e s e e n a s th e u p p e r b o u n d

f o r th e s h o r t e n i n g s i n c e w e a s s u m e d a u n i f o r m c r u s t al

t h i c k e n i n g a l l o v e r t h e S i e rr a s P a m p e a n a s p r o v i n c e . I f

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

w e s t , s t a r t i n g w i t h a 4 0 - k i n - t h i c k c r u s t a l o n g t h e e a s t e r n

m a r g i n ( i . e . , n o t h i c k e n i n g ) , t h e i n i t i a l w i d t h b e c o m e s

5 5 3 k m a n d t h e to t a l s h o r t e n i n g 7 2 k m o r 1 5 % ( F i g . 1 5 ) .

A p e r i o d i c c r u s t al t h i c k e n in g w i t h m a x i m a b e n e a t h e a c h

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

t o t a l s h o r t e n i n g a s t h e r a m p m o d e l . T h e s e s h o r t e n i n g

v a l u e s a r e m u c h l a r g e r t h a n t h e 2 t o 4 % e s t i m a t e o f J o r -

d a n a n d A l l m e n d i n g e r ( 1 9 8 6) b a s e d o n c u m u l a t i v e s h o r t-

e n i n g a c r o s s t h e e n t i r e S i e r r a s P a m p e a n a s f a u l t s y s t e m .

S n y d e r et al. ( 1 9 9 0 ) s u g g e s t e d , f r o m i n t e r p r e t a t io n o f

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

t h a t J o r d a n a n d A l l m e n d i n g e r ' s e s t i m a t e m i g h t b e t o o

l o w . H o w e v e r , t h e i m p o r t a n t d i s c r e p a n c y b e t w e e n t h e

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

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

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

d e r p l a t i n g o r c r u s t a l i n t r u s i o n . B o t h a p o s i t i v e d e n s i t y

a n o m a l y ( I n t r o c a so , 1 9 80 ) a n d a h i g h c o m p r e s s i o n a l w a v e

_ _- - - - T - - [ - - - s ~ I s I ~ - -

1 0

2O

30 I /

r5 0 _ _ , l /

80 _~ ~ ~ \9 0 _ r _ t J I

0 5 1 0 1 5 2 0 2 5 3 0

t (sec )

F ig u re 1 1 . M u l t ip le ev en ts reco rd sec t io n o f t h e v e r t ica l co m p o n e n t a t s t a -t ions on the cen tral P record i l lera 012 , o l 1 , and o10) . The seismo gram s are low-pass f i l tered below 3 Hz. A clear compressional phase is observed th roughou tthe sect ion at the t ime o f the S-to-P co n v er t ed p h ase fo r a 5 9 -k m -d eep M o h o .Traces are l ined up on the observed S t ime and are cu t befo re the S arr ival .

Ev 4 0#

"O

3 5 4 0

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1108 M. Regnier, J.-M. Chiu, R. SmaUey, Jr., B. L. Isacks, and M. Araujo

"6 "

G L

O9

05

1 4

1 2

1 0

- 2 0 0 d e l t a ( k m ) 2 0 0

14. . . -- _

" , . .. . ~ " v p = 7 . 2 _ ~ ................... 5 1 2

N . . . .. . . .. ~ = t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vs=4.7 / .......

t I I i I I I I I I I ] I t I I ] [ 1 I ] I I I ] I t I I ~ I I I I I I I I

- 2 0 0 - 1 6 0 - 1 2 0 - 8 0 - 4 0 0 4 0 8 0 1 2 0 1 6 0 2 0 0

d e l t a ( k m )

Figure 12. Computed S-Sp relative travel time as a function of the epicentral

distance for a flat (solid curve) and a 5° dipping (dashed curve) Moho. Th e inset

shows the crustal model used for the computation. Note the large time differences

between the two curves at distances greater than 100 km and their opposite sign

on both sides of the station.

Nt

"" " • .... 4 - •° o ...... .... •... " ,

+ • " . , ' . . " ,%

• *. .. .: ................. 0 ".,. - '

. + * " - . + t / ~ ~ " ~ ., " + + • . + + . . ~ , ~ .

l ;" . o - 0 • , "

.4-.;++~++ - ,~- + 9

• : P • . + ~ ' + ~ ; + ° i" S ' , " 2 ~ o . + / : =

' + J - , . + + . 4 0 , . ~ o .- ' ,• . . . J r - * + . * . . , "

z _ . ~ . . . . . . . .. 6 0 ., "

°÷ . .. . o . . 04 -- I - ~_ .............. ° " '~ 6 - } ..... "

o . . "...... 1 0 0 , ' .....

4 - o i - .................. , " ' 0

÷ ~ " .................. ,.-" 1 2 0 . '. . . . . . . •

. . . . . . . . . . 1 4 0 ...... . . . . . . . . . . . . . b + .

. . . . . . . . . o . - . "

" " " " " . - " E r R

e , - " "

o

• . . . . . . I . . . . . . . . .

o !o

4 - 1 . 4

+ 1 . 2

+ 1 . 0

+ 0 . 8

+ 0 . 6

+ 0 . 4

0 . 2

0 . 0

- 0 . 2

° - 0 . 4

o - 0 . 6

o - 0 . 8

o - I . 0

0 - 1 . 2

0 - 1 . 4

s e o

Figure 13. Polar plot of the S-Sp residual times at all Precordillera stations

for a horizontal crust 55- or 60-kin thick depending on station location (eastern

or central Precordillera). The size of the symbols is proportional to the residual.A plus is a positive residual and an open circle a negative residual. The triangle

shows the station location. The increase of the residual amplitudes away fromthe station and the distribution of positive and ne gative residuals indicate that the

Moho is dipping westward beneath the Precordillera.

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Crustal Thickness Variation in the Andean F oreland, Argentina, from Converted Waves 1109

0 km

1O0 krn

200 km

300 km

Precordillera terrane

Pampeanas terrane

o /

Chilena terrane ~ " _~ # .~"

- - < - L L . . . . . .:. Nazca Plate

p m 200 km 400 "-~m 600 km

Fig ure 14. Regional cartoon incorporating results of this study (Moho depthand ductile layer thickness). The crossed area is the Sierra Pampeanas province;dashed area, Main A ndes and Chilena terrane; dark grey area, Precordillera thin-skinned tectonic province; light grey area, lithosphere. The dotted strip on topof the Sierras Pampeanas and Main Andes province represents the ductile de-formation zone.

Precordiliera S i e r r a s P a m p e a n a s

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

7 0 % 10 50 km

c

~kmd

Fig ure 15. Schematic cross section of the Pre-cordillera and Sierras Pampeanas terranes before(top) and after (bottom) shortening. The shorten-ing values are indicated for each unit. The M ohoslope is 5 ° in the Precordillera province. The ver-

tical exaggeration is x 2.

ve loc i ty (Volponi , 1968) in the lower c rus t suppor t the

m ode l o f unde rp la ted m af ic m agm a s in the P am p ean c rus t .

Thus , the th icken ing of the P am pean c rus t i s due to the

c o m b i n a t i o n o f a d d it i o n o f m a f i c m a g m a s a n d h o r i z o n t al

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

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

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

f i cu l t to e s tab l i sh . The S p coda (F igs . 4 and i0 ) usua l ly

shows tha t the lower c rus t i s l aye red and l a rge am pl i -

tudes a re p re sen t , ind ica t ing w ave conve rs ions a t h ighe r

ve loc i ty con t ra s t s than the Mo ho d i scon t inu i ty . These

ve loc i ty con t ra s t s cou ld represen t the t rans i t ions f rom

underp la ted m agm as to s t anda rd duc t i l e lower c rus t .

The shor ten ing ac ros s the P record i l l e ra has been e s -

t i m a t e d b y A l l m e n d i n g e r e t a l . (1990) to 95 km or 70%,

wi th an o r ig ina l 135-km -wide be l t . Us ing s t ra igh t fo r -

ward a rea ba lanc ing t echn ics on the whole c rus t , th i s

am o unt o f shor ten ing , to ge the r wi th an in i ti a l 40-km -

th ick c rus t , g ives an e s t im a te o f the p re sen t c rus ta l th ick-

nes s tha t i s to ta l ly unrea l i s t i c (>100 km ) . The uncou-

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

p r o v i d e s a s i m p l e w a y t o a c c o m m o d a t e d i f f e r e n t d e f o r -

m a t ion ra te s fo r the two dom ains . Be for e shor ten ing , the

th icknes s o f the c rus ta l dom ain be low the d rco l l em e nt

was about 35 k in , a s sum ing the o r ig ina l s ed im enta ry

w e d g e w a s 5 - k m t h i c k ( A l l m e n d i n g e r e t a l . , 1990) and

the c rus t 40-k in th ick . The p resen t dep th to the

d r c o l l e m e n t i s e s t i m a t e d t o b e 1 5 k m b y A l l m e n d i n g e r

et al . (1990) wi th s e i sm ic re f l ec t ion da ta . Toge the r wi th

the crus ta l thicknesses found in this s tudy, that leads to

th icknes ses o f 40 and 45 k m for the c rus t be low the

d6co l lem ent fo r the eas te rn and cen t ra l P record i l l e ra , re -

spec t ive ly . Assum ing an o r ig ina l wid th o f 135 km and

a 10 ° wes tw ard M oho s lope , the ac tua l wid th o f the do-

m a i n b e l o w t h e d 4 c o l l e m e n t s h o u l d b e 8 6 k m t o m a t c h

the s am e a rea a f t e r de form a t ion . Tha t i s a to ta l shor t -

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1 1 1 0 M . R e g n i e r , J . - M . C h i u , R . S m a l l e y , J r . , B . L . I s a c k s , a n d M . A r a u j o

e n i n g f o r t h e c ru s t b e l o w t h e d d c o l l e m e n t o f 4 9 k m o r

5 7 % . A 5 ° M o h o s l o p e w o u l d g i v e a p r e s e n t w i d t h o f

9 7 . 4 k m , a s h o r t e n i n g o f 3 7 k m o r 3 8 % ( F i g . 1 5 ) . I n

t h e s e m o d e l s , t h e M o h o a t t h e w e s t e rn b o u n d a r y o f t h e

P r e c o r d i l l e r a l o w e r c r u s t i s a t a d e p t h o f 7 0 . 1 6 k m a n d

6 3 . 5 2 k m f o r a 1 0 ° a n d 5 ° M o h o s l o p e , r e s p e c t i v e l y .

T h i s m o d e l a l s o i m p l i e s th e P r e c o r d i l l e r a l o w e r c r u s t a n d

t h e d 6 c o l l e m e n t s h o u l d n o w e x t e n d b e n e a t h t h e p r i n c ip a l

m o u n t a i n b e lt s w e s t w a r d o f th e w e s t e r n m o s t l i m i t o f t h e

P r e c o r d i l l e r a ( F i g s . 1 4 a n d 1 5 ) . N o l o w e r c r u s t a l v e l o c -

i t y i s k n o w n b e n e a t h t h e P r e c o r d i l l e r a , s o t h e p r e s e n c e

o f u n d e r p l a t e d m a f i c m a g m a s c a n n o t b e d i s c u s s ed . I t is

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

b e c o m e s m o r e i m p o r t a n t a s in l s a c k s ' ( 1 9 8 8 ) s h o r te n i n g

m o d e l .

C o n c l u s i o n s

A n a l y s i s o f c o n v e r t e d w a v e t r a v e l t i m e s f r o m i n te r -

m e d i a t e - d e p t h l o c a l e a r t h q u a k e s i n t h e S i e r r a s P a m -p e a n a s a n d P r e c o r d i l l e r a p r o v i n c e s c l e a r ly s h o w s a c o n -

t i n u o u s e a s t - w e s t d e e p e n i n g o f t h e M o h o f r o m 5 0 t o 6 0

k m , r e s p e c t i v e l y , w h i c h w e i n t e r p r e t a s t h e u n d e r t h r u s t -

i n g o f th e S o u t h A m e r i c a n c r a t o n b e n e a t h t h e A n d e a n

m o u n t a i n b e l t s . A 5 ° t o I 0 ° w e s t w a r d d i p o f t h e M o h o

u n d e r t h e A n d e a n f o r e l a n d i s e s t i m a t e d . T h i c k e n i n g i n -

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

P a m p e a n f o r e l a n d c ru s t h a s u n d e r g o n e a 3 0 % i n c r e a se ,

o r a b o u t 1 2 k i n , d u r i n g t h e l a st 1 0 m . y . I n t h e c e n t r a l

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

t h i c k e n e d b y 1 0 k m . I n b o t h a r ea s o f th e f o r e l a n d , m o s t

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

o f t h e b r i tt le u p p e r c r u s t i m a g e d b y u n u s u a l l y d e e p c r u s-t a l s e i s m i c i t y , w h i l e t h e d u c t i l e l o w e r c r u s t s h o w s n o

s i g n i f i c a n t t h ic k n e s s v a r i a t i o n w i t h r e s p e c t t o t h a t o f s t a -

b l e c o n t i n e n t a l c r u s t . T h i s r h e o l o g i c a l b e h a v i o r o f th e

c rus t i s l i ke l y due t o an i nc r ease i n s t r a i n r a t e and t o

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

s la b b e n e a t h t h e A n d e a n f o r e l a n d . T h e s h o r t e n i n g a c r o s s

t h e S i e r r a s P a m p e a n a s i s e s t i m a t e d t o b e a r o u n d 1 5 % f o r

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

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

d e p e n d i n g o f t h e a m o u n t o f m a g m a a d d i t io n i n th e l o w e r

c r u s t . T h e s h o r t e n i n g a c r o s s t h e P r e c o r d i l l e r a p r o v i n c e

w o u l d c h a n g e f r o m 7 0 % i n th e s e d i m e n t s o f th e t h in -

s k i n n e d t h r u s t b e lt a b o v e t h e d r c o l l e m e n t t o a t l e a s t 3 8 %

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

m a t i o n s a r e p r e s e n t .

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Vo lp o n i , F . S . ( 1 9 6 8 ) . L o s t e r r e m o to s d e M e n d o z a d e l 21 d e Oc tu b r e

d e 1 9 68 y l a e s t r u c tu r a d e l a c o r t e z a t e r r e s t r e , i n A c t a C u y a n a

d e I n g e n i e r ia , Vo l . XI I , I n s t i t u to S i sm o l6 g i c o Z o n d a , Fa c u l t a d

d e I n g e n i e f ia , U n i v e r s i d a d N a c i o n a l d e C u y o , S a n J u a n , A r g e n -

t i n a , 9 5 - 1 1 0 .

W h i tn e y , R . A . , H . E . B a s t i a s , a n d D . B . S l e m m o n s ( 1 9 8 4 ). T h r u s t

f a u l t n e a r - su r f a c e g e o m e t r y a n d a g e o f t h e l a s t d i sp l a c e m e n t d e -

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

Sa n Ju a n p r o v in c e , Ar g e n t in a ( a b s t r a ct ) , Ge o l . S o c . A m. 16, 693.

I n s t i t u t F r a n q a i s d e R e c h e r c h e Sc i e n t i f i q u e

p o u r l e D 6 v e l o p p e m e n t e n C o o p 6 r a t i o n ( O R S T O M )

B P A 5 N o u m e a C e d e x

N e w C a l e d o n i a , S o u t h W e s t P a c i f i c

( M . R . )

C e n t e r f o r E a r t h q u a k e R e s e a r c h a n d I n f o r m a t i o n ( C E R I)

M e m p h i s S t a t e U n i v e r s i t y

M e m p h i s , T e n n e s s e e

( J . - M . C . , R . S . J . )

I n s t i t u t e f o r t h e S tu d y o f t h e C o n t in e n t s ( I NST OC )

C o r n e l l Un iv e r s i t y

I t h a c a , N e w Y o r k

( B . L . I . )

I n t it u t o N a c i o n a l d e P r e v e n c f o n S f s m i c a ( I N P R E S )

( M . A . )

M a n u sc r ip t r e c e iv e d 1 6 Ap r i l 1 9 9 3 .


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