in: neotectonics and sea level vnriotionr in the 71-23

. -

- .

In: Neotectonics and sea level vnriotionr in the Gulf of Californh aren, CI Symposium Hermosillo, Son., April 71-23, 1984), Ma l ica Cruz, 1.. Celis-Gutiérrez, S. Guerrera-Garcia, B-Ortlieb. L. leds.). Univ. Nol: Auth . México, Inst. Geologia, Mé'xico,' D. F.

DEFORMATIONS OF MIDDLE AND LATE PLEISTOCENE DELTAIC DEPOSITS THE MOUTH OF THE R I O COLORADO, NORTHWESTERN GULF OF CALIFORNIA

AT

COLLETTA, Bernard - I n s t i t u t F r a n c a i s du P é t r o l e , 1 e t 4 avenue d e B o i s - P r é a u , B.P.311 92506 R u e i l Malmaison (FRANCE)

O R T L I E B , Luc ORSTOM ( I n s t i t u t F r a n c a i s d e R e - c h e r c h e S c i e n t i f i q u e pour l e D é - veloppement en C o o p é r a t i o n ) 2 4 r u e Bayard , 75008 P a r i s (FRANCE)

R E S U M E N

L o s márgenes m e r i d i o n a l y s u r o c c i d e n t a l d e l a Mesa d e Sono- r a , a l este d e l a desembocadura d e l Rio C o l o r a d o , e s t á n forma- dos p o r po te r l t e s s e c u e n c i a s d e s e d i m e n t o s d e l t á i c o s . En l o s a l - rededores d e G o l f o d e S a n t a C l a r a , c a s i l a t o t a l i d a d d e e s t a s a r e n a s y l i m o s c o n g l o m e r á t i c o s d e o r i g e n f l u v i a l , s o n a t r i b u i b l e s a l P l e i s t o c e n o Medio. La p a r t e m e r i d i o n a l d e l a Mesa d e S o n o r a , e n t r e Punta Gorda y Bahia A d a i r , e s t á c o n s t i t u i d a p o r u n i d a d e s i n t e r e s t r a t i f i c a d a s d e s e d i m e n t o s m a r i n o s , e s t u a r i n o s y f l u v i o - d e l t 6 i c o s ; se c o r r e l a c i o n a l a un idad más r e c i e n t e con t e s t i g o s d e l máximo n i v e l d e l mar d u r a n t e e l Úl t imo p e r i o d o i n t e r g l a c i a l , q u e se o b s e r v a n en v a r i a s l o c a l i d a d e s d e l n o r t e d e l G o l f o d e C a l i f o r n i a .

E s t a s s e c u e n c i a s d e l t a i c a s p l e i s t o c é n i c a s han s i d o f a l l a d a s y l e v a n t á d a s . Dos f a s e s d e f a l l a m i e n t o pueden ser d i s t i n g u i d a s : una más a n t i g u a d u r a n t e l a c u a l f u e r o n a s o c i a d a s f a l l a s n o r m a l e s y d e rumbo, y una más r e c i e n t e y más d é b i l en l a c u a l se produ- j e r o n s o l a m e n t e f a l l a s n o r m a l e s . La d i s p o s i c i ó n d e l a s f a l l a s más a n t i g u a s , a t r i b u i d a s a l f i n a l d e l P l e i s t o c e n o Medio, ,se i n - t e r p r e t a como una r e d d e f a l l a s d e segundo o r d e n a s o c i a d a s con l a F a l l a d e C e r r o P r i e t o , l a c u a l c o r r e r c o n rumbo NW-SE e n e l a c t u a l e s t u a r i o d e l Rio C o l o r a d o . La d e f o r m a c i 6 n d e edad C u a t e r - n a r i o T a r d i o e n e l 6 r e a ha s i d o puramente d i s t e n s i v a .

L o s movimientos l a t e r a l e s e n t r e l a s p l a c a s d e N o r t e América y d e l P a c i f i c o , en e s t a r e g i ó n , han s i d o c o n c e n t r a d o s en l a Fa- l l a d e C e r r o P r i e t o . En a l g u n a época a l f i n a l d e l P l e i s t o c e n o Medio, l a zona d e F a l l a d e C e r r o P r i e t o a b a r c ó un b r e a d e v a r i o s b i l 6 m e t r o s d e a n c h o , p e r o d e s d e el p r i n c i p i o d e l P l e i s t o c e n o T a r d i o , e l f a l l a m i e n t o a rumbo ha e s t a d o r e s t r i n g i d o e n l a t r a z a

- 8 00

.T.O.+f. Fonds u;cuixj:.*?:principal d e l a F a l l a d e C e r r o P r i e t o .

-? --?

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

bte : B

32

C o l l e t t a & O r t l i e b ;i

t a l de l a Mesa de Sonora se r e l a c i o n a n c o n l a a c t i v i d a d d e l f a - l l a m i e n t o a rumbo de l a F a l l a de Cerro P r i e t o . E l l e v a n t a m i e n t o e n forma de domo de l a mesa, c e r c a de Golfo de S a n t a C la ra , se p r o d u j o a n t e s de l P l e i s t o c e n o T a r d í 6 y p r o b a b l e m e n t e es contem- po ráneo a l e p i s o d i o de mayor a c t i v i d a d t e c t ó n i c a e n e l á r e a , d e l f i n a l de l P l e i s t o c e n o Medio; es ta s u r e c c i ó n se ha p ro longado d u r a n t e e l C u a t e r n a r i o T a r d i o . La a l t u r a c r e c i e n t e a p a r t i r de B a h i a Adair y h a c i a e l NW, e n l a que se o b s e r v a n l a s c a p a s mari- n a s a t r i b u i d a s a l p r i n c i p i o d e l P l e i s t o c e n o T a r d i o p e r m i t e n de- t e r m i n a r un l e v a n t a m i e n t o r e c i e n t e ( C u a t e r n a r i o T a r d í o ) d e l bor- de s u r o c c i d e n t a l de l a Mesa de Sonora . A s í se estimaron t a z a s de l e v a n t a m i e n t o de cerca de 1 3 0 mm/1000 a ñ o s e n e l á r e a de Pun- t a Gorda. E s t o s movimientos v e r t i c a l e s q u e s o n de una a m p l i t u d i n f e r i o r a l a q u e se s u p o n í a a n t e r i o r m e n t e (Merr iam, 19651, se l o c a l i z a n e n una pequeña á r e a c e r c a n a a l a t r a z a de l a F a l l a de Cerro P r i e t o .

A B S T R A C T

The s o u t h w e s t e r n and s o u t h e r n edges of t h e Mesa of S o n o r a , e a s t of t h e e s t u a r y of t h e Rio C o l o r a d o , show t h i c k s e q u e n c e s o f d e l t a i c s e d i m e n t s . I n t h e s u r r o u n d i n g s o f Golfo de S a n t a C l a r a a l m o s t a l l t h e s e c o n g l o m e r a t i c s a n d s and s i l t s , of f l u v i a l o r i g i n , a r e a t t r i b u t e d t o t h e Middle P l e i s t o c e n e . The s o u t h e r n Mesa of S o n o r a , be tween P u n t a Gorda and B a h i a Adai r , i s formed by i n t e r - l a y e r e d m a r i n e e s t u a r i n e and f l u v i o - d e l t a i c u n i t s ; t h e most r e c e n t u n i t i s c o r r e l a t e d w i t h r emnan t s o f t h e h i g h d u r i n g t h e l a s t i n t e r g l a c i a l , o b s e r v e d i n s e v e r a l l o c a l i t i e s of t h e n o r t h e r n G u l f of C a l i f o r n i a .

- These P l e i s t o c e n e d e l t a i c s e q u e n c e s have been f a u l t e d and

s e a s t a n d

u p l i f t e d . Two f a u l t i n g e v e n t s a re o b s e r v e d : a n o lder o n e i n which normal and s t r i k e - s l i p f a u l t s a r e assoc ia ted and a younger o n e , much weaker and p r o d u c i n g o n l y normal f a u l t s . The a r r a n g e - ment of t h e o lde r f a u l t s , of l a t e Middle P l e i s t o c e n e age, is i n t e r p r e t e d as a s e c o n d order f a u l t p a t t e r n a s s o c i a t e d w i t h t h e Cerro P r i e t o F a u l t which r u n s NW-SE i n t h e p r e s e n t R i o Co lo rado e s t u a r y . The l a t e Q u a t e r n a r y d e f o r m a t i o n o f t h e a r e a h a s been p u r e l y d i s t e n s i v e .

The l a t e r a l movements be tween t h e Nor th American and P a c i f i c p l a t e s , i n t h i s r e g i o n , have been t a k e n u p by t h e Cerro P r i e t o F a u l t . Dur ing some time a t t h e end of t h e Middle P l e i s t o c e n e , t h e C e r r o P r i e t o F a u l t zone h a s been s e v e r a l k i l o m e t e r s w i d e , b u t s i n c e t h e e a r l y L a t e P l e i s t o c e n e is restricted t o t h e main t race of t h e Cerro P r i e t o F a u l t .

t h e s t r i k e - s l i p a c t i v i t y

V e r t i c a l movements of t h e S o u t h w e s t e r n p a r t of t h e Mesa of Sonora 3re a l s o r e l a t e d t o t h e ma in ly s t r i k e - s l i p Cer ro P r i e t o

3 3

C o l l e t t a , & O r t l i e b

F a u l t a c t i v i t y . The doming of t h e mesa, n e a r Go l fo de S a n t a C l a - r a , o c u r r e d before t h e Late P l e i s t o c e n e and is most p r o b a b l y c o e v a l w i t h t h e l a t e Middle P l e i s t o c e n e e p i s o d e of l a rge r tec- t o n i c a c t i v i t y of t h e a r e a . T h i s doming migh t have been s l i g h t l y a c c e n t u a t e d d u r i n g t h e L a t e Q u a t e r n a r y . The i n c r e a s i n g e l e v a t i o n , from Bah ia A d a i r n o r t h w e s t w a r d s , of m a r i n e beds a t t r i b u t e d t o t h e e a r l y L a t e P l e i s t o c e n e documents a L a t e Q u a t e r n a r y u p l i f t of t h e s o u t h w e s t e r n edge of t h e Mesa of Sonora . A t P u n t a Gorda mean u p l i f t r a tes of a b o u t 1 3 0 mm/1000 y e a r s a r e t h u s i n f a r e d . These v e r t i c a l movements, which a re of lower a m p l i t u d e th.an p r e v i o u s l y supposed (Merriam, 1 9 6 5 1 , a re res t r ic ted t o a s m a l l a r e a c lose t o t h e o f f s h o r e C e r r o P r i e t o F a u l t t r a c e .

'

'

INTRODUCTION 4

The Gulf of C a l i f o r n i a and t h e San Andreas f a u l t s y s t e m form a major f r a c t u r e zone a l o n g which t h e Nor th American and t h e P a c i f i c p l a t e s a re moving i n o p p o s i t e d i r e c t i o n s . A c o n s i d - erable work h a s been d o n e r e c e n t l y t o describe and be t te r unde r - s t a n d t h e p a s t and p r e s e n t mo t ions o c c u r r i n g a l o n g t h i s p l a t e boundary . Though, t h e r e g i o n of t h e n o r t h e r n m o s t Gulf of C a l i - f o r n i a and of t h e Co lo rado d e l t a i n m e d i a t e l y s o u t h o f t h e Mexican- U.S. border, h a s been much less s t u d i e d , f rom a s t r u c t u r a l p o i n t of v iew, t h a n t h e I m p e r i a l V a l l e y and t h e S a l t o n Trough o f s o u t h - e a s t e r n C a l i f o r n i a .

One of t h e main r e a s o n f o r t h i s l a c k o f d a t a is t h e p a u c i t y of o u t c r o p s and t e c t o n i c f e a t u r e s , which i t s e l f is re la ted t o t h e g e n e r a l s u b s i d e n c e of t h e a r e a and t o t h e h i g h s e d i m e n t a t i o n r a t e o f t h e d e l t a . I n t h i s a rea , u p t o 6 ,000 meters o f d e l t a i c sediments have been accumula t ed s i n c e Miocene t i m e , and m o s t l y d u r i n g t h e Q u a t e r n a r y ( B i e h l e r e t a l . , 1 9 6 4 ) . I n t h e Co lo rado d e l t a r e g i o n , t h e o n l y zone where Q u a t e r n a r y s e d i m e n t s a re l a r g e l y o u t c r o p p i n g i s loca ted n e a r G o l f o de S a n t a C l a r a , e a s t o f t h e r i v e r mouth. A p r e l i m i n a r y t e c t o n i c s t u d y of t h i s area w a s comple t ed by Merriam, i n 1 9 6 5 . T h i s a u t h o r mapped a series of f a u l t t races which were c o n s i d e r e d t o be p r o l o n g a t i o n s o f t h e San J a c i n t o f a u l t s y s t e m of s o u t h e r n C a l i f o r n i a ; h e a l s o ment ioned a l o c a l u p l i f t which h e t h o u g h t was v e r y r e c e n t ( H o l o c e n e ) .

I n t h e p r e s e n t work , wd a re c o n c e r n e d w i t h a more p r e c i s e a n a l y s i s of t h e f a u l t mechanisms and t h e v e r t i c a l movements e v i d e n c e d i n t h e same r e g i o n namely between t h e Co lo rado r i v e r mouth and t h e area of S a l i n a Grande i n t h e s o u t h e r n m o s t p a r t of t h e Mesa de Sonora ( f i g . 1) . The a n a l y s i s of f a u l t i n g was done by measu r ing s e v e r a l parameters of f a u l t p l a n e s ( s t r i k e , d i p , r a k e of s t r i a e , and s e n s e of s l i p ) , i n an a t t e m p t t o e v a l u a t e t h e pa leos t ress f i e l d . An i n s i g h t i n t h e v e r t i c a l movements h a s been made p o s s i b l e t h r o u g h m o r p h o l o g i c a l o b s e r v a t i o n s and t h r o u g h

34

C o l l e t t a .S O r t l i e b

F i g u r e 1.- S k e t c h map o f t h e n o r t h e r m n o s t Gu l f o f C a l i f o r n i a a n d t h e s o u t h e r n p a r t o f t h e R i o C o l o r a d o d e l t a . Numbers i n smal l h e x a g o n s i n d i c a t e maximum e l e v a t i o n o f t h e l a s t i n t e r - g l a c i a l s h o r e l i n e ( O r t l i e b , 1982351.

a s t u d y of t h e P l e i s t o c e n e m a r i n e and d e l t a i c d e p o s i t s of t h e n o r t h e r n Gulf of C a l i f o r n i a .

The Mesa of S o n o r a

The r e c e n t C o l o r a d o r i v e r d e l t a , b u i l t s in ' ce t h e l a s t r ise of s e a l e v e l i n t h e Ho locene , is composed o f a - d e l t a i c c o n e (be tween Yuma and C e r r o P r i e t o ) , an a l l u v i a l p l a i n , and an e s t u a r y bo rde red ' by e x t e n s i v e mud f l a t s . To t h e w e s t , t h e d e l t a is l i m - i t e d by t h e sierras Cucapas and E l Mayor; t o t h e e a s t , t h e p r e - s e n t d e l t a a b u t s a g a i n s t t h e c l i f f s of t h e mesas of Yuma, San L u i s and S o n o r a . L i k e o t h e r s i n t h e s o u t h e r n p a r t of t h e S a l t o n d e p r e s s i o n , t h e s e mesas a r e f l u v i a l t e r r a c e s which have been b u i l t i n t h e P l e i s t o c e n e d u r i n g p e r i o d s of h i g h s t a n d of s e a l e v e l and of i n t e n s e a l l u v i a t i o n . The mesas of Yuma and San L u i s were most p r o b a b l y formed d u r i n g t h e l a s t i n t e r g l a c i a l , a t t h e b e g i n n i n g of t h e L a t e P l e i s t o c e n e ( a r o u n d 1 2 0 , 0 0 0 y e a r s B P I (olmsted e t a l . , 1 9 7 3 ) . The Mesa of Sonora i s a much l a r g e r geo - morphic u n i t , l i m i t e d by t h e p r e s e n t e s t u a r y of t h e C o l o r a d o r i v e r t o t h ë w e s t , Bah ia A d a i r t o t h e e a s t , and t h e Gulf of C a l i f o r n i a t o t h e s o u t h ( f i g . 1); t o t h e n o r t h , n e a r t h e b o r d e r between n o r t h w e s t e r n S o n o r a and s o u t h w e s t e r n A r i z o n a , t h e Mesa o f S o n o r a merges w i t h t h e mesas of Yuma and San L u i s . I.t i s a p l a t e a u , w i t h a mean e l e v a t i o n o f a b o u t +50 m , c o v e r e d w i t h e x t e n s i v e f i e l d s of s a n d d u n e s ( " A l t a r deser t" o r "Gran D e s i e r t o " ) . I n t h e a r e a of G o l f o de S a n t a C l a r a , t h e s u r f a c e of t h e Mesa of So- n o r a i s l o c a l l y e l e v a t e d t o an a l t i t u d e of a p p r o x i m a t e l y +150 m .

The h i g h c l i f f s c u t i n t h i s s o u t h w e s t e r n l i m i t of t h e Mesa of Sonora and t h e c o a s t a l c l i f f s a l o n g t h e s o u t h e r n m o s t boundary of t h e mesa, show t h a t t h e s u b s t r a t e of t h e p l a t e a u c o n s i s t s o f t h i c k s e q u e n c e s o f d e l t a i c s a n d s and g r a v e l s . These s e d i m e n t s had been s u c c e s s i v e l y i n t e r p r e t e d a s Miocene m a r i n e beds ( K n i f f e n , 1 9 3 2 1 , t h e n a s P l i o c e n e d e l t a i c d e p o s i t s ( ß e a l , 19481 , and f i n a l - l y d e l t a i c series of Q u a t e r n a r y a g e (Merr iam, 1 9 6 5 ) .

S e v e r a l e x p l o r a t i o n w e l l s d r i l l e d n e a r Yuma and San L u i s , o r o f f s h o r e i n t h e S a l i n a Granda a r e a , i n d i c a t e t h a t Q u a t e r n a r y s t r a t a i n t h i s l a r g e d e l t a i c b a s i n have a t h i c k n e s s i n e x c e s s of one k i l o m e t e r ( O l m s t e d et al., 1973 ; M a t t i c k et &., 1973 ; E b e r l y & S t a n l e y , 1 9 7 8 ; C o n t r e r a s F l o r e s , 1982 ; T r i n i d a d Reyes & Rueda G a x i o l a , 1982 : Vi.ñas, 1 9 8 2 ) . Such a t h i c k n e s s of m a t e r n a r y depos - its below t h e who le r e g i o n of t h e Mesa of Sonora i n d i c a t e s t h a t , u n l e s s i m p o r t a n t f a u l t movements w e r e i n v o l v e d , t h e u p p e r v i s i b l e p a r t of t h e mesa must have been d e p o s i t e d r e l a t i v e l y r e c e n t l y .

T e c t o n i c s e t t i n g

A t t h e j u n c t i o n be tween t h e San Andreas f a u l t s y s t e m and t h e d i s t e n s i v e - t r a n s f o r m zone of t h e Gulf of C a l i f o r n i a , t h e region of t h e Colorado d e l t a , i n c l u d i n g t h e I m p e r i a l V a l l e y , i s t e c t o -

36

C o l l e t t a .& O r t l i e b

n i c a l l y a c t i v e ; t h i s s m a l l a r e a i s c h a r a c t e r i z e d by L a t e Q u a t e r - n a r y v o l c a n i c r o c k s , g e o t h e r m a l f i e l d s , r e c e n t f a u l t i n g a c t i v i t y and an i m p o r t a n t s e i s m i c i t y .

The s o u t h e a s t e r n e x t r e m i t y of t h e San Andreas sys t em i s e s s e n t i a l l y composed o f t h r e e f a u l t b r a n c h e s : E l s i n o r e , San J a - c i n t o and Banning-Miss ion C r e e k f a u l t s ( f i g . 2). The l a s t t w o f a u l t s l i m i t t h e S a l t o n Trough, which i s a s t r u c t u r a l b a s i n and a l s o a t o p o g r a p h i c a l d e p r e s s i o n , l y i n g a t s e v e r a l t e n s o f meters u n d e r s e a l e v e l and s e p a r a t e d from t h e g u l f by t h e r e c e n t Colo- r a d o d e l t a i c cone . I n t h e s o u t h e r n I m p e r i a l V a l l e y and i n t h e d e l t a i c p l a i n s o u t h of t h e i n t e r n a t i o n a l b o r d e r , t h e two main f a u l t s a r e t h e I m p e r i a l and C e r r o P r i e t o f a u l t s ( f i g . 21.. A l l t h e s e f a u l t s , n o r t h and s o u t h of t h e b o r d e r , are NW-SE o r i e n t e d and show e v i d e n c e s of r i g h t l a t e r a l s t r i k e s l i p mot ion . Al though t h e p r e c i s e r e l a t i o n s h i p s had n e v e r been s t r o n g l y e s t a b l i s h e d , t h e I m p e r i a l f a u l t had been i n t e r p r e t e d a s t h e p r o l o n g a t i o n t o t h e s o u t h of t h e Banning-Mission Creek f a u l t , and t h e C e r r o P r i e - t o f a u l t a s t h e p r o l o n g a t i o n o f t h e San J a c i n t o f a u l t (Merr iam, 1965 ; Lomnitz e t s., 1 9 7 0 ; D e l a F u e n t e & Sumner, 1 9 7 4 ; C o l L e t t a & O r t l i e b , 1 9 8 n . But some a u t h o r s c o n s i d e r e d t h e I m p e r i a l f a u l t r a t h e r as' a l i n k be tween b o t h t h e San J a c i n t o and Banning-Mission C r e e k f a u l t s on one hand, and t h e C e r r o P r i e t o f a u l t on t h e o t h e r hand ( E l d e r s e t a l . , 1972 ; F u i s e t a l . , 1 9 8 2 ) .

The two g e o t h e r m a l f i e l d s of t h e a r e a a r e l o c a t e d on t h e s o u t h e r n s h o r e of t h e S a l t o n S e a , i n C a l i f o r n i a , and s o u t h o f Me- x i c a l i , i n Mexico; t h e y a r e a s s o c i a t e d w i t h v o l c a n i c domes of r e l a t i v e l y young age : 110 ,000 t o 1 0 , 0 0 0 y e a r s f o r C e r r o P r i e t o ( B a r n a r d , 1 9 6 8 ; Robinson & E l d e r s , 1 9 7 1 ; D e Boe r , 1979 , 1 9 8 0 ) and a b o u t 1 6 , 0 0 0 y e a r s f o r t h e S a l t o n B u t t e s ( K e l l e y & S o s k e , 1936 ; M u f f l e r & White , 1969 ; Robinson -- e t a l . , 1 9 7 6 ) . These a r e a s of r e c e n t v o l c a n i c a c t i v i t y and h i g h h e a t f l o w a r e supposed t o c o r r e s p o n d t o two newly formed s p r e a d i n g c e n t e r s ~ ( L o m n i t z e t a l . , 1 9 7 0 ; E l d e r s e t a l . , 1972 ; F u i s e t a l . , 1 9 8 2 ) . Accord ing t o t h e g e n e r a l l y a c c e p t e d s t r u c t u r a l model of t h e r e g i o n , t h e t h r e e s p r e a d i n g c e n t e r s of Brawley , C e r r o P r i e t o and Wagner B a s i n , would be l i n k e d by two t r a n s f o r m f a u l t s , t h e I m p e r i a l and Cerro P r i e t o f a u l t s ( f i g . 2 1 .

Almost a l l t h e p r i n c i p a l seisms registered i n t h e l a s t 60 y e a r s i n s o u t h e a s t e r n C a l i f o r n i a and i n t h e Co lo rado d e l t a r e g i o n had t h e i r e p i c e n t e r s a l o n g t h e above ment ioned f a u l t s ( f i g . 2 ) . The h y p o c e n t e r s of t h e s e seisms are l o c a t e d a t v a r i o u s d e p t h s b u t most of them seem t o be o r i g i n a t e d a t a b o u t a 5-km d e p t h (Vonder Haar & Howard, 19811 , t h a t i s i n t h e t r a n s i t i o n zone be tween t h e s e d i m e n t a r y l a y e r s and t h e basement ( B i e h l e r e t a l . , 1 9 6 4 ; F u i s e t a l . , 1983') . I n s o u t h e a s t e r n C a l i f o r n i a , f o c a l mechanisms i n d i c a t e t h a t t h e r e g i o n a l stress f i e l d h a s a N-S o r i e n t e d c o m p r e s s i v e a x i s (0- 1 and an E-W o r i e n t e d t e n s i o n a x i s 1

--

37 -

Col le t ta & O r t l i e b .

F i g u r e 2.- L o c a l i z a t i o n of m a j o r a c t i v e f a u l t s and r e l a t e d seismic e v e n t s w i t h m a g n i t u d e o v e r 6 ( b e t w e e n 1 9 1 2 and 1 9 8 0 ) a t t h e j u n c t u r e b e t w e e n t h e S a n Andreas F a u l t s y s t e m and t h e Gu l f of C a l i f o r n i a (modified from A l l e n , 1 9 7 5 ) .

-..

38

C o l l e t t a & O r t l i e b 1

( J o h h s o n & Hadley , 1 9 7 6 ; F u i s & Schnapp, 1977 ; A l b o r e s e t a l . , 1 9 8 0 1.

STRATIGRAPHY OF THE SOUTHERN MESA DE SONORA

The Go l fo de S a n t a C l a r a r e g i o n

I n t h e r e g i o n of G o l f o de S a n t a C l a r a , most of t h e o u t c r o p s o f t h e Mesa of Sonora s u b s t r a t e show f l u v i o - d e l t a i c s a n d s and Silts, w i t h i n t e r b e d d e d c o n g l o m e r a t i c l e n s e s and l a y e r s of mud-

b o n e s of v e r t e b r a t e s . Downs ( i n Merr iam, 1 9 6 5 ) and C a r r a n z a Cas- t a n e d a ( p e r s . comm., 1 9 7 9 ) i n t e r p r e t e d t h a t t h i s v e r t e b r a t e f a u n a , i n c l u d i n g r e m a i n s of Equus s p . , Gomphoterium s p . and B i son ( ? ) s p . , i s of I r v i n g t o n i a n ( P l e i s t o c e n e ) a g e .

' b a l l s . These s e d i m e n t s c o n t a i n s i l i c i f i e d wood f r a g m e n t s and

T h i s t h i c k s e q u e n c e p r o b a b l y c o r r e s p o n d s t o s e v e r a l a l l u v i a l c y c l e s o f t h e C o l o r a d o r i v e r , b u t i t i s u n e a s y t o d i s t i n g u i s h c h r o n o s t i a t i g r a p h i c u n i t s . The s e d i m e n t a r y f a b i e s of t h e whole s e q u e n c e a r e of a l l u v i a l t y p e , and t h u s d o n o t i n d i c a t e a c l o s e p r o x i m i t y of t h e s e a s h o r e . No m a r i n e o r l i t t o r a l beds, l i k e t h o s e o u t c r o p p i n g n e a r S a l i n a Grande, a r e o b s e r v e d i n t h e v i c i n i t y of '

G o l f o de S a n t a C l a r a ( o n l y one o u t c r o p o f " t i d a l i t e s " , whose s t r a t i g r a p h i c p o s i t i o n i s u n c e r t a i n , c a n be s e e n a t 5 km n o r t h of G o l f ò de S a n t a C l a r a ) . A l l t h i s d a t a s u g g e s t t h a t a t t h e t i m e of t h e d e p o s i t i o n o f t h e s e q u e n c e , t h e r i v e r mouth was l o c a t - ed i n a more s o u t h e r n p o s i t i o n t h a n t h e p r e s e n t one .

B e s i d e t h e main f l u v i o - d e l t a i c s e q u e n c e making u p a l m o s t a i l t h e s u b s t r a t e o f t h e Mesa of S o n o r a i n t h i s r e g i o n , a younger l i t h o s t r a t i g r a p h i c u n i t c o u l d be d i s t i n g u i s h e d ( C o l l e t t a & O r t l i e b , 1979 , 1 9 8 1 ) . It is o n l y o b s e r v e d n o r t h of G o l f o de S a n t a C l a r a , and d i s c o r d a n t l y o v e r l i e s t h e main s e q u e n c e d e p o s i t s . T h i s u n i t of l imi ted t h i c k n e s s and e x t e n t a l s o c o n s i s t s of f l u v i a l sedi- m e n t s , b u t t h e s e are more c o n g l o m e r a t i c , and less c e m e n t e d , t h a n t h e main s e q u e n c e . The g e o m e t r i c form of t h e d e p o s i t and t h e s e d i m e n t a r y facies o f t h i s u n i t , a r e t y p i c a l l y t h o s e o f a f l u - v i a l c h a n n e l f i l l .

C o n s i d e r i n g t h a t t h e younger l i t h o s t r a t i g r a p h i c u n i t i s o u t c r o p p i n g a t an e l e v a t i o n of a b o u t +40 m , and t h a t i s s h o u l d be c o r r e l a t i v e w i t h an i n t e r g l a c i a l h i g h s t a n d of s e a l e v e l ( h i g h base l e v e l ) , o n e may i n f e r t h a t it is p r e - h o l o c e n e , and t h a t

. i t i d most p r o b a b l y c o e v a l w i t h t h e e a r l y l a s t i n t e r g l a c i a l . ( a round IZO k a ) . C o n s e q u e n t l y , i f t h i s a s s u m p t i o n i s c o r r e c t ,

t h e main s e q u e n c e , o l d e r t h a n t h e L a t e P l e i s t o c e n e , w o u l d be of Middle P l e i s t o c e n e age. It s h o u l d n o t be o l d e r t h a n Middle P l e i s t o c e n e ( c o n s i d e r e d h e r e as t h e 700,000-120,000 y e a r s BP

. p e r i o d ) , s i n c e it is l a y i n g a t o p s e v e r a l hundred metres of Qua- t e r r i a r y s e d i m e n t s .

' I . I . .

39

C o l l e t t a & O r t l i e b

A s w i l l be shown l a t e r , t h e main s e q u e n c e and t h e younger l i t h o s t r a t i g r a p h i c u n i t r e g i s t e r e d d i s t i n c t e p i s o d e s of f a u l t i n g .

. P u n t a Gorda -Sa l ina Grande r e g i o n

Along t h e c o a s t l i n e t h e c l i f f s a r e p r a c t i c a l l y u n i n t e r r u p t e d from G o l f o d e S a n t a C l a r a s o u t h e a s t w a r d s t o t h e s h o r e s of Bah ia A d a i r . Between G o l f o de S a n t a C l a r a and P u n t a Gorda , n o t a s i n g l e o u t c r o p of m a r i n e s t r a t a i s o b s e r v e d ; i n t h i s c o a s t a l s egmen t t h e s e d i m e n t s f o r m i n g t h e s u b s t r a t e o f t h e Mesa of S o n o r a , a r e o n l y f l u v i a l / a l l u v i a l s a n d s and c o n g l o m e r a t i c s a n d s t o n e s , c o r r e - l a t e d w i t h t h e o l d e r main s e q u e n c e of t h e G o l f o d e S a n t a C l a r a a r e a .

I n t h i s a r e a of P u n t a Gorda and e a s t w a r d s t o S a l i n a Grande , t h e c o a s t a l c l i f f s show numerous i n t e r l a y e r e d m a r i n e and d e l t a i c s e d i m e n t a r y u n i t s . Along t h e 5 km l o n g c o a s t a l segment e a s t of

( f o s s i l i f e r o u s i n f r a l i t t o r a l s a n d s , p o i n t b a r d e p o s i t s , s a n d / s i l t r h y t h m i t e s , f l u v i a l and l i t t o r a l c h a n n e l f i l l s , e t c . ) p o i n t t o a d e l t a i c and e s t u a r i n e e n v i r o n n e m e n t . The upper-most l i t t o r a l u n i t of t h e s e q u e n c e , which is l o c a l l y o v e r l a i d by c o n t i n e n t a l r e d s i l ts and L a t e Q u a t e r n a r y e o l i a n s a n d s , is a c o q u i n a , made o f beach-worn s h e l l s of Chione c o r t e z i ( f i g . 4 ) . The presumed a g e of t h i s Ch ione c o q u i n a i s l a s t i n t e r g l a c i a l (0rt l . ieb & Mal- p i c a 1 9 7 8 ; O r t l i e b , 1 9 7 8 , 1 9 8 0 , 1982b ; C o l l e t t a & O r t l i e b , 1 9 8 1 ) . The p r e c i s e c h r o n o l o g i c a l r e l a t i o n s h i p s between t h e d i s t i n c t l i t h o l o g i c u n i t s o u t c r o p p i n g i n t h e s e c l i f f s a r e n o t y e t comple t e - l y e s t a b l i s h e d ; i n p a r t i c u l a r , i t i s n o t known w h e t h e r t h e e s t u a - r i n e s e q u e n c e and t h e Ch ione c o q u i n a a r e penecontemporaneous ( a n d were b o t h d e p o s i t e d d u r i n g t h e e a r l y p h a s e of t h e l a s t i n t e r - g l a c i a l t r a n s g r e s s i o n ) , o r i f t h e o b s e r v e d d i s c o r d a n c e which s e p - a r a t e s them ( f i g . 4 ) c o r r e s p o n d s t o a t i m e s p a n o f a whole g l a - c i a l / i n t e r g l a c i a l c y c l e . T h i s p o i n t i s i m p o r t a n t b e c a u s e t h e P u n t a Gorda F a u l t ( F i g . 3 ) h a s been a c t i v e a f t e r t h e d e p o s i t i o n o f t h e e s t u a r i n e s e q u e n c e b u t p r i o r t o t h e f o r m a t i o n of t h e Chione c o q u i n a .

. P u n t a Gorda ( f i g . 3 1 , a g r e a t v a r i e t y of s e d i m e n t a r y fac ies

I n t h e c o a s t a l s egmen t between S a l i n a Grande and Bah ia A d a i r , t h e c l i f f s e s s e n t i a l l y c u t m a r i n e s t r a t a . I n t h i s a r e a , a r e l a - t i v e l y young f o s s i l i f e r o u s m a r i n e u n i t , t e n t a t i v e l y c o r r e l a t e d w i t h t h e Chione c o q u i n a o f P u n t a Gorda, d i s c o r d a n t l y o v e r l i e s a n o l d e r s e q u e n c e of i n f r a l i t t o r a l s a n d s c o n t a i n i n g numerous e c h i n o i d s and b a d l y p r e s e r v e d f o s s i l s h e l l s . A shelJ..qf Chione s p from t h e most r e c e n t bed h a s b e e n d a t e d 1 4 6 , 0 0 0 -11 y e a r s by Io/U r a d i o m e t r i c method (Geotop Lab . , M o n t r e a l ; O . C e r r o , pers. cmm, 1 9 8 3 ) . Four o t h e r Chione s h e l l s , sampled i n d i s t i n c t l o c a l i t i e s o f t h i s s o u t h e r n boundary of t h e Mesa of S o n o r a , have been s u b m i t t e d t o amino-acid g e o c h r o n o l o g i c a l a n a l y s i s ; a l t h o u g h t h i s d a t i n g method is n o t q u i t e r e l i a b l e i n t h e warm e n v i r o n m e n t

4 0


1 f 1 D E G

t

F i g u r e 3 . - S t r u c t u r a l map of t h e c o a s t a l a r e a i n t h e n o r t h e r n - most Gulf of C a l i f o r n i a , between S a l i n a Grande and Go l fo d e San- t a C l a r a .

1- Sand b e a c h e s and l i t t o r a l d u n e s ; 2- S a l t mar sh ; 3- Dune; 4- T i d a l f a l t s of t h e Co lo rado d e l t a ; 5- Colluvium and s u r f a c e o f t h e Mesa of Sonora ; 6- A l l u v i a l d e p o s i t s of L a t e P l e i s t o c e n e a g e ( ? ) ; 7- Main d e l t a i c s e q u e n c e o f t h e pa l eo -Rio Co lo rado (Middle P l e i s t o c e n e ) ; 8- Major f a u l t s ; 9- F l e x u r e ; 10- Secondary f a u l t s ; 11- I n f e r r e d t r a c e of t h e o f f s h o r e c o n t i n u a t i o n of t h e Cerra P r i e t o F a u l t .

s e c t i o n s of f i g . 9 .

.

NE-SW o r i e n t e d l i n e a s A t o K r e p r e s e n t t h e t o p o g r a p h i c a l c r o s s -

41


‘ f f t I I F G ti

42


of t h e n o r t h e r n Gulf ( O r t l i e b , 1 9 8 2 a 1 , t h r e e of t h e f o u r s a m p l e s p r o v i d e d a l l o / i s o l e u c i n e r a t i o s s u g g e s t i n g t h a t t h e s h e l l s a r e n o t o l d e r t h a n t h a n t h e l a s t i n t e r g l a c i a l t r a n s g r e s s i o n ( O r t l i e b , Fn p r e p . 1 .

Thus t h e whole a v a i l a b l e d a t a seem t o i n d i c a t e t h a t t h e Chione c o q u i n a , o b s e r v e d i n s e v e r a l l o c a l i t i e s i n t h e s o u t h e r n - most Mesa o f S o n o r a , is of l a s t i n t e r g l a c i a l age ( e a r l y L a t e P 1 , e i s t o c e n e 1 .

---

ANALYSIS OF DEFORMATION

Two t e c t o n i c e v e n t s have been regis tered by t h e Q u a t e r n a r y d e p o s i t s of t h e G o l f o de S a n t a C l a r a - P u n t a Gorda a r e a : e a c h p r e v i o u s l y men t ioned s t r a t i g r a p h i c u n i t s n e a r G o l f o de S a n t a C l a r a shows d i s t i n c t d e f o r m a t i o n s . The main d e l t a i c s e q u e n c e . (Midd le P l e i s t o c e n e ? ) which forms most of t h e o u t c r o p s of t h e Clolfo d e S a n t a C l a r a a r e a , h a s been s t r o n g l y f a u l t e d and f l e x u r - e d , whereas t h e younger c h a n n e l d e p o s i t s ( n o r t h of G o l f o d e San- t a C l a r a ) a r e o n l y s l i g h t l y f a u l t e d . I n t h e P u n t a Gorda a r e a o n l y t h e m a r i n e and f l u v i a l d e l t a i c / b e d s o l d e r t h a n t h e Chione c o q u i n a d i d register t e c t o n i c d e f d r m a t i o n s .

- D e f o r m a t i o n s registered by t h e Middle P l e i s t o c e n e main d e l t a i c s e q u e n c e . ---

Major s t r u c t u r a l f e a t u r e s i n t h e main d e l t a i c s e q u e n c e have a NNW-SSE o r i e n t a t i o n ( F i g . 3 ) . A s described by Merriam (19651 , f a u l t s p l a n e s a p p e a r as brecciated and cemen ted z o n e s w e l l e v i - d e n c e d by d i f f e r e n t i a l e r o s i o n ; t h e y a r e g e n e r a l l y accompanied by small s u b m e r i d i a n cemen ted v e i n l e t s and g y p s e o u s t e n s i o n g a s h e s .

r a i n more c l a y e y and c o n s e q u e n t l y more d u c t i l e d e p o s i t s , a p p e a r a s s u p e r f i c i a l e x p r e s s i o n o f d e e p no rma l f a u l t s w i t h which t h e y l o c a l l y merge ( F i g . 5 1 .

F l e x u r e s , p r i n c i p a l l y o b s e r v e d n o r t h of G o l f o de S a n t a C la -

W e pe r fo rmed 282 measurements of f r a c t u r e s i n c l u d i n g f a u l t s , small v e i n l e t s and t e n s i o n g a s h e s . R e s u l t s of t h e g e o m e t r i c a l a n a l y s i s are resumed i n F i g . 6 . The s t r i k e a z i m u t h s r a n g e between N115E and N180E, t h e most f r e q u e n t l y measured o r i e n t a t i o n being N160-165E ( F i g . 6-11 (it must be n o t i c e d t h a t t h e f r e q u e n - c y a n a l y s e s h a v e b e e n pe r fo rmed w i t h o u t w e i g h t and t h a t t h e r e s u l t i n g N160-165E m a j o r 0 r i e n t a t i o n . h l a r g e l y d e p e n d e n t o f t h e numerous s u b m e r i d i a n cemented v e i n l e t s ) . F i g u r e 6-3 shows t h a t a l l t h e f a c t u r e s a r e s t e e p l y d i p p i n g and t h a t 95% o f t h e f a u l t s have a d i p o v e r 4 5 0 .

4 3


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F i g u r e 4 . - S k e t c h o f t h e P u n t a Gorda c l i f f showing an i m p o r t a n t '

d e x t r a l f a u l t p l a n e s e p a r a t i n g s a n d y f l u v i a l / e s t u a r i n e d e p o s i t s (1) and f i n e r i n t e r l a y e r e d e s t u a r i n e and t i d a l f l a t s s e d i m e n t s (2) (Middle P l e i s t o c e n e ? ) . The f a u l t s is s e a l e d by red b r o w n i s h and g r e e n c l a y e y d e p o s i t s ( 3 ) o v e r l a i n a c o q u i n a e x c l u s i v e l y made of Ch ione s h e l l s , a t t r i b u t e d t o t h e e a r l y L a t e P l e i s t o c e n e ( h i g h e s t sea l e v e l of t h e l a s t i n t e r g l a c i a l ) ( 4 ) .

sw

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F i g u r e 5.- C r o s s s e c t i o n n o t h e a s t o f Boca e l Abr igo showing f l e x u r e d and f a u l t e d main d e l t a i c s e q u e n c e (Middle P l e i s t e c e n e ) o v e r l a i n i n a n g u l a r u n c o n f o r m i t y by a l l u v i a l d e p o s i t s of p r o - b a b l e L a t e P l e i s t o c e n e a g e ( h a t c h e d ) .

4 4

rEL GOLFO DE SANTA CLARAS,

2 -- F i g u r e 6 . - G e o m e t r i c a l a n a l y s i s of f a u l t s : d i s t r i b u t i o n of s t r i k e (1) d i p O r i e n t a t i o n ( 2 ) and d i p v a l u e s (3) f o r 282

f r a c t u r e s i n c l u d i n g f a u l t s , t e n s i o n g a s h e s and s m a l l cemented v e i n l e t s . Diagram 4 c o r r e s p o n d s t o t h e d i s t r i b u t i o n of r a k e s f o r t h e 52 r e g i s t e r e d sli .cckenside l i n e a t i o n s .

. wan " b l l I

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' a l e o s t r e s s a n a l y s i s o f s t r i a t e d f a u l t p l a n e s i n t h e F i g u r e 7.- main d e l t a i c s e q u e n c e . --

1- P l o t o f measurements on S c h m i d t ' s l o w e r hemisphe re - p r o j e c t ï o n : f a u l t p l a n e s a s c u r v e s , and s l i c k e n s i d e l i n e a t i o n s a s d o t s w i t h a r r o w s ; 2 D e t e r m i n a t i o n of d i r e c t i o n s of compa t i - b i l i t y of Q1 ( low p e r c e n t a g e s ) and flg ( h i g h p e r c e n t a g e s ) ; w h i t e open a r r o w s i n d i c a t e G3 and b l a c k a r r o w s i n d i c a t e c l .

4 5


To estimate p a l e o s t r e s s f i e l d from f a u l t measurements e a c h f a u l t p l a n e must be d e f i n e d by s t r i k e , d i p , r a k e of s t r i a e and s e n s e of s l i p ( A n g e l i e r , 1 9 7 9 ) . U n f o r t u n a t e l y t h e d e l t a i c s a n d s a r e p o o r l y cemented and s l i p p l a n e s r a r e l y bear s l i - c k e n s i d e li- n e a t i o n s . Thus o n l y 52 c o m p l e t e l y d e f i n e d f a u l t mechanisms w i t h c l e a r s e n s e o f s l i p h a v e been r e g i s t e r e d . Rake of s t r i a e a r e d i s t r i b u t e d i n two famil ies ( F i g . 6-41: a . s t r i k e - s l i p f a m i l y w i t h r a k e s c o m p r i s e d between O and 1 0 0 , and a no rma l d i p - s l i p f a m i l y w i t h r a k e s o v e r 700. A f e w f a u l t s h a v e o b l i q u e s l i p s and most of t h e s t r i k e - s l i p movements a r e d e x t r a l . No r e v e r s e f a u l t was f o u n d , and f a u l t s w i t h a p p a r e n t r e v e r s e o f f s e t g e n e r a l l y p r e s e n t i m p o r t a n t h o r i z o n t a l l a t e r a l d i s p l a c e m e n t s .

The t w o f a u l t fami l ies h a v e d i s t i n c t a v e r a g e o r i e n t a t i o n s ( F i g . 7 ) : normal f a u l t s s t r i k e N135-180E, w h i l e s t r i k e s l i p f a u l t s have a p r e d o m i n e n t N135E o r i e n t a t i o n . F i e l d o b s e r v a t i o n s d o n o t p e r m i t t o s e p a r a t e i n t i m e t h e two f a m i l i e s which t h u s seem t o be penecon temporaneous . N ,eve r the l e s s , s e p a r a t e mechan ica l p r e s s u r e and t e n s i o n d i h e d r a a n a l 5 6 s ( A n g e l i e r , 1 9 7 9 ) i n d i c a t e t h a t t h e two famil ies c o u l d r e s u l t from two r a t h e r d i s t i n c t stress régimes ( F i g . 7 ) . On one hand , t h e normal f a u l t s a r e re la ted t o an e x t e n s i o n a l stress f i e l d w i t h v e r t i c a l ma jo r p r i n - c i p a l stress ( c1) and h o r i z o n t a l l e a s t p r i n c i p a l stress ( C3); o r i e n t a t i o n of 6, is p o o r l y c o n s t r a i n e d and c a n r a n g e between N40E and N85E ( F i g . 7-11. On t h e o t h e r hand , s t r i k e - s l i p f a u l t s a r e r e l a t e d t o a be t te r d e f i n e d stress r é g i m e where b o t h Cl and c3 a r e h o r i z o n t a l and r e s p e c t i v e l y o r i e n t e d N05E and N95E.

T h i s l a t t e r stress régime is i n good ag reemen t w i t h f o r m e r deter- m i n a t i o n s from foca l mechanism of seisms ( J o h n s o n & Hadley , 1 9 7 6 ; F u i s & Schnapp, 1977 ; A l b o r e s e t a l . , 1 9 8 0 ) and p r o b a b l y r e p r e s e n t t h e r e g i o n a l t e c t o n i c régime t h a t o p e r a t e d d u r i n g most o f t h e P l e i s t o c e n e and t h a t i s st i l l a c t i v e nowadays.

The e x t e n s i o n a l mechanism a s s o c i a t e d w i t h normal f a u l t s i s somewhat p r o b l e m a t i c . It i s common t o o b s e r v e d i s t e n s i v e and s t r i k e - s l i p mechanisms i n a s s o c i a t i o n o r i n c l o s e s u c c e s s i o n ( A n g e l i e r & B e r g e r a t , l 9 8 3 ) , b u t g e n e r a l l y t h e s e m o t i o n s have t h e same c3 o r i e n t a t i o n . I n t h i s s t u d y , t h e r e is 3 los ! t o 5 5 0 a n g u l a r d i s c r e p a n c y between t h e two computed l e a s t p r i n c i p a l stresses. Two h y p o t h e s e s c a n be f o r m u l a t e d t o e x p l a i n t h i s d i s - c r e p a n c y ; e i t h e r 1) t h e two sets of f a u l t s r e s u l t from two i n - d e b e n d e n t t e c t o n i c e p i s o d e s , o r 2 ) t h e two s e t s of f a u l t s were c r e a t e d penecon temporaneous ly , i n t h e same r e g i o n a l s t r i k e - s l i p régime, and l o c a l stress v a r i a t i o n s (due t o ma jo r h o r i z o n t a l d i s - p l acemen t o r d e e p r e a c t i v a t e d s t r u c t u r e ) i n d u c e l o c a l normal f a u l t i n g . F u r t h e r measurements and f a u l t a n a l y s i s w i l l be nec- e s s a r y t o u n d e r s t a n d t h e p r e c i s e r e l a t i o n s h i p between no rma l and s t r i i k e s l i p f a u l t s .

N e v e r t h e l e s s s e v e r a l f e a t u r e s l e a d u s t o i n t e r p r e t t h e whole f a u l t p a t t e r n a s a s econd o r d e r R i e d e 1 s h e a r zone - - * * . I

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C o l l e t t a & O r t l i e b .'

normal f a u l t s a s s o c i a t e d w i t h t h e o f f s h o r e p r o l o n g a t i o n o f t h e , major C e r r o P r i e t o F a u l t ( F i g . 8 ) . Among t h e s e f e a t u r e s , w e s h a l l

ment ion t h e f o l l o w i n g : '

- The main f a u l t s have a l e f t s t e p p i n g e n é c h e l o n p a t t e r n , and s t r i k e o b l i q u e l y , w i t h an a n g l e of a b o u t 1 5 0 , t o t h e C e r r o P r i e t o F a u l t d i r e c t i o n ;

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

( F i g . 3 and 4 1 , which h a s a h o r i z o n t a l l a t e r a l th row of s e v e r a l t e n s of meters, no m a j o r d i s p l a c e m e n t s were r e c o g n i z e d . It s h o u l d be n o t i c e d t h a t t h e P u n t a Gorda F a u l t i s n e a r l y p a r a l l e l t o t h e C e r r o P r i e t o t r a n s f o r m f a u l t .

l e s s n o r t h - s o u t h and make an a n g l e of 30-459 w i t h t h e C e r r o P r i e t o F a u l t .

- S e v e r a l f a u l t s have s i g m o i d a l t r a c e s and t e r m i n a t e n o r t h -

- Except f o r t h e d e x t r a l s t r i k e s l i p f a u l t a t Pun ta Gorda

- T e n s i o n g a s h e s and s m a l l cemented v e i n l e t s s t r i k e more o r

I n c o n c l u s i o n , w e i n t e r p r e t t h a t t h e f a u l t s c u t t i n g t h e s o u t h - w e s t e r n margin of t h e Mesa of Sonora n e a r Go l fo d e S a n t a C l a r a w e r e p roBably g e n e r a t e d d u r i n g a major s t r i k e - s l i p e p i s o d e a l o n g t h e C e r r o P r i e t o F a u l t (which e n t e r s t h e g u l f a t Boca e l A b r i g o ) . A s normal and s t r i k e - s l i p f a u l t s a r e a l l s e a l e d by p r o b a b l e ( e a r l y ) L a t e P l e i s t o c e n e a l l u v i a l d e p o s i t s ( n o r t h of Go l fo de S a n t a - C l a r a ) o r l i t t o r a l s e d i m e n t s ( t h e Chione c o q u i n a of P u n t a Gorda-Bahia Adai r a r e a ) i t c a n be e s t a b l i s h e d t h a t most of t h e s e s t r u c t u r e s a r e p r e s e n t l y i n a c t i v e .

D e f o r m a t i o n s r e g i s t e r e d by t h e L a t e P l e i s t o c e n e d e p o s i t s .

F i g u r e 8.- I n t e r p r e t a t i o n s k e t c h o f t h e o b s e r v e d f a u l t p a t t e r n a s a s e c o n d a r y R i e d e 1 s h e a r zone r e l a t e d t o t h e o f f s h o r e con- t i n u a t i o n of t h e m a j o r C e r r o P r i e t o F a u l t . R e g i o n a l s t r i k e - s l i p stress f i e l d w i t h 6 and c3 h o r i z o n t a l and r e s p e c t i v e l y o r i e n - 1 ted N-S and E-W is s u p p o s e d t o i n d u c e l o c a l d i s t e n s i v e mechanisms w i t h G3 o r i e n t e d NE-SW t o E-W.

The L a t e P l e i s t o c e n e d e p o s i t s , which a r e unconformable on t h e o l d e r s e q u e n c e and which s e a l t h e p r e v i o u s l y ment ioned s t r u c t u r e s , a r e seldom and s l i g h t l y deformed ( F i g . 5). The o n l y k i n d of d e f o r m a t i o n f e a t u r e s o b s e r v e d i n t h i s u n i t a r e NNW-SSE t o NW-SE normal f a u l t s w i t h metric th rows : t h i s younger e v e n t is e s s e n t i a l l y d i s t e n s i v e . No e v i d e n c e of l a t e r a l movement was found .

V e r t i c a l movements

A series of NNE-SSW o r i e n t e d t o p o g r a p h i c a l c r o s s - s e c t i o n s ( F i g . 9 ) shows t h a t t h e s t u d i e d a r e a a p p e a r s a s an e l o n g a t e d dome p a r a l l e l t o t h e C e r r o P r i e t o F a u l t , w i t h a maximum e l e v a t i o n of a b o u t 150 m n e a r Go l fo d e S a n t a C l a r a . An e s t i m a t i o n of t h e a m p l i t u d e and r a t e of t h e L a t e ' Q u a t e r n a r y v e r t i c a l m o t i o n s c a n b e o b t a i n e d by l a t e r a l c o r r e l a t i o n s of t h e e l e v a t e d s h o r e l i n e s remnants c o e v a l w i t h t h e b e g i n n i n g of t h e l a s t i n t e r g l a c i a l . Such remnants have been i d e n t i f i e d i n t h e whole Gulf of C a l i f o r - n i a and p a r t i c u l a r l y s o u t h w e s t a f t h e p r e s e n t d e l t a and s o u t h e a s t of Bahia Adai r ( H e r t l e i n & Emerson, 1956; Walker & Thompson, 1968 ;

l

Col 48

e t t a & O r t l i e b

F i g u r e 9.- S e r i e s o f t o p o g r a p h i c a l c r o s s - s e c t i o n s ( l o c a l i z a t i o n i n f i g . 3 ) showing t h e doming of t h e Mesa of S o n o r a i n t h e a r e a of G o l f o de S a n t a C l a r a ( v e r t i c a l s c a l e x 1 0 ) .

4 9


O r t l i e b & M a l p i c a , 1 9 7 8 ; Malp ica e t a l . , 1 9 7 8 ; O r t l i e b 1 9 8 2 b ) . I n t h e n o r t h e r n Gulf of C a l i f o r n i a , t h e s t u d y of m a r i n e t e r r a c e s showed t h a t t h e paleo-mean s e a l e v e l of t h e maximum of t h i s t r a n s g r e s s i o n i s r e g i s t e r e d a t a b o u t +8 m above t h e p r e s e n t s i m i l a r datum ( O r t l i e b , 1 9 8 2 b ) ( F i g . 1) . I n t h e d e l t a i c p l a i n of t h e r i o C o l o r a d o , t h e p r i n c i p a l i n d i c a t o r s of a f o r m e r h i g h s e a l e v e l a r e u p p e r t i d a l f a l t d e p o s i t s and f i n e f l o o d p l a i n s e d i m e n t s which a r e f o u n d a t t h e f o o t of t h e s i e r r a s Cucapas and E l Mayor on t h e r i g h t bank of t h e r i v e r ; t h e s e d e p o s i t s , o u t c r o p p i n g a t u p t o t e n meters above t h e p r e s e n t d e l t a i c p l a i n i n d i c a t e t h a t t h e r i o C o l o r a d o had i t s b a s e l e v e l a b o u t t e n meters above t h e p r e s e n t o n e . These s e d i m e n t s a r e presumably of e a r l y L a t e P l e i s t o c e n e age .

Along t h e s o u t h e a s t e r n Mesa of S o n o r a , t h e bes t d e f i n e d l i t t o r a l f a c i e s and t h e most r e c e n t P l e i s t o c e n e m a r i n e u n i t i s t h e Chione c o q u i n a . T h i s u n i t is o b s e r v e d a t i n c r e a s i n g e l e v a - t i o n s from t h e e a s t t o t h e w e s t : +10 m i n s o u t h w e s t e r n Bah ia Ada i r + 15 m s o u t h w e s t of S a l i n a Grande, and +25 m a t P u n t a Gorda. We assume t h a t t h i s u n i t is c o r r e l a t i v e w i t h t h e +8 m s h o r e l i n e s i n t h e San F e l i p e and t h e P u e r t o Penasco a r e a ( f i g . 1) .

--

These l a t e r a l c o r r e l a t i o n s o f t h e l a s t i n t e r g l a c i a l s h o r e - l i n e s , document a r e c e n t t e c t o n i c u p l i f t of t h e P u n t a Gorda a r e a . With r e s p e c t t o t h e +8 m datum of b o t h t h e San F e l i p e and P u e r t o Penasco a r e a s , t h e e l e v a t i o n of t h e Chione c o q u i n a a t P u n t a Gorda s u g g e s t s an u p l i f t a m p l i t u d e of a b o u t 1 7 meters i n 1 2 0 , 0 0 0 o r 1 4 5 , 0 0 0 y e a r s (mean u p l i f t r a t e of 130 mm/103 y e a r s ) . T h i s r e c e n t v e r t i c a l movement c o n c e r n s t h e a r e a of P u n t a Gorda, and r a p i d l y d i e s down t o w a r d s B a h i a A d a i r . Northwestwards from P u n t a Gorda, t h e r e a r e no m a r i n e s t r a t a which would e n a b l e an e v a l u a t i o n of t h e u p l i f t a m p l i t u d e , but t h e dome s h a p e and t h e +150 m maximum h e i g h t of t h e Mesa of S o n o r a n e a r G o l f o d e S a n t a C l a r a p o i n t s t o a n even s t r o n g e r u p l i f t movement t h a n a t P u n t a Gorda.

I f t h e c h a n n e l d e p o s i t o u t c r o p p i n g n o r t h of G o l f o d e S a n t a C l a r a , referred t o a s t h e younger P l e i s t o c e n e s t r a t i g r a p h i c u n i t , w e r e c o e v a l w i t h t h e Chione c o q u i n a , i t s +40 m F r e s e n t e l e v a t i o n would s u g g e s t s a maximum u p l i f t a m p l i t u d e of a b o u t 30 meters ( t a k i n g i n t o acccmnt t h e h i g h t i d a l r a n g e of t h e n o r t h e r n Gulf of C a l i f o r n i a ) , d u r i n g t h e l a s t 1 2 0 , 0 0 0 y e a r s . Even i f s u c h amount of u p l i f t h a s been o c c u r r i n g i n the l a t e Quaternary, :he main v e r t i c a l m o t i o n s r e s p o n s i b l e f o r t h e doming of t h e s o u t h w e s t e r n b o r d e r of t h e Mesa of Sonora p r e d a t e t h e L a t e P l e i s t c c e n e . These Middle P l e i s t o c e n e m o t i o n s are most p r o b a b l y penecontemporaneous with the above mentioned t h e f a u l t i n g and f l e x u r i n g e p i s o d e and have been a c t i v e i n t h e same a r e a t h a n t h e o t h e r d e f o r m a t i o n s h e r e d e s c r i b e d . These v e r t i c a l movements a r e t h u s i n t e r p r e t e d a s some s o r t of d r a g - f o l d i n d u c e d by a r e i n f o r c e d s t r i k e - s l i p a c t i v i t y , a t t h e end of t h e Middle P l e i s t o c e n e , of t h e C e r r o P r i e t o F a u l t .

50

Collet ta & O r t l i e b

CONCLUSIONS

The deformations registered by the Pleistocene d e l t a i c sediments out- cropping e a s t of t h e Rio Colorado mouth include f au l t i ng and ve r t i ca l u p l i f t . Strat igraphic interpretat ion of the d e l t a i c sequences provide a chronological framework f o r the evolution of the deformations.

Two episodes of f au l t i ng are distinguished: an older and st roger one, associating normal and s t r ike - s l ip motions, and a Younger and weaker one with only normal f a u l t s . Similarly the ve r t i ca l motions have been stronger i n the past than i n the l a t e Quaternary even i f u p l i f t r a t e s up t p 130 mm/103 years seem t o have been effect ive i n sme p a r t of the coastal area.

A l l the deformations a re re la ted t o the ac t iv i ty of t he Cerro Prieto Faul t which l i n k s t he Salton Trough (and fu r the r north the San' Andreas Fault system) and the Gulf of Cal i f -mia province. The l a t e r a l motions i n t he area of t he Rio Colorado mouth are concentrated along the Cerro Prieto Fault. The second order f a u l t pattern described i n the Golfo de Santa Clara region and the evidence of a strong u p l i f t reveal t h a t the motions along the Cerro P r i e - t o Fault were dis t r ibuted i n a much wider perturbated zone than presently, a t sane t i m e a t t he end of t he Middle Pleistocene, than presently.

Aknowledgements:

This study has been realized while both authors were invi ted investiga- t o r s a t the Estación Regional d e l Noroeste, I n s t i t u t o de Geología, U.N.A.M. (Hermosillo, Son.). B.C. benef i ted from the ATP "Géodynamique I & II from the C.N.R.S. (France); L.O. was developing t h e GEOCORTE2 Program (O.R.S.T.O.M. I n s t i t u t o Geología U.N.A.M.). The authors thank f o r their appreciated collabo- r a t ion J. Angelier, O. Carranza C., S. Celis G. , and M.T. Zambrano.

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Contributed papers for the Symposium on Neotectonics and sea level variations in the Gulf of California area, held in Her- mosillo, Sonora, Mexico, April 21-23,

1984.

Contribuciones para el Symposium sobre Neoteccónica y variaciones del nivel del mar en el área del Golfo de California, Ile- vado a cabo en Hermosillo, Sonora, Mé-

xico, 21-23 de abril de 1984.

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Edited by Editado por L

t- V. Malpica-CJuz S. Celis-Gutiérrez

J. Guerrero G x í a L. Ortlieb

Published by Publicado por

Instituto de Geología Universidad Nacional Autónoma de México

México, D. F.

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in: neotectonics and sea level vnriotionr in the 71-23

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