Andean reactivation of the Cretaceous Salta rift, northwestern Argentina

8/10/2019 Andean reactivation of the Cretaceous Salta rift, northwestern Argentina

1/22

Journa l of South American Earth Sciences Vol. 4. No. 4, pp. 351 372, 1991 089 5-98 } 1/91 $3. 00+ .00

Printed in Great Britain , 1991 Pergamon Press plc

& Earth Sciences & Resources Institute

A n d e a n r e a c t i v a t i o n o f t h e C r e t a c e o u s S a l t a r if t

n o r t h w e s t e r n A r g e n t i n a

M. E. GRIER 1, J. A. SALFITY2, a n d R. W. ALLMENDINGER ~

14-86 Nelson Street, K ingston, Ontario K7L 3W8, Canada; 2Facultad de Ciencias Naturales, Univer sidad

Naciona l de Salta, 4400 Sa lta, Reptlblica de Argenti na; ~Department of Geological Sciences, Corncll

Unive rsity , Ithaca, NY 14853-1504, USA

Received May 1991; Accepted November 1991)

Abstra ct--- Throug hout the Andes, foreland geometries are correlated with the or ientation of the sub-

ducting Nazca Plate: fold-and-thrust belts with steep subduction and basement uplifts with flat. The

geometries observed in the southern Cordillera Oriental and no rthern Sierras Pampeanas do not fit this

pattern. Instead, in version of the Cretaceous Salta Rift Basin and mechanical differences between rift and

non- rift domains are proposed as the pr imary controls on both the t imin g of late Tertia ry uplift and defor-

mation, and foreland geometries. The influence of the rift basi n is documented through field observations of

structures and lithologies, kinematic analysis of minor fault data, and published data on local stratigraphy.

The southe rn Cordillera Oriental developed within the southwestern subbasin of the Salta rift and is a

basement-i nvolved fold-and-thrust belt. The Sierras Pampeanas developed to the south of the rift and are

basemen t uplifts. Dominant structures in both regions are N/S-trending reverse or thrust faults. They are

cut by oblique strike -slip faulL~. Older deformation is Mio-Pliocene in age a nd is characterized by thrus t

kinematics with E-W to NW-SE shortening. Younger deformation is Plio-Quaternary in age and is char-

acterized by strike-slip kinematics with NE-SW shortening, except ahmg the boundary between the

Cordillera Oriental and the Sierras Pampeanas where thrust kinematics with N-S shortening prevail. The

simi lar ki nematics but di fferent geometries in the two provinces durin g Mio-Pliocene deformation and the

anomalous thrust kinematics observed during Plio-Quaternary deformation suggest that the Salta rift is

the mai n control on stru ctur al geometries. A rift invers ion model is developed and applied to the souther n

Cordillera Oriental.

Re su me n- -A lo largo de los Andes, la geometria estruc tural del antepais generalmente se correlaciona con

la incl inac i6n de subducci6n de la placa Nazca. Las franjas de corrimientos se encu ent ran asociadas con

subducci6n de alto ~ngulo y los levan tami entos de basamento con subducci6n de bajo ~ngulo. En el noroeste

argentino entr e la Cordillera Oriental austral y las Sierras Pampeanas septentrionales, las geometrias no

siguen el modelo. En cambio, para esta zona proponemos que la inver si6n estructural de la cuenca rift

cret~cica de Salta y la diferencia mec~nica entre terrenos con y sin estructuras de rift controlan tanto el

tiempo de levanta miento terciario como la geometria estructural del antepais. La influencia de la cuenca

rift se muestra por observaciones estructurales y litol6gicas de campo, an~lisis quinem~tico de fallas, y

datos publicados de la estrati graf ia local. La Cordillera Orient al austra l se desarroll6 en la subcuenca sud-

occidental del rift Salt a y se clasifica como una fran ja de corrimientos que involucran al basamento. Las

Sierras Pampoanas septentrionale s se desarrollaron al sur del rift y son levantamientos de basamento. Las

estr uctu ras predominan tes en ambas regiones son corrimientos meridiona les de alto o bajo ~ngulo. Fall as

de desplaza miento de rumbo cortan oblicuamente a los corrimientos. La deformaci6n ante rior tiene edad de

Mioceno a Plioceno y se caracteriza por la quinem~tica de corrimiento con acortamiento este-oeste a

noroeste-sudeste. La deformaci6n posterior tiene edad de Plioceno a Cuate rnar io y se caracteriza por la

quinemgttica de desplazamiento de rumbo con acortamiento noreste-sudoeste, excepto por la frontera entr e

la Cordillera Orie ntal y las Sierras Pampeanas en la cual predomina la quinem~tica de corrimiento con

acortamiento norte-sur. Las quinemfiticas semejantes y geometrias distintas de l s dos provincias durante

la deformaci6n mioc6nica-plioc6nica y la quinem~tica an6mala de corrimiento entre las dos provincias

dura nte la deformaci6n plioc6nica-cuaternaria sugieren que el rift Salta control6 la geometria estructural.

Desarroll amos un modelo de inversi 6n estructu ral y lo aplicamos a la Cordillera Orien tal austral.

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

THE PRESENT BENIOFF ZONE be ne at h the Ce nt ra l

Andes i s s egmen t ed i n t o r eg i ons o f s t eep and f l a t

subd uct ion (F ig . 1) (Ba raz ang i an d Isacks , 1976) .

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

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

a r e d e v e l o p e d i n t h e f o r e l a n d ( J o r d a n e t a l . , 1983).

R eg i ons o f s t eep s ubduc t i on a re co r r e l a t ed wi t h t h i n -

*Work done while at Cornell University.

Address l l correspondence and repr int requests to:

Dr. Martha E. Grier; telephone [1] (613) 542-3656.

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

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

men t up l i f t s . The geo l ogy o f no r t hwe s t e rn Argen -

t i na , however , s ugges t s t ha t t h i s co r re l a t i on may be

over l y s i mp l i s t i c i n s ome pa r t s o f t he C e n t ra l Andes

and t ha t p re -ex i s t i ng s t ruc t u res con t ro l t he s t y l e of

f o r e l a n d d e f o r m a t i o n ( A l l m e n d i n g e r e t a l . , 1982;

A l l m e n d i n g e r e t a l . , 1983).

To the no r th of 24S , the fore land co ns i s t s of the

C ord i l l e ra Or i en t a l and t he S i e r ras S uband i nas (F ig .

2). Thes e moun t a i n s ys t ems a re N/ S - t r e nd i ng , domi -

n a n t l y e a s t - v e r g i n g f o l d - a n d - t h r u s t b e l ts w i t h l o ca l

wes t -ve rg i ng back - t h rus t s . To t he s ou t h o f 24 S,

bas eme n t i s i nvo l ved i n t he de fo rmat i o n , t he C or -

35


2/22

352 M. E. GRIER, J. A. SALFITY, and R. W. ALLMENDINGER

14

t~

t

O

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74 62

c -

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Fig. 1. The Cent ral Andes showing the Altip lano /Puna Plat eau and the major geological provinces of the foreland. The contours

show the depth to the s ubduc ting Nazca plate i n kilometers (from Isacks, 1988); regions of steep and flat subduction are indicated.

dillera Oriental broadens markedly, and zones of

both west- and east-verging thrusting are common

both in the Cordillera Oriental and in the southern

extension of the Sierras Subandinas known as the

Sistema de Sa nta B~rbara (Mon, 1976; Rolleri,

1976). In th e reg ion of 2615'S, across a NW/SE-

trending boundary, the geometries of the fold-and-

thrust belt change to the thick-skinned basement

uplifts of the Sierras P ampe anas ( Pampean Ranges).

To the no rth of 24S and to the south of 27S, the

correlation between foreland geometries and the

orientation of the subducting Nazca plate holds.

However, between 24S and 27S, the correlation is

confused, if not inconsistent, in that the complex

foreland overlies a broad, gentle flexure in the

subducting plate (Fig. 1) (Bevis and Isacks, 1984).

No short wavelength change in the contours of the

Benioff zone is observed tha t can explain the abrupt

change from fold-and-thrust geometries to basement

uplifts that occurs in t he region of 2615'S (T. Cahill,

pers. comm., 1989). In addition, underly ing plate

geometries do not account for the broadening of the

Cordillera Oriental, the basement involvement in

the deformation, and the zones of reversed vergence

within the thrust belt that are observed immediately

to the north of the st ructural transition.

Field observations in the southern Cordillera

Oriental suggest instead that foreland development

has been controlled by reactivation of pre-existing

structures. The present-day southern Cordillera

Oriental and Sistema de Santa B~rbara developed

within the southern Alemania and Met~n subbasins

of the Cretaceous Salta rift basin (Figs. 2 and 3). The

vergence of Andean st ructures tra cks the basin mar-

gins: in the southern Cordillera Oriental, on the

west side of the rift, Andean faults are west-verging;

in the southern Sistema de Santa B~rbara, on the

east side of the rift, Andean faults are east-verging.

Vergence changes within these regions may also be

inheri ted from rift structures. In addition, reacti-

vated normal faults are observed along the seuth-

western rift margin. Finally, the abrupt N-S struc-


3/22

Andea n reacti vation of the Cretaceous Salta rift, northwes tern Argent ina 353

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F i g . 2 . T h e g e o lo g i c al p r o v i n c e s o f n o r t h w e s t e r n A r g e n t i n a .

tural transition bet ween the Cordillera Oriental and

the Sie rras Pa mpea nas occurs across the rift margin:

the fold-and-thrust geomet ries of the southern Cord-

illera Oriental and the Sistema de Santa B~rbara

are developed within the rift and the basement up-

lifts of the Sierras Pampeanas are developed to the

south and west of the rift.

A simple rift inversion model, in which Andean

shortening is sub-parallel to rift extension, best ex-

plains the st ructural geometries that are observed in

this part of north west ern Argen tin a (Fig. 4). The

model assumes that crustal extension during the

Cretaceous was ac commodated along listric normal

faults that sole into a zone of quasi-plastic decoup-

ling at a depth of 10-12 km. It furt her assumes that,

during Andean shortening from Miocene to Recent

S A E S ~ 4 / ~ - - F

times, the basal decollement of the deformation re-

used the substructure of the old rift and that listric

normal faults were reactivated in their entirety as

thrus t faults. The model predicts tha t the foreland

has been shortened 25% during Andean deforma-

tion.

RIFT REACTIVATION

In order for old structures to have controlled

later structural geometries they must not only exist

but have been favorably oriented for reactivation

during latest deformation. In any area in which the

possibility of fault reactivat ion arises, several ques-

tions must be answered.


4/22


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6/22

3 5 6 M . E . G R IE R , J . A . S A L F I T Y , a n d R . W . A L L M E N D IN G E R

Qu a te rn a ry t :

Miocene

- Pl iocene

a l eocene

- Eocene

Late

Cre taceous

Early I

Cre taceous

Late Precambr ian

-Ear ly Camb r ian

surficial alluvial deposits

Andean fo re land bas in s t ra ta

(Payogas t il la and Santa M ar ia Groups)

Santa B rbara Subgroup

Ba lbuena Subgroup

M 3 : 6 3 - + 2 M a

Intrusive)

irgua Sugroup

M 2 : 7 6 . 4 3 .5 a a

to 78 _+ 5 M a

Salta

Group

M I : 9 7 5 M a t o

I

28 5 M a - - I

Pu nco viscan a Fo rm ation and I o. 1,0oom

metamorphic equivalents

Fig. 5 . St ra t igraphy of the so uthern Cordi l le ra Or ienta l and n or thernmo st Sier ras Pampean as be twee n 2515 'S and 2630 'S. M1,

M2, and M3 represent three phase s of magm at ism. In th is region , M1 and M2 are rep resented by vo lcanic rocks, and M3 by

int rus ives . A ges of vo lcanic and igneou s rocks f rom Gal l i sk i and Viram onte (1988) .

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

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

t i o n ( M o n , 1 9 7 9 ) .

T h e P u n c o v i s c a n a F o r m a t i o n . T h e o l d e s t ex -

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

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

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

b o n a t e s ( F i g . 5) ( O m a r i n i , 1 98 3) . P r o v e n a n c e o f t h e

e l a s t i c s w a s t o t h e e a s t ( J e z e k a n d M i l le r , 1 9 85 ). T h e

s e q u e n c e is la t e P r e c a m b r i a n t o C a m b r i a n i n a g e

( A c e f o l a z a , 1 9 79 ) a n d , o n t h e b a s i s o f i t s s t r a t i -

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

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

t h e s t a b l e p r o t o - P a c i f ic m a r g i n o f G o n d w a n a ( J e z e k

a n d M i l l e r , 1 9 85 ).

T h e s e q u e n c e w a s d e f o rm e d a n d m e t a m o r p h o s e d

p r i m a r i l y d u r i n g t h e e a r l y P a l e o z o ic (W i l l n e r

e t a l .

1 98 7) . I n t h e L a t e C a m b r i a n , t h e c o n t i n e n t a l

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

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

i n g . S h o r t e n i n g c o n t i n u e d i n t o t h e O r d o v i c ia n w i t h

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

l a r g e - s c a l e c r u s t a l i m b r i c a t i o n ( W i l l n e r e t a l . 1 9 8 7 ) .

I n t h e l a t e s t O r d o v i c ia n , t h e r e g i o n w a s p r o b a b l y t h e

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

s e n t e d b y th e F a j a E r u p t i v a d e la P u n a ( B ah l b u r g ,

1 98 9). H o w e v e r , w e s p e c u l a t e t h a t , t o t h e e a s t o f t h e

F a j a E r u p t i v a , t h e f i n a l e x t e r n a l f o r m o f t h e P u n c o -

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

o f s u b m e r i d i o n a l t re n d ; t h e f in a l i n t e r n a l f o r m t h a t

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

c a n a F o r m a t i o n h a s b e e n m o d i f ie d s in c e t h e e n d o f

t h e O r d o v i c i a n b y e r o s io n a n d b y s u b s e q u e n t d e -

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

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

m e t a m o r p h i c z o n e s (F i g . 6) ( W i l l n e r a n d M i l le r ,

1 9 8 5 ) .

T h e S a l t a B a s i n .

T h e S a l t a G r o u p r e p r e s e n t s a

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

P u n c o v i s c a n a w e d g e (F i g s . 3 a n d 5 ) ( B i a n u c c i a n d

H o m o v c , 19 8 2 ; S a l f i t y , 1 98 2 ). T h e b a s i n ' s f o r m c o u l d

h a v e b e e n c o n t r o ll e d b y s t r u c t u r e s w i t h in t h e w e d g e


7/22

Andea n reactiv ation of the Cretaceous Salta rift, northwes tern Argent ina 357

660 65

,_ K:;i:~i:;if St u d y r e a

= A

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s t r u c t u r a l d e p t h

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Fig. 6. The tectonometamorphic ones of he PuncoviscanaFormation(modified romWillner

t a l .

1987); int = intermediate.

and, in turn, its development modified the Punco-

viscana wedge itself, creating new structures that

may have been favorably oriented for reactivation

during subs equent Andean deformation. The form of

the basin at present reflects uplift and deformation

since the end of the Cretaceous; the pre sent margi ns

may be erosional and not depositional.

This basin is one of a series th at formed during

the Cre taceous from the Atl antic coast to Peru along

a northwe sterl y tren d (Fig. 7). The series includes

the Chaco- Paranense and Salta Basins of Argentina,

and the Subandean and Andean Basins of Bolivia

and Per u (Fig. 7). The links between the Salta, Sub-

andean, and Andean Basins are well established

(Riccardi, 1988; Macellari, 1989), but that between

the Salta Basin and the Chaco-Paranense Basin is

more tenuous, being based on subsurface data (Russo

e t a l .

1979). During the late Campani an through

the Maastrichtian-Paleocene, the Salta Basin was

also linked to the back-arc Andea n Basin of north ern

Chile (Salfity e t a l . 1985; Riccardi, 1988; Macellari,

1989).

These basins were active during the Cretaceous

and early Tertiary, and their plate tectonic setting

altere d substantiall y during this time. Subduction

of the oceanic plate was established along the

western margin of Gondwana in the Jurassic, and

the e aster n margin of South America was rifted from

Africa during the Cretaceous. The basins could thus

be subduction-related back-arc basins, a failed rift

system associated with the opening of the South

Atlant ic, or a combination of the two. The overall

trend of the basin system makes a purely back-arc

origin unlikely. The basins' trend and the time-

transgressive nature of alkalin e volcanism along the

trend~128_ 5 Ma in the Salta Basin (Reyes

e t a l .

1976) and 82.5 Ma in the Andean Basin (Cherroni

Mendieta, 1977)--are consistent, however, with a

failed rift sys tem (Gallisky and Viramonte, 1985).

The Salta Basin itself consists of several sub-

basins grouped around the Salta-Jujuy high (Fig. 3).

The shape of the basin and the position of depo-

centers within the basin are thought to have been

controlled by NE/SW- and NW/SE-trending linea-


8/22

358 M . E . GR I E R , J . A . S A LF I TY, a n d R . W. ALLMENDINGE R

m e n t s (S a l fi ty , 1 98 2) . T h e b a s i n s t r a t a c a n b e

d i v i d e d i n t o s y n - r i f t a n d p o s t - r i f t s e q u e n c e s ( F ig . 5 )

( B i a n u c c i

e t a l .

1 98 1) . T h e s y n - r i f t s e q u e n c e i s

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

e n t a l c l a s t ic s o f t h e P i r g u a S u b g r o u p . T h e p o s t - r i f t

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

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

p o s t -r i ft r e g r e s s i v e c o n t i n e n t a l s e q u e n c e o f th e

S a n t a B f i r b a r a S u b g r o u p ( M o r e n o , 1 9 7 0 ; S a l f i t y ,

19 82 ). R i f t i n g t o o k p l a c e f r o m t h e N e o c o m i a n to

C a m p a n i a n , a n d t h e p o s t - r i f t s t r a t a w e r e d e p o s i t e d

f r o m th e l a t e C a m p a n i a n t o m i d d l e E o c e n e (M a r -

q u i l l a s a n d S a l f i t y , 1 98 8 ).

T h e f o r m a t i o n s t h a t c o m p r i s e th e s y n r i ft P i r g u a

S u b g r o u p a r e t h o u g h t to r e p r e s e n t p r o x i m a l a l lu v i a l

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

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

f a n a p r o n ( M o r e n o , 1 97 0; R e y e s a n d S a l f i ty , 1 9 73 ;

S a l f i t y a n d M a r q u i l l a s , 1 9 81 ). T h e f a c i e s c o m b i n a -

t i o n i s c o n s i s t e n t w i t h d e p o s i t io n i n f a u l t - b o u n d e d

b a s i n s ( S a lf i t y , 1 98 2 ). T h e o r i e n t a t i o n a n d s h a p e o f

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

t h e p r o x i m a l f a n f a c ie s . O n t h e s o u t h w e s t s id e o f t h e

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

o f t h e L a Y e s e r a F o r m a t i o n a r e o v e r 1 00 0 m e t e r s

t h i ck n e a r t h e m a r g i n s o f t h e b a s i n b u t a r e a b s e n t

f r o m t h e s e q u e n c e i n t h e c e n t e r o f t h e b a s i n ( F i g . 8).

I n b o t h t h e A l e m a n i a a n d M e t f i n s u b b a s i n s , t h e s e

F i g . 7. U p p e r m o s t C r e t a c e o u s b a s i n s o f w e s t - c e n t r a l S o u t h A m -

e r i c a sh o w i n g s y n - r i f t s e q u e n c e s in n o r t h e r n A r g e n t i n a a n d

B o l i v i a ( m o d i f i e d f r o m M a r q u i l l a s a n d S a l f i t y , 19 88 ): S B , S a l t a

B a s i n ; C H PB , C h a c o - P a r a n e n s e B a s i n ; A B , A n d c a n B a s i n ; S A B ,

S u b a n d e a n B a s i n ; B A B , b a c k - a r c b a s i n .

f a c i e s a r e e l o n g a t e N - S o r , b u t w i t h l e s s f r e q u e n c y ,

E - W ( M o r e n o , 1 97 0; G b m e z O m i l

e t a l .

1 9 8 9 ) . G i v e n

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

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

t r e n d s a n d w e r e o f fs e t b y E / W - t r e n d i n g t r a n s v e r s e

f a u l t s .

T h e s y n - r i ft f a c ie s a r e o v e r l a i n b y t h e f o r m a t i o n s

o f t h e B a l b u e n a S u b g r o u p w h i ch , in t h e s o u t h e r n

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

l a c u s t r i n e s e q u e n c e ( S a l f i t y , 1 9 8 0 ) a n d l i m e s t o n e s

d e p o s i t e d i n a s h a l l o w ( 1 0 m d e p t h o n a v e r a g e ) ,

r e s t r i c t e d b r a c k i s h b a s i n w i t h lo c a l ly h y p e r s a l i n e

a n d f r e s h - w a t e r c o n d i t i o n s ( M a r q u i ll a s , 1 9 86 ). T h e

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

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

n e s s o f 4 0 0 m e t e r s i n t h e m a j o r d e p o c e n t e r s (M a r -

q u i l l a s , 1 9 8 6 ).

T h e B a l b u e n a S u b g r o u p i s o v e r l a i n b y t h e S a n t a

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

c l a st ic s , th e e a r l i e r l a c u s t r i n e i n f l u e n c e s g r a d u a l l y

d e c r e a s i n g i n i m p o r t a n c e o v e r t i m e (F i g . 5) (M o r e n o ,

1 9 70 ; S a l f i ty , 1 98 2 ). T h e s e q u e n c e i s c a p p e d b y a n

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

r e a c h e s a m a x i m u m o f 1 50 0 m e t e r s i n t h e b a s i n

d e p o c e n t e r s ( M o r e n o , 1 97 0) .

T h e S a l t a G r o u p i n c lu d e s i n t r u s i o n s a n d v o l-

c a n i cs r e p r e s e n t i n g t h r e e p h a s e s o f m a g m a t i s m (F ig .

5 ) ( R e y e s

e t a l .

1 9 7 6 ; B i a n u c c i

e t a l .

1 9 8 1 ; B e r c -

k o w s k i , 1 98 7). T h e f i r s t e v e n t o c c u r r e d f r o m m i d d l e

N e o c o m i a n t o l a t e A l b i a n t i m e a n d i s r e p r e s e n t e d b y

i n t r u s i v e s i n t h e T r e s C r u c e s s u b b a s i n o f t h e r i f t a n d

b y l a v a f lo w s , d i k e s , a n d m i n o r p y r o c l a s t i c fl o w s t h a t

a r e i n t e r c a l a t e d w i th t h e L a Y e s e ra F o r m a t i o n - - t h e

p r o x i m a l f a n f a c ie s o f t h e r i ft s e q u e n c e - - a l o n g t h e

m a r g i n s o f t h e s o u t h e r n s u b b a s i n s o f t h e r i ft ( R e ye s

e t a l .

1 97 6; S a l f i ty , 1 98 2). T h e i n t r u s i v e s a r e a l k a l i

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

t i o n f r o m r h y o l i t e s t o b a s a l t s ( R e y e s e t a l . 1976;

S a l f i t y , 1 98 2 ; G a l l i s k i a n d V i r a m o n t e , 1 98 8).

T h e s e c on d p h a s e o f m a g m a t i s m o c cu r re d d u r i n g

C o n i a c i a n - S a n t o n i a n t i m e a n d i s r e p r e s e n t e d p r i -

m a r i l y b y th e L a s C o n c h a s B a s a l t ( F ig s . 3 a n d 5). I t

i n c l u d e s l a v a s a n d p y r o c l a s t ic f lo w s , d i k e s , a n d s i l l s

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

a r e i n t e r c a l a t e d w i t h t h e L a s C u r t i e m b r e s F o r m a -

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

q u e n ce . T h e y o c c ur p r i m a r i l y i n t h e c e n t e r o f t h e

A l e m a n i a su b b a s i n , a l t h o u g h s o m e b a s a l t s t h a t m a y

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

a l o n g w h a t w e c o n s i de r to b e a b a s i n m a r g i n f a u lt .

T h e t h i r d p h a s e o f m a g m a t i s m o c cu r re d d u r i n g

t h e P a l e o c e n e ( F ig . 5). I t is r e p r e s e n t e d i n t h e s o u t h -

e r n p a r t o f t h e S a l t a B a s i n b y a s i n g l e l a m p r o i t i c si ll

( O m a r i n i

e t a l .

1 98 7) . T h e s il l , a p o t a s s i c t r a c h y t e ,

i n t r u d e s t h e Lo s B l a n q u i t o s F o r m a t i o n . E l s e w h e r e

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

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

( B i a n u c c i

e t a l .

1 9 8 1 ; B e r c o w s k i , 1 9 8 7 ).

T h e v o l c a n i c r o c k s o b s e r v e d i n t h e S a l t a G r o u p

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

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

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


9/22

A n d e a n r e a c t i v a t i o n o f t h e C r e t a c e o u s S a l t a r if t, n o r t h w e s t e r n A r g e n t i n a 3 59

J

f

/

23

Bo,,v,a

I

Argent ina l

Salta

Study

Area

7

T

J u ju y

Shale

Cong lomera te 50% San ds tone

Firs t cyc le

volcanics

~ S e c o n d c y c le

volcanics

Tucum&n 641 100 km 6 /o_ ]

Fig. 8 . The d is t r ibut ion of syn- r i f t conglom erate , sandston e, and shale (Pi rgua Sub group ) in the Sal ta Basin (modif ied f rom

Moreno, 1970) : A, Alema nia su bbas in; M, Met~n subba s in .

o r i g in f o r t h e S a l t a B a s i n ( B ia n u c c i e t a l . 1 9 8 1 ;

S a l f i t y , 1 98 2 ; G a l l i s k i a n d V i r a m o n t e , 1 9 8 8 ).

A n d e a n F o r e l a n d B a s i n S t ra t a .

T h e r i f t b a s i n

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

o v e r l a i n u n c o n f o r m a b l y b y 4 0 0 0- 6 00 0 m e t e r s o f s e d i-

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

t h e A n d e s M o u n t a i n s ( F ig s . 5 a n d 9 ). T h e s t r a t a

e x p o s e d i n t h e s o u t h e r n C o r d i l le r a O r i e n t a l a r e

e a r l y M i o c e n e t o l a t e P l i o c e n e i n a g e ( D i az a n d

M a l i z z i a , 1 9 8 3 ; G r i e r a n d D a l l m e y e r , 1 9 9 0 ); t h o s e

e x p o s e d i n t h e n o r t h e r n S i e r r a s P a m p e a n a s a r e l a t e

M i o c e n e t o P l i o c e n e i n a g e ( B o s s i e t a l . 1 9 8 7 ) . T h e

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

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

t h e p r e - P li o c e n e s t r a t a e x p o s e d i n t h e s o u t h e r n

C o r d i ll e r a O r i e n t al a r e m u c h c o a r s er t h a n t h o s e

e x p o s e d i n t h e n o r t h e r n S i e r r a s P a m p e a n a s G r i er

a n d D a l l m e y e r , 1 9 9 0 ) .

T h e u n c o n f o r m i ty b e t w e e n t h e A n d e a n s t r a t a

a n d t h e S a l t a G r o u p i s v a r i a b l e ( F i g . 4 ). J u s t t o t h e

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

( F ig . 9), A n d e a n s t r a t a l ie d i r e c t l y o v e r s t r a t a f r o m

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

f o r m i t y is 2 7 a n d t h e s t r a t a b e n e a t h t h e r e s t o r e d

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

i n t h e T o n c o V a l l e y , th e a n g l e o f u n c o n f o r m i t y

b e t w e e n A n d e a n a n d S a l t a G r o u p s t r a t a i s 2 -3 . T h e

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

b e g a n b e f o r e A n d e a n f o r e l a n d b a s in s t r a t a w e r e

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

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

t io n , a t l e a s t i n t h e w e s t e r n m o s t p a r t o f t h e b a s i n .

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

v a r i a b l e s u b s t r a t e . D u r i n g t h e e a r l y s t a g e s o f d e p o s i -

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

a l l y on th e w e s t s i d e. D u r i n g l a t e r d e p o s i t io n , t h e

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

d e a n d e f o r m a t io n m o v e d e a s t w a r d .


10/22

3 6 0 M .E . G R TER , J . A . SA LFITY , a n d R . W . A LLMEND IN G ER

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F i g . 9 . G e o l o g i c a l m a p o f t h e s o u t h e r n C o r d i l le r a O r i e n t a l a n d t h e n o r t h e r n m o s t S i e r r a s P a m p e a n a s .

Ge om et r y a n d K i n em a t i c s o f h e A n d e a n F o r el a n d ,

255 S to 260 S

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

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

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

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

f o u n d in t h e R o c k y M o u n t a i n s o f C a n a d a ( D a h l -

s t r o m , 1 9 6 9) . T h e f o r e l a n d h a s t h r e e i n t e r a c t i n g

c o m p o n e n t s : t h e P u n c o v i s c a n a w e d g e , t h e S a l t a ri f t

b a s i n s t r a t a , a n d t h e A n d e a n f o r e l a n d b a s i n s t r a t a ,

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

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

T h e p o s s i b i l i ty o f p a l e o t e c t o n i c c o n t r o l o n A n -

d e a n d e f o r m a t i on t h u s e x i s t s, a l t h o u g h t h e d e g r ee t o

w h i c h c o n d i t i o n s w e r e f a v o r a b le t o r e a c t i v a t i o n i s

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

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


11/22

Andean reacti vation of the Cretaceous Salta rift, northwes tern Argent ina 361

the ext ent ofpaleotectonic control. That is, given the

present-day geology, could reactivat ion of a suite of

old struc ture s or lithologic inhomogeneities account

for the configuration of modern structures, and is

ther e any di rect evidence of fault reactivation?

G e o m e t r y . The study area is divided into two

structural domains across a NW-SE trend that

crosses the Calchaqui Valley several kilometers to

the south of San Carlos (Fig. 9). The southern part of

the Cordillera Orie ntal lies to the north of this tren d

and the northern part of the Sierras Pampeanas to

the south. The struc tural geometries observed in the

two mount ain systems are different: the Sierras

Pampeanas are baseme nt uplifts and the Cordillera

Oriental is a basement-i nvolved fold-and-thrust belt.

The basement uplifts that are observed in the

Sierras Pampeanas are Precambrian to Cambrian

schists and gneisse s of the Puncoviscana Formation

or equivalents and have been uplifted along N- to

NE-stri king reverse faults. They are typically asym-

metric, with a steep eroded slope on the fault-

bounded margin. Remn ants of a peneplain are ex-

posed, in some cases, on the tilted tops of the blocks

where the Cenozoic cover has been eroded (Caminos,

1979; Jordan e t a l . 1990). The block-bounding

faults, in general , dip steeply where the y are exposed

at the surface although, locally, shallow dips have

been observed. They are observed to paral lel major

planes of schistosity (Gonz~lez Bonorino, 1950).

Their orientation at depth is uncertain, but it has

been suggested, for geometric considerations, that

the faults must flatten with depth (Gonz~ilez Bono-

rino, 1950; Jor dan and Allmendinger, 1986). The

blocks have been thru st over as much as 4000 meters

of Tertiar y and Quaterna ry strata. Minor folding

and fault ing is observed in the basin strat a, and the

terminations of the mountain-bounding faults are

often expressed as anticlines in thes e strata.

The two northernmost Pampean ranges--Sierra

de Quilmes and Cumbres Calchaquies--lie in part

with in the study are a (Fig. 9). The Sierr a de

Quilmes is uplifted along its east side by a series of

faults that are exposed only at the north end of the

block and are inferred along the length of the

nor the as te rn par t of the block (Ruiz Huidobro, 1972;

Vile la and Garcia, 1978; Galv~n, 1981). Cumbres

Calchaquies is fault bounded to the east and west

(Caminos, 1979), and a num ber of minor blocks are

observed at its nor thern end: Filo Las Minas, Filo

Paranilla and Loma Negra (Fig. 10). Remnants of

the basemen t-cappin g peneplain have been observed

on both rang es (Strecker, 1987). The ranges are

separated by the Calchaqui or Santa Maria Valley,

which is 20-25 km across.

The mountain-bounding faults trend N-S, as do

faults and fold axes that are observed in the de-

formed Tertiary strata immediately to the west of

Cumbres Calchaquies (Fig. 9). This suggests tha t

major str uctur es were produced durin g an episode of

E-W shortening. Other feat ures indicate that shor-

tening continues in a subsequent episode of deforma-

tion. Quat erna ry fans along the west side of

Cumbres Calchaquies (at the latitude of Colalao del

Valle, Fig. 9) are tilt ed 2-3 toward the moun ta in

front. A fault observed in Quebrada La Vifia at t he

northea st corner of Sierra de Quilmes has an orien-

tati on of 052/52 SE and thr ust s par t of the Quilmes

block over a Quate rna ry conglomerate. In both

cases, the Quaternary strata unconformably overlie

deformed Terti ary strata.

In contrast to the Sierras Pampeanas, the

southern Cordillera Oriental has been described as a

basement-involved fold-and-thrust belt (Vilela a nd

Garcia, 1978; Turner and Mon, 1979). Major struc-

tures are more closely spaced than they are in the

Sierras Pampeanas (10-15 km spacing as opposed to

20-30 km spacing) and the y are developed not only in

the Puncoviscana Formation and the Andean fore-

land basin strata but in strata that are associated

with the Salta rift basin.

In the study area, major faults trend N-S to

NNE-SSW and dip to the east (Fig. 9). Only one

major east-verging thrust fault--in the Amblayo

Val ley --i s observed. Most minor faul ts also have

submeridional trends but dip to either east or west.

Syn-rift Salta Group stra ta or Puncoviscana Forma-

tion are commonly exposed in the han ging wal ls of

the large-scale faults and post-rift Salta Group, and

Andean foreland basin strata in the footwalls (Fig.

9). Major fold axes parallel the faul t trends. Major

anticlines are found in the hanging walls of the

faults, and major synclines and minor anticlines in

the footwalls. Many of the folds are overs teepened

on their west-facing limbs as a result of out-of-the-

syncline thrusting, particula rly in thick sequences of

syn-rift strata. The N/S-trending folds and thru st

faults do not involve Quatern ary strata.

As in the Sierras Pampeanas, the orientation of

the dominant structures suggests that they were

produced during an episode of E-W to WNW-ESE

shortening. Again, minor cross-cutting feature s

show tha t one or more other episodes of deformation

also occurred. One set of features shows a consis tent

NW-SE to WNW-ESE trend (Fig. 9). The major fault

that bounds the east side of the Calchaqui Valley

north of Rio de Las Conchas is scalloped. The faul t

consistently steps to the west along WNW-ESE to

NW-SE trends. E/W-trending minor folds are obser-

ved in the hanging wall of the main thrust at the

corners of some of the scallops and may represent

fault terminations or transpressional features rela-

ted to later deformation. A left-lateral strike-slip

fault that runs along the Rio Calchaqui, to the east

of Angastaco, parallels the trend of these scallops

(Fig. 11). The abrupt t ermina tions of the Tonco and

Amblayo synclines are aligned along a similar tren d

(Fig. 9). On a minor scale, near-vertical , NW/SE- to

NNW/SSE-trending left-lateral strike-slip fault s are

observed in the vicinity of Mina Don Otto on the east

side of the Tonco syncl ine (Raskovsky, 1968).

Other trends are also observed in the cross-cut-

ting st ructures (Fig. 10). In Quebrada Las Chacra s a

NNE/SSW-trending, low-angle strike-slip fault cuts


12/22


Fig. 10 . Cross-cut t ing s t ruc tures in the region of Queb rada La Yesera . See Fig . 9 for locat ion.

Fig . 11 . Cross-cut t ing s t ruc tures in the reg ion of he Q uebrad a de

Piscuyacu. See Fig . 9 for locat ion.

a N / S - t r e n d i n g t h r u s t f a u l t . T h e s t r i k e - s li p f a u l t is

o r i e n t e d p a r a l l e l t o b e d d i n g a n d m a y b e a r e a c t i-

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

L a Y e s e r a , t h e E 1 Z o r r i t o r e v e r s e f a u l t t r e n d s E - W t o

N E - S W a n d c u t s a N / S - t r e n d i n g t h r u s t f a u l t . I n t h e

q u e b r a d a i ts e lf , th e m a j o r N -S f o ld s a n d f a u l t s h a v e

t h e m s e l v e s b e e n f o ld e d a b o u t a n E - W a xi s . F i n a l l y ,

a N / S - t re n d i n g f a u l t s c a r p w i t h r i g h t - l a t e r a l d i s-

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

f a n s t o t h e w e s t o f C e r r o E 1 Z o r r i t o ( F i g . 9 ).

C r o s s - c u t ti n g a n d s t r a t i g r a p h i c r e l a t io n s , a s d e s -

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

t h e p o s s i b le e x c e p t io n o f t h e s c a l l o p s i n t h e m a i n

t h r u s t , p o s t - d a t e m a j o r E - W A n d e a n s h o r t e n i n g .

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

d e f o r m a t i o n w i t h N E - S W t o N - S s h o r t e n i n g t h a t

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

i n t o t h e Q u a t e r n a r y .

K i n e m a t i c s .

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

a r e d e r i v e d f r o m t h e a n a l y s i s o f 3 04 f a u l t - s li p m e a -

s u r e m e n t s w i t h in t h e s t u d y a r e a a n d a r e c o n s i s t e n t

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

o v e r 1 2 00 m e a s u r e m e n t s f r o m a d ja c e n t a r e a s

( M a r r e t t

e t a l .

1 98 9 ; G r i e r , 1 9 90 ). T h e r e g i o n a l d a t a

s e t c o m p r is e s t w o g r o u p s o f f a u l t s t h a t a r e t h o u g h t t o

r e p r e s e n t t w o e p i s o d e s o f d e f o r m a t io n : o n e t h a t i s

M i o - P li o c e n e in a g e , a n d o n e t h a t i s y o u n g e r a n d

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


13/22

Andea n reactivat ion of the Cretaceous Salta rift, northwest ern Argent ina 363

23 -

I

1 O 0 k m v v . . ~

~~ v ~ / ~ '~ A R G E N T I N A

i::i

: : : : : 9

~ v v V v V v V v V ~

vvvvvVvVvVv '

::iiiii iii iil

V v

P u n a

Sie rra . . .~ .s

:S a : d n . a s~ : ." . i /

I

S is te m a e

S a n t a & r b a r a

~ v v v v v ~ ( ~ / J : :: ~ : i l: : S a l ta . . . . . .

i i i i i i i

t i i i i i ;

P u n a

rea

2 7

T u c u m & n

[ ~ C e n o z o i c

vo lcan ics

S i e r ras

P a m p e a n a s

I I I I I

6 8 6 6 6 4

Fig. 12. The ki nemat ics of Mio-Pliocene deformation (modified from Marrett e t a l . 1989). Kinemat ic analyses are displayed as

fault p lane solutions. Solid boxes represent ave rage s horte ning directiorL~; open boxes, average exten sion directions.

stratigraphic relations outside the study area sug-

gest that the chang e in kinematics occurred between

2.35 and 0.78 Ma (Marret t e t a l . 1989).

The kinema tics of Mio-Pliocene deformation are

homogeneous throughout the southern Cordillera

Oriental, the northern Sierras Pampeanas, and the

adjacent Puna margin (Fig. 12). The maximum

shortening direction is betwe en E-W and NNW-SSE.

Shortening is horizontal and extension is vertical.

Maximu m shorte ning is perpendicular to major An-

dean structures. The kinema tics of Plio-Quat ernary

deformation are less homogeneous (Fig. 13). Strike-

slip kinematics prevail, although thrust faults and

normal fault s are observed locally. Maximum shor-

tening directions typically lie between NE-SW and

E-W.

Within the study area, deformation has also

occurred in two kinematically distinct episodes; the

division into two episodes is based on cross-cutting

relationships between faults, fault stratigraphic

relationships, and considerations of kinematic com-

patibility as described in Marret t and Allmendinge r

(1990). The older deformat ion shows th rus t k inema -

tics with maximum shortening directions between

WNW-ESE and E-W (Fig. 12). The younger defor-

mation shows strike-slip kinematics with NE-SW

shortening in the north ern and southern parts of the

study area ~Fig. 13) and thrust kinematics with N-S


14/22


15/22

A

ault

B

Andean reactiva tion of the Cretaceous Salta rift, northwest ern Argenti na 365

W

S k y l i n e

s t ra t a - -~ S a l t a G r o u p

s y n - r if t s t r a ta |

P u n c o v is c a n a F m .

F i g . 1 4 . A ) P h o t o g r a p h o f t h e L a s C h a c r a s f a u l t ( s e e F i g . 1 0 f o r l o c a t io n ) . B ) S k e t c h o f t h e f a u l t / s t r a t i g r a p h i c r e l a t i o n s , w h i c h

s u g g e s t t h a t t h e f a u l t is a r e a c t i v a t e d n o r m a l f a u lt .

Chacras suggest that this is unlikely (Figs. 10 and

14).

Quebrada Las Chacras is little more that 100

mete rs wide. On the east side of the Las Chacras

fault, syn-rift stra ta lie directly on basement; on the

west side, only post-rift strata lie directly on base-

ment. Strat igraphi c relations across the fault sug-

gest that the syn-rift strata onlapped an erosional

basement surface in the hanging wall of a normal

fault and th at the normal fault was reactivated dur-

ing Andean deformation (Fig. 14). The fault system

that forms the boundary between the Cordillera

Oriental and the Sierras Pampeanas therefore pro-

bably represent s the original rift margin.

A map view of fault tre nds and vergences in the

combined Alemania and Met~n subbasins shows

that Andean fault trends track the old basin margin

(Fig. 15). Faul t vergences correlate with position in

the basin. On the west side of the basins, major

faults are west-verging and on the east side they are

east-verging; vergence changes in the Andean struc-

tures are associated with interbasin highs. This,

combined with evidence of reactiva ted basin-margin

faults along the Cordillera Oriental/Sierras Pam-

peanas boundary, suggests that the southern Cor-

dillera Oriental developed within the rift domain

and that the northern Sierras Pampeanas developed

outside the rift.


16/22


66 65

J

I I

Fig. 15. T he vergen ce of An dean s t ruc tures w i th in the southern subb ass ins o f he S al ta r i f t bas in .

- 250

2 6

T h e e v i d e n c e a ls o s u g g e s t s t h a t r e a c t i v a t i o n

w i t h i n t h e r i f t d o m a i n w a s e x t e n s i v e a n d w a s n o t

c o n f in e d to b a s i n m a r g i n f a u l ts . S e i s m i c d a t a f r o m

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

m a l f a u l t s in t h e l e s s - d e f o r m e d L o m a s d e O l m e d o

s u b b a s i n ( B i a n u c c i e t a l . 1 9 8 1 ) .

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

C o r d i l le r a O r i e n t a l a n d n o n - r i f t d o m a i n w i t h t h e

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

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

r e g i o n (F i g . 16). T h e p r e s e n t - d a y s o u t h e r n C o r -

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

n o r t h e r n S i e r r a s P a m p e a n a s , a n d t h e r e la t i v e a g e s

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

s u g g e s t t h a t a s i m i l a r r e la t i o n s h i p e x i s t e d d u r i n g

t h e e a r l y M i o c en e . F o r e l a n d b a s i n s t r a t a w e r e de -

p o s i t e d 5 -6 m il l io n y e a r s e a r l i e r i n t h e s o u t h e r n

C o r d i l le r a O r i e n t a l t h a n i n t h e n o r t h e r n S i e r r a s

P a m p e a n a s ( G r i e r a n d D a l l m e y e r , 19 90 ). B e c a u s e

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

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

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

i n c i d e s w i t h t h e p r e s e n t - d a y s o u t h e r n C o r d i l le r a

O r i e n t a l . T h e S a l t a r i f t b a s i n i s t h e o n l y p a l e og e o -

g r a p h i c f e a t u r e t h a t c o u l d g i v e r is e t o s u c h a lo w .

T h e c r u s t b e n e a t h t h e b a s i n m u s t a l s o h a v e b e e n

t h i n n e d d u r i n g r i f t in g , a n d t h i s e f fe c t m a y h a v e

p e r s i s te d t o t h e p r e s e n t d e s p i t e s h o r t e n i n g d u r i n g

A n d e a n d e f o r m a t i o n .

T h e b o u n d a r y b e t w e e n t h e C o r d i l l e ra O r i e n t a l

a n d t h e S i e r r a s P a m p e a n a s i s n o t a k i n e m a t i c b o u n -

d a r y w i t h r e s p e c t to m a jo r l a t e T e r t i a r y d e f o r m a t i o n ,

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

m e t r ie s . D u r i n g l a te P l i o c e n e - Q u a t e r n a r y d e f o r m a -

t io n , t h e b o u n d a r y h a s a c t e d a s a l o cu s o f a n o m a l o u s

s h o r t e n i n g d i r e ct io n . T h e d i s t i n c t g e o m e t r i e s b u t

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

a r e a t t r i b u t a b l e t o r e a c t i v a t i o n o f r i f t - r e l a te d n o r -

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

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

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

P u n c o v i s c a n a w e d g e a n d o n e th a t i n c l u d e s o n l y

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

P u n c o v i s c a n a w e d g e . T h e a n o m a l o u s s h o r te n i n g

d i re c t i o n s a lo n g t h e b o u n d a r y b e t w e e n t h e t w o

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

i n t e r a c t i o n b e t w e e n th e t w o d o m a i n s . R e a c t i v a t i o n

o f P a le o z o ic s t r u c t u r e s w i t h i n t h e P u n c o v i s c a n a

w e d g e is n o t p r e c l u d e d b u t i s d i ff i c u lt t o d o c u m e n t .


17/22

Andean reactivation of the Cretaceous Salta rift, northwestern Argentina 367

J u j u y

S a l t a

O_

i

i : : : : : : :

: o i :

. . . . ~:~:~:~;~;:~ ~ : ~ , ~ Tucu m ,~n

: : : : : . . : . 6 6 o

6 o

Ele v a t io n

> 6 ,00 0 m

r o

4,500-6 ,000

3,000-4 ,5oo

~-] 1,500-3,000

I I o - 1 5 o o

S a l t a R i ft B a s i n

~ limitof syn-rift

s t r a t a t i c k s o n

b a s i n s i d e o f

boundary

Fig. 16. The relationship between opographyand the southern par~ of the Salta rift basin.

THE RIFT INVERSION MODEL

Field evidence shows that structural inversion of

the Salta Basin is a major controlling factor in the

develo pment of the Andean foreland. Inversion of

the Alemania and Met~n subbasins is well deve-

loped; in the Lomas de Olmedo subbasin it is inci-

pient. To und erst and the effect of inversion on

Andean deformation, it is useful to consider the

geometric po ssibilities of simple rift inversion.

Given a rift an d the existence of the conditions

that will allow inversion through the reactivation of

rift str uctu res (see Sibson, 1985), the suite of struc-

tures observed in an inverted rift depends on several

factors: 1) the rift setting; 2) the suite of extensional

structures developed within the rift; 3) the relative

alignment of the direction of rift extension and the

direction of sub sequ ent shortening; and 4) the degree

to which the syn-rift strata have been overlain by

post-rift strata. The degree to which the rift has

been inverted will affect the scale of the inverted

structures but not the suite of structures itself.

The i f t Se t t ing

Rift structures are developed along passive mar-

gins and within failed rift systems. All may be re-

activated during shortening but the potential for and

the na ture of the reactiv ation will vary according to

tectonic setti ng (Dewey 1969). In a passive margin

in which a thick miogeoclinal wedge overlies the rift

structures, the basal decollement during shortening

may follow the unconformity between the basement

and the sedimentary cover, ramping up to higher

stratigraphic levels along the pre-existing normal

faults. Fold-and-thrust g eometries would develop,

but the reactivation of rift structures would be

limited. Where a failed rift is being shortened, the

basal decollement of the deformati on may re use the

rift detachment and rift-controlling normal faults

may be reactiv ated in their entirety. In this case,

thrust faults will be listric in form within the rift

basement, and the basement itself will be involved

in the deformation. The decollement may also follow

the unconformity between the basement and the


18/22

368 M. E. GRIER, J. A. SALFITY, and R. W. ALLMENDINGER

sedimentary cover on the foreland side of the rift

where the basem ent steps up in the direction of fault

propagation. Where reactivation of rift structures is

extensive, va riabl e fau lt cut-offs will lead to higher

amplitude-shorter wavelength geometries than is

typical of ramp and flat thr ust ing (Suppe, 1983).

T he Su i t e o f S t r uc t u r es Deve l oped w i t h i n t he R i f t

Rifts are developed along one or more sets of

normal faults and associated lateral- or oblique-

trending transvers e faults. Normal faults dominate

the rift g eometry, tra nsve rse faults oriented at 90 to

the normal faults are rare, and the oblique trans-

verse faults decrease in number as their obliquity to

the rift trend increas es (Harding, 1984). The normal

faults are curved in plan view and die out along

strike into monoclines. They are usua lly listric in

form, typ ically dip bet ween 30 and 60 at the surface,

and have cut-off angles of 60-65 betw een t he fault

and syn-rift bedding (Harding and Lowell, 1979;

Gibbs, 1984; Ch~net et al. 1987). The late ral and

oblique fault s dip steeply. Strain compatibility

requires that antithetic normal faults not cross the

main normal faults and that movement on the

linking fau lts is controlled by the mov ement on the

associated normal faults (Jackson and McKenzie,

1983).

The relative o rientat ions of the normal and

transverse faults will affect the stru ctures th at deve-

lop durin g inversio n (Fig. 17a). If the direction of

shortening parallels the original extension direction

of the rift, for example, tr ansv erse faults tha t are

perpendicular to the normal faults will act as tear

faults. An oblique-tren ding tran sver se fault will re-

activate with a thrust component and act in the

manner of a lateral ramp.

T he Re l a t i ve A l i gn me n t o f t he Di rec t ion o f R i f t Ex -

t ensi on and t he Di r ec ti on o f Subs eq uen t Shor t en i ng

Given a suite of rift structures , the struc tures

tha t will develop during rift inversion will depend on

the orientation of the sh ortening direction relative to

the rift axis (Fig. 17b). If shorteni ng is sub-parallel

to the direction of rift extension, no rmal faults will

be reactivated as thrust faults. As the shortening

direction becomes oblique to the rift axis, there will

be an increasing component of strike-slip moveme nt

along reactivated normal faults and an increasing

thr ust compo nent across tran sver se faults. If shor-

tening parallels the rift axis, rift inversion is not

possible although reactivation of transfer faults

within the rift may occur and the entire rift may

control the position of first order latera l ramps.

The Degree to W hich the Ri f t has been Over la in by

Y o u n g e r S t r a t a

The syn-rift strata of rift basins may be overlain

by strata associated with the thermal subsidence of

the basin and by strata related to a later geologic

event. A thick sequence of overlying stra ta will

permit the tr anslati on of the rift fill well beyon d the

rift margins through the propagation of reactivated

faults into the overlying stra ta (Anderson, 1951). It

will also increase the possibility of fault develop-

ment in the post-rift strata. These faults are not, in

themselves, reactivated structures and will have

lower cut-off angles than the rift structures. This

will again lead to higher amplitude-shorter wave-

length geometries than is typical of ramp and flat

thrusting (Suppe, 1983).

THE RIFT INVERSION MODEL APPLIED

TO THE ANDEAN FORELAND

A rift inversion model in which shortening par-

allels the rift extension direction during major de-

formation and is oblique to the extension direction

during subsequent minor deformation explains the

geometries observed in the southern Cordillera Ori-

ental and the Sistema de Santa B~rbara. The rift

extended E-W along N/S-trending normal faults.

One set of transv erse faults tren ds WNW-ESE to

NW-SE. The trend of the southern rift margin was

NW-SE in the Alemania subbasin and NE-SW in the

Met~n subbasin. During major Andea n deforma-

tion, the short ening direction was sub-parallel to the

rift-phase extension direction, and the N/S-trending

normal faults were reactivated as thrust faults.

Transverse structures ma y have been reactivated as

tear faults, although such reactivation has not been

documented. During subs equen t deformation, the

shortening direction was oblique to the rift exten sion

direction and thrust faults (reactivated normal

faults) were in turn reactivated as strike-slip faults.

Because the strike lengt h of the norma l faul ts is

much greater than that of the transverse faults,

strain compatibility requires that the latter were

reactivated principally as thrust faults instead of

strike-slip faults as the model predicts. Movement

on these faults created the transpressional struc-

tures that are observed adjacent to oblique trending

faults (Figs. 9 and 11). Burial of the rift durin g ear ly

Andean deforma tion allowed the tran slatio n of the

rift fill beyond the rift's western margin and ac-

counts for the develo pment of low-angle thrust faults

within the Salta Group and the overlying Andean

foreland basin strata.

In addition to explaining the geometries of the

Andean foreland, the rift inversion model has been

used to infer the subsurface geometry of the Salta

Basin in cross-sectional form. Some assum ptions

were made about the original rift and about Andean

deformation because no subsurface data are avail-

able. Where this was necessary, the simplest pos-

sible assumptions were made.

The cross-section runs E-W at appro ximate ly 25

30'S latitude and extends from 6625'W on the west

side of the Valle Calchaqui to 6352'W to the east of

Met~n (Figs. 2 and 4). Presen t-day surface geo-

metries can be generated by interactively extending


19/22

Andea n reactivati on of the Cretaceous Salta rift, northwes tern Argent ina 369

A

}

Transv erse fau l ts

perpendicular to normal faul ts

T ransverse fau l ts

obl ique to normal faul ts

B

i f fmarg in

Shortening paral le l

to the direct ion of

original r ift extension

Shortening obl ique


or ig inal r i f t extension

Shortening perpendicular


original r ift extension

Fig. 17. Rift nversion: a) The effecton inversiongeometries of he relative orientation of ransverse structures and rift-controlling

normal faults, b) The effecton inversion geometries of he relative alignment of he shorteningdirection o the original direction of

rift extension.

one section to generate the rift and shortening a

second section in two stages to invert it (Fig. 4).

Each section is area balanced. Shortening and,

therefore, degree of inversion decrease to the east

but, w ith in th e confines of the rift, the section is

shortened by 25%. It is assume d that the basin-

controlling normal faults are listric and sole into a

quasi-plastic zone of decoupling, that the normal

faults and their orientations correspond to major

thrust faults observed in the southern Cordillera

Oriental and Sistema de Santa B~rbara, and that

basin extension is reflected in known t hicknesses of

syn-rift strata. The actual amoun t of extension tha t

occurred in the southe rn s ubbasins of the rift is un-

known. It was sufficient, however, to allow as much

as 2 km of tectonic subside nce and to acco mmodate

3000-4000 met ers of compacted syn-ri ft fill. In re-

constr ucting the rift, extensio n of 10% was needed to

create room for known syn-rift stratigraphic thick-

nesses given a m ax imu m of 2 km tectonic subsi-

dence. It is assumed that the rift strat a restore the

full thickness of the crust above the detach ment. It

is also assumed t hat the basal d~collement of An-

dean deformation reused the basal rift structure and

tha t t he rift-controlling normal fa ults have been re-

activated in their entirety. Shortening is sub-paral-

lel to rift extension during major late Tertiary An-

dean deformation, and it is assumed that movement

in and out of the pl ane of the cross-section duri ng

younger Plio(?)-Quaternary deformation is negli-

gible.

The cross-section illustrates the rift inversion

model and shows that it is possible to generate ob-

served surface geometries almost entirely through


20/22

370 M.E. GRIER, J. A. SALFITY, and R. W. ALLMENDINGER

rift reactivation. Some of the par amet ers used in the

cross-section will be modified as more is known

about the region. However, the rift inversion model

allows an interpretation of the southern Cordillera

Oriental and Sistema de Santa Bfirbara that in-

corporates the complex paleogeology of the region.

CONCLUSIONS

The paleogeology of the Andean foreland

between 2515'S and 2630'S is the primary control

on the structural geometries that have formed

during Andean deformation from late Tertiary to

Recent times. The foreland has thre e components:

the deformed passive ma rgin sequence of the Punco-

viscana wedge, the rift basin sequence of the Salta

Group, and the continental clastics of the Andean

foreland basin. Each component has a dist inct litho-

logy, a distinct form, and distinct internal struc-

tures. The structures developed in each component

include all structures that are contemporaneous or

younger. The Puncoviscana wedge and the Andean

stra ta are exposed throu ghout the region. The Salta

Group is exposed within the southern Cordillera

Oriental and Si stema de Santa B~irbara.

The thrust front that bounds the eastern side of

the Calchaqui Valley marks the southwestern limit

of the syn-rift strata in the Cordillera Oriental. The

thrust is a reactivated normal fault and is probably

the translated southwestern margin of the rift. It

divides the foreland into rift and non-rift domains.

To the n orth of the margin, the foreland is comprised

of all paleogeologic components; to the south, the rif t

sequence and st ructu res are absent.

The northern domain includes the basement-

involved fold-and-thrust belts of the southern Cor-

dille ra Orienta l and Sistema de Sant a B~irbara. The

southern domain includes the basement uplifts of

the north ernmo st Sierras Pampeanas. The geome-

tries observed in these regions are very different, but

the kinematics of major late Tertiary deformation

throu ghout the region are homogeneous. The dis-

tinct geometries but similar kinematics during late

Tertiary deformation can be attributed to reactiva-

tion of old structures and to lithologic differences

between the two domains. It is difficult to document

reactivation of structures associated with deforma-

tion of the Puncovi scana wedge. However, re-

activated normal faults are observed in the Cor-

dillera Oriental and the correlation of thrust-fault

orientation with location in the rift suggests that

reactivation of rift structures has been extensive.

Subsequent minor deform ation again shows region-

ally homogeneous kinematics but anomalous kine-

matics along the rift boundary. This is again attri-

butable to reactivation of older structures but, in

this case, reactivation of oblique structures in the

rift margin and strain compatibility requirements

have contr ibuted to a boundar y effect.

The original orie ntations of rift stru ctures in the

southern subbasins of the Salta rift have been in-

ferred from the distribution of syn-rift facies and the

orientation of known reacti vated faults. Basin-

controlling normal faults probably had N-S trends

and one set of oblique struc ture s WNW-ESE to NW-

SE trends. A rift inversion model in which the direc-

tion of Andean shortening is sub-parallel to the

direction of rift extension is used to generate the

present-day structural geometries tha t are observed

within the rift domain. It is illustrated in an E/W-

tre nding cross-section. The model predicts that the

rift has been shortened by 25%. This figure is con-

siderably less than is predicted for classic foreland

fold-and-thrust belts and may reflect the relatively

high-angle trajectories of reactivated, rift-related

faults.

The param ete rs of the rift model will undoubted-

ly be modified as more is learned of this region.

However, the model incorporates both paleogeology

and recent geology. In addition, it explains the

structural anomalies observed in the Andean fore-

land that cannot be explained by variations in the

underlying, subducting Nazca plate. The broaden-

ing of the Cordillera Oriental t hrust belt, t he

basement involvement in the deformation, and the

zones of east- and west-vergence within the thrust

belt are attributable to inversion of the Salta rift

basin. The abrupt change from fold-and-thrust

geometries to basement uplifts that occurs in the

region of 2615'S marks the transition from rift to

non-rift domain.

A c k n o w l e d g m e n t s q W e

are grateful to Theresa Jordan, Randall

Marrett, Ricardo Mon, Stella Montes-Weber, Apolo Ortiz, Victor

Ramos, and Manfred Strecker for their assistance in the field and

for discussions of this work. We also tha nk Miles Shaw, Nivaldo

Rojas, Mary Gilzean, and Gonzalo Bravo of BHP Utah Inter-

national for providing us with a field vehicle, field support, and

much hospitality, and Yacimientos Petroliferos Fiscales and the

Comicion Nacional de Energia At6mica for their assistance in the

field. We are also very grateful to the Flores famil y of Salta for

the ir hospitality. The work was supported by the Divisio n of

Earth Sciences, National Science Foundation, through grants to

R. W. All men din ger (EAR-8206172, EAR-8519037, and EAR-

8816287), the Geological Society of America, Sigma Xi, and the

Marty Memorial Scholarship from Queen's University, Canada.

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Andean reactivation of the Cretaceous Salta rift, northwestern Argentina

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