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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
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352 M. E. GRIER, J. A. SALFITY, and R. W. ALLMENDINGER
14
t~
t
O
g ,
Q. .
3
74 62
c -
O
o ~
c -
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-
8/10/2019 Andean reactivation of the Cretaceous Salta rift, northwestern Argentina
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Andea n reacti vation of the Cretaceous Salta rift, northwes tern Argent ina 353
1 0 0 k m
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v v v v v v v v v V
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A R G E N T I N
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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.
8/10/2019 Andean reactivation of the Cretaceous Salta rift, northwestern Argentina
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8/10/2019 Andean reactivation of the Cretaceous Salta rift, northwestern Argentina
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8/10/2019 Andean reactivation of the Cretaceous Salta rift, northwestern Argentina
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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
8/10/2019 Andean reactivation of the Cretaceous Salta rift, northwestern Argentina
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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
P u n c o v l s c a n a F ro .
s t r u c t u r a l d e p t h
s h a l l o w
i n t . - s h a l l o w
~
d e e p - i n t .
d e e p
g r a n i t i c
I i n t r u s i o n s
/ f a u l t s
C C a f a y a t e
A A n g a s t a c o
A I A l e m a n [ a
T u c u m n
0 2 5 k m
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-
8/10/2019 Andean reactivation of the Cretaceous Salta rift, northwestern Argentina
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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
8/10/2019 Andean reactivation of the Cretaceous Salta rift, northwestern Argentina
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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 .
8/10/2019 Andean reactivation of the Cretaceous Salta rift, northwestern Argentina
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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
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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
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3 6 2 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
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 /
8/10/2019 Andean reactivation of the Cretaceous Salta rift, northwestern Argentina
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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
8/10/2019 Andean reactivation of the Cretaceous Salta rift, northwestern Argentina
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8/10/2019 Andean reactivation of the Cretaceous Salta rift, northwestern Argentina
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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.
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3 6 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
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 .
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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
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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
8/10/2019 Andean reactivation of the Cretaceous Salta rift, northwestern Argentina
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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
to the direct ion of
or ig inal r i f t extension
Shortening perpendicular
to the direct ion of
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
8/10/2019 Andean reactivation of the Cretaceous Salta rift, northwestern Argentina
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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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