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Structural evolution of the Andes in a transitional zone between ¯at andnormal subduction (33830 0 ±33845 0S), Argentina and Chile

L.B. Giambiagia,b,*, V.A. Ramosa

a Laboratorido de TectoÂ nica Andina, Universidad de Buenos Aires, Ciudad Universitaria s/n, PabelloÂ n II, Buenos Aires 1428, ArgentinabCentro Regional de Investigaciones CientõÂ ®cas y TecnoloÂ gicas, CONICET, Parque San MartõÂ n s/n, Mendoza 5500, Argentina

Received 1 November 2001; accepted 1 November 2001

Abstract

The sector of the Andes studied in this paper (33830 0 ±33845 0S) presents a key region to study the relationship between tectonic setting anddeformation history in a transitional zone between ¯at (north of 338S) and normal (south of 33845 0S) subduction segments. The Andes at

these latitudes are primarily composed of the Neogene Aconcagua fold-and-thrust belt and the basement-block uplift of the Cordillera

Frontal. Detailed mapping has revealed that the structure within the inner part of the fold-and-thrust belt resulted from both thin- and thick-

skinned tectonic interactions. In the outer part, displacement is transferred to Mesozoic deÂcollement levels, accounting for a thin-skinned

architecture. Geographically, the switch from thick- to -thin skinned tectonics occurs near the border between Chile and Argentina.

Although the geometry of the subducted Nazca Plate may have in¯uenced the timing and style of deformation in the foreland, plate

geometry alone does not adequately explain the style of deformation within the fold-and-thrust belt. Here it is shown that the change in

deformation style in the fold-and-thrust belt correlates with the location of pre-existing Mesozoic structure, and speci®cally that Neogene-age

thick-skinned thrusting was controlled by the presence of Mesozoic margin-boundary normal faults. Deformation in this region of the

Andean fold-and-thrust belt was thus controlled by a combination of tectonic setting and pre-existing extensional structure. q 2002 Elsevier

Science Ltd. All rights reserved.

Keywords: Andes; Fold-and-thrust belt; Subduction; Mesozoic anisotropy; Neogene deformation

Resumen

El sector de los Andes estudiado en este trabajo (30830 0 ±33845 0S) constituye un aÂrea clave para el estudio de la relacioÂn entre el ambiente

tectoÂnico actual y la deformacioÂn, en una regioÂn situada en la transicioÂn entre el segmento de subduccioÂn subhorizontal (al norte de los 338S)

y el segmento de subduccioÂn normal (al sur de los 33845 0S).

A estas latitudes, los Andes estaÂn constituidos por la faja plegada y corrida del Aconcagua y la Cordillera Frontal. El presente trabajo

demuestra que la deformacioÂn del sector interno de la faja plegada y corrida corresponde a un estilo hõÂbrido de deformacioÂn de piel ®na y

gruesa. En el sector externo de la faja el desplazamiento es transferido a la cobertura sedimentaria generando una regioÂn de piel ®na. La

transicioÂn entre los dos estilos de deformacioÂn coincide con el lõÂmite entre Argentina y Chile y con el margen este de las fallas directas

mesozoicas, sugiriendo que la estratigrafõÂa y las estructuras controlaron el desarrollo de la faja.

El ambiente tectoÂnico particular, junto con la existencia de estructuras extensionales previas, constituyen un factor de primer orden en la

historia neoÂgena de la regioÂn. La geometrõÂa de la placa subducida in¯uencioÂ el estilo y el tiempo de deformacioÂn en el antepaõÂs, pero el estilo

de deformacioÂn dentro de la faja plegada y corrida estuvo controlado principalmente por las estructuras mesozoicas pre-existentes. q 2002

Elsevier Science Ltd. All rights reserved.

Palabras clave: Andes; Fold-and-thrust belt; Subduction; Mesozoic anisotropy; Neogene deformation

1. Introduction

The Andes of central Argentina and Chile are a linear

orogenic belt formed at the convergent plate margin

between the Nazca and South American Plates. Although

the orogen is continuous along strike, a tectonic segmentation

Journal of South American Earth Sciences 15 (2002) 101±116

0895-9811/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.

PII: S0895-9811(02) 00008-1

www.elsevier.com/locate/jsames

* Corresponding author. Address: Centro Regional de Investigaciones

CientõÂ®cas y TecnoloÂgicas, CONICET, Parque San MartõÂn s/n, Mendoza

5500, Argentina.

E-mail addresses: [email protected] (L.B. Giambiagi),

[email protected] (V.A. Ramos).



related to normal and ¯at subduction geometries of the

Nazca Plate has been broadly recognized (Barazangi and

Isacks, 1976; Isacks et al., 1982; Jordan et al., 1983a,b).

This tectonic segmentation interacts with pre-existinginhomogeneities in the South American Plate, giving

rise to a more complex evolution of the different segments

(Mpodozis and Ramos, 1989; Kley et al., 1996; Ramos et

al., 1996a).

The study area, located between 33830 0 and 33845 0S,

corresponds to the transition zone between a normal subduc-

tion segment (south of 33845 0S) and a ¯at subduction

segment (north of 338S) (Fig. 1). The aim of this paper is

to investigate the relationship between this subduction zone

geometry and the structural evolution of the region, and to

evaluate the role of pre-existing structures.

At the studied latitudes, the Andes are characterized by

the east-vergent Aconcagua fold-and-thrust belt of Neogene

age and the basement-block of the Cordillera Frontal. The

foreland is marked by the inversion of Triassic extensionalfaults of the Cuyo basin (Legarreta et al., 1992; DellapeÂ and

Hegedus, 1995) and the easternmost minor exposures of the

Sierras Pampeanas.

North of 33830 0S, traditional models of the Aconcagua

fold-and-thrust belt have invoked a thin-skinned tectonic

style with complete detachment of a deformed sedimen-

tary cover from a gently west-dipping basement slope

(Ramos, 1985, 1988; Cegarra, 1994; Cegarra and

Ramos, 1985; Ramos et al., 1996a). South of 348S, in

the MalarguÈe fold-and-thrust belt, the main deformation

mechanism is a thick-skinned style related to tectonic

L.B. Giambiagi, V.A. Ramos / Journal of South American Earth Sciences 15 (2002) 101±116 102

Fig. 1. Morphological map of the Andes between 32 and 358S (modi®ed from Giambiagi et al., 2001). The boxed areas correspond to (a) structural map

presented in Fig. 4 and (b) the Alto TunuyaÂn foreland basin.



inversion of Mesozoic rift structures (Kozlowski et al.,

1993; Manceda and Figueroa, 1995; Zapata et al., 1999).

In the transition zone between these two different struc-

tural styles, the ®rst balanced cross section of the fold-

and-thrust belt showed a typical thin-skinned style with a

complete detachment of Mesozoic strata above the base-

ment (PaÂngaro et al., 1996). However, this contribution

will show that the basement is involved in the transition

zone deformation in the inner part of the belt, and the

border between Chile and Argentina is a region where

deformation switches from thick-skinned to thin-skinned

tectonics.

In this paper, we present a new regional structural map

and a structural cross section of the Aconcagua fold-and-

thrust belt. These provide the basis for evaluating the

amount of tectonic shortening, the possible control of


Fig. 2. Location of the morphological units on a shaded relief map of the Andes processed by the Cornell Andes project, between 31 and 35 8S, and schematic

block diagrams showing subducted plate segments (modi®ed from Jordan, 1995). Note that the study area is located within the transition zone (boxed area).

Fig. 3. Generalized stratigraphic column of the Cordillera Principal and Cordillera Frontal between 33 830 0 and 33845 0S.



pre-existing structures on fold-and-thrust belt develop-

ment, and the interaction of thick- and thin-skinned

deformation.

2. Plate tectonic setting

The distribution of seismicity along the contact between

the Nazca and South American Plates suggests that the plate

boundary is characterized by alternating ¯at and normal

segments of subduction (Fig. 2) (Stauder, 1973, 1975; Bara-

zangi and Isacks, 1976; Cahill and Isacks, 1992; Gutscher et

al., 2000). Changes in large-scale tectonic features of the

Andean orogenic system appear to correlate with these

geometries, such as volcanism and distribution of morphos-

tructural belts (Barazangi and Isacks, 1976; Isacks et al.,

1982; Jordan et al., 1983a,b). North of 338S a ¯at subductionsegment lacking arc magmatism has underlain the

Cordillera Principal, Cordillera Frontal, Precordillera, and

Sierras Pampeanas structural provinces since the Middle

Miocene (Kay et al., 1991). South of 33845 0S a normal

subduction segment with arc magmatism is present beneath

the Cordillera Principal and Cordillera Frontal. Neither the

Precordillera fold-and-thrust belt nor Sierras Pampeanas

style basement-block uplifts have developed in this

segment.

The study area is located in the transition zone between

the ¯at slab to the north of 338S and the normal-dipping slab

to the south of 33845 0S. The transition appears to occur as a

smooth ¯exure of the subducted slab rather than a tear

(Cahill and Isacks, 1992; YaÂnez et al., 2001).

3. Stratigraphic background

The study area is characterized by a deformed belt of

Mesozoic and Cenozoic sedimentary and volcanic rocks

that unconformably overlie pre-Jurassic basement rocks.

The stratigraphy, examined in previous studies by Darwin

(1846), Armando (1949), Pascual (1949), Polanski (1964)

and Ramos et al. (2001), can be divided into three main

stratigraphic intervals (Fig. 3): (1) pre-Jurassic rocks; (2)

Jurassic to Paleocene marine and nonmarine sedimentary

rocks; (3) Neogene foreland basin deposits and Cenozoic

volcanic rocks.

3.1. Pre-Jurassic rocks

The basement, which crops out in the Cordillera Frontal

(Fig. 1), is composed of Precambrian metamorphic rocks

and upper Paleozoic marine black shales partially metamor-

phosed of the Alto RõÂo TunuyaÂn Formation. These units are

intruded by Carboniferous and Permian granitoids

(Polanski, 1964; Ramos et al., 2001), which are unconform-

ably overlain in the eastern ¯anks of the Cordillera Frontal

by a thick succession of Permian±Triassic volcanic rocks of

the Choiyoi Group. The widely exposed granitoids indicate


Fig. 4. Structural map of the Aconcagua fold-and-thrust belt in the Yeso and Palomares rivers area, showing the main structures. For section A±A 0 see Fig. 8.

The boxed area shows location of Fig. 9.



that a large batholith underlies the Proterozoic to late Paleo-

zoic metamorphic and metasedimentary rocks of Cordillera

Frontal (Polanski, 1964). From this observation, we infer a

rigid behavior to the rocks beneath this range and accord-

ingly consider them to be the basement to the fold-and-

thrust belt.

3.2. Jurassic to Paleocene rocks

Widespread Triassic±Jurassic rifting in the South

American Plate was related to the fragmentation of Gond-

wana and opening of the South Atlantic Ocean (Uliana et

al., 1989). Palinspastically restored isopach maps of the

Jurassic succession in the NeuqueÂn basin show a suite of

asymmetric depocenters interpreted as half-grabens (Uliana

and Biddle, 1988; Legarreta and Gulisano, 1989; Manceda

and Figueroa, 1995). Mesozoic sequences described in this

paper were deposited in the northernmost part of the

NeuqueÂn retroarc basin, as part of an extensional systemthat was subjected to several pulses of extension (Mpodozis

and Ramos, 1989). Correlations with the NeuqueÂn basin

allow identi®cation of lower-middle Mesozoic synrift and

sag-phase successions exposed in the Yeguas Muertas anti-

cline zone (Fig. 4) (Alvarez et al., 1997, 1999, 2000). These

successions are made up of Lower to Middle Jurassic black

shales (Nieves Negras Formation), Middle Jurassic evapor-

ites and redbeds (TaÂbanos and Lotena Formations), marine

platform limestones (La Manga Formation), and extensive

OxfordianÐlower Kimmeridgian evaporites of the

Auquilco Formation.

Upper Jurassic red continental sandstone and conglomer-ate of the Tordillo Formation are preserved in half-grabens

as indicated by thickness variations from 50 to more than

1000 m in a 2 km-long area. This thickness variation has

also been observed in the Aconcagua area further north by

Cegarra (1994), Cegarra and Ramos (1996), and in the study

area by PaÂngaro et al. (1996).

Above the Jurassic strata, the Mendoza Group was

deposited as a stable platform succession during post-rift

Titho-Neocomian thermal subsidence. It consists of three

lithostratigraphic units: black shales of the Vaca Muerta

Formation; mudstones and limestones of the Chachao

Formation; and limestones and sandstones of the Agrio

Formation (PaÂngaro, 1995; Aguirre-Urreta, 1996).A major plate tectonic reorganization took place by the

end of the Early Cretaceous (Somoza, 1998), and as a

response, the extensional regime of the marine intra-arc

and retro-arc basins ended (Mpodozis and Ramos, 1989).

The Upper Cretaceous Diamante and Colimapu Formations,

which unconformably lie above the Mendoza Group, consist

of continental redbeds. These are overlain by siltstones and

carbonates deposited during the Late Cretaceous ®rst

Atlantic transgression (SaldenÄo Formation) (Tunik, 1999,

2001), and continental sediments of the Paleocene Pircala

Formation.

3.3. Neogene foreland basin deposits and Cenozoic volcanic

rocks

The Contreras Formation, the oldest Cenozoic volcanic

unit, is located at the base of the synorogenic strata and

consists of basaltic lava ¯ows and breccias dated as

18.3 Ma (whole rock K/Ar age, Giambiagi, 2000).

Geochemical analyses of these volcanics suggest a typical

retro-arc setting prior to crust thickening (Ramos et al.,

1996b), predating the beginning of thrusting in the Cordil-

lera Principal at these latitudes.

Synorogenic sediments ®lled up the Alto TunuyaÂn fore-

land basin located east of the fold-and-thrust belt (Fig. 1).

Three Miocene units recorded uplift and the eastward

migration of the Andean thrust front (Giambiagi, 1999a).

The oldest one, the TunuyaÂn Conglomerate, consists of

1400 m of coarse sedimentary rocks deposited in an allu-

vial-fan setting. Clast counts and paleocurrent data indicate

that they are derived from the Cordillera Principal (Giam-

biagi, 1999a; Giambiagi et al., 2001). The TunuyaÂnConglomerate represents a proximal and coarser basin

facies linked to coeval distal synorogenic deposits known

as the MarinÄo Formation, which crops out to the east of

Cordillera Frontal foothills (Fig. 1). These two units were

deposited in the same foreland basin, in response to thrust-

ing and uplift in the Aconcagua fold-and-thrust belt. During

the deposition of the TunuyaÂn Conglomerate and MarinÄo

Formation, the Cordillera Frontal was not uplifted (Irigoyen

et al., 2000; Giambiagi et al., 2001). Based on correlation of

these two units, and radiometric and magnetostratigraphic

data from the MarinÄo Formation (Irigoyen et al., 1998,

2000), it has been proposed that the TunuyaÂn Conglomeratewas deposited between 16 and 10 Ma (middle Miocene)

(Giambiagi et al., 2001).

The Palomares Formation, which overlies the TunuyaÂn

Conglomerate, consists of 200 m of volcaniclastic and clas-

tic sediments deposited in an alluvial-fan setting. The paleo-

current measurements and clast composition show

provenance from the northeast, indicating that initial uplift

of Cordillera Frontal took place during the late Miocene (8.5

to 7 Ma, Giambiagi et al., 2001).

The ButaloÂ Formation records ®nal in®lling of an inter-

montane trough between the Cordillera Principal and Cordil-

lera Frontal. It reaches a thickness of more than 300 m and is

made up of medium- to ®ne-grained ¯uvial and lacustrinedeposits (Giambiagi, 1999a). A study of clast provenance

indicates that both Cordillera Principal and Cordillera Frontal

acted as source areas (Giambiagi et al., 2001). These rocks are

unconformably overlain by andesitic volcanic rocks dated as

5.9 Ma (whole rock K/Ar age, Ramos et al., 1998). Therefore,

an age range between 7 and 6 Ma has been proposed for the

ButaloÂ Formation (Giambiagi et al., 2001).

The Marmolejo and San Juan Formations comprise Plio-

ceneÐQuaternary volcanic arc rocks that are separated

from underlying Miocene synorogenic deposits by an angu-

lar unconformity.




4. Structure

4.1. Description of the different structural zones

The Andes at the study latitudes can be divided into two

distinct domains, the Cordillera Frontal and Cordillera Prin-

cipal (Fig. 1). The Cordillera Frontal consists of a basement-

block that forms a forestop to the Aconcagua fold-and-thrust

belt located in the eastern side of the Cordillera Principal.

During the Andean orogeny this morphological unit

behaved as a rigid block that was uplifted by high-angle

faults.

The Aconcagua fold-and-thrust belt presents an overallgeometry of a low-angle eastward-tapering wedge. It can be

divided into four major zones of distinct structural style,

delimited here from east to west (Fig. 4). (I) The eastern

sector comprises thin-skinned fold-thrust sheets above a

deÂcollement developed in the Upper Jurassic Vaca Muerta

Formation and Upper Cretaceous SaldenÄo Formation, and is

characterized by exposures of Neogene strata. (II) The

central sector, which comprises the CordoÂn de Jorge range

and Cerro Palomares area, involves a dense array of thin-

skinned imbricate thrust sheets above a deÂcollement in the

Jurassic evaporite (Auquilco Formation). (III) Toward the


Fig. 5. Photograph, looking south, of the thrust front represented by the Campanario thrust, south of Palomares River.

Fig. 6. (a) View of the Cerro Palomares, showing the Palomares thrust system which places Mesozoic strata over the upper part of the Miocene TunuyaÂn

Conglomerate and Palomares Formation. (b) Field sketch of (a), showing relationship between structures and the synorogenic units (modi®ed from Giambiagi

et al., 2001).



west, the CordoÂn del LõÂmite and Yeguas Muertas anticline

areas constitute a basement-involved thrust complex where

thin-skinned tectonics are combined with high- and low-

angle basement-involved faults. (IV) The western sector is

represented by a series of thin-skinned out-of-sequence

thrusts with a detachment in Jurassic units.

4.1.1. Zone I

This zone corresponds to the Alto TunuyaÂn foreland

basin, where Neogene synorogenic strata crop out (Giam-biagi et al., 2001). It represents a broken foreland basin, as

indicated by the presence of basement exposures immedi-

ately to the east in the Cordillera Frontal (Fig. 1). Four main

thrusts are responsible for deformation of these synorogenic

strata: the Campanario, Chileno, Miranda, and CajoÂn Chico

thrusts (Fig. 4). The thrust front is represented by the

Campanario thrust (Fig. 5), ®rst described by Polanski

(1964), which contains a basal detachment in the base of

the Upper Cretaceous SaldenÄo Formation. It trends north-

ward north of the Palomares River and northwestward south

of the river. This change in trend is interpreted to relate to

the uplift of the basement in the Cordillera Frontal. Folds

developed south of the Palomares River are probably fault-bend folds related to this thrust.

The Chileno thrust, located south of the Palomares River,

is detached above Cretaceous limestones and ramps through

Upper Cretaceous to Neogene strata. North of the Palomares

River, the Miranda and CajoÂn Chico thrusts cut Neogene

strata, and are interpreted to be detached in the Lower

Cretaceous units. These thrusts lose displacement south-

ward and disappear south of the Palomares River.

4.1.2. Zone II

In this area major structures are exceptionally well

exposed, and detailed mapping revealed the relationships

between structural features and synorogenic strata (Giam-

biagi et al., 2001). The region is composed of several east-

verging low-angle thrusts, most with presently steep dips

(varying from 33±908W) as a result of subsequent rotation

during the eastward migration of the thrust front (Fig. 4).

Upper Jurassic evaporites of the Auquilco Formation, which

form an extensive horizon near the basement-cover inter-

face at the base of thin-skinned thrusts, provide a suitable

detachment level. This unit pinches out to the east where theblack shales of the Vaca Muerta Formation act as a decolle-

ment. These structural features point to a thin-skinned

deformational style in this zone.

The easternmost thrusts form an imbricate fan, named the

Cerro Palomares thrust system, with dips varying from 28±

808W (Fig. 6). This system, ®rst described by PaÂngaro

(1995), is composed of several stacked thrust sheets that

involve the Mendoza Group limestones and share a common

basal detachment in the black shale of the Vaca Muerta

Formation. It is formed by ®ve east-vergent thrusts

numbered in sequence of development (Fig. 4): CordoÂn de

Jorge (I), CaletoÂn (II), Del Pozo (III), Quebrada Seca (IV),

and Palomares (V) thrusts. The Palomares thrust is a low-angle fault that splits into two splay thrusts (Va and Vb) and

is responsible for the transport of Mesozoic rocks over

Neogene strata (Fig. 6). The CordoÂn de Jorge, CaletoÂn,

and Del Pozo thrusts are in-sequence thrusts. The Quebrada

Seca and Palomares thrusts are out-of-sequence thrusts

because they cut in-sequence thrusts located toward the

east. Sedimentological analyses of the Neogene TunuyaÂn

Conglomerate show that deformation was synchronous

with sedimentation of synorogenic strata in the adjacent

foreland basin (Giambiagi et al., 2001).

Toward the west, out-of-sequence thrusts referred to as


Fig. 7. Photograph of the Morado out-of-sequence thrust that emplaced the easternmost sheet of Upper Jurassic Tordillo Formation on top of the Upper

Cretaceous redbeds and carbonates of the Diamante and SaldenÄo Formations.



the Ruinas and Oveja thrusts truncate previously developed

structures of the Cerro Palomares thrust system (Fig. 4).

These faults were detached in the Auquilco evaporites and

placed redbeds of the Diamante Formation over previously

deformed thrust sheets of the Mendoza Group.

4.1.3. Zone III

The Morado thrust outlines the border between zones II

and III (Fig. 4). This thrust forms a major tectonic feature in

this zone, emplacing Upper Jurassic Tordillo Formation on

top of Upper Cretaceous SaldenÄo Formation (Fig. 7). It is an

out-of-sequence thrust that propagated within the Auquilco

evaporite horizon. To the north it has a north trend and splits

into the Guanaco thrust. To the south, this fault has a north-

west trend and cuts off the hangingwall of the Palomares

thrust, placing Mesozoic rocks over Neogene strata.

The CordoÂn del LõÂmite range is composed of two thrust

sheets uplifted by two backthrusts (BI and BII, numbered in

order of emplacement), which can be observed in the

Chilean side of the range (Fig. 4). Backthrust BI dipsbetween 478 and 558W and backthrust BII between 50±

678W. The Piuquenes thrust cuts through a pre-existing

syncline generated by the emplacement of the Morado thrust

and the backthrust BI. Backthrusts are interpreted as out-of-

the-syncline thrusts as a consequence of the rigid rheology

of the limestones and the lack of space at the core of the

structure.

The Yeguas Muertas anticline is an important structural

feature in this sector of the belt and is made up of open

folded and thrust Jurassic strata (Godoy, 1993; Alvarez et

al., 1999). It is a broad north-northwest-trending structure,

approximately 4 km wide and 13 km long, with subordinatefolding. Surface data document thick- and thin-skinned

structural interactions in this area, dominated by east-

verging basement-involving structures and the set of back-

thrusts, which de®ne a frontal basement wedge. The

involvement of the basement is inferred from the geometry

of the structure and rift stratigraphy of the deformed

Mesozoic succession exposed in this area.

4.1.4. Zone IV

The deformation of this zone is characterized by thin-

skinned east-vergent thrusts, most with a northerly trend.

They are interpreted as out-of-sequence thrusts because

they cut previously emplaced thrust sheets and overridethe Yeguas Muertas anticline structure, inducing younger-

over-older relationships (Fig. 4).

The Yeguas Muertas anticline is bounded on the west by

the Estero Caballos thrust. This thrust is an out-of-sequence

north-trending structure that outlines the boundary between

zones III and IV. To the north, this fault truncates the

Yeguas Muertas anticline, emplacing the Lower Cretaceous

rocks over the Jurassic strata.

One of the main structural features of this region is the

Chacayal thrust, ®rst described by Godoy (1993). This

thrust places the thickest sheet of Upper Jurassic Tordillo

Formation over the Cretaceous section. West of this thrust

the Upper Jurassic thickness in the hanging-wall varies

between 2000 and 1500 m, while immediately eastward

the thickness is 400 m. Abrupt changes in thickness of the

Upper Jurassic redbeds correlate with major tectonic bound-

aries, suggesting that pre-existing extensional faults have

in¯uenced the generation of Chacayal thrust. In the proxi-

mity of the Yeso River this thrust cuts off the hangingwall of

the Estero Caballos thrust.

The westernmost structure corresponds to the Las LenÄas

thrust (Fig. 4). Slip along this fault occurred after the devel-

opment of zone III structures but prior to development of

Chacayal and Estero Caballos thrusts. Its detachment is

located in the Auquilco evaporites and pinches out north-

ward beneath the Aparejo folds. Out-of-sequence thrusts of

zone IV formed after the uplift of the basement high located

in the Yeguas Muertas anticline zone and are interpreted to

be the response to horizontal shortening against this base-

ment high.

4.2. Regional cross section

The deformation history and quanti®cation of minimum

shortening across the fold-and-thrust belt were calculated

using a restored and balanced cross section (A±A 0, Fig.

4). The section (Fig. 8) runs approximately west± east,

parallel to the direction of maximum Neogene compression,

and was constructed at a scale of 1:25, 000. In the absence of

seismic and well data, the section is constrained by surface

geology for the shallow levels, and a combination of down-

plunge projection, geometrical constraints, and extrapola-

tion of relationships among the structural levels exposedfor balancing the deeper levels. The assumptions required

in the construction of cross section A±A 0 included: defor-

mation by plane strain with no movement out of the plane of

the section; conservation of area in basement-involved

structures; and preservation of line lengths before and

after shortening (Woodward et al., 1989). Since the cross

section does not extend to the undeformed foreland, the

eastern pin line was placed on the edge of the eastern thrust

sheet. The dip of the basement-cover interface varies, from

208W in the eastern sector of zone I (Fig. 5) to 68W below

zone II, as calculated by the dip of synorogenic strata. The

steeper dips are the result of later tilting of the Cordillera

Frontal block.In zones I and II, classical ramp-¯at tectonics developed

in relation to the stratigraphy, including ramps, with cut-off

angles between 18 and 208, located in the redbed and carbo-

nate units, and ¯ats in the evaporites of the Auquilco Forma-

tion and black shales of the Vaca Muerta Formation. In zone

III, the available data suggest that the geological structure

can be interpreted in terms of structural inversion of earlier

extensional faults. The structural restoration demonstrates

the role of these pre-existing faults in the structural devel-

opment of the region. First, the Jurassic normal faults

explain the variation in thickness of the Jurassic strata.




Fig. 8. Balanced structural cross section and restoration of the Aconcagua fold-and-thrust belt at 33 840 0S. Location given in Fig. 4. The numbe



Second, they allow basement deÂcollements to ramp upward

during forward propagation. And ®nally, these structures

were inverted to create a particular structural style in the

Yeguas Muertas anticline area where the presence of back-

thrusts forms a tectonic wedge.

The steep basement-involved ramps located in theYeguas Muertas anticline area merge upsection into ¯ats

where they reach the evaporites of the Auquilco Formation.

However, as predicted by analogue models (McClay and

Buchanan, 1992) with large values of shortening during

inversion, pre-existing extensional listric faults are too

steep to be reactivated as reverse faults, thus thrust faults

may short-cut the half-graben. As a consequence, a new

low-angle basement-involving fault was created during or

immediately after the inversion processes.

The inferred deep detachment may have played a major

role in the basin inversion, but it cannot account for the

amount of shortening recorded in the thrust belt, especially

in the thin-skinned tectonic zone. We believe that shorten-

ing was transferred to a shallower basement detachment (8±

10 km in depth). However, we do not have direct evidence

for such a detachment. The principal constraint for this

interpretation was the need to match shortening of the

cover with that of the basement. It is likely that this

shallower fault, named here Yeguas Muertas thrust, joins

at depth to a common rift deÂcollement. Its trajectory was

probably controlled by the extensional fault architecture

with ramps located at pre-existing extensional fault steps.

The uplift of the Yeguas Muertas area is attributed to move-

ment on this thrust, which probably cuts basement rocks

above the Carboniferous black shales of the Alto RõÂo

TunuyaÂn Formation. This fault branches into two faults,

generating a duplex structure, and continues eastward

above the Auquilco Formation as a basal detachment of

zones I and II.

At least two main decollements underlie strata of zone IV.

The lowermost is situated in basement rocks. The seconddeÂcollement is recorded in the evaporites of the Auquilco

Formation. The ®ll between the two detachment horizons

has been interpreted to be a single basement thrust sheet,

although surface geology data cannot con®rm this.

Shortening estimates are minimum values due to the

uncertainty involved in calculating the amount of fault

displacement where hangingwall cut-offs are eroded. In

order to restore the foreland basin area (zone I), the base

of the synorogenic deposits was used as a datum horizon.

The base of the Vaca Muerta Formation was used in the

fold-and-thrust belt in zones II±IV.

Section balancing and restoration show that the totalshortening within the southern Aconcagua fold-and-thrust

belt is 47 km and represents 57% of its original length (Fig.

8). Most of the shortening was localized across the thin-

skinned zone (24 km or 78% of the original length of

zone II), while in the thick-skinned zones the calculated

shortening is signi®cantly less (19 km or 48% of the original

length). The foreland basin zone accommodated only a total

shortening of 3.5 km.

5. Discussion

5.1. Effect of pre-existing structures

The restored cross section suggests that the evolution of

the fold-and-thrust belt and the resulting structural style

were predetermined by conditions established in earlier

tectonic events. The data provided in the earlier sections

imply that the structure of the area is in¯uenced by the

reactivation of basement extensional faults, which resulted

in a hybrid thick- and thin-skinned style of deformation.

The Yeguas Muertas Anticline zone can be explained by

tectonic inheritance of Jurassic extensional structures.

Structural inversion is conspicuous in this region, which


Fig. 9. (a) Structural map of spatial relations of structures in the thin-

skinned zone II and its boundary with the thick-skinned zone III related

to pre-existing faults. (b) The changes in strike of the Morado thrust near

the Jorge and Morado arroyos are thought to be evidence for extensional

block faulting of Jurassic age. In the Morado arroyo area this change could

be explained also by the presence of a lateral ramp.



has been interpreted as a result of the inversion of the crestal

collapse graben faults. These faults develop above the upper

listric portion of the detachment (McClay, 1995). Cross-

strike variations in facies and thickness of the Jurassic strata

indicate that this area is located at the western margin of the

inverted lower-middle Jurassic basin and at the eastern

margin of the upper Jurassic basin (Fig. 8).

Whereas the original rift basin may have been composedof numerous normal faults, only ®ve faults have been

considered for reactivation along the structural section

(Fig. 8). The changes in the stratigraphic thickness of the

Tordillo Formation provide critical evidence for the Late

Jurassic normal faulting. The great variation in the thickness

of this unit is best explained by the presence of a normal

fault between the Chacayal thrust and the Yeguas Muertas

anticline. The inferred west-dipping fault 1 is assumed to

preserve a normal offset at depth. Faults 2±4 are interpreted

to be crestal collapse graben faults situated westward of a

rollover anticline associated with fault 5, with a similar

geometry as that described by Gibbs (1984) in the NorthSea extensional basin. Fault 2 is an antithetic east-dipping

structure overlain by the thin-skinned, out-of-sequence

Estero Caballos thrust. Faults 3 and 4 are synthetic west-

dipping structures. The pre-existing extensional and transfer

faults may trigger ramp generation and complicate thrust

sequence and trend variation, as appears to be the case

with fault 5. This fault is inferred to be segmented by

strike-slip faults, which control the abrupt change in strike

of the Morado thrust in zone II (Fig. 9). The inferred strike-

slip faults that segment the CordoÂn de Jorge and Palomaresranges have been interpreted as an en echelon transfer

system related to the extensional period. The change in

strike only affects out-of-sequence structures located toward

the east of Morado thrust, suggesting a reactivation of

inferred fault 5 before the emplacement of these thrusts,

yet after the generation of in-sequence thrusts of the Cerro

Palomares system.

The rift model presented here corresponds to a listric fault

system in which the faults nucleated and propagated above a

major detachment. An extensional ramp-̄ at detachment

geometry is proposed to account for the inversion of the

crestal collapse graben faults, with the extensional roll-over structure preserved despite subsequent compression

(e.g. McClay and Buchanan, 1992) (Fig. 10). The basal


Fig. 10. (a) Characteristic hangingwall geometry of a ramp-¯at listric fault. (b) Reactivation of the lower listric segment of the fault; which resulted in the

preservation of the extensional roll-over structure unaffected by the contraction (based on McClay and Buchanan, 1992, analog models).

Fig. 11. Schematic structural cross section of the Aconcagua fold-and-thrust belt, the Alto TunuyaÂn foreland basin, the Cordillera Frontal and the Cuyo basin,

at 33840 0S. In this model the basal detachment of the fold-and-thrust belt is the same for the Cordillera Frontal and the rift system of the Cuyo basin.



detachment of the Jurassic rift may be related to the Triassic

Cuyo basin detachment (Fig. 11).

5.2. Timing of deformation

The tectonic history of the Aconcagua thrust belt has been

constrained by geometric relationships between structures

and syntectonic strata (Giambiagi et al., 2001). The onset of the deformation is con®rmed by the retro-arc geochemical

signature of the early Miocene Contreras Formation, which

suggests magma generation prior to crustal thickening

(Ramos et al., 1996b). The ®rst indication of tectonism

and uplift of the Cordillera Principal is the deposition of

the TunuyaÂn Conglomerate in the early-middle Miocene.

This unit, which comprises lower middle Miocene to

lower upper Miocene strata (16±10 Ma), contains clasts

derived from the Cordillera Principal (Giambiagi, 1999a).

Uplift of the Cordillera Frontal, which began at 8.5 Ma

and continued until 6 Ma, is demonstrated by (1) the uncon-

formity separating the Palomares Formation from the under-

lying TunuyaÂn Conglomerate (Giambiagi, 1999b), (2) the

change in paleocurrent directions from west±east to north-

east± southwest, and (3) the presence of locally derived

conglomerates in the synorogenic deposits of the Palomares

Formation (Giambiagi et al., 2001).

Deformation in the fold-and-thrust belt and the foreland

basin continued after the uplift of the Cordillera Frontal.

Furthermore, since lower Pliocene±Pleistocene volcanic

rocks unconformably cover the deformed belt, the main

deformation event must have occurred before early Plio-

cene. In addition, a stock in the Paso Colinas area (34 8S),

interpreted to be an early post-tectonic intrusive (Godoy,

1998) and dated as 3.4 Ma (whole rock K/Ar age, Ramoset al., 1997), indicates that the structure must be older than

3.4 Ma.

5.3. Structural evolution

Accurate recording of surface structural and sedimento-

logical data from the Aconcagua fold-and-thrust belt has

been integrated to show the reconstruction of the Andean

deformation of the Cordillera Principal and Cordillera

Frontal at the latitudes 33830 0 to 33845 0S. Overprinting rela-

tionships indicate that the fold-and-thrust belt evolved by a

forward propagating thrust sequence with several periods of

out-of-sequence thrust emplacement.The compressional deformation in the belt started around

17 Ma and continued until about 4 Ma. We propose three

compressive deformational stages during the Andean

orogeny in this region. A ®rst phase of deformation is

responsible for the tectonic inversion of earlier normal

faults, which achieved only 8% of the present total short-

ening. Uplift associated with inversion is assumed to be

concentrated in crestal collapse normal faults, as predicted

by analogue models (McClay, 1995). At around 15 Ma there

was an important change in provenance of synorogenic

strata from volcanic clasts to the ®rst appearance of Meso-

zoic clasts (Irigoyen et al., 2000; Giambiagi et al., 2001).

This is interpreted to represent initial thrusting and uplift of

Mesozoic strata following the inversion of the pre-existing

structures. This sequence involved the generation of thrusts

and backthrusts and the creation of a triangle zone with 55%

of the total amount of contraction achieved.

The shift of the thrust front to the east generated the uplift

of the Cordillera Frontal between 8.5 and 6 Ma. This uplift

produced the westward tilting of the structural basement of

the fold-and-thrust belt, the generation of a broken foreland

basin, and the formation of a `sticking point' that prevented

the propagation of the thrust belt towards the foreland. The

next phase of deformation, which accounts for 37% of the

shortening, occurred during the late Miocene, after the uplift

of the crystalline basement of Cordillera Frontal. It corre-

sponds to formation of the out-of-sequence thrusts and

deformation of the foreland basin.

5.4. Comparison with the deformation in the ¯at-slabsegment

Based on the different structural styles, the fold-and-

thrust belt of the Cordillera Principal has been divided

into three distinct segments: La Ramada, between 308 and

32830 0S (Ramos et al., 1996a; Cristallini and Ramos, 2000),

Aconcagua, between 32830 0 and 348S (Ramos, 1988); and

MalarguÈe, between 34 and 368S (Kozlowski et al., 1993). La

Ramada and MalarguÈe fold-and-thrust belts are thick-

skinned belts formed by tectonic inversion of the Mesozoic

rift systems. The northern sector of the Aconcagua fold-and-

thrust belt is located above the ¯at-slab subduction segmentand is thought to be a thin-skinned system. By contrast, the

southern sector, located above the transition segment,

behaved as both thin- and thick-skinned structural systems

due to the in¯uence of pre-existing Triassic±Jurassic exten-

sional structures.

To analyze the role of subducted slab geometry in the

evolution of the Andes, the style and timing of deforma-

tionÐand rates of shorteningÐare considered for both the

¯at-slab segment (32830 0 ±33830 0S) and the transition

segment (338±33845 0S) (Fig. 12). In the northern sector of

the Aconcagua fold-and-thrust belt, thin-skinned tectonics

is promoted by the presence of an undisturbed sedimentary

cover and a weak basal layer. This sector has a greateramount of shortening (55±60 km) and higher rates of thrust-

ing (4.7±6 mm/yr) (Cegarra, 1994; Ramos et al., 1996a)

than the southern sector (47 km and 3.9±4.3 mm/yr, respec-

tively). It must be noted, however, that the main difference

between the two sectors is the style of deformation.

The fact that the Triassic±Jurassic rift system dies out at

33830 0S (Alvarez et al., 2000) is one of the main differences

between the thin-skinned northern part and the thick-

skinned southern part of the belt. This feature cannot be

controlled by the geometry of the subducted slab, since

north of 328S, where the rift intersects the Cordillera




Principal, there is again tectonic inversion (Cristallini and

Ramos, 2000).

Deformation progressed eastward in both regions from

early Miocene to present, but it started earlier (,20 Ma)

in the north (Fig. 12). Synchronism of thrusting in the

Cordillera Principal and Cordillera Frontal is well documen-

ted in both sectors by the relationship of thrusting and

syntectonic sedimentation (Perez and Ramos 1996; Irigoyen

et al., 2000; Giambiagi et al., 2001). In the southern sector

there is a close correspondence between the termination of deformation in the Aconcagua belt and the beginning of

reactivation of normal faults in the Cuyo basin. The same

could have happened in the north, although the onset of the

deformation in the Precordillera is poorly constrained. This

deformation has been interpreted to begin at 7 Ma when the

deformation in the Cordillera Frontal ceased.

The structural style and magnitude of the foreland defor-

mation varies strongly along strike. The foreland north of

338S is characterized by a thick-skinned thrusting of the

Precordillera range, which accommodates a substantial

part of the total amount of shortening (Ramos et al.,

1996a). This range contains thrusts, high-angle faults, and

strike-slip faults as a consequence of Neogene structural

inversion of the Triassic half-graben basins and reactivation

of oblique Paleozoic fracture zones (CorteÂs and Costa,

1995; Folguera et al., 2001). In the southern end of the

Precordillera (t338S), Sarewicz, 1988 proposed a model

of thin-skinned deformation with signi®cant shortening

and an unusual rate of deformation (between 6 and

16 mm/yr). However, this model is not consistent with

recent thick-skinned models based on seismic information(Legarreta et al., 1992; DellapeÂ and Hegedus, 1995; Chiar-

amonte et al., 2000). These models demonstrate that the

foreland foothills of the transition zone are characterized

by an array of folds and thrust faults produced by inversion

of the Cuyo basin structures and generation of new inter-

mediate to high-angle faults, with minor amount of short-

ening (5%; Cristallini and Ramos, 2000, pers. com.). The

boundary between the highly deformed foreland north of

338S and the folded southern sector is remarkably sharp,

although the Cuyo basin extends northward into the present

Precordillera. This leads to the interpretation that the


Fig. 12. Comparison of the style, timing of deformation, and shortening rates of the different morphostructural units composing the Andes between the

southern part of the ¯at-slab segment and the study area within the transition zone.



foreland deformation is controlled by the geometry of the

subducted slab.

6. Conclusions

The segment of the Andes located between 33830 0 and

33845

0

S records Cenozoic deformation that resulted fromthe interaction between the geometry of the subducted

slab and pre-existing extensional structures. The develop-

ment of this segment above the transition zone from ¯at to

normal subduction segments differs from segments located

to the north and south.

The Aconcagua fold-and-thrust belt can be divided into

two sectors: a northern thin-skinned sector between 32830 0

and 33830 0S, and a southern thick- and thin-skinned sector

between 33830 0 and 33845 0S where the study area is located.

Deformation in this sector was the result of basement-

involved thrusts in the hinterland of the belt, which

propagated into the sedimentary cover and generated a

thin-skinned tectonic style in the outer part.

Analyses of the tectonic distinctions between ¯at, normal,

and transition subduction zones permit us to conclude that a

good correlation may be established between variation in

the dip of the Wadati-Benioff zone and the amount of

orogenic shortening of the foreland. However, the structural

style of the different segments is primarily controlled by the

previous geological history and associated crustal hetero-

geneity (e.g. basin structure and structural boundaries across

strike).

Acknowledgements

This research was supported by grants from the Univer-

sity of Buenos Aires (UBACYT TW87), ANPCYT (Pict 07-

06729), and CONICET (PIP 4162). We wish to thank Maisa

Tunik, Carla Buono, Sergio Orts, Patricio Vazques Calvo,

Emilio Rocha, Cochi Kim, and MartõÂn Pereyra for their help

in the ®eld. Special thanks are due to TomaÂs Zapata, Pirzio

Godoy, Leonardo Legarreta and Pamela Alvarez for their

comments and discussions; and to Ernesto Cristallini for

information about the Cuyo basin. We also thank Brendan

McNulty, Alain Lavenu and Brian Horton for their valuable

comments, which contributed to the improvement of the

manuscript.

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