Giambiagi Et Al 2008 Temporal

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Temporal and spatial relationships of thick- and thin-skinned deformation: A casestudy from the Malargüe fold-and-thrust belt, southern Central Andes

Laura Giambiagi a,⁎, Florencia Bechis a, Víctor García b, Alan H. Clark c

a Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales —CCT-CONICET, Parque San Martín s/n, Mendoza, 5500, CC 330, Argentinab Laboratorio de Modelado Geológico (LaMoGe), Universidad de Buenos Aires, Pabellón II, Ciudad Universitaria, Capital Federal, 1428, Argentinac Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario, Canada K7L 3N6

A B S T R A C TA R T I C L E I N F O

Article history:

Received 5 June 2006

Received in revised form 21 December 2006

Accepted 15 November 2007

Available online xxxx

Keywords:

Southern Central Andes

Malargüe fold-and-thrust belt

Thick- and thin-skinned tectonics

Inversion and thrusting

Simultaneous thrusting

In this paper we analyse two end-member models of temporal and spatial interactions between thick- and

thin-skinned structures in a thrust front with pre-existing rift structures. In the most commonly accepted

model, a hinterland-to-foreland sequence of inversion of pre-existing normal faults is proposed. As a result,

the emplacement of shallow thrust sheets in the sedimentary cover occurs before the basement inversion in

the foreland. In the other model, basin inversion occurs early in the deformation history of the external part

of a fold-and-thrust belt, as the result of a foreland-to-hinterland sequence of inversion.

The Malargüe fold-and-thrust belt (34–36°S) formed in response to compression of the Mesozoic Neuquén

basin during Neogene to Pleistocene times. Integrating detailed structural data from the northern part of this

belt with new Ar/Ar dating, we propose a revised kinematic model of thick- and thin-skinned interaction and

define the temporal-spatial evolution of the belt. Comparison of the timing of deformation in the thick- and

thin-skinned areas strongly supports the hypothesis that the reactivation of normal faults was coeval with

the insertion of shallow detachments and low-angle thrusting along the migrating front of the thrust belt

and occurred from the foreland to the hinterland. Detachments occur at several stratigraphic horizons,

including a deep basement decóllement related to the basement-involved thrusting and shallow detachments

located within the Jurassic and Cretaceous beds. These shallow and deep detachments were coeval producing

simultaneous development of thrusts during the complex deformation of the thrust front between 15 and8 Ma.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Many thrust belts are combinations of both thin- and thick-

skinned thrustings as a result of reactivation of pre-existing

anisotropies and weakness zones in the upper crust. The presence of

pre-existing rift structures widely exerts an important control on

thrust-belt geometry and evolution. However, the extent to which

these anisotropies control regional patterns and the kinematics of

deformation in a subsequently developed fold-and-thrust belt is

controversial. The manner in which thin and thick-skinned related

structures interact in time remains poorly constrained. This paper

sheds some light on these topics by analysing the kinematic evolution

of the Malargüe fold-and-thrust belt of the Southern Central Andes.

The Andes of Argentina and Chile between latitudes 33° and 36° S

are superimposed to the Triassic–Jurassic Neuquén basin. The north-

ern part of this extensional trough comprises a series of NNW-

trending depocentres (Fig. 1). At the latitude of the study area, the

Neogene geology of the Cordillera Principal is dominated by the

Malargüe fold-and-thrust belt (Malargüe FTB) involving the Mesozoic

rift sequences of the Atuel depocentre. The Malargüe FTB has been

classically identified as a hybrid fold-and-thrust belt with basement

thrust sheets transferring shortening to the Meso-Cenozoic sedimen-

tary cover (Kozlowski et al., 1993; Manceda and Figueroa, 1995; Rojas

et al., 1999; Zapata et al., 1999; Silvestro and Kraemer, 2005 ). This

study establishes the kinematics of thin- and thick-skinned interac-

tion and hence defines the temporal-spatial evolution of the northern

Malargüe FTB. We present the results of newly acquired field

observations, integrated with subsurface data acquired from oil

exploration. A new kinematic model, which integrates the structural

data and new Ar/Ar geochronology with previous surface data and Ar/

Ar dating, is proposed for the thrust front of the northern part of the

belt. A chronological study of the deformation has been used to test

how thin- and thick-skinned deformational zones interact. Attention

has been paid to the timing of basement fault reactivation and coeval

activation of a shallow detachment in the foreland. From these

observations we address the wider questions of the geometric

evolution and kinematics of fold-and-thrust belts and the role of

extensional structures in generating variable deformational styles.

Thus, does tectonic inversion of normal faults precede thin-skinned

deformation of the sedimentary sequence in the foreland, or does

Tectonophysics xxx (2008) xxx-xxx

⁎ Corresponding author. Fax: +54 261 5244201.

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

[email protected] (F. Bechis), [email protected] (V. García).

TECTO-124155; No of Pages 17

0040-1951/$ – see front matter © 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.tecto.2007.11.069

Contents lists available at ScienceDirect

Tectonophysics

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Please cite this article as: Giambiagi, L., et al., Temporal and spatial relationships of thick- and thin-skinned deformation: A case study fromthe Malargüe fold-and-thrust belt, southern Central Andes, Tectonophysics (2008), doi:10.1016/j.tecto.2007.11.069

http://dx.doi.org/10.1016/j.tecto.2007.11.069

http://www.sciencedirect.com/science/journal/00401951


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basement inversion occur out-of-sequence after the emplacement of

shallow thrust sheets. Our research demonstrates that in the northern

part of the Malargüe FTB, deformation began with inversion of the rift

master fault, in the foreland, and subsequently migrated to the

hinterland with the simultaneous development of inverted high-angle

faults, thrust faults and basement short-cut and by-pass faults.

2. Tectonic setting

The tectonic setting and evolution of southern South America is

controlled by the subduction regime at the western margin of the

South American plate and the Mid-Atlantic Ridge spreading rates

along its eastern margin (Uliana and Biddle, 1988). During the

Mesozoic, the western margin was the site of an active trench, a

relatively narrow magmatic arc and a series of back-arc extensional

basins (Charrier, 1979; Uliana and Biddle, 1988; Legarreta and Uliana,

1991). The most important of these basins was the Neuquén basin,

which comprised several NNW-elongated depocentres implanted on

pre-Jurassic continental crust (Vergani et al.,1995). It was initiated asa

rift basin in the Late Triassic, when Chilean and central western

Argentina underwent extensional tectonism (Digregorio et al., 1984;

Legarreta and Uliana, 1991). Marine and continental sediments were

deposited in isolated depressions during the Late Triassic to Early

Jurassic and are presently exposed in the Cordillera Principal

(Gulisano, 1981; Uliana and Biddle, 1988; Legarreta and Gulisano,

1989). One example of these troughs is the Atuel depocentre, where

the northern part of the Malargüe FTB was developed (Fig. 1).

By the end of the Early Cretaceous, a major plate tectonicreorganization took place (Somoza, 1998), ending the development

of the marine intra-arc and back-arc basins (Mpodozis and Ramos,

1989). Compressive tectonics along the western margin of southern

South America began in the late Early Cretaceous ( Mpodozis and

Ramos, 1989; Cobbold and Rosello, 2003; Zapata and Folguera, 2005).

There is, however, no evidence of this early compression in the study

area, probably reflecting its eastern position. At the study latitude,

convergence was oblique during the Paleogene but became progres-

sively more perpendicular to the trench during the Neogene with a

concomitant increase in convergence rate (Pardo Casas and Molnar,

1987; Somoza, 1998).

The main components of the tectonic setting of the region are a

magmatic arc along the Argentina–Chile border and a fold-and-

thrust belt, which goes from the Cordillera Principal (Malargüe FTB)

Fig. 1. Regional location map and morphostructural map of the Andes between 32° and 36° S. The location of the Malargüe fold and thrust belt, the northernmost sector of the

Neuquén Basin, and the Atuel depocentre in the present-day Cordillera Principal are highlighted. The box indicates the location of the study-area and Fig. 2.

2 L. Giambiagi et al. / Tectonophysics xxx (2008) xxx-xxx

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to a series of uplifted basement blocks in the Cordillera Frontal. The

Malargüe FTB extends from 34° to 36°S and has developed since

Miocene times in a thick-skinned style related to tectonic inversion

of Mesozoic rift structures (Kozlowski, 1984; Manceda and Figueroa,

1995). Deformation involves pre-Jurassic basement rocks and

Mesozoic rift and back-arc basin deposits. The Cordillera Principal

is underlain by Proterozoic to Paleozoic metamorphic and plutonic

rocks of the Cordillera Frontal uplifted by high-angle faults along its

eastern flank. The southern part of this range is uplifted by the

Carrizalito fault which dies out alongside a SW-plunging anticline

south of the Río Diamante (Fig. 2) (Kozlowski, 1984; Turienzo and

Dimieri, 2005).

Fig. 2. Simplified geologicalmapof theMalargüeFTB, between 34°30′ and 35°00′S, showing majorstructuralfeatures andlocationof crosssection in Fig.11. Theareahas been divided

into two sectors: an eastern sector where the Upper Triassic to Upper Jurassic rocks crop out, and a western sector where the Lower Cretaceous to Neogene rocks crop out. Only the

major faults havebeen drawn. Boxes indicate location of Figs5 and6 . Based on Kozlowskiet al. (1981), Cruz et al.(1991), Scaricabarozzi (2003), Kimet al.(2005), Turienzo and Dimieri

(2005), Giambiagi et al. (2005a,b), Bechis et al. (2005), Giambiagi et al. (2008). D2, D3, D6, D8, D9, D10, D12, D13 and D14: location of Ar/Ar dating samples. B-B ′: balanced cross

section of the Malargüe FTB on Fig. 11.

3L. Giambiagi et al. / Tectonophysics xxx (2008) xxx-xxx

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3. Stratigraphic framework

The lithostratigraphic units of the Malargüe FTB are: Proterozoic to

Triassic metamorphic, plutonic and volcanic rocks which constitute

the basement of the belt; Upper Triassic to Lower Jurassic marine and

continental rift sequences deposited in the Neuquén back-arc basin;

Middle Jurassic to Cretaceous platform sequences; and Cenozoic

sedimentary and volcanic rocks.

3.1. Basement rocks

Basement rocks crop out in the Cordillera Frontal, northeast of the

study area (Fig. 2), and in the San Rafael block, east of the study area.

They consist of Proterozoic metamorphic rocks unconformably over-

lain by Upper Paleozoic marine black shales and continental

sandstones, intruded by Upper Paleozoic granitoids (Volkheimer,

1978). Permian–Triassic intermediate and acid volcanic rocks uncon-

formably overlie the previously deformed rocks (Japas and Kleiman,

2004).

3.2. Neuquén basin infill

The lowermost Mesozoic sequences are Late Triassic to Early

Jurassic marine and fluvial synrift strata, unconformably deposited

over deformed basement rocks (Fig. 3). These strata crop out in the

western part of the study area (Fig. 2). The deposition of the marine

massive mudstonesand shalesof theArroyo Malo Formation (Riccardi

et al., 1997; Riccardi and Iglesia Llanos, 1999; Lanés, 2005) marked the

onset of extensional activity in the rift basin. The El Freno Formation

crops out in the eastern sector of the Atuel depocentre and is

represented by braided alluvial deposits with a predominant eastern

provenance. The Puesto Araya Formation consists of slope-type fan

delta deposits (lower section) related to the braided alluvial systems

of the easterly El Freno Formation, and storm-dominated shelf

deposits (upper section) (Lanés, 2005). Off-shore shelf black clays-tones were conformably deposited over the marine strata of the

Puesto Araya Formation, and correspond to the Tres Esquinas

Formation of Toarcian–Bajocian age (Gulisano and Gutiérrez Pleiml-

ing,1994). There is no evidence of faulting duringthe deposition of the

marine platform strata, indicating that the boundary between fluvial

and marine strata in the eastern part of the depocentre marks the end

of the extensional phase, as was suggested by Lanés (2005).

The middle Callovian to Oxfordian interval comprises clastics,

carbonates and evaporites of the Tábanos Formation and the Lotena

Group (Gulisano and Gutiérrez Pleimling,1994). During Kimmeridgian

times, alluvial, fluvial and eolian continental clastic deposition was

controlled by normal faults (Tordillo Formation) (Ramos, 1985;

Cegarra and Ramos,1996; Giambiagi et al., 2003a,b). These continental

deposits were followed by accumulation of calcareous shelf facies

(Mendoza Group). Aptian to Cenomanian red continental deposits

overlying these strata are associated with evaporites and marine

carbonates (Rayoso Group) and Late Cenomanian to Early Campanian

Fig. 3. Generalized stratigraphic column of the Meso-Cenozoic units exposed in the Malargüe FTB (from Gulisano and Gutiérrez Pleimling, 1994, and Legarreta and Gulisano, 1989).

Rift-related units, cropping out in the Atuel depocentre, are defined on the basis of the biostratigraphic zonation and correlation of Riccardi et al. (1997, 1999) and Lanés (2005).


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continental red beds (Neuquén Group: Gulisano and Gutiérrez

Pleimling, 1994, Riccardi et al., 1999). Subsequently, a transgression

from the Atlantic Ocean allowed the accumulation of clastics and car-

bonates (lower Malargüe Group: Barrio, 1990; Tunik, 2004), followed

by fine-grained Paleocene to Eocene sedimentary rocks of lacustrine

and playa lake origin (upper Malargüe Group).

3.3. Synorogenic deposits

Synorogenic sediments and volcanic and volcaniclastic rocks filling

a foreland basin are represented by the Miocene Agua de la Piedra and

Loma Fiera Formations, the Pliocene Río Diamante Formation, and

three Pleistocene coarse conglomerate units (Mesones, La Invernada

and Las Tunas Fms.). These rocks crop out in the Cuchilla de la Tristeza

range (Fig. 2) and are separated by angular unconformities. Foreland

basin sedimentation began with deposition of alluvial fan and fluvial

systems of the Agua de la Piedra Formation over an angular uncon-

formity (Combina et al., 1994; Combina and Nullo, 2005). This unit

is composed of interbedded coarse conglomerate and sandstone

with clasts from volcanic and sedimentary rocks derived from the

Cordillera Principal (Yrigoyen, 1993). The base of this formation is

composed of andesitic clasts in a tuffaceous sandstone matrix. 40Ar/39Ar ages for two boulders (12.83±0.10 and 13.44±0.08 Ma) at the

base of the Agua de la Piedra Formation suggest that the unit is

younger than 13 Ma (Baldauf, 1997).

The Loma Fiera Formation unconformably overlies the Agua de la

Piedra Formation. This unit consists of cross-bedded tuffs containing

clasts of pumice and granite, overlain by volcanic breccia, conglom-

erates and tuffaceous sandstones and andesitic tuffs (Yrigoyen, 1993;

Combina and Nullo, 2000), interpreted as pyroclastic and laharic

deposits (Combina and Nullo, 2000). Conglomerates of this unit

appear to interfinger with andesite flows of the Huincan Formation

(Dessanti, 1959) and incorporate granitic and volcanic clasts from the

Cordillera Frontal, indicating that by the time the Loma Fiera

Formation was deposited the basement was already exposed. 40Ar/39Ar ages for two boulders (9.51±0.07 and 10.68±0.11 Ma) at the base

of the Loma Fiera Formation (Baldauf, 1997) imply a maximum age of

9.5 Ma. The overlying conglomerates and sandstones of the RíoDiamante Formation exhibit gradational contacts with the Loma Fiera

Formation, indicating deposition during a time of decreasing volcanic

and tectonic activity (Combina and Nullo, 1997).

3.4. Cenozoic volcanism

The older Cenozoic igneous rocks, referred as Molles Suite Intrusives

(Groeber, 1951; Volkheimer, 1978), are composed of lower Miocene

basaltic and andesitic porphyry stocks associated with dacitic hypabys-

sal bodies (Baldauf, 1997), exposed in the western and eastern parts of

the Malargüe FTB. Intense volcanism in the Middle Miocene to Early

Pliocene (Stephens et al., 1991; Baldauf et al., 1992; Ramos and Nullo,

1993; Baldauf, 1997) is grouped in the Huincan Formation. This igneous

activity took place between 10.5 and 5.5 Ma (Baldauf, 1997) andcomprises basaltic andesites and andesites similar in chemistry to the

Teniente Volcanic Complex located tens of kilometres to the west (Nullo

et al., 2006). This magmatic event has been proposed by Baldauf (1997)

to have occurred during the waning stages of, or after compressive

deformation in the eastern sectorof the Malargüe FTB. However,we will

show that this volcanic unit has the same age as the main episode of

deformation.

4. Structural setting

4.1. Rift architecture

The northern part of the Neuquén basin is a predominantly NNW-

trending rift comprising a series of narrow depocentres (Fig. 1). The

Atuel depocentre exhibits an asymmetric architecture interpreted by

Manceda and Figueroa (1995) as representing a half-graben with west-

facing polarity. Elsewhere (Giambiagi et al., 2005a, 2008; Bechis et al.,

2005), we demonstrated that the principal normal faults of the Atuel

depocentre have been inverted and moreover, we documented a

detailed characterization of the depocentre architecture through the

integration of our structuralanalysis of rift-relatedfaults with previous

stratigraphic and paleogeographic studies (Lanés, 2005). The depo-

centrecomprised the Arroyo Malo and RíoBlanco half-grabens (Fig. 4),where the former is interpreted as a completely submerged sub-basin

filled with marinesyn-rift strata(Arroyo Malo Fm.and lowersectionof

the Puesto Araya Fm.) and sag deposits (Tres Esquinas Fm.). Its master

fault, the west-dipping NNW-trending Alumbre fault, is well exposed

in the headwaters of the Alumbre creek, where it dips at a high angle

towards the west with no evidence of structural inversion at shallow

levels. In contrast, the Río Blanco half-graben was filled with con-

tinental syn-rift strata(El FrenoFm.) andsag deposits (uppersectionof

the Puesto Araya Fm. and Tres Esquinas Fm.), and was bounded along

its eastern margin by the NNW-trending La Manga master fault. Both

Alumbre and La Manga faults have been interpreted as pre-existing

structures reactivated during the rifting event. This reactivationwould

have generated an oblique rift with WNW- and NNE-striking oblique

normal faults.

4.2. Andean deformation

During Miocene to Pleistocene times, the Atuel depocentre was

inverted and incorporated into the thrust sheets of the thick-

skinned Malargüe FTB (Kozlowski et al., 1993; Manceda and

Figueroa, 1995) exerting its structural architecture a profound

influence on the development of the belt. This influence is reflected

in a variety of structural styles in the study area. We identify several

trends of regional structures, significant changes in fold wavelengths

and multiple detachments (Fig. 5), indicating that the present-day

structure of the belt is controlled by major rift-related basement-

rooted faults. We argue that the mid-crustal weak zone above which

basement thrusting occurs was inherited from a previous mid-

crustal extensional flat detachment. The propagation of invertedbasement faults into the sedimentary cover generated complex

structures that are restricted to narrow belts characterized by tight

Fig. 4. Block diagram illustrating the structural architecture of the Atuel depocentre,

where the main normal faults have been delineated. Note that the scale is approximate.

From Giambiagi et al. (2005a, 2008).


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folding and faulting. Deformation in these areas could have been

complicated by basement short-cut faults which generated several

detachment levels in the sedimentary cover. Towards the foreland

the Andean deformation developed a thin-skinned system using

incompetent layers from the Neuquén and Malargüe Groups as

detachment levels.

Fig. 6. Geological map of the eastern sector of the Malargüe FTB.Modified after Kozlowski etal. (1981) and Cruzet al. (1991) and ourown observations. A-A′: seismic line16029on Fig.8.

Fig. 5. Geological map of the western sector of the Malargüe FTB, based on new field observations and previous stratigraphical studies carried out by Lanés (2005).


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5. Spatial relationship between thick- and thin-skinned structures

The Malargüe FTB can be divided into western and eastern sectors

on the basis of palaeoenvironmental and tectonic relationships. Their

mutual boundary is defined by the NNW-trending Borbollón–La

Manga lineament, related to the La Manga master fault of the

Mesozoic rift system (Figs. 2 and 4).

5.1. Eastern sector

The eastern sector is an emergent thrust-front system, made up of

several N–S to NNW-trending thrust sheets involving Cretaceous to

Neogene strata in a thin-skinned tectonic style (Fig. 6). The oldest

sedimentary rocks involved in the deformation are Cretaceous shales,

evaporites and red beds. The stratigraphic section is dominated by

several incompetent evaporite and black shale units alternating with

competent sandstone units. At least two main décollements are

regionally developed in the eastern zone and account for the thin-

skinned architecture. The lowermost is located in the lower part of the

Upper Cretaceous red beds and is present in the northern part of the

study area, whereas the shallowest is recorded in the uppermost

Cretaceous beds. In the northern part of the belt, in the Río Diamante

area, a third decóllement is located at the base of the Upper Jurassic–

Lower Cretaceous black shale succession (Kim et al., 2005; Broens and

Pereira, 2005).

Three main thin-skinned thrusts have been identified in this

sector: the Sosneado, Mesón, and Alquitrán faults (Kozlowski, 1984)

uplifted from the upper decóllement in the uppermost Cretaceous

beds (Fig. 6). The Sosneado and Mesón faults uplift the Cuchilla de la

Tristeza range and are thrust-rooted into this shallow detachment.

The Mesón thrust repeats the Neogene Agua de la Piedra Formation,

and is a low-angle, west-dipping, fault with N–S trend. This fault is

associated with a hanging wall syncline, which acted as a Neogene-

Quaternary foreland basin depocentre, in which thick synorogenic

deposits record the growth history of the belt. The Sosneado thrust

transposes the Paleogene units on top of the Agua de la Piedra and

Pleistocene fanglomerates (Fig. 7). It strikes N–S and dips 24° west.

The Alquitrán fault is inferred to generate an open anticline thataffects UpperCretaceous to Neogene stratain theCerroAlquitrán area.

Fig. 8 sketches the present-day configuration of the eastern sector

of the belt along the section A-A′ of Fig. 6, as constrained by field and

subsurface (seismic and well) data. A migrated reflection seismic

dataset constrained by well log information from the Cuchilla de la

Tristeza range was available in this study. Two interpretations of the

seismic line 16029 have been made to identify the spatial relationship

between thick- and thin-skinned structures. Interpretation A (Fig. 8A)

assumes that the inversion of the La Manga normal fault accounts for

the detachment in the cover and generation of the Mesón, Sosneado

and Alquitrán thrusts. An alternative approach is shown in inter-

pretation B (Fig. 8B), where the shallow detachment developed in an

initial episode of thin-skinned deformation, not related to the

inversion of the master fault, and was folded in the ensuing episodeof tectonic inversion, in agreement with previous models of the

northern part of the Malargüe FTB (Pereira, 2003; Kim et al., 2005).

Both alternatives are geometrically plausible and the low resolution of

seismic lines along the border between the thick- and thin-skinned

zones does not allow us to discriminate between them. As we will see

in next sections, we favour interpretation A because of the timing of

movement of the basement and thin-skinned faults.

5.2. Western sector

In the western sector, outcropping rocks are predominantly Upper

Triassic–Lower Jurassic rift sequences overlain by Middle Jurassic to

Lower Cretaceous deposits (Fig. 5). TheUpperCretaceous and Paleogene

rocks have been eroded in this domain, and Neogene synorogenic strata

were not deposited (Fig. 3). This sector has previously been studied by

Fortunatti and Dimieri (2002, 2005), who outlined several backthrustsrelated to the basement involvement in the deformation. The Andean

structural pattern shows two predominant trends (Fig. 5): NNE-striking

folds andsubordinate faults;and N to NNW-strikingfoldsand faults.The

western sector is also characterized by a combination of two deforma-

tional styleswith large-scaleopen folds andnarrow belts of intense east-

vergent folding and faulting (Figs. 5 and 9). Large-scale anticlines with

associated synclines suggest regional-scale basement uplift. In the

frontal part of these inferred basement-cored folds, we propose that the

displacement was mainly transferred to the sedimentary cover,

generating narrow belts of intense foldingof syn-rift and post-rift strata

(Fig. 9). Broad, long-wavelength folds developed in the hanging walls of

moderate-to-high-angle reverse faults and are considered to have

formedby inversion of older normal faults (Fig.10, A–B). Two structures,

the La Manga and El Freno faults, are interpreted as reactivated rift-related normal faults on the basis of the highly variable thicknesses and

facies of the rift sequences (Lanés, 2005), the high cut-off angles along

the faults, the presence of antithetic and synthetic faults reactivated in a

reverse sense (Giambiagi et al., 2005b), and syn-extensional unconfor-

mities preserving the original extensional geometry.

Fig. 11 is a cross-section incorporating a projection of the

interpretation A of the seismic line 16029 (Fig. 8A). The cross-section

has been restored with a line-length balance and constant thickness

hypothesis for the sedimentary cover, and an area-balanced method

for the basement. In this section, the previously identified (Fig. 4)

three main basement faults are interpreted to be the principal

structures of the western sector. The faults propagated upwards into

the sedimentary strata, producing shortening accommodated by

thrusting at depth and by folding in the upper levels of the pile, as

Fig.7. TheSosneadothrustin theCuchillade la Tristezarange. Thefault placesthe upper

part of the Malargüe Group on top of Pleistocene fanglomerates and it is covered by

Holocene deposits. See map on Fig. 6 for location.


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Fig. 8. Seismic line 16029 located in the southeastern sector of the Atuel depocentre, and its structural interpretation (see Figs. 2 and 6 for location). Time to depth conversion was

done using Ernesto Cristallini's “Pliegues 2D” program and subsurface data from the YPF.Md.NPQ.x-1 well. Middle J + K: Middle Jurassic to Cretaceous strata (Lotena Group, Tordillo

Fm., and Mendoza, Rayoso and Neuquén Groups); UpperJ+K: Upper Jurassic to Cretaceous strata (Mendoza, Rayoso and Neuquén Groups), Upper K+Paleogene: Upper Cretaceous to

Paleocene (Malargüe Group),AP: Agua de la PiedraFm., LF: Loma Fiera Fm. andRD: Río Diamante Fm. (A) and (B):Two kinematic models for the interaction between thin- and thick-

skinned deformational zones. Interpretation A assumes that the inversion of the master fault accounts for the detachment in the cover. An alternative approach is shown in

Interpretation B, where a shallow detachment in the sedimentary cover developed first, before the inversion of the master fault.


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fault-propagation folds. The areas of intense folding and faulting are

located in front of these large-scale anticlines, as in the region east of

the El Freno anticline, where the marine sag deposits are strongly

deformed by kink and box folds (Fig. 10, C–D).

The La Manga fault system is the most signifi

cant structure in thefoothills, uplifting the Lower Mesozoic sequences on top of the

Neogene synorogenic units, and has a throw of several kilometres

(Kozlowski, 1984). We interpret this fault system as comprising three

related structures, i.e., the Arroyo Blanco fault, the La Manga inverted

normal fault, and a basement by-pass fault (Fig. 11). This highlights an

important characteristic of the basement-cover interaction along the

Triassic–Jurassic master fault, where multiple basement thrusts have

been stacked along the eastern limit of the former rift basin. The La

Manga fault can be interpreted as an inverted, west-dipping, normal

fault, because rift-related Upper Triassic–Lower Jurassic rocks are

present in itshanging wall andabsent in thefootwallblock (Fig.8). Weinfer that this fault has a convex-up geometry, cutting the basement-

cover interface at a high angle and progressively decreasing in dip

upwards. This geometry strongly implies the inversion of a high-angle

pre-existing normal fault by upward propagation of a steep basement

fault into the sedimentary cover. The La Manga by-pass fault has been

inferred in the seismic line (Fig. 8). It runs along the Arroyo La Manga

Fig. 9. A) Two different tectonic styles observed in the western sector of the Malargüe FTB: narrow belts of intense folding associated with a broad open fold. B) Interpretation of A:

Large-scale anticlines with associated synclines are interpreted as regional basement uplifts during inversion of preexisting normal faults. In the frontal part of these folds,

displacement is mainly transferred to the sedimentary cover generating intense folding in rift-related strata. See map on Fig. 5 for location.

Fig.10. Examples of two broad open anticlines (A and B), and narrow tightly folded belts located in front of these anticlines (C and D). See map on Fig. 5 for location.


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Fig. 11. Balanced cross section B-B′ of the Malargüe FTB at 34° 45′S. See Fig. 2 for location. The cross section shows the relationship between the western thick-skinned sector and th

restitution showsthe location of the main normal faults developed during the Triassic–Jurassic extension.During the Neogene inversion, thesestructures wereinverted in associationw

short-cut fault (ASF) and El Freno short-cut fault (ESF). The inversion of the La Manga fault is inferred to be associated with the generation of the La Manga by-pass fault (LMBF).

P l e a s e c i t e t h i s a r t i c l e a s :

G i a mb i a g i ,L .,e t a l .,T e mpor a l a nd s pa t i a l r e l a t i ons h i ps of t h i c k -a nd t h i n-s k i nne d

d e f or ma t i on: A c a s e s t ud yf r om




with a NNW-strike (Fig. 5) and overturns Mesozoic beds in the Loma

del Medio range (Kozlowski et al.,1981). The Arroyo Blanco fault crops

out in the Arroyo Blanco creek (Fig. 5), where it transposes Lower

Jurassic sag deposits over Upper Jurassic red beds and evaporites.

Open folds in the hanging wall of this moderate-to-high angle reverse

fault have been disturbed by two associated backthrusts. These faults

have previously been described by Fortunatti et al. (2004) and

Turienzo et al. (2004) as thin-skinned backthrusts and can be

distinguished in the seismic lines (Fig. 8).The El Freno fault has been interpreted as a NNE-striking high-

angle, reactivated fault with an associated basement short-cut fault

(Fig. 11). An abrupt stratigraphic change (Lanés, 2005) correlates with

the boundary between areas of open folding and intense folding and

faulting (Giambiagiet al.,2008). Theinversion of this fault ismarkedby

the broad El Freno anticline in its hanging wall (Fig. 10B). Its curved

axial plane has been interpreted to reflect the configuration of this

normal fault at depth. Associated with this thick-skinned structure,

small-scale anticlines and synclines with angular hinges (kinks and

box-folds) deform the Lower Jurassic sequences, and low-angle thrusts

formed above shallow detachments, in thin-skinned tectonic style

(Fig. 10, C–D). The steeply-dipping to overturned beds shown by the

outcrops east of the Arroyo El Freno creek reveal structural complexity.

Associated with the inversion of this fault, we have inferred the

presence of the El Freno basement short-cut fault to account for the

generation of a broad open syncline and a low-angle thrust ( Fig. 11).

The Alumbre fault is an approximately 15 km-long, NNW-striking

fault with a continuous trace. It was passively uplifted in the hanging

wall of the El Freno fault, preserving the inherited pattern of extensional

structure at shallow levels. This fault is exposed in the headwaters of

the Arroyo Alumbre creek (Fig. 5). Its orientation is consistent with the

NNW-trending paleocoast and with paleocurrents ranging from SSW to

NW documented by Lanés (2005). Although in outcrop it presents no

evidence of structural inversion, its lower segment is inferred to have

been inverted during Andean compression and to be responsible for a

series of backthrusts affecting the sedimentary cover. The generation of

a short-cut fault is associated with a basement wedge and oppositely

verging cover-detached underthrusts (Figs. 5 and 11). This complex

zone may have formed as a response to buttressing against a basementhigh, previously uplifted by the inversion of the El Freno fault.

We therefore infer that the tectonic evolution of the Malargüe FTB

involved both thin-skinned tectonics along several shallow detach-

ments within the Jurassic rift sequences (western sector) and

Cretaceous strata (eastern sector) and basement involvement along

a deeper detachment which accommodated stacking of basement

thrust units. This model predicts that steep, basement-involved

thrust-ramps in the western sector migrated upsection through

cover and evolved into flats when they reached the incompetent

syn-rift strata. A combination of extensional fault inversion and

development of new basement short-cut faults accounts for the

complex structure in the sedimentary cover.

6. Chronology of deformation

In order to constrain the age of the deformation and to choose

between both interpretations of thick- and thin-skinned interaction

(interpretations A and B – Fig. 8), we analyse the timing of

deformation of the principal structures, based on structural relation-

ships, 40Ar/39Ar dating of tectonic and post-tectonic volcanic and

subvolcanic rocks, and the age of foreland basin deposits and

discontinuities separating the different sequences (Fig. 12). Nine

volcanic rocks were sampled and studied by laser-induced 40Ar/39Ar

step-heating procedures on hornblendes and whole-rocks (Figs. 2

and 12). We integrated our data with previous Ar/Ar dating studies by

Baldauf (1997) and proposed a four-stage temporal model for thrust-

belt development. The four phases are illustrated by cross-sections

that represent time-slices from 15 to 1 Ma (Fig. 13, A-E).

6.1. Inversion of the Río Blanco half-graben (15–11 Ma)

We have previously documented the La Manga thrust system as

comprising three main faults: the inverted La Manga normal fault and

an associated by-pass fault,and theArroyo Blanco fault (Figs. 5 and11).

A maximum age for displacement on the La Manga thrust is given by

the age of pre-tectonic subvolcanic rocks, cropping out in the Las

Bardas creek, dated at 14.48±0.61 (2σ error) Ma (Fig. 5). These rocks

are folded and affected by the deformation in the hanging wall of thefault. In the thick-skinned domain, deformationwas accommodated by

movement along the La Manga fault prior to 10.84 Ma, the age of the

CerroTordilla post-tectonic volcanicrocks (Fig. 5). The ages of porphyry

dikes in the Río Salado area, south of the Río Atuel, assumed to be

syntectonically emplaced by Baldauf (1997), indicate that displace-

ment on theLa Manga fault took place between 13.57± 0.12and 13.43±

0.09 Ma (Baldauf, 1997). Initial movement on the La Manga fault

therefore would have occurred between 15 and 11 Ma (Fig. 12).

We propose that contractional reactivation of the Río Blanco half-

graben began with rigid displacement of thewedge of rift deposits and

the underlying crystalline basement rocks along the La Manga fault,

being fault displacement dissipated in the cover units by folding. The

syntectonical deposition of the syn-rift strata of the Agua de la Piedra

Formation indicates that the anticline associated with the first

movement on the La Manga fault system would have formed between

15 and 11 Ma (Fig. 13B).

6.2. Breakthrough of the La Manga fault onto the sedimentary cover and

reactivation of the El Freno fault (11–9 Ma)

After the partial inversion of the Río Blanco half-graben, faults

emanating from the master fault, such as the La Manga bypass fault

(Fig. 11) broke through the entire sedimentary section andreached the

surface (Fig.13C). The time of breakthrough is well constrained by the

age of the post-tectonic volcanics and by the angular unconformities

between the synorogenic strata (Fig. 12). The Loma Fiera Fm. strata

have filled depressions developed during the generation of the Mesón

fault showing wedge geometry and internal unconformities related to

the uplift of the La Manga fault system. The timing of thrusting of theMesón fault postdates deposition of the Agua de la Piedra Formation,

although was synchronous with the deposition of the Loma Fiera

Formation in its hanging wall. The angular unconformity between

these two synorogenic units (Fig. 8) indicates that this thrust de-

veloped between 10.5 and9.5 Ma, theage of theLoma Fiera Formation

(Baldauf, 1997).

At the same time, the internal deformation of the Río Blanco half-

graben occurred through the inversion of the El Freno fault system.

The age of movement along this system, related at depth to the

inversion of the pre-existing El Freno normal fault, is determined by

the ages of pre-tectonic volcanic rocks (11.16±0.28 Ma) and post-

tectonic volcanics of the Tres Lagunas hill (9.07± 0.24 Ma) (Fig. 2). This

indicates that movement along this fault was contemporaneous with

the development of the Mesón thrust and La Manga bypass thrust, i.e.,between 10.5and 9 Ma,and coincided with the ageof emplacement of

the Cerro Blanco porphyry copper centre (10.54 Ma — Gigola, 2004)

located in its hanging wall (Fig. 5).

6.3. Inversion of the Arroyo Malo half-graben and generation of the

Sosneado thrust (9–8 Ma)

Timing of displacement along the thin-skinned thrusts has

previously been studied by Baldauf (1997). He pointed out that

several stocks were emplaced along thetrace of theSosnedo fault after

the main pulse of compressive deformation. He dated three of these

stocks (Fig. 2), Cerro La Brea (5.97± 0.08 Ma), Cerro Media Luna (6.52±

0.04 Ma) and Cerro Ventana (7.25±0.32 Ma), indicating that the

Sosneado thrust had moved before 7.25 Ma (Fig. 12). Although these


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stocks are mainly post-tectonic, there is evidence for reactivation of

the Sosneado thrust after their emplacement. In the Cerro La Brea

area, Baldauf (1997) identified brecciated zones parallel to the fault, in

the margin of the stock, and suggested that they are fault zones

generated during the reactivation of the thrust. To the south, on the

eastern slope of the Cuchilla de la Tristeza range, the thrust plane is

exposed along a petroleum platform. In this region, the Sosneado

thrust displaces the Paleogene Upper Malargüe Group over Pleisto-

cene fanglomerates (Fig. 7). Baldauf (1997) suggested that the Laguna

Amarga stock (10.56±0.04) was not affected by the Sosneado thrust.

Our alternative explanation is that the thrust was split by the rigid,

pre-existent stock into branches along its western and eastern

margins. The eastern branch is inferred to havepropagated northward

to generate the brecciated zone in the Cerro La Brea area. Moreover,

seismic data indicate that the displacement along the Sosneado thrust

took place after deposition of the Agua de la Piedra Formation. Major

activity on the Sosneado fault followed deposition of the Loma Fiera

Formation but preceded that of the Río Diamante Formation, so we

conclude that it occurred between 9.5 and 7 Ma (Fig.13D). Toward the

east, cross-cutting relationships, together with emplacement ages,

Fig.12. Chartshowing thechronology of thick-skinned andthin-skinned thrustingin theMalargüe FTBas determinedby radiometric data of pre-,syn- andpost-tectonic volcanicand

subvolcanic rocks (D2, D3, D6, D8, D9, D10, D12, D13, D14), relationships of synorogenic units, angular unconformities, and crosscutting structural relationships. The terms pre-, syn-and post-tectonic are related to relationship between extrusion and movement along the closest fault or fold. Times of displacement along individual faults are represented by the

shaded zone. ⁎1 From Gigola (2004); ⁎2 From Baldauf (1997). Three major pulses of deformation are highlighted. See Fig. 2 for location of radiometric data.


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indicate that deformation and uplift in the Cerro Alquitrán area must

have occurred after 10.42 Ma, the emplacement age of the Cerro

Alquitrán stock (Baldauf, 1997).

In the western zone, displacement on the lower part of the

Alumbre fault occurred after the uplift and generation of the El Freno

anticline, because the related short-cut thrust decapitates the

anticline. The western structures are not rotated by the El Freno

anticline and folding of earlier décollements has not been recognized.

Therefore, the Alumbre fault inversion couldhave beenresponsible for

the final uplift of the Cerro Blanco porphyry copper centre, after 9 Ma.

This indicates that the internal deformation of the Atuel depocentre

occurred after the inversion of the La Manga normal fault.

6.4. Internal deformation of the Río Blanco half-graben and reactivation

of the Sosneado thrust (8–1 Ma)

The main phase of deformation in the Malargüe FTB occurred before

8 Ma, and after that time only minor fault movements have been

identified.We infer that the Arroyo Blanco fault was generated after the

main deformation on the La Manga fault system had ended. Structural

relationships indicate that this fault has moved after the generation

of the La Manga by-pass fault, i.e., between 9 and 8 Ma. There is no

evidenceof subsequent deformation in thewesternzone,whereasin the

eastern zonereactivation of the Sosneado and Mesón thruststook place

after the deposition of Lower Pleistocene fanglomerates (Fig. 13E).

Fig.13. Kinematic model of theevolutionof thenorthernpartof theMalargüefold andthrust belt showingthe four-phase evolution of thebelt.A) Distribution of pre-existingnormal

faults before compression. B) Inversion of the Río Blanco half- graben by reactivation of the basement-seated decóllement . During this time, synorogenic deposits of the Agua de la

Piedra Fm. were deposited in a newly developed foreland basin. C) Maximum episode of deformation, between 10.5 and 9 Ma, coincident with the peak of volcanism of the Huincan

Fm. (Baldauf, 1997). Several basement and thin-skinned faults are interpreted to have simultaneously moved. D) Waning of deformation with inversion of the Arroyo Malo half-

graben. The La Manga fault system was still active. E) After 8 Ma only minor deformation occurred with generation of the Arroyo Blanco fault and movement along the Sosneado

thrust.


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7. Discussion: temporal relationship between thick- and thin-

skinned structures

Many fold-and-thrust belts are combinations of both thin- and

thick-skinned thrusting as a result of reactivation of preexisting

anisotropies and weakness zones in the crust. In orogenic fronts with

influence of previous rift structures, the temporal relationship

between thick- and thin-skinned deformation is currently a topic of

controversy between two kinematic models (Fig. 14—

zone C). In themost commonly proposed model, cover detachment on low-friction

horizons in the sedimentary cover occurs before, and basement

inversion occurs afterward, as a result of hinterland-to-foreland

sequence of inversion of preexisting normal faults (Fig. 14A). In the

other model, basin inversion occurs early in the history of the fold-

and-thrust belt, in the thin and thick-skinned interaction zone, as a

result of foreland-to-hinterland sequence of inversion (Fig. 14B). The

main factors favouring one model or the other are the orientation and

dip of preexisting faults with respect to the superimposed compres-

sional stress field (Sibson, 1985), the fluid overpressure (Turner and

Williams, 2004), and the strength of the frictional basal detachment

(Buiter and Pfiffer, 2003). The first model is also favoured by the

occurrence of low-friction horizons in the cover, such as the presence

of thick evaporate layers.

In the Andes of central Argentina and Chile, the first model was

postulated for the Agrio FTB (Zapata et al., 2002; Zamora Valcarce

et al., 2006), located southward of the Malargüe FTB, where hinter-

land-to-foreland sequence of inversion of previous normal faults is

inferred to have generated a first phase of thin-skinned deformation

followed by a thick-skinned phase in the thrust front. The second

model was postulated for the southern part of the Aconcagua FTB

(Giambiagi et al., 2003a,b) where the preexisting Jurassic normal

faults were completely inverted during the first phase of Andean

compression. In the Malargüe FTB previous studies have postulated a

classic hinterland-to-foreland sequence of inversion of extensional

faults, with the generation of an early phase of thin-skinned defor-

mation in the thrust front, followed by basement inversion tectonics

(e.g., Manceda and Figueroa, 1995; Rojas et al., 1999; Giampaoli et al.,

2002; Silvestro and Kraemer, 2005; Kim et al., 2005; Broens and

Pereira, 2005).

For the inversion of the Atuel depocentre, located in the northern

part of the Malargüe FTB, we have demonstrated that inversion of

previous normal faults occurred from the master fault, in this case

located in the foreland, to the hinterland. The reactivation of themaster fault and the coeval activation of the inferred deep-seated

detachment were synchronous with the activation of shallow

detachments and low-angle thrusting in the thin-skinned area. This

indicates that the most plausible kinematic model for the northern

part of the Malargüe FTB incorporates inversion during an early

episode of compression. Our chronology of deformation in this sector

of the belt indicates that the main phase of deformation occurred

during a brief episode of important shortening, mainly between 10.5

and 8 Ma, when displacement occurred simultaneously on several

major faults detached from different decóllement levels.

8. Conclusions

The Malargüe FTB study yields insight into fold-and-thrust belt

evolution. It illustrates the progressive evolution of the thrust front

and the synchronous movement on a number of thrust sheets. The

question whether shortening in the basement occurred first and was

transmitted to the cover, or the cover detached first and basement

thrusting occurred afterwards, has been elucidated through pre-, syn-,

and post-tectonic relations among volcanics and subvolcanic rocks,

structural relationships and foreland basin deposits. Comparison of

the timing of deformation in the thick- and thin-skinned deforma-

tional areas strongly supports the hypothesis that the reactivation of

normal faults was coeval with the activation of shallow detachments

and low-angle thrusting at the thrust front of the Malargüe FTB. Low-

Fig. 14. Two kinematic models for the temporal relationship in the interaction zone (dashed box C) between thick- and thin-skinned deformations in fold and thrust belts influenced

by the presence of preexisting normal faults. A) Cover detachment on low-friction horizons occurs before, and basement inversion occurs afterward, as a result of hinterland to

foreland inversion of preexisting normal faults. B) Basin inversion occurs early in the history of the fold and thrust belt, in the thin and thick-skinned interaction zone, as a result of

foreland-to-hinterland sequence of inversion.


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angle thrusts interacted with high-angle faults related to inversion of

basement normal faults inherited from the extensional history of the

foreland, indicating a mechanics of deformation characterized by

superimposed shallow and deep detachment tectonics. Along the

thrust belt, detachments occur at several stratigraphic horizons: a

deep basement detachment related to the basement-involved thrust-

ing, and shallow detachments located within the Jurassic and

Cretaceous sequences. We propose that these detachments were

active during the complex deformation of the thrust belt, between 15and 8 Ma with a peak of deformation between 10.5 and 8 Ma.

Acknowledgements

This research was supported by grants from the Agencia Nacional

de Promoción Científica y Tecnológica (PICT 07-10942) and CONICET

(PIP 5843).We wish to thank Julieta Suriano, José Mescua, MaisaTunik,

Carla Terrizzano and Marilin Peñalva for their help in the field. Special

thanksare dueto SilviaLanés fordiscussions andcomments. TheAr/Ar

analyses were carried out by L. Giambiagi in the Geochronology

Laboratory at Queen's University, with the assistance of J.K.W. Lee and

D.J. Archibald, and funded by N.S.E.R.C. grants to A.H. Clark. Thierry

Nalpas and Tomás Zapata are sincerely thanked for their critical and

helpful reviews.

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