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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 2006Received in revised form 21 December 2006Accepted 15 November 2007Available online 1 April 2008

Keywords:Southern Central AndesMalargüe fold-and-thrust beltThick- and thin-skinned tectonicsInversion and thrustingSimultaneous thrusting

In this paper we analyse two end-member models of temporal and spatial interactions between thick- andthin-skinned structures in a thrust front with pre-existing rift structures. In the most commonly acceptedmodel, 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 inthe foreland. In the other model, basin inversion occurs early in the deformation history of the external partof 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 Neuquénbasin during Neogene to Pleistocene times. Integrating detailed structural data from the northern part of thisbelt with new Ar/Ar dating, we propose a revised kinematic model of thick- and thin-skinned interaction anddefine the temporal-spatial evolution of the belt. Comparison of the timing of deformation in the thick- andthin-skinned areas strongly supports the hypothesis that the reactivation of normal faults was coeval withthe insertion of shallow detachments and low-angle thrusting along the migrating front of the thrust beltand 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 detachmentslocated within the Jurassic and Cretaceous beds. These shallow and deep detachments were coeval producingsimultaneous 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-existinganisotropies and weakness zones in the upper crust. The presence ofpre-existing rift structures widely exerts an important control onthrust-belt geometry and evolution. However, the extent to whichthese anisotropies control regional patterns and the kinematics ofdeformation in a subsequently developed fold-and-thrust belt iscontroversial. The manner in which thin and thick-skinned relatedstructures interact in time remains poorly constrained. This papersheds some light on these topics by analysing the kinematic evolutionof the Malargüe fold-and-thrust belt of the Southern Central Andes.

The Andes of Argentina and Chile between latitudes 33° and 36° Sare 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, theNeogene geology of the Cordillera Principal is dominated by the

Malargüe fold-and-thrust belt (Malargüe FTB) involving the Mesozoicrift sequences of the Atuel depocentre. The Malargüe FTB has beenclassically identified as a hybrid fold-and-thrust belt with basementthrust sheets transferring shortening to the Meso-Cenozoic sedimen-tary cover (Kozlowski et al., 1993; Manceda and Figueroa, 1995; Rojaset al., 1999; Zapata et al., 1999; Silvestro and Kraemer, 2005). Thisstudy establishes the kinematics of thin- and thick-skinned interac-tion and hence defines the temporal-spatial evolution of the northernMalargüe FTB. We present the results of newly acquired fieldobservations, integrated with subsurface data acquired from oilexploration. A new kinematic model, which integrates the structuraldata and new Ar/Ar geochronology with previous surface data and Ar/Ar dating, is proposed for the thrust front of the northern part of thebelt. A chronological study of the deformation has been used to testhow thin- and thick-skinned deformational zones interact. Attentionhas been paid to the timing of basement fault reactivation and coevalactivation of a shallow detachment in the foreland. From theseobservations we address the wider questions of the geometricevolution and kinematics of fold-and-thrust belts and the role ofextensional structures in generating variable deformational styles.Thus, does tectonic inversion of normal faults precede thin-skinneddeformation of the sedimentary sequence in the foreland, or does

Tectonophysics 459 (2008) 123–139

⁎ Corresponding author. Fax: +54 261 5244201.E-mail addresses: [email protected] (L. Giambiagi),

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

0040-1951/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.tecto.2007.11.069

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basement inversion occur out-of-sequence after the emplacement ofshallow thrust sheets. Our research demonstrates that in the northernpart of the Malargüe FTB, deformation beganwith inversion of the riftmaster fault, in the foreland, and subsequently migrated to thehinterlandwith the simultaneous development of inverted high-anglefaults, thrust faults and basement short-cut and by-pass faults.

2. Tectonic setting

The tectonic setting and evolution of southern South America iscontrolled by the subduction regime at the western margin of theSouth American plate and the Mid-Atlantic Ridge spreading ratesalong its eastern margin (Uliana and Biddle, 1988). During theMesozoic, the western margin was the site of an active trench, arelatively narrow magmatic arc and a series of back-arc extensionalbasins (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 onpre-Jurassic continental crust (Vergani et al., 1995). It was initiated as arift basin in the Late Triassic, when Chilean and central westernArgentina underwent extensional tectonism (Digregorio et al., 1984;

Legarreta and Uliana, 1991). Marine and continental sediments weredeposited in isolated depressions during the Late Triassic to EarlyJurassic 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, wherethe 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 developmentof the marine intra-arc and back-arc basins (Mpodozis and Ramos,1989). Compressive tectonics along the western margin of southernSouth America began in the late Early Cretaceous (Mpodozis andRamos, 1989; Cobbold and Rosello, 2003; Zapata and Folguera, 2005).There is, however, no evidence of this early compression in the studyarea, 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 aconcomitant increase in convergence rate (Pardo Casas and Molnar,1987; Somoza, 1998).

The main components of the tectonic setting of the region are amagmatic 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 theNeuquén 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.

124 L. Giambiagi et al. / Tectonophysics 459 (2008) 123–139


to a series of uplifted basement blocks in the Cordillera Frontal. TheMalargüe FTB extends from 34° to 36°S and has developed sinceMiocene times in a thick-skinned style related to tectonic inversionof Mesozoic rift structures (Kozlowski, 1984; Manceda and Figueroa,1995). Deformation involves pre-Jurassic basement rocks andMesozoic rift and back-arc basin deposits. The Cordillera Principal

is underlain by Proterozoic to Paleozoic metamorphic and plutonicrocks of the Cordillera Frontal uplifted by high-angle faults along itseastern flank. The southern part of this range is uplifted by theCarrizalito fault which dies out alongside a SW-plunging anticlinesouth of the Río Diamante (Fig. 2) (Kozlowski, 1984; Turienzo andDimieri, 2005).

Fig. 2. Simplified geological map of theMalargüe FTB, between 34°30′ and 35°00′S, showingmajor structural features and location of cross section in Fig.11. The area has been dividedinto 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 themajor faults have been drawn. Boxes indicate location of Figs 5 and 6. Based on Kozlowski et al. (1981), Cruz et al. (1991), Scaricabarozzi (2003), Kim et 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 crosssection of the Malargüe FTB on Fig. 11.

125L. Giambiagi et al. / Tectonophysics 459 (2008) 123–139


3. Stratigraphic framework

The lithostratigraphic units of the Malargüe FTB are: Proterozoic toTriassic metamorphic, plutonic and volcanic rocks which constitutethe basement of the belt; Upper Triassic to Lower Jurassic marine andcontinental rift sequences deposited in the Neuquén back-arc basin;Middle Jurassic to Cretaceous platform sequences; and Cenozoicsedimentary and volcanic rocks.

3.1. Basement rocks

Basement rocks crop out in the Cordillera Frontal, northeast of thestudy 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 continentalsandstones, 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 EarlyJurassic marine and fluvial synrift strata, unconformably depositedover deformed basement rocks (Fig. 3). These strata crop out in thewestern part of the study area (Fig. 2). The deposition of the marine

massive mudstones and shales of the ArroyoMalo Formation (Riccardiet al., 1997; Riccardi and Iglesia Llanos, 1999; Lanés, 2005) marked theonset of extensional activity in the rift basin. The El Freno Formationcrops out in the eastern sector of the Atuel depocentre and isrepresented by braided alluvial deposits with a predominant easternprovenance. The Puesto Araya Formation consists of slope-type fandelta deposits (lower section) related to the braided alluvial systemsof the easterly El Freno Formation, and storm-dominated shelfdeposits (upper section) (Lanés, 2005). Off-shore shelf black clays-tones were conformably deposited over the marine strata of thePuesto Araya Formation, and correspond to the Tres EsquinasFormation of Toarcian–Bajocian age (Gulisano and Gutiérrez Pleiml-ing,1994). There is no evidence of faulting during the deposition of themarine platform strata, indicating that the boundary between fluvialand marine strata in the eastern part of the depocentre marks the endof 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 LotenaGroup (Gulisano and Gutiérrez Pleimling,1994). During Kimmeridgiantimes, alluvial, fluvial and eolian continental clastic deposition wascontrolled by normal faults (Tordillo Formation) (Ramos, 1985;Cegarra and Ramos,1996; Giambiagi et al., 2003a,b). These continentaldeposits were followed by accumulation of calcareous shelf facies(Mendoza Group). Aptian to Cenomanian red continental depositsoverlying these strata are associated with evaporites and marinecarbonates (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).



continental red beds (Neuquén Group: Gulisano and GutiérrezPleimling, 1994, Riccardi et al., 1999). Subsequently, a transgressionfrom the Atlantic Ocean allowed the accumulation of clastics and car-bonates (lower Malargüe Group: Barrio, 1990; Tunik, 2004), followedby fine-grained Paleocene to Eocene sedimentary rocks of lacustrineand playa lake origin (upper Malargüe Group).

3.3. Synorogenic deposits

Synorogenic sediments and volcanic and volcaniclastic rocks fillinga foreland basin are represented by the Miocene Agua de la Piedra andLoma Fiera Formations, the Pliocene Río Diamante Formation, andthree Pleistocene coarse conglomerate units (Mesones, La Invernadaand Las Tunas Fms.). These rocks crop out in the Cuchilla de la Tristezarange (Fig. 2) and are separated by angular unconformities. Forelandbasin sedimentation began with deposition of alluvial fan and fluvialsystems of the Agua de la Piedra Formation over an angular uncon-formity (Combina et al., 1994; Combina and Nullo, 2005). This unitis composed of interbedded coarse conglomerate and sandstonewith clasts from volcanic and sedimentary rocks derived from theCordillera Principal (Yrigoyen, 1993). The base of this formation iscomposed 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 thebase of the Agua de la Piedra Formation suggest that the unit isyounger than 13 Ma (Baldauf, 1997).

The Loma Fiera Formation unconformably overlies the Agua de laPiedra Formation. This unit consists of cross-bedded tuffs containingclasts 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 laharicdeposits (Combina and Nullo, 2000). Conglomerates of this unitappear to interfinger with andesite flows of the Huincan Formation(Dessanti, 1959) and incorporate granitic and volcanic clasts from theCordillera Frontal, indicating that by the time the Loma FieraFormation 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 baseof the Loma Fiera Formation (Baldauf, 1997) imply a maximum age of9.5 Ma. The overlying conglomerates and sandstones of the RíoDiamante Formation exhibit gradational contacts with the Loma FieraFormation, indicating deposition during a time of decreasing volcanicand tectonic activity (Combina and Nullo, 1997).

3.4. Cenozoic volcanism

The older Cenozoic igneous rocks, referred asMolles Suite Intrusives(Groeber, 1951; Volkheimer, 1978), are composed of lower Miocenebasaltic and andesitic porphyry stocks associated with dacitic hypabys-sal bodies (Baldauf, 1997), exposed in the western and eastern parts ofthe Malargüe FTB. Intense volcanism in the Middle Miocene to EarlyPliocene (Stephens et al., 1991; Baldauf et al., 1992; Ramos and Nullo,1993; Baldauf, 1997) is grouped in the Huincan Formation. This igneousactivity took place between 10.5 and 5.5 Ma (Baldauf, 1997) andcomprises basaltic andesites and andesites similar in chemistry to theTeniente Volcanic Complex located tens of kilometres to thewest (Nulloet al., 2006). This magmatic event has been proposed by Baldauf (1997)to have occurred during the waning stages of, or after compressivedeformation in the eastern sector of theMalargüe FTB. However, wewillshow that this volcanic unit has the same age as the main episode ofdeformation.

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 byManceda and Figueroa (1995) as representing a half-grabenwithwest-facing polarity. Elsewhere (Giambiagi et al., 2005a, 2008; Bechis et al.,2005), we demonstrated that the principal normal faults of the Atueldepocentre have been inverted and moreover, we documented adetailed characterization of the depocentre architecture through theintegration of our structural analysis of rift-related faultswith previousstratigraphic and paleogeographic studies (Lanés, 2005). The depo-centre comprised the ArroyoMalo and Río Blanco half-grabens (Fig. 4),where the former is interpreted as a completely submerged sub-basinfilledwithmarine syn-rift strata (ArroyoMalo Fm. and lower section ofthe Puesto Araya Fm.) and sag deposits (Tres Esquinas Fm.). Its masterfault, the west-dipping NNW-trending Alumbre fault, is well exposedin the headwaters of the Alumbre creek, where it dips at a high angletowards the west with no evidence of structural inversion at shallowlevels. In contrast, the Río Blanco half-graben was filled with con-tinental syn-rift strata (El Freno Fm.) and sag deposits (upper section ofthe Puesto Araya Fm. and Tres Esquinas Fm.), and was bounded alongits eastern margin by the NNW-trending La Manga master fault. BothAlumbre and La Manga faults have been interpreted as pre-existingstructures reactivated during the rifting event. This reactivationwouldhave generated an oblique rift with WNW- and NNE-striking obliquenormal faults.

4.2. Andean deformation

During Miocene to Pleistocene times, the Atuel depocentre wasinverted and incorporated into the thrust sheets of the thick-skinned Malargüe FTB (Kozlowski et al., 1993; Manceda andFigueroa, 1995) exerting its structural architecture a profoundinfluence on the development of the belt. This influence is reflectedin a variety of structural styles in the study area. We identify severaltrends of regional structures, significant changes in fold wavelengthsand multiple detachments (Fig. 5), indicating that the present-daystructure of the belt is controlled by major rift-related basement-rooted faults. We argue that the mid-crustal weak zone above whichbasement thrusting occurs was inherited from a previous mid-crustal extensional flat detachment. The propagation of invertedbasement faults into the sedimentary cover generated complexstructures 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).





folding and faulting. Deformation in these areas could have beencomplicated by basement short-cut faults which generated severaldetachment levels in the sedimentary cover. Towards the foreland

the Andean deformation developed a thin-skinned system usingincompetent layers from the Neuquén and Malargüe Groups asdetachment levels.

Fig. 6. Geological map of the eastern sector of theMalargüe FTB. Modified after Kozlowski et al. (1981) and Cruz et al. (1991) and our own observations. A-A′: seismic line 16029 on 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).



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

The Malargüe FTB can be divided into western and eastern sectorson the basis of palaeoenvironmental and tectonic relationships. Theirmutual boundary is defined by the NNW-trending Borbollón–LaManga lineament, related to the La Manga master fault of theMesozoic rift system (Figs. 2 and 4).

5.1. Eastern sector

The eastern sector is an emergent thrust-front system, made up ofseveral N–S to NNW-trending thrust sheets involving Cretaceous toNeogene strata in a thin-skinned tectonic style (Fig. 6). The oldestsedimentary rocks involved in the deformation are Cretaceous shales,evaporites and red beds. The stratigraphic section is dominated byseveral incompetent evaporite and black shale units alternating withcompetent sandstone units. At least two main décollements areregionally developed in the eastern zone and account for the thin-skinned architecture. The lowermost is located in the lower part of theUpper Cretaceous red beds and is present in the northern part of thestudy area, whereas the shallowest is recorded in the uppermostCretaceous beds. In the northern part of the belt, in the Río Diamantearea, a third decóllement is located at the base of the Upper Jurassic–Lower Cretaceous black shale succession (Kim et al., 2005; Broens andPereira, 2005).

Three main thin-skinned thrusts have been identified in thissector: the Sosneado, Mesón, and Alquitrán faults (Kozlowski, 1984)uplifted from the upper decóllement in the uppermost Cretaceousbeds (Fig. 6). The Sosneado and Mesón faults uplift the Cuchilla de laTristeza 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 isassociated with a hanging wall syncline, which acted as a Neogene-Quaternary foreland basin depocentre, in which thick synorogenicdeposits record the growth history of the belt. The Sosneado thrusttransposes the Paleogene units on top of the Agua de la Piedra andPleistocene fanglomerates (Fig. 7). It strikes N–S and dips 24° west.The Alquitrán fault is inferred to generate an open anticline thataffects Upper Cretaceous to Neogene strata in the Cerro Alquitrán area.

Fig. 8 sketches the present-day configuration of the eastern sectorof the belt along the section A-A′ of Fig. 6, as constrained by field andsubsurface (seismic and well) data. A migrated reflection seismicdataset constrained by well log information from the Cuchilla de laTristeza range was available in this study. Two interpretations of theseismic line 16029 have been made to identify the spatial relationshipbetween thick- and thin-skinned structures. Interpretation A (Fig. 8A)assumes that the inversion of the La Manga normal fault accounts forthe detachment in the cover and generation of the Mesón, Sosneadoand Alquitrán thrusts. An alternative approach is shown in inter-pretation B (Fig. 8B), where the shallow detachment developed in aninitial episode of thin-skinned deformation, not related to theinversion of the master fault, and was folded in the ensuing episodeof tectonic inversion, in agreement with previous models of thenorthern part of the Malargüe FTB (Pereira, 2003; Kim et al., 2005).Both alternatives are geometrically plausible and the low resolution ofseismic lines along the border between the thick- and thin-skinnedzones does not allow us to discriminate between them. As we will seein next sections, we favour interpretation A because of the timing ofmovement of the basement and thin-skinned faults.

5.2. Western sector

In the western sector, outcropping rocks are predominantly UpperTriassic–Lower Jurassic rift sequences overlain by Middle Jurassic toLower Cretaceous deposits (Fig. 5). The Upper Cretaceous and Paleogenerocks have been eroded in this domain, and Neogene synorogenic strata

were not deposited (Fig. 3). This sector has previously been studied byFortunatti and Dimieri (2002, 2005), who outlined several backthrustsrelated to the basement involvement in the deformation. The Andeanstructural pattern shows two predominant trends (Fig. 5): NNE-strikingfolds and subordinate faults; andN toNNW-striking folds and faults. Thewestern sector is also characterized by a combination of two deforma-tional styleswith large-scale open folds andnarrowbelts of intenseeast-vergent folding and faulting (Figs. 5 and 9). Large-scale anticlines withassociated synclines suggest regional-scale basement uplift. In thefrontal part of these inferred basement-cored folds, we propose that thedisplacement was mainly transferred to the sedimentary cover,generating narrow belts of intense folding of syn-rift and post-rift strata(Fig. 9). Broad, long-wavelength folds developed in the hangingwalls ofmoderate-to-high-angle reverse faults and are considered to haveformed by 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 andfacies of the rift sequences (Lanés, 2005), the high cut-off angles alongthe faults, the presence of antithetic and synthetic faults reactivated in areverse 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 theinterpretation A of the seismic line 16029 (Fig. 8A). The cross-sectionhas been restored with a line-length balance and constant thicknesshypothesis for the sedimentary cover, and an area-balanced methodfor the basement. In this section, the previously identified (Fig. 4)three main basement faults are interpreted to be the principalstructures of the western sector. The faults propagated upwards intothe sedimentary strata, producing shortening accommodated bythrusting at depth and by folding in the upper levels of the pile, as

Fig. 7. The Sosneado thrust in the Cuchilla de la Tristeza range. The fault places the upperpart of the Malargüe Group on top of Pleistocene fanglomerates and it is covered byHolocene deposits. See map on Fig. 6 for location.



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 wasdone 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, TordilloFm., and Mendoza, Rayoso and Neuquén Groups); UpperJ+K: Upper Jurassic to Cretaceous strata (Mendoza, Rayoso and Neuquén Groups), Upper K+Paleogene: Upper Cretaceous toPaleocene (Malargüe Group), AP: Agua de la Piedra Fm., LF: Loma Fiera Fm. and RD: 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 inInterpretation B, where a shallow detachment in the sedimentary cover developed first, before the inversion of the master fault.



fault-propagation folds. The areas of intense folding and faulting arelocated in front of these large-scale anticlines, as in the region east ofthe El Freno anticline, where the marine sag deposits are stronglydeformed by kink and box folds (Fig. 10, C–D).

The La Manga fault system is the most significant structure in thefoothills, uplifting the Lower Mesozoic sequences on top of theNeogene synorogenic units, and has a throw of several kilometres(Kozlowski, 1984). We interpret this fault system as comprising threerelated structures, i.e., the Arroyo Blanco fault, the La Manga invertednormal fault, and a basement by-pass fault (Fig. 11). This highlights animportant characteristic of the basement-cover interaction along the

Triassic–Jurassic master fault, where multiple basement thrusts havebeen stacked along the eastern limit of the former rift basin. The LaManga fault can be interpreted as an inverted, west-dipping, normalfault, because rift-related Upper Triassic–Lower Jurassic rocks arepresent in its hangingwall and absent in the footwall block (Fig. 8).Weinfer that this fault has a convex-up geometry, cutting the basement-cover interface at a high angle and progressively decreasing in dipupwards. This geometry strongly implies the inversion of a high-anglepre-existing normal fault by upward propagation of a steep basementfault into the sedimentary cover. The La Manga by-pass fault has beeninferred 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.



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 the eastern thin-skinned sector of the belt. The palinspasticrestitution shows the location of themain normal faults developed during the Triassic–Jurassic extension. During the Neogene inversion, these structures were inverted in associationwith the generation of basement short-cut faults: Alumbreshort-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).

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with a NNW-strike (Fig. 5) and overturns Mesozoic beds in the Lomadel Medio range (Kozlowski et al., 1981). The Arroyo Blanco fault cropsout in the Arroyo Blanco creek (Fig. 5), where it transposes LowerJurassic sag deposits over Upper Jurassic red beds and evaporites.Open folds in the hanging wall of this moderate-to-high angle reversefault have been disturbed by two associated backthrusts. These faultshave previously been described by Fortunatti et al. (2004) andTurienzo et al. (2004) as thin-skinned backthrusts and can bedistinguished 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 withthe boundary between areas of open folding and intense folding andfaulting (Giambiagi et al., 2008). The inversion of this fault ismarked bythe broad El Freno anticline in its hanging wall (Fig. 10B). Its curvedaxial plane has been interpreted to reflect the configuration of thisnormal fault at depth. Associated with this thick-skinned structure,small-scale anticlines and synclines with angular hinges (kinks andbox-folds) deform the Lower Jurassic sequences, and low-angle thrustsformed above shallow detachments, in thin-skinned tectonic style(Fig. 10, C–D). The steeply-dipping to overturned beds shown by theoutcrops east of the Arroyo El Freno creek reveal structural complexity.Associated with the inversion of this fault, we have inferred thepresence of the El Freno basement short-cut fault to account for thegeneration of a broad open syncline and a low-angle thrust (Fig. 11).

The Alumbre fault is an approximately 15 km-long, NNW-strikingfault with a continuous trace. It was passively uplifted in the hangingwall of the El Freno fault, preserving the inherited pattern of extensionalstructure at shallow levels. This fault is exposed in the headwaters ofthe Arroyo Alumbre creek (Fig. 5). Its orientation is consistent with theNNW-trending paleocoast and with paleocurrents ranging from SSW toNW documented by Lanés (2005). Although in outcrop it presents noevidence of structural inversion, its lower segment is inferred to havebeen inverted during Andean compression and to be responsible for aseries of backthrusts affecting the sedimentary cover. The generation ofa short-cut fault is associated with a basement wedge and oppositelyverging cover-detached underthrusts (Figs. 5 and 11). This complexzone 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 FTBinvolved both thin-skinned tectonics along several shallow detach-ments within the Jurassic rift sequences (western sector) andCretaceous strata (eastern sector) and basement involvement alonga deeper detachment which accommodated stacking of basementthrust units. This model predicts that steep, basement-involvedthrust-ramps in the western sector migrated upsection throughcover and evolved into flats when they reached the incompetentsyn-rift strata. A combination of extensional fault inversion anddevelopment of new basement short-cut faults accounts for thecomplex structure in the sedimentary cover.

6. Chronology of deformation

In order to constrain the age of the deformation and to choosebetween both interpretations of thick- and thin-skinned interaction(interpretations A and B – Fig. 8), we analyse the timing ofdeformation of the principal structures, based on structural relation-ships, 40Ar/39Ar dating of tectonic and post-tectonic volcanic andsubvolcanic rocks, and the age of foreland basin deposits anddiscontinuities separating the different sequences (Fig. 12). Ninevolcanic rocks were sampled and studied by laser-induced 40Ar/39Arstep-heating procedures on hornblendes and whole-rocks (Figs. 2and 12). We integrated our data with previous Ar/Ar dating studies byBaldauf (1997) and proposed a four-stage temporal model for thrust-belt development. The four phases are illustrated by cross-sectionsthat 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 ascomprising three main faults: the inverted La Manga normal fault andan associated by-pass fault, and the Arroyo Blanco fault (Figs. 5 and 11).A maximum age for displacement on the La Manga thrust is given bythe age of pre-tectonic subvolcanic rocks, cropping out in the LasBardas creek, dated at 14.48±0.61 (2σ error) Ma (Fig. 5). These rocksare folded and affected by the deformation in the hanging wall of thefault. In the thick-skinned domain, deformationwas accommodated bymovement along the La Manga fault prior to 10.84 Ma, the age of theCerroTordilla post-tectonic volcanic rocks (Fig. 5). The ages of porphyrydikes in the Río Salado area, south of the Río Atuel, assumed to besyntectonically emplaced by Baldauf (1997), indicate that displace-ment on the LaManga fault took place between 13.57±0.12 and 13.43±0.09 Ma (Baldauf, 1997). Initial movement on the La Manga faulttherefore would have occurred between 15 and 11 Ma (Fig. 12).

We propose that contractional reactivation of the Río Blanco half-graben beganwith rigid displacement of the wedge of rift deposits andthe underlying crystalline basement rocks along the La Manga fault,being fault displacement dissipated in the cover units by folding. Thesyntectonical deposition of the syn-rift strata of the Agua de la PiedraFormation indicates that the anticline associated with the firstmovement on the La Manga fault systemwould have formed between15 and 11 Ma (Fig. 13B).

6.2. Breakthrough of the La Manga fault onto the sedimentary cover andreactivation of the El Freno fault (11–9 Ma)

After the partial inversion of the Río Blanco half-graben, faultsemanating from the master fault, such as the La Manga bypass fault(Fig.11) broke through the entire sedimentary section and reached thesurface (Fig. 13C). The time of breakthrough is well constrained by theage of the post-tectonic volcanics and by the angular unconformitiesbetween the synorogenic strata (Fig. 12). The Loma Fiera Fm. stratahave filled depressions developed during the generation of the Mesónfault showing wedge geometry and internal unconformities related tothe 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 FieraFormation in its hanging wall. The angular unconformity betweenthese two synorogenic units (Fig. 8) indicates that this thrust de-veloped between 10.5 and 9.5Ma, the age of the Loma 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 theinversion of the pre-existing El Freno normal fault, is determined bythe 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). Thisindicates that movement along this fault was contemporaneous withthe development of the Mesón thrust and La Manga bypass thrust, i.e.,between 10.5 and 9Ma, and coincidedwith the age of emplacement ofthe 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 theSosneado thrust (9–8 Ma)

Timing of displacement along the thin-skinned thrusts haspreviously been studied by Baldauf (1997). He pointed out thatseveral stockswere emplaced along the trace of the Sosnedo fault afterthe main pulse of compressive deformation. He dated three of thesestocks (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 theSosneado thrust had moved before 7.25 Ma (Fig. 12). Although these



stocks are mainly post-tectonic, there is evidence for reactivation ofthe Sosneado thrust after their emplacement. In the Cerro La Breaarea, Baldauf (1997) identified brecciated zones parallel to the fault, inthe margin of the stock, and suggested that they are fault zonesgenerated during the reactivation of the thrust. To the south, on theeastern slope of the Cuchilla de la Tristeza range, the thrust plane isexposed along a petroleum platform. In this region, the Sosneadothrust displaces the Paleogene Upper Malargüe Group over Pleisto-cene fanglomerates (Fig. 7). Baldauf (1997) suggested that the LagunaAmarga 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 easternmargins. The eastern branch is inferred to have propagated northwardto generate the brecciated zone in the Cerro La Brea area. Moreover,seismic data indicate that the displacement along the Sosneado thrusttook place after deposition of the Agua de la Piedra Formation. Majoractivity on the Sosneado fault followed deposition of the Loma FieraFormation but preceded that of the Río Diamante Formation, so weconclude that it occurred between 9.5 and 7 Ma (Fig. 13D). Toward theeast, cross-cutting relationships, together with emplacement ages,

Fig.12. Chart showing the chronology of thick-skinned and thin-skinned thrusting in theMalargüe FTB as determined by radiometric data of pre-, syn- and post-tectonic volcanic andsubvolcanic 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 theshaded zone. ⁎1 From Gigola (2004); ⁎2 From Baldauf (1997). Three major pulses of deformation are highlighted. See Fig. 2 for location of radiometric data.



indicate that deformation and uplift in the Cerro Alquitrán area musthave occurred after 10.42 Ma, the emplacement age of the CerroAlquitrán stock (Baldauf, 1997).

In the western zone, displacement on the lower part of theAlumbre fault occurred after the uplift and generation of the El Frenoanticline, because the related short-cut thrust decapitates theanticline. The western structures are not rotated by the El Frenoanticline and folding of earlier décollements has not been recognized.Therefore, the Alumbre fault inversion could have been responsible forthe final uplift of the Cerro Blanco porphyry copper centre, after 9 Ma.This indicates that the internal deformation of the Atuel depocentreoccurred after the inversion of the La Manga normal fault.

6.4. Internal deformation of the Río Blanco half-graben and reactivationof the Sosneado thrust (8–1 Ma)

Themain phase of deformation in theMalargüe FTB occurred before8 Ma, and after that time only minor fault movements have beenidentified.We infer that the Arroyo Blanco fault was generated after themain deformation on the La Manga fault system had ended. Structuralrelationships indicate that this fault has moved after the generationof the La Manga by-pass fault, i.e., between 9 and 8 Ma. There is noevidence of subsequentdeformation in thewestern zone,whereas in theeastern zone reactivation of the Sosneado andMesón thrusts took placeafter the deposition of Lower Pleistocene fanglomerates (Fig. 13E).

Fig.13. Kinematic model of the evolution of the northern part of theMalargüe fold and thrust belt showing the four-phase evolution of the belt. A) Distribution of pre-existing normalfaults 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 laPiedra 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 HuincanFm. (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 Sosneadothrust.



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

Many fold-and-thrust belts are combinations of both thin- andthick-skinned thrusting as a result of reactivation of preexistinganisotropies and weakness zones in the crust. In orogenic fronts withinfluence of previous rift structures, the temporal relationshipbetween thick- and thin-skinned deformation is currently a topic ofcontroversy between two kinematic models (Fig. 14 — zone C). In themost commonly proposed model, cover detachment on low-frictionhorizons in the sedimentary cover occurs before, and basementinversion occurs afterward, as a result of hinterland-to-forelandsequence of inversion of preexisting normal faults (Fig. 14A). In theother model, basin inversion occurs early in the history of the fold-and-thrust belt, in the thin and thick-skinned interaction zone, as aresult of foreland-to-hinterland sequence of inversion (Fig. 14B). Themain factors favouring one model or the other are the orientation anddip of preexisting faults with respect to the superimposed compres-sional stress field (Sibson, 1985), the fluid overpressure (Turner andWilliams, 2004), and the strength of the frictional basal detachment(Buiter and Pfiffer, 2003). The first model is also favoured by theoccurrence of low-friction horizons in the cover, such as the presenceof thick evaporate layers.

In the Andes of central Argentina and Chile, the first model waspostulated for the Agrio FTB (Zapata et al., 2002; Zamora Valcarceet al., 2006), located southward of the Malargüe FTB, where hinter-land-to-foreland sequence of inversion of previous normal faults isinferred to have generated a first phase of thin-skinned deformationfollowed by a thick-skinned phase in the thrust front. The secondmodel was postulated for the southern part of the Aconcagua FTB(Giambiagi et al., 2003a,b) where the preexisting Jurassic normalfaults were completely inverted during the first phase of Andeancompression. In the Malargüe FTB previous studies have postulated aclassic 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 andPereira, 2005).

For the inversion of the Atuel depocentre, located in the northernpart of the Malargüe FTB, we have demonstrated that inversion ofprevious normal faults occurred from the master fault, in this caselocated in the foreland, to the hinterland. The reactivation of themaster fault and the coeval activation of the inferred deep-seateddetachment were synchronous with the activation of shallowdetachments and low-angle thrusting in the thin-skinned area. Thisindicates that the most plausible kinematic model for the northernpart of the Malargüe FTB incorporates inversion during an earlyepisode of compression. Our chronology of deformation in this sectorof the belt indicates that the main phase of deformation occurredduring a brief episode of important shortening, mainly between 10.5and 8 Ma, when displacement occurred simultaneously on severalmajor faults detached from different decóllement levels.

8. Conclusions

The Malargüe FTB study yields insight into fold-and-thrust beltevolution. It illustrates the progressive evolution of the thrust frontand the synchronous movement on a number of thrust sheets. Thequestion whether shortening in the basement occurred first and wastransmitted to the cover, or the cover detached first and basementthrusting occurred afterwards, has been elucidated through pre-, syn-,and post-tectonic relations among volcanics and subvolcanic rocks,structural relationships and foreland basin deposits. Comparison ofthe timing of deformation in the thick- and thin-skinned deforma-tional areas strongly supports the hypothesis that the reactivation ofnormal faults was coeval with the activation of shallow detachmentsand 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 influencedby 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 toforeland 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 offoreland-to-hinterland sequence of inversion.



angle thrusts interacted with high-angle faults related to inversion ofbasement normal faults inherited from the extensional history of theforeland, indicating a mechanics of deformation characterized bysuperimposed shallow and deep detachment tectonics. Along thethrust belt, detachments occur at several stratigraphic horizons: adeep basement detachment related to the basement-involved thrust-ing, and shallow detachments located within the Jurassic andCretaceous sequences. We propose that these detachments wereactive 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 Nacionalde Promoción Científica y Tecnológica (PICT 07-10942) and CONICET(PIP 5843).Wewish to thank Julieta Suriano, JoséMescua,Maisa Tunik,Carla Terrizzano and Marilin Peñalva for their help in the field. Specialthanks are due to Silvia Lanés for discussions and comments. The Ar/Aranalyses were carried out by L. Giambiagi in the GeochronologyLaboratory at Queen's University, with the assistance of J.K.W. Lee andD.J. Archibald, and funded by N.S.E.R.C. grants to A.H. Clark. ThierryNalpas and Tomás Zapata are sincerely thanked for their critical andhelpful reviews.

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