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Structural style of the Chos Malal fold and thrust belt, Neuquén Basin, Argentina: relationship between thick- and thin-skinned tectonics ARTICLE in JOURNAL OF SOUTH AMERICAN EARTH SCIENCES · JULY 2015 Impact Factor: 1.37 · DOI: 10.1016/j.jsames.2015.07.001

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Journal of South American Earth Sciences xxx (2015) 1e19

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n basin, Structural style of the Chos Malal fold and thrust belt, Neuque Argentina: Relationship between thick- and thin-skinned tectonics
nchez a, *, Martín Turienzo a, Fernando Lebinson a, Vanesa Araujo a, Natalia Sa Isabelle Coutand b, Luis Dimieri a a b


gico del Sur (INGEOSUR), CONICET, Departamento de Geología, Universidad Nacional del Sur, Bahía Blanca, Argentina Instituto Geolo Department of Earth Sciences, Dalhousie University, Halifax, Canada

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 December 2014 Received in revised form 23 June 2015 Accepted 2 July 2015 Available online xxx

The Chos Malal fold and thrust belt (FTB) is a thick-skinned mountain belt formed by Mesozoic deposits
n Basin during the Andean orogeny. Four structural cross-sections in the entire deformed of the Neuque area, supported by field and subsurface data, suggest a strong link between thick and thin-skinned structures. Major Andean thrusts branching from a detachment placed 12 km into the crust created large basement wedges, which were inserted in the cover producing minor order structures. The westernmost of these wedges is exposed forming the Cordillera del Viento, while others basement slices at depth were interpreted from seismic lines. These thick-skinned structures transferred deformation to the cover along the Auquilco Formation and contributed to create all thin-skinned structures surveyed in the Chos Malal FTB. We recognized half-graben geometries in the seismic lines, preserving their extensional configuration, which suggests that the main normal faults were not inverted. Shortenings calculated from the restoration of the four cross-sections are 16.9 km (29.7%), 16.9 km (29.7%), 14.7 km (26.9%) and 14.15 km (26.3%), which evidence a slight diminution of the contraction toward the south probably associated with the plunge of the Cordillera del Viento structure in this segment of the Chos Malal FTB. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Thick-skinned structures Shortening
n basin Neuque Andean tectonics

1. Introduction The Andes are the outcome of a complex tectonic evolution that involved different processes along the continental margin of the South American Plate. The Andean orogeny deformed Paleozoic and Mesozoic rocks giving birth to different fold and thrust belts
n Precordillera. In particular, the that together form the Neuque Chos Malal fold and thrust belt (FTB) is located between 37
and 37

n Province (Fig. 1). 300 S, in the northwestern sector of the Neuque This part of the Andes is the southern prolongation of the Malargüe FTB and the northern expression of the Agrio FTB. The latter is separated from the Chos Malal FTB by the Cortaderas lineament (Ramos, 1978). This orogenic belt bears deformed Mesozoic strata
n basin and is limited by two structural highs: the of the Neuque

* Corresponding author. Departamento de Geología, Universidad Nacional del Sur, San Juan 670, 8000, Bahía Blanca, Argentina.
nchez), [email protected] E-mail addresses: [email protected] (N. Sa (M. Turienzo), [email protected] (F. Lebinson), [email protected]. ar (V. Araujo), [email protected] (I. Coutand), [email protected] (L. Dimieri).

Cordillera del Viento on the west side and the Macizo del Tromen on the east side (Bracaccini, 1970). Since the rocks involved in this belt are part of one of the most important hydrocarbon-producing basins in Argentina, many workers paid particular attention to this area. Structural contributions are numerous being most of them of a regional character focusing on geotectonic structures. Among €llner and Amos (1973); Ramos and these we can mention Zo Barbieri (1988); Booth and Coward (1996); Chauveau et al. (1996); Cobbold et al. (1999); Nocioni (1996); Kozlowski et al. (1996, 1998); Zapata et al. (1999); Cobbold and Rossello (2003); Folguera et al. (2007); Rojas Vera et al. (2014); Sagripanti et al. (2014). The Chos Malal FTB is a thick-skinned belt because the basement rocks exposed in the Cordillera del Viento are involved in the deformation. However, there are different points of view about the mechanism by which this basement structure was uplifted. Some authors proposed the tectonic inversion of normal faults as the preponderant mechanism (Vergani et al., 1995; Booth and Coward, 1996; Chauveau et al., 1996; Zapata et al., 1999; Cobbold and Rossello, 2003; Zapata and Folguera et al., 2005; Ramos and Folguera, 2005; Ramos and Kay, 2006). Others suggested in their

http://dx.doi.org/10.1016/j.jsames.2015.07.001 0895-9811/© 2015 Elsevier Ltd. All rights reserved.


nchez, N., et al., Structural style of the Chos Malal fold and thrust belt, Neuque
n basin, Argentina: Please cite this article in press as: Sa Relationship between thick- and thin-skinned tectonics, Journal of South American Earth Sciences (2015), http://dx.doi.org/10.1016/ j.jsames.2015.07.001

2


nchez et al. / Journal of South American Earth Sciences xxx (2015) 1e19 N. Sa


n Province, in the convergent margin between the South American and Nazca Plates. b) Fig. 1. a) Location of the Chos Malal fold and thrust belt (FTB) in the northwest of the Neuque
n River. Yellow lines are the interpreted cross-sections, Digital Elevation Model (DEM) of the study area, from the Cordillera del Viento to the Tromen Volcano, north of the Neuque purple lines are the available 2D seismic lines and black dots show the wells locations. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

models that the main mechanism of deformation is by thrust fault ~ es, systems typically with low dip angles (Poszkiewicz and Vin
nchez et al., 2014; Turienzo 1987; Kozlowski et al., 1996, 1998; Sa et al., 2014). The third model is a hybrid one that considers the combination of the above mentioned models (Folguera et al., 2007; Rojas Vera et al., 2014; Sagripanti et al., 2014). In this paper, we analyze the structures between the Cordillera del Viento and the Tromen volcano to comprehend the structural configuration of this portion of the Chos Malal FTB (Fig. 1). Four balanced structural cross-sections were constructed based on detailed field mapping, information of seismic lines and well data. In order to understand the deformation history of the area, we focus our survey on different scales of folds and thrusts developed in the sedimentary cover. This survey helps us to understand the relationship between basement wedges and the thin-skinned structures. It also let us quantify more accurately the shortenings calculated from the balanced cross-sections. This work also attempts to elucidate the role of the reactivation of the Mesozoic normal faults, compared with Andean thrust systems, as responsible for the building of the Andean orogen. Finally, it leads us to establish the main characteristics of the structural style of this region of the Andes.

2. Tectonic setting This portion of the Andean Cordillera is the result of several tectonic cycles that occurred from the Early Paleozoic to recent times (Ramos, 1999, 2010). The oldest rocks are Paleozoic (Fig. 2) and they are exposed in the Cordillera del Viento (Fig. 3). These rocks show contractional and extensional structures that can be attributed to deformations during the Chanic and Gondwanan € llner and Amos, 1973; Llambías et al., 2007; orogenic cycles (Zo Zappettini et al., 2012; Giacosa et al., 2014). The first of these events took place during the Late Devonian-Early Carboniferous and the second occurred during the Late Carboniferous-Early Permian. The collapse of the orogen generated the magmatic province of Choiyoi (Llambías and Sato, 1995). This mesosiliceous to siliceous magmatic event occurred during the Late Permian and part of the Triassic at the active continental margin of Gondwana (Llambías et al., 2003). All these tectonic and magmatic events form part of the collisional history of terrains during the Paleozoic (Franzese and Spalletti, 2001) and consti
n basin developed. The tute the basement over which the Neuque extensional stage began during the Triassic to Early Jurassic, connected with the early breakup of Gondwana, and it created


nchez, N., et al., Structural style of the Chos Malal fold and thrust belt, Neuque
n basin, Argentina: Please cite this article in press as: Sa Relationship between thick- and thin-skinned tectonics, Journal of South American Earth Sciences (2015), http://dx.doi.org/10.1016/ j.jsames.2015.07.001


nchez et al. / Journal of South American Earth Sciences xxx (2015) 1e19 N. Sa

3


rrez Fig. 2. Stratigraphic column summarizing the nomenclature, age and tectonic setting of the units exposed in this part of the Chos Malal FTB (based on Gulisano and Gutie Pleimling, 1995; Leanza, 2003, 2009; Leanza et al., 2005, 2013; Llambías et al., 2007; Tunik et al., 2010).


nchez, N., et al., Structural style of the Chos Malal fold and thrust belt, Neuque
n basin, Argentina: Please cite this article in press as: Sa Relationship between thick- and thin-skinned tectonics, Journal of South American Earth Sciences (2015), http://dx.doi.org/10.1016/ j.jsames.2015.07.001

4


nchez et al. / Journal of South American Earth Sciences xxx (2015) 1e19 N. Sa

Fig. 3. Regional geological map of the Chos Malal FTB based on own field data (simplified from S
anchez, 2015). Paleozoic to Triassic basement rocks are exposed in the Cordillera del Viento anticlinorium, northwest of the study area. To the east of this basement-involved structure, NeS to NNW trending folds and thrusts of different scale and vergence characterize the Chos Malal FTB. Numbers in white circles identify the main thin-skinned structures mapped in the area: 1: El Alamito backthrust; 2e3: Mayal syncline-anticline; ~ ada Seca anticline-syncline; 6: Chacay Melehue backthrust, 7: Chacay Melehue syncline, 8e9: Cerro Negro anticline-syncline; 10e11: Chos Malal anticline-syncline; 12: Las 4e5: Can
quinas anticline; 13e14: Curí Leuvú syncline-anticline; 15e16: Chapúa anticline-syncline; 17e18: Tilhue
Oeste anticline-syncline; 19e20: Tilhue
anticline-syncline. Ma

NW-oriented half grabens (Vergani et al., 1995; Franzese et al., 2003, 2007). The beginning of subduction at the western margin of Gondwana occurred during the Lower Jurassic resulting in the development of a magmatic arc (Mpodozis and Ramos, 1989; Ramos, 1999; Ramos and Kay, 2006). Related to this process a new cycle of sedimentation took place, with an alternation of marine and continental deposits, and the basin evolved as a back arc basin associated with thermal subsidence (Fig. 2). The end of this period is connected with the closure of the basin and the uplift of this area due to the commencement of the Andean Tectonic Cycle (Groeber, 1946; Leanza, 2009). This compressional stage is linked with the subduction of the Nazca plate under the South American plate, in a direction approximately WSW-ENE, which originated several phases of magmatic activity since the Late Cretaceous (Cobbold and Rossello, 2003; Ramos and Kay, 2006; Zamora Valcarce et al., 2006a, 2007; Folguera et al., 2007). The Andean contraction provoked the Agrio and Chos Malal fold-and-thrust belts in the eastern foothills of the Andes, and also the formation of a wide foreland basin further to the east (Manceda and Figueroa, 1995; Kozlowski et al., 1996, 1998; Cobbold and Rossello, 2003; Zamora Valcarce et al., 2006b; Messager et al., 2010). There is no consensus about which tectonic regime was dominant during the Andean orogeny. Some authors interpret an alternation of extensional and compressional stages in relation to the shallowing or steepening of the angle of subduction of the Nazca Plate (Ramos, 1999, 2010; Ramos and Folguera, 2005; Kay et al., 2006; Oncken et al., 2006;

Mosquera and Ramos, 2006; Ramos and Kay, 2006; Boekhout et al., 2012; Spagnuolo et al., 2012; Ramos et al., 2014). On the other hand, Cobbold and Rossello (2003), Galland et al. (2007) and Messager et al. (2010) among others, postulate that the stress regime remains mainly compressive. The Chos Malal FTB reached its current configuration as a result of different deformation events that occurred since the Late Cretaceous (Cobbold and Rossello, 2003; Tunik et al., 2010) until recent times (Cobbold and Rossello, 2003; Galland et al., 2007; Messager et al., 2010). 3. Stratigraphy of the Chos Malal fold and trust belt The rocks exposed in the Chos Malal FTB consist of a PaleozoicTriassic basement and a MesozoiceCenozoic sedimentary sequence that reaches up to 7.000 m (Fig. 2). The basement is exposed on the western side of the Cordillera del Viento (Fig. 3). It comprises volcanic, plutonic and sedimentary rocks associated with different tectonic and magmatic events during Carboniferous-Permian times (Llambías et al., 2007). An extensional tectonic regime from the Late Triassic to the Early Jurassic affected the oldest rocks, which were unconformably covered by syn-rift deposits of the Precuyano Cycle (Vergani et al., 1995; Franzese and Spalletti, 2001; Franzese et al., 2007; Llambías et al., 2007). This Group is composed by interbedded conglomerates, fanglomerates, fossil bearing marine sediments, andesites and ignimbrites of La Primavera Formation and by
Formation (Fig. 2). The rifting stage was shales of the Milla Michico followed by the Early Jurassic marine ingression of the Cuyo Group,


nchez, N., et al., Structural style of the Chos Malal fold and thrust belt, Neuque
n basin, Argentina: Please cite this article in press as: Sa Relationship between thick- and thin-skinned tectonics, Journal of South American Earth Sciences (2015), http://dx.doi.org/10.1016/ j.jsames.2015.07.001


nchez et al. / Journal of South American Earth Sciences xxx (2015) 1e19 N. Sa

thus changing from localized rifting to a generalized subsidence (Legarreta and Gulisano, 1989). The Pliensbachian Chachil Limestone constitutes the basal unit (Leanza et al., 2013), which is covered by around 800 m of anoxic black shales of the Los Molles Formation deposited in a turbiditic regime (Llambías and Leanza, 2005). The end of the Cuyo Group is marked by evaporites of the T
abanos Formation that reveals an important shallowing of the marine conditions (Legarreta and Uliana, 1996). During Late Jurassic the sedimentation was dominated by sandstones, limestones and evaporites of the Lotena, La manga and Auquilco Formations that form the Lotena Group. A new sedimentary cycle started with the deposition of the Mendoza Group. This group has a wide distribution in the study area and includes the Tordillo, Vaca Muerta, Mulichinco, and Agrio Formations (Figs. 2 and 3). The Tordillo Formation (Kimmeridgian) is composed of red sandstones, conglomerates and claystones of continental origin (Gulisano and
rrez Pleimling, 1995; Spalletti and Veiga, 2007). The thickGutie ness of this formation varies from 600 to 900 m near the Cordillera del Viento, and it decreases to the east reaching a few tens of meters in the region of the Pampa Tril. The Vaca Muerta Formation (Tithonian-Valanginian) is formed by approximately 650 m of black shales, with interbedded limestones (Leanza et al., 1977), deposited during one of the most extended marine transgressions in the basin. The Mulichinco Formation contains approximately 500 m of clastic deposits and banks with a high content of fossils suggesting a shallow marine environment for the area of interest (Rodríguez et al., 2007). The Agrio Formation (Late Valanginian-Barremian) is one of the most prevailing and best exposed units in the region, and
, Avile
and Agua de la it was divided into three members: Pilmatue
Member consists of Mula (Leanza et al., 2005). The Pilmatue 570e680 m of shales, with subordinated sandstones and limestones, deposited in a shoreface platform (Spalletti et al., 2001). Sedimentation in the marine environment was interrupted by the
Member, mainly accumulation of sandy layers defined as the Avile deposited under fluvial and eolian conditions (Veiga et al., 2002). The thickness of this member varies between 10 and 50 m and it constitutes an excellent key bed. The Agua de la Mula Member,
Member, consists of fine-grained clastics thinner than the Pilmatue and minor bioclastic limestones, accumulated in a marine environment (Leanza et al., 2005). During most of the Barremian up to the Aptian, the sedimentation in the basin was characterized by evaporites and continental deposits of the Bajada del Agrio Group (Leanza, 2003; Leanza et al., 2005). The Huitrín Formation is composed of limestones, sandstones and evaporites, while the overlaying Rayoso Formation mainly consists of red siltstones and evaporites that represent the disconnection of the Pacific Ocean and the continentalization of the area (Fig. 2). These units are unconformably covered by a reddish succession of sandstones, mudstones and conglomerates of continental origin, forming the
n Group (Late Cretaceous). At this time, the Neuque
n Basin Neuque evolved to a foreland basin related to the onset of the Andean compressional tectonics (Cobbold and Rossello, 2003; Ramos and Folguera, 2005; Ramos and Kay, 2006; Tunik et al., 2010; Di Giulio et al., 2012). Despite of the magmatic activity was profuse in the region, particularly in the Eocene and the Miocene, the unique evidence of magmatism in the study area are the Sierra del Mayal andesites (Fig. 3) intruded during the Late Eocene (39.7 ± 0.2 Ma, Cobbold and Rossello, 2003). 4. Structure of the Chos Malal fold and thrust belt 4.1. Field observations
n is characterized by its excelThe northwest region of Neuque lent outcrops, which allow us to study in great detail the tectonic

5

structures in the field. During several field trips we collected around 900 GPS points, each one including geological descriptions and structural data. These data were plotted over high resolution satellite images and the digital elevation model (DEM) with 30 m of resolution. From this information we constructed a detailed geological map (1:50.000), using Arcgis software, which covers an
nchez, 2015). area of approximately 1.600 km2 (Sa The first-order structure Cordillera del Viento range is a large NeS trending anticlinorium of more than 30 km of length along strike, where the altitudes reach almost 3.000 m. Widespread exposures of pre-Jurassic rocks in its southern plunge (northwestern corner of the study area, Fig. 3) suggest the participation of the basement in the development of the Chos Malal FTB. Cenozoic volcanic rocks cover partially the western limb of this structure. On the eastern side the sedimentary layers from the Cuyo to Mendoza Groups dips 25ºe35
E forming the forelimb of this NeS trending anticline. The westernmost thin-skinned structure recognized is the El Alamito backthrust (Fig. 3). In the eastern flank of the Cordillera del Viento, the NNE trending the El Alamito backthrust laid the Tordillo Formation over the shales of the Vaca Muerta Formation. Toward the south, other minor east-dipping faults affecting beds of the Tordillo Formation produced small scale folds. These structures involving the Tordillo Formation indicate the existence of an underlying detachment. Based on this evidence we consider the evaporites of the Auquilco Formation as the main detachment horizon for the thin-skinned structures developed toward the foreland. To the east of the El Alamito Backthrust other notable structure is observed in the field, the Mayal syncline (Fig. 3). This fold contains rocks of the Agrio Formation in the limbs and of the Huitrín Formation in the core. The axis of the Mayal syncline shows a strong curvature around the Sierra del Mayal (Fig. 3). We agree with the interpretation of Cobbold and Rossello (2003) suggesting that Eocene andesitic rocks were intruded piercing pre-existing folded structures. Towards the east of the Cordillera del Viento, we mapped several anticlines and synclines involving the Late Jurassic-Early
n Basin (Fig. 3), which can be Cretaceous sequences of the Neuque differentiated according to their scale. The major folds have a wavelength of 4e5 km and involve the whole Mendoza Group. Kozlowski et al. (1996) described these kilometer scale structures as Chorriaca type folds (Herrero- Ducloux, 1948), interpreting them as isoclinal folds with steep flanks. We classify these folds as second order structures, which were detached from the Auquilco Formation. On the other hand, minor folds have around 1 km of wavelength. Their continuity along strike is not as long as those of the major folds and commonly involves the Agrio and Huitrín Formations. These hectometer-scale folds were described by Kozlowski et al. (1996, 1998) as “Codo del Chapúa” type. These folds are third and fourth order structures, which were detached from the Agrio or Huitrin Formations. We interpret these structures of variable scale as thin-skinned fault-related folds. The strike of the largest folds is generally NeS or NNW and they have variable ver
anticlines have an east gence. Mayal, Las M
aquinas and Tilhue vergence while Chos Malal, Curí Leuvú and Tilhúe Oeste anticlines have an opposite vergence to the west (Fig. 3). The western and central folds present axis with south-directed plunge while the folds localized in the eastern side of the map have axis with north
quinas anticline, situated in the central part directed plunge. Las Ma of the studied zone, is the most noticeable kilometer scale fold. This anticline extends for more than 15 km along strike with a significant topographic expression. This remarkable relief is mainly caused by the resistance to erosion of the sandstones of the Mulichinco Formation placed in the flanks, while the shales of the Vaca Muerta Formation lay in the core (Fig. 3).


nchez, N., et al., Structural style of the Chos Malal fold and thrust belt, Neuque
n basin, Argentina: Please cite this article in press as: Sa Relationship between thick- and thin-skinned tectonics, Journal of South American Earth Sciences (2015), http://dx.doi.org/10.1016/ j.jsames.2015.07.001

6


nchez et al. / Journal of South American Earth Sciences xxx (2015) 1e19 N. Sa

In general the second order thin-skinned structures show great continuity along strike and they can be followed for 10e20 km, as ~ ada Seca the El Alamito backthrust, the Mayal syncline, the Can
quinas anticline and the Loma Tilhue
anticlines syncline, the Las Ma (Fig. 3). Other structures do not persist too much, for example the Cerro Negro syncline-anticline, the Chos Malal anticline, the Curí Leuvú anticline-syncline, among others. The Cerro Negro is an east vergent anticline observed only in the northern area. Toward the south, the Chos Malal folds replace the Cerro Negro structures although they have a vergence to the west. This change in the transport direction of the folds seems to be controlled by a tear fault located along the Chacay Melehue stream (Fig. 3). The Chacay Melehue tear fault is a right-lateral strike slip fault, with WSW trend, which also separates the third order structures in the Chacay Melehue area in an east-vergent north domain from a west-vergent south domain (Turienzo et al., 2014). Other tear fault, with WeE strike, normal to the axes of the structures, was interpreted along the Chapúa stream close to the union with the Curí Leuvú River (Fig. 3). This fault creates a long and straight lineament and dis
quinas anticline, defining dextral kiplaces the axis of the Las Ma nematics. The Codo del Curí Leuvú folds are well represented southward of this fault and they are absent toward the north. This tear fault also allows us to explain the change of vergence observed in the Chapúa anticline, to the east in the north and to the west in the south of the study area. During the field work, we also recognized and mapped several structures of minor scale localized in between the larger structures (Fig. 3). In general, they are folds, thrusts and backthrusts that affect the rocks of the Agrio Formation up to the younger rocks of the

Huitrín and Rayoso Formations. Some of these structures generated complex structural zones that were described in great detail by Turienzo et al. (2014). Throughout the study area we have recognized and mapped many of these structures with N-NNW strike. ~ ada Seca anticline, the Chacay Some of these structures are the Can Melehue anticline, the Chacay Melehue backthrust, the Blanco and Chapúa structures. 4.2. Balanced cross-sections Based on the information collected in the field, several 2D seismic lines and oil wells, we constructed four EeW, 40 km long, balanced cross-sections, normal to the main NeS trend of the Andean structures (Fig. 3). These sections were made in order to analyze the complete geometry of the structures in depth and to comprehend the structural style of the Chos Malal FTB. From the seismic lines B-RB-025, 15001, B-RB-007, B-RB-001 and information of the B.Nq.Ch.X-1 and B.Nq.ChE.X-1 wells, it was possible to know the position of the basement in depth as well as compare the thicknesses obtained in the field with stratigraphic thicknesses documented in these two wells (Fig. 3). In the four cross-sections, we interpret a close link between the large basement-involved structures and the smaller fault-related folds developed in the sedimentary cover. We consider that the basement-involved thrusts are detached from 10 to 12 km depth, probably located along the brittleeductile transition within the crust. The hipocentre of an earthquake situated at a depth of ~10 km in the region suggests that this fault system could be currently active. The insertion of different basement wedges along the evaporites of the Auquilco

Fig. 4. Balanced structural cross-section 1. (see location in Fig. 3 and explanation in the text). In our reconstructions we interpret a close relationship between thick and thinskinned structures. The main basement-involved thrust form first order wedges that insert in the cover producing second order fault-related folds. Previous normal faults were passively transported and/or displaced by the Andean thrusts, but they were not inverted. The numbers represent the sequence of faulting and the relationship between the thickskinned with the thin-skinned structures.


nchez, N., et al., Structural style of the Chos Malal fold and thrust belt, Neuque
n basin, Argentina: Please cite this article in press as: Sa Relationship between thick- and thin-skinned tectonics, Journal of South American Earth Sciences (2015), http://dx.doi.org/10.1016/ j.jsames.2015.07.001


nchez et al. / Journal of South American Earth Sciences xxx (2015) 1e19 N. Sa

Formation produced the displacement necessary to generate all the thin-skinned structures that affect the Mesozoic units. The Cordillera del Viento is the unique thick-skinned structure exposed in the study area (Figs. 4 and 5). We reconstructed it as a first-order fault-bend anticline related to the Cordillera del Viento thrust (CdV thrust). Layers of the Cuyo and Lotena Groups form a homoclinal structure, with dips between 20ºe30
SE (Fig. 6), forming the forelimb of the first basement wedge as shown in cross-sections 1 and 2 (Figs. 4 and 5). This first order structure is observed in the 15001 seismic line, where east-dipping reflectors represent the continuity at depth of the layers mapped in the field (Fig. 7). The Cordillera del Viento structure plunges to the south, which explains a non-exposed basement along the areas covered by cross-sections 3 and 4 (Fig. 3). Part of the displacement related to the first basement wedge is absorbed by the El Alamito backthrust. This backthrust is well represented in the cross-section 3 where the Tordillo Formation thrusts over the Vaca Muerta Formation (Figs. 3, 8 and 9). Minor folds affect the Tordillo Formation in the southern area (Fig. 3). We interpreted them as backthrust-related folds developed in front of the basement-involved structure that acts as an intracutaneous wedge. The excess of displacement related to the CdV basement-involved thrust is transferred toward the foreland generating some of the second order anticlines in the cover rocks. The westernmost of these structures is the Mayal anticline (Fig. 3), an asymmetric fold with a long, gently-dipping backlimb (25
W) and a short, steeply-dipping forelimb (80
E). The anticline wavelength is around 2.5 km and we interpreted it as an east-vergent fault-bend fold, with the lower detachment in the Auquilco

7


Formation and the upper detachment at the middle of the Pilmatue Member (Figs. 5 and 8). The Mayal syncline, a broad fold cored by the Huitrín Formation, is placed between the Mayal anticline and the El Alamito backthrust (Figs. 3 and 9). The Cerro Negro anticline is other second order structure observed in the north of the study area. It is an east-vergent anticline with strata of the Agrio Formation with a backlimb dipping about 50
W and a forelimb with dips ranging from 75
E to overturned (Figs. 3, 4 and 10). In crosssection 1 we interpreted the Cerro Negro anticline as created in two stages (Fig. 4). Initially, the structure was formed as a fault-bend fold associated with the first basement wedge (CdV thrust). Subsequently other basement-involved thrust, named Lower Cordillera del Viento thrust (LCdV), transmitted displacement to the same detachment level in the cover and this additional deformation shaped the actual geometry of the Cerro Negro anticline (Figs. 4 and 11). Thus, this anticline was displaced considerably and so becoming an east-vergent fault-propagation fold with an overturned forelimb. In the southern area, the Cerro Negro anticline is replaced by another second order structure, the Chos Malal anticline (Figs. 3, 12 and 13). This fold contains the Mulichinco Formation in its core and the Agrio Formation in the limbs (Fig. 13b). The western limb dips 75ºe85
W, but locally it becomes overturned (Fig. 3), while the eastern limb dips 14ºe23
E. We reconstructed the Chos Malal anticline as a west vergent faultpropagation fold, clearly defined in the southern area and interpreted in cross-section 4 (Fig. 12). The Cerro Negro and Chos Malal synclines are notable structures, exposed in the northern and southern area respectively, and they are placed to the east of the

Fig. 5. Balanced structural cross-section 2. (see location in Fig. 3 and explanation in the text). From field and seismic data, we interpreted two basement-involved structures. In the west, the Cordillera del Viento forms a large basement wedge that is inserted in the cover along the Auquilco Formation producing second order structures such the El Alamito
quinas anticlines. A second thick-skinned structure is formed in depth and it creates the other thin-skinned fault-related folds backthrust and the Mayal, Chos Malal and Las Ma observed in the eastern sector of the cross-section. The numbers represent the sequence of faulting and the relationship between the thick-skinned with the thin-skinned structures.


nchez, N., et al., Structural style of the Chos Malal fold and thrust belt, Neuque
n basin, Argentina: Please cite this article in press as: Sa Relationship between thick- and thin-skinned tectonics, Journal of South American Earth Sciences (2015), http://dx.doi.org/10.1016/ j.jsames.2015.07.001

8


nchez et al. / Journal of South American Earth Sciences xxx (2015) 1e19 N. Sa

homonymous anticlines (Fig. 3). The Cerro Negro syncline is well developed as can be seen in cross-section 1 (Fig. 4) and it is
Member wonderfully exposed due to the sandstones of the Avile that form its flanks (Fig. 10a). The southern syncline is a broad fold involving the Agrio and Huitrín Formations outcropping near Chos
Malal city (Figs. 3 and 12). Minor folds and thrusts affect the Avile Member in both syncline limbs (Fig. 3). They are interpreted as
third order structures detached along the shales of the Pilmatue
quinas anticline is the most important second Member. The Las Ma order fold in the Chos Malal FTB because it is the only structure observed in the whole area and reconstructed in the four crosssections (Figs. 4, 5, 8 and 12). This anticline has an east vergence defined by overturned layers measured in its forelimb and in the northern region, between the Cerro Negro and the Chapúa stream; it contains the Vaca Muerta Formation in the core (Figs. 3 and 10c). We interpreted the Las M
aquinas anticline as the result of two superimposed faults at depth (Figs. 4, 5, 8 and 12). In a first

instance, a west-dipping thrust produced an incipient fault-bend fold, with the lower detachment located in the Auquilco Forma
Member of the tion and the upper detachment along the Pilmatue Agrio Formation. In a second stage, a branch of the initial fault affected the forelimb of the fault-bend anticline. The propagation of this second fault increased the dip of the overlying strata to produce the final fold configuration (Figs. 4 and 5). The Codo del Curí Leuvú syncline and anticline are east-vergent folds involving the Mulichinco and Agrio Formations (Figs. 8 and 12). The geometry of these folds is visible in the valley of the Curí Leuvú River, near the confluence with the Blanco stream (Fig. 3), characterized by a broad and rounded syncline and a tight anticline with an angular hinge. The Mulichinco Formation beds in the forelimb of the Codo del Curí Leuvú anticline shows high angle dips and the layers of the Agrio Formation are locally overturned (Fig. 3). We interpreted this anticline as the last second-order structure associated with the displacement of the CdV basement wedge (Figs. 8 and 12). The


n Basin. This structure plunges to the Fig. 6. Panoramic views of the Cordillera del Viento, a huge NeS trending anticlinorium that exposes the oldest basement rocks of the Neuque south and the Mesozoic layers in its eastern flank dip to the east with moderate angle, forming an extended homoclinal.


nchez, N., et al., Structural style of the Chos Malal fold and thrust belt, Neuque
n basin, Argentina: Please cite this article in press as: Sa Relationship between thick- and thin-skinned tectonics, Journal of South American Earth Sciences (2015), http://dx.doi.org/10.1016/ j.jsames.2015.07.001


nchez et al. / Journal of South American Earth Sciences xxx (2015) 1e19 N. Sa

kinematics of this anticline can be explained in the same manner as the previously described second-order folds, through the interaction of two faults in depth. An initial flat-ramp-flat trajectory generated by a fault-bend fold and the produced displacement
Member allows us to along the upper detachment in the Pilmatue explain the Chapúa anticline involving younger units. Subsequently, a secondary fault splays from the initial thrust and cut the duplicated layers in the thrust sheet. Propagation of this fault increased the structural relief of the anticline and produced its
quinas anticline was overturned forelimb (Fig. 8). The Las Ma transported and rotated on these faults in a normal (piggyback) sequence of faulting. In all cross-sections we have interpreted most of the second order folds formed initially as fault-bend folds, which implies the transmission of displacement from a lower to an upper detachment. This upper detachment is generally located along the
Member of the Agrio Formation. In the Chos Malal FTB Pilmatue there are several third order structures formed over this shallow detachment, commonly repeating the members of the Agrio Formation. Both a detailed description and the kinematic evolution of these complex structures were provided by Turienzo et al. (2014). Examples of these third order structures are the minor folds

9

~ ada Seca, Chacay Melehue, Chapúa and Blanco recognized at the Can stream areas (Fig. 3). Based on a series of east-dipping reflectors over subhorizontal reflections observed in the west end of the B-RB-025 seismic line (Fig. 11), we interpreted the front of other basement wedge associated with the LCdV thrust. In the north of the study area, this wedge forms a second thick-skinned structure below the Cordillera del Viento anticline (Fig. 4). Although the displacement on the LCdV basement-involved thrust sheet is small, it is enough to accomplish
quinas anticlines. Addithe easternmost Cerro Negro and Las Ma tionally, this deeper wedge contributed to the uplift of the Cordillera del Viento to higher altitudes. The seismic line 15001, which it is arranged along section 2, does not show evidence of this second structure underlying the Cordillera del Viento (Fig. 7). Although there are not seismic lines in the southern area we interpreted the existence of the LCdV basement-involved thrust in order to explain the intrusion of the Sierra del Mayal andesites (Fig. 12). Moreover,
n River, Lebinson et al. few kilometers to the south of the Neuque (2014) interpreted comparable structures based on field and seismic information. In the B-RB-025 seismic line, under the Las M
aquinas anticline, we observed a marked angularity between continuous east-

Fig. 7. Uninterpreted and partially interpreted two-way traveltime seismic line 15011, situated between cross-sections 2 and 3 (see location in Fig. 1). East-dipping reflectors to the west of the line correlate with the eastern flank of the Cordillera del Viento anticlinorium. Strong reflections below the Mayal and Chacay Melehue areas are interpreted as the base of the sedimentary sequence, uplifted over a second basement wedge formed at depth.


nchez, N., et al., Structural style of the Chos Malal fold and thrust belt, Neuque
n basin, Argentina: Please cite this article in press as: Sa Relationship between thick- and thin-skinned tectonics, Journal of South American Earth Sciences (2015), http://dx.doi.org/10.1016/ j.jsames.2015.07.001

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nchez et al. / Journal of South American Earth Sciences xxx (2015) 1e19 N. Sa

dipping reflectors assigned to the base of the sedimentary sequence (Cuyo Group) and underlying subhorizontal reflectors (Fig. 11). This lower reflections show a thickening toward the west, which suggest that they can be syn-rift deposits controlled by an east-dipping normal fault. Taking into account these features it is possible to recognize harpoon geometry probably related to the partial reactivation of the normal fault during the Andean compression. Subsequently, this half-graben was cut by a west-dipping thrust that generates a third basement wedge. This thrust puts the eastdipping layers of the Cuyo and Lotena Groups over horizontal reflectors of the same units, as seen below the Chapúa area at approximately 3.300 ms (Fig. 11). In the 15001 seismic line, below the Chacay Melehue area, it is possible to observe a strong contrast of reflections assignable to the basement-cover contact (Fig. 7). These strong reflectors are considerably higher than in the underformed adjacent region, revealing a basement-involved structure at depth. Reflectors gently-dipping to the east in the hangingwall of this basement slice are thrusted over horizontal reflectors in the footwall. We interpreted this relationship as the front of a basement wedge (Fig. 7). We named this basement-involved fault as the
n thrust (Figs. 4, 5, 8 and 12). In the hangingwall of the Neuque
n thrust, the seismic line shows reflectors displaced by two Neuque normal faults that have not evidence of tectonic inversion (Fig. 7).
n thrust In all cross-sections we interpreted that the Neuque transmitted the deformation toward the foreland along the Auquilco Formation producing second order structures in the cover. In the northern cross-sections, these thin-skinned structures are interpreted as two east-vergent fault-bend anticlines below both

the Chapúa and the Blanco streams (Figs. 4 and 5). The B.Nq.ChE.x-1 well drilled the backlimb of one of these anticlines, detecting the beds of the Mulichinco Formation with dips of 45ºe55
W. This fold has not surficial expression and it was interpreted by Kozlowski et al. (1996, 1998) based on subsurface information. The easternmost anticline outcrops along the Blanco stream involving the whole Agrio Formation (Fig. 3). In order to complete the structural reconstruction of cross-sections 1 and 2, we projected this fold toward the north and south respectively. In the southern region of
n basement-involved thrust created a the study area the Neuque series of second order folds, which have opposite vergence
(Figs. 8 configuring a pop-up structure exposed in the Loma Tilhue
Oeste anticline is asymmetric, with vergence to and 12). The Tilhue
Member dipping 70e85
W the west. It has layers of the Pilmatue and 40e50
E in the western and eastern flanks respectively (Figs. 3, 8 and 12). The rocks of the Mulichinco Formation are contained in the fold core and steeply-dipping strata of the Agrio and Huitrín Formations form the forelimb (Fig. 14). The Tilhúe anticline has a vergence to the east and shales of the Vaca Muerta Formation in its core (Figs. 3 and 12). In the forelimb the layers of the Mulichinco Formation dip with high angle to the east and toward the south they are overturned. Between the west-vergent
Oeste anticline and the east-vergent Codo del Curí Leuvú Tilhue anticline a triangular zone is formed (Figs. 8 and 12). Inside this zone there are third order folds, as the west-vergent Chapúa anticline. We interpreted this structure as a fault propagation fold de
Member related to the westward push tached along the Pilmatue
Oeste anticline (Figs. 8 and 12). generated by a Tilhue

Fig. 8. Balanced structural cross-section 3. (see location in Fig. 3 and explanation in the text). Basement rocks are not exposed along this cross-section due to the plunges of the Cordillera del Viento toward the south. Wide outcrops of the Tordillo Formation at surface are explained by means of a set of backthrusts formed in front of the westernmost wedge.
n thrust at depth contributes to create the structures that form the fold and thrust belt. The hipocentre of an earthquake As in cross-section 2, a basement-slice related to the Neuque situated at a depth of ~10 km in this area suggests that the deep fault system could be currently active. The numbers represent the sequence of faulting and the relationship between the thick-skinned with the thin-skinned structures.


nchez, N., et al., Structural style of the Chos Malal fold and thrust belt, Neuque
n basin, Argentina: Please cite this article in press as: Sa Relationship between thick- and thin-skinned tectonics, Journal of South American Earth Sciences (2015), http://dx.doi.org/10.1016/ j.jsames.2015.07.001


nchez et al. / Journal of South American Earth Sciences xxx (2015) 1e19 N. Sa

In the eastern portion of the study area, below the thin-skinned structures, we interpreted different basement-involved structures based on the seismic information. The B-RB-007 seismic line shows wedge-shaped reflectors, underlying continuous reflections that represent the base of the Jurassic sedimentary sequence, which we interpret as syn-rift deposits controlled by normal faults (Fig. 11). The main normal fault dips to the east with high angle and does not show evidence of positive inversion despite being located in a thorough compressive structural framework. A set of minor west-dipping normal faults affects both syn-rift and overlying units, suggesting a younger age or reactivation. Toward the south, the B-RB-001 seismic line also displays evidences of normal faulting. Below a set of strong and continuous horizons there are folded reflectors with a westward-increasing thickness that disappears abruptly in the same direction (Fig. 15). As in the previously described seismic line, we interpret this geometry as syn-rift deposits filling a half-graben limited by an east-dipping normal fault. The observed folding in the syn-rift layers near the main fault constitutes a drag syncline that reveals the normal displacement on the fault plane. Secondary high-angle normal faults displaced the strata of the overlying Cuyo and Lotena Groups, establishing their younger age. Similar structures were

11

described by Cristallini et al. (2009) to the east of the study area. They made a series of analogue models and concluded that younger normal faults were formed due to subsidence related to differential compaction of the syn-rift sequences. All the observed extensional structures were not inverted during the Andean compression. In the eastern end of the B-RB-007, 15001 and B-RB- 001 seismic lines the reflectors located at the basement-cover boundary dip to the west (Figs. 7, 11 and 15). This uplift of basement rocks is related to major thrusts developed in a normal sequence toward the foreland, out of the study area. These west-dipping thrusts produced the basement-cored Las Yeseras and Pampa Tril anticlines observed in the eastern slope of the Tromen volcano (Kozlowski et al., 1996, 1998). This inclined substratum corresponds to the backlimb of the Las Yeseras basement-involved structure, which translated passively the Late Triassic half-grabens. The seismic sections in the area show that, despite of the strong compression affecting this region, the normal faults controlling the syn-rift accumulations were not inverted. From the line-length restoration of the basement and cover structures interpreted in our balanced cross-sections, we calculated shortenings of 16.9 km (29.7%), 16.9 km (29.7%), 14.7 km (26.9%)

Fig. 9. Southward view of the intrusive rocks that form the Sierra del Mayal. These Eocene rocks (39.7 Ma, Cobbold and Rosello, 2003) are located in the core of the Mayal syncline and they displaced several fold axes during its emplacement, thus post-dating the affected structures (see Fig. 3).


nchez, N., et al., Structural style of the Chos Malal fold and thrust belt, Neuque
n basin, Argentina: Please cite this article in press as: Sa Relationship between thick- and thin-skinned tectonics, Journal of South American Earth Sciences (2015), http://dx.doi.org/10.1016/ j.jsames.2015.07.001

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nchez et al. / Journal of South American Earth Sciences xxx (2015) 1e19 N. Sa

and 14.15 km (26.3%) for the northern, central and southern crosssections respectively (Figs. 5, 8, 12 and 14). These percentages of contraction in the Chos Malal FTB are comparable with other previous estimations from regional cross-sections (Booth and Coward,

1996; Nocioni, 1996; Zapata et al., 1999). The observable decreasing shortenings from northern to southern cross-sections coincide with the absence of basement rocks exposures in the south of the study area (Fig. 3). These features can be explained due to the

Fig. 10. View toward the north of the more noticeable structures placed between cross-sections 1 and 2. The Cerro Negro is a late Miocene andesite intruded contemporaneously with the Neogene pulse of Andean contraction (Gürer et al., 2012). a) The Cerro Negro syncline is a NeS fold, spectacularly exposed due to the contrasting lithology of the Agrio Formation members in its flanks. b) 3D image from Google Earth which shows the topographic expression of the mapped folds. c). Panoramic photography of the east-vergent Las
quinas anticline, a second order fold cored by shales of the Vaca Muerta Formation. Ma


nchez, N., et al., Structural style of the Chos Malal fold and thrust belt, Neuque
n basin, Argentina: Please cite this article in press as: Sa Relationship between thick- and thin-skinned tectonics, Journal of South American Earth Sciences (2015), http://dx.doi.org/10.1016/ j.jsames.2015.07.001


nchez et al. / Journal of South American Earth Sciences xxx (2015) 1e19 N. Sa

southward plunge of the Cordillera del Viento structure in the cross-sections 3 and 4. 5. Age of deformation in the Chos Malal FPC In order to better comprehend the development of the Chos Malal FTB it is very important to integrate the available information of the age of Andean deformation in the region with the structures interpreted in our cross-sections. There is a general consensus that this segment of the Andes was affected by more than one compressive episode. Most authors propose three contractional pulses, the first one in the Late Cretaceous, the following in the Paleocene and the last one in the Early to Middle Miocene (Cobbold and Rossello, 2003; Ramos and Folguera, 2005; Kay et al., 2006; Zamora Valcarce et al., 2006a, 2007; Folguera et al., 2007, 2012; Folguera and Ramos, 2011; Rojas Vera et al., 2014). The continen
n Group, probably accumulated in a tal red beds of the Neuque
n Basin, suggest that the Andean uplift foreland stage of the Neuque started during the Late Cretaceous (Cobbold and Rossello, 2003; Ramos and Folguera, 2005). Tunik et al. (2010) propose the onset of exhumation between 98.6 Ma and 88 Ma based on analysis of
n Group that indicate a change in the detrital zircons in the Neuque provenance area. Zamora Valcarce et al. (2006a) showed dykes Ar40/Ar39 ages of 100 Ma over that cut previous structures in the Agrio FTB. Cretaceous deformation was also supported by Ar40/Ar39 cooling ages of plutons of the Cordillera del Viento (Kay, 2002) and fission track analysis (Burns, 2002; Burns et al., 2006), which suggest an initiation of the exhumation at 70 Ma. Our field mapping in the area shows that the intrusions of the Sierra del Mayal produced a clear flexure in the fold axis of the Mayal syncline (Fig. 3). These

13

rocks have an Ar40/Ar39 age of 39.7 ± 0.2 Ma (Cobbold and Rossello, 2003), which evidences a pre-Eocene folding event. A younger compressive episode during the late Miocene was evidenced by the folding and faulting of the Middle to Late Miocene volcanic sequences of the Charilehue Formation (Folguera et al., 2007). Some workers postulate that compressive tectonics in the region was continuous until recent times (Cobbold and Rossello, 2003; Galland et al., 2007; Messager et al., 2010). Others authors proposed an extensional event due to crustal collapse during the 1.7e0.7 Ma time interval (Folguera et al., 2007). In our study area, the only evidence to constrain the age of the structures is the intrusion of the andesitic rocks of the Sierra del Mayal that affects the Mayal syncline. We interpret that this syncline and some others second-order folds in cover rocks are related to the CdV basement-involved thrust (Cordillera del Viento firstorder anticline), and thus they would be of pre-Eocene age. In our cross-sections we considered that the major thick-skinned structures, and their related thin-skinned structures, were formed in a normal sequence of thrusting from hinterland to foreland. In the southern cross-section we inferred a basement slice with a small backthrust, related to the LCdV thrust, underneath the Sierra del Mayal (Fig. 12). Analogue models and natural examples show that under compressive stress regimes magmas can be emplaced through thrusts and backthrusts (Dimieri, 1992; Galland et al., 2007; Araujo et al., 2013). Taking into account these antecedents we think that the backthrust interpreted in cross-section 4 was created contemporaneously with the intrusion of the Sierra del
n thrust formed a Mayal during the Eocene. Afterwards, the Neuque basement slice in depth that moved toward the east creating sec
folds observed in the eastern ond order structures, as the Tilhue

Fig. 11. Uninterpreted and partially interpreted two-way traveltime seismic lines BRB-025 and BRB-007, between cross-sections 1 and 2 (see location in Fig. 1). These lines allow us to interpret the basement geometry at depth in the northern structural cross-sections. In the eastern sector of the BRB-007 line, we can observe a lower package of discontinuous reflectors, wedging to the east, unconformably covered by strong and continuous reflections. This geometry can be interpreted as a syn-rift filling controlled by a high-angle east dipping normal fault.


nchez, N., et al., Structural style of the Chos Malal fold and thrust belt, Neuque
n basin, Argentina: Please cite this article in press as: Sa Relationship between thick- and thin-skinned tectonics, Journal of South American Earth Sciences (2015), http://dx.doi.org/10.1016/ j.jsames.2015.07.001

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nchez et al. / Journal of South American Earth Sciences xxx (2015) 1e19 N. Sa

sector of cross-sections. Unfortunately the study area does not have exposures of Miocene volcanic rocks, notwithstanding they are exposed and folded few kilometers to the north (Folguera et al., 2007) and we correlate this younger episode with the develop
n thrust and its associated structures (Fig. 12). ment of the Neuque 6. Discussion Different structural models have been proposed in order to explain the major basement-involved structures in the Chos Malal FTB. The tectonic inversion of Mesozoic normal faults is one of the mechanisms used to explain the uplift of the thick-skinned structures, as the Cordillera del Viento, in this orogenic belt (Booth and Coward, 1996; Chauveau et al., 1996; Cobbold and Rosello, 2003; Zapata et al., 1999; Zamora Valcarce et al., 2006b). Most of these models suggest that the vergence of the structures would have been inherited from the Jurassic half grabens. Zapata et al. (1999) described two west-vergent faults in the west slope of the Cordillera del Viento that rose the pre-Jurassic rocks to the surface. Additionally, they interpreted an inverted west-dipping blind fault beneath the eastern slope, which propagated in the sedimentary cover forming minor order structures. Other authors proposed that NeS trending thrusts formed during the Andean compression gave birth to basement slices that transferred their displacements along detachment horizons in the cover and produced minor order

~ es, 1987; Kozlowski et al., 1996; structures (Ploszkiewicz and Vin Nocioni, 1996; Allmendinger et al., 2004; Turienzo et al., 2014; S
anchez et al., 2014). These first order basement-involved structures were interpreted as basement wedges or fault-bend folds. Other reconstructions considered both mechanisms responsible for basement uplifts (Folguera et al., 2007, 2011; Rojas Vera et al., 2014; Sagripanti et al., 2014). Sagripanti et al. (2014) interpreted previous extensional structures that were reactivated as contractional and transfer faults in the forelimb of Cordillera del Viento and NeS thrusts without previous controls. Rojas Vera et al. (2014) interpreted a west-dipping inverted fault in the east slope of the Cordillera del Viento, in agreement with Zapata et al. (1999), but they included a short-cut fault in the footwall that explains the transference of displacement to the cover and the creation of the thin-skinned folds in the Chapúa area. In the hanging-wall of the inverted fault Rojas Vera et al. (2014) proposed three backthrusts affecting basement rocks. In the analyzed seismic lines, we recognized syn-rift geometries associated with normal faults, preserving their extensional configuration and without evidences of appreciable tectonic inversion (Figs. 11 and 15). The 15001 seismic line (Fig. 7) records east-dipping strong reflectors that we interpret as the front of two major basement wedges. The reflections observed on the west side of this seismic line can be correlated with the east-dipping forelimb of the Cordillera del Viento anticline at surface (Fig. 5). The strong

Fig. 12. Balanced structural cross-section 4. (see location in Fig. 3 and explanation in the text). Early development of the CdV basement-involved thrust produced some of the folds in cover rocks, as the Mayal syncline, whose fold axis was displaced during the intrusion of the Eocene rocks that form the Sierra del Mayal. This relationship evidences a pre-Eocene
n thrust as post-Eocene. The contractional event, in agreement with observations of previous workers. Following a normal sequence of faulting we considered the Neuque
folds. The numbers represent the sequence of faulting and the displacement of this last basement-involved slice created the easternmost thin-skinned structures as the Tilhue relationship between the thick-skinned with the thin-skinned structures.


nchez, N., et al., Structural style of the Chos Malal fold and thrust belt, Neuque
n basin, Argentina: Please cite this article in press as: Sa Relationship between thick- and thin-skinned tectonics, Journal of South American Earth Sciences (2015), http://dx.doi.org/10.1016/ j.jsames.2015.07.001


nchez et al. / Journal of South American Earth Sciences xxx (2015) 1e19 N. Sa

15

Fig. 13. a) 3D image from Google Earth of the structures placed along cross-section 4, to the west of Chos Malal (see Fig. 3 for location). The Mulichinco Formation is exposed in the
n River. core of the Chos Malal anticline producing a high relief. b) The Chos Malal anticline plunges to the south and subsequently, the Agrio Formation is exposed across the Neuque


Oeste anticline (16 and 18 in Fig. 3), which are exposed to the east of Chos Malal Fig. 14. Photography toward the south of the Chapúa syncline and the western limb of the Tilhue city. Anhydrites of the Huitrín Formation are well preserved in the core of the syncline.


nchez, N., et al., Structural style of the Chos Malal fold and thrust belt, Neuque
n basin, Argentina: Please cite this article in press as: Sa Relationship between thick- and thin-skinned tectonics, Journal of South American Earth Sciences (2015), http://dx.doi.org/10.1016/ j.jsames.2015.07.001

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nchez et al. / Journal of South American Earth Sciences xxx (2015) 1e19 N. Sa

reflections below the Mayal and Chacay Melehue area represent the
n thrust. This wedge top of the basement slice related to the Neuque was inserted into the cover producing the fault-related folds in the Chapúa region. This lower basement-involved thrust slice was also observed in the BRB-025 seismic line underlying the Cerro Negro

and Las M
aquinas anticlines (Fig. 11), several kilometers to the east of the Cordillera del Viento. Our observations support that the thinskinned structures of the Chos Malal FTB are related to several thick-skinned structures. This is a different model compared to previous regional models that proposed the Cordillera del Viento as

Fig. 15. Uninterpreted and partially interpreted two-way traveltime seismic line BRB-001, parallel to the eastern segment of cross-section 4 (see location in Fig. 1). In this line, the dip toward the west of the basement-cover contact is evident, in the backlimb of the eastward-directed Las Yeseras and Pampa Tril thrusts. In the middle of the section, below a set of strong reflections, we recognize folded and wedging reflectors interpreted as syn-rift deposits accumulated in a half-graben. Based on this geometry, it is possible to infer a main east-dipping normal fault controlling the syn-extensional rocks and a set of minor normal faults that affects the overlying reflectors, suggesting a younger age or activity on these latter faults. A remarkable point is that although all these normal faults are located in a region dominated by strong compressive structures (e.g. Las Yeseras thrust), they preserve their extensional geometry and were not inverted during the Andean compression.


nchez, N., et al., Structural style of the Chos Malal fold and thrust belt, Neuque
n basin, Argentina: Please cite this article in press as: Sa Relationship between thick- and thin-skinned tectonics, Journal of South American Earth Sciences (2015), http://dx.doi.org/10.1016/ j.jsames.2015.07.001


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the unique basement structure (Kozlowski et al., 1996, 1998; Booth and Coward, 1996; Chauveau et al., 1996; among others). Shortenings calculated by previous workers were based on regional cross-sections, which included only some of the larger structures. Nocioni (1996) interpreted a section of ~150 km and a second section of ~136 km, both extending from the Chilean side of
n foreland. The resulting shortenings were the Andes to the Neuque 48.4 km (24%) and 72.4 km (34%) respectively. Booth and Coward (1996) analyzed a ~115 km long cross-section that extends from the Cordillera del Viento to the Huantrainco area and obtained 20% of shortening. The structural cross-section at 37
S made by Zapata et al. (1999) implies a shortening of 36 km (26%). The shortening calculated by Rojas Vera et al. (2014) for the sector between Cordillera del Viento and Pampa Tril is 12.5 km (15.10%). From the restitution of our four cross-sections that encompass the area between the Cordillera del Viento and the Tromen volcano, we obtained shortenings ranging from 16.9 km (29.7%) to 14.15 km (26.3%). In the building of our cross-sections we considered all the structures, including fault-related folds of different order developed in the sedimentary cover. In order to obtain more accurate values of shortenings, we balance these major and minor structures mainly based on field observations. This allowed us to reconstruct more appropriately the basement-involved structures. Turienzo et al. (2014) described a backthrust system in the Chacay Melehue area involving the Agrio Formation, which implies 3 km of shortening, demonstrating the relevance of such minor order structures. Therefore, it is straightforward that detailed interpretations considering structures of all scales in fold and thrust belts are essential to obtain realistic shortening values. 7. Conclusions Based on a detailed field mapping of the area between the Cordillera del Viento and the Tromen volcano, and with the assistance of well and seismic data, we elaborated four regional balanced cross-sections in order to characterize the structural style of the Chos Malal FTB. According to our interpretations, Andean deformation in this area is controlled by several basement wedges related to a major detachment placed~12 km into the crust. These wedges were inserted into the cover, along the evaporites of the Auquilco Formation, and they transmitted displacement producing several fault-related folds. The Cordillera del Viento anticlinorium is the westernmost wedge, which exposes Paleozoic-Triassic rocks. Interpretation of 2D seismic lines allows us to recognize other basement slices at depth that also were inserted in the cover, contributing to form the thin-skinned structures interpreted on the eastern side of cross-sections. Seismic lines allowed interpreting syn-rift geometries controlled by high-angle east-dipping normal faults, without evidence of positive inversion tectonics. From the restoration of cross-sections, we calculated shortenings of 16.9 km (29.7%), 16.9 km (29.7%), 14.7 km (26.9%) and 14.15 km (26.3%) for the northern, central and southern crosssections respectively, which evidence a small diminution of the contraction toward the south probably associated with the plunge of the Cordillera del Viento structure in this segment of the Chos Malal FTB. Acknowledgments The present work forms part of the doctoral studies carried out
nchez) and they were supported by several by the first author (N. Sa grants from CeCyT-UNS (24/H091), ANPCyT (Pict 0166) and CONICET (PIP 0390). Seismic and well data were provided by the Sub
n. secretaría de Minería e Hidrocarburos de la Provincia de Neuque
, Joaquín Lavios and Candela We kindly acknowledge Mauricio Espie

17

Gorza for their valuable help during field works. We would like to thank the INGEOSUR, Departamento de Geología-UNS and Municipalidad de Chos Malal. We are also deeply grateful for the comments and suggestions made by Gonzalo Zamora Valcarce and an anonymous reviewer who provided a detailed critique of the manuscript which considerably improved its quality.

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