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ARTICLE IN PRESSG ModelEOD-1302; No. of Pages 17

Journal of Geodynamics xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Journal of Geodynamics

jou rn al hom ep age: ht tp : / /www.e lsev ier .com/ locate / jog

ranstensional tectonics induced by oblique reactivation of previousithospheric anisotropies during the Late Triassic to Early Jurassicifting in the Neuquén basin: Insights from analog models

lorencia Bechisa,∗, Ernesto O. Cristallinib, Laura B. Giambiagi c, Daniel L. Yagupskyb,ecilia G. Guzmánb, Víctor H. Garcíad

Instituto de Investigaciones en Diversidad Cultural y Procesos de Cambio (IIDyPCa), CONICET – Universidad Nacional de Río Negro, Mitre 630, CP 8400 Sanarlos de Bariloche, ArgentinaLaboratorio de Modelado Geológico, Instituto de Estudios Andinos (IDEAN), CONICET – Universidad de Buenos Aires, Pabellón 2, Ciudad Universitaria,P 1428 Buenos Aires, ArgentinaInstituto Argentino de Nivología, Glaciología y Ciencias Ambientales (IANIGLA), CONICET, Centro Científico Tecnológico Mendoza, Parque General Sanartín, CC 330, CP 5500 Mendoza, ArgentinaInstituto de Investigación en Paleobiología y Geología (IIPG), Universidad Nacional de Río Negro, Avenida General Roca 1242, CP 8332 General Roca,rgentina

r t i c l e i n f o

rticle history:eceived 20 October 2013eceived in revised form 22 April 2014ccepted 23 April 2014vailable online xxx

eywords:estern Gondwana

euquén basinentral Andesblique riftingtructural inheritance

a b s t r a c t

The goal of this study was to determine the main factors that controlled the kinematic evolution and thestructural architecture developed during the Late Triassic to Early Jurassic rifting that led to the openingof the Neuquén basin in the southwestern sector of Gondwana. We carried out a series of analog modelsto simulate an extensional system with a bent geometry similar to the northeastern border of the basin.In different experiments, we varied the extension direction between NNE (N10◦E) and NE (N45◦E) ori-entations, inducing rift systems with different degrees of obliquity in each sector of the extended area.We compared the kinematic evolution and the final structural architecture observed in the experimentswith data from two selected representative areas of the basin: (1) the Atuel depocenter, situated in thenorthern Andean sector, and (2) the Entre Lomas area, situated in the northeastern Neuquén Embayment.In both cases, the good match between the field and subsurface data and the results of the analog modelssupports a NNE orientation of the regional extension (N30◦E–N20◦E) during the synrift stage. Our exper-

nalog modeling imental results suggest that lithospheric weakness zones of NNW to NW trend could have controlledand localized the extension in the Neuquén basin. These previous anisotropies were linked to the suturesand rheological contrasts generated during the collision of terranes against the southwestern margin ofGondwana during the Paleozoic, as well as further modifications of the thermo-mechanical state of thelithosphere during the Late Paleozoic to Early Mesozoic evolution.

. Introduction

Please cite this article in press as: Bechis, F., et al., Transtensional teanisotropies during the Late Triassic to Early Jurassic rifting in the Nhttp://dx.doi.org/10.1016/j.jog.2014.04.010

The Neuquén basin is one of the main hydrocarbon-bearingasins of southern South-America, covering a wide area inest-central Argentina and east-central Chile (Fig. 1). Its infill is

∗ Corresponding author at: IIDyPCa, CONICET – Universidad Nacional de Río Negro,itre 630, 5◦ piso, CP 8400 San Carlos de Bariloche, Argentina.

el.: +54 294 4429350x53.E-mail addresses: [email protected] (F. Bechis), [email protected]

E.O. Cristallini), [email protected] (L.B. Giambiagi),[email protected] (D.L. Yagupsky), [email protected]. Guzmán), [email protected] (V.H. García).

ttp://dx.doi.org/10.1016/j.jog.2014.04.010264-3707/© 2014 Elsevier Ltd. All rights reserved.

© 2014 Elsevier Ltd. All rights reserved.

up to 7000 m thick, and it constitutes an almost continuous recordfrom the Late Triassic until the Paleocene (Legarreta and Gulisano,1989; Legarreta and Uliana, 1996; Howell et al., 2005; and refer-ences therein). These characteristics make it an excellent case studythat registers the most relevant tectonic stages of southern SouthAmerica during the Mesozoic (Vergani et al., 1995; Franzese et al.,2003). This contribution focuses on the Late Triassic to Early Juras-sic period, when it was initiated as a continental rift basin in thecontext of a widespread extensional stage that affected westernGondwana and culminated with the break-up of the supercontinent

ctonics induced by oblique reactivation of previous lithosphericeuquén basin: Insights from analog models. J. Geodyn. (2014),

(Uliana et al., 1989; Vergani et al., 1995; Manceda and Figueroa,1995; Franzese and Spalletti, 2001). The basin is divided in twomain areas, the Neuquén Embayment, which includes part of theHuincul High and the Platform, and the Andean sector (Fig. 1). While

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Fig. 1. Digital elevation model showing the location of the Neuquén basin. Notethe marked curvature of its northeastern border, which shows a NNW trend in theAndean sector, and curves into a NW trend in the Neuquén Embayment sector.

Fig. 2. Regional maps showing the main stages of the tectonic evolution since the late PalePampia, Cuyania, Chilena and Patagonia terranes, accreted during the Paleozoic (based on2011; Tomezzoli, 2012). The northern boundary of the Upper Paleozoic Orogen (Gondwá2009; Giambiagi et al., 2011, 2012; Pángaro and Ramos, 2012). On the top-right the locatGondwana (based on Rapalini, 2005). (b) Location of the Triassic to Early Jurassic rift basinon Ramos and Kay, 1991; Ramos, 1994; Alvarez and Ramos, 1999). The inferred suturesunits at the current Andean convergent margin. Convergence direction and rate between

PRESSamics xxx (2014) xxx–xxx

the Andean sector was affected by the shortening that led to thebuilding of the southern Central Andes since the Late Cretaceous,the Embayment underwent little deformation due to its forelandposition (Fig. 2). The basin has a nearly triangular shape, with itsnortheastern border forming a concave bend (Fig. 1). This bordershows a NNW trend in the northern Andean sector, and it curvestoward a NW orientation in the Neuquén Embayment.

The initial synrift deposits, Norian to Toarcian in age, arerestricted to isolated depocenters limited by normal faults, withvariable orientations according to their geographical locationwithin the basin (Vergani et al., 1995). The normal faults weretraditionally interpreted as forming part of three groups that runparallel to the basin borders: N-trending in the Andean sector, NW-trending in the Platform, and ENE-trending in the Huincul High(Vergani et al., 1995; Mosquera and Ramos, 2006). Recently, theextensional architecture of the Neuquén basin has been partiallymodified by detailed structural studies, with most normal faultsranging between WNW and NNW trends (Fig. 3; Pángaro et al.,2006; Franzese et al., 2006, 2007; Yagupsky et al., 2007, 2008;Silvestro and Zubiri, 2008; Giambiagi et al., 2008a, 2009; Cristalliniet al., 2006, 2009; Bechis et al., 2009, 2010; Bechis and Giambiagi,2009; García Morabito et al., 2011; D’Elia et al., 2012). Althougheach sector or depocenter shows particular characteristics andevolution and should be studied in detail, NNW- to NW-trendingnormal faults predominate in the northern Andean sector, whilethe Platform area of the Neuquén Embayment shows NW to WNWoriented faults (Fig. 3).

Rift systems with multiple fault sets like the ones described forthe Neuquén basin can be interpreted as the result of polyphasicrifting with changes in the extension direction (Bonini et al., 1997;Keep and McClay, 1997), reactivation of upper crustal inherited fab-


rics (Morley et al., 2004; Bellahsen and Daniel, 2005), oblique riftingwith extension oblique to the margins of the rift system (Withjackand Jamison, 1986; Tron and Brun, 1991; McClay and White, 1995;Clifton et al., 2000) or orthorhombic fault systems related to a

ozoic. (a) Inferred sutures between Western Gondwana (Río de la Plata craton) and Chernicoff and Zappettini, 2003; Pankhurst et al., 2006; Ramos, 2008; Varela et al.,nides – San Rafael Orogen) is also shown in the map (based on Kleiman and Japas,ion of the map is shown in a simplified paleogeographic reconstruction of Westerns, and the distribution of outcrops of the Choiyoi Group and equivalent units (based

between Paleozoic terranes are also shown in the map. (c) Main morphotectonic the South America and Nazca plates is based on Somoza (1998).

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Fig. 3. Compiled up to date map showing the main normal faults and depocentersfo

tAitmMttltB

mocs

terized by a widespread silicic magmatism of Late Permian to

ormed during the Late Triassic to Early Jurassic synrift stage that led to the openingf the Neuquén basin (modified from Bechis, 2009).

ridimensional strain regime (Reches, 1978; Nieto-Samaniego andlaniz-Alvarez, 1997). In the Neuquén basin, Vergani et al. (1995)

nterpreted the general structural arrangement of normal faults andransfer zones as formed under a uniform stress regime, with the

inimum horizontal stress (�3) oriented in a NE direction. Later,osquera and Ramos (2006) interpreted the variable normal fault

rends as being related to a strong control of the basement struc-ural grain. Oblique rifting and tridimensional strain have also beenocally interpreted for the kinematic evolution of distinct depocen-ers of the northern Andean sector (Giambiagi et al., 2008b, 2009;echis et al., 2009, 2010).

The general goals of this study were to determine the kine-atics of the Late Triassic to Early Jurassic rifting that led to the


pening of the Neuquén basin, and to identify the main factors thatontrolled the structural architecture developed during this exten-ional phase. The orientation and location of previous Permian to

PRESSamics xxx (2014) xxx–xxx 3

Triassic rift systems in southern South America were interpreted ascontrolled by suture zones between terranes accreted during thePaleozoic (Fig. 2; Ramos and Kay, 1991; Ramos, 1994; Giambiagiand Martinez, 2008; Giambiagi et al., 2011). Considering someof the most recent reconstructions of the Paleozoic terranes, thecurved northeastern border of the Neuquén basin remarkably fol-lows the western limit of the Cuyania terrane (Fig. 2; Chernicoffand Zappettini, 2003; Ramos, 2008; Varela et al., 2011; Tomezzoli,2012). This suggest that the suture between the Cuyania and Chile-nia terranes could have acted as a previous lithospheric weakness oras a boundary between lithospheric blocks with contrasting rheo-logical behavior during the Late Triassic to Early Jurassic rifting thatopened the Neuquén basin. Thus, a possible control by a wider anddeeper lithospheric weakness is incorporated in the analysis of thesynrift structure of the basin. In order to explore this possibility, wedesigned a series of analog models to simulate an extensional sys-tem with a bent geometry similar to the northeastern border of thebasin (Fig. 4). We postulated that the deformation observed in thenortheastern sector can be related to a unique regional NE to NNEextension direction, following the original proposal of Vergani et al.(1995), and that the variability of the observed structural trends isdue to the extension of a complex basin boundary controlled byoblique lithospheric anisotropies inherited from previous tectonicstages. In different experiments, we varied the extension directionbetween NE and NNE orientations. We selected two representativeareas to compare the prototype (in this case the Neuquén basin)with our modeling results: (1) the Atuel depocenter, situated inthe northern Andean sector (Manceda and Figueroa, 1995; Lanés,2005; Giambiagi et al., 2008b; Bechis, 2009; Bechis et al., 2009,2010), and (2) the Entre Lomas area, situated in the northeast-ern Neuquén Embayment (Fig. 3; Vergani et al., 1995; Veiga et al.,2002; Mosquera and Ramos, 2006; Cristallini et al., 2006, 2009). Theresults obtained from the analog models and their comparison withstructural data from the selected areas allowed us to make someinferences about the regional extension direction, the control ofprevious anisotropies, and the kinematic evolution of the Neuquénbasin during the Late Triassic to Early Jurassic synrift stage.

2. Tectonic setting

2.1. Terrane accretions and shortening – Early Paleozoic to EarlyPermian

The basement of the Neuquén basin is affected by structuresand rheological contrasts inherited from previous tectonic stages(Mosquera and Ramos, 2006). The evolution of this sector ofGondwana during most of the Paleozoic was marked by tectonicshortening associated with the accretion of the Pampia, Cuya-nia, Chilenia and Patagonia allochthonous terranes (Fig. 2a; Ramoset al., 1986; Ramos, 1988, 2008; Astini et al., 1995; Rapalini, 2005;Pankhurst et al., 2006; Varela et al., 2011; Tomezzoli, 2012; Pángaroand Ramos, 2012; see discussion and references therein). Thesecollisions were later followed by the onset of a classical Andean-type margin during the Carboniferous to Early Permian, which gaveplace to the San Rafael orogenic phase (Mpodozis and Ramos, 1989).

2.2. Extension and rift basin development – Late Permian to EarlyCretaceous

From the Late Permian until the Early Cretaceous the regionunderwent extensional tectonics. The initial stages were charac-


Early Triassic age, represented by the upper section of the ChoiyoiGroup (Fig. 2b; Llambías, 1999). This magmatism was progres-sively followed by the opening of several NNW-trending narrow

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Fig. 4. (a) Comparison between the limits of the Neuquén basin, and the area subjected to extension in the analog models. The orientation of the applied extension in theexperiments is also shown. (b) Diagrams showing the configuration of the experimental apparatus and the deformed materials in upper and lateral views. (c) Shear stress( shearo f intera

csCotz2dtRgC

wmAtt2sa

�) plotted as a function of normal stress (�N) for tested materials in a Hubbert-typef the line with the ordinate represents the cohesion C0, and � is the coefficient onalysis; R2 is the coefficient of determination.

ontinental rift systems during the Early to Middle Triassic, corre-ponding to the Cuyo, Ischigualasto and other coeval basins (Fig. 2b;harrier, 1979; Uliana et al., 1989). The orientation and locationf these rift systems were interpreted to have been controlled byhe suture zones between the terranes accreted during the Paleo-oic (Ramos and Kay, 1991; Ramos, 1994; Giambiagi and Martinez,008; Giambiagi et al., 2011). A younger stage of rifting took placeuring the Late Triassic to Early Jurassic. Remarkably, most ofhe basins that opened during this period, such as the Neuquén,amada and other coeval basins, formed new depocenters not geo-raphically linked with the earlier ones (Alvarez and Ramos, 1999;harrier et al., 2007).

Since the late Early Jurassic, the southwestern margin of Gond-ana was characterized by active subduction and a well developedagmatic arc (Mpodozis and Ramos, 1989; Charrier et al., 2007).ccording to several authors, the regional extensional regime con-

inued until the Early Cretaceous, related to the retreat of the Pacific


rench (Mpodozis and Ramos, 1989; Ramos and Kay, 2006; Ramos,010). From the Middle Jurassic onward, the Neuquén basin wasubjected to thermal subsidence related to the sag phase (Legarretand Uliana, 1996; Vergani et al., 1995; Howell et al., 2005). In

box. The slope gradient of the line represents the friction angle ˚, the intersectionnal friction. Error estimations for �, and C0 were obtained by linear regression

addition to the Late Triassic to Early Jurassic one, a second phase ofrifting was proposed for the basin during the Late Jurassic (Verganiet al., 1995; Giambiagi et al., 2003a; Charrier et al., 2007; Mescuaet al., 2008). On the other hand, local episodic shortening phaseswith selective inversion were registered in the southern sector ofthe basin during the Jurassic, mainly in the Huincul High (Verganiet al., 1995; Pángaro et al., 2006; Mosquera and Ramos, 2006;Silvestro and Zubiri, 2008; Cristallini et al., 2009).

2.3. Andean shortening – Late Cretaceous to Present

Thrusting and basin inversion in the southern Central Andesstarted in the Late Cretaceous (Cobbold and Rossello, 2003; Zapataand Folguera, 2005; Zamora Valcarce et al., 2006; Tunik et al.,2010; García Morabito and Ramos, 2012; Mescua et al., 2013), anda new pulse of deformation and uplift took place from the Mioceneto Recent (Irigoyen et al., 2000; Giambiagi et al., 2003b, 2008a,


2014; Spagnuolo et al., 2012). During these shortening phasesthe infill of the Neuquén basin was deformed and exhumed inthick- and thin-skinned fold and thrust belts that developed in thewestern sector of the basin (Fig. 2c; Manceda and Figueroa, 1995;

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liana et al., 1995; Zapata et al., 1999; Giambiagi et al., 2003a,008a; Zapata and Folguera, 2005; Zamora Valcarce et al., 2006).

Shortening and topographic uplift in the Andes are interpreteds governed by a combination of factors: the geometric and kine-atic setting at the subduction zone in the western margin of Southmerica plate (Jordan et al., 1983; Sobolev and Babeyko, 2005;chellart et al., 2007; Somoza and Zaffarana, 2008; Ramos, 2010;apitanio et al., 2011); rheological variations related to the thermo-echanical structure of the South America continental plate and

he anisotropies inherited from previous tectonic stages (Kley et al.,999; Tassara et al., 2006; Giambiagi et al., 2012); and changes inrosion rates linked to climatic variations (Lamb and Davis, 2003;homson et al., 2010).

. Experimental method

.1. Model setup

We designed a series of analog models of a bent extensionalystem; this configuration simulated a lithospheric weakness witheometry similar to the Paleozoic suture between the Cuyania andhilenia terranes in the western sector of Gondwana (Figs. 2 and 4).he modeling apparatus had two walls, one fixed and anotheroveable, and two open lateral boundaries, to avoid lateral fric-

ional effects (Fig. 4). The initial dimensions of the device were0 cm long by 50 cm wide. The moveable wall was connected to aomputer-controlled step motor. Following Withjack and Jamison1986), the extension was transmitted to the analog materials bysing a basal latex sheet, attached to two basal rigid acetate sheets.ach of the acetate sheets was respectively attached to the fixed andoveable walls of the apparatus. As the wall moved, the attached

cetate sheet moved jointly and the latex stretched uniformly,mposing distributed deformation to the overlying modeling mate-ials, and producing a rift zone slightly wider than the latex sheet.

e designed the geometry of the latex sheet in order to mimic theend observed in the northeastern border of the Neuquén basin,ith two arms 15 and 17 cm wide intersecting at an angle of 145◦

Fig. 4). This configuration simulated the eastern border of north-rn Andean sector (Az 170◦) and the northeastern border of theeuquén Embayment (Az 135◦) respectively. Considering each armf the bent extensional system separately, our models resembledrevious oblique rifting experiments (Withjack and Jamison, 1986;ron and Brun, 1991; McClay and White, 1995; Clifton et al., 2000).owever, in this work we evaluated an extensional system with aurved geometry in order to simulate extension of a complex basinoundary controlled by oblique lithospheric anisotropies inheritedrom previous tectonic stages.

We carried out a series of five models. In different experiments,e varied the direction of displacement between NNE (N10◦E) andE directions (N45◦E), in order to evaluate the effect of the exten-

ion direction on the fault architecture of the rift system. The angle between the rift axis and the direction of displacement was differ-nt for each of the system arms (Fig. 5; Table 1). For each model run,he moving wall was displaced a total of 6 cm at a constant rate of

cm/h. The models surface was photographed every minute using digital camera.

We designed our experimental setting to analyze the orienta-ion and kinematic evolution of the faulting at the models surface,ithout considering isostatic and thermal effects during rifting.

.2. Experimental materials and scaling


We used well sorted quartz sand with well rounded grainsmaller than 600 �m as the modeling material. We deformed aand pack composed by two layers: a 1 cm thick lower layer of


wet sand simulating stronger basement rocks, and a 2.5 cm thickupper layer of dry sand simulating a weak cover rock (Fig. 4).Dry sand is commonly used to simulate the brittle upper crustin analog experiments, because it fails according to a linear Mohrenvelope (Hubbert, 1937, 1951; Sandford, 1959; Krantz, 1991). vanMechelen (2004) demonstrated that the cohesive strength of drysand can be increased through adding a small amount of liquid,while other mechanical properties are maintained. In our models,we moistened the dry sand using a liquid composed of 50% waterand 50% ethanol, with a mass percentage of 2.5% liquid respect thedry material. The dry sand and the liquid were blended using amixing machine for about 5 min. We measured the dry and wetsand mechanical properties using a modified Hubbert-type shearapparatus (Hubbert, 1951), which enables the determination ofthe ratio of normal to shear stress at failure. Our test results indi-cated similar values of friction angle for dry and wet sand, and ahigher value of cohesion in the case of wet sand (Fig. 4; Table 2).Schellart (2000) demonstrated that the behavior of granular mate-rials for very small normal stresses is more complex than previouslyassumed. As the normal stresses used to measure the properties ofour granular materials were higher than 350–400 Pa (Fig. 4), weobtained a maximum extrapolated cohesion (Schellart, 2000). Wemanually distributed the wet sand, while we poured the dry sandusing a sedimentation device, allowing a uniform thickness anda random distribution of grains for the upper layer. An orthogonalgrid was marked on the model surface in order to track the induceddeformation.

An analog experiment must be scaled in order to get geomet-ric, kinematic and dynamic similarities with the natural prototype(Hubbert, 1937; Duarte et al., 2011). The cohesion must satisfy thefollowing dynamic similarity criterion:

�∗ = C∗ = �∗g∗x∗ (1)

where �*, C*, �*, g* and x* are the model to natural prototype ratiosfor stress, cohesion, density, gravity and length respectively. Ouranalog models were scaled such that 1 cm in the model simulatedbetween 1 and 10 km in the field (x* ∼ 1 × 10−5–1 × 10−6). The den-sity ratio between the used granular materials and natural rocksis �* ∼ 0.6 (Table 2), while both the prototype and the model aresubjected to the same value of gravitational acceleration (g* = 1).Considering that values for cohesion of natural rocks range between15 and 110 MPa (Schellart, 2000), the analog granular materialswith cohesion ∼40–136 Pa (Fig. 4, Table 2) are appropriate for work-ing at the selected scale (C* ∼ 3 × 10−6–1 × 10−6).

4. Analog modeling results

The results of the analog modeling are shown in Fig. 5. The vari-ation in the orientation of the rift system introduced in our modelspromoted particular interference patterns in the transition areabetween the two arms simulating the northern Andean and theNeuquén Embayment sectors of the basin respectively. Most of themajor faults were curved in the interference sector, especially thoseformed near the latex sheet borders (Fig. 5). In all experiments weobserved a border effect at the sides of the models, with genera-tion of two conjugated fault systems with strike-slip displacements(Fig. 5). This deformation was related to the free lateral borders ofthe latex sheet, and it has not been considered in our analysis.

4.1. Deformation observed in the northern arm

In model A (N45◦E extension) we applied a moderately oblique


extension to the northern arm of the bent rift system, which sim-ulated the northern Andean sector of the Neuquén basin (˛a = 55◦;Fig. 5; Table 1). We observed the generation of normal faults withorientations varying between NNW and NW trends. NNW-trending

dx.doi.org/10.1016/j.jog.2014.04.010

Please cite this article in press as: Bechis, F., et al., Transtensional tectonics induced by oblique reactivation of previous lithosphericanisotropies during the Late Triassic to Early Jurassic rifting in the Neuquén basin: Insights from analog models. J. Geodyn. (2014),http://dx.doi.org/10.1016/j.jog.2014.04.010



Fig. 5. On top, photographs showing the final configuration at the surface of all the analog models after deformation. The white arrows point the applied extension in eachmodel, and ˛a and ˛e correspond to the angular obliquity between the rift axis and the direction of displacement in the arms simulating the northern Andean and theNeuquén Embayment sectors respectively. The orthogonal grid on the model surface is always aligned to the direction of extension. Note that the photographs were rotatedwith the north direction pointing to the top of the page in order to facilitate the comparison between the different models. The interpretation of the structure observed at thesurface of each model is shown in the diagrams below the photographs. Red lines correspond to west and southwest dipping normal faults, green lines correspond to eastand northeast dipping normal faults, and yellow lines correspond to strike-slip faults. The fault trends measured in each sector were plotted in the rose diagrams shown atthe bottom of the figure. The direction r corresponds to the orientation of the rift axis in each arm, and pA, pB, pC, pD and pE mark the direction orthogonal to the appliedextension in each model. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 1Table showing the direction of extension applied on the experiments, the obliquity of the extension for each arm of the bent rift system, and the type of transtensionalstrain observed in each case. The angle corresponds to the angle between the applied extension and the orientation of the rift border in each arm (˛a = Andean sector,˛e = Neuquén Embayment).

Model Extension direction ˛a (◦) Model deformation ˛e (◦) Model deformation

A N45◦E 55 Pure-shear dominated 90 Extension (ortogonal rift)B N30◦E 40 Simple-shear dominated 75 Pure-shear dominatedC N25◦E 35 Simple-shear dominated 70 Pure-shear dominatedD N20◦E 30 Simple-shear dominated 65 Pure-shear dominatedE N10◦E 20 Simple-shear dominated 55 Simple-shear dominated?

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F. Bechis et al. / Journal of Geodynamics xxx (2014) xxx–xxx 7

Table 2Table showing the coefficient of internal friction, internal friction angle, cohesion and density of the granular materials used in the analog models. The range of values fornatural rocks is also shown for comparison and scaling purposes (compilation from Schellart, 2000). The parameters of the modeled granular materials were measured witha modified Hubbert-type shear apparatus (Fig. 4).

Parameters Modeling materials Natural rocks

Dry sand Wet sand

Coefficient of internal friction 0.863 ± 0.019 0.820 ± 0.047 0.49–1.00◦ ◦ ◦ ◦ ◦ ◦

rsmcg

abssmtaftawstfiWg

eFlrf

4

gwTnotn

atfbmotftouowm

Internal friction angle 40.79 ± 1.06Cohesion 39.9 ± 13.1 Pa

Density 1.84 g/cm3

ift-parallel faults were more common near the border of the riftystem, while NW-trending extension-orthogonal faults formedainly toward its center. The final structural arrangement was

omposed of short, sinuous and segmented faults, resulting in smallrabens limited by curved normal faults with en echelon array.

In models B, C and D (N30◦E, N25◦E, N20◦E extension) we applied highly oblique extension to this arm of the bent rift system (˛a

etween 40◦ and 30◦; Fig. 5; Table 1). A complex transtensionaletting characterized by the development of normal, oblique andtrike-slip faults was obtained. The final fault arrangement wasarkedly bimodal. It consisted of a more numerous group of NNW-

rending oblique faults, slightly oblique to the rift borders, and less represented population of WNW- to NW-trending normalaults, nearly orthogonal to the extension direction. At the ini-ial stages, straight and long NNW-oriented faults formed, with

predominantly left-lateral strike-slip movement (Fig. 6b). Later,e observed an increasing normal component of slip localized in

ome segments of the initial strike-slip faults. Thus, these NNW-rending faults become more segmented and sinuous, showing anite oblique-slip at the end of the experiments. On the other hand,NW- to NW-oriented faults formed after the NNW-trending

roup, showing a predominantly normal displacement.In model E (N10◦E extension) we applied an extension

xtremely oblique to the latex border of the northern arm (˛a = 20◦;ig. 5; Table 1). In this case we observed the generation of left-ateral strike-slip faults, with an NNW orientation slightly obliqueespect the rift border, and a few antithetic right-lateral strike-slipaults of WNW-trend.

.2. Deformation observed in the southern arm

In model A (N45◦E extension) we applied an extension ortho-onal to the borders of the southern arm of the bent rift system,hich simulated the Neuquén Embayment area (˛e = 90◦; Fig. 5;

able 1). In this case, we observed the generation of long, straightormal faults, with an orientation parallel to the rift borders andrthogonal to the applied extension. The final distribution of faultrends was unimodal, with an average direction of NW-orientedormal faults.

In models B, C and D (N30◦E, N25◦E, N20◦E extension) wepplied a moderately oblique extension to this arm of the rift sys-em (˛e between 75◦ and 65◦; Fig. 5; Table 1). We observed theormation of faults with predominantly normal slip, and trendsetween rift-parallel and extension-orthogonal directions, with aaximum close to intermediate orientations. Rift-parallel faults

f NW trend were more common near the border of the rift sys-em, while extension-orthogonal faults of WNW trend tended toorm toward its center. At the initial stages we observed the forma-ion of major fault-bound grabens limited by normal faults parallelr slightly oblique to the rift borders. As the extension contin-


ed, these grabens were segmented and connected by faults nearlyrthogonal to the extension direction (Fig. 6a). This segmentationas more marked for lower values of ˛. The final structural arrange-ent was composed by short, sinuous and strongly segmented

39.36 ± 2.70 26.1 –45.0137.1 ± 37.9 Pa 24–110 MPa1.88 g/cm3 2.3–3.4 g/cm3

faults. In most cases, rift border faults formed en echelon arrays.Rift border faults and intra-rift faults in the models were segmentedalong-strike with relay ramp structures formed between overlap-ping fault tips (Fig. 7). These relay ramps were aligned in transferzones cross-cutting most of the rift width, showing a trend notnecessarily parallel to the applied extension direction (Fig. 7).

In model E (N10◦E extension) we also applied a moderatelyoblique extension to the southern arm (˛e = 55◦; Fig. 5; Table 1),but we observed a complex transtensional strain, probably causedby the highly oblique stretching imposed to the northern arm.In the southern arm, the initial deformation was accommodatedby left-lateral NW-trending strike-slip faults, and right-lateralENE-trending strike-slip faults. Later, NW-trending normal faultsformed near the borders of the rift system, delimiting smalldepocenters with en echelon array. Near the center of the stretchedarea, the ENE-trending faults changed their slip from strike-slip tonormal. The final fault trend distribution was bimodal, with twogroups of NW- and ENE-trending faults.

5. Strain analysis of the experimental results

We grouped the modeling results in two general types oftranstension: pure-shear dominated and simple-shear dominated,following previous proposals of oblique strain analysis (Tikoff andTeyssier, 1994; Teyssier et al., 1995; De Paola et al., 2005a, 2005b).The limit between both types depends on the obliquity betweenthe rift system and the applied extension direction (˛), and onthe mechanical properties of the deformed material (Withjack andJamison, 1986; De Paola et al., 2005b). In our models, the limitbetween pure-shear and simple-shear dominated transtensionoccurred at an obliquity value of between 45◦ and 40◦ (Table 1).The structures we observed on the models surface allowed us toestimate the orientation of the main axes of the finite strain ellip-soid (�1: maximum, �2: intermediate, �3: minimum) during eachstep of their evolution.

For moderately oblique rift systems ( between 90◦ and 45◦)we observed a pure-shear dominated transtension (Figs. 6 and 8;Table 1). Faults with predominantly normal slip were recognized,with a dispersion of faults trending between rift-parallel andextension-orthogonal directions, and a maximum near interme-diate orientations. Rift-parallel faults were more common near theborder of the rift system, while extension-orthogonal faults formedtoward its center. The formation of normal faults evidenced anextensional tectonic strain regime (horizontal �1 and �2, and verti-cal �3). The main axis of the strain ellipsoid (�1) had an intermediateorientation between rift-orthogonal and extension-parallel direc-tions (Withjack and Jamison, 1986), explaining why most of thefaults showed an intermediate orientation.

For the orthogonal rift system ( = 90◦) we observed the gen-eration of poorly segmented, long normal faults, parallel to the


rift borders and orthogonal to the applied extension. The observedfaults evidenced that the main axis of the strain ellipsoid (�1) wasparallel to the applied extension and orthogonal to the rift borders,maintaining this orientation along the model evolution.

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Fig. 6. (a) Photographs showing a close view of the evolution of the deformation at the surface of the southern arm. In this model, a N25◦E extension was applied, generatinga moderately oblique rift system (˛e = 70◦), characterized by pure-shear dominated transtension. (b) Photographs showing a close view of the evolution of the deformationa neratit ases tt

oTsfattmttwiaoi

t the surface of the northern arm. In this model, a N30◦E extension was applied, geranstension. The white arrows point the applied extension in each model. In both che time each photograph was taken.

For highly oblique rift systems ( between 45◦ and 20◦) webserved a simple-shear dominated transtension (Figs. 6 and 8;able 1). In these models we noted the generation of a complextructural pattern constituted by normal, oblique and strike-slipaults. The fault-trend distribution was markedly bimodal, with

more numerous population of faults slightly oblique to the riftrend, and a less represented group of faults nearly orthogonal tohe extension direction. The particular evolution observed in the

odels indicates some important changes in the orientation ofhe main axes of the finite strain ellipsoid during the evolution ofhe transtensional strain. The initial generation of strike-slip faultsith an orientation slightly oblique to the rift trend evidences an


nitial phase of transcurrent deformation (horizontal �1 and �3,nd vertical �2). Later, the generation of new normal faults nearlyrthogonal to the extension direction, together with the increas-ng component of normal slip in some segments of the previously

ng a highly oblique rift system (˛a = 40◦), characterized by simple-shear dominatedhe value e indicates the displacement applied in the border of the extended area at

developed strike-slip faults, indicates a change to an extensionalstrain regime (horizontal �1 and �2, and vertical �3). This switchbetween the �2 and �3 main axes of the finite strain ellipsoidoccurred later for smaller values of ˛, that is, for higher obliquitybetween the rift system and the applied extension direction. Thismay occur because the instantaneous and finite strain ellipsoidscan be non-parallel during the evolution of a transtensional sys-tem (Tikoff and Teyssier, 1994). Initial strike-slip faults form inresponse to an instantaneous strain controlled by the border con-ditions, which impose a transcurrent tectonic regime. However,vertical strike-slip faults are not capable of accommodating theextension imposed by finite strain across the deforming zone when


deformation progresses. Therefore, new normal faults must form inorder to accommodate the finite strain. In our experiments, we alsoobserved a change in the slip of the initial faults, from strike-slip tooblique.

dx.doi.org/10.1016/j.jog.2014.04.010



F -sheara tely ob d exte

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ig. 7. Photographs showing relay zones developed in the southern arm under purend related relay ramps showing an en echelon array at the borders of the moderaetween normal faults. Note the obliquity between the transfer belt and the applie

. Comparison between the analog modeling results andhe Neuquén basin

We compared the kinematic evolution and the final struc-ural architecture observed in the analog models with structuralata from two selected areas of the Neuquén basin: (1) the Atuelepocenter, located in the northern Andean sector of the basinFig. 9), and (2) the Entre Lomas area, situated in the northern sectorf the Neuquén Embayment (Figs. 10 and 11). The main parame-ers used in these comparisons include fault geometry, orientationnd sense of slip, and overall structural and kinematic evolution. Inoth areas, we observed a good match between the field data andhe results of the analog models B, C and D, on which we applied aNE-directed regional extension (N30◦E–N20◦E extension).

.1. Case study 1: Andean sector – Atuel depocenter

In the Andean sector, the Late Cretaceous to Neogene short-ning makes it difficult to identify earlier Mesozoic extensionaltructures. Many basement-involved reverse faults with nearly N–Srend have been interpreted as related to the inversion of Mesozoicormal faults in thick-skinned fold and thrust belts (Manceda andigueroa, 1995; Uliana et al., 1995; Zapata et al., 1999; Giambiagit al., 2003a, 2008a; Zamora Valcarce et al., 2006). However, recentontributions have highlighted the importance of previous normalaults oblique to the Andean thrusts, mainly showing NW and NErends (Franzese et al., 2006, 2007; Yagupsky et al., 2007, 2008;iambiagi et al., 2008a, 2009; Bechis et al., 2009, 2010; Bechis andiambiagi, 2009; García Morabito et al., 2011).

The Atuel depocenter originated as a Late Triassic to Early Juras-ic rift system, later inverted during the Andean thrusting (Fig. 9;anceda and Figueroa, 1995; Lanés, 2005; Lanés et al., 2008;iambiagi et al., 2005, 2008b; Tunik et al., 2008; Bechis et al., 2009,010). It has a general NNW orientation, controlled by the devel-pment of two major faults of NNW trend, Alumbre and La MangaGiambiagi et al., 2008b). Using kinematic data, Bechis et al. (2009,010) characterized the Alumbre fault slip as left-lateral normal-blique, while the La Manga fault could have had a similar slipuring the early synrift, although it underwent a predominantlyormal slip during the late synrift stage. The internal structure ofhe Atuel depocenter is characterized by a marked bimodal distri-


ution of WNW and NNW trending normal faults (Figs. 9 and 12;iambiagi et al., 2008b). Kinematic indicators measured on outcropcale faults indicate an extensional strain regime during most of thevolution of this depocenter (horizontal �1 and �2, and vertical �3),

dominated transtension (model B). (a) General and detailed views of normal faultsblique rift system. (b) The dotted white line marks an alignment of transfer zonesnsion, marked by the white arrow.

with a mean NE internal extension direction (Bechis, 2009; Bechiset al., 2009, 2010). The kinematic conditions during the start of therifting are unknown, since the base of the synrift infill is unexposed.The Atuel depocenter was interpreted as an oblique rift system,evidenced by the bimodal distribution of major faults and the obliq-uity between the NE internal extension direction and its NNWgeneral trend (Giambiagi et al., 2008b; Bechis, 2009; Bechis et al.,2009, 2010). Two mechanisms were proposed in order to explainthe development of this transtensional system: (1) reactivation ofupper-crustal NNW-oriented Paleozoic shear zones, and (2) obliquestretching of a previous NNW-oriented lithospheric weakness zone(Giambiagi et al., 2008b; Bechis et al., 2009, 2010).

The structural architecture of the Atuel depocenter and itskinematic evolution are comparable to the ones observed in theanalog models B, C and D (Fig. 9). In these models we applied ahighly oblique extension to the northern arm of the bent rift sys-tem ( between 40◦ and 30◦), inducing a simple-shear dominatedtranstension. The most remarkable common elements between theAtuel depocenter and the analog models are the bimodal distribu-tion of NNW and WNW faults (Fig. 12), and the generation of majorNNW faults with finite normal and left-lateral oblique slip. Weobserved a closer match with the fault trend distribution obtainedin model B (N30◦E extension). The results of the analog modelsallow us to explain the formation of long, oblique-normal, NNW-trending faults, with no need to account for reactivation of previousdiscrete structures with NNW orientation. Therefore, the majorAlumbre and La Manga faults could have originated as structuresrelated to the oblique stretching of a wide NNW-trending litho-spheric weakness zone, obliquely oriented respect a NNE regionalextension, during the early stages of rifting in the Late Triassic.

6.2. Case study 2: Neuquén Embayment – Entre Lomas sector

Upper Triassic to Lower Jurassic sequences do not crop out inthe Neuquén Embayment, but they show an important subsur-face development. The high quality and availableness of subsurfacegeophysical information acquired by the petroleum industry,together with the low influence of the Andean deformation,have favored a detailed identification of the synrift structuresof this sector. In the Platform area, normal faults show WNWto NW orientations, being segmented by NE- to NNE-trending


transfer zones (Figs. 3 and 10; Vergani et al., 1995; Veigaet al., 1999; Mosquera and Ramos, 2006; Cristallini et al., 2006,2009). In the Huincul High, ENE-trending reverse faults andfolds were first interpreted as related to the inversion of Early

dx.doi.org/10.1016/j.jog.2014.04.010



Fig. 8. Conceptual models showing the structural evolution of the two types of transtensional strain observed in the analog models. ˛: angle between the rift axis and thedirection of displacement, e: displacement applied in the border of the extended area, p: rift-parallel component of the displacement, o: rift-orthogonal component of thed ximum

MHJs2

EbrarWehtt

isplacement. The orientation of the main axes of the finite strain ellipsoid (�1: ma

esozoic half-grabens (Vergani et al., 1995; Veiga et al., 2002).owever, according to recent contributions, Late Triassic to Early

urassic normal faults would present NW to WNW orientations,imilar to the ones observed in the Platform area (Pángaro et al.,006; Silvestro and Zubiri, 2008; Cristallini et al., 2009).

Although normal faults in the northern sector of the Neuquénmbayment show a continuous distribution of trends rangingetween WNW and NW orientations (Figs. 10–12), the main syn-ift depocenters are limited by two groups of faults oriented in NWnd WNW directions (Cristallini et al., 2006, 2009). Half-grabenselated to NW-trending faults are relatively longer and deeper than

NW-trending half-grabens, and are located near the northeast-


rn border of the basin (e.g. El Santiagueno and Estancia Viejaalf-grabens; Fig. 10). On the other hand, half-grabens relatedo WNW-trending faults are relatively smaller, and developedoward the basin interior (e.g. Lindero Atravesado and Río Neuquén

, �2: intermediate, �3: minimum) is also shown. See discussion in the text.

half-grabens; Fig. 10). In the Entre Lomas sector, faults of WNWtrend cut and displace the NW-trending faults that limit the south-ern active border of the El Santiagueno half-graben (Fig. 11),suggesting that the WNW fault system would have been generatedafter the NW one (Cristallini et al., 2006, 2009). NE-directed trans-fer zones are either faults or zones where main normal faults looseslip or terminate. These transfer zones are characterized by relayramp structures formed between overlapping fault tips, with somecases of polarity change between adjacent half-grabens (Veiga et al.,1999; Cristallini et al., 2006).

The structural architecture of the Neuquén Embayment, andin particular of the Entre Lomas sector, is comparable to the


fault arrangement observed in the analog models B, C and D(Figs. 10 and 11). In these models we applied a moderatelyoblique extension to the southern sector of the bent rift sys-tem ( between 75◦ and 65◦), inducing a pure-shear dominated

dx.doi.org/10.1016/j.jog.2014.04.010



Fig. 9. In the left side of the figure, maps showing the kinematic evolution and the structural architecture described for the synrift phase in the Atuel depocenter (modifiedfrom Bechis et al., 2010). See location of the map in Fig. 1. Gray arrows represent the local extension directions obtained from small-scale faults, white arrows representt train

m in andt cturew ended

tmfbrcm(ftSonttea

he inferred regional extension, the dotted line shows a possible refraction of the sargin and the external extension vector, and �t is the angle between the rift marg

he figure, photographs showing the kinematic evolution and the structural architeas applied. The value e indicates the displacement applied in the border of the ext

ranstension. Among the similarities between the Neuquén Embay-ent structure and the model results, the most outstanding

eature is the continuous range of faults with trends rangingetween rift-parallel NW orientations, more common toward theift border, and extension-orthogonal WNW orientations, moreommon toward the rift center (Fig. 12). We observed a closeratch with the fault trend distribution obtained in model D

N20◦E extension). The segmentation of NW major faults by WNWaults during the late stages of the experiments is also analogo the segmentation observed in the southern border of the Elantiagueno half-graben. This correlation explains the formationf different normal fault sets under transtensional strain, witho need to interpret changes in the regional extension direc-


ion or reactivation of previous discrete structures. In addition,he transfer zones of NE trend observed in the analog mod-ls are similar to those observed in the Neuquén Embaymentrea.

due to the oblique extension inside the rifted area, is the angle between the rift the maximum instantaneous elongation inside the rift system. In the right side of

observed in the northern arm of the analog model B, on which a N30◦E extension area at the time each photograph was taken.

7. Regional and tectonic implications

7.1. Regional extension during the Neuquén basin opening

The good match between the field data and the results of theanalog models supports a NNE orientation of the regional exten-sion (N30◦E–N20◦E) during the Late Triassic to Early Jurassic riftingin the Neuquén basin. This orientation of the regional extensionis concordant with kinematic data from the synrift deposits thatcrop out in the northern Andean sector of the Neuquén basin.Giambiagi et al. (2009) carried out detailed kinematic analysis usingslip data of small-scale faults from the Malargüe and Cara Cura-Reyes depocenters. They proposed a tridimensional strain regime


for the synrift stage, with generation of an orthorhombic fault pat-tern symmetric with respect to a principal regional NNE extensiondirection, interpreted as responsible for the basin opening, and asecondary WNW extension, probably related to retro-arc extension

dx.doi.org/10.1016/j.jog.2014.04.010



F ent oS

paN(t(seid

mtamaTflt

ig. 10. Map showing the main normal faults and transfer zones affecting the basemee location in Fig. 1.

rocesses. The obtained main regional extension direction presents N17◦E orientation in the case of the Malargüe depocenter, while a10◦E extension was calculated using data from the Cara Cura area

Giambiagi et al., 2009). Kinematic data of small-scale faults fromhe Atuel depocenter indicated an internal NE extensional strainmean �1 = Az 047◦), oblique to the general NNW orientation of theub-basin (Giambiagi et al., 2008b; Bechis et al., 2009, 2010). Bechist al. (2010) estimated an angle of 24◦ for the Atuel oblique rift-ng, which corresponds to a regional extension oriented in a N14◦Eirection.

Then, the regional extension direction estimated from kine-atic data of small-scale faults in previous contributions shows

rends between N10◦E and N20◦E. On the other hand, we observed good match between the trend distribution and kinematics ofajor faults and the results of the analog models on which we

pplied a regional extension direction between N30◦E and N20◦E.


his small discrepancy could be related to several reasons, like localactors related to the rheological characteristics and structural evo-ution of each particular depocenter, analytical factors related tohe kinematic models applied in each case to estimate the regional

f the basin in the Neuquén Embayment area (modified from Cristallini et al., 2009).

extension from small-scale fault data, or possible small clockwiserotations of the major faults during the Andean deformation inresponse to a southward decrease of crustal shortening (Giambiagiet al., 2012). In any case, it is noteworthy that a strongly consis-tent NNE regional extension was obtained by applying independentmethods in the northern Andean sector.

In the Neuquén Embayment area, a NE regional extensiondirection (NE-directed �3) was previously estimated from theapproximate NW trend of the normal faults and the NE orien-tation of the transfer zones (Vergani et al., 1995; Silvestro andZubiri, 2008). The results of our analog models demonstrate thatthe transfer zones could be oblique to the regional extension(Fig. 7), and thus they cannot be used as reliable indicators ofthe stress field. Moreover, a closer analysis shows that there isa whole range of fault trends between WNW and NW orienta-tions (Figs. 10–12). The structural architecture of the Neuquén


Embayment area can be easily explained by a moderately obliqueextension of a NW-trending rift system under a regional NNE-directed extension ranging between N20◦E and N30◦E orientations.Also, the results of the models show that en echelon arrangements

dx.doi.org/10.1016/j.jog.2014.04.010

Please cite this article in press as: Bechis, F., et al., Transtensional tectonics induced by oblique reactivation of previous lithosphericanisotropies during the Late Triassic to Early Jurassic rifting in the Neuquén basin: Insights from analog models. J. Geodyn. (2014),http://dx.doi.org/10.1016/j.jog.2014.04.010



Fig. 11. Comparison between normal faults and transfer zones described for the synrift structure in the Entre Lomas area (modified from Cristallini et al., 2009) and thestructure developed in the southern arm of the model D, on which a N20◦E extension was applied. The value e indicates the displacement applied in the border of the extendedarea at the time each photograph was taken. See location in Fig. 10.

Fig. 12. Rose diagrams showing the distribution of normal fault trends in the Atuel depocenter and the Neuquén Embayment. The direction r marks the orientation of thebasin in each sector. The direction orthogonal to the extension applied in each analog model (pA, pB, pC, pD and pE) is also shown for facilitating the comparison with thefault populations observed in the experiments, which are presented in the rose diagrams of Fig. 5. N indicates the number of faults in each case.

dx.doi.org/10.1016/j.jog.2014.04.010

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f half-grabens could be formed under extensional strain and stresselds in a pure-shear dominated transtensional system, in contra-osition with previous interpretations of a horizontal orientationf the maximum axis of the stress ellipsoid (�1) under transcurrentonditions (Rossello and Barrionuevo, 2005).

The results of the analog experiments allow us to explainreat part of the structural architecture of the synrift stage of theeuquén basin in a simple and integrated model. This is particularlyalid for the northeastern border of the basin. However, it is prob-bly not sufficient enough to explain the detailed structure of eacharticular depocenter or the occurrence of other fault trends likehe NE ones described in the southern sector of the basin (Verganit al., 1995; Mosquera and Ramos, 2006; Franzese et al., 2006, 2007;’Elia et al., 2012). Another consideration that should be taken intoccount is the age of the synrift phase in the basin. Recent U–Pbges suggest that some of the half-grabens of the Platform areaould have been formed earlier than previously thought, duringhe Middle Triassic (Barrionuevo et al., 2013). The application ofhe results of our models does not necessarily imply that all thetructures should have been formed at the same time, but underhe same kinematic conditions.

The NNE extension proposed for the Neuquén basin is alsoonsistent with recurrent data indicating a regional extension ori-nted between NNE and NE directions in other rift systems ofouthwestern Gondwana that were active from the latest Paleo-oic until the Early Mesozoic. Kinematic data from extensionalaults affecting volcanic rocks from the Upper Permian to Lowerriassic upper section of the Choiyoi Group and the clastic infill ofhe Triassic Cuyo basin indicate NNE to NE extension directionsGiambiagi and Martinez, 2008; Japas et al., 2008; Kleiman andapas, 2009; Giambiagi et al., 2011). Similar orientations for thextension have also been reported for Triassic rocks from northernatagonia (Giacosa et al., 2007).

.2. Lithospheric controls

In oblique rift systems, the direction of extension is obliqueo the trend of the basin, controlled either by the presence of aithospheric weakness or by the previous upper crust basementabric (Tron and Brun, 1991; Holdsworth et al., 1997; Vauchezt al., 1998; Morley et al., 2004; Chenin and Beaumont, 2013).ithospheric weaknesses can be related to ancient terrane sutures,revious orogenic belts, or areas thermally weakened by magmaticctivity. Previous major tectonic events can also imprint a latticereferred orientation of olivine crystals in the lithospheric mantle,hich may induce directional softening and strain localization dur-

ng subsequent extensional stages (Tommasi and Vauchez, 2001).hese lithospheric-scale pre-existing weaknesses generally controlhe location, extension and orientation of the rift systems (Corti,012). On the other hand, in areas where the previous fabric of thepper crustal basement is weak and susceptible to reactivation,his fabric can be the main controlling factor over the orientationf the extensional system. In this case, reactivated structures canvolve as major normal faults during the early stages of the riftevelopment, eventually controlling the general basin orientationMorley et al., 2004; Bellahsen and Fournier, 2006; Withjack et al.,010).

The good match between our experimental results and theelected prototypes from the Neuquén basin suggests that a litho-pheric weakness zone of NNW to NW trend could have controllednd localized the extension during the Late Triassic to Early Juras-ic. Thus, a possible control by a wider and deeper lithospheric


eakness is incorporated in the analysis of the synrift structuref the basin, which has been previously centered in the inter-retation of fault variability only as related to the reactivationf upper-crustal basement fabrics (Mosquera and Ramos, 2006).


According to available geologic, geochronologic, isotopic and geo-physical data, the suture between the Cuyania and Chilenia terranesshows a curved geometry near its southern end (Fig. 2; Chernicoffand Zappettini, 2003; Ramos, 2008; Varela et al., 2011; Tomezzoli,2012). The NW orientation of the basin border in the Platform areaof the Neuquén Embayment was more likely controlled by thesouthwestern limit of the Cuyania terrane. The close coincidencebetween the inferred suture and the basin border in this area sug-gests that the rifting took place on the Chilenia side of the suture(Fig. 2).

The change between NNW and NW orientations that charac-terizes the northeastern border of the Neuquén basin occurs atnearly 36◦ south latitude (Fig. 3). This curved geometry is notexclusive of this basin, since both the Carboniferous-Permian SanRafael orogenic belt and the Permo-Triassic Choiyoi magmaticbelt show a similar shape (Fig. 2; Mpodozis and Kay, 1990, 1992;Kleiman and Japas, 2009; Tomezzoli, 2012). All these morphotec-tonic units have also been interpreted as controlled by the suturesand rheological contrasts between the terrains accreted duringthe Paleozoic (Giambiagi and Martinez, 2008; Giambiagi et al.,2012).

The eastern border of the northern Andean sector of the basinis marked by the NNW trending Borbollón-La Manga lineament,which was interpreted as an important lithospheric structure thatcontrolled the orientation of major normal faults in the Malargüe,Atuel and Yeguas Muertas-Nieves Negras depocenters (Fig. 3;Giambiagi et al., 2008b, 2009; Bechis et al., 2010). To the east of thislineament runs the inferred suture between Chilenia and Cuyaniaterranes, with a similar NNW trend (Ramos et al., 2000). The suturewas interpreted as a lithospheric weakness zone which inducedstrain localization and guided lithospheric reworking during thePermo-Triassic extensional stage (Giambiagi et al., 2011). Duringthis period, a progressive crustal thinning and changes in the crustcomposition recorded by the volcanic rocks of the Choiyoi Groupmay have increased the lithospheric strength along the suture zone(Giambiagi et al., 2011). Extension then migrated immediately tothe east, where the Cuyo basin developed in the inferred hanging-wall of the suture (Ramos and Kay, 1991; Ramos, 1994). While theCuyo basin was under thermal subsidence related to its final sagphase in the Late Triassic, extensional deformation shifted west-wards, and normal faulting started to take place in the northernsector of the Neuquén basin. Again, this process could be relatedto an increase in the lithospheric strength related to crustal thin-ning in the Cuyo basin. In this way, the continental plate wassubjected to a continuous extensional state of stress during theLate Permian to Early Jurassic, but the localization of the exten-sion and normal faulting varied in time in response to changes inthe thermo-mechanical state of the lithosphere. The Borbollón-LaManga lineament could be marking a rheological change inheritedfrom the Permo-Triassic reworking of the suture zone between theChilenia and Cuyania terranes, or could either correspond to a weakprevious lithospheric structure that was reactivated during the LateTriassic rifting.

8. Concluding remarks

We successfully carried out a series of analog models to simulatean extensional system with a bent geometry similar to the north-eastern border of the Neuquén basin. In different experiments,we varied the extension direction between NNE (N10◦E) and NE(N45◦E) orientations, inducing rift systems with different degreeof obliquity in each sector of the extended area. We grouped the


results observed in our models in two general types of transten-sional strain: pure-shear dominated, characteristic of extensionalto moderately oblique rift systems, and simple-shear dominated,for highly oblique rift systems.

dx.doi.org/10.1016/j.jog.2014.04.010

ING ModelG

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We compared the kinematic evolution and the final struc-ural architecture observed in the experiments with structuralata from two selected representative areas of the basin. In bothases, we observed a good match with the results of the ana-og models on which we applied an extension between N30◦End N20◦E orientations. The Atuel depocenter was selected ashe case study for the northern Andean sector, and its structuralrchitecture and kinematic evolution are consistent with a highlyblique rift system, with simple-shear dominated transtension. Thentre Lomas area was selected as the case study for the northerneuquén Embayment, and its structural configuration is consistentith a moderately oblique rift system, with pure-shear dominated

ranstension.The good match between the field data and the results of the

nalog models supports a NNE orientation of the regional exten-ion (N30◦E–N20◦E) during the Late Triassic to Early Jurassic synrifthase. This orientation is concordant with previous estimationsased on kinematic analysis of field data from the northern Andeanector. The results of the analog experiments allow us to explainreat part of the structural architecture of the synrift stage of theeuquén basin in a simple and integrated model, which is particu-

arly valid for the northeastern border of the basin.A nearly uninterrupted NNE to NE extension characterized the

volution of the southwestern sector of Gondwana from the Lateermian until the Early Jurassic. While the regional extensionemained nearly stationary, the locus of deformation, magmatismnd rift basin opening shifted through time. Rift basins were con-rolled by lithospheric anisotropies that were linked to the suturesnd rheological contrasts generated during the collision of terranesgainst the southwestern margin of Gondwana during the Paleo-oic, as well as further modifications of the thermo-mechanicaltate of the lithosphere during the Late Paleozoic to the Early Meso-oic evolution. Our experimental results suggest that a lithosphericeakness zone of NNW to NW trend related to the inferred suture

etween the Cuyania and Chilenia terranes could have controllednd localized the extension in the Neuquén basin during the Lateriassic to Early Jurassic.

cknowledgments

This research was funded by Agencia Nacional de Promociónientífica y Tecnológica (PICT-07-10942, PICT-38295, PICT-2010-441, PICT-2011-1079), Consejo Nacional de Investigacionesientíficas y Técnicas (PIP 5843), and Universidad de Buenos AiresUBACYT 855). This is the contribution R-133 of the Instituto destudios Andinos Don Pablo Goeber (UBA-CONICET). The Editor-n-Chief Wouter Schellart, Joao Duarte and an anonymous reviewerre sincerely thanked for their critical and helpful comments anduggestions.

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