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_Journal of Structural Geology 32 (2010) 886-899_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com/locate/jsg_ _Kinematic analysis of a transtensional fault system: The Atuel depocenter of the Neuquén basin, southern Central Andes, Argentina_ _Florencia Bechis a,*, Laura Giambiagi b, Víctor García c, Silvia Lanús d, Ernesto Cristallini c, Maisa Tunike a _CONICET e IIDyPCA, Universidad Nacional de Río Negro (UNRN), Sarmiento Inferior 3974, CP 8400, San Carlos de Bariloche, Argentina_ _b CONICET e IANIGLA, CCT Mendoza, CC 330, CP 5500, Mendoza, Argentina_ _c CONICET e Laboratorio de Modelado Geológico, FCEN, UBA, Pabellón 2, Ciudad Universitaria, CP 1428, Buenos Aires, Argentina_ _d E505 East Tower, Knightsbridge, Esplanade Road, Century City, 7441, Cape Town, South Africa_ _e CONICET e CIMAR, UNCOMA, Av. Buenos Aires 1300, CP 8300, Neuquén, Argentina_ _article info_ _Article history: Received 4 December 2009; Received in revised form 16 March 2010; Accepted 19 March 2010; Available online 7 April 2010_ _Keywords: Oblique rift Transtension Kinematic analysis Neuquén basin Central Andes_ _abstract_ _The Atuel depocenter of the Neuquén basin originated as an Upper Triassic to Lower Jurassic rift system, later inverted during the Andean contractional deformation. To study the extensional architecture and kinematic evolution of this depocenter, we collected a large amount of field and subsurface data, including slip data from outcrop-scale normal faults, thickness and facies distribution within synrift deposits, and structural data from angular and progressive unconformities. The Atuel depocenter has an NNW trend, showing a bimodal distribution of NNW and WNW major faults (first and second order faults). From kinematic indicators measured on outcrop-scale faults (third and fourth order faults), we found a mean NE internal extension direction, which is oblique to the general trend of the sub-basin. Taking these particular characteristics into account, we interpreted the Atuel depocenter as an oblique rift system. We evaluated two mechanisms to explain the development of this transtensional system: 1) reactivation of upper-crustal NNW-oriented Paleozoic shear zones, and 2) oblique stretching of a previous NNW-oriented lithospheric weakness zone._ _© 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _The Neuquén basin began as a rift basin during Late Triassic to Early Jurassic times (Uliana et al., 1989; Vergani et al., 1995; Franzese and Spalletti, 2001). The early synrift sequences are restricted to isolated depocenters limited by normal faults, with variable orientations according to their geographical position within the basin (Vergani et al., 1995). Mosquera and Ramos (2006) interpreted this variability as being related to a strong control of the basement structural grain. According to several authors (Ramos, 2010; and references therein), the regional extensional regime continued during Middle Jurassic to Early Cretaceous times, related to a negative roll-back of the Pacific subduction trench line. By the end of the Early Cretaceous period, the opening of the South Atlantic Ocean and the consequent separation of South America from Africa marked the start of contractional deformation and basin inversion in the southern Central Andes (Cobbold and Rosello, 2003; Zamora Valcarce et al., 2006; Somoza and Zaffarana, 2008)._ _The Neuquén basin has two well-defined sectors: the Neuquén embayment, and the Andean sector (Fig. 1). The main difference between them is that the embayment has not been affected by the Andean contractional deformation, due to its foreland position. We focused our research on the Atuel depocenter, situated in the northern Andean sector (Fig. 1). Its Late Triassic to Early Jurassic sedimentary infill records the first stages of the basin opening as a rift system (Manceda and Figueroa, 1995; Lanús, 2002, 2005; Lanús et al., 2008; Giambiagi et al., 2005, 2008a; Bechis et al., 2009; Tunik et al., 2008; Bechis, 2009). In Neogene times, the Andean orogeny deformed and uplifted the depocenter infill, favoring the exhumation of the synrift deposits and their exposure in the northern sector of the Malargüe fold and thrust belt (Fig. 1b; Giambiagi et al., 2008b; Bechis et al., 2008)._ _In this paper, we analyze the extensional architecture and kinematic evolution of the Atuel depocenter from the integration of multi-scale structural data. Faults of different scales can give different types of information. Major faults control the location of the main depocenters and the distribution of sedimentary environments, but they can be the result of previous anisotropies reactivation; undergo a progressive syntectonic rotation along their activity history; or change their sense of slip through time. In addition, subsequent tectonic phases often reactivate or cover them, making recognizing and studying them difficult. On the other hand, small-scale faults often escape subsequent reactivation and are generally abundant and well exposed. Moreover, strain or stress inversion methods based on the analysis of slip data from small-scale faults can give key information about the kinematics and dynamics of previous tectonic events (Angelier, 1984; Marrett and Allmendinger, 1990; Twiss and Unruh, 1998; Gapais et al., 2000)._ _In our study, we integrate new field data (from geological and structural mapping), and previous stratigraphic, sedimentological and structural data (Manceda and Figueroa, 1995; Lanús, 2002, 2005; Lanús et al., 2008; Giambiagi et al., 2005, 2008a, 2008b; Bechis et al., 2008, 2009; Tunik et al., 2008; Bechis, 2009), as well as seismic and well-log data acquired by the oil industry. We present a kinematic model for the evolution of the Atuel depocenter as an oblique rift system, taking into account data from minor and major normal faults. We also evaluate the possible factors controlling the transtensional deformation of the sub-basin, such as reactivation of previous anisotropies or lithospheric weakness, and the direction of regional extension._ _2. Stratigraphic setting_ _The structural basement of the Neuquén basin crops out in the northeastern sector of the study area, at the southern end of the Frontal Cordillera (Figs. 1 and 2). In this sector, the basement is composed of Paleozoic metasedimentary rocks, Carboniferous plutons, and widespread outcrops of the Choiyoi Group, a Permian to Triassic volcanic and plutonic complex (Volkheimer, 1978; Llamás et al., 1993; Sruoga et al., 2005)._ _The Upper Triassic to Lower Jurassic infill of the Atuel depocenter consists of siliciclastic marine and continental sedimentary rocks, grouped in the Arroyo Malo, El Freno, Puesto Araya and Tres Esquinas Formations (Figs. 2 and 3; Reijenstein, 1967; Volkheimer, 1978; Riccardi et al., 1991, 1997; Lanús, 2002, 2005; Lanús et al., 2008; Spalletti et al., 2007). Lanús (2002, 2005) and Lanús et al. used the accommodation versus sedimentary supply ratio to identify two stages of differing tectonic behavior (Fig. 3). The first one corresponds to the synrift stage, from Rhaetian to late Early Sinemurian times, showing greater accommodation than sedimentary supply (Lanús, 2002, 2005). This period was characterized by deposition of slope-type fan deltas in the western sector, while in the eastern sector a coeval braided fluvial system developed (Fig. 3; Lanús, 2002, 2005; Lanús et al., 2008; Tunik et al., 2008). A paleogeography dominated by active normal faulting controlled the marked lateral variations of the depositional systems during this stage. It is important to note that the base of the synrift units is always below the present exposure level, making it impossible to observe the contact between the structural basement and the sedimentary infill of the Atuel depocenter. The second stage, from late Early Sinemurian to Toarcian times, shows varying accommodation. During the late Early Sinemurian period, accommodation was outpaced by sedimentary supply, marking the start of the sag phase. An intermediate-type fan delta prograded over the slope-type fan deltas in the western sector, while in the eastern sector the fluvial systems increased their lateral migration (Lanús, 2002, 2005; Lanús et al., 2008). Later, during late Early Sinemurian to Toarcian times accommodation exceeded the supply, allowing transgression of a marine shelf and a marked increase in the marine depositional area (Fig. 3)._ _In the study area, the Middle Jurassic to Paleogene infill of the Neuquén basin consists of a thick pile of evaporitic, carbonatic and clastic marine and continental sedimentary rocks, assigned to the Lotena, Mendoza, Bajada del Agrio, Neuquén and Malargüe Groups (Fig. 2; Legarreta et al., 1993; Gulisano and Gutiérrez Pleimling, 1994; Legarreta and Uliana, 1996)._ _Cenozoic magmatic rocks correspond to Miocene to Quaternary andesitic to basaltic volcanic and subvolcanic rocks (Fig. 2; Volkheimer, 1978; Baldauf, 1997; Nullo et al., 2002; Sruoga et al., 2005; Giambiagi et al., 2008b). Thick clastic and volcaniclastic deposits crop out in the eastern sector, representing the Miocene to Pleistocene synorogenic infill of the Andean foreland basin (Fig. 2; Yrigoyen, 1993; Baldauf, 1997; Combina and Nullo, 2000; Giambiagi et al., 2008b)._| Ключевые слова: kinematic data, stage, franzese, normal-oblique fault, study area, south, transtensional setting, map, evolution, relatorio, continental, large number, spalletti, extensional architecture, geological, ciencias exactas, trend ranging, belt, recursos naturales, triassic, uliana, ax, baldauf, giambiagi, revista, synrift unit, progressive, la, thesis facultad, journal, secondary fracture, eds, fold, rift margin, nullo giambiagi, gulisano, geol?gico, sedimentary, cretaceous, el, upper, time, thrust, angular unconformity, bechis, study, transfer zone, angelier, cobbold, middle jurassic, bechis journal, nnw trend, regional, angle corresponds, oblique rift, probable refraction, ne, alumbre fault, andean deformation, scale, nnw orientation, journal structural, sector, eld data, structural basement, geology, wide dispersion, tankard, controlled, strain, nne orientation, marine, station, kinematic evolution, tectonics, stratum, distribution, case, atuel depocenter, progressive unconformities, extension direction, elsevier, neuqu?n basin, trend, order fault, fourth, western sector, asociaci?n, outcrop-scale fault, naturales universidad, thesis, structural, neuqun basin, measurement station, contractional, andean, spatial variability, kozlowski, location, nne, tectonic, small-scale fault, relatorio mendoza, geographic distribution, colleta bechis, buenos aires, magmatic view, geol?gica, allmendinger, nullo, orientation, sruoga, alumbre, strain ellipsoid, oblique extension, geol?gica argentina, andean margin, synrift stratum, teyssier, late, eds evolution, manga, journal structural geology, slip data, area, early stage, neuqun embayment, structural geology, showing, normal fault, rift, slip, angular unconformities, argentina, kinematic, fault, original, jurassic inll, major fault, eastern, tectonophysics, nnw, basement, synrift deposit, fault-slip data, jurassic, neuqu?n, dimieri, upper triassic, lithospheric weakness, extensional, map showing, malarge fold, order, atuel, sinemurian, previous, depocenter, mendoza, synrift stage, oblique, thrust belt, deformation, northeastern sector, bimodal distribution, figs, structure, previous fabric, indicating, lan?s, basin, northern sector, normal, synrift, data, direction, major, lans lans, sedimentary environment, eastern sector, holdsworth, mendoza revista, tunik, deposit, kinematic model, unconformities, early synrift, model, ramos, control, zone, los, hanging-wall, welsink, asociaci?n geol?gica, angular, early, volkheimer, jur?sico, transtensional, kinematic characteristic, extension, fourth order, depocenter evolved, iglesia llanos, late triassic, morley, del