Journal of South American Earth Sciences 26 (2008) 252–260 Contents lists available at ScienceDirect Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames Permo-Triassic oblique extension in the Potrerillos-Uspallata area, western Argentina Laura Giambiagi a,*, Amancay N. Martinez b a CONICET-IANIGLA Centro Regional de Investigaciones Científicas y Tecnológicas, Parque San Martín s/n, 5500 Mendoza, Argentina b Laboratorio de Tectónica Andina, Universidad de Buenos Aires, Departamento de Ciencias Geológicas, Ciudad Universitaria, Pabellón II, 1428, Capital Federal, Argentina article info abstract Keywords: The Permo-Triassic evolution of southwestern South America was characterized by the development of a Permo-Triassic volcanism great amount of volcanism under extensional conditions. Structural analyses of faults developed contem- Choiyoi Group poraneously with this volcanism in the key area of Potrerillos-Uspallata suggest the existence of an obli- Lithospheric anisotropy que extensional setting controlled by the presence of a pre-existing lithospheric anisotropy. A clear Oblique extension parallelism between the trace of an inferred Devonian suture zone, the Late Paleozoic San Rafael orogenic Andes belt and the Permo-Triassic rifting suggests that Early and Late Paleozoic tectonic inheritance permitted the reactivation of a NNW-trending zone of lithospheric weakness. The reactivation of this pre-existing weak zone during Late Permian to Early Triassic times has resulted in the generation of a new complex fault system, which concentrated the oblique-slip normal displacement related to a NNE–SSW stretching (N23°E). Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction zone, has been traced from the Atlantic coast to the San Rafael block (Urien et al., 1995; Jacques, 2003), and it represents a pre- Several major zones of intraplate deformation, also referred to existing network of structures that was formed or reactivated dur- as megafractures or megashears, occur as major structural aniso- ing a Late Paleozoic compressional event (Fig. 1A). This anisotropy tropies throughout the upper crustal rocks of southern South can be traced northward, along the tectonic depression between America (Coira et al., 1975; Rapela et al., 1991; Rapela and the Cordillera Frontal and Precordillera, running through the Pankhurst, 1992; Tickyj et al., 1997; Visser and Praekelt, 1998; Uspallata-Calingasta valley, or northwestward along the Río Blanco Franzese and Spalletti, 2001; Aceñolaza et al., 2002; Jacques, lineament (Fig. 1A and B). 2003). Several of these deformational zones are believed to be re- In this paper, we present evidence of the presence of the San lated to deep-crustal or even trans-lithospheric structures (Rapela Rafael anisotropy cross-cutting the Andes and its reactivation in and Pankhurst, 1992; Jacques, 2003). Although no clear proof of an oblique extensional mode during the development of the Late transcurrent movement along these anisotropies has been re- Permian to Middle Triassic magmatic event, the rocks of which ported, as was pointed out by von Gosen and Loske (2004) for are known as the Choiyoi Group. Detailed structural analysis, com- the Gastre fault system, they have been inferred to correspond to bined with stratigraphic, petrological and geochemical studies, was pre-existing heterogeneities in the continental lithosphere which carried out in an area where excellent exposures of Permo-Triassic act as planes of weakness during subsequent tectonic events. These faults are preserved. This area is located close to the towns of Potr- anisotropies have been traced from the Atlantic coast to the erillos and Uspallata, at 33°S latitude, and covers the eastern part Andean foothills with WNW to NNW trends, but the complex of the Cordillera Frontal and western part of the Precordillera structural history of the Andes makes them difficult to recognize (Fig. 1B). Our goals are (1) to define the Permo-Triassic structural across this orogen. Attempts to quantify pre-Cenozoic deformation framework of the area with field data and kinematic indicators, in the Andes face the difficulty of recognizing pre-Andean faults in- and (2) to discuss the role of a lithospheric inherited anisotropy side this Cenozoic orogeny, because ancient faults are often over- in the development of the Permo-Triassic extensional system. printed or obscured by intense Andean tectonics. One of these major anisotropies, the NW-trending San Rafael deformational 2. Geological setting The study region is located between two parallel north–south * Corresponding author. E-mail addresses: [email protected] (L. Giambiagi), [email protected] trending mountain ranges of the Andes formed during the Neogene (A.N. Martinez). Andean orogeny: the Cordillera Frontal, composed of pre-Jurassic 0895-9811/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2008.08.008 L. Giambiagi, A.N. Martinez / Journal of South American Earth Sciences 26 (2008) 252–260 253 Fig. 1. (A) Location map of the study area. The San Rafael anisotropy was outlined. (B) Regional map of the study area, showing the morphostructural units, the inferred boundary between Chilenia and Cuyania terranes (from Ramos, 2004), and the boundary between Cuyania and western Gondwana. rocks uplifted by high-angle faults, and the Precordillera fold and (Ramos, 1988). In Early Permian times a widespread compressive thrust belt characterized by west- and east-vergent basement-in- event known as the San Rafael phase (Azcuy and Caminos, 1987; volved faults affecting Proterozoic to Neogene metamorphic and Ramos, 1988), which occurred between 280 and 270 Ma (Sato sedimentary rocks. The pre-Permian rocks correspond to metamor- et al., 1990; Martinez, 2005), generated a NNW-trending orogenic phic, igneous and sedimentary rocks of Proterozoic to Early Car- belt and important crustal thickening in the southwestern part of boniferous age. They are thought to belong to at least two the Gondwana supercontinent (Llambías and Sato, 1995; Mpodozis different terranes, Cuyania and Chilenia, suspected to have collided and Kay, 1990). Following the dominantly compressional setting, against the old cratonic basement of western Gondwana during the regional tectonic regime underwent a major change towards Early Paleozoic times (Ramos et al., 1986; Ramos, 2004). Although an extensional regime that was reflected by strong magmatic activ- the precise extent of both terranes remains unclear, the suspected ity, known as the Choiyoi Group (Caminos, 1965; Stipanicic et al., suture zone between them has been located roughly between the 1968; Rolleri and Criado Roque, 1969; Kay et al., 1989; Mpodozis Cordillera Frontal and Precordillera with a NNW–SSE trend in the and Kay, 1992; Llambías and Sato, 1995; Llambías et al., 2003), south, where the study area is located, and a N–S to NNE–SSW during the Late Permian to Middle Triassic (270–230 Ma, Martinez, trend in the north (Fig. 1B) (Haller and Ramos, 1984; Ramos 2005). In the study area and southward, the Choiyoi rocks are ex- et al., 1986; Davis et al., 2000). This suture zone is marked by out- posed in a narrow NNW-trending zone (Fig. 1B) and have a strong crops of mafic and ultramafic rocks, with geochemistry and isoto- spatial relationship with the San Rafael orogenic belt, suggesting pic signatures indicating an oceanic crust obducted during that the latter influenced the location of the Permo-Triassic Devonian times (Ramos et al., 1986). Toward the south, the San extension. Rafael block has been assigned as the southwards extension of the Cuyania terrane (Cingolani et al., 2003). Aeromagnetic studies 2.1. Choiyoi Group carried out in the San Rafael area by Chernicoff and Zappettini (2003) depict a NNW-oriented outstanding discontinuity which In southern South America, the Permo-Triassic magmatism of has been inferred to run parallel and adjacent to the Chilenia- the Choiyoi Group extends along a NNW- to NW-trending belt Cuyania boundary. from at least 28°S in the Cordillera Frontal to 41°S in the North Pat- The Late Paleozoic tectonic cycle began with the inception of agonian Massif. This magmatism is characterized by extrusive subduction along the western continental margin of Gondwana rocks and shallow level batholiths. The bottom of this sequence 254 L. Giambiagi, A.N. Martinez / Journal of South American Earth Sciences 26 (2008) 252–260 is composed of andesitic lavas and breccias, while the upper part to alkaline and have geochemical characteristics attributed to a corresponds to acid pyroclastics flows. These rocks are subalkaline magmatic arc of post-orogenic character. This magmatism has Fig. 2. Geological map of the Potrerillos-Uspallata area where distinction between non-reactivated and reactivated Permo-Triassic faults is made. Location shown on Fig. 1. L. Giambiagi, A.N. Martinez / Journal of South American Earth Sciences 26 (2008) 252–260 255 been associated with an extensional regime, probably related to tion have high levels of K2O and SiO2 and are peraluminous with the final stage of a subduction process (Llambías and Sato, 1995; a calc-alkaline to alkaline imprinter. This trace-element pattern Llambías, 1999). The heat source has been proposed to be injection could indicate a low-pressure with a plagioclase-bearing source. of extension-related basalt into the lower crust (Kay et al., 1989), These rocks present medium La/Yb ratios and can be related to creating a mafic underplating which would have contributed