Geology and Structural History of the Southwest Precordillera Margin, Northern Mendoza Province, Argentina

Journal of South American Earth Sciences 14 (2002) 821±835 www.elsevier.com/locate/jsames Geology and structural history of the southwest Precordillera margin, northern Mendoza Province, Argentina C. Gerbia,*, S.M. Roeskea, J.S. Davisb aDepartment of Geology, University of California, One Shields Avenue, Davis, CA 95616, USA bExxonMobil Production Co, 800 Bell Street, Houston, TX 77002, USA Received 1 March 2001; accepted 1 August 2001 Abstract Rocks and structures in the southwest Precordillera terrane, located in western Argentina, constrain the Paleozoic distribution of continents and the development of the western margin of Gondwana. Detailed mapping of an area in the southwest Precordillera allowed identi®cation of several pre-Carboniferous rockunits formed in distinct tectonic environments and were later tectonically juxtaposed. The pre-Carboni- ferous rockunits comprise carbonate metasiltstone, metasandstone, massive diabase, and quartzo-feldspathic gneiss intruded by ultrama®c rocks and layered gabbro. Preliminary structural analysis indicates that the present distribution of units is due to two contractional deformation episodes, an east-directed Devonian ductile event and a west-directed Tertiary brittle event. The metasedimentary rocks, which form the structural base of the area and are part of the western Precordilleran passive margin sequence, were juxtaposed along minor ductile shear zones early in the ductile event. Their contact was then folded during continued ductile deformation; at this time the ultrama®c/layered gabbro complex and the massive diabase were emplaced over the metasedimentary units along narrow ductile shear zones. Brittle deformation, associated with the Andean orogeny, involved open folding, thrust faulting, and reactivation of some ductile features. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Precordillera; ductile shear zone; ultramatic 1. Introduction basement, age, etc.) as well as the deformational events related to the closing of that basin. Despite their importance, The Precordillera terrane of western Argentina (Fig. 1) few detailed structural and tectonic studies have been plays a key role in Early Paleozoic global paleogeographical conducted in the early Paleozoic rocks of the western reconstructions. Most workers agree that stratigraphic (e.g. Precordillera. Thus, existing tectonic interpretations for Thomas and Astini, 1996), geochemical (Abbruzzi et al., the Precordillera are largely unsupported by the structural 1993), paleontologic (e.g. Astini et al., 1995), and paleo- data. magnetic data (Rapalini and Astini, 1998) indicate that the Most early mapping of the southwestern Precordillera Precordillera terrane originated in the Laurentian Ouachita was of reconnaissance style (AveÂ Lallement, 1892; embayment (in the present day southeastern United States) Stappenbeck, 1910; Keidel, 1939; Harrington, 1971) and (Dalziel et al., 1996). Substantial debate remains, however, did not suf®ciently describe the structures and units present. regarding the timing and nature of the transfer of the Pre- The majority of the studies in the Precordillera since then cordillera from Laurentia to Gondwana and of the closing of have been detailed paleontologic (e.g. Cuerda et al., 1985; the ocean basin that developed as the Precordillera rifted Lehnert and Keller, 1992), sedimentologic (e.g. Astini, from Laurentia. 1988; Spalletti et al., 1989), and petrologic (e.g. Dias and Early Paleozoic rocks on the Western margin of the de Tonel, 1987; Kay et al., 1984) investigations. Structural Precordillera terrane formed in the controversial marine studies have been limited primarily to regional investi- basin between the Precordillera and Laurentia, and as gations (e.g. Allmendinger et al., 1990; Ramos et al., such, they offer the greatest possibilities for understanding 1986). Some early detailed structural studies were under- the nature of that basin (its geometry, subsidence history, taken, but most of them concentrated on very small areas in the southwest Precordillera and all were hampered by a lack * Corresponding author. Address: Department of Geological Sciences, of geochronologic control (e.g. Cosentino, 1968; Cucchi, University of Maine, Orono, ME 04469, USA. 1972; De Roemer, 1964). Recently, von Gosen (1992, 0895-9811/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0895-9811(01)00080-3 822 C. Gerbi et al. / Journal of South American Earth Sciences 14 (2002) 821±835 Fig. 1. Location of the Precordillera terrane in Western Argentina. (Generalized geography after Ramos, 1988; terrane boundaries after Astini, 1996a). 1995) documented in detail the deformation in the Rio San western Precordillera. The present distribution of units in Juan area and in part of the Bonilla area (Fig. 2), and Davis the Cortaderas and Pozos areas resulted almost exclusively et al. (1999) mapped the Cortaderas and Bonilla areas. from the Devonian and Tertiary contractional deformation Our present study involved lithologic and structural events. mapping in the Pozos area and continued mapping and tectonic analysis in the Cortaderas area (Fig. 2); together, these areas make up the northern end of the Cortaderas 2. Geological setting mining district. The mapping, integrated with tectonic, structural, and petrologic analyses, adds to the data of The Pozos and Cortaderas areas lie in the southwestern previous workin order to provide further constraints on portion of the Precordillera terrane, near its western margin the structural and tectonic history of the Precordillera (Figs. 1 and 2). The Precordillera terrane, one of a series of terrane. Speci®cally, this workaddresses: (1) the present accreted terranes that make up much of southern South and former distribution of rockunits that formed in the America (Ramos, 1996), is located between the Sierras ocean basin west of the Precordillera, and (2) the structural Pampeanas to the east and the Cordillera Frontal to the history of the Cortaderas and Pozos areas during and since west. Lithologic units in the Precordillera include an exten- the Devonian deformation that juxtaposed all the pre- sive early Paleozoic passive margin marine sedimentary Carboniferous mapped units. Results of this workindicate sequence (Astini et al., 1995), a late Paleozoic continental that the carbonate and clastic metasedimentary rocks, which and marine sedimentary sequence, early Mesozoic rift formed on the early Paleozoic west-facing Precordilleran deposits, and late Paleozoic through Cenozoic volcanic passive margin, make up the structural base of the section, and associated plutonic rocks (Caminos et al., 1993; and that more distally derived ma®c and ultrama®c crystal- Ramos et al., 1986). The unexposed crystalline basement line suites were thrust over the margin from the west. of the Precordillera is known from gneissic xenoliths in Discrete Devonian ductile shear zones, rather than meÂlange Miocene volcanic rocks from the central Precordillera style mixing, juxtaposed these ma®c and ultrama®c crystal- (Leveratto, 1968; Abbruzzi et al., 1993; Kay et al., 1996). line suites, which have independent tectonic histories, with The xenoliths have ages (1.1 ^ 0.1 Ga) and lead isotopic the passive margin metasedimentary rocks after the meta- signatures similar to those of Grenville basement in North sedimentary rocks were juxtaposed. Tertiary west-directed America (Abbruzzi et al., 1993; Kay et al., 1996). As brittle thrust faults minimally reorganized the units of the no basement of the Precordillera is exposed, the early C. Gerbi et al. / Journal of South American Earth Sciences 14 (2002) 821±835 823 Fig. 2. Generalized map of the geology of the southwest Precordillera. This study focuses on rocks from the Cortaderas and Pozos areas. Cambrian through Devonian sedimentary rocks, including those hosting the ma®c and ultrama®c belt, are metamorphosed to at most low-greenschist facies (Keller et al., 1993). All other units are unmetamorphosed. (Geology after Caminos et al., 1993). Paleozoic sedimentary rocks and metasedimentary rocks facies metamorphism (Keller et al., 1993; Spalletti et al., form the structurally lowest observed units. Major defor- 1989). mational events affecting the Precordillera include A thin, discontinuous belt of ma®c and ultrama®c rocks Devonian and early Permian contraction, Mesozoic crops out along nearly the entire western margin of the extension, and late Tertiary to recent contraction associated Precordillera, including in the Pozos and Cortaderas areas. with the uplift of the Andes (Ramos, 1988; von Gosen, Although known since the earliest geologic investigations in 1992, 1995; Davis et al., 1999). Andean deformation is the area (AveÂ Lallement, 1892; Stappenbeck, 1910), until responsible for major east-vergent thrust faults in the eastern recently most studies have reported the rocks as intrusive and central Precordillera, which have shortened the terrane into or `inter®ngering' (Haller and Ramos, 1984) with the by more than 50% (Allmendinger et al., 1990; von Gosen, metasedimentary rocks and/or as fragmented parts of an 1992). ophiolite. Where ma®c and ultrama®c rocks are imbricated Much of the eastern and central Precordillera is an areally with the metasedimentary rocks they have been previously extensive Cambrian through Ordovician carbonate plat- assigned to a tectonic meÂlange (Ramos et al., 1986). Recent form, where many of the key Precordilleran fauna have workhas shown that the ma®c and ultrama®c bodies are been found

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