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,

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 (summarized in Astini et al., 1995). The western geochemically and geochronologically unrelated to each Precordillera sedimentary section is primarily made up of other, are tectonically juxtaposed, and cannot represent turbiditic submarine fan and basinal clastic rocks. Meta- fragmented parts of a single ophiolite (Davis, 1997; Davis morphism of the stratigraphic succession increases mark- et al., 1999, 2000). edly from east to west, from diagenetic to low greenschist In the southwest Precordillera, near the ultrama®c and 824 C. Gerbi et al. / Journal of South American Earth Sciences 14 (2002) 821±835 7) and Gerbi (1999) for more detailed geologic maps of the Fig. 3. Geologic map of the Pozos and Cortaderas areas, both of which are part of the Cortaderas mining district. Refer to Davis et al. (1999); Davis (199 areas. C. Gerbi et al. / Journal of South American Earth Sciences 14 (2002) 821±835 825

Fig. 4. Cross-sections A±A0 and B±B0; section lines are shown in Fig. 3. Topography is estimated, with no vertical exaggeration. ma®c bodies, unmetamorphosed Carboniferous through of Davis et al. (1999). We have chosen not to use the latter Triassic sedimentary rocks and late Paleozoic volcanic term because although the unit may represent oceanic rocks form an angular unconformity with underlying crust (Haller and Ramos, 1984), it may not (Davis et al., metasedimentary rocks (Keidel, 1939; Ramos et al., 1986; 1999). Caminos et al., 1993). The detailed distribution of lithologic units, particularly in the Cortaderas area, is complicated, but a gross pattern exists for the mapped region. Except for the monzodioritic 3. Map features intrusive rocks, units in the ®eld area tend to form north± south trending belts. Field relations indicate that the carbo- During mapping, we recognized ®ve rockunits and ®ve nate metasiltstone and metasandstone form the structural intrusive rocktypes (Fig. 3). The two most abundant units base of the area and that the ultrama®c/layered gabbro are metasandstone and carbonate metasiltstone. The next complex and massive diabase are structurally separated two most abundant units, the ultrama®c/layered gabbro units emplaced over the metasedimentary rocks (Figs. 3 complex and massive diabase, structurally overlie the meta- and 4). All rockunits, including the monzodioritic plutonic sedimentary rocks. Minor amounts of Carboniferous to rocks, are deformed, and all pre-Carboniferous rock units Triassic clastic sedimentary rocks crop out in the Cortaderas are metamorphosed to at least low greenschist facies. area. Ma®c sills, ranging in thickness from 1 to 30 m, Evidence for both ductile and brittle deformation is present intrude the metasedimentary rocks, and monzodioritic throughout the area. plutonic rocks intrude all bedrock units. The ultrama®c/ layered gabbro complex comprises abundant serpentinized ultrama®c rocks, ultrama®c cumulate bodies, ultrama®c 4. Unit descriptions and intrusive rocks dikes, and layered gabbro, which intrude host quartzo-feldspathic gneiss. 4.1. Metasandstone We divided the lithologies of the Pozos area in agree- ment with the units proposed by Davis et al. (1999) for The metasandstone unit crops out on both the eastern and the Cortaderas area. Previous authors (Cucchi, 1972; Western margins of the map area, and reconnaissance work Harrington, 1971) chose to subdivide the metasedimentary immediately north of the Pozos area indicates that meta- rocks in this area into three units, but a division into two sandstone becomes by far the dominant unit. Toward the metasedimentary units more accurately represents the south, in the Cortaderas area, carbonate metasiltstone natural depositional environments, with local facies becomes abundant. The metasandstone comprises alter- changes, indicated by these rocks. The `massive diabase' nating coarse- to medium-grained layers (10 cm±1 m unit of this study correlates with the Ma®c Ophiolite section thick) and ®ne-grained layers (5±30 cm thick). The 826 C. Gerbi et al. / Journal of South American Earth Sciences 14 (2002) 821±835 boundary between the coarse- and ®ne-grained layers is metasiltstone contain calcite, quartz, white mica, chlorite, sharp. Scarce, thin (1±5 cm thick), carbonate-rich bands and graphite, with minor apatite, Fe-oxide, and rutile. The parallel textural layering in the Pozos area, but are less mica and chlorite are neocrystalline rather than detrital, and common in the Cortaderas area. Scarce (5±30 cm) chert the entire matrix is recrystallized. The sandstone lenses, layers crop out in the metasandstone in the southern portion which are compositionally similar to the metasandstone of the map area. Throughout the area, the sandstone com- unit, consist of medium-grained massive sandstone with position, grain-size range, and bedding style are relatively no well-developed bedding plane; grain size throughout uniform. In the southern Cortaderas area, several inter- the lenses is uniform. The sandstone lenses contain equant, mediate and ma®c ¯ows and tuffs are interlayered with subrounded detrital clasts surrounded by a ®ne-grained the metasandstone. Most ¯ows are in the order of 1 m quartz and white mica matrix. Micaceous minerals in the thickand occur near the contact with the carbonate metasandstone matrix are aligned. siltstone. Although the stratigraphic base of the unit is not The ma®c ¯ows are similar to those in the metasandstone exposed and there are numerous structural repetitions, unit, consisting of ®ne-grained, vesicular, darkgreen meta- outcrop width suggests the unit has a thickness of at least basalts, with minor relict plagioclase phenocrysts, quartz, several hundred meters. clinopyroxene, and neocrystalline chlorite, white mica, At the microscopic scale, the coarser-grained layers of the actinolite, and carbonate. metasandstone consist of subrounded to subangular detrital Only weakdirect data constrain the age of the carbonate clasts surrounded by a very ®ne grained white mica 1 metasiltstone. The one fossil age reported for the carbonate chlorite 1 quartz ^ carbonate matrix. All chlorites and metasiltstone, in the Cortaderas area, relies on poorly much of the white mica are neocrystalline. Morpho- preserved specimensÐa protoconodont, a conodont, and a logically, the dominantly monocrystalline or polycrystalline phosphatized bryozoanÐwhose ranges overlap in the Early quartz detrital clasts appear to have been variably affected to Middle Ordovician (Davis et al., 1999). One diabase sill by pressure solution. The composition and texture of the that intrudes the carbonate metasiltstone in the Pozos area ®ne-grained layers within the metasandstone are com- has a single fraction concordant zircon U±Pb crystallization parable to the matrix material in the coarser-grained layers age of 418 ^ 10 Ma (Davis et al., 2000), thus providing a (i.e. very ®ne grained phyllosilicate and quartz). The relict minimum age for the carbonate metasiltstone. layering, textures, and sandstone protolith compositions, which vary from graywacke to quartz arenite, are consistent 4.3. Diabase sills with a sequence of turbidite deposits. Ma®c sills in both the Cortaderas and Pozos areas intrude No one has directly determined either the depositional or the metasedimentary rocks and can be categorized into the metamorphic age of the metasandstone unit. Several small (1±5 m thick) and large (15±30 m thick) bodies. authors (Cuerda et al., 1987; Harrington, 1971; Ramos et The larger sills are concentrated in the carbonate meta- al., 1984; von Gosen, 1995) have proposed a possible siltstone within 50 m of the contact with the metasandstone. regional correlation, however, based on the lithologic simi- The ma®c bodies are darkgreen, ®ne grained to very ®ne larity between the metasandstone and the Alcaparrosa and grained, and massive. Mineralogically, the sills consist of Villavicencio Formations to the north and east, which are all relict phenocrysts of plagioclase and clinopyroxene in a Middle Ordovician to Early Devonian in age (Cuerda et al., quartz and plagioclase matrix. The igneous mineralogy 1985, 1987, 1988; Cuerda, 1988). has been altered in part to sericite, epidote, actinolite, and chlorite. 4.2. Carbonate metasiltstone 4.4. Massive diabase Carbonate metasiltstone occupies the center portion of the map area. Three lithologiesÐbedded carbonate, meta- The massive diabase unit, identi®ed only in the siltstone, and lenses of massive sandstoneÐeach of which Cortaderas area, crops out as 0.1±1 km long and tens to grades into the other two, make up the unit. The southern hundreds of meters thick, elongate, homogeneous diabase portion of the Pozos area is predominantly bedded carbonate to microgabbro bodies. Diabase portions of the unit are (the calcareous schist of Cucchi, 1972), but the majority of indistinguishable from the diabase sills in outcrop, but in the unit consists of very ®ne grained quartz±clay±graphite some areas this unit constitutes a gabbro with grains as metasiltstone layers interbedded with 10 cm±1 m thick coarse as 1 cm. Internal features such as magma ¯ow carbonate bands. Irregularly spaced lenses of medium- features, small plagiogranite bodies, and probable half- grained sandstone, several meters long, occur in carbo- dikes with pillow screens are also present. Relict clino- nate-poor parts of the Pozos area and constitute only pyroxene and plagioclase occur as phenocrysts in an a minor portion of the unit. Rare ma®c ¯ows, 3±5 m isotropic plagioclase and quartz matrix; poikiolitic textures thick, are also found in the Pozos area, but not in the are common. Fine-grained neocrystalline phases include Cortaderas. sericite, epidote, actinolite, and chlorite. These bodies Individual compositional layers within the carbonate are everywhere in tectonic contact with the carbonate C. Gerbi et al. / Journal of South American Earth Sciences 14 (2002) 821±835 827

Fig. 5. (a) Southward view of a hilltop in the Cortaderas area. Massive diabase overlies carbonate metasiltstone along a subhorizontal contact. This geometry is common throughout the area and suggests that the massive diabase does not extend far into the subsurface. (b) Photomicrograph of a quartz±feldspar±garnet gneiss within the ultrama®c/layered gabbro complex. Features of quartz, with ribbon morphology and undulose extinction, and garnet (at the top of the ®gure), with brittle fractures, suggest that signi®cant deformation occurred well below the granulite facies conditions that the rockexperienced during i ntrusion of the ma®c and ultrama®c igneous rocks. We interpret this lower temperature deformation as synchronous with the development of ductile shear zones responsible for the emplacement of this unit over the metasiltstone. (c) Photomicrograph of the mylonitic fabric in metasiltstone from the ductile shear zone at the contact between metasiltstone and the ultrama®c/layered gabbro complex. Elongate quartz crystals in the strain shadow of the pyrite porphyroblast indicate a top-to- the-east sense of shear. (d) View down onto the contact between the metasandstone and the metasiltstone. The outcrop scale folding pattern is similar along the length of the contact. Neither unit has universally developed a high strain or mylonitic fabric along the contact. Foliation in both units parallels contact. Hammer (circled) is 30 cm long. metasiltstone unit. The contacts are well exposed in the al., 1999). Later retrograde recrystallization occurred Cortaderas area and indicate that these bodies do not extend under greenschist facies conditions. far into the subsurface (Fig. 5(a)). The massive diabase has The ultrama®c bodies have a cumulate texture, are dark crystallization age of 576 ^ 17 Ma, determined by U±Pb green to black, are generally coarse grained, and range in dating of several zircon fractions (Davis et al., 2000). size from meters to tens of meters across. In the majority of the ultrama®c bodies, serpentine ^ talc ^ anthophyllite ^ chlorite ^ tremolite has partially or wholly replaced the 4.5. Ultrama®c/layered gabbro complex original igneous mineralogy; at the contacts with the metasedimentary rocks, sheeted talc is common. Some of the Cropping out in low hills as a tan- to brown-weathering ultrama®c rocks, however, are only partly serpentinized or rock, the ultrama®c/layered gabbro complex lies in a frag- contain serpentine growth textures that preserve the original mented belt within the carbonate metasiltstone west of the igneous cumulate textures. In the partially serpentinized ma®c sills in the Pozos area and west of the massive diabase bodies, relict igneous mineralogy includes orthopyroxene ^ in the Cortaderas area. The ultrama®c/layered gabbro clinopyroxene ^ spinel, with orthopyroxene most com- complex consists of a quartzo-feldspathic gneiss and irregu- monly the dominant cumulate phase. larly distributed metamorphosed ma®c to ultrama®c igneous Weakly foliated, medium to coarse grained layered rocks. More than 50% of the complex is made up of layered gabbro, comprising clinopyroxene 1 plagioclase ^ garnet, gabbro and serpentinized intrusive ultrama®c rocks. Petro- is compositionally banded, with feldspar-rich layers alter- logic studies of the ultrama®c cumulates and layered nating with clinopyroxene-rich layers on a centimeter scale. gabbros indicate that they recrystallized at granulite facies The mineralogy represents the prograde metamorphic conditions (850 , T , 1000 8C and P . 9 kb) (Davis et assemblage, but cumulate textures and the presence of 828 C. Gerbi et al. / Journal of South American Earth Sciences 14 (2002) 821±835

Fig. 6. Equal area stereographic plots of measured structural data (small symbols) and calculated fold axes (large symbols) from the Pozos area. The calculated fold axes assume cylindrical folding. (a) Poles to carbonate metasiltstone foliation .10 m from unit contacts. (b) Poles to foliation measured in carbonate metasiltstone and metasandstone within 10 m of their mutual contact and their calculated fold axes. (c) Hinge lines in metasiltstone and metasandstone within 10 m of their mutual contact. The hinge lines lie in the axial plane de®ned in (b). (d) Poles to metasiltstone and metasandstone foliation for folds a and b, two small-scale folds (,3 m amplitude) and their calculated fold axes. The two folds are located along the eastern metasandstone±carbonate metasiltstone contact; fold a is shown in Fig. 5(d). Note similar orientation of fold axes with those from (b). some undeformed crystals indicate that the compositional fabric and are compositionally banded on a centimeter layering is texturally igneous. In places, clinopyroxene scale. The quartz exhibits a ribbon texture with signi®cant crystals contain bent lamellae indicating a high temperature subgrain development and almost universal undulose deformation. Partially overprinting the prograde assem- extinction (Fig. 5(b)). Feldspar crystals, commonly blage, retrograde phases include albite, epidote, actinolite, subgrains within feldspathic domains, are arranged in pumpellyite, chlorite, and calcite. The retrograde overprint bands parallel to compositional banding. Individual crystals is more developed near the contacts with the metasedi- display no preferential orientation. Garnet crystals are mentary units. Due to poor exposure, the contacts of commonly fractured and ¯attened parallel or oblique to the larger intrusive bodies cannot be traced farther than the banding. 5±10 m. Observable relations indicate that the layered gabbro intrudes the quartzo-feldspathic gneiss either as irregularly shaped bodies up to tens of meters across or as 4.6. Intermediate intrusive rocks thin sills a few centimeters thick. The small intrusive sills have a preferred orientation parallel to the foliation plane of Melanocratic to leucocratic andesitic and monzodioritic the gneiss, but they do truncate gneissic foliation in places. rocks occur as thin dikes (50 cm thick) to elongate plutons U±Pb dating of zircon crystals from a layered gabbro body (.1 km long) and intrude all rock units throughout the map in the southern Pozos area indicates a crystallization age of area. 40Ar/39Ar dates of hornblende and biotite crystals from 450 ^ 20 Ma (Davis et al., 2000). one sill in the Cortaderas area and a tuff in the Bonilla area The host quartzo-feldspathic gneiss occurs throughout the (located 30 km to the south) indicated Miocene cooling ages Pozos area, stitching together separated bodies of ultrama®c of these igneous rocks (Davis et al., 1999). A U±Pb zircon rocks. The gneiss contains varying modal abundances of date for a monzodiorite pluton in the Cortaderas area also quartz 1 plagioclase ^ garnet. All rocks have a gneissic indicates Miocene crystallization (Davis et al., 1999). C. Gerbi et al. / Journal of South American Earth Sciences 14 (2002) 821±835 829 lower strain, including most of the unit in the Pozos area, still preserve some sedimentary features, such as homogeneous thickness of beds, sharp contacts between ®ne- and coarse-grained layers, and locally graded bedding. Folds in the lower strain areas are small scale (less than 1 m in amplitude) and open, and penetrative foliation in the ®ne-grained layers is commonly oblique to bedding. Highly deformed zones, on the order of tens of meters wide, contain thinned beds, dynamically recrystallized contacts between bedding layers, one or more foliations, and multiple generations of tight to isoclinal similar folds that fold both bedding and an early foliation. The foliation on the limbs of the folds and the spaced axial planar cleavage in these high-strain zones parallel the north±south strike of the regional foliation. The microscopic fabric is anisotropic and contains asymmetric quartz grains within the ®ner-grained matrix. Zones of high degrees of deformation grade into areas of low degrees of deformation. There is no spatial correlation between the degree of deformation and unit contacts in the Pozos area, but in the Cortaderas Fig. 7. Equal area stereographic plots of measured structural data (small the metasandstone is generally more deformed near the symbols) and calculated fold axes (large symbols) from the Pozos and Cortaderas areas. The calculated fold axes assume cylindrical folding. contacts with the other units. (a) Poles to foliation in carbonate metasiltstone and metasandstone in the Cortaderas area. Orientations of the structures and calculated fold axes are 5.1.2. Carbonate metasiltstone similar to those from the Pozos area (compare to Fig. 6(a)). (b) Poles to Like the metasandstone, the carbonate metasiltstone has ductile shear zones and calculated fold axis from the Cortaderas area. experienced variable degrees and styles of deformation (c) Poles to foliation and calculated fold axes in metasiltstone and metasandstone within thrust sheet at north end of map area. The distribution of throughout the unit, with intensity increasing southwards orientations is dissimilar from that along unit contacts elsewhere in the into the Cortaderas area. The majority of the carbonate Pozos area Fig. 6(b) but similar to that of the moderately deformed portions metasiltstone is moderately deformed and well foliated, of the carbonate metasiltstone Fig. 6(a). but there are distinct zones of greater and of lesser strain. The least deformed rocks in the unit are located within 15 m 5. Structural analysis of the large diabase sills in the Pozos area. There, con- centrically folded compositional layering with a weak The structural development of the Cortaderas and Pozos layer-parallel cleavage is preserved. Areas of moderate areas occurred primarily during two major deformation deformation are characterized by open to close, disharmonic events, one ductile and one brittle. Ductile features include folds with amplitudes in the order of 1 m and with a single tight to isoclinal folds, folded shear zones, and multiple penetrative foliation de®ning the folds (i.e. the foliation is generations of foliation. Shear zones, which are common folded). Throughout the Pozos area, the penetrative foliation along unit contacts, are identi®ed by a mylonitic texture strikes approximately NNE±SSW and dips shallowly to and planar geometry. They are discussed later in the context steeply in both directions around a calculated fold axis of in which they appear in the ®eld. In outcrop and in thin 1948/158 (Fig. 6(a)). Data from the Cortaderas area are section, all ductile structures are consistent with a single similar (Fig. 7(a)). Zones of high degrees of deformation progressive ductile deformational event. Brittle features characteristically feature a highly planar fabric, isoclinal include reverse and thrust faults, normal faults, and open folds with a well-developed axial planar cleavage parallel to tight folds. These brittle structures everywhere cross-cut to the fold limbs, and compositional banding transposed or reactivate ductile structures. Where brittle thrust parallel to the foliation. The highest strain fabrics (Fig. fault orientations are discernable, they are uniformly east- 5(c)) typically are present at the contacts with the ultra- dipping. The map pattern is dominated by north±south ma®c/layered gabbro complex, massive diabase, and meta- trending features, the result of east±west directed shortening sandstone, but are also found away from unit contacts. during both contractional deformations. 5.1.3. Contact between metasandstone and carbonate 5.1. Ductile structures metasiltstone The contact between the metasandstone and the carbonate 5.1.1. Metasandstone metasiltstone is a distinct lithologic break, marked by a The metasandstone was variably affected by ductile sharp transition from bedded sandstone to compositionally deformation, increasing in intensity to the south. Areas of banded siltstone. In the Pozos area, much, but not all, of the 830 C. Gerbi et al. / Journal of South American Earth Sciences 14 (2002) 821±835 carbonate metasiltstone near the contact is highly deformed, massive diabase, and their contacts, are consistent with having developed a planar structure and protomylonitic to deformation during emplacement at low greenschist facies mylonitic texture. Metasandstone near the contact only conditions. In the quartzo-feldspathic gneiss, quartz grains locally exhibits deformed clasts and well-foliated matrix commonly exhibit ribbon textures around porphyroblasts characteristic of highly deformed zones. The ductile contact (Fig. 5(b)), and in the layered gabbro and massive diabase between the metasedimentary units is folded along its length near the contact with the carbonate metasiltstone, both into meter-scale open to close folds (Fig. 5(d)) and in places deformed and undeformed calcite veins are present. Orien- is overprinted by brittle thrust faults. In the Cortaderas area tation of foliation in the tight to isoclinally folded fabric the contact is partially intruded by a pluton but is preserved within the ductile shear zones, which lie almost exclusively as a brittle thrust fault along much of its length. within the metasiltstone, is oblique to the regional foliation Data from the carbonate metasiltstone±metasandstone in the metasedimentary rocks, and instead of being cut by contact indicate that the two units were folded as one struc- the shear zones, the regional foliation curves into them. tural package during the ductile deformation (Fig. 6(b)). Additionally, the low-greenschist metamorphic grade of Foliation in both units parallels the contact along asym- the ductile shear zone foliation is equivalent to that of the metric, close to tight, meter- to ten-meter-scale folds. regional foliation. Parasitic folds within these larger folds contain hinge lines oriented subparallel to the megascopic north±south strike of 5.2. Brittle structures the contact (Fig. 6(c)), and axial planar cleavage developed in the hinges of the folds within the carbonate metasiltstone Whereas most ductile features vary between units is coplanar with the spaced axial planar cleavage in the and between contacts, brittle features in the Pozos and metasandstone. Poles to carbonate metasiltstone and meta- Cortaderas areas cut unit contacts and are not lithology sandstone foliation measured along the contact de®ne dependent. The three major styles of brittle deformation girdles that re¯ect tight folds with axes of 1688/438 and include thrust faults, normal faults, and minor subvertical 1558/498, respectively (Fig. 6b). Two individual meter- strike-slip faults. scale folds are tightly folded about similar axes of 1698/ West-vergent thrust faults crop out throughout the map 428 and 1658/638 (folds a and b, respectively, in Fig. area. Some of these thrust faults clearly reactivated ductile 6(d)), indicating that the small-scale features represent the shear zones. These reactivations of indeterminate but nature of the contact as a whole. small (meters or less) displacement are common along the contacts between carbonate metasiltstone and meta- 5.1.4. Ductile shear zones sandstone as well as between carbonate metasiltstone and The contacts between the carbonate metasiltstone and the the crystalline rocks. ultrama®c/layered gabbro complex and massive diabase are At the northern end of the map area, a major thrust sheet narrow ductile shear zones, de®ned by an isoclinally folded, has been emplaced over carbonate metasiltstone (Fig. 3). planar, mylonitic fabric in the metasiltstone, locally reacti- The thrust sheet comprises carbonate metasiltstone, meta- vated as brittle faults. In general, the deformational fabrics sandstone, diabase sills, and Miocene intrusive rocks. In the of the massive diabase and the ultrama®c/layered gabbro metasandstone unit, a strong foliation in the ®ne-grained complex near their contacts does not differ greatly from portions of the unit is oblique to bedding and strikes north- the fabric within the units as described earlier. A few east with moderate southeast dips (Fig. 7(c)). No highly localities exist, however, where a weakfoliation has deformed portions exist in the thrust sheet that correspond developed in the massive diabase or in the ultrama®c/ to those of the metasandstone unit elsewhere in the map layered gabbro complex subparallel to the contact with the area. Foliation in the carbonate metasiltstone within the metasiltstone. Where the contacts are not subvertical, the thrust sheet is relatively planar and not tightly folded. All carbonate metasiltstone unit forms the footwall. Away rocks of the thrust sheet, including the Miocene intrusive from the contacts, the metasiltstone commonly exhibits rocks, have been folded openly on a scale of tens of meters the characteristics of moderately deformed portions of the about an axis plunging shallowly to the south-southwest unit, with open to close folds and an anastamosing foliation. (Fig. 6(d)). Foliation is folded, and tight to isoclinal folds In some locations along the contact between the carbo- are absent. The style and orientation of folding within the nate metasiltstone and the ma®c bodies, both outcrop and metasedimentary units of the thrust sheet are remarkably thin section-scale kinematic indicators strongly indicate similar to those of the internal, lower strain portions of the top-to-the-east emplacement of the ma®c bodies over metasedimentary rocks in the footwall (compare Figs. 6(b), the carbonate metasiltstone (Fig. 5(c)). However, in other 7(a) and (c)) yet signi®cantly different from the folding style localities along the contact the kinematics are indeter- and orientation along the contact between the metasedi- minate, either because of the lackof kinematic indicators mentary rocks in the footwall (compare to Fig. 6(b)). Brittle or because kinematic indicators yield con¯icting senses of tear faults cut the complete thrust sheet, including the shear. Miocene intrusive rocks, in at least two places and do not Features of the ultrama®c/layered gabbro complex and appear to extend into the footwall below the thrust sheet. In C. Gerbi et al. / Journal of South American Earth Sciences 14 (2002) 821±835 831 the Cortaderas area, several normal faults cut ductile where the Alojamiento Formation overlies the equivalent structures. These normal faults are all brittle, with north to of the metasandstone unit. Additionally, Stappenbeck northwest strikes and steep dips. Two normal faults in the (1910) described lithologies similar to the carbonate meta- southwest Cortaderas area de®ne a graben in which Carbo- siltstone in Quebrada Santa Clara, which lies to the north of niferous to Triassic sedimentary rocks are preserved (Fig. 3) the Pozos area. The tentative correlations between these two (Davis et al., 1999). areas and the metasiltstone in our study area are compatible Although ductile features dominate throughout the map with the fossil age reported by Davis et al. (1999) suggesting area, subvertical brittle faults, too small to map, are that the carbonate metasiltstone formed in the Early to common. These faults uniformly cut foliation, zones of Middle Ordovician. varying degrees of deformation, and contacts. Offset along As well as providing a depositional environment for the these brittle faults is dif®cult to estimate, but the lackof metasandstone, the west-facing slope of the Precordillera continuity of these faults suggests that total offset could likely also provided the depositional environment for the not exceed a few meters. carbonate metasiltstone. In the Ordovician, the Precordil- leran continental margin was an uneven surface cut by a signi®cant number of normal faults and horst and graben 6. Discussion structures (Keller, 1995). A site conducive to local carbonate-rich shale development in proximity to a turbidite 6.1. Origins of units sequence may have existed within this irregular continental slope environment. Burial or possibly middle Paleozoic Metasedimentary rocks crop out extensively along the thrust loading metamorphosed both metasedimentary units western margin of the Precordillera. Many of these sedi- to low-greenschist facies. mentary rocks are turbiditic (Keller, 1995; Spalletti et al., In contrast to the metasedimentary rocks, which formed 1989), and their composition is similar to that of the on the western margin of the Precordillera, the igneous and metasandstone unit in the Pozos and Cortaderas areas. In high-grade metamorphic rocks formed in disparate tectonic previous studies, the metasandstone of the Pozos and settings and have traveled far to their present locations. Cortaderas areas have been correlated with the nearby Davis (1997) proposed that the massive diabase formed at Alcaparrosa (Haller and Ramos, 1984; Ramos et al., 1986) an oceanic spreading center as the Precordillera separated and Villavicencio (Harrington, 1971) Formations; the from Laurentia and that the ultrama®c/layered gabbro Portezuelo del Tontal Formation (Cuerda et al., 1985; complex represents underplated continental crust, forming Spalletti et al., 1989) located to the north and east of the at approximately 30 km depth. The ultrama®c/layered Pozos area is another candidate for correlation with the gabbro complex may have originated in a suprasubduction metasandstone unit. These three formations have each zone setting at the base of Chilenia, the terrane West of the been dated as Middle to/or Late Ordovician (Cuerda et al., Precordillera (Davis et al., 1999, 2000), or at the base of the 1985, 1987, 1988; Cuerda, 1988). Without more detailed Precordillera (Gerbi, 1999) during Ordovician extension mapping to the north and east we cannot test the proposed (Keller et al., 1998). Regardless of its exact origin, because correlations, but the lithologic similarity and geographic the metasedimentary rocks on which the ultrama®c/layered proximity of the metasandstone unit to the several similar gabbro complex thrust did not experience greater than formations suggests that the metasandstone was deposited in greenschist facies conditions, the complex must have been an analogous setting. Stratigraphic models of the develop- exhumed from granulite to greenschist facies conditions ment of the Villavicencio, Portezuelo del Tontal, and prior to the Devonian deformation. Alcaparrosa Formations place them all in a submarine fan environment (Cuerda et al., 1985; Spalletti et al., 1989) on a 6.2. Nature of unit contacts continental slope (Keller, 1995). Thus we interpret the protolith of the metasandstone unit as a sequence of tur- The contacts between the carbonate metasiltstone and bidites deposited along or at the base of the westward-facing both the massive diabase and the ultrama®c/layered gabbro continental slope of the Precordillera, most likely in the complex exhibit strongly deformed, planar, syn-meta- Middle or Late Ordovician. morphic foliation parallel to the curviplanar contacts. No published workproposes any correlation between the These structural features suggest that, despite local brittle carbonate metasiltstone and any other nearby formation. faulting, the contacts are fundamentally ductile shear zones. Sedimentological similarities exist, however, between the Further evidence that signi®cant displacement caused the carbonate metasiltstone and parts of the Villavicencio and juxtaposition of these units is that the contact between Alcaparrosa Formations (Cingolani et al., 1987). Harrington the ultrama®c/layered gabbro complex and the carbonate (1971) and Cucchi (1972), who mapped in the Cortaderas metasiltstone juxtaposes granulite facies rocks with a low- and Pozos areas and eastward, correlated the calcareous greenschist overprint against low-greenschist facies meta- portions of the carbonate metasiltstone with the Aloja- sedimentary rocks, and the contact between the massive miento Formation to the east and describes a contact diabase and the carbonate metasiltstone juxtaposes 832 C. Gerbi et al. / Journal of South American Earth Sciences 14 (2002) 821±835 Proterozoic ma®c rocks (Davis et al., 2000) against meta- within the carbonate metasiltstone at the contact and the sedimentary rocks of probable Ordovician depositional age probability that the carbonate metasiltstone is older, yet (Davis et al., 1999). Where brittle faults are present between structurally higher, than the metasandstone indicate that the ultrama®c/layered gabbro complex and the meta- the contact must be structural rather than stratigraphic. sedimentary units, a strong ductile foliation in the carbonate Because the metasandstone and the carbonate metasiltstone metasiltstone near, and in places cut by, the faults indicate likely formed in adjacent depositional settings, their struc- that ductile deformation had occurred prior to brittle tural contact may not represent a shear zone of major faulting. These interpretations are consistent with those of tectonic signi®cance. Because both the metasandstone and Davis et al. (1999), who found that the ultrama®c/layered the metasiltstone exhibit the same fold geometry (Fig. 6), gabbro complex and the massive diabase are in similar tight folding of the contact between the metasedimentary top-to-the-east ductile shear zone contact, on originally units must postdate the juxtaposition of the metasedi- west-dipping planes, with the underlying metasedimentary mentary units. rocks. Davis et al. (1999) based their kinematic interpretation on an analysis of shear sense indicators within the 6.3. Number of deformational episodes ductile shear zones. The similar metamorphic and structural histories of the ductile shear zones beneath the ultrama®c/ One penetrative low-temperature foliation is developed layered complex and the massive diabase, including one throughout the entire Pozos and Cortaderas areas in the phase of deformation forming a mylonite with foliation metasedimentary units. This foliation within the metasedi- parallel to the unit contacts and no evidence of meta- mentary units, as well as that developed in the ductile shear morphism higher than greenschist facies, indicate that zones, predominantly strikes north-northwest to north- emplacement of both units occurred during a single event. northeast and dips steeply. In the hinge zones of many Further support for a top-to-the-east sense of shear is folds, the foliation is folded tightly to isoclinally and cut presented immediately below. We found no evidence of by a spaced axial plane cleavage parallel to the limbs of the west-vergent ductile features noted by Ramos et al. (1986) fold; in isoclinal folds the axial planar cleavage and the and von Gosen (1995). foliation are nearly indistinguishable. The similarity of After the Precordillera docked with Gondwana, an ocean the distribution and orientation of the axial planar cleavage basin existed to the west of the terrane (e.g. Ramos, 1988). and the penetrative foliation, as well as the occurrence of Deposition into this ocean, along a passive margin, formed neocrystalline white mica within both the penetrative folia- the sedimentary rocks of the Western Precordillera (e.g. tion and the axial plane foliations suggests that the two Keller, 1995). Concurrently, central and eastern portions developed during successive generations of a single defor- of the terrane were undergoing uplift, possibly as a fore- mational period (i.e. progressive deformation) rather than bulge, to a subaerial setting (Astini et al., 1995), and the during two distinct periods of deformation. eastern side of the Precordillera was already welded to In the upper plate of the large thrust sheet at the northern the Sierras Pampeanas. Moreover, no location exists within end of the map area, the close to open folds of the unmeta- the Precordillera where the massive diabase and ultrama®c/ morphosed igneous dikes and metasedimentary rock folia- layered gabbro complex could have formed, and, no tion (Fig. 7(c)) are similar in style and orientation to the evidence exists that the massive diabase and ultrama®c/ folds observed in the internal portions of the carbonate layered gabbro complex could have been transported from metasiltstone (Fig. 6(a)). This style of folding is, however, east of the terrane across the relatively undeformed carbo- distinct from that of the folds along unit contacts in the nate platform. Instead, these two allocthonous units must remainder of the Pozos area (Fig. 6(b)). Such a discrepancy have arrived on the Precordillera margin from the west. in fold style and orientation requires two different deforma- Top-to-the-east motion is thus the only mechanism by tional events: in the ®rst, penetrative foliation developed in which the massive diabase and the ultrama®c/layered all areas and the unit contacts (particularly the metasand- gabbro complex could have been thrust upon the passive stone±carbonate metasiltstone contact) were folded; in the margin sediments. This geographical argument corroborates second deformation the thrust sheet and the internal portions the kinematic data of Davis et al. (1999). of the metasedimentary rocks were warped or broadly Although some of the structural features are ambiguous, folded. The folded carbonate metasiltstone±metasandstone the data from the contact between the carbonate meta- contact was probably reoriented at this time. No additional siltstone and the metasandstone are consistent with minor foliation developed during this second folding event. displacement having occurred across the interface. Most brittle features are consistent with a single major Ambiguous features, consistent with little or no displace- contractional event that postdated all ductile deformation. ment, include a lackof ubiquitous high degree of defor- The only brittle features that we can con®dently assign to an mation along the contact, the presence of compositional extensional regime are associated with the normal faults on layering parallel to the contact, the tightly folded nature of either side of the sole exposure of unmetamorphosed sedi- the contact, and the lackof a breakin metamorphic grade mentary rocks, located in the southwestern corner of the across the contact. The presence of highly deformed zones Cortaderas area. Because the thrust fault under the large C. Gerbi et al. / Journal of South American Earth Sciences 14 (2002) 821±835 833 thrust sheet at the northern end of the map truncates 1995). No direct evidence for the age of deformation and bedding, and because no ductile features cut brittle features, metamorphism exists for rocks of the study area. Nearby in all brittle deformation must have occurred after all ductile the Bonilla area, ductile deformation occurred during deformation. The open folding of the thrust sheet and its Middle Devonian time, as evidenced by a 385 Ma Miocene intrusive rocks must have occurred after the intru- 40Ar/39Ar date of neocrystalline white mica in a shear sion of the intermediate dikes. The open folding most likely zone in the Bonilla area (Davis et al., 1999). Assuming occurred during the time of brittle faulting that deformed that the deformations in the Bonilla area and the study intermediate igneous intrusions in the Cortaderas. Thus, the area were broadly synchronous, this date provides an age distribution of foliation orientations in the thrust sheet and in for deformation in the Pozos and Cortaderas areas as well. the internal portions of the metasedimentary rocks (Figs. Undeformed Carboniferous to Triassic sedimentary rocks 6(a) and 7(c)) re¯ects the deformational event that occurred that overlie the deformed early Paleozoic rocks provides during the time of brittle deformation. a complementary minimum age constraint. Because the The rocks in the brittle thrust sheet are less deformed and ductile deformation in the Pozos and Cortaderas areas of a lower metamorphic grade than the rocks underlying the must have postdated the intrusion of the ma®c sills into thrust sheet. The logical origin of the thrust sheet is from the the carbonate metasiltstone at 418 Ma, and was contem- east, where the regional metamorphic grade is lower (Keller poraneous with a single low-temperature metamorphic et al., 1993). This observation, coupled with the east- event, we interpret the age of the major ductile deformations dipping minor brittle thrust faults observed throughout the in this region as Middle Devonian. map area, strongly suggests that the brittle deformation was At least three major brittle deformational periods have west- directed in this area. affected the Precordillera (Ramos, 1988; von Gosen, Structural evidence from the Pozos area requires at least 1995), but because weakstratigraphic control exists in the four periods of deformation. The earliest deformation is Cortaderas and Pozos areas only the timing of the single con®ned to the ultrama®c/layered gabbro complex and major brittle contractional deformation is well constrained. was a high-temperature event in which the gneissic fabric Brittle top-to-the-west thrust faults cut dated Miocene intru- developed and in which the clinopyroxene crystals sive bodies in the Cortaderas (Davis et al., 1999). Because deformed. In addition to the bent lamellae described earlier, we can correlate the unmetamorphosed andesitic and Davis et al. (1999) describe lozenge-shaped clinopyroxene monzodioritic igneous dikes intruding the metasedimentary crystals with tails and interpret those features as evidence of rocks of the thrust sheet in the northern map area with the high-temperature solid-state deformation. This event must dated Miocene intrusive bodies, the contractional brittle have occurred before or during the exhumation of the deformation, which folded and truncated the dikes, must complex and prior to its juxtaposition against the metasedi- be Miocene or younger. These observations suggest that mentary rocks (Davis et al., 1999). During the second defor- the majority of the brittle deformation occurred during the mation, which affected all the pre-Carboniferous rocks in late Tertiary Andean orogeny. the map area, foliation in the metasedimentary rocks formed and was folded. Additionally, a minor ductile shear zone 6.5. Regional implications developed in which the metasiltstone was thrust over the metasandstone. Also at this time, the ultrama®c/layered Based on the fact that stratigraphic and sedimentary gabbro complex and massive diabase were thrust over the features of the Cortaderas and Pozos areas are extremely metasedimentary rocks along separate ductile shear zones. similar to those described along strike in the Sierra del During the third deformation, regional extension produced Tontal to the north (Cuerda et al., 1985; Spaletti, et al., minor normal faulting. With the fourth deformation, brittle 1989) and to the Bonilla area to the south (Davis et al., faults reactivated ductile shear zones, minor brittle faults 1999), we suggest that interpretations derived from data in developed, and rocks within the thrust sheet at the northern the Cortaderas and Pozos areas are likely applicable to the end of the map area were folded and thrust over the other western margin of the Precordillera as a whole. Some units in the Pozos area. authors report structural data that differ from that presented here (e.g. von Gosen, 1992, 1995), but their investigations 6.4. Ages of deformational episodes tookplace away from the line of ma®c and ultrama®c bodies that are critical to any interpretation of the western margin In structural studies in the Cortaderas, Bonilla, and Rio of the terrane. San Juan areas, as well as in other areas in the southwest This investigation enhances our understanding of the Precordillera, many workers have identi®ed several periods timing and nature of the deformation of the western margin of regional deformation. These include Ordovician, of the Precordillera terrane, but it does not suggest any Silurian, Devonian, and Permian contraction, Mesozoic driving force for the deformation. Most studies assume the extension, and Tertiary contraction (Astini, 1996b; presence of the outboard Chilenia terrane that collided with Buggisch et al., 1994; Cucchi, 1971; Davis et al., 1999; the Precordillera to drive the deformation (e.g. Ramos, Ramos, 1988; Ramos et al., 1986; von Gosen, 1992, 1988; Astini et al., 1995; Davis et al., 1999), but no evidence 834 C. Gerbi et al. / Journal of South American Earth Sciences 14 (2002) 821±835 in the study area requires the presence of an outboard National Science Foundation (grant EAR-9614826 to SMR terrane. Attempted subduction of the Precordillera margin and E.M. Moores). CG was supported by the Geology under oceanic crust (rather than under Chilenia) in a west- Department of the University of California, Davis, and dipping subduction zone would provide enough force to by a National Science Foundation Graduate Research drive the deformation. Unfortunately, the best evidence Fellowship. While undertaking the ®eldwork in Argentina, for understanding the causes of the Devonian deformation we received valuable logistical help and ®eld assistance lies buried under the volcanic rocks of the high Andes. from Victor Ramos, Patricio Figueredo, Matias Giglione, and Fabian Suarez, all of the Universidad de Buenos Aires. E.M. Moores and R.J. Twiss provided critical advice 7. Conclusion and assistance during this project. We thankRichard A. Allmendinger for use of his stereonet plotting program. The Cortaderas and Pozos areas, located in the southwest Reviews by Daniel Gregori and an anonymous reviewer portion of the Precordillera terrane, contain valuable signi®cantly improved this manuscript. This is a contri- evidence that provides constraints on the development and bution to International Correlation Project 376. tectonic history of the western margin of the Precordillera terrane. The rocks that formed in the Cortaderas and Pozos areas during the early Paleozoic include carbonate meta- References siltstone of probable early Middle Ordovician age, metasandstone of probable Middle to Late Ordovician age, a Abbruzzi, J., Kay, S.M., Bickford, M.E., 1993. Implications for the nature quartzo-feldspathic gneiss intruded by ultrama®c rocks of the Precordilleran basement from Precambrian xenoliths in Miocene and Ordovician layered gabbro, and late Proterozoic volcanic rocks, San Juan province, Argentina. XII Congreso Geolgico massive diabase. Silurian ma®c sills intrude the metasedi- Argentino y II Congreso de Exploracion de Hidrocarburos, Actas III, pp. 331±339. mentary rocks. Both the metasandstone and the carbonate Allmendinger, R.W., Figueroa, D., Snyder, D., Beer, J., Mpodozis, C., metasiltstone formed in related environments along the Isacks, B.L., 1990. Foreland shortening and crustal balancing in the west-facing passive margin of the Precordillera terrane Andes at 308S latitude. Tectonics 9, 789±809. that existed from Early Cambrian through Silurian time. Astini, R.A., 1988. Yerba Loca Formation, an Ordovician clastic wedge in Correlative formations whose sedimentological histories Western Argentina. Fifth International Symposium on the Ordovician System, Abstracts 3. are well known crop out to the north. The ultrama®c/layered Astini, R.A., 1996a. The Argentine Precordillera-lower plate of the gabbro complex formed at the base of continental crust Ouachita conjugate-pair rift; a suitable explanation for their strati- where ultrama®c cumulates and layered gabbro intruded graphic differences. Abstracts with ProgramsÐGeological Society of deformed gneiss. The massive diabase may have formed America 2. at an ocean spreading center or in another rift-related Astini, R.A., 1996b. Las fases diastro®cas del Paleozoico medio en la Precordillera del oeste Argentino-evidencias estratigra®cas. XIII environment. Congreso GeoloÂgico Argentino y III Congreso de ExploracioÂnde Two primary contractional deformation events have Hidrocarburos V, pp. 509±526. affected the rocks exposed in the Cortaderas and Pozos Astini, R.A., Benedetto, J.L., Vaccari, N.E., 1995. The early Paleozoic area. The earliest regional deformational period was ductile evolution of the Argentine Precordillera as a Laurentian rifted, drifted, and affected all units in the area present at the time. During and collided terrane; a geodynamic model. Geological Society of America Bulletin 107, 253±273. this deformation, which occurred in the Middle Devonian, Ave Lallement, G., 1892. Observaciones sobre el mapa del Departamento the metasedimentary units developed a penetrative foliation de Las Heras: Anales del Museo de la Plata. Seccion Geologica y and were juxtaposed against each other along a ductile shear Mineria I, 5±20. zone. Their contact was then tightly folded. Subsequent to Buggisch, W., von Gosen, W., Henjes-Kunst, F., Krumm, S., 1994. The age the juxtaposition of the metasedimentary rocks, but during of Early Paleozoic deformation and metamorphism in the Argentine Precordillera; evidence from K±Ar data. Zentralblatt fuer Geologie the same deformational event, the massive diabase and the und Palaeontologie I, 275±286. ultrama®c/layered gabbro complex were thrust on ductile Caminos, R., Nullo, F.E., Panza, J.L., Ramos, V.A., 1993. Mapa Geologico shear zones over the carbonate metasiltstone. All four de la Provincia de Mendoza. Secretaria de Mineria, Direccion Nacional rockunits exhibit evidence of a low-greenschist facies del Servicio Geologico, Buenos Aires, Argentina. metamorphism associated with the pervasive ductile defor- Cingolani, C., Varela, R., Cuerda, A.J., Schauer, O., 1987. Estratigra®a y estructura de la Sierra del Tontal, Precordillera de San Juan, Argentina. mation. Later brittle deformation associated with late Decimo congreso geologico argentino 3, pp. 95±98. Tertiary uplift of the Andes reactivated some ductile struc- Cosentino, J.M., 1968. ContribucioÂn al conocimiento geoloÂgico del CordoÂn tures and modi®ed some unit contacts but did not change de Bonilla (UspallataÐMendoza). AsociacioÂn GeoloÂgica Argentina, the fundamental distribution of units resulting from the Revista 23, 21±31. Paleozoic deformation. Cucchi, R.J., 1971. Edades radimetricas y correlacion de metamor®tas de la Precordillera, San Juan±Mendoza, Rep. Argentina. Revista de la Asociacion Geologica Argentina 26, 503±515. Acknowledgements Cucchi, R.J., 1972. Geologia y estructura de la sierra de Cortaderas, San Juan±Mendoza, Republica Argentina. Revista de la Asociacion Argentina 27, 503±515. Funding for ®eldworkand research was provided by the Cuerda, A.J., 1988. Investigaciones estratigra®cas en el Grupo Villavicencio, C. Gerbi et al. / Journal of South American Earth Sciences 14 (2002) 821±835 835

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