Geological setting of the Argentine Frontal Cordillera in the flat-slab segment (30°00′–31°30′S latitude)

Journal of South American Earth Sciences 15 (2002) 79±99

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Geological setting of the Argentine Frontal Cordillera in the ¯at-slab segment (30800 0 ±31830 0S latitude) N. Heredia a,*, L.R. RodrõÂguez FernaÂndez b, G. Gallastegui a, P. Busquets c, F. Colombo c a

Instituto GeoloÂgico y Minero de EspanÄa, Avda. RepuÂblica Argentina 30, 1B, 24004 LeoÂn, Spain b Instituto GeoloÂgico y Minero de EspanÄa, RõÂos Rosas 23, 28003 Madrid, Spain c Facultat de GeologõÂa, Universitat de Barcelona, MartõÂ i FranqueÂs s/n Barcelona, Spain Received 1 November 2001; accepted 1 November 2001

Abstract The Argentine Frontal Cordillera, between latitudes 30820 0 and 31820 0 S, presents an eastward-verging polycyclic structure. Two large groups of rocks with markedly different structural nature can be distinguished: a Paleozoic basement with characteristic thin-skinned structure and the Andean cover showing remarkable extensional Mesozoic structures inverted by Tertiary compressional tectonic events. These groups are divided by a large tectonic unconformity separating not only rocks with different style and degree of deformation but also different paleogeographic settings. The Cenozoic tectonic inversion is linked with the subhorizontal subduction of the Nazca Plate in the Pampean ¯at-slab segment and subsequent uplift of the Andean Cordillera. The Paleozoic basement is composed of Devonian and Permo-Carboniferous marine sedimentary rocks, separated by an angular unconformity. These rocks were deposited in siliciclastic and mixed sedimentary platforms, respectively, and intruded by Permo-Triassic granitoid rocks. The Devonian rocks were deformed by the Famatinian orogeny in Late Devonian times. The most important Paleozoic structures are related to the Gondwanic orogeny, of Permian age. These include east-vergent thrusts and related folds that record a large shortening component. The Andean cover, lying unconformably over the Paleozoic basement, is primarily volcanic and volcanoclastic, with some interbedded continental sedimentary rocks. Two main stratigraphic groups can also be de®ned here: an older sequence linked to an extensional tectonic event and a synorogenic sequence connected to a compressional tectonic event that produced an inversion of previous extensional features. The lower unit (Choiyoi Group: Permian and Triassic) is affected by normal faults with downthrown western blocks and intruded by Jurassic granodioritic rocks. The faults in this unit involve the Gondwanic basement in a typical thick-skinned tectonic style and are grouped in bands with N±S strike. The uppermost units (Vizcachas, Melchor, and Olivares groups: Tertiary and Plio-Quaternary) also lie unconformably over the latter. The normal faults were inverted in the upper Miocene by the uplift of the western block, deforming the lower and upper units in a compressional context during the Andean orogenic cycle. Displacement on reverse faults, measured in the Tertiary synorogenic rocks, is usually less than 1 km. The Tertiary crustal shortening calculated from cross-sections is about 10%, in contrast with the estimated 50% shortening of the Precordillera unit. All this shows that most of the crustal shortening of the Andean Cordillera during the compressional stage has been transferred to the Precordillera unit through the lower detachment fault. These facts also show that the Cordillera Frontal unit is an uplifted block in which the extensional structures have been preserved, and that the Rodeo±Upsallata basin is a piggy-back type. q 2002 Published by Elsevier Science Ltd. Keywords: Frontal Cordillera; Andean orogeny; Gondwanic orogeny; Inversion tectonic; Andean igneous rocks

Resumen El aÂrea investigada en este trabajo abarca un sector de la unidad morfoestructural conocida como Cordillera Frontal, si bien el extremo oriental de la misma comprende parte de la DepresioÂn de Rodeo±Upsallata. En la Cordillera Frontal se diferencian dos grandes conjuntos de rocas con naturaleza y estructuracioÂn diferentes: un substrato Paleozoico y una cobertera Mesozoico terciaria. El substrato Paleozoico esta constituido por rocas sedimentarias depositadas en ambientes marinos o de transicioÂn, deformadas en el DevoÂnico superior y el PeÂrmico inferior durante los Ciclos OrogeÂnicos Famatiniano y GondwaÂnico, e intruidas por rocas granõÂticas permotriaÂsicas. Las rocas relacionadas con el Ciclo OrogeÂnico Famatiniano tienen una edad devoÂnica y aparecen cubiertas en discordancia

* Corresponding author. Tel.: 134-98-726-2171; fax: 134-98-726-2183. E-mail address: [email protected] (N. Heredia). 0895-9811/02/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S 0895-981 1(02)00007-X

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angular por los depoÂsitos del Ciclo OrogeÂnico GondwaÂnico, constituidos por la FormacioÂn Agua Negra (CarbonõÂfero superior±PeÂrmico inferior). Dentro de la FormacioÂn Agua Negra se ha separado una sucesion preorogeÂnica, predominantemente siliciclaÂstica, y otra sinorogeÂnica (PeÂrmico inferior) en la que aparecen numerosas intercalaciones de calizas estromatolõÂticas y rocas volcaÂnicas; ambas sucesiones aparecen separadas por una discordancia de orden menor. Las principales estructuras relacionadas con la Orogenia GondwaÂnica son sistemas de cabalgamientos y pliegues relacionados, dando lugar a un acortamiento que oscila entre el 60±70%. Sobre el substrato Paleozoico se apoya discordantemente una cobertera Mesozoico±Cenozoica caracterizada por un predominio de rocas volcaÂnicas, con rocas sedimentarias subordinadas, depositadas en ambientes continentales, que aparecen intruidas por granitoides Mesozoicos y Cenozoicos. Dentro de este conjunto superior se pueden diferenciar dos secuencias estratigra®cas originadas en contextos tectoÂnicos distintos y separadas por discordancias de diversa magnitud: una inferior compuesta por el Grupo Choiyoi, ligada a un proceso extensional, y otra superior ligada a un evento compresional posterior. Todas las rocas se encuentran deformadas durante el Ciclo OrogeÂnico Andino, por fallas de direccioÂn N± S, con geometrõÂa lõÂstrica, enraizadas en un nivel de despegue comuÂn. La mayor parte de las fracturas presentan un juego normal, relacionado con un evento extensional Mesozoico, encontraÂndose soÂlo parcialmente invertidas por la compresioÂn Cenozoica andina. La mayor parte de la compresioÂn Cenozoica aparece ligada a la subduccioÂn subhorizontal de la placa de Nazca, a lo largo del segmento Pampeano. El acortamiento cortical calculado en la Cordillera Frontal es del orden del 10%, en contraste con el acortamiento de la Precordillera que se situÂa en el 50%. Este hecho implica que la mayor parte del acortamiento cortical Cenozoico de la Cordillera Frontal, se ha transferido hacia la Precordillera a traveÂs de un cabalgamiento basal. La Cordillera Frontal constituye un bloque levantado y trasladado pasivamente hacia el este, en el que se ha preservado la estructura extensional Mesozoica y gran parte de la estructuracioÂn gondwaÂnica anterior. La depresioÂn de Rodeo-Calingasta constituye una cuenca transportada (piggy-back basin). q 2002 Published by Elsevier Science Ltd. Palabras clave: Cordillera Frontal; Orogenia GondwaÂnica; Orogenia Andina; InversioÂn tectoÂnica; Rocas Âõgneas andinas

1. Introduction In recent papers, different tectonic processes are invoked to explain the tectonic evolution of the Andean Cordillera between 30800 0 and 31830 0 S (Fig. 1). These include collision of exotic terranes or oceanic ridges, plate-motion variations, segmentation of the subducted oceanic (Nazca) plate with several sectors of the Andes recording subhorizontal subduction, and the presence of oblique or normal subduction. However, due to insuf®cient ®eld constraints, geological maps and balanced cross-sections are often not available, and therefore most of these papers lack geometric and kinematic analysis.

The aims of this paper are then to de®ne representative main stratigraphic units, to discuss how they relate genetically to the proposed tectonic events, and to describe some of the geometric and kinematic features of the structures that are connected to them. Finally, the amount of approximate shortening that derives from each tectonic event is evaluated and interpreted considering the data available from nearby areas. The study area is located within the morphostructural unit known as Cordillera Frontal (Groeber, 1938), although part of the Rodeo±Upsallata basin (Keidel, 1921) is also covered here (Fig. 1). The Rodeo±Upsallata basin, which divides the Cordillera Frontal and the Precordillera (Figs. 1 and 2), is

Fig. 1. Major geological provinces of the Central Andes with the location of the study area.

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Fig. 2. Geological map of the study area. Modi®ed after Heredia et al. (1996), Espina et al. (1998), Cardo et al. (2000), Ragona et al. (1995) and RodrõÂguez FernaÂndez et al. (1996a). Legend and symbols in Fig. 3.

essentially ®lled by Tertiary and thick clastic Quaternary deposits linked to alluvial fans; these are less deformed than Frontal Cordillera and Precordillera sectors. In this sector, the Cordillera Frontal presents a polycyclic structure verging eastwards. Due to variation in the structural style, rocks and tectonic structures from three different orogenic cycles can be differentiated in the study area (Figs. 2±4).

In sum, a Paleozoic basement, constituted by sedimentary rocks, was strongly deformed during the Famatinian and Gondwanic orogenic cycles and is intruded by upper Paleozoic granitoid rocks. An Andean cover lies unconformably over the Paleozoic basement, and is comprised of PermoTriassic and Tertiary volcanic and volcanoclastic rocks and intruded by Mesozoic and Tertiary granitoids. This cover

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Fig. 3. Legend and symbols of the geological map (Fig. 2).

was deformed in Cenozoic (Miocene) times, during the Andean orogenic cycle. The main Andean orogeny uplift is linked with the Nazca Plate subhorizontal subduction, located between 278 and 33830 0 S latitude along the Pampean ¯at-slab segment (Gutscher et al., 2000). The absence of recent volcanism in the Pampean ¯at-slab segment allows the observation of the Paleozoic, Mesozoic, and Cenozoic structures. 2. The Paleozoic basement The oldest rocks in the area are of Devonian age (GutieÂrrez, 1983), and consist of an alternating sequence of sandstones, conglomerates, and lutites (Fig. 4). These rocks have been affected by the so-called Chanic phase of the Famatinian orogenic cycle (Ramos et al., 1984, 1986), during the Late Devonian. Several folds with chevron geometries and western vergence can be recognized in the Agua Negra valley (Fig. 5(f)). The presence of a strong unconformity between the Devonian and Carboniferous±Permian deposits (Figs. 5(f) and 6) of the Agua Negra Formation con®rm the presence of Chanic tectonic structures in this zone during this period. 2.1. The Gondwanic orogenic cycle The Gondwanic orogenic cycle spans from the Late Devonian to Early Permian, although in the study area, only preserved Late Carboniferous±Early Permian rocks are preserved. 2.1.1. Stratigraphy/sedimentology The Carboniferous±Permian rocks outcropping in the east border of the Cordillera Frontal (Fig. 2) can be correlated with Agua Negra Formation (Polanski, 1970) or La Puerta Formation (CaballeÂ, 1986). This series can be divided in two tectonostratigraphic units (Figs. 4 and 7) with different sedimentary characteristics separated by a clearly visible erosive surface marked with channels in®lled with sedimentary breccias and conglomerates. The two units

represent pre-orogenic and synorogenic successions related with a main tectonic event: the Gondwanic orogeny. The lower or pre-orogenic unit in the Quebrada de las  nimas section (Fig. 8) consists of lutites, sandstones, A conglomerates, and microconglomerates (with high-grade metamorphic and sedimentary pebbles), derived from a clastic shallow-marine platform environment with some storm deposition. This unit has an estimated total thickness in the Frontal Cordillera of around 2000 m, and its age is Late Carboniferous±Early Permian (Aparicio, 1969; Gonzalez, 1977). The upper limit of this succession is marked by an important erosive surface (Fig. 6), which can be very clearly observed at the head of the Quebrada Ä ipas section (Fig. 8). de las N The upper or synorogenic unit has been identi®ed in the Ä ipas quebrada, showing some volcanic rocks intercalated N with the sedimentary rocks. At the base of the unit (Fig. 7) polymictic orthoconglomerates with metric scale olistholiths often occur. Sandstones, lutites, and paraconglomerate, with volcanic and sedimentary pebbles, are found in the middle part, and limestones arranged as tidal plain cycles with abundant fossil ¯ora remains to appear at the top. The top of the synorogenic succession (Fig. 7), is mainly formed by limestones. The limestones correspond to microbioliths that form large interconnected stromatolithic domes. The stromatolithic limestones are organized in sequences of metric thickness (from 1 to 6 m) with paleosoils at the top, suggesting shallowing upward processes. This unit has an estimated total thickness of around 400 m, and its age is probably Permian due to its stratigraphic position between the lower unit of Agua Negra Formation and Choiyoi Group. The lower succession is characterized by shallow-marine sediments deposited during the late Paleozoic (Carboniferous to Early Permian). The source area appears to have been to the east (Precordillera ranges, Fig. 1), where the shallower and/or continental facies are located. The presence of high-grade metamorphic pebbles (gneissic and anphibolitic) suggests source areas located eastward of the Precordillera, probably in the Sierras Pampeanas (Fig. 1).

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Fig. 4. Synthetic lithostratigraphic succession of the study area and related tectonic events.

During this time the Frontal Cordillera was located in a retroarc position (Ramos, 1988). In this geotectonic setting, the most likely depositional environment would have been the propagation of the deltaic system from the Precordillera and Sierras Pampeanas to the Frontal Cordillera, where the environment would have evolved towards open-marine conditions.

The upper succession corresponds to distributary channels, delta lobes, and delta plain deposits, with less terrigenous content towards the upper part. The sequences were developed on top of terrigenous strata, which in some cases are volcanoclastic. A dramatic change of the main source area of sediments took place in the Frontal Cordillera during the deposition of

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Fig. 5. (a) Unconformity between CastanÄo Formation and Vega de los Machos Formation. Surroundings of CastanÄo Viejo mine. (b) Calcareous olistolithes (ol) of the Agua Negra Formation in the basal part of the CastanÄo Formation. (c) Gondwanic duplex with west vergence in the San Ignacio Formation, near the CastanÄo River (rt ˆ roof thrust and ft ˆ ¯oor thrust). (d) Assymetric folds related to a Gondwanic thrust of Fig. 9(a), involving limestones of the upper unit of the Agua Negra Formation. (e) Andine thrust (Cortadera fault of Fig. 4, II±II 0 ) involving the Melchor Group (hangingwall) and Olivares Group (footwall). (f) Chevron folds in Devonian sediments fossilized by the basal unconformity of the Agua Negra Formation in the quebrada Agua Negra.

the upper succession. At this time, the uplift of a volcanic arc and a Famatinian basement located in the west is shown by the presence of volcanic and sedimentary pebbles in the conglomerates from this provenance (or source origin). This event can be related to the Gondwanic orogenic uplift. Moreover, the existence of stromatolithic pebbles in the conglomerates at the top of succession (cycle 2, Fig. 7) represent the cannibalization of the earlier foreland sedimentary basin and the progradation of the deformation to the east. The most likely geotectonic

setting is for the Frontal Cordillera, a retroarc foreland basin at this time. 2.1.2. Structure The Gondwanic structures preserved in the study area were generated during the San Rafael compressive phase in the early Permian (Ramos, 1988). The structures resulting from the deformation are thin-skinned with very low-grade metamorphism and cleavage, and exist on numerous thrust levels, with folds related to thrust surface geometry (Figs. 6, 9 and 10).

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Fig. 6. Geological sketch map of the western border of the Frontal Cordillera in the area around the Chita and Bauchaceta valleys. See location in Fig. 2.

Thrust surfaces usually occupy favourable stratigraphic levels, such as the Permian limestones of the upper unit of the Agua Negra Formation (Fig. 9(a)). The main geometric structures at different scales (metre to kilometre) are imbricate fans or duplexes (Fig. 9(b)). Within the latter, some antiformal stacks can be recorded, especially metric to hectometric duplexes (Fig. 5(c)). Some different scales of folds seem to be related to the Gondwanic thrusts. These folds, facing east or southeast, have different geometric features, and asymmetric folds with interlimb angles lower than 708 and an axial plane

dipping 20±408W are the most common. Some symmetric folds, or folds with west vergences, appear in the eastern part of Frontal Cordillera (Fig. 11) around the Gondwanic orogenic front. This west vergence is probably related to the presence of Gondwanic backthrusts (Fig. 12, cross-section I±I 0 , and Fig. 5(c)). The folds are generally cylindrical, but sometimes non-cylindrical folds can also be found next to the thrust surfaces (Fig. 5(d)). Fault-bend folds related with hangingwall ramps and minor antiformal stacks have also been recorded at small and medium scale. The observation of different kinematic criteria shows in

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Fig. 7. Stratigraphic succession of the Agua Negra Formation in the CastanÄo River surroundings.

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Fig. 8. Geological sketch map with position of the Agua Negra stratigraphic Ä I ˆ Quebrada de las sections: An ˆ Quebrada de las Animas section; N Ä ipas section. See location in Fig. 2. N

most cases an east tectonic transport direction for the Gondwanic thrusts (RodrõÂguez FernaÂndez et al., 1996a). This information is in agreement with other kinematic indicators such as ramp trajectories and cut-off and branch lines measured in some of these thrusts. Although restoring the Gondwanic deformation was not possible, the shortening measured over minor structures in several localities, such as duplex structures of the Atutia river area (Fig. 4(b)) or the imbricate fan thrusts of CastanÄo River (Fig. 4(a)), has been estimated at up to 70%, using a bed-length balance method (Heredia et al., 1996b; RodrõÂguez FernaÂndez et al., 1996a,b). The calculated shortening in some fold reconstructions at Las Burras stream is about 60% (Espina et al., 1998; Fig. 11). The estimated shortening should not be too different from the regional shortening generated by the Gondwanic deformation in the study area. 2.2. Permo-Triassic igneous rocks Because igneous rocks may track the evolving nature of plate boundaries, as discussed by Kay et al. (1989) and LlambõÂas and Sato (1995), among others, we focus particular attention here on the mineralogy, compostion, and structure of Permo-Triassic igneous rocks found in the Cordillera

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Frontal. Upper Paleozoic and Triassic granodioritic and granitic rocks that make up part of the ColanguÈil batholith outcrop mainly along the eastern side of the Cordillera Frontal (Fig. 2). The granodioritic assembly includes the Tocota pluton, located in the central-eastern part of the study area and is the largest pluton, and other minor plutons such as Los Leones, Las Vacas, Casposo, and La Alumbrera. The granite assembly in this area includes Los Patos pluton (Patos Norte Valley, Fig. 2), which is the westernmost and largest one, and other minor plutons such as La Totora, Vallecito, Colorados, Agua Blanca, and Chita. Their shapes are elongate parallel to the batholith axis in N±S direction. The plutons sometimes show subvolcanic characteristics and were apparently emplaced at shallow levels. These plutons can be related to extensive andesitic and rhyolitic volcanism of the Choiyoi Group (Caminos, 1979; RodrõÂguez FernaÂndez et al., 1996a). The correlation between the ColanguÈil batholith and the Choiyoi Group is based on petrological and geochemical data (LlambõÂas and Sato, 1990; Sato et al., 1990; Sato and LlambõÂas, 1993; RodrõÂguez FernaÂndez et al., 1996b). The granodiorite plutons intrude the Agua Negra Formation with clean-cut intrusive contacts. Thermal metamorphism is documented by hornfels formation. They show great compositional variation resulting in multi-intrusive plutons. The Tocota pluton unit presents a centripetal irregular zonation, with the earlier and more basic facies occupying the external parts and the later granitic facies located in the core of the pluton (RodrõÂguez FernaÂndez et al., 1996b). Some other examples of irregular or centripetal zonation are the Casposo and Las Vacas plutons (Espina et al., 1998). The more basic facies contain numerous microgranular ma®c enclaves, angular or subrounded xenoliths, and ma®c cumulates (pyroxene and amphibole). Granitic dikes and hydrothermal veins or dikes are common. These plutons range from quartz-diorites, monzodiorites, tonalites, biotite, or amphibole granodiorites, to granites and leucogranites. Their textures are hypidiomorphic equigranular, sometimes intersertal, or coarse- to medium-grained xenomorphic heterogranular with porphyritic tendency. They are composed of plagioclase (An26 ± 45), hornblende, biotite, tremolite±actinolite aggregates and rare clinopyroxene. Except in the granodiorites, quartz and alkali feldspar are scarce, interstitial in nature, or occur as graphic intergrowths. They also contain quartz with subvolcanic af®nities. The granitic plutons, which may include more than one granitic facies, intrude the Agua Negra Formation and the Choiyoi Group as well as the aforementioned granodioritic plutons, which are incorporated as angular fragments. They exhibit subhorizontal upper contacts with roof pendants, which point to stoping mechanisms during the later stages of emplacement (LlambõÂas and Sato, 1995; Espina et al., 1998), as well as many aplitic and pegmatitic dikes and subvertical lateral edges. They may present numerous miarolitic cavities and develop pegmatoid areas. On the

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Fig. 9. (a) Geological map around the San Ignacio mine, showing the outcrops of the upper unit of the Agua Negra Formation and Gondwanic thrusts. See location in Fig. 2. (b) Gondwanic duplex in the Atutia River. Location in (a).

whole, these are pink-coloured granites and leucogranites, with ®ne- to coarse-grained xenomorphic equigranular texture or porphyritic tendency. They are formed by plagioclase (An5 ± 8 to An35), quartz and alkali feldspar graphic intergrowths, and scarce ma®c minerals. In addition to biotite and muscovite, some granites also contain cordierite and andalusite evidencing their peraluminous nature, while others show late interstitial amphibole (agpaitic texture) and pyroxene (Vallecito) typically of hypersolvus A type anorogenic granites (LlambõÂas and Sato, 1995; Espina et al., 1998). According to geochemical data, the Permo-Triassic intrusives are high-K calc-alkaline rocks (Fig. 13). Granodiorites are metaluminous to peraluminous …A=CNK ˆ 0:82±1:17† and more sodic than potassic in character; whereas granites are more potassic than sodic and mainly peraluminous …A=CNK ˆ 1:02±1:23†: The compositional gap between granodiorites and granites, highlighted by the lack of rocks with 68±72% SiO2 in content, and the evolution of some major and trace elements, point to the genetic independence of both granodiorites and granites (LlambõÂas and Sato, 1995; RodrõÂguez FernaÂndez et al., 1996b). Thus, according to the A ˆ Al 2 …K 1 Na 1 2Ca† and B ˆ Mg 1 Fe 1 Ti parameters of Debon and Le Fort (1988) granodiorites and granites constitute, respectively, cafemic and aluminous associations, corresponding to i-type and s-type granites of Chapell and White (1974). However, some granites have TiO2/MgO . 1 ratios typical of anorogenic A-type granites, in addition to high Y and Nb contents (hypersolvus granites Vallecito and Chita). Granodiorites exhibit normalized patterns similar to the calc-alkaline continental magmatic arcs of Pearce (1983) (Fig. 14) consistent with their plot in the volcanic arc granite ®eld (VAG) on the tectonic setting discrimination diagrams of Pearce et al. (1984) (Fig. 15). They result from mixing mantle or lower crustal derived magmas and acid crustal melts, as shown by their typical hybrid petrographic characteristics (RodrõÂguez FernaÂndez et al., 1996b) and their

Fig. 10. Geological cross-section showing the structure of the Paleozoic basement between the Chita and Bauchaceta valleys. See location in Fig. 6.

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Fig. 11. Geological cross-section across the Las Burras Creek (Espina et al., 1998). See location in Fig. 2.

isotopic signature (Las Piedritas: 0.7041±0.7048 ^ 0.0002; Tocota: 0.7052±0.7059 ^ 0.0004; LlambõÂas and Sato, 1995). Some granites show normalized patterns of Pearce (1983) similar to the granodiorites, although strongly depleted in P and Ti, while others exhibit signi®cant negative Sr and Ba anomalies, and Ta, Nb, Y, Zr enrichment (Fig. 14). They plot in the plate granite ®eld (WPG) on the tectonic setting discrimination diagrams of Pearce et al. (1984) (Fig. 15), consistent with their hypersolvus nature (agpaitic textures). Their isotopic signature (0.7072 ^ 0.0005±0.7163 ^ 0.0001) points to a crustal source (LlambõÂas and Sato, 1995). One characteristic shared by all the Permo-Triassic plutons is the almost complete absence of deformation fabric, being exclusively affected by Andean structures. This suggests that they intruded at the end of the Gondwanic orogeny during a regional extensional postorogenic deformation, as claimed in LlambõÂas and Sato (1990, 1995) in pervasive conditions (RodrõÂguez FernaÂndez et al., 1996b). With the one exception of the Tabaquito pluton (Early Carboniferous), the Rb±Sr age of the ColanguÈil batholith would be of 272±247 Ma (Sato and LlambõÂas, 1993; LlambõÂas and Sato, 1995), although Linares and LlambõÂas (1974) estimate an age of 283 ^ 15 Ma for the Tocota pluton. Espina et al. (1998) obtain K±Ar ages of 261 ^ 6 and 250 ^ 8 Ma for the Las Vacas and Casposo granodiorite intrusions. Their geochemical characteristics show the evolution from a calc-alkaline magmatism, typical from an orogenic environment, to an alkaline and peralkaline (Vallecito) anorogenic one. The Permo-Triassic igneous rocks were intruded in an extensional period related to an orogenic collapse, developed after docking of exotic terranes to the Gondwana margin (Kay et al., 1989) during the Gondwanic orogeny. The foregoing petrology discussion showed that the igneous rocks were formed in an active margin, which concluded with a widespread general extension. This last stage was associated with alkaline and peralkaline assemblages emplaced during a late anorogenic episode.

3. The Andean cover The Andean cover lies unconformably over the Paleozoic basement (Figs. 2 and 4). It is composed of late Permian± Triassic and Tertiary volcanic, volcanoclastic, and some interbedded continental sedimentary rocks with a remarkable stratigraphic gap that accounts for most of the Mesozoic. These rocks are intruded by Mesozoic and Tertiary granitoids and deformed by the Andean orogenic cycle. Two main stratigraphic sequences can be de®ned in the Andean cover: a Permian±Triassic sequence linked to an extensional tectonic event, and a Neogene sequence connected to a compressional tectonic event (Fig. 4) producing the inversion of the previous extensional features. 3.1. The synextensional stratigraphic units: the Choiyoi Group The Choiyoi Group (Groeber, 1946) forms a complex volcanic and volcaniclastic series with compositional variations both laterally and vertically, and lies unconformably over the previously structured Gondwanic basement. Three main stratigraphic units can be de®ned there (Fig. 4): 1. The Lower Choiyoi unit (CastanÄo Formation, RodrõÂguez FernaÂndez et al., 1996a) consists in a sequence of tuffs, agglomerates, lava ¯ows, tobaceous sandstones, and ignimbrites with some epiclastic and calcareous intercalations of local character (Manrique Member). The lower part is mainly andesitic, while the upper part is predominantly dacitic to rhyolitic. 2. The Middle Choiyoi unit lies unconformably over the Lower Choiyoi (Fig. 5(a)). It is volcanic and has a lower part (Vega de los Machos Formation, CaballeÂ, 1986) of andesitic nature, while the upper part is rhyolitic (Palque Formation, CaballeÂ, 1986).

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Fig. 12. Geological cross-sections of the Frontal Cordillera in the study area. See location in Fig. 2.

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Fig. 13. Q ˆ Si=3 2 …K 1 Na 1 2Ca=3† versus P ˆ K 2 …Na 1 Ca† of Debon and Le Fort (1988) and K2O versus SiO2 of Peccerillo and Taylor (1976) plot of Permo-Triassic and Mesozoic intrusive rocks and representative ®elds of andesites and rhyolites of the Choiyoi Group. CALK ˆ calcalkaline association; SALKD ˆ dark subalkaline association; SALKL ˆ light-coloured subalkaline association; Ca ˆ calc-alkaline associations; K±Ca ˆ high-K calc-alkaline associations; Sh ˆ shoshonite associations. Symbols used are: (solid circle) granodiorites, (blank circle) granites Permo-Triassic, and (blank square) Mesozoic Vizcachas pluton. Geochemical data from LlambõÂas and Sato (1995), RodrõÂguez FernaÂndez et al. (1996a), Espina et al. (1998), and Malizia et al. (2002).

3. The Upper Choiyoi (Atutia Formation, Espina et al., 1998) also lies unconformably over the Middle Choiyoi and has an andesitic nature with a thin rhyolitic intercalation at the upper part. A series of intrusive bodies occur associated with the Choiyoi Group (Mesozoic intrusives, Figs. 2 and 3) whose nature is either rhyolitic, andesitic, or granodioritic. Only two geochemical data are available from the lower part of the CastanÄo Formation that correspond to metaluminous±peraluminous …A=CNK ˆ 0:99±1:21† andesites (60.13±62.14% SiO2). The Vega de los Machos Formation is constituted by metaluminous to weakly peraluminous …A=CNK ˆ 0:80±1:07† andesites and dacites (54.8± 67.26% SiO2), whereas the Palque Formation is mainly formed of weakly peraluminous …A=CNK ˆ 0:99±1:15† rhyolites (72.6±77.84% SiO2) and rare metaluminous …A=CNK ˆ 0:89±0:92† andesites (56.7±60.6% SiO2). The Atutia Formation is constituted by both metaluminous …A=CNK ˆ 0:87±0:97† andesites (57.7±62% SiO2) and peraluminous …A=CNK ˆ 1:09±1:27† rhyolites (Fig. 16). As a whole, the Choiyoi Group is a high-K calc-alkaline association (Fig. 16), distinguished by a compositional gap shown by the non-existence of 68±72% SiO2 compositions, as in the Permo-Triassic intrusive rocks. Diagrams in Fig. 16

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reveal the correlation between both granodiorites±andesites and granites±rhyolites of the Choiyoi Group and ColanguÈil batholith intrusive rocks, as proposed in LlambõÂas and Sato (1990), Sato et al. (1990), Sato and LlambõÂas (1993), RodrõÂguez FernaÂndez et al. (1996b), and Espina et al. (1998). Most of the andesites, dacites, and rhyolites exhibit normalized patterns that are characteristic of calc-alkaline continental magmatic arcs of Pearce (1983) (Fig. 17). However, some rhyolites of the Palque and Atutia Formations are strongly depleted in Sr and Ba, and show high contents in Nb, Ta, HFSE, Y, and Yb, similar to the anorogenic volcanism (Fig. 17). These geochemical features are in agreement with their respective plot on the volcanic arc (VAG) and within plate granite (WPG) ®elds on the tectonic setting discrimination diagrams of Pearce et al. (1984) (Fig. 18). The volcanism of the Choiyoi Group shows, as the Permo-Triassic intrusive rocks, the evolution of an orogenic to anorogenic setting also claimed in other papers aforementioned. The largest Mesozoic intrusions associated with the Choiyoi Group in the study area are the Vizcachas pluton in the southeast of the Tocota pluton, and a large subvolcanic body of andesite composition in the south (Fig. 2). The subvolcanic porphyritic andesite body mainly affects the Vega de los Machos Formation and is composed of plagioclase (andesine) phenocrysts, pyroxene (augite), hornblende, biotite, and rare quartz, in a ®ne-grained plagioclase, quartz and subordinate alkali feldspar groundmass (Espina et al., 1998). These authors believe it to be Triassic in age, contemporary with the Choiyoi Group. The Vizcachas pluton intrudes the Vega de los Machos Formation and is uncomformably covered by the Melchor Group (Miocene). These rocks range from tonalite to granite, with numerous microgranular ma®c enclaves and xenoliths of basalt, andesite, and rhyodacite. These rocks exhibit medium- to coarse-grained inequigranular xenomorphic texture, and are sometimes porphyritic, with plagioclase (oligoclase-andesine), graphic textures of quartz and alkali feldspar, biotite, and amphibole, which may not occur in some granodiorites and granites. The only geochemical data available from the Vizcachas pluton is from RodrõÂguez FernaÂndez et al. (1996b). Contents and behaviour of major and trace elements are very similar to those in the Permo-Triassic intrusions. These high-K calcalkaline rocks (Fig. 13) are metaluminous …A=CNK ˆ 0:93±1† in character, with compositional gaps between enclaves (62% SiO2), granodiorites (65±69% SiO2), and granites (75% SiO2). According to their plot in the VAG on the tectonic-setting discrimination diagrams of Pearce et al. (1984) (Fig. 15), these rocks correspond to calc-alkaline continental magmatic arcs (Fig. 14). 3.2. Structures related to the extensional stage The structures related to the Mesozoic extensional tectonic process are normal faults, grouped in N±S bands,

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Fig. 14. Geochemical patterns of Pearce (1983) for Permo-Triassic and Mesozoic intrusive rocks. Symbols as shown in Fig. 13.

with E±W or NW±SE intercalated bands that represent transfer zones (Figs. 2 and 19). These faults are only partially inverted by the later Andean compression, which allows their structure and original slip to be easily recognized. These faults control the deposit of the Permo-Triassic volcanosedimentary rocks, which show minimal thicknesses to the east (Figs. 2 and 12). Westward migration of the extension is con®rmed by the movement of depocenters in the same direction. In this way, the depocenter of the CastanÄo Formation is located in the easternmost half-graben (Figs. 12 and 19), while that of the Atutia Formation is one of the westernmost (Atutia graben). The westernmost halfgraben of the area under study, Cortaderas graben, contains  lvarez et the Triassic±Jurassic Rancho de Lata Formation (A al., 1995) further to the south. In the easternmost part of the study area, the elevated Tocota horst (Fig. 19), where rocks of the Paleozoic basement and ColanguÈil and Tocota batholiths crop out, restricts the Choiyoi Group, its volcanic equivalent, to the aforementioned graben areas. Although data on the geometry of these fractures at depth is lacking, their reconstruction (Fig. 12) can be attempted on the basis of both experimental extensional patterns and the patterns followed by some natural examples from other orogenic units. Thus, it is possible to assume that the normal faults are listric, merging to a common westward dipping

Fig. 15. Distribution of Permo-Triassic and Mesozoic intrusive rocks, and representative ®elds of andesites and rhyolites of the Choiyoi Group, on the Nb versus Y and Rb versus (Nb 1 Y) tectonic setting discrimination diagrams of Pearce et al. (1984). Fields for syn-collision (syn-COLG), volcanic arc (VAG), within plate (WPG), and ocean ridge (ORG) granites are indicated. Symbols as shown in Fig. 13.

detachment level. Since the Andean compressional structures are generated by the inversion of the extensional faults, it is also possible to assume that the Andean compressional detachment may have used the extensional detachment level. Data published about the depth of the Andean detachment in nearby areas (Gosen, 1992; Kozlowski et al., 1993) show that the upper detachment would be situated at around 24000 m in the frontal zone of the Precordillera, between 26000/28000 m at the Cordillera foreland, and between 212,000 and 216,000 m at its back (Chilean Andes). On the other hand, the detachment of the basal part would take place at a depth of 20 km. These data have been used to construct the cross-sections shown in Fig. 12. The crosssections show the likely geometry of the Andean extensional prism, with the Gondwanic basement dipping to the west and each fault block dipping to the east. The presence, from east to west, of relative maximum and minimum thicknesses of the Choiyoi Group allows the assumption that this geometrical con®guration de®nes a half-graben model. 3.3. The extensional stage: nature and discussion An important extensional stage began in the upper Permian with the end of the Gondwanic orogeny (MalumiaÂn et al., 1983; Uliana and Biddle, 1988). Here, crustal

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Fig. 16. SiO2 versus Zr/TiO2 of Winchester and Floyd (1977) and K2O versus SiO2 of Peccerillo and Taylor (1976) plot of Choiyoi, Melchor, and Olivares groups, and representative ®elds of Permo-Triassic intrusive rocks. Symbols used for Choiyoi Group are: (solid square) andesites of CastanÄo Formation, (solid dot) andesites and rhyolites of Vega de los Machos Formation, (solid triangle) andesites and rhyolites of Palque Formation, (solid diamond) andesites and rhyolites of Atutia Formation; Melchor Group: (blank square) basalts, andesites, and rhyolites; Olivares Group: (blank diamond) basalts and andesites. Geochemical data from Sato and LlambõÂas (1993), RodrõÂguez FernaÂndez et al. (1996a), Espina et al. (1998), and Malizia et al. (2002).

thickening was followed by extensional collapse and significant volcanism (Choiyoi volcanic episode). The overall teconic setting was within early breakup of Gondwana, prior to the inception of the subduction in the present trench.

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Extension increased through the Triassic to the Lower Jurassic, but slowed again during the rest of the Jurassic. In the Cretaceous, rifting was more intense and temporally linked with the opening of the South Atlantic margin at these latitudes (Rosello and Mozetic, 1999). This rifting is marked by the ®rst marine deposits connected to innerarc and backarc basins (Uyeda, 1983; Mpodozis and Ramos, 1990; Ramos and Kay, 1991). This last Mesozoic basin would have originated during extensional periods due to the increasingly steep dip of the subduction zone (Ramos, 1988). Mesozoic sedimentation depocenters migrated to the west (Principal Cordillera), concurrent with migration of extensional deformation in the same direction (Uliana and Biddle, 1988). The latter resulted in the lack of post-Triassic and Mesozoic deposits in the Frontal Cordillera. The Choiyoi Group has been related to a rifting event (Heredia, 1996a; RodrõÂguez FernaÂndez et al., 1996a, 1999). Higher blocks are inferred to the east and north, where the thickness of the unit is smaller. This minimal thickness essentially corresponds to the Tocota pluton outcrop, which would have acted as an elevated block (horst) during the extensional process. To the west, the thickness increases up to 2500 to 3000 m (Fig. 12). The basal part probably corresponds to the initial extensionrelated denudation phase, which would have locally generated thick detrital deposits with clasts of sedimentary or plutonic Paleozoic rocks. The middle and upper parts of the section originated during widespread volcanic activity, and thus the basin contains a large amount of volcanic rock. Most of the volcanoes and cauldrons, well preserved in the CastanÄo area, are in close proximity to the normal faults, whereas areas further away from these faults contain laketype carbonate sedimentary deposits with volcanic and volcanosedimentary intercalations (Manrique Member in Fig. 4). Occasionally, the base of the Choiyoi Group

Fig. 17. Geochemical patterns of Pearce (1983) for Choiyoi, Melchor, and Olivares groups. Symbols as shown in Fig. 16.

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Fig. 18. Distribution of Choiyoi, Melchor, and Olivares groups, and representative ®elds of Permo-Triassic intrusive rocks, on the Nb versus Y and Rb versus (Nb 1 Y) tectonic setting discrimination diagrams of Pearce et al. (1984). Fields for syn-collision (syn-COLG), volcanic arc (VAG), within plate (WPG), and ocean ridge (ORG) granites are indicated. Symbols as shown in Fig. 16.

shows syntectonic unconformities (Riba, 1976) as well as large Carboniferous olistholiths detached from the higher block (Fig. 5(b)). These olistholiths are formed by very hard Carboniferous rocks (sandstones, conglomerates, or limestones). They reach lengths of over 1 km, with thicknesses of less than 100 m. 3.4. Compressional stage: synorogenic stratigraphic units A thick, gently west-dipping volcanic and volcaniclastic sequence that includes the Melchor and Olivares groups (Figs. 2 and 4) lies above a strong paleorelief. This paleorelief formed over previous units during a non-deposit and/or deformation period, ranging in this region from the Mesozoic to the lower Tertiary periods. These units, whose main source area is to the west, are linked to the Cenozoic compression of the Andean orogen and are estimated to have originated during the Tertiary period (Heredia et al., 1996b; RodrõÂguez FernaÂndez et al., 1996a, 1999). These rocks have a strong compositional variation and are intruded by Tertiary granitoids and dikes of different nature. Data about the Tertiary intrusions are scarce in comparison with Permo-Triassic homologous, due to extensive alteration processes. The largest outcrop, located in the southwestern part of the study area, exhibits monzogranites, granodiorites, diorites, and gabbros (Super Unidad RõÂo Grande; Ragona et al., 1995). The Entrecordilleras intru-

sion, located to the south of the Vizcachas pluton, intrudes the Las Vizcachas and Arroyo de las Chinches Formations and is composed of a porphyritic quarz-monzodiorite with a ®ne-grained tonalitic outer facies. These rocks are composed of plagioclase (oligoclase±andesine) phenocrysts in a groundmass quartz and alkali-feldspar-rich, with more scarce biotite, clinopyroxene and amphibole (CaballeÂ, 1986; Espina et al., 1998). The Melchor Group (RodrõÂguez FernaÂndez et al., 1996a) is composed of three formations: the Vizcachas and Puntas Negras, which are mainly volcanic, and the RõÂo Mondaca, which is mainly sedimentary. The lower unit is an andesitic lava ¯ow sequence called Las Vizcachas Formation (CaballeÂ, 1986). The strong unconformity located at the base of these andesites (Figs. 2 and 4) appears to re¯ect fossilization of the prior extensional period. The lithological characteristics and stratigraphic position of the Vizcachas Formation allow its correlation with the Escabroso member (18 ^ 1.7 Ma) of the DonÄa Ana Formation (Mpodozis and Cornejo, 1986). The Puntas Negras Formation is formed by some 1200± 1400 m of tuffs, agglomerates, lavas, and ignimbrites of andesitic nature, with intercalated rhyolitic lava ¯ow that is more abundant towards the top (Fig. 4). The estimated K± Ar age of 15.6 ^ 1.2 and 13.4 ^ 2 Ma (RodrõÂguez FernaÂndez et al., 1996b) for the unit and its geographic spreading towards the Chilean border suggests a possible correlation with Cerro de las ToÂrtolas Formation (Mpodozis and Cornejo, 1986). In the upper part of the Melchor Group, there are about 800 m of sedimentary rocks with volcanic clasts, typical of subaerial alluvial fans (RõÂo Mondaca Formation, RodrõÂguez FernaÂndez et al., 1996a; Figs. 2 and 4). On the Zr/TiO2 diagram of Winchester and Floyd (1977), the Melchor Group is made up of subalkaline basalt, andesites, rare dacites-rhyodacites and rhyolites, with remarkable compositional gaps (Fig. 16). According to the K2O content, andesites and most of the rhyolites are high-K calc-alkaline rocks on the K2O versus SiO2 diagram of Peccerillo and Taylor (1976) (Fig. 16). The olivine basalt is medium-K, as the basalts and basaltic andesites of homologous DonÄa Ana and Cerro de la ToÂrtolas Formations (Kay et al., 1991). The olivine basalt (48.67% SiO2) is metaluminous …A=CNK ˆ 0:75† and TiO2 (1.41%) and Al2O3 (18.72%) rich. The andesites and dacites (61.9±64.6% SiO2) are metaluminous to weakly peraluminous …A=CNK ˆ 0:89±1:03†; and rhyolites (70.1±75.8% SiO2) are peraluminic …A=CNK ˆ 1±1:23† in character. Andesites and some rhyolites exhibit normalized patterns similar to the calc-alkaline continental magmatic arc of Pearce (1983) (Fig. 17) and plot in the VAG ®eld of Pearce et al. (1984) (Fig. 18). By contrast, some rhyolites of the lower Melchor Group are strongly depleted in Sr and Ba, and show high contents in Nb, Ta, HFSE, Y, and Yb, similar to the anorogenic volcanism (Fig. 17). As some rhyolites of the Choiyoi Group, they plot in the WPG ®eld on the

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Fig. 19. Geological sketch map with the position of main normal faults and grabens in the study area, previously to the Andean compressional stage.

tectonic setting discrimination diagrams of Pearce et al. (1984) (Fig. 18). The olivine basalt has normalized pattern of Pearce (1982) like certain anomalous volcanoes developed above subduction zones with geochemical characteristics of within-plate series (Fig. 17). The alkaline character of this basalt is the same as other early Miocene basalts, such as MaÂquinas alkali basalts (Kay et al., 1991; Kay and Abbruzzi, 1996). The Olivares Group (RodrõÂguez FernaÂndez et al., 1996a) lies unconformably over all the previous units (Figs. 2 and 4). It comprises a 300 m thick sequence of andesitic basalts, arranged as massive lava ¯ows, intruded by abundant dacitic and rhyodacitic bodies. Above this group, a sedimentary series of about 600 m (La Puentecilla Formation, Fig. 4) comprises several alluvial-fan-type sequences of conglomerate, sandstone and siltstone. The conglomerate rocks include clasts from the underlying formations, and from granitic rocks probably from early Paleozoic and Tertiary granitoids of the Chilean slope. The age of the Olivares Group is estimated to be Miocene±Pliocene (RodrõÂguez FernaÂndez et al., 1996a).

Three geochemical data from the Olivares Group correspond to subalkaline basalt and andesites in the Zr/TiO2 versus SiO2 Winchester and Floyd (1977) diagram. Andesites are high-K calc-alkaline in the SiO2 versus K2O Peccerillo and Taylor (1976) diagram (Fig. 16). Both basalt (47.3% SiO2) and andesites (58.9±60.6% SiO2) are metaluminic in nature (A=CNK ˆ 0:70 and 0.89±0.93, respectively). The basalt of Olivares Group is more K rich than the basalt of Melchor Group and more alkaline in nature. Andesites exhibit normalized patterns similar to the calcalkaline continental magmatic arc of Pearce (1983) (Fig. 17) and plot in the VAG ®eld of Pearce et al. (1984) (Fig. 18), like the andesites of the Melchor Group. Basalt has normalized pattern of Pearce (1982), as in the Melchor Group, similar to certain anomalous volcanoes developed above subduction zones with geochemical characteristics of within-plate series (Fig. 17). The source and evolution of early to late Miocene magmas have been explained extensively by Kay and Abbruzzi (1996), according to geochemical, geological, and geophysical data.

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The Quaternary deposits of the Rodeo±Upsallata basin consist of slightly consolidated and heterometric sediments, with predominantly thick facies derived from erosion of an elevated mountain range. The general megasequence involves the forward movement of the cordillera front, later reversing as a result of the stabilization and backward motion of the front towards the west. 3.5. Structures related to the compressional stage: nature and discussion The beginning of the Andean compressional stage is not yet well documented. Ramos (1993) claims that it could have begun in the upper part of the Lower Cretaceous (Albian±Cenomanian). Crustal thickening from the Upper Cretaceous onwards is unanimously accepted (Turonian± Santonian) (see Groeber, 1946). This would have given rise to the subsequent development on the westernmost part (Chilean Andes) of an incipient foreland basin during the Campanian±Paleocene period (Legarreta and Uliana, 1991). During the rest of the Cenozoic, the orogenic front migrated to the foreland with the corresponding cannibalization of the foreland basin; the traces of which are recorded both over the Main Cordillera and Precordillera sectors (Ramos et al., 1996). However, in the study area, this probably starts in Miocene times (,20 Ma), since this is the age of the ®rst synorogenic strata (Melchor Group). Signi®cantly, the main deformation is synchronous with the start of the ¯at-slab subduction at about 15 Ma (Pilger, 1981, 1984), and also with the deposition of thick siliciclastic sediments of the Andean synorogenic series (Rio Mondaca and Puentecilla Formations, Fig. 4). The Melchor Group lays unconformably over the preorogenic successions and the Gondwanic basement (Figs. 2,4 and 12), and its depocenters migrate from west to east, opposite to the extensional stage depocenters. The geometrical con®guration of the Melchor Group resulted from pre-existing extensional structure and erosional surfaces developed over the different fault blocks from the Jurassic to the Oligocene. In the upper edge of the fault blocks, the Melchor Group rests on the lower Andean stratigraphic units (see cross-sections in Fig. 12), and in the east of the studied area, it rests on the Paleozoic basement. The most important compressional structures are reverse faults and thrusts (Fig. 5(e)) and scarce related folds. The deep geometry of these reverse faults was in¯uenced by the geometry of the Mesozoic extensional system, which would be only partially inverted, as stated above. Most of the faults are generated by the inversion of the extensional faults during compressional tectonic processes. However, some later faults cut across the pre-existing faults (Figs. 2, 6, 9, 10 and 12). Some lateral compensation structures occur in relation to the reverse faults, tear faults, or oblique ramps for instance. The tear faults have a NNW±SSE direction and subvertical, or slightly ESE dipping, fault plane (Fig. 2).

Folds from the Andean compressional stage are scarce, mainly due to insigni®cant shortening and the lack of competency contrast among the rocks. Only close to some of the larger faults, decametric to hectometric folds occur that are slightly further developed. The folded structures in most cases correspond to fault-bend folds related with extensional inverted faults. The observation of different kinematic markers (cut-off and branch lines, fold axis, etc.) shows an east (90±1008E) tectonic transport direction for the Andean compressional structures. This is in agreement with the direction that Ramos (1988) and Allmendiger et al. (1990) have stated for Andean thrust sheets, both at the Main Cordillera and at the Precordillera. The general propagation mechanism seems to be forwards, as evidenced by the migration towards the east of the synorogenic depocenters. The displacement on reverse faults measured in the Tertiary synorogenic rocks is usually less than 1 km (Fig. 12). The crustal shortening calculated in cross-sections I±I 0 and II±II 0 over the synorogenic formations (Fig. 12) is of about 10%, so the pre-existing extensional and Gondwanic structures have only been slightly modi®ed. This fact contrasts with the more than 50% calculated shortening of the Precordillera unit (Allmendiger et al., 1990; Gosen, 1992). Thus, most of the crustal shortening of the Andean Cordillera during the compressional stage has been transferred to the Precordillera unit (Ramos et al., 1996; Heredia, 1996a), through the lower detachment fault (Fig. 12). These facts also show that tectonic compression during the Cenozoic took place with little tectonic inversion in the Frontal Cordillera. According to this interpretation, the Cordillera Frontal unit is an uplift block in which the extensional structures have been preserved and the Rodeo±Upsallata basin is of piggy-back type (Allmendiger et al., 1990; Beer et al., 1990; Heredia, 1996a). The shortening along the Andean Cordillera exhibits remarkable lateral and transverse variation. The main shortening is located in either Principal Cordillera or Precordillera, and the total Andean shortening decreases to the south (Ramos et al., 1996). 4. Conclusions The Argentine Frontal Cordillera between 30830 0 and 31820 0 shows polycyclic structures with a more or less generalized vergence of its structures towards the east. Two large tectonostratigraphic groups of rocks with different nature and structure can be de®ned: a Paleozoic basement and Andean cover. The Paleozoic basement, formed by sedimentary rocks deposited between the Devonian and the lower Permian periods, was deformed by the Famatinian and Gondwanic orogenic cycles and separated by a major unconformity. They would have developed in marine or transitional conditions and were intruded by upper Paleozoic granitoids.

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The Carboniferous±Permian rocks of the Agua Negra Formation can be divided into two tectonostratigraphic units with different sedimentary characteristics separated by a clearly visible erosive surface. The two units represent pre-orogenic and synorogenic successions related to a main tectonic event: the Gondwanic orogeny. At the Carboniferous±early Permian time, the Frontal Cordillera had a retroarc position (Ramos, 1988). Under this geotectonic setting, the most likely sedimentary evolution would have been the propagation of the deltaic system from the Precordillera and Sierras Pampeanas to the Frontal Cordillera, where they would initially have met more open-marine conditions. During the deposit of the Permian synorogenic succession, a dramatic change of the main source area of sediments took place in the Frontal Cordillera. At this time, the uplift of a volcanic arc and a Famatinian basement is shown by the presence of volcanic and sedimentary pebbles in the conglomerates with western provenance. The existence of stromatolithic pebbles in the conglomerates of the top of the succession also represents the cannibalization of the earlier foreland sedimentary basin and its progradation to the east. The most likely geotectonic setting of the Frontal Cordillera at this time is a retroarc foreland basin. This event can be related to the Gondwanic orogeny, with the same sense of propagation. All these rocks were strongly deformed by thin-skinned tectonics during the Gondwanic orogenic cycle. The most important structures are thrust sheets and related folds with east tectonic transport direction, producing very important shortening (60±70%). The upper Paleozoic±Triassic granitoids show the evolution from a calc-alkaline magmatism, typical of an orogenic environment, linked to an Andean-type subduction, to an alkaline and peralkaline anorogenic one. They intruded in an extensional period (orogenic collapse) that followed the docking of exotic terranes to the Gondwana margin during the Gondwanic orogeny. The Andean cover lies unconformably over the Paleozoic basement. It is characterized by the presence of volcanic and volcaniclastic rocks, with some interbedded sedimentary rocks deposited in strictly continental conditions. Their ages range from late Permian to Quaternary, and they are intruded by Jurassic and Miocene granitoid rocks. Within this upper group, two synorogenic sequences can be differentiated: a lower one formed by the Choiyoi Group connected to an extensional process, and an upper sequence integrated by Melchor and Olivares groups. This second sequence is related to a later compressional episode producing the inversion of the previous extensional features. All the units are separated by unconformities of varying magnitude. The lower unit (Choiyoi Group, Permian and Triassic) can be related with the upper Paleozoic and Triassic granitoids. It is cut by normal faults with downthrown western blocks, and intruded by Jurassic granodioritic rocks. The faults in this unit involve the Gondwanic base-

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ment in a typical thick-skinned tectonic style and are grouped in bands with N±S direction. Their geometry is listric, and they share a common detachment level. The uppermost units (Melchor and Olivares groups, Tertiary and Plio-Quaternary) also lie unconformably over the latter. The normal faults were inverted in the upper Miocene by the uplifting of the western block, deforming the lower and upper units in a compressional context during the Andean orogenic cycle. The displacement of reverse faults measured in the Tertiary synorogenic rocks is usually less than 1 km. The crustal shortening calculated in cross-sections I±I 0 and II± II 0 (over the synorogenic formations: Fig. 4) is about 10%. This fact contrasts with the estimated shortening of over 50% at the Precordillera unit (Gosen, 1992). All this shows that most of the crustal shortening of the Andean Cordillera at the compressional stage has been transferred to the Precordillera unit through the lower detachment fault. These facts also show that the Cordillera Frontal unit is an uplift block in which the extensional structures have been preserved, and that the Rodeo±Upsallata basin is a piggyback type. Acknowledgements We thank V.A. Ramos, R. Allmendinger, M. Cegarra, D. Ragona and R.G. Espina for their suggestions and valuable comments. We thank I. ZamarrenÄo for her suggestions in the limestone study, and A. Cuesta for his suggestions and valuable comments in the petrological study. Thanks also due to Ana Ojanguren and Jorge Gallastegui for helping with the English version of the manuscript and Inma Carmena for the ®gure design. This paper contains part of the results obtained for the CastanÄo Viejo and CastanÄo Nuevo sheets (1:100,000) and Rodeo (1:250,000) of Argentine Geological Map, funded by the SEGEMAR (Argentine Geological Survey), AECI (ICI) (Spanish Cooperation Agency) and IGME (Spanish Geological Survey). It has also been funded by DGICYT PB 98-1189 (Spanish Science and Technology Agency) and by Comissionat per Universitats i Recerca, Generalitat de Catalunya, Grup de Qualitat GRQ94-1048. References Allmendiger, R.W., Figueroa, D., Synder, D., Beer, J., Mpodozis, C., Isacks, B.L., 1990. Foreland shortening and crustal balancing in the Andes of 308S latitude. Tectonics 9, 789±809. Â lvarez, P.P., Benoit, S.V., Ottone, E.G., 1995. Las formaciones Rancho de A Lata, Los Patillos y otras unidades mesozoicas de la Cordillera Principal de San Juan. Revista AsociacioÂn GeoloÂgica Argentina 49 (1±2), 123± 142. Aparicio, E.P., 1969. ContribucioÂn al conocimiento de la edad de los sedimentos del arroyo de Agua Negra, Departamento de Iglesia, San Juan, RepuÂblica Argentina. Revista AsociacioÂn GeoloÂgica Argentina 31 (3), 190±193. Beer, J.A., Allmendinger, R.W., Figueroa, D.A., Jordan, T.E., 1990.

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Seismic stratigraphy of a Neogene piggy-back basin, Argentina. American Association of Petroleum Geologists Bulletin 74, 1183± 1202. CaballeÂ, M.F., 1986. Estudio geoloÂgico del sector oriental de la Cordillera Frontal entre los rõÂos Manrique y Calingasta (Provincia de San Juan). Tesis Doctoral, Universidad Nacional de La Plata (unpublished). Caminos, R., 1979. Cordillera Frontal. In: Leanza, A.F. (Ed.). GeologõÂa Regional Argentina. Academia Nacional de Ciencias, CoÂrdoba, pp. 41±80. CardoÂ, R., DõÂaz, I.N., Cegarra, M.I., RodrõÂguez FernaÂndez, L.R., Heredia, N., 2000. Hoja del Mapa GeoloÂgico de Argentina a E. 1:250.000 no 3169-1, Rodeo. Servicio GeoloÂgico y Minero Argentino in press. Chapell, B.W., White, A.J.R., 1974. Two contrasting granite types. Paci®c Geology 8, 173±174. Debon, F., Le Fort, P., 1988. A cationic classi®cation of common plutonic rocks and their magmatic association: principles, method, application. Bulletin of Mineralogy 111, 493±510. Espina, R.G., Cegarra, M.I., Ragona, D., 1998. Hoja del Mapa GeoloÂgico de Argentina a E. 1:100.000 no 3169-20, CastanÄo Nuevo. Servicio GeoloÂgico y Minero Argentino. Gonzalez, C.R., 1977. Oriocrassatella y Stutchburia (Bivalvia) en la ingresioÂn marina del PeÂrmico inferior de la quebrada del Agua Negra, provincia de San Juan (Argentina). Ameghiniana 13 (2), 127±140. Gosen, W.V., 1992. Structural evolution of the Argentine Precordillera: the RõÂo San Juan section. Journal of Structural Geology 14 (6), 643±667. Groeber, P., 1938. MineralogõÂa y GeologõÂa. Espasa-Calpe Argentina, 492. Groeber, P., 1946. Observaciones geoloÂgicas a lo largo del meridiano 708; Hoja Chos Malal. Revista de la AsociacioÂn GeoloÂgica Argentina 1 (3), 177±208. GutieÂrrez, P.R., 1983. GeologõÂa del tramo medio de la quebrada de Agua Negra. Depto. de Iglesia, Provincia de San Juan. Trabajo Final de Licenciatura, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires (unpublished). Gutscher, M.A., Spakman, W., Bijwaard, H., Engdahl, E.R., 2000. Geodynamic of ¯at subduction: seismicity and tomographic constraints from the Andean margin. Tectonics 19 (5), 814±833. Heredia, N., RodrõÂguez FernaÂndez, L.R., Ragona, D., 1996a. Estructura extensional e inversioÂn tectoÂnica en los Andes argentinos entre los 30830 0 y 31800 0 S. Geogaceta 20 (4), 864±866. Heredia, N., RodrõÂguez FernaÂndez, L.R., Ragona, D., 1996b. Structure of the Argentine Andean Cordillera between 30830 0 y 31800 0 S. Andean Geodynamics, ORSTOM, Paris pp. 379±382. Heredia, N., RodrõÂguez FernaÂndez, L.R., Quesada, C., MarõÂn, G., CardoÂ, R., 1996. Hoja del Mapa GeoloÂgico de Argentina a E. 1:100.000 no, Castano Viejo. Servicio Geologico y Minero Argentino in press. Kay, S.M., Abbruzzi, J.M., 1996. Magmatic evidence for Neogene lithospheric evolution of the central Andean ¯at-slab between 308S and 328S. Tectonophysics 259, 15±28. Kay, S.M., Ramos, V.A., Mpodozis, C., Sruoga, P., 1989. Late Paleozoic to Jurassic silicic magmatism at the Gondwana margin: analogy to the Middle Proterozoic in North America? Geology 17, 324±328. Kay, S.M., Mpodozis, C., Ramos, V.A., Munizaga, F., 1991. Magma source variations for mid-late Terciary magmatic rocks associated with a shallowing subduction zone and a thickening crust in the central Andes (28 to 338S). Geological Society of America, Special Paper 265, 113±137. Keidel, J., 1921. Sobre la distribucioÂn de los depoÂsitos glaciares del PeÂrmico conocidos en la Argentina y su signi®cacioÂn para la estratigrafõÂa de la serie de Gondwana y la paleogeografõÂa del Hemisferio Austral. BoletõÂn de la Academia Nacional de Ciencias 25, 239±368. Kozlowski, E.E., Manceda, R., Ramos, V.A., 1993. In: Ramos, V.A. (Ed.). GeologõÂa y Recursos Naturales de Mendoza, Relatorio XII Congreso GeoloÂgico Argentino AsociacioÂn GeoloÂgica Argentina. Instituto Argentino del PetroÂleo, pp. 236±256. Legarreta, L., Uliana, M.A., 1991. Jurassic±Cretaceous marine oscillations and geometry of back-arc basin ®ll, Central Argentine Andes. Inter-

national Association of Sedimentologists, Special Publication 12, 429± 450. Linares, E., LlambõÂas, E.J., 1974. Edad K±Ar de la granodiorita de la quebrada de Tocota, Departamento de Iglesia, San Juan. Revista de la AsociacioÂn GeoloÂgica Argentina 29 (1), 135±141. LlambõÂas, E.J., Sato, A.M., 1990. El Batolito de ColanguÈil, Cordillera Frontal, Argentina: Estructura y marco tectoÂnico. Revista GeoloÂgica de Chile 17 (1), 89±108. LlambõÂas, E.J., Sato, A.M., 1995. El Batolito de ColanguÈil: transicioÂn entre orogeÂnesis y anorogeÂnesis. Revista de la AsociacioÂn GeoloÂgica Argentina 50 (1±4), 111±131. Malizia, D., Limarino, C.O., Sosa-GoÂmez, J., Kokot, R., Nullo, F.E., GutieÂrrez, P.R., 2002. Texto explicativo de la Carta GeoloÂgica No 3169-2 a escala 1: 100,000 (Paso del Agua Negra). Servicio GeoloÂgico y Minero Argentino, 166 in press. MalumiaÂn, N., Nullo, F., Ramos, V.A., 1983. The cretaceous of Argentina, Chile, Paraguay and Uruguay. In: Moullade, M., Nairn, A.E.N. (Eds.). The Phanerozoic Geology of the World, II: The Mesozoic. Elsevier, Amsterdam, pp. 265±304. Mpodozis, C., Cornejo, P., 1986. Carta GeoloÂgica de Chile a E. 1:250,000 no 68, (Pisco Elqui). Servicio Nacional de GeologõÂa y MinerõÂa, Santiago de Chile, 165. Mpodozis, C., Ramos, V.A., 1990. The Andes of Chile and Argentina. In: Eriksen, G.E., CanÄas Pinochet, M.T., Reinemund, J.A. (Eds.). Geology of the Andes and its relation to Hydrocarbon and Mineral Resources. Circumpaci®c Council for Energy and Mineral Resources, Earth Sciences Series, 11. , pp. 59±90. Pearce, J.A., 1982. Trace element characteristics of lavas from destructive plate boundaries. In: Thorpe, R.S. (Ed.). Andesites. Wiley, New York, pp. 525±548. Pearce, J.A., 1983. Role of the sub-continental lithosphere in magma genesis at active continental margins. In: Hawkesworth, C.I., Norry, M.J. (Eds.). Continental basalts and mantle xenoliths. Nantwich, Shiva, pp. 230±249. Pearce, J.A., Harris, N.B.W., Tindle, A.G., 1984. Trace element discriminations for the tectonic interpretation of granitic rocks. Journal of Petrology 25, 956±983. Peccerillo, A., Taylor, S.R., 1976. Geochemistry of Eocene calc-alkaline volcanic rocks from the Kastamonu area, northern Turkey. Contributions to Mineralogy and Petrology 58, 63±81. Pilger, R.H., 1981. Plate reconstructions, aseismic ridges, and low-angle subduction beneath the Andes. Geological Society of America Bulletin 92, 448±456. Pilger, R.H., 1984. Cenozoic plate kinematics, subduction and magmatism, South American Andes. Journal Geological Society of London 141, 793±802. Polanski, J., 1978. CarboÂnico y PeÂrmico en la Argentina. Second ed. Eudeba, Buenos Aires pp. 216. Ragona, D., Anselmi, G., GonzaÂlez, P., Vujovich, G., 1995. Mapa GeoloÂgico de la Provincia de San Juan, RepuÂblica Argentina. Servicio GeoloÂgico y Minero Argentino, Buenos Aires. Ramos, V.A., 1988. The tectonic of the Central Andes: 308 to 338S latitude. Processes in Continental Lithospheric Deformation, Clark, S., Burch®el, D. (Eds.). Geological Society of America, Special Paper 218, 31± 54. Ramos, V.A., 1993. InterpretacioÂn TectoÂnica. In: Ramos, V.A. (Ed.). GeologõÂa y Recursos Naturales de Mendoza. Relatorio XII Congreso GeoloÂgico Argentino. AsociacioÂn GeoloÂgica Argentina. Instituto Argentino del Petroleo, pp. 257±266. Ramos, V.A., Kay, S.M., 1991. Triassic rifting and associated basalts in the Cuyo Basin, Central Argentina. Andean magmatism and its tectonic setting, Harmon, R.S., Rapela, C.W. (Eds.). Geological Society of America, Special Paper 265, 79±91. Ramos, V.A., Jordan, T.A., Allmendinger, R.W., Kay, S.M., Cortes, J.M., Palma, M.A., 1984. Chilenia: un terreno aloÂctono en la evolucioÂn Paleozoica de los Andes Centrales. Actas IX Congreso GeoloÂgico Argentino 2, 84±106.

N. Heredia et al. / Journal of South American Earth Sciences 15 (2002) 79±99 Ramos, V.A., Jordan, T.A., Allmendinger, R.W., Mpodozis, S., Kay, S.M., CorteÂs, J.M., Palma, M.A., 1986. Paleozoic Terranes of the Central Argentine±Chilean Andes. Tectonics 5, 855±880. Ramos, V.A., Cegarra, M., Cristallini, E., 1996. Cenozoic tectonics of the High Andes of western-central Argentina (308±368S latitude). Tectonophysics 259, 185±200. Riba, O., 1976. Syntectonic unconformities of the Alto Cardener, Spanish Pyrenees: a genetic interpretation. Sedimentary Geology 15, 213±233. RodrõÂguez FernaÂndez, L.R., Heredia, N., MarõÂn, G., Quesada, C., Robador, A., Ragona, D., CardoÂ, R., 1996a. TectonoestratigrafõÂa y estructura de los Andes Argentinos entre los 30830 0 y 31800 0 de latitud Sur. Actas XIII Congreso GeoloÂgico Argentino 2, 111±124. RodrõÂguez FernaÂndez, L.R., Heredia, N., Gallastegui, G., Quesada, C., Robador, A., MarõÂn, G., CardoÂ, D.R., 1996. Texto explicativo de la Carta GeoloÂgica No 3169-14 a escala 1: 100,000 (Paraje de CastanÄo Viejo). Servicio GeoloÂgico y Minero Argentino, pp. 145 (unpublished). RodrõÂguez FernaÂndez, L.R., Heredia, N., Espina, R.G., Cegarra, M.I., 1999. EstratigrafõÂa y estructura de los Andes Centrales argentinos entre los 308 y 318 de latitud Sur. In: Busquets, P., Colombo, F., PeÂrez EstauÂn, A. (Eds.). GeologõÂa de los Andes Centrales. Acta GeologõÂa HispaÂnica, 32. , pp. 93±102.

99

Rosello, E., Mozetic, M.E., 1999. CaracterizacioÂn estructural y signi®cado geotectoÂnico de los depocentros cretaÂcicos continentales del centrooeste argentino. 58 Simposio sobre el CretaÂceo do Brasil (Rio Claro). Boletim, 107±113. Sato, A.M., LlambõÂas, E.J., 1993. El grupo Choiyoi, provincia de San Juan: equivalente efusivo del batolito de ColanguÈil. Actas XII Congreso GeoloÂgico Argentino, II Congreso de ExploracioÂn de Hidrocarburos 4, 156±165. Sato, A.M., LlambõÂas, E.J., Shaw, S.E., Castro, C.E., 1990. El Batolito de ColanguÈil: modelo del magmatismo neopaleozoico de la Provincia de San Juan. Relatorio de GeologõÂa y Recursos Naturales de la Provincia de San Juan. XI Congeso GeoloÂgico Argentino, AsociacioÂn GeoloÂgica Argentina pp. 100±123. Uliana, M.A., Biddle, K.T., 1988. Mesozoic±Cenozoic paleogeographic and geodynamic evolution of southern South America. Revista Brasilera de Geociencias 18, 172±190. Uyeda, S., 1983. Comparative subductology. Episodes 1983 (2), 19±24. Winchester, J.A., Floyd, P.A., 1977. Geochemical discrimination of different magma series and their differentiation products using inmobile elements. Chemical Geology 20, 326±348.