Paleozoic to Mesozoic Geodynamic Evolution of the Sierras Pampeanas

The metamorphic, magmatic and structural history of the Eastern and Western Sierras Pampeanas is the result of several accretional phases. Accretion occurred from Late Proterozoic to Devonian times and corresponds with the docking of different allochthonous and parautochthonous terranes along the southwestern continental margin of Gondwana. According to the terranes involved, these phases can be divided into the Pampean, Famatinian and Achalian cycles (e.g. Ramos 1988; Rapela et al.

1998; Sims et al. 1998; Astini and Thomas 1999; Astini and Dávila 2004; Steenken et al. 2004; 2006, Miller and Söllner 2005). Potential sutures indicate the margins of adjacent terranes, e.g. between the Río de la Plata Craton and the Pampia, Famatina and Cuyania terranes (Fig. 1.5).

Fig. 1.5: Simplified geo-logical sketch map of the Sierras Pampeanas. Dashed lines indicate inferred position of Precambrian to Paleozoic terrane boundaries (modified according to Ramos 2010, based on van Gosen 1998).

Post-accretional basement cooling in the eastern and western Sierras Pampeanas is traced by Ar-Ar and K-Ar cooling ages (see also Figs. 1.1 and 8.1). In general, high-temperature cooling propagated from the east to west (e.g. Steenken et al. 2004, 2010). In the Sierra de Córdoba, Ar-Ar ages from Muscovite document passage through Ar-retention temperatures (ca. 400°C; McDougal and Harrison 1999) in Cambrian times (500-480 Ma; Krol and Simpson 1999; Martino et al. 2007). Subsequent cooling is documented by Ar-Ar and K-Ar biotite cooling ages, recording passage through temperatures between 350°C and 300°C, respectively (McDougal and Harrison 1999; Willigers et al.

2001), in Ordovician to Silurian times (480-420 Ma; Krol et al. 2000, Martino et al. 2007). In the more western Sierras de San Luis and Chepes, passage through Ar-retention temperatures occurred in

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Ordovician to Devonian times (460-380 Ma; Sims et al. 1998; Steenken et al. 2004, 2007a). The youngest K-Ar biotite ages even yield Early Carboniferous ages (Steenken et al. 2004, 2010). Those differences in thermal evolution between the eastern (Sierra de Córdoba) and western (Sierras de San Luis and Chepes) basement domains were caused by differential movement. The latter was accommodated by westward thrusting along north-south-striking crustal shear zones, developed during the Famatinian Orogenic cycle, e.g. in the most western parts of the Sierra de Córdoba (Fig. 1.6; Whitmeyer and Simpson 2003, 2004; Simpson et al. 2003; Martino 2003). Ar-Ar and K-Ar muscovite ages of mylonites from these shear zones indicate that deformation persisted until the Late Devonian (Krol and Simpson 1999; Martino et al. 2005). In addition, K-Ar-dating on Illite fine-fraction ages from phyllitic rocks of the San Luis Formation (Sierra de San Luis; Fig. 1.6) indicates that differential movement between basement blocks were reactivated in Carboniferous to Permian times (Wemmer et al. 2011). The authors note that ages are synchronous to a widespread period of intra-Carboniferous compressional tectonism, e.g. the Toco Orogeny of Bahlburg and Breitkreuz (1991). The latter defines a compressional tectonic phase expressed, for example, by a hiatus between Lower Carboniferous (Mississippian) and Permian sequences of the Paganzo Formation (e.g.

Bahlburg and Breitkreuz 1991).

In the Late Paleozoic, the Paganzo basin was formed due to the final orogenic collapse and extension as a consequence of Early Paleozoic mountain building (Salfity and Gorustovic 1983; Mpodozis and Ramos 1989; Ramos et al. 2002). Sediments of the Paganzo basin were widely developed in northwest Argentina (Fig. 1.7; e.g. Gonzalez and Aceñolaza 1972), comprising mainly Carboniferous alluvial sediments, tillites, claystones and lacustrine sediments which are overlain by Lower Permian (Cisuralian) arid to semi-arid aeolian sequences (Limarino and Césari 1988; Césari and Guitiérrez 2000; Limarino et al. 2002; Martins-Neto et al. 2007). Additionally, relicts of alkaline volcanism are associated with the formation of the Paganzo basin (e.g. Ramos et al. 2002).

In the northern Sierras Pampeanas, the maximum accumulated thickness of Paganzo strata is about 1800 m (Azcuy 1999), while, in the study region, the record of Paganzo strata is scarce. Trapped and preserved sedimentary records of this time period can be found in minor outcrops at the foot of the Sierra de Pocho and the Sierra de Chepes, as well as in the Bajo de Véliz region in the northeastern Sierra de San Luis (Fig. 1.7 and 1.8; Hünicken and Penza; 1980; Hünicken et al. 1981; Limarino and Spalletti 2006). The latter region is also considered to represent the southeasternmost branch of the Paganzo basin (Fig. 1.7; e.g. Fernandez-Seveso and Tankard 1995; Césari and Guitiérrez 2000;

Limarino and Spalletti 2006; Martins-Neto et al. 2007).

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Fig. 1.6: Simplified geological map of the Eastern Sierras Pampeanas with indicated location of main brittle and ductile fault zones.

The re-onset of compressional tectonics after Late Paleozoic extension is documented by the rhyolitic volcanism of the Choiyoi Group, which discordantly overlay the Paganzo Formation to the west of the Sierras Pampeanas (e.g. Llambías et al. 2003; Japas and Kleimann 2009). Additional indications come from the scarce remnants of clastic sediments and the emplacement of granitoids in the area of the Cuyania terrane (see Llambías and Sato 1990; Mpodozis and Kay 1992).

A second phase of extensional tectonism during the Late Triassic to Early Jurassic led to reactivation of Late Proterozoic to Plaeozoic terrane boundaries (Fig. 1.5). Extensional movement along these faults resulted in the formation of localized, NNW-trending basins and the deposition of non-marine, clastic sediments (Figs. 1.7 and 1.8). Remnants of these depocenters can be found along the Valle Fértil fault in the Western Sierras Pampeanas, i.e. the Ischigualasto (Valle de la Luna; Fig, 1.4 i), Marayes, Las Salinas and Beazley basins (Criado Roque et al. 1981; Aceñolaza and Toselli 1988;

Ramos et al. 2002), where thicknesses of up to 2000 m can be observed (Fig. 1.8). In the Eastern Sierras Pampeanas, significant lower thicknesses are developed (e.g. Jordan et al. 1989), of which potential relicts can be found south of the Sierra de San Luis (Costa et al. 2001). In addition to sedimentation, rifting was accompanied by alkaline intraplate volcanism (e.g. Ramos et al. 2002).

A third extensional episode, linked to the opening of the South Atlantic Ocean, occurred during Early Cretaceous times (e.g. Schmidt et al. 1995, Rossello and Mozetic 1999). Major but narrow rift basins developed, which are mainly located along the eastern and western boarder of the Pampia terrane (Fig. 1.7), partially superimposing the Triassic basins. In the northern Sierras Pampeanas, cretaceous extension caused the formation of the Salta Rift (Salfity and Marquillas 2000). In the Eastern and

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Western Sierras Pampenas, Cretaceous basins accumulated non-marine sediments of up to 2500m thicknesses (e.g. Santa Cruz 1979; Gordillo and Lencinas 1979; Criado Roque et al. 1981; Battaglia 1982; Jordan et al. 1983; Schmidt et al. 1995; Gardini et al. 1996; Gardini et al. 1999; Costa et al.

2000; Costa et al. 2001; Ramos et al. 2001; Ramos et al. 2002). Sedimentary remnants of those basins can be found in the Sierra de Norte and along the western side of the Sierra Chica and Sierra de Valle Fértil (Fig. 1.7 and 1.8). In the Western Sierras Pampeanas, extension and basin formation was accompanied by alkaline magmatism (Ramos et al. 2002).

Late Triassic to Early Jurassic as well as Early Cretaceous sedimentation caused highly variable sediment distribution throughout the Sierras Pampeanas and resulted in a quite irregular sedimentary basement cover (Fig. 1.8). Although the original extent of the Mesozoic depocenters is masked by erosion of the basin remnants (Furque 1968; Gonzalez and Aceñolaza 1972; Lucero Michaut and Olsacher 1981; Ramos 1982; Salfity and Gorustovich 1983; Jordan et al. 1983; Schmidt et al. 1995) the rather asymmetrical pattern of Mesozoic sediment distribution (Fig. 1.8) indicates that large areas of the central and eastern Sierras Pampeanas had little or no sedimentary cover (e.g.

Jordan et al. 1983, Schmidt et al. 1995). Additionally, the continuance of a pronounced positive relief in the Sierras Pampeanas is indicated by the absence of marine sediments of the Paleozoic or Mesozoic age (e.g. Lencianas and Timonieri 1968).

Fig. 1.7: A) Late Paleozoic Paganzo basin with main depocentres (based on Salfity and Gorustovic (1984) and Koukharsky et al. (2002), modified from Ramos et al. 2002). B) Location of major rift systems of TriassicEarly Jurassic and Early Cretaceous ages (based on Ramos 2000, modified from Ramos et al. 2002). BV: Bajo de Véliz.

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Fig. 1.8: Geographical distribution of Mesozoic sediments in the Sierras Pampeanas. Sedimentary thicknesses according to Jordan et al. 1989 and Carignano et al. 1999.