Cenozoic Geodynamic Evolution

In the Cenozoic, the almost orthogonal subduction of the Nazca Plate beneath the South American Plate resulted in the onset of Andean compression, which led to the development of the Andean orogen (e.g. Windley 1995). ENE-directed subduction with rates of up to 85 mm/a led to crustal shortening and, in general, to an east-verging geometry of the Andean Orogen. Eastward propagation of crustal shortening resulted in the progressive exhumation of individual mountain ranges which are oriented almost parallel to the Andes (e.g. Mpodozis and Ramos 1989; Russo and Silver 1996; Yañez et al. 2002; Ramos et al. 2002). The present-day Sierras Pampeanas south of 26°S comprise an exception to this trend (Figs. 1.3 and 1.9). Here, the Cenozoic reactivation of Paleozoic and Mesozoic basement structures resulted in generally west-verging geometry (see above).

During Miocene times (18-11 Ma), the Juan Fernández ridge was incorporated into the subduction of the Nazca plate, which resulted in a shallowing of the subduction angle from around 30° to about 5-10° in the area between 27°S and 33°S, just below the extent of the present-day Sierras Pampeanas (Fig. 1.9; Barazangi and Isacks 1976; Pilger 1981; Jordan and Allmendinger 1986; Yañez et al. 2001;

Ramos et al. 2002). During Upper Miocene to Pliocene times, the subduction-related volcanic activity shifted, associated with the shallowing of the subduction angle, from the Andes eastward to the Sierras de San Luis and Pocho, more than 650 km away from the Peru-Chile trench (e.g. Pilger 1984;

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Isacks 1988; Gütscher et al. 2000; Kay and Mpodozis 2002; Ramos et al. 2002). The eastward shift of magmatic activity towards the Sierras Pampeanas left a gap in the chain of active arc volcanism of the Andes from 28°S to 33°S, between the central volcanic zone in the north and the southern volcanic zone in the south (Fig. 1.9).

In the Sierra de San Luis, magmatic activity formed a WNW trending belt between La Carolina and El Morro (Fig. 1.9; Kay et al. 1991). Volcanic rocks of this belt comprise andesites, dacites, latites and trachytes with a typical subduction signature (Kay et al. 1991; Kay and Gordillo 1994). Along this ”San Luis Volcanic Belt”, ages decrease from 9.5-6.3 Ma in the west to 1.9 Ma in the east, documenting an ongoing eastward propagation of volcanic activity, which is correlated with the eastward propagation of the flat-slab beneath. The latter age represents the youngest magmatic manifestation associated with this flat-slab subduction event (Llambías and Brogioni 1981; Sruoga et al. 1996; Urbina et al.

1997; Urbina 2005; Ramos et al. 1991, 2002). A similar trend of eastward rejuvenating Miocene to Pliocene volcanic activity is also reported for the Sierras de Aconquija and Cumbres Calcaquies, the Sierra de Famatina and for the Sierra de Pocho (see Ramos et al. 2002 and references therein).

Fig. 1.9: Location of the Pampean flat-slab segment in Argentina and Chile and associated volcanism.

Contours indicate depth to Wadati-Benioff zone, indicating the oceanic slab, modified from Ramos and Folguera 2009 and Ramos et al. 2002; Location of neotectonic faults associated to flat-slab subduction according to Costa at al. 2006; outline of Wadati-Benioff zone according to Cahill and Isacks (1992).

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Fig. 1.10: Evolution of arc magmatism through time in the Pampean flat slab segment. Ages after Ramos et al. (2002) and references therin. Countourline indicate 200 km depth corresponding to the oceanic slab.

Modified, from Ramos and Folguerea 2009

Isacks et al. (1982) and Jordan et al. (1983) were the ones to first introduce the hypothesis that the flat-slab subduction segment and associated Neogene compressional tectonic can be linked to the formation of the present-day Sierras Pampeanas. This idea was corroborated by further studies, e.g.

Jordan and Allmendinger (1986), Ramos et al. (1991), Kay and Abbruzzi (1996) and Ramos et al.

(2002). In short, models suggest that the uplift of the Sierras Pampeanas is related to an increase in heat flow associated with the eastward migration of the magmatic arc activity due to the flat-slab subduction. After a residence time of 2.6 – 4 Ma, the increased heat flow led to thermal weakening of the crust which caused an elevation of the brittle–ductile deformation transition. This led to the development of décollments and, together with pre-existing crustal anisotropies, to a failure of the crust. The latter resulted in the thick-skinned basement uplifts of the Sierras Pampeanas through tilt and rotation of the basement-cored blocks (González Bonorino 1950; Jordan and Allmendinger 1986;

Intracaso et al. 1987; Ramos et al. 2002). Seismic data reveal the position of the main décollment near the crust-mantle boundary at 38 km depth (Fig. 1.11; e.g. Snyder et al. 1990; Regnier et al. 1992;

Cristallini et al. 2004; Fromm et al. 2004; Brooker et al. 2004).

The uplift of the Pampean basement blocks was mainly controlled by older structures such as Proterozoic to Paleozoic sutures and the extensional Mesozoic fault systems (see Chapter 1.3), as well as inherited crustal weaknesses (e.g. Criado Roque et al. 1981; Schmidt et al. 1995; Gardini et al.

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1996, 1999; Costa et al. 2000, 2001). The Neogene inverse reactivation of these structures (Fig. 1.9) generally resulted in a listric fault shape with a dominant dip to the east, which is expressed by a distinct morphologic asymmetry of the Pampean mountain ranges, i.e. a steep western and gentle eastern slope (see Figs. 1.4 b,c,d; e.g. González Bonorino 1950; Gordillo and Lencinas 1979; González Diaz 1981; Jordan and Allmendinger 1986; Introcaso et al. 1987; Costa and Finzi 1996; Urreiztieta et al. 1996). The main uplift phase of the Sierras Pampeanas, due to the flat-slab subduction period, is thought to have been completed in Late Pliocene times (Ramos et al. 2002). Additionally, it is inferred that Neogene block uplift led to the development of the present-day mountainous landscape with peak elevations of 2200-2800 m in the Sierras de San Luis and Córdoba, which tower the surrounding Pampa by 2300 m (see Chapter 1.1).

Fig. 1.11: Schematic crustal cross-section of the Andes and Sierras Pampeanas at 31° and 33°S; see Fig. 1.3 (modified, from Ramos et al. 2002).

After the Mesozoic, sedimentation did not resume until the Eocene (Santa Cruz 1972). Sediments of the Late Miocene age are scarce in the Eastern Sierras Pampeanas but can be found in the Valle de Punilla between the Sierras Grande and Chica. Those sediments are interpreted as synorogenic deposits, indicating the existence of a positive relief in this region prior to Neogene Andean Orogeny.

In Neogene times, sedimentation continued due to the onset of Andean compression and the uplift

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of the Eastern Sierras Pampeanas (see Chapter 1.4). Upper Miocene to Pliocene sedimentation in the eastern Sierras Pampeanas led to deposition of fluvial and alluvial piedmont sediments of up to 180 m thicknesses (Casa Grande Formation in the Valle de Punilla in between the Sierras Grande and Chica; Lencinas 1971). Andean deformation can also be traced in Pleistocene piedmont deposits derived from the Pampean ranges, which are mostly comprised of conglomerates. The thickness of sediments above the Casa Grande Formation is restricted to a maximum of 90 m (Kull and Methol 1979; Sayago 1979; Carignano 1997).