Was the Norpatagonian Plateau tilted in the Neogene??

ARAGÓN EUGENIO, AGUILERA EMILIA Y, CAVAROZZI CLAUDIA E., RIBOT ALEJANDRO

Cordoba

Congreso; XIX Congreso Geológico Argentino; 2014

Asociación Gelogica Argentina

The Northpatagonian Plateau (a surface of 100,000 km2) is bounded by four major lineaments: the Gastre lineament to the southwest, the Chacays lineament to the southeast, the Gualicho lineament to the northeast and the Limay lineament to the northwest (Fig. 1), standing 500 metres above the rest of the North Patagonian Massif and surrounding basins (Rabassa, 1978; González Díaz and Malagnino, 1984; Franchi et al., 1984; Corbella, 1984; Aguilera, 2006; Aragón et al., 2011). The surface morphology of the Northpatagonian Plateau has at least three well preserved nearly flat surfaces: (a) the Gondwana planation surface caused by protracted erosion from middle Jurassic to late Cretaceous times; (b) the surface of the Maastrichtian-Danian marine sediments which flooded much of the surface of the Gondwana planation surface (Profile in Fig. 1); (c) the late Oligocene-early Miocene basalt lava flows, that now stand as the Somun Cura basaltic plateau, spilling from the top of the plateau to the surrounding lower lands, and constraining therefore the age of uplift of the Plateau.. This uplift of the Gondwana planation surface from sea level (the Maastrichtian-Danian marine sediments) to the present altitude of 1200 m.a.s.l. could have started in the late Eocene, but the main uplift of the unit has been Oligocene just before the major Somon Cura basaltic lava flows that spilled from the top of the plateau to the surroundings. The tectonic scenario was extensional, and uplift was related to normal faulting (Aragón et al., 2011). Was the plateau tilted after it formed?: To answer this question it is relevant to consider the topography of the plateau previous to the large basaltic Somon Cura lava flows that spilled from the plateau. From figure 1 it can be observed that most basaltic lava flows have radial motions with respect to the emission centres, showing that there was no preferential slope at the time of eruption. Nevertheless, if the topographic position of lavas and underlying Maastrichtian-Danian marine sediments boundary is considered, from El Cain (1100 metres) to the eastern boundary of the Plateau (600 metres) within a distance of 145 km, it can be observed that today there is a regional eastward slope that is post-lava flows. Furthermore, the eastern boundary of the plateau is surrounded by the depressions of Bajo de la Tierra Colorada, Bajo del Gualicho, and Bajo de San Matias (Fig. 2). Geomorphology also suggests an eastward slope for the basaltic surface (Aguilera, 2006). The present eastward slope with the coupled depressions, suggest that the Northpatagonian Plateau was tilted in the Neogene (post-Somon Cura lava flows). If so, then the deformation efforts should have come from the west (the Andes). The extensional scenario of the Eocene-Oligocene is interrupted by the tectonic inversion of the Miocene (Giacosa and Heredia, 2004; Costa et al., 1996). To this respect, a recent study by Bilmes et al. (2013) in the south-western boundary (Gastre Lineament) of the Plateau, show a significant uplift and deformation on the south-western side of the Plateau originated by reverse faulting and inversion of pre-existing normal faults, constraining this event to 14-16 Ma ago. Our interpretation of the new constrains published by Bilmes et al., (2013) suggest that the old Paleogene Norpatagonian Plateau, could have been tilted eastwardly in the Neogene as a consequence of the middle Miocene tectonic inversion that affected the Andes developing a fold and thrust belt to the west of the Plateau. With present time information, it is difficult to make a balance model trying to relate uplift with tilting. The main problem arises from the fact that it is not possible to discriminate to which extent the Neogene uplift by tectonic inversion exceeded the Paleogene uplift of the Northpatagonian plateau. Unfortunately the work of Bilmes et al., (2013) does not consider this situation.