CONICET | Buscador de Institutos y Recursos Humanos

The forces that drove rock uplift of the low-relief, high-elevation, tectonically stable North Patagonian Massif Plateau (NPMP) are the subject of debate in this study. While the adjacent Ñirihuau Basin underwent extension during Oligocene time and compression deformation, reverse faulting and shortening during the Mio-Pliocene (Bechis and Cristallini, 2005), the NPMP only experienced 1200 m of rock uplift without significant internal deformation during the Paleogene, remaining since that time as an elevated Plateau.Present back arc at northern Patagonia (37º to 44º S) shows a heterogeneous continental crust, at which, several basins of different ages (Jurassic to Cenozoic), with some degree of deformation (folding, tilted block faulting) surround an almost undeformed massif (strike and slip faulting).The area bounded by Limay, Gastre, Los Chacays and Gualicho alignments is known as the core of the NPMP. With a surface of 100.000 km2, is a large plateau that stands at an average altitude of 1200 meters above sea level and surpasses the surrounding country rocks by 400 up to 700 meters. The massif is built by early and late Paleozoic metamorphic complexes intruded by Ordovician, Devonian, Carboniferous Permian, and Triassic plutonic rocks (Pankhurst et al., 2006), large Triassic and Jurassic volcanic rhyolitic complexes (Rapela et al., 2005), few Triassic sediments and a thin sedimentary cover that overlay an extended planation surface developedfrom late Jurassic to early Cretaceous time, that include Jurassic, upper Cretaceous (marine) and Tertiary (continental) sediments. Most of the south-eastern side of the Massif is covered by Oligocene plateau basalts that were erupted from the top of the Massif onto the surrounding lower land. Deformation in the massif is restricted to faulting (mostly strike and slip), no tilted block faults, and the thin sedimentary cover shows no folding.The NPMP has a quite different Cenozoic tectono-magmatic history from that of its surrounding country rocks, since it shows a sudden Paleogene uplift from below sea level at Danian time to above 1200 meters at early Oligocene (the 30 Ma Somun Cura basalt plateau lavas spilled from the top of the already uplifted Massif). As a consequence of this uplift the Massif has no internal deformation and most Danian marine sediments stand sub-horizontal at an average altitude of 1200 meters above sea level. Instead the surrounding country rocks show different degrees of deformation that include faulting and folding related to an upper Miocene tectonic reversal (Bechis and Cristallini, 2005). Geophysical properties such as Bouguer gravity anomalies and P wave velocities may provide a robust geodynamic interpretation of observed contrasts between the NPMP and the surrounding country, as preliminary shown in Fig.1, where lower seismic velocities and more negative Bouguer gravity are present on the massif side. Here we propose to study present day North Patagonian Plateau and surrounding country lithosphere and to make further advance on the characterization of the rheological structure of the massif through the 3D densityand thermal modelling and elastic thickness estimations that may help to better understand the present day isostatic support of the massif, and then its possible relationship with the Cenozoic geodynamic evolution. This modelling will be constrained using upper mantle and lower crust xenoliths data. Present day information suggest an upper Proterozoic (Re-Os modelled TRD minimum depletion ages) core at the NPM (Praguaniyeu xenoliths locality) is surrounded by an outer boundary of Mesozoic and Cenozoic (Re-Os TRD ages) xenoliths localities (Schilling and Tassara, 2008). Finally, a consistent processing of repeated GPS observations from an ad hoc network installed in the area will provide a direct measurement of the relative motion of the massif with respect to its surroundings, including vertical movements? estimators and their formal uncertainties.