POST-PLIOCENE DEFORMATION AND UPLIFT OF THE PATAGONIAN ANDES IN RESPONSE TO THE SUBDUCTION OF THE ACTIVE CHILE SPREADING RIDGE (CSR)

The region of Monte Zeballos-Paso Roballos south of Lago General Carrera Buenos Aires (LGCBA) is a key area to understand the Neogene tectonic evolution of the frontal Patagonian Cordillera. New tectonic analysis combined with new geochronological data allow us to provide a more detailed chronology of the deformation stages of the Andean front and to better constrain its morphological evolution. The Upper Jurassic ignimbrites of the Cordillera are thrusted over the continental sandstones of the Miocene Molasse (Zeballos Group, 20-15 Ma). Alkali basalts of the Meseta del Lago Buenos Aires (MLBA, 12 Ma-Present) unconformably overlie the corresponding tectonic contact. The age of the thrust is now well constrained by calc-alkaline volcanics dated at 16-14 Ma (Espinoza et al., this issue) that conformably overlie the upper beds of the Zeballos Group. Since the basal MLBA flows are 12 Ma old, this implies that the frontal Cordillera has been rapidly eroded between 14 and 12 Ma. Moreover, the LGCBA region is characterized by important erosion and relief inversion that is supposed to have started since 5 Ma. Ages of 3 Ma are obtained from an alkalic pluton (Mifeldi pluton) intruding the top of the MLBA basalts and now exposed along the western scarp of the meseta close to Paso Roballos. In this region, the edge of the meseta is tilted along vertical faults trending N140-170 and lava flows emplaced in paleovalleys incised within the molasse are dated at 3 Ma. We therefore confirm that the front of the Cordillera has been rejuvenated recently along transpressional faults. Faulting induced uplift (up to 1 km) and subsequent regional relief inversion associated with exhumation of young plutons. This phase, that started around 3 Ma, can be correlated with the subduction of segment CSR0 of the Chile ridge at the latitude of the LGCBA.