STRATIGRAPHIC STAGES OF BERMEJO BASIN: NEW INSIGHTS FROM SEDIMENTOLOGY AND U/PB DATING

The south central Andes at 30°S represent a key area to understand Andean geodynamics due to its location within the Pampean flat slab subduction segment and the development of the following morphotectonic units from west to east: Coastal Range, Principal Cordillera, Frontal (or High) Cordillera, Rodeo-Iglesia Valley, Precordillera, Bermejo foreland basin and Sierras Pampeanas. , New data demonstrate that more work is necessary in order to unravel the geological history of this classical cross-section. New stratigraphic and and geochronologic data from proximal synorogenic deposits of Precordillera indicate that uplift of this unit is younger than previously proposed. Two zircon U-Pb maximum depositional ages from these deposits indicate that the onset of uplift of the Precordillera at 30°S occurred around 14 Ma. This suggests that Precordillera uplift is nearly synchronous along strike, challenging the hypothesis of a north to south progression of uplift linked to the migration of the subducted Juan Fernandez ridge. On the other hand, maximum ages of deposits previously interpreted as a distal synorogenic environment have ages older than 22 Ma and contain abundant juvenile volcanic clasts. This could indicate either sedimentation in an earlier foreland basin setting related to pre-Miocene uplift of Frontal and Principal Cordilleras, or alternatively sedimentation in retro-arc extensional basins associated explosive magmatism east and west of the studied zone.