EVOLUTION OF THE PAMPEAN FLAT-SLAB SEGMENT OF THE SOUTHERN CENTRAL ANDES

Oceanic and seismological data constrain the Cenozoic interaction between the Nazca and South American plates, pointing out the timing and latitude of the collision of the Juan Fernandez aseismic ridge. Examination of the changes in the magmatism, time of uplift of the Main Andes, and the rupture of the Pampean foreland, show a close link between shallowing of the subducted oceanic plate and these processes. As a result of that, the Sierras Pampeanas have been uplifted in Late Miocene times. Fission track data on the basement rocks, geochronologic and magnetostratigraphic studies in the synorogenic deposits, together with some isotopic studies, show a wave of deformation from north and west, to south and east through time. The effects of shallowing subduction are first recorded by the changes in the composition of the magmatic rocks, and the expansion and migration of the arc-related rocks to the foreland until the final cessation of magmatism. Deformation started as a thin-skinned thrust-belt in the Principal Cordillera of the High Andes in the Early Miocene. It changed to the Frontal Cordillera as a thick-skinned thrust-belt after 9 Ma, at about one million year later than the time of collision of the Juan Fernandez oceanic ridge against the trench at these latitudes. Expansion and migration of the magmatism preceded tectonic uplift of the Sierras Pampeanas, as if a crustal thermal weakening was required to develop brittle-ductile transitions in the crust to foster rupture and uplift. Present neotectonics is concentrated in the interaction between Precordillera and Sierras Pampeanas, although minor ruptures occur in most of the Sierras Pampeanas broken foreland. Inception of the Quaternary and previous tectonic activity has been enhanced by weakness zones in the crust such as sutures, that have been reactivated as rift systems during the opening of the South Atlantic ocean, and later on contracted during Andean orogeny.