Resumen
The eastern Andean orogenic front at 36ºS shows unique attributes when compared to neighbor segments, from an anomalous eastward development of the orogenic front area due to the uplift of a zone of reduced topography in the foothills constituting a pre-Andean belt, to the occurrence of great Late Oligocene to Early Miocene within-plate series. The study of this eastern pre-Andean belt from extensive field mapping and analysis of seismic lines reveals a complex evolution that encompasses a pre-Late Oligocene stage of exhumation prior to the Late Oligocene-Miocene within-plate volcanic products which are coetaneous with an extensional relaxation phase, and one last period of contractional deformation that uplifted the pre-Andean sector in Miocene times. This last stage of orogenic construction is associated with synorogenic strata that show coarsening up sedimentary cycles with an eastward provenance. Contractional mechanisms involved deep ramps rooted into the basement, some of which are linked to inverted Late Oligocene-Miocene normal faults, to shallower detachments located in the Upper Cretaceous and Miocene incompetent layers.