THE ACTIVE ANDEAN OROGENIC FRONT AT THE SOUTHERNMOST PAMPEAN FLAT-SLAB

The surficial expression of the Andean thrust front at the southernmost Pampean flat-slab (32º-33ºS) is characterized by thrust systems reflecting the overprint of the Precordillera and Sierras Pampeanas inherited structures. Quaternary deformation are concentrated along two NNW-trending east-verging Precordilleran-type thrusts (Las Higueras and Las Peñas thrusts). These thrusts are expressed by monoclinal scarps and propagation folds affecting the youngest alluvial cover. Progressive unconformities and cumulative deformation of several terraces across the Las Peñas thrust suggest a continuous interaction between the range front uplift and alluvial fan sedimentation.

The west-verging Pampean-type thrusts are represented by two NNE-trending basement-cored structures (Pedernal and Cerro Salinas thrusts). The western one belongs to the Sierra de Pedernal where recent uplift and fault displacement have been concentrated along the range bounding Pedernal thrust. However, Quaternary deformation are buried by coarse gravitational deposits. The Cerro Salinas thrust has concentrated a considerably lower uplift and neotectonic features are related to several parallel flexural-slip secondary faulting. Interactions between these thrust systems determines two Y-shaped junctions. In the northern one, compressive Quaternary deformation between Las Peñas and Pedernal thrusts might suggests a surficial expression of transfer zones and fault linkages. Whereas the Montecito growing anticline highlights the southern end of the Cerro Salinas thrust system against Las Peñas (32º30'S). South of this junction the Las Peñas thrust loses its topographic signature becoming a south-plunging anticline. Main Quaternary deformation are here expressed by the Precordilleran-type Las Higueras-La Cal thrust and gentle quaternary growing anticlines (Capdevila-Borbollón).

Historic earthquakes as well as the northward migration of a major drainage as the Mendoza river, suggest that the eastern margin of the current S-N (formerly W-E) Mendoza river alluvial plain, might roughly indicates the location of an easternmost active thrust (Pampean type?) in its initial stage of landscape expression.