Geometry and kinematics of the Andean thick-skinned thrust systems: Insights from the Chilean Frontal Cordillera (28 degrees-28.5 degrees S), Central Andes

Abstract

The structure of the Chilean Frontal Cordillera, located over the Central Andes flat-slab subduction segment (27 degrees-28.5 degrees S), is characterized by a thick-skinned deformation, affecting both the pre-rift basement and the Mesozoic and Cenozoic infill of the NNE-SSW Lautaro and Lagunillas Basins, which were developed during the Pangea-Gondwana break-up. The compressive deformation show a complex interaction between Mesozoic rift structures and thrust systems, affecting a suite of Permo-Triassic (258 -245 Ma) granitic blocks. We used a combination of geological mapping, new structural data, balanced and restored cross sections and geochronological data to investigate the geometry and kinematics of the Andean thick-skinned thrust systems of the region. The thrust systems include double-vergent thick-skinned thrust faults, basement-cored anticlines and minor thin-skinned thrusts and folds. The presence of Triassic and Jurassic syn-rift successions along the hanging wall and footwall of the basement thrust faults are keys to suggest that the current structural framework of the region should be associated with the shortening of previous Mesozoic half grabens. Based on this interpretation, we propose a deformation mechanism characterized by the tectonic inversion of rift-related faults and the propagation of basement ramps that fold and cut both, the early normal faults and the basement highs. New U-Pb ages obtained from synorogenic deposits (Quebrada Seca and Dona Ana formations) indicate at least three important compressive pulses. A first pulse at 80 Ma (Late Cretaceous), a second pulse related to the K-T phase of Andean deformation and, finally, a third pulse that occurred during the lower Miocene.