Deciphering the controlling mechanism for Andean mountain building in Central Chile: Preliminary Results

The Andes has been widely considered as an orogenic belt formed under an ocean-continent subduction. During the last decade, however, there is a tendency to accept that the Andes are the result of intraplate shortening as the expression of an intracontinental subduction, with long debate between two proposed opposite models, with the Andean subduction synthetic and antithetic to the oceanic plate subduction. In the debatable place, located in Central Chile-Argentina (33.3°-34.4°S), much of the evidence, which supports the westward subduction of the eastern South American lithosphere beneath the Andes, has been obtained by surface geological studies and some geophysical survey. Nevertheless, the prolongation at depths is still unknown. Our study objective is find evidence at depths of the structural architecture of the Andes in order to unveil the polarity of the main intracontinental subduction. Using wedge theory, we anticipate that the Coastal Cordillera corresponds to the oceanic subduction wedge, while the Andes are an orogenic intracontinental wedge with a main detachment dipping to the west. In this contribution, we test the proposed wedge model using a 3D seismic waves tomography with data coming from a 33 short period, 3 components, continuous recording stations that were in operation during the first semester of 2018 in the Chilean Andes. More than 6000 microearthquakes were recorded, corresponding to approximately 60.000 P and S wave arrival times; with this dataset we obtained a 3D body wave velocity model in the study area, which preliminary shows clearly the differential geometry of the Coastal and Andes wedges. It is interesting note that the distance between both wedges decreases to the north, in coincidence with the lack of a forearc basin (Central Depression) further north. This coincides with the better developed frontal accretionary complex to the south, while tectonic erosion has been prevailed to the north. We explore and interpret jointly surface structures, deformation timing and exhumation with seismological model the subsurface prolongation of this structures, giving new data supporting the westward subduction of the South American lithosphere beneath the Andes.