Abstract

The San Ramon Fault (SRF) runs for 30 km along the western flank of the Andes in front of the city of Santiago, Chile (33.5 degrees S). Geological studies have highlighted the SRF role in the Neogene uplift of the Andes at this latitude, but the fault geometry at depth is not well constrained. Here we infer the structure of the sedimentary cover and bedrock up to a depth of 500 m along the SRF by integrating gravimetric, electrical resistivity, seismic, and magnetic methods. In a section crossing the central segment of the SRF, lateral variations inPwave velocity models indicate that the sedimentary cover is more deformed close to SRF scarps, while coincident low-density and low-resistivity zones suggest that the presence of two depocenters in the sedimentary cover below the SRF scarps. Gravimetric profiles distributed along the entire trace of the SRF show the same configuration of two depocenters, but the geometry of these depocenters is complex and varies along strike. Our findings suggest that the bedrock and sediments deformation along the SRF exhibit a complex geometry, which can be explained by the interplay of reverse and strike-slip movements during the late Cenozoic. Based on our results, the distribution of crustal seismicity and shortening rates interpreted across the Andes, we estimated that SRF deformation represents 5-20% of the whole Andean shortening for the last 4 Ma. Using empirical length scales for crustal seismicity, this deformation pattern is consistent with a maximum earthquake magnitude in the range of 6.0-6.5 Mw.