Crustal‐scale structural architecture in central Chile based on seismicity and surface geology: Implications for Andean mountain building

Abstract

We document a crustal‐scale structural model for the central Chile Andes based on seismicity and surface geology, which consists in a major east verging ramp‐detachment structure connecting the subduction zone with the cordillera. The ramp rises from the subducting slab at ∼60 km depth to 15–20 km below the western edge of the cordillera, extending eastward as a 10 km depth flat detachment. This structure plays a fundamental role in the Andean orogenesis because most of the shortening has been accommodated by structures rooted in it and allows the distribution of crustal thickening in a “simple shear deformation mode.” Indeed, despite shortening distribution being very asymmetric (∼16 km versus ∼70 km in the western and eastern side, respectively), the western side is higher and thicker than what is expected. Yield strength envelopes show strong rheological control on this structure. Vp and Vp/Vs variations in the upper mantle and in the deepest limit of the seismogenic interplate contact mark the intersection of the ramp with the slab, which coincides with the blueschist‐eclogite transition. Therefore, subduction processes would control the depth where the major east verging structure may merge with the slab. Such a ramp‐flat structure is observed in other parts of the Chilean margin; hence, it seems to be a first‐order feature in the Andean subduction zone. This structure delimitates upward the rocks, transmitting part of the plate convergence stress from the plate interface, and controls mountain‐building tectonics, thus playing a key role in the Andean orogeny.