Seismic structure of the north‐central Chilean convergent margin: Subduction erosion of a paleomagmatic arc

Abstract

Abstract We study the erosive convergent margin of north‐central Chile (at ~31°S) by using high‐resolution bathymetric, wide‐angle refraction, and multichannel seismic reflection data to derive a detailed tomographic 2‐D velocity‐depth model. In the overriding plate, our velocity model shows that the lowermost crustal velocities beneath the upper continental slope are 6.0–6.5 km/s, which are interpreted as the continental basement composed by characteristic metamorphic and igneous rocks of the Coastal Cordillera. Beneath the lower and middle continental slope, however, the presence of a zone of reduced velocities (3.5–5.0 km/s) is interpreted as the outermost fore arc composed of volcanic rocks hydrofractured as a result of frontal and basal erosion. At the landward edge of the outermost fore arc, the bathymetric and seismic data provide evidence for the presence of a prominent trenchward dipping normal scarp (~1 km offset), which overlies a strong lateral velocity contrast from ~5.0 to ~6.0 km/s. This pronounced velocity contrast propagates deep into the continental crust, and it resembles a major normal listric fault. We interpret this seismic discontinuity as the volcanic‐continental basement contact of the submerged Coastal Cordillera characterized by a gravitational collapse of the outermost fore arc. Subduction erosion has, most likely, caused large‐scale crustal thinning and long‐term subsidence of the outermost fore arc.