The State of Subduction in Southern Peru

Abstract:

The crustal thickness in southern Peru has doubled to 70-75 km in the last 40Ma. Various causes are thought to contribute the additional volume including subduction related volcanism and compression and the curvature of the Arica Bend, but the sum of these appears to fall short of accounting for the needed volume. In a recent detailed seismic survey in southern Peru (PeruSE), we have been able to image the suduction zone, including the overriding continent and the slab in the regions of both flat and normal subduction in southern Peru. One surprising aspect of the image is the presence of a mid-crustal layer boundary that is pervasive in the back-arc of both the normal and flat subduction regimes. Our interpretation of this feature is an extension of an older model in which the Brazilian Shield is underthrusting the Andes, with the difference that this process (according to our image) extends under the Western Cordillera, almost to the current subduction arc. This would explain the missing crustal thickness.