[formula omitted] wave teleseismic tomography of the subducted Chile rise

Abstract

We investigate the crustal and upper mantle structure of the Aysén region of Chile from 44° to 49° S, a place where the diverging oceanic Nazca and Antarctic plates subduct beneath the South American continent. The Seismic Experiment in the Aysén Region of Chile (SEARCH) project operated a network of up to 60 land-based seismometers in this region between 2004 and 2006, centered over a 6 Myr old subducted spreading center. The data are used to examine the P wave velocity structure beneath the region using relative-arrival teleseismic travel time tomography, using 2534 P wave residuals from 173 teleseismic earthquakes. It is possible to image the velocity structure beneath the seismic network from ∼30 down to ∼300 km depth. The model can resolve structures at a spatial scale between ∼60 and ∼200 km and shows a large difference between the northern and southern parts of the region. To the north, a ∼100 km thick fast anomaly exists which dips away from the subduction trench; likely to be related to the subducting Nazca plate. Going to the south, as the age of this plate at the subduction trench decreases and arc volcanism shuts off, the fast anomaly migrates further from the trench, suggesting that the Nazca plate subducts at a reduced angle over a larger distance before the subduction angle steepens. The distinct sections of the fast anomaly suggest that slab tears exist across the fracture zones between subducted plate segments. Where the 6 Myr old subducted ridge segment is predicted to lie, there is a region of low velocity between ∼100 and ∼300 km depth, and no fast region associated with a subducting slab is present, indicating the presence of an asthenospheric window between the subducted Nazca and Antarctic plates.