Deep source regions for Patagonia backarc volcanism imaged by finite frequency body wave tomography

The subduction of Chile Ridge beneath South America beginning 12-16 Myr ago opened a gap in the subducting slab beneath southern Patagonia, which migrated northward and is located today at 46oS. Geodynamic processes associated with the slab window are poorly understood. Here we apply finite-frequency P and S body wave tomography to seismic data from several temporary arrays as well as regional stations to image seismic heterogeneities down to 650 km depth. The results show strong low velocity anomalies extending to 400 km depth beneath recent back-arc volcanism between 46oS and 48oS, suggesting a link to thermal upwelling in the upper mantle. The southern edge of the Nazca slab extends aseismically down to at least 350 km and has steeper dip than previously suggested. We also image low upper mantle seismic velocities beneath the Patagonia icefields, suggesting low viscosity modulates the patterns of uplift and horizontal deformation observed by GNSS.