Complex Anisotropy of Lowermost Mantle Beneath the Southeastern Pacific : Evidence from SKS–SKKS Splitting Discrepancies

Abstract:

The D″ region, the lowermost 200–300 km of the mantle, exhibits seismic anisotropy often in both radial and azimuthal geometries and displays remarkable properties in contrast to the bulk of the lower mantle (usually assumed isotropic) above. Shear wave splitting of SK(K)S phases is often used as a common tool to evaluate upper mantle anisotropy. However, splitting of these phases can also constrain anisotropy in the lowermost mantle. SK(K)S phases have paths through lowermost mantle that are more vertical (incidence angles of ~30°–55°), potentially providing different sampling of D″ anisotropy and a different pattern of the anisotropic geometry than only using S, ScS, or Sdiff phases. Here we report observations of SKS–SKKS splitting measurements observed at10 stations (GSN network)in South America that sample D″ beneath Southeastern Pacific Ocean. About 10% of these show significantly different splitting between the two phases, which likely reflects a contribution from lowermost mantle anisotropy. Phases that have sampled the boundary of the Pacific large low shear velocity province (LLSVP) exhibit quite different splitting . Our results can be explained by deformation concentrations discrepancies of D″ region just outside the LLSVP boundary and its interior.