Subduction of oceanic asthenosphere: Evidence from sub-slab seismic anisotropy

Abstract

The oceanic asthenosphere is characterized as a low viscosity channel down to 200-300 km depth separating the cold lithosphere from above, and it is intimately linked to a layer of low seismic velocity and prominent seismic anisotropy observed globally beneath ocean basins. While subduction of tectonic plates in convergent margins is well recognized, the fate of oceanic asthenosphere remains enigmatic. We demonstrate that subduction of the oceanic asthenosphere characterized by weak azimuthal anisotropy and strong radial anisotropy explains the essence of sub-slab shear-wave splitting patterns, where the fast splitting direction changes from predominantly trench-parallel (or sub-parallel) under relatively steep subduction zones to frequently trench-normal under shallow subduction zones. To explain the observed splitting time, the thickness of the subducted asthenosphere is estimated to be 100 ± 50 km. © 2012. American Geophysical Union. All Rights Reserved.