Subslab anisotropy in the Andaman subduction zone controlled by slab dip?

Abstract

The mechanisms that govern anisotropy in subduction zones are debated. In this study, we present evidence of subslab mantle, crustal and mantle wedge anisotropy in the Andaman Nicobar subduction zone using 52 teleseismic and 103 local shear wave splitting (SWS) measurements. The local SWS measurements reveal a dominant trench-parallel anisotropy in the crust that can be attributed to SPO associated with arc-parallel fault planes of normal earthquakes and strike-slip faults parallel to the trench. 3D-flow from the subslab mantle possibly caused by a tear in the subducting slab is a plausible mechanism for anisotropy in the mantle wedge. Synthesis of the present results with the previous ones from the Andaman-Sumatra-Java arc reveals a clear relation between the fast polarization azimuths in a ray coordinate system (ϕ∗) and slab dip. The sub-slab anisotropy is trench-parallel in steeply subducting (>40° in Java) to trench-normal in gently subducting segments in the Andaman and Sumatra regions.