Seismic anisotropy above and below the subducting Nazca lithosphere in southern South America

摘要

The goal of this study is to better constrain anisotropy and mantle flow above and below the Nazca slab from 28S to 42S through modeling of shear wave splitting in local S, SKS and SKKS (SK(K)S) phases. Comparisons of local S splitting times and path lengths in the slab, mantle wedge, and upper plate indicate that splitting times for arc and back-arc stations are consistent with anisotropy in the mantle wedge, but long slab paths to fore-arc stations imply that slab anisotropy is also significant. SK(K)S shear wave splitting observations and models for sub-slab anisotropy show that significant anisotropy is present below the slab, and that the orientation of sub-slab anisotropy sometimes differs from anisotropy above the slab. Anisotropy both above the slab and below the slab in the South American subduction zone is consistent with mantle flow that is driven by a combination of entrainment with downgoing slab motion and flow complexity related to variations in slab shape and slab rollback.