The Deformational Journey of the Nazca Slab From Seismic Anisotropy

Abstract

The Andean subduction zone is an excellent place to study deformation within a subducting slab as a function of depth, owing to the varying and well-resolved geometry of the subducting Nazca slab beneath South America. Here we combine the results of source-side shear wave splitting with the latest regional tomography model to isolate intraslab raypaths and determine the spatial distribution of anisotropy within the Nazca slab. We observe that in the upper mantle, the intraslab anisotropy appears strongest where the slab is most contorted, suggesting a strong link between anisotropy and subduction-related slab deformation. We identify a second source of anisotropy (delta t similar to 1 s) within the subducting slab at lower mantle depths (660-800 km). The surrounding mantle and transition zone appear largely isotropic, with deep anisotropy concentrated within the slab as it deforms while entering the higher-viscosity lower mantle.