Seismic characterization of mantle flow in subduction systems: Can we resolve a hydrated mantle wedge?

Abstract

This study provides new constraints on the resolvability of mantle flow in subduction zone settings as inferred by observations of seismic anisotropy. We are motivated by the broad range of shear wave splitting observations in subduction systems that suggest complex flow geometries, changes in the deformation state of mantle minerals, or a combination of both. While shear wave splitting fast polarization directions are typically interpreted as a proxy for flow or maximum finite extension, experimental studies suggest that olivine slip systems change under higher stress and hydration states, conditions likely appropriate for subduction systems. In this study, we predict shear wave splitting as a result of mantle silicate lattice-preferred orientation development in steady-state two-dimensional mantle flow models using a textural development theory that incorporates the combined effects of intracrystalline slip and dynamic recrystallization. We utilize the resulting textures to predict shear wave splitting for populations of seismic raypaths traversing the model within the subduction zone mantle wedge. The results of our study demonstrate that combined observations of variations in fast polarization directions and splitting times make it possible to resolve a shift from anhydrous to hydrous mantle insubduction zone settings provided very good sampling of the mantle wedge. Our models are generally consistent with observed splitting variations for several subduction zones with dense data sampling, including Tonga, Japan, and Kamchatka. The implications of our work suggest that, provided adequate data sampling, shear wave splitting measurements can provide the necessary information to evaluate potential competing effects between variations in mantle flow direction and changes in the stress and hydration states of subduction zone mantle wedges.