Geodynamic insights into patterns of shear wave anisotropy in subduction zones.

Abstract

Shear wave splitting observations from many subduction zones show complex patterns of seismic anisotropy that have trench-parallel fast directions and abrupt rotations. Several hypotheses have been proposed to explain these unexpected patterns. This work shows tests of the olivine fabric transition and three-dimensional flow hypotheses. The tool used to carry out this investigation is geodynamic modeling with seismological constraints on subduction zone geometry and rheological parameters based on rock deformation experiments. The geodynamic models presented in this work show that a transition to B-type olivine fabric can produce trench-parallel shear wave splitting with delay times greater than 1 s above the cold fore-arc mantle of subduction zones. The olivine fabric transition model adequately reproduces local-S splitting parameters associated with phases that sample the deep fore-arc mantle of the Ryukyu subduction system. The three-dimensional subduction zone models presented in this work show that both a transition to flat-slab subduction and strong trench curvature generate trench-parallel stretching in the warm are and back arc mantle. These models may explain trench-parallel shear wave splitting and abrupt rotations in fast direction in the are and back arc-mantle of the Andean and Marianas subduction systems. This thesis demonstrates that a variety of mechanisms may be necessary to account for trench-parallel anisotropy in subduction zones.