Effect of three-dimensional slab geometry on deformation in the mantle wedge: Implications for shear wave anisotropy

Abstract

Shear-wave splitting observations from many subduction zones show complex patterns of seismic anisotropy that commonly have trench-parallel fast directions. Three-dimensional flow may give rise to trench-parallel stretching and provide an explanation for these patterns of seismic anisotropy. Along-strike variations in slab geometry produce trench-parallel pressure gradients and are therefore a possible mechanism for three-dimensional flow. In this study we quantify the effects of variable slab dip, curved slabs, oblique subduction, and slab edges on flow geometry and finite strain in the mantle wedge of subduction zones. Temperature, dynamic pressure, velocity, and strain are calculated with high-resolution three-dimensional finite element models. These models include temperature-and stress-dependent rheology and parameterized slab and trench geometry. Thick layers (20-60 km) with strong trench-parallel stretching are observed in the mantle wedge when slab geometry involves a transition to slab dip less than 15° or strong curvature in the slab. In these cases, strong trench-parallel stretching develops when flow lines have an oblique to trench-normal orientation. This suggests that trench-parallel seismically fast directions may not indicate trench-parallel flow lines in systems with large along-strike variations. An oblique component of stretching is confined to a 20-30 km layer above the slab in systems with oblique subduction. The effects of slab edges include strong toroidal flow and focusing in the mantle near slab edges and trench-parallel flow that extends 50-100 km into the core of the mantle wedge. Copyright 2008 by the American Geophysical Union.