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Abstract: Recent shear wave splitting measurements from the fore-arc region of the Ryukyu subduction system show large magnitude (0.3–1.6 s) trench-parallel splitting in both local and teleseismic phases. The similarity of splitting parameters associated with shallow local-S and teleseismic phases suggests that the source of anisotropy is located in the fore-arc mantle. One explanation for this pattern of shear wave splitting involves a transition from commonly observed high-temperature olivine fabrics with flow-parallel seismically fast directions to a flow-normal B-type olivine fabric in the cold fore-arc mantle of the Ryukyu wedge. We test the B-type fabric hypothesis by comparing observed splitting parameters to those predicted from geodynamic models that incorporate olivine fabric development. The distribution of olivine fabric is calculated with high-resolution thermomechanical models of the Ryukyu subduction zone that include realistic slab geometry and an experimentally based wet olivine rheology. We conclude that B-type fabric can explain the magnitude and trench-parallel orientation of deep local-S phases that sample the core of the fore-arc mantle. However, our calculations show that B-type fabric alone cannot account for large magnitude trench-parallel splitting associated with teleseismic phases that sample the shallow tip of the fore-arc mantle. Alternative models for trench-parallel teleseismic splitting in the shallow tip of the fore-arc mantle involve the addition of crustal or slab anisotropy and highly anisotropic foliated antigorite serpentinite. [Copyright &y& Elsevier]

Abstract: Several hypotheses have been proposed to explain trench-parallel shear wave splitting in the mantle wedge of subduction zones. These include 3-D flow effects, parallel melt filled cracks, and B-type olivine fabric. We predict the distribution of B-type and other fabrics with high-resolution thermal and stress models of subduction zones. A composite viscous rheology is used that incorporates wet diffusion creep, wet dislocation creep, and a stress-limiting approximated Peierls creep law. Rheological parameters for dislocation and diffusion creep are based on deformation experiments. The effects of variable viscous coupling between the slab and mantle are explored using kinematic boundary conditions that change along the top of the slab. Two end-member models are observed, which depend on the depth of partial viscous coupling between the slab and mantle: (1) deep partial coupling which gives rise to cold, high-stress conditions in the forearc mantle and high-temperature, low-stress conditions in the arc and back-arc mantle (2) full viscous coupling at the base of a 40 km conducting lid which is characterized by high temperature and low stress. The case with deep partial coupling, which produces a better match with attenuation tomography and heat flow, shows a large region with suitable conditions for B-type fabric in the forearc mantle and a rapid transition toward the back-arc to conditions that are more suitable for A-, C-, or E-type fabrics. Full viscous coupling gives rise to high-temperature, low-stress conditions unsuitable for B-type fabric. Our modeling predicts that the best candidates for regions with B-type fabric are the forearc mantle and a cold 10–15 km layer above the slab. [Copyright &y& Elsevier]