Implications of Subduction Obliquity to the Inferences of Seismic Anisotropy in the Mantle Wedge

This study aims to systematically quantify the effect of subduction obliquity on the patterns of mantle flow, olivine crystal preferred orientation (CPO), and seismic anisotropy in the creeping part of the overriding mantle wedge in subduction zones. Previous studies of mantle wedge flow patterns using numerical models have shown that when the subduction direction is oblique to the margin, coupling between the subducting slab and the overriding mantle induces complex 3-D mantle wedge flow pattern [e.g., Kneller and van Keken, 2008; Wada et al., 2015]. In this study, we develop a series of 3-D kinematic-dynamic models for generic subduction systems by varying subduction obliquity and other subduction parameters, such as slab dip and slab age. We pay special attention to adopting a model domain that minimizes the effects of boundary conditions on the mantle wedge flow pattern. The models show that the directions of trench-ward inflow and down-dip outflow of the mantle in the mantle wedge are both oblique to the margin, and their obliquity increases with subduction obliquity. For a given mantle flow pattern, we compute the pattern of olivine CPO in the mantle wedge, using the crystallographic code [Kaminski et al., 2004]. Preliminary results indicate that the CPO in the inflowing mantle is generally sub-parallel to the mantle flow direction whereas that in the outflowing mantle is oblique the flow direction, resulting in a 3-D pattern of CPO and giving rise to a spatial variation in the seismic anisotropy of the mantle wedge. The latter has been inferred largely from S-wave splitting observations, in which the computed splitting parameters represent the integrated effect of the anisotropy in the mantle wedge along the length of the ray path and are dependent on the spatial variation of the CPO. We plan to calculate S-wave splitting parameters using the calculated CPO patterns and quantify the variations in the splitting parameters with subduction obliquity.