Mantle Wedge Seismic Anisotropy and Shear Wave Splitting: Effects of Oblique Subduction

We investigate the evolution of olivine crystal preferred orientation (CPO) and its effect on local shear wave splitting (SWS) in the mantle wedge of oblique subduction zones. Based on model-predicted 3-D mantle wedge flow fields, we compute the A-type and E-type olivine CPO distribution for a range of subduction obliquity. The results show that the seismically fast axis does not necessarily align with the flow direction. To model the local SWS parameter distribution for oblique subduction zones, we apply a full range of initial polarization to multilayer models that approximate the model-predicted CPO distributions. These models result in a bimodal SWS parameter distribution, which relaxes as subduction obliquity increases. Unlike non-oblique subduction models, these models indicate considerable variations in the SWS parameters with subduction obliquity and initial polarization and also among the forearc, arc, and backarc regions. Because of this variability, a single SWS measurement cannot constrain the CPO distribution, and shear waves with a range of initial polarization are required to interpret the SWS parameters in oblique subduction zones. Our results indicate that 3-D mantle wedge flow due to oblique subduction cannot explain commonly observed margin-parallel fast direction in the forearc region but can explain margin-normal fast directions that are observed in the arc and backarc regions of oblique subduction zones.