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 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 CPO distribution for a range of subduction obliquity. The results show that the seismically fast axis does not necessarily align with the flow direction, particularly in the mantle outflow and transition regions, contrary to the common assumption. We first systematically test the effects of the olivine a-axis orientation and initial polarization in one-layer and two-layer models. The results indicate that in contrast to one-layer models, two-layer models result in peak delay times at two different fast directions. To model the SWS parameter distribution for oblique subduction zones, we apply a full range of initial polarization to multi-layer models that approximate the model-predicted CPO distributions. These models also result in a bimodal SWS parameter distribution, which relaxes as subduction obliquity increases. Unlike normal subduction models, these models indicate considerable variation in the SWS parameter distribution among the forearc, arc and back-arc regions and with subduction obliquity, with the fast direction ranging from margin-normal to margin-parallel. Because of this variability, a single SWS measurement cannot constrain the CPO distribution and the mantle flow, and shear waves with a range of initial polarization are required to interpret the SWS parameters for oblique subduction zones. Our results indicate that olivine CPO distribution cannot explain the margin-parallel fast directions that are commonly observed in many subduction zones.