Freezing of Crystal Preferred Orientation in the Mantle Wedge Corner and Shear Wave Splitting

Using numerical models, we compute the evolution of the mantle flow field and the crystal preferred orientation (CPO) of mineral aggregates in the mantle wedge of generic subduction systems from their nascent to mature stage and investigate shear wave splitting (SWS) through the forearc mantle wedge corner and overriding crust. Upon subduction initiation, the maximum depth of slab-mantle decoupling (MDD) is relatively shallow (∼20 km depth), resulting in mantle flow and CPO development in the wedge corner. As subduction continues, the MDD deepens, the wedge corner cools and stagnates, and the olivine CPO becomes frozen-in. In the cool wedge corner, antigorite can form if water is available. In non-deforming mantle, antigorite CPO develops relative to the host olivine CPO through topotactic growth. We calculate splitting parameters of synthetic local S waves based on the model-predicted A- and B-type olivine CPOs and topotactically grown antigorite CPO that replaces A-type olivine CPO in the wedge corner. The fast direction is trench-normal for A-type olivine and antigorite CPOs and trench-parallel for B-type. When the delay times are long enough (>0.1 s), we find them positively correlated with the thickness of the mantle wedge corner. In NE Japan, where the results of detailed analyses on the spatial variation of the SWS parameters are available, such correlation is not observationally reported. However, the addition of an anisotropic overriding crust provides delay times (∼0.1 s) and trench-normal fast directions that are consistent with the local SWS observations.