SOURCE-SIDE SHEAR-WAVE SPLITTING OF THE KAMCHATKA SLAB TERMINATION REGION

We present new measurements of source-side shear-wave splitting from the Kamchatka portion of the Pacific plate subduction zone. The study region includes the abrupt lateral termination of subducted Pacific lithosphere at the Bering fault, termed the slab edge. Source-side splitting results from shear-wave propagation across an anisotropic upper-mantle region lying strictly beneath the subducting Pacific plate. Using observations of splitting of core shear phases (SKS, SKKS) at a suite of distant stations, we calibrate the upper mantle anisotropy directly below the stations and then correct the splitting of teleseismic S phases from Kamchatka earthquakes, thereby isolating splitting from the sub-slab regions sampled by these S waves. Preliminary results show that the fast axis of anisotropy beneath the Pacific slab is strongly trench parallel (i.e., NE); splitting delay times range from 1-3 s, indicating long (100-350 km) anisotropic travel paths through the sub-slab asthenosphere. Fast-axis measurements for one event are consistently trench-normal in a region of slab contortion or possibly tearing defined by seismicity. Shear-wave splitting observations made using core shear phases recorded at stations deployed on Kamchatka (Side Edge Kamchatka Slab experiment) are also strongly trench-parallel in the region south of the slab edge, but become trench-normal near the slab edge itself. We interpret both sets of splitting measurements to indicate the presence of northeastward upper mantle flow parallel to the trench beneath the Pacific plate. These results indicate that the primary upper mantle anisotropy observed in the study region lies beneath the Pacific slab and not in the subduction zone's upper mantle wedge, and that the slab base guides flow until the slab edge is reached, whereupon asthenosphere flows around the slab edge. Such flow may thus mix potentially long-separated geochemical reservoirs from beneath and above the Pacific slab.