Insights into mantle structure and flow beneath Alaska based on a decade of observations of shear wave splitting

SKS shear wave splitting measurements from three Program for Array Seismic Studies of the Continental Lithosphere experiments (Broadband Experiment Across the Alaska Range, Alaska Receiving Cross Transect of the Inner Core, and Multidisciplinary Observations Of Subduction), which form a north/south transect across Alaska, show a remarkably simple pattern of two large anisotropy domains. In the northern domain, extending from the 70km contour of the subducting Pacific plate north to the Arctic Ocean, fast directions are consistently in the NE-SW direction. These directions are essentially parallel to the absolute plate motion direction in northern Alaska and parallel to the strike of the subducting plate above the mantle wedge, suggesting that they represent some combination of plate-scale asthenospheric flow in the upper mantle and flow along the subducting plate in the mantle wedge. A strong wedge component beneath the Alaska Range is required to explain systematics of splitting delay times. In the southern domain, which extends south from the 70km depth contour to the subducting plate, fast directions are in the NW-SE direction, a 90 rotation from the northern domain. These fast directions are parallel to the dip of the subducting plate in the direction of convergence and represent entrained flow beneath the subducting slab; the Pacific Plate absolute motion approximately parallels local convergence. Two major factors seem to control flow in these regions, absolute plate motion in the north and the subduction of the Pacific plate in the south, although both subduction-driven wedge flow and absolute plate motion contribute to the southern part of the northern regime.