Mantle Flow Systems Associated with Slab Subduction and Absolute Plate Motion in Alaska: Constraints from Shear Wave Splitting Analyses

Seismic azimuthal anisotropy beneath Alaska, an ideal locale for investigating mantle flow systems in a complicated subduction setting, is quantified by splitting of teleseismic PKS, SKKS, SKS ("XKS") phases. A total of over 5,300 pairs of well defined XKS splitting parameters were obtained at 570 USArray and other stations that recorded data between 1989 and 2019. Based on the characteristics of the resulting splitting parameters, the entire region can be divided into 4 subareas. The fast orientations observed on the Aleutian Islands are mostly trench parallel, probably suggesting along-strike flow in the mantle wedge. Those observed in southcentral Alaska are mostly NW-SE which are parallel to the subduction direction, indicating that entrained and corner flows induced by the subducting oceanic slab may play an important role in the formation of the observed anisotropy. The fast orientations observed in this area clearly exhibit a rotation which is consistent with several previous studies, and may suggest a slab-edge-induced toroidal asthenospheric flow. In southeastern Alaska, the splitting parameters are complicated with numerous N-S and E-W fast orientations and anomalously small splitting times, probably due to the interaction between the subducting slab and the Yakutat terrane. In northern Alaska, most of the stations show NE-SW fast orientations which is consistent with the absolute plate motion direction of the North American Plate in the area. The uniform set of numerous splitting parameters provides the foundation for further analyses including estimating the optimal depth of the source of anisotropy using spatial coherency analysis, and investigating multi-layered anisotropy.