Shear wave splitting and mantle flow in Alaska

Abstract

We explore the nature of mantle anisotropy and flow under Alaska by presenting 2389 SKS shear wave splitting observations from 547 earthquakes recorded at 384 broadband stations deployed in Alaska since 2010. We expand upon the results of Seismic Anisotropy under central Alaska from SKS splitting observations by Christensen and Abers (2010) and Insights into mantle structure and flow beneath Alaska based on a decade of observations of shear wave splitting by Perttu, Christensen, Abers, and Song (2014) to better understand the effect of flat slab subduction on mantle anisotropy and flow. Shear wave splitting is a common tool to investigate anisotropy in the upper mantle, which is often assumed to be caused by mantle flow or preexisting fabrics in the lithosphere. In Alaska, splitting appears to be controlled by the absolute plate motion of the North American and Pacific plates, and the interaction between the two plates. In particular, the subducting Pacific plate acts as a barrier to flow. Directly north of the slab, fast directions are oriented along the strike of the slab with large δts, and are caused by along strike flow in the mantle wedge. Stations further to the north, outside of the influence of the mantle wedge, gradually see fast directions parallel to the absolute plate motion direction of the North American plate. South of the slab, fast directions depend on the geometry of the subducting plate. South of the Alaska Peninsula, the fast directions are parallel to the trench, which represent along strike flow under the Pacific plate. To the east, however, flat slab subduction dominates. Here the fast directions are perpendicular to the trench (parallel to the absolute motion of the Pacific plate) and are indicative of entrained flow from the motion of the Pacific plate. Fast directions near the Fairweather- Queen Charlotte transform system are parallel to the faults, and are likely caused by the deformation associated with large lithospheric blocks moving past each other. The region between the inferred east end of the Pacific plate and the transform boundary is dominated by the collision and accretion of the Yakutat terrane. The tectonics of this region are still in debate and the fast directions are difficult to interpret.