SKS splitting beneath Alaska: Regional variability and implications for subduction processes at a slab edge

Abstract

Many aspects of subduction zone geodynamics remain poorly understood, including the pattern of flow in the mantle surrounding subducting slabs. Because of the relationship between deformation and seismic anisotropy, measurements of shear wave splitting in subduction zones can shed light on this pattern of flow. While observations of splitting in subduction zones are numerous and robust, the source and geodynamical implications of the anisotropy remain imperfectly constrained. The Alaska subduction zone, associated with the subduction of the Pacific plate beneath North America, is a particularly complex tectonic setting. Dramatic lateral variations in slab morphology, seismicity, and volcanic character are present beneath the region, and previous splitting observations and modeling studies have suggested the possibility of significant along-strike mantle flow in the region. Here we present SKS splitting measurements from more than 50 stations of the permanent broadband AK network located throughout Alaska. Splitting patterns show significant regional variability, but proximate stations often exhibit similar splitting behavior. Stations in southern Alaska exhibit nearly trench-normal fast directions, while in southeasternmost Alaska fast directions trend parallel to the plate boundary. A group of stations in the eastern part of the array exhibit predominantly null splitting, while stations to the northwest of this group tend to exhibit ~1s or more of splitting with fast directions that are roughly NE–SW. Stations east of the Kenai Peninsula region exhibit complicated splitting patterns that suggest complex anisotropy. We discuss a number of potential factors that may contribute to the complexity in splitting patterns observed in Alaska, including frozen anisotropic structure in the overriding plate lithosphere, asthenospheric shear due to absolute plate motion, trench-parallel flow in the mantle wedge, two-dimensional entrained flow beneath the slab, and mantle upwelling at the slab edge.