Mapping large-scale mantle flow driving surface motions in southern and central Alaska through the integration of GPS and shear-wave splitting data

Unlike other continental convergent margins where compressional deformation is propagated inboard over the entire deforming region, the majority of Pacific-North America relative plate motion in Alaska is accommodated within a few 100s km of the margin. Furthermore north of the Denali fault surface velocities are directed towards the margin generating extension in northern Alaska. Due to the fact that northern and western plate boundaries in Alaska do not produce significant relative motions, it has been hypothesized that surface motions within central and northern Alaska are driven by basal tractions associated with the down going plate. To study the extent of continental deformation driven by large-scale mantle flow and the level of lithosphere-mantle coupling within central and southern Alaska we have utilized a joint inversion of shear-wave splitting from published studies and the surface deformation field inferred from GPS, earthquake, and geologic data. First, it was determined that there was no correlation between the shear-wave splitting data and the surface deformation field indicating that the anisotropy was not located within the lithosphere and instead represented the differential motion between the lithosphere and convection mantle. In order to solve for flow of the deeper mantle, we divided the anisotropy into two groups: a northern group, near and north of the Denali fault, consisting shear-wave splitting data trending NE-SW that has been determined to come from the mantle wedge, and a southern group, south of the Denali fault, trending NNW-SSE located beneath the slab (Christensen and Abers, 2010). Following the method of Silver and Holt (2002) we solved for a rotation of the sub-asthenospheric mantle for each region. Inversion of the northern group produces an ~30 mm/yr southwestward-directed mantle flow that leads the surface arguing for large-scale mantle flow driving surface velocities in central Alaska. This result is consistent with the torridal flow hypothesis, however, the two stations to the eastern side are poorly fit and are only fit with a southeastward-directed mantle flow. This result argues for a leading southward-directed mantle flow in northern Alaska that is diverted by and flows to the east and west around the shallowly subducting Yakutat slab in central Alaska. More shear-wave splitting data in eastern Alaska will be needed to confirm this result.