The Role of Variable Slab Dip in Driving Mantle Flow at the Eastern Edge of the Alaskan Subduction Margin: Insights From Shear-Wave Splitting

Alaska provides an ideal tectonic setting for investigating the interaction between subduction and asthenospheric flow. Within the span of a few hundred kilometers along strike, the geometry of the subducting Pacific plate varies significantly and terminates in a sharp edge. Furthermore, the region documents a transition from subduction along the Aleutian Arc to strike-slip faulting along the Pacific Northwest. To better understand mantle interactions within this subduction zone, we conduct an SKS shear-wave splitting analysis on passive-source seismic data collected between 2011 and 2018 at 239 broadband seismometers, including those from the Transportable Array. Anisotropic fast directions in the east of our study area parallel the Queen Charlotte and Fairweather transform faults, suggesting that the ongoing development of lithospheric anisotropy dominates the results there. However, our observed delay times (δt = 1-1.5 s) obtained across the study region may also imply an asthenospheric contribution to the splitting pattern. Our splitting observations exhibit slab-parallel fast directions northwest of the trench and a rotation of fast directions around the northeastern slab edge. These observations suggest the presence of toroidal asthenospheric flow around the edge of the downgoing Pacific plate. We suggest that Wrangell Volcanic Field volcanism might be caused by mantle upwelling associated with this flow. Splitting observations closer to the trench can be explained by fossil anisotropy within the downgoing Pacific-Yakutat plate combined with entrained subslab mantle. The geometry of the slab, including its variable dip and its abrupt eastern edge, thus plays an important role in governing mantle flow beneath Alaska.