Slab-driven flow at the base of the mantle beneath the northeastern Pacific Ocean

Flow in the mantle's bottom boundary layer plays an important role in shaping structures and processes in the deep mantle; however, knowledge of lowermost mantle flow patterns remains elusive. In particular, the influence of remnant slabs on lowermost mantle flow is poorly known, although it is likely that slabs play an important role in driving flow and thus in controlling key aspects of lowermost mantle behavior. Measurements of seismic anisotropy can yield relatively direct constraints on slab-induced lowermost mantle flow; however, such observations are challenging to make. We take advantage of the excellent raypath coverage beneath the northeastern Pacific Ocean provided by the USArray deployment in North America to provide detailed sampling of a region that has a long subduction history, with remnant slabs likely impinging on the core-mantle boundary. We present observations of coherent, strong shear wave splitting of SKKS and Sdiff phases across USArray stations and show through global wavefield modeling that the splitting is due to lowermost mantle anisotropy. A stacking approach enables us to make robust estimates of lowermost mantle splitting parameters, which we model by considering realistic mineral physics scenarios. Under the assumption of simple horizontal shear deformation, our observations are consistent with generally north-south flow directions for either a post-perovskite or a bridgmanite mineralogy; ferropericlase cannot explain observations. We speculate that this flow is driven by subducting slab remnants impinging on the core-mantle boundary.