Mapping Mantle Flows and Slab Anisotropy in the Cascadia Subduction Zone

The Cascadia margin is an unusual subduction zone characterized by the downdip movement of young and thin oceanic plates, where mantle flow and intraslab deformation are still unclear. Here we present new anisotropic tomography of the Cascadia subduction zone, in which the hexagonal symmetry axis of anisotropy is tilting rather than horizontal or vertical as assumed in previous studies of seismic anisotropy. Subduction-induced entrained and toroidal flows under the Cascadia margin are discriminated well by the spatial relationship between tilting-axis anisotropy and slab geometry. The obliquely entrained flow is trapped in a narrow zone (<100 km wide) above and below the subducting slab and reaches similar to 200 km depth, which is surrounded by large-scale sub-horizontal toroidal flow. The intraslab anisotropy is trench-normal above 80 km depth but changes to trench-parallel at 100-400 km depths, which may reflect fossil anisotropy overprinted by deep deformation beneath the arc, or joint effect of serpentinization and hydrous faulting.