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 ∼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. The slab deformation and mantle flow under the Cascadia margin are controversial. We obtain new anisotropic tomography of the Cascadia subduction zone, which can reveal tilting 3‐D fast velocity directions (FVDs) and planes. Because the observed trench‐normal FVD in the Cascadia margin can be explained by both 3‐D toroidal and 2‐D entrained flows, we discriminate the two types of mantle flow with 3‐D FVDs that contain dip angle information. The 2‐D entrained flow above and below the subducting slab is trapped in a narrow zone of ∼100 km wide and is surrounded by large‐scale sub‐horizontal toroidal flow. The slab anisotropy above 80 km depth is consistent with that in the Juan de Fuca plate before subduction, but it is reshaped beneath the arc due to the slab deformation or phase transition. Tilting‐axis anisotropy can reconcile contradictory assumptions of azimuthal and radial anisotropiesEntrained flow occurs within ∼100 km depth above and below the subducting slab and is surrounded by toroidal flowFossil anisotropy in the slab is overprinted beneath the arc due to intraslab deformation or phase transition Tilting‐axis anisotropy can reconcile contradictory assumptions of azimuthal and radial anisotropies Entrained flow occurs within ∼100 km depth above and below the subducting slab and is surrounded by toroidal flow Fossil anisotropy in the slab is overprinted beneath the arc due to intraslab deformation or phase transition