Poloidal- and Toroidal-Mode Mantle Flows Underneath the Cascadia Subduction Zone

Abstract

Several hypotheses have been proposed to explain intriguing circular shear wave splitting patterns in the Pacific Northwest, invoking either 2-D entrained flows or 3-D return flows. Here, we present some hitherto unidentified, depth-dependent anisotropic signatures to reconcile different conceptual models. At depths shallower than 200 km, the fast propagation directions of seismic waves to the west of the Rocky Mountain are aligned sub-parallel to the subduction direction of the Juan de Fuca and Gorda Plates. This pattern is consistent with previous onshore/offshore shear wave splitting measurements and indicates that 2-D entrained flows dominate at shallower depths. From 300 to 500 km, two large-scale return flows are revealed, one circulating around Nevada and Colorado and the other running around the edge of the descending Juan de Fuca slab. These observations suggest the development of toroidal-mode mantle flows, driven by the fast rollback of the narrow, fragmented Juan de Fuca and Gorda slabs.