Seismic evidence for rotating mantle flow around subducting slab edge associated with oceanic microplate capture

Abstract

Tectonic plate reorganization at a subduction zone edge is a fundamental process that controls oceanic plate fragmentation and capture. However, the various factors responsible for these processes remain elusive. We characterize seismic anisotropy of the upper mantle in the Explorer region at the northern limit of the Cascadia subduction zone from teleseismic shear wave splitting measurements. Our results show that the mantle flow field beneath the Explorer slab is rotating anticlockwise from the convergence-parallel motion between the Juan de Fuca and the North America plates, re-aligning itself with the transcurrent motion between the Pacific and North America plates. We propose that oceanic microplate fragmentation is driven by slab stretching, thus reorganizing the mantle flow around the slab edge and further contributing to slab weakening and increase in buoyancy, eventually leading to cessation of subduction and microplate capture. Key Points Evidence for rotating mantle flow in northern Cascadia from seismic anisotropy Coincides with hypothesized slab stretching between Explorer and Juan de Fuca Suggests mantle reorganization during plate capture at subduction zone edge ©2014. American Geophysical Union. All Rights Reserved.