Pacific‐North America plate boundary reorganization in response to a change in relative plate motion: Offshore Canada

Abstract

The transition from subduction in Cascadia to the transform Queen Charlotte fault along western Canada is often drawn as a subduction zone, yet recent studies of GPS and earthquake data from northern Vancouver Island are not consistent with that model. In this paper we synthesize seismic reflection and gravity interpretations with microseismicity data in order to test models of (1) microplate subduction and (2) reorganization of the preexisting strike‐slip plate boundary. We focus on the critical region of outer Queen Charlotte Sound and the adjacent offshore. On much of the continental shelf, several million years of subsidence above thin crust are a counterindicator for subduction. An undated episode of compression uplifted the southernmost shelf, but subsidence patterns offshore show that recent subduction is unlikely to be responsible. Previously unremarked near‐vertical faults and a mix of extensional and compressional faults offshore indicate that strike‐slip faulting has been a significant mode of deformation. Seismicity in the last 18 years is dominantly strike‐slip and shows large amounts of moment release on the Revere‐Dellwood fault and its overlap with the Queen Charlotte fault. The relative plate motion between the Pacific and North American plates rotated clockwise ∼6 Ma and appears to have triggered formation of an evolving array of structures. We suggest that the paleo‐Queen Charlotte fault which had defined this continental margin retreated northward as offshore distributed shear and the newly formed Revere Dellwood fault propagated to the northwest.