Plate Deformation at Cascadia's Northern Terminus

Date: 4/24/2019

Time: 05:00 PM

Room: Puget Sound

The Juan de Fuca (JDF) plate system, a vestige of the former Farallon plate, is breaking apart as a result of resistance to subduction. At its northern end, the Explorer plate moves independently of the JDF plate along the Nootka Fault Zone (NFZ) which forms an unstable triple junction with the JDF ridge and the Sovanco Fracture Zone. We examine the evolution of the NFZ from ocean basin to Vancouver Island using double difference tomography and earthquake relocation. Two independent inversions were undertaken: one focused on seismicity and structure along the NFZ near the deformation front using OBS recordings from the SeaJade1 experiment, and a second landward study employing permanent CNSN stations and data from portable POLARIS and SeaJade2 experiments. A margin-perpendicular seismicity profile extending landward from the offshore NFZ reveals a strong bend in the subducting lithosphere, comparable to but shorter wavelength (50 vs 150 km) than a similar structure at Cascadia's southern terminus by the Mendocino triple junction. The NFZ near the deformation front exhibits low-magnitude seismicity that extends well into oceanic mantle with low Vp/Vs (<1.7) values. This anomaly, continuous across both velocity models, extends to the NE below Vancouver Island with a more northerly trajectory than a linear subsurface extrapolation of the NFZ through the seismicity concentration off Nootka Island. We propose a model for the landward evolution of the NFZ that includes an increasingly oblique and extended zone of mechanically compromised oceanic plate that lies between seismicity concentrations at Nootka Island and Brooks Peninsula. This model is supported by independent constraints on the NW limit of the JDF plate afforded by LFEs and receiver functions. Accordingly, the Nootka Island and Brooks peninsula seismicity concentrations are ascribed to stress concentrations on either side of the NFZ associated with deformation at the edges of more competent JDF and Explorer plates.

Presenting Author: Michael G. Bostock

Authors