RECENT CHANGES AND SEGMENTATION OF THE PACIFIC - NORTH AMERICA PLATE BOUNDARY OFFSHORE SOUTHERN QUEEN CHARLOTTE ISLANDS, CANADA

A change in relative motions between the Pacific and North America plates resulted in transpression offshore western Canada on this right lateral strike-slip plate boundary. The Queen Charlotte fault (QCF) and the Revere Dellwood fault (RDF) accommodate much of the plate motions; off southern Haida Gwaii (the Queen Charlotte Islands) both faults are evolving into configurations more parallel to the strike-slip plate motion vector.

Synthesizing geomorphology, seismicity, sidescan sonar, gravity and vintage seismic reflection profiles shows that the RDF has propagated northward and overlaps the QCF by ~120 km. Additionally part of the southern QCF is being reoriented through a series of short steps to the west resulting in a less oblique orientation relative to the plate motions.

Northward propagation of the RDF is defined by a dozen strike-slip earthquakes 3.9< M_W<6.4, 500 m uplift of oceanic crust, and lineations in sidescan sonar data. Recent basalt flows and near-vertical faulting attest to the deformation of the crust lying between the RDF and QCF. The RDF has further propagated into the Queen Charlotte terrace disrupting margin parallel structures.

Reorientation of the QCF is mapped through sidescan sonar and multibeam data. The recent trace of the fault is easily seen in soft flat sediments; it leads into and borders a series of east to east-northeast trending hills and faults that are sharply defined on their eastern side by a lineation often depicted as the main trace of the QCF. Compression on left steps is as expected on a right lateral strike-slip fault.

This mapping allows us to define two segments of the Pacific – North America plate boundary: one that ruptured in the 1949 M8.1 strike-slip event and the 2012 M7.8 oblique thrust event and a second southern segment consisting of overlap between the QCF and RDF faults that is thought to be a seismic gap.