Plate Boundary Adjustments of the Southernmost Queen Charlotte Fault

A revision of the plate boundary configuration offshore southern Haida Gwaii, which is the locus of the 2012 M-w 7.8 thrust earthquake, is based on a reevaluation of regional geophysical data and leads to recognition of fault segments critical for seismic-hazard analysis. Off western Canada, the strike-slip Queen Charlotte fault (QCF) and Revere-Dellwood fault (RDF) constitute the transpressive plate boundary between the Pacific and North American plates. The southernmost 80 km of the QCF strikes at the highest angle (similar to 20 degrees) to the relative plate motion, yet geomorphology adjacent to the fault is the most subdued. Synthesizing geomorphology, seismicity, sidescan sonar, gravity, and seismic reflection profiles indicates that the plate boundary is evolving to a less transpressive configuration than previously interpreted. The RDF is 100 km longer than previously mapped; it has propagated northward and overlaps the QCF by 120 km. Propagation is defined by strike-slip earthquakes 3: 9 < M-w < 6.4, 500 m uplift of oceanic crust, and a fault scarp that disrupts margin parallel structures of the Queen Charlotte terrace. Part of the southern QCF has been reoriented through a series of short restraining steps, resulting in a less oblique orientation relative to the plate motions; the recent fault trace is clearly seen in previously uninterpreted sidescan sonar data. Overall this mapping delineates a segment of the plate boundary characterized by overlap between the QCF and RDF and defines the southern limit of a segment of the plate boundary characterized by historic pure shear deformation and rupture during the 2012 M-w 7.8 and the 1949 M 8.1 earthquakes.