Abstract

This article provides a summary of the structure and tectonic history of the Queen Charlotte transform fault zone off western Canada, as background to understanding the 2012 Mw 7.8 thrust earthquake off Haida Gwaii. There was margin subduction prior to the Eocene. The fault zone then became the mainly transcurrent Pacific–North America boundary. There was mid‐Tertiary oblique extension, then 15°–20° oblique convergence from ∼6  Ma to the present that resulted in underthrusting and subduction initiation. The total underthrusting has been too small for Benioff–Wadati seismicity or arc volcanics but is indicated by (1) a trench, the Queen Charlotte Trough, into which the oceanic plate bows downward and an offshore flexural bulge, the Oshawa rise; (2) the Queen Charlotte terrace, an accretionary sedimentary prism; (3) seismic receiver function delineation of the underthrusting Pacific plate; (4) heat flow decreasing landward as predicted for underthrusting; (5) low gravity offshore and high onshore, consistent with underthrusting; and (6) late Tertiary uplift and erosion of the west coast of the islands. Oblique convergence is partitioned into nearly margin‐normal underthrusting (i.e., Mw 7.8 event) relative to the terrace, which is moving along the margin, and margin parallel on the Queen Charlotte strike‐slip fault just off the coast that produced the 1949 Ms 8.1 earthquake. Landward on the mainland, Global Positioning System data suggest slow coast‐parallel shear with no historical seismicity. The convergence rate decreases to the north of Haida Gwaii off Dixon Entrance, but large thrust earthquakes are possible. To the south, underthrusting of the Winona basin margin also could generate large earthquakes.

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