Structure, seismicity, and thermal regime of the Queen Charlotte Transform Margin

[1] This study examines structural, seismicity, thermal, and current deformation constraints on the tectonic regime of the Queen Charlotte Margin located off the northwest coast of Canada. The margin is primarily a transcurrent boundary between the oceanic Pacific Plate and the continental North America Plate, but plate motion models indicate ∼20° of current oblique convergence that initiated ∼5 Myr. The estimated 120 km of convergence has been variously explained as accommodated by underthrusting or by crustal shortening. The near-vertical Queen Charlotte transcurrent fault and lack of Wadati-Benioff earthquakes argue against underthrusting. However, structural models for the margin, from receiver function and refraction studies, show little evidence of crustal thickening. The pattern of landward decreasing heat flow, gravity anomalies, an accretionary prism landward of the trench or trough, down-bowing ocean crust and sediments horizons, and a variety of other geophysical evidence support underthrusting. Receiver function analysis on teleseismic data indicates a 10 km thick low-velocity layer at 35 km depth, dipping 20° to the east, which is interpreted as underthrusting oceanic crust. The heat flow across the fault zone rapidly decreases landward, in agreement with predictions from a finite element thermal model of underthrusting.