Seismic velocity structure of the Queen Charlotte Basin beneath Hecate Strait

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Abstract
Seismic refraction data across Hecate Strait in the northern Queen Charlotte Basin were collected in a coincident reflection and refraction survey. Crustal velocity models provide a framework to help understand the formation of the sedimentary basin and the processes occurring near the Queen Charlotte Fault, a major ocean–continent transform fault. Beneath the sediments, which have a maximum thickness of 6 km, a velocity gradient extends from about 5 to 8 km depth, within which velocities increase typically from 6.3 to 6.4 km∙s−1. A thick constant-velocity region was found down to a depth varying from 14 to 22 km, with the smallest depths located beneath the central basin. The base of the constant-velocity layer was marked by a distinct mid-crustal interface, across which velocities increased from 6.4–6.5 km∙s−1 to approximately 6.8–6.9 km∙s−1. Moho was interpreted to be at a near-uniform depth of 26–28 km beneath Hecate Strait and the eastern Queen Charlotte Islands. The associated variation in crustal thickness beneath the basin implies crustal thinning, perhaps caused by extension, of 30% or more.The mid-crustal interface may mark the change to a more mafic and perhaps ductile lower crust. The interface appears to be about 1–4 km deeper than the brittle–ductile transition, as indicated by the estimated depth to the 450 °C isotherm and by the moderate increase in reflectivity on the seismic reflection sections. Ductile flow may also occur in the lower crust near the Queen Charlotte Fault, where the relative motion of the oceanic plate induces lithospheric flow and thinning beneath both the ocean and the continent. The observed decrease in Moho depth from 28 to 21 km near the fault is consistent with recent (1989) numerical predictions of I. Reid for lithospheric flow near ocean–continent transforms.

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