SEISMIC VELOCITY STRUCTURE OF THE QUEEN CHARLOTTE BASIN BENEATH HECATE STRAIT

Seismic refraction data across Hecate Strait in the northern Queen Cha rlotte 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 ne ar the Queen Charlotte Fault, a major ocean-continent transform fault. Beneath the sediments, which have a maximum thickness of 6 km, a velo city gradient extends from about 5 to 8 km depth. within which velocit ies increase typically from 6.3 to 6.4 km . s-1. A thick constant-velo city region was found down to a depth varying from 14 to 22 km, with t he 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 approxima tely 6.8-6.9 km . s-1. Moho was interpreted to be at a near-uniform de pth of 26-28 km beneath Hecate Strait and the eastern Queen Charlotte Islands. The associated variation in crustal thickness beneath the bas in implies crustal thinning , perhaps caused by extension, of 30% or m ore. 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 esti mated depth to the 450-degrees-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 th e relative motion of the oceanic plate induces lithospheric flow and t hinning beneath both the ocean and the continent. The observed decreas e in Moho depth from 28 to 21 km near the fault is consistent with rec ent (1989) numerical predictions of 1. Reid for lithospheric flow near ocean-continent transforms.