1988 Queen Charlotte Basin Experiment: Seismic refraction and gravity models of line 3

Abstract

The Queen Charlotte Basin is situated off Canada's west coast. It lies between the mainland, a large plate-boundary transform fault, a triple-junction and a subduction zone. In 1988, a large seismic refraction and wide-angle reflection survey was carried out in order to study the basin's structure and evolution. Line 3 of that survey is the focus of this thesis. The velocity model of line 3 was obtained using refraction data from three land-based receivers. The initial sediment thickness and velocity was determined by the reflection section of line 3 and Harlequin Well data. Results show that the upper crust velocities increase from 5.5 km/s to 6.3 km/s at 8 km. Velocities increase in the mid crust from 6.5km/s to 6.7 km/s at 18 km depth. In the lower crust, a velocity of 6.8-6.9 km/s was found with Moho depths ranging from 26-28 km. These velocities correspond well with the results found in the southern sound, and the depth of the Moho is similar to that found in the Hecate Strait. The present Moho depth of line 3 shows that the northern Queen Charlotte Sound is only 80% of the original crustal thickness, strongly indicating an extensional history in the area. The gravity model defined a transition from oceanic to continental crust over a horizontal distance of 75 km (+/- I 0 km) corresponding with a Moho dip of 14 degrees.