INTERPRETATION OF 3-DIMENSIONAL SEISMIC-REFRACTION DATA FROM WESTERN HECATE-STRAIT, BRITISH-COLUMBIA - STRUCTURE OF THE QUEEN CHARLOTTE BASIN

The Queen Charlotte Basin consists of up to 6 km of Tertiary clastic sediments in a complex sequence of fault-bounded subbasins. The tectonic evolution of the basin in still being debated, with recent interpretations including distributed strike-slip extension, oblique or en echelon rifting, simple extension orthogonal to the plate margin, and block faulting and vertical tectonics. A combined seismic reflection and retraction survey was carried out in 1988 to investigate the structure and tectonic evolution of the basin and underlying crust. While the marine multichannel reflection data are being collected. refracted and wide-angle reflected energy from the large air-gun array was recorded at surrounding land sites in with two-dimensional (in-line) and three-dimensional (broadside) geometries. The broadside refraction data recorded on the Queen Charlotte Islands provide good three-dimensional coverage of western Hecate Strait. These data are labelled to determine the three-dimensional structure of the Queen Charlotte Basin. The reflection data indicate that the sedimentary Queen Charlotte Basin beneath the shotpoints varies rapidly in thickness and is highly three-dimensional. First-arrival traveltimes from the broadside refraction data are inverted to find the three-dimensional structure of the basement interface beneath the shots and out of the planes of the reflection sections. A map of basement depth is derived for a region several kilometres wide adjacent to the reflection lines. Basin thickness varies rapidly between approximately 200 m and approximately 6 km. The model is consistent with the seismic reflection and potential field data sets. Although most of the basin is modelled as sediments overlying rocks with crustal velocities, a thick sequence of interbedded sedimentary and volcanic rocks is interpreted to underlie the shot lines in one region that lies east of the central Queen Charlotte Islands. Four major faults are also interpreted. These are based on sharp vertical relief of over 2 km on the map of basement depth. The orientation and topography across the faults and the small lateral scale and large topographic changes of the subbasins support the distributed strike-slip extension evolutionary model for the basin.