Crust-penetrating multichannel seismic data imaged crustal features of late Tertiary extension in east Dixon Entrance, British Columbia. The data show grabens as much as 3 km deep, mid-crustal west-dipping reflecting packages interpreted as normal sense shear zones, middle to lower crustal subhorizontal reflecting horizons, a generally reflective Moho, and arches of the Moho with relief of as much as 3 km (Moho depths 24.8-27.5 km). Based on dated extension-related features reported for the region, east-west extension occurred between 40 and 20 Ma and may have been concentrated between 25 and 20 Ma. If the pre-late Tertiary crustal thickness of east Dixon Entrance was the same as that of the mainland to the east (34 km), the minimum amount of crustal thinning in the study area was 30%.Thinning of the whole crust above a stronger mantle lithosphere is implied by the extension-related features that we describe from the brittle upper crust down through the ductile lower crust to the Moho. Our data confirm and extend the interpretation of crustal extension by Lowe and Dehler and Dehler et al. for Hecate Strait and Queen Charlotte Sound. Integration of our results with the geologic history across the study area leads to a conclusion that preexisting zones of weakness may have controlled the pattern of crustal thinning during the extension.
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