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Structural characteristics of a subducting oceanic plate off Western Canada Waldron, David Anthony


The plate tectonic regime off the southwest coast of British Columbia is convergent; the oceanic Juan de Fuca and Explorer plates are obliquely subducting beneath the continental America plate. To investigate the structural complexity of this region, the Vancouver Island Seismic Project was conducted in August 1980. The principal component of this project was a reversed refraction profile perpendicular to the continental margin, extending 350 km from the America plate to the deep ocean of the Juan de Fuca plate. This work deals with the marine section of the profile. Data from large explosive sources and an airgun were recorded on three ocean bottom seismographs (in the deep ocean, on the continental rise and on the shelf). Continuous seismic reflection profiles complemented the refraction information. To adequately model the seismic structure of this complex region required the application of ray-tracing procedures and a synthetic seismogram technique, based on asymptotic ray theory, for laterally varying media. Consistency between the seismic interpretation and previously published gravity anomaly variations across the margin was verified with the aid of empirical velocity-density relations. Additional seismic constraints were provided by multichannel reflection sections and sonic log data from a nearby well. The aim of all procedures for data modelling was to obtain velocity and density models which had the least structure consistent with all available geophysical information. The interpreted structural section indicates that the sediments thicken from 1 km in the deep ocean to 2 km at the base of the continental rise, where the eastward dip of the oceanic basement increases from 1.4° to about 3° and sediment velocities increase landwards. A stratified upper crustal velocity sequence has been derived below the deep ocean which is similar to that deduced from other studies on the Juan de Fuca plate. This stratification is discordant with the descending oceanic plate further east; it is replaced by a block of relatively low velocity material beneath the continental rise. This unit is interpreted to be a highly sheared and compressed melange. Lower crustal structure remains constant over the entire marine profile, suggesting that the melange material may have been formed as upper crustal layers were scraped off the descending oceanic plate. The lower crust has been modelled as a constant velocity gradient region, extending down to approximately 9 km below sea floor in the deep ocean. At this depth there is a strong decrease in velocity gradient, interpreted to be the Mohorovicic discontinuity. Resolution of Moho structure is poor, but no velocity discontinuity at the boundary is required by the data. There is an increase in the dip of the Moho under the continental rise from about 1° to 6°. The refraction data are thus explicable in terms of a relatively simple, two-dimensional velocity model, which is consistent with multichannel, well-log, and gravity information.

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