THE NORTHERN CASCADIA SUBDUCTION ZONE AT VANCOUVER ISLAND - SEISMIC STRUCTURE AND TECTONIC HISTORY

A series of new multichannel seismic lines have been acquired across the continental shelf to the deep sea. The top of the downgoing oceanic crust is imaged from the deep ocean basin to a depth of 40 km beneath Vancouver Island. The reflection depths are in good agreement with seismic refraction models and Benioff-Wadati seismicity. Two broad reflective bands imaged as dipping gently landward at depths of about 15 and 30 km on the land lines merge to a single reflector band offshore. They may represent underplated oceanic material or, they may be zones of contrasting physical properties. Two narrow terranes have been thrust beneath, and accreted to, the margin in the Eocene, about 42 Ma, near the start of the present phase of subduction. They provide a landward-dipping backstop to the large sediment wedge accreted since that time. The deformation front is characterized by mainly landward-dipping thrust faults that cut close to basement. This, and the mass balance of the incoming sediment, suggest that there is little subduction of sediment into the mantle. The Tofino Basin sediments, up to 4 km in thickness, have been deposited on the continental shelf over the accreted terranes and the developing accretionary wedge. -from Authors