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Abstract: In this study, we have produced P- and S-wave velocity models for western Canada using 23,420 delay times measured on vertical component seismograms, and 15,805 delay times measured on transverse component seismograms, respectively, from a range of permanent and temporary networks. Resolution is best in southwestern British Columbia, and along the CANOE (northwestern Alberta, southern Yukon and Northwest Territories) and BATHOLITHS (northwestern BC) arrays where the station density is the highest, and fair elsewhere. We focus our attention on two distinct features 1) the transition from Phanerozoic to Cratonic mantle in northwestern Canada, and 2) the complex tectonic environment at the northern terminus of the Cascadia subduction zone where the plate boundary changes from convergent to transform. We find that the main transition from Phanerozoic to Cratonic mantle in northwestern Canada occurs at the Cordilleran deformation front and represents a sharp jump in seismic velocity from −2% to +2% over a distance of ~50km. In northern Cascadia, we have imaged and characterized the signature of the subducting Juan de Fuca plate and observed evidence of subduction beyond the northern edge of the slab. We also demonstrate that the Anahim hotspot track is underlain by a −2% low-velocity zone possibly extending to 400km beneath Nazko cone that appears to be the source of volcanism in this area. Consequently, we associate the source of magmatism in this area to a mantle-scale rather than lithospheric-scale process. [Copyright &y& Elsevier]

The Explorer plate is a young oceanic microplate that accommodates relative motion between the Pacific, Juan de Fuca, and North America plates near northern Vancouver Island, Canada. The northern limit of Explorer plate-Juan de Fuca subduction and the fate of the slab in northern Cascadia are poorly understood. We use passive teleseismic recordings from an array of POLARIS broadband seismic stations to image crustal and upper mantle structure beneath northern Vancouver Island into the interior of British Columbia. A clear signature of subducted material extends northeast from the Brooks Peninsula at crustal levels, beneath Georgia Strait and the mainland deep into the mantle to 300 km depth. Complexity in slab morphology results from Juan de Fuca ridge subduction and toroidal flow around the slab edge, in agreement with geophysical and geological data. We propose a tectonic model for the Explorer plate in which its separation from the Juan de Fuca plate is caused by the thermomechanical erosion of the slab edge and slab thinning at shallow levels, both of which slow convergence with North America and lead eventually to plate capture. [ABSTRACT FROM AUTHOR]