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TitleTopology of the Explorer/Juan de Fuca slab edge in northern Cascadia: Imaging plate capture at a ridge-trench triple junction
AuthorAudet, PORCID logo; Bostock, M G; Mercier, J -P; Cassidy, J FORCID logo
SourceCanadian Geophysical Union Annual Meeting, abstracts volume; 2008, 1 pages
Alt SeriesEarth Sciences Sector, Contribution Series 20090171
MeetingCanadian Geophysical Union Annual Meeting; Banff; CA; May 11-14, 2008
Mediaon-line; digital
ProvinceWestern offshore region
Subjectstectonics; geophysics; geophysical surveys; seismic surveys; seismic interpretations; seismicity; lithosphere; continental crust; oceanic crust; subduction; plate tectonics; tectonic interpretations; tectonic environments; Explorer/Juan de Fuca slab; Cascadia; POLARIS
Released2008 01 01
AbstractThe Explorer region is a rapidly evolving oceanic micro-plate fragment that accomodates relative motion between the Pacific, Juan de Fuca, and North America plates in the vicinity of northern Vancouver Island, Canada. The northern limit of Explorer/Juan de Fuca subduction along the margin and the fate of the slab in northern Cascadia are poorly known. We use passive teleseismic recordings from a dense cross-shaped array of portable broadband seismic stations as part of the POLARIS-BC experiment to image upper mantle structure underneath northern Vancouver Island and the interior of British Columbia. One arm of the array trends NW-SE in the direction parallel to strike and straddles the assumed northern end of the subduction zone (LINE 1), and the second arm trends SW-NE in the direction perpendicular to strike, just north of the extension of the Nootka fault beneath Vancouver Island where subduction is observed (LINE 2). The array has been in operation since June 2005 and to date each station recorded an average of 50 events with high signal-to-noise ratio. The NVI array is complemented with data from a few permanent stations operated by the GSC and data from a previous experiment. Station spacing is approximately 5 km along both lines across northern Vancouver Island.
We find a clear signature of subducted material extending northeast from Brooks Peninsula at crustal levels, into Georgia Strait and beyond deep into the mantle down to 300 km depth. The location of the sharp slab edge and complexities in slab topology result from Juan de Fuca ridge subduction and opening of a slab window, in agreement with heat flow and gravity modelling, geochemical data, and fault patterns in northern Vancouver Island. We propose a model of plate evolution for the Explorer region in which its separation from the juan de Fuca plate is caused by the thermal erosion of the slab edge and slab thinning at shallow levels, which act to decrease convergence with North America and lead eventually to plate capture. Our model postulates that Juan de Fuca subduction is still active north and east of the detached Explorer slab, implying a revision of earthquake hazard potential along the northern Cascadia margin.

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