GEOSCAN, résultats de la recherche


TitreTopology of the Explorer/Juan de Fuca slab edge in northern Cascadia: Imaging plate capture at a ridge-trench triple junction
AuteurAudet, P; Bostock, M G; Mercier, J -P; Cassidy, J F
SourceCanadian Geophysical Union Annual Meeting, abstracts volume; 2008, 1 pages
Séries alt.Secteur des sciences de la Terre, Contribution externe 20090171
RéunionCanadian Geophysical Union Annual Meeting; Banff; CA; mai 11-14, 2008
Mediaen ligne; numérique
ProvinceRégion extracotière de l'ouest
Sujetslevés géophysiques; levés sismiques; interpretations sismiques; sismicité; lithosphère; croûte continentale; croûte océanique; subduction; tectonique de plaques; interprétations tectoniques; milieux tectoniques; Cascadie; tectonique; géophysique
Résumé(disponible en anglais seulement)
The 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.