Abstract

Three plate boundaries, the Queen Charlotte transform, the Cascadia subduction zone, and the Juan de Fuca Ridge, meet in a complex triple junction offshore Vancouver Island. Some interpretations of the plate tectonics of the region have included the Explorer ridge, the Dellwood Knolls, and the Tuzo Wilson volcanic field as part of the spreading system, requiring up to three triple junctions in the region. New SeaBeam bathymetry interpreted with existing regional data sets indicate that the Dellwood Knolls and Tuzo Wilson volcanic field are not independent plate boundaries, but the result of leaky transform tectonics between two overlapping transform faults. Seismicity shows that the Explorer plate is being deformed by Pacific–North American relative motion as a new transform plate boundary forms and cuts off the Explorer ridge. The system is evolving to a single triple junction at the northern terminus of the Juan de Fuca Ridge where it meets the Nootka deformation zone. Thus the Explorer microplate, which was spawned ≈5 Ma, is an ephemeral adjustment to mechanical difficulties at the triple junction. This new model implies that the Explorer subduction zone is no longer active. Ephemeral oceanic microplates have also existed at Pacific–North American triple junctions off southern California and Baja California.

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