EPHEMERAL PLATE-TECTONICS AT THE QUEEN-CHARLOTTE TRIPLE JUNCTION

Three plate boundaries, the Queen Charlotte transform, the Cascadia su bduction zone, and the Juan de Fuca Ridge, meet in a complex triple ju nction offshore Vancouver Island. Some interpretations of the plate te ctonics of the region have included the Explorer ridge, the Dellwood K nells, and the Tuzo Wilson volcanic field as part of the spreading sys tem, 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 independen t plate boundaries, but the result of leaky transform tectonics betwee n 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 ter minus of the Juan de Fuca Ridge where it meets the Nootka deformation zone. Thus the Explorer microplate, which was spawned similar to 5 Ma, is an ephemeral adjustment to mechanical difficulties at the triple j unction. This new model implies that the Explorer subduction zone is n o longer active. Ephemeral oceanic microplates have also existed at Pa cific-North American triple junctions off southern California and Baja California.