EVIDENCE FOR THE SILETZIA TERRANE'S ORIGIN AS A RIDGE-CENTERED OCEANIC PLATEAU

The Siletzia terrane forms a thick package (up to 32 km) of basaltic crust in western Oregon, western Washington, and on southern Vancouver Island. Several lines of evidence suggest that this terrane represents an accreted oceanic plateau, including the volume of erupted basalt, isotopic data suggestive of a plume-like source, and geophysical evidence that the terrane remains connected to subducted oceanic crust. This plateau likely accreted to North America ca. 50 Ma based on regional shortening documented from southern Oregon to Vancouver Island. In this abstract I discuss the geology of the Metchosin (Vancouver Island) and Bremerton (Washington state) complexes, which lie at the northern end of the terrane. Both complexes have a partial ophiolite stratigraphy and likely represent the oceanic basement onto which Siletzia was emplaced. Paleomagnetic data suggests that neither complex has experienced significant northward translation or vertical axis rotation since the Eocene and that the inferred WNW-ESE extensional direction from both complexes is primary. U-Pb zircon geochronology shows that the more northwesterly Metchosin complex is ca. 51 Ma, while the more southeasterly Bremerton complex is ca. 50 Ma. If these complexes were generated at an oceanic spreading center, then this age relationship suggests that a NE-striking spreading center was near Bremerton ca. 50 Ma. This ridge position is consistent with previous suggestions that Siletzia was emplaced as a ridge-centered oceanic plateau, similar to modern Iceland, as well as plate reconstructions that place the Kula-Farallon ridge along this part of the North American margin during the middle Eocene. One consequence of this tectonic setting is that Siletzia was extremely young during its attempted subduction (0-5 Myr). The buoyancy of this young crust may explain why it jammed the subduction zone. The accretion of this terrane may be linked to several changes along this part of the North American margin, including potential slab breakoff and a potential jump in triple-junction location.