SILETZIA, YELLOWSTONE, AND THE FARALLON PLATE BENEATH NORTH AMERICA (Invited Presentation)

Collision and accretion of two Paleogene oceanic basalt plateaus, Siletzia and the Yakutat terrane, have had profound and lasting effects on the evolution of the Cascadia and SE Alaska convergent margins. Plate motion models, isotopic ages, and onlap relations show that both terranes may have originated between 56 and 49 Ma at a long-lived Yellowstone hotspot (YHS) centered on the northeast-trending Kula/Resurrection-Farallon ridge, not far offshore of Oregon and Washington. Both plateaus are up to 30 km thick and together form a Large Igneous Province (LIP). The time between eruption and collision with North America was very short, less than 5 m.y., and large parts of the plateaus were accreted, presumably because their hot, buoyant crust hindered subduction. In fact, part of Siletzia was obducted onto the margin of North America, as indicated by potential field data. Locally, adjacent Franciscan-related accretionary rocks are 25-50 m.y. older than Siletzia, suggesting subduction erosion or prior truncation of the margin. Beneath Cascadia, some of Siletzia may be underplated. Accretion between 51 and 49 Ma produced contemporary deformation throughout the Pacific Northwest, from the Klamath Mountains to Vancouver Island and North Cascades. For the Yakutat terrane, cooling during its Pacific Northwest residence and northward transport by the Pacific plate has allowed it to subduct beneath Alaska, although not without considerable deformation of North America. Beneath the Northwest, the subducted Farallon slab between 100 and 1100 km depth is marked by a pronounced, NE-trending slab gap visible in tomographic imagery (Sigloch, 2011). In a plate motion model with a fixed deep slab root and YHS, the gap coincides with the subducted Kula-Farallon ridge segment and the nearly sub-parallel 50-17 Ma track of the YHS on the Farallon plate. This may reflect some combination of slab window and slab heating by the YHS.