A kinematic model for the formation of the Siletz terrane by capture of coherent fragments of the Farallon and Resurrection plates (Invited)

The details of the formation of the mostly Eocene basaltic Siletz terrane in the coast ranges of Oregon and Washington have remained enigmatic. Early models [e.g. Duncan, 1982 JGR], noting younger ages for basalts and overlying foramifera in the center of the terrane near the Columbia River and older ages to the north and south, called for an interpretation of a captured fossil spreading ridge between the Kula and Farallon plates. However, later work showed two problems with this simple model [Wells et al., 1984 Tectonics]. First, the difference between old and young ages was larger than predicted by the relatively fast spreading rate. Second, the presence of proximal continental sediments interbedded with some of the older basalts conflicted with the large amounts of convergence predicted by the generally used models for motion of the Kula and Farallon plates relative to North America. The recent suggestion that an additional oceanic plate, the Resurrection plate, was present between the Kula and Farallon plates and subducting beneath eastern Alaska and western Canada during the early Cenozoic [Haeussler et al., 2003 GSAB] permits important revisions to the interpretation of regional tectonic history. We propose a model in which the older (~55 Ma) parts of the Siletz terrane formed at the Resurrection-Farallon ridge, not far offshore from the Klamath Mountains. Proximity to the Yellowstone hotspot could have generated thick oceanic crust in a setting analogous to Iceland. When the Juan de Fuca (Vancouver) plate separated from the Farallon plate at ~52 Ma, we propose that a small Siletz plate also separated from the northern Juan de Fuca plate, eventually to be captured in the Oregon coast range. A fragment of the Resurrection plate may have separated later (~48 Ma), eventually captured in the Washington coast range and constituting the southward-younging part of the terrane. Cessation of motion of these fragments relative to adjacent parts of North America may have been achieved by ~40 Ma. Slower motion of the Resurrection plate and the eventually captured fragments compared with the Kula and Farallon plates can resolve previous problems with rates of age progression and amount of convergence.