RECONSIDERING THE ORIGIN AND EMPLACEMENT OF SILETZIA

The basalt basement of the Oregon and Washington Coast Ranges is an accreted oceanic terrane sutured to North America in Eocene time. Also known as Siletzia, it consists of Paleocene to early Eocene submarine and subaerial tholeiitic and alkalic basalt thrust beneath the continental margin. Siletzia is 19 km thick on the Olympic Peninsula and may exceed 30 km in Oregon based on seismic studies. Paleomagneticically-determined paleolatitudes are not significantly different from those expected for Oregon and Washington. Sedimentary interbeds in the basalt near Roseburg and in the Olympic Mountains locally include cobbles apparently derived from the continent. These data argue for eruption and emplacement close to its present location. Siletzia was first interpreted as an accreted ocean island chain, but proximity to the margin led other geologists to consider Siletzia the product of oblique continental margin rifting and slab window magmatism during ridge subduction. Rifting might explain prolonged coastal mafic magmatism lasting until about 35 Ma, and near-trench Eocene magmatism along much of the NE Pacific margin has been explained in a similar manner. Geologic mapping near Roseburg, U/Pb, Ar/Ar, and coccolith ages provide constraints on Siletzia's emplacement at 51 Ma. Vitrinite reflectance indicates low thermal maturity for Tyee basin fill deposited on Siletzia, incompatible with an active rift basin. Dike swarms expected from rifting are absent in the adjacent continental margin. Strike-slip emplacement from a distant source is unlikely given the proximal paleolatitude. Instead, slip vectors from the basin-bounding faults indicate margin-normal thrusting which began prior to basin filling. The great thickness of Siletzia is more consistent with an accreted oceanic plateau. Potential field data suggest partial obduction onto the continent, similar to the Ontong Java Plateau collision with the Australian plate. Collision-related thrusting was overprinted by a regional extensional event during Tillamook-Yachats time (41-35 Ma), producing mafic and alkalic magmatism, regional dike swarms, and normal faulting. Siletzia may have been created at an oceanic hot spot. Following accretion, it was driven southwest over the hot spot as part of N. America, causing the extensional pulse and continued magmatism.