ON THE CAUSE OF VOLUMINOUS MAGMATISM IN THE NORTHWESTERN UNITED STATES

The Columbia River - Snake River Plain (SRP) - Yellowstone magmatic provinces are but one expression of Cenozoic volcanism in northwestern North America. The plume model for the SRP explains a variety of features ranging from topographic swells to direction and speed of volcanic migration relative to plate motion, to low 4He/3He in the erupted basalts. Less easily explained by the plume model are: 1) the counter migrating volcanic trace of the High Lava Plains (HLP) in central Oregon, 2) localization of voluminous flood basalt eruption some 300-400 km north of the hypothesized plume impact point, 3) continuation of basaltic volcanism to Recent time along the whole HLP and SRP traces, and 4) the relationship to other patterns in the Cenozoic magmatic and tectonic history of the northwestern US. Attempts to explain the broader volcanic history of the northwestern US concentrate on the interaction of North America with the failing subduction system along its western margin. For example, shallow dip subduction can effectively hydrate the overlying mantle without leading to melting, yet should the slab dip increase and expose this wet mantle to normal asthenospheric mantle, conduction can raise the temperature of the bottom 10-20 km of the wet lithosphere by hundreds of degrees in under 10 Myr leading to rapid extensive melting. Slab roll back also predicts migrating volcanic trends that move counter to plate motion, as in the early Cenozoic ignimbrite sweep and the late Cenozoic HLP trace. Another factor that may focus volcanism is flow of hot mantle around the edge of the subducting plate that might occur with the opening of a slab window or around the southern edge of the subducting Juan de Fuca as this boundary changes to a transform margin. Unlike Yellowstone where a conduit of low velocity mantle has now been imaged from the surface to a depth approaching 600 km, the position of the Juan de Fuca/Farallon plate is known only up to the Cascade front, or at great depth in the mantle beneath central North America. We hope to remedy this situation through a multidisciplinary project recently funded by the Continental Dynamics program. Our work will focus on seismic imaging of the mantle beneath eastern Oregon and exploring the connection between mantle and crustal structure and the magmatism of this area.