Abstract
K-Ar and (super 40) Ar- (super 39) Ar geochronologic data reveal the Paleocene to Eocene eruptive history of volcanic centers which produced the basaltic basement rocks of the Coast Range of Oregon and Washington. Volcanism was short lived at each center and migrated with time from the northern and southern ends toward the center, near the present mouth of the Columbia River. A plot of crystallization ages against geographic location produces a distinct V-shaped pattern. An origin by hot spot volcanism centered on or near a spreading ridge in early Tertiary time best explains the observed age distribution. Limited geochemical data support this model, as the basalts exhibit attributes of both spreading ridge and hot spot-related volcanism. Icelandic volcanism may be a presentday analog. Absolute motion modeling shows that the Yellowstone hot spot could have generated an island chain with approximately the correct paleoazimuth and volcanic propagation rate. Shortly after formation, this oceanic volcanic lineament collided with North America and rotated into its present marginal setting. The western edge of North America subsequently crossed over the hot spot, perhaps triggering late Eocene to Oligocene volcanism in the Coast Range, the Miocene Columbia River basalts in eastern Washington and Oregon and most recently, the basalts of the Snake River Plain and the Yellowstone area.