A captured island chain in the coast range of Oregon and Washington

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