A POSSIBLE DISLOCATED TRACK OF THE YELLOWSTONE MANTLE PLUME IN THE PACIFIC NORTHWEST

Over the past 20 years or more significant interest has developed in understanding the relationship between the origin of the middle Miocene Columbia River Basalt Group, the Yellowstone mantle plume, and the Siletzia terrane underlying the coast ranges of Oregon and Washington. Several models have shown a genetic relationship between the lavas of the Columbia Plateau, the mantle plume, and the hotspot track along the Snake River Plain connecting the two. However, few proposals attempt to explain the trace of the plume from Siletzia to the western Snake River Plain. To address this problem, we propose (1) that the Yellowstone mantle plume has been in existence well before being overrun about 50 Ma near Cape Mendocino, California, along the western margin of North America, and (2) that a hotspot track of Yellowstone exists between the point of overrun and the McDermitt caldera, at 16.1 Ma, at the western end of the Snake River Plain. This apparent obscure track is complicated because it has been dislocated by diverse tectonic movements in the region—westward extension in eastern Oregon, southwest movement of the North American plate, and northward movement along the western margin of North American. The point of overrun was relocated 660 km northward to about the mouth of the Columbia River. From here the relocated track extends southeast to McDermitt caldera. Four eruptive centers, showing chemical and isotopic affinities to the mantle plume, delineate this track. The first is a cluster of basalts exposed in the lower Columbia River area—Tillamook, Grays River, Waverly Heights, and Kalama River between 43 and 38 Ma. The second is a possible caldera in the Warm Springs Reservation, which erupted tuffs of the John Day Formation. The third is the Crooked River caldera, which erupted basalt and rhyolite lava flows and tuffs between 32 and 27 Ma of the John Day Formation. And the fourth is Steens Mountain, which erupted Steens Basalt, the lower member of the Columbia River Basalt Group, at 16.7 Ma.