A TWO-STAGE SPREADING MODEL FOR THE YELLOWSTONE MANTLE PLUME HEAD

Generic plume models predict that flood-basalt provinces are generated above starting plume heads, and that hotspot tracks are generated above stationary plume tails. There is a lack of consensus, however, on whether these models are applicable to the Columbia River Flood Basalts and the Snake River Plain hotspot track. Many workers prefer nonplume interpretations, such as back-arc extension and/or small-scale convection associated with the EGDE model (Edge-Driven Gyres and Eddies) of King and Anderson (1978). Such alternative mechanisms, however, do not adequately resolve the abrupt onset and short duration of flood basalt volcanism in the Pacific Northwest. An argument in support of a plume model lies in the recognition of a series of radial volcanic migrations in the eruptive sequence, which we believe represents the emplacement and spreading of the Yellowstone mantle-plume head away from a focal point in southeasterm Oregon. These outwardly propagating trends are difficult to reconcile with nonplume interpretations, all of which lack a clear mechanism for both the radial orientation and the temporal pattern of flood-basalt eruption. Initial spreading from 16.6-15.0 Ma resulted in the rapid propagation (~10-100 cm/yr) of volcanism along three coeval, radial trends: the Chief Joseph, the Steens-Picture Gorge, and the Northern Nevada Rift trends. At ~15 Ma, the plume head was sheared off by the North American craton, thus allowing the plume tail to generate the SRP hotspot track on the over-riding plate. Decapitated from its feeder tail, the plume head continued to spread westward, but at a much reduced rate (1-5 cm/yr) due to asthenospheric drag and counterflow above the subducting plate. The plume head currently underlies much of the inland Pacific Northwest, where it is manifested in a broad zone of regional uplift, slow p-wave travel times, high surface-heat flow, and young volcanism. The edge of the plume head is marked by a gravity discontinuity and an adjacent belt of wrinkle-ridges to the north, the Precambrian margin of North America to the east, a broad, ill-defined zone of transitional lithosphere to the south, and a prominent belt of young HAOT basalts and active volcanoes lying adjacent to the Cascades volcanic arc to the west.