LITHOSPHERIC TOPOGRAPHY, TILTED PLUMES, AND THE TRACK OF THE SNAKE RIVER-YELLOWSTONE HOTSPOT

The trace of the Snake River-Yellowstone hotspot is the world's best example of a mantle plume that has been overridden by continental lithosphere. The “standard model” that is now accepted by most workers has the Snake River-Yellowstone plume impacting the lithosphere around 17 Ma under northern Nevada (McDermitt caldera) and melting to form a plume head flood basalt province (Columbia River Basalt Group: CRBG), followed by the initiation of time-transgressive volcanism trending NE from its initial position under McDermitt caldera to its final position under the Yellowstone caldera. The track of the plume tail is assumed to be marked by the position of rhyolite caldera complexes that become younger to the northeast (from oldest to youngest): McDermitt, Owyhee-Humboldt, Bruneau-Jarbidge, Twin Falls, Picabo, Heise, and Yellowstone. The standard model poses two significant conundrums: 1) the plume head flood basalt province located far to the north of the purported plume track and 2) the purported plume track deviates significantly from North American plate motion in both trend and velocity prior to 12 Ma. Previous models have address the first issue, but none can explain the apparent plume prior to 12 Ma.

We present a new conceptual model for the origin of this feature that resolves inconsistencies in the current standard model and explains the recent documentation of a thermal anomaly in the mantle below Yellowstone today that plunges 65º NW. Our model implies that the plume tail was forced beneath thinned cratonic lithosphere to the SE along with part of the plume head, and has remained in this orientation for the last 12 Ma. We infer that almost all of the volcanism in northern Nevada, SE Oregon and SW Idaho prior to 12 Ma results from over-riding the southern extension of the plume head, not the plume tail, and that a distinct plume tail hotspot track was not established until formation of the Bruneau-Jarbidge eruptive center around 12 Ma. The plume tail track may also be controlled by a preexisting structural boundary in lithosphere that is thinner than adjacent lithosphere. This model resolves long-standing problemes with previous models for the plume track. It also demonstrates the importance of lithospheric topography on controlling the surface manifestation of plume volcanism and the complexity that may arise when lithospheric thickness is non-uniform.