THREE-DIMENSIONAL GEOPHYSICAL STRUCTURE OF THE YELLOWSTONE / SNAKE RIVER PLAIN HOTSPOT SYSTEM: IS A DEEP MANTLE PLUME REQUIRED?
Nearly all new body wave tomographic models utilizing EarthScope data show a distinct swath of strongly reduced seismic wavespeeds extending laterally from the central SRP to Yellowstone, extending to depths of no greater than ~200 km. There is no evidence for a singular, concentrated conduit of reduced velocities below 200 km, as expected from a focused mantle plume upwelling. Surface wave tomography shows similar patterns for the YSRP region, with shear wavespeeds consistent with partial melt zones within the YSRP crust and uppermost mantle extending to depths of ~125 km, and aligned with the widespread distribution of Quaternary basaltic volcanism all along the SRP. Results from regional 3D MT models show focused zones of highly conductive crust and upper mantle, with the strongest conductivities in the uppermost mantle residing beneath the central Snake River Plain and the largest contrasts extending to ~100km depth.
Given the paucity of evidence for a present-day plume, we explore geophysical proxies in the mantle flow field for past plume-related dynamics, appealing to proxies for mantle flow. Data from several seismic anisotropy studies confirm that the Yellowstone region exhibits little evidence for vertical mantle flow across the region. Further, the downgoing Juan de Fuca plate, imaged clearly in the tomographic studies, would provide a barrier to an upwelling mantle plume.
An alternative to the plume model involves mantle flow around a stranded fragment of the Farallon plate whose northern edge parallels the SRP, and whose eastern edge is beneath Yellowstone. Flow of deep mantle around this sinking portion of the Farallon would introduce ascending mantle beneath the whole of the YSRP, not just Yellowstone, and could also explain the significant tectonomagmatism of the Columbia River flood basalt event and continuing volcanic activity on the High Lava Plains.