SURFACE WAVE TOMOGRAPHY OF THE YELLOWSTONE HOTSPOT AND WYOMING CRATON

A high resolution fundamental mode Rayleigh wave inversion for VSV structure of the crust and uppermost mantle of the Yellowstone Hotspot shows 6.5% crustal velocity variations, with an E-W slow velocity trend not aligned with mantle low velocities. A region of high crustal velocities occurs to the NE of Yellowstone, and probably delimits a region of mafic underplating. Throughout the model, there is a relatively high velocity mantle lid, ranging in thickness from 20 km along the hotspot track, to 40 km NE of the Hotspot. Variations in lid thickness are inconsistent with a zone of preexisting lithospheric weakness along the Hotspot trend. Along the Hotspot track, a well-defined low velocity channel has a minimum velocity of 3.6+/-0.2 km at 75-85 km. This minimum velocity is 8-10% lower than found under Hawaii, Iceland, or the East Pacific Rise suggesting Yellowstone melt resides in a different topology. Outside the Hotspot track, the lowest mantle VS is 4.15 +/- 0.15 km/s, at 90 km depth. This velocity is similar to that found under 4-20 Ma oceanic lithosphere and implies that below 90 km the Wyoming mantle is sub-solidus adiabatic asthenosphere, not lithosphere.