The uniform southwesterly trend of fast-traveling split SKS waves that traverse the upper mantle beneath the Yellowstone swell provides good evidence for both southwesterly motion of North America over a relatively stable deep Earth interior, and young strain accommodation within the Yellowstone swell mantle via dislocation creep of olivine. These results contrast with the many SKS splits recorded across the western United States, the splitting behavior of which often is very complex at individual sites, and the orientations of which, as a set, cannot be attributed simply to plate motion or to small-scale convection beneath North America lithosphere. These data suggest that, away from Yellowstone, (1) mantle fabric is complex, and (2) North America motion is accommodated by strain at depths greater than those typically associated with asthenosphere.

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