THE YELLOWSTONE HOTSPOT: PLUME OR PLUM

On behalf of the Yellowstone Hotspot Team (G. P. Waite, M. Jordan, C. Puskas, D. Schutt, E. Humphreys, K. Dueker, J. Hernlund and P. Tackley)

Origin of the Yellowstone hotspot and Yellowstone-Snake River Plain (YSRP) volcanic system has been hypothesized by various mechanisms: a plume, the tip of a propagating lithospheric fracture, decompression-mantle melting related to lithosphere extension, etc. On the basis of these ideas, the above team formed an interdisciplinary project (1999-2003) focusing on the dynamics and kinematics of the Yellowstone hotspot and its modification of continental structure employing seismic and GPS imaging. Crustal structure reveals a pronounced low velocity, magmatic system at Yellowstone that is underlain by a low velocity plume-like body at mantle depths beneath Yellowstone, beginning at ~80 km that extends to up to 400 km. This body, the Yellowstone hotspot, may be a partial-melt of depleted mantle, while thinning of the transition zone between the 440- and 600- km discontinuities favors a plume source. The GPS-derived kinematic field exhibits large secular changes of cm/yr of uplift and subsidence of the Yellowstone caldera that are related to its crustal magma system. These motions are superimposed on more uniform NE-SW extension of 4 mm/yr across Yellowstone, that notably decreases across the eastern SRP to ~2 mm/yr implying intra-block compression. Upper mantle fast S-wave splitting directions imply generally uniform NE asthenosphere flow which parallels the direction of plate motion. Dynamic models will be constrained by seismic data and GPS-derived deformation supplemented by geologic and potential field data. One model suggests that strain concentration at the NE edge of the Basin-Range corresponds to a zone of mantle shear induced by plate motion where melting occurs due to extension and the presence of a hot mantle. In this model, focused extension moves eastward in a wave, i.e. producing progressively younger volcanism from an upwelling beneath the YSRP. While these data favor a “Yellowstone plum” of low velocity, depleted mantle directly beneath Yellowstone, enhanced by Basin-Range extension, they can not rule out a mid mantle source. Notably, these upper mantle decompressive sources would affect tectonics and magmatism of the region in much the same way as a deep CMB plume.