George P. Woollard Technical Lecture: Dynamics and Evolution of the Yellowstone Hotspot

The Yellowstone hotspot results from the interaction of a mantle plume with the overriding N. America plate producing a ~300-m high topographic swell centered on the Quaternary Yellowstone volcanic field. The Yellowstone area is dominated by earthquake swarms including a deadly M7.3 earthquake, extraordinary high heat flow up to ~40,000 mWm-2, and unprecedented episodes of crustal deformation. Seismic tomography and gravity data reveal a crustal magma reservoir, 6 to 15 km deep beneath the Yellowstone caldera but extending laterally ~20 km NE of the caldera. Kinematically, deformation of Yellowstone is dominated by regional crustal extension of up to ~0.4 cm/yr but with superimposed decadal-scale uplift and subsidence episodes, of ~2 cm/yr from 1923 to 2003. From 2004 to 2009 Yellowstone experienced an unprecedented episode of accelerated uplift of up to 7 cm/yr whose source is modeled as magmatic recharge of a sill near the top of the crustal magma reservoir at 8-10-km depth. Mantle tomography reveals that Yellowstone volcanism is fed by an upper-mantle plume low-velocity body extending from ~60 km to 700 km in depth, tilting 60° NW, but then reversing tilt to ~60° SE to a depth of ~1000 km. Moreover, images of upper mantle conductivity from MT data reveal a high conductivity annulus around the north side of the plume to resolved depths of ~300 km. On a larger scale, mantle circulation Yellowstone is characterized by eastward flow at 5 cm/yr that deflects the plume to the west, and is underlain by a deeper zone of westerly return flow in the lower mantle reversing the deflection of the plume body to the SE. Dynamic models of the Yellowstone plume reveal low, excess plume temperatures, up to 200°K, consistent with a weak buoyancy flux of ~0.25 Mg/s. Integrated kinematic modeling of GPS, Quaternary fault slip, and seismic data suggest that the gravitational potential of the Yellowstone swell creates regional extension affecting much of the western U.S. We extrapolate the original location of the Yellowstone plume source southwestward 800 km to an initial position at 17 million years ago beneath eastern Oregon and Washington where the original plume head ascended vertically behind the subducting Juan de Fuca plate, but was entrained and deflected at ~12 Ma ago in faster mantle flow and tilted into its present configuration.