THE YELLOWSTONE HOTSPOT

Direct evidence for a plume-plate interaction as the mechanism respons ible for the Yellowstone-Snake River Plain (YSRP), 16-Ma volcanic syst em is the observation of a linear age-progression of silicic volcanic centers along the Snake River Plain 800 km to the Yellowstone caldera - the track of the Yellowstone hotspot. Caldera-forming rhyolitic volc anism, active crustal deformation, extremely high heat flow (about 30 times the continental average), and intensive earthquake activity at Y ellowstone National Park mark the surface manifestations of the hotspo t. Anomalously low P-wave velocities in the upper crust of the Yellows tone caldera are interpreted as solidified but still hot granitic rock s, partial melts, hydrothermal fluids and sediments. Unprecedented def ormation of the Yellowstone caldera of up to 1 m of uplift from 1923 t o 1984, followed by subsidence of as much as approximately 12 cm from 1985 to 1991, clearly reflects a giant caldera at unrest. The regional signature of the Yellowstone hotspot is highlighted by an anomalous, 600-m-high, topographic bulge centered on the caldera and that extends across a approximately 600-km-wide region. We suggest that this featu re reflects long-wavelength tumescence of the hotspot. Yellowstone is also the center of a + 10 m to + 12 m geoid anomaly, the largest in No rth America, and extends about 500 km laterally from the caldera, simi lar in width to the geoid anomalies of many oceanic hotspots and swell s. The 16-Ma trace of the Yellowstone hotspot, the seismically quiesce nt Snake River Plain, is surrounded by ''bow-wave'' or parabolic-shape d regions of earthquakes and high topography. The systematic topograph ic decay along the Snake River Plain, totaling 1,300 m, fits a model o f lithospheric cooling and subsidence which is consistent with passage of the North American plate across a mantle heat source. The 16-0 Ma rate of 4.5 cm/yr silicic volcanic, age progression of the YSRP includ es a component of southwest motion of the North American plate, modele d at approximately 2.5 cm/yr, and a component of concomitant crustal e xtension estimated to be 1 to 2 cm/yr. The YSRP also exhibits anomalou s crustal structure which we believe is inherited from magmatic and th ermal processes associated which the Yellowstone hotspot. This include s a thin, 2-5-km-thick surface layer composed of basalts and rhyolites and an unusually high-velocity (6.5 km/s), mid-crustal mafic layer th at we suggest reflects extinct ''Yellowstone'' magma systems that have replaced much of the normal granitic upper crust. Direct evidence for a mantle connection for the YSRP system is from anomalously low, P-wa ve velocities that extend from the crust to depths of approximately 20 0 km. These properties and the kinematics of the YSRP are consistent w ith an analytic model for plume-plate interaction that produces a ''bo w-wave'' or parabolic pattern of upper-mantle flow southwesterly from the hotspot, similar to the systematic patterns of regional topography and seismicity. Our unified model for the origin of the YSRP is consi stent with the geologic evidence where basaltic magmas ascend from a m antle plume to interact with a silicic-rich continental crust producin g partial melts of rhyolitic composition and the characteristic calder a-forming volcanism of Yellowstone. Cooling and contraction of the lit hosphere follows the passage of the plate over the hotspot with contin uing episodic eruptions of mantle-derived basalts along the SRP.