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Crustal Structure and Evolution of an Explosive Silicic Volcanic System at Yellowstone National Park
Large volumes of Quaternary silicic volcanics (~ 6700 km3), associated explosive caldera-forming eruptions, and high heat flow (in excess of 1800 mW m-2 infer the presence of silicic magmas within the crust and upper mantle beneath the Yellowstone Plateau. Seismic refraction-reflection data, analyses of earthquake hypocenters, and seismic attenuation have revealed a laterally inhomogeneous upper crust with low P-wave velocities but a more seismically homogeneous lower crust. The upper crust beneath the Yellowstone caldera is characterized by P-wave velocities of 5.7 km/sec and 4.0 km/sec—values that are anomalously low compared with that of the surrounding thermally undisturbed crystalline basement of 6.0 km/sec. The 5.7 km/sec body generally underlies the Yellowstone caldera (35 km x 65 km) and coincides with a regional -60 mgal gravity low, suggesting concomitant low density and low velocity. The 5.7 km/sec low-velocity body is interpreted to represent a hot but relatively solid body approximately 8 to 10 km thick that was probably the reservoir for the silicic magmas. A 4.0 km/sec low-velocity body located beneath the northeast boundary of the caldera coincides with a local -20 mgal gravity low and has a tenfold increase in seismic attenuation—properties that can be interpreted to result from a steam-water-dominated system to a body of 10-50 percent silicic partial melt. The P-wave velocity of the upper 100 to 250 km of the mantle beneath the Yellowstone region, analyzed from teleseismic arrivals, is reduced by ~5 percent, suggesting the presence of a basaltic partial melt that is probably the source of heat that drives the Yellowstone hydrothermal system. In comparison, the lower crust of the Yellowstone region appears seismically homogeneous to the horizontally propagating refracted rays and similar to that of the thermally undisturbed lower crust of the surrounding Rocky Mountains. This suggests that the seismic properties of the lower crust were relatively unaffected by the ascension of the parental basaltic magmas that are hypothesized to have intruded and partially melted the crust producing the voluminous rhyolite and ash flow tuffs of the Yellowstone Quaternary volcanic system. Maximum focal depths of earthquakes in Yellowstone systematically shallow from ~20 km outside the caldera to ~5 km beneath the caldera, suggesting the influence of high-temperature abnormal pore pressure, compositional changes that restrict brittle failure to the upper crust. Orthometrically corrected reobservations of level lines across the Yellowstone caldera show an area of crustal uplift, up to 15 mm/yr, that generally coincides with the outline of the 5.7 km/sec low velocity layer. These data are consistent with a model in which an upper-crustal low-velocity/low-density layer, 75 km x 25 km, appears to be plastically deforming. Taken together with the geologic data this crustal model is interpreted to reflect the structure and properties of a thermally deforming Archean crust and the initial stages of the bimodal rhyolitic/basaltic volcanism of the Yellowstone- Snake River Plain volcano-tectonic system. While the interpretations are not unique; the youthfulness and volume of Quaternary volcanism, the high heat flow, the high rates of contemporary uplift, and the upper-crustal low-velocity layers infer the presence of hot crustal material and possible partial melts that underlie the Yellowstone Plateau. These properties cannot yet be evaluated to indicate temporal variations in volcanism, but the geologic record and the new geophysical models suggest future volcanic activity in the Yellowstone Plateau.
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