HR: 09:50h
AN: V51F-08 INVITED    [Abstracts]
TI: Yellowstone and Long Valley – A Comparison of Two Restless Calderas
AU: * Hill, D P
EM: hill@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, United States
AU: Smith, R B
EM: rbsmith@mines.utah.edu
AF: Dept. of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, United States
AB: Three large, silicic calderas in the conterminous United States have explosively erupted volumes > 300 km³ within in the last 2 million years -- Yellowstone caldera (Wyoming) Long Valley caldera (California) and the Vallez caldera (New Mexico) all located in extensional tectonic environments. All have shown varying levels of historic unrest. Pronounced unrest episodes at Yellowstone and Long Valley calderas over the past three decades stimulated extensive research on these two closely monitored calderas, and we explore some emerging similarities and differences. Yellowstone caldera is underlain by a long-lived (> 17 my) upper-mantle hot-spot that has fed a series of caldera-forming, extending to the southwest across southern Idaho to central Oregon including three caldera-forming eruptions from the Yellowstone caldera system in the last 2 my, the most recent at 600,000 ybp. It is marked by relatively low density and low seismic velocities extending to depths of at least 400 km and a regional topographic swell with elevations exceeding 2000 m. The extensive Yellowstone hydrothermal system has a thermal output of 5 GW. The most recent magmatic eruption dated at 70,000 ybp. By comparison, Long Valley caldera is underlain by a relatively modest "hot-spot", the locus of which appears to be influenced by a dilatational jog between the dextral Eastern California Shear Zone and the Walker Lane and westward delamination of the dense lithospheric root of the adjacent Sierra Nevada. The Long Valley system has fed multiple eruptions of over the past 4 my and a single caldera-forming eruption at 760,000 ybp. It is marked by a limited topographic swell but with the elevation of the caldera floor and adjacent basins comparable to the 2000-plus m elevation of the Yellowstone swell. Long Valley caldera hydrothermal system has a thermal output of 0.3 GW (including a 40 MW geothermal power plant). The most recent eruptions from the Long Valley Caldera- Mono Domes volcanic field occurred 600 and 250 ybp. Unrest at both calderas is characterized by strong regional seismicity, recurring intra-caldera earthquake swarms, deformation measured in decimeters, changes in the hydrothermal systems, and elevated CO2 emissions. Extra-caldera seismicity at Yellowstone includes the 1959 M=7.5 Hebgen Lake earthquake, and at Long Valley, four M~6 earthquakes in May 1980 immediately south of the caldera. In both cases, the extra-caldera seismicity is more energetic and persistent than intra-caldera activity underscoring the importance of tectonic-magmatic interactions. Long Valley seismicity includes long-period (LP) "volcanic" earthquakes, spasmodic bursts, and very-long-period (VLP) earthquakes. Seismicity in Yellowstone is limited to brittle-failure earthquakes. Deformation in Yellowstone has shown alternating episodes of inflation and subsidence while that in Long Valley has shown several episodes of rapid uplift with negligible subsidence. In both cases, peak uplifts reached 70 to 80 cm. Among the many outstanding questions for both calderas include 1) the nature of the process driving deformation and whether the active agent involves hydrous magmatic fluids and volatiles or the intrusion of magma into the upper crust, 2) the configuration of the magmatic plumbing system within the crust, and 3) regional tectonic-magmatic interactions.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7280 Volcano seismology (8419)
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
DE: 8440 Calderas
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting