Abstract: Hydrogeology of Long Valley Geothermal Area, California

F. H. Olmsted

The Long Valley geothermal area is within a 450-sq km elliptical caldera on the east flank of the Sierra Nevada in east-central California. The caldera formed 700,000 years ago after the eruption of ~600 cu km of rhyolite ash, the Bishop Tuff. Seismic and gravity studies and data from a deep test hole indicate an average thickness of 2.4 km of caldera fill, of which densely welded Bishop Tuff occupies most of the lower 1.4 km. Extensively fractured Bishop Tuff probably forms the principal hydrothermal reservoir.

Hydrologic, isotopic, and thermal data indicate recharge to the hydrothermal reservoir along the western and northeastern margins of the caldera. Chemical geothermometers and mixing-model calculations indicate source temperatures of at least 210° and possibly 280°C for the 200 to 300 kg/second hot-water discharge from the reservoir. Source of heat is inferred on the basis of heat-flow and teleseismic studies, to be a residual body of magma at a depth below 6 to 8 km in the western part of the caldera.

Total present discharge of heat from the caldera is estimated at 6.9 × 10⁷ cal/second. Numerical simulations indicate that fluid circulation to depths of 4 to 5 km--far below the Bishop Tuff--would be required to attain the present steady-state heat flow and temperatures after the period of hydrothermal activity of 300,000 years that is suggested by some geologic evidence. However, other geologic evidence suggests a much shorter period of hydrothermal discharge, perhaps only 30,000 years. The shorter period would require depths of fluid circulation of 1.5 to 2.5 km--well within the Bishop Tuff. The paradox could be explained by assuming that discharge from the hydrothermal system has been intermittent during the past 300,000 years as a result of climatic variations and self-sealing processes in discharge channels.

AAPG Search and Discovery Article #90962©1978 AAPG 2nd Circum-Pacific Energy and Minerals Resource Conference, Honolulu, Hawaii