Thermal Gradient Holes At Long Valley Caldera Geothermal Area (Sorey, Et Al., 1991)
Exploration Activity: Thermal Gradient Holes At Long Valley Caldera Geothermal Area (Sorey, Et Al., 1991)
|Exploration Activity Details|
|Location||Long Valley Caldera Geothermal Area|
|Exploration Technique||Thermal Gradient Holes|
|Activity Date||1985 - 1988|
Sorey et al. (1991) integrated information from previous scientific and private industry investigations with new data obtained from fluid sampling, test drilling, and geological and geophysical studies conducted between 1985-1988 into a comprehensive conceptual model of the present-day hydrothermal flow system at Long Valley caldera. Lithology and temperature gradient data from wells drilled prior to 1988 are summarized in detail in the compilation, which includes information from numerous wells described in previous studies, and data from many of the wells are available online through the U.S. Geological Survey (Farrar et al., 2010). Thermal conductivity, XRD, and isotopic analyses of core cuttings from several of the wells discussed have been completed in several studies, and seem to prove useful in most cases (Flexser, 1991; Goff et al., 1991; Smith and Suemnicht, 1991). Results from these studies are also summarized in Sorey et al. (1991). Relevant data from chemical and isotopic studies published during the same year are also considered in the review.
"Data collected since 1985 from test drilling, fluid sampling, and geologic and geophysical investigations provide a clearer definition of the hydrothermal system in Long Valley caldera than was previously available. This information confirms the existence of high-temperature (> 200°C) reservoirs within the volcanic fill in parts of the west moat. These reservoirs contain fluids which are chemically similar to thermal fluids encountered in the central and eastern parts of the caldera. The roots of the present-day hydrothermal system (the source reservoir, principal zones of upflow, and the magmatic heat source) most likely occur within metamorphic basement rocks beneath the western part of the caldera. Geothermometer-temperature estimates for the source reservoir range from 214 to 248°C. Zones of upflow of hot water could exist beneath the plateau of moat rhyolite located west of the resurgent dome or beneath Mammoth Mountain. Lateral flow of thermal water away from such upflow zones through reservoirs in the Bishop Tuff and early rhyolite accounts for temperature reversals encountered in most existing wells. Dating of hot-spring deposits from active and inactive thermal areas confirms previous interpretations of the evolution of hydrothermal activity that suggest two periods of extensive hot-spring discharge, one peaking about 300 ka and another extending from about 40 ka to the present. The onset of hydrothermal activity around 40 ka coincides with the initiation of rhyolitic volcanism along the Mono-Inyo Craters volcanic chain that extends beneath the caldera's west moat."
- Michael L. Sorey, Gene A. Suemnicht, Neil C. Sturchio, Gregg A. Nordquist (December 1991) New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits