A transient model of the geothermal system of the Long Valley Caldera, California

Publisher Website
Google Scholar
Abstract
The present‐day geothermal system underlying the southern one third of the Long Valley caldera is discussed, based on interpretation of the complex shapes of temperature‐depth curves and the correlation of this interpretation with hydrologic and geologic data. The temperature‐depth curves are interpreted using a simple analytical model for transient flow of hot water in a confined aquifer. The results give information on the transient nature of the geothermal system operating in the Long Valley caldera and on the application of the analytical model to a complex geothermal system. At the present time there is major west‐east flow of hot fluid in two aquifers. The first aquifer occurs at a depth of 20–120 m. The aquifer is charged with water at a temperature greater than 175°C west of Casa Diablo Hot Springs which then flows eastward at a velocity of the order of 100–200 m/yr to exit in the hot springs along Hot Creek and at the edge of Lake Crowley. Temperatures systematically decrease eastward in the shallow aquifer. In order to interpret the temperature data in Long Valley caldera a model of the thermal effects of flow in a shallow aquifer is discussed, and the results are found to be consistent with geologic and hydrologic information. The shapes of inversions of temperature versus depth curves indicate that the age of this aquifer ranges between 50 and 700 years; the best estimate is 500–700 years. The initiation of hot water flow in the shallow aquifer coincides with extrusive activity associated with the Mono/Inyo craters chain of rhyolitic centers, which was active over a linear distance of about 40 km during the period 500–600 years ago. A second aquifer occurs just below the top of the Bishop Tuff at a depth of approximately 700 m in hole Mammoth 1. Less is known about the flow velocity and other characteristics of this aquifer. Modeling of the temperature‐depth curve in Mammoth 1 suggests that this aquifer is of the order of 3000 years old. Temperatures associated with both aquifers are evidence for transient events which caused a sudden input of hot water along the west side of the caldera into an existing geothermal system that was circulating at temperatures below 120°C. Apparently, previous to the input of hot water into these aquifers, the geothermal system in the caldera was in a waning phase. The waning of the system may have been due to loss of porosity and permeability by mineral deposition in the circulation pathways and/or cooling of the caldera and subjacent intrusive rocks and a consequent decrease in the thermal drive. There is no evidence at the present time of a large‐scale, high‐temperature geothermal system associated with the magma chamber identified by seismic studies beneath the resurgent dome. Whether the high temperatures in the current geothermal system reflect cooling of magma associated with Mono/Inyo craters or merely fracturing and reconnection of a preexisting source of hot water to the shallow subsurface is not known. Although the direction of flow of hot water is from west to east, deep temperature data suggest return flow of cold water from east to west in the lower half of the caldera fill (in the eastern part of the caldera at least). The velocity of this return flow is probably of the order of a meter per year, or two orders of magnitude below the velocity of flow in the shallow aquifers.