Geologic History of the Coso Geothermal System

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Conference Paper: Geologic History of the Coso Geothermal System


The Coso geothermal system is developed within a PlioPleistocene volcanic field. At least three episodes of geothermal activity have affected the area during the last 300,000 years. Travertine deposits that formed at 307 ka represent the earliest thermal activity. These deposits may have been related to a large low- to moderate-temperature geothermal system. The second episode produced sinter at 238 ka. Fluid inclusions document a large high-temperature system with an upflow zone in the southern part of the present-day geothermal field. Temperatures up to 328 C and N2 /Ar ratios of inclusion fluids provide evidence of magmatic activity during this episode. Fluid originating within this upwelling zone was progressively diluted by low-salinity groundwater as it flowed laterally and upward to the north. The modern geothermal system consists of several weakly connected or isolated reservoirs that are being heated by young intrusions. At least three reservoirs can be distinguished based on the relationships between the production fluid compositions and temperatures. These relationships indicate that flow between the north and south had all but ceased in the pre-exploitation system, and that heating of the East Flank has been so recent that the salinity of the fluid has not yet increased. Both the production chemistry and the fluid-inclusion data indicate that the lowsalinity water that once blanketed the system and provided a

low-temperature mixing end-member has disappeared, presumably due to lower recharge rates.

Michael C. Adams, Joseph N. Moore, Steven Bjornstad and David I. Norman

World Geothermal Congress; Kyushu-Tohoku, Japan; 2000/05/28

World Geothermal Congress, 2000

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Michael C. Adams,Joseph N. Moore,Steven Bjornstad,David I. Norman. 2000. Geologic History of the Coso Geothermal System. In: Proceedings. World Geothermal Congress; 2000/05/28; Kyushu-Tohoku, Japan. Kyushu-Tohoku, Japan: World Geothermal Congress; p. 205–210