Low-altitude aeromagnetic survey of a portion of the Coso Hot Springs KGRA, Inyo County, California

Fox, R.C.

doi:10.2172/6548832

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“…Most present-day activity is confined to an east-northeast-trending zone between Sugarloaf Mountain and Coso Hot Springs. This zone is characterized geophysically by low telluric J value [Jackson and O'Donnell, 1980] and low resistivity [Fox, 1978b] and has been mapped as a fault by Hulen [1978]. The east-northeast trend is parallel to some left-lateral, strike-slip, fault-plane solutions reported by Walter and Weaver [1980].…”

Section: The Geothermal Systemmentioning

confidence: 97%

Late Cenozoic volcanism, geochronology, and structure of the Coso Range, Inyo County, California

Duffield¹,

Bacon²,

Dalrymple³

1980

J. Geophys. Res.

167

123

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The Coso Range lies at the west edge of the Great Basin, adjacent to the southern part of the Sierra Nevada. A basement complex of pre-Cenozoic plutonic and metamorphic rocks is partly buried by --35 km 3 of late Cenozoic volcanic rocks that were erupted during two periods, as defined by K-At dating: (1) 4.0-2.5 m.y., ~31 km 3 of basalt, rhyodacite, dacite, andesite, and rhyolite, in descending order of abundance, and (2) _<1.1 m.y., nearly equal amounts of basalt and rhyolite, most of the rhyolite being _<0.3 m.y. old. Vents for the volcanic rocks of the younger period are localized on and near a horst of basement rocks within a concavity defined by the distribution of vents of the older period. The alignment of many vents and the presence of a considerable number of roughly north-trending normal faults of late Cenozoic age reflect basin and range tectonics dominated by roughly east-west lithospheric extension. Fumaroles, intermittently active thermal springs, and associated altered rocks occur within and immediately east of the central part of the field of Quaternary rhyolite, in an area characterized by various geophysical anomalies that are evidently related to an active hot-water geothermal system. This system apparently is heated by a reservoir of silicic magma at _>8-kin depth, itself produced and sustained through partial melting of crustal rocks by thermal energy contained in mantle-derived basaltic magma that intrudes the crust in response to lithospheric extension.The youthfulness of some of the volcanic rocks and the presence of fumaroles, intermittently active hot springs, and associated hydrothermally altered rock prompted the selection of this area for study by the U.S. Geological Survey in its Geothermal Research Program. In conjunction with ongoing and planned investigations by workers at universities and at the China Lake Naval Weapons Center, within whose bounds most of the volcanic field lies, a comprehensive program of geologic, geophysical, geochemical, geodetic, and hydrologic studies was formulated in 1974; several progress reports have been published and are cited below. This report summarizes our present knowledge of the geology of the area, based principally on information gained from field mapping, rock chemistry, K-Ar dating, and study of thin sections; it also synthesizes our understanding of the geothermal system on the basis of all types of data now available and is intended to provide background for the related papers in this issue.Earlier work in the Coso Range included several topical studies. Ross and Yates [1943] and Dupuy [1948] described mercury occurrences that were once worked commercially from altered rocks in a few fumarolic areas. Power [1958, 1959] and geologists of Lucius Pitkin, Inc. [1976], examined the general geology and uranium mineralization in the northwestern part of the range. Chesterman [1956] described late Cenozoic pyroclastic deposits, some of which have been mined for pumice intermittently for many years. Schultz [1937] studied a volcaniclastic-rich s...

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Section: The Geothermal Systemmentioning

confidence: 97%