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

Abstract

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...

Quaternary alkalinity fluctuations recorded in rock varnish microlaminations on western U.S.A. volcanics

“…This is related to either the biotic or abiotic mechanism of manganese enhancement; but what factor(s) might govern the fluctuations in alkalinity levels recorded in Coso varnishes? For any Duffield et al (1980) and Duffield and Bacon (1981). When no standard error is listed, all the profiles had the same number of peaks for a given volcanic, excluding profiles that were discontinuous or lacked laminations due to local influences discussed in the text.…”

“…Approximate age is also given, based on the assumption of a constant rate of varnish deposition. The K-Ar dates from Duffield et al (1980) provide the endpoint for the bottom of the varnish; the surface (present) provides the other end point.…”

“…The Coso volcanic field in eastern California (Duffield et al, 1980;Duffield and Bacon, 1981) was selected as the major region to determine whether changes in Mn:Fe ratios over depth yield a reproducible signal. Abundant K-Ar dated lava flows with stable constructional surfaces and excellent varnish development made the Coso region ideal.…”

Paleomagnetic Analyses of Short Normal Polarity Magnetic Anomalies in the Matuyama Chron

“…It has also been observed from basalt flows (Mankinen et al, 1978;Mankinen and Grommé, 1982;Chauvin et al, 1990). K/Ar radiometric analyses (Mankinen et al, 1978;Duffield et al, 1980;Chauvin et al, 1990) give an age of about 1.12 m.y. for this subchron.…”

Characterization of EGS Fracture Network Lifecycles

“…WNW-trending and minor NE-trending faults, which are probably dextral and sinistral strike-slip faults respectively [Duffield et al, 1980]. These faults may currently be inactive as they are not observed to offset Quaternary sediments.…”