MULTIDISCIPLINARY CHARACTERIZATION OF THE COSO VOLCANIC AND GEOTHERMAL FIELDS FORTY YEARS AGO

Coso was one of the first areas extensively studied under the USGS Geothermal Research Program. The multidisciplinary assault began in the mid 1970s by researchers from USGS, Naval Air Weapons Station, universities, and the private sector, in part supported by DOE. Initial papers appeared in the June 1975 Geology. Results of much of the work were published as a 1981 geologic map and in the May 1980 Journal of Geophysical Research that included papers on volcanism, geochronology, structure, tectonics, stress field, rhyolite vent distribution, thermal waters, heat flow, seismicity, seismic attenuation, seismic tomography, and aeromagnetic, gravity, and electrical surveys. Geologic mapping and K‑Ar dating showed that volcanic rocks were erupted in two periods: (1) 4.0–2.5 Ma, ~35 km³ of basalt, rhyodacite, dacite, andesite, and rhyolite, in decreasing volumetric order, and (2) ~4 km³ of basalt and rhyolite ≤1.1 Ma. The young rhyolites sit atop a horst in Mesozoic granitic rocks bounded by ~N–S normal faults. The region is transected by WNW dextral strike-slip faults, consistent with gravity and aeromagnetic data. Maximum horizontal compressive stress is ~N–S, as reflected in structure and coeval vent alignments; Quaternary vents of multiple ages tend to coincide with major WNW structures. Heat flow implied a ratio of intruded to erupted silicic magma ≤1.1 Ma as much as 100:1. Waters from two drill holes indicated presence of a hot-water geothermal system in the east-central part of the rhyolite field, consistent with low apparent resistivity. Earthquake focal mechanisms were consistent with the geologically defined stress field. Focal depths shallowed to ≤7 km in the geothermal area and center of the rhyolite field where teleseismic P‑delays indicated relatively low velocities in a cylindrical volume that extended NNE and SSW with increasing depth. Spinoffs from Duffield’s and my own work included rhyolite geochemistry and petrology, the age of the Coso Formation, origin of reversely graded bedding in pumice, deformation of sediment by invasive lava, and characteristics of rhyolites in the adjacent Sierra Nevada. Erupted volume versus time suggested that Pleistocene bimodal volcanism was time-predictable, and mafic enclaves in rhyolite were shown to be basalt contaminated with crystals from an intrusive complex.