ISSN:0148-0227    

DOI:10.1029/JB085iB05p02379

年代:1980

数据来源: WILEY

摘要:

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 km3of late Cenozoic volcanic rocks that were erupted during two periods, as defined by K‐Ar dating: (1) 4.0–2.5 m.y., ∼31 km3of 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‐km 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

ISSN:0148-0227    

DOI:10.1029/JB085iB05p02381

年代:1980

数据来源: WILEY

摘要:

Wellhead and downhole water samples were collected and analyzed from a 114.3‐m well at Coso Hot Springs (Coso No. 1) and a 1477‐m well (CGEH No. 1) 3.2 km to the west. The same chloride concentration is present in hot waters entering both wells (about 2350 mg/kg), indicating that a hot‐water‐dominated geothermal system is present. The maximum measured temperatures are 142°C in the Coso No. 1 well and 195°C in the CGEH No. 1 well. Cation and sulfate isotope geothermometers indicate that the reservoir feeding water to the Coso Hot Spring well has a temperature of about 240°–250°C, and the reservoir feeding the CGEH well has a temperature of about 205°C. The variation in the chemical composition of water from the two wells suggests a model in which water‐rock chemical equilibrium is maintained as a convecting solution cools from about 245°C to 205°C by co

ISSN:0148-0227    

DOI:10.1029/JB085iB05p02405

年代:1980

数据来源: WILEY

摘要:

Obvious surface manifestations of an anomalous concentration of geothermal resources at the Coso geothermal area, Inyo County, California, include fumarolic activity and associated hydrothermally altered rocks. Pleistocene volcanic rocks associated with the geothermal activity include 38 rhyolite domes occupying a north trending structural and topographic ridge and numerous basaltic cinder cones and lava flows partly surrounding its southern half. In an investigation of the thermal regime of this Basin and Range geothermal area, temperature measurements were made in 25 shallow and 1 intermediate depth borehole. Geothermal gradients ranged from 25.3°C/km to 906.°C/km. The high gradients arise from convecting hot water and former convective transport of heat by dikes that fed the domes and flows. Thermal conductivity measurements were made on 312 samples from cores and drill cuttings. The resultant terrain‐corrected heat flow values range from 1.6 to 23. HFU. The actual process by which heat is transferred is rather complex; however, the heat flow determinations can be divided into two groups. The first group, less than 4.0 HFU, are indicative of regions with primarily conductive regimes, although deep‐seated mass transfer is implied. The second group, greater than 4.0 HFU, are characteristic of regions with considerable convective heat transfer in the shallow subsurface. The general shapes of the 3‐and 5‐HFU contours in the upper 35 m of the subsurface are essentially aligned with the major regional fault zones, suggesting that the high heat flows are the result of convective heat transfer caused by circulation of hot groundwater in these vertically permeable subsurface channels. The high heat flow values (>10 HFU) are essentially restricted to the rhyolite dome field and the associated surface thermal manifestations. Heat transferred by convection of water would be rapidly exhausted if it were not intermittently supplied with heat energy from depth; therefore the heat flow data substantiate the concept that the surface hydrothermal activity and associated volcanic rocks are products of a long‐lived magmatic system that has periodically erupted lava during the past 0.3–1.0 m.y. This magmatic system is the crustal heat source for the present‐day Coso geothermal system (1 HFU

ISSN:0148-0227    

DOI:10.1029/JB085iB05p02411

年代:1980

数据来源: WILEY

摘要:

Thirty‐eight separate domes and flows of phenocryst‐poor, high‐silica rhyolite of similar major element chemical composition were erupted over the past 1 m.y. from vents arranged in a crudelyS‐shaped array atop a granitic horst in the Coso Range, California. Most of the extrusions are probably less than about 0.3 m.y. old. The area is one of Quaternary basaltic volcanism and crustal extension. The central part of the rhyolite field is characterized by high heat flow, low apparent resistivity, and substantial fumarolic activity indicative of an active geothermal system. The immediate source of heat for the surficial geothermal phenomena is probably a silicic magma reservoir that may still contain molten or partially molten material at a depth of at least 8 km beneath the central part of the rhyolite field. Outlying rhyolite extrusions probably reflect the presence of feeder dikes emanating from the reservoir beneath the central region. Azimuths of dikes appear to be parallel to the regional tectonic axis of maximum horizontal compression, analogous to some dike‐fed flank eruptions on basaltic shields and andesitic stratovolcanoes. The areal extent of a magma reservoir and the present total heat content of the silicic magma system at Coso may be less than was previously estimated. However, the area is still considered to be one of significant geothermal

ISSN:0148-0227    

DOI:10.1029/JB085iB05p02425

年代:1980

数据来源: WILEY

摘要:

The tectonics of the Coso Range has been described as having arcuate and ring faults both suggesting the presence of a circumscribed subsidence bowl or calderalike feature. New information suggests the Coso Range is situated in an area of transition between the stress of the right slip San Andreas fault‐plate interaction and the extensional tectonics of the Basin and Range. Arcuate faults in the Coso Range are interpreted to have been produced by the regional stress field rather than to have been of volcanogenic origin. Focal mechanisms of small‐magnitude earthquakes support the stress directions indicated by local fault patterns. Fumeroles in the area are primarily associated with oblique slip faults rather than with arcuate or ring faults. The geothermal reservior is therefore much different from that of a caldera or subsidence bowl, and the overall geothermal potential is probably less than earlier estima

ISSN:0148-0227    

DOI:10.1029/JB085iB05p02434

年代:1980

数据来源: WILEY

摘要:

A 16‐station seismographic network, approximately 40 km north‐south by 30 km east‐west, was installed in the Coso Range, California, in September 1975 as part of a geological and geophysical assessment of the geothermal resource potential of the range. During the first 2 years of network operations, 4216 local earthquakes (0.5 ≤m≤ 3.9) defined zones of seismicity that strike radially outward from a Pleistocene rhyolite field located near the center of the Coso Range. Most earthquakes were located in zones showing a general northwest trend across the range. Six earthquake swarms occurred within the area that includes the rhyolite field. Fault plane solutions show regional north‐south compression: earthquakes located in northwest striking zones generally had right lateral strike slip focal mechanisms, those in northeast striking zones left lateral strike slip focal mechanisms, and those in north‐south striking zones both normal and strike slip focal mechanisms. Earthquake depths showed little variation across the Coso Range; the depth distribution is similar to that of several carefully studied segments of the central San Andreas fault. Thebvalue calculated for the entire range is 0.99±0.08. The rhyolite field has a significantly higherbvalue of 1.26±0.16; if only the shallow events (depth<5 km) are used in the calculation, thebvalue for this area becomes even higher, 1.34±0.24. The higherbvalues were interpreted as reflecting the existence of short average fault lengths (<5 km) within the rhyolite field. The seismic data and other data suggest that the fault system lying between the rhyolite field and the adjacent Coso Basin is an important tectonic boundary. Present information is insufficient to determine the geothermal production capability of this fault system, but it does suggest that the system is a good target for fur

ISSN:0148-0227    

DOI:10.1029/JB085iB05p02441

年代:1980

数据来源: WILEY

摘要:

Observations of teleseismicPwaves above geothermal systems exhibit travel time delays and anomalously high seismic attenuation, which is extremely useful in estimating the thermal regime and the potential of the system. A regional telemetered network of sixteen stations was operated by the U.S. Geological Survey in the Coso Hot Springs Known Geothermal Resources Area (KGRA) for such studies from September 1975 to October 1976. Subsequently, they deployed a portable Centipede array of 26 three‐component stations near the center of the anomaly. The seismograms of 44 events recorded by the telemetered array and nine events by the Centipede array were analyzed using the reduced spectral ratio technique to determine the differential attenuation factor δt* for the events recorded with the highest signal‐to‐noise ratio. The δt* variation observed across the Coso Hot Springs KGRA were small (<0.2 s). A three‐dimensional generalized linear inversion of the δt* observations was performed using a three‐layer model. A shallow zone of high attenuation exists within the upper 5 km in a region bounded by Coso Hot Springs, Devils Kitchen, and Sugarloaf Mountain probably corresponding to a shallow vapor liquid mixture or ‘lossy’ near surface lithology. No zones of significantly high attenuation occur between 5‐ and 12‐ km depth. Between the depth of 12–20 km a thick zone of high attenuation (Q<50) exists, offset toward the east fr

ISSN:0148-0227    

DOI:10.1029/JB085iB05p02459

年代:1980

数据来源: WILEY

摘要:

TeleseismicPwave arrivals were recorded by a dense array of seismograph stations located in the Coso geothermal area, California. The resulting pattern of relative residuals reveals an area showing approximately 0.2‐s excess travel time that migrates with changing source azimuth, suggesting that the area is the ‘delay shadow’ produced by a deep, low‐velocity body. Inversion of the relative residual data for three‐dimensional velocity structure determines the lateral variations in velocity to a depth of 22.5 km beneath the array. An intense low‐velocity body, which coincides with the surface expressions of late Pleistocene rhyolitic volcanism, high heat flow, and hydrothermal activity, is resolved between 5‐ and 20‐ km depth. It has maximum velocity contrast of over 8% between 10 and 17.5 km. The shallowest part of this body is centered below the region of highest heat flow; at depth it is elongate in approximately the N‐S direction. The hypothesis that this low‐velocity body is caused by the presence of partial melt in the middle crust is consistent with the local seismic, geologi

ISSN:0148-0227    

DOI:10.1029/JB085iB05p02471

年代:1980

数据来源: WILEY

摘要:

Observations of the dc magnetic field resulting from a temporary single‐pole operation of the Pacific Northwest‐Southwest DC Power Intertie indicate an unexpectedly large concentration of current on the eastern Sierran Front and the possibility of a secondary concentration of current associated with the Coso Range. High electrical conductivity in central and eastern Oregon, in the Basin and Range province, and on the eastern Sierran Front would account for these observati

ISSN:0148-0227    

DOI:10.1029/JB085iB05p02484

年代:1980

数据来源: WILEY