摘要:

During 1972 and 1973 the U.S. Geological Survey (U.S.G.S.) conducted detailed geological, geophysical, hydrological, and geochemical investigations in Long Valley, California, as part of a new geothermal research program. The goal of these investigations was to understand a typical hot water geothermal system, thus providing a basis for extrapolation to other hot water areas and for regional exploration and assessment of geothermal resources. Although the U.S,G.S. investigations have thoroughly characterized the surface expression and geophysical signatures of the Long Valley geothermal system, our understanding of the geothermal system at depth is incomplete. The available data allow us to make only a crude estimate of 350–700 MW cent, for the electric power generation potential. Refinement of this estimate must await exploration of the area by deep drill hole

ISSN:0148-0227    

DOI:10.1029/JB081i005p00721

年代:1976

数据来源: WILEY

摘要:

Long Valley caldera, a 17‐ by 32‐km elliptical depression on the east front of the Sierra Nevada, formed 0.7 m.y. ago during eruption of the Bishop tuff. Subsequent intracaldera volcanism included eruption of (1) aphyric rhyolite 0.68‐0.64 m.y. ago during resurgent doming of the caldera floor, (2) porphyritic hornblende‐biotite rhyolite from centers peripheral to the resurgent dome at 0.5, 0.3, and 0.1 m.y. ago, and (3) porphyritic hornblende‐biotite rhyodacite from outer ring fractures 0.2 m.y. ago to 50,000 yr ago, a sequence that apparently records progressive crystallization of a subjacent chemically zoned magma chamber. Holocene rhyolitic and phreatic eruptions suggest that residual magma was present in the chamber as recently as 450 yr ago. Intracaldera hydrothermal activity began at least 0.3 m.y. ago and was widespread in the caldera moat; it has since declined due to self‐sealing of near‐surface caldera sediments by zeolitization, argillization, and silicification and has become localized on recently reactivated north‐west‐trending Sierra Nevada frontal faults that tap h

ISSN:0148-0227    

DOI:10.1029/JB081i005p00725

年代:1976

数据来源: WILEY

摘要:

Two seismic refraction profiles crossing the Long Valley caldera in approximately east and north directions indicate that the crystalline basement withPwave velocities of 6.0±0.4 km/s has been downdropped by 2.5–3 km across normal faults along the north and northwest sides of the caldera and 1–2 km along the south and east sides. Basement depths beneath the caldera floor range from between 3 and 4 km in the north and east sections to about 2 km in the central and south sections. Relief on the basement within the caldera suggests that the caldron block was partially disrupted during collapse, although a steplike offset in the eastern part of the basement may be due in part to precollapse displacement along the Hilton Creek fault. The distribution ofPwave velocities in the caldera fill suggests that the Glass Mountain rhyolite and Bishop tuff have velocities of 4.0–4.4 km/s and the postcollapse rhyolite, rhyodacite, and basalt flows have velocities of 2.7–3.4 km/s. Domelike relief on the 4.0‐ to 4.4‐km/s horizon indicates that postcollapse resurgence elevated the west central part of the caldera by about 1 km. Evidence for the roof of the magma chamber is contained in later arrivals tentatively identified as reflections from a low‐velocity horizon at a depth of 7–8 km. Evidence for anomalous scattering or absorption properties associated with the region of shallow hydrothermal alteration and hot spring activity is contained in relative attenuation of high frequencies in a guided wave propagating thr

ISSN:0148-0227    

DOI:10.1029/JB081i005p00745

年代:1976

数据来源: WILEY

摘要:

Gravity studies show that the subsurface part of the Long Valley caldera is a coincident steep‐sided depression filled with porous epiclastic and volcanic materials to a depth of as much as 3 km. The depression contains two major basins, a larger and deeper one making up much of the eastern part and a somewhat smaller, more shallow one to the west; a positive feature underlying the central part of the depression separates the two basin areas. The east side of this feature is linear in plan and coincides with the extension of the Hilton Creek fault which is mapped within and beyond the south edge of the caldera. The indicated relief on the postulated subsurface Hilton Creek fault together with difference in depth of the eastern and western basinal areas indicates that the eastern basin is downdropped in relation to the western one. Gentle gravity gradients outside the caldera but sloping towards it are interpreted as evidence of a low‐density mass located below the caldera fill. We conclude that it is probably related to the magma source. Aeromagnetic data indicate that a northwest‐trending belt of metasedimentary rocks on the south flank of Long Valley may extend into the caldera proper and form much of the bedrock floor of the western part of the caldera. A magnetic low of shallow source in the hot spring region in the southwest is thought to be caused by hydrothermal alteration of the ferrimagnetic minerals in the underlying rocks. A broad positive magnetic anomaly near the center of the caldera may be caused by a thick section of magnetic volcanic flow lying east of the projected Hilton Creek fault and underlying much of the eastern

ISSN:0148-0227    

DOI:10.1029/JB081i005p00754

年代:1976

数据来源: WILEY

摘要:

Temperatures at the 5‐ to 10‐m depth from 29 shallow holes in Long Valley caldera can be contoured systematically; they correlate well with the character of the thermal gradient to 30 m. Where the temperature at a depth of 10 m is less than 11°C (group I), the gradients to 30 m are practically zero; where the 10‐m temperature is between 11°C and 16°C (group II), the gradients are 2000°–400°C/km and uniform, corresponding to conductive heat flows of 4–8 HFU (1 HFU = 1 × 10−6cal/cm2s). Where the 10‐m temperatures exceed 16°C (group III), gradients are larger and irregular, with local heat flows to 50 HFU. Thermal considerations suggest that the first group is characteristic of regions of hydrologic recharge, that the second group is probably characteristic of regions with conductive regimes to substantial depth, and that the third group is characteristic of regions of hydrologic discharge. This interpretation is supported by limited drilling to depths up to 300 m. Regimes in group I occur in the peripheral portion of the caldera, suggesting that this is an area of recharge. The hot springs discharge in a fault zone characterized by near‐surface regimes in groups II and III; chemical evidence indicates that their source reservoir is at about 200°C. Evidently, the springs are fed by local fractures; if the background regime is conductive, their reservoirs are probably less than 1 km deep. Hydrologic and isotopic data indicate that gross circulation in the hydrothermal system is from west to east, suggesting that the hot springs gain their heat in the western caldera. The large estimates of heat being removed from the caldera by flowing water and the geologic inference that hydrothermal activity was more intense in the past support the view that the Long Valley system was resupplied with heat from deep magmatic sources duri

ISSN:0148-0227    

DOI:10.1029/JB081i005p00763

年代:1976

数据来源: WILEY

摘要:

Heat flow and heat production measurements have been made in the vicinity of Long Valley from 0–30 km from the rim of the caldera and up to 30 km on either side of the boundary of the Basin and Range province at the eastern scarp of the Sierra Nevada. The search for a thermal anomaly associated with magma is complicated by the location of the caldera at the boundary between these two provinces with strongly contrasting regional heat flows and by unknown effects of hydrothermal circulation. The data show no conspicuous effect of the province transition, possibly a small local heat flow anomaly near the east rim of the caldera, and a very substantial anomaly near the west rim. Simple heat conduction models suggest that Long Valley caldera is the surface expression of a deep magmatic system; an upper crustal magma chamber could not have sustained molten material throughout the 2‐m.y. eruptive history unless it were resupplied with heat from deep crustal or subcrustal magmatic sources. If the heat were supplied by crustal intrusion of mantle basalt, the crust would thicken rapidly unless magmatic activity were accompanied by accelerated local crustal spreading. To generate a viable silicic magma chamber by sill injection in the upper 5–8 km of crust, minimum intrusion rates of the order of l m per century are probably required. Thermal models for the near‐normal heat flow at the east rim suggest that magma beneath the eastern part of Long Valley caldera might have been exhausted during eruption of the Bishop Tuff 0.7 m.y. ago and that the resurgent dome, which subsequently formed in the west central caldera, marks the location of a residual chamber more circular in plan. High heat flow indicated by the single measurement hear the west rim can be attributed to a simple shallow magma chamber beneath the western caldera or to recent local magmatism along the Sierra frontal fault system. Additional heat flow and hydrologic measurements are necessary for a confident interpretation of the thermal history and the present state of the caldera

ISSN:0148-0227    

DOI:10.1029/JB081i005p00769

年代:1976

数据来源: WILEY

摘要:

The heat discharged by the hot spring system in Long Valley, California, has been estimated from measured spring discharges by using geochemical mixing models. Of the total flow of 1300 1/s from 11 thermal springs in the caldera, approximately 20% or 250 1/s is contributed by the hydrothermal system at depth with temperatures near 210°C. The effects of heat loss by conductive cooling, mixing, and boiling are quantified for the springs in Hot Creek Gorge, which are the major source of hot water discharge in the caldera. The estimated total convective heat discharge is 4.3×107cal/s, which is in agreement with an estimate obtained from the rate of boron discharge from the caldera into Lake Crowley. To supply heat conductively to circulating water of meteoric origin at a rate of 4.3×107cal/s requires a heat flux at depth in excess of 10 μcal/cm

ISSN:0148-0227    

DOI:10.1029/JB081i005p00785

年代:1976

数据来源: WILEY

摘要:

Thermal springs and wells in Long Valley, California, issue sodium bicarbonate‐chloride waters containing 1000–1420 mg/l of dissolved solids. Thermal waters of sodium bicarbonate‐chloride composition are usually associated with hot‐water reservoirs. Chloride concentrations and stable isotope data indicate that the thermal waters have had varied histories. All of the thermal springs issue a mixture of fluid from the thermal reservoir and less saline, cooler water from one or more shallow aquifers. The composition of springs in Hot Creek Gorge may have been further altered by minor subsurface boiling. Thermal springs between Hot Creek and Lake Crowley issue mixtures of fresh and thermal waters which have lost heat by conductive cooling and changed composition by reaction with rock in the shallow aquifer. The silica content of water from Magma Richie 5 and mixing calculations based on the concentrations of silica in thermal waters collected from springs in Hot Creek Gorge and along Little Hot Creek indicate a temperature of at least 200°C in the thermal reservoir. The sodium‐potassium‐calcium geothermometer yields a reservoir temperature estimate near 200°C for most of the thermal springs. If geothermal energy is developed in Long Valley, the high concentrations of arsenic (up to 2.2 mg/l), boron (up to 15 mg/l), and total dissolved solids in the thermal fluids will make it necessary to isolate the effluent of production wells from the fres

ISSN:0148-0227    

DOI:10.1029/JB081i005p00792

年代:1976

数据来源: WILEY

摘要:

An audiomagnetotelluric (AMT) sounding system developed by the U.S. Geological Survey appears to be an effective technique for reconnaissance exploration to detect shallow resistivity anomalies associated with geothermal reservoirs. The equipment operates within the frequency range of 8–18,600 Hz by using nine logarithmically spaced narrow band filters. The technique has been evaluated in Long Valley, California, where the results from dc resistivity and time domain electromagnetic surveys were available for control. The AMT method outlines two linear zones of low resistivity that correlate well with known hot springs in the area. Generally, good agreement was obtained with the results of other electrical method

ISSN:0148-0227    

DOI:10.1029/JB081i005p00801

年代:1976

数据来源: WILEY

摘要:

Direct current resistivity and time domain electromagnetic techniques were used to study the electrical structure of the Long Valley geothermal area, A resistivity map was compiled from 375 total field resistivity measurements. Two significant zones of low resistivity were detected, one near Casa Diablo Hot Springs and one surrounding the Cashbaugh Ranch—Whitmore Hot Springs area. These anomalies and other parts of the caldera were investigated in detail with 49 Schlumberger dc soundings and 13 transient electromagnetic soundings. An extensive conductive zone of 1‐ to 10‐Ωm resistivity was found to be the cause of the total field resistivity lows. Drill hole information indicates that the shallow parts of the conductive zone in the eastern part of the caldera contain water of only 73°C and consist of highly zeolitized tuffs and ashes in the places that were tested. A deeper zone near Whitmore Hot Springs is somewhat more promising in potential for hot water, but owing to the extensive alteration prevalent in the caldera the presence of hot water cannot be definitely assumed. The resistivity results indicate that most of the past hydrothermal activity, and probably most of the present activity, is controlled by fracture systems related to regional Sierran f

ISSN:0148-0227    

DOI:10.1029/JB081i005p00810

年代:1976

数据来源: WILEY