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Chapter 5— Silicic Domes: Heat Flow around Small, Evolved Magma Bodies
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Coso California Geothermal Field

The Coso volcanic field is in eastern California on the western edge of the basin and range province of the western United States. About 35 km3 of volcanic rocks have erupted and overlie principally Mesozoic plutons associated with the Sierra Nevada batholith; The Coso volcanics range in age from ~4 to 0.04 Ma and vary in composition from basalt to rhyolite. Pleistocene rhyolitic domes are the major volcanic features, and these are cut by numerous normal faults that may reflect late Cenozoic extension. Along some of these faults within the dome field, fumaroles and hot springs provide evidence of a high geothermal gradient. Recent geothermal development by the California Energy Company, Inc., resulted in nearly 90 wells. The first production well, drilled to a depth near 2000 m, had a bottomhole temperature of ~340°C and produced dry steam. At this time, with drilling operations still under way, nine power plants have been completed and are online with a net capacity of 230 MWe. The Coso volcanic field exploration is considered one of the best documented investigations of a silicic dome system in the world. We summarize here the work of Bacon and Duffield (1980) at Coso as an example of a geothermal system developed in Mesozoic basement rocks below silicic domes.

Geologic Setting

The Coso range is a horst block immediately east of the Sierra Nevada range in eastern California. It is covered by a veneer of ~400 km2 of mostly lava flows and domes of late Cenozoic age. Early geologic exploration (Ross and Yates, 1943; Dupuy, 1948) identified mercury, which has been mined in fumarolically altered rocks near rhyolite domes; numerous subsequent studies have described pyroclastic and volcaniclastic deposits, the general stratigraphy, potassium-argon ages, and geothermal phenomena. Duffield et al . (1980) documented both the relationship of the volcanic rocks to an underlying granitic basement (Fig. 5.20), which is exposed within the volcanic field and along its margins, and the nature of a late Cenozoic extension, which is marked by north-northeast-trending normal faults that have produced considerable uplift of horst block under the range.

The geothermal system has developed in a Mesozoic basement of dominantly granitic plutons and subordinate mafic plutons and metamorphic rocks associated with the Sierra Nevada composite batholith. Late Tertiary and Quaternary volcanic rocks drape over the basement. Duffield et al . (1980) used potassium-argon and obsidian-rind techniques to determine that ~35 km3 were erupted between 4.0 and 0.04 Ma (Fig. 5.21). These volcanic rocks include 38 separate domes and flows of phenocryst-poor, high-silica rhyolite, most of which are likely younger than 0.3 Ma.

The oldest lavas, alkalic basalt flows, are the most voluminous and widespread of Pliocene volcanic rocks (Fig. 5.22) and were erupted from cinder cones onto a relatively subdued terrain. They occur as notable stepfaulted flows in the eastern portion of the field. These lavas are overlain by Pliocene andesite, dacite, rhyodacite, and rhyolite flows and tuff. Andesite and dacite occur as parts of polygenetic volcanoes in which dacite flows, shallow intrusive masses, and pumice are interlayered with andesite flows and


Fig. 5.19
Basin-filling clastic rocks shed from a growing
silicic dome can provide a permeable formation
for a hydrothermal reservoir—especially when
continued down-warping of the basin allows
it to be filled with impervious sediments such
as lahars. These lahars efficiently hide the
geothermal reservoir, which develops at
some distance from the volcano.

cinders. Rhyodacite is found in widespread pumice fall and lava flows, and Pliocene rhyolite forms a nonwelded pumiceous ash flow that is intercalated with sedimentary volcaniclastic rocks of the Coso Formation.

The youngest volcanic rocks are Pleistocene in age and consist of contemporaneous alkalic basalt and high-silica rhyolite (Bacon et al ., 1980). Most basalt vents are marked by partly eroded cinder cones that fed one or more lava flows on the east, south, and west sides of the rhyolite field. Pleistocene rhyolites compose the 38 steep-sided domes as well as some short, thick flows whose surfaces are notably perlitic and pumiceous. Bacon et al . (1981; 1984) inferred that the rhyolite magma (total extruded volume @ 1.6 km3 ) erupted from a chemically stratified magma chamber, which formed when mantle-derived basalts partially melted crustal rocks. Most domes have extrusive volumes of <0.3 km3 and are located within and/or above tephra rings that were formed by the initial explosive phases of dome eruption. The tephra from these dome eruptions have a total volume of ~0.3 km3 and consist of well-bedded obsidian, pumice, and rhyolite clasts and minor amounts of lithic fragments from basement rocks. Accretionary lapilli and impact sags provide evidence that the tephra are in part hydrovolcanic. Tuff rings average about 600 m in diameter, and rim deposits range from several meters to 30 m thick. Most of the rhyolite field is mantled by tephra similar in character to those in the tuff rings. The first production well completed by California Energy is collared within the tuff ring of dome 53 near the Devil's Kitchen fumarolic area (Fig. 5.23). Intensely fractured Mesozoic basement rock encountered by the well can be attributed to several processes, including hydraulic fracturing associated with hydrovolcanic explosions that occurred during the initial eruptions of this dome, thermal stresses exerted by elevated heat flow, and ongoing tectonic movement. Other geothermal wells are being bored in the area around dome 53.

Using the distribution of silicic vents, the volume of extruded magma, gravity and seismic surveys, and heat flow measurements, Bacon et al . (1980) predicted that a silicic magma body ~5 km in diameter and >1 km thick (a total volume of at least 20 to 30 km3 ) underlies the Coso volcanic field at a depth of at least 8 km. It is possible that the silicic magma body may still be partially molten, if one applies the reasoning that the most recent basaltic eruption occurred as late as a few thousand years ago and that such extrusions are evidence that heat was supplied to the magma body from an underlying mafic reservoir. Bacon (1982) indicated that the ages of extrusive rocks (Friedman, 1976), plotted in Fig. 5.21 with respect to cumulative volumes, show a trend that suggests these eruptions will continue in the future.

Figure 5.24 illustrates three sets of faults in the Coso Range (Duffield and Bacon, 1979; Roquemore, 1980) that indicate principally late Cenozoic crustal extension; outward dips of the Coso Formation demonstrate considerable uplift of the range during Pliocene time. A west-northwest-trending set of apparently vertical faults are well developed in the southern and western parts of the range. This fault set is an expression of


Fig. 5.20
Geologic map of the Coso volcanic field of California, showing distribution of major rock units and
faults. Abbreviated locations: CP = Coso Peak, LCF = Lower Cactus Flat, UCF = Upper Cactus Flat,
SP = Silver Peak, CHS = Coso Hot Springs, SM = Sugarloaf Mountain, VB = Volcano Butte,
VP = Volcano Peak, AL = Airport Lake, and LL = Little Lake.
(Adapted from Duffield et al ., 1980.)


Fig. 5.21
Rock volumes and compositions vs radiometric age for the Coso volcanic field. Volumes were
calculated by using the geological map area and cross-sectional exposures and allowing for
material removed by erosion. Volumes of pyroclastic rocks were converted to dense rock
equivalence by multiplying by 0.5. Exponential thickness decreases with distance were
applied to pyroclastic deposits. In the last 4 Ma, ~35 km3  of lava has been erupted, of
which 31 km3  erupted before 2.5 Ma ago.
(Adapted from Duffield et al ., 1980.)


Fig. 5.22
Distribution of eruptive vents in the Coso volcanic field of California. The locations of vents are
shown by letters designating the composition of materials erupted: B = basalt, A = andesite,
D = dacite, and Rd = rhyodacite. Asterisks denote the location of
Late Pliocene and early Pleistocene vents.
(Adapted from Duffield et al ., 1980.)

regional structure and, although these faults do not offset Pleistocene rhyolite, some of the silicic domes are aligned along their strike. The basin and range morphology of the Coso range is developed along north-to northeast-trending normal faults. These faults cut the horst onto which the rhyolites have been erupted and form en echelon sets that are consistent with north-northwest right lateral shear. In Fig. 5.25, information from Bacon et al . (1980) shows that the Quaternary maximum horizontal compression follows this northeast trend. This interpretation arose from consideration of the distribution of domes and application of the stress analysis suggested by Nakamura (1977). Arcuate faults present in the northern and northeastern parts of the field are approximately


Fig. 5.23
Generalized geological map of the principal geothermal area in the Coso geothermal area. Coso Hot
Springs emanate along major graben-bounding faults, whereas geothermal drilling has focused on
the region around Sugarloaf Mountain rhyolite dome and the Devil's Kitchen fumarolic area. Hulen
(personal communication) reports that drilling in regions around these
vents has encountered intensely fractured basement rock.
(Adapted from Bacon et al ., 1980.)

concentric around the geographic center of the field and dip inward toward this center. This set of faults was originally interpreted as part of a caldera structure (Austin et al ., 1971; Koenig et al ., 1972), and more recently, Roquemore (1980) has attributed their origin to strike-slip movement; however, Duffield (personal communication, 1990) is not convinced by either of these interpretations. The step-faulted terrane in the eastern part of the volcanic field near Airport Lake (see Fig. 5.23) is attributed to downwarping and down-faulting in response to late Cenozoic crustal extension that caused an effective decoupling of that terrane from a block-faulted terrane to the west and south. Figure 5.26 depicts the step-faulted terrane that forms a graben structure and its relationship to the horst on its west side onto which the rhyolite domes have been extruded. An ongoing study by California Energy indicates that new interpretations of structural relationships will be required to fully understand Coso's geothermal system.



Most present-day surface thermal activity is concentrated within and immediately east of the Pleistocene rhyolite field, apparently along an east-northeast-trending zone between Sugarloaf Mountain and Coso Hot Springs—a zone mapped as a fault by Hulen (1978). Coso Hot Springs consists of fumaroles and intermittently active, acid-sulfate springs and mud lakes that emanate from a north-northeast-trending fault along the east side of the main horst block. Surface flow is related to local precipitation, but water samples from a 125-m-deep well are alkaline and chloride rich (~3,000 ppm of chloride); the bottomhole temperature was 142°C (Austin and Pringle, 1970). South of this area are laminated siliceous sinter and travertine exposures that are evidence of older, widespread thermal springs. Fumaroles of Devil's Kitchen, occurring in the tuff ring of dome 53, are noted for their present-day deposition of sulfates, sulfur, and cinnabar. Although these surface expressions are not chloriderich and are typical of a high-level, vapordominated system, the chloride-rich waters from wells in this immediate vicinity indicate that at depth there is a hot-water-dominated hydrothermal reservoir (White et al ., 1971).

Fig. 5.24
Detailed structural map of the Coso Range and adjacent area, showing distribution of faults.
Location abbreviations are those used in Fig. 5.20; shaded patterns denote rhyolite domes.
(Adapted from Roquemore, 1980.)


The hydrothermal system at Coso is apparently controlled by fractures in the Mesozoic granitic and older metamorphic basement rocks. Water samples from two wells of this system were analyzed by Fournier et al . (1980), and their chemistry is summarized in Table 5.4. Although the chemical analyses show variability that can be attributed to evaporative concentration, water/rock reactions at different temperatures, and different sample preservation and laboratory procedures, the samples exhibit essentially the same chloride content, and water of relatively uniform composition is found throughout the permeable rock underlying the Coso area sampled. The chloride content also indicates a hot-water-dominated rather than a vapor-dominated system.

Geophysical Character

Numerous geophysical techniques have been applied to the geothermal exploration of the Coso area: heat flow measurements, microseismicity and teleseismicity, gravity, magnetics, and electrical resistivity. Taken together, these methods have provided mutually supportive data that promote the development of a subsurface model to locate and define the nature of the magma

Fig. 5.25
Idealized axis of maximum horizontal tectonic compression of the Coso area is determined from
(a) the orientation of normal faults, location of rhyolite domes, and sense of strike-slip displacement
(inset) in zones of seismic epicenters (Walter and Weaver, 1980). This relationship is compared to
(b) the idealized horizontal cross section of a large volcano (Nakamura, 1977), which shows
the effect of a differential horizontal stress on dike propagation (curves) from a region of magma
storage and ascent (shaded) underlying the Sugarloaf Mountain area. Dikes fed the outlying domes
around the Sugarloaf Mountain area (shown by heavier lines).
(Adapted from Bacon et al ., 1980.)


Fig. 5.26
Generalized block diagram illustrating the step-faulted terrane (dark lines and wedges) and block
faulting in the Coso Basin near Airport Lake (see Fig. 5.20). Pleistocene rhyolite domes have
erupted on the horst; the step faulting has developed in response to Cenozoic crustal extension
(arrows). The cenozoic basement is shown by light stipple and Pliocene volcanic rocks are dark
stippled. Coso Hot Springs emanate along the northeastern extension of the western graben-
bounding fault, and geothermal drilling is mostly on the crystalline horst
(Adapted from Duffield et al ., 1980.)

heat source as well as determine the structural and lithological control of heat flow.

Combs (1980) reported the results of thermal gradients measured in 25 shallow-depth and 1 intermediate-depth boreholes in addition to thermal conductivity measurements on 312 core and cutting samples from the igneous and metamorphic basement rocks. Figure 5.27 shows equilibrated thermal gradients for shallow boreholes where temperatures ranged from 25.3 to 906°C/km. The high gradients are a product of thermal convection by hot water, and the low gradi-


Table 5.4. Composition of Geothermal Waters from Coso, Californiaa


Coso Geothermal
Exploration Hole 1

Coso Well 1

Sample Temperature (°C)






Constituent (mg/kg)

















































































Temperatures (°C)






a From Fournier et al . (1980). Coso Geothermal Exploration Hole 1 was drilled ~3.2 km west of Coso Hot Springs and 1.9 km north of Devil's Kitchen to a depth of 1477 m in granitic and metamorphic rocks (Goranson and Schroeder, 1978); it was sampled at a depth of 835 m. Coso Well 1 was drilled at Coso Hot Springs to a depth of 114.3 m in altered alluvium and granitic rock (Austin and Pringle, 1970); it was sampled at the surface.

b TDS = total dissolved solids.

ents are caused by conduction of heat away from dikes that fed domes and lava flows. Figure 5.28 shows isotherms at 5- and 10-m depths. These data correspond to terrain-corrected heat flows that range from 1.6 to 23 heat-flow units (HFU; 1 HFU = 41.84 mW/m2 ). Background measurements for the region are between 1.6 and 2.4 HFU. Heat-flow contours enclose the area being developed for geothermal energy near dome 53 and Devil's Kitchen (Fig. 5.29). Heat flows near 4 HFU divide the convective regimes of high heat flow from conductive regimes; the 3- and 5-HFU contours in Fig. 5.29 generally parallel regional structure, which suggests that convective heat flow is controlled by the circulation of hot water along fault and fracture systems in the rhyolite dome fields.

As is shown by Fig. 5.30, microearthquakes are common in the Coso Range (Walter and Weaver, 1980); a magnitude 1.0 or greater earthquake occurs almost every day in the region. Zones of seismicity strike radially outward from the rhyolite field, and earthquake swarms show a general northwest trend across the field. Fault-plane solutions show a regional north-south compression. Earthquake depth varies little across the field: most quakes are around 5 to 6 km deep. This trend suggests that the brittle-to-ductile transition does not rise under the field as would be the case if near-liquidus temperatures occurred at a shallow level. However, Young and Ward (1980) did find a shallow zone of high teleseismic P-wave attenuation within the upper 5 km in a region under Coso Hot Springs, Devil's


Kitchen, and Sugarloaf Mountain—probably corresponding to vapor and liquid in near-surface lithologies. Furthermore, Reasenberg et al . (1980) found significant teleseismic-wave delays that were likely caused by a low-velocity body of partially molten rock concentrated from 8 to 17.5 km directly below the region of highest heat flow.

Although gravity data from Plouff and Isherwood (1980) reveal the regional tectonic patterns, they do not predict a mass deficit for a magma reservoir underlying the Coso Range. A magnetic-low area that corresponds to the area of high heat flow near Coso Hot Springs can be explained by a poorly magnetized silicic pluton that crops out inthe area; much of its magnetite may have been destroyed by hydrothermal fluids. Jackson and O'Donnell (1980) reported telluric current and 7.5-Hz audio-magnetotelluric data that reveal major resistivity lows associated with conductive basinfill materials (such as those underlying the region directly east of Coso Hot Springs) and a secondary low trough that extends across the geothermal area (Fig. 5.31).

Volcanological Interpretations

Although none of the data sets is conclusive in itself, taken together, the geological, hydrogeochemical, and geophysical evidence pinpoints the area now being developed as a geothermal resource. However, most of the geophysical conclusions—and

Fig. 5.27
Idealized equilibrium thermal gradient profiles for 24 shallow boreholes at the Coso geothermal area.
Profile numbers refer to boreholes shown in Fig. 5.28. In general, boreholes with high thermal gradients
are located in the immediate vicinity of the Devil's Kitchen and
Sugarloaf Mountain thermal manifestations.
(Adapted from Combs, 1980.)


some of the geochemical ones—are based upon geological field observations. As Duffield et al . (1980) showed in a schematic east-west cross section of the Coso Range (Fig. 5.32), the concentration of young, rhyolitic volcanism on a horst block, the prevalence of phreatic and phreatomagmatic tephra in tuff rings below the silicic domes, and through-going regional faults all point to a region that should be characterized by high heat flow, convective water circulation, and fractured rock. Such is the case for the Coso geothermal field.

previous sub-section
Chapter 5— Silicic Domes: Heat Flow around Small, Evolved Magma Bodies
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