Dixie Valley is an active Basin and Range graben in west-central Nevada. It is roughly 120 km long and 19 km wide, and is bounded to the west by the Stillwater Range and to the east by the Clan Alpine Mountains. The valley supports a broad geothermal area and maintains a heat flow which is high in relation to the surrounding Great Basin (Blackwell, 1992). The adjacent ranges, which have been uplifted along range-bounding normal faults, reveal a complicated history including:
* Triassic and Jurassic marine sedimentation
*Jurassic mafic igneous intrusions
*Jurassic metamorphism and thrust faulting
*Cenozoic volcanism and sedimentation
*Cenozoic normal faulting.
At the north end of the valley, fumaroles and hot springs occur along the Stillwater range front fault, or Dixie Valley fault. The commercial development of a geothermal field began in 1979 with exploratory drilling, and today the field supports one 60Mw plant which became operational in 1989. .
The reservoir rocks of the down-dropped Dixie Valley block, which are buried beneath 1800 meters of Tertiary and Quaternary basin fill deposits, reflect the complex pre-Tertiary geology exposed in the Stillwater and Clan Alpine ranges. At least two and possibly as many as four separate tectonic events have affected the pre-Tertiary rocks. These events spanned the range from brittle to ductile deformation, and they involved sedimentary, metamorphic, and intrusive and extrusive igneous rocks. .
The geologic development of the geothermal area began in Middle Triassic time with the deposition of shelf carbonates of the Star Peak Group (Silberling and Roberts, 1962) and basinal strata of the northwestern Nevada marine province (Speed, 1978). The shelf strata were deposited upon rocks of the both the Golconda allochthon, which was emplaced during the Early Triassic Sonoma orogeny, and silicic volcanic rocks of the Koipato Group. Jurassic deposits include quartz arenites of the Boyer Ranch Formation, and a major gabbroic intrusive complex, the Humboldt gabbroic complex, which reportedly intruded sediments of the Boyer Ranch Formation. During Jurassic contractional deformation, the basinal deposits were thrust to the northeast over shelf strata along the Fencemaker thrust, a major structure in western Nevada which subtends the Fencemaker allochthon (Speed, Elison, and Heck, 1988). Basinal pelites and shelf carbonates adjacent to the Fencemaker thrust are locally imbricated and metamorphosed and define a high-strain zone of greenschist-grade marble tectonite, phyllite, slate, and mica schist. The Fencemaker thrust may have been active at a depth of between nine and twelve kilometers (Speed, 1978). .
Quartz arenite, dolomitic conglomerate, and sandy limestone of the Jurassic Boyer Ranch Formation (Speed, 1969) overlie the Fencemaker allochthon along the low angle Boyer fault. .
The structurally highest unit includes gabbro, diorite, granodiorite, monzogranite, basaltic to intermediate dike swarms, and associated volcanic rocks of the Jurassic Humboldt igneous complex (Dilek and Moores, 1995). The complex locally intrudes the Boyer Ranch Formation and structurally overlies pelitic rocks of the Fencemaker allochthon. All the rocks within the complex have been metamorphosed to greenschist facies, and altered by subsequent hydrothermal systems, notably by scapolitic alteration of plagioclase (Speed, 1963). .
Miocene basaltic to andesitic dikes dated between 14.9 and 21.1 Ma (K/Ar, Dilek, 1991) intrude the tectonostratigraphy at all levels. However, the largest and most abundant dikes intrude the Triassic phyllite. The dikes typically trend N-NE and dip steeply to the west. .
During the early to mid Tertiary, around 550 m of andesitic to basaltic lava flows and rhyolitic tuffs accumulated depositionally upon the units described above (Speed, 1976). These volcanic units cap the Stillwater and Clan Alpine ranges and similar thicknesses of volcanic rocks underlie the Tertiary and Quaternary basin fill of Dixie Valley. Some of the tuffs have been identified and correlated with regional volcanic centers, but others remain to be investigated (Dave John, personal communication). Paleomagnetic studies of these rocks south and west of the field area in White Rock Canyon and Kitten Springs Canyon, respectively, have shown that between 10 and 30 degrees of counterclockwise vertical axis rotation has occurred since their deposition, with the majority of rotation during the Oligocene to Miocene epochs (Hudson and Geissman, 1991). .
Dixie Valley formed over the last 15 m.y., since the early Miocene (Okaya and Thompson, 1985). Hastings (1979) suggests, however, that upper Miocene basalt (8 +/- 4 ma) was deposited on a surface of low relief and thus predates the extensional faulting that caused the formation of Dixie Valley. .
Dixie Valley is characterized by active extensional faulting along its margins. The valley occupies a part of the Central Nevada Seismic Belt, a generally north-northeast trending zone of late Quaternary faulting and historical seismicity that has been the locus of several moderate to large magnitude earthquakes during the past 100 years. These earthquakes include the 1954 Dixie Valley and 1915 Pleasant Valley earthquakes (Savage and others, 1995). The geothermal field is located between the endpoints of the 1954 and 1915 surface ruptures (Slemmons, 1954) along a segment of the Dixie Valley fault which has not ruptured in historic time. This segment, known as the Stillwater seismic gap (Wallace and Whitney, 1984), may be kinematically influenced by several regional structural features. First of all, it falls within a major tilt domain boundary or transverse zone between the east-tilted Tobin Range to the north and the west-tilted Stillwater Range (Slemmons, 1967). Secondly, it coexists with a possible pre-Cenozoic fault-barring structure within the Sou Hills to the north (Fonseca, 1988). Third, the gap occurs just north of a large bend in the trace of the Dixie Valley fault (Caskey 1996) and in an area where total fault displacement begins to decrease (Fonseca, 1988).
