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During 2010, the INL Seismic Monitoring Program evaluated 11,606 earthquakes from around the world, the western United States, and local region of the eastern Snake River Plain (ESRP). INL located 2,085 earthquakes and man-made blasts within the local region outside and within a 161-km (or 100-mile) radius of INL. Of these events, 53 were small-to-moderate size earthquakes ranging in magnitude from 3.0 to 4.8. 672 earthquakes occurred within the 161-km radius of INL and the majority of these earthquakes were located in active regions of the Basin and Range Province that surrounds the ESRP. There were 10 microearthquakes within the boundary of the ESRP, all of magnitude less than or equal to 2.0. Five of those were located within and near the ESRP at Craters of the Moon National Monument (COM) at mid- and lower-crust depths and are interpreted to be related to fluid movement. Since 1972, INL has recorded 48 small-magnitude, microearthquakes (M = 2.2) within the ESRP (not including COM events) and 22 deep microearthquakes (M = 2.3) in the vicinity of Craters of the Moon National Monument.

Medicine Lake Volcano is a Quaternary shield volcano located in a tectonically complex and active zone at the transition between the Basin and Range Province and the Cascade Range of the Pacific Province. The volcano is topped by a 7x12 km elliptical depression surrounded by a discontinuous constructional ring of basaltic to rhyolitic lava flows. This thesis explores the possibility that the depression may have formed due to regional extension (rift basin) or dextral shear (pull-apart basin) rather than through caldera collapse and examines the relationship between regional tectonics and localized volcanism. Existing data consisting of temperature and magnetotelluric surveys, alteration mineral studies, and core logging were compiled and supplemented with additional core logging, field observations, and fault striae studies in paleomagnetically oriented core samples. These results were then synthesized with regional fault data from existing maps and databases. Faulting patterns near the caldera, extension directions derived from fault striae P and T axes, and three-dimensional temperature and alteration mineral models are consistent with slip across arcuate ring faults related to magma chamber deflation during flank eruptions and/or a pyroclastic eruption at about 180 ka. These results are not consistent with a rift or pull-apart basin. Limited subsidence can be attributed to the relatively small volume of ash-flow tuff released by the only known major pyroclastic eruption and is inconsistent with the observed topographic relief. The additional relief can be explained by constructional volcanism. Striae from unoriented and oriented core, augmented by striae measurements in outcrop suggest that Walker Lane dextral shear, which can be reasonably projected from the southeast, has probably propagated into the Medicine Lake area. Most volcanic vents across Medicine Lake Volcano strike north-south, suggesting they are controlled by crustal weakness related to Basin and Range extension. Interaction of dextral shear, Basin and Range extension, and the zone of crustal weakness expressed as the Mount Shasta-Medicine Lake volcanic highland controlled the location and initiation of Medicine Lake Volcano at about 500 ka.

During 2008, the INL Seismic Monitoring Program evaluated 7,284 earthquakes from around the world, the western United States, and local region of the eastern Snake River Plain. 2,396 earthquakes and man-made blasts were evaluated within the local region outside and within a 161-km (or 100-mile) radius of INL. Of these events, 25 were small to moderate size earthquakes ranging in magnitude from 3.0 to 3.9. 823 earthquakes occurred within the 161-km radius of INL and over 300 events were associated with eight different earthquake swarms which were located in active regions of the Basin and Range Province that surrounds the eastern Snake River Plain. Eight microearthquakes in 2008 of magnitude (M) 2.0 and less were located within the eastern Snake River Plain, seven at or near the Craters of the Moon National Monument and one within the INL boundary. Further analyses of the anomalously deep focal depths (15 to 42 km) and different waveform characteristics of all Craters of the Moon National Monument events (1999-2008) suggest association with magmatic processes. From 1972 to 2008, INL located 36 other small-magnitude microearthquakes (M < 2.0) at depths (< 11 km) within the eastern Snake River Plain and attributes these events to regionalmore » tectonic tensional stresses.« less

During 2007, the INL Seismic Monitoring Program evaluated 2,515 earthquakes from around the world, the western United States, and local region of the eastern Snake River Plain. 671 earthquakes and man-made blasts occurred within the local region outside and within a 161-km (or 100-mile) radius of INL. Of these events, eleven were small to moderate size earthquakes ranging in magnitude from 3.0 to 4.8. 341 earthquakes occurred within the 161-km radius of INL and the majority of these earthquakes were located in active regions of the Basin and Range Province that surrounds the ESRP. Three earthquakes were located within the ESRP at Craters of the Moon National Monument. The earthquakes were of Mc 0.9, 1.4, and 1.8. Since 1972, INL has recorded 36 small-magnitude microearthquakes (M < 2.0) within the ESRP.

During 2006, the Idaho National Laboratory (INL) recorded 1998 independent triggers from earthquakes both within the region and from around the world. Fifteen small to moderate size earthquakes ranging in magnitude from 3.0 to 4.5 occurred within and outside the 161-km (100-mile) radius of INL. There were 357 earthquakes with magnitudes up to 4.5 that occurred within the 161-km radius of the INL. The majority of earthquakes occurred in the Basin and Range Province surrounding the eastern Snake River Plain (ESRP). The largest of these earthquakes had a body-wave magnitude (mb) 4.5 and occurred on February 5, 2006. It was located northeast of Spencer, Idaho near the east-west trending Centennial fault along the Idaho-Montana border. The earthquake did not trigger SMAs located within INL buildings. Three earthquakes occurred within the ESRP, two of which occurred within the INL boundaries. One earthquake of coda magnitude (Mc) 1.7 occurred on October 18, 2006 and was located southeast of Pocatello, Idaho. The two earthquakes within the INL boundaries included the local magnitude (ML) 2.0 on July 31, 2006 located near the southern termination of the Lemhi fault and the Mc 0.4 on August 6, 2006 located near the center of INL. The MLmore » 2.0 earthquake was well recorded by most of the INL seismic stations and had a focal depth of 8.98 km. First motions were used to compute a focal mechanism, which indicated normal faulting along one of two possible fault planes that may strike N76oW and dip 70±3oSW or strike N55oW and dip 20±13oNE. Slip along a normal fault that strikes N76oW and dips 70±3oSW is consistent with slip along a possible segment of the NW-trending Lemhi normal fault.« less

Basaltic volcanism poses a potential hazard to the proposed Yucca Mountain nuclear waste repository because multiple episodes of basaltic volcanism have occurred in the Yucca Mountain region (YMR) in the past 11 Ma. Intervals between eruptive episodes average about 1 Ma. Three episodes have occurred in the Quaternary at approximately 1.1 Ma (5 volcanoes), 350 ka (2 volcanoes), and 80 ka (1 volcano). Because Yucca Mountain lies within the Basin and Range Province, a significant portion of the pre-Quaternary volcanic history of the YMR may be buried in alluvial-filled basins. An exceptionally high-resolution aeromagnetic survey and subsequent drilling program sponsored by the U.S. Department of Energy (DOE) began in 2004 and is gathering data that will enhance understanding of the temporal and spatial patterns of Pliocene and Miocene volcanism in the region (Figure 1). DOE has convened a ten-member expert panel of earth scientists that will use the information gathered to update probabilistic volcanic hazard estimates originally obtained by expert elicitation in 1996. Yucca Mountain is a series of north-trending ridges of eastward-tilted fault blocks that are bounded by north to northeast-trending normal faults. Topographic basins filled with up to 500 m of alluvium surround it to the east, southmore » and west. In the past several decades, nearly 50 holes have been drilled in these basins, mainly for Yucca Mountain Project Site Characterization and the Nye County Early Warning Drilling Program. Several of these drill holes have penetrated relatively deeply buried (300-400 m) Miocene basalt; a Pliocene basalt dated at 3.8 Ma was encountered at a relatively shallow depth (100 m) in the northern Amargosa Desert (Anomaly B in Figure 1). The current drilling program is the first to specifically target and characterize buried basalt. Based on the new aeromagnetic survey and previous air and ground magnetic surveys (Connor et al. 2000; O'Leary et al. 2002), at least eight drill holes are planned with the goal of sampling each geographic subpopulation of magnetic anomalies in the region (Figure 1). This will result in a more complete characterization of the location, age, volume and composition of buried basaltic features for the purpose of updating the volcanic hazard assessment. Smith and Keenan (2005) suggested that volcanic hazard estimates might be 1-2 orders of magnitude higher than estimated by the DOE expert elicitation in 1996, based on (1) a proposed relationship between recurrence rates in the YMR and the Reveille-Lunar Crater volcanic field to the north, and (2) the implication that a number of so-far-undiscovered buried volcanoes would have a significant impact on hazard estimates. This article presents the new aeromagnetic data and an interpretation of the data that suggests magnetic anomalies nearest the proposed repository site represent buried Miocene basalt that will likely have only a minor impact on the volcanic hazard.« less

Most producing geothermal fields and known geothermal resources in the Basin and Range province are associated with Quaternary active fault systems, within which hydrothermal fluids are presumed to circulate from depth to relatively shallow production levels through high permeability fractures. Research at the Dixie Valley field by Barton et al. (1997) indicates that hydraulically conductive fractures within the Stillwater fault zone are those that have orientations such that the fractures are critically stressed for normal shear failure under the regional tectonic stress field. In general, therefore, we might expect geothermal resources to occur in areas of high inter-seismic strain accumulation, and where faults are favorably oriented with respect to the regional strain tensor; in the case of Basin and Range normal faults, these would generally be faults striking normal to the direction of maximum extension. Expanding this hypothesis, Blewitt et al. (2003), based on preliminary, broad-scale analysis of regional strain and average fault strike in the northwestern Basin and Range, have proposed that geothermal resources occur in areas where fault-normal extension associated with shear strain is the greatest. Caskey and Wesnousky (2000) presented evidence that the Dixie Valley field occupies a 10 km-long gap between prehistoric Holocene ruptures of themore » fault segments on either side. Modeled maximum shear and Coulomb failure stress are high within the gap owing to the stress concentrations at the ends of the ruptures. These results suggest that a major contributing factor to the enhanced permeability at fault-hosted geothermal systems may be localized stress and strain concentrations within fault zone segments. This notion is generally consistent with the common occurrence of geothermal fields within fault offsets (pull-aparts) along strike-slip fault systems, where the local strain field has a large extensional component (e.g., Salton Sea and Coso). Blewitt et al. (2003) suggested that resources correlate with abrupt changes in fault orientation and with changes in the direction of extensional strain.« less

There have been great advances in the last 20 years in understanding the volcanic, structural, geophysical, and petrologic development of the Coso Range and Coso geothermal field. These studies have provided a wealth of knowledge concerning the geology of the area, including general structural characteristics and kinematic history. One element missing from this dataset was an understanding of the sedimentology and stratigraphy of well-exposed Cenozoic sedimentary strata - the Coso Formation. A detailed sedimentation and tectonics study of the Coso Formation was undertaken to provide a more complete picture of the development of the Basin and Range province in this area. Detailed mapping and depositional analysis distinguishes separate northern and southern depocenters, each with its own accommodation and depositional history. While strata in both depocenters is disrupted by faults, these faults show modest displacement, and the intensity and magnitude of faulting does no t record significant extension. For this reason, the extension between the Sierran and Coso blocks is interpreted as minor in comparison to range bounding faults in adjacent areas of the Basin and Range.

This study characterizes the hydrogeologic system of the Death Valley region, an area covering approximately 100,000 square kilometers. The study also characterizes the effects of faults on ground-water movement in the Death Valley region by synthesizing crustal stress, fracture mechanics,a nd structural geologic data. The geologic conditions are typical of the Basin and Range Province; a variety of sedimentary and igneous intrusive and extrusive rocks have been subjected to both compressional and extensional deformation. Faulting and associated fracturing is pervasive and greatly affects ground-water flow patterns. Faults may become preferred conduits or barriers to flow depending on whether they are in relative tension, compression, or shear and other factors such as the degree of dislocations of geologic units caused by faulting, the rock types involved, the fault zone materials, and the depth below the surface. The current crustal stress field was combined with fault orientations to predict potential effects of faults on the regional ground-water flow regime. Numerous examples of fault-controlled ground-water flow exist within the study area. Hydrologic data provided an independent method for checking some of the assumptions concerning preferential flow paths. 97 refs., 20 figs., 5 tabs.

In support of Yucca Mountain site characterization studies, a hydrogeologic framework was developed, and a hydrogeologic map was constructed for the Death Valley region. The region, covering approximately 100,000 km{sup 2} along the Nevada-California border near Las Vegas, is characterized by isolated mountain ranges juxtaposed against broad, alluvium-filled valleys. Geologic conditions are typical of the Basin and Range Province; a variety of sedimentary and igneous intrusive and extrusive rocks have been subjected to both compressional and extensional deformation. The regional ground-water flow system can best be described as a series of connected intermontane basins in which ground-water flow occurs in basin-fill deposits, carbonate rocks, clastic rocks, and volcanic rocks. Previous investigations have developed more site-specific hydrogeologic relationships; however, few have described all the lithologies within the Death Valley regional ground-water flow system. Information required to characterize the hydrogeologic units in the region was obtained from regional geologic maps and reports. Map data were digitized from regional geologic maps and combined into a composite map using a geographic information system. This map was simplified to show 10 laterally extensive hydrogeologic units with distinct hydrologic properties. The hydraulic conductivity values for the hydrogeologic units range over 15 orders of magnitude due to the variability in burial depth and degree of fracturing.

Geodetic surveys provide important information for estimating recent ground movement in support of seismotectonic investigations of the potential nuclear-waste storage site at Yucca Mountain, Nevada. Resurveys of established level lines document up to 22 millimeters of local subsidence related to the 1992 Little Skull Mountain earthquake, which is consistent with seismic data that show normal-slip rupture and with data from a regional trilateration network. Comparison of more recent surveys with a level line first established in 1907 suggests 3 to 13 centimeters of subsidence in the Crater Flat-Yucca Mountain structural depression that coincides with the Bare Mountain fault; small uplifts also were recorded near normal faults at Yucca Mountain. No significant deformation was recorded by a trilateration network over a 10-year period, except for coseismic deformation associated with the Little Skull Mountain earthquake, but meaningful results are limited by the short temporal period of that data set and the small rate of movement. Very long baseline interferometry that is capable of measuring direction and rates of deformation is likewise limited by a short history of observation, but rates of deformation between 8 and 13 millimeters per year across the basin and Range province are indicated by the available data.

The Basin and Range (BR aggregate current production from the Blackburn field in Pine Valley commonly exceeds 1000 BO per day. These two and several other Nevada oil fields are unusually hot at reservoir depth--up to 130{degrees}C at depths as shallow as 1.1 km, up to three times the value expected from the prevailing regional geothermal gradient.

The Radioactive Waste Management Complex (RWMC) is the disposal and storage facility for low-level radioactive waste at the Idaho National Engineering Laboratory (INEL). Transuranic waste and mixed wastes were also disposed at the RWMC until 1970. It is located in the southwestern part of the INEL about 80 km west of Idaho Falls, Idaho. The INEL occupies a portion of the Eastern Snake River Plain (ESRP), a low-relief, basalt, and sediment-floored basin within the northern Rocky Mountains and northeastern Basin and Range Province. It is a cool and semiarid, sagebrush steppe desert characterized by irregular, rolling terrain. The RWMC began disposal of INEL-generated wastes in 1952, and since 1954, wastes have been accepted from other Federal facilities. Much of the waste is buried in shallow trenches, pits, and soil vaults. Until about 1970, trenches and pits were excavated to the basalt surface, leaving no sediments between the waste and the top of the basalt. Since 1970, a layer of sediment (about 1 m) has been left between the waste and the basalt. The United States Department of Energy (DOE) has developed regulations specific to radioactive-waste disposal, including environmental standards and performance objectives. The regulation applicable to all DOE facilities is DOE Order 5820.2A (Radioactive Waste Management). An important consideration for the performance assessment of the RWMC is the long-term geomorphic stability of the site. Several investigators have identified geologic processes and events that could disrupt a radioactive waste disposal facility. Examples of these {open_quotes}geomorphic hazards{close_quotes} include changes in stream discharge, sediment load, and base level, which may result from climate change, tectonic processes, or magmatic processes. In the performance assessment, these hazards are incorporated into scenarios that may affect the future performance of the RWMC.

This report synthesizes the results of volcanism studies conducted by scientists at the Los Alamos National Laboratory and collaborating institutions on behalf of the Department of Energy's Yucca Mountain Project. Chapter 1 introduces the volcanism issue for the Yucca Mountain site and provides the reader with an overview of the organization, content, and significant conclusions of this report. The hazard of future basaltic volcanism is the primary topic of concern including both events that intersect a potential repository and events that occur near or within the waste isolation system of a repository. Future volcanic events cannot be predicted with certainty but instead are estimated using formal methods of probabilistic volcanic hazard assessment (PVHA). Chapter 2 describes the volcanic history of the Yucca Mountain region (YMR) and emphasizes the Pliocene and Quaternary volcanic record, the interval of primary concern for volcanic risk assessment. The distribution, eruptive history, and geochronology of Plio-Quaternary basalt centers are described by individual center emphasizing the younger postcaldera basalt (

Yucca Mountain, a proposed site for a high-level nuclear-waste repository, is located in southern Nevada, 20 km east of Beatty, and adjacent to the southwest comer of the Nevada Test Site (NTS) (fig. 1). Yucca Mountain is located within the Basin and Range province of the western United States. The climate is semiarid, and the flora is transitional between that of the Mojave Desert to the south and the Great Basin Desert to the north. As part of the evaluation, hydrologic conditions, especially water levels, of Yucca Mountain and vicinity during the Quaternary, and especially the past 20,000 years, are being characterized. In 1982, the US Geological Survey, in cooperation with the US Department of Energy (under interagency agreement DE-A104-78ET44802), excavated twenty-six bulldozer and backhoe trenches in the Yucca Mountain region to evaluate the nature and frequency of Quaternary faulting (Swadley and others, 1984). The trenches were oriented perpendicular to traces of suspected Quaternary faults and across projections of known bedrock faults into Quaternary deposits. Trench 14 exposes the Bow Ridge Fault on the west side of Exile Hill. Although the original purpose of the excavation of trench 14 was to evaluate the nature and frequency of Quaternary faulting on the Bow Ridge Fault, concern arose as to whether or not the nearly vertical calcium carbonate (the term ``carbonate`` in this study refers to calcium carbonate) and opaline silica veins in the fault zone were deposited by ascending waters (ground water). These veins resemble in gross morphology veins commonly formed by hydrothermal processes.

Results of our research on the oil fields of the Basin and Range province of the western USA continue to support the following concept: Convecting, moderate-temperature geothermal systems in this region have fostered and in some cases critically influenced the generation, migration, and entrapment of oil. At one Basin-Range field (Grant Canyon), oil-bearing and aqueous fluid inclusions in late-stage hydrothermal quartz were entrapped at temperatures comparable to those now prevailing at reservoir depths (120--130{degrees}C); apparent salinities of the aqueous varieties match closely the actual salinity of the modern, dilute oil-field waters. The inclusion-bearing quartz has the oxygen-isotopic signature for precipitation of the mineral at contemporary temperatures from modern reservoir waters. Measured and fluid-inclusion temperatures define near-coincident isothermal profiles through the oil-reservoir interval, a phenomenon suggesting ongoing heat and mass transfer. These findings are consistent with a model whereby a still-active, convectively circulating, meteoric-hydrothermal system: (1) enhanced porosity in the reservoir rock through dissolution of carbonate; (2) hydrothermally sealed reservoir margins; (3) transported oil to the reservoirs from a deep source of unknown size and configuration; and (4) possibly accelerated source-rock maturation through an increase in the local thermal budget. Grant Canyon and other Basin-Range oil fields are similar to the more » oil-bearing, Carlin-type, sediment-hosted, disseminated gold deposits of the nearby Alligator Ridge district. The oil fields could represent either weakly mineralized analogues of these deposits, or perhaps an incipient phase in their evolution. « less

The Pahrump fault system is an active fault system located in Pahrump and Stewart Valleys, Nevada and California, in the southern part of the Basin and Range Province. This system is 50 km long by 30 km wide and is comprised of three fault zones: the right-lateral East Nopah fault zone, the right-oblique Pahrump Valley fault zone, and the normal West Spring Mountains fault zone. All three zones have geomorphic evidence for late Quaternary activity. Analysis of active fault patterns and seismic reflection lines suggests that the Pahrump basin has had a two-stage genesis, an early history associated with a period of low angle detachment faulting probably active 10-15 Ma, and a more recent history related to the present dextral shear system, probably active post-4 Ma.

The Pahrump fault system is an active fault system located in Pahrump and Stewart Valleys, Nevada and California, in the southern part of the Basin and Range Province. This system is 50 km long by 30 km wide and is comprised of three fault zones: the right-lateral East Nopah fault zone, the right-oblique Pahrump Valley fault zone, and the normal West Spring Mountains fault zone. All three zones have geomorphic evidence for late Quaternary activity. Analysis of active fault patterns and seismic reflection lines suggests that the Pahrump basin has had a two-stage genesis, an early history associated with a period of low angle detachment faulting probably active 10-15 Ma, and a more recent history related to the present dextral shear system, probably active post-4 Ma.

The Baltazor Hot Springs KGRA and nearby Painted Hills thermal area are situated in Humboldt County, northwestern Nevada along the northwestern margin of the Basin and Range province. The oldest rocks exposed in the Baltazor area are eugeosynclinal metasedimentary and subordinate metavolcanic rocks of Permian to Triassic age intruded by Cretaceous diorite and quartz diorite. These are overlain by a thick volcanic and volcaniclastic sequence of Miocene through Pliocene age. Pre-Tertiary rocks are not exposed in the Painted Hills. Principal structures in the Baltazor area are intersecting high-angle normal faults which trend northerly and northwesterly. Quaternary landslides are dominant in the Painted Hills, although northerly- and northwesterly-trending high-angle faults are also present. Hydrothermal alteration and mineralization at Baltazor and in the Painted Hills are of several different styles and ages. Copper-bearing quartz veins in pre-Tertiary rocks antedate Cenozoic volcanism and sedimentation. The heat source for thermal phenomena and alteration in both areas is probably deep fault-controlled fluid circulation coupled with an abnormally high regional thermal gradient. (MHR)

Geologic factors in the Basin and Range province in Utah are more favorable for the occurrence of geothermal resources than in other areas on the Colorado Plateaus or in the Middle Rocky Mountains. These geologic factors are principally crustal extension and crustal thinning during the last 17 million years. Basalts as young as 10,000 years have been mapped in the area. High-silica volcanic and intrusive rocks of Quaternary age can be used to locate hydrothermal convection systems. Drilling for hot, high-silica, buried rock bodies is most promising in the areas of recent volcanic activity. Southwestern Utah has more geothermal potential than other parts of the Basin and Range province in Utah. The Roosevelt Hot Springs area, the Cove Fort-Sulphurdale area, and the area to the north as far as 60 kilometers from them probably have the best potential for geothermal development for generation of electricity. Other areas with estimated reservoir temperatures greater than 150/sup 0/C are Thermo, Monroe, Red Hill (in the Monroe-Joseph Known Geothermal Resource Area), Joseph Hot Springs, and the Newcastle area. The rates of heat and water discharge are high at Crater, Meadow, and Hatton Hot Springs, but estimated reservoir temperatures there are less than 150/sup 0/C.more » Additional exploration is needed to define the potential in three additional areas in the Escalante Desert. 28 figs., 18 tabs.« less

A total of five hot springs and 25 thermal wells are located within the combined counties. The water discharged from these hot springs and wells may be suitable for applications such as process heat and space heating and cooling. Within Cochise county there are two large firms which are capable of using 70/sup 0/C (158/sup 0/F) geothermal water for their process heat requirements but the potential use of geothermal energy in Santa Cruz county is limited due to the absence of industry within the county. The amount of geothermal energy on line as a function of time under both private and city-owned utility development is also predicted using a computer simulation model.

The geology and hydrology of the Basin and Range Province of the western conterminous United States are characterized in a series of data sets depicted in maps compiled for evaluation of prospective areas for further study of geohydrologic environments for isolation of high-level radioactive waste. The data sets include: (1) average precipitation and evaporation; (2) surface distribution of selected rock types; (3) tectonic conditions; and (4) surface- and ground-water hydrology and Pleistocene lakes and marshes. Rocks mapped for consideration as potential host media for the isolation of high-level radioactive waste are widespread and include argillaceous rocks, granitic rocks, tuffaceous rocks, mafic extrusive rocks, evaporites, and laharic breccias. The unsaturated zone, where probably as thick as 150 meters (500 feet), was mapped for consideration as an environment for isolation of high-level waste. Unsaturated rocks of various lithologic types are widespread in the Province. Tectonic stability in the Quaternary Period is considered the key to assessing the probability of future tectonism with regard to high-level radioactive waste disposal. Tectonic conditions are characterized on the basis of the seismic record, heat-flow measurements, the occurrence of Quaternary faults, vertical crustal movement, and volcanic features. Tectonic activity, as indicated by seismicity, is greatest in areas bordering the western margin of the Province in Nevada and southern California, the eastern margin of the Province bordering the Wasatch Mountains in Utah and in parts of the Rio Grande valley. Late Cenozoic volcanic activity is widespread, being greatest bordering the Sierra Nevada in California and Oregon, and bordering the Wasatch Mountains in southern Utah and Idaho. 43 refs., 22 figs.

Some possible hydrogeologic effects of disruptive events that may affect repositories for nuclear wastte are described. A very large number of combinations of natural events can be imagined, but only those events which are judged to be most probable are covered. Waste-induced effects are not considered. The disruptive events discussed above are placed into four geologic settings. Although the geology is not specific to given repository sites that have been considered by other agencies, the geology has been generalized from actual field data and is, therefore, considered to be physically reasonable. The geologic settings considered are: (1) interior salt domes of the Gulf Coast, (2) bedded salt of southeastern New Mexico, (3) argillaceous rocks of southern Nevanda, and (4) granitic stocks of the Basin and Range Province. Log-normal distributions of permeabilities of rock units are given for each region. Chapters are devoted to: poresity and permeability of natural materials, regional flow patterns, disruptive events (faulting, dissolution of rock forming minerals, fracturing from various causes, rapid changes of hydraulic regimen); possible hydrologic effects of disruptive events; and hydraulic fracturing.

The Hyder area consists of the Palomas Plain, Hyder Valley, and the northern portion of the Sentinel Plain, Part of the area is in western Maricopa County and the remainder is in Yuma County (Figure I-1). The Gila River crosses the south-central portion of the area in an east to west direction on its way to the Colorado River. Highway 80 parallels the river to connect the agriculture communities of Gila Bend and Yuma. Highway 80 is also the major route to southern California. Secondary dirt roads provide access to the numerous farms in the southern portion of the Palomas Plain and Hyder Valley. A review of existing geological, hydrological and temperature data from the Dateland-Hyder area was undertaken. This was judged to be the first step in assessing the area's geothermal potential. Additional information was gathered from firsthand field observations and discussions with the local population. Temperature measurements and chemical geothermometry suggest three areas where 40 degrees C to 100 degrees C waters may be found at depths of 500 to 1,000 meters. Projection of thermal gradients indicate 200 degrees C temperatures may be encountered at depths as shallow as 2,000 meters. Such a projection assumes continuation of the most favorable thermal gradients. Additional site specific work will be required to delineate these more favorable areas. (33 pages)

The Yuma area has had a long and complex tectonic history. The most southwesterly corner of the area presently comprises a small segment of the Salton Trough, a deep sediment-filled structural depression. Known geothermal anomalies in the Salton Trough make the Yuma area a favorable exploration target even though spreading-center heat sources are not expected to occur there. Geological and geophysical investigations reveal that the area is made up of low, rugged northwest-trending mountains separated by deep sediment-filled basins. Relief is a result of both erosional and structural activity. Northwest-trending en-echelon faults bound the range fronts and the basins, and have created several horst blocks (basement highs) that crop out at or near the surface. The Algodonnes fault is inferred to represent the northeast margin of the Salton Trough and apparently an inactive extension of the San Andreas fault system. Extensive well-pumping and applications of irrigation waters in recent years have created an unnatural state of flux in the hydrologic regime in the Yuma area. Gravity and aeromagnetic anomalies trend strongly northwest through the region as do lineaments derived from Landsat and Skylab photos. Electrical resistivity values in the Bouse Formation are exceptionally low, about 3 ohn-m. Heat flow appearsmore » to be normal for the Basin and Range province. Ground-water temperatures indicate zones of rising warm water, with one such warm anomaly confirmed by sparse geothermal-gradient data.« less

Southern Grass Valley is a fairly typical extensional basin in the Basin and Range province. Leach Hot Springs, in the southern part of the valley, represents the discharge end of an active hydrothermal flow system with an estimated deep aquifer temperature of 163 to 176/sup 0/C. Results of geologic, hydrologic, geophysical and geochemical investigations are discussed in an attempt to construct an internally consistent model of the system.

A Geothermal Reservoir Assessment Case Study was conducted in the Leach Hot Springs Known Geothermal Resource Area of Pershing County, Nevada. The case study included the drilling of twenty-three temperature gradient wells, a magnetotelluric survey, seismic data acquisition and processing, and the drilling of one exploratory well. Existing data from prior investigations, which included water geochemistry, gravity, photogeologic reports and a hydrothermal alteration study, was also provided. The exploratory well was drilled to total depth of 8565' with no significant mud losses or other drilling problems. A maximum temperature of 260/sup 0/F was recorded at total depth. The relatively low temperature and the lack of permeability (as shown by absence of mud loss) indicated that a current, economic geothermal resource had not been located, and the well was subsequently plugged and abandoned. However, the type and extent of rock alteration found implied that an extensive hot water system had existed in this area at an earlier time. This report is a synopsis of the case study activities and the data obtained from these activities.

The Baltazor KGRA and McGee/Painted Hills geothermal prospects are located in northern Humboldt County, Nevada along the northwestern margin of the Basin and Range province. Exploration work other than drilling has included groundwater sampling, a microearthquake study, a geologic literature search and photogeologic mapping, compilation of aeromagnetic and gravity mapping, soil mercury surveying, electrical resistivity and self-potential surveys and detailed hydrothermal alteration mapping. Exploration drilling included 27 shallow temperature gradient holes, four intermediate-depth gradient wells and one 3703-foot deep test, Baltazor 45-14. The deep test penetrated Miocene rhyolite, andesite, basalt and andesitic basalt flows before excessive hold deviation forced an end to drilling and completion as a deep temperature observation well. A temperature survey two weeks after completion obtained a 119.7/sup 0/C (247.4/sup 0/F) reading at survey total depth, 1110 m (3640 feet).

The Castle Dome Mountains and surrounding ranges constitute a voluminous pile of silicic volcanic rocks within the Basin and Range province of southwestern Arizona. Previously reported as Cretaceous and Quaternary in age, these volcanics all are of late Oligocene to early Miocene age as indicated by five new K-Ar dates. Reconnaissance field studies indicate that the volcanic section locally has undergone large rotations that contrast with the usual structural style of the Basin and Range and resemble the thin-skinned rotational tectonics documented for earlier, mid-Tertiary extensional deformation in ranges to the north and northeast. Significant geothermal potential of the Castle Dome area is suggested by a shallow depth to the Curie isotherm and by the apparent presence of a good electrical conductor at anomalously shallow depth in the crust. Warm wells exist in the area and Shearer (1979) reported a geothermal gradient of about 70/sup 0/C/km in a dry well near the center of the gravity low. Radiogenic heat production in the silicic batholith inferred above constitutes a reasonable candidate for a shallow regional heat source.

Local seismic networks were established at the Roosevelt Hot Springs geothermal area, utah and at Raft River geothermal area, Idaho to monitor the background seismicity prior to initiation of geothermal power production. The Raft River study area is currently seismically quiet down to the level of approximately magnitude one. The Roosevelt Hot Springs area has low-level seismic activity for M/sub L/ greater than about two; however, microearthquake (M/sub L/ less than or equal to 2) swarms appear to be relatively common. One swarm occurred adjacent to the Roosevelt geothermal area during the summer of 1981. From June 27 to August 28, 1044 microearthquakes (M/sub L/ less than or equal to 1.5) were recorded from which 686 earthquakes were located and analysed. The main cluster of microearthquakes was located about 2 km east of the production field at a depth of about 5 km. A few small events were located in the production field at shallow depths (< 2 km). Three of the four largest earthquakes in the swarm (M/sub L/ 1.5-2.0) were located 4 to 5 km further east along a n-NW trend beneath the flank of the adjacent Mineral Mountains. Focal mechanism solutions indicate primarily normal faulting due tomore » the regional E-W extension which characterizes this portion of the eastern Basin and Range province. Hence, the Mineral Mountain swarm appears to be a natural release of tectonic stress in this area. Nevertheless, the occurrence of natural earthquake swarms indicates a potential for induced seismicity at Roosevelt Hot Springs after major production operations are initiated.« less

Numerous thermal springs are present along the Wasatch Front from Utah valley on the south to the state line on the north. These systems are just west of the Wasatch Mountains at the eastern edge of the Basin and Range physiographic province and within the active seismic zone referred to as the Intermountain Seismic Belt. This Report of Investigation is a summary of UGMS investigations at four northern Wasatch Front geothermal systems: Utah, Crystal (Madsen), and Udy hot springs; and the Little Mountain - south system. All of these resources are deep circulation systems and the water is heated by the normal heat flow of the Basin and Range Province. Heat from volcanic sources is not believed to contribute to the warming of any northern Wasatch Front springs. Data collected under the DOE/DGE state coupled program are presented for use by individuals interested in these systems.

This continuing research is designed to provide an integrated geological, geophysical, and geochemical study of the geothermal energy potential of promising thermal anomalies in the Rio Grande rift, Basin and Range province, the Mogollon--Datil volcanic field of New Mexico. Specific objectives undertaken in this study include the following: (a) reconnaissance and detailed geologic mapping (Animas Valley, Radium Springs, Alum Mountain, Truth or Consequences, Ojo Caliente, Albuquerque---Belene basin, and San Ysidro); (b) geochemical studies including reconnaissance water sampling (Animas Valley, Radium Springs and Alum Mountain); and (c) geophysical surveys using deep electric-resistivity, gravity, and magnetic techniques (Radium Springs, Animas Valley and Truth or Consequences). The results of one and one-half summer field seasons and approximately two years of analytical work, laboratory research, and development of research equipment and facilities are covered. Publications, communications, and public service resulting from the first years of U.S.G.S. and State funding are listed in Appendix A.

Ground motion parameters are necessary for structural damage assessments and dynamic effects prediction in Las Vegas, Nevada. To develop these, a model of tectonic activity in the southern Basin and Range province was constructed on the basis of late Cenozoic patterns of crustal deformation, estimates of regional strain rates in Holocene time, and historic seismicity. From this information, the region surrounding Las Vegas was subdivided into six seismotectonic zones. Historic seismicity was analyzed on the basis of seismographically recorded earthquakes and compared with long-term seismotectonic activity. Apparent agreement between the two sets of data indicates that the average rates of historic seismicity observed in the areas analyzed are reasonably representative of long-term seismicity. Magnitude-recurrence relationships were developed for each of the six seismotectonic zones, and probable maximum values of peak ground acceleration in Las Vegas were calculated using a computer program (HAZARD) developed for the study. Probable causative earthquake magnitudes in each source zone, probable values of duration of seismic shaking, and predominant periods likely to be associated with various peak accelerations were also determined.

Union Oil Company of California drilled two exploratory geothermal wells in the Stillwater geothermal prospect area in northwestern Nevada to obtain new subsurface data for inclusion in the geothermal reservoir assessment program. Existing data from prior investigations, which included the drilling of four earlier deep temperature gradient wells in the Stillwater area, was also provided. The two wells were drilled to total depths of 6946 ft and 10,014 ft with no significant drilling problems. A maximum reservoir temperature of 353 F was measured at 9950 ft. The most productive well flow tested at a rate of 152,000 lbs/hr with a wellhead temperature of 252 F and pressure of 20 psig. Based upon current economics, the Stillwater geothermal prospect is considered to be subcommercial for the generation of electrical power. This synopsis of the exploratory drilling activities and results contains summary drilling, geologic, and reservoir information from two exploratory geothermal wells.

As part of the national hot dry rock (HDR) geothermal program conducted by Los Alamos Scientific Laboratory, a regional deep magnetotelluric (MT) survey of Arizona and New Mexico was performed. The main objective of the MT project was to produce a regional geoelectric contour map of the pervasive deep electrical conductor within the crust and/or upper mantle beneath the Colorado Plateau, Basin and Range Province, and Rio Grande rift. Three MT profiles cross the Jemez lineament. Preliminary one-dimensional analysis of the data suggest the lineament is associated with anomalously high electrical conductivity very shallow in the crust. An MT/audiomagnetotelluric (AMT) study of a 161 km/sup 2/ HDR prospect was performed on the Zuni Indian Reservation, New Mexico. Two-dimensional gravity modeling of a 700-km gravity profile at 34/sup 0/30'N latitude was used to study the crust and upper mantle beneath the Rio Grande rift. Several models of each of three consecutive layers were produced using all available geologic and geophysical constraints. Two short-wavelength anomalies along the gravity profile were analyzed using linear optimization techniques.

A 15-phase strategy of exploration for high temperature convective hydrothermal resources in the Basin and Range province features a balanced mix of geologic, geochemical, geophysical, hydrologic, and drilling activities. The strategy is based on a study of data submitted under the Department of Energy's Industry Coupled Case Study Program. Justification for inclusion in or exclusion from the strategy of all pertinent geoscientific methods is given. With continuing research on methods of exploration for and modeling of convective hydrothermal systems, this strategy is expected to change and become more cost-effective with time. Variations on the basic strategy are to be expected where the geology or hydrology requires it. Personal preferences, budgetary constraints, time nd land position constraints, and varied experience may cause industrial geothermal exploration managers to differ with our strategy. For those just entering geothermal exploration, the strategy is expected to be particularly useful. End_of_Article - Last_Page 799