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Research about Nevada

Source-linked reports with geographic coverage including Nevada.

At least 325 records · Page 18Linked to original sources
USGS2011

Episodic growth of a Late Cretaceous and Paleogene intrusive complex of pegmatitic leucogranite, Ruby Mountains core complex, Nevada, USA

Gneissic pegmatitic leucogranite forms a dominant component (>600 km 3 ) of the midcrustal infrastructure of the Ruby Mountains–East Humboldt Range core complex (Nevada, USA), and was assembled and modified episodically into a batholithic volume by myriad small intrusions from ca. 92 to 29 Ma. This injection complex consists of deformed sheets and other bodies emplaced syntectonically into a stratigraphic framework of marble, calc-silicate rocks, quartzite, schist, and other granitoids. Bodies of pegmatitic granite coalesce around host-rock remnants, which preserve relict or ghost stratigraphy, thrusts, and fold nappes. Intrusion inflated but did not disrupt the host-rock structure. The pegmatitic granite increases proportionally downward from structurally high positions to the bottoms of 1-km-deep canyons where it constitutes 95%–100% of the rock. Zircon and monazite dated by U-Pb (sensitive high-resolution ion microprobe, SHRIMP) for this rock type cluster diffusely at ages near 92, 82(?), 69, 38, and 29 Ma, and indicate successive or rejuvenated igneous crystallization multiple times over long periods of the Late Cretaceous and the Paleogene. Initial partial melting of unexposed pelites may have generated granite forerunners, which were remobilized several times in partial melting events. Sources for the pegmatitic granite differed isotopically from sources of similar-aged interleaved equigranular granites. Dominant Late Cretaceous and fewer Paleogene ages recorded from some pegmatitic granite samples, and Paleogene-only ages from the two structurally deepest samples, together with varying zircon trace element contents, suggest several disparate ages of final emplacement or remobilization of various small bodies. Folded sills that merge with dikes that cut the same folds suggest that there may have been in situ partial remobilization. The pegmatitic granite intrusions represent prolonged and recurrent generation, assembly, and partial melting modification of a batholithic volume even while the regional tectonic environment varied dramatically from contractile thickening to extension and mafic underplating.

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USGS2011

Digitized generalized areas where surface-water resources likely or potentially are susceptible to groundwater withdrawals in adjacent valleys, Great Basin National Park area, Nevada

Abstract Polygons delineate generalized areas in and around Great Basin National Park where surface-water resources likely or potentially are susceptible to groundwater withdrawals in adjacent valleys. Purpose This data set was created as part of a U.S. Geological Survey study, done in cooperation with the National Park Service, to characterize surface-water resources in and around Great Basin National Park. The intended uses of this data set include, but are not limited to, natural resource modeling, mapping, and visualization applications. Source Information SIR 2006-5099, Plate 1: Generalized areas where surface-water resources likely or potentially are susceptible to ground-water withdrawals in adjacent valleys, Great Basin National Park area, Nevada.

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USGS2011

Geochemical and stable isotopic data on barren and mineralized drill core in the Devonian Popovich Formation, Screamer sector of the Betze-Post gold deposit, northern Carlin trend, Nevada

The Devonian Popovich Formation is the major host for Carlin-type gold deposits in the northern Carlin trend of Nevada. The Popovich is composed of gray to black, thin-bedded, calcareous to dolomitic mudstone and limestone deposited near the carbonate platform margin. Carlin-type gold deposits are Eocene, disseminated, auriferous pyrite deposits characterized by acid leaching, sulfidation, and silicification that are typically hosted in Paleozoic calcareous sedimentary rocks exposed in windows through siliceous sedimentary rocks of the Roberts Mountains allochthon. The Carlin trend currently is the largest gold producer in the United States. The Screamer ore zone is a tabular body on the periphery of the huge Betze-Post gold deposit. Screamer is a good place to study both the original lithogeochemistry of the Popovich Formation and the effects of subsequent alteration and mineralization because it is below the level of supergene oxidation, mostly outside the contact metamorphic aureole of the Jurassic Goldstrike stock, has small, high-grade ore zones along fractures and Jurassic dikes, and has intervening areas with lower grade mineralization and barren rock. In 1997, prior to mining at Screamer, drill core intervals from barren and mineralized Popovich Formation were selected for geochemical and stable isotope analysis. The 332, five-foot core samples analyzed are from five holes separated by as much as 2000 feet (600 meters). The samples extend from the base of the Wispy unit up through the Planar and Soft sediment deformation units into the lower part of the upper Mud unit of the Popovich Formation.

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USGS2011

Preliminary analysis of Greater Sage-grouse reproduction in the Virginia Mountains of northwestern Nevada

Relationships between habitat selection and population vital rates of greater sage-grouse (Centrocercus urophasianus; hereafter sage-grouse), recently designated as a candidate species under the Endangered Species Act, within the Great Basin are not well-understood. The growing development of renewable energy infrastructure within areas inhabited by sage-grouse is thought to influence predator and vegetation communities. For example, common ravens (Corvus corax), a synanthropic sage-grouse nest predator, are increasing range-wide and select transmission lines and other tall structures for nesting and perching. In the Virginia Mountains of northwestern Nevada, we collected preliminary information of space-use, habitat selection, and population vital rates during the nesting and brood-rearing period over two years on 56 sage-grouse. Additionally, videography at nest sites (n = 22) was used to identify sage-grouse nest predators. The study area is a potential site for renewable energy developments (i.e., wind and solar), and we plan to continue monitoring this population using a before-after-control-impact study design. The results reported here are preliminary and further data are required before conclusions can be drawn from this population of sage-grouse.

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USGS2011

Geologic framework and hydrogeology of the middle Carson River Basin, Eagle, Dayton, and Churchill Valleys, West-Central Nevada

Changes in land use and water use and increasing development of water resources in the middle Carson River basin may affect flow of the river and, in turn, affect downstream water users dependent on sustained river flows to Lahontan Reservoir. The U.S. Geological Survey, in cooperation with the Bureau of Reclamation, began a study in 2008 of the middle Carson River basin, extending from Eagle Valley to Churchill Valley. Various types of geologic and hydrologic data were compiled from previous studies, collected for this study, and compiled and analyzed to provide a framework for development of a numerical model of the groundwater and surface-water flow systems of the basin. Geologic units that are assumed to have similar hydrologic characteristics were grouped into hydrogeologic units comprised of consolidated rocks of pre-Cenozoic age that underlie a unit of consolidated volcanic rock and semi-consolidated sediments of Tertiary age. The principal aquifer in the study area is comprised of unconsolidated sediments of Quaternary age. The Quaternary sediments include alluvial fan, fluvial, and lake sediments, and were grouped into a basin-fill hydrogeologic unit that overlies the pre-Cenozoic and Tertiary hydrologic units. The thickness of the combined section of Tertiary volcanic and sedimentary rocks and Quaternary basin-fill deposits previously was estimated to range from zero where pre-Cenozoic rocks are exposed to greater than 10,000 feet in the Bull Canyon subbasin, and greater than 6,000 feet on the western side of Churchill Butte and beneath the Desert Mountains. The thickness of Quaternary basin-fill sediments was estimated using gravity data and lithologic descriptions from driller’s logs. The most permeable parts of basin-fill sediments are greater than 1,000 feet thick in the Carson Plains subbasin, greater than 800 feet and 600 feet thick in the western and northeastern parts of the Stagecoach subbasin, and greater than 1,000 feet and 800 feet thick in the northern and southern parts of Churchill Valley, respectively. The distribution of aquifer properties was estimated for basin-fill sediments using slug-test and aquifer test data, and the lithologic descriptions of previously mapped geologic units. Slug-test data show hydraulic conductivity is greater than 10 to greater than 100 feet per day for fluvial sediments near the flood plain, less than 10 feet per day for basin-fill sediments outside the flood plain, and less than 1 foot per day for consolidated rocks. Estimates of transmissivity exceed 20,000 feet squared per day near the Carson River in Dayton, Churchill, and western Lahontan Valleys and in the northern part of the Stagecoach subbasin, and exceed 10,000 feet squared per day in the western part of Churchill Valley. A transmissivity of 90,000 feet squared per day was estimated from results of an aquifer test in the Carson Plains subbasin, indicating that permeable gravel and cobble zones at depths greater than 400 feet supplied water to the pumping well. Estimates of specific yield ranged from less than 1 to 2 percent for most consolidated rocks, from 1 to 15 percent for semi-consolidated Tertiary sediments, and from 10 to 40 percent for unconsolidated basin-fill sediments. Water-level altitude maps based on measurements at about 300 wells in 2009 show water levels have declined as much as 70 feet since 1964 on the northwestern side of Eagle Valley, about 10 feet since 1995 near Dayton in the Carson Plains subbasin, and from 5 to 10 feet since 1982 in the western and northeastern parts of the Stagecoach subbasin and the northwestern part of Churchill Valley. The declines are likely the result of municipal and agricultural pumping. The maps show a groundwater divide between the Carson Plains and Stagecoach subbasins, and a continuous hydraulic gradient between the Stagecoach subbasin and Churchill Valley. Groundwater flow directions are uncertain beneath parts of the boundary of Churchill Valley. The altitude of the top of pre-Cenozoic rocks shows thick sections of saturated Tertiary rocks and sediments south of the Dead Camel Mountains and beneath the eastern part of the Desert Mountains through which groundwater flow between Churchill Valley, Mason Valley, and Lahontan Valley may take place. North of Lahontan reservoir, beneath the Dead Camel Mountains, and beneath the southern part of Adrian Valley, the altitude of pre-Cenozoic rocks indicates groundwater flow between the three valleys is minimal. Streamflow measurements, supported by data on the deuterium content and specific conductance of surface-water samples, indicate a loss of Carson River streamflow in the Riverview subbasin, streamflow gains in the Moundhouse subbasin and the eastern part of the Carson Plains subbasin, and streamflow losses in the Bull Canyon subbasin. Comparisons of fluctuations in groundwater levels to those in stream stage in the Carson Plains subbasin indicate that streamflow lost to infiltration from the Carson River, from irrigation ditches, and from irrigated fields is an important source of groundwater recharge. Fluctuations in groundwater levels compared with the stage of Lahontan Reservoir in Churchill Valley indicate losses to infiltration from the reservoir during high stage and groundwater seepage to the reservoir during low stage.

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USGS2011

Precipitation and runoff simulations of select perennial and ephemeral watersheds in the middle Carson River basin, Eagle, Dayton, and Churchill Valleys, west-central Nevada

The effect that land use may have on streamflow in the Carson River, and ultimately its impact on downstream users can be evaluated by simulating precipitation-runoff processes and estimating groundwater inflow in the middle Carson River in west-central Nevada. To address these concerns, the U.S. Geological Survey, in cooperation with the Bureau of Reclamation, began a study in 2008 to evaluate groundwater flow in the Carson River basin extending from Eagle Valley to Churchill Valley, called the middle Carson River basin in this report. This report documents the development and calibration of 12 watershed models and presents model results and the estimated mean annual water budgets for the modeled watersheds. This part of the larger middle Carson River study will provide estimates of runoff tributary to the Carson River and the potential for groundwater inflow (defined here as that component of recharge derived from percolation of excess water from the soil zone to the groundwater reservoir). The model used for the study was the U.S. Geological Survey's Precipitation-Runoff Modeling System, a physically based, distributed-parameter model designed to simulate precipitation and snowmelt runoff as well as snowpack accumulation and snowmelt processes. Models were developed for 2 perennial watersheds in Eagle Valley having gaged daily mean runoff, Ash Canyon Creek and Clear Creek, and for 10 ephemeral watersheds in the Dayton Valley and Churchill Valley hydrologic areas. Model calibration was constrained by daily mean runoff for the 2 perennial watersheds and for the 10 ephemeral watersheds by limited indirect runoff estimates and by mean annual runoff estimates derived from empirical methods. The models were further constrained by limited climate data adjusted for altitude differences using annual precipitation volumes estimated in a previous study. The calibration periods were water years 1980-2007 for Ash Canyon Creek, and water years 1991-2007 for Clear Creek. To allow for water budget comparisons to the ephemeral models, the two perennial models were then run from 1980 to 2007, the time period constrained somewhat by the later record for the high-altitude climate station used in the simulation. The daily mean values of precipitation, runoff, evapotranspiration, and groundwater inflow simulated from the watershed models were summed to provide mean annual rates and volumes derived from each year of the simulation. Mean annual bias for the calibration period for Ash Canyon Creek and Clear Creek watersheds was within 6 and 3 percent, and relative errors were about 18 and -2 percent, respectively. For the 1980-2007 period of record, mean recharge efficiency and runoff efficiency (percentage of precipitation as groundwater inflow and runoff) averaged 7 and 39 percent, respectively, for Ash Canyon Creek, and 8 and 31 percent, respectively, for Clear Creek. For this same period, groundwater inflow volumes averaged about 500 acre-feet for Ash Canyon and 1,200 acre-feet for Clear Creek. The simulation period for the ephemeral watersheds ranged from water years 1978 to 2007. Mean annual simulated precipitation ranged from 6 to 11 inches. Estimates of recharge efficiency for the ephemeral watersheds ranged from 3 percent for Eureka Canyon to 7 percent for Eldorado Canyon. Runoff efficiency ranged from 7 percent for Eureka Canyon and 15 percent at Brunswick Canyon. For the 1978-2007 period, mean annual groundwater inflow volumes ranged from about 40 acre-feet for Eureka Canyon to just under 5,000 acre-feet for Churchill Canyon watershed. Watershed model results indicate significant interannual variability in the volumes of groundwater inflow caused by climate variations. For most of the modeled watersheds, little to no groundwater inflow was simulated for years with less than 8 inches of precipitation, unless those years were preceded by abnormally high precipitation years with significant subsurface storage carryover.

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USGS2011

Meteoric precipitation at Yucca Mountain, Nevada: Chemical and stable isotope analyses, 2006-09

Meteoric precipitation samples collected in 2006-09 at Yucca Mountain, Nevada, were analyzed for chemistry and stable isotope composition. Precipitation is the major source of infiltration to the unsaturated zone and of recharge to the saturated zone at Yucca Mountain. On February 28, 2005, seepage of water was observed about 40 to 80 meters below the ground surface within the Tiva Canyon Tuff in the South Ramp of the Exploratory Studies Facility tunnel within Yucca Mountain. This seepage was preceded by a 5-month period of above-average precipitation. Chemical and isotopic analysis of this seepage could not be used to estimate travel time, extent of water-rock interaction, and (or) evaporative water loss during percolation through the unsaturated zone due to the lack of corresponding chemical and isotopic analyses of precipitation inputs to the infiltration events that produced the seepage. In February 2006, collection of precipitation samples for chemical (major ions), delta oxygen-18 (&delta 18 O), and delta deuterium (&deltaD) analyses began at seven meteorological monitoring stations to provide baseline isotopic and chemical analyses. The sampling stations range in elevation from 1,131 to 1,562 meters. Each site has two collectors-one for chemical analysis and the other for isotopic analysis of precipitation. The collectors were sampled and emptied after each precipitation event. In 2006-09, 36 distinct precipitation events, with an average 3- to 4-day duration and an average 9.9 millimeters of accumulation, have been analyzed. The chemical composition of these samples of Yucca Mountain precipitation is relatively dilute but contains measurable and variable concentrations of Na + , Ca 2+ , NH + , NO 3 - , and SO 4 2- . Dust transported by typical winter storms and generated from soil carbonates is the main contributor to this precipitation chemistry. Elevated nitrate and ammonium concentrations may be linked to agriculture in the nearby Amargosa River valley to the south and west of Yucca Mountain. Cumulatively, &delta 18 O values range from 3.0 to -20.4 per mil (%o) and &deltaD values range from 10 to -14%o. Winter-season precipitation commonly has isotopically lighter compositions. The cumulative &delta18O plotted against &deltaD shows that precipitation samples define a line with slope of 6.4, less than the 8 of the Global Meteoric Water Line. This difference in slope, typical of arid environments, is chiefly the result of evaporation of falling raindrops due to warmer air temperatures. ;

Scientific Investigations Report↗
USGS2011

Assessing hydraulic connections across a complex sequence of volcanic rocks - Analysis of U-20 WW multiple-well aquifer test, Pahute Mesa, Nevada National Security Site, Nevada

Groundwater beneath Pahute Mesa flows through a complexly layered sequence of volcanic rock aquifers and confining units that have been faulted into distinct structural blocks. Hydraulic property estimates of rocks and structures in this flow system are necessary to assess radionuclide migration near underground nuclear testing areas. The U.S. Geological Survey (USGS) used a 12 month (October 1, 2008&mdash; October 1, 2009) intermittent pumping schedule of well U-20 WW and continuously monitored water levels in observation wells ER-20-6 #3, UE-20bh 1, and U-20bg as a multi-well aquifer test to evaluate hydraulic connections across structural blocks, bulk hydraulic properties of volcanic rocks, and the hydraulic significance of a major fault. Measured water levels were approximated using synthetic water levels generated from an analytical model. Synthetic water levels are a summation of environmental water-level fluctuations and a Theis (1935) transform of the pumping signal from flow rate to water-level change. Drawdown was estimated by summing residual differences between measured and synthetic water levels and the Theis-transformed pumping signal from April to September 2009. Drawdown estimates were used in a three‑dimensional numerical model to estimate hydraulic properties of distinct aquifers, confining units, and a major fault. A maximum water-level drawdown of nearly 0.4 foot in well UE-20bh 1, which is more than 1 mile from the pumping well, was detected across a major fault. Drawdown estimates in the observation well nearest to (ER-20-6 #3, less than 1 mile) and within the same structural block as the pumping well were less than detection (<0.1 foot). Minimal drawdown within the same structural block indicates that lava units are likely separated by bedded tuff confining units. Hydraulic property estimates indicate that wells U-20 WW, UE-20bh 1, and ER-20-6 #3 produce water from moderately permeable fractured lava, as hydraulic conductivity and specific storage estimates average 4.8 feet per day and 2.1&times;10 &ndash;6 per foot, respectively, and transmissivity estimates range from 1,200 to 3,600 feet squared per day. Sensitivity analyses indicate that the major fault is hydraulically similar to the permeable host rock and connects flow between structural blocks.

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USGS2011

Seroprevalence of West Nile Virus in feral horses on Sheldon National Wildlife Refuge, Nevada, United States

We screened 1,397 feral horses ( Equus caballus ) on Sheldon National Wildlife Refuge, Nevada, United States, for IgM and IgG against flavivirus during 2004&ndash;2006, 2008, and 2009. Positive serum samples were tested for neutralizing antibodies to West Nile virus (WNV) and St. Louis encephalitis virus (SLEV). One animal was positive for antibody against WNV in 2004, but all others tested in 2004&ndash;2006 were negative. In 2008 and 2009, we found evidence of increasing seropositive horses with age, whereas seroprevalence of WNV decreased from 19% in 2008 to 7.2% in 2009. No horses were positive for antibody against SLEV. Being unvaccinated, feral horses can be useful for WNV surveillance.

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USGS2011

Paleozoic stratigraphy and kinematics of the Roberts Mountains allochthon in the Independence Mountains, northern Nevada

New biostratigraphic, stratigraphic, and structural data from deformed lower Paleozoic sedimentary units of the Roberts Mountains allochthon (RMA), Independence Mountains, Nevada, have enabled the identification of stratigraphic units within the allochthon that range in age from Late Cambrian through Late Devonian. The McAfee thrust fault emplaced a relatively thick (>200 m) sheet of Ordovician Valmy quartzite (locally termed the McAfee Quartzite) on complexly deformed units of the RMA (locally termed the Snow Canyon Formation) comprised of the Vinini Formation, Elder Sandstone, and Slaven Chert. The rocks were affected by multiple phases of deformation during and after the Devonian-Mississippian Antler orogeny, including: emplacement of the Roberts Mountains allochthon with south-vergent folding and faulting of the upper and lower plate rocks to the Roberts Mountains thrust; emplacement of Upper Mississippian (324 Ma) basalt dikes that cross-cut the upper (allochthonous) and lower (para-autochthonous) plate rocks and south-vergent folds; emplacement of the McAfee Quartzite; and upright north-trending folding after the deposition of Antler overlap rocks. Recognition of this history is required to determine the internal structure and thickness of the allochthon and location of high-angle structures within it that, respectively, may conceal and localize Carlin-type gold deposits in underlying carbonate rocks.

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USGS2011

Global Positioning System constraints on crustal deformation before and during the 21 February 2008 Wells, Nevada M6.0 earthquake

Using Global Positioning System (GPS) data from permanent sites and U.S. Geological Survey (USGS) campaign data we have estimated co-seismic displacements and secular background crustal deformation patterns associated with the 21 February 2008 Wells Nevada earthquake. Estimated displacements at nearby permanent GPS sites ELKO (84 km distant) and GOSH (81 km distant) are 1.0±0.2 mm and 1.1±0.3 mm, respectively. The magnitude and direction are in agreement with those predicted from a rupture model based on InSAR measurements of the near-field co-seismic surface displacement. Analysis of long GPS time series (>10 years) from the permanent sites within 250 km of the epicenter indicate the eastern Nevada Basin and Range undergoes steady tectonic transtension with rates on the order of 1 mm/year over approximately 250 km. The azimuth of maximum horizontal crustal extension is consistent with the azimuth of the Wells earthquake co-seismic slip vector. The orientation of crustal shear is consistent with deformation associated with Pacific/North America plate boundary relative motion seen elsewhere in the Basin and Range. In response to the event, we deployed a new GPS site with the capability to telemeter high rate, low latency data that will in the future allow for rapid estimation of surface displacement should aftershocks or postseismic deformations occur. We estimated co-seismic displacements using campaign GPS data collected before and after the event, however in most cases their uncertainties were larger than the offsets. Better precision in co-seismic displacement could have been achieved for the campaign sites if they had been surveyed more times or over a longer interval to better estimate their pre-event velocity.

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USGS2011

Geophysical setting of the February 21, 2008 Mw 6 Wells earthquake, Nevada, and implications for earthquake hazards

We utilize gravity and magnetic methods to investigate the regional geophysical setting of the Wells earthquake. In particular, we delineate major crustal structures that may have played a role in the location of the earthquake and discuss the geometry of a nearby sedimentary basin that may have contributed to observed ground shaking. The February 21, 2008 M w 6.0 Wells earthquake, centered about 10 km northeast of Wells, Nevada, caused considerable damage to local buildings, especially in the historic old town area. The earthquake occurred on a previously unmapped normal fault and preliminary relocated events indicate a fault plane dipping about 55 degrees to the southeast. The epicenter lies near the intersection of major Basin and Range normal faults along the Ruby Mountains and Snake Mountains, and strike-slip faults in the southern Snake Mountains. Regionally, the Wells earthquake epicenter is aligned with a crustal-scale boundary along the edge of a basement gravity high that correlates to the Ruby Mountains fault zone. The Wells earthquake also occurred near a geophysically defined strike-slip fault that offsets buried plutonic rocks by about 30 km. In addition, a new depth-to-basement map, derived from the inversion of gravity data, indicates that the Wells earthquake and most of its associated aftershock sequence lie below a small oval- to rhomboid-shaped basin, that reaches a depth of about 2 km. Although the basin is of limited areal extent, it could have contributed to increased ground shaking in the vicinity of the city of Wells, Nevada, due to basin amplification of seismic waves.

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USGS2011

The regional structural setting of the 2008 Wells earthquake and Town Creek Flat Basin: implications for the Wells earthquake fault and adjacent structures

The 2008 Wells earthquake occurred on a northeast-striking, southeast-dipping fault that is clearly delineated by the aftershock swarm to a depth of 10-12 km below sea level. However, Cenozoic rocks and structures around Wells primarily record east-west extension along north- to north-northeast-striking, west-dipping normal faults that formed during the middle Miocene. These faults are responsible for the strong eastward tilt of most basins and ranges in the area, including the Town Creek Flat basin (the location of the earthquake) and the adjacent Snake Mountains and western Windermere Hills. These older west-dipping faults are locally overprinted by a younger generation of east-dipping, high-angle normal faults that formed as early as the late Miocene and have remained active into the Quaternary. The most prominent of these east-dipping faults is the set of en-échelon, north-striking faults that bounds the east sides of the Ruby Mountains, East Humboldt Range, and Clover Hill (about 5 km southwest of Wells). The northeastern-most of these faults, the Clover Hill fault, projects northward along strike toward the Snake Mountains and the approximately located surface projection of the Wells earthquake fault as defined by aftershock locations. The Clover Hill fault also projects toward a previously unrecognized, east-facing Quaternary fault scarp and line of springs that appear to mark a significant east-dipping normal fault along the western edge of Town Creek Flat. Both western and eastern projections may be northern continuations of the Clover Hill fault. The Wells earthquake occurred along this east-dipping fault system. Two possible alternatives to rupture of a northern continuation of the Clover Hill fault are that the earthquake fault (1) is antithetic to an active west-dipping fault or (2) reactivated a Mesozoic thrust fault that dips east as a result of tilting by the west-dipping faults along the west side of the Snake Mountains. Both alternatives are precluded by the depths of the earthquake and aftershocks, about 8 km and as deep as 12 km, respectively. These depths are below where an antithetic fault would intersect any main fault, and a tilted, formerly shallow and sub-horizontal thrust fault would not extend to depths of more than about 5–6 km. The east-dipping, high-angle, earthquake fault cuts older west-dipping faults rather than reactivating them, highlighting a change in the structural style of Basin and Range extension in this region from closely-spaced, west-dipping faults that rotated significantly during slip and accommodated large-magnitude extension, to widely-spaced, high-angle faults that accommodate much less total strain over a long time span.

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USGS2011

Geologic map of the Caetano caldera, Lander and Eureka counties, Nevada

The Eocene (34 Ma) Caetano caldera in north-central Nevada offers an exceptional opportunity to study the physical and petrogenetic evolution of a large (20 km by 10–18 km pre-extensional dimensions) silicic magma chamber, from precursor magmatism to caldera collapse and intrusion of resurgent plutons. Caldera-related rocks shown on this map include two units of crystal-rich intracaldera tuff totaling over 4 km thickness, caldera collapse breccias, tuff dikes that fed the eruption, hydrothermally altered post-eruption rocks, and two generations of resurgent granitic intrusions (John et al., 2008). The map also depicts middle Miocene (about 16–12 Ma) normal faults and synextensional basins that accommodated >100 percent extension and tilted the caldera into a series of ~40° east-dipping blocks, producing exceptional 3-D exposures of the caldera interior (Colgan et al., 2008). This 1:75,000-scale map is a compilation of published maps and extensive new mapping by the authors (fig. 1), and supersedes a preliminary 1:100,000-scale map published by Colgan et al. (2008) and John et al. (2008). New mapping focused on the margins of the Caetano caldera, the distribution and lithology of rocks within the caldera, and on the Miocene normal faults and sedimentary basins that record Neogene extensional faulting. The definition of geologic units and their distribution within the caldera is based entirely on new mapping, except in the northern Toiyabe Range, where mapping by Gilluly and Gates (1965) was modified with new field observations. The distribution of pre-Cenozoic rocks outside the caldera was largely compiled from existing sources with minor modifications, with the exception of the northeastern caldera margin (west of the Cortez Hills Mine), which was remapped in the course of this work and published as a stand-alone 1:6000-scale map (Moore and Henry, 2010).

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USGS2011

Coulomb stress analysis of the 21 February 2008 Mw= 6.0 Wells, Nevada, earthquake

Static Coulomb stress changes imparted by the February 21, 2008 Wells, Nevada earthquake are calculated, using an 8 x 6 km rectangular patch with a uniform slip as a source fault. Stress changes are resolved on nearby active faults using their rake, dip, and strike direction, assuming a fault friction of 0.4. The largest Coulomb stress increase (0.2 bars) imparted to surrounding major active faults from the Wells earthquake occurs on the Clover Hill fault, which may be the southern continuation of the ruptured fault. A 0.1 bar Coulomb stress increase is calculated on the western Snake Mountains fault. Coulomb stress decreases of 0.5 bars are calculated for the northern parts of the Independence and Ruby Mountains faults. The Coulomb stress change is calculated on relocated aftershocks assuming that they have the same strike, dip, and rake, as the source fault. Under this assumption, 75% of the aftershocks received a Coulomb stress increase.

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USGS2011

Coluber (= Masticophis) flagellum piceus (Red Racer). Arboreal/nocturnal behavior

Many species of snakes display arboreal behavior and are often found in vegetation many meters above ground. Mojave Desert snake species rarely get very far above the ground surface and are perhaps limited by predominantly low growing vegetation. Coluber flagellum piceus is considered strictly diurnal and although may ascend vegetation while active, is thought to retreat to subterranean refugia at night. Werler and Dixon (2000. Texas Snakes: Identification, Distribution, and Natural History. University of Texas Press, Austin. 437 pp.) stated “The western coachwhip is active above ground only during daylight hours. It almost invariably retires to depths of an animal burrow before sundown, where it spends the night.” The normal morning activity period for C. flagellum appears to be from approximately 0700–1000 h from June through September (Jones and Whitford 1989. Southwest. Nat. 34:460–467). Secor and Nagy (1994. Ecology 75:1600–1614), and Secor (1995. Herpetol. Monogr. 9:169–186) determined that the average body temperature for active above ground activity of C. flagellum was (33.1°C + 0.1°C, range of 24.0°C–40.8°C, N = 502), stating that coachwhip snakes were seldom active on the surface with body temperatures below 28°C. Here, I report a C. f. piceus that apparently spent the night draped in a Creosote Bush ( Larrea tridentata ).

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