The Surficial Geology and Quaternary Fault Map of the Las Vegas Valley, Clark County, Nevada is a 1:50,000-scale compilation of published 1:24,000-scale geologic maps integrated with new field and desktop mapping. This geologic map compilation and GIS database are part of a broader study on the Quaternary faults in the Las Vegas Valley, the results of which are summarized in this report. The map compilation utilized a variety of digital base maps to evaluate the characteristics of surficial deposits and Quaternary fault scarps including lidar data in undeveloped parts of the valley and orthorectified historical aerial photos in the urbanized center of the valley. The map distinguishes twenty surficial map units including alluvial-fan and ephemeral-wash deposits, groundwater discharge deposits of the Las Vegas Formation, and alluvium deposits of Las Vegas Wash. Quaternary faults include the Quaternary Las Vegas Valley fault system in the center of the valley and the Frenchman Mountain fault system along the eastern margin. All Quaternary fault traces in the valley were evaluated, and the previously published fault mapping was modified as needed. The new mapping and uniform correlation of surficial deposits displaced by Quaternary faults yields a consistent valley-wide characterization of the recency of fault activity. The map and GIS database include a Quaternary geochronology compilation of 119 previously published surficial deposit ages of various types, 35 new luminescence ages, and 3 new radiocarbon ages. The new age data further bracket the ages of the Las Vegas basin Quaternary stratigraphy and provide new constraints on the timing of Quaternary fault activity. This geologic map was supported by the Clark County Building Department, the Southern Nevada Chapter of the International Building Council, the Nevada Bureau of Mines and Geology, and the U.S. Geological Survey, and is part of the Earthquake Analysis of Las Vegas Valley Project.

The Las Vegas Valley is home to a fast-growing metropolitan area with more than 2 million residents. The Quaternary geology of the valley impacts land use planning initiatives, water resource management, and the characterization of both seismic and flood hazards. Our new 1:50,000 scale geologic map involved the compilation of twenty published 1:24,000 scale 7.5’ quadrangles, in conjunction with new field and desktop mapping, and geochronology. The compilation utilized a variety of digital base datasets including pre-development historical aerial imagery and lidar topographic data to validate existing geologic mapping, edge match adjacent maps, and digitize new geologic linework. To facilitate the updated mapping in the largely urbanized center of Las Vegas Valley we produced a new orthophoto mosaic and digital elevation model from 1965 aerial photos using structure from motion photogrammetric software. The map compiles 130 unique, named map units from the original source mapping into a uniform list of 20 geologic units. Linework from the existing mapping was evaluated, and wherever possible the published linework was preserved, though in many locations the original linework was modified to integrate new map units, incorporate field and remote observations, update geologic interpretations, and to preserve map style consistency. This map compilation was produced in support of a recent investigation into the seismic hazard of the Las Vegas basin, with a primary goal being the review and refinement of Quaternary fault mapping and the production of a consistent characterization of surficial units displaced by the faults. The mapping was accompanied by thirty-seven new luminescence ages collected from fine-grained ground water discharge deposits of the Las Vegas Formation and alluvial fan deposits to better constrain the age of surficial deposits and the recency of fault activity. This study was perfumed in conjunction with paleoseismic investigations of the Las Vegas Valley fault system and Frenchman Mountain fault system which have yielded new constraints on fault slip rates and the age of paleo-earthquakes in Las Vegas Valley.

The Great Basin region of the western USA is capable of generating much greater amounts of geothermal energy than currently produced. Most geothermal resources in this region are blind, and thus the favorable characteristics for geothermal activity must be synthesized and methodologies developed to discover new commercial-grade systems. The geothermal play fairway concept involves integration of multiple parameters indicative of geothermal activity as a means of identifying promising areas for new development. In the Nevada play fairway project, nine geologic, geochemical, and geophysical parameters were initially synthesized to produce a new geothermal potential map of 96,000 km. Granite Springs Valley in western Nevada is a particularly promising site selected for detailed study. It contains several favorable structural settings, including terminations of major Quaternary normal faults and fault intersections. Geologic, geophysical, and geochemical techniques were employed to define the most likely sites for high permeability and select drilling targets for temperature-gradient holes. Local and intermediate permeability models were revised to reflect results of detailed analyses and generate new detailed play fairway maps of the area. The most promising site lies in the northeastern part of the basin directly east of Adobe Flat, where the horse-tailing termination of a major normal fault (as revealed by new gravity data), opaline sinter deposits, warm shallow wells, and a low-resistivity anomaly are collocated. Geothermometry suggests a blind system with temperatures as high as ~130°C in this area. Four new temperature-gradient holes document temperatures of ~80°C at ~150 m depth. Lessons learned in the detailed studies of this project include: 1) initially identified sites commonly include multiple favorable settings at a finer scale; 2) promising sites in Cenozoic basins cannot be well defined without detailed geophysical surveys; and 3) play fairway analysis is critical at multiple scales, providing a means to select regional prospects as well as vectoring into drilling targets at individual sites.

PRELIMINARY GEOLOGIC MAP OF THE BATEMAN SPRING QUADRANGLE, LANDER COUNTY, NEVADA Alan R. Ramelli, Chester T. Wrucke, and P. Kyle House 2000 OPEN-FILE REPORT 2000-8(rev. 9-3-02)MPRELIMINARY GEOLOGIC MAP OF THE BATEMAN SPRING QUADRANGLE, LANDER COUNTY, NEVADA Prepared as part of the STATEMAP component of the National Cooperative Geologic Mapping Program in cooperation with the U.S. Geological Survey NEVADA BUREAU OF MINES AND GEOLOGY 40°37'30" R45E R46E

The preliminary geologic map of the Griffith Canyon quadrangle, first released in 1999 in PDF format, was originally drafted in Adobe Illustrator. This map was converted and modified in ArcMap Version 9.3 in 2009. Most of the Quaternary unit descriptions were only slightly modified from that original map. Some changes were made in the descriptions of various pediment deposits, and landslide deposits in Hungry Valley that were originally included in the Quaternary were later determined (see Garside and others, 2003) to be part of the Tertiary sedimentary rocks. Tertiary units older than the Nine Hill Tuff were completely remapped in the quadrangle, based on a more complete understanding of the stratigraphic sequence (e.g., Henry and others, 2004). Bedrock unit descriptions were written, modified and corrected, and a short section was added on Structural Geology. Because a variety of map compilation methods were used, the accuracy of location of some contacts is only 1–2 mm (24–48 m on the ground at 1:24,000 scale). Additionally, the preliminary and incomplete mapping of faults and contacts between surficial units in Spanish Springs Valley, done in more detail by Alan R. Ramelli, is displayed as a separate data layer. Mineral contents of igneous rocks were estimated visually from thin sections and rock slabs (which were stained for potassium feldspar). Igneous rock names are based on the IUGS classification (e.g., Le Maitre, 1989). Unit thicknesses were estimated from the geologic map and topographic base map. Strikes and dips were averaged or rounded to the nearest 5° unless accuracy was better than that. Dike widths on the map are commonly exaggerated. Thin surficial units were omitted from cross sections; thicker deposits were combined into a single unit. Photolineaments in Spanish Springs Valley were mapped by examination of low sun-angle photographs. They could not be examined on the ground because of urbanization disturbance or because they were not visible from the ground. The lineaments do not necessarily represent faults. For example, trenching by a consulting company in 2001 did not find a fault at the north end of a long lineament west of Sugarloaf Peak. Field work was completed in 1996–1997, and 2005. More detailed mapping of surficial deposits in southern Spanish Springs Valley was done by Alan R. Ramelli in 2006. Geologic mapping was partly supported by the U.S. Geological Survey STATEMAP Program (Agreement No. 1434-HQ-96-AG-01502 and G09AC00116).

Qt1 Alluvial terrace deposits (late and middle Holocene) Cobbly and pebbly coarse to medium sands, mostly poorly sorted and matrix supported, but locally clast supported; intercalated with well-sorted, medium-grained sand deposits up to 10 cm thick. Surfaces are generally smooth, but commonly have muted bar-andswale morphology. Soils are typically absent or show a weak cambic horizon. Deposits typically 1 m thick or less.

Qt1 Alluvial terrace deposits (late and middle Holocene) Cobbly and pebbly coarse to medium sands, mostly poorly sorted and matrix supported, but locally clast supported; intercalated with well-sorted, medium-grained sand deposits up to 10 cm thick. Surfaces are generally smooth, but commonly have muted bar-and-swale morphology. Soils are typically absent or show a weak cambic horizon. Deposits typically 1 m thick or less.

This report describes the results of a comprehensive surficial geologic mapping effort undertaken to support the development of a series of relative flood hazard maps of the Ivanpah Valley area, Clark County, Nevada. The study area spans approximately 1030 km2 (398 mi2) and is defined by all of three and part of one internally drained basins between Las Vegas and Primm, Nevada. The proximity of the study area to Las Vegas makes it a likely site of urban and suburban development in the near future, and it is currently being considered for the construction of a major airport facility. The geologic study focused on the delineation of surficial deposits of alluvial, aeolian, and playa sediments ranging in age from recent to late Miocene (approximately 5.6 million years old). Mapping emphasized the discrimination of active alluvial and playa surfaces from relict, inactive surfaces for the purpose of developing a relative flood hazard characterization to provide a baseline physical data set to guide floodplain management and more detailed studies related to hazard mitigation efforts in the area. Ideally, the maps will help planners understand the distribution of flood hazard conditions in the valley and direct mitigation efforts and engineering studies to areas with the highest potential for flooding. Study results indicate that of the 60% of the total study area (236 mi2) is composed of surficial geologic deposits. Within that subset, nearly 75% (175 mi2) is subject to a relative flood hazard level deemed greater than ‘none’, and nearly 53% of that area (125 mi2) is classified as having a hazard status high List of Figures List of Tables Abstract Introduction Piedmont Geomorphology and Related Flood Hazards Geomorphology of Alluvial Fans The Problem of Flooding on Desert Piedmonts and the Role of Geologic MappingIntroduction Piedmont Geomorphology and Related Flood Hazards Geomorphology of Alluvial Fans The Problem of Flooding on Desert Piedmonts and the Role of Geologic Mapping 1/18/11 1:21 PM NBMG Report XX, Geologic Assessment of Piedmont and Playa Flood Hazards in the Ivanpah Valley Area, Clark County, Nevada Page 2 of 58 http://dev.nbmg.unr.edu/Pubs/Reports/rXX/ greater than ‘none’, and nearly 53% of that area (125 mi2) is classified as having a hazard status high enough to represent a significant concern for floodplain management. A series of 9 maps and a complete digital GIS dataset accompany this report. Introduction This report describes the application of surficial geologic mapping to flood hazard assessment on desert piedmont and playa surfaces in the Ivanpah Valley area along the I-15 corridor between Las Vegas, Nevada and the California border. It is a descriptive summary of and companion to a GIS data set that has been previously published as 9 separate paper maps (2 maps of the entire area and 7 maps of parts of the area that correspond generally to 7.5 minute quadrangle boundaries). The first map (House and others, 2006) is the surficial geologic map of the study area, and the second (House, 2007) is a qualitative flood hazard assessment of the study area (both at printed scale of 1:50,000). The seven remaining maps (House, 2006b through 2006h) are depictions of the flood hazard classifications on a series of 1:24,000 panels that cover the entire study area. The maps and their underlying data will serve as a useful guide for addressing flood-hazard management issues. At a minimum the mapping provides a useful template for determining the location and extent of areas of greatest concern to engineers and land managers responsible for flood hazard assessment and mitigation. These maps are not regulatory maps or actuarial flood hazard maps, but were developed with the intention of illustrating the spatial array of flood hazardous areas in the valley through the geologic interpretation of landforms, sedimentary deposits, soil characteristics, and surface morphology. Flood-hazard management on desert piedmonts and terminal playas is a particularly challenging task. Desert piedmonts are crossed by ephemeral flow networks that occasionally convey high-velocity flows through a complex array of steep, potentially unstable and highly mobile, alluvial channels. These channels may follow a single, commonly braided thread or may be part of a multi-branched distributary network. Playa surfaces are inundated sporadically with shallow, standing water from a wide range of possible sources. Playa inundation may persist for days to weeks. Difficulties in characterizing floods in these settings also arise from limited amounts of measured data on flow frequency and hydraulics. Rapid urban and suburban growth often further exacerbate the problem by impacting already poorly understood temporal and spatial patterns of runoff and sediment transport while also creating a pressing demand for effective flood hazard mitigation. Conventional concepts of floodplain management (i.e., as related to perennial streams) do not transfer to desert piedmonts. Numerous studies over the last 20 years have indicated that regulatory models for and approaches to flood hazard assessment on desert piedmonts can produce erroneous results when they ignore geologic information (e.g., Baker and others, 1990; Fuller, 1990; Pearthree, 1991; House and others, 1991, 1992; House, 2005; NRC, 1996; Robins et al., 2009). Piedmont Geomorphology and Related Flood Hazards Piedmont is the non-genetic term for a typically broad, generally low-relief area extending from the base of a mountain range toward the center or, axis, of a valley. The valley axis may host an axial stream, river, or wash; or a lake or playa. The latter is the situation for piedmonts in Ivanpah Valley. Piedmonts are composed mostly of alluvial sediment shed from adjacent highlands by streamflows and debris flows that form complexly coalescing and overlapping deposits and landforms of alluvial fans; but they may also include complex mixtures of eroded bedrock and other kinds of surficial geologic deposits, including: well-defined washes, inactive alluvial fan remnants, river terraces, pediments, sand dunes, sand sheets, spring mounds, and lacustrine beach forms. The mosaic of surficial geologic deposits and geomorphic surfaces that form desert piedmonts contain a geologic record of long-term effects of flooding, climate change, and tectonic activity. The various components of the piedmont can have widely varying ages. The array of landforms and geologic deposits on a particular piedmont is dictated by local conditions and the cumulative effects of geologic and climatic history on overall topography, sediment yield, and the types, rates, and magnitudes of surficial processes. With respect to floodplain management concerns, the distinction between active and inactive alluvial fans is of greatest and most immediate relevance. Awareness and recognition of other types of deposits and landforms can be very useful for interpreting piedmont history, geomorphic process dominance, and long-term stability. Thus, describing, mapping and understanding the geomorphology and recent geologic history of desert piedmonts is an important part of understanding their flood hazard. Geomorphology of Alluvial Fans Alluvial fan flooding is the greatest concern of piedmont flood hazard management. Alluvial fans have been the topic of geologic research for a long time, and there is a correspondingly rich record of related scientific literature (c.f., comprehensive summary in Blair and McPherson, 1994). In the last twenty-five years or so, descriptions and analyses of alluvial fans and desert piedmonts have appeared frequently in the context of floodplain management because of the growing awareness and impact of the problem of alluvial fan flooding on piedmonts (e.g., Dawdy, 1979; French, 1986, 1987; Hjalmarson and Kemna, 1991; NRC, 1996, FEMA, 2000; House, 2005; Robins et al., 2009). The Field Area Expected Land Use Issues in Ivanpah Valley, Nevada The Geologic Map Geologic Maps vs. Soil Maps for Flood Hazard Assessment Previous Work Compilation Methods and Base Data Quickbird Data Field Data Collection Mapping Criteria Stratigraphic Relationships Topographic Characteristics Drainage Pattern Surface Morphology Characteristics of Desert Soil Development and Surface Clast Weathering Soil Carbonate Development Desert Pavement Development Surface Clast Weathering Descriptions and Examples of Ivanpah Valley Map Units (House and others, 2006) Statement on Nomenclature Anthropogenic Features and Deposits Playa and Related Deposits Aeolian Deposits and Related Deposits Hillslope Deposits Alluvial Deposits Bedrock Units Age Estimates of the Surficial Units Las Vegas and Pahrump Valleys Eastern Mohave Desert Nevada Test Site and Vicinity Mormon Mesa Area Flood Hazard Interpretation of the Geologic Map Relative Flood Hazard Classes General Geologic Assessment of Flood 1/18/11 1:21 PM NBMG Report XX, Geologic Assessment of Piedmont and Playa Flood Hazards in the Ivanpah Valley Area, Clark County, Nevada Page 3 of 58 http://dev.nbmg.unr.edu/Pubs/Reports/rXX/ The Federal Emergency Management Agency (FEMA) has a vested interest in the characterization and management of piedmont flood hazards and the definition of alluvial fans and alluvial fan flooding. FEMA (2000, p. 6; and NRC, 1996 pp. 6–7) formally defined an alluvial fan as “...a sedimentary deposit located at a topographic break such as the base of a mountain front, escarpment, or valley side, that is composed of stream flow and/or debris flow sediments and has the shape of a fan, either fully or partially extended.” This definition is accompanied by physically based distinctions between active and inactive alluvial fans and their respective flood hazards. These distinctions reflect the ranges of fan geometry and geomorphology on most piedmonts by emphasizing different types of flooding characteristic of active and inactive fans, including: stable channel flooding (inactive f

Shoreline features formed by the late Pleistocene pluvial Lake Dixie in Dixie Valley, central Nevada, record crustal deformation resulting from isostatic rebound of the Lake Lahontan basin, and from Holocene and historic surface faulting. Constructional beach bars on the east side of Dixie Valley show eastward tilt of 0.16 m/km, indicating that lithospheric flexure due to isostatic rebound is symmetrical with the west side of the Lahontan basin. The tilt signal is potentially complicated by post-Lake Dixie fault displacements on the west side of the valley. However, elevation changes recorded geodetically across the analogous 1983 Borah Peak, Idaho earthquake ruptures suggest that coseismic deformation is probably not significant on the east side of the valley relative to the shoreline elevation uncertainties and the overall tilt signal. A survey of faulted shorelines on the west side of the valley suggest that previous fault slip rates estimated from an earlier survey of these same shorelines are in error by nearly a factor of two. Better constrained slip rates from elsewhere along the fault indicate Holocene vertical slip rates of 0.3-0.5 mm/a, consistent with estimates of long term slip rates on the Dixie Valley fault. (C) 2004 Elsevier Ltd. All rights reserved.

The central Nevada seismic belt (CNSB) is a concentration of historical (1915-1932-1954) surface faulting in the western Basin and Range province, forming a linear, nearly continuous 300-km-long rupture zone. Previous results are integrated in this study with new paleoseismic and exploratory trenching data from the historical zones to look for evidence of older, similar beltlike patterns or elevated slip rates that could indicate whether the CNSB is a zone of focused, long-term crustal strain. The data show that the continuous rupture belt produced by the seven earthquakes occurring between 1915 and 1954 is unique in the available paleoseismic record. At the 1954 Fairview Peak fault, the lack of prehistorical faulting in deposits containing the Wilson Creek bed 19 tephra eliminates the possibility of an identical seismic belt in the past 35.4 ka. Our studies also show that the faults have net slip rates ranging from a low of 0.09 mm/yr on the Fairview Peak fault to a high of 0.7 mm/yr on the 1932 Cedar Mountain fault. These are considered moderate to low rates that are similar to most late Quaternary faults in the western Basin and Range province. A space-time comparison shows that the paleoseismic histories for these multiple rupture zones are diverse, and the number and timing of events in each of the zones indicate that there is little evidence for older contemporaneous ruptures of these same faults. Based on these results we reach several conclusions regarding the longer term (≈Holocene) behavior of the CNSB. Although paleoseismic data preclude an older identical rupture belt among the historical zones, consideration of associated Holocene faults within the greater CNSB region indicates that several similar, but not identical, beltlike rupture patterns are plausible during the past 13 ka, although each requires seismic gaps in the along-strike pattern. Although long-term strain (represented by density of young faults) does appear to increase from east to west into the CNSB, the slip-rate data demonstrate that the CNSB is not a belt of concentrated or elevated crustal strain compared with areas that extend west to the Sierra Nevada. The increase in the distribution of Holocene fault activity from east to west into the CNSB is consistent with a marked increase in the 1992-2002 GPS velocity field at the latitude of the 1954 rupture sequence. However, a comparison of the geologic rates across the belt at this same latitude indicates that the extension rates (0.59-1.37 mm/yr) are systematically lower than both the campaign and continuous GPS rates (2.20-3.13 mm/yr) by factors of 2-5. These discrepancies may be due to postseismic strain, or to some form of off-fault deformation. We conclude that the results of our study of fault behavior in the CNSB best support the belt migration model proposed by Wallace (1987) for the western Basin and Range province in which temporal tectonic pulses are believed to migrate regionally, activating different beltlike combinations of late Quaternary faults in an as yet unknown pattern of migration.

The Rainbow Mountain area was the site of three surface-rupturing earthquakes on 6 July and 23 August 1954. More than 50 field measurements of surface offsets constrain the distribution of slip along the discontinuous and distrib- uted rupture zone that formed during the earthquake sequence. Vertical offsets reach a maximum of 0.8 m with the average vertical offset being 0.2 m. In contrast to original reports, we see evidence for a right-lateral component of slip along portions of the rupture zone, including offset stream channels (0.5-1.0 m), left-stepping en echelon scarps, and a well-preserved, 100-m-long mole track. The right-slip com- ponent is consistent with focal plane solutions for the events and recent geodetic results. Previously unmapped surface ruptures now extend the known rupture length of the sequence by 25 km to a total of 70 km. Surface ruptures along the previously unmapped Fourmile Flat fault are subparallel to and form a 10-km left step to the southeast of the Rainbow Mountain fault. Event locations and anecdotal information indicate that the Fourmile Flat ruptures represent minor, primary surface rupture associated with the large 6 July aftershock, triggered 11 hr after the initial 6 July Rainbow Mountain event. The paleoseismic histories of the Rainbow Mountain and Fourmile Flat faults, as recorded in natural and trench exposures, are different although both faults experi- enced three post 15-ka surface rupturing events, including 1954. Bracketing ages for triultimate events on both faults do not overlap. However, constraints on the penultimate event for the Rainbow Mountain and triultimate event for the Fourmile Flat fault do overlap slightly, allowing the possibility that they may have ruptured close in time as in 1954. The Holocene slip rate for the Fourmile Flat fault (0.40 mm/yr) is similar to the post-latest Pleistocene rate for the Rainbow Mountain fault (0.20-0.46 mm/yr) even though the total length of the Fourmile flat (10 km) is much shorter than the overall length of the Rainbow Mountain rupture zone (60 km), indicating that even minor faults can be important for assessing regional strain rates and patterns.

The 1994 Double Spring Flat earthquake (M-W 5.8) occurred within a densely faulted step-over between the Genoa and Antelope Valley faults, two principal normal faults of the transition zone between the Basin and Range Province and the northern Sierra Nevada. The earthquake created zones of ground cracks from 0.1 to 2.8 km long along at least five northwest- to north-northwest-striking faults in the epicentral area. Individual cracks had extensional openings generally from 1 to 10 mm wide. No cracks displayed obvious vertical separation, and only one zone showed permissive evidence of right-lateral separation. Over the 8 days following the main-shock (the period over which the cracks were found), aftershocks formed a dominant northeast trend suggesting the earthquake occurred along a northeast-striking structure. However, no ground breakage was found along faults striking parallel to this northeast aftershock alignment, and subsequent aftershocks formed a conjugate northwest trend. Based on the location and character of the five zones, the observed cracks are attributed to secondary fault slip and shaking effects. The earthquake also created ground cracks along at least two faults 15-25 km from the epicenter. In both of these cases, the faults had documented histories of prior ground cracking, indicating that they are particularly susceptible to such triggered deformation.

We present a methodology for conducting a site-specific probabilistic analysis of fault displacement hazard. Two approaches are outlined. The first relates the occurrence of fault displacement at or near the ground surface to the occurrence of earthquakes in the same manner as is done in a standard probabilistic seismic hazard analysis (PSHA) for ground shaking. The methodology for this approach is taken directly from PSHA methodology with the ground-motion attenuation function replaced by a fault displacement attenuation function. In the second approach, the rate of displacement events and the distribution for fault displacement are derived directly from the characteristics of the faults or geologic features at the site of interest. The methodology for probabilistic fault displacement hazard analysis (PFDHA) was developed for a normal faulting environment and the probability distributions we present may have general application in similar tectonic regions. In addition, the general methodology is applicable to any region and we indicate the type of data needed to apply the methodology elsewhere.