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David M. Miller

Publications and source records attributed to David M. Miller.

At least 37 records · Page 2Linked to original sources
USGS2019

Geophysical characterization of a Proterozoic REE terrane at Mountain Pass, eastern Mojave Desert, California

Mountain Pass, California (USA), located in the eastern Mojave Desert, hosts one of the world’s richest rare earth element (REE) deposits. The REE-rich terrane occurs in a 2.5-km-wide, northwest-trending belt of Mesoproterozoic (1.4 Ga) stocks and dikes, which intrude a larger Paleoproterozoic (1.7 Ga) metamorphic block that extends ∼10 km southward from Clark Mountain to the eastern Mescal Range. To characterize the REE terrane, gravity, magnetic, magnetotelluric, and whole-rock physical property data were analyzed. Geophysical data reveal that the Mountain Pass carbonatite body is associated with an ∼5 mGal local gravity high that is superimposed on a gravity terrace (∼4 km wide) caused by granitic Paleoproterozoic host rocks. Physical rock property data indicate that the Mountain Pass REE suite is essentially nonmagnetic at the surface with a magnetic susceptibility of 2.0 × 10 −3 SI ( n = 57), and lower-than-expected magnetizations may be the result of alteration. However, aeromagnetic data indicate that the intrusive suite occurs along the eastern edge of a distinct northwest-trending aeromagnetic high along the eastern Mescal Range. The source of this magnetic anomaly is ∼1.5–2 km below the surface and coincides with an electrical conductivity zone that is several orders of magnitude more conductive than the surrounding rock. The source of the magnetic anomaly is likely a moderately magnetic pluton. Combined geophysical data and models suggest that the carbonatite and its associated REE-enriched ultrapotassic suite were preferentially emplaced along a northwest-trending zone of weakness, which has potential implications for regional mineral exploration.

California↗
USGS2019

Ten-million years of activity within the Eastern California Shear Zone from U-Pb dating of fault-zone opal

Reconstructions of long-term fault activity are essential for understanding both the mechanisms controlling fault behavior and accurate earthquake hazard assessments. Increasing evidence for temporal variations in strain accumulation suggests non-uniform strain rates over a range of historic to geologic timescales. The paucity of long-term records of fault activity has limited our ability to resolve these variations. We present a method for constraining long-term fault activity based on U-Pb dating of fault-related opal from secondary fault segments within the Eastern California Shear Zone (ECSZ). The presence of sheared and breccia-cemented opaline silica within well-exposed faults at near-surface conditions suggest that opal formation is associated with high-magnitude earthquakes capable of surface rupture (>6 M). Temporal constraints from sheared syntectonic opal (n=74) on related secondary faults from this study provide new insights on the timing of fault initiation, reactivation, and longevity. The oldest dates obtained indicate that ECSZ activity commenced at or before 10 Ma. Multiple deformation events dated within a single structure on episodically deposited and sheared opal (up to six generations), demonstrate that fault reactivation occurred over 105 year timescales (0.7-0.1 Ma). Relative probabilities of dated deformation events can be used to evaluate changes in fault activity in the past 2.5 Ma (n=60). This analysis indicates enhanced fault activity starting at 2 Ma and peaking around 1 Ma, possibly due to fault-interactions and distribution of deformation between the ECSZ and the San Andreas Fault.

California↗
USGS2019

Strike-slip fault interactions at Ivanpah Valley, California and Nevada

Ivanpah Valley is flanked by high mountain ranges, and represents one of the most imposing valleys of the eastern Mojave Desert. Its sinuous shape implies a complex origin as does the fact that it is not bordered by prominent range-front normal faults like valleys of the Basin and Range Province. In Addition, its deepest sedimentary basin is restricted to a small part of the valley near Nipton that does not coincide with the lowest part of the valley at Ivanpah Lake. The deep basin was caused by pull-apart at the intersection of two major strike-slip faults, the Stateline and Nipton faults. The northern part of the valley, in Nevada, probably resulted from normal faulting, and much of the normal faulting may have predated the strike-slip faulting. The southern valley, in California, is underlain by bedrock at shallow depths and is of uncertain origin. The dextral Stateline fault terminates at the sinistral Nipton fault, indicating that eastern California shear zone tectonics in this area consists of interwoven synthetic and antithetic faults, rather than through-going strike slip faults of a single orientation.

California, Nevada↗
USGS2019

Distributed fault slip in the eastern California shear zone: Adding pieces to the puzzle near Barstow, California

We investigate the dextral Lockhart and Mt. General faults, which are among four active structures in the northwestern portion of the eastern California shear zone (ECSZ). Early mapping depicts the Lockhart and Mt. General faults as discontinuous fault traces that continue northwest of the Lenwood Fault. Recent work indicates that the Lenwood Fault slips at ~0.2-1.0 mm/yr over the past ~8 ka and 0.8 ± 0.2 mm/yr since ~37 ± 7 ka. We reconstruct the record of fault slip for the Lockhart and Mt. General faults using high-resolution Structure-from-Motion built topography, field observations, geochronology, and gravity data. Geomorphic offsets along a Holocene-active trace of the Lockhart Fault indicate dextral displacement between ~4 and 6 m. A feldspar infrared stimulated luminescence (IRSL) age implies surface abandonment and at least one earthquake after 3540 ± 880 ka (2σ). The implied Holocene fault slip rate on the Lockhart Fault is between ~0.9 and 2.3 mm/yr. Holocene-active traces of the 19-km-long Mt. General Fault are marked by southwest-facing scarps and dextral offsets of ~4–5 m on alluvial fans, with down-to-the-southwest vertical offset of ~0.3 m. Summing dextral displacements across subparallel fault strands yields a maximum of ~7–8 m. A feldspar IRSL age indicates deposition of the alluvial fans since 11,380 ± 1700 ka (2σ). This results in a Holocene slip ~0.3–0.6 mm/yr, possibly ranging up to 1.0 mm/yr. Taken together, these observations imply a net Holocene dextral slip rate for active faults in Hinkley Valley at 1.2–3.3 mm/yr―higher than expected given published fault slip rates along-strike to the southeast.

California↗
USGS2018

Cenozoic geology of Fort Irwin and vicinity, California

The geology of the Fort Irwin National Training Center in the north-central Mojave Desert, California, provides insights into the hydrology and water resources of the area. The Fort Irwin area is underlain by rocks ranging in age from Proterozoic to Quaternary that have been deformed by faults as young as Quaternary. Pre-Tertiary sedimentary, igneous, and metamorphic bedrock and Miocene volcanic and sedimentary rocks are exposed in the mountains and ridges, between which are basins containing Quaternary to Pliocene deposits. During the Miocene, in the western part of Fort Irwin, development of the Eagle Crags volcanic field resulted in a complex assemblage of lava flows, pyroclastic flow and fallout tephra deposits, and volcaniclastic sedimentary rocks that were deposited in alluvial, fluvial, and locally lacustrine environments; in the eastern part of Fort Irwin, epiclastic sedimentary rocks and minor tuffaceous rocks were deposited in alluvial, fluvial, and locally lacustrine environments. In the Pliocene and Quaternary, sandstone and conglomerate were deposited in alluvial and fluvial environments, and locally fine-grained materials were deposited in lacustrine, eolian, playa, and groundwater discharge environments. The Fort Irwin area is transected by Neogene to Holocene northwest- and east-striking (and fewer northeast-striking) strike-slip, normal, and locally thrust faults. Structural blocks between faults are broadly warped, and locally rocks adjacent to the faults are folded and sheared. Many of these faults influenced the formation or modification of basins, especially after about 11 million years, when the Eastern California Shear Zone developed in this area. The three-dimensional geologic framework produced by the late Cenozoic stratigraphic and structural history is represented by the continuity or spatial limitations of lithostratigraphic and correlative hydrogeologic properties. The continuity or limitations of rocks and properties influence how water moved (and moves) through the hydrogeologic system.

California↗
USGS2018

Ecohydrologic changes caused by hydrologic disconnection of ephemeral stream channels in Mojave National Preserve, California

Emplacement of highways and railroads has altered natural hydrologic systems by influencing surface-water flow paths and biotic communities in Mojave National Preserve. Infiltration experiments were conducted along active and abandoned channels to evaluate changes in hydrology and related effects on plant water availability and use. Simulated rainfall infiltration experiments with vegetation monitoring were conducted along an active channel upslope and a comparable abandoned channel down slope of the transportation corridor. We also conducted 90 single-ring, ponded infiltration experiments in adjacent channels to evaluate field-saturated hydraulic conductivity and particle size distributions. The abandoned channels are still morphologically evident, but are disconnected from runoff sources at higher elevations. Infiltration test results show that water infiltrates twice as fast in the active channels. Excavation showed weak soil development with fewer plant roots beneath the abandoned channel. SEM analysis on surface samples showed the presence of cyanobacteria only in abandoned channels. Plants up to three meters away from both channels showed physiological responses to channel water applied in a simulated pulse of rain. The response was short-lived and less pronounced for plants adjacent to the abandoned channel, whereas those adjacent to the active channel showed responses up to two months after the pulse. These responses may explain observed lower plant densities and fewer deep-rooted species along abandoned channels compared to active channels. We infer that the deeper-rooting plants are more abundant where they are able to take advantage of the increased soil-water storage resulting from greater infiltration and flow frequency in active stream channels.

California↗
USGS2018

Geohydrology, geochemistry, and numerical simulation of groundwater flow and land subsidence in the Bicycle Basin, Fort Irwin National Training Center, California

Groundwater pumping from Bicycle Groundwater Basin (referred to as Bicycle Basin) in the Fort Irwin National Training Center, California, began in 1967. From 1967 to December 2010, about 46,000 acre-feet of water had been pumped from the basin and transported to the Irwin Basin. During this time, not only did water levels in the basin decline by as much as 100 feet, the quality of the groundwater pumped from the basin also deteriorated in some wells. The U.S. Geological Survey collected geohydrologic data from existing wells, test holes, and 16 additional monitoring wells installed at 6 sites in Bicycle Basin during 1992–2011 to determine the quantity and quality of groundwater available in the basin. Geophysical surveys, including electrical, gravity, and seismic refraction surveys, were completed to help determine the geometry of the structural basin, delineate depths to the interface between Quaternary and Tertiary rocks, map the depth to the water table, and used to develop a geohydrologic framework and groundwater-flow model for Bicycle Basin. Water samples were used to determine the groundwater quality in the basin and to delineate potential sources of poor-quality groundwater. Analysis of stable isotopes of oxygen and hydrogen in groundwater indicated that presentday precipitation is not a major source of recharge to the basin. Tritium and carbon-14 data indicated that most of the groundwater in the basin was recharged prior to 1952 and had an apparent age of 15,625–39,350 years. Natural recharge to the basin was not sufficient to replenish the groundwater pumped from the basin. Interferograms from synthetic aperture radar data (InSAR), analyzed to evaluate land-surface subsidence between 1993 and 2010, showed 0.23 to 1.1 feet of subsidence during this period near one production well north of Bicycle Lake (dry) playa. A groundwater-flow model of Bicycle Basin was developed and calibrated using groundwater levels for 1964– 2010, and a subsidence model using land-surface deformation data for 1993–2010. Between January 1967 and December 2010, the simulated total recharge from precipitation runoff and underflow from adjacent basins was about 5,100 acre-feet and pumpage from the Bicycle Basin was about 47,000 acrefeet of water. Total outflows exceeded natural recharge during this period, resulting in a net loss of about 42,100 acre-feet of groundwater storage in the basin. The Fort Irwin National Training Center is considering various groundwater-management options in the Bicycle Basin. The groundwater-flow model was used to (1) evaluate changes in groundwater levels and subsidence with the addition of capture and recharge of simulated runoff in retention basins (scenario 1) for predevelopment through 2010; (2) simulate a base case (scenario 2) for reference; and (3) compare projections of alternative future pumping strategies for 2011–60 (scenarios 3–5). Model results from the runoff-capture simulation (scenario 1) indicated that total recharge, including runoff captured using retention basins, locally increased water levels, which partially offset, but did not mitigate, groundwater depletion associated with pumping. Groundwater-storage depletion in scenario 1 was about 14 percent less than without runoff capture. Simulated-drawdown results in model layer 1 in the eastern part of the basin indicated that, because of the captured runoff, simulated heads were as much as 100 feet higher in December 2010 than prior to the onset of development in 1967. In contrast, simulated drawdown for model without runoff capture indicated that, without captured runoff, simulated heads for December 2010 in this area were 80–90 feet lower than during the predevelopment period. Subsidence was mitigated slightly in scenario 1 compared to without runoff capture; the largest decrease in subsidence at observation sites was about 0.07 feet.

California↗
USGS2018

Middle Pleistocene infill of Hinkley Valley by Mojave River sediment and associated lake sediment: Depositional architecture and deformation by strike-slip faults

Hinkley Valley in the Mojave Desert, near Barstow about 140 km northeast of Los Angeles and midway between Victorville Valley and the Lake Manix basin, contains a thick sedimentary sequence delivered by the Mojave River. Our study of sediment cores drilled in the valley indicates that Hinkley Valley was probably a closed playa basin with stream inflow from four directions prior to Mojave River inflow. The Mojave River deposited thick and laterally extensive clastic wedges originating from the southern valley that rapidly filled much of Hinkley Valley. Sedimentary facies representing braided stream, wetland, delta, and lacustrine depositional environments all are found in the basin fill; in some places, the sequence is greater than 74 m (245 ft) thick. The sediment is dated in part by the presence of the ~631 ka Lava Creek B ash bed low in the section, and thus represents sediment deposition after Victorville basin was overtopped by sediment and before the Manix basin began to be filled. Evidently, upstream Victorville basin filled with sediment by about 650 ka, causing the ancestral Mojave River to spill to the Harper and Hinkley basins, and later to Manix basin. Initial river sediment overran wetland deposits in many places in southern Hinkley Valley, indicating a rapidly encroaching river system. These sediments were succeeded by a widespread lake (“blue” clay) that includes the Lava Creek B ash bed. Above the lake sediment lies a thick section of interlayered stream sediment, delta and nearshore lake sediment, mudflat and/or playa sediment, and minor lake sediment. This stratigraphic architecture is found throughout the valley, and positions of lake sediment layers indicate a successive northward progression in the closed basin. A thin overlapping sequence at the north end of the valley contains evidence for a younger late Pleistocene lake episode. This late lake episode, and bracketing braided stream deposits of the Mojave River, indicate that the river avulsed through the valley, rather than continuing toward Lake Manix, during the late Pleistocene. Two dextral strike-slip fault zones, the Lockhart and the Mt. General, fold and displace the distinctive stratigraphic units, as well as surficial late Pleistocene and Holocene deposits. The sedimentary architecture and the two fault zones provide a framework for evaluating groundwater flow in Hinkley Valley.

Conference Paper↗
USGS2018

Against the current— The Mojave River from sink to source: The 2018 Desert Symposium field trip road log

The Mojave River evolved over the past few million years by “fill and spill” from upper basins near its source in the Transverse Ranges to lower basins. Each newly “spilled into” basin in the series? sustained a long-lived lake but gradually filled with Mojave River sediment, leading to spill to a yet lower elevation? basin. The Mojave River currently terminates at Silver Lake, near Baker, CA, but previously overflowed this terminus onward to Lake Manly in Death Valley during the last glacial cycle. The river’s origin and evolution are intricately interwoven with tectonic, climatic, and geomorphic processes through time, beginning with San Andreas fault interactions that created a mountain range across a former externally draining river. We will see and discuss the Mojave River’s predecessor streams and basins, its evolution as it lengthened to reach the central Mojave Desert, local and regional tectonic controls, groundwater flow, flood history, and support of isolated perennial stream reaches that host endemic species. In association with these subjects are supporting studies such as paleoclimate records, location and timing for groundwater and wetlands in the central Mojave Desert, and effects of modern water usage. The trip introduces new findings for the groundwater basin of Hinkley Valley, including an ongoing remediation project that provides a wealth of information on past and present river flow and associated development of the groundwater system.

California↗
USGS2018

Middle Pleistocene infill of Hinkley Valley by Mojave River sediment and associated lake sediment: Depositional architecture and deformation by strike-slip faults

Hinkley Valley in the Mojave Desert, near Barstow about 140 km northeast of Los Angeles and midway between Victorville Valley and the Lake Manix basin, contains a thick sedimentary sequence delivered by the Mojave River. Our study of sediment cores drilled in the valley indicates that Hinkley Valley was probably a closed playa basin with stream inflow from four directions prior to Mojave River inflow. The Mojave River deposited thick and laterally extensive clastic wedges originating from the southern valley that rapidly filled much of Hinkley Valley. Sedimentary facies representing braided stream, wetland, delta, and lacustrine depositional environments all are found in the basin fill; in some places, the sequence is greater than 74 m (245 ft) thick. The sediment is dated in part by the presence of the ~631 ka Lava Creek B ash bed low in the section, and thus represents sediment deposition after Victorville basin was overtopped by sediment and before the Manix basin began to be filled. Evidently, upstream Victorville basin filled with sediment by about 650 ka, causing the ancestral Mojave River to spill to the Harper and Hinkley basins, and later to Manix basin. Initial river sediment overran wetland deposits in many places in southern Hinkley Valley, indicating a rapidly encroaching river system. These sediments were succeeded by a widespread lake (“blue” clay) that includes the Lava Creek B ash bed. Above the lake sediment lies a thick section of interlayered stream sediment, delta and nearshore lake sediment, mudflat and/or playa sediment, and minor lake sediment. This stratigraphic architecture is found throughout the valley, and positions of lake sediment layers indicate a successive northward progression in the closed basin. A thin overlapping sequence at the north end of the valley contains evidence for a younger late Pleistocene lake episode. This late lake episode, and bracketing braided stream deposits of the Mojave River, indicate that the river avulsed through the valley, rather than continuing toward Lake Manix, during the late Pleistocene. Two dextral strike-slip fault zones, the Lockhart and the Mt. General, fold and displace the distinctive stratigraphic units, as well as surficial late Pleistocene and Holocene deposits. The sedimentary architecture and the two fault zones provide a framework for evaluating groundwater flow in Hinkley Valley.

Hinkley Valley↗
USGS2017

Geologic map of the Providence Mountains in parts of the Fountain Peak and adjacent 7.5' quadrangles, San Bernardino County, California

Introduction The Providence Mountains are in the eastern Mojave Desert about 60 km southeast of Baker, San Bernardino County, California. This range, which is noted for its prominent cliffs of Paleozoic limestone, is part of a northeast-trending belt of mountainous terrain more than 100 km long that also includes the Granite Mountains, Mid Hills, and New York Mountains. Providence Mountains State Recreation Area encompasses part of the range, the remainder of which is within Mojave National Preserve, a large parcel of land administered by the National Park Service. Access to the Providence Mountains is by secondary roads leading south and north from Interstate Highways 15 and 40, respectively, which bound the main part of Mojave National Preserve. The geologic map presented here includes most of Providence Mountains State Recreation Area and land that surrounds it on the north, west, and south. This area covers most of the Fountain Peak 7.5′ quadrangle and small adjacent parts of the Hayden quadrangle to the north, the Columbia Mountain quadrangle to the northeast, and the Colton Well quadrangle to the east. The map area includes representative outcrops of most of the major geologic elements of the Providence Mountains, including gneissic Paleoproterozoic basement rocks, a thick overlying sequence of Neoproterozoic to Triassic sedimentary rocks, Jurassic rhyolite that intrudes and overlies the sedimentary rocks, Jurassic plutons and associated dikes, Miocene volcanic rocks, and a variety of Quaternary surficial deposits derived from local bedrock units. The purpose of the project was to map the area in detail, with primary emphasis on the pre-Quaternary units, to provide an improved stratigraphic, structural, and geochronologic framework for use in land management applications and scientific research.

California↗
USGS2017

Directly dated MIS 3 lake-level record from Lake Manix, Mojave Desert, California, USA

An outcrop-based lake-level curve, constrained by ~ 70 calibrated 14 C ages on Anodonta shells, indicates at least 8 highstands between 45 and 25 cal ka BP within 10 m of the 543-m upper threshold of Lake Manix in the Mojave Desert of southern California. Correlations of Manix highstands with ice, marine, and speleothem records suggest that at least the youngest three highstands coincide with Dansgaard–Oeschger (D–O) stadials and Heinrich events 3 and 4. The lake-level record is consistent with results from speleothem studies in the Southwest that indicate cool wet conditions during D–O stadials. Notably, highstands between 43 and 25 ka apparently occurred at times of generally low levels of pluvial lakes farther north as interpreted from core-based proxies. Mojave lakes may have been supported by tropical moisture sources during oxygen-isotope stage 3, perhaps controlled by southerly deflection of Pacific storm tracks due to weakening of the sea-surface temperature gradient in response to North Atlantic climate perturbations.

California↗
USGS2017

Early Holocene Great Salt Lake

Shorelines and surficial deposits (including buried forest-floor mats and organic-rich wetland sediments) show that Great Salt Lake did not rise higher than modern lake levels during the earliest Holocene (11.5–10.2 cal ka BP; 10–9 14 C ka BP). During that period, finely laminated, organic-rich muds (sapropel) containing brine-shrimp cysts and pellets and interbedded sodium-sulfate salts were deposited on the lake floor. Sapropel deposition was probably caused by stratification of the water column — a freshwater cap possibly was formed by groundwater, which had been stored in upland aquifers during the immediately preceding late-Pleistocene deep-lake cycle (Lake Bonneville), and was actively discharging on the basin floor. A climate characterized by low precipitation and runoff, combined with local areas of groundwater discharge in piedmont settings, could explain the apparent conflict between evidence for a shallow lake (a dry climate) and previously published interpretations for a moist climate in the Great Salt Lake basin of the eastern Great Basin.

Utah↗
USGS2017

Models of invasion and establishment of African Mustard ( Brassica tournefortii )

Introduced exotic plants can drive ecosystem change. We studied invasion and establishment of Brassica tournefortii (African mustard), a noxious weed, in the Chemehuevi Valley, western Sonoran Desert, California. We used long-term data sets of photographs, transects for biomass of annual plants, and densities of African mustard collected at irregular intervals between 1979 and 2009. We suggest that African mustard may have been present in low numbers along the main route of travel, a highway, in the late 1970s; invaded the valley along a major axial valley ephemeral stream channel and the highway; and by 2009, colonized 22 km into the eastern part of the valley. We developed predictive models for invasibility and establishment of African mustard. Both during the initial invasion and after establishment, significant predictor variables of African mustard densities were surficial geology, proximity to the highway and axial valley ephemeral stream channel, and number of small ephemeral stream channels. The axial valley ephemeral stream channel was the most vulnerable of the variables to invasions. Overall, African mustard rapidly colonized and quickly became established in naturally disturbed areas, such as stream channels, where geological surfaces were young and soils were weakly developed. Older geological surfaces (e.g., desert pavements with soils 140,000 to 300,000 years old) were less vulnerable. Microhabitats also influenced densities of African mustard, with densities higher under shrubs than in the interspaces. As African mustard became established, the proportional biomass of native winter annual plants declined. Early control is important because African mustard can colonize and become well established across a valley in 20 yr.

California↗
USGS2017

Water-resources and land-surface deformation evaluation studies at Fort Irwin National Training Center, Mojave Desert, California

The U.S. Army Fort Irwin National Training Center (NTC), in the Mojave Desert, obtains all of its potable water supply from three groundwater basins (Irwin, Langford, and Bicycle) within the NTC boundaries (fig. 1; California Department of Water Resources, 2003). Because of increasing water demands at the NTC, the U.S. Geological Survey (USGS), in cooperation with the U.S. Army, completed several studies to evaluate water resources in the developed and undeveloped groundwater basins underlying the NTC. In all of the developed basins, groundwater withdrawals exceed natural recharge, resulting in water-level declines. However, artificial recharge of treated wastewater has had some success in offsetting water-level declines in Irwin Basin. Additionally, localized water-quality changes have occurred in some parts of Irwin Basin as a result of human activities (i.e., wastewater disposal practices, landscape irrigation, and/or leaking pipes). As part of the multi-faceted NTC-wide studies, traditional datacollection methods were used and include lithological and geophysical logging at newly drilled boreholes, hydrologic data collection (i.e. water-level, water-quality, aquifer tests, wellbore flow). Because these data cover a small portion of the 1,177 square-mile (mi2 ) NTC, regional mapping, including geologic, gravity, aeromagnetic, and InSAR, also were done. In addition, ground and airborne electromagnetic surveys were completed and analyzed to provide more detailed subsurface information on a regional, base-wide scale. The traditional and regional ground and airborne data are being analyzed and will be used to help develop preliminary hydrogeologic framework and groundwater-flow models in all basins. This report is intended to provide an overview of recent water-resources and land-surface deformation studies at the NTC.

California↗
USGS2017

Connecting the Soda–Avawatz and Bristol–Granite Mountains faults with gravity andaeromagnetic data, Mojave Desert, California

The Soda–Avawatz and Bristol–Granite Mountains faults are considered by some to form the northeastern margin of the eastern California shear zone yet their connectivity and extents are obscured by surficial deposits and the estimates of total right-lateral offset from geologic data range from 0 to as much as 24 km. We use gravity and recently released detailed aeromagnetic data to map strands of these faults, examine structure within the fault zones and provide estimates of right-lateral offset. Gradients in gravity and aeromagnetic data define physical property contrasts that coincide with mapped strands of the faults and allow for extension of these faults, where concealed, to indicate continuity between the Soda–Avawatz and Bristol–Granite Mountains faults. Gravity data reveal local tectonic basins west of Silver Lake, beneath Soda Lake, and southwest of the Marble Mountains that are approximately 9–15 km long, 3–5 km wide, and 1–1.5 km deep. The basins are located where the local fault traces strike more northerly than the overall fault zone strike, suggesting that these basins are transtensional (pull-apart). If the lengths of these basins can be used as a proxy for rightlateral offset, the Soda–Avawatz and Bristol–Granite Mountains faults may have up to 9–15 km of post-early Miocene offset, consistent with our offset estimates from correlative magnetic anomalies across the fault zone.

California↗