Three-dimensional geologic map of the San Andreas Fault Zone between Gold Hill and Pinnacles National Park, California
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Scientific Investigations Map↗
Geology topics
Darcy McPhee
Publications and source records attributed to Darcy McPhee.
Renewing the National Cooperative Geologic Mapping Program as the Nation’s authoritative source for modern geologic knowledge
This document presents the renewed vision, mission, and goals for the National Cooperative Geologic Mapping Program (NCGMP). The NCGMP, as authorized by the National Cooperative Geologic Mapping Act of 1992 (Public Law 102-285, 106 Stat. 166 and its reauthorizations), is tasked with expediting the production of a geologic database for the Nation based on modern geologic maps and their supporting data. In addition to highlighting the benefits of geologic maps for economic prosperity, national security, and environmental quality, the report describes the NCGMP structure and components. A renewed vision and mission for the NCGMP are stated, and three goals for guiding the program toward that vision for the next ten years are established. The vision of creating an integrated, three-dimensional, digital geologic map of the United States and its territories to address the changing needs of the Nation by 2030 is thereby defined to drive the activities of all NCGMP components for the next ten years. The strategic actions required to realize the NCGMP vision are identified for each of its components.
Open-File Report↗
Assessment of a claimed ultra-low frequency electromagnetic (ULFEM) earthquake precursor
The rate of occurrence of anomalous ultra-low frequency electromagnetic (ULFEM) pulses has been claimed to have increased days to weeks prior to the M5.4 2007 and M4.0 2010 Alum Rock earthquakes. We re-examine the previously reported ultra-low frequency (ULF: 0.01–10 Hz) magnetic data recorded at a QuakeFinder site located 9 km from the earthquake hypocentre, and compare to data from a nearby Stanford-USGS site located 42 km from the hypocentre, to analyse the characteristics of the pulses and assess their origin. Using pulse definitions and pulse-counting algorithms analogous to those previously reported, we corroborate the increase in pulse counts before the 2007 Alum Rock earthquake at the QuakeFinder station, but we note that the number of pulses depends on chosen temporal and amplitude detection thresholds. These thresholds are arbitrary because we lack a clear physical model or basis for their selection. We do not see the same increase in pulse counts before the 2010 Alum Rock earthquake at the QuakeFinder or Stanford-USGS stations. In addition, the majority of pulses in the QuakeFinder data and Stanford-USGS data do not match temporally, indicating the pulses lack a common origin and are not from lightning or solar-driven ionospheric/magnetospheric disturbances. Our assessment of the temporal distribution of pulse counts shows pulse counts increase during peak human activity hours, suggesting these pulses result from local cultural noise and are not tectonic in origin. The many unknowns about the character and even existence of precursory earthquake pulses means that standard numerical and statistical tests cannot easily be applied. Yet here we show that exhaustive investigation of many different aspects of ULFEM signals can be used to properly characterize their origin.
California↗
U.S. Geological Survey STATEMAP Program—Geologic mapping for the public good
As of 2020, STATEMAP has invested more than $150 million in 48 State geological surveys, matched dollar for dollar, to complete geologic mapping projects crucial to the health and security of State natural resources and residents. For more information about STATEMAP and other geologic mapping efforts supported by the National Cooperative Geologic Mapping Program, visit https://ncgmp.usgs.gov .
Fact Sheet↗
U.S. Geological Survey EDMAP Program—Training the next generation of geologic mappers
Detailed geologic maps are the basis of nearly every Earth-science investigation and can be used for natural hazard mitigation, resource identification and exploration, infrastructure planning, and more. A component of the congressionally mandated National Cooperative Geologic Mapping Program, EDMAP is a partnership among the U.S. Geological Survey, the Association of American State Geologists, and participating colleges and universities that provides mentorship and training opportunities to geology students nationwide. Under the guidance of a faculty member, EDMAP supports upper level undergraduate and graduate students to gain meaningful experience working on 1-year geologic-mapping projects. Between 1996 and 2019, EDMAP funded research projects for more than 1,200 students at more than 160 universities. Every Federal dollar awarded through the EDMAP program is matched by the student’s university.
Fact Sheet↗
Three-dimensional electrical resistivity model of the hydrothermal system in Long Valley Caldera, California, from magnetotellurics
Though shallow flow of hydrothermal fluids in Long Valley Caldera, California, has been well studied, neither the hydrothermal source reservoir nor heat source has been well characterized. Here a grid of magnetotelluric data were collected around the Long Valley volcanic system and modeled in 3-D. The preferred electrical resistivity model suggests that the source reservoir is a narrow east-west elongated body 4 km below the west moat. The heat source could be a zone of 2–5% partial melt 8 km below Deer Mountain. Additionally, a collection of hypersaline fluids, not connected to the shallow hydrothermal system, is found 3 km below the medial graben, which could originate from a zone of 5–10% partial melt 8 km below the south moat. Below Mammoth Mountain is a 3 km thick isolated body containing fluids and gases originating from an 8 km deep zone of 5–10% basaltic partial melt.
California↗
Imaging the magmatic system of Mono Basin, California with magnetotellurics in three--dimensions
A three–dimensional (3D) electrical resistivity model of Mono Basin in eastern California unveils a complex subsurface filled with zones of partial melt, fluid–filled fracture networks, cold plutons, and regional faults. In 2013, 62 broadband magnetotelluric (MT) stations were collected in an array around southeastern Mono Basin from which a 3D electrical resistivity model was created with a resolvable depth of 35 km. Multiple robust electrical resistivity features were found that correlate with existing geophysical observations. The most robust features are two 300 ± 50 km3 near-vertical conductive bodies (3–10 Ω·m) that underlie the southeast and north-eastern margin of Mono Craters below 10 km depth. These features are interpreted as magmatic crystal–melt mush zones of 15 ± 5% interstitial melt surrounded by hydrothermal fluids and are likely sources for Holocene eruptions. Two conductive east–dipping structures appear to connect each magma source region to the surface. A conductive arc–like structure (< 0.9 Ω·m) links the northernmost mush column at 10 km depth to just below vents near Panum Crater, where the high conductivity suggests the presence of hydrothermal fluids. The connection from the southernmost mush column at 10 km depth to below South Coulée is less obvious with higher resistivity (200 Ω·m) suggestive of a cooled connection. A third, less constrained conductive feature (4–10 Ω·m) 15 km deep extending to 35 km is located west of Mono Craters near the eastern front of the Sierra Nevada escarpment, and is coincident with a zone of sporadic, long–period earthquakes that are characteristic of a fluid-filled (magmatic or metamorphic) fracture network. A resistive feature (103–105 Ω·m) located under Aeolian Buttes contains a deep root down to 25 km. The eastern edge of this resistor appears to structurally control the arcuate shape of Mono Craters. These observations have been combined to form a new conceptual model of the magmatic system beneath Mono Craters to a depth of 30 km.
California↗
Summer of Applied Geophysical Experience (SAGE): Training for our future geoscientists
Energy and natural resources are crucial to the sustainability of worldwide economies, security, and overall well-being. However, the future workforce in the energy and natural-resources sector is at risk, and meeting the challenges of this dwindling workforce requires well-educated geoscientists in exploration and applied geophysics and related geoscience and technology disciplines. Programs such as geophysical field courses that are supported by SEG and industry, in partnership with academic institutions and government laboratories, are important approaches to maintaining and enhancing expertise in exploration geophysics. One example of a geophysical field course devoted to educating our future workforce is the Summer of Applied Geophysical Experience (SAGE), a four-week program based in Santa Fe, New Mexico, designed to actively engage students in all phases of applied geophysical research. SAGE is a unique educational experience that combines teaching and research as a partnership among universities, industry, government agencies, and professional societies. SAGE teaches the principles and applications of refraction and reflection seismology, magnetics, gravity, GPS, heat flow, several electromagnetic (EM) methods, and ground-penetrating radar (GPR) in a field-based, hands-on setting. More than 850 students and qualified professionals have attended SAGE, many of whom have gone on to become leaders in academia, industry, and government. SAGE students are exposed to the exciting challenges that face earth scientists today, and they develop skills that are necessary to address the world's growing energy demands. Examples of SAGE research projects include mapping archaeological sites and tectonic structure and investigating water and geothermal resources in the Rio Grande rift.
Leading Edge (Tulsa, OK)↗
Eruptions at Lone Star geyser, Yellowstone National Park, USA: 2. Constraints on subsurface dynamics
We use seismic, tilt, lidar, thermal, and gravity data from 32 consecutive eruption cycles of Lone Star geyser in Yellowstone National Park to identify key subsurface processes throughout the geyser's eruption cycle. Previously, we described measurements and analyses associated with the geyser's erupting jet dynamics. Here we show that seismicity is dominated by hydrothermal tremor (~5–40 Hz) attributed to the nucleation and/or collapse of vapor bubbles. Water discharge during eruption preplay triggers high-amplitude tremor pulses from a back azimuth aligned with the geyser cone, but during the rest of the eruption cycle it is shifted to the east-northeast. Moreover, ~4 min period ground surface displacements recur every 26 ± 8 min and are uncorrelated with the eruption cycle. Based on these observations, we conclude that (1) the dynamical behavior of the geyser is controlled by the thermo-mechanical coupling between the geyser conduit and a laterally offset reservoir periodically filled with a highly compressible two-phase mixture, (2) liquid and steam slugs periodically ascend into the shallow crust near the geyser system inducing detectable deformation, (3) eruptions occur when the pressure decrease associated with overflow from geyser conduit during preplay triggers an unstable feedback between vapor generation (cavitation) and mass discharge, and (4) flow choking at a constriction in the conduit arrests the runaway process and increases the saturated vapor pressure in the reservoir by a factor of ~10 during eruptions.
Wyoming↗
Structural controls on geothermal circulation in Surprise Valley, California: A re-evaluation of the Lake City fault zone
Faults and fractures play an important role in the circulation of geothermal fluids in the crust, and the nature of that role varies according to structural setting and state of stress. As a result, detailed geologic and geophysical mapping that relates thermal springs to known structural features is essential to modeling geothermal systems. Published maps of Surprise Valley in northeastern California suggest that the “Lake City fault” or “Lake City fault zone” is a significant structural feature, cutting obliquely across the basin and connecting thermal springs across the valley. Newly acquired geophysical data (audio-magnetotelluric, gravity, and magnetic), combined with existing geochemical and geological data, suggest otherwise. We examine potential field profiles and resistivity models that cross the mapped Lake City fault zone. While there are numerous geophysical anomalies that suggest subsurface structures, they mostly do not coincide with the mapped traces of the Lake City fault zone, nor do they show a consistent signature in gravity, magnetics, or resistivities that would suggest a through-going fault that would promote connectivity through lateral fluid flow. Instead of a single, continuous fault, we propose the presence of a deformation zone associated with the growth of the range-front Surprise Valley fault. The implication for geothermal circulation is that this is a zone of enhanced porosity but lacks length-wise connectivity that could conduct fluids across the valley. Thermal fluid circulation is most likely controlled primarily by interactions between N-S–trending normal faults.
California↗
Tectonic evolution of the Tualatin basin, northwest Oregon, as revealed by inversion of gravity data
The Tualatin basin, west of Portland (Oregon, USA), coincides with a 110 mGal gravity low along the Puget-Willamette lowland. New gravity measurements (n = 3000) reveal a three-dimensional (3-D) subsurface geometry suggesting early development as a fault-bounded pull-apart basin. A strong northwest-trending gravity gradient coincides with the Gales Creek fault, which forms the southwestern boundary of the Tualatin basin. Faults along the northeastern margin in the Portland Hills and the northeast-trending Sherwood fault along the southeastern basin margin are also associated with gravity gradients, but of smaller magnitude. The gravity low reflects the large density contrast between basin fill and the mafic crust of the Siletz terrane composing basement. Inversions of gravity data indicate that the Tualatin basin is ∼6 km deep, therefore 6 times deeper than the 1 km maximum depth of the Miocene Columba River Basalt Group (CRBG) in the basin, implying that the basin contains several kilometers of low-density pre-CRBG sediments and so formed primarily before the 15 Ma emplacement of the CRBG. The shape of the basin and the location of parallel, linear basin-bounding faults along the southwest and northeast margins suggest that the Tualatin basin originated as a pull-apart rhombochasm. Pre-CRBG extension in the Tualatin basin is consistent with an episode of late Eocene extension documented elsewhere in the Coast Ranges. The present fold and thrust geometry of the Tualatin basin, the result of Neogene compression, is superimposed on the ancestral pull-apart basin. The present 3-D basin geometry may imply stronger ground shaking along basin edges, particularly along the concealed northeast edge of the Tualatin basin beneath the greater Portland area.
Oregon↗
Geophysical investigations of the geologic and hydrothermal framework of the Pilgrim Springs Geothermal Area, Alaska
Pilgrim Hot Springs, located on the Seward Peninsula in west-central Alaska, is characterized by hot springs, surrounding thawed regions, and elevated lake temperatures. The area is of interest because of its potential for providing renewable energy for Nome and nearby rural communities. We performed ground and airborne geophysical investigations of the Pilgrim Springs geothermal area to identify areas indicative of high heat flow and saline geothermal fluids, and to map key structures controlling hydrothermal fluid flow. Studies included ground gravity and magnetic measurements, as well as an airborne magnetic and frequency-domain electromagnetic (EM) survey. The structural and conceptual framework developed from this study provides critical information for future development of this resource and is relevant more generally to our understanding of geothermal systems in active extensional basins. Potential field data reveal the Pilgrim area displays a complex geophysical fabric reflecting a network of intersecting fault and fracture sets ranging from inherited basement structures to Tertiary faults. Resistivity models derived from the airborne EM data reveal resistivity anomalies in the upper 100 m of the subsurface that suggest elevated temperatures and the presence of saline fluids. A northwest trending fabric across the northeastern portion of the survey area parallels structures to the east that may be related to accommodation between the two major mountain ranges south (Kigluaik) and east (Bendeleben) of Pilgrim Springs. The area from the springs southward to the range front, however, is characterized by east-west trending, range-front-parallel anomalies likely caused by late Cenozoic structures associated with north-south extension that formed the basin. The area around the springs (~10 km2 ) is coincident with a circular magnetic high punctuated by several east-west trending magnetic lows, the most prominent occurring directly over the springs. These features possibly result from hydrothermal alteration imposed by fluids migrating along intra-basin faults related to recent north-south extension. The Pilgrim River valley is characterized by a NE-elongate gravity low that reveals a basin extending to depths of ~300 m beneath Pilgrim Springs and deepening to ~800 m to the southwest. The margins of the gravity low are sharply defined by northeasttrending gradients that probably reflect the edges of fault-bounded structural blocks. The southeastern edge of the low, which lies very close to the springs, also corresponds with prominent NE-striking anomalies seen in magnetic and resistivity models. Together, these features define a structure we refer to as the Northeast Fault. The location of the hot springs appears to be related to the intersection of the Northeast Fault with a N-oriented structure marked by the abrupt western edge of a resistivity low surrounding the hot springs. While the hot springs represent the primary outflow of geothermal fluids, additional outflow extends from the springs northeast along the Northeast fault to another thaw zone that we interpret to be a secondary region of concentrated upflow of geothermal fluids. The Northeast Fault apparently controls shallow geothermal fluid flow, and may also provide an important pathway conveying deep fluids to the shallow subsurface. We suggest that geothermal fluids may derive from a reservoir residing beneath the sediment basin southwest of the springs. If so, the shape of the basin, which narrows and shallows towards the springs, may funnel fluids beneath the springs where they intersect the Northeast Fault allowing them to reach the surface. An alternative pathway for reservoir fluids to reach intermediate to shallow depths may be afforded by the main Kigluaik range front fault that coincides with a resistivity anomaly possibly resulting from fluid flow and associated hydrothermal mineralization occurring within the fault zone.
Conference Paper↗
Superimposed extension and shortening in the southern Salinas Basin and La Panza Range, California: A guide to Neogene deformation in the Salinian block of the central California Coast Ranges
We synthesized data from geologic maps, wells, seismic-reflection profiles, potential-field interpretations, and low-temperature thermochronology to refine our understanding of late Cenozoic extension and shortening in the Salinian block of the central California Coast Ranges. Data from the La Panza Range and southern Salinas Basin document early to middle Miocene extension, followed by Pliocene and younger shortening after a period of little deformation in the late Miocene. Extension took place on high-angle normal faults that accommodated ∼2% strain at the scale of the ∼50-km-wide Salinian block (oriented perpendicular to the San Andreas fault). Shortening was accommodated by new reverse faults, reactivation of older normal faults, and strike-slip faulting that resulted in a map-view change in the width of the Salinian block. The overall magnitude of shortening was ∼10% strain, roughly 4–5 times greater than the amount of extension. The timing and magnitude of deformation in our study area are comparable to that documented in other Salinian block basins, and we suggest that the entire block deformed in a similar manner over a similar time span. The timing and relative magnitude of extension and shortening may be understood in the context of central Coast Range tectonic boundary conditions linked to rotation of the western Transverse Ranges at the south end of the Salinian block. Older models for Coast Range shortening based on balanced fault-bend fold-style cross sections are a poor approximation of Salinian block deformation, and may lead to mechanically improbable fault geometries that overestimate the amount of shortening.
California↗
Application and evaluation of electromagnetic methods for imaging saltwater intrusion in coastal aquifers: Seaside Groundwater Basin, California
Developing effective resource management strategies to limit or prevent saltwater intrusion as a result of increasing demands on coastal groundwater resources requires reliable information about the geologic structure and hydrologic state of an aquifer system. A common strategy for acquiring such information is to drill sentinel wells near the coast to monitor changes in water salinity with time. However, installation and operation of sentinel wells is costly and provides limited spatial coverage. We studied the use of noninvasive electromagnetic (EM) geophysical methods as an alternative to installation of monitoring wells for characterizing coastal aquifers. We tested the feasibility of using EM methods at a field site in northern California to identify the potential for and/or presence of hydraulic communication between an unconfined saline aquifer and a confined freshwater aquifer. One-dimensional soundings were acquired using the time-domain electromagnetic (TDEM) and audiomagnetotelluric (AMT) methods. We compared inverted resistivity models of TDEM and AMT data obtained from several inversion algorithms. We found that multiple interpretations of inverted models can be supported by the same data set, but that there were consistencies between all data sets and inversion algorithms. Results from all collected data sets suggested that EM methods are capable of reliably identifying a saltwater-saturated zone in the unconfined aquifer. Geophysical data indicated that the impermeable clay between aquifers may be more continuous than is supported by current models.
California↗
ULFEM time series analysis package
This manual describes how to use the Ultra-Low-Frequency ElectroMagnetic (ULFEM) software package. Casual users can read the quick-start guide and will probably not need any more information than this. For users who may wish to modify the code, we provide further description of the routines.
Open-File Report↗
Resistivity imaging in eastern Nevada Using the audiomagnetotelluric method for hydrogeologic framework studies
Inversion of audiomagnetotelluric (AMT) sounding data collected in eastern Nevada shows significant structure within the upper kilometer of the subsurface that defines the geologic framework from which hydrologic models will be developed. We collected AMT data along two profiles in Spring and Cave valleys in 2004–2005, using the Geometrics StrataGem EH4 system, a four‐channel, natural and controlled‐source tensor system recording in the range of 10–92,000 Hz. Profiles were 12 and 3 km in length with station spacing of 200–400 m. Two‐dimensional inverse models show detailed structure within the alluvial basin including clear transitions between unsaturated and saturated alluvium/volcanic rocks, highly‐resistive (>1000 ohm‐m) carbonate rocks, and the locations of range‐front and intra‐basin faults. In addition, our results define the shape of and the depth to the basement surface, which correlates well with depth to basement estimates derived from the inversion of gravity data.
Conference Paper↗