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Thomas G. Hildenbrand

Publications and source records attributed to Thomas G. Hildenbrand.

15 recordsLinked to original sources
USGS2011

Unique U.S. magnetic anomaly data base forthcoming

The year 2004 will offer an exciting and cost-effective opportunity to acquire a new U.S. magnetic anomaly data base. High Altitude Mapping Missions Inc. (HAMM) is currently planning an airborne mission to collect high-resolution Interferometric Synthetic Aperture Radar (IFSAR) imagery at an altitude of about 15 km, with a flight-line spacing of about 14 km over the conterminous United States and Alaska. Total and vector magnetic field data will also be collected with a “piggy-back” magnetometer system as a secondary mission objective. Because HAMM would fund the main flight costs of the mission, the geomagnetic community would acquire invaluable magnetic data at a nominal cost. These unique data will provide new insights on fundamental tectonic and thermal processes and give a new view of the structural and lithologic framework of continental areas and offshore regions.

Eos Science News↗
USGS2011

Control of Precambrian basement deformation zones on emplacement of the Laramide Boulder batholith and Butte mining district, Montana, United States

What are the roles of deep Precambrian basement deformation zones in the localization of subsequent shallow-crustal deformation zones and magmas? The Paleoproterozoic Great Falls tectonic zone and its included Boulder batholith (Montana, United States) provide an opportunity to examine the importance of inherited deformation fabrics in batholith emplacement and the localization of magmatic-hydrothermal mineral deposits. Northeast-trending deformation fabrics predominate in the Great Falls tectonic zone, which formed during the suturing of Paleoproterozoic and Archean cratonic masses approximately 1,800 mega-annum (Ma). Subsequent Mesoproterozoic to Neoproterozoic deformation fabrics trend northwest. Following Paleozoic through Early Cretaceous sedimentation, a Late Cretaceous fold-and-thrust belt with associated strike-slip faulting developed across the region, wherein some Proterozoic faults localized thrust faulting, while others were reactivated as strike-slip faults. The 81- to 76-Ma Boulder batholith was emplaced along the reactivated central Paleoproterozoic suture in the Great Falls tectonic zone. Early-stage Boulder batholith plutons were emplaced concurrent with east-directed thrust faulting and localized primarily by northwest-trending strike-slip and related faults. The late-stage Butte Quartz Monzonite pluton was localized in a northeast-trending pull-apart structure that formed behind the active thrust front and is axially symmetric across the underlying northeast-striking Paleoproterozoic fault zone, interpreted as a crustal suture. The modeling of potential-field geophysical data indicates that pull-apart?stage magmas fed into the structure through two funnel-shaped zones beneath the batholith. Renewed magmatic activity in the southern feeder from 66 to 64 Ma led to the formation of two small porphyry-style copper-molybdenum deposits and ensuing world-class polymetallic copper- and silver-bearing veins in the Butte mining district. Vein orientations parallel joints in the Butte Quartz Monzonite that, in turn, mimic Precambrian deformation fabrics found outside the district. The faults controlling the Butte veins are interpreted to have formed through activation under shear of preexisting northeast-striking joints as master faults from which splay faults formed along generally east-west and northwest joint plane orientations.

Scientific Investigations Report↗
USGS2006

Inversion of Gravity Data to Define the Pre-Cenozoic Surface and Regional Structures Possibly Influencing Groundwater Flow in the Rainier Mesa Region, Nye County, Nevada

A three-dimensional inversion of gravity data from the Rainier Mesa area and surrounding regions reveals a topographically complex pre-Cenozoic basement surface. This model of the depth to pre-Cenozoic basement rocks is intended for use in a 3D hydrogeologic model being constructed for the Rainier Mesa area. Prior to this study, our knowledge of the depth to pre-Cenozoic basement rocks was based on a regional model, applicable to general studies of the greater Nevada Test Site area but inappropriate for higher resolution modeling of ground-water flow across the Rainier Mesa area. The new model incorporates several changes that lead to significant improvements over the previous regional view. First, the addition of constraining wells, encountering old volcanic rocks lying above but near pre-Cenozoic basement, prevents modeled basement from being too shallow. Second, an extensive literature and well data search has led to an increased understanding of the change of rock density with depth in the vicinity of Rainier Mesa. The third, and most important change, relates to the application of several depth-density relationships in the study area instead of a single generalized relationship, thereby improving the overall model fit. In general, the pre-Cenozoic basement surface deepens in the western part of the study area, delineating collapses within the Silent Canyon and Timber Mountain caldera complexes, and shallows in the east in the Eleana Range and Yucca Flat regions, where basement crops out. In the Rainier Mesa study area, basement is generally shallow (< 1 km). The new model identifies previously unrecognized structures within the pre-Cenozoic basement that may influence ground-water flow, such as a shallow basement ridge related to an inferred fault extending northward from Rainier Mesa into Kawich Valley.

Open-File Report↗
USGS2002

The Commerce geophysical lineament and its possible relation to Mesoproterozoic igneous complexes and large earthquakes in the central Illinois Basin

New inversions of existing magnetic and gravity data lead to insights on upper crustal structures in the central Illinois basin. The results of 2D and 3D inversion techniques suggest that the source of the Commerce geophysical lineament follows the southeast boundary of a dense and magnetic, northeast-trending igneous center, named the Vincennes igneous center. Thus the Commerce geophysical lineament, defining the 5- to 10-km-wide Commerce deformation zone, appears to have influenced the structural development of the Vincennes igneous center. Overlying this igneous center is the Centralia seismic-reflection sequence, expressed as highly coherent reflectors ( McBride and Kolata, 1999 ). We suggest that the buried Vincennes igneous center is the source of inferred volcanic units of the Centralia sequence and is related to a rifted margin or a Proterozoic plate boundary. A comparison of gravity and magnetic fields over the Vincennes igneous center and the St. Francois Mountains igneous center in southeastern Missouri suggests that the associated sources in these regions are similar in composition and perhaps origin. Such a conclusion, although speculative, suggests that ∼1.47 Ga-year-old volcanic fields of high-silica rocks, with more abundant mafic components at depth, characterize basement in the regions of the Vincennes igneous center and the St. Francois Mountains. Further, we conclude that the Commerce deformation zone evolved in the Mesoproterozoic (1.1 to 1.5 Ga) as a major cratonic rheological boundary and was the focus of episodic reactivation related to varying stress regimes throughout its history. Spatial relations of the Commerce deformation zone with large Pleistocene and Holocene earthquakes suggest that the zone represents a major rheological boundary intimately related to both surface and deep structures and to the seismic hazard of the Illinois basin region. Assuming recent right-lateral slip along the Commerce deformation zone, we propose that a jog or left step in the Vincennes area leads to thrusting or a restraining bend, where associated stress accumulations may have resulted in nearby large prehistoric earthquakes and may also be the nucleation sites of future large earthquakes.

Illinois, Indiana, Kentucky↗
USGS2002

Interpreting the earthquake source of the Wabash Valley seismic zone (Illinois, Indiana, and Kentucky) from seismic-reflection, gravity, and magnetic-intensity data

Reprocessing of seismic-reflection data reveals new images of upper- to middle-crustal structures beneath the Wabash Valley seismic zone, located north of the New Madrid seismic zone within the seismically active southern Illinois basin. Four intersecting deep seismic profiles (243 km total) indicate an anomalous, 5–10-km-wide zone of dipping reflections and diffractions below the western flank of the Wabash Valley fault system (WVFS). The zone corresponds in places to gently arched regions of Paleozoic strata. The reflector zone can be interpreted as a result of either (or a combination of) magmatic intrusion or structural deformation. The area encompassing the reflection profiles has experienced several moderate magnitude (3.0 ≤ m bLg ≤ 5.5) earthquakes during the past 50 years, defining the central part of the Wabash Valley seismic zone. The hypocenter of the largest 20th-century earthquake in the central USA midcontinent (9 November 1968, m bLg 5.5) corresponds to the most prominent zone of dipping middle-crustal reflections, just west of the WVFS. Both the focal mechanism (moderately dipping reverse fault) and the expected rupture zone size (∼2.9 km fault length) of this earthquake are consistent with the orientation and size of observed reflectors. Dipping reflector patterns in the Precambrian crust are not collinear with fault surfaces updip in the Paleozoic sedimentary section. This indicates that shallow Paleozoic structures are effectively “decoupled” from deeper, possibly seismogenic structure, which suggests that understanding Paleozoic structure is not the key to understanding the earthquake source. The complex dipping crustal reflectivity beneath the WVFS is typical of Paleozoic continental convergent zones observed elsewhere ( e.g. , Appalachian orogen) and thus may suggest a preserved Proterozoic suture, possibly associated with the distal Grenville orogeny or an older event. Although magnetic intensity, Bouguer gravity, and seismic-reflection data present different means of understanding the deep geology of the area, their integration aids in limiting the number of admissible interpretations. The reflection profiles indicate a variable zone of anomalous crustal structure, including the dipping reflector zone, along a trend of northeast-trending gravity and magnetic highs locally defining the Commerce geophysical lineament (CGL), which is a suspected source of seismic hazard in the central USA midcontinent. Three-dimensional inverse modeling of the residual isostatic gravity anomaly values indicates that the upper part of the dipping reflector zone beneath the CGL lies near an important density boundary in the upper Precambrian crust. The results of our study suggest that the seismogenic source just north of the New Madrid seismic zone consists, in part, of a pre-existing fabric of blind thrusts localized along pre-existing igneous intrusions, locally coincident with the CGL. This suggests that the CGL may be seismogenic in places and thus a potential seismic hazard. The variation in the expression of the CGL using reflection and potential-field data sets is probably partly related to the differing geologic features by which it is expressed, but would be also consistent with its reactivation numerous times under varying stress regimes.

Illinois, Indiana, Kentucky↗
USGS2002

Rationale and operational plan for a U.S. high-altitude magnetic survey

On August 8, 2002, twenty-one scientists from the federal, private and academic sectors met at a workshop in Denver, Co., to discuss the feasibility of collecting magnetic anomaly data on a Canberra aircraft (Figure 1). The need for this 1-day workshop arose because of an exciting and cost-effective opportunity to collect invaluable magnetic anomaly data during a Canberra mission over the U.S. in 2003 and 2004. High Altitude Mapping Missions (HAMM) is currently planning a mission to collect Interferometric Synthetic Aperture Radar (IFSAR) imagery at an altitude of about 15 km and with a flight-line spacing of about 18 km over the conterminous U.S. and Alaska. The additional collection of total and vector magnetic field data would represent a secondary mission objective (i.e., a "piggy-back" magnetometer system). Because HAMM would fund the main flight costs of the mission, the geomagnetic community would obtain invaluable magnetic data at a nominal cost. These unique data would provide new insights on fundamental tectonic and thermal processes and give a new view of the structural and lithologic framework of the crust and possibly the upper mantle. This document highlights: (1) the reasons to conduct this national survey and (2) a preliminary operational plan to collect high-altitude magnetic data of a desired quality and for the expected resources. Although some operational plan issues remain to be resolved, the important conclusions of the workshop are that the Canberra is a very suitable platform to measure the magnetic field and that the planned mission will result in quality high-altitude magnetic data to greatly expand the utility of our national magnetic database.

Open-File Report↗
USGS2002

Rationale and operational plan to upgrade the U.S. gravity database

A concerted effort is underway to prepare a substantially upgraded digital gravity anomaly database for the United States and to make this data set and associated usage tools available on the internet. This joint effort, spearheaded by the geophysics groups at the National Imagery and Mapping Agency (NIMA), University of Texas at El Paso (UTEP), U.S. Geological Survey (USGS), and National Oceanic and Atmospheric Administration (NOAA), is an outgrowth of the new geoscientific community initiative called Geoinformatics (www.geoinformaticsnetwork.org). This dominantly geospatial initiative reflects the realization by Earth scientists that existing information systems and techniques are inadequate to address the many complex scientific and societal issues. Currently, inadequate standardization and chaotic distribution of geoscience data, inadequate accompanying documentation, and the lack of easy-to-use access tools and computer codes for analysis are major obstacles for scientists, government agencies, and educators. An example of the type of activities envisioned, within the context of Geoinformatics, is the construction, maintenance, and growth of a public domain gravity database and development of the software tools needed to access, implement, and expand it. This product is far more than a high quality database; it is a complete data system for a specific type of geophysical measurement that includes, for example, tools to manipulate the data and tutorials to understand and properly utilize the data. On August 9, 2002, twenty-one scientists from the federal, private and academic sectors met at a workshop to discuss the rationale for upgrading both the United States and North American gravity databases (including offshore regions) and, more importantly, to begin developing an operational plan to effectively create a new gravity data system. We encourage anyone interested in contributing data or participating in this effort to contact G.R. Keller or T.G. Hildenbrand. This workshop was the first step in building a web-based data system for sharing quality gravity data and methodology, and it builds on existing collaborative efforts. This compilation effort will result in significant additions to and major refinement of the U.S. database that is currently released publicly by NOAA’s National Geophysical Data Center and will also include an additional objective to substantially upgrade the North American database, released over 15 years ago (Committee for the Gravity Anomaly Map of North America, 1987).

Open-File Report↗
USGS2002

Aerial Magnetic, Electromagnetic, and Gamma-ray Survey, Berrien County, Michigan

This publication includes maps, grids, and flightline databases of a detailed aerial survey and maps and grids of satellite data in Berrien County, Michigan. The purpose of the survey was to map aquifers in glacial terrains. This was accomplished by using a DIGHEMVRES mufti-coil, mufti-frequency electromagnetic system supplemented by a high sensitivity cesium magnetometer and 256-channel spectrometer. The information from these sensors was processed to produce maps, which display the conductive, magnetic and radioactive properties of the survey area. A GPS electronic navigation system ensured accurate positioning of the geophysical data. This report also includes data from the advanced spaceborne thermal emission and reflection (ASTER) radiometer. ASTER measures thermal emission and reflection data for 14 bands of the spectrum.

Open-File Report↗
USGS2001

Implications for the formation of the Hollywood Basin from gravity interpretations of the northern Los Angeles Basin, California

Gravity data provide insights on the complex tectonic history and structural development of the northern Los Angeles Basin region. The Hollywood basin appears to be a long (> 12 km), narrow (up to 2 km wide) trough lying between the Santa Monica Mountains and the Wilshire arch. In the deepest parts of the Hollywood basin, the modeled average thickness ranges from roughly 250 m if filled with only Quaternary sediments to approximately 600 m if Pliocene sediments are also present. Interpretations of conflicting drill hole data force us to consider both these scenarios. Because of the marked density contrast between the dense Santa Monica Mountains and the low-density sediments in the Los Angeles Basin, the gravity method is particularly useful in mapping the maximum displacement along the Santa Monica-Hollywood-Raymond fault zone. The gravity-defined Santa Monica–Hollywood fault zone deviates, in places, from the mapped active fault and fold scarps located with boreholes and trenching and by geomorphological mapping by Dolan and others (1997). Our models suggest that the Santa Monica–Hollywood fault zone dips northward approximately 63°. Three structural models are considered for the origin of the Hollywood basin: pull-apart basin, flexural basin, and a basin related to a back limb of a major fold. Although our preferred structural model involves flexure, the available geologic and geophysical data do not preclude contributions to the deepening of the basin from one or both of the other two models. Of particular interest is that the distribution of red-tagged buildings and structures damaged by the Northridge earthquake has a strong spatial correlation with the axis of the Hollywood basin defined by the gravity data. Several explanations for this correlation are explored, but two preferred geologic factors for the amplification of ground motion besides local site effects are (1) focussing of energy by a fault along the axis of the Hollywood basin and (2) focussing effects related to differential refraction of seismic rays across the basin.

California↗
USGS1999

Gravity and magnetic study of the Pahute Mesa and Oasis Valley region, Nye County, Nevada

Regional gravity and aeromagnetic maps reveal the existence of deep basins underlying much of the southwestern Nevada volcanic field, approximately 150 km northwest of Las Vegas. These maps also indicate the presence of prominent features (geophysical lineaments) within and beneath the basin fill. Detailed gravity surveys were conducted in order to characterize the nature of the basin boundaries, delineate additional subsurface features, and evaluate their possible influence on the movement of ground-water. Geophysical modeling of gravity and aeromagnetic data indicates that many of the features may be related to processes of caldera formation. Collapse of the various calderas within the volcanic field resulted in dense basement rocks occurring at greater depths within caldera boundaries. Modeling indicates that collapse occurred along faults that are arcuate and steeply dipping. There are indications that the basement in the western Pahute Mesa - Oasis Valley region consists predominantly of granitic and/or fine-grained siliceous sedimentary rocks that may be less permeable to groundwater flow than the predominantly fractured carbonate rock basement to the east and southeast of the study area. The northeast-trending Thirsty Canyon lineament, expressed on gravity and basin thickness maps, separates dense volcanic rocks on the northwest from less dense intracaldera accumulations in the Silent Canyon and Timber Mountain caldera complexes. The source of the lineament is an approximately 2-km wide ring fracture system with step-like differential displacements, perhaps localized on a pre-existing northeast-trending Basin and Range fault. Due to vertical offsets, the Thirsty Canyon fault zone probably juxtaposes rock types of different permeability and, thus, it may act as a barrier to ground-water flow and deflect flow from Pahute Mesa along its flanks toward Oasis Valley. Within the Thirsty Canyon fault zone, highly fractured rocks may serve also as a conduit, depending upon the degree of alteration and its effect on porosity and permeability. In the Oasis Valley region, other structures that may influence ground-water flow include the western and southern boundaries of the Oasis Valley basin, where the basement abruptly shallows.

Nevada↗
USGS1999

Magnetotelluric study of the Pahute Mesa and Oasis Valley regions, Nye County, Nevada

Magnetotelluric data delineate distinct layers and lateral variations above the pre-Tertiary basement. On Pahute Mesa, three resistivity layers associated with the volcanic rocks are defined: a moderately resistive surface layer, an underlying conductive layer, and a deep resistive layer. Considerable geologic information can be derived from the conductive layer which extents from near the water table down to a depth of approximately 2 km. The increase in conductivity is probably related to zeolite zonation observed in the volcanic rock on Pahute Mesa, which is relatively impermeable to groundwater flow unless fractured. Inferred faults within this conductive layer are modeled on several profiles crossing the Thirsty Canyon fault zone. This fault zone extends from Pahute Mesa into Oasis Valley basin. Near Colson Pond where the basement is shallow, the Thirsty Canyon fault zone is several (~2.5) kilometers wide. Due to the indicated vertical offsets associated with the Thirsty Canyon fault zone, the fault zone may act as a barrier to transverse (E-W) groundwater flow by juxtaposing rocks of different permeabilities. We propose that the Thirsty Canyon fault zone diverts water southward from Pahute Mesa to Oasis Valley. The electrically conductive nature of this fault zone indicates the presence of abundant alteration minerals or a dense network of open and interconnected fractures filled with electrically conductive groundwater. The formation of alteration minerals require the presence of water suggesting that an extensive interconnected fracture system exists or existed at one time. Thus, the fractures within the fault zone may be either a barrier or a conduit for groundwater flow, depending on the degree of alteration and the volume of open pore space. In Oasis Valley basin, a conductive surface layer, composed of alluvium and possibly altered volcanic rocks, extends to a depth of 300 to 500 m. The underlying volcanic layer, composed mostly of tuffs, fills the basin with about 3-3.5 km of relief on basement. A fault zone, related to the southern margin of the basin, appears to extend up to a depth of about 500 m. The path of groundwater encountering this fault zone is uncertain but may be either to the southwest towards Beatty or to the south towards Crater Flat.

Nevada↗
USGS1999

Digital Geologic Map of the Nevada Test Site and Vicinity, Nye, Lincoln, and Clark Counties, Nevada, and Inyo County, California

This digital geologic map of the Nevada Test Site (NTS) and vicinity, as well as its accompanying digital geophysical maps, are compiled at 1:100,000 scale. The map compilation presents new polygon (geologic map unit contacts), line (fault, fold axis, metamorphic isograd, dike, and caldera wall) and point (structural attitude) vector data for the NTS and vicinity, Nye, Lincoln, and Clark Counties, Nevada, and Inyo County, California. The map area covers two 30 x 60-minute quadrangles-the Pahute Mesa quadrangle to the north and the Beatty quadrangle to the south-plus a strip of 7.5-minute quadrangles on the east side-72 quadrangles in all. In addition to the NTS, the map area includes the rest of the southwest Nevada volcanic field, part of the Walker Lane, most of the Amargosa Desert, part of the Funeral and Grapevine Mountains, some of Death Valley, and the northern Spring Mountains. This geologic map improves on previous geologic mapping of the same area (Wahl and others, 1997) by providing new and updated Quaternary and bedrock geology, new geophysical interpretations of faults beneath the basins, and improved GIS coverages. Concurrent publications to this one include a new isostatic gravity map (Ponce and others, 1999) and a new aeromagnetic map (Ponce, 1999).

Open-File Report↗