USGS Science⌕ Search

SEARCH · USGS Science

Results for “GSA Memoirs”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 recordsLinked to original sources
USGS2010

The tectono-thermal evolution of the Waterbury dome, western Connecticut, based on U-Pb and 40 Ar/ 39 Ar ages

The Waterbury dome, located in the Rowe-Hawley zone in western Connecticut, is a triple window exposing three terranes: parautochthonous or allochthonous peri-Laurentian rocks in its lowest level 1, allochthonous rocks of the Rowe-Hawley zone in its middle level 2, and allochthonous cover rocks, including Silurian-Devonian rocks of the Connecticut Valley Gaspé trough, in its highest level 3. Levels 1 and 2 are separated by the Waterbury thrust, a fault equivalent to Cameron's Line, the Taconic suture in southwestern New England. Relict mesoscopic folds and foliation in levels 1 and 2 are truncated by a dominant D 2 migmatitic layering and are likely Taconic. U-Pb zircon crystallization ages of felsic orthogneiss and tonalite, syntectonic with respect to the formation of S 2 , and a biotite quartz diorite that crosscuts level 2 paragneiss are 437 ± 4 Ma, 434 ± 4 Ma, and 437 ± 4 Ma, respectively. Level 3 nappes were emplaced over the Waterbury dome along an Acadian décollement synchronous with the formation of a D 3 thrust duplex in the dome. The décollement truncates the Ky + Kfs-in (migmatite) isograd in the dome core and a St-in isograd in level 3 nappes, indicating that peak metamorphic conditions in the dome core and nappe cover rocks formed in different places at different times. Metamorphic overgrowths on zircon from the felsic orthogneiss in the Waterbury dome have an age of 387 ± 5 Ma. Rocks of all levels and the décollement are folded by D 4 folds that have a strongly developed, regional crenulation cleavage and D 5 folds. The Waterbury dome was formed by thrust duplexing followed by fold interference during the Acadian orogeny. The 40 Ar/ 39 Ar ages of amphibole, muscovite, biotite, and K-feldspar from above and below the décollement are ca. 378 Ma, 355 Ma, 360 Ma (above) and 340 (below), and 288 Ma, respectively. Any kilometer-scale vertical movements between dome and nappe rocks were over by ca. 378 Ma. Core and cover rocks of the Waterbury dome record synchronous, post-Acadian cooling.

Connecticut↗
USGS2010

A model for Iapetan rifting of Laurentia based on Neoproterozoic dikes and related rocks

Geologic evidence of the Neoproterozoic rifting of Laurentia during breakup of Rodinia is recorded in basement massifs of the cratonic margin by dike swarms, volcanic and plutonic rocks, and rift-related clastic sedimentary sequences. The spatial and temporal distribution of these geologic features varies both within and between the massifs but preserves evidence concerning the timing and nature of rifting. The most salient features include: (1) a rift-related magmatic event recorded in the French Broad massif and the southern and central Shenandoah massif that is distinctly older than that recorded in the northern Shenandoah massif and northward; (2) felsic volcanic centers at the north ends of both French Broad and Shenandoah massifs accompanied by dike swarms; (3) differences in volume between massifs of cover-sequence volcanic rocks and rift-related clastic rocks; and (4) WNW orientation of the Grenville dike swarm in contrast to the predominately NE orientation of other Neoproterozoic dikes. Previously proposed rifting mechanisms to explain these features include rift-transform and plume–triple-junction systems. The rift-transform system best explains features 1, 2, and 3, listed here, and we propose that it represents the dominant rifting mechanism for most of the Laurentian margin. To explain feature 4, as well as magmatic ages and geochemical trends in the Northern Appalachians, we propose that a plume–triple-junction system evolved into the rift-transform system. A ca. 600 Ma mantle plume centered east of the Sutton Mountains generated the radial dike swarm of the Adirondack massif and the Grenville dike swarm, and a collocated triple junction generated the northern part of the rift-transform system. An eastern branch of this system produced the Long Range dike swarm in Newfoundland, and a subsequent western branch produced the ca. 554 Ma Tibbit Hill volcanics and the ca. 550 Ma rift-related magmatism of Newfoundland.

GSA Memoirs↗
USGS2014

Geophysical framework of the Peninsular Ranges batholith—Implications for tectonic evolution and neotectonics

The crustal structure of the Peninsular Ranges batholith can be divided geophysically into two parts: (1) a western mafic part that is dense, magnetic, and characterized by relatively high seismic velocities (>6.25 km/s), low heat flow (<60 mW/m 2 ), and relatively sparse seismicity, and (2) an eastern, more felsic part that is less dense, weakly magnetic, and characterized by lower seismic velocities (<6.25 km/s), high heat flow (>60 mW/m 2 ), and abundant microseismicity. Potential-field modeling indicates that the dense, mafic part of the batholith extends to depths of at least 20 km and likely to the Moho. The magnetic anomalies of the western part of the batholith extend south beyond the spatially extensive exposures of the batholith to the tip of the Baja California peninsula, which suggests that the mafic part of the batholith projects beneath Cenozoic volcanic cover another 400 km. The linearity and undisrupted nature of the magnetic belt of anomalies suggest that the western part of the batholith has behaved as a rigid block since emplacement of the batholith. The batholith may have influenced not only the development of the Gulf of California oblique rift, but also strike-slip faulting along its northern margin, and transtensional faulting along its western margin, likely because it is thermally and mechanically more resistant to deformation than the surrounding crust.

California↗
USGS2021

Late Cenozoic tephrochronology of the Mount Diablo area within the evolving plate-tectonic boundary zone of northern California

We present a tephrochronologic/chronostratigraphic database for the Mount Diablo area and greater San Francisco Bay region that provides a spatial and temporal framework for geologic studies in the region, including stratigraphy, paleogeography, tectonics, quantification of earth surface processes, recurrence of natural hazards, and climate change. We identified and correlated 34 tephra layers within this region using the chemical composition of their volcanic glasses, stratigraphic sequence, and isotopic and other dating techniques. Tephra layers range in age from ca. 65 ka to ca. 29 Ma, as determined by direct radiometric techniques or by correlation to sites where they have been dated. The tephra layers are of Quaternary or Neogene age except for two that are of Oligocene age. We correlated the tephra layers among numerous sites throughout northern California. Source areas of the tephra layers are the Snake River–Yellowstone hotspot trend of northern Nevada, southern Idaho, and western Wyoming; the Nevadaplano caldera complex of central Nevada; the Jemez Mountains–Valles Caldera in northwestern New Mexico; the Southern Nevada volcanic field and related source areas in eastern California and west-central Nevada; the Quien Sabe–Sonoma volcanic centers of the California Coast Ranges; and the young Cascade Range volcanic centers of northeastern California and Oregon.

GSA Memoirs↗
USGS2021

Late Cenozoic paleogeographic reconstruction of the San Francisco Bay Area from analysis of stratigraphy, tectonics, and tephrochronology

The Neogene stratigraphic and tectonic history of the Mount Diablo area is a consequence of the passage of the Mendocino Triple Junction (MTJ) by the San Francisco Bay area between 12 and 6 Ma, volcanism above a slab-window trailing the MTJ, and crustal transpression beginning ~8-6 Ma, when the Pacific Plate and Sierra Nevada microplate began to converge obliquely. Between ~12-6 Ma, parts of the Sierra Nevada microplate were displaced by faults splaying from the main trace of the San Andreas Fault and incorporated into the Pacific Plate. The Mount Diablo anticlinorium was formed by crustal compression within a left-stepping, restraining bend of the eastern San Andreas Fault system (SAF), with southwest-verging thrusting beneath, and with possible clockwise rotation between faults on its southeast and northwest. At ~10,5 Ma, a drainage divide formed between the northern Great Central Valley (GCV) and the ocean. Regional uplift accelerated at ~6 Ma with onset of transpression between the Pacific and North American plates. Marine deposition ceased in the eastern Coast Range basins as a consequence of the regional uplift accompanying passage of the MTJ, and trailing slab-window volcanism. From ~11 to ~5 Ma, andesitic volcanic intrusive rocks and lavas were erupted along the northwest crest of the central to northern Sierra Nevada and were deposited on its western slope, providing abundant sediment to northern Great Central Valley (GCV) and the northeastern Coast Ranges. Sediment filled the GCV, overtopped the Stockton fault and arch forming one large, south-draining system that flowed into a marine embayment at its southwestern end, the ancestral San Joaquin Sea. This marine embayment shrunk with time and by ~2.3 Ma was eventually cut off from the ocean. Fluvial drainage continued southwest in GCV until it was cut off in turn, probably by some combination of sea level fluctuations and transpression along the SAF that uplifted, lengthened and narrowed the outlet channel. As a consequence, a great lake, Lake Clyde, formed in the GCV at ~1.4 Ma, occupying all of the ancestral San Joaquin Valley and part of ancestral Sacramento Valley. The lake rose and fell with global glacial and interglacial cycles. After a long, extreme glacial period, Marine Oxygen Isotope Stage (MOIS) 16, it overtopped Carquinez sill at 0.63 Ma and drained via San Francisco valley (now Bay) and the Colma gap, into the Merced marine embayment of the Pacific Ocean. Later, a new outlet for GCV drainage formed between ~75 and ~130 ka ago., when the Colma gap closed due to transpression and right-slip on the SAF, and Duxbury Point at the south end of Pt. Reyes Peninsula moved sufficiently northwest along the SAF to unblock a bedrock notch, the feature we now call the Golden Gate.

California↗
USGS2022

Circa 1.50–1.45 Ga metasedimentary rocks in southwestern Laurentia provide distinctive records of Mesoproterozoic regional orogenesis and craton interactions

The discovery of multiple deformed and metamorphosed sedimentary successions in southwestern Laurentia that have depositional ages between ca. 1.50 and 1.45 Ga marked a turning point in our understanding of the Mesoproterozoic tectonic evolution of the continent and its interactions with formerly adjacent cratons. Detrital zircon U-Pb ages from metasedimentary strata and igneous U-Pb zircon ages from interbedded metavolcanic rocks in Arizona and New Mexico provide unequivocal evidence for ca. 1.50–1.45 Ga deposition and burial, followed by ca. 1.45 and younger deformation, metamorphism, and plutonism. These events reflect regional shortening and crustal thickening that are most consistent with convergent to collisional orogenesis—the Mesoproterozoic Picuris orogeny—in southwestern Laurentia. Similar metasedimentary successions documented in the midcontinent of the United States and in eastern Canada help to establish ca. 1.45 Ga orogenesis as a continent-scale phenomenon associated with a complex and evolving convergent margin along southern Laurentia. Metasedimentary successions of similar age are also exposed across ~5000 km of the western Laurentian margin and contain distinctive 1.6–1.5 Ga detrital zircon populations that are globally rare except in select cratonic provinces in Australia and Antarctica. The recognition of these distinctive detrital zircon ages provides a transient record of plate interactions prior to breakup of Nuna or Columbia ca. 1.45 Ga and provides key constraints on global plate reconstructions.

GSA Memoirs↗
USGS1969

Mineral layering in the Twin Lakes granodiorite, Colorado

The Twin Lakes intrusion is composed mainly of coarse-grained porphyritic granodiorite, and is zoned from a felsic core to a slightly more mafic border. Steeply dipping mineral layers, typically a few inches to 5 feet thick and several tens of feet long, occur in discontinuous marginal zones as wide as 5000 feet. Four main types of layers are defined by increased abundances of orthoclase, quartz, plagioclase, and mafic minerals. The characteristic minerals of each type of layer differ markedly in size (orthoclase, average length about 10 cm; quartz, average diameter about 1 cm; plagioclase, average length .45 mm; and mafic minerals, average length .15 mm). Textural evidence from fine-grained granodiorite porphyry and deformed mafic layers indicates that the magma contained 50 to 60 volume percent suspended crystals during emplacement. Structures in the mafic layers such as size and concentration grading normal to the plane of layering, wedge layering, and cross layering superficially resemble sedimentary structures. Inspection of these structures, however, reveals a number of features that are difficult to explain by a process of sedimentation, but which are consistent with a flow sorting process accompanied by deformation. The layering probably formed by size sorting of the suspended crystals in marginal zones of the intrusion by essentially vertical shear flow during emplacement.

Colorado↗
USGS1970

North American Devonian conodont biostratigraphy

The Lower Devonian of Nevada provides a reference sequence of nine conodont faunas, five of which are also at Royal Creek, Yukon Territory. The first appearance of Icriodus woschmidti is comparable to that in the lower Gedinnian in Europe; the Polygnathus dehiscens- P. foveolatus lineage correlates with the Emsian. Directly associated graptolite and brachiopod zones in Nevada date the intervening conodont succession as upper Gedinnian and Siegenian. Five faunas characterize the lower Middle Devonian (Eifelian) in New York ; at least two of the lower three are present in the Illinois Basin. Illinois Basin faunas correlative to the upper two New York faunas (Seneca and Cherry Valley) have Icriodus angustus, which is absent in New York, but lack Polygnathus kockelianus, which marks the top of the New York Eifelian. Three faunas subdivide the upper Middle Devonian (Givetian) in the Illinois Basin; the lower two occur in New York and Michigan. The Middle Devonian-Upper Devonian boundary possibly falls at the base of or within beds carrying the Spathognathodus insitus fauna. Alberta and Iowa sequences from the Lower Polygnathus asymmetricus through Upper Palmatolepis gigas Zones closely correspond to the lower Upper Devonian in Germany. An Iowa section yields the standard lower Famennian zonal succession (Middle Palmatolepis triangularis through Upper Palmatolepis quadrantinodosa Zones). Higher Devonian faunas, in the western and central United States, are assigned with difficulty to the Scaphignathus velifer, Upper Polygnathus styriacus, and Spathognathodus costatus Zones. The faunal sequence of the S. costatus Zone differs significantly from that in Germany.

GSA Memoirs↗
USGS1983

Introduction

Tectonic and Stratigraphic Studies in the Eastern Great Basin is compiled as a tribute to Max D. Crittenden, Jr., whose geologic leadership and exemplary studies in the eastern Great Basin region have stimulated significant scientific advances. Many of the authors in this volume conducted their studies with assistance from Max, and much of the foundation for these studies is his work on the foreland thrust belt in the Wasatch Mountains of Utah.

GSA Memoirs↗
USGS1983

Intrusive rocks and plutonic belts of southeastern Alaska, U.S.A

About 30 percent of the 175,000-km 2 area of southeastern Alaska is underlain by intrusive igneous rocks. Compilation of available information on the distribution, composition, and ages of these rocks indicates the presence of six major and six minor plutonic belts. From west to east, the major belts are: the Fairweather-Baranof belt of early to mid-Tertiary granodiorite; the Muir-Chichagof belt of mid-Cretaceous tonalite and granodiorite; the Admiralty-Revillagigedo belt of porphyritic granodiorite, quartz diorite, and diorite of probable Cretaceous age; the Klukwan-Duke belt of concentrically zoned or Alaskan-type ultramafic-mafic plutons of mid-Cretaceous age within the Admiralty-Revillagigedo belt; the Coast Plutonic Complex sill belt of tonalite of unknown, but perhaps mid-Cretaceous, age; and the Coast Plutonic Complex belt I of early to mid-Tertiary granodiorite and quartz monzonite. The minor belts are distributed as follows: the Glacier Bay belt of Cretaceous and(or) Tertiary granodiorite, tonalite, and quartz diorite lies within the Fair-weather-Baranof belt; layered gabbro complexes of inferred mid-Tertiary age lie within and are probably related to the Fairweather-Baranof belt; the Chilkat-Chichagof belt of Jurassic granodiorite and tonalite lies within the Muir-Chichagof belt; the Sitkoh Bay alkaline, the Kendrick Bay pyroxenite to quartz monzonite, and the Annette and Cape Fox trondhjemite plutons, all interpreted to be of Ordovician(?) age, together form the crude southern southeastern Alaska belt within the Muir-Chichagof belt; the Kuiu-Etolin mid-Tertiary belt of volcanic and plutonic rocks extends from the Muir-Chichagof belt eastward into the Admiralty-Revillagigedo belt; and the Behm Canal belt of mid- to late Tertiary granite lies within and next to Coast Plutonic Complex belt II. In addition, scattered mafic-ultramafic bodies occur within the Fairweather-Baranof, Muir-Chichagof, and Coast Plutonic Complex belts I and II. Palinspastic reconstruction of 200 km of right-lateral movement on the Chatham Strait fault does not significantly change the pattern of the major belts but does bring parts of the minor mid-Tertiary and Ordovician(?) belts closer together. The major belts are related to the stratigraphic-tectonic terranes of Berg, Jones, and Coney (1978) as follows: the Fairweather-Baranof belt is largely in the Chugach, Wrangell (Wrangellia), and Alexander terranes; the Muir-Chichagof belt is in the Alexander and Wrangell terranes; the Admiralty-Revillagigedo belt is in the Gravina and Taku terranes; the Klukwan-Duke belt is in the Gravina, Taku, and Alexander terranes; the Coast Plutonic Complex sill belt is probably between the Taku and Tracy Arm terranes; and the Coast Plutonic Complex belts I and II are in the Tracy Arm and Stikine terranes. Significant metallic-mineral deposits are spatially related to certain of these belts, and some deposits may be genetically related. Gold, copper, and molybdenum occurrences may be related to granodiorites of the Fairweather-Baranof belt. Magmatic copper-nickel deposits occur in the layered gabbro within that belt. The Juneau gold belt, which contains gold, silver, copper, lead, and zinc occurrences, parallels and lies close to the Coast Plutonic Complex sill belt; iron deposits occur in the Klukwan-Duke belt; and porphyry molybdenum deposits occur in the Behm Canal belt. The Muir-Chichagof belt of mid-Cretaceous age and the Admiralty-Revillagigedo belt of probable Cretaceous age are currently interpreted as possible magmatic arcs associated with subduction events. In general, the other belts of intrusive rocks are spatially related to structural discontinuities, but genetic relations, if any, are not yet known. The Coast Plutonic Complex sill belt is probably related to a post-Triassic, pre-early Tertiary suture zone that nearly corresponds to the boundary between the Tracy Arm and Taku terranes. The boundary between the Admiralty-Revillagigedo and Muir-Chichagof belts coincides nearly with the Seymour Canal-Clarence Strait lineament and also is probably a major post-Triassic suture.

Alaska↗
USGS1986

Blueschist metamorphism of the Eastern Franciscan belt, northern California

Rocks of the Eastern Franciscan belt, northern California, are divided into two tectonostratigraphic terranes metamorphosed to the blueschist facies, both with a distinct lithologic association and deformational history. The easternmost terrane, the Pickett Peak terrane of Early Cretaceous isotopic age, consists of crenulated mica schist and gneissic to schistose metagraywacke, with lesser alkalic mafic metaigneous rocks and scarce metachert. The Pickett Peak terrane retains evidence of three periods of penetrative deformation, the first of which is characterized by segregation layering, and the second and third by crenulation cleavages. Blueschist-facies conditions persisted during the first two deformations. The Yolla Body terrane of Late Jurassic and Early Cretaceous paleontologic age lies structurally below and to the west of the Pickett Peak terrane. It is characterized by voluminous metagraywacke and lesser argillite, coherent interbedded radiolarian chert, and alkalic gabbroic dikes and sills. The Yolla Bolly terrane retains evidence for two phases of penetrative deformation that were coaxial with the second and third phases of deformation in the Pickett Peak terrane. The first phase of deformation (parallel to the second phase in the Pickett Peak terrane) was also accompanied by blueschist-facies metamorphism.

California↗
USGS1988

Deformation along the northeast side of Blacktail Mountains salient, southwestern Montana

The Blacktail Mountains salient is a convex-eastward area of stacked Laramide-age thrust faults that trend north and dip west at moderate angles. The thrusts occur in Mississippian to Cretaceous strata above a basement of Archean metamorphic rocks. The northern margin of the salient is delimited by the Jake Canyon fault, a northwest-trending, northeast-dipping Laramide reverse fault. During the Laramide orogeny, the fault formed a common boundary of the present-day Blacktail Mountains and a structural high that existed in the area of the present-day valley of Blacktail Deer Creek. The fault juxtaposed Archean metamorphic rocks upon Phanerozoic strata in the northwest-era half of its extent, and against other Archean metamorphic rocks in the southeastern half. General structural relationships and study of small-scale structures in local areas show that movement along the Jake Canyon fault caused deformation of the north-trending thrust faults and associated folds. During Cenozoic extensional faulting, the Blacktail fault developed northeast of the Jake Canyon fault, and generally delimits the southwestern side of the basin of sedimentary rocks that lies beneath the valley of Blacktail Deer Creek.

Idaho, Montana↗
USGS1989

Seismic methods for determining earthquake source parameters and lithospheric structure

The seismologic methods most commonly used in studies of earthquakes and the structure of the continental lithosphere are reviewed in three main sections: earthquake source parameter determinations, the determination of earth structure using natural sources, and controlled-source seismology. The emphasis in each section is on a description of data, the principles behind the analysis techniques, and the assumptions and uncertainties in interpretation. Rather than focusing on future directions in seismology, the goal here is to summarize past and current practice as a companion to the review papers in this volume. Reliable earthquake hypocenters and focal mechanisms require seismograph locations with a broad distribution in azimuth and distance from the earthquakes; a recording within one focal depth of the epicenter provides excellent hypocentral depth control. For earthquakes of magnitude greater than 4.5, waveform modeling methods may be used to determine source parameters. The seismic moment tensor provides the most complete and accurate measure of earthquake source parameters, and offers a dynamic picture of the faulting process. Methods for determining the Earth’s structure from natural sources exist for local, regional, and teleseismic sources. One-dimensional models of structure are obtained from body and surface waves using both forward and inverse modeling. Forward-modeling methods include consideration of seismic amplitudes and waveforms, but lack the formal resolution estimates obtained with inverse methods. Two- and three-dimensional lithospheric models are derived using various inverse methods, but at present most of these methods consider only traveltimes of body waves. Controlled-source studies of the Earth’s structure are generally divided by method into seismic refraction/wide-angle reflection and seismic reflection studies. Seismic refraction profiles are usually interpreted in terms of two-dimensional structure by forward modeling of traveltimes and amplitudes. The refraction method gives excellent estimates of seismic velocities, but relatively low resolution of structure. Formal resolution estimates are not possible for models derived from forward modeling, but informal estimates can be obtained by perturbing the best-fitting model. Inversion methods for seismic refraction data for one-dimensional models are well established, and two- and three-dimensional methods, including tomography, have recently been developed. Seismic reflection data provide the highest resolution of crustal structure, and have provided many important geological insights in the past decade. The acquisition and processing of these data have been greatly advanced by the hydrocarbon exploration industry. However, reliable crustal velocity control is generally lacking, and the origin of deep crustal reflections remains unclear, resulting in nonunique interpretations. A new form of lithospheric seismology has recently emerged that combines the advantages of seismic refraction and seismic reflection profiles, and the distinction between the two methods is steadily diminishing. Major challenges for future work will be the collection of data that are more densely sampled in space, and the development of interpretation methods that provide quantitative estimates of the uncertainties in the calculated models.

GSA Memoirs↗
USGS1989

Regional crustal structure and tectonics of the Pacific Coastal States: California, Oregon, and Washington

The Pacific Coastal States form a complex geologic environment in which the crust and lithosphere have been continuously reworked. We divide the region tectonically into the southern transform regime of the San Andreas fault and the northern subduction regime, and summarize the geophysical framework with contour maps of crustal thickness, lithospheric and seismicity cross sections, and results from site-specific geophysical studies. The uniformity of crustal thickness (30 ± 2 km) in southern California is remarkable, and appears to be primarily the result of crustal extension in the Mojave Desert and ductile shear of the lower crust along the plate transform boundary. Southern California seismicity defines a broad zone of deformation that extends from the Borderland to the Mojave Desert (about 300 km). The geophysical framework of central and northern California records magmatism and accretion associated with the Mesozoic and Cenozoic subduction, late Cenozoic transform faulting, and in the Basin and Range to the east, extension. The crust thickens from about 20 km at the coast to as much as 55 km in the Sierra Nevada, and thins to about 30 km in the Basin and Range. Cross sections of the crust show that seismic velocities and densities vary significantly over short distances perpendicular to the coast, reflecting processes that include the accretion of oceanic sediments and igneous crust, and significant lateral motion of crustal blocks. Maximum hypocentral depths in central California become deeper as the crust thickens to the west, but seismicity is low beneath the Great Valley and Sierra Nevada, which together appear to form a relatively undeforming block. The lower crust of the Pacific Coastal States has a high average seismic velocity (6.7 km/sec or greater), which probably is the product of tectonic underplating of oceanic crust and/or magmatic underplating by a basaltic melt.

California, Oregon, Washington↗
USGS1989

Isostatic residual gravity and crustal geology of the United States

A new isostatic residual gravity map of the conterminous United States presents continent-wide gravity data in a form that can be readily used, with geologic information and other geophysical data, in studies of the composition and structure of the continental crust. This map was produced from the gridded gravity data on which the recently released Gravity Anomaly Map of the United States is based. About 1 million onland and 0.8 million offshore gravity observations interpolated to a 4- by 4-km grid serve as the basis for both maps. The Airy-Heiskanen model of isostatic compensation of topography applied to topographic and bathymetric data averaged over 5- by 5-min compartments was used to remove, to first order, the large, long-wavelength Bouguer gravity anomalies caused by deep density distributions that support topographic loads. The parameters used in the Airy-Heiskanen model were topographic density, 2.67 g/cm 3 ; sea-level crustal thickness, 30 km; and density contrast across the base of the model crust, 0.35 g/cm 3 . Many of the conspicuous short-wavelength anomalies (widths less than several hundred kilometers) on the isostatic residual gravity map correlate with mapped or near-surface geologic features, and primarily reflect shallow-density distributions rather than any departures from isostatic equilibrium. In general, gravity highs occur over (1) mafic igneous bodies emplaced in rift or magmatic arc settings or as isolated intrusions controlled by structures; (2) accreted slices of mafic oceanic, island-arc, or transitional crust; and (3) uplifted crystalline basement. Gravity lows are found over (1) thick bodies of felsic intrusive or extrusive rocks; (2) sedimentary deposits in extensional, convergent, or transform settings; and (3) depressed crystalline basement. Anomalies with widths as much as 1,000 km or more also appear to reflect crustal properties in many cases—several broad gravity highs are associated with crust having a high average seismic wave velocity, and comparable broad gravity lows occur over areas of low average seismic velocity. Alternative ways of viewing the isostatic residual gravity data provide additional information about density distributions in the crust. The first-vertical derivative map accentuates gravity anomalies over shallow, abrupt density changes at the expense of those resulting from deeper or more gradual density transitions. The maximum horizontal gradient map contains information about the locations of pronounced density boundaries. Two-dimensional spectral analysis of the gravity data provides a quantitative means for identifying dominant fabrics in the gravity field and for distinguishing various terranes from each other. Neither Bouguer nor isostatic residual gravity anomalies are particularly well suited for practical modeling of deep structure in conjunction with deep seismic information. However, a scheme in which the entire Earth outside the area of interest is approximated by laterally homogeneous layers and isostatically compensated topography, and in which the area of interest is modeled using the seismic constraints applied in a two-and-one-half-dimensional geometry, holds promise for exploiting useful features of both the Bouguer and isostatic residual gravity anomalies.

conterminous United States↗
USGS1989

Crustal studies using magnetic data

The magnetic method plays an important role in mineral, petroleum, and geothermal exploration. It also has made important contributions to geologic mapping, structural geology, and plate-tectonic theory. In particular, magnetic measurements using aircraft provide a relatively inexpensive way to trace magnetic rock units beneath covered areas, to reveal the shape of subsurface magnetic bodies, and to interpolate subsurface geologic information between widely spaced seismic data and other localized geophysical measurements. Computerized interpretation procedures currently fall into two categores: techniques designed to enhance the data, which include various display and filtering procedures, and modeling experiments, which may be either forward (trial-and-error) or inverse in nature.

GSA Memoirs↗
USGS1989

Geophysical framework of the continental United States: Progress, problems, and opportunities for research

Significant progress has been made over the past five decades in determining the geophysical framework of the continental United States. Highlights include detailed maps of gravity and aeromagnetic anomalies, heat flow, crustal thickness, seismicity, state of stress, and paleomagnetic pole positions. Important tectonic insights have come from earthquake studies, and from knowledge of lithospheric structure derived from seismic reflection, refraction/wide-angle reflection, surface-wave, and teleseismic data. Additional major advances in lithospheric geophysics will depend on four key factors: the reduction of uncertainties in the measurement and interpretation of geophysical data, the widespread application of coincident geophysical methods in concert with geological investigations, the collection of a more uniform continent-scale data base for all geophysical measurements, and the investigation of topical geophysical questions regarding the physical state and properties of the lithosphere. The impracticality of repeating most geophysical field measurements introduces poorly known, but likely large, uncertainties. Since most measurements are not repeated, high priority must be given to the reduction and quantification of uncertainties in measurements and interpretations. The most productive future investigations, in terms of resolution and minimum uncertainties in interpretation, will be those that apply different geophysical methods along identical profiles or areas, and that include geological investigations as a vital ingredient. Important gaps remain in our knowledge of the geophysical framework of the United States on a continent-wide scale, including the deep conductivity structure, the nature of the Moho discontinuity, the structure of the subcrustal lithosphere, and the depth of the lithosphere/asthenosphere boundary. Most transition zones separating geologic or physiographic provinces are poorly studied, yet these zones are likely to be the locations of the most profound changes in the physical properties of the lithosphere. Application of coincident geophysical techniques is needed to study these transition zones. Several topical geophysical questions warrant special emphasis in the future. These questions include the rheology of the crust and subcrustal lithosphere; the distribution, composition, and abundance of fluids in the crust; the genesis and evolution of the Moho; the origin of crustal conductivity zones and deep crustal reflections; the evidence for seismic anisotropy; and the short-term prediction of earthquakes.

GSA Memoirs↗
USGS1990

Chapter 14: Middle Cretaceous silicic metavolcanic rocks in the Kings Canyon area, central Sierra Nevada, California

Metamorphosed silicic volcanic and hypabyssal rocks of middle Cretaceous (110 to 100 Ma) age occur in two roof pendants in the Kings Canyon area of the central Sierra Nevada. The metavolcanic remnants are similar in age to or are only slightly older than the voluminous enclosing batholithic rocks. Thus, high to surface levels of the batholith are implied for this region. This is interesting considering that deep-level (∼25 km) batholithic rocks of the same age as the metavolcanic rocks occur at the southern end of the range. Apparent structural continuity between these two regions suggests that the southern half of the range offers an oblique section through young (˜100 Ma) sialic crust. The middle Cretaceous ages of the two volcanic sequences are indicated by U/Pb zircon and Rb/Sr bulk-rock isochron data. The two isotopic systems agree very closely with one another. Some of the U/Pb systems within the Boyden Cave pendant are discordant due to the inheritance or entrainment of Proterozoic zircon. This is a common phenomenon in volcanic or plutonic rocks erupted or emplaced within the Kings sequence metamorphic framework, a belt of distinct pendants with abundant continent-derived sedimentary protoliths. In conjunction with other petrochemical parameters, lavas and magmas of this framework domain are shown to be contaminated with sedimentary admixtures. The contaminated domain of the batholith reflects the bounds of the Kings sequence framework, which along its eastern margin probably represents a major pre-batholith to early batholith tectonic break. The middle Cretaceous metavolcanic sequences were apparently built on two distinctly different early Mesozoic substrates separated by a major tectonic break. In the Boyden Cave pendant, the substrate may be represented by the shallow to deep-marine Kings sequence; to the east in the Oak Creek pendant, the substrate consists of a thick silicic ignimbrite sequence. In both areas the middle Cretaceous rocks and adjacent sequences share intense ductile deformation fabrics. Earlier views that considered these fabrics as an expression of Jurassic orogenic deformation are in error. Structural and age relations indicate that the fabrics developed between 105 and 100 Ma and during the medial phases of Cretaceous composite batholith growth.

California↗