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At least 127 records · Page 7Linked to original sources
USGS2012

Including foreshocks and aftershocks in time-independent probabilistic seismic hazard analyses

Time‐independent probabilistic seismic‐hazard analysis treats each source as being temporally and spatially independent; hence foreshocks and aftershocks, which are both spatially and temporally dependent on the mainshock, are removed from earthquake catalogs. Yet, intuitively, these earthquakes should be considered part of the seismic hazard, capable of producing damaging ground motions. In this study, I consider the mainshock and its dependents as a time‐independent cluster, each cluster being temporally and spatially independent from any other. The cluster has a recurrence time of the mainshock; and, by considering the earthquakes in the cluster as a union of events, dependent events have an opportunity to contribute to seismic ground motions and hazard. Based on the methods of the U.S. Geological Survey for a high‐hazard site, the inclusion of dependent events causes ground motions that are exceeded at probability levels of engineering interest to increase by about 10% but could be as high as 20% if variations in aftershock productivity can be accounted for reliably.

Bulletin of the Seismological Society of America↗
USGS1963

Gibson peak pluton: A discordant composite intrusion in the southeastern Trinity Alps, northern California

Gibson Peak pluton is the most discordant of several dominantly granitic intrusions in the Trinity Alps of northern California . It formed during Nevadan (Late Jurassic) deformation by emplacement of at least five discrete rock units that define a successively more silicic series, ranging from hypersthene gabbro to trondhjemitic tonalite. Contact features suggest that several units were incompletely crystalline when intruded by succeeding phases. Deformation of wall rocks, mainly partly serpentinized peridotite, indicates forceful intrusion , despite remarkable discordance of the pluton to regional structures. The discordance probably was controlled by regional extension fracturing during late stages of Nevadan deformation. Chemical compositions, computed from average modes of the intrusive units, are characterized by high Fe 2 O 3 -FeO and Na 2 O-K 2 O ratios. Plots of normative feldspar define a trend of trondhjemitic differentiation that diverges markedly from typical calc-alkaline trends. Contact metamorphism to mineral assemblages of pyroxene hornfels facies has been largely obscured by later low-grade hydration reactions, resulting in a net increase in serpentinization of most country-rock peridotite within the contact aureole.

California↗
USGS1985

Isotopic studies of the late Archean plutonic rocks of the Wind River Range, Wyoming

Isotopic studies of the Rb-Sr and U-Th-Pb systems in whole-rock samples and the U-Pb systematics for zircons document the existence of two late Arehean intrusive events in the Wind River Range. All of the systems examined indicate an age of ∼2,630 ± 20 m.y. for the Louis Lake batholith. Apparent ages for the Bears Ears pluton range from 2,504 ± 40 m.y. to 2,575 ± 50 m.y. The scatter in apparent ages for the Bears Ears pluton does not appear to be primarily the result of disturbance by postintrusive events, but it may be explained by an isotopically inhomogenous magma at the time of intrusion. Data for a few samples indicate that the Wind River Range was affected locally by a postmagmatic hydrothermal event that was approximately Tertiary in age. This event lowered δ 18 O values and disturbed parent-daughter relationships in most of the isotopic systems investigated, but it was recent enough that there is no demonstrable effect in the Pb-Pb system. The Bears Ears pluton has some chemical and petrologic features that are similar to those reported for the granites in the Granite Mountains to the east. These granites are spatially associated with low-temperature uranium deposits of Tertiary age and have been shown to have lost large amounts of uranium during the early to middle Tertiary. U-Pb systematics indicate, however, that the low to moderate uranium contents and highly variable Th/U values noted for the Bears Ears pluton are best interpreted as being primary features. If uranium was lost after magma generation, the loss most likely occurred at the time of intrusion. Such a loss could account for uraniferous Precambrian pegmatites southwest of the main part of the Range. The two intrusive units apparently were derived from different protoliths that were formed during early to middle Archean. Initial isotopic ratios and petrochemistry for the Louis Lake batholith are consistent with an early Archean trondhjemitic to tonalitic source. The protolith for the Bears Ears pluton must have been more evolved and somewhat younger. Inconsistencies as to the degree of evolution of this protolith, as inferred from isotopic and trace-element data, suggest that the protolith may have been subjected to high-grade meta-morphism that caused loss of Rb and U prior to generation of the magma.

Wyoming↗
USGS1993

Aspects of three-dimensional strain at the margin of the extensional orogen, Virgin River depression area, Nevada, Utah, and Arizona

The Virgin River depression and surrounding mountains are Neogene features that are partly contiguous with the little-strained rocks of the structural transition to the Colorado Plateau province. This contiguity makes the area ideally suited for evaluating the sense, magnitude, and kinematics of Neogene deformation. Analysis along the strain boundary shows that, compared to the adjacent little-strained area, large-magnitude vertical deformation (both uplift and depression, locally exceeding 10 km) greatly exceeds extensional deformation and that significant amounts of lateral displacement approximately parallel the province boundary. Extensional kinematics, the directions of lateral displacements, and the position and orientation of uplifts and one major depression are such that isostatic rebound following tectonic denudation is an unlikely direct cause of the strong vertical structural relief adjacent to the strain boundary. Instead, the observed structures are first-order features defining a three-dimensional strain field produced by approximately east-west extension, vertical structural attenuation, and extension-normal shortening. All major structural elements of the strain-boundary strain field are also found in the adjacent Basin and Range. Especially evident is a discontinuous and inhomogeneous distribution of vertical structural relief resulting from uplift and tilting of the footwalls on convex-upward normal faults, combined with formation of extension-parallel folds. Deformation resulted in a bewilderingly complex mixture of coeval folds, normal faults, strike-slip faults, and, in some areas, reverse faults, thrust faults, and thrust faults reactivated as extensional detachment faults. These complexly mixed structures exhibit a tectonically important aspect of scale independence, ranging from micro- structures to mountain-range scale. Many intensely extended domains appear to reflect a process of surface-relief minimization above uplifted and tilted blocks. Some intensely extended zones are depth-limited by relatively undeformed rocks above, and intact tilted blocks below, and are best viewed as subhorizontal zones of accommodation for the uplift and tilt of subjacent blocks. Whether or not the intense deformation extends to the surface, its depth distribution commonly reflects lithologic or paleostructural controls. Whatever the cause for the concentration of extensional strain, many gently dipping zones are more accurately viewed as local features recording small to moderate extension-parallel translations than a regionally interconnected detachment faults recording enormous extension-parallel translations. Although the total Neogene strain field in the Nevada-Utah-Arizona tricorner area has unique aspects, other parts of the Basin and Range contain generally similar structures that presumably formed from similar processes. These processes include lateral mass transfer, probably by viscoelastic flow, of the material needed to inflate areas of strong uplift and to translate blocks laterally by tectonic rafting. Directions of mass transfer vary but are commonly both parallel and normal to the direction of extension or parallel to the boundaries of extended areas. Much of the strong vertical structural relief probably reflects boundary-condition or localized perturbations (in the form of restraints or releases) of the lateral flow system. These conclusions are consistent with modern concepts of the flexural strength and rheological stratification of the crust in regions undergoing extension.

Geological Society of America Bulletin↗
USGS2001

Detrital zircon provenance of Mesoproterozoic to Cambrian arenites in the western United States and northwestern Mexico

U-Pb isotopic dating of detrital zircon from supracrustal Proterozoic and Cambrian arenites from the western United States and northern Mexico reveal three main age groups, 1.90 to 1.62 Ga, 1.45 to 1.40 Ga, and 1.2 to 1.0 Ga. Small amounts of zircons with ages of 3.1 to 2.5 Ga, 1.57 Ga, 1.32 Ga, 1.26 Ga, 0.7 Ga, and 0.5 Ga are also present. Detrital zircons ranging in age from 1.90 to 1.62 Ga and from 1.45 to 1.40 Ga are considered to have been derived from Proterozoic crystalline basement rocks of these known ages, and probably in part from reworked Proterozoic supracrustal sedimentary rocks, of the western United States. The 1.2 to 1.0 Ga detrital zircon ages from California, Arizona, and Sonora are characterized by distinct spikes (1.11 Ga, in particular) in the age-probability plots. These spikes are interpreted to indicate the influx of zircon from major silicic volcanic fields. Igneous rocks such as the Pikes Peak Granite (1.093 Ga) of Colorado, and the Aibo Granite (1.110 Ga) of Sonora, Mexico, may represent the deeply eroded roots of such volcanic fields. Samples from farther north along the Cordilleran margin that contain abundant 1.2–1.0 Ga detrital zircons do not show spikes in the age distribution, but rather ages spread out across the entire 1.2–1.0 Ga range. These age spectra resemble those for detrital zircons from the Grenville province, which is considered their source. Less common detrital zircons had a variety of sources. Zircons ranging in age from 3.36 to 2.31 Ga were apparently derived from inland parts of the North American continent from Wyoming to Canada. Zircons of about 1.577 Ga are highly unusual and may have had an exotic source; they may have come from Australia and been deposited in North America when Australia and North America were juxtaposed as part of the hypothetical Rodinian supercontinent. Detrital zircon of ∼1.320 Ga apparently had the same source as that for tuff (1.320 Ga) in the Pioneer Shale of the Apache Group in Arizona. Detrital zircons of about 1.26 Ga in the Apache Group and Troy Quartzite appear to be related to local, approximately coeval volcanic fields. Zircons of about 0.7 Ga may have had a source in igneous rocks related to rifting of the Proterozoic supercontinent of Rodinia, and 0.5 Ga zircons a source in relatively small areas of granitic rocks of this known, or inferred, age in Oklahoma, Texas, New Mexico, and Colorado.

Geological Society of America Bulletin↗
USGS1998

The seismic response of the Los Angeles basin, California

Using strong-motion data recorded in the Los Angeles region from the 1992 ( M w 7.3) Landers earthquake, we have tested the accuracy of existing three-dimensional (3D) velocity models on the simulation of long-period (≧2 sec) ground motions in the Los Angeles basin and surrounding San Fernando and San Gabriel Valleys. First, the overall pattern and degree of long-period excitation of the basins were identified in the observations. Within the Los Angeles basin, the recorded amplitudes are about three to four times larger than at sites outside the basins; amplitudes within the San Fernando and San Gabriel Valleys are nearly a factor of 3 greater than surrounding bedrock sites. Then, using a 3D finite-difference numerical modeling approach, we analyzed how variations in 3D earth structure affect simulated waveforms, amplitudes, and the fit to the observed patterns of amplification. Significant differences exist in the 3D velocity models of southern California that we tested (Magistrale et al. , 1996; Graves, 1996a; Hauksson and Haase, 1997). Major differences in the models include the velocity of the assumed background models; the depth of the Los Angeles basin; and the depth, location, and geometry of smaller basins. The largest disparities in the response of the models are seen for the San Fernando Valley and the deepest portion of the Los Angeles basin. These arise in large part from variations in the structure of the basins, particularly the effective depth extent, which is mainly due to alternative assumptions about the nature of the basin sediment fill. The general ground-motion characteristics are matched by the 3D model simulations, validating the use of 3D modeling with geologically based velocity-structure models. However, significant shortcomings exist in the overall patterns of amplification and the duration of the long-period response. The successes and limitations of the models for reproducing the recorded ground motions as discussed provide the basis and direction for necessary improvements to earth structure models, whether geologically or tomographically derived. The differences in the response of the earth models tested also translate to variable success in the ability to successfully model the data and add uncertainty to estimates of the basin response given input “scenario” earthquake source models.

California↗
USGS1973

Isotopic and paleontologic evidence for correlating three volcanic sequences in the Maine coastal volcanic belt

The correlation of the Castine Volcanics, the Thorofare Andesite–Vinalhaven Rhyolite sequence, and the Cranberry Island Series of Shaler (1889), suggested by their similar lithologies, appearance, and structural histories, is supported by the results of Rb-Sr whole-rock isotopic analyses and by the faunal assemblages from old and new fossil localities in the Castine Volcanics, Ames Knob Formation, and Thorofare Andesite. The volcanic rocks are partly Late Silurian, but mostly Early Devonian in age and yield an average radiogenic age of 390 ± 5 m.y. The use of calcite-bearing volcanic samples for whole-rock Rb-Sr dating degrades the method by greatly increasing the uncertainty of the isochron and initial Sr 87 /Sr 86 . Lower to Middle Devonian granitic plutons have initial ratios of Sr 87 and Sr 86 similar to those in the volcanic formations. The Castine Volcanics and the Lower Devonian granite of Sedgwick may be comagmatic, but the time interval between the extrusion of the Vinalhaven Rhyolite and its intrusion by the Middle Devonian granite of Vinalhaven Island is too long to support the comagmatic hypothesis.

Maine↗
USGS2006

Terrestrial cosmogenic nuclide surface exposure dating of the oldest glacial successions in the Himalayan orogen: Ladakh Range, northern India

Terrestrial cosmogenic nuclide surface exposure dating of moraine boulders and alluvial fan sediments define the timing of five glacial advances over at least the last five glacial cycles in the Ladakh Range of the Transhimalaya. The glacial stages that have been identified are: the Indus Valley glacial stage, dated at older than 430 ka; the Leh glacial stage occurring in the penultimate glacial cycle or older; the Karglacial stage, occurring during the early part of the last glacial cycle; the Bazgo glacial stage, at its maximum during the middle of the last glacial cycle; and the early Holocene Khalling glacial stage. The exposure ages of the Indus Valley moraines are the oldest observed to date throughout the Himalayan orogen. We observe a pattern of progressively more restricted glaciation during the last five glacial cycles, likely indicating a progressive reduction in the moisture supply necessary to sustain glaciation. A possible explanation is that uplift of Himalayan ranges to the south and/or of the Karakoram Mountains to the west of the region may have effectively blocked moisture supply by the south Asian summer monsoon and mid-latitude westerlies, respectively. Alternatively, this pattern of glaciation may reflect a trend of progressively less extensive glaciation in mountain regions that has been observed globally throughout the Pleistocene. ?? 2006 Geological Society of America.

Geological Society of America Bulletin↗
USGS1994

Late Quaternary normal faulting of the Hat Creek basalt, northern California

The Hat Creek fault is a major, young, north-striking, normal fault along the western boundary of extensional Basin and Range deformation in the Lessen region of northeastern California. Volcanic rocks of Quaternary and late Pliocene age are displaced a total of >500 m down to the west along west-facing, en echelon scarps now retreated to ∼35° slopes. Fresh, young scarps as much as 30 m high cut the Hat Creek Basalt (erupted between 15 and ∼40 ka) a few tens of meters west of the retreated scarps. Prior to the late 1980s, these young scarps were interpreted as lava slump scarps formed as the Hat Creek Basalt ponded against the older fault scarps and then drained away to the northwest. Numerous pieces of geologic evidence, however, show that the young scarps formed after the Hat Creek Basalt solidified and cooled and are true fault features formed by the youngest displacements of the Hat Creek fault. Structural details are remarkably well preserved along the series of left-stepping scarps cutting the Hat Creek Basalt. Near the central parts of individual segments, the fault is displayed as a single, vertical scarp. Near the ends of the segments, the scarp decreases in height and becomes a monodinal flexure on which the recent dip separation has been taken up by small-scale offset along columnar cooling joints in the basalt These monodinal flexures commonly rotate into east-west monoclines that join adjacent north-south segments. Displacement of outwash gravel overlying the Hat Creek Basalt shows that vertical separation on the Hat Creek fault has averaged ∼1.3 mm yr −1 for the past 15,000 yr. The Hat Creek fault thus represents a potential earthquake hazard, despite the low level and diffuse nature of modern seismidty in the region.

Geological Society of America Bulletin↗
USGS1940

Stratigraphic measurements in parallel folds

Folded rocks having bedding surfaces which are approximately parallel are said to lie in parallel folds. Utilizing the principle of evolutes and involutes, the author offers a more precise definition of parallel folds and points out inconsistencies in other concepts. With the idea of classifying parallel folds and possibly of deducing the mechanics of their formation, methods are presented for obtaining the differential equations of the families of involutes which, in certain cross sections, represent the traces of stratigraphic surfaces; and for obtaining the equations of the corresponding evolutes. Geometric methods are also given. These equations and geometric constructions may be deduced either from assumed structural postulates or from actual field data. Another part of the paper deals with the application of mean trigonometric functions to the measurement of thickness of strata, depth and distance to a stratum, and other stratigraphic dimensions in sections oblique to the strike of the rocks. This topic is considered under two headings: (1) where such measurements can be made from data collected at several stations along a line of traverse, and (2) where they must be made from a series of structural observations, considered in pairs. In the first case, no assumption is made regarding the curvature of the strata, but instead the mean values of the required functions are derived by mechanical integration. In the second case, the usual assumption of circular curvature is made, and the necessary functions are obtained by the use of definite integrals. Tables of the logarithms of these mean functions, with an increment of 5 degrees for the argument, are also presented. © 1940 Geological Society of America.

Geological Society of America Bulletin↗
USGS1952

Pimpled plains of Eastern Oklahoma

Patterns formed by the networks of furrows separating the natural mounds of pimpled plains in eastern Oklahoma are regarded as attributable to shrinkage-polygon systems of coarse texture comparable to those occurring (1) in mound-studded parts of the northwestern United States that are underlain by vertically jointed basalt, (2) in tracts of Alaskan tundra that are occupied by ice-wedge networks, and (3) in beds of playa lakes in the arid southwest that are cut by systems of giant desiccation fissures. While the inter-mound furrow networks characteristic of pimpled plains in the mid-continent region may owe their origin to former frigid climatic conditions, one of a number of alternative possibilities is that the peculiar configuration of these surfaces, as exemplified in eastern Oklahoma , has resulted from erosion of systems of giant soil polygons caused by desiccation. The time of origin of the pimpled plains of eastern Oklahoma is believed to have been not earlier than late Pleistocene.

Oklahoma↗
USGS1966

Magnetic data on the structure of the central Arctic Region

A study of 23,000 miles of total intensity aeromagnetic profiles in the central Arctic has been made by the U. S. Geological Survey and the U. S. Coast and Geodetic Survey. The profiles were flown at 20,000 feet above sea level and cover approximately 1,350,000 square miles of the Arctic Ocean between the North Pole and the North American continent. When the profiles are smoothed to remove crustal anomalies, the resulting contoured values differ from the U. S. Hydrographic Office Chart 1703 N for 1955 corrected to 1951 by as much as 2000 gammas in the northern part of the Arctic Archipelago. A nondipole regional focus east of Greenland has decreased in amplitude but has changed very little in position since 1907.5. There is a profound difference in the magnetic characteristics of the rocks on either side of the underwater Lomonosov Ridge across the Arctic Ocean. In the Eurasian Basin the high-altitude profiles are relatively smooth or show only minor anomalies, but on the North American side of the ridge there is a large area of closely spaced, high-amplitude anomalies which has been designated the Central Magnetic Zone. Although the anomaly trends parallel the Alpha Rise, this zone is far more extensive, including nearly half of the Canadian Basin on one side and probably all the Central Arctic Basin on the other side of the rise. The Lomonosov Ridge is marked by a persistent anomaly of moderate size that indicates the presence of magnetic material in the ridge. Probable block-fault structures along the flanks of the Alpha Rise are associated with blocklike magnetic anomalies of comparable widths. A characteristic magnetic pattern occurs over an area of jagged bottom topography in the Eurasian Basin. A similar magnetic pattern over part of the Lena Trough may indicate another area of jagged topography. The belt of epicenters associated with the Mid-Atlantic Ridge continues through this rugged part of the Eurasian Basin, but the absence of the typical high magnetic anomaly makes it doubtful that the mid-oceanic ridge extends through this part of the Arctic . Magnetic data indicate that the thick sections of sedimentary rocks in the Paleozoic geosynclinal belts of northern Ellesmere Island and northern Greenland continue out under the adjacent continental shelves north of Greenland, west of the Arctic Archipelago, north of the part of Alaska east of Barrow, and under part of the Chukchi Shelf, and that they make up the bulk of the Nansen Swell off Spitsbergen. Thick sedimentary fill is indicated in the magnetically flat areas of the Eurasian Basin next to the Lomonosov Ridge and in the southern part of the Canadian Basin. The magnetic profiles on the Eurasian side of the Lomonosov Ridge closely resemble typical magnetic profiles over both Atlantic and Pacific oceans, where as the profiles of the Central Magnetic Zone on the North American side of the Lomonosov Ridge are completely unlike the oceanic data and show a striking similarity to typical profiles over the Precambrian rocks of the Canadian Shield and its buried equivalent under the Central Stable Region of the United States. Therefore, it is concluded that the Arctic region consists of a probable oceanic area on the Eurasian side and a basin formed by downdropped continental rocks, presumably a Precambrian complex similar to that of the Canadian Shield, on the North American side of the ridge.

Geological Society of America Bulletin↗
USGS2003

Hydrothermal and tectonic activity in northern Yellowstone Lake, Wyoming

Yellowstone National Park is the site of one of the world's largest calderas. The abundance of geothermal and tectonic activity in and around the caldera, including historic uplift and subsidence, makes it necessary to understand active geologic processes and their associated hazards. To that end, we here use an extensive grid of high-resolution seismic reflection profiles (∼450 km) to document hydrothermal and tectonic features and deposits in northern Yellowstone Lake. Sublacustrine geothermal features in northern Yellowstone Lake include two of the largest known hydrothermal explosion craters, Mary Bay and Elliott's. Mary Bay explosion breccia is distributed uniformly around the crater, whereas Elliott's crater breccia has an asymmetric distribution and forms a distinctive, ∼2-km-long, hummocky lobe on the lake floor. Hydrothermal vents and low-relief domes are abundant on the lake floor; their greatest abundance is in and near explosion craters and along linear fissures. Domed areas on the lake floor that are relatively unbreached (by vents) are considered the most likely sites of future large hydrothermal explosions. Four submerged shoreline terraces along the margins of northern Yellowstone Lake add to the Holocene record of postglacial lake-level fluctuations attributed to “heavy breathing” of the Yellowstone magma reservoir and associated geothermal system. The Lake Hotel fault cuts through northwestern Yellowstone Lake and represents part of a 25-km-long distributed extensional deformation zone. Three postglacial ruptures indicate a slip rate of ∼0.27 to 0.34 mm/yr. The largest (3.0 m slip) and most recent event occurred in the past ∼2100 yr. Although high heat flow in the crust limits the rupture area of this fault zone, future earthquakes of magnitude ∼5.3 to 6.5 are possible. Earthquakes and hydrothermal explosions have probably triggered landslides, common features around the lake margins. Few high-resolution seismic reflection surveys have been conducted in lakes in active volcanic areas. Our data reveal active geothermal features with unprecedented resolution and provide important analogues for recognition of comparable features and potential hazards in other subaqueous geothermal environments.

Wyoming↗
USGS1965

Structural geology of aconcagua province and its relationship to the central Valley Graben, Chile

Aconcagua Province is herein divided into three major structural provinces which, for the sake of simplicity, are named the Coastal Cordillera, Central Valley graben , and Andean Cordillera structural provinces to correspond to the three geomorphic provinces recognized farther south. The coastal structural province includes the Coastal Cordillera which is underlain mainly by layered sedimentary and effusive rocks that strike north and dip homoclinally to the east, range from Triassic to Late Cretaceous in age, and are intruded by Cretaceous granodioritic and dioritic rocks. Igneous and metamorphic rocks largely of Paleozoic age comprise the western coastal margin, Along the eastern edge of the province is the Los Angeles fault zone, a wide, poorly defined band of semiparallel, arcuate faults which show downward displacement to the east and which appear to have resulted mainly from intrusion and uplift on the west. The Coastal Cordillera, therefore, may be considered a large horst with an intrusive granodiorite core. The Central Valley graben is bounded on the west by the Los Angeles fault zone and on the east by the Pocuro fault zone. Between these two fault zones is an area 20-30 km wide in which volcanic rocks of Late Cretaceous age are flat lying to gently folded and block faulted. In places, pipelike stocks of andesitic to dioritic igneous rock intrude the area. The Pocuro fault zone is a prominent lineament that marks the eastern limit of the Central Valley at Santiago and has been traced northward for 150 km. It may, however, have a mappable length of more than 1200 km. Vertical displacement downward to the west has been measured near Los Andes to be at least 2000 m. Consequently, this fault zone may rank among the major faults of the world. The Andean structural province is subdivided into the Las Ollas and Juncal subprovinces. The Las Ollas is a mountainous front range that lies east of the Pocuro fault and extends eastward for about 25 km. Upper Cretaceous volcanic strata are gently warped into broad, open, north-trending folds, with local sharp flexures and faulted mainly by normal faults. The eastern part of the subprovince is cut by a narrow belt of Tertiary plutonic rocks, some of which are associated with porphyry copper deposits. The Juncal structural subprovince extends eastward into Argentina. It is typified by close, overturned folding, vertical bedding, and thrust faulting. In general, structural deformation increases in intensity from west to east, with imbricate overthrusting to the east in Argentina. Present evidence indicates that major graben formation began during early Tertiary (pre-Miocene) time as a result of tensional stress developed by strain release of earlier compressional forces that folded the Andes. Post-Miocene uplift of the central Andean region renewed tensional stress and opened deep-seated, north-striking fractures and reinitiated volcanism. Most of the active volcanos of Chile may be aligned along such fractures.

Geological Society of America Bulletin↗
USGS1952

Sedimentary volumes in Gulf Coastal Plain of the United States And Mexico: Part I: Volume of Mesozoic Sediments In Florida and Georgia

Mesozoic sedimentary rocks are present throughout Florida and the Coastal Plain of Georgia, but chiefly in the subsurface in an area of approximately 93,500 square miles. The Mesozoic rocks in this area belong, for the most part, to the Gulf and Comanche series of the Cretaceous system. Rocks tentatively classified as part Jurassic and part Triassic underlie the Comanche series in parts of Florida and Georgia. In the northern part of the Georgia Coastal Plain, an irregular outcrop belt of sandstone and shale belonging to the early Upper Cretaceous Tuscaloosa formation borders on the south the crystalline rocks of the Piedmont. Southward from its outcrop in western Georgia, the Tuscaloosa formation dips under roughly parallel belts of the progressively younger Eutaw formation and formations equivalent to the Selma group of the Gulf series. In central and eastern Georgia, the Eutaw formation and formations equivalent to the Selma group are covered by overlaps of Tertiary formations. Conclusions in regard to the stratigraphy, structure, and thickness of the Mesozoic rocks are based largely on interpretations of the records of approximately 200 oil test wells. The total volume of the Mesozoic rocks in Florida and southern Georgia is estimated at 60,000 to 75,000 cubic miles.

Florida, Georgia↗
USGS1965

Chemical characteristics of oceanic basalts and the upper mantle

Tholeiitic basalts ( oceanic tholeiites) that form most of the deeply submerged volcanic features in the oceans are characterized by extremely low amounts of Ba, K, P, Pb, Sr, Th, U, and Zr as well as Fe 2 O 3 /FeO < 0.2 and Na/K > 10 in unaltered samples. Oceanic tholeiites also have rare earth abundance-distribution patterns and ratios of K/Rb (1300) and Sr 87 /Sr 86 (0.702) similar to or overlapping those of calcium-rich (basaltic) achondritic meteorites. The close compositional similarities between the oceanic tholeiites and calcium-rich achondrites indicates the relatively primitive nature of the oceanic tholeiites. In contrast, the alkali-rich basalts that cap submarine and island volcanoes are relatively enriched in Ba, K, La, Nb, P, Pb, Pb 206 , Rb, Fe 2 O 3 , Sr, Sr 87 , Ti, Th, U, and Zr; i.e. in the same elements and isotopes that are concentrated in the sialic continental crusts by factors of 5 to 1000 more than the amounts readily inferred in the upper mantle . These analytical data coupled with the field relationships indicate that the alkali-rich basalts are derivative rocks, fractionated from the oceanic tholeiites by processes of magmatic differentiation, and that the oceanic tholeiites are the principal or only primary magma generated in the upper mantle under the oceans. Studies of the abundances and compositions of continental basalts show that essentially identical tholeiitic lavas, contaminated with Si, K, and the chemically coherent trace elements and radiogenic isotopes from the sial, also have been the predominant or only magma generated in the mantle under the continents. The chemical properties of oceanic tholeiites suggest that the upper mantle probably contains less than (in parts per million): Ba, 10; K, 1000; Pb, 0.4; Rb, 10; Th, 0.2; and U, 0.1. The Sr 87 /Sr 86 must be less than 0.7015; Th/U about 2; K/Rb about 1500-2000; and Fe 2 O 3 /FeO less than 0.1. The integration of field and petrochemical data with seismic, density, and shock-wave studies suggests that the oceanic tholeiites are either complete melts of the upper mantle or are generated from a mix of this tholeiite and a magnesium-rich peridotite or dunite in proportions up to perhaps 1:4. The Mohorovičić discontinuity under the oceans appears to mark the transition downward from a largely tholeiitic oceanic crust to either tholeiite reconstituted to blueschist or greenschist or to the ultramafic residue left after expulsion of oceanic tholeiite.

Geological Society of America Bulletin↗
USGS1987

The case for tectonic denudation by the Heart Mountain fault - A response

T wo basic concepts pertaining to the history of the Heart Mountain fault of northwestern Wyoming have recently been challenged; one, that there was tectonic denudation, and two, that volcanic rock of the Wapiti Formation was deposited on the exposed fault surface. Tectonic denudation is believed to have occurred as a consequence of the upper plate having broken into numerous blocks that separated as movement progressed along a nearly horizontal fault surface, thus leaving the fault surface exposed between blocks. Volcanic rocks of the Wapiti Formation were then deposited both on the exposed fault surface and against and over the upper-plate blocks. Two formations of Eocene volcanic rocks are involved. The older volcanic unit, the Cathedral Cliffs Formation, and the Paleozoic carbonate rocks are part of the upper plate of the Heart Mountain fault and moved with it, whereas the younger Wapiti Formation was deposited on the fault surface after movement had ceased. In an alternate interpretation recently advanced by T. A. Hauge, subdivisions of the Absaroka Volcanic Supergroup, of which the Cathedral Cliffs and Wapiti Formations are units, are not recognized. The upper plate of the Heart Mountain detachment is interpreted as having been a single, continuous allochthon composed largely of volcanic rocks with small amounts of Paleozoic rocks. During Heart Mountain faulting, extension of the once-continuous slab of Paleozoic sedimentary rock is alleged to have been accompanied by the formation of ten or more grabens, now filled predominantly by Absaroka volcanic rocks. This interpretation further proposes that the volcanic rocks were emplaced while the separating blocks of Paleozoic strata were still moving and that the basal part of the volcanic rock between these blocks is in fault contact rather than depositional contact with the strata beneath the Heart Mountain fault. Many lines of geologic field evidence indicate that the Wapiti Formation is younger than the Heart Mountain fault and was deposited on the technically denuded fault surface. (1) Wapiti rocks bury the break-away fault. (2) Fault breccia at the base of the upper-plate carbonate blocks is composed entirely of carbonate fault breccia and has no volcanic component. (3) Small blocks of upper-plate rocks have been displaced by gravity from the upper part of the allochthon to the detachment fault surface. (4) Eocene stream-channel deposits locally cut into the surface of tectonic denudation and also have been displaced on the Heart Mountain fault. (5) The volume of Wapiti Formation filling the spaces between allochthonous blocks in proportion to the volume of those blocks is much too great for the Wapiti to have been allochthonous. (6) Clastic dikes of carbonate fault breccia penetrate Wapiti volcanic rocks. (7) Some of these clastic dikes of fault breccia contain Precambrian xenoliths and wood phenoclasts requiring surface exposures of the fault breccia before injection as dikes. (8) Wapiti volcanic rocks having chilled borders are in tightly bonded contact with upper-plate Paleozoic rocks. (9) Faults present in the upper-plate blocks do not penetrate the overlying Wapiti Formation. (10) Volcanic fault breccia is absent where volcanic rocks overlie carbonate fault breccia. (11) A mound of carbonate fault breccia is not mixed with overlying Wapiti Formation. The continuous allochthon interpretation is based on several erroneous assumptions that cannot be supported by field observations. (1) Faults to transport and emplace the Wapiti Formation onto and along the Heart Mountain fault do not exist. (2) The contact between volcanic rocks and the allochthon west of Corral Creek at the west end of Cathedral Cliffs, cited by Hauge as a fault in an extending allochthon, is a depositional contact. (3) The volcanic rock adjoining allochthonous Paleozoic rocks north of Pilot Creek cannot be part of an extending allochthon because (a) it is Cathedral Cliffs Formation, which is pre–Heart Mountain fault, and (b) its direction of movement is horizontal rather than down dip, as required in an extending allochthon. (4) Most of the igneous dikes were intruded after the Heart Mountain fault movement ceased, and so they could not accommodate significant extension of the upper plate. (5) Striae reported as indicating fault emplacement of volcanic rock (Wapiti Formation) on the Heart Mountain fault actually lire flow features, formed as the Wapiti Formation was deposited on the exposed fault surface. Tectonic denudation is the only model that is consistent with evidence observable in the field. Although the process by which tectonic denudation was accomplished remains enigmatic, tectonic denudation remains a constraining fact in any model for the origin of the Heart Mountain fault.

Wyoming↗
USGS1995

Chemically diverse, sporadic volcanism at seamounts offshore southern and Baja California

Compositions of lavas from seven small to medium-sized seamounts, between lat 34.0°N and 30.5°N offshore southern and Baja California, include low-K 2 O tholeiitic, transitional, and mildly to moderately alkalic basalt and their differentiates. The low-K 2 O tholeiites resemble primitive (>9% MgO) mid-oceanic-ridge basalt (MORB) with low incompatible element abundances and very depleted, concave-downward, chondrite-normalized rare-earth-element (REE) patterns and lower 87 Sr/ 86 Sr and higher 143 Nd/ 144 Nd ratios than typical MORB from the East Pacific Rise. The seamounts with these MORB-like lavas are inferred to have formed at or near the spreading center. Transitional and mildly to moderately alkalic basalts have higher abundances of incompatible elements and steeper slopes for chondrite-normalized REE patterns with light REE enrichment up to 150 times chondrites. The alkalic compositions indicate more variably enriched mantle sources than those of most seamounts presently located near the East Pacific Rise, but the compositions are within the mantle array defined by other ocean-island basalts. Volcanic rocks from the upper part of Rocas Alijos, a much larger and morphologically more complex edifice than the northern seamounts, located offshore central Baja California at lat ∼25°N, are all highly differentiated trachyte and trachyandesite. Based on 40 Ar/ 39 Ar laser fusion techniques, MORB-like lava from one of the northern edifices is as old as the underlying oceanic crust (>20 Ma), indicating that it originated at a spreading center. Other seamount lava ages are much younger than the oceanic crust on which they reside, ranging from 16.8 ± 0.3 to <7 Ma for some of the northern seamounts to 270 ± 16 ka for the trachyte from Rocas Alijos. Similar highly evolved lavas cap fossil spreading centers like Guadalupe and Socorro Islands, but Rocas Alijos, based on magnetic anomalies, is not an abandoned spreading center but may instead have formed on a leaky transform fault. Some of the seamounts with transitional and alkalic lavas may have formed as part of a short, age-progressive chain formed by a short-lived mantle plume. Many others, aligned along abandoned spreading centers or faults and fracture zones which are abundant in the tectonically complex region offshore southern and peninsular California, may have resulted from upwelling mantle diapirs in response to localized extension. Some of the episodes of volcanism appear to have been contemporaneous with volcanism in the continental borderland and coastal southern California, suggesting linkage between extension along the continental margin and the seamount province farther offshore. The data available for the abundant volcanic edifices of varying sizes, shapes, and orientations in this region suggest that the seamounts formed from multiple episodes of chemically diverse volcanism, tapping variably enriched, heterogeneous mantle, which occurred sporadically from early Miocene to late Pleistocene.

Baja California↗