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At least 1,243 records · Page 69Linked to original sources

USGS1997

Färoe-Iceland Ridge Experiment: 1. Crustal structure of northeastern Iceland

Results from the Färoe-Iceland Ridge Experiment (FIRE) constrain the crustal thickness as 19 km under the Northern Volcanic Zone of Iceland and 35 km under older Tertiary areas of northeastern Iceland. The Moho is defined by strong P wave and S wave reflections. Synthetic seismogram modeling of the Moho reflection indicates mantle velocities of at least 8.0 km/s beneath the Tertiary areas of northeastern Iceland and at least 7.9 km/s beneath the neovolcanic zone. Crustal diving rays resolve the structure of the upper and lower crust. Surface P wave velocities are 1.1–4.0 km/s in Quaternary rocks and are rather higher, 4.4–4.7 km/s, in the Tertiary basalts that outcrop elsewhere. The highest crustal P wave velocities observed directly from diving rays are 7.1 km/s, from rays that turn at 24 km depth. Velocities of 7.35 km/s at the base of the crust are inferred from extrapolation of the lower crustal velocity gradient (0.024 s −1 ). A Poisson's ratio of approximately 0.27, equivalent to an S wave to P wave travel time ratio of 1.78, is measured throughout the crust east of the neovolcanic zone. The Poisson's ratio and the steep Moho topography (in places up to 30° from the horizontal) indicate that the entire crust outside the neovolcanic zone is cool (<800°C). Gravity data are well matched by a velocity/density conversion of our seismic crustal model and indicate a region of low mantle density beneath the neovolcanic zone, believed to be due to elevated mantle temperatures. The crustal thickness in the neovolcanic zone is consistent with geochemical estimates of the melt generation, placing constraints on the flow within the Iceland mantle plume.

Journal of Geophysical Research B: Solid Earth↗

USGS2011

Data on past climate warmth may lead to better model of warm future

Weather bureaus around the world have accumulated daily historical records of atmospheric conditions for more than a century to help forecast meteorological conditions 3 to 5 days ahead. To gain insight into the impact of possible future climate warming and constrain predictive models for a warm future, climatologists are seeking paleoclimatologic and paleoceanographic records from recent intervals of the late Quaternary, when conditions were demonstrably warmer than today. Results of past research on Brunhes-age paleoclimate suggest that Marine Isotope Stage 11 (MIS 11), an interval between 420 and 360 ka, was the longest and warmest interglacial interval of the past 500 kyr (see the recent review by Howard [1997]). This interval was characterized by warmer seasurface temperatures in high latitudes, strong thermohaline circulation, unusual carbonate plankton blooms in high latitudes, higher than present sea level, coral reef expansion resulting in large accumulation of neritic carbonates, and overall poor pelagic carbonate preservation.

Eos Science News↗

USGS2012

Group takes a fresh look at the lithosphere underneath southern Kenya

Since the turn of the century the well-developed Kenya rift has been a crucial location for studying the interrelationships between extension, uplift, and magmatism. In 1989–1990, an experiment conducted by the Kenya Rift International Seismic Project (KRISP) focused on the central and northern portions of Kenya (Figure 1) and provided a rich base of information regarding the structure and evolution of the rift, which answered many key questions and raised others. Does the crust thin to the south of the Kenya dome as it does to the north?, and if it does, which crustal layer would be mainly affected? How is the Chyulu Hills Quaternary volcanism related to the rifting process? What is the relationship between the crust and lithospheric mantle during extension?

East African Rift, Great Rift Valley, Gregory Rift↗

USGS2012

Seismicity and volcanism in the Pacific Northwest: Evidence for the segmentation of the Juan De Fuca Plate

The distributions of earthquakes and late Cenozoic and Quaternary volcanism in Washington and northern Oregon change markedly across two northeast-striking lines, one near Mount Rainier and one near Mount Hood. On the basis of these observations and a comparison with the Nazca subduction zone, we propose that the Juan de Fuca subduction zone is divided into two segments. Landward of the coastal thrust zone, we suggest the Juan de Fuca plate dips more steeply beneath the southern segment than beneath the northern segment.

Geophysical Research Letters↗

USGS2012

Late Cenozoic volcanism, geochronology, and structure of the Coso Range, Inyo County, California

The Coso Range lies at the west edge of the Great Basin, adjacent to the southern part of the Sierra Nevada. A basement complex of pre‐Cenozoic plutonic and metamorphic rocks is partly buried by ∼35 km 3 of late Cenozoic volcanic rocks that were erupted during two periods, as defined by K‐Ar dating: (1) 4.0–2.5 m.y., ∼31 km 3 of basalt, rhyodacite, dacite, andesite, and rhyolite, in descending order of abundance, and (2) ≤1.1 m.y., nearly equal amounts of basalt and rhyolite, most of the rhyolite being ≤0.3 m.y. old. Vents for the volcanic rocks of the younger period are localized on and near a horst of basement rocks within a concavity defined by the distribution of vents of the older period. The alignment of many vents and the presence of a considerable number of roughly north‐trending normal faults of late Cenozoic age reflect basin and range tectonics dominated by roughly east‐west lithospheric extension. Fumaroles, intermittently active thermal springs, and associated altered rocks occur within and immediately east of the central part of the field of Quaternary rhyolite, in an area characterized by various geophysical anomalies that are evidently related to an active hot‐water geothermal system. This system apparently is heated by a reservoir of silicic magma at ≥8‐km depth, itself produced and sustained through partial melting of crustal rocks by thermal energy contained in mantle‐derived basaltic magma that intrudes the crust in response to lithospheric extension.

California↗

USGS2012

The growth of geological structures by repeated earthquakes: 2, Field examples of continental dip-slip faults

A strong test of our understanding of the earthquake cycle is the ability to reproduce extant fault-bounded geological structures, such as basins and ranges, which are built by repeated cycles of deformation. Along strike-slip faults, the coseismic and interseismic deformation can be nearly equal in magnitude and opposite in sign, resulting in little permanent deformation except for the fault offset. For dip-slip faults, portions of the crust are lifted and dropped, and so buoyancy forces are exerted. The seismic and interseismic deformations do not balance, and structures grow and become subject to erosion and deposition. We consider three examples for which the structure and fault geometry are well known: the White Wolf reverse fault in California, site of the 1952 Kern County M=7.3 earthquake, the Lost River normal fault in Idaho, site of the 1983 Borah Peak M=7.0 earthquake, and the Cricket Mountain normal fault in Utah, site of Quaternary slip events. Basin stratigraphy and seismic reflection records are used to profile the structure, and coseismic deformation measured by leveling surveys is used to estimate the fault geometry. To reproduce these structures, we add the deformation associated with the earthquake cycle (the coseismic slip and postseismic relaxation) to the flexure caused by the observed sediment load, treating the crust as a thin elastic plate overlying a fluid substrate. The cumulative deformation is principally dependent on the elastic plate thickness, modestly sensitive to the sediment-substrate density difference, and insensitive to the fluid viscosity for the 4- to 8-Ma structures. We deduce a longterm flexural rigidity of 2–15 × 10 19 Nm; this is equivalent to an elastic plate thickness of 2–4 km for a Young's modulus of 2.5 × 10 10 Nm −2 . This value is found where independent estimates of the elastic thickness from the coherence between surface topography and gravity yield values of about 4 km, but where coseismic fault slip extends to a depth of 10–15 km. Thus much of the seismogenic crust must weaken substantially during the life of active faults, causing the fault-bounded basins to narrow over time.

Journal of Geophysical Research Solid Earth↗

USGS2012

Stratigraphic, lithologic, and major element geochemical constraints on magmatic evolution at Lassen volcanic center, California

The Lassen volcanic center is the most recent of several long-lived volcanic centers in the southernmost Cascade Range. These centers have erupted products ranging from basaltic andesite to rhyolite and are superimposed on a background of regional basaltic to andesitic volcanism. The evolution of the Lassen volcanic center is described in three stages. Stages I and II comprise the Brokeoff volcano, and 80 km 3 andesitic stratocone, active from 600 to 400 ka. Brokeoff volcano is compositionally equivalent to the regional basaltic andesite to andesite volcanism in the Lassen region and is the result of structurally controlled focusing of the diffuse regional mafic magmatism. Stage III comprises a silicic dome field and adjacent area of hybrid andesites and has a total volume of about 100 km 3 . Volcanism during stage III was episodic and is subdivided into four sequences of lithologically and temporally distinct lavas. Stage III began at 400 ka with a rhyolitic, caldera-forming pyroclastic eruption and chemically related lavas. Additional sequences of dacite erupted between 250–200 ka and 100–0 ka. Hybrid andesites erupted adjacent to the silicic dome field between 300 and 0 ka. Porphyritic andesite and dacite with high Al 2 O 3 , low TiO 2 , medium K 2 O and FeO/MgO ratios of 1.5–2.0 are the most abundant rock types in the Lassen volcanic center. However, the single most voluminous unit is sparsely phyric rhyolite pumice. In general, the lavas of Lassen volcanic center form a single coherent trend on major element variation diagrams and in pseudo-quaternary phase space, consistant with an origin either by fractional crystallization or magma mixing. In detail, however, the lack of systematic temporal change in silica and subtly crossing trends indicate a complex origin. A variety of statistically successful fractional crystallization models can be constructed that derive Brokeoff andesites from regional magmas. An important conclusion of the modeling is that if fractional crystallization is the process responsible for generation of Brokeoff andesite, then the parent magma must be low to medium K in geochemical affinity in order to explain the variation in K 2 O. However, although major element variation can be modeled by fractional crystallization, petrographic and stratigraphic evidence indicates that magma mixing is an important but subtle process in Brokeoff lavas and suggests that lavas evolved in small independent batches. Lavas erupted during stage III, while predominantly silicic, range from 53 to 75% SiO 2 . Disequilibrium mineral assemblages in the stage III lavas indicate that they are not directly derived from Brokeoff andesite by fractional crystallization. Mixing of silicic magma with regional mafic magma and disaggregation of andesitic quenched magmatic inclusions play dominant roles in the compositional diversity of stage III lavas.

Journal of Geophysical Research Solid Earth↗

USGS2010

Milankovitch cycles in Neocene deep‐sea sediment

Pelagic carbonate sediments from the world ocean basins commonly show cyclic variations in amount and/or degree of preservation of biogenic calcite, with periodicities of several tens to several hundreds of thousands of years. The direct causes of these cycles are fluctuations in noncarbonate dilution, carbonate production, carbonate dissolution, and/or current winnowing. The overall driving force, however, is variation in the earth's orbital characteristics (Milankovitch cycles) through their influence on global climate and depositional processes. The main inferred climatic effects of orbital perturbations are on global ice volume, global temperature, ocean circulation, and distribution of climatic patterns. Eustatic sea level is directly related to ice volume, as is the rate of erosion of clastic material from continental margins. Changes in volume of sea ice affect the volume and intensity of bottom water flow, which in turn may cause changes in the intensity of sediment winnowing by bottom currents, in the intensity of upwelling of nutrient‐rich bottom waters, and in the depth of carbonate dissolution. Change in productivity of calcareous plankton are difficult to prove as a cause of carbonate cycles but may have contributed to the formation of carbonate cycles off northwest and southwest Africa. Fluctuations in winnowing of fine‐grained components have been demonstrated as a cause of cyclic variations in the coarse‐fraction component of carbonate sediments in the southwest Pacific. Dilution of carbonate by clastic material probably was a major cause of fluctuations in carbonate content of deep‐sea sediments off northwest and southwest Africa. Carbonate dissolution cycles probably are the most common manifestation of fluctuations in bottom water flow. Dissolution cycles are common in Quaternary and Neogene sediments of the North Atlantic, Caribbean, and eastern equatorial Pacific. The main cause of the carbonate dissolution was shoaling of the carbonate compensation depth during the early Neogene in response to climatically induced fluctuations in the thickness of Antarctic Bottom Water.

Paleoceanography↗

USGS2010

Changes in redox conditions in deep‐sea sediments of the subarctic North Pacific Ocean: Possible evidence for the presence of North Pacific Deep Water

Cores of upper Quaternary and Holocene sediment from the subarctic North Pacific north of about 48°N contain one or more layers of oxidized brown sediment interbedded within predominantly reduced green sediment. The brown layers are enriched in several trace elements, especially Mn, Mo, Ni, and Co, relative to the green layers. Where multiple oxidized layers are present, the intensity of the brown coloration and the magnitude of trace element enrichment often decrease with depth, suggesting that the oxidized layers are unstable and are being chemically reduced at depth. The oxidized layers represent a change in redox conditions between the North Pacific red clay province and the subarctic biosiliceous green clay province. The redox change may have been caused by an increase in supply of dissolved oxygen to bottom waters during glacial‐interglacial transitions as the result of the periodic formation of a seasonal bottom water mass in the northeastern Pacific Ocean.

Alaska↗

USGS2010

A kinematic model of southern California

We propose a kinematic model for southern California based on late Quaternary slip rates and orientations of major faults in the region. Internally consistent motions are determined assuming that these faults bound rigid blocks. Relative to North America, most of California west of the San Andreas fault is moving parallel to the San Andreas fault through the Transverse Ranges and not parallel to the motion of the Pacific plate. This is accomplished by counterclockwise rotation of California south of the San Andreas fault and by the westward movement of central California north of the Gar lock fault. The velocities of the blocks are calculated along several paths in southern California that begin in the Mojave Desert and end off the California coast. A path that crosses the western Transverse Ranges accumulates the accepted relative North America-Pacific plate velocity, whereas paths to the north and south result in a significant missing component of motion. This implies the existence of a zone of active deformation in southern California that is interpreted to include the western Transverse Ranges and northwest trending, predominately strike-slip faults close to the coast both north and south of the Transverse Ranges. Strain on this system accounts for about a third of the total North America-Pacific plate motion.

California↗

USGS2010

Stress orientation determined from fault slip data in Hampel Wash area, Nevada, and its relation to contemporary regional stress field

Fault-slip data were collected from an area of relatively young faulting in a seismically active part of the Nevada Test Site 12 km NW of Mercury, Nevada. The data come primarily from intensely faulted Miocene tuffaceous sedimentary rocks in Hampel Wash, which is bounded on the north by the Quaternary ENE trending Rock Valley fault and on the south by a parallel unnamed fault. Data from faults with known sense of displacement exhibit a bimodal distribution of slip angles (rakes). Faults exhibiting steep rakes (typically 75° to 90°) cluster about a N30°–35°E strike; most dip 65° to 80°. Faults having shallow rakes (generally less than 20°) exhibit a wide range of strikes (from N6°W to N80°E) and mostly dip between 80° and 90°. The predominant N30°–35°E strike of the steep-rake faults and the quasi-conjugate nature of a consistent subset of the shallowrake faults suggest a maximum horizontal stress orientation of about N30°–35°E and a least horizontal principal stress direction of N55°–60°W. Analysis of the data using a least squares iterative inversion to determine a mean deviatoric principal stress tensor indicates a normal-faulting stress regime (S 1 vertical) with principal stress axes in approximately horizontal and vertical directions (S 1 , trend = N 19°E and plunge = 82°N; S 2 , N30°E and 8°S; and S 3 , N60°W and 2°E). The maximum horizontal stress, S 2 , was found to be nearly intermediate in magnitude between S 1 and S 3 . The N60°W least horizontal principal stress orientation obtained from the fault-slip inversion agrees with our geometric analysis of the data and is consistent with a modern least horizontal principal stress orientation of N50°–70°W inferred from earthquake focal mechanisms, well bore breakouts, and hydraulic fracturing measurements in the vicinity of the Nevada Test Site. This solution fits all the data well, except for a subset of strike-slip faults that strike N30°–45°E, subparallel to the normal faults of the data set. Nearly pure dip-slip and pure strike-slip movement on similarly oriented faults, however, cannot be accommodated in a single stress regime. Superposed sets of striae observed on some faults suggest temporal rotations of the regional stress field or local rotations within the region of the fault zone.

Nevada↗

USGS2010

The style of late Cenozoic deformation at the eastern front of the California Coast Ranges

The 1983 Coalinga earthquake occurred at the eastern boundary of the California Coast Ranges in response to northeast directed thrusting. Such movements over the past 2 Ma have produced Coalinga anticline by folding above the blind eastern tip of the Coalinga thrust zone. The 600-km length of the Coast Ranges boundary shares a common structural setting that involves westward upturn of Cenozoic and Cretaceous strata at the eastern front of the Coast Ranges and a major, southwest facing step in the basement surface beneath the western Great Valley. Like Coalinga anticline, Pliocene and Quaternary folding and faulting along the rest of the boundary also result from northeast–southwest compression acting nearly perpendicular to the strike of the San Andreas fault. We suggest that much of this deformation is related to active thrusts beneath the eastern Coast Ranges. The step in the basement surface beneath the Great Valley seems to have controlled the distribution of this deformation and the shape of the Coast Ranges boundary.

California↗

USGS2014

Nitrate in the ground water of Texas

Ground water in many parts of Texas contains nitrate in excess of 20 ppm (parts per million) as NO 3 . About 3,000 of the 20,000 nitrate determinations made of water from wells in Texas showed more than 20 ppm of nitrate . The public water supplies of 27 Texas towns and cities contained more than 50 ppm of nitrate . Recent medical research indicates that methemoglobinemia or infant cyanosis (“blue babies”) may be caused by nitrate in water used in formula mixtures. Most of the high nitrate in ground water is found in wells less than 200 ft deep and mainly in water from late Tertiary and Quaternary formations; however, high nitrate was found in water from all kinds of rocks of all ages. The presence of high nitrate in ground water appears to be unrelated to rainfall, geography, or cultivation. A hypothesis of bacterial origin of the high nitrate is supported by strong evidence but is weakened by the presence of the correspondingly high content of associated salts. The disappearance of nitrate at depth presents additional problems.

Texas↗

USGS2010

Ground-water flow related to streamflow and water quality

A ground-water flow system in southwestern Minnesota illustrates water movement between geologic units and between the land surface and the subsurface. The flow patterns indicate numerous zones of ground-water recharge and discharge controlled by topography, varying thicknesses of geologic units, variation in permeabilities, and the configuration of the basement rock surface. Variations in streamflow along a reach of the Yellow Medicine River agree with the subsurface flow system. Increases and decreases in runoff per square mile correspond, apparently, to ground-water discharge and recharge zones. Ground-water quality variations between calcium sulfate waters typical of the Quaternary drift and sodium chloride waters typical of the Cretaceous rocks are caused by mixing of the two water types. The zones of mixing are in agreement with ground-water flow patterns along the hydrologic section.

Water Resources Research↗

USGS2013

Future distribution of tundra refugia in northern Alaska

Climate change in the Arctic is a growing concern for natural resource conservation and management as a result of accelerated warming and associated shifts in the distribution and abundance of northern species. We introduce a predictive framework for assessing the future extent of Arctic tundra and boreal biomes in northern Alaska. We use geo-referenced museum specimens to predict the velocity of distributional change into the next century and compare predicted tundra refugial areas with current land-use. The reliability of predicted distributions, including differences between fundamental and realized niches, for two groups of species is strengthened by fossils and genetic signatures of demographic shifts. Evolutionary responses to environmental change through the late Quaternary are generally consistent with past distribution models. Predicted future refugia overlap managed areas and indicate potential hotspots for tundra diversity. To effectively assess future refugia, variable responses among closely related species to climate change warrants careful consideration of both evolutionary and ecological histories.

Alaska↗

USGS2002

Interactions between onshore bedrock-channel incision and nearshore wave-base erosion forced by eustasy and tectonics

We explore the response of bedrock streams to eustatic and tectonically induced fluctuations in base level. A numerical model coupling onshore fluvial erosion with offshore wave‐base erosion is developed. The results of a series of simulations for simple transgressions with constant rate of sea‐level change ( SLR ) show that response depends on the relative rates of rock uplift ( U ) and wave‐base erosion (ɛ w ). Simple regression runs highlight the importance of nearshore bathymetry. Shoreline position during sea‐level fall is set by the relative rate of base‐level fall ( U‐SLR ) and ɛ w , and is constant horizontally when these two quantities are equal. The results of models forced by a realistic Late Quaternary sea‐level curve are presented. These runs show that a stable shoreline position cannot be obtained if offshore uplift rates exceed ɛ w . Only in the presence of a relatively stable shoreline position, fluvial profiles can begin to approximate a steady‐state condition, with U balanced by fluvial erosion rate (ɛ f ). In the presence of a rapid offshore decrease in rock‐uplift rate ( U ), short (∼5 km) fluvial channels respond to significant changes in rock‐uplift rate in just a few eustatic cycles. The results of the model are compared to real stream‐profile data from the Mendocino triple junction region of northern California. The late Holocene sea‐level stillstand response exhibited by the simulated channels is similar to the low‐gradient mouths seen in the California streams.

California↗

USGS2002

Sea level controls on the textural characteristics and depositional architecture of the Hueneme and associated submarine fan systems, Santa Monica Basin, California

Hueneme and Dume submarine fans in Santa Monica Basin consist of sandy channel and muddy levee facies on the upper fan, lenticular sand sheets on the middle fan, and thinly bedded turbidite and hemipelagic facies elsewhere. Fifteen widely correlatable key seismic reflections in high-resolution airgun and deep-towed boomer profiles subdivide the fan and basin deposits into time-slices that show different thickness and seismic-facies distributions, inferred to result from changes in Quaternary sea level and sediment supply. At times of low sea level, highly efficient turbidity currents generated by hyperpycnal flows or sediment failures at river deltas carry sand well out onto the middle-fan area. Thick, muddy flows formed rapidly prograding high levees mainly on the western (right-hand) side of three valleys that fed Hueneme fan at different times; the most recently active of the lowstand fan valleys, Hueneme fan valley, now heads in Hueneme Canyon. At times of high sea level, fans receive sand from submarine canyons that intercept littoral-drift cells and mixed sediment from earthquake-triggered slumps. Turbidity currents are confined to ‘underfit’ talweg channels in fan valleys and to steep, small, basin-margin fans like Dume fan. Mud is effectively separated from sand at high sea level and moves basinward across the shelf in plumes and in storm-generated lutite flows, contributing to a basin-floor blanket that is locally thicker than contemporary fan deposits and that onlaps older fans at the basin margin. The infilling of Santa Monica Basin has involved both fan and basin-floor aggradation accompanied by landward and basinward facies shifts. Progradation was restricted to the downslope growth of high muddy levees and the periodic basinward advance of the toe of the steeper and sandier Dume fan. Although the region is tectonically active, major sedimentation changes can be related to eustatic sea-level changes. The primary controls on facies shifts and fan growth appear to be an interplay of texture of source sediment, the efficiency with which turbidity currents transport sand, and the effects of delta distributary switching, all of which reflect sea-level changes.

California↗

USGS2008

Site characterization of Italian strong motion recording stations

A dataset of site conditions at 101 Italian ground motion stations with recorded motions has been compiled that includes geologic characteristics and seismic velocities. Geologic characterization is derived principally from local geologic investigations by ENEL that include detailed mapping and cross sections. For sites lacking such detailed geologic characterization, the geology maps of the by Servizio Geologico d'Italia are used. Seismic velocities are extracted from the literature and the files of consulting engineers, geologists and public agencies for 33 sites. Data sources utilized include post earthquake site investigations (Friuli and Irpinia events), microzonation studies, and miscellaneous investigations performed by researchers or consulting engineers/geologists. Additional seismic velocities are measured by the authors using the controlled source spectral analysis of surface waves (SASW) method for 18 sites that recorded the 1997–1998 Umbria Marche earthquake sequence. The compiled velocity measurements provide data for 51 of the 101 sites. For the remaining sites, the average seismic velocity in the upper 30 m (V s30 ) "> (V s30 ) is estimated using a hybrid approach. For young Quaternary alluvium, V s30 "> 𝑉 s30 an existing empirical relationship for California sites by Wills and Clahan (2006) is used, which we justify by validating this relationship against Italian data. For Tertiary Limestone and Italian Mesozoic rocks, empirical estimates of V s30 "> 𝑉 s30 are developed using the available data.

Conference Paper↗