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

USGS2009

Reconstructing Rodinia by Fitting Neoproterozoic Continental Margins

Reconstructions of Phanerozoic tectonic plates can be closely constrained by lithologic correlations across conjugate margins by paleontologic information, by correlation of orogenic belts, by paleomagnetic location of continents, and by ocean floor magmatic stripes. In contrast, Proterozoic reconstructions are hindered by the lack of some of these tools or the lack of their precision. To overcome some of these difficulties, this report focuses on a different method of reconstruction, namely the use of the shape of continents to assemble the supercontinent of Rodinia, much like a jigsaw puzzle. Compared to the vast amount of information available for Phanerozoic systems, such a limited approach for Proterozoic rocks, may seem suspect. However, using the assembly of the southern continents (South America, Africa, India, Arabia, Antarctica, and Australia) as an example, a very tight fit of the continents is apparent and illustrates the power of the jigsaw puzzle method. This report focuses on Neoproterozoic rocks, which are shown on two new detailed geologic maps that constitute the backbone of the study. The report also describes the Neoproterozoic, but younger or older rocks are not discussed or not discussed in detail. The Neoproterozoic continents and continental margins are identified based on the distribution of continental-margin sedimentary and magmatic rocks that define the break-up margins of Rodinia. These Neoproterozoic continental exposures, as well as critical Neo- and Meso-Neoproterozoic tectonic features shown on the two new map compilations, are used to reconstruct the Mesoproterozoic supercontinent of Rodinia. This approach differs from the common approach of using fold belts to define structural features deemed important in the Rodinian reconstruction. Fold belts are difficult to date, and many are significantly younger than the time frame considered here (1,200 to 850 Ma). Identifying Neoproterozoic continental margins, which are primarily extensional in origin, supports recognition of the Neoproterozoic fragmentation pattern of Rodinia and outlines the major continental masses that, prior to the breakup, formed the supercontinent. Using this pattern, Rodinia can be assembled by fitting the pieces together. Evidence for Neoproterozoic margins is fragmentary. The most apparent margins are marked by miogeoclinal deposits (passive-margin deposits). The margins can also be outlined by the distribution of continental-margin magmatic-arc rocks, by juvenile ocean-floor rocks, or by the presence of continent-ward extending aulacogens. Most of the continental margins described here are Neoproterozoic, and some had an older history suggesting that they were major, long-lived lithospheric flaws. In particular, the western margin of North America appears to have existed for at least 1,470 Ma and to have been reactivated many times in the Neoproterozoic and Phanerozoic. The inheritance of trends from the Mesoproterozoic by the Neoproterozoic is particularly evident along the eastern United States, where a similarity of Mesoproterozoic (Grenville) and Neoproterozoic trends, as well as Paleozoic or Mesozoic trends, is evident. The model of Rodinia presented here is based on both geologic and paleomagnetic information. Geologic evidence is based on the distribution and shape of Neoproterozoic continents and on assembling these continents so as to match the shape, history, and scale of adjoining margins. The proposed model places the Laurasian continents?Baltica, Greenland, and Laurentia?west of the South American continents (Amazonia, Rio de La Plata, and Sa? Francisco). This assembly is indicated by conjugate pairs of Grenville-age rocks on the east side of Laurentia and on the west side of South America. In the model, predominantly late Neoproterozoic magmatic-arc rocks follow the trend of the Grenville rocks. The boundary between South America and Africa is interpreted as the site of a Wilson cycle

Open-File Report↗

USGS2007

Ground-water recharge in the arid and semiarid southwestern United States: Climatic and geologic framework

Ground-water recharge in the arid and semiarid southwestern United States results from the complex interplay of climate, geology, and vegetation across widely ranging spatial and temporal scales. Present-day recharge tends to be narrowly focused in time and space. Widespread water-table declines accompanied agricultural development during the twentieth century, demonstrating that sustainable ground-water supplies are not guaranteed when part of the extracted resource represents paleorecharge. Climatic controls on ground-water recharge range from seasonal cycles of summer monsoonal and winter frontal storms to multimillennial cycles of glacial and interglacial periods. Precipitation patterns reflect global-scale interactions among the oceans, atmosphere, and continents. Large-scale climatic influences associated with El Niño and Pacific Decadal Oscillations strongly but irregularly control weather in the study area, so that year-to-year variations in precipitation and ground-water recharge are large and difficult to predict. Proxy data indicate geologically recent periods of multidecadal droughts unlike any in the modern instrumental record. Anthropogenically induced climate change likely will reduce ground-water recharge through diminished snowpack at higher elevations, and perhaps through increased drought. Future changes in El Niño and monsoonal patterns, both crucial to precipitation in the study area, are highly uncertain in current models. Land-use modifications influence ground-water recharge directly through vegetation, irrigation, and impermeable area, and indirectly through climate change. High ranges bounding the study area—the San Bernadino Mountains and Sierra Nevada to the west, and the Wasatch and southern Colorado Rocky Mountains to the east—provide external geologic controls on ground-water recharge. Internal geologic controls stem from tectonic processes that led to numerous, variably connected alluvial-filled basins, exposure of extensive Paleozoic aquifers in mountainous recharge areas, and distinct modes of recharge in the Colorado Plateau and Basin and Range subregions.

southwestern United States↗

USGS2009

Continental margin sedimentation: From sediment transport to sequence stratigraphy

This volume on continental margin sedimentation brings together an expert editorial and contributor team to create a state-of-the-art resource. Taking a global perspective, the book spans a range of timescales and content, ranging from how oceans transport particles, to how thick rock sequences are formed on continental margins. - Summarizes and integrates our understanding of sedimentary processes and strata associated with fluvial dispersal systems on continental shelves and slopes - Explores timescales ranging from particle transport at one extreme, to deep burial at the other - Insights are presented for margins in general, and with focus on a tectonically active margin (northern California) and a passive margin (New Jersey), enabling detailed examination of the intricate relationships between a wide suite of sedimentary processes and their preserved stratigraphy - Includes observational studies which document the processes and strata found on particular margins, in addition to numerical models and laboratory experimentation, which provide a quantitative basis for extrapolation in time and space of insights about continental-margin sedimentation - Provides a research resource for scientists studying modern and ancient margins, and an educational text for advanced students in sedimentology and stratigraphy

Book↗

USGS2025

From sample to sonde to Sentinel-2: Insights from a multi-scale chlorophyll-a monitoring effort in the Hudson River, New York

Monitoring cyanobacteria and other nuisance phytoplankton in the Hudson River is of great interest given its societal and ecological importance. Satellite remote sensing provides a cost-effective method to monitor chlorophyll- a (chl-a), a common proxy for algal biomass; however, the dynamic nature of rivers complicates approaches traditionally applied to lakes and oceans. During 2021–2023, we collected discrete samples for laboratory measurement of chl-a and measured in situ chl-a fluorescence during a series of longitudinal boat surveys along a 220-km reach of the lower Hudson River. Surveys were timed to coincide with Sentinel-2 satellite overpasses. We first investigated relations between laboratory-measured chl-a concentration and field-measured chl-a fluorescence, observing a weak correlation ( r 2 = 0.25) that improved substantially after splitting data by day (mean r 2 = 0.53). Separately, to estimate chl-a fluorescence using satellite data, we developed a series of random forest models leveraging the rich fluorescence dataset collected. We tested three model types: individual day models, leave-one-out models trained on all days except a holdout test day, and a single pooled model trained on all days. Generally, individual day models exhibited lowest error (mean of mean absolute error [MAE] = 0.16 relative fluorescence units [RFU]), followed by the single pooled model (MAE = 0.22 RFU). Daily holdout models showed highest error (mean MAE = 0.40 RFU); this approach was intended to represent model performance on a day unseen in the training set, providing a more conservative estimate of performance than the more traditional pooled approach. Findings from both analyses emphasize the importance of considering temporal variability when modeling riverine systems.

New Jersey, New York↗

USGS2004

The relationship between gorgonian coral (Cnidaria: Gorgonacea) diseases and African dust storms

The number of reports of coral diseases has increased throughout the world in the last 20 years. Aspergillosis, which primarily affects Gorgonia ventalina and G. flabellum, is one of the few diseases to be characterized. This disease is caused by Aspergillus sydowii, a terrestrial fungus with a worldwide distribution. Upon infection, colonies may lose tissue, and ultimately, mortality may occur if the infection is not sequestered. The spores of A. sydowii are <5 ??m, small enough to be easily picked up by winds and dispersed over great distances. Aspergillosis is prevalent in the Caribbean, and it appears that this primarily terrestrial fungus has adapted to a marine environment. It has been proposed that dust storms originating in Africa may be one way in which potential coral pathogens are distributed and deposited into the marine environments of the Caribbean. To test the hypothesis that African dust storms transport and deposit pathogens, we collected air samples from both dust storms and periods of nondust in St. John, U.S. Virgin Islands. Because we focused on fungal pathogens and used A. sydowii as a model, we isolated and cultured fungi on various types of media. Fungi including Aspergillus spp. were isolated from air samples taken from dust events and non-dust events. Twenty-three separate cultures and seven genera were isolated from dust event samples whereas eight cultures from five genera were isolated from non-dust air samples. Three isolates from the Virgin Islands dust event samples morphologically identified as Aspergillus spp. produced signs of aspergillosis in seafans, and the original pathogens were re-isolated from those diseased seafans fulfilling Koch's Postulates. This research supports the hypothesis that African dust storms transport across the Atlantic Ocean and deposit potential coral pathogens in the Caribbean.

Aerobiologia↗

USGS1997

Mass balance approach to selenium cycling through the San Joaquin Valley, sources to river to bay

Surface and ground waters of the Central Valley of California (e.g., rivers, dams, off-stream storage reservoirs, pumping facilities, irrigation and drinking water supply canals, agricultural drainage canals) are part of a hydrologic system that makes up a complex ecosystem extending from the riparian wetlands of the Sacramento and San Joaquin Rivers through the San Francisco Bay/Delta Estuary to the Pacific Ocean (Fig. 1). Water quality concerns center on elevated selenium (Se) and salt concentrations in irrigation drainage water discharged into the waterways of the relatively arid San Joaquin Valley (SJV), including the San Joaquin River (SJR). These waters are made unique by dissolved Se, weathered from marine sedimentary rocks of the Coast Ranges to the west, being ultimately concentrated to toxic levels in aquatic wildlife in the wetlands of the SJV/SJR trough (Figs. 1 and 2) (Presser and Ohlendorf, 1987; Presser, 1994). Scientific and environmental concerns focus on the bioreactive properties of Se and its partitioning among biota, water, and sediment, and on whether simple dilution models can be applied to an element that bioaccumulates. Because of state and federal commitments to provide water for irrigation, as well as drainage of irriga tion wastewater by the year 2000 drainage from over 180,000 ha of seleniferous, salinized farmland within the western SJV will create approximately 387 million cubic meters of potentially toxic drainage water annually (i.e., “problem water,” as defined by the San Joaquin Valley Drainage Program, 1990), thus lending urgency to an understanding of the biogeochemistry of Se in this environment.

California↗

USGS2015

The modelling and assessment of whale-watching impacts

In recent years there has been significant interest in modelling cumulative effects and the population consequences of individual changes in cetacean behaviour and physiology due to disturbance. One potential source of disturbance that has garnered particular interest is whale-watching. Though perceived as ‘green’ or eco-friendly tourism, there is evidence that whale-watching can result in statistically significant and biologically meaningful changes in cetacean behaviour, raising the question whether whale-watching is in fact a long term sustainable activity. However, an assessment of the impacts of whale-watching on cetaceans requires an understanding of the potential behavioural and physiological effects, data to effectively address the question and suitable modelling techniques. Here, we review the current state of knowledge on the viability of long-term whale-watching, as well as logistical limitations and potential opportunities. We conclude that an integrated, coordinated approach will be needed to further understanding of the possible effects of whale-watching on cetaceans.

Ocean and Coastal Management↗

USGS2011

Evidence for competition at sea between Norton Sound chum salmon and Asian hatchery chum salmon

Increasing production of hatchery salmon over the past four decades has led to concerns about possible density-dependent effects on wild Pacific salmon populations in the North Pacific Ocean. The concern arises because salmon from distant regions overlap in the ocean, and wild salmon populations having low productivity may compete for food with abundant hatchery populations. We tested the hypothesis that adult length-at-age, age-at-maturation, productivity, and abundance of a Norton Sound, Alaska, chum salmon population were influenced by Asian hatchery chum salmon, which have become exceptionally abundant and surpassed the abundance of wild chum salmon in the North Pacific beginning in the early 1980s. We found that smaller adult length-at-age, delayed age-at-maturation, and reduced productivity and abundance of the Norton Sound salmon population were associated with greater production of Asian hatchery chum salmon since 1965. Modeling of the density-dependent relationship, while controlling for other influential variables, indicated that an increase in adult hatchery chum salmon abundance from 10 million to 80 million adult fish led to a 72% reduction in the abundance of the wild chum salmon population. These findings indicate that competition with hatchery chum salmon contributed to the low productivity and abundance of Norton Sound chum salmon, which includes several stocks that are classified as Stocks of Concern by the State of Alaska. This study provides new evidence indicating that large-scale hatchery production may influence body size, age-at-maturation, productivity and abundance of a distant wild salmon population.

Environmental Biology of Fishes↗

USGS2003

Serpentinization of abyssal peridotites from the MARK area, Mid-Atlantic Ridge: Sulfur geochemistry and reaction modeling

The opaque mineralogy and the contents and isotope compositions of sulfur in serpentinized peridotites from the MARK (Mid-Atlantic Ridge, Kane Fracture Zone) area were examined to understand the conditions of serpentinization and evaluate this process as a sink for seawater sulfur. The serpentinites contain a sulfur-rich secondary mineral assemblage and have high sulfur contents (up to 1 wt.%) and elevated ??34Ssulfide (3.7 to 12.7???). Geochemical reaction modeling indicates that seawater-peridotite interaction at 300 to 400??C alone cannot account for both the high sulfur contents and high ??34Ssulfide. These require a multistage reaction with leaching of sulfide from subjacent gabbro during higher temperature (???400??C) reactions with seawater and subsequent deposition of sulfide during serpentinization of peridotite at ???300??C. Serpentinization produces highly reducing conditions and significant amounts of H2 and results in the partial reduction of seawater carbonate to methane. The latter is documented by formation of carbonate veins enriched in 13C (up to 4.5???) at temperatures above 250??C. Although different processes produce variable sulfur isotope effects in other oceanic serpentinites, sulfur is consistently added to abyssal peridotites during serpentinization. Data for serpentinites drilled and dredged from oceanic crust and from ophiolites indicate that oceanic peridotites are a sink for up to 0.4 to 6.0 ?? 1012 g seawater S yr-1. This is comparable to sulfur exchange that occurs in hydrothermal systems in mafic oceanic crust at midocean ridges and on ridge flanks and amounts to 2 to 30% of the riverine sulfate source and sedimentary sulfide sink in the oceans. The high concentrations and modified isotope compositions of sulfur in serpentinites could be important for mantle metasomatism during subduction of crust generated at slow spreading rates. ?? 2003 Elsevier Science Ltd.

Geochimica et Cosmochimica Acta↗

USGS2007

South San Francisco Bay, California

The U.S. Geological Survey, in cooperation with the California Coastal Conservancy and the National Oceanic and Atmospheric Administration, mapped the floor of south San Francisco Bay and adjoining land using single-beam sonar and airborne lidar (light detection and ranging). To learn more, visit http://pubs.usgs.gov/sim/2007/2987/. View eastward. Elevations in mapped area color coded: purple (approx 15 m below sea level) to red-orange (approx 90 m above sea level). South San Francisco Bay is very shallow, with a mean water depth of 2.7 m (8.9 ft). Trapezoidal depression near San Mateo Bridge is where sediment has been extracted for use in cement production and as bay fill. Land from USGS digital orthophotographs (DOQs) overlaid on USGS digital elevation models (DEMs). Distance across bottom of image approx 11 km (7 mi); vertical exaggeration 1.5X.

California↗

USGS2009

International year of planet earth 7. Oceans, submarine land-slides and consequent tsunamis in Canada

Canada has the longest coastline and largest continental margin of any nation in the World. As a result, it is more likely than other nations to experience marine geohazards such as submarine landslides and consequent tsunamis. Coastal landslides represent a specific threat because of their possible proximity to societal infrastructure and high tsunami potential; they occur without warning and with little time lag between failure and tsunami impact. Continental margin landslides are common in the geologic record but rare on human timescales. Some ancient submarine landslides are massive but more recent events indicate that even relatively small slides on continental margins can generate devastating tsunamis. Tsunami impact can occur hundreds of km away from the source event, and with less than 2 hours warning. Identification of high-potential submarine landslide regions, combined with an understanding of landslide and tsunami processes and sophisticated tsunami propagation models, are required to identify areas at high risk of impact.

Geoscience Canada↗

USGS2023

Spatial distribution and variability of lobe facies in a large sand-rich submarine fan system: Neoproterozoic Zerrissene Group, Namibia

The deposits of the upper Neoproterozoic Zerrissene Group of central-western Namibia represent a large siliciclastic deep-water depositional system that showcases the intricacies of facies and architectural relationships from bed-scale to fan-system-scale. The lack of vegetation in the Namib Desert and regular east–west repetition of folded stratigraphy (reflecting ca 50% tectonic shortening) provides quasi-three-dimensional exposure over a current area of approximately 2700 square kilometres. The Brak River Formation, the middle sand-rich unit of the Zerrissene Group, consists of nearly 600 m of strata exposed in multiple parallel continuous outcrops up to ca 10 km in length and oriented obliquely to depositional dip. Ten stratigraphic sections are correlated ca 32 km ( ca 64 km restored) across the basin and offer exposure comparable in scale to modern submarine fans. Six sedimentary facies are identified and grouped into four facies associations that represent axial-to-marginal portions of deep-water lobes in an unconfined submarine fan system. Spatial facies patterns, regional thickness variations, and palaeocurrents indicate that Brak River Formation sediments were transported primarily from the north to south–south-west through a trough-like basin, and deposited within an unconfined basin plain at the junction of the Adamastor and Khomas oceans. The unique outcrop exposure and extent permits the documentation of system-scale architecture and basin configuration of the Brak River submarine fan system. A transition from the sand-rich lower Brak River Formation to more intercalated mudstone-dominated intervals in the middle and upper Brak River Formation is interpreted to record a change from aggradational to compensational stacking of lobe deposits. This records the evolution of a large submarine fan as it filled the subtle seafloor topography and became less confined at the system-scale. The documentation of these deep-water deposits from centimetre-scale to basin-scale provides a new model for a system with extensive long-distance transport of sand-rich sediment gravity flows to submarine lobes without apparent channelization.

Sedimentology↗

USGS2008

Joint variability of global runoff and global sea surface temperatures

Global land surface runoff and sea surface temperatures (SST) are analyzed to identify the primary modes of variability of these hydroclimatic data for the period 1905-2002. A monthly water-balance model first is used with global monthly temperature and precipitation data to compute time series of annual gridded runoff for the analysis period. The annual runoff time series data are combined with gridded annual sea surface temperature data, and the combined dataset is subjected to a principal components analysis (PCA) to identify the primary modes of variability. The first three components from the PCA explain 29% of the total variability in the combined runoff/SST dataset. The first component explains 15% of the total variance and primarily represents long-term trends in the data. The long-term trends in SSTs are evident as warming in all of the oceans. The associated long-term trends in runoff suggest increasing flows for parts of North America, South America, Eurasia, and Australia; decreasing runoff is most notable in western Africa. The second principal component explains 9% of the total variance and reflects variability of the El Ni??o-Southern Oscillation (ENSO) and its associated influence on global annual runoff patterns. The third component explains 5% of the total variance and indicates a response of global annual runoff to variability in North Aflantic SSTs. The association between runoff and North Atlantic SSTs may explain an apparent steplike change in runoff that occurred around 1970 for a number of continental regions.

Journal of Hydrometeorology↗

USGS2015

Basement and regional structure along strike of the Queen Charlotte Fault in the context of modern and historical earthquake ruptures

The Queen Charlotte fault (QCF) is a dextral transform system located offshore of southeastern Alaska and western Canada, accommodating &sim;4.4  cm/yr of relative motion between the Pacific and North American plates. Oblique convergence along the fault increases southward, and how this convergence is accommodated is still debated. Using seismic reflection data, we interpret offshore basement structure, faulting, and stratigraphy to provide a geological context for two recent earthquakes, an M w 7.5 strike‐slip event near Craig, Alaska, and an M w 7.8 thrust event near Haida Gwaii, Canada. We map downwarped Pacific oceanic crust near 54° N, between the two rupture zones. Observed downwarping decreases north and south of 54° N, parallel to the strike of the QCF. Bending of the Pacific plate here may have initiated with increased convergence rates due to a plate motion change at &sim;6  Ma. Tectonic reconstruction implies convergence‐driven Pacific plate flexure, beginning at 6 Ma south of a 10° bend the QCF (which is currently at 53.2° N) and lasting until the plate translated past the bend by &sim;2  Ma. Normal‐faulted approximately late Miocene sediment above the deep flexural depression at 54° N, topped by relatively undeformed Pleistocene and younger sediment, supports this model. Aftershocks of the Haida Gwaii event indicate a normal‐faulting stress regime, suggesting present‐day plate flexure and underthrusting, which is also consistent with reconstruction of past conditions. We thus favor a Pacific plate underthrusting model to initiate flexure and accommodation space for sediment loading. In addition, mapped structures indicate two possible fault segment boundaries along the QCF at 53.2° N and at 56° N.

Alaska↗

USGS2015

Climate change in the Northeast and Midwest United States

The climate is changing rapidly in ways that have already impacted wildlife and their habitats. Here, we present a summary of the observed past and projected future climate changes in the region that are relevant to wildlife and ecosystems, as well as what we know and don’t know in order to raise managers’ confidence in their planning. A number of large-scale regional changes affect the overall terrestrial landscape within the Northeast and Midwest United States: Warming is occurring in every season, particularly in winter, at higher latitudes, at higher elevations, and inland (i.e. away from the ocean and lake coasts). Heatwaves may become more frequent, more intense, and last longer. Precipitation amounts are increasing, particularly in winter and with respect to highintensity events in summer. Snow is shifting to rain, leading to reduced snowpacks and extent of snow cover, as well as harder, crustier snowpacks. Atmospheric moisture content is likely to increase. Wind speeds are declining, though wind gusts may be intensifying. Streamflows are intensifying. Streams are warming. Thunderstorms may become more severe. Floods are intensifying, yet droughts are also on the rise as dry periods between events get longer. Blizzards and ice storms are occurring more often in some areas, though most areas experiencing milder winters (i.e., warmer and with less snow). Growing seasons are getting longer, with more growing degree days accumulating earlier in the season. In addition, localized climate change is occurring in specific regions: U.S. Atlantic coast Sea level is rising at an accelerating rate. Tropical cyclones and hurricanes may be intensifying and storm tracks have been shifting northward along the coast. Oceans are warming and becoming more acidic. Great Lakes The lakes are warming. Winter maximum lake ice extent is shrinking. Lake evaporation rates are increasing. Lake-effect snow events are becoming more severe, longer lasting, and shifting to rain, but occurring less often. Water levels have decreased, but may not be linked to anthropogenic climate change. Appalachians Warming may be occurring more rapidly at higher elevations. Greater intensification of heavy rainfall events may be occurring. In the short term (i.e., over the next 5-20 years), the direction and magnitude of warming in the global climate are mostly consistent across all emissions scenarios and with strong agreement across models. Accordingly, we are certain that the Northeast and Midwest will see longer growing seasons. We are likely to see shifts from snow to rain, though shifts in the amount of total precipitation (rain and snow) are less certain. Severe weather events (e.g., thunderstorms, tornadoes) are challenging to detect. Soil moisture and evapotranspiration trends are neither robustly observed nor consistent amongst modeling studies.

Connecticut, Delaware, Iowa, Illinois, Indiana, Ke↗

USGS2020

Dendritic reidite from the Chesapeake Bay impact horizon, Ocean Drilling Program Site 1073 (offshore northeastern USA): A fingerprint of distal ejecta?

High-pressure minerals provide records of processes not normally preserved in Earth’s crust. Reidite, a quenchable polymorph of zircon, forms at pressures >20 GPa during shock compression. However, there is no broad consensus among empirical, experimental, and theoretical studies on the nature of the polymorphic transformation. Here we decipher a multistage history of reidite growth recorded in a zircon grain in distal impact ejecta (offshore northeastern United States) from the ca. 35 Ma Chesapeake Bay impact event which, remarkably, experienced near-complete conversion (89%) to reidite. The grain displays two distinctive reidite habits: (1) intersecting sets of planar lamellae that are dark in cathodoluminescence (CL); and (2) dendritic epitaxial overgrowths on the lamellae that are luminescent in CL. While the former is similar to that described in literature, the latter has not been previously reported. A two-stage growth model is proposed for reidite formation at >40 GPa in Chesapeake Bay impact ejecta: formation of lamellar reidite by shearing during shock compression, followed by dendrite growth, also at high pressure, via recrystallization. The dendritic reidite is interpreted to nucleate on lamellae and replace damaged zircon adjacent to lamellae, which may be amorphous ZrSiO 4 or possibly an intermediate phase, all before quenching. These results provide new insights on the microstructural evolution of the high-pressure polymorphic transformation over the microseconds-long interval of reidite stability during meteorite impact. Given the formation conditions, dendritic reidite may be a unique indicator of distal ejecta.

Virginia↗

USGS1997

Transform push, oblique subduction resistance, and intraplate stress of the Juan de Fuca plate

The Juan de Fuca plate is a small oceanic plate between the Pacific and North America plates. In the southernmost region, referred to as the Gorda deformation zone, the maximum compressive stress σ 1 constrained by earthquake focal mechanisms is N-S. Off Oregon, and possibly off Washington, NW trending left-lateral faults cutting the Juan de Fuca plate indicate a σ 1 in a NE-SW to E-W direction. The magnitude of differential stress increases from north to south; this is inferred from the plastic yielding and distribution of earthquakes throughout the Gorda deformation zone. To understand how tectonic forces determine the stress field of the Juan de Fuca plate, we have modeled the intraplate stress using both elastic and elastic-perfectly plastic plane-stress finite element models. We conclude that the right-lateral shear motion of the Pacific and North America plates is primarily responsible for the stress pattern of the Juan de Fuca plate. The most important roles are played by a compressional force normal to the Mendocino transform fault, a result of the northward push by the Pacific plate and a horizontal resistance operating against the northward, or margin-parallel, component of oblique subduction. Margin-parallel subduction resistance results in large N-S compression in the Gorda deformation zone because the force is integrated over the full length of the Cascadia subduction zone. The Mendocino transform fault serves as a strong buttress that is very weak in shear but capable of transmitting large strike-normal compressive stresses. Internal failure of the Gorda deformation zone potentially places limits on the magnitude of the fault-normal stresses being transmitted and correspondingly on the magnitude of strike-parallel subduction resistance. Transform faults and oblique subduction zones in other parts of the world can be expected to transmit and create stresses in the same manner.

Journal of Geophysical Research B: Solid Earth↗

USGS2004

Estimating hydraulic properties of the Floridan Aquifer System by analysis of earth-tide, ocean-tide, and barometric effects, Collier and Hendry Counties, Florida

Aquifers are subjected to mechanical stresses from natural, non-anthropogenic, processes such as pressure loading or mechanical forcing of the aquifer by ocean tides, earth tides, and pressure fluctuations in the atmosphere. The resulting head fluctuations are evident even in deep confined aquifers. The present study was conducted for the purpose of reviewing the research that has been done on the use of these phenomena for estimating the values of aquifer properties, and determining which of the analytical techniques might be useful for estimating hydraulic properties in the dissolved-carbonate hydrologic environment of southern Florida. Fifteen techniques are discussed in this report, of which four were applied. An analytical solution for head oscillations in a well near enough to the ocean to be influenced by ocean tides was applied to data from monitor zones in a well near Naples, Florida. The solution assumes a completely non-leaky confining unit of infinite extent. Resulting values of transmissivity are in general agreement with the results of aquifer performance tests performed by the South Florida Water Management District. There seems to be an inconsistency between results of the amplitude ratio analysis and independent estimates of loading efficiency. A more general analytical solution that takes leakage through the confining layer into account yielded estimates that were lower than those obtained using the non-leaky method, and closer to the South Florida Water Management District estimates. A numerical model with a cross-sectional grid design was applied to explore additional aspects of the problem. A relation between specific storage and the head oscillation observed in a well provided estimates of specific storage that were considered reasonable. Porosity estimates based on the specific storage estimates were consistent with values obtained from measurements on core samples. Methods are described for determining aquifer diffusivity by comparing the time-varying drawdown in an open well with periodic pressure-head oscillations in the aquifer, but the applicability of such methods might be limited in studies of the Floridan aquifer system.

Florida↗