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USGS2022

Defining fine-scaled population structure among continuously distributed populations

Understanding wildlife population structure and connectivity can help managers identify conservation strategies, as structure can facilitate the study of population changes and habitat connectivity can provide information on dispersal and biodiversity. To facilitate the use of wildlife monitoring data for improved adaptive management, we developed a novel approach to define hierarchical tiers (multiple scales) of population structure. We defined population structure by combining graph theory with biological inference about dispersal capability (based on movement, gene flow, and habitat condition) and functional processes affecting movement (e.g. habitat selection across scales of landscape preferences). First, we developed least-cost paths between high fidelity sites (habitat patches) using a cost surface, informed from functional processes of habitat characteristics to account for resistance of inter-patch movements. Second, we combined the paths into a multi-path graph construct. Third, we used information on potential connectivity (dispersal distances) and functional connectivity (permeability of fragmented landscapes based on selection preferences) to decompose the graph into hierarchical tiers of connected subpopulations, denoting the degree that dispersal affected population structure. As a case study, we applied our approach across the greater sage-grouse ( Centrocercus urophasianus ) range, a species of conservation concern in western United States. We described the relative importance of local populations and where to potentially avoid landscape disturbances that may negatively affect population connectivity using centrality measures supported by graph theory, and we demonstrated close alignment of the resulting population structure with population densities. This method can be adapted for other species with site fidelity and used as a management tool to evaluate population trends and responses to landscape changes across different temporal and spatial scales.

Methods in Ecology and Evolution↗

USGS2013

Distance, dams and drift: What structures populations of an endangered, benthic stream fish?

Spatial population structure plays an important role in species persistence, evolution and conservation. Benthic stream fishes are diverse and frequently imperilled, yet the determinants and spatial scaling of their population structure are understudied. We investigated the range-wide population genetic structure of Roanoke logperch ( Percina rex ), an endangered, benthic stream fish of the eastern United States. Fish were sampled from 35 sites and analysed at 11 microsatellite DNA loci. Clustering models were used to sort individuals into genetically cohesive groups and thereby estimate the spatial scaling of population structure. We then used Bayesian generalized linear mixed models (BGLMMs) to test alternative hypotheses about the environmental factors most responsible for generating structure, as measured by the differentiation statistic F ST . Clustering models delineated seven discrete populations, whose boundaries coincided with agents of fragmentation, including hydroelectric dams and tailwaters. In the absence of hydrological barriers, gene flow was extensive throughout catchments, whereas there was no evidence for contemporary dispersal between catchments across barriers. In the best-supported BGLMM, F ST was positively related to the spatial distance and degree of hydrological alteration between sites and negatively related to genetic diversity within sites. Whereas the effect of tailwaters was equivocal, dams strongly influenced differentiation: the effect of a dam on F ST was comparable to that of a between-site distance of over 1200 km of unimpounded river. Overall, the effect of distance-mediated dispersal was negligible compared to the combined effects of fragmentation and genetic drift. The contemporary population structure of P. rex comprises a few geographically extensive ‘islands’ that are fragmented by hydroelectric projects. This information clarifies the importance of a catchment-scale perspective on conserving the species and suggests that its recovery may require genetic and/or demographic reconnection of presently isolated populations.

North Carolina, Virginia↗

USGS2018

Spatial extent of analysis influences observed patterns of population genetic structure in a widespread darter species (Percidae)

Connectivity among stream fish populations allows for exchange of genetic material and helps maintain genetic diversity, adaptive potential and population stability over time. Changes in species demographics and population connectivity have the potential to permanently alter the genetic patterns of stream fish, although these changes through space and time are variable and understudied in small‐bodied freshwater fish. As a spatially widespread, common species of benthic freshwater fish, the variegate darter ( Etheostoma variatum ) is a model species for documenting how patterns of genetic structure and diversity respond to increasing isolation due to large dams and how scale of study may shape our understanding of these patterns. We sampled variegate darters from 34 sites across their range in the North American Ohio River basin and examined how patterns of genetic structure and diversity within and between populations responded to historical population changes and dams within and between populations. Spatial scale and configuration of genetic structure varied across the eight identified populations, from tributaries within a watershed, to a single watershed, to multiple watersheds that encompass Ohio River mainstem habitats. This multiwatershed pattern of population structuring suggests genetic dispersal across large distances was and may continue to be common, although some populations remain isolated despite no apparent structural dispersal barriers. Populations with low effective population sizes and evidence of past population bottlenecks showed low allelic richness, but diversity patterns were not related to watershed size, a surrogate for habitat availability. Pairwise genetic differentiation ( F ST ) increased with fluvial distance and was related to both historic and contemporary processes. Genetic diversity changes were influenced by underlying population size and stability, and while instream barriers were not strong determinants of genetic structuring or loss of genetic diversity, they reduce population connectivity and may impact long‐term population persistence. The broad spatial scale of this study demonstrated the large spatial extent of some variegate darter populations and indicated that dispersal is more extensive than expected given the movement patterns typically observed for small‐bodied, benthic fish. Dam impacts depended on underlying population size and stability, with larger populations more resilient to genetic drift and allelic richness loss than smaller populations. Other darters that inhabit large river habitats may show similar patterns in landscape‐scale studies, and large river barriers may impact populations of small‐bodied fish more than previously expected. Estimation of dispersal rates and behaviours is critical to conservation of imperilled riverine species such as darters.

Freshwater Biology↗

USGS2010

Rayleigh-wave phase-velocity maps and three-dimensional shear velocity structure of the western US from local non-plane surface wave tomography

We utilize two-and-three-quarter years of vertical-component recordings made by the Transportable Array (TA) component of Earthscope to constrain three-dimensional (3-D) seismic shear wave velocity structure in the upper 200 km of the western United States. Single-taper spectral estimation is used to compile measurements of complex spectral amplitudes from 44 317 seismograms generated by 123 teleseismic events. In the ﬁrst step employed to determine the Rayleigh-wave phase-velocity structure, we implement a new tomographic method, which is simpler and more robust than scattering-based methods (e.g. multi-plane surface wave tomography). The TA is effectively implemented as a large number of local arrays by deﬁning a horizontal Gaussian smoothing distance that weights observations near a given target point. The complex spectral-amplitude measurements are interpreted with the spherical Helmholtz equation using local observations about a succession of target points, resulting in Rayleigh-wave phase-velocity maps at periods over the range of 18–125 s. The derived maps depend on the form of local ﬁts to the Helmholtz equation, which generally involve the nonplane-wave solutions of Friederich et al. In a second step, the phase-velocity maps are used to derive 3-D shear velocity structure. The 3-D velocity images conﬁrm details witnessed in prior body-wave and surface-wave studies and reveal new structures, including a deep (>100 km deep) high-velocity lineament, of width ∼200 km, stretching from the southern Great Valley to northern Utah that may be a relic of plate subduction or, alternatively, either a remnant of the Mojave Precambrian Province or a mantle downwelling. Mantle seismic velocity is highly correlated with heat ﬂow, Holocene volcanism, elastic plate thickness and seismicity. This suggests that shallow mantle structure provides the heat source for associated magmatism, as well as thinning of the thermal lithosphere, leading to relatively high stress concentration. Our images also conﬁrm the presence of high-velocity mantle at 100 km depth beneath areas of suspected mantle delamination (southern Sierra Nevada; Grande Ronde uplift), low velocity mantle underlying active rift zones, and high velocity mantle associated with the subducting Juan de Fuca plate. Structure established during the Proterozoic appears to exert a lasting inﬂuence on subsequent volcanism and tectonism up to the Present.

Geophysical Journal International↗

USGS2008

Population genetic structure in Atlantic and Pacific Ocean common murres ( Uria aalge ): Natural replicate tests of post-Pleistocene evolution

Understanding the factors that influence population differentiation in temperate taxa can be difficult because the signatures of both historic and contemporary demographics are often reflected in population genetic patterns. Fortunately, analyses based on coalescent theory can help untangle the relative influence of these historic and contemporary factors. Common murres ( Uria aalge ) are vagile seabirds that breed in the boreal and low arctic waters of the Northern Hemisphere. Previous analyses revealed that Atlantic and Pacific populations are genetically distinct; however, less is known about population genetic structure within ocean basins. We employed the mitochondrial control region, four microsatellite loci and four intron loci to investigate population genetic structure throughout the range of common murres. As in previous studies, we found that Atlantic and Pacific populations diverged during the Pleistocene and do not currently exchange migrants. Therefore, Atlantic and Pacific murre populations can be used as natural replicates to test mechanisms of population differentiation. While we found little population genetic structure within the Pacific, we detected significant east-west structuring among Atlantic colonies. The degree that population genetic structure reflected contemporary population demographics also differed between ocean basins. Specifically, while the low levels of population differentiation in the Pacific are at least partially due to high levels of contemporary gene flow, the east-west structuring of populations within the Atlantic appears to be the result of historic fragmentation of populations rather than restricted contemporary gene flow. The contrasting results in the Atlantic and Pacific Oceans highlight the necessity of carefully considering multilocus nonequilibrium population genetic approaches when reconstructing the demographic history of temperate Northern Hemisphere taxa. ?? 2008 The Authors.

Molecular Ecology↗

USGS2022

Species-specific responses to landscape features shaped genomic structure within Alaska galliformes

Aim Connectivity is vital to the resiliency of populations to environmental change and stochastic events, especially for cold-adapted species as Arctic and alpine tundra habitats retract as the climate warms. We examined the influence of past and current landscapes on genomic connectivity in cold-adapted galliformes as a critical first step to assess the vulnerability of Alaska ptarmigan and grouse to environmental change. We hypothesize that the mosaic of physical features and habitat within Alaska promoted the formation of genetic structure across species. Location Alaska, United States of America. Taxa Ptarmigan and Grouse (Galliformes: Tetraoninae). Methods We collected double digest restriction-site-associated DNA sequence data from six ptarmigan and grouse species ( N = 13–145/species) sampled across multiple ecosystems up to ~10 degrees of latitude. Spatial genomic structure was analysed using methods that reflect different temporal scales: (1) principal components analysis to identify major trends in the distribution of genomic variation; (2) maximum likelihood clustering analyses to test for the presence of multiple genomic groupings; (3) shared co-ancestry analyses to assess contemporary relationships and (4) effective migration surfaces to identify regions that deviate from a null model of isolation by distance. Results Levels of genomic structure varied across species (Φ ST =0.009–0.042). Three general patterns of structure emerged: (1) east-west partition located near the Yukon-Tanana uplands; (2) north-south split coinciding with the Alaska Range and (3) northern group near the Brooks Range. Species-specific patterns were observed; not all landscape features were barriers to gene flow for all ptarmigan and grouse and temporal contrasts were detected at the Brooks Range. Main conclusions Within Alaska galliformes, patterns of genomic structure coincide with physiographic features and highlight the importance of physical and ecological barriers in shaping how genomic diversity is arrayed across the landscape. Lack of concordance in spatial patterns indicates that species behaviour and habitat affinities play key roles in driving the contrasting patterns of genomic structure.

Alaska↗

USGS2011

Differences in macroinvertebrate community structure in streams and rivers with different hydrologic regimes in the semi-arid Colorado Plateau

Aquatic macroinvertebrates are sensitive to changes in their chemical and physical environment, and as such, serve as excellent indicators of overall ecosystem health. Moreover, temporal and spatial differences in macroinvertebrate community structure can be used to investigate broad issues in aquatic science, such as the hypothesis that changes in climate are likely to have disproportionately large effects on small, intermittent stream ecosystems. We quantified macroinvertebrate community structure and abiotic conditions at ten stream sites with different dominant hydrologic regimes in the Colorado Plateau, ranging from small, intermittent desert streams to large perennial mountain rivers. Considerable differences were observed in community structure between sites with differing hydrologic regimes. Quantitative results of non-metric multidimensional scaling (NMDS) ordination and Spearman rank correlations between physical habitat and macroinvertebrate resemblance matrices indicate that discharge, geomorphic channel unit type (% pools vs. % riffles), percent of substrate composed of sand, and velocity were the subset of measured habitat variables that best explained the differences in macroinvertebrate community structure among sites. Of the 134 taxa identified, nine taxa explained 95 % of the variability in community structure between sites. These results add to a growing base of knowledge regarding the functioning of lotic ecosystems in the Colorado Plateau, and provide timely information on anticipated changes in the structure and function of aquatic ecosystems in response to predicted future environmental conditions.

Colorado Plateau↗

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↗

USGS1969

Experiments on formation of contorted structures in mud

Contorted structures can be formed in mud or sand as a result of differential loading. Fifteen sets of experiments were conducted in water tanks to test various factors of possible significance in the contortion of mud by loading. Of six factors tested, the most significant was distribution of load, but others affecting the type of structure under certain conditions were (1) the manner of depositing the mud, (2) the form of the underlying surface, (3) the direction of loading, and (4) the movement or lack of movement of water during loading. Organic material was shown to be unneccessary in forming conical structure or convolute bedding. Strength of base had little or no influence on convolute-structure development. Contortions ranged from the simple anticlinal type with vertical axial plane, commonly referred to as convolute, to structures with gently dipping axial planes, to others with lateral extensions or “flames” from the apexes, and, finally, to those with complex overturned folds. Causes of these variations were determined in terms of the factors listed above. Some additional forms of contorted bedding result from other types of penecontemporaneous deformation such as slumping from undermining or from oversteepening, differential lateral movement, and surface drag; these forms differ from those structures formed by loading.

Geological Society of America Bulletin↗

USGS1998

40Ar/39Ar age of the Manson impact structure, Iowa, and correlative impact ejecta in the Crow Creek member of the Pierre Shale (Upper Cretaceous), South Dakota and Nebraska

A set of 34 laser total-fusion 40 Ar/ 39 Ar analyses of sanidine from a melt layer in crater-fill deposits of the Manson impact structure in Iowa has a weighted-mean age of 74.1 ± 0.1 Ma. This age is about 9.0 m.y. older than 40 Ar/ 39 Ar ages of shocked microcline from the Manson impact structure reported previously by others. The 74.1 Ma age of the sanidine, which is a melt product of Precambrian microcline clasts, indicates that the Manson impact structure played no part in the Cretaceous-Tertiary (K-T) mass extinction at 64.5 Ma. Moreover, incremental-heating 40 Ar/ 39 Ar ages of the sanidine show that it is essentially free of excess 40 Ar and has not been influenced by postcrystallization heating or alteration. An age spectrum of the matrix of the melt layer shows effects of 39 Ar recoil, including older ages in the low-temperature increments and younger ages in the high-temperature increments. At 17 places in eastern South Dakota and Nebraska, shocked quartz and feldspar grains are concentrated in the lower part of the Crow Creek Member of the Pierre Shale (Upper Cretaceous). The grains are largest (3.2 mm) in southeastern South Dakota and decrease in size (0.45 mm) to the northwest, consistent with the idea that the Manson impact structure was their source. The ubiquitous presence of shocked grains concentrated in a thin calcarenite at the base of the Crow Creek Member suggests it is an event bed recording an instant of geologic time. Ammonites below and above the Crow Creek Member limit its age to the zone of Didymoceras nebrascense of earliest late Campanian age. Plagioclase from a bentonite bed in this zone in Colorado has a 40 Ar/ 39 Ar age of 74.1 ± 0.1 Ma commensurate with our sanidine age of 74.1 Ma for the Manson impact structure. 40 Ar/ 39 Ar ages of bentonite beds below and above the Crow Creek are consistent with our 74.1 ± 0.1 Ma age for the Manson impact structure and limit its age to the interval ±74.5 0.1 to 73.8 ± 0.1 Ma. Recently, two origins for the Crow Creek have been proposed—eastward transgression of the Late Cretaceous sea and a Manson impact-triggered tsunami. We conclude that most data are in accord with an impact origin for the Crow Creek Member and are at odds with the marine transgression hypothesis.

Iowa, Nebraska, South Dakota↗

USGS2005

Structural evolution of fault zones in sandstone by multiple deformation mechanisms: Moab fault, southeast Utah

Faults in sandstone are frequently composed of two classes of structures: (1) deformation bands and (2) joints and sheared joints. Whereas the former structures are associated with cataclastic deformation, the latter ones represent brittle fracturing, fragmentation, and brecciation. We investigated the distribution of these structures, their formation, and the underlying mechanical controls for their occurrence along the Moab normal fault in southeastern Utah through the use of structural mapping and numerical elastic boundary element modeling. We found that deformation bands occur everywhere along the fault, but with increased density in contractional relays. Joints and sheared joints only occur at intersections and extensional relays. In all locations , joints consistently overprint deformation bands. Localization of joints and sheared joints in extensional relays suggests that their distribution is controlled by local variations in stress state that are due to mechanical interaction between the fault segments. This interpretation is consistent with elastic boundary element models that predict a local reduction in mean stress and least compressive principal stress at intersections and extensional relays. The transition from deformation band to joint formation along these sections of the fault system likely resulted from the combined effects of changes in remote tectonic loading, burial depth, fluid pressure, and rock properties. In the case of the Moab fault, we conclude that the structural heterogeneity in the fault zone is systematically related to the geometric evolution of the fault, the local state of stress associated with fault slip , and the remote loading history. Because the type and distribution of structures affect fault permeability and strength, our results predict systematic variations in these parameters with fault evolution. ?? 2004 Geological Society of America.

Geological Society of America Bulletin↗

USGS2023

Structural discontinuities and their control on hydrothermal systems in the Great Basin, USA

Faults are important controls on hydrothermal circulation worldwide. More specifically, structural discontinuities, i.e. locations where faults interact and intersect, host many hydrothermal systems. In the Great Basin, western USA, an extensive characterization effort demonstrated that hydrothermal systems are controlled by one (or more) of eight types of structural discontinuities. Presumably, specific attributes of these structural settings control the generation and maintenance of permeability and porosity, and therefore localize hydrothermal processes. Herein, I examine representative examples of the eight structural settings that host hydrothermal systems in the Great Basin. For each setting, I use a boundary element method to model fault slip on the major faults and track the distribution of stress and strain in the surrounding crust. Results demonstrate that the largest magnitude and most localized stress and strain effects occur in the structural settings that host the largest number of hydrothermal systems; fault stepovers and fault terminations. Structural settings that are common in areas of strike-slip faulting also show localized stress and strain effects. The modelling presented provides process-based explanations for the empirical and conceptual results of regional characterization of Great Basin hydrothermal systems.

Great Basin↗

USGS2024

Evidence on the ecological and physical effects of built structures in shallow, tropical coral reefs: A systematic map

Shallow, tropical coral reefs face compounding threats from climate change, habitat degradation due to coastal development and pollution, impacts from storms and sea-level rise, and pulse disturbances like blast fishing, mining, dredging, and ship groundings that reduce reef height and complexity. One approach toward restoring coral reef physical structure from such impacts is deploying built structures of artificial, natural, or hybrid (both artificial and natural) origin. Built structures range from designed modules and repurposed materials to underwater sculptures and intentionally placed natural rocks. Restoration practitioners and coastal managers increasingly consider incorporating – and in many cases have already begun to incorporate – built structures into coral reef-related applications, yet synthesized evidence on the ecological (coral-related; e.g., coral growth, coral survival) and physical performance of built structures in coral ecosystems across a variety of contexts (e.g., restoration, coastal protection, mitigation, tourism) is not readily available to guide decisions. To help fill this gap and inform management decisions, we systematically mapped the global distribution and abundance of published evidence on the ecological (coral-related) and physical performance of built structure interventions in shallow (≤ 30 m), tropical (35°N to 35°S) coral ecosystems.

Environmental Evidence↗

USGS2010

Assessing the effects of catch and release regulations on a quality adfluvial brook trout population using a computer based age-structure model

Assessing the Effects of Catch-and-Release Regulations on a Brook Trout Population Using an Age-Structured Model: North American Journal of Fisheries Management: Vol 30, No 6  var _prum=[['id','54ff88bcabe53dc41d1004a5'],['mark','firstbyte',(new Date()).getTime()]];(function(){var s=document.getElementsByTagName('script')[0],p=document.createElement('script');p.async='async';p.src='//rum-static.pingdom.net/prum.min.js';s.parentNode.insertBefore(p,s);})(); (function (i, s, o, g, r, a, m) { i['GoogleAnalyticsObject'] = r; i[r] = i[r] || function () { (i[r].q = i[r].q || []).push(arguments) }, i[r].l = 1 * new Date(); a = s.createElement(o), m = s.getElementsByTagName(o)[0]; a.async = 1; a.src = g; m.parentNode.insertBefore(a, m) })(window, document, 'script', '//www.google-analytics.com/analytics.js', 'ga'); ga('create', 'UA-3062505-5' , 'auto' ); ga('set', 'dimension1', 'ujfm20'); ga('send', 'pageview'); As populations of wild brook trout Salvelinus fontinalis decline across the species' native range in North America, angling regulations such as fly-fishing only and catch and release are being used by management agencies to conserve fish while maintaining angling opportunities. Postrelease (hooking) mortality may significantly shift the age structure and size structure of populations. To assess the possible influence of catch-and-release fishing on brook trout population structure, we applied data from several sources to build a deterministic population model that included age-classes up to age 5. To assess the potential effect of fishing, we examined the changes in population density and age structure at varying levels of angler effort (0–400 angler-hours·ha −1 ·year −1 ) and hooking mortality rates (0–14%). Assuming a low (5%) hooking mortality rate, trophy brook trout density (ages 4 and 5) decreased by 50% at an angling intensity of 160 angler-hours·ha −1 ·year −1 . As angling effort increased, the proportion of older fish (ages 3–5) declined further. At very high levels of angling effort (>300 angler-hours·ha −1 ·year −1 ), age-4 and age-5 fish were eliminated from the population. Increases in postrelease mortality rates resulted in similar declines for older age-classes. The results of this simulation indicate that hooking mortality rates as might be common in catch-and-release fisheries may significantly shift the age structure of a population, thus reducing trophy angling potential.

North American Journal of Fisheries Management↗

USGS2005

Effects of managed impoundments and herbivory on wetland plant production and stand structure

Managed impoundments, a form of structural marsh management, have been used to enhance plant production in the rapidly-eroding marshes of coastal Louisiana, USA, yet few studies have quantified their effects by measuring plant production before and after impoundment construction. We tested the effects of structural marsh management on the annual aboveground production and plant stand structure (stem density and stem height) of Spartina patens and Schoenoplectus americanus by collecting measurements before and after the construction of two shallow impoundments. We manipulated the water level in each impoundment by adjusting a single flap-gated culvert fitted with a variable crest weir. Because nutria herbivory also seemed to have a strong influence on plant production in these marshes, we tested the effects of nutria herbivory on the annual aboveground production and plant stand structure of both plant species by collecting data from fenced (ungrazed) and unfenced (grazed) plots located in both managed and unmanaged areas. There were no significant differences in Spartina annual production, stem density, and stem height between managed and unmanaged areas, and Schoenoplectus annual production, stem density, and stem height were greater in unmanaged marsh, indicating that the management method used in this study was not effective in promoting plant production in the rapidly-eroding, brackish, deltaic marshes of coastal Louisiana. Nutria herbivory dramatically reduced the annual aboveground production, stem density, and stem, height of Schoenoplectus , a preferred forage species, and thus altered the structure of the mixed species stand. Herbivory had no significant effect on the annual aboveground production and stem density of Spartina . In the absence of herbivory, the stem height of Spartina increased significantly and coincided with significant increases in the stem density and height of Schoenoplectus . The changes in plant stand structure caused by nutria herbivory may facilitate marsh erosion and ultimately contribute to wetland loss.

Louisiana↗

USGS1992

Structure of the Reelfoot rift as interpreted from 2-D magnetotelluric models

The results of magnetotelluric (MT) surveys reveal structures associated with the Reelfoot rift, including an axial high-resistivity structure that may be related to intrusions in the central part of the rift or to a previously unrecognized horst. The axis of this resistivity high generally follows the central part of the Reelfoot rift, but its orientation is offset several degrees from the enigmatic Blytheville arch. The MT structural high follows the main part of a northeast-trending seismicity belt, but in the New Madrid, Missouri area electrical structures are more complicated. In this northern part of the study area, the strike of resistivity structures determined from the MT data indicate a nearly north-south direction, rather than the N45°E direction indicated for most of the data. If the axial high-resistivity structure represents a horst, then it may form an interior divide between thick shale basins. Faults bounding the proposed horst may be a controlling factor for the northeast trending seismic zone.

Arkansas, Kentucky, Missouri, Tennessee↗

USGS1947

Preliminary report on stratigraphy and structure of the Wolf Creek anticline, Alaska

In 1946 U. S. Geological Survey Party 2 was assigned the task of carrying out detailed structural and stratigraphic investigations in the area of Maybe Creek*. Field studies indicated the presence of a large anticlinal structure whose axis lies just south of Wolf Creek. Later structural data compiled from aerial photographic studies indicated that the anticline is a closed structure 8-9 miles long, and as much as 6 miles wide. The evidence for a closure on the east end of the structure appears very strong, but not enough evidence was obtained to prove closure on the west end. In 1947 U. S. Geological Survey Party 1 spent two weeks at Wolf Creek, mapping bedding traces on the west end of the structure with plane table and alidade, in an attempt to prove or disprove west closure on the anticline. Party 1, consisting of 3 geologists, cook, and a "weasel" mechanic, left Umiat on July 21, and returned on August 5. Two "weasels" were used for the transportation of the party and equipment, and were also used extensively in mapping the area.

Alaska↗

USGS1980

Distribution of salt structures in the Gulf of Mexico map and descriptive text

Slender diapiric stocks, broad massifs, anticlinal masses, low-relief swells, and pillowy lobes of Middle to Upper Jurassic salt (Viniegra, 1971; Kirkland and Gerhard, 1971; Imlay, 1980) dominate the structural fabric of large parts of the continental margins and deep basin of the Gulf of Mexico (sht. 1, A). These structures, known collectively as "salt domes," are almost exclusively features of the terrigenous elastic margins of the northern and southwestern Gulf and the central Sigsbee Plain (sht. 2, A). Some salt structures may be present in the East Mexico Slope province of the western gulf, but they are difficult to distinguish from diapiric and nondiapiric shale anticlines, which are the dominant structures on this margin. Except for some piercement domes and nonpiercement uplifts in the northern Florida shelf region and in the eastern Golfo de Campeche, salt structures are not known in the extensive carbonate platform provinces off western Florida and the Yucatan Peninsula. The accompanying compilation from seismic-reflection data documents the distribution of Gulf of Mexico salt features and helps to define boundaries of structural provinces formed mainly, or in part, by tectonic movement of salt.

Miscellaneous Field Studies Map↗