Geology Reports⌕ Search

SEARCH · Geology Reports

Results for “Evolution and Development”

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

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

At least 235 records · Page 13Linked to original sources
USGS2010

Sampling in ecology and evolution - bridging the gap between theory and practice

Sampling is a key issue for answering most ecological and evolutionary questions. The importance of developing a rigorous sampling design tailored to specific questions has already been discussed in the ecological and sampling literature and has provided useful tools and recommendations to sample and analyse ecological data. However, sampling issues are often difficult to overcome in ecological studies due to apparent inconsistencies between theory and practice, often leading to the implementation of simplified sampling designs that suffer from unknown biases. Moreover, we believe that classical sampling principles which are based on estimation of means and variances are insufficient to fully address many ecological questions that rely on estimating relationships between a response and a set of predictor variables over time and space. Our objective is thus to highlight the importance of selecting an appropriate sampling space and an appropriate sampling design. We also emphasize the importance of using prior knowledge of the study system to estimate models or complex parameters and thus better understand ecological patterns and processes generating these patterns. Using a semi-virtual simulation study as an illustration we reveal how the selection of the space (e.g. geographic, climatic), in which the sampling is designed, influences the patterns that can be ultimately detected. We also demonstrate the inefficiency of common sampling designs to reveal response curves between ecological variables and climatic gradients. Further, we show that response-surface methodology, which has rarely been used in ecology, is much more efficient than more traditional methods. Finally, we discuss the use of prior knowledge, simulation studies and model-based designs in defining appropriate sampling designs. We conclude by a call for development of methods to unbiasedly estimate nonlinear ecologically relevant parameters, in order to make inferences while fulfilling requirements of both sampling theory and field work logistics. ?? 2010 The Authors.

Ecography↗
USGS2021

Movement of sediment through a burned landscape: Sediment volume observations and model comparisons in the San Gabriel Mountains, California, USA

Post-wildfire changes to hydrologic and geomorphic systems can lead to widespread sediment redistribution. Understanding how sediment moves through a watershed is crucial for assessing hazards, developing debris flow inundation models, engineering sediment retention solutions, and quantifying the role that disturbances play in landscape evolution. In this study, we used terrestrial and airborne lidar to measure sediment redistribution in the 2016 Fish Fire, in the San Gabriel Mountains in southern California, USA. The lidar areas are in two adjacent watersheds, at spatial scales of 900 m 2 to 4 km 2 , respectively. Terrestrial lidar data were acquired prior to rainfall, and two subsequent surveys show erosional change after rainstorms. Two airborne lidar flights occurred (1) 7 months before, and (2) 14 months after the fire ignition, capturing the erosional effects after rainfall. We found hillslope erosion dominated the overall sediment budget in the first rainy season after wildfire. Only 7% of the total erosion came from the active channel bed and channel banks, and the remaining 93% of eroded sediment was derived from hillslopes. Within the channelized portion of the watershed erosion/deposition could be generally described with topographic metrics used in a stream power equation. Observed sediment volumes were compared with four empirical models and one process-based model. We found that the best predictions of sediment volume were obtained from an empirical model developed in the same physiographic region. Moreover, this study showed that post-wildfire erosion rates in the San Gabriel Mountains attain the same magnitude as millennial time scale bedrock erosion rates.

California↗
USGS2020

Compositional layering in Io driven by magmatic segregation and volcanism

The compositional evolution of volcanic bodies like Io is not well understood. Magmatic segregation and volcanic eruptions transport tidal heat from Io's interior to its surface. Several observed eruptions appear to be extremely high temperature (≥ 1600 K), suggesting either very high degrees of melting, refractory source regions, or intensive viscous heating on ascent. To address this ambiguity, we develop a model that couples crust and mantle dynamics to a simple compositional system. We analyse the model to investigate chemical structure and evolution. We demonstrate that magmatic segregation and volcanic eruptions lead to stratification of the mantle, the extent of which depends on how easily high temperature melts from the more refractory lower mantle can migrate upwards. We propose that Io's highest temperature eruptions originate from this lower mantle region, and that such eruptions act to limit the degree of compositional stratification.

Journal of Geophysical Research↗
USGS2021

Basalt geochemistry and mantle flow during early backarc basin evolution: Havre Trough and Kermadec Arc, southwest Pacific

The Havre Trough (HT) backarc basin in the southwest Pacific is in the rifting stage of development. We distinguish five types of basalt there based on their amount and kind of slab component: backarc basalts (BAB) with little or no slab component, modified BAB with slight amounts, reararc (RA) with more, remnants of the preexisting arc (Colville Ridge horsts), and arc front volcanoes within the HT. Previous subarc mantle is quickly removed and replaced by more fertile mantle with less slab component. The ambient mantle is “Pacific” isotopically, and more enriched in Nb/Yb and Nd and Hf isotope ratios north of the Central Kermadec Discontinuity at 32°S than to the south. The contrast may reflect inheritance in the south of mantle that was depleted during spreading that formed the southern South Fiji Basin and a higher degree of melting because of a wetter slab-derived flux. The slab component also differs along strike, more like a dry melt in the north and a supercritical fluid in the south. The mass fraction of slab component increases southward in the backarc as well as the arc front. RA volcanoes have the most slab component (1%–2%) and form indistinct ridges at high angles to, and <50 km behind, frontal volcanoes. Backarc basalts have less and occur throughout the basin. Slab components are distributed further into the backarc, and more irregularly, during the rifting than spreading stage of backarc basin development. The rifting stage is disorganized geochemically as well as spatially.

Geochemistry Geophysics Geosystems↗
USGS2010

Regional seismic stratigraphy and controls on the Quaternary evolution of the Cape Hatteras region of the Atlantic passive margin, USA

Seismic and core data, combined with amino acid racemization and strontium-isotope age data, enable the definition of the Quaternary stratigraphic framework and recognition of geologic controls on the development of the modern coastal system of North Carolina, U.S.A. Seven regionally continuous high amplitude reflections are defined which bound six seismic stratigraphic units consisting of multiple regionally discontinuous depositional sequences and parasequence sets, and enable an understanding of the evolution of this margin. Data reveal the progressive eastward progradation and aggradation of the Quaternary shelf. The early Pleistocene inner shelf occurs at a depth of ca. 20-40 m beneath the western part of the modern estuarine system (Pamlico Sound). A mid- to outer shelf lowstand terrace (also early Pleistocene) with shelf sand ridge deposits comprising parasequence sets within a transgressive systems tract, occurs at a deeper level (ca. 45-70 m) beneath the modern barrier island system (the Outer Banks) and northern Pamlico Sound. Seismic and foraminiferal paleoenvironmental data from cores indicate the occurrence of lowstand strandplain shoreline deposits on the early to middle Pleistocene shelf. Middle to late Pleistocene deposits occur above a prominent unconformity and marine flooding surface that truncates underlying units, and contain numerous filled fluvial valleys that are incised into the early and middle Pleistocene deposits. The stratigraphic framework suggests margin progradation and aggradation modified by an increase in the magnitude of sea-level fluctuations during the middle to late Pleistocene, expressed as falling stage, lowstand, transgressive and highstand systems tracts. Thick stratigraphic sequences occur within the middle Pleistocene section, suggesting the occurrence of high capacity fluvial point sources debouching into the area from the west and north. Furthermore, the antecedent topography plays a significant role in the evolution of the geomorphology and stratigraphy of this marginal system. ?? 2009 Elsevier B.V.

Marine Geology↗
USGS2021

Influence of antecedent geology on the Holocene formation and evolution of Horn Island, Mississippi, USA

Horn Island, one of the two most stable barriers along the Mississippi-Alabama chain (Cat, East and West Ship, Horn, West Petit Bois, Petit Bois, and Dauphin), provides critical habitat, helps regulate estuarine conditions in the Mississippi Sound, and reduces wave energy and storm surge before they reach the mainland shore. However, important details of the formation and evolution of the island in response to sea-level rise, storms, and antecedent geology remain unclear. This study integrates 2200 km of high-resolution geophysical data, 35 sediment cores, and 18 radiocarbon ages to better understand the geologic history of the island. Incised valleys of the Biloxi and Pascagoula Rivers underlie Horn Island and played a profound role in the evolution of the system. Within the incised valleys, sandy paleochannel deposits represent potential sediment sources during island development. Scour associated with wave and tidal ravinement processes liberated sand from the paleochannels and along with numerous other sizable sand sources on the shelf contributed to the formation and continued maintenance of Horn Island. Based on radiocarbon ages, transgressive ephemeral islands/shoals with no preserved shoreface existed at least 8000 cal yr BP and were frequently overwashed when sea-level rise rates were ~ 4–5 mm/yr. Approximately 5000 cal yr BP, coinciding with a deceleration in sea-level rise to about 1.4 mm/yr and attendant increased sand supply, radiocarbon ages associated with Horn Island's barrier complex and lower shoreface indicate a period of island stabilization. Seismic and sediment core data show a long history of westward lateral migration by longshore currents through tidal ravinement and inlet fill. Subsurface sand packages associated with tidal inlet fill and paleochannels are available for ravinement and may be important sand sources for Horn Island to maintain subaerial exposure with the expected accelerated future sea-level rise.

Mississippi↗
USGS1997

Population analysis of faint galaxies with mixture modeling

We investigate the use of spectral mixture modeling to decompose the integrated spectra of several galaxies. This technique uses a linear combination of template spectra to model over 93% of the variance in Kennicutt's sample of galaxy spectra. One result of our model is a stellar population parameter for each galaxy, which correlates with both star formation rate and the population's mass-to-light ratio. Armed with the template spectra, we use mixture modeling to develop a photometric system that is independent of redshift, in order to more precisely measure the color evolution of galaxies. We show how to use the photometric system to model the composition of high-redshift galaxies in terms of the template spectra, and hence estimate their star-formation history. This photometric technique avoids being biased by the nucleus, unlike techniques which rely on slit spectra.

Astronomical Journal↗
USGS2004

Transmission of Neospora caninum between wild and domestic animals

To determine whether deer can transmit Neospora caninum , brains of naturally infected white-tailed deer ( Odocoileus virginianus ) were fed to 4 dogs; 2 of these dogs shed oocysts. Oocysts from 1 of the dogs were tested by polymerase chain reaction and found to be positive for N. caninum and negative for Hammondia heydorni . The internal transcribed spacer 1 sequence of the new strain (designated NC-deer1) was identical to N. caninum from domestic animals, indicating that N. caninum is transmitted between wild and domestic animals, often enough to prevent divergent evolution of isolated populations of the parasite. NC-deer1 oocysts were administered to a calf that developed a high antibody titer, providing evidence that N. caninum from wildlife can infect cattle. In addition, N. caninum antibody seroprevalence was detected in 64/164 (39%) free-ranging gray wolves ( Canis lupus ), 12/113 (11%) coyotes ( Canis latrans ), 50/193 (26%) white-tailed deer, and 8/61 (13%) moose ( Alces alces ). These data are consistent with a sylvatic transmission cycle of N. caninum between cervids and canids. We speculate that hunting by humans favors the transmission of N. caninum from deer to canids, because deer carcasses are usually eviscerated in the field. Infection of canids in turn increases the risk of transmitting the parasite to domestic livestock.

Illinois, Wisconsin↗
USGS1987

The geologic story of Yosemite National Park

Within 150 years, Yosemite has moved from great obscurity to worldwide fame as one of the most visited of our national parks. As a remarkable place where people can enjoy unparalleled scenes of natural beauty and where many easily observed geologic features are concentrated, the park is rivaled by few other areas on the planet. The majesty and immense variety of these features have inspired artists and photographers, intrigued tourists, and stirred controversy among geologists. Field studies in the Yosemite area have contributed to the development of our ideas about geologic processes, including the different actions of streams and glaciers in the evolution of the landscape, and the formation of granite, the basic bedrock of much of the Earth's continents. The park's role as a natural laboratory for geologic research cannot be overemphasized, and its investigation has led to many landmark studies by U.S. Geological Survey geologists over the past 70 years. In 1913, the first detailed program of research on the geology of the park and the origin ofYosemite Valley was begun by Francois Matthes and Frank Calkins. Their work, along with that of later generations of Survey geologists, myself included, serves as the basis for our present understanding of the geologic history of Yosemite and of the processes that formed and continue to mold its landscape. This book, which makes available in one volume a comprehensive summary of the current geologic knowledge of Yosemite National Park, is an excellent example of the Survey's continuing effort to provide earth-science information in the public service.

California↗
USGS2015

Trans-Amazon Drilling Project (TADP): origins and evolution of the forests, climate, and hydrology of the South American tropics

This article presents the scientific rationale for an ambitious ICDP drilling project to continuously sample Late Cretaceous to modern sediment in four different sedimentary basins that transect the equatorial Amazon of Brazil, from the Andean foreland to the Atlantic Ocean. The goals of this project are to document the evolution of plant biodiversity in the Amazon forests and to relate biotic diversification to changes in the physical environment, including climate, tectonism, and the surface landscape. These goals require long sedimentary records from each of the major sedimentary basins across the heart of the Brazilian Amazon, which can only be obtained by drilling because of the scarcity of Cenozoic outcrops. The proposed drilling will provide the first long, nearly continuous regional records of the Cenozoic history of the forests, their plant diversity, and the associated changes in climate and environment. It also will address fundamental questions about landscape evolution, including the history of Andean uplift and erosion as recorded in Andean foreland basins and the development of west-to-east hydrologic continuity between the Andes, the Amazon lowlands, and the equatorial Atlantic. Because many modern rivers of the Amazon basin flow along the major axes of the old sedimentary basins, we plan to locate drill sites on the margin of large rivers and to access the targeted drill sites by navigation along these rivers.

Amazon region↗
USGS2003

Large carnivores, moose, and humans: A changing paradigm of predator management in the 21st century

We compare and contrast the evolution of human attitudes toward large carnivores between Europe and North America. In general, persecution of large carnivores began much earlier in Europe than North America. Likewise, conservation programs directed at restoration and recovery appeared in European history well before they did in North America. Together, the pattern suggests there has been an evolution in how humans perceive large predators. Our early ancestors were physically vulnerable to large carnivores and developed corresponding attitudes of respect, avoidance, and acceptance. As civilization evolved and man developed weapons, the balance shifted. Early civilizations, in particular those with pastoral ways, attempted to eliminate large carnivores as threats to life and property. Brown bears (Ursus arctos) and wolves (Canis lupus) were consequently extirpated from much of their range in Europe and in North America south of Canada. Efforts to protect brown bears began in the late 1880s in some European countries and population reintroductions and augmentations are ongoing. They are less controversial than in North America. On the other hand, there are no wolf introductions, as has occurred in North America, and Europeans have a more negative attitude towards wolves. Control of predators to enhance ungulate harvest varies. In Western Europe, landowners own the hunting rights to ungulates. In the formerly communistic Eastern European countries and North America, hunting rights are held in common, although this is changing in some Eastern European countries. Wolf control to increase harvests of moose (Alces alces) occurs in parts of North America and Russia; bear control for similar reasons only occurs in parts of North America. Surprisingly, bears and wolves are not controlled to increase ungulates where private landowners have the hunting rights in Europe, although wolves were originally exterminated from these areas. Both the inability of scientific research to adequately predict the effect of predator control on ungulate populations and a shift in public attitudes toward large carnivores have resulted in an accelerating number of challenges to predator management in places where it is still espoused. Utilitarian attitudes towards wildlife are declining in Western cultures and people now increasingly recognize the intrinsic value of wildlife, including large predators. In the future, agencies responsible for managing resident wildlife will face increased pressure to balance the needs of the hunting public with the desires of non-hunting publics. We suggest that in the next century we will witness a continued shift in how wildlife agencies manage both moose and large carnivores. More attention will be paid to maintaining and restoring intact ecosystems and less toward sustainable yield of meat.

Alces↗
USGS2011

Native Americans, regional drought and tree Island evolution in the Florida Everglades

This study uses palynologic data to determine the effects of regional climate variability and human activity on the formation and development of tree islands during the last ~4000 years. Although prolonged periods of aridity have been invoked as one mechanism for their formation, Native American land use has also been hypothesized as a driver of tree island development. Using pollen assemblages from head and near tail sediments collected on two tree islands and documented archeological data, the relative roles of Native Americans, climate variability, and recent water-management practices in forming and structuring Everglades tree islands are examined. The timing of changes recorded in the pollen record indicates that tree islands developed from sawgrass marshes ~3800 cal. yr BP, prior to human occupation. Major tree island expansion, recorded near tail sediments, occurred ~1000 years after initial tree island formation. Comparison of the timing of pollen assemblages with other proxy records indicates that tree island expansion is related to regional and global aridity correlated with southward migration of the Intertropical Convergence Zone. Local fire associated with droughts may also have influenced tree island expansion. This work suggests that Native American occupation did not significantly influence tree island formation and that the most important factors governing tree island expansion are extreme hydrologic events due to droughts and intense twentieth century water management.

Florida↗
USGS2010

Paleozoic ophiolitic assemblages within the southern New England orogen of eastern Australia: Implications for growth of the Gondwana margin

Several ophiolitic assemblages occur in the southern New England orogen. The development of these rocks and their relations to the rest of the orogen have major implications for the tectonic evolution of eastern Gondwana. A major, narrow but elongate belt of Early Cambrian suprasubduction zone ophiolite crops out along and near the PeelManning Fault System and is juxtaposed against younger arc and subduction complex terranes. No pre-Permian links with the rest of the New England orogen have been established for this terrane. It may represent portions of Lachlan Fold Belt basement which underlies younger, westward overthrust New England terranes, and has been diapirically emplaced at higher crustal levels as serpentinite-matrix melange. Middle to Late Devonian ophiolitic rocks in the Yarras Complex comprise basement to the Birpai subterrane and represent a crustal cross section through a rifted island arc. Correlatives of this terrane also occur within the more extensive Gamilaroi terrane to the west of which deeper crustal levels are not exposed. The various components of serpentinite-matrix melange in the Ngamba terrane at Port Macquarie superficially appear to represent a dismembered ophiolite association. However, the various components of the melange exhibit a wide range of ages, metamorphism, and tectonic affinities, rendering a genetically related origin unlikely. This terrane includes fragments of ocean floor accreted into a Late Devonian subduction complex, which was later affected by Early Carboniferous forearc serpentinite diapirism and high Mg series magmatism. Zircon inheritance in Triassic dikes, which intrude the melange attest to the development or later emplacement of this forearc region over an older Lachlan Fold Belt basement. Ultramafic rocks of the Bundjalung terrane in the east of the New England region probably formed at deep levels in an intraoceanic island arc and are intruded by boninitic dikes. The tectonic development of the NEO was significantly more complicated than has been suggested in earlier published models. Periodic accretion of island arc systems, some of which are now represented by suprasubduction zone ophiolites, to the eastern margin of Gondwana suggests multiple phases of subduction with the possibility of polarity reversals throughout the history of accretion. Lateral accretion was not the only means by which Gondwana continental crust grew, and there was considerable postaccretion continentward overthrusting of younger terranes.

New South Wales↗
USGS2014

Parameter optimization, sensitivity, and uncertainty analysis of an ecosystem model at a forest flux tower site in the United States

Ecosystem models are useful tools for understanding ecological processes and for sustainable management of resources. In biogeochemical field, numerical models have been widely used for investigating carbon dynamics under global changes from site to regional and global scales. However, it is still challenging to optimize parameters and estimate parameterization uncertainty for complex process-based models such as the Erosion Deposition Carbon Model (EDCM), a modified version of CENTURY, that consider carbon, water, and nutrient cycles of ecosystems. This study was designed to conduct the parameter identifiability, optimization, sensitivity, and uncertainty analysis of EDCM using our developed EDCM-Auto, which incorporated a comprehensive R package—Flexible Modeling Framework (FME) and the Shuffled Complex Evolution (SCE) algorithm. Using a forest flux tower site as a case study, we implemented a comprehensive modeling analysis involving nine parameters and four target variables (carbon and water fluxes) with their corresponding measurements based on the eddy covariance technique. The local sensitivity analysis shows that the plant production-related parameters (e.g., PPDF1 and PRDX) are most sensitive to the model cost function. Both SCE and FME are comparable and performed well in deriving the optimal parameter set with satisfactory simulations of target variables. Global sensitivity and uncertainty analysis indicate that the parameter uncertainty and the resulting output uncertainty can be quantified, and that the magnitude of parameter-uncertainty effects depends on variables and seasons. This study also demonstrates that using the cutting-edge R functions such as FME can be feasible and attractive for conducting comprehensive parameter analysis for ecosystem modeling.

Massachusetts↗
USGS2011

Climate, Fire and Geology in the Convergence of Mediterranean-type Climate Ecosystems

Integrating Climate, Fire and Geology in a Fire-prone World Fire challenges the long-standing hegemony of ecology, biogeography and paleoecology that climate and soils are sufficient to explain the origin and distribution of plant species. In a world where half of the land surface is fire-prone (Krawchuk et al . 2009), understanding the past and predicting the future requires a close integration of climate, fire and geology. The dogma that fire is an anthropogenic phenomenon of little use in understanding paleoecology (Axelrod 1980, 1989), or merely incidental to vegetation development (Hopper 2009), is rapidly being replaced with a better understanding of paleofire's impact on land plant evolution (Scott 2000; Pausas & Keeley 2009). Attempts to model future global vegetation patterns have been demonstrated to be inadequate without including both natural and anthropogenic fire regimes (Bond et al . 2005). Bond and Keeley (2005) outlined the conundrum posed by alternative explanations for the present distribution of vegetation and assembly of communities. Classical explanations have invoked resource-based mechanisms that are driven by climate and soils. There are ecosystems where resource-based mechanisms may be sufficient, but on many seasonally dry landscapes ecosystem processes such as fire play a major role in the organization and evolution of vegetation.

Book chapter↗
USGS2020

Formation criteria for hyporheic anoxic microzones: Assessing interactions of hydraulics, nutrients and biofilms

Recent experimental studies have detected the presence of anoxic microzones in hyporheic sediments. These microzones are small‐scale anoxic pores, embedded within oxygen‐rich porous media and can act as anaerobic reaction sites producing reduction compounds such as nitrous oxide, a potent greenhouse gas. Microbes are a key control on nutrient transformation in hyporheic sediment, but their associated biomass growth is also capable of altering hydraulic flux, leading to potential bioclogging. Here, we developed one of the first computational modeling approaches that combined hydraulics and microbial conditions to explore the continuous evolution of microzones in stream sediments. The model assessed stream and sediment conditions with different hydraulic flux (0.1–1.0 m/day Darcy flux), nutrient concentrations (O 2 = 8 mg/L, OrgC = 20 mg/L, NO − 3 = 1.5–3 mg/L, and NH 3 = 0.5–1 mg/L), and biomass scenarios (with and without). The model domain is a pore network model with random sized pore‐throat radii creating heterogeneous and anisotropic flow that is representative of a natural streambed composed of medium sand with a hydraulic conductivity of 0.8 m/day. Results from 30 day simulations indicate that hyporheic microzone formation will occur and microzone distributions are not simply controlled by residence time alone, rather by the complex interactions of hydraulic flux, nutrient concentrations, and biomass, with bioclogging having strong feedbacks on both hydraulics and nutrients. Under all conditions with biomass growth, anoxic microzones were unstable, perishing a few days after formation, because bioclogging primarily occurs near the influent (downwelling) area of the hyporheic zone. In turn, this bioclogging shifts transport conditions from advection‐dominated to diffusion‐dominated transport, removing all oxic regions in the hyporheic zone. Overall, results from the modeling show that anoxic microzones are likely to form under many hyporheic zone conditions, and be dynamic through space and time as they are dependent on both hydraulic flux and nutrient transport.

Water Resources Research↗
USGS2024

Hydrodynamic model of the Colorado River, Glen Canyon Dam to Lees Ferry in Glen Canyon National Recreation Area, Arizona

The U.S. Geological Survey constructed a two-dimensional hydrodynamic model that was applied to a 15.8-mile tailwater reach of the Colorado River in Glen Canyon that begins 0.25 mile downstream from Glen Canyon Dam and extends to Lees Ferry in Glen Canyon National Recreation Area, Arizona. The model used the Flow and Sediment Transport with Morphological Evolution of Channels (FaSTMECH) solver in the International River Interface Cooperative (iRIC) modeling interface. The model grid was developed from a full channel digital elevation model derived by combining bathymetric and topographic data collected from March 2013 to February 2016. The model was used to predict water-surface elevations, depths, depth-averaged flow velocities, and bed shear stresses for discharges ranging from 1,000 to 70,000 cubic feet per second. Modeled water-surface elevations matched well with measured values at cross sections throughout the reach, with a mean absolute error of 0.14 meter over the range of typical discharge releases from Glen Canyon Dam. The mean error on discharge, a measure of how well the model solution converged, averaged 0.6 percent and did not exceed 2 percent over the range of discharges modeled. These results indicate that model predictions of hydraulic parameters are reasonably accurate and suitable for use for a variety of purposes, such as ecological and geomorphic modeling.

Arizona↗
USGS2007

Break-up of Gondwana and opening of the South Atlantic: Review of existing plate tectonic models

The opening history of both the Weddell Sea and South Atlantic Ocean is critical to understanding the break- up of Gondwana and the evolution of Antarctica since Early Jurassic times. The dispersal of the Gondwanide fragments is important to understanding the development of past ocean circulation. Unfortunately the tectonics of the Weddell Sea region is difficult to resolve because of the paucity of data coverage and the region’s inherent complexity. Although considerable progress has been achieved in the past 20 years with new marine aeromagnetic data and satellite derived gravity anomaly maps, there are still several models in consideration which differ in crustal ages and schemes of opening. In this paper we present a review of four of those models. Focusing on poles of rotation, synthetic isochrons and flowlines, we proceed backward in time beginning at Chron 34 (83.5 Ma), as it is relatively well defined ocean wide, and present crustal age maps that display the estimated trace of the South America-Antarctica-Africa triple junction for each model. We also plot reconstructions at four selected epochs for all models using the same projection and scale to facilitate comparison. The diverse simplifying assumptions that need to be made in every case regarding plate fragmentation to account for the numerous syn-rift basins and periods of stretching are strong indicators that rigid plate tectonics is too simple a model for the present problem.

Open-File Report↗