• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 7827
  • 1241
  • 645
  • 358
  • 342
  • 342
  • 342
  • 342
  • 342
  • 340
  • 162
  • 61
  • 54
  • 53
  • 53
  • Tagged with
  • 14783
  • 1854
  • 1393
  • 1311
  • 1256
  • 991
  • 987
  • 915
  • 629
  • 625
  • 540
  • 526
  • 522
  • 513
  • 487
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

The Mississippi River-Gulf Outlet and Land Changes in Louisiana Deltaic Plain, Saint Bernard Parish, Louisiana

Brasseaux, Shawn 04 February 2016 (has links)
<p> Louisiana contains approximately 40 percent of the contiguous United States&rsquo; wetlands, and as much as 80 percent of America&rsquo;s coastal land loss occurs there. This land loss is occurring at an astounding rate&mdash;a rate that is accelerating. The Mississippi River-Gulf Outlet (MRGO) is a 120-km- (75-mi-) long canal initially dredged in the 1950s and 1960s and maintained by the United States Army Corps of Engineers. It was intended to connect the Port of New Orleans directly to the Gulf of Mexico and bypass the time-consuming, tortuous route via the Mississippi River and Delta. Intially, environmental groups and citizens greatly opposed the MRGO as a potential ecological disaster. While in operation, the channel doubled and even tripled in width in some areas; it merged with Lake Borgne; it amplified storm surge, especially during Hurricanes Betsy and Katrina; and it caused saltwater intrusion in nearby wetlands and forests. Channel dredging ceased after Hurricane Katrina. In 2009, MRGO maritime traffic ended when a rock closure structure was installed. By comparing series of aerial photographs and satellite images, this study will summarize, highlight, and evaluate the land changes associated with the MRGO, with respect to its inland &ldquo;Mile Markers.&rdquo; Additionally, this study compares the roles of human activities versus natural processes in causing land loss in the MRGO area. Aerial photographs and satellite images of the MRGO also were compared and contrasted to see what is presently occurring at the channel concerning the closure structure. Lastly, possible solutions are offered for the mitigation of land loss and/or for creating new land area in the region.</p>

Augmenting flood frequency estimates using flood-scarred trees.

McCord, Virgil Alexander Stuart. January 1990 (has links)
Flood-damaged trees along streams and rivers in 20 localities in Arizona, Colorado, New Mexico, and Utah were examined to assess the feasibility of reconstructing the frequency and magnitude of floods over the last several hundred years. Tree-ring dating of the flood damage produced evidence for 17 floods during the last 125 years, and for at least four floods prior to 1866. Most of the flood-scar dates from the historic period were found to coincide with the dates of major floods on the waterways from which they were collected, or from nearby streams. Flood damage to trees was found to be very abundant and accessible, with the most productive situation probably being that of seasonal streambeds in narrow deep canyons with relatively steep gradients. Amount of effort required to produce a flood chronology appears to be relatively small. Collections at most sites involved sampling from 1 to 5 trees, by extracting 3 to 6 increment cores from each visibly scarred tree, and 2 cores from undamaged trees to facilitate crossdating. At two sites larger collections were made, including 3 cross sections (V-cuts), which were very helpful in establishing dates of scars. For a given channel gradient, scars seem to develop only when the flood depth is above a particular threshold level. This threshold was defined by plotting scar height for scarred trees and flow depth for other floods not producing scars against the logarithm of channel slope. The scarred and unscarred individual points were separated into two well-defined fields, separated by a fairly straight line. Discriminant function analysis showed the separation to be highly significant, and classification of individual points as scarred or unscarred was usually correct. The implication of the scar threshold is that even in drainages with no gage or historical record at all, the presence or absence of flood scars on channelside trees would indicate the occurrence or nonoccurrence of floods of a certain depth over the lifetime of the trees. Basin analysis and field measurements of appropriate channel characteristics would allow this depth and the corresponding discharge to be calculated.

Models of plate kinematics along the western margin of the Americas: Cretaceous to present.

Cole, Gregory Lawrence. January 1990 (has links)
We provide a realistic assessment of the uncertainties associated with plate reconstructions by creating empirical probability density functions (PDFs) for rotations between plate pairs based upon uncertainties in the positions of observed fracture zone and magnetic anomaly crossings. We determine the PDF for a sequence of rotations through Monte Carlo sampling of the PDFs for these plate pairs. We assign confidence limits within PDFs and find that worst-case uncertainty regions for global reconstruction poles can be twice as large as 95% confidence regions. We illustrate our technique with the anomaly 6 rotation between the Pacific and North American plates and demonstrate that realistic uncertainties for summations of rotations are apt to be significantly smaller than those previously predicted by earlier techniques. We develop revised apparent polar wander paths for the North American and Pacific plates through paleomagnetic Euler pole analysis. We determine motions of the Pacific plate within the hotspot reference frame for the past 80 Ma through a similar analysis and extend the model back to 145 Ma through study of anomalous basement depths and application of the paleomagnetic constraints. The match between the paleomagnetic and hotspot reference frames within uncertainty limits imposed by the available data, contradicts the notion of true polar wander during the late Cretaceous and/or motion between distinct Pacific plates. Applying realistic limits on the expected error in global plate reconstructions, we determine that either the South Atlantic hotspot track is improperly defined, or that there has been more than 1,000 kilometers of relative motion between the Pacific and South Atlantic hotspots since 80 Ma. Our model of plate interactions for the western Americas indicates subduction perpendicular to the margins from 78 Ma to 33 Ma. Northward transpression and transport of allochthonous terranes is limited to regions north of the evolving Pacific-Izanagi (Kula) ridge. We present a series of reconstructions which allows quantification of subduction and lateral transport along the margins. We model the generation of possible oceanic plateaus by known hotspots in the Pacific region and find little evidence to indicate that their subduction had an impact on the magmatic history of North America.

Structural evolution of the northeastern Santa Catalina Mountains, Arizona: A glimpse of the pre-extension history of the Catalina complex.

Bykerk-Kauffman, Ann January 1990 (has links)
The Catalina complex, like the other Cordilleran metamorphic core complexes, experienced an episode of profound middle Tertiary extension that resulted in low-angle mylonite zones and detachment faults. However, the northern and eastern parts of the complex escaped significant middle Tertiary deformation and preserved a record of the complicated Mesozoic and early Cenozoic tectonic history that preceded middle Tertiary extension. A detailed examination of the northeastern Santa Catalina Mountains and a reconnaissance study of the rest of the Catalina complex reveal evidence for Cretaceous-earliest Tertiary (Laramide) thrusting and an enigmatic episode of early Tertiary (Eocene?) magmatic intrusion, metamorphism and ductile deformation. Laramide thrusting is represented by the Edgar and Youtcy thrusts. The Edgar thrust, a southwest(?)-vergent bedding-subparallel fault that repeats 300-500 m of section, is intruded by the 64 Ma Leatherwood Quartz Diorite. The northeast(?)-vergent Youtcy thrust repeats at least 1300 m of section and is intruded by the Eocene(?) Wilderness Granite. The early Tertiary orogenic event is represented in the northeastern Santa Catalina Mountains by the Eocene(?) Wilderness Granite, its metamorphic aureole, widespread low-grade metamorphism, bedding-subparallel foliations, east-trending lineations, and several bedding-subparallel stretching(?) faults that omit strata. Numerous kinematic indicators show that the ductile deformation involved top-to-the-east shear. Cross-cutting relationships between the foliation, the faults and the Wilderness Granite and coincident gradients in strain magnitude, metamorphic grade, rock ductility, and fault offset demonstrate that intrusion, metamorphism, ductile flow and faulting were all contemporaneous and that the heat source for the metamorphism and ductile flow was the Wilderness Granite. Evidence for the early Tertiary event extends into the rest of the Catalina complex but lineation trends and shear directions vary considerably from region to region. Furthermore, abundant steep east-striking foliations within the Wilderness Granite batholith appear to have formed during this event. The tectonic significance of the early Tertiary orogenic episode is unclear. Three hypotheses may explain the wide variety of early Tertiary structures: forceful diapiric intrusion of the Wilderness Granite, late-stage Laramide thrusting imposed on a tilted section, or early-stage crustal extension.

Tectonics of one-plate planets.

Janes, Daniel Mark. January 1990 (has links)
The Voyager 2 encounter with Neptune and its moons in August of 1989 completed the discovery phase of planetary exploration. In the 25 years since Mariner 4 returned the first images of another planet, geophysical models for such basic processes as mantle convection and loading which were developed for the Earth have been strained beyond their limits by features such as the Tharsis rise on Mars and the coronae of Miranda which cover as much as a quarter of their planetary circumference. In this work I develop a general planetary shell model in spherical coordinates that is capable of treating shells of arbitrary thickness and driving forces of arbitrary breadth. I then present a methodology for finding the forces exerted on the shell from two processes. I first develop a treatment for mantle convection driven by a density anomaly within a viscous mantle. This model is applied to the small moon of Uranus, Miranda, to study the three large coronae which dominate its surface and for which several competing hypotheses were offered, two of which invoked mantle convection driven by density anomalies of opposite sign. I then develop a general model for loading of the lithosphere and examine the effects of a range of load breadths and lithosphere thicknesses. I map out the combinations of these two variables where classical approximations such as the flat-plate and thin-shell models are applicable as well as determine the nature and extent of the transition between these two regimes. Finally, I employ finite element modeling to investigate the coronae on Venus, showing that morphological aspects of these features reported in the literature can be produced by flexure of the lithosphere beneath a volcanic load and gravitational sliding of a cooled crust off these volcanic mounds. I then, however, produce independent characteristic topographic profiles for three of the more regular coronae which question how typical the reported morphologies are in the coronae in general.

Paleoenvironmental and stratigraphic implications of taphonomic processes: Case studies from Recent and Pleistocene shallow marine environments.

Meldahl, Keith Heyer. January 1990 (has links)
Taphonomic data can be applied to problems in paleoenvironmental analysis, stratigraphy and paleobiology. Ecologic and taphonomic data from molluscan assemblages in Recent clastic shallow marine environments (northern Gulf of California, Mexico, and Cape Cod, Massachusetts, U.S.A.) furnish different and complementary types of environmental information. Ecological data (species composition, trophic and life habit data) are regulated principally by substrate variation. In contrast, taphonomic data (abrasion, fragmentation, corrosion, bioerosion and encrustation) variously track shifts in surface residence time of shells, water energy, and tidal submergence time. Taphonomic contrasts between sedimentary environment arise because shells in different environments are altered along distinct "taphonomic pathways". Variation in residence time, water energy and tidal submergence time elicit responses in unique suites of taphonomic attributes. Taphonomic processes affect the distribution of fossils in strata, and this has important stratigraphic and paleobiologic ramifications. Shell concentrations in Pleistocene shallow marine strata in the northern Gulf of California formed either as beach ridge accumulations, tidal channel lags, autochthonous communities, or "unconformity beds". The latter are significant stratigraphic markers, capping angular unconformities. The "unconformity beds" are identified taphonomically as transgressive lags derived from beach face reworking during erosion of structural bulges that formed by periodic deformation along the Pleistocene shoreline. These shell beds are products of sedimentary processes along tectonically active continental margins. Preservational incompleteness of fossils hampers reconstruction of patterns of mass extinction, because biostratigraphic last occurrences nearly always underestimate times of lineage extinction. The distribution of biostratigraphic last occurrences of mollusc species in sediment cores from a Recent tidal flat indicates that sudden extinction can appear gradual, due to error in biostratigraphic range endpoints (Signor-Lipps effect). Extinction is typically not accurately recorded for species with less than 15% stratigraphic abundance (i.e. occurring in less than 15% of the sample intervals). Extinction simulations demonstrate that stratigraphic abundance and last occurrence data (readily available in the fossil record) can be used together to distinguish between sudden, stepwise and gradual patterns of mass extinction.

Tectonic analysis of the Nisling, northern Stikine and northern Cache Creek terranes, Yukon and British Columbia.

Jackson, Jay Loren. January 1992 (has links)
Lower Mesozoic strata in the northern Cache Creek terrane range in age from Ladinian to Pliensbachian as shown by conodont and radiolarian collections from chert. Chert beds are interlayered with argillite that has ε(Nd)(t) values of -8.8 to -7.4, indicating detritus eroded from Precambrian source areas. These ε(Nd)(t) values are similar to those of fine-grained Middle Triassic sedimentary strata of the miogeocline (-10.5 to -6.7) and to sediments of the modern Pacific Ocean floor. Volcanic-lithic sandstone interbedded with the chert and argillite is petrographically similar to coeval sandstone from the northern Stikine terrane. ε(Nd)(t) values for northern Cache Creek sandstone are -1.1 to +5.8, similar to most coeval northern Stikine strata (-0.4 to +4.7). These observations, coupled with limited paleocurrent indicators, suggest that northern Cache Creek sandstone was deposited in the distal parts of clastic fans derived from the Late Triassic northern Stikine arc. Quartzofeldspathic sandstone layers in northern Stikine have ε(Nd)(t) values of -8.1 to -1.8 and are associated with conglomerate containing metasedimentary clasts similar to rocks of the Nisling terrane. Nisling rocks have ε(Nd)(t) values of -4.6 to +0.5 (Boundary Ranges suite) and -20.6 to -15.6 (Florence Range suite). These data and existing sedimentologic evidence corroborate interpretations that northern Stikine clastic rocks were derived in part from the Nisling terrane in Late Triassic time. Stacks of thrust sheets containing Cache Creek strata are floored by the Nahlin and King Salmon faults, and are bound on the north by the Squanga-Crag Lake tear fault system which has at least 90 km of right-lateral slip and 2.2 km of south-side-up slip. These strata were thrust southwestward over northern Stikine between Middle Jurassic and mid-Cretaceous time. North and west of this nappe, Stikine and Cache Creek strata are shortened by open, upright folds and only minor thrust faults. Structural, stratigraphic and isotopic data are consistent with a minimum-displacement model for development of the western Canadian Cordillera, in which terranes located east of the Coast Mountains batholith developed and remained in the eastern Pacific Ocean throughout their histories.

A geologic, tectonic and metallogenic evaluation of the Siuna terrane.

Venable, Margaret Ellen. January 1994 (has links)
The Siuna terrane, defined here for the first time, is composed of slices of limey sedimentary strata, intermediate volcanic rock, serpentinite, and ultramafic cumulates, thrust together in the lower Upper Cretaceous. The terrane is intruded by intermediate dikes, plugs and plutons dated from 75 to 60 Ma, and is cut by Tertiary normal and strike-slip faults. The Siuna terrane volcanic and sedimentary strata probably formed in a Lower Cretaceous volcanic arc and associated back-arc or inter-arc basin. They host a Zn-Cu-Au volcanogenic massive sulfide occurrence and a Au-Cu skarn (the Siuna Mine), as well as several Au prospects. The serpentinite bodies host podiform chromite and disseminated Au occurrences. The Siuna terrane is distinct from the Chortis block to the north, and may have formed far from it. Evidence to date indicates that the low grade metamorphic "continental" basement of the Chortis block does not extend beneath the Siuna area. Isotopic evidence, as well as the presence of fragments of oceanic crust (serpentinite and ultramafic cumulates), indicate that the Siuna belt represents a fragment underlain by oceanic crust, appended to the Chortis block in the Upper Cretaceous; it may originally have been part of Greater Antilles Arc. The interpretation of the Siuna terrane presented here constitutes a significant departure from previous interpretations of the geology and tectonics history of the area.

Structural observations and stratigraphic variability in Jurassic strata, Upheaval Dome, Canyonlands National Park, Utah, USA

Geesaman, Patrick J. 23 January 2014 (has links)
<p> Upheaval Dome is a structurally deformed topographic depression located in Canyonlands National Park, southeast Utah. Multiple hypotheses for its origin have been proposed by various scientists over many years of research. The two remaining viable hypotheses are at opposite ends of the geologic spectrum, one proposing long-term deformation of the structure, while the other proposes a catastrophic meteorite impact. (1) The seminal paper by Jackson et al. (1998) suggests that Upheaval Dome was created due to the growth and subsequent pinch-off of a salt diapir sourced from the Pennsylvanian Paradox Formation. Their conclusions were based on various growth geometries in Jurassic age strata. (2) Perhaps the most influential paper proposing a meteorite impact at Upheaval Dome is by Buchner and Kenkmann (2008), titled "Upheaval Dome, Utah, USA: Impact origin confirmed". In this paper only two grains of shocked quartz are identified, out of 120 standard thin sections. Based on these thin sections comprising medium-coarse sand grains, only ~0.00043% of grains displayed evidence of high-pressure deformation. For shocked quartz to confirm a meteorite impact there must be abundant shocked grains (2-5%), and ~0.00043% cannot be considered abundant (French and Koeberl, 2010).</p><p> Prior to this study there has been no attempt made to combine an in depth stratigraphic investigation of exposed, accessible formations with structural and lithologic observations in the Upheaval Dome area. Analysis of stratigraphic field data for Triassic to Jurassic-aged strata reveals: (1) stratigraphic thicknesses from measured sections range from 7 meters to 224 meters in the Kayenta Formation, and projected thicknesses in cross sections can exceed 400 meters; (2) distinct changes in facies distributions in relation to mapped structural features; (3) localized angular discordances, such as angular unconformities and onlaps, at the contact between formations or within individual formations. </p><p> Analysis of structural features at Upheaval Dome reveals: (1) synclinal axes and associated depositional centers shift throughout the Jurassic; (2) stratigraphic thicknesses across normal faults from hanging to footwall blocks are unequal on the scale of meters to tens of meters; (3) thrust faults verge dominantly to the southeast regardless of the side of the dome they are located on; (4) blocks of Triassic Chinle Formation encased in the younger Jurassic Wingate Sandstone adjacent to dog tongues suggests the involvement of a brief period of allochthonous salt break out after the deposition of the Chinle. Petrographic analysis was inconclusive, as there were no shocked grains, nor any clasts of the Paradox Formation present in younger formations.</p><p> The research presented in this study strongly indicates that long-term deformation occurred at Upheaval Dome during the Early Jurassic and possibly in older less well exposed units. Evidence supporting long-term deformation includes growth strata, changes in facies distributions, shifting formation depocenters, angular discordances, and growth faults. Sparse indicators of catastrophic are also present in the form of sparse shocked quartz grains and poorly developed shatter cones. To accommodate these juxtaposing deformational regimes an evolution of Upheaval Dome is presented here that relies on an early meteorite impact to initiate active diapirism leading eventually to a passively growing salt diapir. This explanation would account for the petrographic evidence supporting meteorite impact, as well as the growth geometries in the Triassic-Jurassic aged strata surrounding Upheaval Dome.</p><p> Upheaval Dome has historically been one of the most controversial geologic features in the United States. It is important for geologists to understand the genesis for this structure as it is an extremely well exposed field example of a meteorite impact, pinched-off salt diapir, or a combination of the two, and can help further understand similar structures found around the world at the Earth's surface, or in the subsurface.</p>


Kottke-Levin, Janine 17 May 2013 (has links)
The study in hand reports on compositional variations in mineral and whole-rock geochemistry of the chromitite and silicate layers occurring in the Middle Group of the eastern Bushveld Complex. Special attention is paid to the platinum-group element (PGE) content and mineralization as well as the nature of platinum-group minerals (PGM) within the MG sequence. A general progressive evolution of the MG chromitite layers can be deduced from chromite composition showing decreasing Mg# and enrichment of Fe and Al relative to Cr as well as from the decreasing whole-rock Mg#. At the LCZ/UCZ transition no marked change in mineral and whole-rock geochemistry can be observed, indicating that the MG sequence derives from a continuously progressive evolving melt. The presence of one parental magma for the formation of the MG is further substantiated by the chondrite-normalized PGE patterns of the MG chromitite layers, which resemble each other. They further resemble that of the UG2, which suggests that they derive from the same magma and a similar style of mineralisation applied. One marked reset to compositions even more primitive than the MG1 chromitite layer is present at the level of the MG4A chromitite layer, which is illustrated by a low Mg#chr, low whole-rock Mg#, low mineral and whole-rock Cr3+/(Cr3++Fe3+) ratios and increasing mineral and whole-rock Cr3+/(Cr3++Al3+) ratios and TiO2 contents. It strongly suggests the addition of hot and primitive magma at this level of the MG stratigraphy. Whole-rock geochemistry of the silicate layers is strongly governed by mutual influence of co-precipitating minerals competing for major elements like Mg, Fe, Al or Cr, and hence a statement to general trend with respect to evolution from bottom to top of the stratigraphic column of the MG sequence canât be made. Nevertheless, a strong decrease in whole-rock Mg# and low whole-rock Al2O3 concentrations at the level of the MG4A pyroxenite is illustrated, which can be ascribed to the same event of addition of primitive magma concluded for the MG4A chromitite layer. The existence of Na-rich silicate inclusions occurring in chromite of all the MG chromitite layers most likely proves chromitite formation by mixing of primitive melt with a siliceous melt. Hence, the general process for the formation of the chromitite layers and their corresponding silicate layers in the MG seems to be mixing of a primitive (mafic-ultramafic) parental melt with siliceous roof-rock melt deriving from the granophyric Rooiberg felsites. Although Cu deriving from the base metal sulphides (BMS) seems to migrate away from the chromitite layers, local Cu enrichment in the chromitite layers to concentrations up to >6000 ppm can be observed. This excess Cu most likely derives from an external source e.g. country rocks, which could have âgeneratedâ metal-loaded hydrothermal fluids. Excess S occurring in the silicate layers may result from limited, probably hydrothermal, dissolution of BMS from the respective chromitite layer below. Chromitite samples have been investigated with the mineral liberation analyzer (MLA) for their PGM. The study focused on the mineral association of the PGM, i.e. whether they occur liberated, locked or attached to gangue or the BMS, since the mineral association is important to conclude on PGE mineralization and PGM formation. The majority of the PGM occurring in the chromitite layers of the MG sequence are Pt- Rh -sulfides (26.2%), followed by laurite (25%), Pt-Pd -sulfides (24.3%) and Pt -sulfides (13.8%). The remaining 10.7% comprise PGE âsulphoarsenides and PGE- arsenides, Pt - and Pd âalloys and Pt - and Pd âtellurides. Except laurite, which is commonly locked in chromite (66%), the PGM are dominantly associated with silicate minerals, and to a lesser extend with the BMS only. According to this discrepancy in the PGM association, PGE mineralization of the MG chromitite layers most likely canât be modelled in terms of the R-factor and therefore PGE concentration by the cluster model is favoured by the author. Alteration of the primary silicate minerals in the MG chromitite layers to amphibole, chlorite, talc, mica and quartz can be observed locally. Since the primary BMS assemblage (chalcopyrite, pyrite and pentlandite) shows losses of Fe, Cu and S, and millerite, a Ni-rich sulphide of secondary origin, occurs, the influence of hydrothermal fluids on the chromitite layers was concluded. Besides affecting the BMS, the fluid most likely also redistributed the PGE occurring in solid solution in the BMS, i.e. Pt and Pd, as especially the negative slope from Pt to Pd in the chondrite normalized PGE patterns of the MG chromitite layers suggests. Enrichment of the high-temperature PGE (HT-PGE) over the low-temperature PGE (LT-PGE) is depicted in the chondrite normalized PGE patterns of the MG chromitite and silicate layers. The fact that the HT-PGE are enriched relative to the LT-PGE in the lowermost MG chromitite layers as well as in the MG4A suggests that temperature could play a role in PGE fractionation. Temperature control on PGE fractionation has also been concluded from changing Pt/Ir ratio in dependence of the whole-rock Al2O3 content from bottom to top of the MG sequence, with increasing Al2O3 concentrations considered to point to decreasing temperature. Hence, Al-depletion, i.e. decreasing Al2O3 content, of chromite relative to Cr may result in enrichment of the HT-PGE relative to the LT-PGE. The LT-PGE are preferentially concentrated by increasing amounts of plagioclase within the chromitite layers.

Page generated in 0.0442 seconds