Geology Theses and Dissertations

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End date: 2009Subject: search.filters.subject.Geology

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    CONSTRAINTS ON THE DEPOSITIONAL AGES OF LESSER HIMALAYAN ROCKS IN CENTRAL NEPAL AND THEIR STRUCTURAL IMPLICATIONS
    (2009) Burgy, Katherine Diane; Martin, Aaron; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The lack of good exposures and paucity of datable horizons in central Nepal has hindered the ability of geologists to piece together a relatively cohesive and straightforward stratigraphic succession within the Lesser Himalaya. U-Pb isotopic analyses of detrital zircons from the Modi Khola valley indicates maximum depositional ages of ~1875 Ma for the Kuncha Formation, ~1800 Ma for the Fagfog Formation, and ~ 1780 Ma for the Kushma Formation. The intrusive 1831 ± 17 Ma Ulleri augen gneiss provides a minimum depositional age bound for the Kuncha. Combined, these data suggest the Kuncha Formation is the oldest member of the Lesser Himalayan series in central Nepal. Additionally, 13C data suggest the Malekhu Formation of the Lakharpata Group was deposited before ca. 1250 Ma. A field mapping comparison based on the redefined stratigraphy indicates the Ramgarh thrust is located >10 km farther south than previously mapped, potentially reducing regional shortening estimates.
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    EFFECTS OF BAR FORMATION ON CHANNEL STABILITY AND SEDIMENT LOADS IN AN URBAN WATERSHED
    (2009) Blanchet, Zachary; Prestegaard, Karen L; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This study investigates channel adjustment due to urbanization in the Little Paint Branch creek of the Anacostia River watershed. In the past 15 years, large gravel bars have formed in the channels, more than doubling the active channel width of some reaches. Field data was collected to analyze downstream hydraulic geometry and the effects of gravel bars on shear stress, turbidity, and morphological change. The watershed was gauged at three locations to document the contributions of discharge and sediment to the downstream Anacostia Estuary. The results indicate that Little Paint Branch Creek generates proportionally more runoff per basin area than the watershed does as a whole, even though the impervious surface area is lower in the upstream tributaries, like Little Paint Branch Creek. Bar formation induces channel widening, which decreases flow depth and thus shear stress for bankfull and higher stages. This shoaling limits bed transport and will eventually limit bank erosion.
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    Advection-Diffusion Controlled Lithium Isotopic Distribution in Contact Aureoles: A Case Study from the Florence County Pegmatites, Wisconsin
    (2009) Liu, Xiaoming; Hier-Majumder, Saswata; Rudnick, Roberta L; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Stable isotopes are useful tracers of fluid-rock interactions in contact aureole settings. To date, only a few case studies have used Li isotopes to study fluid-rock interactions in contact aureole settings. These studies highlight the very large Li isotopic fractionation that can be generated in these settings via diffusion of Li from the pluton into the country rocks, but none of these studies have generated a complete and detailed section of the contact aureole needed to understand the Li distributions. Here, I report the results from a combination of Li isotope analyses and 2-D advection-diffusion modeling for two detailed profiles through country rocks adjacent to Li-rich pegmatite dikes in the Florence County pegmatite field. The results show that the Li concentration and isotopic distribution in the two contact profiles is consistent with fluid infiltration and diffusion of Li through a grain boundary fluid from the pegmatites into their country rocks.
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    Highly Siderophile Element and Tungsten Systematics of Hawaiian Picrites
    (2009) Ireland, Thomas James; Walker, Richard J; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A suite of Hawaiian picrites (MgO > 13 wt.%), and associated basalts, that represent some of the most primitive melts from the Hawaiian mantle source regions were analyzed for their W, highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd and Re) and 186Os-187Os isotope systematics. These picritic samples are among the most primitive samples produced from the Hawaiian main-shield stage volcanoes. As such, they may preserve considerable information about the mantle source regions from which they were derived. Hawaii is of particular interest because there is geochemical and geophysical evidence that suggest that the Hawaiian plume may originate at the core-mantle boundary. If any outer core material is incorporated into plume lavas, it could carry important geochemical information. The primary goal of this study is to improve our understanding of the processes and materials that may affect the mantle source regions of the Hawaiian volcanoes. Abundances of HSE and W, as well as Os isotopes, are useful tools for evaluating the mantle source regions of ocean island basalts because their absolute and relative abundances may be affected by various mantle processes, including the recycling of oceanic crust and sediment, mantle metasomatism, and other forms of crystal-liquid fractionation. In addition, these elements may be suitable for addressing the question of core-mantle interaction, because the core is highly concentrated in both the moderately siderophile and highly siderophile elements, and may have a distinct Os isotopic composition relative to the mantle. The collected data imply that W abundances in the Hawaiian mantle sources are similar for all volcanic centers, and enriched relative to depleted MORB mantle. This suggests that W may be controlled by a primary source component that is less depleted in incompatible elements than the depleted mantle. HSE abundances in the picrites are controlled predominantly by crystal-liquid fractionation processes, and may reflect the presence of residual sulfides in the mantle sources. Lastly, the 187Os/188Os variations are consistent with some proportion of a recycled oceanic crust component; however, variations in 186Os/188Os require another process, such as the incorporation of variable Pt-enriched base-metal sulfides, or mixing with an 186Os-187Os enriched reservoir.
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    Scales of Bank Roughness and Their Relationship to Bank Erosion Processes
    (2009) Hankin, Erik Ravnholt; Prestegaard, Karen L; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Stream bank erosion rates and the stabilization of channel width are poorly understood processes. There have been two distinct approaches to the study and prediction of bank erosion rates in natural streams. In order to predict bank shear stresses, scientists either define a reach as being meandering or straight, even though most river channels are neither meandering nor straight but a combination of the two. This thesis aims to determine if river segments can be divided into straight reaches and curved reaches with different bank erosion prediction approaches applied to each as well as investigating the role of bank roughness element size and spacing in bank erosion. The results show that straight reaches are affected by upstream curvature and that large isolated bank protrusions that are widely spaced generate erosion-causing, stable, macroturbulent eddies. The thesis has implications for stream restoration practices regarding bank stability and erosion.
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    Topographic signatures in the Himalaya: A geospatial survey of the interaction between tectonics and erosion in the Modi Khola valley, central Nepal
    (2009) Walsh, Lisa Schleicher; Martin, Aaron J; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Spurred by the recognition that lithologic resistivity to erosion influences the steepness of terrain, the purpose of this study is to enhance our understanding of feedbacks between erosion and tectonics in the Himalaya. Using spatial statistics, within a Geographic Information Systems (GIS) framework, this study extracts steepness and curvature values from the terrain to identify the unique spatial signature of lithologic units in the Annapurna Range. The spatial relationship of faults and significant changes in river steepness (ksn) and concavity (θ) are examined in this project using high resolution digital elevation models (DEMs), derived from a variety of interpolation methods. Through these quantifications, I explore the possibility of a new model for tectonic activity in central Nepal, in an effort to improve our understanding of how surface processes sculpt the landscape.
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    Melting and Phase Relations in Iron-Silicon Alloys with Applications to the Earth's Core
    (2009) Miller, Noah Andrew; Campbell, Andrew; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Experiments were performed on iron-silicon alloys to determine their suitability as analog compositions for the Earth's core. Starting compositions with 9 wt.% silicon and 16 wt.% silicon were compressed in diamond anvil cells and laser-heated. The melting temperatures of the alloys were measured up to 52 GPa using a recently developed optical system. Both curves show a melting point depression from pure iron but intersect at ~50 GPa. The two starting compositions were also studied up to 90 GPa and over 3500 K in synchrotron x-ray diffraction experiments, and phase diagrams were constructed for both compositions that show significant deviation from the pure iron phase diagram. Based on this synchrotron data, a model was produced which predicts the core to contain 8.6 to 11.1 wt.% silicon for a core-mantle boundary temperature of 4000 K.
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    Diffusion of Oxygen and Lithium Isotopes at a Contact between the Bushveld Complex and Metasedimentary Rock: Implications for the Timescale of Phepane Dome Diapirism
    (2009) Potter, Rachel; Penniston-Dorland, Sarah; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Within the Eastern Lobe of the Bushveld Complex, the Phepane Dome is a circular structure of metasedimentary rock hypothesized to have formed as a wallrock diapir. To constrain the duration of Phepane Dome formation using one-dimensional diffusion models of oxygen and lithium exchange between the Bushveld Complex and the Phepane Dome, samples taken across the contact between these two lithologies were measured for their O and Li isotopic compositions and Li concentrations. Models of O and Li diffusion through melt and through aqueous fluid were fit to the data, resulting in a diffusive distance of 1.0 m for oxygen and 14.1 m for lithium. Using experimentally constrained parameters for O and Li diffusion, a range of 2 kyrs to 2 Myrs was calculated from the diffusive distances. This is consistent with previous studies of the time for crystallization of the Bushveld Complex and a model of Phepane diapir development.
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    THE P-T-t HISTORY OF A BARROVIAN SEQUENCE IN DUTCHESS COUNTY, NEW YORK, AND THE ADJACENT PART OF CONNECTICUT
    (2009) Chen, Yan; Brown, Michael; Piccoli, Philip; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The P-T-t history of a Barrovian sequence in Dutchess County, New York and the adjacent part of Connecticut was studied. Using the average P-T mode in THERMOCALC, the following P-T results were calculated for the garnet, staurolite, kyanite and sillimanite-K-feldspar zones (1sigma uncertainties): 5.2±0.8 kbar, 570±12°C; 5.9±1.1 kbar, 557±12°C; 6.4±1.1 kbar, 600±29°C; and 7.4±1.4 kbar, 752±68°C, respectively. Phase equilibria modeling in the MnNCKFMASHTO system yields peak P-T results consistent with the average P-T results. Monazite ages were obtained for the garnet, staurolite, kyanite and sillimanite-K-feldspar zones (2 sigma uncertainties): 455±6 and 453±10 Ma; 451±9 and 438±8 Ma; 478±6, 431±4 and 425±8 Ma; 533±7 Ma, 507±6, 506±4 Ma and 472±5 Ma, respectively. These ages help to constrain the timing of the Taconic and Salinic orogenic events.
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    Timing of Orogenesis in the Southern Brasília Belt
    (2009) Reno, Barry L; Brown, Michael; Piccoli, Philip M; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The Neoproterozoic-Cambrian Brasília Orogeny records suturing of the passive margin on the western side of the São Francisco Craton with magmatic arcs and the Paranapanema Block in the south or the Amazon Craton in the north during the assembly of West Gondwana. In the southern sector of the Southern Brasília Belt, the Andrelândia Nappe Complex comprises a stack of passive margin-derived nappes metamorphosed to high-pressure granulite facies during subduction-to-collision orogenesis and overlain by an arc-derived high-pressure granulite facies nappe. I present the results of an integrated petrologic and chronologic study on the metamorphic conditions and the timing and duration of orogenesis in the Andrelândia Nappe Complex. I also report on my implementation of the (U-Th)-Pb monazite EPMA dating technique at the University of Maryland, and propose a protocol for statistical analysis and interpretation of monazite ages. Using the U-Pb zircon chronometer in conjunction with Ti-in-zircon thermometry, REE distribution data and petrologic observations, a minimum age for timing of initial nappe detachment is constrained to 678±29 Ma, whereas just-post-peak-T metamorphism of the uppermost passive margin-derived Três Pontas-Varginha Nappe is interpreted to have occurred at ca. 648 Ma. (U-Th)-Pb monazite ages in the Três Pontas-Varginha and underlying Carmo da Cachoeira Nappes indicate that they were accreted to the hanging wall of the subduction zone at depths corresponding to near-peak-P by 635-624 Ma. High-P granulite facies metamorphism of the arc-derived nappe is dated by a U-Pb zircon age of 622±28 Ma. Two Rb-Sr multimineral-whole rock isochrons record cooling of the Três Pontas-Varginha Nappe below ~700˚C at ca. 590 Ma, suggesting slow cooling from peak-T. 40Ar/39Ar biotite ages between 591.4±6.6 and 568.9±6.1 for the Três Pontas-Varginha Nappe indicate an increased cooling rate interpreted to be due to tectonically-driven exhumation. 40Ar/39Ar biotite ages of ca. 540 Ma in the Carmo da Cachoeira Nappe indicate that this nappe was exhumed after the Três Pontas-Varginha Nappe had cooled below ~300˚C. In the southeast part of the Nappe Complex, the Andrelândia Nappe yields 40Ar/39Ar biotite ages consistent with emplacement directly on the São Francisco Craton and cooling from the bottom up.