Geology Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2774

Browse

End date: 2019

Search Results

Now showing 1 - 10 of 111
  • Thumbnail Image
    Item
    Pressure-Temperature-time-Deformation (P-T-t-D) History of High-Grade Gneisses of the Port aux Basque Area, Southwest Newfoundland, Canada
    (1994) Burgess, Jerry Lee; Brown, Michael; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
    A polyphase deformation history (D1-D4) and upper amphibolite facies metamorphism characterize the Port aux Basques Gneisses. Late-D1 to early D2 kyanite porphyroblasts each contain inclusion trails that preserve S1. Reaction out of muscovite, staurolite and kyanite in favor of sillimanite + garnet + alkali feldspar-bearing assemblages in the matapelitic gneisses record syn- to late- D2 peak metamorphic conditions. Isograd surfaces related to syn-D2 metamorphism were probably subhorizontal to inclined but not their metamorphism were probably subhorizontal to incline but now their map pattern reflects subsequent deformation by D3. Fluid-present melting initiated in the kyanite zone and continued into the sillimanite zone. Metamorphic conditions increase to the southeast with 'peak' temperatures of c. 700-750° at 8-9 kbar associated with the D2/M2 thermal regime. A Pb207/Pb206 date of c. 417 Ma was obtained from titanite in high-grade rocks of the Harbour le Cou Group. This date provides a minimum contraint for the M2 event. Hornblende from a nearby amphibolite yields an 40Ar/39Ar isotope correlation date of c. 419 Ma. Muscovite at the same locality records a 40Ar/39Ar plateau date of 391 Ma. Hornblende and muscovite separates from rocks of the Port aux Basques Complex yield similar 40Ar/39Ar dates. Calculations indicate that post- D3 cooling rates of approximately 8-1°C/Ma are required for the area. They kyanite to sillimanite transition and D2 structures suggest a clockwise trajectory in P-T space as a result of Silurian orogenesis.
  • Thumbnail Image
    Item
    ROCK FLUID INTERACTION AND ITS EFFECT ON BRITTLE ROCK DEFORMATION
    (2019) Xing, Tiange; Zhu, Wenlu; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Accurate assessments of Earth’s dynamic processes, which produce earthquakes and volcanoes, require better understanding of rock deformation. All rocks, to some extent, contain pores. In the Earth’s crust, the pore space is usually filled with water and other fluids such as CO2. Interactions between a rock and the interstitial fluids can significantly alter the physical and chemical properties of the rock and consequently how the rock deforms. My dissertation research focuses on how fluid-rock interactions affect brittle rock deformation including fracture growth and frictional slip that are central to earthquake mechanics, energy exploration and waste deposits. I use both conventional experimental methods and the state-of-the-art synchrotron-based X-ray tomography to quantify the changes of mechanical properties and 3-dimensional pore structures of deforming rocks. The two major findings are: 1) olivine carbonation reactions, in which carbon dioxide is chemically incorporated into silicates to form carbonate, can produce nano- to micro-scale dissolution channels as well as expansion cracks in the host rocks, suggesting that olivine carbonation can be self-sustaining despite its large positive volume change. By identifying the mechanisms that generate porosity during olivine carbonation, this work provides new insights into the application of CO2 mineral sequestration; 2) increasing pore pressure impedes fracture propagation in intact rocks and stabilizes slip along gouge-bearing faults. The stabilizing effect is positively correlated with pore volume increases, suggesting that dilatant hardening is responsible for the observed strengthening. These results provide new physical understanding of the observed spatial correlation between slow slip events and high pore pressure in many subduction zones where tsunami-generating mega earthquakes occur.
  • Thumbnail Image
    Item
    The Redox History of the Earth's Mantle: Evidence from Ultramafic Lavas
    (2019) Nicklas, Robert William; Walker, Richard J; Puchtel, Igor S; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In order to determine the evolution of the redox state of the mantle, the oxygen fugacities of sixteen mantle-derived komatiitic and picritic systems, ranging in age from 3.55 Ga to present day, were determined using the redox-sensitive partitioning of V between olivine and komatiitic/picritic magma, a method refined by this study. The oxygen fugacity data for the studied systems was determined to reflect that of their respective mantle source regions. The dataset defines a well-constrained trend indicating an increase in oxygen fugacity of the bulk convecting mantle of 1.33±0.43 FMQ log units from 3.48 to 1.87 Ga, and nearly constant oxygen fugacity from 1.87 Ga to the present. The oxygen fugacity data for the 3.55 Ga Schapenburg komatiites, the mantle source region of which was shown to have been isolated from mantle convection within the first 30 Ma of the Solar System history, plot well above the trend defined by the data for the contemporaneous lavas. This anamolous data point likely reflects preservation of early-formed magma ocean redox heterogeneities until at least the Paleoarchean. The observed increase in the oxygen fugacity of the mantle requires admixture of a likely geochemically detectable amount of oxidized material. Three mechanisms were considered to account for the observed change in mantle redox state. The first two mechanisms: recycling of altered oceanic crust and venting of oxygen from the core due to inner core crystallization, were found to be unfeasible due to the large mass of recycled crust required, and the likely young age of the inner core, respectively. The third accessible mantle oxidation mechanism: convection-driven homogenization of an initially redox-heterogeneous mantle, is the most likely given available geochemical constraints. The new data presented here provide evidence for the mantle having triggered the Great Oxidation Event at ~2.4 Ga. We have additionally determined the Os isotopic and HSE systematics of 89 Ma komatiites from Gorgona Island, Colombia. The subset of these Gorgona samples that were also analyzed for oxygen fugacity shows BSE-like Os isotopes and HSE abundances in their mantle source, showing that their oxygen fugacity is likely representative of the mantle at 89 Ma.
  • Thumbnail Image
    Item
    Using Shear Waves to Characterize a Firn Aquifer on the Helheim Glacier in Southeast Greenland
    (2019) Guandique, Jonathan Alexander; Schmerr, Nicholas C.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The Greenland ice sheet is melting at an accelerating rate due to increasing global average temperatures. Ice penetrating radar surveys and ice cores extracted from the southeastern margin of the Greenland ice sheet near Helheim glacier discovered that a liquid water aquifer has formed within the near surface recrystallized and compacted snow (firn). Here we use active source seismology to probe the structure of this aquifer in the firn, and present results from a joint inversion technique that uses S-waves, P-waves, and surface waves to constrain the attenuation and seismic velocities that inform on the liquid water stored within the aquifer. Confirming past studies, we find that the aquifer lies at 27.9 +/- 3.5 m and has an approximate thickness of 10 +/- 4 m. We determine there is 1565 +/- 769 kg m-2 of water within the aquifer, a downward revision from past studies. Our study of S-waves and surface waves identified a complex structure in the aquifer layer and future work must incorporate full waveform modeling to use these waves for understanding firn aquifers.
  • Thumbnail Image
    Item
    STRUCTURE OF CONTINENTAL LITHOSPHERE FROM TRANSDIMENSIONAL BAYESIAN INVERSION OF SURFACE WAVES AND RECEIVER FUNCTIONS
    (2019) Gao, Chao; Lekić, Vedran; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Continental crust and the underlying lithospheric mantle make up the continental lithosphere of the Earth. Our understanding of its structure and composition is limited by the inaccessibility of Earth’s deep interior. Seismic imaging utilizing complementary seismic data provides unique constraints on the present-day structure of continental lithosphere. However, while recent efforts have improved the resolution of seismic images, the quantification of uncertainties remains challenging due to the non-linearity and the non-uniqueness of the geophysical inverse problem. To gain insights into the composition, formation, and evolution of the continental lithosphere, an interdisciplinary approach that incorporates seismological, geodynamical, and geochemical contributions is needed. In this dissertation, I implement a model-space search approach – transdimensional Bayesian inversion – to explore seismological constraints of continental lithosphere. I utilize seismic observables including Rayleigh and Love wave dispersion, Rayleigh wave ZH ratio, and Ps receiver functions to invert for shear velocity (Vs), compressional velocity (Vp), density (ρ), and radial anisotropy (ξ) profiles of lithospheric structure. I begin by systematically investigating the effects of parameterization choices on inversion results using synthetic tests. Then, I proceed to tackle several technical challenges regarding the accurate retrieval of multi-parameter velocity structures from large seismic arrays in the presence of sediment layers. Finally, I apply these techniques to create a shear velocity model (TBI-NGP) of the lithosphere across the Northern Great Plains of the United States using data from the EarthScope Transportable Array. This probabilistic seismic model enables statistical assessment of the elastic properties in an Archean craton and Paleoproterozoic orogen. Subsequently, I incorporate seismic constraints with geophysical and geochemical measurements to infer the composition of continental crust in the region.
  • Thumbnail Image
    Item
    THE SEDIMENT AND CRUSTAL STRUCTURE OF THE SOUTHEASTERN UNITED STATES
    (2019) Cunningham, Erin; Lekic, Vedran; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The Southeastern United States (SEUS) preserves a detailed geologic record of the continental collision and rifting that has shaped it over billions of years. Currently the SEUS lies on a passive margin far away from ongoing tectonics, yet retains the ability to produce damaging earthquakes. It has long been suspected that the current seismicity in the SEUS is related to zones of weakness inherited from past structural boundaries; however, the mechanisms that dictate the size and location remain poorly understood and the ground shaking hazard posed by widespread sedimentary basins remains poorly quantified. P-to-S converted waves, analyzed through the receiver function technique, enable detailed imaging of lithospheric structure, but suffer from contamination in sedimented regions. Therefore, I implement a method for constraining average crustal thickness and seismic velocities while correcting for the effects of sediments. I then use high-frequency constraints on sediment structure to construct a reference velocity model for the Atlantic Coastal Plain, discussing its implications for seismic hazard. Finally, using Sp and sediment-corrected Ps receiver functions, I image the structure of the crust and mantle lithosphere across the SEUS, applying sediment corrections and identifying abrupt changes in crustal thickness that appears to be associated with active seismic zones. I discuss these structures in the context of the region’s tectonic past and future seismic hazards
  • Thumbnail Image
    Item
    APPLYING GEODESY TO MODEL POSTSEISMIC SLIP OF THE 2016 MW 6.4 MEINONG EARTHQUAKE
    (2019) Butcher, Rebecca; Huang, Mong-Han; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In regions of rapid convergence such as southwest Taiwan, unmapped active structures at multiple depths increase the uncertainty of seismic hazard estimates. The 2016 Mw 6.4 MeiNong earthquake occurred below the main Taiwan detachment, and may have illuminated some preexisting, but undocumented, fault structures. In this study, I use geodetic measurements to constrain afterslip on the main fault for 15 months following the MeiNong earthquake. The inverted afterslip is concentrated around the peak coseismic slip asperity without significant aftershock correlation, which implies heterogeneous frictional properties on the fault. Additionally, slip model misfit indicates shallower faults that are critically stressed before the earthquake creep due to the MeiNong coseismic stress. My results can help identify active faults located at shallower depth as well as their seismic potential in southwest Taiwan.
  • Thumbnail Image
    Item
    The Kinetics and Mechanism of Sedimentary Iron Sulfide Formation
    (1976) Pyzik, Albert John; Sommer, Sheldon E.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
    The reaction between goethite, ∝-FeOOH, and aqueous bisulfide ion, HS, was studied under conditions representative of estuarine sediments. The concentration-time curves of the following species were determined by spectrophotometric methods: total sulfide, dissolved sulfide, precipitated sulfide, thiosulfate ion, sulfite ion, elemental sulfur , and dissolved (<0.1μ) iron. Polysulfides were monitored by ultraviolet absorbance measurements, while the hydrogen ion concentration was determined with a pH electrode. Elemental sulfur, both as free and polysulfide sulfur was found to be the major sulfide oxidation product. Thiosulfate ion comprised about 14±8% (electron balance-wise) of the oxidation products. Concentration-time curves of precipitated sulfide sulfur were analyzed by the initial rate method to determine the rate expression. The rate expression for the reaction between ∝-FeOOH and HS- is d [FeS]/dt = k [HS-]i^97 (H+)i^82 A1.1FeOOHi where d [FeS]/dt is the rate of precipitated iron sulfide formation, (H+)i is the initial hydrogen ion activity, AFeOOHi is the initial geothite surface are in m^2/1, and k is the rate constant with the value 31±10 M^-1 1^-1 m^-2 min ^-1. 0.97, 0.82, and 1.1 are the reaction orders for the species bisulfide ion, hydrogen ion, and goethite surface area respectively. A combination of hydrogen balance and electron transfer balance and stoichiometric reactions were studied in view of the rate expression to yield a mechanism. The multistep mechanism consisted of several parallel and consecutive reactions: (1) the protonation reaction of the goethite surface, (2) the parallel reduction reactions of ferric iron to yield elemental sulfur and thiosulfate as oxidation products, (3) the dissolution of the ferrous hydroxide, and (4) the precipitation reaction of dissolved ferrous species and aqueous bisulfide ion. The rate determining step in the reaction sequence was the dissolution step. Results of this study indicate that the oxidation of sulfide species by ferric iron may be a significant source of elemental sulfur in the sediment. Elemental sulfur is necessary for the formation of pyrite (FeS2), the thermodynamically stable iron sulfide. In addition, the previous studies of the interstitial waters of anoxic sediments showed an excess of "dissolved" iron which was greater than calculated from equilibrium solubility products. It is suggested from particle size studies of the precipitated iron sulfide that these high concentration are a result of the submicron particles of ferrous sulfide (<0.1μ). These particles would obviously pass through the .45μ filters which are traditionally used as the dividing line for dissolved and particulate species.
  • Thumbnail Image
    Item
    Characterization of the regional, crustal, and global distribution and abundance of the heat producing elements and their geoneutrino flux
    (2019) Wipperfurth, Scott Alan; McDonough, William F; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The amount and distribution of radiogenic power generation from the heat producing elements (HPE) U, Th, and K in the Earth is not well constrained. Compositional estimates of these elements vary by a factor of three in the bulk-Earth and 30 in the mantle after removal of the continental crust contribution. Understanding the total power derived from these elements is critical to understanding the power driving the Earth as they supply fuel to the geodynamo and mantle convection. The decay of HPE's produce particles called geoneutrinos and the measurement of the geoneutrino flux reveals the frequency of decay and the abundances of these elements in the Earth. The total geoneutrino flux can be categorized into three major contributors: the dominant component from the nearest 500 km of continental crust surrounding the detector and slightly smaller sub-equal contributions from the remaining global continental crust and the mantle. The negligible amount of HPE's within the core was tested by a mass-balance of the Th/U derived from Pb isotopes (κ_Pb). Each Earth layer was attributed a κ_Pb from representative samples with associated weighting factor from the estimated mass of U in each reservoir. The radiogenic power in the core from U and Th was constrained to ~0.03 terra-watts (median), emphasizing the core's negligible geoneutrino luminosity. To unravel the contribution from the inaccessible mantle to the signal at a detector one must build a physical and chemical description of the local and global crust. The 50x50 km regional geoneutrino flux surrounding the SNO+ detector (Sudbury, Canada) was modeled. 112 geologic samples were analyzed for their U, Th, K abundances and combined with a 3D physical model of the region. To supplement this, the methodology of Huang et al. (2013) was applied to an updated geophysical model for the bulk-crust to predict the global crustal signal at SNO+ and other detectors. Variable correlation is addressed and uncertainties from density, seismic velocity, crustal thickness, and abundances propagated. This dissertation explores the amount and distribution of HPE's within the Earth and their geoneutrino flux through geochemical and geophysical modeling on regional, crustal, and global scales. Together, the results update our understanding of the Earth's geoneutrino flux and the uncertainties still in the system.
  • Thumbnail Image
    Item
    Origin of the anomalous sulfur isotope composition of the Rustenburg Layered Suite (Bushveld Complex), South Africa
    (2019) de Assis Magalhaes, Nivea Maria; Penniston-Dorland, Sarah C; Farquhar, James; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The 2.06 Ga Bushveld Magmatic Province (BMP) hosts the largest platinum group element (PGE) reserve of the world that occurs mainly as sulfide-rich layers within the Rustenburg Layered Suite (RLS), and also in mineralized layers of the Waterberg Project (WP). Despite extensive studies, many questions remain on the origin and evolution of this large igneous province, and on the source of sulfur that allowed for the extensive PGE mineralization. This study looks systematically into the multiple sulfur isotope composition of the RLS, finding that all layers show the presence of a mass-independently fractionated sulfur component (Δ33S≠0), which are all distinguishable from the expected Δ33S value of the mantle. The exogenic sulfur reflects contamination by Archean surface-derived material (e.g. sediments, altered oceanic crust). Such contamination can occur in many different stages of the evolution of these intrusions: either by assimilation of wall rock during ascent and emplacement, or in a staging chamber in the lower crust, or by recycling of crustal material in an ancient subduction zone. The WP, an intrusion related to the BMP that was emplaced off-craton, has a similar sulfur composition to the Main Bushveld Series of the RLS. It is, however, a separate intrusion that crystallized in a separate magma chamber and was emplaced in a different unit than the RLS, which suggests that the contamination of the parental magma occurred at a deeper level, prior to emplacement of magma in the upper crust. Rocks from the Vredefort Dome, used as a proxy for the sulfur composition of the lower crust underneath that region, yield a sulfur composition that cannot account for the composition of the RLS or the WP. Finally, the sub-continental lithospheric mantle has been studied through xenoliths carried by the Premier Kimberlite. These xenoliths, such as what was observed in sulfide inclusions in diamond, also have Δ33S≠0, evidencing that the sub-continental lithospheric mantle may contain recycled sulfur that contributed this sulfur to primitive magmas during the Bushveld magmatic event.