Geology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2774
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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.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.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 hazardsItem 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.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.Item Seismic Observations of Fluvial Energy Dissipation(2018) Goodling, Phillip James; Prestegaard, Karen; Lekic, Vedran; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Observing microseismic waves excited by turbulent flow is an emerging way to document river dynamics during extreme flood events. This thesis records fluvial-seismic observations in two contrasting systems at different scales. Two single-seismometer particle motion methods are introduced to characterize the seismic signal produced by rivers. In the large-scale system, the Oroville Dam spillway is observed when it is a simple rectangular channel and when it is damaged by erosion. The small-scale system is along the cobble-bed Northwest Branch of the Anacostia River. Particle motion analyses and the scaling between seismic power and discharge are suitable to characterize flow turbulence at the large-scale system. In the small-scale system, particle motion methods are found to be unsuitable and the scaling of seismic power is unable to resolve observed variability in flow dynamics within the study reach. This work suggests that methods of fluvial seismology are best suited to large-scale systems.Item Melt extraction and crustal thickness variations at segmented mid-ocean ridges(2017) Bai, Hailong; Montesi, Laurent G. J.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Mid-ocean ridges are underwater volcanic mountains extending more than 55,000 km in ocean basins worldwide, accounting for nearly 80% of the Earth’s volcanism. They are the birthplace of new seafloor, resurfacing two thirds of the planet over about 100 million years. At mid-ocean ridges, tectonic plates move away from each other, a phenomenon known as seafloor spreading, at rates ranging from slow (~10 cm/yr) to fast (~100 cm/yr). Plate divergence induces the underlying mantle to rise and melt. Buoyant melts segregate from the mantle and collect toward axes of mid-ocean ridges, where they are extracted and solidify into new oceanic crust. The thickness of oceanic crust, the final product of ridge magmatism, contains integrated information about plate motion, mantle flow, mantle temperature, melt generation, melt extraction and crustal accretion. In this dissertation, I investigate three types of crustal thickness variations at mid-ocean ridges to provide insights into the Earth’s deep, less accessible interior. Mid-ocean ridges are broken into segments bounded by transform faults. At fast-spreading ridges, transform faults exhibit thicker crust than adjacent ridge segments, while the crust along transform faults at slow-spreading ridges is thinner. I show that these observations are compatible with melt being extracted along fast-slipping transform faults, but not at the slow-slipping ones. The plates on either side of a ridge axis may move away from the ridge at different rates. I reveal a discrepancy between the expected and observed topography at such asymmetrically spreading ridges, and argue that the discrepancy is best explained by asymmetric crustal thickness, with thicker crust on the slower-moving plate and thinner crust on the faster-moving plate. Crustal thickness may differ between ridge segments separated by a transform fault, in a way that correlates with the relative motion between the ridge and the underlying mantle. I study the three-dimensional effects of background mantle flow, and demonstrate that the pattern of along-axis crustal thickness variations is controlled by the relative angle between ridge and background mantle flow. This dissertation systematically examines the origins of crustal thickness variations at mid-ocean ridges, and provides constraints on mantle and melt dynamics.Item The Role of Water and Strain Rate in the Deformation of Limestone(2017) Kibikas, William; Zhu, Wen-lu; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fluids are pervasive throughout the Earth’s crust. Fluid-rock interaction can significantly alter the mechanical and petrophysical properties of host rocks. This study focuses on the role of fluids in weakening porous carbonate rocks. High solubility of calcite can cause chemically-induced weakening and lead to time-dependent deformation in carbonate rocks. To quantify the effect of hydro-chemo-mechanical coupling on the deformation behavior and failure mode of carbonate rocks, limestone was deformed under both dry and water-saturated conditions. To elucidate the deformation mechanisms, the deformation experiments were conducted at different strain rates. The experimental data shows that while the shear strengths of water-saturated limestone increase with increasing strain rate, the effect of strain rate on the dry samples is negligible. Quantitative microstructural analyses reveal that the grain-scale damage is primarily in forms of stress-induced microcracking and mechanical twinning under both dry and wet conditions. However, with the presence of water, the extent of intergranular pressure solution increases with increasing confinements. The positive correlation between the extent of intergranular pressure solution and the magnitude of weakening suggests that intergranular pressure solution exerts important controls in time-dependent deformation of carbonate rocks.Item Patterns and Mechanisms of Melt Distribution in Partially Molten Harzburgite under Hydrostatic Pressure(2017) Hou, Jiangyi; Zhu, Wenlu; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Geophysical and geochemical properties of partially molten regions of the Earth’s upper mantle are strongly affected by the distribution of melt. I investigated the mechanisms affecting the 3-dimensional (3D) melt distribution of partially molten harzburgite samples. The 3D melt distributions of experimental charges of various melt fractions were recorded using synchrotron-based X-ray microtomography. In most samples the melt fraction increases along the axial direction of the sample, but the mineral phases exhibit no systematic heterogeneities. Analysis of time-series samples confirmed that the melt fraction heterogeneity was generated during sintering and took more than 84 hours to develop. To elucidate the mechanisms of melt focusing, I evaluated the importance of gravity, surface tension, lithologic partitioning and thermal migration as potential driving forces. The scale of the melt fraction variations and the speed at which they develop appear compatible with the thermal migration model.Item Lithospheric Extension on Venus: how to form narrow rifts(2017) Martone, Alexis Ann; Montesi, Laurent G. J.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Venusian rifts of Devana and Ganis Chasmata have been noted for their similar morphology to some rifts on Earth (i.e., the East African rift system). These are narrow rifts that are associated with localized deformation. This thesis aims to explore the link between lithospheric structure and rift style using a force analysis model, following previous work by Buck (1991), in order to determine under what conditions narrow rifts are predicted for Venus conditions. Results for two cases, one using a constant lithospheric thermal conductivity and another using a depth dependent thermal conductivity, are initially determined; Devana and Ganis Chasmata are predicted to be wide rifts rather than narrow rifts. Lithospheric weakening mechanisms (rheological weakening and diking) are implemented to determine their effect on localizing deformation and, thus, forming narrow rifts. Diking did not produce any effect on forming narrow rifts. Rheological weakening, likely due to a combination of melt and a transition to grain size sensitive creep, appears necessary to produce narrow rifts.
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