Browsing by Author "Morgan, Julia K"
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Item Controls on Uplift Patterns in Raukumara Peninsula, New Zealand: Insights from DEM Modeling(2018-11-28) Farrell, William; Morgan, Julia KDiscrete element numerical simulations are used to understand how underplating, the subcretion of subducted sediments from the top of the downgoing plate to the bottom of the overriding plate, drives upper plate faulting and uplift patterns. Our simulations are modeled on processes occurring at Raukumara Peninsula, New Zealand, a location of proposed underplating. We also apply differential surface erosion, landsliding, and multiple versions of upper plate faulting, motivated by processes occurring on Raukumara Peninsula, to test how these factors affect the underplating system. We find that antiformal uplift generally occurs above an underplated body, accompanied by trenchward displacement outboard of the uplifting region. These two zones are separated by a normal fault, the location of which determines the relative amounts of uplift and horizontal trenchward displacement. Most of the tested factors in this system (landsliding, faulting, upper plate zones of weakness) have minimal effect on the form of uplift and overall system evolution. Differential surface erosion, however, shifts the zone of most rapid uplift towards the area with the greatest amount of erosion. This work suggests that, in the absence of a strong differential erosion forcing, antiformal uplift should be expected on Raukumara Peninsula but if differential erosion occurs, then asymmetrical uplift would be predicted. Both uplift patterns have been proposed by different workers there based on field measurements.Item Effects of Seamount Subduction on Tectonic Accretion, and Erosion, and Forearc Evolution: Discrete Element Simulations(2017-11-30) Foo, Wey Yi; Morgan, Julia KA series of deep imbricate thrust structures associated with a period of margin accretion, have been interpreted in seismic reflection data offshore Costa Rica. This interpretation for accretion is inconsistent with the long-term margin subsidence recorded in that setting. Using Discrete Element Method simulations, we test the idea that thinning of the incoming sedimentary blanket exposed underlying seamounts that interacted with the overriding plate, and drove a change in tectonic regime from margin accretion to erosion. Our modeling results show seamounts can enhance the rates of wedge growth, at least transiently, even as they cause erosion during later stages of forearc evolution. Seamounts also impart considerable changes to forearc structure as they pass by, rotating faults, initiating a cross-cutting backthrusts, activating landslides, and steepening the forearc slopes. We also show seamounts partitioning strain within the wedge by landward reactivation of structures, which may have implications to seismogenesis.Item Last 2000 Year Climate Sediment Record from the Belize Central Shelf Lagoon: A Detailed Archive of Droughts and Floods Linked to the Collapse of the Mayan Civilization and Caribbean Historical Famines(2014-05-30) Agar Cetin, Ayca; Droxler, Andre W; Anderson, John B; Morgan, Julia KIn the past several decades, climate change linked to increasing anthropogenic CO2 emission to the atmosphere, has resulted not only in steady global warming but also in extreme climate events. Heat and cold waves, flash floods and droughts, and catastrophic hurricanes, are some of the extreme climate events the Earth has been experiencing. In the future, those events are expected to become common rather than exceptional. Understanding processes linked to extreme climate is becoming more crucial and analyzing extreme climate paleo-records have become more important. This study is focusing on the last 2000 yr precipitation record archived in the mixed carbonate/siliciclastic sediments accumulated in the Belize Central Shelf Lagoon, partially filling the Rhomboid Reef lagoons and English Caye Channel. The Belize climate is described as subtropical, largely influenced by the seasonal migration of the Intertropical Convergence Zone (ITCZ), triggering alternating winter dry and summer wet seasons. In the late Holocene, the ITZC has been reported to have reached higher latitudes during the Medieval Climate Anomaly (MCA) producing high precipitation on the Yucatan Peninsula, contrasting with periods when the ITCZ remained in low latitudes, generating years of low precipitation and even dramatic droughts, as during the couple of centuries just preceding the MCA, corresponding to the Mayan Terminal Classic (TCC) Collapse and the Little Ice Age (LIA). Two submersible vibrocores, BZE-RH-SVC-58 from Elbow Caye Lagoon, and BZE-ECC-SVC-68 from English Caye Channel, were retrieved, among several additional cores, from the Belize Central Shelf Lagoon. Carbonate content values were determined by carbonate bomb and element (Ti, Si, K, Fe, Al, and Sr) counts via X-Ray Fluorescence (XRF) scans. This study is mainly based upon the detailed analyses of two of these cores with well-constrained timeframe, established by accelerator mass spectrometry (AMS) radiocarbon dating of benthic foraminifera, Quinqueloculina. The mixed sediments in these two cores, based upon the variations in the past 2000 years of elements such as Ti and K counts, have recorded the weathering rate variations of the adjacent Maya Mountain, highly influenced by alternating periods of high precipitation and droughts, linked to large climate fluctuations and extreme events. The 800-900 CE century just preceding the MCA, characterized by unusually low Ti and K counts and interpreted to be triggered by low precipitation and resulting in severe droughts in the Yucatan Peninsula, corresponds well with the Mayan Terminal Classic collapse (TCC). High Ti and K counts, although highly variable, during the MCA (CE 900-1350) are interpreted as an unusually warm period characterized by two 100-to-250 years-long intervals of higher precipitation when the number of tropical storms peaked, separated by a century (CE 1000-1100) of severe droughts and low tropical storm frequency coinciding with the collapse of Chichen Itza (CE 1040-1100). During the LIA (CE 1400-1850), Ti and K counts reach minimum values, with extreme minima during two historical drought times and related Caribbean-wide famines in the year CE 1535 and the last third of the 18th century (CE 1765-1800).Item On the Evolution of Planets: From Convective Bi-stability to Volcanic Edifice Instability(2015-11-24) Weller, Matt; Lenardic, Adrian; Morgan, Julia K; Sawyer, Dale S; McGovern, Patrick J; Johns-Krull, Christopher MThe Eastern Olympus Mons Basal Scarp (EOMBS) is conditionally stable when the edifice contains pore fluid, and critically stable, or in failure, when the edifice contains a dipping-overpressured-confined aquifer and mechanical sublayer at depth. Failure of the fault bounded portion of the flank results in estimated volumes of material ranging from 5600–6900km3, or 32–39% of the estimated volume of the “East” Olympus Mons aureole lobe. We suggest that the EOMBS faults may be an expression of early stage flank collapse and aureole lobe formation. Ages of deformed volcano adjacent plains indicate that this portion of the edifice may have been tectonically active at <50Ma, and may be coeval with estimated ages of adjacent outflow channels, 25–40Ma. This observation suggests that conditions that favor flank failure, such as water at depth below the edifice, existed in the relatively recent past and potentially could drive deformation to the present day. Coupled 3D mantle convection and planetary tectonics models are used to explore the links between tectonic-regimes, the level of internal heating (Q) within the mantle, planetary surface-temperature, and planetary lithospheric-strength. At high and low values of Q, for moderate to high yield, hot and cold single-plate planets prevail. For intermediate Q, multiple stable tectonic-states exist. In this parameter space, the specific evolutionary path of the system has a dominant role in determining its tectonic state. For low to moderate lithospheric yield strength, mobile-lid behavior (a plate tectonic-like mode of convection) is attainable for high degrees of internal heating (i.e., early in a planet’s thermal evolution). This state is sensitive to climate driven changes in surface-temperatures. Relatively small increases in surface-temperature can be sufficient to usher in a transition from a mobile- to a stagnant-lid regime. Once stagnant, a return to mobile-lid is not attainable by a reduction of surface-temperatures alone. For lower levels of Q, the tectonic regime becomes less sensitive to surface-temperature changes. These results indicate that terrestrial planets can alternate between multiple tectonic-states over giga-year timescales. Within parameter space regions that allow for bi-stable behavior, any model-based prediction as to the current mode of tectonics is inherently non-unique in the absence of constraints on the geologic and climatic histories of a planet.Item Shear Fracture Growth in Granular Rocks and Porosity-Permeability Relationships in Mudstones(2019-08-07) Vora, Harsha; Morgan, Julia K; Dugan, BrandonI employ the Discrete Element Method to analyze the micromechanical response of granular rocks to unstable failure. Calibrated granular models of sandstone and granite are subjected to biaxial experiments under confining pressures of 0–50 MPa, leading to the development of shear fractures through interaction of emergent microcracks occurring in shear and tensile modes. I document the mode and energy associated with microcracks to analyze fracture growth patterns and quantify energy release. Shear fracture growth in my sandstone model occurs through cooperative interaction between shear and tensile microcracks, with shear microcracks accounting 4-44% of total microcracks and 31-92% of fracture energy. Shear fracture growth in my granite models occurs through coalescence of tensile microcracks, which account for 96-98% of total microcracks and 87 -93% of fracture energy. My model results show that fracture energy increases with confining pressure, accounting for 10-15% of the total input mechanical energy in sandstone vs. 16-47% in granite. I estimate that the work done against friction from intergranular and fracture sliding accounts for 69-86% of total input energy in sandstone models and 46-81% in granite models. My results indicate that frictional deformation is a significant term in the energy budget during rock deformation. I employ the Discrete Element Method to analyze statistical indicators of critical failure during shear fracture growth in calibrated models of sandstone. To investigate the precursory signatures of critical failure, I document the location, mode and stress associated with emergent tensile and shear microcracks during biaxial experiments under confining pressures of 0-45 MPa. I employ the documented microcracking activity to calculate temporal trends of microcracking variance, fraction of shear microcracks to total microcracks, fractal dimension of microcrack locations, acoustic energy variance and seismic b-value. My results indicate that each parameter is a function of strain-to-failure and can be treated as an indicator of critical point. Pre-failure damage evolution in our sandstone model is characterized by: 1)increase in microcracking variance of one to two orders of magnitude, 2)peak shear microcrack fraction 3) peak fractal dimension of microcrack locations, 4) increase in acoustic energy variance of an order of magnitude, and 5) peak b-values. We employ the five microcracking indicators and confining pressure as inputs for an artificial neural network (ANN) to predict critical failure. Over confining pressures of 0-45 MPa, our ANN architecture exhibits good prediction capability for stress-to-failure and strain-to-failure for our sandstone model. Our machine learning approach reveals that microcracking variance, seismic b-value and fractal dimension are the most important indicators of critical failure. Thus, we develop an integrated analysis of microcracking indicators to understand signatures of critical point and combine them with machine learning to predict failure in granular rock. I model mudstone permeability during consolidation and during fluid injection by simulating porous media flow using the lattice Boltzmann method. I define the mudstone structure using clay platelet thickness, aspect ratio, orientation and pore widths. Over the representative range of clay platelet lengths (0.1 – 3 µm), aspect ratios (length/thickness=20-50) and porosities (0.07 – 0.80), my permeability results show good correlation with natural mudstone datasets. Over =0.32-0.58, the porosity-permeability trends of two mudstone models of heterogenous mineralogy match experimental datasets well. I extend my methodology to evaluate how mudstone permeability might evolve during microfracture network growth or macrofracture propagation upon fluid injection. My results suggest that the growth of a distributed microfracture network results in greater vertical permeability increase than a single macrofracture upon fluid injection in compacted mudstones. My modeling approach provides a simple means to estimate permeability during burial and compaction or fluid injection based on knowledge of porosity and mineralogy.