Theses and Dissertations at Montana State University (MSU)
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Item Structural control of the Norris Hot Springs and associated geothermal system(Montana State University - Bozeman, College of Letters & Science, 2023) Wafer, Mia Peyton; Chairperson, Graduate Committee: Andrew K. LaskowskiMany active and productive geothermal systems are structurally controlled, suggesting that certain structural configurations have higher potentials for geothermal energy. Analyzing potential productive geothermal wells is known as geothermal play fairway analysis (PFA). Identifying these high potential structural configurations is another way to locate geothermal systems without surface expressions, which represent an unquantified natural resource for geothermal energy. By studying geothermal systems with surface expressions, I move closer to being able to locate additional natural resources. The Norris Hot Springs, located in the foothills of the Tobacco Root Mountains, Southwestern Montana, presents a novel study area to test structural control on geothermal system development. I combine geologic mapping, UAV-based photogrammetry, zircon U-Pb geochronology, seismic data, and 3D geologic modeling to date and interpret the multistage development of local structures and their influence on the geothermal system beneath the Norris Hot Springs. A comprehensive structural model suggests the geothermal system associated with the Norris Hot Springs is hosted by a deep network of faults and fractures in Precambrian gneiss basement. A nearby fault system primarily comprised of high angle NNW-SSE trending normal faults, which often display pervasive alteration and polymetallic vein mineralization, likely connects surficial features with deeper reactivated Laramide-era structures. A primary goal of this study was to investigate the benefits of supplementing traditional field work with novel methods. Results from this study demonstrate the utility of combining geologic mapping, 3D modeling, structural analysis, 3D modeling and UAV surveys for geothermal exploration and constrain the timing and influence of local faulting on geothermal activity at the Norris Hot Springs.Item The kinematics and dynamics of rifting in south-central Tibet(Montana State University - Bozeman, College of Letters & Science, 2023) Reynolds, Elizabeth Aislin Nicole; Chairperson, Graduate Committee: Andrew K. Laskowski; This is a manuscript style paper that includes co-authored chapters.Southern Tibet is a unique location to study complex interactions between continental collision and extension, or stretching, of the Earth's crust which forms linear structures called rifts. The study of rifts is important because the rocks they expose can record thermal changes in the Earth's crust related to large-scale processes such as shifts in tectonic plates which occur over long timescales and are difficult to observe. Rifts also interact with topography, can influence river systems, and cause changes in rainfall distribution across a landscape by forming topographic drainage divides. Despite their importance, the kinematics and dynamics of rifting, or processes that occur during rift formation and evolution, are not well understood. This study uses field and radiometric dating techniques to investigate the shape, orientation, and timing of extension in southern Tibet by testing kinematic models for two classes of rifts: (1) Tibetan rifts which are defined as rifts that are >150 km in length and crosscut the Lhasa Terrane, and (2) Gangdese rifts that are defined as rifts <50 km long that are isolated within the high topography of the Gangdese Range. Evaluation of rift age across the Tangra Yumco rift and three Gangdese rifts suggests the TYC rift formed through the linkage of smaller normal fault segments into larger and longer structures over time, while Gangdese rifts may have relatively constant lengths. Additionally, interactions between rifts and contractional structures have likely influenced the evolution of topography and drainage patterns in southern Tibet for at least the past sixteen million years. To further investigate structural interactions, a broader compilation of thermochronology ages expands results to include another Tibetan rift, the Lunggar rift. Trends in the data reveal all samples from Gangdese rifts and Tibetan rifts that spatially overlap the Gangdese Range yield ages between ~28-16 Ma, whereas samples north of the Gangdese Range yield ages between ~12-4 Ma. I interpret these results to reflect Gangdese rift initiation at ~28 Ma in conjunction with, and perhaps balancing, uplift driven by the India-Asia collision, while young ages North of the Gangdese Range (~12-4 Ma) reflect extension along Tibetan rifts.Item Change in provenance and sediment routing history of the Miocene-Pleistocene Bengal Fan, Indian Ocean using detrital zircon (U-Th)/He thermochronology(Montana State University - Bozeman, College of Letters & Science, 2023) Dixon, Timothy Spencer; Chairperson, Graduate Committee: Devon A. Orme; This is a manuscript style paper that includes co-authored chapters.The Ganges-Brahmaputra river system carries thousands of tons of sediment from the Himalaya each year to the submarine Bengal Fan within the Indian Ocean. Deposition of sediments to the fan has been ongoing since at least late-Oligocene time (Krishna et al., 2016), preserving a record of Himalayan tectonics and topographic evolution (Blum et al., 2018). IODP Expedition 354 (2015) collected 1.7 km of sediment core from the Bengal Fan to expand the record of Himalayan sediment routing (France-Lanord et al., 2016). Detrital zircon U-Pb data from core samples reflect five major tectonostratigraphic sequences from the Himalaya-Tibet Orogen (Blum et al., 2018). Age populations appear to vary temporally, suggesting change in erosion rates and sediment routing through time, especially apparent during the Pliocene-Pleistocene transition. This research uses detrital zircon (U-Th)/He thermochronology, double dating analyses, and sediment mixing model approaches to fingerprint changes in sediment routing and river deposition to the Bengal Fan. (U-Th)/He analyses are used to quantify rock uplift and exhumation experienced in the Himalayas, facilitating lag-time interpretations across Miocene-Pleistocene time. To interpret the Bengal Basin sink record, a compilation of mid to low-temperature thermochronology datasets across the Himalaya-Tibet orogen is presented for characterization of modern-day thermochronologic age signals. This database highlights orogen wide trends in cooling dates that show younging apatite/zircon (U-Th)/He and fission tracks in the frontal Himalaya and syntaxes, and a younging trend in 40Ar/39Ar dates towards the orogen center. Zircon (U-Th)/He analyses of Bengal Fan sediments deposited 6.20 - 0.13 Ma reveal cooling dates from ~0.28 + or - 0.03 - 540.15 + or - 6.13 Ma. Age populations vary greatly between Miocene-Pleistocene time, with notable loss of >23 Ma populations between Miocene-Pliocene time and increasing 5.3 - 23 Ma populations across the Pliocene-Pleistocene transition with decreasing depositional lag-times. Comparison of age populations to previously recorded cooling dates from the orogen reveal shifts from Brahmaputra to Ganges sources between Miocene-Pliocene time before reaching a mixed drainage signal in the middle Pleistocene. Observed shifts in sediment provenance are attributed to coupled tectonic-climatic forces with regional acceleration of exhumation in the Himalaya, onset of northern hemisphere glaciation and increased denudation of the frontal-Himalaya during Miocene-Pleistocene time.Item Record of crustal thickening and synconvergent extension from the Dajiamang Tso Rift, southern Tibet(Montana State University - Bozeman, College of Letters & Science, 2021) Burke, William Brian; Chairperson, Graduate Committee: Andrew K. Laskowski; Andrew K. Laskowski, Devon A. Orme, Kurt E. Sundell, Michael H. Taylor, Xudong Guo and Lin Ding were co-authors of the article, 'Record of crustal thickening and synconvergent extension from the Dajiamang Tso Rift, southern Tibet' submitted to the journal 'MDPI geosciences -- special volumes' which is contained within this thesis.Gangdese Rifts such as the Dajiamang Tso Rift of south-central Tibet provide an opportunity to study the dynamics of synconvergent extension in contractional orogenic belts. In this study, we present quantitative crustal thickness estimates calculated from Trace/Rare Earth Element zircon data paired with U-Pb geochronology and zircon-He thermochronology. These data constrain the timing and rates of exhumation in the Dajiamang Tso Rift and provide a basis for evaluating dynamic models of synconvergent extension. Our results also provide a semi-continuous record of Mid-Cretaceous to Miocene evolution of the Himalayan-Tibetan orogenic belt along the India-Asia suture zone. We report igneous zircon U-Pb ages of ~103 Ma and 70-42 Ma for samples collected from the Xigaze forearc basin and Gangdese Batholith/Linzizong Formation, respectively. Zircon-He cooling ages of forearc rocks in the hanging wall of the Great Counter Thrust are ~28 Ma while Gangdese arc samples in the footwalls of the Dajiamang Tso Rift are 16-8 Ma. These data reveal the approximate timing of the switch from contraction to extension along the India-Asia suture zone (minimum 16 Ma). Crustal-thickness trends from zircon geochemistry reveal possible crustal thinning (to ~40 km) immediately prior to India-Asia collision onset (100-70 Ma). Following collision onset, crustal thickness increases to 50 km by 40 Ma with continued thickening until the early Miocene supported by regional data from the Tibetan Magmatism Database. Modern crustal thickness estimates based on geophysical observations show no evidence for crustal thinning following the onset of E-W extension (~16 Ma), suggesting that modern crustal thickness is likely facilitated by underthrusting Indian lithosphere balanced by upper plate extension.