Theses and Dissertations at Montana State University (MSU)
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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 The Montana Alps: kilometer-scale recumbent folding and tectonic attenuation in the Anaconda Range, southwestern Montana(Montana State University - Bozeman, College of Letters & Science, 2022) Neal, Bryce Alan; Chairperson, Graduate Committee: Andrew K. Laskowski; This is a manuscript style paper that includes co-authored chapters.The Eocene Anaconda metamorphic core complex (AMCC) is the most recently documented metamorphic core complex in the North American Cordillera. While much work has focused on constraining the nature and timing of core complex extension, earlier deformation preserved in its footwall is not as well understood. The AMCC footwall contains an anomalously thin, mid-crustal section of Mesoproterozoic Belt Supergroup and Paleozoic strata. While the tectonic nature of this thinning is generally accepted, the mechanisms behind it remain enigmatic. Geologists from the Montana Bureau of Mines and Geology hypothesize that footwall strata were attenuated along the upper limb of the Fishtrap recumbent anticline (FRA), a kilometer-scale NW-verging recumbent fold exposed throughout the southwestern AMCC footwall. New geologic mapping in the Carpp Ridge 7.5' quadrangle and U-Pb geochronology better constrain the nature and timing of tectonic attenuation in this complex area. Two generations of folds deformed rocks in the quadrangle: F 1 recumbent folds with S 1 axial planar fabrics associated with the FRA, and F 2 upright folds with S 2 axial planar fabrics that refold the FRA. These deformations are likely Late Cretaceous in age based on dates from cross-cutting intrusions, although a foliation sub-parallel to S 1 in a 51.87 Ma granodiorite stock in the FRA hinge suggests localized Eocene deformation. Elsewhere in the field area, the same granodiorite crosscuts S1 fabrics. F 1 folds and S 1 fabrics transpose, attenuate, and omit Belt strata in the southeastern portion of the quadrangle, suggesting that recumbent folding is intimately associated with tectonic attenuation. Further, west-vergent F 1 and F 2 folds may be decoupled from regional east-vergent tectonics and instead related to gneiss-doming in the ~75-74 Ma Lake of the Isle shear zone. Gneiss-doming and associated development of the FRA may have been driven by widespread decompression of the Cordilleran middle crust during Late Cretaceous time, perhaps in response to delamination of lithospheric mantle or arc-root foundering beneath the Cordilleran magmatic arc of SW Montana.Item Orogens of Big Sky Country: reconstructing the deep-time tectonothermal history of the Beartooth Mountains, Montana and Wyoming, USA(Montana State University - Bozeman, College of Letters & Science, 2021) Ronemus, Chance Baylor; Chairperson, Graduate Committee: Devon A. Orme; Devon A. Orme, William R. Guenthner, Stephen E. Cox and Christopher A. L. Kussmaul were co-authors of the article, 'Orogens of Big Sky Country: reconstructing the deep-time tectonothermal history of the Beartooth Mountains, Montana and Wyoming, USA' submitted to the journal 'MDPI Geosciences Special Issue: Evolution of Modern and Ancient Orogenic Belts' which is contained within this thesis.The southwestern Montana region has experienced a protracted history of orogeny, burial, and erosion recording the development of the western margin of Laurentia, the core of the North American continent. This > 2.5 Gyr record contains clues about the nature of Precambrian tectonism, the development of economic mineral and hydrocarbon reserves, and the long-term geodynamic evolution of Earth. However, aspects of this history remain enigmatic, with events in the geologic record obscured by erosion and thermal overprinting. The manuscript presented herein, bound by introductory and concluding chapters, comprises a deep-time thermochronologic investigation of the Beartooth Mountains. New biotite 40 Ar/39 Ar, and zircon U-Pb and (U-Th)/He data are presented from 14 samples collected from the Montana part of the range. These data indicate that thermal effects of Paleoproterozoic thermotectonism associated with the Big Sky orogeny (ca. 1.8-1.71 Ga) and/or related mantle metasomatism or mafic underplating penetrated into the core of these mountains. Thermal history model results indicate that this region of the craton experienced multi-phase Proterozoic cooling. The first phase of this cooling is generally coeval with the collapse of the Big Sky orogen. A second phase of Proterozoic cooling culminated in the development of the Great Unconformity surface, across which > 2 Gyr is regionally 'missing' from the stratigraphic record. Constraints placing this latter phase between 1.4 Ga and 0.8 Ga preclude mechanisms predicting later Neoproterozoic-Cambrian cooling, such as erosion associated with Snowball Earth glaciation, and support diachronous development of the Great Unconformity surface in Laurentia. Thermal models resolve a Phanerozoic thermal history involving maximum burial temperatures by late Pennsylvanian time and cooling throughout Mesozoic time. This Phanerozoic thermal history, broadly out of sync with nearby basins, underscores the effects of interactions between far-field tectonism and inherited crustal weaknesses in the Beartooth Mountains and reconciles previous interpretations of pre-Late Cretaceous cooling with other evidence only constraining later phases of uplift. Finally, model results suggest Cenozoic reheating--likely due to burial by volcanics--and later cooling to surface temperatures due to erosional removal of these rocks--potentially related to encroachment of the Yellowstone hotspot and/or regional Basin and Range extension.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.Item Scarp analysis of the Centennial Normal Fault, Beaverhead County, Montana and Fremont County, Idaho(Montana State University - Bozeman, College of Letters & Science, 2008) Petrik, Falene Elizabeth; Chairperson, Graduate Committee: David R. LagesonThe Centennial Mountains are an east-west trending mountain range in southwest Montana. The Centennial Mountains are bound on the south by the Eastern Snake River Plane, the north-trending Madison Range and fault on the east and the Centennial Valley on the north. The Centennial normal fault offsets the Centennial Mountains on the north down-dropping the Centennial Valley. Approximately 3000 meters of offset along the Centennial normal fault creates the Centennial Mountains. The present Centennial Mountains are subdivided into two stratigraphically different blocks by the Odell Creek normal fault. The eastern Centennial Mountains are interpreted as the upthrown block of the Odell Creek normal fault exposing Precambrian and Paleozoic rock along the northern face of the range. The western Centennial Mountains are interpreted as the downthrown block of the Odell Creek normal fault exposing Cretaceous and younger rocks. Both eastern and western segments of the Centennial Mountains are then offset along the range bounding Centennial normal fault. Offset along the Centennial normal fault started approximately 2.1 Ma as evidenced by the displacement of the 2.1 Ma Huckleberry Ridge tuff. It is believed that prior to the emplacement of the 2.1 Ma Huckleberry Ridge tuff, the Centennial Mountains had minimum to no surface relief. The majority of offset along the Centennial normal fault has occurred with in the late Pleistocene with estimated slip rates of 0.65-0.82 mm/yr. The late Pleistocene surface offsets along the Centennial Mountains have an average of 9.1-9.6 meters with similar offset seen along the eastern and western segments.