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Item Structural analysis of the Gaurishankar Lineament to understand its role as a cross fault in the east-central Nepal Himalaya(Montana State University - Bozeman, College of Letters & Science, 2023) Baral, Nischal; Chairperson, Graduate Committee: Mary S. HubbardThe Gaurishankar Lineament (GL) in east-central Nepal is a topographic feature in eastern Nepal that has been proposed to be a cross-fault (faults perpendicular to the regional E-W strike). This feature has also been proposed to have blocked the lateral propagation of the thrust rupture that was activated during the Mw 7.8 Gorkha earthquake. To understand whether the GL is a cross fault, I conducted a field study along ~E-W traverses south of Gaurishankar peak in the Rolwaling (Greater Himalayan Sequence (GHS)) and in the central Tamakoshi and Jiri regions (Lesser Himalayan Sequence (LHS)). I augmented my fieldwork with petrographic and kinematic analysis of oriented samples. As circumstantial evidence for the GL representing the topographic expression of a cross fault, I identified multiple steep fracture patterns orthogonal to regional E-W striking fabric in the Rolwaling region and an additional system of NW striking fabric orthogonal to the NNE trending GL. In the Jiri region, the offset in a thin band of graphitic schist within the rocks of the LHS aligns along the GL and may represent part of a cross-fault system. An eastward plunging synclinal fold that covers the broad region between Tamakoshi and Jiri is disrupted to the east along the GL, also consistent with the presence of a cross fault. Several NW striking faults in the Jiri region end along the GL, as suggested by the topographic expression from the satellite imagery. Further study is needed to recognize and develop a comprehensive understanding of the GL as a cross fault and to explore a newly identified system of NW striking fault.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 Syn-orogenic magmatism, mid-crust exhumation, and placer gold deposition: the Anaconda metamorphic core complex of western Montana(Montana State University - Bozeman, College of Letters & Science, 2020) Howlett, Caden James; Chairperson, Graduate Committee: Andrew K. Laskowski; Andrew K. Laskowski was a co-author of the article, 'Determining the source of placer gold in the Anaconda metamorphic core complex supradetachment basin using detrital zircon U-PB geochronology, pioneer district, western Montana' submitted to the journal 'Geosphere' which is contained within this thesis.; Aislin N. Reynolds and Andrew K. Laskowski were co-authors of the article, 'Farallon slab-removal as a driving force of metamorphic core complex formation in the western USA: details from the Anaconda metamorphic core complex of western Montana' submitted to the journal 'Tectonics' which is contained within this thesis.Since their initial discovery in the late 1960's, metamorphic core complexes have remained of high interest in tectonics research. Early uncertainty regarding the mechanics of slip along low-angle normal (detachment) faults is now accompanied by controversy surrounding the relationship between magmatism and large-magnitude extension. As deeply exhumed geologic structures that record lithospheric thermomechanical processes, investigating core complex formation is crucial to understanding how the mid-crust behaves in extensional tectonic settings. In some regions, the exhumation of these structures is also linked to the formation of economically valuable mineral deposits, making them of notable societal importance. This thesis is a two-part investigation of core complex evolution that addresses the concepts above, formatted with introductory and concluding chapters that bound two main chapters prepared for publication. Chapter two consists of a study that tests the utility of using detrital zircon (DZ) U-Pb geochronology and DZ unmixing models to determine the source of placer gold. New zircon U-Pb (n=1,058) and Lu-Hf (n=61) isotopic data are presented from four placer deposit samples extracted from the Pioneer District of western Montana. Geochronology and DZ unmixing modeling suggest that gold from the placer deposits was derived from vein and skarn lode sources in northern footwall of the Anaconda metamorphic core complex (AMCC). Our data offers the first DZ-based support for previous interpretations that the Late Cretaceous Royal Stock pluton precipitated gold along its contact with overlying Proterozoic through Mesozoic supracrustal rock, and was subsequently weathered, transported, and deposited in the AMCC supradetachment basin during the Late Oligocene-Early Miocene. Chapter three consists of an integrated geologic, geochronologic, thermochronologic, and isotopic investigation of the AMCC footwall. Results suggest that the AMCC is an example of a core complex that was primed for large-magnitude extension through crustal thickening and voluminous magmatism. It is proposed that buckling of the Farallon slab, marked by the onset of 'ignimbrite flare up' volcanism, was responsible for the initiation of AMCC extension. Furthermore, a compilation of MCC cooling ages and ages of Cenozoic volcanics across the western USA suggest that removal of the Farallon Plate was a primary driver of Cordilleran core complex formation.Item Structural analysis of the Benkar Fault Zone, a cross structure in the higher Himalaya of the Khumbu Region, eastern Nepal(Montana State University - Bozeman, College of Letters & Science, 2019) Seifert, Neil Jordan; Chairperson, Graduate Committee: Mary S. HubbardThe Himalaya are dominated by laterally continuous, range-parallel thrust faults and less frequent extensional structures such as the South Tibetan Detachment system. Recent discovery of range-perpendicular strike-slip and extensional fault zones (cross structures) in the Himalaya has raised questions regarding the significance of these structures in the collisional process. I have analyzed a newly-recognized cross structure in the Khumbu region of eastern Nepal, the Benkar Fault Zone. Structural mapping, petrographic observation, and analytical evidence reveals a zone of consistently NE-striking, SE-dipping metamorphic foliation from the villages of Phakding to Gorak Shep along the Dudh Kosi valley. Deformation within this zone is restricted to foliation-parallel, nonpenetrative, anastomosing sillimanite- and mica-bearing shear zones that wrap around poorly deformed quartz and feldspar enclaves. Kinematics of these shear zones from thin sections and outcrop observations are consistent with normal, right-lateral oblique slip and some local zones of purely extensional displacement. Quartz crystallographic fabric orientation suggests crystal plastic deformation under upper greenschist to lower amphibolite facies conditions. Outcrop observations show evidence for isolated brittle-ductile deformation, suggesting short-lived brittle events that punctuated a history of predominantly plastic deformation. Benkar Fault Zone fabric crosscuts older, thrust-related foliation and suggests that deformation within the zone postdates peak metamorphism and occurred while rocks were on the retrograde path. Crosscutting relationships between Benkar Fault Zone fabric and leucogranites constraints the timing of deformation to be younger than 20.5-21.3 Ma. The Benkar Fault Zone is interpreted to represent a tear fault within the Greater Himalayan sequence associated with differential displacement along a structurally deeper thrust.Item Metalamprophyre dike deformation in the Homestake Reservoir Quadrangle, Leadville, Colorado(Montana State University - Bozeman, College of Letters & Science, 2017) Frothingham, Michael Geoffrey; Chairperson, Graduate Committee: Mary S. Hubbard; Colin Shaw (co-chair)Field relations and microstructural observations show that intrusion of mafic, ultrapotassic, lamprophyre dikes in the Northern Sawatch Range near Leadville, Colorado localized shear zone deformation ca. 1.4 Ga. This suggests that lithologic contrasts and advective heating associated with magmatism contributed to localizing intracontinental deformation associated with a widespread tectonothermal event that affected the southwestern U.S. during the Mesoproterozoic. Dikes are concentrated in a distinct structural domain characterized by east striking, subvertical foliations in a localized zone of Paleoproterozoic hornblende gneiss, calc-silicate gneiss, and biotite gneiss country rock. Observations show that preexisting fabrics in Proterozoic units preferentially localized north south extension, which accommodated dike emplacement. Following emplacement, dikes were metamorphosed and sheared along their margins within relatively unaffected country rock. The TitaniQ titanium and quartz thermometer yields deformation temperatures in dikes of at least 564 + or - 45 °C, hotter than previously calculated deformation temperatures of mylonite in the nearby Homestake Shear Zone (HSZ) and similar to deformation temperatures of the nearby Slide Lake Shear Zone (SLSZ). This implies that during deformation, the dikes were still hot from their recent intrusion and this may have localized early stages of progressive strain accumulation. Dike emplacement and subsequent deformation demonstrates that strain was more widely distributed than in previously mapped, strictly defined 1.4 Ga shear zones. This study identifies how the newly recognized element of strain partitioning in mafic dikes, in addition to coeval emplacement of St. Kevin batholith and deformation in the HSZ and SLSZ, suggests a multi-faceted, kinematically-linked system of deformation ca. 1.4 Ga, which illustrates the dynamic middle continental crustal effects of Proterozoic intracontinental deformation.Item Structural geology of the north-half of the Swift Reservoir culmination, Sawtooth Range, Montana(Montana State University - Bozeman, College of Letters & Science, 2016) Ross, Daniel Landers; Chairperson, Graduate Committee: David R. LagesonThe Sawtooth Range forms a broad salient at the eastern edge of the Sevier fold-and thrust belt along the Rocky Mountain front of northwest Montana. The Swift Reservoir Culmination is a structural high located along the range-front of the Sawtooth Range, just south of Glacier National Park. The range is dominated by steeply-dipping imbricate thrust sheets. The culmination exposes an anomalous suite of Cambrian rocks that are not found elsewhere along the Sawtooth front. The structural complexity of the range is underscored by the high degree of imbrication that has taken place as well as the tight folding. Moreover, units in the SRC are characterized by strike-parallel cutoffs that create a surface expression of a compound eyelid window. The chief objectives of this research were to determine if the structural geometry of the culmination was compatible with an interpretation of a hinterland-dipping duplex in the footwall of the Major Steele Backbone thrust; what factors are responsible for the strike-parallel cutoffs seen in the culmination; and whether the SRC is a viable structural analog to subsurface duplex systems targeted for exploration along the Rocky Mountain front. An in-depth structural investigation of the culmination was conducted through field-based mapping, followed by lab investigations of the data collected in the field. This included geologic mapping followed by the construction of cross-sections, as well as the synthesis of this data with published magnetic and gravity data in order generate an accurate structural model of the culmination from basement to surface. Deformation within the SRC is controlled primarily through the mechanical properties of the units within the culmination, resulting in compartmentalization of the culmination into four distinct lithostratigraphic/structural domains. Moreover, this facilitated the development of duplex fault zones within the culmination. The Heart Butte thrust is the linking fault between three of these domains, and is a reflection of pre-orogenic basement lineaments that controlled the structural development of not only the Sawtooth Range, but the SRC as well.Item Structural geology and history of the Buck Mountain fault and adjacent intra-range faults, Teton Range, Wyoming(Montana State University - Bozeman, College of Letters & Science, 1991) Smith, Daniel JosephItem Tectonic significance of the pass fault, central Bridger Range, southwest Montana(Montana State University - Bozeman, College of Letters & Science, 1986) Craiglow, Carol JeanItem Sedimentology, provenance, and paleotectonic significance of the cretaceous Newark Canyon Formation, Cortez Mountains, Nevada(Montana State University - Bozeman, College of Letters & Science, 1988) Suydam, James David
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