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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 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 Tectonic significance of the pass fault, central Bridger Range, southwest Montana(Montana State University - Bozeman, College of Letters & Science, 1986) Craiglow, Carol JeanItem Structural geometry of the Paradise Valley, Park County, southwest Montana(Montana State University - Bozeman, College of Letters & Science, 1995) Wu, ZhangmingItem Structural setting of Teton Pass with emphasis on fault breccia associated with the Jackson thrust fault, Wyoming(Montana State University - Bozeman, College of Letters & Science, 1982) Vasko, Ann MarleneItem Geologic setting and geomorphic analysis of quaternary fault scarps along the Deep Creek fault, Upper Yellowstone Valley, south-central Montana(Montana State University - Bozeman, College of Letters & Science, 1982) Personius, Stephen FrancisItem Fault segmentation control on alluvial fan and fan drainage basin morphometry, Lemhi Range, East-Central Idaho(Montana State University - Bozeman, College of Letters & Science, 2011) Carr, Christina Grace; Chairperson, Graduate Committee: James G. SchmittAlluvial fan and fan drainage basin morphometric parameters are proposed to vary as a function of distance from closest fault segment boundary along the Lemhi fault in east-central Idaho. Large normal faults are broken into segments along strike that have unique rupture histories. Since potential earthquake magnitude is related to rupture area and therefore segment rupture length, accurate delineation of segments can have implications for earthquake hazard assessment. Alluvial fans were mapped on airphotos, and drainage basins were extracted from digital elevation models (DEMs). Morphometric parameters calculated for each drainage basin/fan pair based on DEM-extracted data include: elongation ratio (measures basin roundness in map view), Melton ratio (relates absolute relief and basin area), fan area to drainage basin area ratio, and hypsometric integral. Hypsometric integral is the area under the hypsometric curve, which displays relative elevation as a function of relative area. Standard regressions compared variations in these parameters with distance to fault segment boundaries mapped by previous workers. Drainage basins closer to fault segment boundaries tend to have lower elongation ratios (more elongate shape), higher Melton ratios, higher fan area to drainage basin area ratios, higher hypsometric integrals, and straighter (less sinusoidal) hypsometric curves. No parameter is strongly correlated with distance to closest segment boundary, but the strongest correlation occurs with the Melton ratio. High Melton ratios have been associated with basins dominated by flows with higher sediment-to-water concentrations compared to basins with low Melton ratios. The observed along-strike morphometric variations can influence conceptual models of extensional footwall drainage development and hangingwall basin stratigraphic evolution. However, the relationships are not strong enough to inform seismic hazard or similar studies requiring a high degree of confidence and strong correlations. In agreement with previous studies, fan area and drainage basin area are directly correlated, fan slope and drainage basin area are inversely correlated, and drainage basin slope and area are also inversely correlated. This study recognizes differences between slope calculated conventionally and slope calculated using standard grid-based methodology. This observation requires future studies to consider which slope definition is most representative of gravity-driven hydrogeomorphic processes.