Browsing by Author "Khanal, Subodha"

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    Structural and kinematic evolution of the Himalayan thrust belt, central Nepal
    (University of Alabama Libraries, 2014) Khanal, Subodha; Robinson, D. M.; University of Alabama Tuscaloosa
    Over the last two decades, several competing dynamic models have been proposed to explain the kinematics of the Himalayan thrust belt. The accuracy of dynamic and kinematic models is limited by poorly documented geologic structures. With increased accessibility of the thrust belt and advances in analytical techniques, several new data sets greatly improve our understanding and provide a background to reevaluate the kinematics of the Himalayan thrust belt. In this dissertation, I integrate structural mapping, microstructural analysis, detrital and igneous zircon geochronology, low-temperature thermochronology, Nd isotopic analysis, and structural reconstructions in central Nepal to determine the evolution of the Himalayan thrust belt. Because the role and evolution of the Main Central thrust, the Ramgarh-Munsiari thrust, and the Lesser Himalayan duplex are highly debated, I emphasize these systems to provide a comprehensive structural evolution of the Himalayan thrust belt. U-Pb dating of metamorphic rims of igneous zircons and crystallization ages of cross-cutting pegmatite veins suggest that deformation on the Himalayan thrust belt started with slip on an intra-Greater Himalayan thrust active at ~20-29 Ma that emplaced the now erosionally isolated Kathmandu klippe. These ages predate the slip on the Main Central thrust. Absence of a fault contact between the Greater Himalaya and Tethyan Himalaya in the klippe suggests the South Tibetan Detachment system may have activated after the slip started on the intra-Greater Himalayan thrust. Ductile motion on the South Tibetan Detachment system may have ended prior to the activation of the Main Central thrust. This result and observations contradict the extrusion model that advocates contemporaneous activity with thrust sense shear on the Main Central thrust and normal sense shear on the South Tibetan Detachment system. In addition, there is another orogenic scale thrust, subparallel to the Main Central thrust, the Ramgarh-Munsiari thrust, that only carries lower Lesser Himalayan Paleoproterozoic rock over other Lesser Himalayan rock and accommodates a magnitude of shortening similar to the 100's km of slip on the Main Central thrust. I construct an orogenic scale balanced cross-section along the Marsyangdi River where the entire Lesser Himalayan duplex is exposed, particularly focusing on the architecture of the duplex to determine whether the duplex is forward dipping or hinterland dipping and the presence/absence of an orogenic scale, out-of-sequence thrust. I integrate quartz-feldspar deformation temperatures and zircon (U-Th)/He thermochronology and present a kinematic model that provides the structural context for geophysical, petrological, and geochronological studies in central Nepal. Collectively, this study helps to determine partitioning of strain among the various thrust sheets that account for over 2000 km of shortening in a compressional continental tectonic setting. The results suggest that deformation in the Himalaya began with the activation of an intra-Greater Himalayan thrust and successively moved south with the activation of Main Central thrust, Ramgarh-Munsairi thrust, Lesser Himalayan duplex, and finally the Subhimalayan thrust system. Although there was minor out-of-sequence thrusting in the hinterland, the bulk of the Himalaya evolved in-sequence thrusting from north to south.
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    Upper crustal shortening and forward modeling of the Himalayan fold-thrust belt along the Budhi-Gandaki river, central Nepal
    (University of Alabama Libraries, 2009) Khanal, Subodha; Robinson, D. M.; University of Alabama Tuscaloosa
    Geologic mapping along the Budhi-Gandaki River in central Nepal reveals 6 significant structures: 1) South Tibetan Detachment system; 2) Main Central thrust; 3) Ramgarh thrust; 4) Lesser Himalayan duplex including the Trishuli thrust; 5) Main Boundary thrust; and 6) Main Frontal thrust system. A balanced cross-section between the South Tibetan Detachment system and Main Frontal thrust reveals that the region has a minimum total shortening of 76% or 420 km. The breakdown of the accommodation of shortening on each thrust is as follows: Main Central thrust - 115 km; Ramgarh thrust - 120 km; Lesser Himalayan duplex including the Trishuli thrust - 156 km; Main Boundary thrust - 10 km; Main Frontal thrust system - 19 km. In order to validate the balanced cross-section, a reconstruction program was used to forward model the system. By moving faults with appropriate amounts of displacement over a reasonable configuration of undeformed stratigraphy from the hinterland to foreland, the deformation of the Himalayan thrust belt along the Budhi-Gandaki River cross-section is reproduced. The forward modeling program moves hanging wall rock over stationary footwall rock using each individual fault identified in the balanced crosssection. Hanging wall rock deforms as it is thrust over footwall structures. Using forward modeling, the cross-section has a shortening estimate of 412 km or 75%. The two shortening estimates are virtually identical indicating the balanced cross-section along the Budi-Gandaki River is viable and admissible.