Thesis (Ph. D.)--Lehigh University, 2000. / Includes vita. Includes bibliographical references (leaves 120-131).

Syn-convergent low-angle normal-sense detachments (LANDs) are found in many orogens around the world. However, those tectonic processes which result in their formation are little known. The South Tibetan detachment system (STDS) is the best-studied example worldwide of a syn-convergent LAND, and formed in the Miocene due to the continental collision of India and Asia. In Bhutan, eastern Himalaya, the STDS is duplicated. Here I investigate the tectonic history of the inner STDS and particularly the outer STDS in Bhutan, to determine whether the duplicated STDS can be explained by or used to constrain models of Himalayan orogenesis. A range of geochronometric, thermochronologic, petrologic, structural, thermobarometric, thermometric, and isotopic tools are used to constrain: the onset and cessation of motion on the outer STDS; the cessation of motion on the inner STDS; the peak metamorphic conditions in the hanging wall and footwall of the outer STDS; the pressure-temperature-time paths of tectonites in the hanging wall and footwall of the outer STDS; the structural history of the hanging wall rocks of the outer STDS, and; the paleogeographic affinity of the hanging wall rocks of the outer STDS. The results of these studies are compared to thermo-mechanical models of Himalayan- type continental collision. Similarities in model predictions of the type and timing of structures, peak metamorphic conditions of hanging wall and footwall tectonites, pressure-temperature-time paths, and other regional tectonic observations lead to two main conclusions. 1. The STDS is a system of three main types of LANDs: those that formed during channel flow of low-viscosity mid-crustal rocks, those that formed by extrusion of cooled channel rocks to the surface, and those that formed by destabilization of the upper crust above a dome of mid-crustal channel rocks. 2. The STDS was duplicated by underthrusting of a crustal ramp into the Himalayan orogen since early Miocene. The underthrusting led to extrusion of a dome of weak mid-crustal above a previously-extruded channel. The crustal ramp may be local to the eastern Himalaya due to higher convergence and/or erosion rates, or due to local underthrusting of relatively strong crust behind weaker crust.

Understanding the development of the Himalaya is critical to elucidating continental collisional processes. The Greater Himalayan sequence (GHS), the exhumed mid-crust of the Himalayan orogen, records the tectonometamorphic evolution of the Himalaya from its deep hinterland to its foreland. The GHS in central Nepal and adjacent Tibet is deformed pervasively; quartz c-axis orientation fabrics from across the GHS indicate that it was deformed at high temperatures (~550-650˚C). The asymmetries of these quartz c-axis fabrics confirm field observations that define a reversal in shear sense from top-south shear near the bottom and middle of the GHS to top-north shear near the top of the package proximal to the South Tibetan detachment system (STDS). Estimates of mean kinematic vorticity from across the GHS indicate a pure shear contribution between 33% and 67%. U-Pb geochronologic data from the upper GHS exposed in the Changgo culmination in south Tibet indicate that melt crystallization and metamorphism related to crustal thickening occurred at ca. 35 Ma and was succeeded by a second metamorphic episode and syn-kinematic voluminous anatexis at ca. 22 Ma. The upper GHS was thinned vertically by 50% and extended horizontally during and immediately after the second metamorphic event, in a manner typical of the deep hinterland regions of orogens. In central Nepal, the ductile lateral extrusion of the upper GHS between the Main Central thrust (MCT) below and STDS above ceased by ~19 Ma. The cessation of lateral extrusion followed the collapse of the orogenic wedge and a reduction in the gravitational potential necessary to drive the propagation of deformation southwards towards the foreland. To restore the geometry of the wedge, deformation stepped out-of-sequence into southern Tibet, with the exhumation of the Changgo culmination and the North Himalayan antiform, before migrating incrementally back toward the foreland. Subsequently, the MCT migrated downward structurally adding material to the lower GHS as thrust slices, characteristic of foreland-style deformation. Thus, the transition between the upper and lower GHS in central Nepal records the transition from hinterland-style deformation to foreland-style deformation. / Thesis (Ph.D, Geological Sciences & Geological Engineering) -- Queen's University, 2009-04-13 11:48:45.702

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High-resolution regional climate simulations of the Karakoram, Himalaya have been performed for investigation into the atmospheric dynamics in this region, and their role in the Karakorams snowfall accumulation and glacial evolution. It has been seen through a combination of field measurements and satellite observations that a large number of glaciers in this region are static or advancing whilst other glaciers in the central and eastern Himalaya, as well as around the world, are nearly all retreating. By performing time slice calculations for the Karakoram region through the 21st century, it is found that, despite region wide simulated temperature changes, the highly elevated regions of the Karakoram mountain range experience positive snow mass balance through the 21st century. This result arises from a strong positive correlation between snow mass balance and simulated increases in regional precipitation, which outweighs the negative correlation between snow mass balance and simulated increases in temperature. / None

abstract: The collision of India and Eurasia constructed the Himalayan Mountains. Questions remain regarding how subsequent exhumation by climatic and tectonic processes shaped the landscape throughout the Late Cenozoic to create the complex architecture observed today. The Mount Everest region underwent tectonic denudation by extension and bestrides one of the world’s most significant rain shadows. Also, glacial and fluvial processes eroded the Everest massif over shorter timescales. In this work, I review new bedrock and detrital thermochronological and geochronological data and both one- and two-dimensional thermal-mechanical modeling that provides insights on the age range and rates of tectonic and erosional processes in this region. A strand of the South Tibetan detachment system (STDS), a series of prominent normal-sense structures that dip to the north and strike along the Himalayan spine, is exposed in the Rongbuk valley near Everest. Using thermochronometric techniques, thermal-kinematic modeling, and published (U-Th)/Pb geochronology, I show exhumation rates were high (~3-4 mm/a) from at least 20 to 13 Ma because of slip on the STDS. Subsequently, exhumation rates dropped drastically to ≤ 0.5 mm/a and remain low today. However, thermochronometric datasets and thermal-kinematic modeling results from Nepal south of Everest reveal a sharp transition in cooling ages and exhumation rates across a major knickpoint in the river profile, corresponding to the modern-day Himalayan rainfall transition. To the north of this transition, exhumation histories are similar to those in Tibet. Conversely, < 3 km south of the transition, exhumation rates were relatively low until the Pliocene, when they increased to ~4 mm/a before slowing at ~3 Ma. Such contrasting exhumation histories over a short distance suggest that bedrock exhumation rates correlate with modern precipitation patterns in deep time, however, there are competing interpretations regarding this correlation. My work also provides insights regarding how processes of glacial erosion act in a glacio-fluvial valley north of Everest. Integrated laser ablation U/Pb and (U-Th)/He dating of detrital zircon from fluvial and moraine sediments reveal sourcing from distinctive areas of the catchment. In general, the glacial advances eroded material from lower elevations, while the glacial outwash system carries material from higher elevations. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2017

A database containing previously published geochronologic, geochemical, and isotopic data on Mesozoic to Quaternary igneous rocks in the Himalayan-Tibetan orogenic system are presented. The database is intended to serve as a repository for new and existing igneous rock data and is publicly accessible through a web-based platform that includes an interactive map and data table interface with search, filtering, and download options. To illustrate the utility of the database, the age, location, and Hf-t composition of magmatism from the central Gangdese batholith in the southern Lhasa terrane are compared. The data identify three high-flux events, which peak at 93, 50, and 15 Ma. They are characterized by inboard arc migration and a temporal and spatial shift to more evolved isotopic compositions. Plain Language Summary A new database with a web-based interface is presented that contains compiled geochronologic, geochemical, and isotopic data on igneous rocks in the Himalaya-Tibetan orogen.

Data on erosion processes and other aspects of environmental change in the Himalaya are scarce and unreliable, and consequently policy decisions have been taken in a quantitative vacuum. Published estimates of denudation for large catchments in Nepal vary from 0.51 to 5.14 mm/yr, and indicate a dynamic geomorphological environment A review of the literature on erosion in Nepal revealed a consensus that: (1) mass wasting is the dominant hillslope process; (2) activity is seasonal, with virtually all failures occurring during the monsoon; (3) geological factors are the most important determinants of slope stability; (4) sediment delivery to channels is high; (5) little quantitative evidence exists to link landsliding to deforestation. Although few data exist, loss of forest cover does appear to be related to surface erosion and gullying, and a hypothesis linking the expansion of unmanaged, eroding areas to reduced nutrient subsidies from the forest is proposed. A reconnaissance survey of sediment production and transfer mechanisms in the 122 km² Phewa Valley in the Middle Mountains of Nepal identified a variety of mass movement processes. The commonest events were shallow translational failures on slopes of, typically, 36° to 45°, with volumes ≤1 x 10³ m³ and with recovery taking less than ten years. Larger slides occurred on slopes oversteepened by fluvial action. Flows developed in areas of weak rock and unfavourable structure, and were associated with groundwater discharge. Flow velocities accelerated during the monsoon. The highly fractured and deeply weathered zones around faults were the sites of "mass movement catchments", complex failures responsible for approximately 90% of all sediment production by mass wasting in the watershed. A first estimate of surface lowering by mass movement processes in the Phewa Valley is 2-3 mm/yr. Locally, surface erosion on overgrazed pasture may be 5-6 mm/yr. No data were available on soil losses from cultivated areas, and, similarly, losses due to shallow creep, gullying and solution remain unknown. The fluvial transport system in the valley bottom is unable to transport all the material with which it is supplied. Sediment yield to the lake was not calculated owing to insufficient data. Discharge estimates and intensity-duration-frequency analysis of rainfall records indicate that in Pokhara storms of 275 mm/day have a return period of approximately 10 years. The primary controls on mass movement processes in the Middle Himalaya of Nepal are geological and climatic, and therefore are not amenable to modification by man. However, surface erosion is a consequence of poor land management, and therefore can be controlled, given the right institutional environment / Forestry, Faculty of / Graduate

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No description available.