Tectono-geomorphic model for the landscape evolution of the Kathmandu Basin, Nepal
Creators: name: Pokhrel, Prakash
Publisher: name: The University of Edinburgh
Year: 2024
Subjects: Himalaya
low relief landscape
palaeogeography
tectonics
drainage reorganization

description: The Himalayan Mountain range is a classic example of a transient landscape shaped by the interaction between active tectonics and climate dynamics. As a result, fluvial drainage systems in the region reflect this transient nature, undergoing changes in the geometry of their catchment areas and river channels. The Kathmandu Basin, located in the central Himalaya, exemplifies a 'low-relief, high-elevation' basin. It is situated between two major trans-Himalayan river systems: the Gandaki to the west and the Kosi to the east. Due to its distinct position and geomorphology, the Kathmandu Basin offers an ideal case study for exploring the processes that drive the formation of low-relief surfaces and isolate them from surrounding rivers. In general, ‘low-relief, high-elevation’ basins are often filled with sediments, and studying these trapped sediments allows for the reconstruction of their palaeogeography, as well as providing records of past climate and tectonic activity. Comparing this palaeogeography with modern topography helps in understanding processes such as drainage reorganization. However, establishing these connections requires effective tools for extracting relevant information. This thesis, therefore, focuses on developing and applying techniques to reconstruct the palaeogeography of the Kathmandu Basin and its surrounding river catchments. First, I employ various sedimentary methods to analyse the fluvial sediments preserved at the base of the basin. The basin fill consists of Pliocene fluvial deposits that unconformably overlie the eroded surface of Precambrian basement rocks. While the basement geology and lacustrine sediments have been extensively studied, the fluvial deposits have not been investigated in sufficient detail. By applying a multi-technique sedimentary approach to these fluvial deposits, I test the long-standing hypothesis of drainage reorganization in central Nepal. This study provides the sedimentological evidence of drainage reorganization. I then propose a tectono-stratigraphic model involving localised accelerated rock uplift along the Mahabharat Range, leading to damming and the formation of a perched basin in the Himalayas. Continuing the sedimentary approaches discussed earlier, I aim to estimate sediment transport distance by analysing the roundness of sediment deposited in the Kathmandu Basin. While roundness, along with grain size and shape, has been a critical tool for reconstructing ancient fluvial environments, it is often assessed qualitatively. To address this limitation, I develop a new roundness model that quantitatively estimates sediment transport distance. This study introduces a workflow and a novel model for using sediment roundness as a proxy for transport distance. The proposed roundness model is based on Sternberg’s law of abrasion and incorporates a roundness metric that is independent of pebble elongation. Traditional field methods for measuring roundness are limited in their ability to provide numerous and replicable measurements. In contrast, the workflow presented here relies on 2D field photographs of pebbles, allowing for consistent and replicable measurements when using the same photographs. I demonstrate the applicability of the new roundness model by applying it to clasts from the stratigraphic record and modern pebbles. The model estimates a greater transport distance for pebbles preserved at the base of the Kathmandu Basin compared to the length of the modern river. Finally, I use cosmogenic-derived erosion rates to confirm drainage asymmetry across the main drainage divide (MDD) of the Kathmandu Basin. Additionally, I compare Gilbert metrics for the channel heads and hillslopes along both sides of the MDD. I interpret that the erosional asymmetry observed across the southern MDD can be attributed to significant uplift along the Mahabharat Range. In contrast, the lack of significant erosional asymmetry across the northern MDD, as well as on the eastern and western sides of the MDD, is likely due to the positions of knickpoints and active faults with varied characteristics. The divide asymmetry identified through the Gilbert metrics analysis aligns broadly with the erosional asymmetry observed in the Kathmandu Basin. Overall, the MDD in the south is experiencing significant divide migration, while the divide on the other side of the Kathmandu Basin, which drains through a smaller perched basin, remains relatively stable. Based on sedimentary and geomorphic evidence, I demonstrate that topographic contrast between the low-gradient alluvial channels of the Kathmandu Basin and the steep bedrock channels of the neighbouring Kosi and Gandaki Rivers has been a significant factor in drainage reorganisation and has contributed to shaping the Kathmandu Basin into its current form.