School of Earth Sciences - Theses

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Start date: 2020 × End date: 2022 × Type: PhD thesis × Subject: Thermochronology × Author: Wood, Matthew Peter ×

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    Arc segmentation and landscape evolution in the Bhutan Himalaya
    Wood, Matthew Peter ( 2022)
    The principal structural elements of the Himalayan arc can be traced nearly continuously for 2500 km. Historically, along-strike variations in structure and denudation have not received the same attention as equivalent arc-normal trends. Yet research has demonstrated that arc segmentation can be controlled by lateral variations in the geometry of the Main Himalayan Thrust (MHT). The Bhutan Himalaya has a distinctive physiography and hosts nominal instrumental seismicity despite experiencing long-term strain accommodation comparable to the wider arc. This enigmatic section of the orogen presents an opportunity to test the case for local arc segmentation through applied tectonic geomorphology. By integrating low-temperature thermochronology – including apatite fission track (AFT), and apatite and zircon (U-Th-(Sm))/He thermochronometry of in situ bedrock, synorogenic sediments and modern detrital samples – cosmogenic radionuclide methods (10Be concentrations from detrital quartz samples that add to a nation-wide compilation of published data) and quantitative geomorphometry, this study documents the spatial and temporal variability of denudation to infer partitioning of deformation across crustal structures. Results show prominent along- and across-strike variation in denudation within Bhutan. Contiguous geomorphic zones are defined based on millennial-scale erosion rates and their morphometric proxies, including the physiographically distinct low-relief belt and southeast range front. Profile curvature statistics of central range front ridges are linked to earthquake-triggered landsliding and define the Naka Zone, which overlies a historically seismogenic MHT decollement flat. Synorogenic detrital thermochronometers provide information on source area bedrock cooling and the thermal evolution of the Indo-Gangetic paleo-basin. Linking the depositionally-adjusted age spectra of Siwaliks thermochronometers to an analogous modern detrital suite allows the estimation of sedimentary provenance. The dominant Lesser Himalaya source has been tectonically constructive since at least ~5 Ma, while the secondary Greater Himalaya source is ‘steady state’, as evidenced by a persistent ~4 Ma ZHe age peak. Range front and Siwaliks thermochronometers show that the structural succession is complicated near the eastern border. Southward decreasing detrital AFT exhumation rates in hinterland mountain catchments document progressive, semi-horizontal, post-cooling translation across the basal thrust flat. Decoupled intra-catchment millennial-scale erosion rates are a transient response to geologically recent rock uplift westward of the southeast range front. Exhumation rate modelling of individual in situ samples and elevation sampling transects show that geothermal gradients of at least 35 degrees C km-1 – and long-term erosion rates of ~1 km Myr-1 – have persisted across much of the study area. Multi-thermochronometric thermal history modelling results indicate that the locus of exhumation is offset by ~20 km towards the front southeast of Drangme Chu, predictive of a mid-crustal ramp beneath the Tawang River valley and Lumla window. An oblique ramp is invoked to reconcile differing orogenic sections. A synthesis of findings leads to the proposal of an obliquely oriented, second-order segment boundary within Eastern Bhutan, which may help constrain the seismic potential of adjacent arc segments.