Tectonic-erosion interactions:insights from the paleo-drainage of the Brahmaputra River

Govin, Gwladys (2017) Tectonic-erosion interactions:insights from the paleo-drainage of the Brahmaputra River. PhD thesis, UNSPECIFIED.

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Abstract

This thesis investigates the interactions between tectonics and erosion in the eastern Himalaya through the study of paleo-Brahmaputra deposits in the foreland basin. Sediment depositional dating of two sedimentary sections is performed using magnetostratigraphy, apatite fission-track and luminescence dating. Provenance analysis using zircon and apatite U-Pb dating allows the reconstruction of the Miocene-Quaternary paleo-drainage of the Brahmaputra River and the documentation of the tectonic evolution of two poorly understood Himalayan features: the Shillong Plateau and the Namche Barwa Syntaxis. The Shillong Plateau is the only elevated topography in the Himalayan foreland and the timing of its surface uplift is debated. Decoupling between of the time of rock exhumation and surface uplift has been explained by differences in rock erodibilities of the plateau between the Shillong Precambrian basement and the overlying Cenozoic sedimentary rock. New detrital zircon U-Pb data and lithospheric stress field modelling presented here date the rise of the Plateau between 5.2 Ma and 4.4 Ma leading to the redirection of the Brahmaputra River at that time, and the role of tectonics in the rise of the plateau is invoked. The Namche Barwa syntaxis is located at the eastern Himalayan termination and its development is widely debated. It has been subjected to anomalously young (<10 Ma) peak metamorphism, and unusually high exhumation rates (up to 10 mm/yr), in comparison with the Himalayan main arc of the orogen. However, the timing of the onset of rapid exhumation of the Namche Barwa syntaxis is poorly constrained (between 10 and 3 Ma). Focusing on the proximal detrital record of material eroded from the syntaxis, new rutile U-Pb, white-mica 40Ar/39Ar and zircon fission-track data, together with published data are incorporated in a thermokinematic model which suggests an older onset (>10 Ma) of rapid exhumation, and at high but not extreme rates (<5 mm/yr).