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Determination of the Quaternary evolution of Nepalese-Himalayan faults inferred from luminescence thermochronometry

Author Chloé Bouscary
Director of thesis Dr. King Georgina
Co-director of thesis Prof. Herman Frédéric Prof. Anselmetti Flavio
Summary of thesis The 2015 Mw 7.8 Gorkha earthquake in Nepal, which caused widespread devastation and loss of life, reveals some gaps in our understanding of the deformation of the Himalaya. Here we aim to constrain recent - Quaternary - changes in deformation in Nepal, through quantifying exhumation rates using luminescence thermochronometry. Optically Stimulated Luminescence (OSL)-thermochronometry [Guralnik B. et al., 2015; King G.E. et al., 2016] is a recently developed very-low-temperature thermochronometer, sensitive to temperatures of 30-100°C, based on luminescence dating of quartz and feldspar minerals. It offers the potential for precise constraint of cooling histories over recent timescales, and provides high-resolution cooling histories beyond the range of other thermochronometric systems. Applying this new technique to feldspar extracts of a set of samples from the Nepalese-Himalaya provides insights into the cooling and thus exhumation/erosion history of the Himalayan fold-and-thrust belt, giving a better understanding of the Quaternary dynamics of the Himalayas. The Himalaya mountain belt is the result of compressional orogeny due to the continental collision between the Indian and the Eurasian tectonic plates. Different shear zones and north-dipping crustal-scale thrusts accommodate this convergence. In this project, the objective is to analyse samples collected across some of the most tectonically significant structures in the Himalayan orogen. This allows us to better define the location of the main faults, which is still debated [Searle M. et al., 2008; Parsons A.J. et al., 2016], and by quantifying exhumation rates on each side of the faults, to assess the long-term deformation across the major thrusts. Preliminary results from five samples along the Marshyangdi River give a better idea of the location of the Main Central Thrust (MCT), and indicate variations in exhumation rates between the different tectonic structures. Whilst potentially relating to local surface process and/or climatic differences (i.e. fluvial incision rates, glacial erosion), these data could also indicate recent tectonic deformation, potentially implying Quaternary fault reactivation within this region.
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