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Late Pleistocene - Holocene development of the Tista megafan (West Bengal, India): 10Be cosmogenic and IRSL age constraints

Abstract : The Himalayan proximal foreland is characterized by Quaternary megafans, of which the formational mechanisms remain debated. The Tista megafan spreads over more than 16,000 km2 from the mountain front, where it is strongly incised, to the confluence of the Tista River with the Jamuna/Brahmaputra River, and stores sediments produced in the Sikkim Himalaya. We propose a scenario for the late Pleistocene - Holocene development of the Tista megafan based on new 10Be cosmogenic and Infra-Red Stimulated Luminescence (IRSL) age constraints, and discuss the main potential controls on its evolution. We suggest that two distal lobes developed successively downstream from a common proximal lobe. Deposition in the proximal lobe took place since at least ∼135 ka and incision began at 3.7-0.7+1.0 ka. The western distal lobe of the megafan was deposited early in the history of the megafan, when the Sikkim Himalaya catchment was drained by a tributary of the Ganga River, and was abandoned in the early Holocene (10-11 ka). The eastern, recent (<1 ka), and little incised lobe was built after the main Tista drainage system shifted eastward through nodal avulsions and can be considered still active. Approximately synchronous incision between terraces in the hinterland and megafan surfaces suggests that incision propagated rapidly through the system. Our data do not evidence a direct link between incicion and tectonic processes. Aggradation and incision episodes appear more compatible with a climatic control, through changes in monsoon intensity and associated sediment flux. Depositional episodes in the Tista megafan, as elsewhere in the Himalaya and its foreland, appear to correlate with periods of strong monsoon precipitation and associated high sediment flux toward the foreland. Abandonment and incision of megafan surfaces and hinterland terraces appear associated to both the onset and the ending of phases of strong monsoon precipitation, during which the balance between water and sediment discharge changes rapidly.
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Submitted on : Thursday, March 3, 2022 - 2:06:59 PM
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Rachel Abrahami, Pascale Huyghe, Peter van der Beek, Sally Lowick, Julien Carcaillet, et al.. Late Pleistocene - Holocene development of the Tista megafan (West Bengal, India): 10Be cosmogenic and IRSL age constraints. Quaternary Science Reviews, 2018, 185, pp.69-90. ⟨10.1016/j.quascirev.2018.02.001⟩. ⟨insu-03595911⟩



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