Change in provenance and sediment routing history of the Miocene-Pleistocene Bengal Fan, Indian Ocean using detrital zircon (U-Th)/He thermochronology

Abstract

The Ganges-Brahmaputra river system carries thousands of tons of sediment from the Himalaya each year to the submarine Bengal Fan within the Indian Ocean. Deposition of sediments to the fan has been ongoing since at least late-Oligocene time (Krishna et al., 2016), preserving a record of Himalayan tectonics and topographic evolution (Blum et al., 2018). IODP Expedition 354 (2015) collected 1.7 km of sediment core from the Bengal Fan to expand the record of Himalayan sediment routing (France-Lanord et al., 2016). Detrital zircon U-Pb data from core samples reflect five major tectonostratigraphic sequences from the Himalaya-Tibet Orogen (Blum et al., 2018). Age populations appear to vary temporally, suggesting change in erosion rates and sediment routing through time, especially apparent during the Pliocene-Pleistocene transition. This research uses detrital zircon (U-Th)/He thermochronology, double dating analyses, and sediment mixing model approaches to fingerprint changes in sediment routing and river deposition to the Bengal Fan. (U-Th)/He analyses are used to quantify rock uplift and exhumation experienced in the Himalayas, facilitating lag-time interpretations across Miocene-Pleistocene time. To interpret the Bengal Basin sink record, a compilation of mid to low-temperature thermochronology datasets across the Himalaya-Tibet orogen is presented for characterization of modern-day thermochronologic age signals. This database highlights orogen wide trends in cooling dates that show younging apatite/zircon (U-Th)/He and fission tracks in the frontal Himalaya and syntaxes, and a younging trend in 40Ar/39Ar dates towards the orogen center. Zircon (U-Th)/He analyses of Bengal Fan sediments deposited 6.20 - 0.13 Ma reveal cooling dates from ~0.28 + or - 0.03 - 540.15 + or - 6.13 Ma. Age populations vary greatly between Miocene-Pleistocene time, with notable loss of >23 Ma populations between Miocene-Pliocene time and increasing 5.3 - 23 Ma populations across the Pliocene-Pleistocene transition with decreasing depositional lag-times. Comparison of age populations to previously recorded cooling dates from the orogen reveal shifts from Brahmaputra to Ganges sources between Miocene-Pliocene time before reaching a mixed drainage signal in the middle Pleistocene. Observed shifts in sediment provenance are attributed to coupled tectonic-climatic forces with regional acceleration of exhumation in the Himalaya, onset of northern hemisphere glaciation and increased denudation of the frontal-Himalaya during Miocene-Pleistocene time.