Late Cenozoic evolution of the Nicobar Fan - Insights from U-Pb-Hf in Zircon

International audienceLate Cenozoic deep-sea deposits of the Bengal – Nicobar Fan recorded the events of Himalayan accretion during the India – Asia collision. IODP Expedition 362 – The Sumatra Subduction Zone – recovered a sedimentary section at the Nicobar Fan that comprises Early Miocene to Holocene strata. This work aims to identified potential sources and the evolution of the fluvial system that fed into the Nicobar Fan during that interval. For this, we analyzed zircon U-Pb-Hf in seven samples from sandy and silty turbidite deposits of U1480 and U1481 Sites. U-Pb zircon ages range from 22 to 3358 Ma and εHf −33.7 to +15, originated from Central and Eastern Himalayan, Lhasa Block, peninsular India and Indo Burma Ranges, the Ganges – Brahmaputra rivers drained areas. Our data suggest a significant reorganization of source areas since the Late Miocene. Rapid uplift of the Himalayan syntaxis in the Late Miocene to Pleistocene exposed distinct source rocks, altering the provenance signature and enhancing sediment delivery towards the sink. This is evidenced by gradually switched in first cycle zircons in the Nicobar Fan from the Lhasa Block and Gangdese Batholith to the Bomi-Chayu and Lohit Batholiths. Sediment provenance shifted since the Pliocene, with an increase in Ganges-derived Archean peninsular India, Cambro-Ordovician granites and Greater Himalaya orthogneisses at the Nicobar Fan. We attribute this shift to the rise of the Shillong Plateau, which projected the Brahmaputra delta to the west towards the Ganges river. This rise of the Shillong Plateau and the sea level fall at the Plio-Pleistocene enabled the merge of the Brahmaputra and Ganges deltas for the first time. Our data suggest that the Nicobar Fan was majorly supplied by the Brahmaputra River during the Tortonian, with an only minor contribution from the Ganges river. Following that, an increase in sediment contribution from the Ganges River is observed