The Palaeogene record of Himalayan erosion in the Andaman Basin

The Himalayan orogeny has been recognized as one of the most important Cenozoic events that shaped the geography, climate and ocean chemistry of our planet. The erosion in the Himalayas is believed to have played a critical role in crustal deformation and changes in the chemistry of the ocean water since the Eocene. In spite of the fact that the orogeny began after India–Asia collision at 59±1 Ma, the record of its earliest erosional history is meagre. In an attempt to fill this gap in the knowledge, we studied temporal changes in provenance of Paleogene–Neogene siliciclastic sediments of the Andaman Islands, deposited in a trench-forearc basin in the Bay of Bengal. Using Sr-isotope stratigraphy and tephrochronology we determined the timings of depositions of various lithologies. Sediment sources were identified using trace element and isotopic (Sr–Nd) fingerprinting. Results of our study suggest that the Myanmar Arc had remained a constant sediment source to the Andaman basin during 55–5 Ma, whereas the basin started receiving significant continental sands input after 35 Ma that increased with time until ~20 Ma. Geochemical provenance of these sands suggests their derivation from Precambrian crustal sources in the Himalaya, which probably is an outcome of higher erosional rates subsequent to a rapid exhumation of the orogen in the late Eocene and efficient sediment transport through the palaeo-channels of the rivers Brahmaputra and Ganga under optimal conditions of the Indian monsoon. Such a scenario is consistent with the idea that the Himalayan sediment input is the cause for the conspicuous rise in marine Sr/Sr since ~40 Ma. Our data also suggest that since the Miocene, sediment sources in the Indo-Burman Ranges and the Myanmar arc have become the major contributors to the Andaman Basin through the Irrawaddy river system.