No abstract

The eastern Himalayan syntaxis, where the Yarlung Tsangpo sharply bends, is one of the areas experiencing most rapid exhumation on Earth. The rapid exhumation is often regarded as the result of capture of the Yarlung Tsangpo by the Brahmaputra River.However, both the timing of integration of the Yarlung Tsangpo-Brahmaputra River and initiation of the rapid syntaxial exhumation are debated. As the ultimate sedimentary trap of the Yarlung Tsangpo-Brahmaputra River, the Nicobar Fan is a window to look into the drainage evolution and exhumation history of the eastern Himalaya. International Ocean Discovery Program Expedition 362 drilled the Nicobar Fan for the first time, recovering fan sediments dating back to the Early Miocene (~19 Ma). We apply trace elements and Sr-Nd isotopes to investigate the provenance of the sediments in the Nicobar Fan with the aim of constraining the timing of integration of the Yarlung Tsangpo-Brahmaputra River and initiation of the rapid syntaxial exhumation. The geochemical and Sr-Nd isotope compositions indicate an eastern Himalayan source dominated by the Greater Himalaya, with significant Gangdese arc contribution and primarily carried by the Brahmaputra River. Flux of Gangdese arc material appears to have been continuous from the base of the Nicobar Fan, suggesting that the Yarlung Tsangpo-Brahmaputra River has been established at least since ~19 Ma. Synchronously with the sharp rise in sedimentation rate, the abrupt change of geochemical and isotope compositions at ~9.2 Ma indicates an increase in erosion of the Greater Himalaya as the result of initiation of rapid exhumation in the broad syntaxial region. The proportion of Greater Himalayan material increased again at 3.5-1.7 Ma, consistent with a younger pulse of rapid exhumation focused in the core of the syntaxis since ~3.5 Ma. Our results show that initiation of the rapid syntaxial exhumation postdated integration of the Yarlung Tsangpo-Brahmaputra River by at least ~10 m.y. Therefore, tectonic uplift rather than river capture could be responsible for the initiation of the rapid syntaxial exhumation.