THE DETRITAL RECORD OF QUATERNARY MEGAFLOODS THROUGH THE TSANGPO GORGE, EASTERN HIMALAYA

Holocene and late-Pleistocene deposits in the Yarlung-Brahmaputra River drainage, southeast Tibet and northeast India, represent an extraordinary record of glacial impoundments and outburst flooding in one of the most tectonically active landscapes on Earth. Glacial damming may have inhibited river incision into the margin of the Tibetan Plateau while outburst floods focused erosion in the steep Tsangpo Gorge and deposited slackwater sands along the river valley downstream. However, few flood deposits have been studied, and the specific source area of individual floods remains unknown. Here we report n=1438 new detrital zircon U-Pb and n=1484 single-grain luminescence data from 15 ancient megaflood and historical outburst flood deposits to examine the sources and erosive potential of these events. Bayesian inversion of new and published zircon data (n=3980) confirms that megaflood sediments contain twice the proportion of zircons sourced from the Gorge compared to modern river and historical outburst flood deposits—a finding that has been interpreted to indicate that megafloods disproportionately focused erosion within the Gorge. We evaluate this interpretation using multidimensional scaling analysis and Bayesian inversion of U-Pb and luminescence data. Most of the megaflood deposits have U-Pb age components characteristic of a Gorge/Namche Barwa massif source, but Gorge zircons are present in variable proportions in these samples, and four megaflood deposits show a significant component of ages ~75-200 Ma that are more similar to source regions in the middle reaches of the Yarlung River. Single-grain feldspar luminescence data for megaflood deposits show varying degrees of bleaching, and multiple age components that we interpret as residual luminescence signals from impounded glacial lake sediment transported during megafloods. The U-Pb and luminescence data suggest megaflood sediments were derived from paleo-lake impoundments near Namche Barwa and further upstream at the Dazhuka-Yueju Gorge in the middle reaches of the Yarlung River. This finding supports a direct link between megaflood sources and deposits, and highlights the importance of considering recycling of stored sediment in the interpretation of erosion patterns from megaflood and modern river sediment age distributions.