ANCIENT LAKES UPSTREAM OF THE TSANGPO RIVER GORGE AT NAMCHE BARWA, EASTERN TIBET

Lacustrine and fluvial terraces along a major tributary to the Yarlong Tsangpo River upstream from Namche Barwa at the eastern end of the Himalaya indicate that Holocene glaciers repeatedly advanced far enough to dam the Yarlong Tsangpo. An upper set of lacustrine terraces, together with fluvial terraces graded to them, define the extent of a lake roughly 200 km long and 500 m deep, impounding an estimated 152 km³ of water. A lower set of terraces record a smaller lake that extended almost 100 km upstream from the entrance to the gorge, that was almost 200 m deep, and which impounded about 15 km³ of water. Radiocarbon dating of woody debris and charcoal from lake sediments from the higher lake terraces yield ages of 8860±40 and 9870±50 ¹⁴C yr B.P. Dates from the lower terrace yield ages of 1220±40 and 1640±50 ¹⁴C yr B.P. Field observations of moraine deposits on both sides of the Yarlong Tsangpo immediately upstream of the entrance to the gorge suggest that advances by a glacier from the flank of Namche Barwa repeatedly dammed the Yarlong Tsangpo. Rupturing of the ice dams would have released giant outburst floods down the gorge of the Yarlong Tsangpo. Although the estimated peak discharge for failure of the larger lake dam was 200,000 m³ s^-1, the estimated maximum stream power during the Yarlong Tsangpo floods was comparable to that of the Pleistocene Missoula floods in Washington state (USA). Therefore, it seems likely that substantial downcutting and valley-side erosion during the late Quaternary occurred during these floods, with important implications from models seeking to link mountain uplift, climate, and upper mantle rheology. The timing of the lake forming events is of interest because of these tectonic models, and because of the light they shed on the glacial history of southeastern Asia.