A 2.5 Myr-Old Canyon Beneath the Yarlung Tsangpo Valley, Southern Tibet

Abstract:

Orogenic plateaus like Tibet form when rock uplift and lateral tectonic growth outpace the headward incision of rivers. Although it is generally acknowledged that drainage systems concurrently reorganize, the patterns, timing, and causes of such events are usually poorly defined. Here we report the discovery of a deeply incised canyon of the Yarlung Tsangpo, at the eastern end of the Himalaya, which is now buried under >500m of sediments. Five drillings have recently been conducted at distances of up to 300 km upstream from the Tsangpo Gorge. Each drilling reached bedrock after penetrating through unconsolidated sediments and allowed us to reconstruct the former valley bottom. Results show that prior to aggradation, the Yarlung Tsangpo was able to erode back into the Tibetan Plateau and to develop a nearly graded profile. Cosmogenic nuclide derived burial ages from the base of the valley fill suggest that aggradation initiated at ~2-2.5 Ma. Our results provide clear evidence that accelerated uplift of the Namche Barwa and Gyala Peri massifs started ~2-2.5 Myr ago, blocked the canyon, and formed the modern Tsangpo Gorge. These findings refute existing hypotheses that relate the Tsangpo Gorge to young river capture or glacial damming during the Quaternary. We provide evidence for a similar and synchronous evolution of the Indus and Tsangpo Gorges, suggesting that the Himalayan syntaxes responded to a common forcing, possibly related to accelerated E-W extension of southern Tibet.