The Yarlung Tsangpo is a major river of the world. It drains most of the northern Himalaya and much of the Tibetan Plateau to the Bay of Bengal. Its long profile is kinked, with the easterly portion north of the Himalayas having a low gradient (~2‰) and a much steeper reach (≤10‰) plunging south 2700 m through the Himalaya between the giant peaks Namche Barwa (7782 m) and Gyala Peri (7294 m). The Yarlung Tsangpo hosted giant lakes during the late Quaternary that sourced outburst floods, some of which were among the largest known, with peak discharge as much as 5x10^6 m^3 s^-1, 50x modern seasonal flood levels.
Montgomery et al. 2004 recognized two prominent lacustrine terrace levels on the Yarlung, and 14C-dated these to about 10 ka and 2 ka. The maximum heights were 680 m and 240 m where the eroded ends of the Zelunglung moraines from Namche Barwa met the river. This was an exploratory effort and only four dates were reported, although more have been measured since and are given here. Two episodes of lake filling behind a moraine dam and possibly rapid draining were inferred. Since 2004, a number of Chinese-led studies have used mainly OSL and 14C to date the lower terrace to greater than 13 ka. Their consensus was that one or possibly two paleolakes filled the Yarlong basin for tens of thousands of years. One study measured B concentrations in the sediments, concluding that the paleolakes were saline and the basin closed during the late Pleistocene. These two discrepant views have not been resolved, and likely will not be until detailed field work is conducted.
Dating the Zelunglung moraines might help reconcile the two interpretations, except that OSL of till is prone to incomplete bleaching (overestimating age) and we found no suitable materials for 10Be dating. Two studies suggested that the age could be inferred from nearby datable sequences, but there are unresolved complexities in the telecorrelation. New 10Be dates reported here together with older ones suggest that there was no major early Holocene or latest glacial glaciation in the region that could have caused the Zelunglung glacier to have blocked the Yarlung, yet the moraines must have been large if the 10 ka terrace dates are correct. One possible cause would be the collapse of the western arête of Namche Barwa onto the head of the glacier, causing it to surge. The collapse could have been as large as 5 km in width and 2 km high. A similar event occurred in 2012 on Annapurna IV in Nepal, causing a devastating flood.
Normally a talk such as this one would await more concrete evidence and robust conclusions. However, even discussing the contradictions seems important now for two reasons: 1) western geologists have been excluded from this part of Tibet for several years, and 2) China appears to be planning a dam across the Yarlung near the moraine blockage site, and if a collapsed arête did cause the 10 ka event, the next one is unpredictable but could precipitate a flood disaster for India and Bangladesh downstream.
----------------------------------------------------------------------------------------------------------------------------------------
The Planetary Science Institute is a private, nonprofit 501(c)(3) corporation dedicated to Solar System exploration. It is headquartered in Tucson, AZ, where it was founded in 1972. In 2016 PSI established a second office in Lakewood, CO (near Denver).
PSI scientists and educators are involved in numerous NASA and international missions, the study of Mars, Earth, and other planets, the Moon, asteroids, comets, interplanetary dust, impact physics, the origin of the solar system, extrasolar planet formation, dynamics, the rise of life, and other areas of research. They conduct fieldwork on all continents of the Earth to understand our planet and others. They are also actively involved around the world in science education and public outreach through school programs, children's books, popular science books, podcasts, and art.
To support our education and outreach activities, please donate here: [ Ссылка ]
For the latest in planetary science news and content from PSI, come find us on all our media sites:
- Twitter: [ Ссылка ]
- Facebook: [ Ссылка ]
- Instagram: [ Ссылка ]
- LinkedIn: [ Ссылка ]
Please like, subscribe, and ring the bell for notifications!
