Megafloods eroded steep Tsangpo gorge 2 mln years ago
Washington: Scientists have direct geochemical evidence that the 150-mile Tsangpo Gorge was the medium by which megafloods from glacial lakes drained suddenly and catastrophically through the Himalayas when their ice dams failed at times during the last 2 million years.
The 2005 image shows a concentration of grains of zircon taken from sand deposits, where it occurs with other heavy minerals such as magnetite and ilmenite.
"You would expect that if a three-day long flood occurred, there would be some pretty significant impacts downstream," Karl Lang, a University of Washington doctoral candidate in Earth and space sciences, said.
In this case, the water moved rapidly through bedrock gorge, carving away the base of slopes so steep they already were near the failure threshold.
Because the riverbed through the Tsangpo Gorge is essentially bedrock and the slope is so steep and narrow, the deep flood waters could build enormous speed and erosive power.
As the base of the slopes eroded, areas higher on the bedrock hillsides tumbled into the channel, freeing microscopic grains of zircon that were carried out of the gorge by the fast-moving water and deposited downstream.
Uranium-bearing zircon grains carry a sort of geochemical signature for the place where they originated, so grains found downstream can be traced back to the rocks from which they eroded.
Lang found that normal annual river flow carries about 40 percent of the grains from the Tsangpo Gorge downstream.
But grains from the gorge found in prehistoric megaflood deposits make up as much as 80 percent of the total.
The study is published in the journal Geology.
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