Researchers have made a surprising discovery in Tibet, unearthing an ancient canyon buried deep under sediment along the Yarlung Tsangpo River in the southern part of the country.
The canyon is thousands of feet deep in some places and is believed to have been carved by a river three million to seven million years ago.
"I was extremely surprised," Dr. Jean-Philippe Avouac, professor of geology at Caltech in Pasadena, Calif. and one of the researchers, said in a written statement. "When I first saw the data, I said, 'Wow!' It was amazing to see that the river once cut quite deeply into the Tibetan Plateau because it does not today. That was a big discovery."
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This Google Earth image looks down the Yarlung Tsangpo valley, where the ancient canyon lies about 800 to 900 meters below the present-day river.
Evidence of the buried canyon was first seen in data collected last year by China Earthquake Administration engineers who were drilling along the river. To determine when the canyon formed and when it was buried, the researchers measured levels of two radioisotopes -- beryllium-10 and aluminum-26 -- in samples of sediment collected from the drilling.
The isotope measurements showed that the canyon began filling with sediment around 2.5 million years ago. Around the same time, shifting continental plates triggered an abrupt upward movement of the mountainous terrain in the region, Science magazine reported. That dammed the river, causing the sediment to accumulate until the canyon was buried.
"Most people in the field, including myself, were imagining that rivers flowing out of Tibet were slowly 'eating' the plateau from its edges and had not yet been able to incise much into the plateau interior," Avouac said in the email. But this new discovery proves otherwise.
A description of the ancient canyon was published in the journal Science on Nov. 21, 2014.