We think of canyons as being carved by rivers, as the Colorado River did with the Grand Canyon, so this item is a little mind-boggling.
In a just-published article in the journal Science, researchers from the California Institute of Technology have discovered an vast ancient canyon that lies buried underneath a present-day river that cuts through the Himalayas in Tibet.
The ancient canyon is thousands of feet deep in places, and apparently was carved by a previous river 3 to 7 million years ago.
"I was extremely surprised when my colleagues, Jing Liu-Zeng and Dirk Scherler, showed me the evidence for this canyon in southern Tibet," Caltech geology professor Jean-Philippe Avouac said in a press release "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."
The ancient river "existed in this location prior to about 3 million years ago, but at that time, it was not affected by the Himalayas. However, as the Indian and Eurasian plates continued to collide and the mountain range pushed northward, it began impinging upon the river. Suddenly, about 2 1/2 million years ago, a rapidly uplifting section of the mountain range got in the river's way, damming it, and the canyon subsequently filled with sediment.
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The scientists analyzed core samples collected by the China Earthquake Administration, which were taken from five locations along the Yarlung Tsangpo River. They found that at several locations there were sedimentary conglomerates, rounded gravel and larger rocks cemented together, that are associated with flowing rivers, until a depth of 800 meters or so, where the record clearly indicated bedrock. This indicated that the river once carved deeply into the plateau.
The discovery may force geologists to rethink long-held assumptions about how the Himalayas' dramatic gorges formed.