Ancient Canyon Buried Deep in Tibet

Researchers have just recently discovered evidence of an ancient canyon buried deep in south Tibet, rewriting the history of how the Himalayas came to be, according to a new study.

Researchers relied on the region's rivers to realize that a arcane canyon - thousands of feet deep in places - once ran along the Yarlung Tsangpo River in south Tibet, north of the eastern end of the Himalayas. Their findings put the kibosh on a popular model for explaining how the massive and picturesque gorges of the Himalayas became so steep, so fast.

"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, in my opinion," lead researcher Jean-Philippe Avouac of Caltech said in a statement.

Normally researchers use tools such as GPS and seismology to study today's geological deformations. But for something that occurred millions of years ago, they have to use geomorphic signatures left behind by rivers to learn how these waterways once interacted with the land, when the land changed and by how much.

"In this case, we used a paleocanyon that was carved by a river," Avouac explained. "It's a nice example where by recovering the geometry of the bottom of the canyon, we were able to say how much the range has moved up and when it started moving."

Described in the journal Science, researchers collected cores by drilling into the valley floor at five locations along the Yarlung Tsangpo River. They found gravel and larger rocks cemented together - sings of a flowing river - some 800 meters (2,624 feet) down.

And by measuring two isotopes - beryllium-10 and aluminum-26 - in the lowest sediment layer, they could say when the bedrock turned to depositing sediments - about 2.5 million years ago.

The depth and age of the paleocanyon combined with the geometry of the valley tells geologists that the river existed in this location prior to about three 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 dammed the river and caused the canyon to fill with sediment, covering up almost all evidence of its existence.