Ancient buried canyon in Tibet unearthed

An ancient, deep canyon buried along the Yarlung Tsangpo river in south Tibet has been found.

According to a team of researchers from California Institute of Technology (Caltech) and the China Earthquake Administration, the ancient canyon - thousands of feet deep in places - effectively rules out a popular model used to explain how the massive and picturesque gorges of the Himalayas became so steep and 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," said Jean-Philippe Avouac, the Earle C Anthony Professor of Geology at Caltech.

Last year, civil engineers collected core data by drilling into the valley floor at five locations along the Yarlung Tsangpo river.

Shortly after, Caltech graduate student Jing Liu-Zeng shared the core data with Avouac and Dirk Scherler, then a post-doc in Avouac's group.

They immediately recognised that the new data suggested the presence of a paleocanyon.

After analysing it further, they found that at several locations, there were sedimentary conglomerates, rounded gravel and larger rocks cemented together - associated with flowing rivers until a depth of 800 metres or so.

This suggested that the river once carved deeply into the plateau.

"Combining the depth and age of the paleocanyon with the geometry of the valley, we found 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," Avouac explained.

However, as the Indian and Eurasian plates continued to collide and the mountain range pushed northward, it began impinging on the river.

Suddenly, about 2.5 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.

The new hypothesis rules out a model called tectonic aneurysm that has been around for about 15 years.

It suggests that the rapid uplift was triggered by intense river incision.

The team has reported its findings in the journal Science.