Ancient Canyon Buried in Tibet Reveals How Himalayan Gorges Became So Steep

Researchers have uncovered an ancient, deep canyon buried in Tibet. The latest findings reveal a bit more about how the massive gorges of the Himalayas became so steep so quickly.

"I was extremely surprised when my colleagues, Jing Liu-Zeng and dirk Scherler, showed me the evidence for this canyon in southern Tibet," said Jean-Philippe Avouac, one of the researchers, in a news 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. That was big discovery, in my opinion."

When it comes to studying changes millions of years ago, rivers become a main source of information since they leave behind geomorphic signatures that geologists can then interpret to learn how these rivers once interacted with the land. This, in turn, can help scientists pin down when the land changed and by how much.

In this case, the researchers studied cores that were taken from the region. Once they analyzed the core data, they found that several locations included sedimentary conglomerates, which is rounded gravel and larger rocks cemented together that are associated with flowing rivers. This reveals that a river once carved deeply into the plateau where the cores were taken.

The researchers then reconstructed the former valley floor. This showed that the slop of the river once increased gradually from the Gangetic Plain to the Tibetan Plateau. Today, the river has a steep knickpoint where it reaches the Himalayas. There, the uplift of the mountains is extremely rapid.

In the end, the researchers found that the river itself probably existed prior to about three million years ago. As the Indian and Eurasian plates continued to collide and the mountain range pushed northward, it began impinging on the river. Then about 2.5 million years ago, a rapidly uplifting section of the mountain range got in the river's way, dammed it, and the canyon filled with sediment.

This picture of the river and the Tibetan Plateau is far different from the typically accepted geologic version.

"Now we have discovered that the river was able to cut into the plateau way before the uplift happened, and this shows that the tectonic aneurysm model was actually not at work here," said Avouac. "The rapid uplift is not a response to river incision."

The findings are published in the journal Science.