A huge 3 million-year-old canyon, thousands of feet deep in places, has been discovered buried under a major river in Tibet.
The canyon buried along the Yarlung Tsangpo River (Brahmaputra in India) in south Tibet, north of the eastern end of the Himalayas was discovered by a team of researchers from California Institute of Technology (Caltech) and the China Earthquake Administration.
The geologists say that 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, 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," said Jean-Philippe Avouac, the Earle C Anthony Professor of Geology at Caltech.
"We used a paleocanyon that was carved by a river. 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," Avouac said.
Last year, civil engineers from the China Earthquake Administration collected cores by drilling into the valley floor at five locations along the Yarlung Tsangpo River.
Researchers analysed the core data and 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, at which point the record clearly indicated bedrock.
This suggested that the river once carved deeply into the plateau.
To establish when the river switched from incising bedrock to depositing sediments, they measured two isotopes, beryllium-10 and aluminium-26, in the lowest sediment layer.
The isotopes are produced when rocks and sediment are exposed to cosmic rays at the surface and decay at different rates once buried, and so allowed the geologists to determine that the paleocanyon started to fill with sediment about 2.5 million years ago.
The researchers' reconstruction of the former valley floor showed that the slope of the river once increased gradually from the Gangetic Plain to the Tibetan Plateau, with no sudden changes, or knick-points.
Today, the river, like most others in the area, has a steep knick-point where it meets the Himalayas, at a place known as the Namche Barwa massif.
There, the uplift of the mountains is extremely rapid and the river drops by 2 kilometres in elevation as it flows through the famous Tsangpo Gorge, known by some as the Yarlung Tsangpo Grand Canyon because it is so deep and long.
Combining the depth and age of the paleocanyon with the geometry of the valley, the geologists surmised that the river existed in this location prior to about 3 million years ago, but at that time, it was not affected by the Himalayas.
The findings were published in the journal Science.
