A very old and very deep canyon lies alongside the Yarlung Tsangpo River in southern Tibet, near the eastern end of the Himalayas, and its recent discovery is prompting geologists to rethink a popular model currently used to explain how the gorges of the Himalayas came to form so quickly and steeply. Reporting in the journal Science, researchers from Caltech and the China Earthquake Administration say the paleocanyon is several thousand feet deep in some places and is "now buried under more than [1,600 feet] of sediments." By drilling from several sites along the gorge, they measured two isotopes in the lowest sediment layer to see how long ago it was last exposed to cosmic rays at the Earth's surface, reports Science magazine.
They found that about 2.5 million years ago, one section of the mountain range rose quickly and dammed the river, filling the canyon with sediment and establishing a gorge. This means that a model popularized almost 20 years ago, called tectonic aneurysm, didn't actually play a role in this instance, as it posits that a river can cut into the Earth's crust so quickly it heats the crust, weakening nearby mountains and helping massifs rise suddenly. But in this case, one geologist at Caltech says, "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. The rapid uplift is not a response to river incision." (Geologists are also scratching their heads over a newly observed phenomenon: holes in sand dunes.)