‘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,’ said Jean-Philippe Avouac, Earle C. Anthony professor of geology at Caltech. ‘That was a big discovery, in my opinion.’

Geologists use old watercourses to understand how rivers interact with the land, and how much has changed in a region. ‘In tectonics, we are always trying to use rivers to say something about uplift,’ Avouac said. ‘In this case, we used a paleocanyon that was carved by a river. It’s a nice example of how, 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.’

The discovery came when engineers from the China Earthquake Administration drilled for cores at five locations on the Yarlung Tsangpo River. Researchers at Caltech analysed the samples and identified a paleocanyon. Distinctive sedimentary conglomerates, gravel and large rocks cemented together, helped to confirm the discovery.

Existing models for the region’s geological development could not account for the canyon. As a result, the team discounted the existing model, called tectonic aneurysm. An aneurysm, according to the model, occurs when a river cuts into the Earth’s crust so fast it causes it to heat up, weakening nearby mountain ranges and allowing the region to uplift.

‘But now we have discovered that the river was able to cut into the plateau way before the uplift happened,’ said Avouac. ‘This shows that the tectonic aneurysm model was actually not at work here. The rapid uplift is not a response to river incision.’