Tibetan Plateau Born Before Himalayas

May 7, 2007 — The Tibetan Plateau is not just a byproduct of the Himalayas, according to a new study based on field work in the highest, driest region on Earth.

A rare look at the rocks in the middle of the plateau has revealed that it was already a high desert perhaps 80 million years ago — long before the Indian Plate crashed into Eurasia to build the planet's highest mountain range. Some current models had put the Tibetan Plateau's age at no more than 10 million years.

"In the case of Tibet, we can say with some certainty that things were quite arid in the late Cretaceous," said geologist Peter DeCelles of the University of Arizona, Tucson. That's the same Cretaceous which began 114 million years ago and ended with the extinction of the dinosaurs 65 million years ago.

DeCelles and his colleagues arrived at this conclusion after studying the geological history found in the rocks of the Nima basin. The rocks record sand dunes, dry lakes and braided seasonal streams — all of which are desert features. But was it a high or low desert?

The researchers got at that question with a fairly new technique using oxygen isotopes found in certain minerals. The isotopes suggest that even at a modest 26 million years ago, the Tibetan Plateau was already more than 16,000 feet above sea level (5,000 meters).

Their results appear in the May issue of the Bulletin of the Geological Society of America.

"They are strongly implying that a lot of the thickening (of the crust to make it higher) happened earlier," said Tibetan Plateau researcher Brad Ritts of Indiana University in Bloomington. The question is: How did the Tibetan Plateau get so high so early?

The answer may be on another continent — South America. The central Andes, too, are quite high and have an extremely high and dry plateau just east of their summits. And like Eurasia before the Indian plate plowed into it, there is no continent-on-continent collision in South America to explain the mountains and the plateau.

Instead, the Andes were built from the South American Plate riding over the thinner oceanic plates to the west. The crust in the collision zone crumpled and thickened — as in any crash — and now stands taller.

The Altiplano plateau to the east, however, might be made higher by something a bit more complex going on deep in the Earth, DeCelles explained.

It turns out that when the crust thickens, great pressures at the bottom can cause some minerals to rearrange their atoms into tighter-packed molecules. That makes a lot of rock denser than the hot mantle all around it, so it might peel away from the underside of the crust and sink into the mantle.

When that happens, hot mantle material surges up to fill the void — and the crust buoys higher. This not only encourages a plateau at the surface, but it triggers volcanic eruptions as well.

Of course, the new study is just one snapshot from one small part of the central Tibetan Plateau, Ritts cautions.

"It's still just a tiny area in a place that's twice the size of Texas," Ritts told Discovery News. "It's just the tip of the iceberg."