Washington, March 25 : Geologists have gathered new evidence which indicates that the vast Tibetan Plateau rose to its spectacular height in stages, with uplift occurring first in the central plateau and later in regions to the north and south.
The evidence was obtained from an eight-year study by U.S. and Chinese researchers of the world's highest and largest plateau, which is bordered by the world's highest mountains - the Himalayas.
The researchers discovered volcanic rock in an area of the central plateau south of the Hoh Xil Basin. The flat bed of hardened lava lies on top of tilted and folded layers of sedimentary rocks; geochronology techniques dated it to 40 million years ago.
"The middle part of the plateau was uplifted first at least 40 million years ago, while the Himalayan Range in the south and also the mountains to the north were uplifted significantly later," said Xixi Zhao, a research scientist at the University of California, Santa Cruz (UCSC).
"One of the traditional views of when Tibet became a high plateau is that it's a relatively recent phenomenon that happened in the last 15 million years," said coauthor Peter Lippert, a UCSC graduate student. "The existence of a high plateau at least 40 million years ago could have important climatic implications," he added.
The team also found marine fossils suggesting that the now lofty Himalayas remained below sea level at a time when the central plateau was already at or near its modern elevation.
These fossils turned out to be 5 million years younger than any previously discovered marine fossils from that area. The discovery narrows the window of time during which the Himalayas could have been uplifted.
"When the central part of the Tibetan plateau was uplifted more than 40 million years ago, Mount Everest and the rest of the Himalayas were still part of a deep ocean basin," Zhao said.
Known as "the roof of the world," the Tibetan Plateau was created by the ongoing collision of tectonic plates as India plows northward into Asia.
According to coauthor Robert Coe, a professor of Earth and planetary sciences at UCSC, ideas about how the uplift of the plateau occurred have been evolving since well before his first visit to Tibet in 1988.
"People used to talk about the whole plateau coming up at once, but it has become clear that different parts of the plateau were elevated at different times," said Coe. "Our work shows that the central part of the plateau was uplifted first, and it seems to fit pretty well with other studies," he added.