Himalayan region remained below sea level while central region rose, geologists say
"SANTA CRUZ, CA--The vast Tibetan Plateau--the world's highest and largest plateau, bordered by the world's highest mountains--has long challenged geologists trying to understand how and when the region rose to such spectacular heights. New evidence from an eight-year study by U.S. and Chinese researchers indicates that the plateau rose in stages, with uplift occurring first in the central plateau and later in regions to the north and south.
"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.
The team 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, Zhao said. The average elevation of the plateau today is more than 4,500 meters (14,850 feet).
The researchers published their findings in the Proceedings of the National Academy of Sciences (online the week of March 24 and later in print)......
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. Coauthor Robert Coe, a professor of Earth and planetary sciences at UCSC, said ideas about how the uplift of the plateau occurred have been evolving since well before his first visit to Tibet in 1988....
The team of U.S. and Chinese geologists based their findings on extensive field studies conducted mostly in a remote interior region of the Tibetan Plateau. They focused on an area called the Hoh Xil Basin in the north-central part of the plateau. The area's geologic history is recorded in layers of sedimentary rock 5,000 meters thick. Now a part of the high plateau, it was once a basin on the northern edge of the central plateau, Lippert said.
"The structure of the basin and way the sediments were deposited show that it is the type of basin that forms at the base of large mountains. So we've shown that there was high topography to the south of the Hoh Xil Basin at least 40 million years ago," he said....
In the Himalayas, the team found fossils of marine plankton called radiolarians that 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."........... Read the rest of the article here[link]
images from: http://visibleearth.nasa.gov/view_rec.php?id=5371