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Researchers analyze new data from beneath the 'Rooftop of the World'

Tibet and the Himalayas form the biggest plateau and highest mountains on Earth, but no one clearly understands how they got that way. Now an international team of researchers, including New Mexico State University physics professor James Ni, has put together the most detailed picture yet of the geologic processes at work beneath the plateau.

port published in the Nov. 8 issue of Science magazine, the scientists provide the first evidence that the Eurasian continental plate is probably thrusting under the northern margin of the Tibetan Plateau in a process known as continental subduction.

They also detail variations in the thickness of the Tibetan crust and signs that the upper mantle, at depths above 400 kilometers, is about 300 degrees Celsius hotter beneath northern Tibet than it is under the southern part.

Together, the seismic data analyzed by the scientists show that the plateau is still growing, even though the collision of India and Eurasia began about 50 million years ago, said Ni, who has been researching the Tibetan Plateau for 25 years.

"Tibet was formed by the collision and subsequent penetration of India into Asia," he said. "We know that at least 1,000 kilometers of the Indian continental plate has been consumed beneath Eurasia. Global Positioning System measurements in China indicate more than 90 percent of the relative motion between Indian and Eurasian plates is taken up in the Tibetan Plateau and its margins, the Himalayas and Qilian Mountains. However, what we do not yet know is whether forces that control the deformation and growth of the plateau all come from the push of the Indian plate or from mantle flow below the plateau."

The international collaborators -- from Germany, Russia, France, China and the United States -- combined all available seismic records that have been collected in Tibet to picture the subsurface structure and understand the evolution of the collision between India and Eurasia.

Most of the seismic data comes from a series of projects called INDEPTH (INternational DEep Profiling of Tibet and the Himalayas), which began in 1992. For INDEPTH III, which made the latest findings possible, NMSU physics professor Thomas Hearn and two graduate students spent nine weeks in central Tibet in the summer of 1998, installing 57 seismometers in the ground along a 400-kilometer line running north from a point about 100 kilometers north of the capital of Lhasa.

Traveling through the remote region, which is sparsely populated and has few roads, required special permission of the Chinese government. The researchers contracted with a Chinese oil exploration brigade for camps, food, vehicles and drivers.

The U.S. contributions to the INDEPTH projects over the past decade have been funded by the Continental Dynamic Program of the National Science Foundation.

The Science magazine report is significant because it is the first time a detailed seismological image crossing all of Tibet has been presented, and also because it indicates the Eurasian plate is thrusting beneath northern Tibet, Ni said.

More work remains, though, and the collaborators already are making plans for INDEPTH IV. Field work is planned to begin in 2006 and data would be available in 2007 if the group can obtain support from the NSF, Ni said.

All of this international expertise is focused on the Tibetan Plateau because it is the best example of continental collision the world has to offer, scientists say. The findings of the INDEPTH investigators have led to a better understanding of the plateau-making and mountain-building processes that are still taking place today.

Karl Hill
Nov. 18, 2002