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New Mexico State University

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Nine weeks in Tibet: Probing the mysteries of the Roof of the World

The Tibetan Plateau, known as the Roof of the World because of its lofty altitude, is a remote and unforgiving place. "Just getting there is an ordeal," says Thomas Hearn of New Mexico State University's physics faculty.

Roads are few and utilities even scarcer. Yak dung is the major source of fuel. But getting there is worth the ordeal for an international team of investigators exploring what Hearn describes as "one of the most interesting features on the face of the earth."

Hearn and two NMSU graduate students recently returned from nine weeks in Tibet, where they placed more than 50 seismometers in the ground along a 400-kilometer section in the middle of the world's highest plateau. The signals recorded by those instruments over the next year will help Hearn and other geophysicists determine what lies deep beneath the surface -- and provide new clues about the continent-making and mountain-building processes that the earth is still undergoing.

"The Tibetan Plateau is the largest continental plateau on earth and we really don't have a clear understanding of how it got there," Hearn said. Scientists know the plateau, and the Himalayan Mountains along its southern edge, was created by the collision of India and Asia that started about 40 million years ago. But much of what is happening beneath the surface remains a mystery.

"The earth's crust under the Tibetan Plateau is about 70 kilometers thick," said physics professor James Ni, NMSU's lead scientist in the international research collaboration known as INDEPTH (INternational DEep Profiling of Tibet and the Himalayas). "What made it so much thicker there?"

Scientists also are uncertain whether the plateau rose to its heights of 15,000 to 16,000 feet as a result of the Indian continent thrusting under the southern edge of Asia and pushing it upward, or because of some accordion-like shortening of the two colliding continental plates, Ni said.

One thing they do know, as a result of previous INDEPTH investigations, is that zones of partially molten rock lie in the middle of the crust, at least under the plateau's southern margin. That was the major finding of INDEPTH II, the second phase of the international investigation in 1994. Hearn and NMSU graduate students Richard Rapine and Yunsheng Ma trekked with their international colleagues to central Tibet this summer as part of INDEPTH III, which extends the exploration northward from Lhasa.

Entering the closed region required special permission of the Chinese government. The researchers contracted with the 5th Seismic Brigade, a Chinese outfit that explores for oil, to use its seismic camp, vehicles and drivers.

"Our typical workday consisted of eight hours of bouncing around in a Land Cruiser and about two hours of digging and placing our instruments," Hearn said. At each remote location, a seismometer was buried along with an instrument to record the seismic signals and store the data on a four-gigabyte hard drive. A solar panel above ground provides the power. Each seismometer has a GPS (Global Positioning Satellite) receiver to provide precise time-and-location information, without which the readings would be meaningless.

"We couldn't have done this 10 years ago," Hearn said. "There have been major advances in technology that make this possible."

The group at times encountered a lone sheepherder or yakherder, and occasionally a small village of mud houses, on the otherwise barren plateau. "We didn't see a paved road or tree for two months," Hearn said.

In one respect, the Tibetan Plateau proved similar to the desert Southwest: July and August are the monsoon months. "We were there during the rainy season and it was one of their wettest years ever," he said. The timing of the trip was to accommodate the availability of the German instrumentation.

Occasionally even one of the six-wheel-drive trucks used to haul equipment -- second-hand Chinese army vehicles -- mired in mud and had to be pulled out by another truck.

"It was quite a relief to get back," Hearn said.

But already the researchers are talking of returning. "We plan to make a service trip in late March or early April," Ni said. "On this trip we will make sure the seismic station is working OK and dump seven months of data from the hard disk drives to tape." On yet another trip, possibly next August, the instruments will be removed and INDEPTH III will enter the data processing and analysis stage.

Besides NMSU, INDEPTH III involves the Chinese Academy of Geologic Sciences, the Canadian Geological Survey, German scientists from a university in Potsdam, and researchers from other U.S. universities -- Cornell, Stanford, Syracuse, the University of California at Santa Barbara, State University of New York at Albany, and the University of Washington. Funding the U.S. collaborators is the National Science Foundation.

The NSF is providing about $800,000 for NMSU's portion of the research. The NMSU team is responsible for INDEPTH's passive seismology component, which examines the structure of the deep crust and mantle by measuring the velocity of sound waves underground. Other groups are doing active seismology, which involves setting off small explosive blasts to measure the structure closer to the surface; magnetotelluric surveying, which measures the electrical conductivity of the crust; and surface geological mapping.

All this international expertise is focused on the Tibetan Plateau because it is the best example of continental collision the world has to offer. The findings of the INDEPTH investigators, says Hearn, will lead to a better understanding of the processes that created the Tibetan Plateau, and the making of continents in general.