Pool of Magma Could Explain Why Tibetan Plateau Is Flat
An international team of geophysicists has discovered an enormous pool of partially molten granite, a possible result of the Indian subcontinent's thrust into Asia, in a band 100 miles wide, north to south, underneath southwestern Tibet.
The existence of a partially molten middle crust beneath Tibet had been proposed in the early 1970s on the basis of comparisons between the Himalayas and other mountain ranges, but until now no study had verified the hypothesis.
Such a partially molten region could explain how the Tibetan plateau has remained so flat despite being wedged between the Asian continent and the Indian subcontinent, which has been plowing under Asia for at least 50 million years. Such a pool of liquified rock would absorb some of India's impact and lift the Tibetan plateau, which has an average elevation of about 16,000 feet.
The research, reported in a series of five articles in the Dec. 6 issue of Science magazine, is the result of an ongoing collaboration among more than two dozen geologists and geophysicists in China and at universities in the United States, Germany and Canada. Principal investigators include Zhao Wenjin of the Chinese Academy of Geological Sciences; Douglas Nelson, professor of earth sciences at Syracuse University; Larry Brown, professor of geological sciences at Cornell, and John T. Kuo, the Ewing and Worzel Professor Emeritus of Geophysics at Columbia.
Scientists need to do more work to understand how the pool of granite came to exist and how deep it might be, Professor Kuo said. The Indian continent's subduction, or underthrusting, could be creating friction at the northern end of the Indian plate, but that would not fully explain how the rock stays molten. The pool might be deep-earth magma trapped by India in the upper crust, or the end of the Indian plate could even have broken off, releasing pressure on surrounding rock and causing it to liquefy. "Upper mantle heat could also be causing this," Professor Kuo said. There is no recent history of volcanic activity in the region.
The investigation, known as Project INDEPTH (International Deep Profiling of Tibet and the Himalaya), uses a technique called seismic profiling, in which seismic waves generated by dynamite charges are reflected from deep earth structures and are captured by instruments that detect vibrations in solids. Researchers also used magnetotelluric surveys, which measure conductivity of the crust at depths of at least 12 to 18 miles, and passive source studies, in which small earthquakes provided the seismic waves used to detect deep earth structures.
The scientific team produced a 240-mile-long transect, or cross-section, of the Himalayas, with some gaps, showing deep earth features of the crust to a depth of about 50 miles. They found a series of "bright spots"--loud reflections--at depths of about nine miles in an area nearly 60 miles north of the central Himalayas. The team interpreted those bright spots to mean the reflecting material is liquid, and at those depths the material is almost certainly molten or partially molten rock, Professor Kuo said.
In earlier work, Project INDEPTH scientists found a mid-crustal reflection at 15 miles deep at the southern end of a 60-mile-long transect, falling to about 27 miles at the northern end of the transect. That reflection is called the Main Himalayan Thrust, or MHT, and scientists believe it is the fault along which India is currently slipping under southern Tibet.
A key finding of a second phase, and one that requires further investigation, is that reflections from the MHT drop abruptly from about 15 miles to 30 miles directly beneath a feature called the Kangmar Dome, a belt of high mountains in the central Himalayas. Researchers believe the dome may have been formed by material extruded upwards from the pool of molten rock by the leading edge of the Indian plate. The dome could thus indicate the northernmost edge of the Indian plate.
The group conducted the first phase of its seismic explorations along a 60-mile transect southwest of Lhasa in the summer of 1992, and extended it 120 miles further north in a second phase in 1994 and 1995. A third phase is scheduled to begin in 1998. The work has been funded by the National Science Foundation's Geodynamic Program and by both the National Natural Science Foundation and the Ministry of Geology and Natural Resources in China. The second phase was also funded by the Deutsche Forschungsgemeinschaft and the GeoForschungsZentrum Potsdam (GFZ), both German government agencies that fund scientific research.
Further details and technical reports from Project INDEPTH are available at http://www.geo.cornell.edu/geology/indepth/indepth.html. This document is available at http://www.columbia.edu/cu/pr/. 4.1.97
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