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09/04/1998 07:00:00

Crust of Tibetan Plateau is being squeezed by India and Asia, new study shows

PASADENA—Geophysicists have discovered why there are high plains and mountains in the Himalayas for trekkers to trek on. According to new data, the soft crust of the Tibetan Plateau is being squeezed like an accordion between the harder crusts of India and Asia.

According to Caltech professor of geophysics Donald Helmberger and his doctoral student Lupei Zhu, the results show for the first time that a portion of crust can be squeezed and thickened if plate tectonics is forcing a harder section of crust into another hard section. Before the current study, geophysicists were unsure whether the plateau was formed by actions in the mantle or more shallow movements of the crust.

In the August 21 issue of the journal Science, the researchers show that the northward tectonic motion of India is forcing the softer and younger crust of the Tibetan Plateau into the Qaidam Basin to the north. Like India, the crust of the Qaidam Basin is also old and hard.

Since the seismic data shows there is a dramatic change in the thickness of the crust at the edge of the Qaidam Basin, the researchers infer that the softer crust is being literally forced into a hard vertical wall beneath the surface.

Therefore, the crust of the Tibetan Plateau is being crammed up and thickened in the collision. In addition to providing uplift, the action is also grinding the materials laterally. The horizontal fault lines observed in the region also support this interpretation.

"This gives a different perception about how strong an old crust can be," says Helmberger. "There's a very sharp change in the thickness of the crust, from about 40 kilometers at the Qaidam Basin to about 60 kilometers at the Tibetan Plateau.

"Physically, this means the crust beneath the Qaidam Basin is like a solid wall," he adds. "This thing below the basin is cold and old and very tough."

Zhu and Helmberger's results come from raw data collected by the Institute of Geophysics at Beijing and the University of South Carolina during the joint PASSCAL project in the early 1990s. Zhu did some of the field work before entering Caltech in 1993 as a graduate student.

Zhu says he would like to return to Tibet for additional data at other sites, and both he and Helmberger think the work could herald a new understanding of how the crust figures into plate tectonics. 

Written by Robert Tindol