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Geological demolition derby

17.11.2004


The spectacular rift valleys of the Tibetan plateau don’t all run north-south as previously thought, according to new research.


A satellite’s-eye-view of India and Tibet. Image from NASA’s Terra satellite. Photo credit: NASA



The rift valleys actually curve away -- some to the east, some to the west -- from the point where India is punching into the gut of Tibet. "Everyone looked at the rifts and said they went north-south," said Paul Kapp, assistant professor of geosciences at the University of Arizona in Tucson. "I looked and said -- they’re not." His work contradicts a leading theory that suggests the rifts are a consequence of Tibet flowing slowly out over India’s northern edge.

The new research indicates the Tibetan plateau is being compressed between the Indian subcontinent to the south and the solid wall of the North China block. As a result, Tibet is splitting much like an orange squeezed by a vise. Kapp’s research challenges the idea that the 16,000-foot-high Tibetan plateau, the highest-elevation region on Earth, is losing elevation. Previous research reported the Tibetan plateau reached its highest elevation eight million years ago and is now slowly deflating as it spreads out over India. "My hypothesis predicts that the plateau is getting higher. The other theory suggests the plateau is collapsing," he said. "We’re in a place where continents are slamming against each other. Instead of Tibet crumpling like an accordion, we see these rift valleys. The rifts are from the east-west stretching of the plateau."


The article, "India Punch Rifts Tibet," by Kapp and Jerome H. Guynn, a doctoral candidate in UA’s department of geosciences, is in the November issue of the journal Geology. Although the standard description says Tibet’s rift valleys run north-south, that didn’t square with what Kapp saw when he looked at topographic maps of the area. The problem nagged at him for years.

In fall of 2003, he was teaching structural geology. As he worked on the lecture about stress in the crust from continents colliding, he realized that collisional stress caused the pattern of Tibet’s rift valleys. He remembers thinking, "Yeah, that’s it!" "It took me eight years to recognize the pattern," he said. "It took me two days to come up with an explanation."

Geologists often use digital elevation models, or DEMs, that are developed from satellite imagery. Such maps, which look like a shaded relief map, show the Earth’s current surface in incredible detail. Kapp said that the detailed nature of such maps obscures the underlying pattern of the rifts.

So Kapp and Guynn used a computer to strip away the DEM’s superficial layers to expose the underlying structure of the plateau. Once they created a bare-bones map of the region, the curving patterns of the rifts were clear. "I took away all the secondary faults and then the pattern jumped out," Kapp said. Because India is crashing into Tibet, geologists call India "the indentor." Kapp says that because India is hitting Tibet head-on, the Tibetan plateau is developing splits, or rifts, that curve away from the axis of impact.

Once Kapp figured out what caused the rifts, he and Guynn created mathematical models to test the idea. According to the models, a head-on punch split the plateau just the way Kapp predicted. In addition to punching Tibet directly, a lower portion of the Indian subcontinent is sliding under Tibet and lifting the plateau, Kapp said. Measuring how much Tibet is moving up or down is extremely difficult, although the technology is getting better all the time. "I think there will be some serious arguing for probably the next five years."

The Himalayas and Tibet are an area of active research by many groups of geologists. Kapp said, "If you want to understand mountain-building, you go there." Kapp and Guynn are among them. This summer they’ll be there scanning the region’s rocks for more evidence to support their new theory.

Mari N. Jensen | EurekAlert!
Further information:
http://www.arizona.edu

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