India gets under Eurasia's skin.

India gets under Eurasia's skin

The fate of most oceanic plates has been to plunge into the earth's mantle when they ram into other plates. But what happens when two continents collide? The Tibetan plateau is one result. This 700,000-square-kilometer chunk of land has been elevated 5 kilometers above sea level and its crust has been thickened to twice the typical amount. Scientists think it was created during the last 50 million years by the collision between Indian and Eurasia, but they have long debated the exact mechanism of its formation.

One theory, proposed 60 years ago, suggests that the Indian plate has slid under Eurasia and, because of its buoyancy, has been able to lift the plateau. However, after scientists completed a series of studies using seismic waves to probe the Tibetan crust, this idea lost ground to another theory, which holds that the ramming plates have squeezed together a warm and weak Tibetan crust, thickening it and pushing up the plateau.

Now, in the Sept. 10 JOURNAL OF GEOPHYSICAL RESEARCH, Kin-Yip Chun at the University of Toronto in Ontario and Thomas V. McEvilly at the University of California at Berkeley uplift the earlier idea. In scrutinizing past seismological studies, Chun and McEvilly found that investigators hadn't taken into account the differences in crustal structures between the Tibetan plateau and surrounding areas through which seismic waves had passed. Using an improved data set, the researchers developed a model in which seismic waves travel much faster through the lower crust than previously thought. According to Chun, the new velocity is comparable to that found under cold continental areas such as the Canadian shield. If the squeezing model were correct, he says, one would expect higher temperatures in the lower crust in order for it to be "squeezable." Chun thinks this cold layer is evidence of the underthrusting Indian continent.

The researchers also found that overlying this high-seismic-velocity layer is a low-velocity zone that is much more apparent in their work than in past studies. At first glance this low-velocity zone might appear to support the squeezing model, because similar zones are found in places with high crustal heat flow or recent volcanism. But the low-velocity zone is more pronounced under Tibet than in these other, high-temperature regions, and Chun argues that the slow seismic speeds in Tibet are due not to high temperatures but to water trapped in the cracks of the underthrusting Indian continent, which has increased the pore pressure of the crust.