The India-Asia collision: what gives?

The Tibetan plateau rises above the rest of the planet's surface like a giant welt. And for scientists, this part of Asia is truly a bit of a sore spot. The plateau and mountains to the north reveal an apparent flaw in the elegant theory of plate tectonics, which holds that Earth's thin outer shell, or lithosphere, consists of a broken, shifting patchwork of large segments.

According to the theory, collisions between two plates should cause damage only near the point of contact -- a rule followed nicely by oceanic lithosphere. But the continuing crash between India and Asia has somehow cracked and crinkled the land surface for thousands of kilometers from the plates' edges.

Does the theory of plate tectonics fail on the continents?

Maybe not, say Sean D. Willett and Christopher Beaumont of Dalhousie University in Halifax, Nova Scotia. In the June 23 NATURE, they propose a new idea about the India-Asia collision that could rescue the concept that lithospheric plates deform only at their edges.

Over the last 20 years, scientists have tossed up several different ideas to explain how India has managed to dent Asia so severely. In one early scenario (figure 1), the lower part of the Indian lithosphere (left), made up of mantle rock, subducts beneath Asia (right) and then sinks down into the mantle. In another theory (figure 2), the Asian lithosphere is weak enough that it absorbs the impact by thickening, almost like a chunk of cheese squeezed in a vise.

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Willett and Beaumont suggest instead that the Asian lithosphere takes the impact by splitting apart. The upper coating, the crust, thickens and buckles because it is weak. That explains why the Tibetan plateau, Tien Shan Mountains, and Altai Mountains could form so far north of the collision point. But while the surface compacts, the bulk of the plate, made up of mantle rock, is too strong to give in such a way. Rather, the lower plate redirects its motion and subducts downward, just as an oceanic plate does when it collides with a continent (figure 3).

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The scientists developed this theory while working with computer simulations of two colliding continents. Although they have little firm evidence that Asia ducks below India and plunges into the mantle, Willett and Beaumont say this pattern fits several observations about the geometry of the two continents.

If correct, their model would come to the rescue of plate tectonic theory. Although the crust behaves like a weak material in their scenario, the mantle section of the lithosphere remains strong and does not squash.

"Most people working on continental tectonics say the theory stops working on the continents. We would say that plate tectonics continues to work, but you just don't see it," says Willett. The strong part of the Asian plate is hidden beneath the more easily deformed crust.