As India hammers into Asia, is Tibet behaving rigidly like a watermelon seed squeezed between two fingers or malleably like an impacted waterbed?
For the past 50 million years the Indian subcontinent has been crashing north into the Asian landmass, producing the iconic 2500 km long Himalayan Mountains and the soaring Tibetan plateau – the planet’s highest and largest plateau deserves its popular title of “the Roof of the World” with 2.5 million km2 averaging over 4.5 km above sea level. But how exactly is this, the textbook collision of two continents, happening?
The first hypothesis was proposed in 1975. Two geologists, Paul Tapponnier and Peter Molnar, published the first in a series of academic papers outlining how they thought the Tibetan plateau and southeast Asia were being pushed out eastwards “like a watermelon seed between two fingers” (vivid imagery from Kerr 2004) as the rigid block of India rammed into the softer Asian continent.
The pair used satellite images of southeast Asia acquired by NASA’s Landsat Program to infer the presence of major strike-slip faults (those which occur as rocks grind laterally past each other as oppose to going over or under – like the famous San Andreas fault of California). Tapponnier and Molnar claimed that these huge, deep faults either side of the Tibetan Plateau had moved rocks up to 1000 km in the past and that they are still moving relatively fast today.
The diagrams below show how the pair imagined the collision of India into Asia to have occurred. The behaviour of plasticine in the experiment shown in figure 2 appeared to support this theory of eastward pushing out of Tibet and southeast Asia.
In recent years however, new data have brought Tapponnier and Molnar’s theory into doubt.
GPS measurements across the region (Jade et al. 2004) have shown that the geological faults that are vital to the eastward extrusion story are actually moving very slowly (less than 8 mm/year). These slow movements (or slip-rates, as they are known) are supported by other new technologies such as InSAR (e.g. Wright et al. 2004) which quantifies land movement based on satellite radar data.
Detailed geological studies have also shown that the region’s major faults have actually moved far less than the earlier proposed 1000 km. Mike Searle (1996), of the University of Oxford, concluded that the Karakoram Fault has probably moved by “less than 120 km”. Thus a new model for the collision of India into Asia that doesn’t involve extensive movement on large faults is needed.
It is increasing clear that the collision of two continental plates is not at all a simple process controlled solely by major faults pushing out solid blocks of land. Rather than behaving rigidly, Tibet and southeast Asia are deforming plastically, like viscous fluids.
Mountain ranges, like icebergs, have huge roots extending deep below the Earth’s surface. It is now thought that the great thickness of the roots below the Tibetan plateau and the Himalayas leave the underlying mantle rock so deep, and therefore hot, that the region “can flow under India’s squeeze” (Kerr 2004). To quote Peter Molnar: “it’s as if India were colliding with a waterbed”.
Ph.D. student, Department of Geology, Trinity College Dublin
References and further reading
Jade, S., Bhatt, B. C., Yang, Z., Bendick, R., Gaur, V. K., Molnar, P., Anand, M. B. &Kumar, D. 2004. GPS measurements from the Ladakh Himalaya, India: Preliminary tests of plate-like or continuous deformation in Tibet. Geological Society of America Bulletin, 116 (11-12), 1385-1391.
Kenny, G. 2011. A description of the Tapponnier-Molnar model for major strike-slip extrusion of SE Asia during the late Cenozoic. The Undergraduate Journal of Ireland and Northern Ireland, 3, 38-48.
A good summary of contemporary peer-reviewed publications:
Kerr, R.A. 2004. News of the Week GEOPHYSICS: Hammered by India, Puttylike Tibet Shows Limits of Plate Tectonics. Science, 305, 161.
Molnar, P. & Tapponnier, P. 1975. Cenozoic Tectonics of Asia: Effects of a Continental Collision. Science, 189, 419-426.
Searle, M. P. 1996. Geological evidence against large-scale pre-Holocene offsets along the Karakoram Fault: Implications for the limited extrusion of the Tibetan plateau. Tectonics, 15, 171-186.
Wright, T.J., Parsons, B., England, P.C. & Fielding, E.J. 2004. InSAR observations of lowslip rates on the major faults of western Tibet. Science, 305, 236-239
Image references
Figure 1:
Google maps 2012
Figures 2 and 3 modified from:
Tapponnier, P., Peltzer, G., Le Dain, A.Y., Armijo, R. & Cobbold, P. 1982. Propagating extrusion tectonics in Asia: New insights from simple experiments with plasticine. Geology, 10, 611–616.
On The Rocks 
very cool. Seeing entire mountain ranges as plastic rather than rigid and deforming under the pressures of massive (but slow) impacts is a real eye-opener.