How do continents deform during collision, and what happens to the collided crust? Collision of India and Asia built the Tibetan Plateau, Earth's largest, but this surface expression of collision lacks the volume to accommodate all the material consumed over 55 million years of ongoing orogeny.  There appears to be a striking asymmetry between the "front" of the collision, in the Himalayan mountains that bound southern Tibet; and the "back" of the collision, in the Kunliun mountains that bound northern Tibet. In the south, large volumes of “India” have disppeared below the surtface, in a process akin to oceanic subduction, that gives rise to huge thrust-fault earthquakes, like the devastating 2004 Sumatra earthquake.  In the north, equally large earthquakes occur, but they are largely "strike-slip" showing two plates sliding past eash other, like the well-known San Andreas Fault of California. 

Our project was designed to help map the India-Asia collision in three dimensions, across the northern margin of Tibet.  Starting in 2007 our Chinese-US-German collaboration carried out a series of experiments across the Kunlkun mountains (Figure).  We detonated small explosive charges from which we could observe the propagation of seismic (sound) waves threough the crust to be recorded at hundreds of geophones (analogous to microphones) we had laid out across the surface. We left dozens of sophisticated seismic recorders deployed in the mountains for two years to record even modest earthquakes on the far side of the world.  We used these seismic waves to build up a 3D picture of the boundary of northern Tibet.

Althouth the conventional picture of a vertical plate boundary appears to be correct in the upper crust, we discovered that Tibet is being injected to the north beneath the Qaidsam Basin (Asian continent) at the base of the crust (Figure).  The overlapping Mohos and weak Tibetan crust extending beneath the Qaidam Moho suggest northward injection of Tibetan lower crust beneath the Qaidam crust. These geometries suggest north-directed lower-crustal channel flow or injection is a better description of geodynamics at this plateau margin than intra-continental transform faulting  or southward subduction of Eurasia.

These experiments are a telescope turned into the earth to provide a new understanding about the weays continents deform beneath our feet; and also provide an opportunity to train a new generation of young scientists how to study the earth, and carry out sophisticated computational analyses of the scientific data.

Last Modified: 03/20/2013
Modified by: Simon L Klemperer