Their findings, published this week in the journal Science, help explain the formation of the world's largest mountain range, which is still growing.
The researchers discovered that as the Indian and Eurasian tectonic plates collide, the Indian lower crust slides under the Tibetan crust, while the upper mantle peels away from the crust and drops down in a diffuse manner.
"The building of Tibet is not a simple process," said John Nabelek, an Oregon State University geophysicist and lead author on the Science study. "In part, the mountain building is similar to pushing dirt with a bulldozer except in this case, the Indian sediments pile up into a wedge that is the lesser Himalayan mountains.
"However, an important component of the mass transfer from the upper crust of India to the Himalayas also occurs at depth through viscous processes, while the lower crust continues sliding intact farther north under the Tibet plateau," Nabelek added.
The findings are important because there has been clear scientific consensus on the boundaries and processes for that region's tectonic plates. In fact, the piecemeal images gathered by previous research have led to a series of conflicting models of the lithospheric structure and plate movement.
In this study, the international research team – called Hi-CLIMB (Himalayan-Tibetan Continental Lithosphere during Mountain Building) – was able to create new in-depth images of the Earth's structure beneath the Himalayas.
The interface between the subducting Indian plate and the upper Himalayan and Tibetan crust is the Main Himalayan thrust fault, which reaches the surface in southern Nepal, Nabelek said. The new images show it extends from the surface to mid-crustal depths in central Tibet, but the shallow part of the fault sticks, leading to historically devastating mega-thrust earthquakes.
"The deep part is ductile," Nabelek said, "and slips in a continuous fashion. Knowing the depth and geometry of this interface will advance research on a variety of fronts, including the interpretation of strain accumulation from GPS measurements prior to large earthquakes."
Nabelek, an associate professor in OSU's College of Oceanic and Atmospheric Sciences, said the lower part of the Indian crust slides about 450 kilometres under the southern Tibetan plate and the mantle appears to shear off and break into sub-parallel segments.
The researchers found evidence that subduction in the fault zone has been occurring from both the north and south sides – likely at different times in its geologic history.
In this project, funded primarily by the National Science Foundation, the researchers deployed and monitored about 230 seismic stations for a period of three years, cutting across 800 kilometres of some of the most remote terrain in the world. The lowest-elevation station was at 12 meters above sea level in Nepal; the highest, nearly 5,500 meters in Tibet. In fact, 30 of the stations were higher than 5,000 meters, or 16,400 feet.
"The research took us from the jungles of Nepal, with its elephants, crocodiles and rhinos, to the barren, wind-swept heights of Tibet in areas where nothing grew for hundreds of miles and there were absolutely no humans around," Nabelek said. "That remoteness is one reason this region had never previously been completely profiled."