Thaindian News

Washington, Sep 12 (IANS) Researchers have created the most complete seismic map of the upper mantle beneath the rugged Himalayas, stumbling on some unusual geologic features that may explain how the region came into being.
Researchers discovered that as the Indian and Eurasian tectonic plates collided, the Indian lower crust slid under the Tibetan crust, while the upper mantle pealed away from the crust and dropped down in a diffuse manner.

“The building of Tibet is not a simple process,” said John Nabelek, Oregon State University (OSU) geophysicist and study co-author.

“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.

He said the lower part of the Indian crust slides about 450 kms under the southern Tibetan plate and the mantle appears to shear off and break into sub-parallel segments.

The findings are important because there has been no 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.

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.

In this project, funded primarily by the National Science Foundation, researchers deployed and monitored about 230 seismic stations for a period of three years, cutting across 800 kms of some of the most remote terrain in the world.

The lowest-elevation station was at 12 metres above sea level in Nepal; the highest, nearly 5,500 meters in Tibet. In fact, 30 of the stations were higher than 5,000 metres, or 16,400 feet, said an OSU release.

“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.”

Their findings were published in Science.