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10:18 Sep 14 2009

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Scientists make seismic map of Himalayas
10:17, September 14, 2009  


Photo shows the glaciers in south of the Himalayas. (Xinhua Photo)


An international team of researchers has created the most complete seismic image of the Earth's crust and upper mantle beneath the rugged Himalayan Mountains, discovering some unusual geologic features that may explain how the region has evolved.

Their findings, published recently 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 China's 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 study.


Photo shows Mt. Qomolangma in the mid-section of the Himalayas. (Xinhua Photo)


"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."

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 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, according to Nabelek and his team.

The new images show it extends from the surface to mid-crustal depths in the central Tibet Autonomous Region, 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 at OSU, said the lower part of the Indian crust slid about 450 kilometers under the southern Tibetan plate.

Source: Shanghai Daily

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