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First Complete Image Created Of Himalayan Fault, Subduction Zone

September 13, 2009

ScienceDaily (Sep. 11, 2009) - An international team of researchers has
created the most complete seismic image of the Earth's crust and upper
mantle beneath the rugged Himalaya Mountains, in the process discovering
some unusual geologic features that may explain how the region has evolved.

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
kilometers 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 kilometers 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."

Other authors on the Science study include Gyorgy Hetenyi and Jerome Vergne
of Ecole Normale Superieure in France; Soma Sapkota and Basant Kafle,
Department of Mines and Geology in Kathmandu, Nepal; Mei Jiang and Heping
Su, Chinese Academy of Geologic Sciences; John Chen, Peking University in
Beijing; Bor-Shouh Huang, Academia Sinica in Taiwan; and the Hi-CLIMB Team.
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