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Scientists use seismic waves to locate missing rock under Tibet
James E.
Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@illinois.edu
|
Released
2/8/2007
CHAMPAIGN, Ill. —
Geologists at the University of Illinois at Urbana-Champaign have located
a huge chunk of Earth’s lithosphere that went missing 15 million
years ago. By finding the massive block of errant rock beneath Tibet,
the researchers are helping solve a long-standing mystery, and clarifying
how continents behave when they collide.
The Tibetan Plateau and adjacent Himalayan Mountains were created by
the movements of vast tectonic plates that make up Earth’s outermost
layer of rocks, the lithosphere. About 55 million years ago, the Indian
plate crashed into the Eurasian plate, forcing the land to slowly buckle
and rise. Containing nearly one-tenth the area of the continental U.S.,
and averaging 16,000 feet in elevation, the Tibetan Plateau is the world’s
largest and highest plateau.
Tectonic models of Tibet vary greatly, including ideas such as subduction
of the Eurasian plate, subduction of the Indian plate, and thickening
of the Eurasian lithosphere. According to this last model, the thickened
lithosphere became unstable, and a piece broke off and sank into the
deep mantle.
“While attached, this immense piece of mantle lithosphere under
Tibet acted as an anchor, holding the land above in place,” said
Wang-Ping Chen, a professor of geophysics at the U. of I. “Then, about 15 million years ago, the chain broke
and the land rose, further raising the high plateau.”
Until recently, this tantalizing theory lacked any clear observation
to support it. Then doctoral student Tai-Lin (Ellen) Tseng and Chen
found the missing anchor.
“This remnant of detached lithosphere provides key evidence for
a direct connection between continental collision near the surface and
deep-seated dynamics in the mantle,” Tseng said.
“Moreover, mantle dynamics ultimately drives tectonism, so the
fate of mantle lithosphere under Tibet is fundamental to understanding
the full dynamics of collision.”
Through a project called Hi-CLIMB – an integrated study of the
Himalayan-Tibetan Continental Lithosphere during Mountain Building,
Tseng analyzed seismic signals collected at a number of permanent stations
and at many temporary stations to search for the missing mass.
Hi-CLIMB created a line of seismic monitoring stations that extended
from the plains of India, through Nepal, across the Himalayas and into
central Tibet. “With more than 200 station deployments, Hi-CLIMB
is the largest broadband (high-resolution) seismic experiment conducted
to date,” said Chen, who is one of the project’s two principal
investigators.
Using high-resolution seismic profiles recorded at many stations, Tseng
precisely measured the velocities of seismic waves traveling beneath
the region at depths of 300 to 700 kilometers. Because seismic waves
travel faster through colder rock, Tseng was able to discern the positions
of detached, cold lithosphere from her data.
“We not only found the missing piece of cold lithosphere, but
also were able to reconstruct the positions of tectonic plates back
to 15 million years ago,” Tseng said. “It therefore seems
much more likely that instability in the thickening lithosphere was
partially responsible for forming the Tibetan Plateau, rather than the
wholesale subduction of one of the tectonic plates.”
Other evidence, including the age and the distribution of volcanic rocks
and extrapolation of current ground motion in Tibet, the researchers
say, also indicates the remnant lithosphere detached about 15 million
years ago.
Chen and Tseng present their findings in a paper to appear in the Journal
of Geophysical Research. The National Science Foundation funded the
work.
Editor’s note: To reach Wang-Ping Chen, call 217-333-2744; e-mail: wpchen@illinois.edu.
To reach Tai-Lin (Ellen) Tseng, call 217-244-6048; e-mail: tseng1@illinois.edu.
