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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. Scientists say that 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.
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 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.
(Text copied from
University of Illinois at Urbana-Champaign News Bureau release)
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