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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS014 Oral Presentation 2256
Seismic Structure Beneath Tibet: Implications for Top-down Tectonics
Prof. James Ni
Department of Physics New Mexico State University IASPEI
Recent seismic studies in Tibet indicate that the transition from weak splitting in southern Tibet to
strong azimuthal anisotropy in northern Tibet is exceptionally sharp, suggesting that the northern edge
of the advancing Indian plate has reached ~32 N and plunges vertically downward to ~400 km depth as
revealed from a tomographic image. In northern Tibet, a continental back-arc, the partially melted Asian
mantle is highly anisotropic and is being squeezed and sheared between the Indian continental
lithosphere to the south and Tsaidam and Tarim shields to the north, resulting in eastward mantle flow.
In the eastern Himalayan Syntaxis region, where the Burma subduction zone meets the eastern
Himalayas the fast directions of the split SKS waves are oriented approximately around the syntaxis
which is consistent with the oroclinal bending hypothesis. The approximately N-S fast direction beneath
easternmost Tibet and the E-W fast direction in southern Yunan province are consistent with a coherent
lithosphere-asthenosphere deformation as the Indian plate continues to underthrust beneath southern
Tibet and the oceanic Indian plate subducts beneath Burma and southern Yunan province.
Keywords: tibet, collision, himalayas