HR: 10:50h
AN: T22D-03 INVITED    [Abstracts]
TI: The upper mantle Sv wave speed structure beneath Tibet
AU: * Priestley, K
EM: keith@esc.cam.ac.uk
AF: Bullard Laboratories, University of Cambridge, Madingley Rise, Madingley Road, Cambridge, CB3 0EZ, United Kingdom
AU: Debayle, E
EM: Eric.Debayle@eost.u-strasbg.fr
AF: EOST, IPG, Strasbourg, Universite Louis Pasteur, 5 rue Rene Descartes, Strasbourg, 67084, France
AU: McKenzie, D
EM: mckenzie@esc.cam.ac.uk
AF: Bullard Laboratories, University of Cambridge, Madingley Rise, Madingley Road, Cambridge, CB3 0EZ, United Kingdom
AU: Barron, J
EM: jaab3@cam.ac.uk
AF: Bullard Laboratories, University of Cambridge, Madingley Rise, Madingley Road, Cambridge, CB3 0EZ, United Kingdom
AB: Although it has long been accepted that the uplift of the Himalaya and Tibet result from the collision of India with Eurasia, there is still no agreement on the details concerning the geodynamical processes involved. This lack of consensus arises primarily due to uncertainties in the Tibetan upper mantle structure. To improve understanding of this structure we determine a 3D Sv wave speed and azimuthal anisotropy model for the region constrained by the analysis of more than 22000 vertical component, short propagation path, multi-mode Rayleigh wave seismograms. The dense path coverage, wide azimuthal distribution and rich higher mode content of the data set allow us to build an upper mantle model for Tibet and the surrounding region with a horizontal resolution of a few hundred kilometers and a vertical resolution of a few tens of kilometers extending to about 400 km depth. The surface wave analysis shows that sub-Moho upper mantle shear wave structure is slow down to about 130 km, but at deeper depths, down to about 250 km, the long wavelength pattern of Sv velocity of the mantle beneath the whole of Tibet is fast with respect to PREM. The high wave speed mantle at these depths beneath Tibet merges to the south with high wave speeds beneath northern India and to the east with high wave speeds beneath southeastern China. The most commonly invoked model for the uplift of Tibet involves lithospheric delamination but our results indicate that the Tibetan lithosphere is still largely, if not completely intact. The seismic model suggest that most of the plateau has been underthrust by high wave speed Indian mantle from the south and possibly to a lesser extent, high wave speed Asian mantle from the north. The surface wave results are supported by observations of frequency-dependent Sn propagation in Tibet. High frequency Sn does not propagate beneath northern Tibet but low frequency Sn is observed to propagate across all of the plateau. Low velocities in the upper mantle beneath northern Tibet have previously been noted and these, along with the recent volcanism in northern Tibet, have been cited as evidence for lithospheric delamination beneath the plateau. However, both the surface wave tomography and Sn propagation efficiency studies reported here show that the low wave speeds in the upper mantle below Tibet are only a shallow feature and not the result of lithospheric delamination. Therefore, the elevation of Tibet is not the result of uplift caused by hot, buoyant astenospheric material replacing the Tibetan lithosphere.
DE: 7255 Surface waves and free oscillations
DE: 7270 Tomography (6982, 8180)
DE: 8107 Continental neotectonics (8002)
DE: 8108 Continental tectonics: compressional
SC: Tectonophysics [T]
MN: 2007 Fall Meeting