NEW_Accomplishments.indd - IRIS

SURFACE OF THE EARTH: GLOBAL STUDIES
2006 IRIS 5-YEAR PROPOSAL
Structure and Deformation of the Tibetan Lithosphere
Nikolai M. Shapiro, Michael H. Ritzwoller • University of Colorado at Boulder
Intermediate-period Rayleigh and Love waves propagating across Tibet are affected by strong radial anisotropy within
the mid-to-lower crust, consistent with a thinning of the middle crust by about 30% (Shapiro et al., 2004). The anisotropy is
largest in the western part of the plateau (Figure 1) where moment tensors of earthquakes indicate active crustal thinning. The
preferred orientation of mica crystals resulting from crustal thinning can account for the observed anisotropy. The mid-tolower
crust of Tibet appears to have thinned more than the upper crust, consistent with deformation of a mechanically weak
layer in the mid-to-lower Tibetan crust that flows as if confined to a channel.
Inversion of broadband surface-wave dispersion data (Ritzwoller et al., 2002; Shapiro and Ritzwoller, 2002) reveals a
coherent pattern of high-velocities in the mantle underlying the Tibetan plateau, as shown in Figure 2. The high velocities
in western Tibet appear to be derived from underthrusted Indian lithosphere and a more speculative underthrusting of Asian
lithosphere is apparent in eastern Tibet. In central Tibet, however, the high-speed zone appears to be detached from both the
Indian and Asian lithosphere. Overlying this feature is a low-speed anomaly coincident with high attenuation and the blockage
of regional phases.
Figure 1. Strength of radial anisotropy in the middle crust from the
best-fitting radially anisotropic model, represented as the idealized
travel time difference between SV and SH waves propagating vertically
through the middle crust. Solid lines show selected major active
faults, and dashed lines are approximate locations of sutures.
Figure 2. Smoothed isosurface representation of shear wave speeds
beneath the Tibetan Plateau. The blue and red surfaces are the +2% and
the -1% perturbations, respectively, relative to a regional average at each
depth. Topography on the surface and the Moho is vertically exaggerated.
(Moho topography is estimated along with crust and mantle velocities.)
Ritzwoller, M.H., N.M. Shapiro, M.P. Barmin, and A.L. Levshin, Global surface wave diffraction tomography, J. Geophys. Res., 107(B12), 2335,
doi:10.1029/2002JB001777, 2002.
Shapiro, N.M. and M.H. Ritzwoller, Monte-Carlo inversion for a global shear velocity model of the crust and upper mantle, Geophys. J. Int., 151, 88-105,
2002.
Shapiro, N.M., M.H. Ritzwoller, P. Molnar, and V. Levin, Thinning and flow of tibetan crust constrained by seismic anisotropy, Science, 305, 233-236, 2004.
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