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Science 9 July 2004:
Vol. 305. no. 5681, pp. 233 - 236
DOI: 10.1126/science.1098276
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Reports

Thinning and Flow of Tibetan Crust Constrained by Seismic Anisotropy

Nikolai M. Shapiro,1* Michael H. Ritzwoller,1 Peter Molnar,2 Vadim Levin3

Intermediate-period Rayleigh and Love waves propagating across Tibet indicate marked radial anisotropy within the middle-to-lower crust, consistent with a thinning of the middle crust by about 30%. The anisotropy is largest in the western part of the plateau, where moment tensors of earthquakes indicate active crustal thinning. The preferred orientation of mica crystals resulting from the crustal thinning can account for the observed anisotropy. The middle-to-lower crust of Tibet appears to have thinned more than the upper crust, consistent with deformation of a mechanically weak layer that flows as if confined to a channel.

1 Center for Imaging the Earth's Interior, Department of Physics, University of Colorado at Boulder, Boulder, CO 80309, USA.
2 Department of Geological Sciences, Cooperative Institute for Research in Environmental Science (CIRES), University of Colorado at Boulder, Boulder, CO 80309, USA.
3 Department of Geological Sciences, Rutgers University, Piscataway, NJ 08854, USA.

* To whom correspondence should be addressed. E-mail: nshapiro{at}ciei.colorado.edu

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