HR: 0830h
AN: S11C-0307 INVITED     [PDF]
TI: Seismic anisotropy within the Tibetan crust
AU: * Shapiro, N
EM: nshapiro@fignon.colorado.edu
AF: Department of Physics, University of Colorado at Boulder, Campus Box 390, Boulder, CO 80309-0390 United States
AU: Levin, V
EM: vadim@ldeo.columbia.edu
AF: Department of Geological Sciences,Rutgers University, Wright Geological Laboratory, 610 Taylor Road, Piscataway, NJ 08854-8066 United States
AU: Ritzwoller, M
EM: ritzwoll@merckx.colorado.edu
AF: Department of Physics, University of Colorado at Boulder, Campus Box 390, Boulder, CO 80309-0390 United States
AU: Molnar, P
EM: molnar@cires.colorado.edu
AF: Department of Geological Sciences, University of Colorado at Boulder, Benson Earth Sciences Building, Campus Box 399, Room 462C, Boulder, CO 80309 United States
AU: Smith, D
EM: dansmith@ciei.colorado.edu
AF: Department of Physics, University of Colorado at Boulder, Campus Box 390, Boulder, CO 80309-0390 United States
AB: Three lines of evidence argue for the existence of strong anisotropy within the Tibetan crust. First, Rayleigh waves demonstrate very strong reverse dispersion between periods of 20 and 35 sec, while Love wave dispersion is normal. This discrepancy is explained by radial anisotropy in the middle to lower crust beneath most of Tibet. Second, we map azimuthal anisotropy using surface-wave tomography. The short period maps (e.g., 20 sec), which are particularly sensitive to the upper to middle crust, display predominantly north-south 2$\psi$ fast axes across Tibet whereas these patterns rotate to being predominantly northwest-southeast at longer periods, which are characteristic of the deep crust and/or uppermost mantle. Third, teleseismic body-wave reverberations display strong signals on the transverse-component on Tibetan stations, consistent with the presence of an anisotropic layer (or layers) in the crust. The interpretation of the crustal anisotropy in terms of the models of finite strain is more uncertain than interpreting mantle anisotropy which is mostly produced by the preferred orientation of olivine. Several physical mechanisms relate crustal anisotropy to finite strain. Preferential orientation of faults may be the dominant mechanism in the Tibetan upper crust, whereas in the middle to lower crust beneath Tibet anisotropy may result from preferred mineralogical orientation. Mica, in particular, may play the most significant role in middle to lower crustal anisotropy, with larger shear moduli parallel to the plane of oriented mica crystals than perpendicular to it. These preliminary results on the vertical distribution of radial and azimuthal anisotropy are consistent with a model in which Tibet extends east-west with vertical flattening. This extension would be accommodated in the upper crust by normal faulting with a preferential north-south orientation, which would explain the azimuthal anisotropy observed with short-period Rayleigh waves. In the middle crust, large-scale extension would produce both crustal thinning and viscous east-west flow that would result in a combination of radial and azimuthal anisotropy in which the latter may be oriented differently from that in the upper crust.
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
DE: 7255 Surface waves and free oscillations
DE: 8109 Continental tectonics--extensional (0905)
SC: Seismology [S]
MN: 2003 Fall Meeting