HR: 08:30h
AN: T41G-03    [Abstracts]
TI: Stratification of Anisotropy in the Pacific Upper Mantle
AU: * Ritzwoller, M H
EM: ritzwoller@ciei.colorado.edu
AF: University of Colorado at Boulder, Department of Physics, Boulder, CO 80301 United States
AU: Shapiro, N M
EM: nshapiro@cici.colorado.edu
AF: University of Colorado at Boulder, Department of Physics, Boulder, CO 80301 United States
AU: Smith, D B
EM: dansmith@ciei.colorado.ed
AF: University of Colorado at Boulder, Department of Physics, Boulder, CO 80301 United States
AB: Based on the use of broad-band (25 sec - 150 sec) Rayleigh wave group speeds to estimate the 2-psi component of azimuthal anisotropy, we present evidence for a stratification of anisotropy in the uppermost mantle at large-scales across the Pacific basin. We confirm previous surface wave studies that established that the fast-axis directions of azimuthal anisotropy for intermediate and long period Rayleigh waves approximately align with present-day plate motions. At shorter periods (25 sec - 50 sec), however, fast-axes align nearer to the paleo- or fossil spreading direction which differs from present-day plate motion in the old Pacific. These observations, as well as observations of the age dependence of the amplitude of azimuthal anisotropy, imply that azimuthal anisotropy in the Pacific upper lithosphere is fixed or ``fossilized'', on average, reflecting the strain conditions extant during the early evolution of the lithosphere rather than the current ambient flow direction. In the deeper lithosphere and asthenosphere, anisotropic fast axis directions align nearer to present-day plate motions, apparently having re-oriented to conform to the current conditions of mantle flow. The mechanism of anisotropy stratification remains unclear, but observations are consistent with the anisotropy of the shallow lithosphere being fixed because the shear flows that can produce dislocation creep and a change in anisotropy will occur at increasing depths as the plate ages.
DE: 8180 Tomography
DE: 7255 Surface waves and free oscillations
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting