Stratification of anisotropy in the Pacific upper mantle

On the basis of the use of broadband (25-150 s) Rayleigh wave group speeds to estimate the 2psi 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 (25-50 s), however, fast axes align nearer to the paleospreading 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 (<100 km depth) 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 reoriented 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.