Stratification of anisotropy in the Pacific upper mantle

Abstract

[1] On the basis of the use of broadband (25-150 s) Rayleigh wave group speeds to estimate the 2y 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 presentday 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.

Estimation of azimuthal anisotropy in the NW Pacific from seismic ambient noise in seafloor records

“…is parallel to current plate motion (Tanimoto and Anderson 1984). For investigating the intermediate depth range, recent studies have analysed surface waves at shorter periods of ∼18-50 s either by measuring group velocities from onland records (Smith et al 2004) or by measuring phase velocities from records of ocean bottom seismometers (OBSs; Forsyth & Li 2005;Weeraratne et al 2007).…”

New evidence for dislocation creep from 3-D geodynamic modeling of the Pacific upper mantle structure

“…The thermal structure of the Pacific lithosphere and upper mantle ( Figure 1) is deduced from a 3D seismological model [10] that has been constructed from a large set of broadband surface wave group [11] and phase [12,13] speed dispersion measurements using a two-step inversion procedure. First, diffraction tomography of [14] was used to construct dispersion maps at periods ranging from 18 sec to 200 sec for group speeds and from 40 sec to 150 sec for phase speeds.…”

Multiscale waveform tomography with two‐step model parameterization

“…Anisotropy is a nonnegligible effect in the uppermost mantle, especially for regions underneath the Pacific, as has been pointed out by many studies [e.g., Montagner and Tanimoto, 1991;Ekström and Dziewonski, 1998;Gaboret et al, 2003;Gung et al, 2003;Smith et al, 2004;Panning and Romanowicz, 2006]. Therefore, interpretations of resolved finer details Figures 8a-8c).…”