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  • 1
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Mantle-wide heterogeneity is largely controlled by deeply penetrating thermal convective currents. These thermal currents are likely to produce significant lateral variation in rheology, and this can profoundly influence overall material behaviour. How thermally related lateral viscosity variations impact models of glacio-isostatic and tidal deformation is largely unknown. An important step towards model improvement is to quantify, or bound, the actual viscosity variations that characterize the mantle. Simple scaling of viscosity to shear-wave velocity fluctuations yields map-views of long-wavelength viscosity variation. These give a general quantitative description and aid in estimating the depth dependence of rheological heterogeneity throughout the mantle. The upper mantle is probably characterized by two to four orders of magnitude variation (peak-to-peak). Discrepant time-scales for rebounding Holocene shorelines of Hudson Bay and southern Iceland are consistent with this characterization. Results are given in terms of a local average viscosity ratio, 〈inlineGraphic alt="inline image" href="urn:x-wiley:0956540X:GJI305:GJI_305_mu1" location="equation/GJI_305_mu1.gif"/〉, of volumetric concentration, φi. For the upper mantle deeper than 340 km the following reasonable limits are estimated for 〈inlineGraphic alt="inline image" href="urn:x-wiley:0956540X:GJI305:GJI_305_mu2" location="equation/GJI_305_mu2.gif"/〉. A spectrum of ratios 〈inlineGraphic alt="inline image" href="urn:x-wiley:0956540X:GJI305:GJI_305_mu3" location="equation/GJI_305_mu3.gif"/〉 at concentration level φi≈ 10−6−10−1 in the lower mantle implies a spectrum of shorter time-scale deformational response modes for second-degree spherical harmonic deformations of the Earth. Although highly uncertain, this spectrum of spatial variation allows a purely Maxwellian viscoelastic rheology simultaneously to explain all solid tidal dispersion phenomena and long-term rebound-related mantle viscosity. Composite theory of multiphase viscoelastic media is used to demonstrate this effect.
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