The Dynamical Influences of a Hard Transition Zone on Post-Glacial Uplifts and Rotational Signatures

  • G. Spada
  • R. Sabadini
  • D. A. Yuen
Chapter
Part of the NATO ASI Series book series (ASIC, volume 334)

Abstract

Recent investigations from laboratory (Karato, 1989) and Monte-Carlo inversion of geophysical signatures (Ricard et al., 1989) have suggested that the transition zone of the mantle between 400 and 670 km depth may be stiffer than the lower and the upper mantle. By means of a five-layer viscoelastic spherical Earth model, we have evaluated the displacement fields associated with post-glacial rebound, the induced polar motions, the temporal variations of the coefficients of the geopotential up to degree eight and the stress fields induced by deglaciation in the lithosphere and the upper mantle. Temporal variations of stress fields in the lithosphere reveal a non-monotonic behaviour due to the viscosity stratification. A completely analytical approach to this problem has also been made possible by the direct evaluation of both the eigenspectra and eigenfunctions which characterize the viscoelastic relaxation spectra of the Earth model. We have compared two models, the first with two viscoelastic layers separated at a depth of 670 km, the second with three layers in which a garnet layer Hes between the upper and the lower mantle. The results have shown that it may be possible to discern the existence of the garnet layer with a viscosity of at least 10 times greater than the upper mantle, on the basis of uplift data near the center of the Laurentide ice-sheets from polar wander and gravity coefficient J 2 data. The time-dependence of the higher order coefficients J 6 and J 8 can place tighter constraints on the rheological properties of the transition zone.

Keywords

Secular Variation Polar Motion Lower Mantle Viscoelastic Layer Viscosity Profile 
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Copyright information

© Springer Science+Business Media Dordrecht 1991

Authors and Affiliations

  • G. Spada
    • 1
  • R. Sabadini
    • 1
  • D. A. Yuen
    • 2
  1. 1.Dipartimento di Fisica - Settore GeofisicaUniversita’ di BolognaBolognaItaly
  2. 2.Minnesota Supercomputer Institute and Department of Geology and GeophysicsUniversity of MinnesotaMinneapolisUSA

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