Viscoelastic responses of a hard transition zone: effects on postglacial uplifts and rotational signatures

Recent studies from laboratory (S. Karato, 1989) and Monte Carlo inversion (Y. Ricard et al., 1989) have argued that the transition zone between 400 and 670 km depth may be stiffer than the lower mantle and the upper mantle above it. Using a five-layer viscoelastic spherical model, we have calculated the transient displacements of postglacial rebound, the induced polar motions and the temporal variations of the geopotential up to degree eight of the zonal coefficients. We have compared two models, (a) one with two viscoelastic layers separated at 670 km, and (b) the other with three layers in which a hard garnet layer lies between the upper and lower mantle. Forward modelling shows that it may be possible to discern the presence of a hard garnet layer with a viscosity of at least ten times greater than the upper mantle, on the basis of uplift data near the center of the former Laurentide ice-sheet and from polar wander and j _.₂ data. Temporal variations of higher gravity harmonics, such as j _.₆ and j _.₈, can potentially place even tighter constraints on the rheological properties of the hard transition zone. A lower mantle viscosity between 2 and 4 × 10²² Pa s is generally preferred in models with a garnet layer which may be as large as 50 times more viscous than the upper mantle.