P. Johnston et al., MATERIAL VERSUS ISOBARIC INTERNAL BOUNDARIES IN THE EARTH AND THEIR INFLUENCE ON POSTGLACIAL REBOUND, Geophysical journal international, 129(2), 1997, pp. 252-268
Most previous earth models used to calculate viscoelastic relaxation a
fter the removal of the Late Pleistocene ice loads implicitly assume t
hat there is no exchange of mass across the mantle density discontinui
ties on periods of tens of thousands of years (the material boundary f
ormulation), In the present study, simple incompressible models are us
ed to determine the Earth's behaviour in the case where the density di
scontinuity remains at a constant pressure rather than deforming with
the material (the isobaric boundary formulation). The calculation of t
he movement of the boundary is more rigorous than in earlier studies a
nd uses the local incremental pressure calculated at the depth of the
boundary and allows for the vertical deformation caused by the change
in volume as material changes phase. It is shown that the buoyancy mod
es associated with the density discontinuities decrease in strength an
d increase in relaxation time analogous to what results when the densi
ty contrast is reduced, Also, two viscoelastic modes arise from an iso
baric boundary, which is also predicted when there is a contrast in ri
gidity or viscosity across a material boundary. The difference in pred
icted radial deformation between the isobaric boundary model and the m
aterial boundary model is largest for long-wavelength loads for which
the material incremental pressure at depth is largest. If the isobaric
boundary model is appropriate for the treatment of the mineral phase
changes in the mantle on glacial rebound timescales, then previous inf
erences of the deep-mantle to shallow-mantle viscosity ratio based on
large-scale deformation (spherical harmonic degree <10) of the Earth a
nd including data from the early part of the glacio-isostatic uplift a
re too small.