MATERIAL VERSUS ISOBARIC INTERNAL BOUNDARIES IN THE EARTH AND THEIR INFLUENCE ON POSTGLACIAL REBOUND

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.