Modelling of mantle postglacial relaxation in axisymmetric geometry with a composite rheology and a glacial load interpolated by adjusted spherical harmonics analysis

Although studies on glacial isostatic adjustment usually assume a purely linear rheology, we have previously shown that mantle relaxation after the melting of Laurentide ice sheet is better described by a composite rheology including a nonlinear term. This modelling is, however, based on axially symmetric geometry and glacial forcing derived from ICE-3G and suffers from a certain amount of arbitrariness in the definition of the ice load. In this work, still based on non-selfgravitating axisymmetric flat models, we apply adjusted spherical harmonics analysis (ASHA) to interpolate the ice thicknesses tabulated both in ICE-3G and ICE-1 glaciological models. This filters out the nonaxisymmetric components of the ice load by considering only the zonal terms in the spherical harmonics expansion. The resulting load function is used in finite-element (FE) simulation of postglacial rebound (PGR) to compare composite versus purely linear rheology. Our results confirm a significantly better fit of composite rheology to relative sea level (RSL) data in North America than a purely linear rheology. The performance of composite rheology suggests that in future investigations, it may be better to use this more physically realistic creep law for modelling mantle deformation induced by glacial forcing.