Inverting the Fennoscandian relaxation-time spectrum in terms of an axisymmetric viscosity distribution with a lithospheric root

The Fennoscandian relaxation-time spectrum is an observational data set representing post-glacial land uplift and employed in interpretations of glacial-isostatic adjustment. We invert this data set by adapting the spectral finite-element method, originally developed for a 3D self-gravitating spherical earth model [Martinec, Z., 2000. Spectral-finite element approach to three-dimensional viscoelastic relaxation in a spherical earth. Geophys. J. Int. 142, 117-142], to an axisymmetric viscosity distribution. The free parameters used in the inversion are either the central-lithosphere thickness (below the former Fennoscandian ice sheet) and the upper-mantle viscosity or the peripheral-lithosphere thickness and the peripheral-asthenosphere viscosity (surrounding the former Fennoscandian ice sheet). We demonstrate that a model featuring a central lithosphere with a thickness of 200 km and a peripheral lithosphere with a thickness of 80 km underlain by an asthenosphere satisfies the relaxation-time spectrum. We also show that the spectrum can be explained on the basis of a spherically symmetric model featuring a 100-km thick lithosphere, but no asthenosphere. Whereas this result is essentially consistent with previous interpretations of glacial-isostatic adjustment, the result obtained for the axisymmetric viscosity model also agrees with a large number of interpretations based on seismic, geomagnetic, thermal or rheological evidence for Fennoscandia.