Implications of a ductile crustal layer for the deformation caused by the Fennoscandian ice sheet

Abstract

Previous studies of glacial-isostatic adjustment have usually considered earth models with a purely elastic lithosphere. A possibly significant deviation from this assumption is the presence of a ductile crustal layer embedded in an otherwise elastic lithosphere. Such a layer has been suggested for various continental regions on the basis of seismic and tectonic evidence. The present study investigates the implications of a ductile crustal layer for the interpretation of glacial-isostatic adjustment using a layered, incompressible Maxwell viscoelastic earth model and a simplified representation of the Fennoscandian glaciation. The relaxation-time and amplitude spectra show that, besides the conventional buoyancy mode M0 also present in earth models with a purely elastic lithosphere, the ductile layer supports a slowly decaying mode MC, which is capable of modifying the deformation markedly. Thus, measures of the absolute deformation such as the stress clearly reflect the presence of the ductile layer. In contrast to this are measures of the relative deformation such as the vertical displacement rate and the rate of gravity change. To a good approximation, these rates are inversely proportional to the relaxation time of the more quickly decaying mode M0, and are, therefore, less affected by the ductile layer. Taken together, the present study suggests that, provided a ductile crustal layer exists, its presence should be accounted for in interpretations of the glacial-isostatic adjustment following the Fennoscandian or smaller glaciations.