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

Previous studies of glacial-isostatic adjustment have usually considered ea rth models with a purely elastic lithosphere. A possibly significant deviat ion from this assumption is the presence of a ductile crustal layer embedde d 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, inc ompressible Maxwell viscoelastic earth model and a simplified representatio n of the Fennoscandian glaciation. The relaxation-time and amplitude spectr a show that, besides the conventional buoyancy mode MO also present in eart h models with a purely elastic lithosphere, the ductile layer supports a sl owly decaying mode MC, which is capable of modifying the deformation marked ly. Thus, measures of the absolute deformation such as the stress clearly r eflect 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 MO, a nd are, therefore, less affected by the ductile layer. Taken together, the present study suggests that, provided a ductile crustal layer exists, its p resence should be accounted for in interpretations of the glacial-isostatic adjustment following the Fennoscandian or smaller glaciations.