Viscoelastic stratification of the lithosphere: implications for its effective thickness in response to glacial loads

Volker Klemann, GeoForschungsZentrum, Potsdam, Germany
Prague, Mar 31, 2004

In glacial-isostatic adjustment, the lithosphere usually is only represented as an elastic boundary layer parameterized by its effective thickness, which modifies the viscoelastic response of the earth's mantle. Lateral variations of lithospheric structure or rheological behaviour inside the lithosphere are usually neglected. Therefore, the relation to more regional studies on composition and structure of the lithosphere is weak.
This presentation shows on the base of a lateral homogeneous structure and linear rheology how the flexural behaviour of the lithosphere is influenced by a viscoelastic stratification of the lithosphere. Two regions of potentially viscous behaviour are identified: a ductile layer at the base of the crust and viscous weakening of the lithosphere towards the convective mantle due to temperature activated diffusion creep.
The relaxation of shear stresses in the ductile crustal layer, modelled by Maxwell viscoelastic material, results in a decoupling of the shear stresses in the upper crust and the mantle lithosphere which reduces the effective elastic thickness of the lithosphere drastically. But for GIA processes, this phenomenon appears only if the viscosity in the ductile layer is smaller than the viscosity in the upper mantle.
The decrease of viscosity with depth in the mantle lithosphere leads to a successive stress relaxation of the lower parts, which mainly depends on the time scale of the loading process and results in a successive decrease of effective elastic thickness with loading time.
One lack of the above study is the assumption of linear viscoelasticity which is in dought to appear in the lithosphere. The consideration of a non-linear viscoelastic law by the assumption of power-law creep of material, is the aim of this MAGMA founded cooperation with Institute of Geophysics at the Charles University in Prague, Czech Republic. The implementation of such a rheology into a self-gravitating viscoelastic code is possible due to recent improvements of numerics developed by Zdenek Martinec at GeoForschungsZentrum Potsdam, Germany.

Last edited May 19, 2004