HR: 1330h
AN: T32B-0936    [PDF]
TI: Self-Consistent Formation of a low Viscosity Zone
AU: * Stein, C
EM: stein@earth.uni-muenster.de
AF: Inst. f. Geophysics, WWU, Corrensstr. 24, Muenster, 48149 Germany
AU: Hansen, U
EM: hansen@earth.uni-muenster.de
AF: Inst. f. Geophysics, WWU, Corrensstr. 24, Muenster, 48149 Germany
AB: The role of a low viscosity zone in stabilizing plate motion has been proposed by convection models in which a low viscosity zone (LVZ) below the surface has been prescribed. In this case a plastic yield stress serving as deformation mechanism for the stiff surface is combined with a viscosity drop below the thermal boundary layer. As a result regions of constant velocity and continuous motion was observed.In contrast to models that prescribe the formation of the LVZ, we combine a three-dimensional numerical mantle convection model with a temperature-, stress- and pressure-dependent rheology. The additional variation of viscosity with pressure yields the self-consistent formation of a low viscosity zone. However in pressure-dependent viscosity convection not automatically a low viscosity zone forms. The LVZ only appears under a certain parameter combination. Depending on the parameter combination different regimes of convection arise. A stagnant lid type of convection prevails at high yield stresses and a mobile lid type at low yield stresses. In between an episodic regime occurs in which regions of constant velocity are observed on short timescales. These regimes have already been discussed in studies considering a temperature and stress dependence. But for additional pressure dependence of the viscosity a further regime results. In this regime a plate-like (i.e. rigidly moving) surface is observed and plate motion is stable on long timescales. The variation of viscosity with pressure thus is of capital importance in the generation of the LVZ, but furthermore the interaction of all rheological parameters is relevant. In none of the regimes apart from that showing stable plates a viscosity drop beneath the surface was observed even though a pressure dependence was assumed. Thus the existence of continuously moving plates and the presence of a low viscosity zone are two coupled phenomena.
DE: 3210 Modeling
DE: 3230 Numerical solutions
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8160 Rheology--general
SC: Tectonophysics [T]
MN: 2003 Fall Meeting