Abstract

Kari Moisio, Numerical geodynamical modeling. Geophysical investigations in proterozoic lithosphere of South Finland.
University of Oulu, Department of Geophysics, February 1996.

Basic quantities in geodynamical modeling are stress and strain. Relation between stress and strain is described by rheological (constitutive) equations. Rheological behaviour in lithosphere can be roughly divided in brittle and viscous behaviour. Brittle deformation concentrates on the upper parts of the crust, when the stress reaches a value called yield strength or yield stress. Below this yield strength behaviour is elastic. Viscous deformation concentrates to the deeper parts of the crust and lithosphere. This behaviour is usually called ductile and the concept of creep is essential in this. Ductile deformation can generally be described by the power-law creep equation. Most common mechanisms in the lithosphere are dislocation and diffusion creep.

Equilibrium equation (of forces) is very often the differential equation, whose solution gives the required parameters for geodynamic model. Solution for this equation can be find for example by using finite element method. In finite element method the continuum is divided to number of finite elements, in which each the unknown parameter is solved by approximation functions. If the unknown parameter is displacement, the method is called displacement method.

Thermal equations are used to describe the thermal state of lithosphere. These are important when, used geodynamical model includes the effect of temperature. Rheological equations are very often functions of temperature. Thermal behaviour in lithosphere can be described by equations, which include the effects of heat conduction and convection as well as time variations of temperature.

Geophysical investigations in proterozoic lithosphere of South Finland include especially deep seismic soundings, electromagnetic, heat flow and gravity investigations. Results of geophysical investigations can be used in constructing geodynamic models and defining boundary conditions. In generally, most useful studies are deep seismic soundings, earthquake focus depth determinations and heat flow density studies. Seismic and focus depth studies give information concerning the structure of the crust. Heat flow studies give information about the thermal state of lithosphere, which can be for example used in rheological equations. To put it shortly, all geophysical investigations can be useful, when doing geodynamical modeling.