NUMERICAL LITHOSPHERIC MODELLING: RHEOLOGY, STRESS AND DEFORMATION IN THE CENTRAL FENNOSCANDIAN SHIELD

This thesis deals with the analysis of the rheological structure and tectonic modelling of the Fennoscandian Shield. First, a short introduction to the geology and geophysics of the Fennoscandian Shield is presented followed by a description of rheological concepts. Second, the applied modelling procedures, together with the sources of error are explained. Last a brief summary of each original paper including conclusions is given. Understanding rheological conditions through the entire lithosphere and even deeper is the key for understanding the deformation of the earth's interior. Thus, investigating the rheological structure and possible consequences resulting from tectonic loading are required to some extent when interpreting geophysical data into tectonic models. In this thesis rheological structure is obtained by calculating rheological strength in different locations of the central Fennoscandian Shield. These locations are mainly situated along different deep seismic sounding (DSS) profiles as they provide necessary geophysical information required for model construction. Modelling begins by solving the thermal structure in the lithosphere, as rheological behaviour, mainly ductile flow is strongly controlled by temperature. Results from these calculations show that the rheological structure of the lithosphere depends on the thermal conditions resulting in significant areal variations. Generally, the central Fennoscandian Shield can be considered to be rheologically rather strong. Rheologically weak layers are however usually found in the lower crust. Correlation of the rheological structure with earthquake focal depth data shows that brittle fracture is the relevant mechanism in the earthquake generation and that non-occurrence of deep earthquakes implies low stress or high strength conditions deeper in the crust. Calculated rheological structure is furthermore used as a material parameter in the structural models which are solved next. These results suggest that it is highly unlikely that any considerable ductile deformation in the crust of the central Fennoscandian Shield exists and it seems that the present-day thermal and mechanical conditions in the investigated area do not favour such processes in significant amounts.

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