Geodynamics and rheology of the lithosphere along the DSS profile SVEKA in the central Fennoscandian Shield

Abstract

Numerical modelling was carried out to investigate the rheological structure and properties of the northern part of the deep seismic sounding (DSS) profile SVEKA. This profile is located in central Finland and it crosses the Lake Ladoga–Bothnian Bay zone (LLBB), which is the border between the Proterozoic and the Archaean domains. Data from seismic, thermal, electromagnetic and gravitational studies are used as initial parameters and also for analysing the results. Mainly DSS data are used to construct the subsurface structures. Rheological and thermal models with different material properties are calculated first. These results are then applied to the two-dimensional finite-element model of the SVEKA profile. The rheologically defined model is furthermore subjected to compressive boundary conditions and the resulting response is suggested to give some understanding of the present-day state of stress and deformational conditions in the lithosphere. The rheology of the DSS profile SVEKA seems to be fairly strong. The Proterozoic lower crust of the model is weaker than the Archaean one. Finite-element modelling shows that with moderate stress fields, the crust remains still intact, and only minor deformational areas are found. These include brittle deformation in the uppermost Proterozoic crust and plastic deformation in the lowermost crust. Lower crustal yielding present in few model cases is limited to very narrow zones at depths, which are very close to the Moho boundary. Changing the lower crustal composition from dry to wet reduces the strength in the lower crust remarkably, which enables plastic deformation in the lowermost crust. Similar results are obtained with reduction of the strain rate value.