P. Kaikkonen

Magnetotelluric and audiomagnetotelluric measurements in Finland

Abstract Joint Finnish—Hungarian MT (magnetotelluric) and AMT (audiomagnetotelluric) measurements were carried out in Finland in the framework of the international ELAS project. The conditions for MT measurements are favorable at these latitudes. Five MT and 150 AMT stations gave information on the electrical conductivity distribution in the area: AMT results guided the choice of MT sites with minimal near-surface distortion effects and helped the interpretation of the MT soundings; the MT measurements indicate the presence of large conductivity anomalies and can be best interpreted as lateral induction effects of near-surface dyke structures. This result is confirmed by a certain correspondence between the directions of the maximum impedances and of the tectonic zones of the area. Any information about the upper mantle would require the use of Sq harmonics because of the crustal conductivity anomalies detected by the MT measurements.

The geoelectric model of the POLAR Profile, Northern Finland

Abstract Electromagnetic soundings have been made in order to construct a geoelectrical (conductivity) model of the crust along the European Geotraverse (EGT) POLAR Profile. Forty magnetotelluric (MT) soundings, eighteen audiomagnetotelluric (AMT) soundings and ten magnetohydrodynamic (MHD) soundings were made on the main POLAR Profile (POLAR I) and ten more MT soundings on a parallel profile (POLAR II), 40 km to the southeast of the main profile. Analysis of simultaneous recordings by the EISCAT magnetometer chain, and thin-sheet modelling of the effect of the Barents Sea, indicate that neither the source field effects nor the presence of the ocean are significant at periods below 200 s in the measurement area. The magnetotelluric data have been modelled with two-dimensional models representing the regional structure along the profiles. In addition to the regional structure, a thin inhomogeneous surface layer is included in the models in order to explain some local features of the measured response functions. Although details of the surface electrical structures are poorly resolved, the gross features of the geoelectrical cross section are considered to be reliable. The results divide the POLAR Profile into three different blocks. The better conducting Karasjok-Kittila Greenstone Belt in the south has an average resistivity of less than 10 Ωm. The more resistant Lapland Granulite Belt, with a resistivity between 100 and 200 Ωm, is underlain by conductive ( The geoelectric cross section agrees, in gross detail, with the corresponding gravity, refraction seismic and reflection seismic cross sections of the POLAR Profile. All methods indicated a similar shape for the southwestern part of the Lapland Granulite Belt i.e., granulites have a gently NE-dipping boundary against the underlying Karelian Province. In the northeastern part of the granulite belt the geoelectric model and the gravimetric model show a rather steep S-dipping boundary against the Inari Terrain northeast of the granulite belt.

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.