Aspects of the geoelectric models of the baltic shield

Abstract

The geoelectric structure of the Baltic Shield in Finland has been studied since 1980. Four types of geoelectric model have been considered so far: numerical models for the galvanic distortions of the upper 10 km; horizontal distribution of crustal inductive anomalies; vertical cross-sections above relevant crustal anomalies; and vertical conductivity profiles on a regional scale. The models are based on extensive, on-going measurements being conducted with magnetovariational, magnetotelluric and frequency sounding methods. Geoelectric studies in Finland are a part of both the international ELAS (Electrical conductivity of the asthenosphere) and the EGT (European Geotraverse) projects. In Finland, the most relevant crustal conductivity anomalies south of 67°N are a conductive band surrounding the Central Finland Granite Area in the west and south and the Oulu, Mikkeli and Outokumpu anomalies (N-S, E-W and N-S, respectively). The anomalies are located in the middle crust, typically from 6–12 km downwards. Several regional conductivity boundaries and the existence of a conductive layer in the lower crust, at least in parts of Central Finland, have also been noted. A vertical cross-section across the Oulu anomaly shows a wide crustal conductor, which seems to dip towards the west. The anomaly has exceptionally good conductivity (0.5/5 Ωm) and cannot be attributed to the known sandstone formation or the schist zone on the surface. Along the SVEKA DSS (deep seismic sounding) profile, some increases in conductivity of the upper crust correlate with the Kainuu and the Kuhmo schist zones. The upper crustal parts of the Raahe-Ladoga zone has also a higher conductivity than the surroundings. Some local conductivity anisotropies seem to coincide with deep faults located by DSS. The area east of Kuhmo seems to be more or less free of crustal conductivity anomalies, allowing electric soundings down to the upper mantle. A deep conductivity model based on the measurements in this area differs from earlier models for northern Scandinavia, where the presence of a good conductor at asthenospheric depths has been reported. Variations in the electrical conductivity of the Earth's crust and upper mantle thus yield information about the structural composition and the tectonic development of the Earth. The results now presented are only preliminary and their detailed geological-tectonic importance is not yet fully understood. Later, the models will be completed with the results from work done in both the eastern and the western parts of the Shield.