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-Kittilä 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 (< 5 Ωm), N-dipping layers. The depth for the uppermost conductive layer varies from a few kilometres in the southwestern part of the granulite belt to 13 km in the northeastern part, from where it rises steeply towards the surface close to the boundary between the Lapland Granulite Belt and the Inari Terrain. These features appear to be continuous between the two parallel MT profiles. Within the Inari Terrain a conductive zone at an approximate depth of 10 km and with a resistivity of about 20 Ωm was identified in a resistive upper crust.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.