The Electrical Structure of the Lithosphere and Asthenosphere beneath the Fennoscandian Shield

The electrical structure of the crust and upper mantle beneath three regions of Scandinavia has been delineated by the magnetotelluric (MT) and the horizontal spatial gradient (HSG) techniques. The analyses were applied to data recorded by the Scandinavian IMS magnetometer array complimented by telluric observations. Models compatible with the response functions observed in northern Sweden and northeastern Norway/northern Finland are distinctive by exhibiting: (i) a negligibly small resistivity contrast across the seismic Moho; and (ii) the unequivocal existence of an electrical asthenosphere beneath both regions. In definite contrast, the response function observed in southern Finland demands a highly conducting layer (of resistivity around 10-50 Ωm) in the lower crust, and an order of magnitude increase in resistivity on entering the mantle. This increase is at a depth compatible with the known seismic Moho for the region. It is not possible to make a quantitative estimate of the depth to the electrical asthenosphere beneath southern Finland, due to lack of long period information, but a qualitative measure indicates that the asthenosphere depth increases with increasing distance towards the centre of the north European craton.