Electric lithosphere-astenosphere boundary in the north-west Fennoscandia as revealed from magnetotelluric data (MaSca project)

Abstract:

Magnetotelluric images of the electric lithosphere-astenosphere boundary (LAB) below the north-western Fennoscandian Shield show that thickest lithosphere is in the Proterozoic Domain, not in Archaean. The Magnetotelluric (MT) method is well suited to map the depth to the LAB. The electric lithosphere is defined as the resistive outer shell overlying a highly conducting zone (astenosphere) in the upper mantle. A few extensive field campaigns have been undertaken in the Summers of 2011 to 2014 within the framework of the project “Magnetotellurics in the Scandes”. The MaSca survey crosses two important boundaries: (i) the transition zone from the stable Precambrian cratonic interior to passive continental margin beneath the Caledonian orogen and the Scandinavian Mountains in western Fennoscandia; (ii) the boundary between the Archaean and Proterozoic crust and upper mantle. In this study we present the electrical resistivity models obtained from 279 MT sites (70 long-period stations among them) acquired from 2011 to 2013. We also used other sources of data available in the study area: the IMAGE observatory data, the BEAR array and two small arrays of Finnish Geological Survey (GTK). We present the results of 2D lithospheric-scale inversions obtained along four selected profiles: three profiles are NW-SE directed, thought to be perpendicular to estimated regional strike and one profile crosses the Archaean-Proterozoic boundary, thus NE-SW directed. The final interpretation models revealed thickest lithosphere is in the Palaeoproterozoic Svecofennian province and reaches depth of 300 km. The Archaean lithosphere is thinner, with depth of 200 km to 250 km.