Magnetotelluric array data analysis from north-west Fennoscandia

• Regional (350 × 480 km2) magnetotelluric array data were acquired in NW Fennoscandia.
• The Q-function and phase tensor analyses are compared.
• Data are inverted using 2D approach based on the usage of the determinant of the impedance tensor.
• Crustal conductors are revealed around Skellefteå, Kittilä and Kiruna mining areas.
• The Proterozoic lithosphere is thicker than the Archaean lithosphere.

New magnetotelluric (MT) data in north-west Fennoscandia were acquired within the framework of the project “Magnetotellurics in the Scandes” (MaSca). The project focuses on the investigation of the crustal and upper mantle lithospheric structure in the transition zone from stable Precambrian cratonic interior to passive continental margin beneath the Caledonian orogen and the Scandinavian Mountains in western Fennoscandia. An array of 59 synchronous long period and 220 broad-band MT sites was occupied in the summers of 2011 to 2013. We estimated MT transfer functions in the period range from 0.003 to 105 s.The Q-function multi-site multi-frequency analysis and the phase tensor were used to estimate strike and dimensionality of MT data. Dimensionality and strike analyses indicate generally 2-D behaviour of the data with 3-D effects at some sites and period bands. In this paper we present 2-D inversion of the data, 3-D inversion models are shown in the parallel paper. We choose to invert the determinant of the impedance tensor to mitigate 3-D effects in the data on our 2-D models. Seven crustal-scale and four lithospheric-scale 2-D models are presented.The resistive regions are images of the Archaean and Proterozoic basement in the east and thin Caledonian nappes in the west. The middle and lower crust of the Svecofennian province is conductive. The southern end of the Kittilä Greenstone Belt is seen in the models as a strong upper to middle crustal conductor. In the Caledonides, the highly conductive alum shales are observed along the Caledonian Thrust Front. The thickest lithosphere is in the Palaeoproterozioc Svecofennian Domain, not in the Archaean. The thickness of the lithosphere is around 200 km in the north and 300 km in the south-west.