Structure of the Central Scandinavian Caledonides and the underlying Precambrian basement, new constraints from magnetotellurics

SUMMARY We investigate the structure of the accretionary wedge of the Caledonian orogen, the underlying autochthonous/parautochthonous carbonaceous alum shales and the Precambrian basement. We have conducted 60 broad-band magnetotelluric soundings along a 180 km long profile in J¨ amtland, Sweden, across the eastern section of the Central Scandinavian Caledonides. Dimensionality analysis and regional strike estimates indicate that the conductivity structure can be approximated by a 2-D model having a N40 ◦ E strike direction, consistent with the dominant geological strike. The determinant average of the impedance tensor, together with the tipper transfer function from the best 34 sites, were inverted by the REBOCC 2-D inversion code. An electrically highly conducting layer beneath the Caledonides images alum shales, the autochthonous Cambrian carbon-bearing black shales on top of the Precambrian basement. Based on the comparison of electrical conductivity and seismic reflectivity models, we suggest that the Caledonian accretionary wedge thickens in a step-wise manner from ca. 1 to 5–6 km towards the west. In the east, the wedge is composed of the lower allochthon. In the west, the wedge reaches the thickness of 15 km and is composed of the lower allochthon at the bottom, the middle/upper allochthons at the top and resistive allochthonous basement slices. The upper crust of the autochthonous Precambrian basement is homogeneous and resistive from surface