Magnetotelluric measurements across the Sorgenfrei-Tornquist Zone in southern Sweden and Denmark

• Published in: Geophysical journal international Vol. 176; no. 2; pp. 443 - 456
• Main Authors: Smirnov, M. Yu, Pedersen, L. B.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.02.2009
• Subjects: Composition of the mantle; Cratons; Earth sciences; Electrical properties; Geovetenskap; Magnetotelluric; NATURAL SCIENCES; NATURVETENSKAP
• ISSN: 0956-540X; 1365-246X; 1365-246X
• DOI: 10.1111/j.1365-246X.2008.03987.x

The results of a magnetotelluric experiment across the Sorgenfrei-Tornquist-Zone (STZ) in the southwestern part of the Baltic Shield are presented. The STZ marks the border between the Proterozoic intact part of the shield in Sweden to the north and the reactivated part including the Danish basin and the Ringköbing-Fyn High (RFH) to the south. We deployed 69 magnetotelluric sites along two profiles, each 350 km long. High quality data were collected in the period range 0.003–20 000 s. Data were processed using a robust multiremote-reference technique. Strike and dimensionality analysis as well as directions of induction arrows support the general 2-D character of the conductivity distribution in the area, allowing us to model the data in 2-D. Satisfactory agreement can be obtained between the measured and calculated transfer functions projected onto the profile direction. In order to ensure that the final model is stable we have performed a sensitivity analysis using synthetic models with different hypotheses about mantle conductivity and including a priori models for the sedimentary basin. The crustal part of the model compares well with that of Thybo which is derived from seismic, gravity and magnetic data. In the Danish basin we resolve a thick sedimentary cover that extends to the southwest. At the STZ a resistive body surrounded by more conductive material coincides with a zone of high seismic velocities and densities may be interpreted as a large magmatic intrusion. The STZ manifests itself electrically very clearly in the lower crust and upper lithospheric mantle as a narrow zone of high conductivity. The thickness of the electric lithosphere shows a gradual thinning from The Baltic Shield in Sweden (about 300 km) across the STZ into the Danish basin (about 100 km).