Electrical conductivity of the lithosphere under the western margin of cratonic Fennoscandia
Our study focuses on determining the electrical conductivity of the accretionary wedge of the Caledonian orogen, the underlying autochthonous/parautochthonous carbonaceous alum shales and the Proterozoic basement. We also investigate the deep western margin of the cratonic Fennoscandian lithosphere towards the oceanic lithosphere beneath the Atlantic Ocean. We have conducted 60 broad-band (0.001 10000 s) magnetotelluric soundings along a 180 km long profile in J?mtland, Sweden across the eastern section of the Central Scandinavian Caledonides. The determinant average of the impedance tensor together with the tipper transfer function were inverted by the REBOCC 2D inversion code. An electrically highly conducting layer beneath the Caledonides images alum shales, the autochthonous Cambrian carbon-bearing black shales on top of the Proterozoic 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 c.1 km to 15 km towards the west. In the east, 1-5 km thick wedge is composed of resistive rocks of the 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 Proterozoic basement is homogeneous and resistive from surface down to 15 km and can be associated with the Revsund and R?tan granites. The lower crust and uppermost mantle in the easternmost part of the profile are very resistive whereas in west they are two to three orders of magnitude more conductive. The increase of average crustal conductivity is related to the Caledonian processes or later opening of the Atlantic Ocean that have affected also the lower crust. A region of enhanced conductivity at the depth of c. 100-150 km is detected under the Caledonides in the western border of the Fennoscandian Shield. Yet the conductor is absent beneath the eastern part of the profile in the Fennoscandian Shield, proper, or is detected at the depth of more than 250-300 km. This feature may map the western limit of the lithosphere of the cratonic Fennoscandia Shield. As an extension of the Jamtland profile, 11 new long period magnetotelluric instruments are currently recording in Tr?ndelag, Norway. The profile extends from the Swedish-Norwegian border in the east to the coast of the Norwegian Sea (Atlantic Ocean) in the west. Additional broad band soundings will be carried out along the new extension as well as around the T?nnfors synform, Mullfj?llet antiform and Are synform in Sweden on summer 2008.
Magnetotellurics electrical conductivity crust lithosphere upper mantle Caledonides Fennoscandia
KORJA Toivo SMIRNOV Maxim PEDERSEN Laust B
University of Oulu, Department of Physical Sciences, Geophysics, Oulu, Finland Uppsala University, Department of Earth Sciences, Uppsala, Sweden
国际会议
The 19th International Workshop on Electromagnetic Induction in the Earth(第十九届国际地球电磁感应学术研讨会)
北京
英文
209-214
2008-10-23（万方平台首次上网日期，不代表论文的发表时间）