New geoelectrical model of Lake Ladoga conductivity anomaly: tectonic implications in correlation with potential fields Nina Golubtsova1, Victor Glaznev2, Victor Kulikov1, Maria Kosnyreva1, Mikhail Mints3, Mikhail Nilov4, Pavel Pushkarev1, Pavel Ryazantsev4, Yana Taran1, Maxim Smirnov5, Elena Sokolova6, Andrey Yakovlev1,7, Lyudmila Zolotaya1 1Lomonosov Moscow State University, Moscow, Russia
... [Show full abstract] 2Voronezh State University, Voronezh, Russia 3Geological Institute RAS, Moscow, Russia 4Karelian Reserch Centre RAS, Petrozavodsk, Russia 5Lulea University of Technology, Sweden 6Schmidt Institute of Physics of the Earth, Moscow, Russia 7Nord-West Ltd., Moscow, Russia The paper is focused on the interpretation of new MT/MV data collected at the profile across prominent Lake Ladoga conductivity anomaly (LA). This anomaly was discovered by pioneer MV soundings more than thirty years ago at the boundary between Archaean Karelia craton and Palaeoproterozoic Svecofennian accretionary orogen in the Baltic (Fennoscandian) Shield. Due to increased possibilities of modern synchronous sounding methods the interest to LA was inspired in response to actual demand in solid geophysical constrains for the studies of Precambrian geological evolution. The observations at 200 km long Vyborg-Suoyarvi profile held in 2013-2014 by “Nord-West” Ltd, MSU and KRS RAS with “Phoenix” and “LEMI” stations have resulted in 50 broad-band and 20 long period MT/MV soundings with synchronous recording in remote sites. The data processing was done by modern techniques, ranging from software, provided by equipment manufacturers, to sophisticate remote reference and multi-RR schemes (magnetic and/or electric remotes) basing on the records in distant sites and geomagnetic observatories. Despite industrial noises abundant in the area local and inter-stations MT/MV transfer function were estimated in broad band and long period ranges for most of sounding sites. The invariant analyses of the resulted response functions has provided estimates of general strike (45-50?NE) and dimensionality (quasi-2D with local 3D distortions) and so defined the interpretational approach. To get new model of deep geoelectrical cross-section of LA we used different techniques for 2D inversion of resulted dataset, starting from smooth and successive partial/mutual inversions with suppression of local 3D distortions and finalizing by constrained ones. The obtained geoelectrical model lead to conclusion that LA is caused not by a single anomalous object but complicated ensemble of conductive features of different structural identity. At mid- and lower-crustal levels they are generally characterized by distinct SW dip, most probably reflecting the thrust zones, developed along graphite-bearing slippery surfaces of supracrustal Palaeoproterozoic formations during their accretion/thrusting upon the SW border of Karelian Craton in the late Palaeoproterozoic. At the upper levels they are steepening and connecting to the major faults recognized at the surface, including activated neotectonic ones, which border Ladoga-Bothnian tectonic zone. In the upper crust (5-7km) of NE part of the profile bowl-formed association of conductive faults is imaged, which describes the structure of Raahe-Ladoga suture zone across its _50-km width. In the SW part of the section we found significant inflation of the deepest conductive layer (20-25km) that produces the very Lake Ladoga anomaly originally found by long-period MVS. We believe that it is constituted by deeply metamorphosed complexes of South-Finland Granulite-Gneiss Belt, which include crystal graphite and could be similar to exhumed and exposed formations of Lapland Granulite Belt. While at NE, in pericratonic zone, the enhance upper-crustal conductivity is connected with often exposed graphite- and sulphide-bearing sedimentary-volcanic Kalevian and Ladoga series of lower metamorphic stages. Relevant correspondences of geoelectric, density and magnetization images of deep structures in cross-section of Vyborg-Suoyarvi line are discussed to support obtained tectonic inferences. Acknowledgements: Colleagues from geomagnetic observatories Nurmijarvi, Mekkrijarvi, Suwalki; RFBR grants 13-05-00786, 15-05-00543, 15-05-01214. Keywords: crustal conductivity anomaly, Baltic (Fennoscandian) Shield, complex geophysical and tectonic interpretation