ИСТИНА

Due to increased possibilities of modern magnetotelluric and magnetovariational deep sounding methods: digital synchronous recording, GPS positioning and synchronization, advanced processing and inversion techniques developed and introduced into MTS practice by the end of the first decade of XXI century - the interest to LA has been recently inspired again according to actual needs of Precambrian geological studies. The MT group of A.A. Kovtun (SPbGU) has invited MT researches from Moscow (IPE RAS, MSU) to combine efforts and to initiate new stage of LA instrumental investigations. “Nord-West” Ltd has provided invaluable assistance in the implementation of this idea and large MT/MV sounding project has been organised. The observations at 200 km long Vyborg-Suojarvi profile held in 2013-2014 by “Nord-West” Ltd, MSU and IPE RAS with “Phoenix” and “LEMI” stations have resulted in 50 broad-band and 9 long period MT/MV soundings with synchronous recording in remote bases BASE_2013, 2014 (Fig. 1). Several modern processing techniques (software provided by equipment manufacturers and remote reference and multi-RR schemes with magnetic and/or electric remotes [Varentsov et al., 2003;Varentsov, Sokolova, 2005; Epishkin, 2014] were used to suppress EM noices. The data of geomagnetic observatories Nurmijarvi, Mekkrijarvi, Suwalki and permanent geomagnetic observations of St. Petersburg Branch of IZMIRAN in Krasnoe (Fig.1) were used for long period multi-RR TF estimation and “synchronization” of different campaigns’ data. The resulting local (impedance Z, tipper Wz) and inter-stations (horizontal magnetic tensor M) transfer functions have been estimated in broad band 0.003-2048 (4096)s or combined broad-band+LMT 0.003-8000(10000)s ranges. Their invariant analyses (Fig 1) has defined general strike (45-50˚NE) and dimensionality (quasi-2D with local 3D distortions) of the data and thus approved application of 2D interpretational approach [Sokolova et al., 2016]. The course of profile inversions with [Varentsov, 2007] robust code has produced the resistivity model of Lake Ladoga anomaly, which demonstrates, that LA is caused not by a unique anomalous object but a complicated ensemble of conductive features of different structural identity (Fig. 2) [Sokolova, Ladoga WG, 2017]. The resolution of the cross-section permitted to carry out informative tectonic interpretation. At mid-crustal levels the conductive structures are generally characterized by distinct SW dip and hypothesized to correspond to 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 bordering Ladoga-Bothnian tectonic zone. Probably at these levels the conductance of the layers is increased by mineralized aqueous fluids (due to meteoric water) gathered in weakened fault zones. In the upper crust (5-7km) of NE part of the profile bowl-formed association of conductive features is imaged, which describes the structure of Raahe-Ladoga suture zone across its ~50 -km width. In the SW part of the resistivity cross-section significant inflation of the deepest conductive layer (about 15-25 depth) was found which produces the very long-period Lake Ladoga anomaly originally discovered by pioneering MVS surveys. One can assume that this extremum of the conductivity is caused by deeply metamorphosed complexes of South-Finland Granulite-Gneiss Belt which could be similar to exhumed formations of Lapland Granulite Belt and include crystalline graphite. The formations of the coarse- and medium-flaky graphite exposed at the famous Ikhal'sky deposit, located in the nappe of the South-Finland Belt, probably represent these deep-seated fabric. However at NE segment of the profile, 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 the profile support obtained tectonic inferences [Golubtsova et al., 2016].