Database of geomagnetic induction vectors across the Carpathians and modelling of the regional conductivity distribution

Study of the geoelectrical structure of the Carpathians has a long history. The first induction studies performed in the late fifties revealed reversals in orientation of the geomagnetic induction vectors. Untill the nineties, geoelectric groups from former Czechoslovakia, Poland, Hungary, Ukraine and Romania carried out a series of deep induction experiments and covered the whole Carpathian region by number of field stations clustered along profiles crossing the Carpathian arc. The collected long period electromagnetic induction data made it possible to map the surface trace of the Carpathian geoelectrical anomaly and to estimate the depths range and conductivity of the causative electrical conductor in the crust. Various methods were applied to model the electrical structure beneath the Carpathians. Based on a compilation of previous results as well as on our original modelling experiments we present a generalized conductivity model of the Earth’s crust of the western, northern and eastern parts of Carpathians. Electromagnetic induction data from the period range of 400 to 6000 s collected on the territory of former Czechoslovakia, Poland, Hungary and Ukraine were used to study the crustal structure of the electrically anomalous zone. Along several profiles, magnetotelluric data were acquired which provided information on the directional properties of subsurface structures and on their directionality. 2D electrical cross-sections along the magnetotelluric profiles were constructed by stitching formal 1D anisotropic inverse models obtained from complete magnetotelluric impedance tensors and, further, by employing the 2D REBOCC inversion procedure. The obtained models were used as pilot conductivity distributions for a subsequent quasi 3D modelling. 3D modelling and inversion aimed at fitting the geomagnetic induction data across the whole area by a regional distribution of the integrated electrical conductivity (conductance) and was carried out by applying a thin sheet approximation of the crustal structures as well as by utilizing results of numerical simulations of the horizontal magnetic tensor (HMT) obtained from the observed induction vectors. The electrical model of the Carpathian region is presented and discussed in relation to other geophysical data available and with respect to the basic regional geological units. The modelling allowed us to constrain both the geometry and electrical properties of the anomalous electrical structures beneath the Carpathians. The models show significant spatial variability of the geometry of the Carpathian conductivity anomaly along the Carpathian arc. Particularly, according to the modelling results the maximum width of the electrically anomalous zone reaches 50 to 60 km and the depth of the crustal conductors varies between 10 and 30 km in the West Carpathians and between 6 and 25 km in the northern and eastern part of the Carpathians. A special character of the induction vectors is observed in the western part of the Carpathian region where it makes contact with the Variscean structures of the Bohemian Massif and shows a complex 3D structural pattern generated most likely by a couple of pronounced but discordant anomalous induction features.