Geoelectrical and geological structure of the crust in Western Slovakia

Geoelectrical and geological structure of the crust in Western Slovakia VLADIMÍR BEZÁK1, JOSEF PEK2, JÁN VOZÁR3, MIROSLAV BIELIK1,4 AND JOZEF VOZÁR5 1 2 3 4

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Geophysical Institute, Slovak Academy of Sciences, Dúbravská 9, 848 25 Bratislava, Slovak Republic ([email protected]) Institute of Geophysics, Academy of Sciences of the Czech Republic, Boční II/1401, 141 31 Praha 4, Czech Republic ([email protected]) Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland ([email protected]) Department of Applied and Environmental Geophysics, Faculty of Natural Sciences, Comenius University, Mlynská dolina, 842 48 Bratislava, Slovak Republic ([email protected]) Geological Institute, Slovak Academy of Sciences, Dúbravská 9, P.O.BOX 106, 840 05 Bratislava, Slovak Republic ([email protected])

Received: December 17, 2013; Revised: March 12, 2014; Accepted: April 4, 2014

ABSTRACT Electrical resistivity of the Earth’s crust is sensitive to a wide range of petrological and physical parameters, and it particularly clearly indicates crustal zones that have been tectonically or thermodynamically disturbed. A complex geological structure of the Alpine nappe system, remnants of older Hercynian units and Neogene block tectonics in Western Slovakia has been a target of recent magnetotelluric investigations which made a new and more precise identification of the crustal structural elements of the Western Carpathians possible. A NW-SE magnetotelluric profile, 150 km long, with 30 broad-band and 3 longperiod magnetotelluric sites, was deployed, crossing the major regional tectonic elements listed from the north: Brunia (as a part of the European platform), Outer Carpathian Flysch, Klippen Belt, blocks of Penninic or Oravicum crust, Tatricum and Veporicum. Magnetotelluric models were combined with previous seismic and gravimetric results and jointly interpreted in the final integrated geological model. The magnetotelluric models of geoelectrical structures exhibit strong correlation with the geological structures of the crust in this part of the Western Carpathians. The significant resemblance in geoelectrical and crustal geological structures are highlighted in shallow resistive structures of the covering formations represented by mainly Tertiary sediments and volcanics. Also in the deeper parts of the crust highly resistive and conductive structures are shown, which reflect the original building Hercynian crust, with superposition of granitoids or granitised complexes and lower metamorphosed complexes. Another important typical feature in the construction of the Western Carpathians is the existence of young Neogene steep fault zones exhibited by conductive zones within the whole crust. The most significant fault zones separate individual blocks of the Western Carpathians and the Western Carpathians itself from the European Platform.

Stud. Geophys. Geod., 58 (2014), 473488, DOI: 10.1007/s11200-013-0491-9 © 2014 Inst. Geophys. AS CR, Prague

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K e y w o r d s : applied geophysics, magnetotellurics, MT15 profile, Earth’s crust, Western Carpathians

1. INTRODUCTION Interpretation of crustal structure of orogenic belts is a complex problem. In its interpretation the newest tectonic knowledge and geophysical measurements need to be employed. Complicated tectonic structure and variable lithological composition presented as in the Western Carpathians make the interpretation of geophysical results difficult. The aim of this work is a more precise interpretation of a crustal structure in western Slovakia - western part of the Western Carpathians - mainly based on newest magnetotelluric (MT) measurements which were carried out along the profile MT15 (Fig. 1). On the territory of Slovakia, this MT profile is parallel to the seismic profile S-04 (Hrubcová et al., 2010). The coverage of the western Slovakia by MT measurements is very poor. Previous work describing geoelectrical structures by MT method is presented only in Červ et al. (2001), with 500 km long 2D profile with a coarse site distribution. Geoelectrical section provides deep lithospheric-asthenospheric information in the Bohemian Massif and Carpathian region. Another method based on geomagnetic induction measurements (geomagnetic deep sounding) has been used to study the most significant conductive crustal structure, the Carpathian Conductivity Anomaly (CCA), for the last several decades. The CCA, as a strong quasi-linear regional conductive anomaly, has been consistently detected along the whole Western Carpathian Arc (Jankowski et al., 1977, 2008; Kováčiková et al., 2005). The explanation for CCA is a combination of mineralized water in pores of rocks and of a gradual formation of graphite films in the pores, because motion of the pore fluid is necessary to connect individual cells of the film (Hvoždara and Vozár, 2004). This paper presents a crustal geoelectrical model of the region following from earlier interpretations of the crustal and lithospheric structure based on an integrated geophysical modelling (Bezák et al., 1997; Bielik et al., 2004), on seismic measurements (Tomek, 1993; Vozár and Šantavý, unpublished data; Hrubcová et al., 2010) on gravimetric models (Bielik et al., 2006; Grand et al., 2002).

Fig. 1.

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Location of the profile MT15.

Stud. Geophys. Geod., 58 (2014)

Geoelectrical and geological structure of the crust …

2. GEOLOGICAL SETTING Classification of tectonic units is based on Bezák et al. (2004, 2011) and geological evolution of investigated area is described e.g. in Plašienka et al. (1997). The tectonic elements of the Western Carpathians were originated during two main orogenic stages the Hercynian and the Alpine. The Hercynian orogeny took place during the Palaeozoic between Gondwana and Laurussia. The Alpine orogeny had several stages in the Mesozoic and Tertiary, which we distinguish conventionally according to closing of oceanic domains in the region between the European and African plates. The Paleoalpine stage started by closing of the Meliatic ocean in the Jurassic and ended by a subsequent collision prior to the Late Cretaceous. The Mesoalpine stage was connected with closing of the Southern Penninic - Vahic ocean and subsequent - compressional events to the end of the Cretaceous and at the beginning of the Paleogene. In the Neoalpine stage the Northern Penninic Flysch Basin was closed in the Neogene and following transpressional and transtensional movements took place between the European platform and the Carpathian block. The earliest tectonic elements of the Western Carpathians are fragments of Cadomian blocks in the substratum (Brunia as a part of the European platform in the north, fragments in the substratum in southern Slovakia) and Hercynian tectonic units of the crystalline basement, which are the fundamental structural units of the Western Carpathians crust. The Hercynian units are middle crustal nappes composed of a complex of metamorphosed rocks differing in the grade of metamorphism and lithology. To the close of its development they were intruded by bodies of granitoids in various periods. In later tectonic development the Hercynian structure was disintegrated and fragments of the Hercynian units were incorporated into the new Alpine units and structurally reworked. With the known polarity of the Alpine orogen from the South to the North, the Meliata oceanic basin was closed first. Its existence is testified only by fragments of the oceanic crust preserved in the form of nappes. Its closing and subsequent collision we put into connection with formation of principal crustal Paleoalpine units of the Western Carpathians (Tatricum, Veporicum, Gemericum), but also of near-surface nappes (Fatricum, Hronicum, Meliaticum, Turnaicum, Silicicum). The crustal units are built up of the crystalline basement, which includes fragments of Hercynian tectonic units, and of cover units of the Late Paleozoic and Mesozoic. The long-term Paleoalpine tectonic stage ended in the Middle Cretaceous with a formation of the Carpathian continental block, which collided with another continental block rifted away from the European platform (hypothetical Oravicum or Pieninic basement) in the Mesoalpine stage (roughly to the end of the Cretaceous and the beginning of the Paleogene) probably after the closing of the supposed oceanic basin of Southern Penninic type. The tectonic units from this stage were reworked structurally in further development and became a part of the Neoalpine structure at the boundary of the recent Inner and Outer Carpathians. Therefore, we consider the Mesoalpine stage of development from indirect indications and according to an analogy with the development of adjacent segments, mainly in the Alps only. After the Mesoalpine collision, the Carpathian block gradually closed the oceanic basin of the outer flysch from W to E and an oblique collision with the European platform


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took place. The zone of an accretion prism of flysch nappes formed (Outer Western Carpathians). In Neogene, the Carpathian block (Inner Carpathians) was in Neogene mainly disintegrated by horizontal strike-slip movements and next came extension of crust connected with formation of sedimentation basins and volcanism. Western Slovakia is an area with nearly all main tectonic units of the Western Carpathians present. The cross-section MT15 started to the northwest of the Flysch Belt, it crosses the Pieniny Klippen Belt, core mountains Považský Inovec and Tribeč, which belong to the Tatric units, and ends in Neovolcanic and Tertiary sediments in the southeast. In the deeper morphology, it crosses the contact of the Europrean platform, which is represented by Brunia in this area with Inner Western Carpathians. South of the Klippen Belt there are Paleoalpine units of Tatricum and Veporicum. Brunia represents the Proterozoic crystalline fundament (result of Cadomian orogeny) with supposed Paleozoic and Mesozoic cover. Flysch nappes, represented by the Biele Karpaty group of nappes in this segment, are overthrusted over the Brunia. The Klippen belt represents a Neoalpine structural zone, which incorporates both the Pieniny units and the adjacent Flysch units with units of the Inner Western Carpathians. According to several geotectonic and geophysical indications, the bedrock is expected to include crystalline blocks originally from the Pieninic or Oravic crust with Mesozoic Pieninic cover or Vahicum. Tatricum is composed of both the Hercynian crystalline units and their Upper Paleozoic and Mesozoiuc cover. These units are covered by the superficial nappes of Fatricum and Hronicum. The crystalline foundation has two basic components metamorphic complexes and granitoids. Both environments differ in electrical conductivity. A significant phenomenon manifesting itself in Western Slovakia are NE oriented young shear zones, mostly strike-slips and normal faults. The most important of those that cross the section from the North are the Kátlov-Čachtice, Považie, Mojmírovce, Veľké Zálužie, Pohorelá, Divín and Rapovce faults (Fig. 2).

3. MAGNETOTELLURIC MODELLING As an extension to the CELEBRATION 2000 project MT data were collected along the profile MT15. They are modelled and interpreted further in this paper. The MT method is based on measurements of varying magnetic and electric fields on the Earth surface and provides information about a distribution of the electrical conductivity within the Earth (e.g., Tikhonov, 1950; Cagniard, 1953; Vozoff, 1972). A planar incidence source magnetic wave induces an electric field in the Earth according to Faraday’s Law, and this field drives electric (telluric) currents in conductive structures related by Ohm’s Law. The electromagnetic (EM) signals are processed into an impedance transfer function, which represents a response of geoelectrical structures, and converted to the apparent resistivity and impedance phase sounding curves. These MT data are modelled by multidimensional forward and inversion codes, which connect the MT data with the most likely conductivity models. In this paper we are presenting 2D models of MT data. The raw data consists of broadband time series records within the period range of 0.005 to about 1000 s at 34 sites along the MT15 profile with inter-site spacing of about 5 km on the average. At three sites, with roughly a 50 km inter-site step, long period data were collected up to a period of about

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Fig. 2. Position of measured MT points on the profile MT15 on the geological map of Lexa et al. (2000). Faults after Bezák et al. (2004). 1 - External Magura nappes of the Flysch Belt (FB); 2 - Internal Magura nappes of the FB: Biele Karpaty nappe; 3 - Pieniny Klippen Belt; 4 - Neogene to Quaternary sedimentary rocks of interarc and backarc basins; 5 - alkali basalts (Pannonian Quaternary); 6 - andesitic volcanic rocks (Neogene); 7 - rhyolitic volcanic rocks (Neogene); 8 - Eocene to Early Miocene sedimentary rocks of the Buda Basin; 9 - sedimentary rocks of the Inner Carpathian Paleogene; 10 - sedimentary rocks of the Brezová, Myjava and Gosau formations (Cretaceous to Paleogene); 11 - Tatricum: a - crystalline basement, b - sedimentary cover; 12 - Veporicum: a - crystalline basement, b - sedimentary cover; 13 - Hronicum; 14 - Gemericum; 15 - Silicicum; 16 - a - faults, b - Neogene shear zones (CCZ - Carpathian Conductive Zone, Ka Kátlov-Čachtice, Pv - Povžský Inovec, Mj - Mojmírovce, Vz - Veľké Zálužie, Cz - Čertovice, Ph Pohorelá, Dv - Divín, Rp - Rapovce ; 17 - thrust faults.

104 s, too. In spite of many evident sources of man-made noise in areas crossed by the profile, the data quality was acceptable, though not excellent. Typical MT curves of principal apparent resistivities and impedance phases from three different locations on the profile are shown for illustration in Fig. 3. Vertical magnetic field was recorded at most of the sites, too, but the quality of the induction arrows was rather poor for short and medium period ranges. Therefore, we have not used the induction arrows in the subsequent analysis.


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Fig. 3. Typical raw MT curves of principal apparent resistivities and impedance phases for three locations on the MT15 profile. The data show a consistent course throughout the period range considered, though some scatter and outliers are observed, particularly for very short periods and in the MT dead-band between 1 and 10 s. The curves are shown in the N-E observation coordinate system.

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Prior to modelling, the regional strike analyses were performed to estimate orientation of 2D geoelectrical structures. Point estimates of parameters of the deep, regional structure are often scattered and uninformative if experimental impedances are not of utterly excellent quality. Therefore, we tried to test the hypothesis on the applicability of a composite 3D local/2D regional model by using a stochastic approach, as well as by employing techniques based on a multi-site and multi-frequency aggregation of the data. First, we analysed basic MT dimensionality parameters, specifically the standard (Swift’s) skew (Swift, 1967), Bahr’s regional skew (Bahr, 1988) and impedance anisotropy, which are shown as gray shade plots in Fig. 4. Two dashed lines in each plot indicate period ranges that correspond to Bostick penetration depths of 10 and 40 km, respectively (Niblett and Sayn-Wittgenstein, 1960). The skew and anisotropy plots clearly divide the profile into three distinct zones. In the west, and especially in the east, Swift’s skew and the impedance anisotropy are relatively small for periods penetrating in the upper crust (down to the 10 km line), indicating thus low dimensionality, close to 1D. In the central part of the profile, 3D effects throughout the whole crust have to be expected as indicated by the increased Bahr’s regional skew. The 3D local/2D regional composite model can be considered a reasonable approximation for the middle and lower crust (down to the 40 km line) except in the central part of the profile. To assess the regional strike and static distortion parameters, we first applied a stochastic Monte Carlo fitting of the Groom-Bailey composite model (Groom and Bailey, 1989) to the experimental impedances (Červ et al., 2010). We merged the obtained single-site, single-frequency model samples over depth and profile ranges and tried to find zones in which a common prevailing regional strike is indicated. Polar histograms of the regional strike estimates in Fig. 5. illustrate that the strikes are rather scattered for upper crustal penetration depths, with maxima varying between N0 and N30°E (± 90° ambiguity). A different prevalent strike, about N60°E, is suggested in the easternmost section of the upper crust, but the dimensionality is developed only weakly in this zone.

Fig. 4. Gray-shade plots of basic MT dimensionality parameters as functions of the profile distance and frequency (periods increase from the top to the bottom): Swift’s skew (left), Bahr’s regional skew (middle) and impedance anisotropy Z max Z min (right). The overlayed dashed lines indicate periods along the profile for which 1D Bostick penetration depths for both xy and yx-curves reach 10 km (upper line) and 40 km (lower line). Triangles on the top axes show MT site positions along the profile. The three sites indicated by upward off-set symbols are for the sites MT15-07, MT15-14 and MT15-31 with curves shown in Fig. 3.


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Data penetrating into the middle and lower crust also show prevalent regional strike estimates within the range of N10°E to N30°E in the western and central sections of the profile and about N75°E in the east. The directional pattern derived from the Monte Carlo simulations is similar to that given by the directional distribution of the principal axes of the phase tensor ellipses (not shown here). Further, McNeice and Jones (2001) multi-site and multi-frequency extension to Groom-Bailey MT tensor decomposition technique (Groom and Bailey, 1989) was used to determine the dominant geoelectric regional strike for several sites and frequencies simultaneously, again for the chosen Niblett-Bostick depth ranges that were used above to assess the depth coverage of the induction data (Niblett and Sayn-Wittgenstein, 1960). By grouping the data through various depth and profile ranges, we could identify best fitting composite models with the regional strike of about N20°E (± 90° ambiguity) throughout all penetration depth ranges in the western and central part of the profile as well as in the sub crustal domain below its eastern part. Similarly as in Fig. 5, the crustal strikes in the eastern section of the profile are considerably deflected to the E. Though the regional strike is not completely unique on a site-by-site basis, the strike

Fig. 5. Polar histograms of regional strike estimates along the MT15 profile in three depth ranges demarkated by 1D Bostick penetration depths at individual MT sites. The histograms are constructed from samples of 3D local/2D regional models generated, at each site and for each frequency, by a Monte Carlo regression of the Groom-Bailey composite model to observed impedances. The depth ranges are defined arbitrarily, specifically 110 km (upper crust), 1040 km (middle and lower crust) and 40200 km (subcrustal litosphere). In each of the depth ranges, the model samples are merged throughout profile sections that show roughly similar prevalent regional strikes. The MT15 profile line along with the MT site positions and the geological Sections IIV, defined in the text, are shown on the top of the figure.

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azimuth of N20°E showed as representative of substantial parts of the profile, and thus was accepted for a subsequent 2D treatment of the whole data set. The MT data were decomposed to the estimated regional strike azimuth and the resulting decomposed apparent resistivity and phase sounding curves were used for the modelling. MT impedances, decomposed in the common regional strike direction accepted, were further modelled for the distribution of the electrical conductivity within the crust beneath the profile. Due to the profile distribution of MT sites, and only little electrical information available from the neighbourhood of the profile, we have adopted a purely 2D approximation to the crustal electrical structures with the TE-mode (Transversal Electric) direction defined by the regional strike of about N20°E. For the modelling, we used an inverse algorithm by Pek et al. (2012) which is an extension to the classical non-linear conjugate gradient algorithm of the MT inversion (Rodi and Mackie, 2001) to a fully anisotropic 2D conductivity distribution in the Earth. Though anisotropic inversion is possible by using this algorithm, we did not use that option and applied the inversion in the isotropic modus only since no geological features in the studied region indicate that electrical macro-anisotropy on the scale of the MT sites distribution would be of relevance. The inverse procedure minimizes the roughness of the conductivity distribution, either in the sense of a conductivity spatial gradient or curvature of the conductivity distribution, under the constraint of the misfit between the experimental data and those produced by the model being within the limits of statistical error distribution of the measured data. As a result, we aimed at obtaining the flattest or smoothest conductivity spatial distribution in the crust which fits the experimental MT response functions to a satisfactory degree. A series of inverse runs was carried out which resulted into a final model accepted for a geological interpretation and shown in the top panel of Fig. 6. The overall misfit, characterized by the root mean square (RMS) of the data vs. model residuals normalized by the data variances, is about 4.23 for this model when the error floor for the impedance elements was set to 5%. This relatively large misfit is mainly due to larger systematic positive (experiment model) residuals of the TE-mode apparent resistivities in the western and partly also central segments of the profile for medium and longer periods, typically greater than 1 s, which may reflect an influence of larger scale 3D crustal conductivity variations in the region. Also, the data quality is only moderate in the western half of the profile due to industrial noise effects. To verify structural features of the final model and to avoid artefacts due to the inversion ambiguity and erroneous data points, the data were re-inverted several times from modified starting models in which the tested resistivity features were removed. Only robust features were preserved in the final model. To further reduce the influence of disturbed experimental data values we also inverted the data in a series of runs with always one MT site left out from the complete input data set. Resistivity features due to one sole MT site were further inspected and removed from the model if not found significant enough. Special attention was paid to structures with large vertical extension throughout the whole upper crust which might be artefacts due to insufficient horizontal smoothing at greater depths if one single weight for the roughness penalty is used throughout the model. It especially refers to the resistive feature beneath the profile segment MT15-25 to 28, which proved a robust resistivity structure in all inverse runs.


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Fig. 6. MT15 profile - geoelectric model and geological-tectonical interpretation. IIV interpreted sectors. 1 - Brunia: a - basement, b - cover; 2 - Pieninic crustal block: a - basement, b -cover and units in the frame of Klippen belt; 3 - Flysch belt; 4 - northernmost Tatric Upper Paleozoic and Mesozoic units; 5 - Tatricum basement and cover and superficial nappes; 6 - Veporicum; 7 - Neogene sediments and volcanics; 8 - supposed Neogene subvolcanic intrusions; 9 - Hercynian granitised complexes; 10 - Hercynian metasedimentary complexes; 11 - main crustal thrusts; 12 - thrusts: a - Alpine, b - Hercynian; 13 - main Neogene shear zones (for explanations of marks see Fig. 2).

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4. INTERPRETATION OF GEOLOGICAL-TECTONICAL STRUCTURES The interpretation of the structures is focusing on the upper crust which best reflects the conductive inhomogeneities. The analysis of the electrical resistivity in relation to geological/tectonical units will be performed in four sectors, which correspond to the basic blocks (Fig. 6). Sector I includes the European platform, in this area represented by Brunia. It is separated by a significantly conductive interface, which is associated with Carpathian conductive zone. A block of Brunia is reflected in the magnetic map (Kubeš et al., 2010) as a deep magnetic source, underlying the Flysch nappes. What can also be identified is the shift of the Moho interface, which was previously determined by seismic measurements (Hrubcová et al., 2010). The conductivity contrasts between the Proterozoic basement, its cover and overthrusted Flysch nappes can be identified in the Brunia block. In Sector II we assume the existence of the Pieninic crust block (Hrubcová et al., 2010; Janik et al., 2011) with its cover, above which there are conductive manifestations of the overthrusted Flysch nappes and structures of the Klippen belt, which is just like in other geophysical measurements manifested as a rather not very deep structure. Further south, we find Neoalpine shear zones, accompanying the Klippen belt and frontal units of the Inner Western Carpathians NW of Považský Inovec Mts. These zones also contain some wedged units of Tatricum, including the quite exotic Upper Carboniferous sequences. A block of Pieninic crust is hypothetically assumed to be present in the Tatricum fundament. Range of this block due to the gravimetric model of Grand et al. (2002) and Szalaiová et al. (unpublished data) is far to the South beneath the Tatricum. Sector III includes the main zone of the Inner Western Carpathians, which is built dominantly from the Tatricum and Veporicum crust units. These come to the surface in the horsts of core mountains separated by Neogene grabens which are filled by Tertiary sediments. The Tatricum and Veporicum are made of crystalline complexes and their Upper Paleozoic and Mesozoic cover. These structures are overlaid with superficial nappes of the Fatricum and Hronicum. In the structure of the crystalline basement there are the subhorizontal conductive contrasting zones which reflect the superposition of granitic and metasedimentary units, which are a typical feature of the Hercynian tectonic structure. Horsts are separated of grabens by steep faults, corresponding to the known faults, as defined for example in the Tectonic map of Slovak Republic (Bezák et al., 2004), which are also visibly manifested by high conductivity. The last Sector IV is located in the territory where the units of Tatricum and Veporicum are completely buried under the post-nappes Tertiary sediments and volcanics, which are manifested by higher conductivity in the sub-surface levels. In the deeper structure the famous faults such as Pohorelá, Divín, Rapovce fault are also manifested as good conductors. An interesting feature of this sector is the presence of deep resistive zones which are interpreted as a combination of granitoid bodies in the top part of the crust with assumed Neogene subvolcanic intrusions, which were also interpreted in other places by geophysical studies (e.g. Prutkin et al., 2014).


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5. COMPARISON OF MT RESULTS WITH GRAVIMETRIC AND SEISMIC MODELS The Bouguer gravity anomaly in the surrounding of the profile MT15 (Kubeš et al., unpublished data; Szalaiová et al., unpublished data) consists of the long-wavelength (regional) anomaly and short-wavelength (residual) anomalies. The interpretation of the gravity field is based on quantitative and qualitative interpretations of the gravity field in Slovakia (e.g. Šefara et al., 1987; Lillie et al., 1994; Szalaiová and Šantavý, 1996; Bielik et al., 2006; Dérerová et al., 2006). The regional anomaly is characterized by an increasing linear trend of the gravity field from the NW part (the Bohemian Massif and the Outer Western Carpathians junction) to the SE part of the profile (the Inner Western Carpathians and the Pannonian Basin junction). This regional trend is due to the superposition of the positive Moho and negative lithosphere-asthenospheric gravity effects. Note that the Moho effect is considerable larger in comparison with the lithosphere-asthenospheric effect. It means that the Moho course is dominant for the regional increase of the gravity field (Lillie et al., 1994; Bielik, 1995; Alasonati Tašárová et al., 2009). In the frame of residual anomalies in the north-westernmost part of the profile a relative gravity low can be observed. This significant gravity anomaly zone is a part of the Western Carpathians gravity minimum (Tomek et al., 1979; Bielik et al., 2006). The source of this anomaly can be explained by superposition of the gravity effects coming from the SE dipping of the European Platform (Brunia) and low-density sediments belonging to the Outer Western Carpathian Foredeep and Flysch zone. The largest gravity effect is due to the Outer Western Carpathian Flysch sediments. The Klippen Belt generates a small relative gravity high. The high density body in the lower part of the upper crust to the south of Klippen Belt beneath Tatricum (e.g. Kubeš et al., unpublished data) we interpret as the remnant of the Pieninic heavier crust (Fig. 6). Neogene Trenčianska Kotlina depression is characterized by a very sharp, short-wavelength (local) gravity low. Significant relative gravity high dominates over the Považský Inovec Mts. The main source of this anomaly comes from the large density contrast between the highdensity crystalline rocks of the Považský Inovec Mts. in comparison with bordering of the low-density sedimentary fillings belonging to the Trenčianska Kotlina depression on the North and Rišňovecká depression on the South. Borders of these contrasts are divided by steep low resistive zones (faults) in MT profile (Fig. 6). The crystalline of Tribeč Mts. produce further significant relative gravity high and can be explained similarly as in the case of the Považský Inovec Mts. The gravity high reflects the effect of higher density rocks building the mountains against the low density sedimentary filling of the Rišňovecká depression and the rocks of the Central Slovakian neovolcanics. Kubeš et al. (unpublished data) and Szalaiová et al. (unpublished data) suggested in the massif that the high density anomaly body located in the upper part of the upper crust with apical boundary at the depth of about 3.5 km. This body could represent the crystalline, which is built by the magnetic mica-schist complexes detected in MT measurements as relatively conductive structures. The area of the Central Slovakian neovolcanics is characterized by a large relative gravity minimum. The anomaly source is represented by lower density neovolcanic rocks. In the Central Slovakian neovolcanic

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gravity field the local gravity highs and lows can be observed. The local gravity highs and lows correlate very well with the pre-Tertiary basement elevations and depressions, respectively. The MT section was also confronted with the interpretation of seismic measurements in the South-East stretch of the S-04 section (Hrubcová et al., 2010), which runs parallel with the MT15 section in this part. In the seismic section, a varying depth of Moho under Brunia and Inner Western Carpathians is interpreted. This transition to Moho was also interpreted in the gravimetric sections (Csicsay, 2011; Bielik, 1995) and may be visible also in the MT image. This phenomenon is probably caused by young tectonic approximation of two tectonic blocks with a different evolution of the crust. In this case it is the European Platform from the North and the Inner Western Carpathian block from the South. The contact of these blocks is mediated by steep shear zones, which show high conductivities according to the MT measurements (the CCZ zone in Fig. 6). The highly conductive environment is apparently caused by crushing during the movement of tectonic blocks and the subsequent saturation of the crushed zone with fluids and CO2 from the mantle, which may also be the source of carbon deposits (Hvoždara and Vozár, 2004; Kucharič et al., 2013). Another structure, which has been interpreted in the S-04 seismic section in the interface of the Bohemian Massif and the Carpathians, is the Pieninic crust block (Hrubcová et al., 2010). This block was also interpreted in other seismic sections (e.g. Janik et al., 2011) and it is also reflected in gravimetry (e.g. Grand et al., 2002; Szalaiová et al., unpublished data). We have also accepted it in MT15 interpretation. It is interesting that this block has a similar conductive structure in the depth as the Inner Western Carpathian block itself (the less conductive granitised complexes above or in the overlay of the higher conductive metamorphites). This structure is a remnant of the Hercynian tectonic pattern and it indicates that both blocks were developing in the Paleozoic era in a similar tectonic environment.

6. CONCLUSIONS The structure of the crust in the profile MT15 can be generally divided into major crust blocks, from the North: Brunia, Pieninic crust, Tatric and Veporic crust. The Brunia crust is not structured in contrast with the Pieninic crust, which has a similar structure to the Tatric and Veporic Hercynian crust. This structure is caused by the contrast between the non-conductive and more conductive environments, which we interpret as superposition of granitoids or granitised units, metamorphic mica-schist and gneisses units as a result of Hercynian tectonic structure of the crust. The structure also exhibits crustal Paleoalpine thrusts (especially in the area of Tribeč Mts). In the conductive structure of the MT15 profile, the superficial Neogene sediments and volcanics are apparent. They reach the maximum thickness of up to 23 km. Flysch units of Outer Carpathians and Klippen belt are not deep units, this was also confirmed by other previous geophysical methods. The most salient phenomenon in the conductivity structure is existence the subvertical shear zones (strike-slips). They are the conductive manifestations of deep young shear zones, which directly connect to the known fault zones in the Central Slovakia, where


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they were identified on the surface from geological maps. Significant zones include those separating the blocks of Považský Inovec and Tribeč Mts., Neogene basins and zones in the South Slovakia (e.g. Divín and Rapovce fault zones) which are probably connected to the Hurbanovo fault. These shear zones were probably formed in the process of ejection of the blocks of the inner Western Carpathians toward north-east into the area of the Flysch Basin and in the ongoing oblique collision. Very important shear zone corresponds to the Carpathian Conductive Zone, separating Brunia from the Inner Western Carpathians, where the Moho jump can be also observed. There is a noticeable difference between the diversified structure of the top crust and the monotonous image of the lower crust and the upper mantle, which is likely due to decreasing resolution of the electromagnetic response to structures at greater depths. In the conclusion, the most important contribution of the MT method for the structure of the crust is the differentiation of high-resistivity and low-resistivity structures, which can be interpreted as inhomogeneities in the crust caused by the tectonic superposition of granitoid and metamorphic complexes, which is a typical feature of the Hercynian tectonic structure in Europe. The second major benefit is the significant conductive property of the young steep shear zones, which were not hitherto manifested in any geophysical methods in a significant way. Acknowledgements: This work was supported by the Slovak Grant Agencies APVV (grants No. APVV-0194-10, APVV-0724-11) and VEGA (grants No. 1/0095/12, 2/0088/12).

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