1 Introduction

Due to its deep penetration depth, magnetotellurics (MT) is a useful tool in the exploration of subsurface geology. Combining with information from other geophysical-geological methods, the results gained from MT studies help to refine the geological structure of a region.

In our study, we discuss the interpretation of the MTOA-02Footnote 1 regional magnetotelluric key section and provide a plausible schematic geological model (a static model) based on the resistivity distribution. MT data used in this study is available from the Magnetotelluric Database of the Geological Survey of the Supervisory Authority for Regulatory Affairs, Hungary.

1.1 Dataset

The MTOA-02 magnetotelluric key section is an approximately 210 km long section which was created from archived MT data (Fig. 1). It is composed of 3 archived sub-sections: the MT soundings of KA-3 and SB-1 sections and some stations of the Nagyatád MT network which lies in the track of the key section. In the middle of the created profile, around Lake Balaton there is an approximately 20 km long part with data gap where no MT measurements were made previously. Measurements are in progress to fill this gap and those which have been already completed are also integrated in the dataset.

Fig. 1
figure 1

MT stations of the MTOA-02 magnetotelluric key section. Red lines indicate main tectonic lines. Abbreviations LHP – Little Hungarian Plain, TMR – Transdanubian Mountain Range, BH – Balaton Highlands, TH – Transdanubian Hills, AL – Alpokalja line, RL – Rába line, BL – Balaton line, KL – Kapos line, PU – Penninic Unit, AAU – Austroalpine Unit, TRU – Transdanubian Range Unit, MH MU – Mid-Hungarian Megaunit, T MU – Tisza Megaunit. Tectonic lines were taken from the GIS files of the SARA, digitized from the Pre-Cenozoic geologic map of Hungary (Haas et al. 2010). The names of the geologic units follow the nomenclature of Haas et al. (2010)

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The key section goes through the Transdanubian region in a NW-SE direction. It crosses areas with different geology: a portion of the crystalline rocks of Alpokalja, the basin of the Little Hungarian Plain filled with sedimentary rocks, the Transdanubian Mountain Range composed of mainly carbonate formations, the Balaton Highlands with its volcanic remnants and the area of the Transdanubian Hills with thick sedimentary rock coverage. These formations have different electric and magnetic properties which cause different effects on the measurements and consequently on the behavior of the derived apparent resistivity curves.

1.2 Polarization modes

There are two concepts related to MT data processing that are important to highlight for the better understanding and interpretation of the results summarized in this paper. In an ideal 2D situation the resistivity of the subsurface varies only in two directions: in one horizontal direction and in depth. Regarding the other horizontal direction, there is no resistivity variation. This direction in which the resistivity is constant often coincides with the direction of tectonic elements, for example the strike direction of faults (Niasari 2016). In this case, the measured data depend on the direction and can be decoupled into two polarization modes: the transverse electric (TE) and transverse magnetic (TM) modes.

The electrical component of the electromagnetic field is parallel to the strike direction in TE mode, while in TM mode the electrical component is perpendicular to it. Due to their nature, the two polarization modes are sensitive to the characteristics of the subsurface medium differently. TE mode is influenced more by the flow direction of electric currents, thus it is more suitable for detecting tectonic features like deformation zones. In contrast, electric charges accumulated at layer boundaries affect TM mode to a greater extent, hence TM mode data are better suited for detecting high resistivity formations in the basement (Simpson and Bahr 2005; Novák 2010).

Although ideal 2D cases are seldom found in real geology, rotating data to local or regional strike directions may help to separate data to these polarization modes and this approach is commonly used in MT data processing.

2 Methods and parameters

The main steps of data processing are summarized in the followings. The time series data of the supplementary measurements were processed by using ProcMT (https://manuals.geo-metronix.de/en/procmt/procmt.html). In case of archived data, the original time series data were not available, only the EDI (Electrical Data Interchange) files generated from them. It was necessary to unify these files with respect to the rotation angles. This step was done with the help of the MTpy python library (Krieger and Peacock 2014; Kirkby et al. 2019). MTpy was also used in data analysis. Further processing steps were carried out in WinGLink (Geosystem Srl 2008).

In WinGLink the data was rotated to 65°, i.e. the direction of the Mid-Hungarian line which defines the main tectonic orientation of the area. Apparent resistivity and phase curves were filtered and then D+ smoothing (Beamish and Travassos 1992) was applied to the curves. The phenomenon of static shift (constant shift between apparent resistivity curves) was corrected manually in case of those stations that had static shifts larger than 5–7 Ωm.

For 2D inversions a smoothing parameter of 60 was used. Minimum and maximum resistivity values were set to 1 and 50,000 Ωm, respectively. The effect of the topography was considered. As input data the filtered and smoothed apparent resistivity and phase curves were used. The maximum number of iterations was 60. 2D inversions were made for three cases: (1) using only TE mode data, (2) using only TM mode data and (3) using the combination of TE and TM mode data. The root-mean-square (RMS) error in all cases was less than 3%.

Gridding parameters for displaying apparent resistivity/phase data and inversion results were the following. For visualization purposes only, the sections are displayed from the surface to 25 km depth because the penetration depth of most of the MT soundings was approximately 25 km. This way, the sections do not show data with high uncertainty, interpolated from few data points below this threshold. The size of the individual grid elements was 2 km horizontally and 500 m vertically.

3 Observations from raw data

The apparent resistivity, phase and anisotropy sections are displayed in depth (i.e., Bostick depth—Bostick 1977) instead of frequency in order to facilitate the comparison to other observations.

Interpolated views of the apparent resistivity-depth and phase-depth curves in TE and TM polarization modes are shown in Figs. 2 and 3 respectively. These figures give a general overview of the resistivity variation along the key section but are not suitable for direct interpretation as the values shown on these sections are only apparent values before the inversion. According to the apparent resistivity sections, a low resistivity zone is located below the Little Hungarian Plain. It is followed by a high resistivity area under the Transdanubian Mountain Range. Below the Transdanubian Hills, on the shallower part of the apparent resistivity sections there is a low resistivity zone underlain by a high resistivity region.

Fig. 2
figure 2

2D interpolated view of a apparent resistivity-depth and b phase-depth curves of TE mode MT data. Black triangles indicate MT stations along the profile. The depth of the basement according to Haas et al. (2010) is shown with purple dotted line

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Fig. 3
figure 3

2D interpolated view of a apparent resistivity-depth and b phase-depth curves of TM mode MT data. Black triangles indicate MT stations along the profile. The depth of the basement according to Haas et al. (2010) is shown with purple dotted line

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The phase sections show similar tendency. Higher phase values indicate zones of lower apparent resistivity, while lower phases characterize the regions with higher apparent resistivity. The depth of the basement according to Haas et al. (2010) is shown with purple dotted line on the sections. An important observation—which was also used during the interpretation procedure—is that the depth where the phase values fall below 45° well coincides with the depth of the basement in the area of the Little Hungarian Plain and the Transdanubian Hills.

Figure 4 shows the anisotropy maximum (Kiss et al. 2020) along the key section. Maximum anisotropy values were calculated from the ratios of apparent resistivities (ρ):

$$Anisotropy\,Maximum=~\left| {\frac{{{\rho _{TE}}}}{{{\rho _{TM}}}}+\frac{{{\rho _{TM}}}}{{{\rho _{TE}}}}} \right|$$
(1)
Fig. 4
figure 4

Anisotropy along the section calculated from the ratios of TE and TM mode apparent resistivities. Anisotropy is most common in the area of the Transdanubian Mountain Range as it is characterized by high anisotropy values. Black triangles indicate MT stations along the profile. The depth of the basement according to Haas et al. (2010) is shown with purple dotted line

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Due to structural reasons or changes in rock quality the TE and TM polarization modes may detect different apparent resistivity in the related orthogonal directions which results in anisotropy. The phenomenon can be recognized from the splitting of the resistivity curves as it is shown in Fig. 5. Towards greater depth, the TE and TM mode resistivities do not follow the same trend. These kind of resistivity curves are more common in the area of the Balaton Highlands. Here, the data are also noisier than in other parts of the profile. Anisotropy is the highest between 50 and 90 km along the key section (Fig. 4).

Fig. 5
figure 5

TE and TM mode apparent resistivities plotted in depth. The upper part of the curves shows almost the same resistivity while in greater depth a splitting can be observed between the curves. The highly different apparent resistivity values at a given depth indicate anisotropy. The great number of outlier data points are caused by some source of noise which may be related to the presence of igneous rocks in the area of the MT station

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In most stations’ data static shift was only a few Ωm between the TE and TM mode apparent resistivity curves. This characteristic is more common for the Transdanubian Hills where MT stations were placed above thick layers of sedimentary rocks.

4 Inversion results

The results of the joint TE-TM inversion (Fig. 6) and the inversion of the TE (Fig. 7) and TM (Fig. 8) modes separately show similarities in their main features but differences can be observed in the details due to the nature of the different modes. The results obtained from the different polarization modes complement each other, so it is useful to consider them jointly during interpretation.

Fig. 6
figure 6

Result of the joint TE-TM inversion with double vertical exaggeration. Black triangles indicate MT stations along the profile. The depth of the basement according to Haas et al. (2010) is shown with purple dotted line. White rectangles indicate zones of missing data points

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Fig. 7
figure 7

Inversion section of the TE mode data with double vertical exaggeration. Black triangles indicate MT stations along the profile. The depth of the basement according to Haas et al. (2010) is shown with purple dotted line. White rectangles indicate zones of missing data points

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Fig. 8
figure 8

Inversion section of the TM mode data with double vertical exaggeration. Black triangles indicate MT stations along the profile. The depth of the basement according to Haas et al. (2010) is shown with purple dotted line. White rectangles indicate zones of missing data points

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A low resistivity continuous zone can be observed on the inversion result of the TM mode data which starts at the Little Hungarian Plain and extends below the Transdanubian Mountain Range. This zone seems to be less connected on the TE and joint inversion results. Main similarities among the three inversion results are the followings. A high resistivity anomaly appears between 0 and 20 km along the profile. Another high resistivity region is located below the Transdanubian Mountain Range. However, a difference in this area is visible on the TE inversion section: the high resistivity anomaly under the Balaton Highlands extends to a greater depth than in the other resistivity sections. An extended region with higher resistivity is observed on the TE mode result between Kaposvár and Szigetvár. The image of the resistivity inverted from TM mode data seems to follow best the morphology of the basement in the SE half of the key section.

5 Interpretation

During the interpretation procedure information from other geophysical methods than magnetotellurics were also used in order to strengthen the conclusions based on the resistivity profiles. These were the Bouguer anomaly map (Kiss and Gulyás 2005) and magnetic anomaly map (Kiss and Gulyás 2006) of Hungary (Figs. 9 and 10 respectively). The results were also compared with the Pre-Cenozoic geological map of Hungary (Haas et al. 2010, Fig. 11). For the interpretation of the area North from Lake Balaton the geologic profile from Tari and Horváth (2010) was also considered.

Fig. 9
figure 9

Stations of the MTOA-02 magnetotelluric key section displayed on the Bouguer anomaly map of Hungary (Kiss and Gulyás 2005, modified version). Abbreviations AL – Alpokalja line, RL – Rába line, BL – Balaton line, KL – Kapos line

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Fig. 10
figure 10

Stations of the MTOA-02 magnetotelluric key section displayed on the magnetic ΔZ anomaly map of Hungary (Kiss and Gulyás 2006, modified version). Abbreviations AL – Alpokalja line, RL – Rába line, BL – Balaton line, KL – Kapos line

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Fig. 11
figure 11

Stations of the MTOA-02 magnetotelluric key section displayed on the Pre-Cenozoic geological map of Hungary (Haas et al. 2010). Legend to the geologic units can be found in Haas et al. 2010. Abbreviations AL – Alpokalja line, RL – Rába line, BL – Balaton line, KL – Kapos line

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The 2D inversion sections with interpretation are summarized in Fig. 12. Each geologic unit and structural element could be identified according to their resistivity characteristics and considering the knowledge on rocks’ resistivities from near surface studies. The interpretation in Fig. 12 is discussed from the NW part of the key section towards SE.

Fig. 12
figure 12

The results of the a joint TE-TM inversion, b TE mode inversion, c TM mode inversion and d a schematic view of the interpretation. All sections are displayed without vertical exaggeration. Elements that can be delineated with uncertainty are marked with dashed black lines on each section

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5.1 Lower Austroalpine Unit and the subducted European Passive Margin

An area with high resistivity values (several hundreds of Ωm between 0 and 20 km along profile) is observed at the NW onset of each of the three resistivity sections based on different 2D inversions. Comparing its position with the Pre-Cenozoic geologic map of Hungary (Fig. 11) and with the geologic profile from Tari and Horváth (2010), this zone can be identified as the Lower Austroalpine Unit and the subducted European Passive Margin. According to the Pre-Cenozoic geologic map (Fig. 11) and the work of Haas et al. (2014) the Lower Austroalpine Unit consists of medium grade metamorphic rocks, mainly gneiss and mica schists, whose resistivity range from several hundreds to several thousands of Ωm (based on near surface geophysics and geology studies). The Penninic Unit could also be present in the area (Tari et al. 2010) but probably with relatively small thickness, therefore, it cannot be detected as a separate layer because of the decreasing resolution of the method with depth.

The change between this high resistivity anomaly and the next low resistivity area towards SE may be considered as the Alpokalja line (Novák 2010), a structural line separating medium grade and low grade metamorphites (Haas et al. 2014; Tari et al. 2021) within the Austroalpine Unit.

5.2 Upper Austroalpine Unit and the Variscan basement of the Transdanubian Range Unit

The next anomaly towards SE is a low resistivity area (less than 100 Ωm) starting under the Little Hungarian Plain and extending below the Transdanubian Mountain Range. The Transdanubian conductivity anomaly (Ádám 1997) is more pronounced on the TM and on the joint TE and TM mode inversion results. By comparing the orientation of the anomaly on the inversion sections with the geologic profile of Tari and Horváth (2010) and the Pre-Cenozoic geological map (Haas et al. 2010) this zone can be identified as a part of the Austroalpine Unit (Upper Austroalpine Unit) and the Variscan basement of the Transdanubian Range Unit. Although the mentioned units belong to different nappes, we discuss them in the same subsection of the study since they are characterized by similar resistivity values and produce a coherent low resistivity area on the inversion results.

The Upper Austroalpine Unit is composed of phyllite and partly graphitic formations (Ádám 1997; Haas et al. 2014). The Variscan basement of the Transdanubian Range Unit also consists of phyllites (Haas et al. 2014). The only difference between them is the existence of Alpine metamorphism. The Upper Austroalpine nappes—underlying the Transdanubian Range—underwent low grade Alpine metamorphism. In contrast, the Paleozoic rocks of the Transdanubian Range suffered only Variscan low grade metamorphism (Haas et al. 2014). Phyllites can also be transformed from low resistivity clayey sediments and the temperature increase with depth can further reduce the resistivity of rocks.

Probably, the Rába line—a dominantly SW-NE oriented deformation zone—contributes significantly to the formation of this low resistivity area by channeling currents into its strike direction and forming a well-conducting zone. There is no agreement regarding the location and the kinematics of the Rába fault. In older interpretations it was considered as a strike-slip fault (Kázmér and Kovács 1985). According to the recent studies, the Rába line is a Cretaceous nappe boundary (Haas et al. 2010, 2014) which suffered negative inversion during the Miocene extension (Tari et al. 2021). This model implies that the Rába fault is a SE dipping tectonic contact separating the Variscan basement of the Transdanubian Range Unit from the underlying Austroalpine nappes.

The extent of this low resistivity anomaly appears differently on the TE inversion result (Fig. 12b) compared to the TM mode one (Fig. 12c). This zone extends deeper (below 15 km) than on the TM mode inversion and joint inversion results. Also, it has nearly vertical borders from NW and SE while on the other resistivity sections it dips towards SE.

The reason for this difference may be revealed by a comparison between the resistivity profile and the Bouguer anomaly map (Fig. 9). On the Bouguer anomaly map a dominantly SW-NE oriented structure (the Rába line) can be seen in the region between KA-3-10 and KA-3-14 MT stations. This structure can channel the currents in its strike direction, so the TE mode cannot detect layers in this region of interest. Considering that the electric component of the TE mode points to the direction of structures, this may appear as a relatively sharply defined well-conducting zone on the TE inversion section due to the sensitivity of this polarization mode to the presence of structural zones. Probably this deformation zone could be permeated by fluids and the increase of the geothermal gradient may also contribute to the formation of this well conducting zone. A similar low resistivity anomaly along the Rába line can be tracked through neighboring MT profiles (Ádám 1997; Novák 2010). Besides the presence of the deep deformation zone, the extent of the anomaly in depth in the TE polarization mode may be caused also by the lack of data points. Between the distance of 20–40 km along profile and under 8 km in depth there are only a few MT soundings which reached greater depth. Therefore, interpolation may also contribute to the shape of the anomaly under 8 km.

Summarizing, the Rába line was marked on the schematic interpretation (Fig. 12d) based on the TE and joint TE-TM inversion results. Since it forms a well-conducting zone, its position was considered to be indicated by the lowest resistivities within the anomaly. The NW part of the anomaly was interpreted as the Austroalpine Unit and the other part SE from the Rába line as the Variscan basement of the Transdanubian Range Unit. The alignment of the anomaly on the resistivity sections (especially in TM mode) seems to coincide with the direction of the thrust sheets with NW vergence (Tari and Horváth 2010). Based on the inversion results, the anomaly under discussion seems to be the part of a nappe system with thick-skinned tectonic style. According to this model, thick-skinned thrusts can reach and cross the lower crust forming huge, crustal-scale thrust sheets similar to the thrusts in the Alpine basement nappes (Schmid et al. 2008) or in the Tisza Megaunit (Haas and Péró 2004).

5.3 Half-graben filled with Miocene sediments (N from Lake Balaton)

Above the anomaly which was identified as the Upper Austroalpine Unit and the Variscan basement of the Transdanubian Range Unit a half-graben is situated filled with Miocene sedimentary rocks. This zone does not show significant resistivity contrast with the underlying strata on any of the inverted sections. Hence, its lower boundary was marked on the interpreted sections based on the position of the bedrock (purple dotted line on figures) according to Haas et al. (2010).

5.4 Transdanubian Range Unit (Permian-Mesozoic formations)

Between 50 and 110 km distance along the profile, below the Transdanubian Mountain Range, all three resistivity sections show a high resistivity zone (several hundreds of Ωm). On the TM and joint inversion results this zone can be delineated with a syncline-like shape, although defining its SE part is uncertain due to the lack of data points at this part of the key section. This anomaly was identified as the Permian and Mesozoic formations of the Transdanubian Range Unit because of its position, shape and high resistivity values. The Unit consists of Permian sandstones, Lower Triassic siliciclastic formations and Middle and Upper Triassic shallow marine carbonates (Fig. 11, Haas et al. 2014). These are high resistivity formations, so they are able to cause the high resistivity anomaly in the sections.

Volcanic rocks of the Balaton Highlands may also play a role in the formation of the area with higher resistivities (few thousands of Ωm) between 80 and 100 km along profile. This is confirmed by the magnetic anomaly map in Fig. 10 and some observations from the processing steps: (i) the measurements in the vicinity of the volcanics had high anisotropy (splitting of the TE and TM apparent resistivity curves) and (ii) here the data were noisier than in other parts of the key section. These all may be related to the effect of highly magnetized rocks at the Balaton Highlands near the MT soundings (Kiss et al. 2020; Kiss and Prácser 2021).

Furthermore, high anisotropy values in this area (Fig. 4) may be caused by a combination of several factors: structural changes and variation in electric and magnetic properties. These may also be related to the presence of volcanic vents (Kovács et al. 2020) and still active fluid transport along deformation zones (Hencz et al. 2023).

5.5 Mid-Hungarian Megaunit

At a distance of about 110 km along profile, a boundary appears on each of the three differently determined resistivity sections. The position of this line coincides with the Balaton line. As this boundary is located at the SE edge of the area with data gap, it could be possibly marked more precisely after filling this gap with MT measurements and integrating the new data into the existing dataset. The next major structural line is the Kapos line which can be identified on the TE and joint inversion results, approximately at 140 km.

The region between the Balaton and Kapos lines is identified as the Mid-Hungarian Megaunit. This unit shows different characteristics on the TE and on the TM inversion sections. In TE mode a low resistivity anomaly is observed which can be explained by the highly tectonized nature of the area. Since the electric component of the TE polarization mode coincides with the direction of structures, the system of densely located faults in the area forms a well-conducting zone regarding the sensitivity of the TE mode. So the Mid-Hungarian Tectonic Zone appears with low resistivities on the TE inversion results.

In contrast, the same area shows high resistivity values (more than 100 Ωm) on the TM inversion section. The reason is that the electric component of the TM polarization mode is perpendicular to the direction of structures, therefore, it is less sensitive to the presence of faults and is more suitable to separate thick layers with high resistivity contrast. Rock materials forming the Pre-Cenozoic basement between the Balaton and Kapos line are known from boreholes (Haas et al. 2000). According to these, thick layers of carbonate successions are common in the area of the MT profile (Fig. 11, Haas et al. 2014) which are similar to the carbonate rocks of the Southern Alps and the Dinarides (Schmid et al. 2008). The key section goes through an area where the rocks forming the basement are unknown (Fig. 11). Here, a slight magnetic anomaly can be observed (Fig. 10). As a consequence, the high resistivity values in TM mode could be related to the carbonate formations of the unit and possibly to geological objects with magnetic effects.

No layer boundaries are observable within the zone. It may be caused by non-significant resistivity contrast and the limitations in the resolution of the MT method. The different appearance of this geologic unit on the sections is a good example of using the results from the different polarization modes simultaneously in geologic interpretations.

5.6 Tisza Megaunit

By comparing the three resistivity sections, the high resistivity zone (more than 100 Ωm) ranging from approximately 140 km until the end of the profile was identified as the Tisza Megaunit. The Tisza Megaunit is a nappe system of NW vergent thick-skinned nappes (Haas and Péró 2004) that are made up of Variscan medium grade metamorphic rocks and Permian to Mesozoic sedimentary cover. The pre-Miocene rocks along the investigated section are dominated by Variscan crystallines, dominantly gneiss, granite, and micaschist (Fig. 11, Haas et al. 2014). According to the knowledge from near surface geophysics, these rocks are characterized by high resistivity, so they are able to cause the high resistivity values in the inverted resistivity sections.

In the joint TE-TM inversion section (Fig. 12a) and in the TM inversion result (Fig. 12c) an anomaly can be observed between SB-1-13 and SB-1-19 MT stations. It is characterized by higher resistivities (from several hundreds of Ωm to few thousands of Ωm) than the surrounding media. Variscan granitoid rocks NE from Szigetvár, in the vicinity of the key section (Fig. 11, Haas et al. 2010; Haas et al. 2014) may play a role in the formation of the anomaly. This opinion is supported by the fact that a borehole (Sh-1) in the close vicinity of SB-1-16 reached the granitic rocks of the Mórágy Formation.

High resistivities (several thousands of Ωm) are particularly pronounced on a part of the TE inversion section between SB-1-6 and SB-1-19 MT stations. This area coincides with an extended magnetic anomaly along the profile (Fig. 10). Magnetized geologic features are able to cause increased resistivity values in TE mode (Kiss et al. 2020). Only few boreholes provide information about the geology in this area (e.g., Sh-1 borehole), hence the knowledge about the formations of the Pre-Cenozoic basement is quite limited. Considering that high resistivities and a magnetic anomaly can be observed in this part of the profile and granite was identified in a borehole, we assume that Cretaceous volcanics, dikes or serpentinite bodies may also be found in this part of the profile. This would be similar to the geology of the neighboring area (Balla et al. 2009; Haas et al. 2010; Haas et al. 2014).

At the same place, in the area of stations SB-1-6 and SB-1-19, an uplifted portion of the basement can be identified (Fig. 9). This structure can be observed also in the shape of the resistivity anomalies. Although the Tisza Megaunit also consists of nappe systems (Haas and Péró 2004), the shape of the anomaly differs from the shape of the Austroalpine Unit, the thrusts are not observable so clearly like in the NW part of the profile. Possibly, there is no significant change in rock quality which would cause notable resistivity variation or layers thick enough in order to detect them with the resolution of the method.

Based on the three resistivity sections a boundary can be detected in the shape of the anomalies approximately at 183 km. It is most pronounced on the inversion result of the TE mode data (Fig. 12b) where it separates the anomaly with several thousands of Ωm from a lower resistivity one (approximately 10–300 Ωm). A similar feature can be observed on the joint TE-TM inversion result (Fig. 12a) separating (i) the anomaly with few thousands of Ωm from the area with less than 100 Ωm in the upper 10 km thick portion of the resistivity section and (ii) below 10 km in depth separating the area with approximately 100 Ωm from the SE part with resistivities less than 40 Ωm. The boundary is less pronounced on the TM mode resistivity section (Fig. 12c) where the lower portion of the boundary does not appear clearly. The boundary was interpreted based on the TE mode resistivity section since TE mode data are more sensitive to the presence of structural zones. According to its location it was identified as the Mecsekalja zone. The Mecsekalja deformation zone is a steeply NW dipping Variscan ductile shear zone (Balla et al. 2009) that suffered repeated brittle reactivation during the Permian to Cenozoic (Csontos et al. 2002).

5.7 Basin filled with Miocene sediments (S from Lake Balaton)

Finally, on the top of the sections, to the S from Lake Balaton, a basin is observed filled with Miocene sedimentary rocks of low resistivity. This low resistivity area can be found on all three inverted sections. The main morphology of the basement is well observable on each section. The TM inversion results and the measured phase values plotted in depth (Figs. 2b, 3b and 12c) follow the best the morphology and depth of the basement (indicated by purple dots on the sections).

6 Conclusions

In this paper we summarized our results on the processing and interpretation of the MTOA-02 magnetotelluric key section. Based on the results of the different types of inversions and considering also the observations from raw data, several structural units were identified. Information from other geophysical methods and geological studies were also included in order to give a more accurate explanation of the resistivity anomalies on the sections and hence to make a more reliable interpretation of the area.

The following units and structures were identified on the resistivity sections.

  • The Lower Austroalpine Unit and the subducted European Passive Margin were present in all three inversion results as a high resistivity anomaly.

  • The Upper Austroalpine Unit and the Variscan basement of the Transdanubian Range Unit appeared with low resistivity values but with slightly different shape on the different inversion sections due to the presence of the deformation zone of the Rába line.

  • High resistivity anomaly with a syncline-like shape indicated the Permian and Mesozoic formations of the Transdanubian Range Unit. Volcanic rocks of the Balaton Highlands probably contributed to the observed high resistivity area between 80 and 100 km along the profile. They may be responsible for the noisiness and anisotropy of the data in this region. The more precise delineation of the Unit would be possible after completing the data gap of the profile between 90 and 110 km with supplementary MT measurements.

  • The Mid-Hungarian Megaunit had different characteristics on the inversion results of the different polarization modes due to the different nature and sensitivity of the modes.

  • The Tisza Megaunit was indicated by high resistivities on the sections.

  • Five main structural lines were identified: the Alpokalja line, the Rába line, the Balaton line, the Kapos line, and the Mecsekalja line. They are primarily observed on the inversion section of the TE mode data.

  • Basins filled with low resistivity sedimentary rocks could be identified to varying degrees in the different parts of the profile. South from Lake Balaton they were clearly indicated by low resistivity anomaly. North from Lake Balaton they did not show significant resistivity contrast with the formations of the underlying unit and could not be detected on the resistivity sections. However, phase-depth sections could help to identify them since the depth of the Pre-Cenozoic basement seems to show similarity with the depth where the phase turns below 45°.

The interpretation could be further refined by using also the rotational invariants of the MT impedance tensor (Szarka and Menvielle 1997). In the future, we intend to use this interpretation technique as well to make the geologic model of this complex region even more accurate.