Tectonic Weak Zones Determined by Magnetotellurics Along the Cel-7 Deep Seismics Profile

Acta Geod. Geoph. Hung., Vol. 40(3–4), pp. 413–430 (2005) TECTONIC WEAK ZONES DETERMINED BY MAGNETOTELLURICS ALONG THE CEL-7 DEEP SEISMICS PROFILE

A Ad´ am´ 1,ANovak´ 1,LSzarka1

[Manuscript received June 15, 2005]

In the contact zone of three tectonic units (Pannonian Basin, Eastern Alps and Dinarides), in a complicated — basin and range — geological situation magnetotelluric deep soundings were carried out along a 140 km long profile (CELEBRATION-007) with a site distance of 2 km. In this area deep fractures of the Basin run together in NE-SW direction. In the paper various magnetotelluric images completed with gravity and magnet- ics are provided. In the traditional magnetotelluric approach, the structural indication of the TM and TE mode magnetotelluric sounding curves is clearly separated. The TM mode curves well express the resistive basement structure, already known from dense boreholes and detailed seismic exploration. The TE mode curves on the other hand (together with the induction vectors of very low values) definitely show the conductive root of the deep fractures, where the ductile materials are assumed to be raised into a very shallow depth of about of 8 km. The high heat flow of the area (about 100 mW/m2), which explains the shallowness of the conductive asthenosphere is also well indicated. The asthenosphere has more Alpine character in the NW part of the profile (its depth is about 80 km) and it is at smaller (about 50 km) depth in the SE part of the profile, due to the higher heat flow near the extensional Drava Basin. The induction vectors are also separated into two characteristic regions, according to their general direction, influenced by both local and remote effects. A strong correlation is shown between magnetotelluric and gravity inversion results. A joint interpretation of magnetotelluric, gravity, magnetic results provide a quite comprehensive interpretation about the deep geological structures in SW- Hungary. Keywords: conductive asthenosphere; deep fracture; electric conducting zone; inversion; magnetotelluric sounding; sedimentary basin

1. Introduction In August 2003 deep magnetotelluric (MT) and magnetovariation (MV) soundings were carried out in 72 measuring sites with site distance of 2 km along the CELEBRATION-007 (shortly CEL-7, Guterch et al. 2001) deep seismic proﬁle in SW border of Hungary (Fig. 3) with SPAM and CASTLE type instruments — having frequency range 1000 − 1/1000 Hz — obtained from the magnetotelluric instrument pool of the Geoforschungs-Zentrum (GFZ), Potsdam, Germany (Szarka et al. 2005). The block scheme of the CASTLE instrument is shown in Fig. 1. The data processing was carried out by using a robust program written by Ritter et al. (1998).

1Geodetic and Geophysical Research Institute of the Hungarian Academy of Sciences, H-9401 Sopron, POB 5, Hungary

1217-8977/$ 20.00 c 2005 Akad´emiai Kiad´o, Budapest 414 A AD´ AM´ et al.

Fig. 1. Block scheme of the measuring system CASTLE (from the MT instrument pool of GFZ Potsdam, Germany)

The results: the complex impedance elements with the co-ordinates of the measuring sites determined by GPS have been visualized and inverted by the “WinGLink” geophysical processing and interpretation software. An example of the extreme resistivity (Rho) and phase (ϕ) sounding curves (a), the real and imaginary part of the induction vectors (tippers) (b) and Zxy, Zxx polardiagrams (c) is given in Fig. 2 for MT site No. 35. Using these input values at ﬁrst 1D geoelectric layer sequences were computed. In the next step of the interpretation 1D stitched- and 2D pseudosections were calculated. Then diﬀerent 2D inversion results (TE mode, TM mode and a bimodal inversion, involving the complex induction vectors) were obtained. Question arises whether these very detailed MT and MV soundings could provide any new information to the seismics about the very complicate deep geological structures in the seismologically active border area of the Pannonian Basin, the Eastern Alps and the Dinarides. A positive answer is given and discussed in the followings.

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Fig. 2a. Resistivity (Rho) and phase (ϕ) sounding curves measured at the site No. 35

Fig. 2b. Real (red) and imaginary (blue) parts of the induction vectors vs. period from site No. 35

Fig. 2c. Zxy and Zxx polardiagrams with diﬀerent parameters (see the Legend)

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Fig. 3. Pre-tertiary Basement Contour Map of the Carpathian Basin beneath Austria, Czechoslo- vakia and Hungary (Kil´enyi and Sefaraˇ 1989) with the magnetotelluric measuring sites along the CEL-7 seismic proﬁle

2. Measuring area

The near-surface geological (geophysical) structures of the measuring area are described in different maps. Our measuring sites are shown in the corresponding part of the “Pre-tertiary Basement Contour Map of the Carpathian Basin beneath Austria, Czechoslovakia and Hungary” (Kilényi and Sefaraˇ 1989) (Fig. 3). The measuring profile CEL-7 was selected after the proposal by Németh (1997), who described the extremly varied geology and tectonics of the area, and also after a pilot magnetotelluric measurement by ELGI, along a nearly parallel profile (KA-05, where the distance between measuring sites was 10–15 km). According Németh (1997) our 140 km long CEL-7 profile crosses the following geological tectonic structures:

— the metamorphic belt at Szentgotth´ard (upper East Alpine nappe at the northernmost part of the proﬁle),

— the R´aba line (as continuation of the Insubric line in the Eastern Alps),

— the Preneogene deep zone at K¨ormend,

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— continuation of the Mesozoic, mainly carbonatic, thousands m thick mass of the Transdanubian Central Range at Salomvár-Nagylengyel oil field (Zala basin), — Pre-tertiary graben line structure at Nova and Bak, — Mesozoic carbonatic highland at Hahót, — Balaton-line as a Pre-tertiary graben structure (Nagykanizsa, Oltárc, Budafa) as continuation of the Periadriatic lineament in the Alps, — Mesozoic horst as continuation of the Kalnik Mts (at Nemespátró, Liszó, Pat), — Mid-Hungarian tectonic line (also known as Zagreb-Hernád, or Kapos line) which separates the two great terranes: Alcapa and Tisza composing the Pannonian Basin, — the Gyékényes-Inke graben, — Prealpine polymetamorph crystalline schist complex (Szenta-Kutas) with a northward gravitation creep.

3. Results of the measurements 3.1 A general characterization of the sounding curves and induction vectors The TE (generally Rhomin) and TM (usually corresponding to Rhomax) sounding curves substantially differ from each other from site 01 to site 60∗.Thisdif- ference (“anisotropy”) is due to deep fractures which cut or broke the basement into horst and graben as given in the description of the main geological/tectonical elements. The induction vectors also reflect the complexity of the region. The less distorted TM mode (Rhomax) sounding curves give more reliable values to the thickness of the low resistivity sediment (i.e. to the depth to the basement) than the TE mode ones. It became obvious when comparing these data with borehole sections and seismic horizons. On the other hand, the TE mode sounding curves together with the real induction vectors (Wiese convention) give basic information about the deep tectonics indicating crustal conductors, while the TM mode soundings seem to be not sensitive enough for its indication. Therefore, in a first approximation, we interpret independently the two modes.

∗An essential difference should be emphasized between the sounding curves measured along the CEL-7 profile and those ones obtained in the area of the Transdanubian Conductivity Anomaly where both extreme sounding curves have decreasing branches indicating fractures upwelling from a conducting layer. Type of the transitional sounding curves is shown from the measuring site Gyürüs (Gy) (Fig. 9).

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3.2 Structures in the sedimentary basin The following basin parameters could be derived from the 1D inversion of the TM mode (Rhomax) sounding curves:

1. the depth of the highly resistive basement (or thickness of the low resistivity sediments),

2. (mean) resistivity of the sedimentary layer(s),

3. conductance of the sediment,

4. resistivity of the Preneogene (Mezosoic, Paleozoic) basement.

These data have been collected in a database and they are visualized by means of pseudosections (Figs 4a and 4b) and in form of stitched sections (Figs 5a and 5b). These sections are very informative concerning the anomalous structures both for TE and TM modes. They can also serve as initial models for the 2D inversion. According to the 1D inversion, the TE mode sounding curves lose their connec- tion with the true basement structure when shallow crustal conductors distort the indication of the resistive basement (S-interval). For example, in the region of the Balaton line (see sites 34, 35, 36) the depth to the basement derived from TE mode curves diﬀers with more than 1000 m from that obtained from TM mode curves. At the same time, as mentioned, the 1D TM mode (Rhomax) layer models are conform to the borehole and geophysical (mainly seismic) data. The sedimentary complex generally consists of a thin and high-resistivity layer at the surface, and of an un- derlying thicker, more conductive one. The average resistivity of the sedimentary complex in case of the sites 1–49 is 10.3 ± 4.4Ωm,anditis11.7 ± 2.4Ωmincase of sites 50–72. (The extreme values are 5 and 16 Ωm.) The conductance reaches 400–500 siemens in the deep basins. In the 2D inversion sections (Figs 6 and 7) the basement depth values are in a relatively good agreement with those in the Kil´enyi-Sefaraˇ map (1989), especially intheTMmode(Fig.3).

3.3 Tectonics as indicated by conductivity anomalies The tectonic maps e.g. as shown in Fig. 8, all indicate the Rába-, Balaton- and Mid Hungarian lines, crossing the CEL-7 profile. (It does not show the Balatonf˝o- line, the NE-SW axis of Lake Balaton.) All these features appear in Haas’ (2001) geologic cross-section No. 1, running from Szentgotthárd to Gyékényes-Cún (SW Hungary), showing structures down to a depth of 6000 m. The seismics cannot give any information about the materials of the broken zones. Haas assumes granite intrusion between the Balatonf˝o-line and Balaton-line and Miocene andesite in the Mid Hungarian-line. Both the dry granites and dry andesites are of high-resistivity rocks. In tectonical fracture zones their resistivity is decreased, due to pore fluid (water, electrolyte) (Ad´´ am 1987a). Németh (2005) describes geothermal fluids and gas accumulation (CO2,CH4) upstreaming in the neighbourhood of these deep

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Fig. 4a. TE mode resistivity (Rho) and phase (ϕ) pseudosection tectonic zones. This ﬂuid contains a great quantity of silicate solution. Isotope ratio of CO2 carbon indicates a deep origin of CO2. Conductivity anomalies, as mentioned, appear only in TE mode (Rhomin) sounding curves and in the induction vectors (see Figs 2a, b, c for No. 35). The real part of the induction vectors is less than 0.1. As a function of the period, there is a sig- niﬁcant change both in the ratio of the real and imaginary parts and in the direction of the induction vector. Conductivity anomalies appear at measuring sites between No. 12 and 39 at depths between 7 and 10 km (at a mean depth of 8 km). Conductance anomalies

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Fig. 4b. TM mode resistivity (Rho) and phase (ϕ) pseudosection higher than 2000 siemens represent the “backbone” of the fractures. Outwards from these peaks the conductance decreases in any directions (Fig. 10). 1D inversion gave about 3 Ωm for the resistivity of the Balaton-line conductor and 20–30 Ωm for the R´aba- and Mid Hungarian lines. Taking into account the “backbones” (high conductance values) it can be stated that between sites No. 10 and 25 the crustal conductors are assumed to be in the continuation of these NE-SW directed great fractures, observed in the area of the Transdanubian Conductivity Anomaly (TCA) (Ad´´ am 2001). Moving southeastward along the proﬁle, there are high conductance anomalies beween sites No. 27–30 and No. 33–34 corresponding to the Balatonf˝o-

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Fig. 5a. Stitched layer section on the basis of the 1D inversion of the TE mode (Rhomin) sounding curves

Fig. 5b. Stitched layer section on the basis of the 1D inversion of the TM mode (Rhomax) sounding curves

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Fig. 6. 2D inversion of the TE mode sounding curves with the tectonic indications derived by Kiss (2005) from gravity (black dots) and magnetic data (red dots)

Fig. 7. 2D inversion of the TM mode sounding curves with the tectonic indications derived by Kiss (2005) from gravity (black dots) and magnetic data (red dots) and Balaton-line according to 1D inversion. The 2D inversion cannot separate these anomalies (Fig. 6). It seems that the side eﬀect of the TE mode sounding curves is stronger in the 2D inversion than in case of the 1D one. As shown in the proﬁle of the real values of the induction vectors (Wiese convention) obtained between periods 102 sand103 s, the low values (generally less

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Fig. 8. Tectonic map of Hungary (Haas 2001) than 0.1) appear continuously between sites 12 and 39 (Fig. 12), showing a united effect of the TCA fractures Balatonf˝o- and Balaton-line. The same feature is supported by the large TE mode side effect, as well. The behaviour of a series of conducting dikes in TE mode sections is also shown by Ad´´ am and Zalai’s (2000) model, where the conducting dikes are becoming a part of a quasi conducting layer. The original location of the dikes is referred only from some local peaks (that is from undulations in the thickness or conductance). The Balaton-line has already been detected by Varga (1979), as a 10–15 km wide conducting zone at a depth of 7–9 km south of the eastern basin of the Lake Balaton, separately from the TCA effect. The minor axis of the Zxy polar diagrams — disregarding a few exceptional cases — is directed towards E-W, NE-SW, corresponding to the longitudinal fractures in Transdanubia. The direction of Rhomin curves was found as the strike direction (TE-mode) by using the WinGLink software. In the TM mode inversion (Fig. 7), where the TE mode indicates conductivity anomalies (Fig. 6), resistive bodies appear. This phenomenon can only be explained by the effect of electric charges on the surface of imperfect 2D conducting fractures (dikes), where the resistivity strongly changes. (We return to this question when interpreting the 2D bimodal inversion.) The depth of the conductor in the fractures, as mentioned, is about 8 km. Due to the low viscosity fluid (and/or electronic conduction [graphite?]), the transition zone between the rigid and ductile zones becomes much shallower than without these materials (Ad´´ am 1987a). It would be interesting to study their effect on the earthquake distribution. Note that the same conductor depth was found in the seismically active Diósjen˝o tectonic line (Ad´´ am et al. 2003).

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Fig. 9. MT sounding curves measured in Gyürüs, representing a transition between the Trans- danubian Conductivity Anomaly (TCA) and the CEL-7 profile

3.4 Data about the asthenosphere

About the half of the TE and TM mode sounding curves measured with a maximum period T = 1000 s give information about the conducting asthenosphere by using 1D inversion. The depth estimation for the asthenosphere changes from region to region. At the northwestern part of the proﬁle (between sites 7 and 49) the depth values are as follows: in case of the TM mode: 82 ± 17.5km TE mode: 54.3 ± 15.8km. The ratio of these depth values (anisotropy): 1.5. At the southeastern segment of theproﬁle(betweensites50and72) the depth is in case of the TM mode: 51.2 ± 9.6km TE mode: 36.2 ± 19.6km their ratio: 1.4.

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Fig. 10. Conductance values calculated by using 1D inversion of the TE mode sounding curves. The peaks correspond to the “backbones” of the fractures

The average values calculated from the all values: in case of the TM mode: 63.3 ± 22.8km TE mode: 45 ± 19.8km their ratio: 1.4. The difference in the asthenospheric depth between the northwestern and southeastern segment of the CEL-7 profile cannot be explained by any static shift effect, caused by possible resistivity variations in the sediments. The mean value of sediment resistivity is 10.3 ± 4.4 Ωm at sites 1–49 and it is 11.7 ± 2.4 Ωm at sites 50–72 in average. Nevertheless, there is an essential difference in the asthenospheric resistivity values between the two regions and modes: For sites 1–49: TM mode resistivity: 176.6 ± 114 Ωm (16 data) TE mode resistivity: 9.4 ± 8.4Ωm(17data) For sites 50–72: TM mode resistivity: 75.6 ± 16.9Ωm(17data) TE mode resistivity: 50.6 ± 11.4 Ωm (23 data). Higher asthenospheric depth values (in case of both modes) at the northwestern segment of the CEL-7 profile — following the Ad´´ am’s formula (1978) — accompany with lower heat flow values (84 and 94 mW/m2), while in the southeastern part of the profile, where the heat flow is higher (103, 104, 108, 139 mW/m2) the asthenosphere is at shallower depths (Dövényi et al. 1983). These values give valuable contribution to the asthenospheric map (Ad´´ am and Wesztergom 2001). We assume on the basis of Ledo (2000) — that due to a lot of 3D near- surface structures in the measuring area — the TM mode asthenospheric depth values have more probability. (This “paradigm” is supported by numerical mod- elling, carried out among others also by Berdichevsky and Dmitriev (1976) and Ad´´ am et al. (1993).)

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Fig. 11. The bimodal (TE+TM) inversion proﬁle of the CEL-7 soundings

The upwelling asthenosphere in the southeastern segment, i.e. the nearby Dráva subbasin — similarly to the Békés subbasin (Posgay et al. 1995, Ad´´ am et al. 1996) — can be explained by extensional forces during the Miocene rifting of the basins. The ratio of the depth values derived from the TE and TM modes (1.4–1.5) does not differ from the characteristic mean value of the Pannonian Basin. Its origin (whether it is of near-surface or deep origin) is to be discussed. Finally, we discuss the possible cause of the high resistivity of the TM mode asthenosphere in the northwestern profile segment, where the Balatonf˝o- and Balaton- line conductors lie. The anomalously high resistivity values may be due to electric charge accumulation at the surface of the conductors (see later).

3.5 Characteristics of the induction vectors The small real induction vectors (≤ 0.1) measured in the period range of 102 and 103 s between sites 12 and 39, represent a large conductive body (block), which — according to the TE mode rules — shows the integrated effects of all nearby conductors (TCA, Balatonf˝o-, Balaton-line). At its nothwestern part (see sites 1, 7, 8, 9, 10 and 11) greater values than 0.1 were observed, but this increased value only at site 1 (Szentgotthárd) can be explained by the near-surface metamorphic schist with a shallow sediment above it (Ad´´ am and Koppán 2004). At the other sites, the increased length of the vectors is already due to the periphery of the conductive block (Rokityansky 1982, Takács et al. 2005). At the southeastern rim of the integrated “conducting block” appears an almost symmetric counterpart of the northwestern increased values of the induction

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Fig. 12. a) Magnetic (total field) anomaly along the CEL-7 profile, b) Gravity anomaly along the CEL-7 profile, c) Zxy max/Zxy min anisotropy and TM apparent resistivity values at T = 800 s, d) The length of the induction vectors at periods between 102 sand103 s and their direction vectors (Fig. 12). Since the conductance in the Mid Hungarian line (fracture) is much less than that in the Balatonf˝o- and Balaton-line, the induction vectors are also longer here. There is a change in the direction of the induction vectors in the middle part of the profile, where the “Alpine direction” (W) (Ad´´ am 2002) turns to the Trans- danubian (SE) one (Ad´´ am and Koppán 2004) (Fig. 12). The anisotropy — the ratio between the resistivities of the two modes — decreases only at the site 56, but it does not disappear completely even at the end of the profile. The diagonal elements of the impedance (Zxx,Zyy)herearemuchless than in the highly anomalous and distorted Balaton region.

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4. The bimodal 2D inversion

At first individual TE- and TM mode 2D inversions were carried out, where (Figs 6 and 7) the strike direction was selected on basis of fracture tectonics. The 2D TM mode inversion — similarly to the TM mode 1D inversion — provides reliable images about the basement structures. At the same time, the TE mode inversion merges into one great conductor the Balatonf˝o- and Balaton-line and their populations, as a consequence of the large TE mode side effect. In the measuring area the western continuation of the Transdanubian Central Range also appears as a conductor (sites 12, 13), which may correspond to the middle fracture line (TCA2) in paper by Ad´´ am (2001), running in NE-SW strike slip direction through Transdanubia. The Zagreb-Hernád line seems to be indicated as a conductor around sites 48–50. At the beginning and at the end of the profile the high-resistivity crystalline schists give a frame to the more conductive structures. In the TM mode inversion resistive bodies are obtained even in the place of the tectonic lines. This type of distortion — certainly due to the charge accumulation at sharp resistivity changes — is known from the COPROD2 data processing results (Jones 1993), or e.g. from the paper of Wu et al. (1993) where the behaviour of the two modes are similarly very different. The question arises what is the role of the two modes in a bimodal (TE+TM) inversion? As can be seen in the pseudosections corresponding to one conducting dike model (Ad´´ am 1987b), the TM mode locates the dike very well, while the TE mode can give a very good resistivity estimation for the dike. These features were described by Wu et al. (1993), and also by Ad´´ am and Zalai (2000). In case of CEL-7, in the bimodal (TE+TM) inversion profile phantom structures: highly resistive bodies appear even in places of conductive fractures as derived from the TM mode inversion (Fig. 11). These phantoms surround the TE+TM inverted conductors (see Fig. 5 by Wu et al. 1993). Therefore it is difficult to separate the true or false phantom resistive bodies in the bimodal inversion. In Fig. 11 in the place of the TCA2 fracture a resistive body appears, but the most questionable phantom is the one, which substitutes the conductive Balaton anomalies (around the sites 33–34), where 1D inversion gave conductance values about 2000 siemens, and the length of the induction vectors is almost zero, and furthermore the Zxy max/Zxy min anisotropy has one of the highest values along the profile. The conductors are indicated the best by using either 1D or 2D TE mode inversions. In case of the CEL-7 profile the role of TM mode in the bimodal inversion needs further investigations. Kiss (2005) calculated the discontinuity surface along the profile using Werner’s (1953) deconvolution of gravity and magnetic data (Fig. 11). His results are shown in both TE and TM inversion profiles (Figs 6 and 7). The gravity discontinuities marked by black points well correlate with the 1D conductance section (Fig. 10). The magnetic discontinuities (red points) rarely agree with the MT results, since the deep fractures, mainly filled with geothermal fluid and gas (CO2,CH4), do not contain materials of high magnetic permeability. (Maybe the effusive rocks of the Mid-Hungarian line may contain magnetic material.) Nevertheless, Kis et al. (1999) found some relation between the deep fractures and the vertical magnetic anomalies.

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5. Summary Deep magnetotelluric soundings along the CEL-7 seismic deep refraction line were carried out with the same spacing distance as the seismic measurements. The magnetotelluric results provide information about the very complicate basement structure and deep fracture tectonics in the SW border of Hungary, where three great tectonic units: Pannonian Basin, Eastern Alps and Dinarides meet in a seismically active area. This detailed investigation was able to detect the shallow ductile zones in the great fractures at depths of about 8 km, which certainly influences the depth dependence of earthquake distribution. The depth to the conductive asthenosphere — as shown by using 1D inversion of about the half of the sounding curves — well corresponds to the heat flow variation in the region: its shallowest depth value is obtained in the extensional Dráva basin (similarly as it had been found in the Békés graben). Various images and inversion results provided information about the near-surface sedimentary complex, the basement structures and the deep tectonics. Besides the MT soundings, the induction vectors were also involved into the 2D inversion, being very sensitive to conductivity inhomogeneities. The authors emphasize the special role of the TE and the TM mode inversions, and they call the attention to a possible disturbing effect of the TM mode in the bimodal (TE+TM) inversion, since TM mode generates false structures (phantoms) in form of high resistive bodies around and between the conducting fractures. Discontinuities derived from gravity data (and partly from magnetic ones) are in correlation with the conductive anomalies.

Acknowledgements The research work was carried out in the frames of the project No. T37694 of the Hungarian Scientific Research Fund Programmes (OTKA). The magnetotelluric field measurements were partially supported by OTKA project No. TS 408048, too. The authors highly acknowledge the possibility to use the MT pool instruments of GFZ Potsdam, they are grateful to O Ritter and U Weckmann for their kind co-operation. The authors acknowledge the valuable contribution of V Wesztergom (GGRI) and A Madarasi and G Varga (ELGI) in the field measurement. Their interpretation about the same dataset will be published in another paper.

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