Tectonic Weak Zones Determined by Magnetotellurics Along the Cel-7 Deep Seismics Profile
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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 magnetotel- luric 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 indica- tion 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 astheno- sphere 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 re- sults. 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) sound- ings 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 in- strument 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 measur- ing 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. Acta Geod. Geoph. Hung. 40, 2005 MAGNETOTELLURICS ALONG THE CEL-7 PROFILE 415 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) Acta Geod. Geoph. Hung. 40, 2005 416 A AD´ AM´ et al. 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” (Kil´enyi and Sefaraˇ 1989) (Fig. 3). The measuring profile CEL-7 was selected after the proposal by N´emeth (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 N´emeth (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, Acta Geod. Geoph. Hung. 40, 2005 MAGNETOTELLURICS ALONG THE CEL-7 PROFILE 417 — continuation of the Mesozoic, mainly carbonatic, thousands m thick mass of the Transdanubian Central Range at Salomv´ar-Nagylengyel oil field (Zala basin), — Pre-tertiary graben line structure at Nova and Bak, — Mesozoic carbonatic highland at Hah´ot, — Balaton-line as a Pre-tertiary graben structure (Nagykanizsa, Olt´arc, Budafa) as continuation of the Periadriatic lineament in the Alps, — Mesozoic horst as continuation of the Kalnik Mts (at Nemesp´atr´o, Lisz´o, Pat), — Mid-Hungarian tectonic line (also known as Zagreb-Hern´ad, or Kapos line) which separates the two great terranes: Alcapa and Tisza composing the Pannonian Basin, — the Gy´ek´enyes-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) sound- ing 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 informa- tion 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 Gy¨ur¨us (Gy) (Fig. 9). Acta Geod. Geoph. Hung. 40, 2005 418 A AD´ AM´ et al. 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).