The crustal and upper-mantle geophysical signature of narrow continental rifts in the Pannonian basin

Transcription

1 Geophys. J. Int. (1998) 134, 157^171 The crustal and upper-mantle geophysical signature of narrow continental rifts in the Pannonian basin A. Aè da m 1 and M. Bielik 2 1 Geodetic and Geophysical Research Institute of the Hungarian Academy of Sciences, H^9401 Sopron, POB 5, Hungary 2 Geophysical Institute, Bratislava, Dübravska cesta 9, Slovakia Accepted 1998 January 14. Received 1998 January 7; in original form 1996 June 1 1 INTRODUCTION Under extensional forces linear tectonic features such as fractures, faults and shear zones widen into grabens, which subside during the accumulation of sediments. The extension causes rifting on the surface and at the same time it may trigger material motion, physical changes in the Earth's crust and upper mantle: the mantle material penetrates into the rifts from below. This results in upwelling of the Moho, and the accompanying heat transfer lifts the isotherms and therefore the partially molten asthenosphere moves nearer to the surface. This complex phenomenon can also be initiated from the opposite direction, i.e. mantle convection. In this latter case the upwelling asthenosphere (mantle plume) triggers the rifting. The result is a `narrow rift'. The narrow rift has been described, for example, by Bott (1982), Lachenbruch & Morgan (1990), Buck (1991) and Mechie, Fuchs & Altherr (1994). It represents one of the modes of continental extension tectonics. Its large-scale features are shown in Fig. 1 after Buck (1991) but modi ed with rifting-related intrusion into the lower crust after the authors mentioned above. The rift depression is accompanied by upwelling of the upper mantle (Moho) and asthenosphere. In addition to this uparching, narrow rim uplifts may border the rift as a result of the fault movements. The upwelling asthenosphere plays a decisive role in the plate SUMMARY The Pannonian basin provides a very good opportunity to study extensional tectonics. The image of narrow continental rifts, which represent one of the modes of continental extensional tectonics, was studied in detail in the Be ke s basin. In order to constrain the lithosphere structure beneath the narrow rifts in the Pannonian basin, density models and interpretation of magnetotelluric measurements along the Pannonian Geotraverse are presented. The results obtained agree with the most recent deep seismic data. Most of the narrow rifts in the Pannonian basin are characterized not only by thinner crust but also by thinner lithosphere. A typical phenomenon of the gravity eld over the central part of the narrow rifts in the Pannonian basin is the existence of a relative local gravity high. These gravity highs are probably due to intrusions of high-density masses into the lower crust and lower part of the upper crust beneath the narrow rifts, which are related to the extension of the basins (subbasins). Key words: asthenosphere, continental rift, extension, gravity, lithosphere, magnetotellurics. Figure 1. Scheme of a narrow rift mode of continental extensional tectonics for the Pannonian basin (modi ed after Bott 1982; Buck 1991; Lachenbruch & Morgan 1990; Mechie et al. 1994). motion/mechanism, too (e.g. Ziegler 1992). Therefore the study of these narrow rifts in a suitable area can deepen our knowledge of extensional tectonics, including plate tectonics. It seems that the Pannonian basin (Fig. 2) is a very ß1998RAS 157

2 158 A. Aè da mandm.bielik Figure 2. A simpli ed tectonic map of the Eastern Alpine^Carpathian^Pannonian basin region (from Lillie et al. 1994). The cross-section of pro le A^A' presented in this study is shown by the line. Key: AM~Apuseni Mts, BB~Be kës basin, DB~Danubian basin, DT~Dräva trough, GB~Styrian basin, GHP~Great Hungarian Plain, J~Jäszsa g basin, MB~Makö basin, ST~Säva trough, TB~Transylvanian basin, TCB~Transcarpathian basin, VB~Vienna basin, ZB~Zala basin, 1~East Slovakian basin, 11~South Slovakian^North Hungarian basin, PGT~Pannonian Geotraverse (after Posgay et al. 1995); 2T~pro le 2T (after Tomek et al. 1989). suitable region for studying the image of both narrow and wide continental rifts. Moreover, new deep re ection seismic data (Posgay, Hegedu«s& T ma r 1990; Posgay et al. 1995), gravity data (Sza än, Horväth & Cloetingh 1997) and magnetotelluric results (Aè däm, Szarka & Steiner 1993; Aè da m et al. 1996) allowed an improvement of the former density models (Bielik 1990, 1991). 2 THE STUDY AREA: THE PANNONIAN BASIN In the Pannonian basin (the central part is located in Hungary) there are signi cant narrow deep subbasins that have been generated by the Neogene extensional forces. Tari, Horväth & Rumpler (1992) interpreted a series of seismic sections obtained for oil exploration and deep borehole data to determine the character of the Miocene extensional structures. Among the narrow rifts, two twin subbasins, the Makö and Bëke s grabensöseparated by the Battonya highöin the Great Hungarian Plain (Fig.2), have attracted much attention in recent times from geophysicists. The width of the Bëkës graben is about 40^60 km, and the depth to its bottom (resistive basement) or thickness of the sediments reaches about 7 km. These dimensions ful l the notion of a narrow rift. Similar to these subbasins are those in Fig. 3 having a black core in a schematic isopach map (indicating that the basin depth is greater than 4 km). 3 GEOPHYSICAL OBSERVATIONS IN THE AREA OF THE BEè KEè S GRABEN 3.1 Gravitational investigations Previous studies of the Bouguer gravity anomalies in the Pannonian basin were published by Stegena (1964), Renner & Stegena (1965) and Meskö (1983, 1988). Their interpretations provided the rst information on the crustal structure. Fig. 4 shows the Bouguer gravity anomaly map of the Be kës basin area. The map was drawn after Sza än et al. (1997). The anomaly values are in milligals (1 mgal~10 {5 ms {2 ). An interesting nding is that, in spite of a signi cant thickness (almost 7 km) of the sedimentary basin ll, the Be kës basin is characterized by a gravity high (more than z30 mgal). The maximum is obtained near Socodor in Romania. The anomaly direction is NW^SE. The axis of the anomaly correlates well with the basin direction. The construction of the stripped gravity map of the Pannonian basin, i.e. gravity eld corrected for the gravity e ect of the sedimentary basin ll (Bielik 1988a), was stimulated by the fact that its basement is almost completely covered with Neogene sediments, the thickness of which reaches 7^9 km in certain places. The deepest subbasins of the Pannonian basin correspond to the largest gravity highs (Bielik 1988b). It was suggested (Bielik 1991) that the largest subsidence is characterized by intrusions of higher-density upper-mantle material into the lower crust and lower part of ß1998RAS,GJI 134, 157^171

3 Narrow continental rifts in Pannonian basin 159 Figure 3. Simpli ed isopach map of the Neogene^Quaternary formations (after Jiric ek 1979). Key: 1~1 and 2 km depth isolines, 2~3 and 4 km depth isolines, 3~Pre-Neogene outcrops, 4~Flysch Carpathians and timing of their major deformations, 5~Foredeep Molasse. the upper crust. Similar results have been obtained by Nemesi & Stomfai (1992), Kova csvo«lgyi (1994, 1995) and Sza a n et al. (1997). Bielik (1991) suggested that in all extensional subbasins of the Pannonian basinöin accordance with the narrow rift modelöthe lower crust and the Moho are in an elevated position. Lillie et al. (1994) found that, besides the most prominent anomalous bodies (Moho, sedimentary thickness and topography) re ected in the long-wavelength gravity eld, the lithosphere/asthenosphere boundary also has an in uence upon the regional gravity eld in the Pannonian basin. Thus previous gravity studies that did not take the asthenosphere into account must have distributed its e ect elsewhere in the crust (e.g. through combined e ects of thicker sediments, lower-density sediments, less relief on the Moho, or a lower density contrast across the Moho). In our study we suggest a density contrast between asthenosphere and lower lithosphere ({0.03 g cm {3 ) based on consideration of local isostatic equilibrium (Lillie et al. 1994). Although the nature of the transition from lithosphere to asthenosphere is not well understood, Lillie et al. (1994) suggested that the chosen density contrast accurately portrays the mass de ciency represented by the asthenosphere. Therefore the low-density asthenosphere corresponds to the low-velocity zone used to determine the depth to the `seismic' lithosphere/asthenosphere transition (Posgay 1975; Babus ka et al. 1988). Figure 4. Bouguer gravity anomaly map of the Be kës basin area (after Sza än et al. 1997). Contour interval is 5 mgal. 3.2 Seismic investigations The newest results of the seismic re ection soundings have been published most recently by Posgay et al. (1995). In that paper the upwelling of the Moho and the asthenosphere is described. They refer also to the above-mentioned gravity and magnetic modelling. According to the seismic models ß 1998 RAS, GJI 134, 157^171

4 160 A. Aè da mandm.bielik the Moho is elevated by 5^6 km above the regional level (to a depth of 22 km from 28 km), and the asthenosphere is supposed to be 12^20 km nearer to the surface than its average depth of 60^70 km in the hot Pannonian basin (Aè da m 1965). Posgay et al. (1995) show curved re ecting horizons under the Be kës graben, and assume the upper boundary of the asthenosphere where the mantle becomes transparent (no re ection) due to the strong energy drop (and to the velocity decrease) in partially molten asthenosphere (see Fig. 18). 3.3 Magnetotelluric results Posgay et al.'s (1995) paper includes the RRI 2-D inversion (Smith & Booker 1991) results of the magnetotelluric (MT) data measured by the Eo«tvo«s Geophysical Institute (ELGI) (Budapest) along the PGT1 pro le with an upper period of about 1000 s. According to this inversion, the conducting asthenosphere beneath the Be kës graben lies at a depth of about 40 km, i.e. much shallower than regionally in the Pannonian basin (60^70 km). Taking into account the analytical results of Berdichevsky & Dmitriev (1976) concerning the distortion e ect of a surface 3-D conducting embedding (i.e. sedimentary basin) on the MT o curves, the question emerges to what extent Posgay et al.'s (1995) 2-D inversion results are in uenced by galvanic distortion. Both our thinsheet 3-D (Aè däm et al. 1993) and 3-D forward modelling using Wannamaker, Hohman & Ward's (1984) program (Aè da m et al. 1996) con rmed the existence of such a distortion of the MT o curves, which apparently decrease the depth of the conducting basement, i.e. of the asthenosphere. (See the plane asthenosphere in Fig. 7 characterized by 1 ) m.) Against the distortions of MT curves stated by these models, the following arguments favour the upwelling of the asthenosphere in the Be kës graben. (1) Because in the case of a 3-D surface structure (Be kës graben) the MT curves of E and B polarization cannot be clearly separated, the best approximation of the layer structure including the conducting basement (asthenosphere) could be obtained by calculation of the geometrical mean (h av )ofthe depths derived from o max and o min p curves (h omax and h omin ), similar to the o eff value, i.e. h av ~ h omax h omin as proposed by Berdichevsky & Dmitriev (1976). As the o min curves in the area of the Bëke s graben give smaller depth values than 40 km for the conductive basement (supposedly for the asthenosphere), even around 20 km in some cases (e.g. in the case of No. 7, see the site in Fig. 5), the h av value can also go down below 40 km (Aè däm et al. 1993). This di erence between o max and o min curves is generally caused by structural inhomogeneity and called MT anisotropy (Aè da m 1969, 1996). These results may con rm the probability of the upwelling of the asthenosphere below the Bëkës grabenöand at the same time the validity of the 2-D RRI inversion of Posgay et al. (1995). For demonstration the MT curves measured in the deepest part of the graben in MT site No. 7 are shown in Fig. 6 with their 1-D layer sequences. (2) According to the 3-D modelling by the program of Wannakamer et al. (1984) above the model of the Be kës graben (Fig. 7), the phase indication of the 3-D basin is clearly separated along the period axis from that of the asthenosphere. The indication of the asthenosphere is not distorted by galvanic e ects of the surface basin as in the case of the o curves. If the boundary of the asthenospheric layer is a plane, Figure 5. Isopach map of the eastern part of the Pannonian basin with the MT sites along the Pannonian Geotraverse (PGT1) (Aè däm et al. 1996). ß1998RAS,GJI 134, 157^171

5 Narrow continental rifts in Pannonian basin 161 Figure 6. (a) o min and (b) o max magnetotelluric sounding curves (o and r) and their 1-D inversion in MT site No. 7 (Elek). as it is in our model, the phase curves run together independently from sites and directions (Fig. 8). As the measured phase curves are di erent in two orthogonal directions at the same sites above the Be kës graben, as can be seen in Fig. 9, a 3-D upwelling of the asthenosphere can also be supposed. The reference station outside the graben is the MT site Tu rkeve (Tu) with the least phase anisotropy in Fig. 9. (3) There could be an interesting compensating e ect for the distortion of the o curves caused by the surface sedimentary basin if the asthenospheric boundary is not a plane but has a 2-D or 3-D structure, too, similar to the highly resistive basement in the Be kës graben. According to Berdichevsky & Dmitriev (1976), in the case of a 2-D sinusoidal conductive basement model (asthenosphere), the o curves of B polarization are distorted exactly in the opposite direction as in the case of the resistive basement of the sedimentary basin. As long as the o curves of B polarization give a shallower asthenospheric depth above a 2-D or 3-D sedimentary basin, the 2-D or 3-D elevation of the asthenosphere causes the opposite e ect, i.e. an apparent deepening of the asthenosphere. This compensation, of course, is model-dependent but may exist in thecaseofthebe kës graben. (4) Occam's inversion (Constable, Parker & Constable 1987) of the MT data measured by the Geodetic and Geophysical Research Institute (Sopron) along the PGT1 pro le with much longer periods than ELGI also con rmed the Figure 7. Depth meshes of the 3-D forward modelling representing the Be ke s graben and its geoelectric cross-section. ß 1998 RAS, GJI 134, 157^171

6 162 A. Aè da mandm.bielik [s] Figure 8. Phase curves calculated at points given by the x and y coordinates (see Fig. 7) along the major axis of the model representing the Bëke s graben: in magnetic strike direction (a) and perpendicular to it (b). upwelling of the asthenosphere (see Figs 15 and 16) (Aè da m et al. 1996). (See later in detail.) Of course, the nal and fully adequate solution could be obtained by 3-D inversion techniques. At present we do not have an appropriate method for 3-D inversion and in addition our data are distributed only along the Pannonian Geotraverse and not areally. Therefore the problem has been approximated by a 2-D inversion and 3-D forward program. [s] the sediments: `It was established that in the Be ke s basin the temperature of sediments had not reached the equilibrium state and that the heat ow density of 70^80 mw/m 2 may make up 65^80% of the equilibrium heat ow which for the area of the Be kës basin, is estimated to be 100 mw/m 2.' This value indicates already a shallower depth of the conducting asthenosphere on the basis of Aè da m's (1978) formula, which relates the asthenospheric depth to the heat ow. 3.4 Geothermal data The heat ows measured above the Be kës graben are a little less (70^80 mw m {2 ) than the regional average of the Pannonian basin. The di erence increases in comparison with some other subbasins like the Makö and Transcarpathian basins, where heat ow is about 100 mw m {2. This heat ow decrease is explained by Posgay et al. (1995) by the blanketing e ect of 4 MODELLING OF THE LARGE-SCALE FEATURES OF THE NARROW RIFT MODE IN THE EXTENSIONAL PANNONIAN BASIN The modes of continental extension tectonics from Buck (1991) have been applied and classi ed to the NW Pannonian basin by Tari et al. (1992). On the basis of our previous analyses we suggest that the Bëkës basin is the most available example of ß1998RAS,GJI 134, 157^171

7 Narrow continental rifts in Pannonian basin 163 Figure 9. Phase curves measured in the directions of o max and o min, i.e. in two orthogonal directions along the Pannonian Geotraverse in and near the Be ke s subbasin (see the measuring sites in Fig. 5; in the SE end of the Geotraverse). Figure 10. Isostatic column of the Be ke s basin, relative to `average lithosphere' of the Pannonian basin region. Density contrasts are for the topographic relief relative to air (z2.47 g cm {3 for sediments), and for the other zones relative to typical upper-crustal materials (sediments, {0.20; lower crust, z0.20; lower lithosphere, z0.50; asthenosphere, z0.47 g cm {3 ). Given the topography, sedimentary, upper- and lower-crustal thicknesses shown, column B predicts the relief on the lithosphere/asthenosphere boundary that would bring the region into local isostatic equilibrium. The depth of compensation is 60 km. The value of the `mass column' is 18: gcm {2. ß 1998 RAS, GJI 134, 157^171

8 164 A. Aè da mandm.bielik the narrow rift mode extension in the Pannonian basin. To demonstrate this statement we have to take into consideration that: a new, uni ed Bouguer anomaly map has been compiled by Sza a n et al. (1997); new knowledge has been obtained by density modelling of long-wavelength gravity anomalies taking into account depth variations of the lithosphere/asthenosphere boundary (Lillie et al. 1994); deep seismic soundings along the Pannonian Geotraverse PGT1 have been done (Posgay et al. 1990, 1995); new magnetotelluric deep soundings have been carried out (Aè da m et al. 1993); and new crustal and lithospheric maps (Horväth 1993) have been constructed. For modelling relatively long-wavelength gravity anomalies, models are simpli ed into elements representing major density contrasts (Fig. 10). The analysis of the long-wavelength gravity anomalies by means of the local isostasy has been done without taking into account the high-density intrusions, because they represent the short-wavelength gravity anomalies. Density contrasts are related to upper crustal material. The four anomalous bodies and their density contrasts relative to the upper crust are (Figs 10 and 11): (1) Neogene Pannonian basin sediments ({0.20 g cm {3 ); (2) lower crust (z0.20 g cm {3 ); (3) mantle part of lithosphere (z0.50 g cm {3 ; i.e. z0.30 g cm {3 contrast with the lower crust); (4) asthenosphere (z0.47 g cm {3 ; i.e. {0.03 g cm {3 contrast with the mantle lithosphere). Average values for topographic height, depth to basin, and upper and lower crustal thicknesses were determined from the published papers mentioned above. For the Bëkës basin region, the assumed density contrasts, average topographic height of 0.05 km, depth of 6.5 km for the basin, and 2 and 4 km relief on the lower crust and Moho, respectively, yield a rise of the asthenosphere by about 19 km. A value of 41 km is the lithospheric depth beneath the Be kës basin that results in local isostatic equilibrium for {0.03 g cm {3 contrast between the asthenosphere and lower lithosphere (column B in Fig. 10), consistent with the depth to the lithosphere/asthenosphere boundary determined by Posgay et al. (1995). Changes in gravity due to changes in topography, sediment thickness and depths to the upper/lower crust, crust/ mantle and lithosphere/asthenosphere boundaries are shown in Fig. 11. The Moho and the shallowing of the lower crust contribute approximately z40 mgal to the free-air gravity anomaly (Fig. 11a). The lower topography (about 0.05 km lower than the surrounding region) and the sedimentary basin ll, which averages about 6.5 km (Fig. 10, column B), contribute about {32 mgal (Fig. 11b). The rise of the Figure 11. Local isostatic model for the Be ke s basin region (Fig. 10, column B), illustrating contributions to gravity from di erent levels. Thicknesses of the upper crust, lower crust and lithosphere outside the Be kës basin are assumed to be 17, 27 and 60 km, respectively (Fig. 10, column A). (a) The Moho and lower crust shallowing contribute about z40 mgal to the gravity anomaly. Key: 1~the Moho upwelling by about 4 km contributes about z28 mgal, 2~the anomaly due to the lower crust, which on average rises about 2 km, is approximately z12 mgal. (b) The topographic depression and sedimentary ll of the Bëke s basin together contribute about {32 mgal. (c) The rise of the asthenosphere by about 19 km provides about {8 mgal to the gravity eld (the asthenosphere density contrast of {0.03 g cm {3 is relative to the lower lithosphere). (d) The sum of the three components yields the free-air gravity anomaly. The free-air anomaly (about {3 mgal) predicted agrees well with those observed at the Bëkës basin (Lillie et al. 1994). Computations for isostatic columns A and B are shown in Fig. 10. ß1998RAS,GJI 134, 157^171

9 Narrow continental rifts in Pannonian basin 165 asthenosphere by about 19 km provides the remainder of the isostatic compensation for the shallow mantle and upper crust, contributing about {8 mgal to the gravity eld (Fig. 11c). The total of the three components gives free-air gravity anomalies of about {3 mgal over the Be kës basin (Fig. 11d), close to the values observed (Lillie et al. 1994). In order to constrain the lithospheric structure, 2-D density and electric resistivity distributions are presented along pro le A^A' and pro le PGT1, respectively (Fig. 2). The pro le A^A' is almost identical with the deep seismic pro les 2T (Tomek et al. 1989) and PGT1 (Posgay et al. 1990). Besides the Bëke s graben both pro les cross another deep subbasin, the so-called `Ja szsäg subsidence'. The thickness of its sediment cover is about 4 km (see `J' in Fig. 2, and Fig. 5). A perspective of the current structure of the lithosphere in the Be kës basin region along the pro le A^A' is illustrated in Fig. 12. Although we are mainly interested in this area it has been extended onto the territories of Poland, Slovakia and Romania to demonstrate that the Pannonian basin and the Carpathian arc provide a good opportunity to study the evolution of an extensional basin in a regime of overall convergence of lithospheric plates, and a surrounding compressional mountain chain. Gravity modelling was performed with the GM-SYS programs developed by Northwest Geophysical Associates Inc. Thicknesses of the Pannonian basin sediments are from the map by Jir ic ek (1979) and Kile nyi&sí efara (1989). The thickness of sediments in the Carpathian foreland and thickness of Tatricum are from deep seismic pro le 2T (Tomek et al. 1989). Depths to the upper/lower crust boundary were deduced from Bielik et al. (1990). Depths to the Moho and lithosphere/ asthenosphere boundary were taken from Horva th (1993). Figure 12. Two-dimensional gravity model of the lithosphere along pro le A^A'. Density contrasts are in g cm {3.Key:1~observed anomaly, 2~calculated anomaly, 3~gravity e ect of the Pannonian basin sediments removed, 4~gravity e ect of the asthenosphere removed, 5~gravity e ect of the asthenosphere, 6~gravity e ects of the intrusions. ß 1998 RAS, GJI 134, 157^171

10 166 A. Aè da mandm.bielik Density contrasts of the di erent bodies are from Sza a n et al. (1997). The lithospheric structure beneath the Be ke s basin (Fig. 12) is accompanied by extension not only of the crust (including the upper crust) but also of the lithosphere. To improve the t between the observed and calculated local gravity high over the Be ke s basin we have to assume a striking highdensity anomalous body (density contrast z0.30gcm {3 ). It is located within the lower crust when its apical part reaches a depth of 10^15 km. The maximum 2-D gravity e ect of the anomalous body is z29.5 mgal. On the basis of the results published by Posp s il (1980), Bielik (1991), Nemesi & Stomfai (1992) and Posgay et al. (1995) we suggest that the highdensity anomalous body could be explained by intrusions of upper-mantle (basic and ultrabasic) material into the lower crust and the lower part of the upper crust. We note that these high-density rocks have not been found by boreholes in the Pannonian basin up to the present time. Only in the pre- Tertiary basement of the East Slovakian basin (which belongs to the westernmost part of the Transcarpathian basin) was a complex of metabasalts found within the In ac evo^kric evo unit (Biron et al. 1993). Moreover, no boreholes in the Pannonian basin, which reached the basin basement, revealed this type of rock within the Neogene sediments. Only the Neogene acidic calc-alkaline volcanic rocks were found in the sedimentary ll of several subbasins of the Pannonian basin including the area of the Be ke s subbasin, also indicating its tectonic activity (Biela 1978; Pëcskay et al. 1995). As their thicknesses are very thin and they are located at the bottom of the subbasins, the acidic calc-alkaline volcanic rocks cannot be a source of the mentioned gravity highs. That is why we must look for a source of the gravity highs beneath the pre-tertiary basement. A practically similar result was obtained, independently, by Sza a n et al. (1997). Moreover these authors gave a detailed discussion about the reason for this signi cant mass surplus. After their view, the most plausible mechanism is that Moho updoming and signi cant crustal extension in the Be ke sbasin are accompanied by intrusions (mantle-derived ma c melts) into the thin crust during the Neogene. We think that this conclusion is indeed very likely. Fig. 12 also shows the Bouguer gravity anomalies corrected by the gravity e ect of the Pannonian basin sediments and of the asthenosphere. The gravity e ect of the lithosphere/ asthenosphere boundary changes from 0 to almost {60 mgal. These results indicate that the con guration of this boundary is also an important component in modelling the gravity eld of the Pannonian basin. The gravity e ect of the asthenosphere upwelling beneath the Be kës basin for density contrast {0.03 g cm {3 is only about {8 mgal (Fig. 11c). As the Bëkës basin represents a 3-D geological structure, the gravity e ect of the model of Neogene sediments is treated as a 3-D direct gravimetric problem. The sedimentary basin ll is modelled by vertical prisms (Bielik 1988a). Individual geological bodies are divided into layers. Each layer is de ned by a contour line, upper and lower depth, and constant density. The mean density contrasts for sediments relative to the upper crust vary from {0.62 to {0.01 g cm {3. The particular depth intervals used for calculation were adopted by Bielik (1988a). The total gravity e ect of anomalous bodies is obtained by summation of the gravity e ects of n-sided vertical prisms with horizontal bases. The computations were carried out using Sm s ek, Planc a r & Krs äk's (1970) formula. The result of Figure 13. Map of the 3-D gravity e ect of the Be kës basin sedimentary ll. Contour interval is 5 mgal. computations, carried out in a square km 2 grid, is the map of the gravity e ect of sediments in the Bëke s basin (Fig. 13). The amplitude of the gravity e ect reaches values of about {50 mgal. On the basis of the computed gravity e ect, we then corrected the map of the observed Bouguer anomaly (Fig. 4). An interesting nding is the magnitude of the stripped gravity map (Fig. 14) over the Be kës basin. The gravity high Figure 14. Bouguer gravity map corrected by the gravity e ect of the Be kës basin ll (stripped gravity map). Contour interval is 5 mgal. ß1998RAS,GJI 134, 157^171

11 Narrow continental rifts in Pannonian basin 167 reaches more than z80 mgal. By comparison with the stripped gravity map calculated by Bielik (1988a,b), it was found that the Bëke s basin is characterized by the largest gravity high in the whole Pannonian basin region. We stress that the centre of the anomaly is shifted northwestwards in comparison with the centre of the observed Bouguer anomaly (Fig. 4). It lies at the boundary between Hungary and Romania. As the contribution of the asthenosphere upwelling is negative, we suggest that the source of this gravity high is a signi cant mass surplus represented by Moho updoming and signi cant crustal extension accompanied by intrusions of mantle material into the thin crust. Burda, Vyskoc il & Hu«bner (1988) calculated the 3-D gravity e ect of the lithosphere/asthenosphere boundary for a density contrast of {0.01 g cm {3 in Central Europe. Moreover, Bl z kovsky et al. (1994) also published a 3-D gravity e ect of the Moho discontinuity relief for a density contrast of z0.1 g cm {3 and reference depth 30 km in the eastern part of Central Europe. If we correct the stripped gravity map in the Be ke s basin (Fig. 14) by recalculated gravity e ects of the lithosphere/asthenosphere boundary and Moho for density contrasts of {0.03 and z0.30 g cm {3 (using the maps published by Burda et al. (1988) and Bl z kovskÿ et al. (1994)), we obtain for the 3-D maximum gravity e ect of the highdensity anomalous body values of about z25 to z35 mgal. This rough estimation is compatible with the results obtained by means of 2-D density modelling. The magnetotelluric method as an inductive one rst of all gives an image of the conductive structures in the Earth and hence, besides the sedimentary basin, of the asthenosphere, too. Both the o max and o min 2-D Occam's inversion pro les of the PGT1 (Figs 15 and 16) show the elevation of the asthenosphere beneath the Be kës graben and Ja szsäg subsidence. Nevertheless in the latter case the elevation is less in accordance with the basement depth (thickness of the sediment) of the rift zone (Aè da m et al. 1996). The inversion of the o min curves (yx) and corresponding impedance phase generally gives a shallower depth than the o max curves. Nevertheless, in the case of the Be ke s graben both polarizations indicate a depth of 40^50 km to the top of the asthenosphere with somewhat di erent resistivity values (21.5 and 32 ) m). A greater di erence between the two polarizations appears in the case of the Ja szsäg subsidence where the o min curves and their phases give about 65 km depth, and the o max curves about 75 km for the asthenosphere. Figure D Occam's inversion results of the o max curves along PGT1 (Aè däm et al. 1996). ß 1998 RAS, GJI 134, 157^171

12 168 A. Aè da mandm.bielik Figure D Occam's inversion results of the o min curves along PGT1 (Aè däm et al. 1996). We could not make an inversion for the Makö graben (see Fig. 5) because of the lack of data. Nevertheless, some kind of similarity between the Makö and Be kës grabens is expressed by the MT sounding curves. To illustrate this fact, o min and its phase curves measured in the site HOD (Fig. 5, near a deep borehole) are shown in Fig. 17 with a 1-D layer sequence. The Makö graben is a less developed extensional basin than the Bëkës graben regarding its deep structure. 5 CONCLUSION The large-scale features of the narrow continental rifts (subbasins) in the Pannonian basin correspond to those models elaborated by di erent authors (e.g. Bott 1982; Lachenbruch & Morgan 1990; Buck 1991; Mechie et al. 1994). One of the main characteristic features of this model is the intrusion of highdensity masses within the lower crust and lower part of the upper crust beneath the narrow rift (Fig. 1) as also found in the Be kës basin. Note that the lower crustal intrusions were found in almost all subbasins in the Pannonian basin (Bielik 1988a). These masses may be explained in terms of intrusions of mantle-derived ma c melts. The high-density intrusions were traced on the basis of a detailed gravity eld analysis over the Neogene basins. In spite of a signi cant thickness of the sedimentary basin ll, the narrow rifts are characterized by relative gravity highs. There are indications of upwelling of the asthenosphere, too, in both seismic re ection horizons and magnetotelluric 2-D inversion data. The results of local isostasy and density modelling for the Be kës basin also indicate the rising lithosphere/asthenosphere boundary. The local isostatic model suggests a rise of the asthenosphere by about 19 km. The Be kës basin is accompanied by about 2^4 km shallowing of the upper/lower crust boundary and about 4^6 km updoming of the Moho. The asthenospheric boundary has been determined by those curved seismic horizons below which the mantle becomes transparent (Fig. 18). Posgay's (1975) earlier study con rmed that the seismic velocity decrease is in this depth range in the Pannonian basin. It has been known since the mid-1960s (Aè da m 1965) that there is an asthenospheric diapir (appearing as an electric conductor) in the Pannonian basin determined by magnetotellurics (MT) at a depth of about 60 km. The further upwelling of this diapir beneath narrow subbasins (extensional rift) has been approximated by 1-D and 2-D MT inversions ß1998RAS,GJI 134, 157^171

13 Narrow continental rifts in Pannonian basin 169 Figure 17. MT sounding curves of MT site HOD1 with their 1-D layer sequences. and 3-D forward modelling and by very critical study of the electromagnetic distortions (Aè da m et al. 1996). It has been concluded that the probability of the upwelling (plume) of the asthenosphere below narrow subbasins is high. At present the 3-D inversion technique in magnetotellurics is in a very early stage of development and therefore it cannot be used for solving this problem. In conclusion it appears that as a whole the Pannonian basin re ects a wide rift mode extension, which was de ned by Buck (1991). This wide rift mode extension is also accompanied by about 9 km shallowing of the Moho and almost 60 km updoming of the asthenosphere (Lillie et al. 1994). In the frame of this regional wide rift mode, local narrow rift mode and core complex mode extensions can be observed. For the narrow rift mode the Bëkës graben is the best example in the Pannonian basin, as discussed in this paper in detail. In this way the whole region of the Pannonian basin represents a very complex system of continental extensional tectonics. After Cloetingh et al. (1995) it results from di erences in the pre-rift rheology of the lithosphere. ACKNOWLEDGMENTS The authors are indebted to anonymous referees who greatly helped to improve this paper by their important comments. AAè is grateful to the National Research Fund for the nancial support of the magnetotelluric investigations (OTKA Project T ) and MB thanks VEGA for the partial support of this work (Grant No. 95/5305/418). ß 1998 RAS, GJI 134, 157^171

14 170 A. Aè da mandm.bielik Figure 18. Interpreted line drawing of the migrated depth section of pro le PGT1 (Posgay et al. 1995). REFERENCES Aè da m, A., Einige Hypothesen Ïber den Aufbau des oberen Erdmanteles in Ungarn, Gerlands Beitr. Geophys., 74, 20^40. Aè da m, A., Appearance of electrical inhomogeneity and anisotropy in the results of the complex electrical exploration of the Carpathian Basin, Acta Geod. Geophys. Mon. Hung., 4, 187^ 197. Aè da m, A., Geothermal e ect in the formation of electrically conducting zones and temperature distribution in the Earth, Phys. Earth planet. Inter., 17, 21^28. Aè da m, A., Regional magnetotelluric (MT) anisotropy in the Pannonian basin (Hungary), Acta Geod. Geophys. Hung., 31, 191^216. Aè da m, A., Szarka, L. & Steiner, T., Magnetotelluric approximations for the asthenospheric depth beneath the Be kës Graben, Hungary, J. Geomagn. Geoelectr., 45, 761^773. Aè da m, A., Präcser, E., Szarka, L. & Varga, G., Mantle plumes or EM distortions in the Pannonian basin? (Inversion of deep magnetotelluric (MT) soundings along the Pannonian Geotraverse), Geophys. Trans., 40, 45^78. Babus ka, V., Plomerovä, J., & Pajdus äk, P., Lithosphere^ astheno-sphere in Central Europe: Models derived from P residuals, Proc. 4th EGT Workshop, European Science Foundation, pp. 37^48 Berdichevsky, M.N. & Dmitriev, V.I., Distortion of magnetic and electric elds by near-surface lateral inhomogeneities, Acta Geod. Geophys. Mon. Hung., 11, 447^483. Biela, A., Deep boreholes in the buried regions of the Inner Western Carpathians, Regional Geol. W Carpathians, 10, 1^224 (in Slovakian). Bielik, M., 1988a. A preliminary stripped gravity map of the Pannonian Basin, Phys. Earth planet. Inter., 51, 185^189. Bielik, M., 1988b. Analysis of the stripped gravity map of the Pannonian Basin, Geol. Zborn kögeol. Carpathica, 39, 99^108. Bielik, M., Two-dimensional crustal density models of the Carpatho-Pannonian region, in Advances in Gravimetry, pp. 45^50, ed. Sí korvanek, M., Geophys. Inst. of the SAS, Bratislava, Slovak Republic. Bielik, M., Density modelling of the Earth's crust in the Intra- Carpathian basins, in Geodynamic Evolution of the Pannonian Basin, pp. 123^132, ed. Karamata, S., Acad. Conf. Serbian Acad. Sci. Arts, Beograd, Yugoslavia. Bielik, M., Fusa n, O., Burda, M., Hu«bner, M. & Vyskoc il, V., Density models of the Western Carpathians, Contrib. Geophys. Inst. Slov. Acad. Sci., 20, 103^113. Biron, A., Sotäk, J., Spis iak, J., Bebej, J. & Magyar, J., `Bu«ndnerschiefer' metasediments of the Pozdis ovce-in ac ovce unit: metamorphic petrology data, Geol. Carpathica Ser. CLAY, 44, 65^79. Bl z kovskÿ, M., Sí efara, J., Burda, M. & Vyskoc il, V., Stripped gravity maps in Czechoslovakia, in Bucha, V. & Bl z kovskÿ, M., Crustal Structure of the Bohemian Massif and the West Carpathians, pp. 162^174, eds Academia, Praha. Bott, M.H.P., The Interior of the Earth: Its Structure, Constitution and Evolution, Edward Arnold, London. Buck, W.R., Modes of continental lithospheric extension, J. geophys. Res., 96, ^ Burda, M., Vyskoc il, V. & Hu«bner, M., Density inhomogeneities in the upper mantle of Central Europe and their gravitational e ects, Studia geophys. geod., 32, 54^61. Cloetingh, S., Van Wees, J.D., Van der Beck, P.A. & Spadini, G., Role of pre-rift rheology in kinematics of extensional basin formation: constraints from thermomechanical models of Mediterranean and intracratonic basins, Mar. Petrol. Geol., 12, 793^808. Constable, S.C., Parker, R.L. & Constable, C.G., Occam's inversion: a practical algorithm for generating smooth models from electromagnetic data, Geophysics, 52, 289^300. Horva th, F., Towards a mechanical model for the formation of the Pannonian basin, Tectonophysics, 226, 333^357. Jir ic ek, R., Tectonogenetic development of the Carpathian area in the Oligocene and Neogene (in Slovakian with English summary), in Mahel, M., Tectonic Pro les Through the West Carpathians, pp. 203^214, ed., Publ. Geod. Inst. D. Stur, Bratislava. Kile nyi, E. & Sí efara, J. (eds), Pre-Tertiary Basement Contour Map of the Carpathian Basin Beneath Austria, Czechoslovakia and Hungary, Map 1: , ELGI, Budapest Kova csvîlgyi, S., Interpretation of gravity and magnetic anomalies in the Be kës basin in the light of recent data (in Hungarian), Magyar Geo z., 35, 90^94. Kova csvo«lgyi, S., Interpretation of gravity and magnetic anomalies in SE Hungary (in Hungarian), Magyar Geo z., 36, 198^202. Lachenbruch, A.H. & Morgan, P., Continental extension, magmatism and elevation: formal relations and rules of thumb, Tectonophysics, 174, 39^62. Lillie, R.J., Bielik, M., Babus ka, L. & Plomerovä, J., Gravity modelling of the lithosphere in the Eastern Alpine^ Western Carpathian^Pannonian Basin region, Tectonophysics, 231, 215^235. ß1998RAS,GJI 134, 157^171

15 Narrow continental rifts in Pannonian basin 171 Mechie, J., Fuchs, K. & Altherr, R., The relationship between seismic velocity, mineral composition and temperature and pressure in the upper mantle with an application to the Kenya Rift and its eastern ank, Tectonophysics, 236, 453^464. Mesko, A., Regional Bouguer gravity maps of Hungary, Acta Geod. Geophys. Mon. Hung., 18, 187^200. Mesko, A., Nouvelles donne es sur les rapports structuraux entre les Carpathes Me ridionales et la de pression Gëtique, An. Inst. Geol. Geo z., 60, 147^158. Nemesi, L. & Stomfai, R., Some supplements to the exploration of the basement of the Be kës basin,magyar Geo z., 33, 70^79. Pëcskay, Z., Lexa, J., Szaka cs, A., Balogh, K., Seghedi, I., Konec nÿ,v., Kova cs, M., Märton, E., Kalic ak, M., Sze ky-fux, V., Pöka, T., Gyarmati, P., Edelstein, O., Rosu, E. & Zí ec, B., Space and time distribution of Neogene^Quaternary volcanism in the Carpatho-Pannonian Region, Acta Vulcanol., 7, 15^28. Posgay, K., Mit Re exionsmessungen bestimmte Horizonte und Geschwindigkeitsverteilung in der Erdkruste und im Erdmantel, Geo z. Ko«zl., 23, 13^18. Posgay, K., HegedÏs, E. & T mär, Z., The identi cation of mantle re ections below Hungary from deep seismic pro ling, Tectonophysics, 173, 379^385. Posgay, K., Bodoky, T., HegedÏs, E., KoväcsvÎlgyi, S., Lenkey, L., Sza a n, P., Takäcs, E., T mär, Z. & Varga, G., Asthenospheric structure beneath a Neogene basin in SE Hungary, Tectonophysics, 252, 467^484. Posp s il, L., Interpretation of gravity eld in the East Slovakian Neogene area, Minerälia Slovaca, 12, 421^440 (in Slovakian with English summary).renner, J. & Stegena, L., Gravity research of the deep structure of Hungary, Ann. Univ. Sci., VIII, 153^159. Sm s ek, M., Planc är, J. & Krs äk, J., Computation of the gravity e ect of three-dimensional bodies of arbitrary shape, Contrib. Geophys. Inst. Slov. Acad. Sci., 2, 153^160. Smith, J.T. & Booker, J.R., Rapid inversion of two- and three-dimensional magnetotelluric data, J. geophys. Res., 96, 3905^3922. Stegena, L., The structure of the Earth's crust in Hungary, Acta Geol. Acad. Sci. Hung., VIII, 413^431. Sza än, P., Horva th, F. & Cloetingh, S., Gravity constraints on the crustal structure and slab evolution along a Trans-Carpathian Transect, Tectonophysics, 27, 233^247. Tari, G., Horva th, F. & Rumpler, J., Styles of extension in the Pannonian Basin, Tectonophysics, 208, 203^219. Tomek, C., Ibrmajer, I., Koräb, T., Biely, A., Dvoa kova,l.,lexa,j.& Zboil, A., Crustal structures of the West Carpathians on deep seismic line 2T, Minerälia Slovaca, 21, 3^26. Wannamaker, P.E., Hohman, G.W. & Ward, S.H., Magnetotelluric responses of three-dimensional bodies in layered earths, Geophysics, 49, 1517^1533. Ziegler, P.A., Plate tectonics, plate moving mechanisms and rifting, Tectonophysics, 215, 9^34. ß 1998 RAS, GJI 134, 157^171