Sources of recent tectonic stress in the Pannonian region: inferences from finite element modelling

Transcription

1 Geophys. J. Int. (1998) 134, Sources of recent tectonic stress in the Pannonian region: inferences from finite element modelling Gábor Bada,1,2 Sierd Cloetingh,2 Péter Gerner3 and Frank Horváth3 1 Department of Applied and Environmental Geology, Eötvös University, Budapest, Hungary. bada@iris.elte.hu 2 Faculty of Earth Sciences, Vrije Universiteit, Amsterdam, the Netherlands 3 Department of Geophysics, Eötvös University, Budapest, Hungary Accepted 1997 December 21. Received 1997 November 5; in original form 1997 April 3 1 INTRODUCTION Comparison of recent stress observations and the results of stress modelling provide a powerful approach to enhance our understanding of geodynamic processes (e.g. Zoback 1992). This is especially the case for complex areas such as the Pannonian region, where the state of the lithospheric stress is governed by many distinct tectonic factors. The results of previous stress modelling, either on a theoretical basis (e.g. Richardson et al. 1979; Cloetingh et al. 1985; Bird 1989; Bott 1990; Whitakker et al. 1992; Gölke et al. 1994) or focusing on case studies (e.g. Cloetingh & Wortel 1986; Richardson & Reding 1991; Grünthal & Stromeyer 1992; Meijer & Wortel 1992; Angelier et al. 1994; Beekman 1994; Bassi & Sabadini 1994; Gölke & Coblentz 1996), have demonstrated that plate tectonic processes can be fairly well simulated by finite element modelling. In addition, our knowledge about the state of contemporary stress (Dövényi & Horváth 1990; Brereton & Müller 1991; Gerner 1992; Müller et al. 1992; Rebaï et al. 1992; Becker 1993; Zoback et al. 1993; Gerner et al. 1995, 1997) and the present-day crustal deformation (Anderson & Jackson 1987; Jackson & McKenzie 1988; Robaudo & Harrison 1993; Smith et al. 1994; Ward 1994) in the central Mediterranean SUMMARY We present the results of finite element modelling of the recent stress field in the Pannonian basin and surrounding Alpine orogenic belt. Our results show that the recent, predominantly compressive, stress regime in the Alpine Pannonian Carpathian Dinaric system is governed by distinct tectonic factors. Of great importance is the deformation of crustal blocks with different geometries and rigidities in an overall convergent setting associated with the Africa Europe collision. The most important stress source appears to be the counterclockwise rotation of the Adriatic microplate at the southwest boundary of the Pannonian basin. This plate tectonic unit has been interpreted as moving independently of both the European plate and the African plate. Additional boundary conditions active shortening and compression in the Vrancea zone and the Bohemian Massif, and the effect of the immobile Moesian Platform also significantly influence the modelling results. The incorporation of additional stress sources such as crustal thickness variation and the presence of two main fault zones separating the primary tectonic units in the study area have only locally important effects but improve the fit between the calculated results and the observed stress pattern. Key words: finite element modelling, neotectonics, Pannonian basin, recent tectonic stress. region has greatly improved during the last decade. This enables the construction of simple but geologically reasonable models to analyse how the intraplate stresses are influenced by tectonic processes, that is deformation or direct forces acting at plate edges. The validity of the models can be assessed by comparing their predictions with the extensive stress database of the study area. The complex Tertiary deformational history resulted in a peculiar geometry in the Pannonian region: mountain belts (that is the Alpine, Carpathian and Dinaric chains) surround an internal and relatively flat area. In Fig. 1 we depict the major Late Neogene to Quaternary structural elements of the Pannonian basin and neighbouring orogens. Many of these faults and large-scale lineaments show recent tectonic activity (Gerner et al. 1997). Among these, first-order tectonic lineaments separate the two main structural domains that constitute the pre-neogene basement of the Pannonian basin: the Alcapa terrain in the north and the Tisza-Dacides unit in the south (Fig. 1). These units were moving and were juxtaposed during the Neogene; they were finally locked into their current positions in Late Miocene (Alcapa) and Pliocene (Tisza-Dacides) times (Csontos et al. 1992; Fodor et al. 1997). This is consistent with the gradual eastward younging of the 1998 RAS 87

2 88 G. Bada et al. Figure 1. Simplified Late Neogene to Quaternary structural map and the main geographical units of the Pannonian basin and surrounding orogens. Black heavy line in the upper-left inset indicates the boundary of the modelled area. 1: Molasse belt; 2: Flysch belt; 3: internal units; 4: Tauern window and ophiolitic belt of the Vardar zone; 5: Pieniny Klippen Belt; 6: Neogene and Quaternary volcanites; 7: generalized movement of the main tectonic units; 8: strike-slip faults; 9: normal faults; 10: anticlines and thrust faults. Heavy lines indicate fault zones which separate the primary tectonic units constituting the basement of the Pannonian basin. BM: Bohemian Massif; DB: Drava basin; GHP: Great Hungarian Plane; LHP: Little Hungarian Plane and Danube basin; ST: Sava trough; TB: Transylvanian basin; TR: Transdanubian Range; VB: Vienna basin. last major thrusting along the Carpathian arc (Jiricek 1979). structures of Quaternary age from the internal parts of the The only area now showing a high level of tectonic activity Pannonian basin strongly suggest a compressional origin appears to be the Vrancea zone at the junction of the Eastern (Horváth 1995). It is interesting to note that while the main and Southern Carpathians. The movement of these terranes part of the Pannonian basin system exhibited subsidence relative to each other and to their stable foreland was accom- during Late Neogene times, its southwestern sectors at the modated along NE SW-orientated dextral and NW SE- to Dinaric Southern Alps junction were already slightly com- E W-trending sinistral faults (Csontos 1995; Fodor et al. 1997). pressed and inverted. This gradual shift or propagation of In the southwest-verging fold and thrust belt of the Dinarides, structural inversion into the basin interior from Late Miocene NW SE-striking dextral faults accommodated the movement to recent times was recognized by Tari (1994) and Fodor et al. of the Adriatic unit relative to the Pannonian basin (Horváth (1997). The positive structural inversion of the Pannonian 1993). basin is manifested in the large-amplitude deflection of the The Pannonian basin itself experienced multiphase Neogene underlying lithosphere, that is uplifting areas separate subsiding evolution from a syn-rift ( late early to middle Miocene) to regions. This is most probably the consequence of the increased a post-rift or thermal stage (late middle Miocene Pliocene) compressional intraplate stresses now affecting the whole area (Horváth 1993). During the Quaternary the tectonic regime (Horváth & Cloetingh 1996). has changed significantly. The neotectonics of the Pannonian The principal aim of this study is to examine the nature basin is characterized by the transition from overall extension and relative importance of the different stress sources in the during the late Neogene to transpression or compression Pannonian region, with the emphasis on their geodynamic during the Late Pliocene Quaternary period (Horváth & features and origins. Below, a brief summary is given of the Cloetingh 1996; Bada et al. 1997). Results of both recent and recent stress patterns and crustal deformation in the study palaeostress field measurements (Fodor et al. 1997; Gerner area. We present the results of several sets of finite element et al. 1997), seismotectonic activity (Fuchs et al. 1979; Oncescu models, supporting a counterclockwise rotation of the Adriatic 1987; Gutdeutsch & Aric 1988; Gerner et al. 1997) and vertical domain as the main source of tectonic stress in and around crustal movements (Rónai 1978; Czarecka 1993; Joó 1993) the Pannonian basin. In the bend of the Eastern Carpathians, in the area confirm this interpretation. Furthermore, local the seismically active Vrancea zone contributes to another

3 Map Project were used to obtain the maximum horizontal stress directions (see details in Zoback 1992). The P- and T -axes derived were used to indicate tectonic stress. In the areas of normal faulting (tensional stress field) S values correspond to the intermediate principal stress s2, while in the regions of strike-slip and thrust faulting (compressional and strike-slip type stress regimes, respectively) S coincides with the maximum principal stress s1 (e.g. Isacks et al. 1968; McKenzie 1969). We have also constructed a map showing the smoothed S directions (Fig. 3), interpolating and extra- polating stress directions for areas where no data were accessible. The algorithm of Hansen & Mount (1990) was applied to obtain the smoothed pattern of the maximum horizontal stress from individual points of stress determinations. This was carried out for two reasons. First, we wanted to construct a map which provides a clear presentation of the general trends of S. Second, the grid density was chosen to be #50 50 km, which is very close to the average element size used in the finite element modelling. In addition, this technique provides both the fidelity and the smoothness of the generalized map obtained from the original data (Gerner et al. 1997). In spite of the heterogeneous distribution and quality variation of the available data, we can divide the region into separate areas where the directions of the maximum horizontal stress (S ) reveal good consistency. One of the most charac- teristic features is that the S directions show a clockwise rotation along the Southern Alpine Dinaric front: the roughly NNW SSE-directed compression in the Southern and Eastern Alps becomes NE SW-orientated in the southern regions of the Adriatic sea. This pattern is further traceable in the southern segments of the Pannonian basin, while in its more internal parts the maximum horizontal stress is dominantly NE SW-orientated. Closer to the eastern sectors of the strongly deforming area. The Bohemian Massif in the north and the Moesian Platform at the Southern Carpathians in the east form rigid buttresses: their increased stiffness and immobility strongly influence the resultant stress pattern. In addition, the changes of the crustal thickness (Horváth 1993) and two main discontinuity zones (representing large-scale faults between the primary tectonic units in the study area adopted from Csontos et al. 1992, Fig. 1) were also incorporated. We compare our results with the general stress pattern of the region, and discuss the similarities and discrepancies between models and data. Some of the main characteristics of the present-day geodynamics of the Pannonian basin and its surroundings are then discussed. 2 STATE OF RECENT STRESS AND CRUSTAL DEFORMATION PATTERN IN AND AROUND THE PANNONIAN BASIN: CONSTRAINTS FOR MODELLING PARAMETERS The Pannonian basin and surrounding orogens exhibit a fairly complex present-day stress-field pattern that reflects the main features of the geodynamics of the area. Fig. 2 is a compilation of the maximum horizontal stress (S ) directions. The map contains more than 350 entries including over 200 earthquake focal mechanism solutions, approximately 150 boreholebreakout analyses and some in situ stress measurements (compiled from Dövényi & Horváth 1990; Gerner 1992; Grünthal & Stromeyer 1992; Müller et al. 1992; Becker 1993; Gerner et al. 1997). The determination of stress directions from borehole breakouts and overcoring measurements provide a direct measurement of the orientations of S. In the case of focal mechanisms, the methods of the World Stress Recent tectonic stress in the Pannonian region 89 Figure 2. Maximum horizontal stress directions (S ) observed in and around the modelled area (after Gerner et al. 1997). The length of the symbols reflects the quality ranking of the stress data; that is, longer symbols represent better data quality (after Zoback 1992). For legend see Fig. 1.

4 90 G. Bada et al. Figure 3. Smoothed directions of the observed maximum horizontal stress (S ) obtained using the smoothing and extrapolating algorithm of Hansen & Mount (1990) for the available stress data (after Gerner et al. 1997). For legend see Fig. 1. Carpathian chain this orientation progressively changes to E W. Further to the east, in the Vrancea zone in the Eastern Carpathians, the observed S is dominantly E W orientated. In the Western Carpathians, the easternmost parts of the Eastern Alps and the Transdanubian Range, the stress measurements yield a fairly scattered S pattern with a slight NW SE trend. This region was interpreted by Gerner (1992) as one of the easternmost regions of the Western European stress province, which exhibits a fairly uniform NW-NNW S orientation (Müller et al. 1992). Finally, sparse data from the northern segment of the Carpathian arc indicate N S-directed compression, roughly perpendicular to the mountain belt. Since the displacement (strain) pattern of the Pannonian basin and particularly that of its surroundings is relatively well known, the recent stress field in the study area is modelled by applying displacements at the edges of the study area. Therefore, we outline the main characteristics of the recent deformation pattern of the region and the geological rationale for the boundary conditions examined. These are, in fact, well constrained by geological, geophysical and geodetic observations and provide fairly reliable modelling parameters which were utilized during the modelling procedure (Fig. 4). The high seismicity in the Southern Alps and the Dinarides indicates considerable present-day deformation in the southern part of the study area (e.g. Anderson & Jackson 1987; Del Ben et al. 1991; Console et al. 1993). Recently, Gerner et al. (1997) calculated the spatial pattern of the total seismic energy release in order to estimate the amount of active deformation in this region. Their results clearly show that the most intensely deforming area is the Dinaric chain: this belt exhibits tectonic activity several orders of magnitude higher than any other part of the area of interest. The active shortening in the Dinarides has been interpreted as a consequence of the counterclockwise rotation, and thus the NNE-directed shifting of the Adriatic domain (Gerner et al. 1997; Bada et al. 1997). The role of this tectonic unit has been one of the key concerns of studies of Alpine Mediterranean plate tectonics. Some authors have interpreted it as a promontory of the African plate (Channell et al. 1979; D Argenio et al. 1980; Horváth & D Argenio 1985; Mantovani et al. 1990). Alternatively, independent and counterclockwise rotation of the Adriatic plate as a consequence of the active opening of the Tyrrhenian basin has been proposed (Dercourt et al. 1986; Locardi 1988). Malinverno & Ryan (1986) were the first to propose the rollback effect of the subduction front at the Apenninic arc as a possible explanation for either the opening of the Tyrrhenian Sea or the rotation of the Adriatic crustal block. Studying the seismotectonics of the central Mediterranean region, Anderson & Jackson (1987) and Jackson & McKenzie (1988) suggested that the Adriatic plate is a fairly rigid unit rotating counterclockwise with respect to the Eurasian plate. They proposed that the Adria Europe Euler pole (AEEP) is located at 45.8 N, 10.2 E in the Central Alps; the calculated angular velocity is in the range of /107 yr. The most recent space geodesy results firmly support and confirm the independence of the Adriatic microplate from the African plate. Satellite laser ranging (SLR) and very-long- baseline interferometrics ( VLBI) yielded coherent data from which the position of the AEEP was determined (Cenci et al. 1993; Robaudo & Harrison 1993; Robbins et al. 1993; Smith et al. 1994; Ward 1994). In our models we adopted the values suggested by Ward (1994): the AEEP was taken at 46 N, 6 E, which is very close to the pole calculated by Jackson &

5 Recent tectonic stress in the Pannonian region 91 Figure 4. Model geometry and boundary conditions used in the finite element procedure. Note that a larger framework was created to minimize edge effects and errors. As a result, the free edges are buffered but can be deformed on a small scale. For further discussion see text. The Adria Europe rotation pole was taken from Ward (1994). McKenzie (1988). Console et al. (1993) and Doglioni et al. Massif. Similar conclusions have been made by Gerner et al. (1994) proposed that the Adriatic plate is split into two subunits. (1997) from an analysis of the kinematics of some major faults However, for the sake of simplicity, we regard it as a and recent seismic activity. Ratschbacher et al. (1991) have homogenous, uniformly moving block in the Mediterranean shown the key importance of such a strong foreland in the puzzle. late Oligocene Miocene tectonic evolution of the Eastern Alps. Another strongly deforming area in the Carpathian This unit forms an important element in the finite element Pannonian system is the Vrancea zone at the sharp bend modelling. First, similar to the Moesian Platform, it was of the Eastern Carpathians. The intense intermediate- to considered as a non-deforming and stable crustal block shallow-depth seismic activity shows that this unique area is representing a buttress with high stiffness. Second, SE-directed, seismically the most active part of the Carpathian chain (e.g. relatively slow motion of the Bohemian Massif was assumed Constantinescu & Enescu 1984; Oncescu & Trifu 1987). We in order to simulate the transmission of the Western European accept the results of Oncescu (1987) and Mueller & Kahle stress field (Müller et al. 1992) into the interior of the (1993): they accounted for dominant shortening acting roughly Pannonian basin system. perpendicular to the Carpathian arc at the Vrancea zone. This compressional stress may result from the final detachment of 3 DESCRIPTION OF THE FINITE the subducted lithospheric slab beneath the Vrancea zone ELEMENT MODELS (Wortel & Spakman 1992; Royden 1993). In the vicinity of the Vrancea zone, the Moesian Platform In our study several sets of finite element models were constructed forms stiff foreland which played an important role during the which treat the study area as a 2-D plate with collision and orocline formation of the Southern Carpathians. heterogeneous elastic rheology: a plain stress approximation Ratschbacher et al. (1993) concluded that the corner effect of is adopted for the calculation of the regional horizontal stress the resistant Moesian Platform is responsible for the general field. At this point, we emphasize that our 2-D calculations geometric and kinematic features of the Cenozoic structural ignore several possible stress sources which might significantly evolution in the Southern Carpathians. Therefore, we accept influence the stress pattern in the region. We are well aware their result when modelling this rigid unit as an unmovable of the fact that tectonic factors, such as the effect of topography, crustal buttress with high stiffness, which can be deformed buoyancy forces and the rheological stratification of the lithosphere only to a very limited extent. can generate additional stresses. We also refrain from Mueller & Kahle (1993) pointed out the possibility of N S- simulating inelastic deformation (earthquakes as slip along to NW SE-directed crustal shortening between the western active faults) and use a simple elastic rheology, which is another edge of the Western Carpathians and its foreland, the Bohemian major simplification. Below, however, we will demonstrate that

6 92 G. Bada et al. our relatively simple and straightforward 2-D finite element high heat flow ( mw m 2) of the region (Dövényi & models are quite useful in recognizing and interpreting the Horváth 1988). For the external sectors (that is the mountain gross features of strain stress relationships in the Pannonian chains around the Pannonian basin) we adopted a value of basin and its surroundings. Our approach is similar to that E=75 GPa, while for the rigid buttresses, together with the applied by Grünthal & Stromeyer (1992), but much more external frame around the area of interest (i.e. the Bohemian detailed and better constrained geometries and sets of boundary Massif and Moesian Platform), a value of E=100 GPa was conditions have been utilized. assumed. Poisson s ratio was taken uniformly as n=0.25. The For computation the ANSYS and the MARC finite element initial models consisted of 527 quadrilateral elements and the packages were used in order to cross-check the numerical average element size was about km (MESH I, Fig. 5). quality of our modelling results: both codes yielded nearly In the first set of the models the elastic plate had a reference identical outcomes. Instead of direct forces, which are very thickness of 50 km. Four different boundary conditions poorly constrained in the region, the boundary conditions were applied separately and in different combinations. It were defined as displacements at the model edges. These is emphasized that we analyse and highlight the relative horizontal displacements govern the deformation (strain) importance of the applied boundary conditions representing pattern and, together with the assumed elastic properties of real tectonic processes. As a first approach to employing the the lithosphere, define the state of stress throughout the study latest findings of Gerner et al. (1997) on the total seismic elastic plate (e.g. Zienkiewicz & Cheung 1967). In this way, energy release in and around the Pannonian basin, in our relative plate motions in the Mediterranean system govern the models we incorporated shortening at the Dinaric front by boundary conditions, which are regarded as the principal stress applying displacements one order of magnitude higher than sources of our models. These are influenced by other tectonic those applied at the other model edges. The movement of the elements such as the changing thickness of the elements Adriatic domain was utilized as a permanent boundary condition representing the Moho depth variation or the presence of with a counterclockwise angular displacement of 0.1 fault zones in the Pannonian region. We do not consider the around a pole at 46 N, 6 E. As a consequence, the southern absolute stress magnitudes; we focus instead on the comparison edge of the modelled area (i.e. the Adriatic coastline) was of observed and calculated stress directions. We do so for two displaced by 1000 m to the north at its western end in the reasons. First, our knowledge about the recent stress field in Southern Alps and 2400 m to the NNE at its southernmost the area is restricted to only the orientation of the maximum tip. The edge at the Bohemian Massif was first kept free, then horizontal stress. Consequently, it is difficult or even impossible fixed, and finally displaced by 200 m to the SE, simulating to verify the calculated stress magnitudes. Second, we utilized the transmission of the Western European stress field. The geologically reasonable but still completely arbitrary boundary boundary at the Southern Carpathians was either free or fixed, conditions, that is the absolute values of the applied displace- while the edge at the Vrancea zone was first free and then ments at the model edges are subjective quantities. Therefore, pushed by 200 m perpendicular to the Carpathian arc (Vrancea they are not comparable with those arising from buoyancy push), reflecting the active deformation and compression forces and/or topography. inferred from space geodesy (Mueller & Kahle 1993) and The model boundaries (Fig. 4, see inset map in Fig. 1) are earthquake focal mechanism studies (Fuchs et al. 1979; the northern border of the Eastern Alps, the front of the Oncescu 1987; Oncescu & Trifu 1987). Many other different Carpathian flysch belt and the contact of the Adriatic unit and the Dinarides (i.e. roughly the Adriatic coastline). Two arbitrary boundaries were selected perpendicular to mountain ranges to make a closed unit: one across the Eastern and Southern Alps on the left side, and the other across the Dinarides and Southern Carpathians in the lower-right corner of Fig. 1. After obtaining unrealistic results at unconstrained model edges, a fixed framework was applied at a distance of 500 km from the model edge in order to avoid any edge effect or error and to incorporate the effect of the presence of the stable Eurasian plate. In this way, we had the opportunity to leave the free model edges unconstrained. They bound the area of interest, influence the mesh geometry and, due to the presence of the external frame (Fig. 4), these boundaries are allowed to be deformed only to a certain limit. This approach is further supported by the assumption that no stresses are transmitted perpendicular to the boundaries because these edges are roughly orthogonal to the strike of the Alps in the west and the Dinarides in the southeast. Testing the relative magnitude of the elastic parameters has shown that the calculated stress pattern has little or no sensitivity to the absolute values. Therefore, we kept these values constant. For the internal parts of the modelled area (i.e. the Pannonian basin) Young s modulus was chosen to be Figure 5. Finite element mesh of the model (MESH I): a total of 550 E=50 GPa, reflecting the relative weakness (Lankreijer et al. elements were created, while the area of interest consists of ) of this lithospheric segment owing to the extremely quadrilateral elements with an average size of km.

7 Recent tectonic stress in the Pannonian region 93 displacement vectors were also tested during early runs of the in the vicinity of major fault zones. The model was re-meshed models but yielded unrealistic results. Again, these values are to reflect the changing geometry: 330 quadrilateral elements defined to provide a common basis for the direct comparison were regenerated in the study area (MESH II, Fig. 7). of the relative importance of applied boundary conditions. They were first tested separately and then used in the subsequent model sets. 4 MODELLING RESULTS In the second set of models we examined the stresses First, a detailed analysis is carried out to investigate the influenced by the changes in crustal thickness in the Pannonian relative importance of the boundary conditions applied in region (Fig. 6). We emphasize that in our 2-D numerical the finite element calculation (Figs 8 and 9). Then, the consequences models the changing Moho depth primarily represents a of the changing crustal thickness (Fig. 10) and finally variation in plane thickness. We employed the values of the effects of two main fault zones inside the Pannonian region Horváth (1993) instead of using poorly constrained effective (Fig. 11) are examined. elastic thickness estimates. The purpose of this part of the study is to explore how geometrical irregularities of an at least partly elastic plate can influence the resultant stress pattern. 4.1 The relative importance of the boundary conditions Finally, two main discontinuities were incorporated to The counterclockwise rotation of the Adriatic unit around a simulate the effect of two fault zones separating the main pole in the Western Alps appears to be the main source of the tectonic domains in the Pannonian region (Csontos et al. tectonic stress in the Pannonian region. Special attention was 1992) (Fig. 7). Because Gerner et al. (1997) demonstrated that paid to the amount of rotation. An arbitrary angular displacethese tectonic lines show significant present-day activity, we ment of 0.1 was used, which turns out to be sufficient to examined their behaviour and effect on stress strain relationships. induce reasonable tectonic stresses of the order of tens of or Displacement along faults is controlled by the coefficient even 100 MPa in an elastic plate with a thickness of tens of of friction (Byerlee 1978). In a first simple approach, we kilometres. According to the strain pattern and the amount adopted a high value for the coefficient of friction along these of deformation suggested by Gerner et al. (1997) for the faults in order to investigate how the presence of these major Pannonian basin, this deformation appears to be realistic in inhomogeneities can influence the regional stress pattern and representing the ongoing Adria convergence with respect to thus produce stress deviations. Subsequently, adopting the the Pannonian basin. On the other hand, as evidenced by results of laboratory tests, we incorporated a more realistic verifying a broad range of angular displacements, this rotation value of m=0.5 to allow sliding along faults (e.g. Jaeger & is small enough to allow the stress pattern to be influenced by Cook 1971). According to several authors (e.g. Lacombe et al. 1993; Petit & Mattauer 1995; Homberg et al. 1997), if sliding occurs, stress deviations in the range of can be expected the thickness variations of the modelled plate in the subsequent model sets. As a consequence, the deformation at the Adriatic coastline produces a stress pattern in the area (Fig. 8a) which, Figure 6. Thickness of the crust in and around the modelled area (after Horváth 1993). By attaching these values to the geometrically corresponding elements, the effect of plate thickness variation was modelled. For legend see Fig. 1.

8 94 G. Bada et al. Figure 7. Two main discontinuity zones were introduced to examine the effect of faults separating the primary tectonic units of the study area (Csontos et al. 1992). Faults I and II/a correspond to the Periadriatic and the Mid-Hungarian lineaments, respectively. Fault II/b represents the southern edge of the Tisza-Dacides unit. According to the new geometry, the area of interest was re-meshed (MESH II). A total of 330 quadrilateral elements were regenerated. prediction shows a very characteristic rotation of the S direction around Moesia (Fig. 9c,d). Finally, we applied a somewhat deforming edge (displaced 200 m to the SE) at the Bohemian Massif, together with all the other boundary conditions (Fig. 9d). This shortening at the northwestern edge of the modelled area is meant to simu- late the far-field effect of the well-established Western European stress regime, where the average direction of S is roughly NW SE (e.g. Müller et al. 1992). The divergent fan-like pattern of the calculated stresses becomes pronounced, showing a good correlation with the observed stress field. Local misfits are visible in the western sectors of the Pannonian basin where the modelled S is roughly perpendicular to the measured directions. At this stage, we can conclude that the remarkable fan-like pattern of the maximum horizontal stress directions in the study area is basically governed by the counterclockwise rotation of the Adriatic unit. This first-order stress source of the Pannonian region interacts with the Vrancea push in the Eastern Carpathians and the Transylvanian basin, where S is orientated in an E W direction. The effect of the two rigid blocks at the model edges is also well established. Compression becomes oblique with a high angle in the vicinity of the Bohemian Massif. On the other hand, S directions show a rotational pattern in the Southern Carpathians: further north they become gradually E W orientated at the western side of the Moesian Platform. The Pannonian basin itself displays a at least in the general trends, correlates fairly well with the observed field. If we fix the edge at the Bohemian Massif (Fig. 8b), S becomes more similar to the measured NW SE direction in the north. When no displacements are allowed at the Southern Carpathians (Fig. 8c) the dramatic decrease of relative stress magnitudes is very distinct in the Transylvanian basin. The stresses are now concentrated between the Dinarides and the Moesian Platform, and the maximum horizontal stresses are slightly oblique to the general direction of shortening. The effect of the deformation imposed at the Vrancea zone in order to imitate the active compression in the Eastern Carpathians can be recognized in Fig. 8(d). Comparison with the results displayed in Fig. 9(a) demonstrates that the S directions are closer to the observed stress pattern in the eastern sectors of the modelled area; that is, they become more easterly orientated. If we combine the different boundary conditions and keep the deformation at the Dinaric front constant, the resultant stress pattern becomes even more realistic (Fig. 9). For instance fixing the two stiff buttresses, the Bohemian Massif and the Moesian Platform, results in a stress concentration in the southwestern and central portions (Fig. 9a), although its radial pattern remains more or less the same. Fig. 9( b) suggests that the Vrancea push has a considerable effect on the style of stress in the eastern parts of the study area. This is especially the case if we combine it with a fixed Moesian Platform. In spite of the noticeable decrease of the magnitudes, the model

9 Recent tectonic stress in the Pannonian region 95 Figure 8. Model results showing the effect of the applied boundary conditions indicated by the insets. Lines plotted in the centre of each element represent the direction of the maximum horizontal stress (S ). Their length is proportional to the calculated stress magnitudes. The area displayed corresponds to the outline of the modelled area in Fig. 1. (a) The active deformation at the Dinaric front is tested alone. (b), (c) Rotating Adriatic microplate with fixed Bohemian Massif and Southern Carpathians, respectively. (d) Deforming Dinaric front and Vrancea zone. Figure 9. Model results showing the combined effects of applied boundary conditions (see insets in the lower-left corner). The rotation of the Adriatic microplate is kept constant. (a) Fixed Bohemian Massif and Southern Carpathians. ( b) Fixed Bohemian Massif and deforming Vrancea zone. (c) Fixed Southern Carpathians and deforming Vrancea zone. (d) Deforming Bohemian Massif and Vrancea zone, fixed Southern Carpathians. Figure conventions as in Fig. 8.

10 96 G. Bada et al. Figure 10. Modelling results showing the effect of crustal thickness variations in the study area. (a), ( b) The edge at the Bohemian Massif is fixed and slightly deforming, respectively. Figure conventions as in Fig. 8. stress field with a divergent S geometry, becoming roughly perpendicular and oblique in the Western and Eastern Carpathians, respectively. 4.2 Effect of the changing crustal thickness in the Pannonian region 4.3 Effects of two main fault zones in the Pannonian region Two main fault zones were added to the finite element model to simulate the possible influence of these presently active discontinuities (Gerner et al. 1997) on the stress pattern in the study area. Apart from their recent activity, these structural features (see Fig. 7) represent the most important fault zones separating the main tectonic units of the Alpine Carpathian Pannonian Dinaric system (Csontos et al. 1992). We have plotted the computed stress orientations directly on the smoothed version of the observed S directions in order to allow their direct comparison and to facilitate the evaluation of the modelling results (Fig. 11). Two cases are considered. As a first approach, the faults are completely locked by adopting a high value of the coefficient of friction whilst the edge at the Bohemian Massif is held fixed. The applied Significant changes of the crustal thickness in and around the Pannonian basin are a conspicuous feature of the area (Fig. 6). Generally speaking, the Moho depth decreases towards the centre of the Pannonian basin, while the surrounding mountain chains are characterized by thick crust. Thickness variations in an elastic plate can significantly influence both the magnitude and the orientation of tectonic stress. Stresses are inversely proportional to the effective elastic thickness; that is, they are amplified beneath basins (Kusznir & Bott 1977; Cloetingh 1992). This effect, combined with the boundary conditions, inhomogeneities in the model geometry can induce stress was modelled (Fig. 10). We emphasize that in our 2-D elastic deviations around these faults without any displacement along models the changing Moho depth represents variations in them (Fig. 11a). This effect is more pronounced if the edge at plane thickness. The edge at the Bohemian Massif was first the Bohemian Massif is deforming and we allow sliding by fixed (Fig. 10a) and then deformed (Fig. 10b). In both cases, reducing the coefficient of friction to m = 0.5 along the faults the resultant stress pattern shows minor changes in comparison (Fig. 11b). The S directions become less uniform and more with the previous models. However, in the central and western realistic in the central parts of the Pannonian basin. This is in areas of the Pannonian basin a small-scale stress deviation good agreement with the results of e.g. Zoback (1991), Rebaï can be detected: the S directions are rotated #10 15 et al. (1992) and Petit & Mattauer (1995). All of these authors clockwise, contrasting with the previous models (see Figs 9d emphasized that stress trajectories are strongly dependent on and 10b). This result may be partly a consequence of the fact the geometry of the fault zones, that is they can be rotated to that our 2-D finite element models ignore stresses arising from intersect the faults at a higher angle, which is indeed the case changes of topography and the lateral density inhomogeneities with the northwestern sectors of the Pannonian basin. Most of the lithosphere. Although this type of stress is obviously recently Homberg et al. (1997) have carried out a numerical superimposed on the observed stress field, the effect of density- investigation to examine the effects of pre-existing structures induced stress should be second order compared to the effect in the crust on the stress pattern. They demonstrated the of plate geometry and the rotation of the Adriatic unit (see critical importance of the coefficient of friction on the fault Zoback 1992). According to several authors (e.g. Fleitout & surface and the orientation of the discontinuity relative to Froidevaux 1982; Ranalli 1991), density contrasts in the the far-field stress. Moreover, their results show that stress lithosphere can generate stresses of the order of tens of MPa, deviations can be as large as 50. However, this high value is while stress sources acting at plate boundaries can induce detectable only in the close vicinity of fault tips, which are not stresses of hundreds of MPa (e.g. Turcotte & Schubert 1982; incorporated in our calculation. Cloetingh & Wortel 1986; Richardson 1992). Nevertheless, When all modelling parameters are combined, the calculated the nearly isometric shape of the Pannonian basin system and observed stress patterns are very similar (Fig. 11). and, as a consequence, the gradual decrease of crustal thick- The NNW SSE compression at the Bohemian Massif, the ness towards the basin interior may partly explain why the E W-orientated S in the Eastern Carpathians and the change in elastic plate thickness (i.e. pure geometrical inhomo- Transylvanian basin, and the divergent fan-like pattern at geneity) only has a minor effect on the stress orientation in the Dinarides propagating through the internal parts of the the area. Pannonian basin all reflect a satisfactory fit.

11 Recent tectonic stress in the Pannonian region 97 Figure 11. Best-fitting resultant stress pattern reflecting the combined effects of the applied boundary conditions (see insets), changing crustal thickness and two predefined weakness zones. (a), ( b) The edge at the Bohemian Massif is fixed and slightly deforming, respectively. In order to make direct comparison possible, the smoothed (observed) and calculated stress directions are superimposed. 5 DISCUSSION AND CONCLUSIONS Our model calculations show that the general trends of the recent stress field in the Pannonian region can be simulated applying relatively simple but geologically reasonable boundary conditions (Fig. 12). The models comprise the deformation of crustal blocks with different geometries and rigidities in an overall convergent setting, that is the active Africa Europe collision zone. Recent stress indicator data clearly show that the present-day stress regime in the Pannonian region is

12 98 G. Bada et al. Figure 12. Cartoon summarizing the main stress sources in the Alpine Carpathian Pannonian Dinaric system applied in our finite element models. Buttresses are rigid crustal blocks indenting or blocking their surroundings. Dashed lines represent faults that were included during modelling. The kinematics of some major faults showing present-day activity are also shown (after Gerner et al. 1997) 1: Molasse belt; 2: Flysch belt; 3: internal units; 4: Neogene and Quaternary volcanites; 5: Pieniny Klippen Belt; 6: strike-slip faults; 7: normal faults; 8: thrust faults. For discussion see text. the central sectors, while the flanks of the basin and the Transdanubian Range have been uplifting since the Late Pliocene. This anomalous subsidence pattern was explained by these authors in terms of stress-induced lithospheric deflection; that is, the increasing intraplate stresses cause large- scale bending or folding of the lithosphere (Horváth & Cloetingh 1996). The stress database and the results of stress modelling confirm the notion that compressive stresses in and around the Pannonian basin are capable of producing large-scale deformation of the lithosphere in this area. Another notable feature which influences our modelling results is the effect of the active deformation at the Vrancea zone indicated by the remarkable shallow- to intermediatedepth seismicity ( Vrancea push). The general direction of E W-orientated compression inferred from focal mechanism solutions was simulated as shortening perpendicular to the Carpathian arc. As a consequence, large parts at the eastern sectors of the modelled area (the Eastern Carpathians and the Transylvanian basin) became dominated by E W-orientated maximum horizontal stress. At first sight, this seems to be quite surprising as the displacements defined at the Vrancea zone have a relatively small value (200 m). However, the screening effect of the immobile and stiff Moesian Platform at the Southern Carpathians (Fig. 12) can probably explain this pattern. The minor angular deviation in the directions of shortening and compression between the Dinaric and the Southern Carpathian front (NNE SSW versus NE SW) is also noteworthy. Therefore, as pointed out by several authors (e.g. Richardson & Cox 1984; Cloetingh & Wortel 1986; Richardson & Reding 1991; Zoback & Magee 1991; Grünthal predominantly of strike-slip type or compressive. It appears that the best fit with the observed maximum horizontal stress field can be obtained if we take the rotation of the Adriatic crustal block as a first-order stress-generating component. This interpretation is further verified by the strain pattern in the area of interest: the rate of active deformation in the Dinaric belt is higher by several orders of magnitude than in any other region in and around the Pannonian basin (Gerner et al. 1997). Our results appear to support the idea that the Adriatic unit is moving independently of both the European and the African plates as an individual microplate. This is further evidenced by the results of independent studies using space geodesy (e.g. Ward 1994), seismology (e.g. Anderson & Jackson 1987; Del Ben et al. 1991) and analysis of recent fault kinematics (Gerner et al. 1997) (Fig. 12). As a consequence, a high level of compression is induced at the eastern Adriatic coastline (Adria push), which is manifested in the high level of seismicity and in the Southern Alps and Dinarides. Additional boundary conditions with displacements an order of magnitude less, that is 200 m of shortening in the Vrancea zone (Vrancea push) and/or the Bohemian Massif versus 2000 m of displacement at the Adriatic coastline, can also significantly affect the modelling results. Other stress sources, such as the changing crustal thickness and the presence of two main fault zones in the study area, have only a minor, but still detectable, influence, giving rise to local perturbations of the stress directions. Recently, the recent increase in horizontal stress magnitude was put forward as a means of explaining the Quaternary subsidence anomalies in the Pannonian basin system (Horváth & Cloetingh 1996). Accelerated subsidence is observed in

13 Recent tectonic stress in the Pannonian region 99 & Stromeyer 1992), the geometry of plate boundaries plays a Other parameters, such as the changing crustal thickness and dominant role in determining the intraplate stress field. the presence of two main fault zones in the study area, have The trends of the Western European stress province appear minor influence, giving rise to local perturbations to have a visible expression only in the northwestern sector of of the stress directions. We emphasize that our 2-D models the study area, close to the Alpine Carpathian junction. This ignore stress sources which might play an important role in region is characterized by a stress pattern with a NW SEorientated controlling the recent state of stress in and around the S. In contrast, our models fail to explain the Pannonian basin. Tectonic factors, such as the effects of roughly E W-orientated S directions in the northwestern topography and buoyancy forces, thermal stresses, lateral parts of the Pannonian basin. However, the combined effect density inhomogeneities and rheological stratification of the of the local maximum of the crustal thickness and the presence lithosphere may generate additional stresses. Nevertheless, of two faults near this region provided a better but still poor reasonably simple numerical models such as carried out in this fit. Unfortunately, data coverage in this region is not sufficient study are straightforward and useful in recognizing and under- to develop models to explain the origin of the misfit in this standing the characteristics of the main neotectonic features in area. Hence, we leave this question open for further stress the Alpine Carpathian Pannonian Dinaric system. measurement and modelling studies. The compressional stresses probably at least partly originate as a result of the detachment of the subducted slab below the ACKNOWLEDGMENTS Vrancea zone. Wortel & Spakman (1992) proposed a dynamic model for the evolution of the Mediterranean region in which We are indebted to R. M. Richardson (University of Arizona, they emphasized that the gradual detachment of the subducted Tucson) and M. Gölke ( Vrije Universiteit, Amsterdam) for slab in space and time can give rise to significant changes in their inspiring suggestions on an earlier version of the manu- the stress regime. The evident consequence of slab detachment script. Thanks are due to the valuable and constructive is that the roll-back effect of the relatively dense sinking slab comments of the two journal reviewers. cannot be transmitted to higher crustal levels, and as a result This work was supported by the joint TEMPUS project of the convergence of the plates leads to a gradual increase of Eötvös University, Budapest, Vrije Universiteit, Amsterdam, compressional intraplate stresses. This scenario can also be and University Fridericiana, Karlsruhe (JEP-1506), and the applied to explain the high level of seismicity and compression IBS (Integrated Basin Studies) project of the European at the Dinaric belt. In this region, the detachment of the Community (contract JOU2-CT ). Partial support was Adriatic slab once subducting below the Dinarides is thought provided by the Hungarian National Scientific Research Fund to be responsible for the active crustal shortening and seismicity (OTKA) no. T This is Publication of the (Spakman 1990). These authors have provided a plausible Netherlands Research School of Sedimentary Geology. scenario to address the ultimate question: why has extension terminated in the Pannonian basin and why is it being inverted? The most plausible origin of lithospheric extension seems to REFERENCES be related to the roll-back effect of subduction acting at the Anderson, H. & Jackson, J., Active tectonics of the Adriatic Carpathian front (e.g. Royden 1988; Csontos et al. 1992) and region, Geophys. J. R. astr. Soc., 91, the gravitational collapse of overthickened orogenic terranes Angelier, J., Lee, J.-C. & Hu, J.-C., Construction of trajectory (Ratschbacher et al. 1991; Tari 1994). In the absence of a maps based on local paleostress determinations and their intersubductable high-density plate and/or easily deformable areas pretation: some new insights, in Peri-T ethyan Platforms, for mass transport, these processes cannot facilitate further pp , ed. Roure, F., Éditions Technip, Paris. lithospheric extension. From Late Miocene times onwards, the Bada, G., Horváth, F., Gerner, P. & Fejes, I., Review of the Alcapa and Tisza-Dacides units reached their final position present-day geodynamics of the Pannonian basin: progress and within the Carpathian chain and were buffered by the rigid problems, J. Geodyn., in press. East European Platform. In this way, the still ongoing Adria Bassi, G. & Sabadini, R., The importance of subduction for the Europe convergence resulted in the gradual inversion of the modern stress field in the Tyrrhenian area, Geophys. Res. L ett., 21, Pannonian basin system. Furthermore, additional compression Becker, A., Contemporary state of stress and neotectonic is still influencing the eastern sectors of the study area due to deformation in the Carpathian-Pannonian region, T erra Nova, the effects of Vrancea push. 5, We conclude that our modelling results show that the general Beekman, F., Tectonic modelling of thick-skinned compressional features of the present-day stress field in the Pannonian region intraplate deformation, PhD thesis, Vrije Universiteit, Amsterdam. can be fairly well simulated by applying relatively simple but Bird, P., New finite element techniques for modelling deformation geologically rational boundary conditions. The best fit between histories of continents with stratified temperature-dependent the observed and calculated stress fields (Fig. 11) was obtained rheology, J. geophys. Res., 94, when the rotation of the Adriatic microplate was taken as a Bott, M.H.P., Stress distribution and plate boundary force first-order stress source. It appears that this tectonic unit is associated with collision mountain ranges, T ectonophysics, 182, moving independently of both the African and the Eurasian Brereton, R. & Müller, B., European stress: contribution from plates. Additional and second-order stress-generating boundary borehole breakouts, Phil. T rans. R. Soc. L ond., A, 337, conditions are the active shortening and compression at the Byerlee, J., Friction of rocks, Pure appl. Geophys., 116, Vrancea zone, the slight compression from the Bohemian Cenci, A., Fermi, M., Sciarretta, C. & Devoti, R., Tectonic motion Massif and the presence of the immobile Moesian Platform. in the Mediterranean area from laser ranging to LAGEOS, in They induce additional stresses in the modelled elastic plate Contributions of Space Geodesy T o Geodynamics: Crustal Dynamics, superimposed on the effect of the rotating Adriatic microplate. eds Smith, D.E. & Turcotte, D.L., Geodyn. Ser., 23,