Gábor Bada

Review of the present-day geodynamics of the Pannonian basin: progress and problems

Abstract We present a comprehensive review on what we have learned during the last decade and what we need to know in the future about the present-day crustal deformation and geodynamics of the Pannonian basin and its surroundings. The recent tectonic activity of the region is controlled primarily by the counterclockwise rotation of the Adriatic microplate relative to Europe around a pole in the Western Alps. Due to the indentation of this crustal block against the Southern Alpine–Dinaric fold and thrust belt, intense shortening is effecting these orogens as evidenced by the general seismicity pattern and crustal deformation. The present-day kinematics of the Pannonian basin shows that the area is pushed from the south-southwest. As a result, strike-slip to compressive faulting is observed well inside the Pannonian basin and, furthermore, the nearly complete absence of normal faulting in the whole study area suggests that extension in the Pannonian basin has been finished and structural inversion is in progress. Due to an increase of intraplate compressional stress the Pannonian lithosphere exhibits large-scale bending manifested by the Quaternary subsidence and uplift history. The orientation of the modern tectonic stress field in and around the Pannonian basin shows a remarkable radial pattern of maximum horizontal stress around the Adriatic microplate. N–S directed compression observed at its northern tip in the Southern Alps gradually becomes NE–SW oriented along the Dinaric belt. This pattern is further traceable well inside the Pannonian basin, while in the Vrancea zone of the southeastern Carpathians E–W to ESE–WNW directed compression can be determined. Finite element stress modelling suggests that the stress regime in the Pannonian basin is governed by distinct tectonic factors in the overall convergent setting associated with the Africa–Europe collision. The most important stress source appears to be the active push of the Adriatic microplate. Additional boundary conditions, such as the deformation of crustal blocks with different geometry and rigidity at the margin of the Pannonian–Carpathian area and the effect of active compression in the Vrancea zone, significantly influence the stress regime and pattern. Finally, with a brief overview about the principal aims of the Central Europe Regional Geodynamic Project (CERGOP), we argue for the need of further investigations applying the latest techniques of space geodesy (GPS). This international cooperation can provide an excellent opportunity to further develop our understanding of the recent crustal deformation in Central Europe and to refine concepts and models about the tectonic inversion of sedimentary basins with back-arc origin.

Tertiary tectonic evolution of the Pannonian Basin system and neighbouring orogens: a new synthesis of palaeostress data

Abstract Compilation of a microtectonic observation data base for most of the data measured in the Pannonian Basin and surrounding orogens permits a detailed reconstruction of the Tertiary stress field evolution. Combination of tectonic observations, borehole, gravity and seismic data, palaeogeographic and stratigraphic information led to an understanding of fault kinematics and description of the structural evolution in seven major tectonic episodes. The first two episodes depict the kinematics of the two major separated blocks, the Eastern Alpine-Western Carpathian-Northern Pannonian (Alcapa) and the Southern Pannonian-Eastern Carpathian (Tisza-Dacia) microplates. A Mid-Eocene to Early Oligocene N-S compression led to contractional basin formation both in the foreland (Western Carpathians) and hinterland (Hungarian Palaeogene basins) of the orogenic wedge. Due to oblique convergence, the Palaeogene basins are generally asymmetric and often dissected by dextral tear faults. Northward advance of the Adriatic promontory initiated the separation of the Alcapa from the Southern Alps and its eastward extrusion. This process probably started during latest Oligocene and reached its climax during the Early Miocene. The main displacement was accommodated by dextral slip along the Periadriatic and Mid-Hungarian shear zones and during and after this tectonic episode Alcapa suffered 50° CCW rotation. At about the same time period the Tisza-Dacia block also experienced rotation of 60–80°, but clockwise. These opposite rotations resulted in the marked actual deviation of earlier compression axes, which are now N or NW in the Eastern Alps, WNW-ESE in the Western Carpathian-Pannonian domain and NE-SW in the Tisza-Dacia domains. Termination of rotations can be considered as the time for final amalgamation of the two separate blocks and the beginning of extensional tectonics in a single Pannonian unit. The Pannonian Basin system was born by rifting of back-arc style during the late Early and Mid-Miocene time. Extension was controlled by the retreat and roll-back of the subducted lithospheric slab along the Carpathian arc. Two corners, the Bohemian and Moesian promontories formed gates towards this free space. At both the northern and southern corners, broad shear zones developed. The initial NE-directed tension was gradually replaced by a later E- to SE-directed tension as a consequence of the progressive termination of subduction roll-back along the arc from the Western Carpathians towards the Southern Carpathians. There is growing evidence that an E-W-oriented short compressional event occurred during the earliest Late Miocene but during the most of the Late Miocene extension was renewed. Starting from the latest Miocene roll-back terminated everywhere and a compressional stress field has propagated from the Southern Alps gradually into the Pannonian Basin, and resulted in Pliocene (?) through Quaternary tectonic inversion of the whole basin system.

Role of topography-induced gravitational stresses in basin inversion: the case study of the Pannonian basin

Numerical stress models suggest that gravitational body forces associated with elevated topography around sedimentary basins can significantly influence the stress and strain pattern in basin interiors. In the absence of tectonic forces, basins surrounded by high-altitude mountain ranges experience net horizontal compression. Owing to gravitational forces pointing from areas of high gravitational potential energy to subsided basin areas, further lithospheric extension can eventually terminate, leading to a gradual late stage inversion of the entire basin system. Modeling results suggest that the state of recent stress in the Pannonian basin, particularly in its western part, is controlled by the interplay of plate boundary forces, i.e., the counterclockwise rotation and northward indentation of the Adriatic microplate against the Alpine-Dinaric belt, and buoyancy forces associated with the elevated topography and related crustal thickness variation of the Alpine-Dinaric belt. Model calculations show that uplifted regions surrounding the basin system can exert compression on the thinned Pannonian lithosphere of ~ 40-60 MPa that is of the order of the assumed far-field tectonic stress magnitudes. The combined analysis of stress sources of tectonic and gravitational origin helps estimating the magnitude of maximum horizontal compression. High levels of compressional stresses (up to >100 MPa) are concentrated in the elastic core of the lithosphere, consistent with the ongoing structural inversion of the Pannonian basin system.

Recent tectonic stress and crustal deformation in and around the Pannonian Basin: data and models

Abstract Recent (active) tectonics of the Pannonian Basin and its surroundings has been investigated using data from over 900 earthquake focal mechanism solutions, 200 borehole breakout analyses, some in-situ stress measurements and by applying finite element modelling technique. We have established a database for indicators of recent stress, and analysed the stress state of the region by the methods of the World Stress Map project. The alignments of the largest horizontal stresses have been mapped and the tectonic regimes were also determined. We present a map of seismoactive faults and seismic energy release combining historical and modern seismicity data and results of local seismotectonic studies. The pattern of earthquake slip vectors and the style of faulting are summarised in order to characterise the active deformations. Our results show that the alignment of the largest horizontal stress exhibits a radial pattern around the Adriatic sea. In the Southern Alps and northwestern Dinarides the largest horizontal stress (SH) is aligned N-S and thrust faulting is dominant. Along the southern Dinarides and the Dalmatian coast thrusting with strike-slip component can be observed. Here the trajectories of SH are aligned NE-SW. E-W aligned SH trajectories and normal faulting are characteristic of the Rhodope Massif. Thrust faulting of the Vrancea region seems to be distinct from the compressive regime around the Adriatic sea. In the Pannonian Basin borehole breakout analyses show that the direction of largest horizontal stress is changing from N-S in the western part to NE-SW in the east. Most of focal mechanisms and available hydraulic fracturing measurements indicate strike-slip and thrust faulting inside the basin. The lack of normal faulting mechanisms indicates that the extension of the basin has been terminated and a new compressive stress regime prevails. The crustal deformation of the area is controlled by the counterclockwise rotation of Adria with respect to Europe around a pole at the 45°N latitude and 6–10°E longitudes, which is inferred from satellite geodesy and supported by earthquake slip vectors. This movement can explain the shortening of the Southern Alps, and squeezing eastward the region between the Adriatic sea and the Mur-Murz line. Rotation of Adria generates thrusts along the Dalmatian coast, and this compressive deformation extends into the land far from the coastline, and leads to squeezing of the Pannonian Basin from the southwest. The seismicity pattern in the Pannonian Basin shows that earthquakes are restricted to the crust and the control by pre-existing (mostly Miocene) fault zones is strongly masked by random activity due to general weakness of the lithosphere. Although earthquakes are of small to medium magnitude (M ≤ 6), the cummulative energy release is remarkably higher than in the surrounding Carpathian arc. The Vrancea zone is the only exception, where high energy release in the crust and down to 200 km depth is associated with a relict subducted slab. Finite element stress modelling has been performed in order to simulate the observed stress pattern and, hence, to understand the importance of different possible stress sources in and around the Pannonian Basin. The observed radial stress pattern of the region can be well explained by the counterclockwise rotation of the Adriatic microplate as a first-order stress source. Additional boundary conditions, such as the active deformation at the Vrancea zone and the role of rigid crustal blocks at the Bohemian Massif and the Moesian Platform, can significantly effect the style of deformation and the alignment of the largest horizontal stress. Furthermore, our calculations show that differences in the crustal thickness and the presence of large scale fault zones in the Pannonian region have only local influence on the model results.

Modes of basin (de)formation, lithospheric strength and vertical motions in the Pannonian-Carpathian system: Inferences from thermo-mechanical modelling

Abstract After a rapid multiphase evolution and a transition from passive to active rifting during late Early Miocene to Pliocene times, the Pannonian Basin has been subjected to compressional stresses leading to gradual basin inversion during Quaternary times. Stress modelling demonstrates the significance of the interaction of external plate-boundary forces and the effect of gravitational stresses caused by continental topography and crustal thickness variation. Flexural modelling and fission-track studies have elucidated the complex interplay of flexural downloading during collision, followed by rapid unroofing by unflexing and isostatic rebound of the lithosphere. The stretching and subsidence history of the Pannonian Basin, the temporal and spatial evolution of the flexure of the Carpathian lithosphere, and the lithospheric strength of the region reflect a complex history of this segment of the Eurasia-Africa collision zone. The polyphase evolution of the Pannonian-Carpathian system has resulted in strong lateral and temporal variation in thermo-mechanical properties in the area. Modelling results suggest that, as a whole, the Pannonian Basin has been an area of pronounced lithospheric weakness since Cretaceous time, shedding light on the high degree of strain localization in this region. This basin, the hottest in continental Europe, has a lithosphere of extremely low rigidity, making it prone to multiple tectonic reactivations. Another feature is the noticeable absence of lithospheric strength in the mantle lithosphere of the Pannonian Basin. Modelling studies suggest pronounced lateral variations in lithospheric strength along the Carpathians and their foreland, which have influenced the thrust load kinematics and post-collisional tectonic history. The inferences and models discussed in this paper are constrained by a large geophysical database, including seismic profiles, gravity and heat-flow data.

Slip tendency analysis as a tool to constrain fault reactivation: A numerical approach applied to three‐dimensional fault models in the Roer Valley rift system (southeast Netherlands)

We describe a new numerical approach to constrain the three‐dimensional (3‐D) pattern of fault reactivation. Taking advantage of the knowledge of the tectonic stress field, the ratio of the resolved shear and normal stresses (slip tendency) as well as the direction of the shear stress is calculated at every location on the faults modelled by triangulated surfaces. Although the calculated contact stresses represent only a first order approximation of the real stresses, comparison of the 3‐D pattern of slip tendency with the frictional resistance of the fault can provide useful constraints on the probability of fault reactivation. The method was applied to 3‐D geometrical fault models in the Roer Valley Rift System (southeast Netherlands) which is presently characterized by pronounced tectonic activity. The input stress tensors were constrained by published stress indicators. The analysis demonstrated that the observed fault activity could be explained within a reasonable range of frictional parameters and input stress magnitudes. In addition a fairly good correlation was found between the predicted slip directions and the focal mechanisms of local earthquakes. This suggests that in the study area, fault models being valid in the uppermost part of the crust are suitable to constrain fault reactivation even in the deeper part of the seismogenic layer. The analysis further demonstrated that fault hierarchy and the regional tectonic contexts of the fault system are important factors in fault reactivation. Therefore they always should be taken into account during evaluation of the calculated slip tendency and slip direction patterns.

Probing tectonic topography in the aftermath of continental convergence in central Europe

Continental topography is at the interface of processes taking place at depth in the Earth, at its surface, and above it. Topography influences society, not only in terms of slow processes of landscape change and earthquakes, but also in terms of how it affects climate. The Pannonian Basin-Carpathian Orogen System in Central and Eastern Europe represents a key natural laboratory for the development of a new generation of models for ongoing orogeny and its effect on continental topography development (Figure l). This system comprises some of the best documented sedimentary basins in the world, located within the Alpine orogenic belt, at the transition between the western European lithosphere and the East European Craton. It includes one of the most active seismic zones in Europe, with intermediate depth (50–220 km) mantle earthquakes of significant magnitude occurring in a geographically restricted area in theVrancea zone of southeastern Romania.

Tertiary brittle faulting and stress field evolution in the Gerecse Mountains, northern Hungary

Abstract The Gerecse Mountains of the Transdanubian Range in northern Hungary have suffered polyphase deformation during the Tertiary. The evolution of the stress fields was determined by the detailed structural survey of the area: the statistical analysis of microtectonic observations, the mapping and reinterpretation of fault pattern and the study of basin stratigraphy and geometry. The stress fields show an apparent clockwise rotation from Eocene until Quaternary times. Three main phases were reconstructed. During the Eocene-Early Oligocene E-W- to ENE-WSW-oriented compression and perpendicular tension affected the area. The formation and evolution of several sedimentary basins were controlled by fault systems activated by this phase. After a period of intensive erosion in the Early Oligocene the stress field rotated approx, 30–40°: NW-SE-directed compression and NE-SW-trending tension existed during the Late Oligocene and Early Miocene. After correction with palaeomagnetic data, an original Palaeogene σ 1 orientation is deduced NNE-SSW, reflecting the collisional processes of Europe and the Apulian microplate. The Middle Miocene to Pliocene period was characterised mainly by extensional deformation: the direction of the minimal stress axis ( σ 3 ) varied between E-W and NW-SE. The characteristic clockwise rotation pattern of the Tertiary palaeostress fields in the Gerecse Mts. is explained by the combined effect of the regional stress field changes in the Carpatho-Pannonian region and the rotation of the North Pannonian Unit.

Slope-toe Turbidite Systems Related to Aggradational - Progradational Sequences: Potential Stratigraphic Traps in the Makó Trough, Pannonian Basin, Hungary

Though the Mako Trough is best known as the location of major unconventional gas accumulations, the thick Neogene to Quaternary sedimentary successions may contain conventional HC resources as well. Structurally controlled traps are widespread on the neighbouring basement highs and not likely to occur in the basin interior. However, stratigraphic traps, untested so far, were identified in relation to the basin-filling progradational slope system. The style and pace of slope advancing are the key to understand sand delivery to and formation of potential reservoirs in the deep parts of the basin.

Tectonic Evolution and Burial History of the Makó trough, Hungary: Implications for the Exploration of Juvenile Unconventional Petroleum Systems in the Pannonian Basin

The Mako trough in Hungary is a sedimentary depression formed in the extensional Pannonian basin during the last 15-20 Ma. It represents a young HT/HP system with >6 km thick basin fill and has been recognized as the location of major unconventional hydrocarbon resources. Such accumulations are regarded as “unconventional” when economic production is only possible by means of some sort of stimulation technique, usually hydraulic fracturing. Hydrocarbons in this setting do not accumulate conventionally in structural or stratigraphic traps, but in pervasive cells. Due to the geological setting of the Mako trough, the hydrocarbon cell forms a relatively continuous zone marked by considerable internal lithological and petrophysical variability. Due to its novelty and complexity, the exploration of this unconventional resource demands the concurrent application of a wide range of geological and geophysical methods. Evaluation of such petroleum systems strongly rely on the reconstruction of the tectonosedimentary evolution of the host basin, the understanding of the subsidence, burial, thermal and maturation history, and the timing and mechanism of hydrocarbon generation and related abnormal pressure development. In this contribution, highlights are on the latest models for basin evolution and petroleum system development are presented.