F. Horváth

Tertiary evolution of the Intra-Carpathian area: A model

Abstract The Outer Carpathian flysch nappes encircle an Intra-Carpathian domain which can be divided into two megatectonic units (North Pannonian and Tisza) mostly on the basis of contrasting Mesozoic and Palaeogene facies development. We see two major kinematic problems to be solved: 1. (1) The present distribution of the Mesozoic and Palaeogene facies is mosaic-like, and some belts form exotic bodies within realms of Austroalpine affinity. 2. (2) Late Eocene palinspastic reconstruction of the Outer Carpathian flysch nappes suggest, that the entire Intra-Carpathian area must have been located several hundreds of kilometres to the south and to the west of its present position. Neogene extension can account for shortening in the external Carpathian nappes, but is unable to explain Mesozoic facies anomalies and offsets of Palaeogene formations. We suggest that evolution of the Intra-Carpathian area involved first Late Palaeogene-Early Miocene juxtaposition of the North-Pannonian and Tisza megatectonic units, accompanied by the closure of the external Carpathian flysch troughs; thereafter extension of this amalgamated unit occurred, which was compensated by thrusting of flysch nappes onto the European foreland and formation of molasse foredeeps. Eastward escape of the North-Pannonian unit from the Alpine collisional belt involved left lateral shear along the Pieniny Klippen belt and right lateral shear along the Mid-Hungarian zone. Parts of the Late Palaeogene basin and an Early Miocene volcanic edifice were dissected, offset and elongated by several 100 kms. The driving mechanism of the eastward escape of the Intra-Carpathian area can be related to the collision of Apulia and Europe and the subduction of the external Carpathian crust under the Pannonian units. The escape ceased gradually in the Early Miocene, when oblique collision between the North-Pannonian unit and European continent occurred. Neogene extension of the Pannonian region was an areal deformation. The extension at locally variable rate resulted in the break-up of the heterogenous floor of the Neogene basin. The driving mechanism of basin extension and contemporaneous compressional deformation of the external Carpathians is thought to be related to ongoing subduction, involving the marginal part of the attenuated European continental crust. Tectonic activity in the Carpathians and basin subsidence and volcanism shifted in time and in unison from the west toward the east-southeast.

Towards a mechanical model for the formation of the Pannonian basin

Abstract New maps showing crustal and lithospheric thickness variations in the Pannonian basin and the surrounding East Alpine, Carpathian and Dinaric mountains have been prepared on the basis of recent seismic, seismologic and electromagnetic data. A map presenting Miocene faults of regional significance has been also constructed for the same region by using a wealth of recently available national and/or more local studies. It is suggested that observed crustal and lithospheric structural contrasts have been controlled primarily by Neogene kinematic history of the region. Kinematic history is characterized by the following main patterns: 1. (1) indentation by Adria and large-scale backthrusting in the Southern Alps; 2. (2) eastward escape and extension of the Eastern Alps; 3. (3) extensional collapse in the Pannonian basin area; 4. (4) formation of broad wrench fault systems along strike in the Dinarides and Southern Carpathians, and the Western Carpathians with dextral and sinistral shear, respectively; 5. (5) compression and accretion of the external thrust and fold belt in the Eastern Carpathians. These kinematic patterns are thought to be interrelated and all are manifestations of the late-stage evolution of an overthickened orogenic wedge. In order to arrive at a better understanding of the mechanism of extension, which formed the Pannonian basin, deep crustal seismic profiles, hydrocarbon exploration reflection lines and borehole data have been analyzed in the Little Hungarian Plain. This plain represents the transition zone between the Alps and the Pannonian lowlands, and the results are illustrated by eight interpreted cross-sections. One result of regional importance is the clear recognition that the Transdanubian Central Range at the southeastern flank of the Little Hungarian Plain is composed of Alpine (pre-Senonian) thrust sheets. This finding marks the end of a century of debate: the allochthony of the substrata of the Pannonian basin can be now considered proven. Another result of more general interest is that these cross-sections document the mode of lithospheric extension. Preexisting compressional detachment planes reactivate as low-angle normal faults and lead to tectonic unroofing of deeply buried metamorphic terranes characterized by ductile flow along subhorizontal lineation.

Stress-induced late stage subsidence anomalies in the Pannonian Basin

Abstract Subsidence, sedimentation and tectonic quiescence of the Pannonian basin was interrupted a few million years ago by tectonic reactivation. This recent activity has manifested itself in uplift of the western and eastern flanks, and continuing subsidence of the central part of the Pannonian basin. Low- to medium-magnitude earthquakes of the Carpathian-Pannonian region are generated mostly in the upper crust by reverse and wrench fault mechanisms. There is no evidence for earthquakes of extensional origin. 2-D model calculation of the subsidence history shows that a recent increase in magnitude of horizontal compressional intraplate stress can explain fairly well the observed Quaternary uplift and subsidence pattern. We propose that this stress increase is caused by the overall Europe/Africa convergence. In Late Pliocene, consumption of subductible lithosphere at the eastern margin of the Pannonian basin was completed, and the lithosphere underlying the Pannonian basin became locked in a stable continental frame. Consequently extensional basin formation has come to an end, and compressional inversion of the Pannonian basin is in progress.

Styles of extension in the Pannonian Basin

Abstract Structural interpretation of reflection seismic profiles reveals distinct modes of upper crustal extension in the Pannonian Basin. While some subbasins in the Pannonian Basin complex show little extension (planar rotational normal faults), others are characterized by large magnitude of extension (detachment faults, metamorphic core complexes). Gravitational collapse of the Intra-Carpathian domain, combined with subduction zone roll-back is thought to be the driving mechanism of the Neogene back-arc extension. The very heterogeneously distributed extension is accommodated by transfer faults, which bound regions characterized by different polarity, direction, or amount of extension. In cross section these transfer faults are characterized by flower structures, typical for strike-slip faults. Seismic stratigraphic interpretations indicate that the non-marine post-rift sedimentary fill of the Pannonian Basin can be described in terms of sequence stratigraphy. The exceptionally good seismic sequence resolution allows recognition of third-order and also fourth-order depositional sequences, which may reflect the interplay of tectonics and eustasy, and Milankovitch scale climatic variations, respectively.

The formation of the intra-Carpathian basins as determined from subsidence data

The Carpathian arc is the result of continental collision during subduction of the European plate beneath a Pannonian continental block. In the Early/Middle Miocene, during and after the last stages of thrusting in the Outer Carpathians, several “back-arc” basins started to form within the Carpathian loop. These basins are of two types: (1) those lying in the peripheral regions of the intra-Carpathian lowlands (Vienna, West Danube, Transcarpathian and Transylvanian), and (2) those lying in the central intra-Carpathian region (East Danube, Little Hungarian and Great Hungarian (Pannonian)). Though both groups of basins have thin crust, the subsidence history and the present heat flow are different. The peripheral basins exhibit a rapid initial subsidence followed by a much slower general increase in depth. Their heat flow is close to the average for continental areas. In contrast the central basins have no initial subsidence but do show a fast linear increase in depth which has continued until the present. The heat flow is nearly twice the average for continents. We believe that the basins are thermal in origin and are the direct result of the continental collision which formed the Carpathian arc. The peripheral basins appear to be the result of uniform stretching of the lithosphere by about a factor of two. The rapid initial subsidence is an immediate isostatic adjustment to the stretching, the slower linear subsidence is due to conductive cooling of the thinned lithosphere. In the central basins, uniform stretching by about a factor of 3 could explain the thermal subsidence and the high heat flow. Unfortunately such a simple explanation is not supported by either the geology or the absence of a clearly defined initial subsidence. Alternative explanation involve crustal stretching with additional subcrustal thinning or, alternatively, attenuation of the whole subcrustal lithosphere and part of the crust by melting and erosion. Both explanations create a very thin lithosphere, reduce the initial subsidence to a minimum but still give a rapid thermal subsidence and high heat flow. The subsidence history gives quantitative information concerning the evolution of the inter-Carpathian basins. In other areas, it may place equally important constraints on the development of intercontinental basins and continental shelves.

Phases of compression during the evolution of the Pannonian Basin and its bearing on hydrocarbon exploration

Abstract Progress in understanding the structural evolution of the Pannonian Basin is reported. This has been driven by the application of seismic stratigraphy constrained by magnetostratigraphic data and the recent release of a great amount of hydrocarbon exploration data. This has led to a redefinition and better understanding of the syn-rift period and style of rifting. In addition, a complex structural evolution history during the post-rift phase has been recognized. Two compressive events are defined: one in the early stage and another in the late stage of evolution. The importance of these findings for hydrocarbon exploration includes an improved knowledge of the timing of trap formations and a possible explanation for remarkably variable reservoir pressures in pools of the Great Hungarian Plain.

Rare gas constraints on hydrocarbon accumulation, crustal degassing and groundwater flow in the Pannonian Basin

The isotopic composition and abundances of He, Ne and Ar have been measured in a sequence of vertically stacked gas reservoirs at Hajduszoboszlo and Ebes, in the Pannonian Basin of Hungary. The gas reservoirs occur at depths ranging from 727 to 1331 m, are CH4 dominated and occupy a total rock volume of approximately 1.5 km3. There are systematic variations in both major species abundances and rare gas isotopic composition with depth: CO2 and N2 both increase from 0.47 and 1.76% to 14.1 and 30.5%, respectively, and 40Ar/36Ar and 21Ne/22Ne increase systematically from 340 and 0.02990 at 727 m to 1680 and 0.04290 at 1331 m. A mantle-derived He component between 2 and 5% is present in all samples, the remainder is crustal-radiogenic He. The Ar and Ne isotope variations arise from mixing between atmosphere-derived components in groundwater, and crustally produced radiogenic Ar and Ne. The atmosphere-derived 40Ar and 21Ne decreases from 85 and 97% of the total 40Ar and 21Ne at 727 m to 18 and 68% at 1331 m. The deepest samples are shown to have both atmosphere-derived and radiogenic components close to the air-saturated water and radiogenic production ratios. The shallowest samples show significant fractionation of He/Ar and Ne/Ar ratios in atmosphere-derived and radiogenic rare gas components, but little or no fractionation of He/Ne ratios. This suggests that diffusive fractionation of rare gases is relatively unimportant and that rare gas solubility partitioning between CH4 and H2O phases controls the observed rare gas elemental abundances. The total abundance of atmosphere-derived and radiogenic rare gas components in the Hajduszoboszlo gas field place limits on the minimum volume of groundwater that has interacted with the natural gas, and the amount of crust that has degassed and supplied radiogenic rare gases. The radiogenic mass balance cannot be accounted for by steady state production either within the basin sediments or the basement complex since basin formation. The results require that radiogenic rare gases are stored at their production ratios on a regional scale and transported to the near surface with minimal fractionation. The minimum volume of groundwater required to supply the atmosphere-derived rare gases would occupy a rock volume of some 1000 km3 (assuming an average basin porosity of 5%), a factor of 670 greater than the reservoir volume. Interactions between groundwater and the Hajduszoboszlo hydrocarbons has been on a greater scale than often envisaged in models of hydrocarbon formation and migration.

Lithospheric structure of the Pannonian basin derived from seismic, gravity and geothermal data

Abstract The structure of the Pannonian basin is the result of distinct modes of Mid-Late Miocene extension exerting a profound effect on the lithospheric configuration, which continues even today. As the first manifestation of extensional collapse, large magnitude, metamorphic core complex style extension took place at the beginning of the Mid-Miocene in certain parts of the basin. Extrapolation of the present-day high heat flow in the basin, corrected for the blanketing effect of the basin fill, indicates a hot and thin lithosphere at the onset of extension. This initial condition, combined with the relatively thick crust inherited from earlier Alpine compressional episodes, appears to be responsible for the core complex type extension at the beginning of the syn-rift period. This type of extension is well documented in the northwestern Pannonian basin. Newly obtained deep reflection seismic and fission-track data integrated with well data from the southeastern part of the basin suggests that it developed in a similar fashion. Shortly after the initial period, the style of syn-rift extension changed to a wide-rift style, covering an area of much larger geographic extent. The associated normal faults revealed by industry reflection seismic data tend to dominate within the upper crust, obscuring pre-existing structures. However, several deep seismic profiles, constrained by gravity and geothermal modeling, image the entire lithosphere beneath the basin. It is the Mid-Miocene synrift extension which is still reflected in the structure of the Pannonian lithosphere, on the scale of the whole basin system. The gradually diminishing extension during the Late Miocene/Pliocene could not advance to the localization of extension into narrow rift zones in the Pannonian region, except some deep subbasins such as the Makó/Békés and Danube basins. These basins are underlain coincidently by anomalously thin crust (22–25 km) and lithosphere (45–60 km). Significant departures (up to 130 mW m−2) from the average present-day surface heat flow (c. 90 mW m−2) and intensive Pliocene alkaline magmatism are also regarded as evidence for the initiation of two newly defined narrow rift zones (Tisza and Duna) in the Pannonian basin system. However, both of these narrow rifts failed since the final docking of the Eastern Carpathians onto the European foreland excluded any further extension of the back-arc region.

Late Cenozoic evolution of the Pannonian basin

The Palaeozoic—Mesozoic nucleus of the Pannonian basin is the result of the collision of the European and Gondwanic microplates. The present basin was formed in the Late Cenozoic by the subsidence of this nucleus. The sinking was brought about by an active mantle diapir which was generated by the subduction associated with the formation of the surrounding mountains. The evidence and consequences of the mantle diapir are as follows. 1. (1) Strong Miocene andesitic-rhyolitic and Plio-Pleistocene basaltic volcanism. 2. (2) Geothermal highs and the high value of reduced heat flow (Qo = 1.2−1.7 HFU). 3. (3) Anomalous upper mantle with a lower density and elevated position of LVZ (low-velocity zone) and HCL (high-conductivity layer). The shallow depth of the HCL (40–60 km) shows that the geothermal highs are well developed in the upper mantle; this extra heat could not be the result of heat conduction only because of the young age (about 10 m.y.) of the basin, but convectionally transported heat has to be supposed as well. 4. (4) Continental-type thin crust, thinned out by the subcrustal erosion of the mantle diapir. The primary cause of the basin formation is the isostatic sinking of the thinnedout crust.

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.