László Csontos

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.

Review of Neogene and Quaternary volcanism of the Carpathian-Pannonian region

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The Mid-Hungarian line: a zone of repeated tectonic inversions

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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.

Evolution of the stress fields in the Carpatho-Pannonian area during the Neogene

Abstract Observations of microtectonic faults in and around the Pannonian basin suggest that the Tertiary evolution of the area was controlled by large-scale strike-slip fault activity. Statistical analysis of the fault pattern for different localities makes it possible to separate several conjugate sets of strike-slip faults and related dilatational and compressive structures. These stress fields have a distinct rotation pattern with time. Inversion of fault data suggests a characteristic rotation of the principal stress axes with time. Some rigid body rotations are indicated by paleomagnetic data. There are basically three contrasting mechanisms to explain this observation: 1. (1) Appropriate rotation of the regional (e.g., European) stress field; 2. (2) rotation of larger tectonic units (e.g., microcontinents) under a stable regional stress field; and 3. (3) coherent rotation of smaller, detached continental blocks under a stable regional stress field. Our model for the Neogene tectonic evolution of the Pannonian area combines these mechanisms. Moderate microcontinent rotation may have occurred during the Paleogene-Early Miocene. In the early Middle Miocene, the microcontinents were dissected by a system of strike-slip and normal faults, created by a N-S compressive/E-W extensive stress field. Gravity spreading of the Pannonian units and block rotation within bounding strike-slip faults are suggested to explain rotated magnetic directions and fault patterns. In the Late Neogene, clockwise rotation of the general stress-field by at least 40° is thought to be responsible for the reactivations of older strike-slip faults and for the creation of new sets. This change in the orientation of the stress field is related to the eastward and then southward shift of the Carpathian subduction activity.

Tectonic evolution of the northwestern Internal Dinarides as constrained by structures and rotation of Medvednica Mountains, North Croatia

Abstract This paper attempts to explain the tectonic history and possible reasons for the change of trend of the northwestern part of the Internal Dinarides in a transitional area between the Southeastern Alps, central Dinarides and Tisia, north of Zagreb. Structural and palaeomagnetic data collected in pre-Neogene rocks at Medvednica Mountains, combined with palaeomagnetic data available from Neogene rocks in the surrounding area, point to the following conclusions: (1) The reason for dramatic deflection in structural trend of the Internal Dinarides in the area north of Zagreb is a 130° clockwise rotation and eastward escape of a tectonic block comprising Medvednica Mountains and the surrounding inselbergs, bounded to the north by the easternmost tip of the Periadratic Lineament. In Medvednica Mountains, the main period of tectonic escape and associated clockwise rotation occurred in the Late Palaeogene, possibly in the Oligocene–earliest Miocene. (2) When rotated into the original position, the trend of observed pre-Neogene structures of Medvednica Mountains becomes parallel to the major structural trend of the central Dinarides. In view of their original orientation, these structures are interpreted in the following way: (a) The first D1 deformational event is attributed to the Aptian–Albian nappe stacking in the central–northern Dinarides that was accommodated by a top-to-the-north directed shearing and northward propagation of already obducted ophiolites of the Central Dinaridic ophiolite zone. This nappe stacking, which resulted in a weak regional metamorphism in tectonic units underlying the ophiolites, was orogen-parallel or at a very acute angle to known structural (and possibly palaeogeographic) trends. This implies a major left-lateral shear component along the former Adriatic margin and obducted Dinaridic ophiolite zone. (b) This was followed by Early Albian orogen-perpendicular shortening (D2) that was accommodated by folding and top-to-the-west thrusting. This deformation resulted in gradual cooling of the metamorphic stack and also in uplift and erosion of the higher structural units. (c) The D3 deformational event was driven by renewed E–W shortening that took place after the Paleocene, most probably during the Middle Eocene–Oligocene, i.e. synchronous with the main Dinaridic tectonic phase of the External Dinarides. This shortening was probably triggered by collision and thrusting of Tisia over the northern segment of the Internal Dinarides. (d) This was finally followed by D4 pervasive, right-lateral N–S shearing that is tentatively interpreted as being related to the right-lateral shearing of the Sava zone during the Eocene–Oligocene. (e) Following the main period of tectonic escape and induced clockwise rotation along the Periadriatic fault, possibly in the Oligocene–earliest Miocene, the Medvednica Mountains and the surrounding area were affected by repeated extensions and inversions since the Early Miocene to recent times. Palaeomagnetic data suggest that in the Early Miocene (but probably before the Karpatian) this area was part of a regional block that shifted northwards and rotated in a counter-clockwise sense. A second episode of counter-clockwise rotation occurred at the present latitude in post-Pontian times (since c. 5 Ma), driven by the counter-clockwise rotating Adriatic Plate.

Tertiary deformation history from seismic section study and fault analysis in a former European Tethyan margin (the Mecsek–Villány area, SW Hungary)

Abstract Outcrop-scale structural data and seismic section interpretation are combined to unveil a very complicated Tertiary deformation history of a once Tethyan margin: the Mecsek–Villany area of Hungary. This combination of data helped to reconstruct the possible activity of individual fault zones. At least four ENE–WSW striking zones—the Northern Imbricates, the South Mecsek zone, the Gorcsony–Mariakemend ridge and the Villany Mountains—were confirmed as regional long-lived transpressive zones with very complicated internal deformation, frequently with oppositely dipping thrust faults. Tertiary structural history began with a roughly N–S-directed shortening in the South Mecsek zone. It was followed by a NE–SW-directed transpression activating practically all important wrench zones together with perpendicular transfer faults. Basins were created along some of these deformation zones, but were also affected by major tilts due to inversion. After a relatively quiescent period in the Middle Miocene, the Late Sarmatian inversion followed. Shortly after, this event was relayed by a NE–SW-directed extension–transtension. An important inversion period characterised by NW–SE compression occurred in Late Pannonian (Messinian), when all the former wrench zones were reactivated as right-lateral shear. This event is responsible for the present topography of the region.

Early Tertiary structural evolution of the border zone between the Pannonian and Transylvanian Basins

Abstract The Mid-Hungarian Lineament is the most important tectonic feature of the Intra-Carpathian area. Its evolution is closely related to the Early Tertiary (Palaeogene-Early Miocene) episodes of basin formation. This paper attempts to explain the structural relationships between the Transylvanian Palaeogene Basin and the Mid-Hungarian Lineament. Microtectonic data collected on the field, combined with the available geological and geophysical data point to the following conclusions. (1) The first compressional event is probably Early Oligocene in age, and is characterized by σ1 oriented ENE-WSW. We assume that the Oligocene basin is flexural in origin. (2) The second phase of compression occurred during Early Miocene times with σ1 oriented NNW-SSE. The last thrust emplacement accounts for the Ottnangian-Karpatian overthrusting of the Alcapa block on top of the Tisza-Dacia block. This deformational phase could have induced large-scale block rotations in both units. (3) Late Miocene left lateral faulting occurred along the Dragos-Voda fault system, the effects of which can be traced as far as Hungary. This event most probably corresponds to an eastward escape of the Tisza-Dacia block with respect to surrounding terranes.

The Variscan belt of Northern France–Southern Belgium: geodynamic implications of new palaeomagnetic data

Abstract Palaeomagnetic investigations were carried out in Devonian–early Carboniferous rocks of the Variscan foreland chain of Northern France–Southern Belgium in order to reveal the origin of its arcuate shape. The Brabant Parautochthon was sampled in the Boulonnais (near Calais) and near Tournai, while the Ardenne Allochthon was sampled near Maubeuge and in the Givet area. All the sampled localities yielded characteristic remanent magnetization as a result of stepwise demagnetization and component analysis. Fold or tilt tests were possible for three localities, with negative results indicating pervasive remagnetization. The tectonic position was sub-horizontal at two localities, while the tilt was monoclinal for the rest. Therefore, the acquisition time of the magnetic signals was estimated by comparing the palaeolatitude computed from each magnetic component to the palaeolatitudes of Variscan Europe calculated after Van der Voo (1993) . Three components showing: A, a southern B, a near-Equatorial, and C, a northern palaeolatitude are recognized from our data. Since a pre-Variscan age of component A (observed only in Boulonnais, at 10 sites) is not supported by data, it is assigned to an early phase of deformation. Component B (16 sites) was acquired during the peak of the Variscan tectonics (late Westphalian), while component C (five sites) originated during Permian times. Regardless of the palaeolatitudes, declinations fall between 190 and 210°, thus being conformable with the declinations expected for Variscan Europe. The declinations show no correlation with the arcuate shape of the belt, neither are they different in the Paraauthochthon and in the Allochthon, nor in the different components. Arc formation by moulding of the Allochthon on the Brabant Parautochthon is, therefore, not supported by these data, since this mechanism requires substantial (opposed) rotations on both wings of the arc. The available palaeomagnetic data are conformable with a pre-formed arc, simply docking to the Brabant obstacle of similar shape. Variable offsets along a main thrust make possible a third model, which slightly unfolds the former passive arc.

Compression during extension in the Pannonian Basin and its bearing on hydrocarbon exploration

Formation of a Late Cretaceous thrust and fold belt was followed by major wrench faulting which eventually juxtaposed the originally distant orogenic terranes making up now the substrata of the Pannonian basin.