Ferenc Horváth

The Pannonian Basin : a study in basin evolution

The Pannonian basin system is an integrap part of the Alpine mountain belts of east-central Europe. It is completely encircled by the Carpathian Mountains to the north and east, the Dinaric Alps to the south, and the Southern and Eastern Alps to the west. In 1912, Kober defined the Pannonian basin as one of the type “Zwischengebirge,” a relatively un-deformed region characterized by block faulting and situated between externally vergent thrust belts. More recent studies using subsurface data have shown that the Pannonian area was extensively deformed by Mesozoic thrusting and subsequently disrupted by a complex system of Cenozoic normal and wrench faults. Thus, the Pannonian “massif” has undergone several types of deformation, which are partly hidden by a thick sequence of sedimentary rocks of Neogene-Quaternary age. The Pannonian basin is actually a system of small, deep basins separated by relatively shallow basement blocks. The Neogene-Quaternary sedimentary rocks exceed 7 km in thickness in some areas, and the basin system (including the Transylvanian basin) is about 400 km from north to south and 800 km from east to west. It is currently interpreted by most workers as a Mediterranean back arc extensional basin of the middle Miocene age. The Carpathians, Eastern Alps, and Dinarides, which surround the Pannonian basin, are the result of Mesozoic and Cenozoic continental collision between Europe and several continental fragments to the south, including Africa. Thrusting was direted outward from the present Pannonian basin toward the European platform and the Adriatic region. In all the orogenic belts, the interior parts of the thrust belts were deformed in Mesozoic time, while the outer parts were deformed in Tertiary time. The volume presents 26 papers and eight regional maps resulting from a joint five-day symposium held in Veszprem, Hungary, in 1982 entitled “Evolution of Extensional Basins within Regions of Compression with Emphasis on the Intra-Carpathian Region.” The symposium was sponsored jointly by the Hungarian Oil and Gas Trust, the Hungarian Geological Survey, and the U.S. National Science Foundation.

Mantle dynamics in the Mediterranean

The Mediterranean offers a unique opportunity to study the driving forces of tectonic deformation within a complex mobile belt. Lithospheric dynamics are affected by slab rollback and collision of two large, slowly moving plates, forcing fragments of continental and oceanic lithosphere to interact. This paper reviews the rich and growing set of constraints from geological reconstructions, geodetic data, and crustal and upper mantle heterogeneity imaged by structural seismology. We proceed to discuss a conceptual and quantitative framework for the causes of surface deformation. Exploring existing and newly developed tectonic and numerical geodynamic models, we illustrate the role of mantle convection on surface geology. A coherent picture emerges which can be outlined by two, almost symmetric, upper mantle convection cells. The downwellings are found in the center of the Mediterranean and are associated with the descent of the Tyrrhenian and the Hellenic slabs. During plate convergence, these slabs migrated backward with respect to the Eurasian upper plate, inducing a return flow of the asthenosphere from the backarc regions towards the subduction zones. This flow can be found at large distance from the subduction zones, and is at present expressed in two upwellings beneath Anatolia and eastern Iberia. This convection system provides an explanation for the general pattern of seismic anisotropy in the Mediterranean, first-order Anatolia and Adria microplate kinematics, and may contribute to the high elevation of scarcely deformed areas such as Anatolia and Eastern Iberia. More generally, the Mediterranean is an illustration of how upper mantle, small-scale convection leads to intraplate deformation and complex plate boundary reconfiguration at the westernmost terminus of the Tethyan collision.

Transform faulting, extension, and subduction in the Carpathian Pannonian region

The Carpathian arc formed during the Cretaceous to Miocene continental collision of Europe with a smaller continental fragment following southward and westward subduction of an oceanic terrane. During and after the last stages of thrusting in the outer Carpathians, a set of discrete basins opened up inside the Carpathian loop. These basins are regions of local extension, which appear to be associated with strike-slip faults. In general, northeast- and northwest-trending sets of conjugate shears reflect east-west extension of the intra-Carpathian region during the middle and late Miocene. Palinspastic reconstruction of the basins indicates approximately 75 to 100 km of extension, comparable to the magnitude of synchronous crustal shortening in the East Carpathians. Extension may have occurred to accommodate continued westward (A-type) subduction after plate convergence was prohibited by geometrical constraints and suggests that in this region Miocene subduction is driven by forces acting on the downgoing plate.

The Sinai subplate and tectonic evolution of the northern Red Sea region

Abstract Although the precise boundaries and kinematics of the Sinai subplate are still doubtful, it has a significant role in the tectonic evolution of the northern Red Sea region. On the basis of earthquake distribution, the Sinai region can be considered as a subplate partially separated from the African plate by the Suez rift. The relative motion between Africa, Sinai and Arabia is the main source generating the present-day earthquake activity in the Gulf of Suez and the Gulf of Aqaba regions. According to geological observations, the southern segment of the Dead Sea fault system can be characterized by a left-lateral displacement of about 107km since the Middle Miocene, in contrast to the northern segment where only 25 to 35km offset can be inferred. We think that along the southern segment the total displacement was 72km until the late Miocene (10Ma). The earthquake activity is strongly reduced along the northern segment of the Dead Sea fault segment. Therefore, we suggest that the northern part (Yammouneh fault) evolves through initial cracking of the crust due to build-up of stress since the Pliocene time (5Ma) and propagates northward into Lebanon and Syria. This last 5 million years is the period when the southern and northern segments became linked and formed a single fault system with a new displacement of 35km. According to the proposed model the predicted opening pole of the Red Sea is near 34.0N, 22.0E with an angle of total rotation of 3.4 since the early miocene, providing a 0.82cm/a opening rate in the northern Red Sea. We suggest that the Dead Sea strike-slip fault was active since Middle Miocene time (15Ma) with a slip rate of 0.72cm/a to provide a total displacement of about 107km. This strike slip motion occured about an Euler pole near 33.0N, 21.0E with a rotation angle of about 3.0. It can be inferred from the proximity of the pole and angle of rotations for the Red Sea and Dead Sea fault that more than 85% of the motion has been accommodated on the Gulf of Aqaba and the Dead Sea fault and less than 15% in the Gulf of Suez. This model predicts a normal extensional motion in the Gulf of Suez with a minor left-lateral strike-slip component. We expect the pole of this motion to be at 31.0N, 29.0E, offshore of Alamein city about 320 km west of the Nile Delta. The rate of motion through the last 15Ma (Middle Miocene) is about 0.1 cm/a and the angle of rotation is 0.9. During this period the total opening of the Suez rift is 15 km while the rest of the motion (45 km) occured mainly through the first phase of the development before the Middle Miocene.

Seismicity of the Sinai subplate region:kinematicimplications

Abstract The seismic activity of the Sinai subplate region on the basis of both historical (2200B.C.–1900 A.D.) and recent (1900–1995) earthquake catalogs have been evaluated.Moderateand large earthquakes occurred mainly at the subplate boundaries, Dead Sea Fault (DSF) systemin the east, Cyprean arc in the north, and Suez rift in the southwest. Along the Dead Sea Fault system the activity concentrated at the southern andcentralsegments. The earthquake distribution appears to have a tendency to cluster in time andspace.The swarms (February, 1983; April, 1990; August, 1993 and November, 1995) in the GulfofAqaba indicate that the southern segment of the Dead Sea Fault system is the mostseismogenicthrough the last two decades. North of the Dead Sea depression the seismic activitytends to haveoccurred with NW trend to extend under the Levantine Sea. Although the northernsegment ofthe Dead Sea Fault system is well defined from geological, geophysical and historicalearthquakeactivity recent seismic activity is practically absent especially north of Latitude 34°N. In the eastern Mediterranean the seismicity is much higher in the area of the Hellenicarcthan in the Cyprean arc. Moreover, the activity occurs in a wide belt suggesting that theplateboundary is a deformation zone instead of a single line. The seismic activity in the Gulf of Suez is scattered and does not have any distincttrend.However, three active zones are delineated. At the mouth of the gulf most of activityisconcentrated where the Sinai triple junction (Africa, Arabia, Sinai) is situated. The centralpartand the northern part of the gulf include the adjacent area as far as the river Nile. Actually,theactivity is markedly decreased from south to north. Although there is no seismological evidence that the Suez rift continues into theeasternMediterranean, the activity in the Gulf of Suez region cannot be ignored. The parameters of magnitude-frequency relation ( a , b ) indicate thatthelevel of earthquake activity in the Sinai subplate region is generally moderate. Moreover,theenergy release curve shows a regular trend and reflects occasional high activity.

Recent stress field of the Sinai subplate region

Abstract Quaternary and recent tectonic stress data for the Sinai subplate region have been compiled including focal mechanism solutions, in-situ stress measurements, fault slip data and alignments of young volcanic feeders. The average direction of maximum horizontal stress (SH) has been determined by means of a linear interpolation method. The results indicate that the direction of the average maximum horizontal stress is closely parallel to the direction of the absolute motion of Africa. Analysis of the orientation of principal stress directions suggests that a large part of the Sinai subplate region is subjected to a strike-slip regime characterized by an average maximum horizontal stress trend of NW (54°W). For example, out of 50 earthquake focal mechanism solutions 52% are pure strike-slip, and 10% are either normal or thrust fault with a remarkable strike-slip component. The remaining are pure normal fault (18%), thrust fault (12%), or undetermined (8%). Three different tectonic subprovinces can be delineated. Northwestern Egypt, the eastern Mediterranean, Israel, Lebanon, and northern Syria are characterized by strike-slip often with a remarkable thrust faulting component stress regime. The northern Red Sea, the Sinai peninsula, and a significant part of the Arabian plate are characterized by strike-slip often with a remarkable normal faulting component stress regime. This is also the case for southwestern Turkey.

The link between tectonics and sedimentation in back‐arc basins: New genetic constraints from the analysis of the Pannonian Basin

The architecture of sedimentary basins reflects the relationship between accommodation space and sediment supply, their rates and localization being variable during basin evolution. The mechanisms driving the interplay between tectonics and sedimentation in extensional back-arc basins overlying rheological weak zones inherited from an earlier orogenic evolution are less understood. A typical example is the Pannonian back-arc basin of Central Europe. It is floored by continental lithosphere and was affected by large amounts of extension driven by the subduction rollback that took place in the Carpathians and/or Dinarides. A novel kinematic and seismic sequence stratigraphic interpretation calibrated by wells allows the quantification of the link between the formation of half grabens and coeval sedimentation in the Great Hungarian Plain part of the basin. While the lower order tectonic-induced cycles characterize the main phases of extension in various subbasins, the higher-order cyclicity and associated unconformities define individual moments of fault (re)activation. Our novel interpretation of a temporal and spatial migration of extension during Miocene times explains the contrasting present-day strike of various subbasins as a result of their gradual clockwise rotation. Incorporating the observed asymmetry, in particular the associated footwall exhumation, infers that the amount of extension is much larger than previously thought. The quantitative link between tectonics and sedimentation has allowed the definition of a novel model of sedimentation in asymmetric basins that can be ported to other natural scenarios of similarly hyperextended back-arc basins observed elsewhere.

Strike-slip tectonics in the Pannonian basin based on seismic surveys at Lake Balaton

Strike-slip tectonics has been the dominant style of deformation during the neotectonic (Pliocene and Quaternary) evolution of the Pannonian basin. Main faults are exposed in the “island mountains” of the basin, but strike-slip tectonic features can be best studied in the basin fill by seismic data. Lake Balaton offers the opportunity to carry out high to ultra-high-resolution seismo-acoustic surveys to image stratigraphic and tectonic features in the central part of the Pannonian basin. Several campaigns in the lake using different acquisition techniques have resulted in more than 2000-km seismo-acoustic profiles with a range of resolutions and penetration depths. Interpretation of faults and folds shows a few kilometers wide shear zone below the lake in Late Miocene–Pliocene strata. This zone can be identified as the continuation of the Balatonfő line known onshore to the east of the lake. Mapping revealed a set of duplex structures and highlighted the importance of this shear zone in the formation of Lake Balaton. Comparison of our results to analogue clay models suggests that the observed shear zone is sinistral and the horizontal displacement is on the order of hundreds of meters. Looking at 3D industrial seismic data to the south of the lake, we suggest that the first-order Balaton line, which represents the continuation of Periadriatic line, is also sinistral and characterized by small horizontal displacement of about 1.0–1.5xa0km during Pliocene and Quaternary times. This indicates a 0.2–0.3xa0mm/year average slip rate, which is compatible with recent GPS measurements.

Intraplate volcanism in the Danube Basin of NW Hungary: 3D geophysical modelling of the Late Miocene Pásztori volcano

Three-dimensional geophysical modelling of the early Late Miocene Pásztori volcano (ca. 11–10xa0Ma) and adjacent area in the Little Hungarian Plain Volcanic Field of the Danube Basin was carried out to get an insight into the most prominent intra-crustal structures here. We have used gridded gravity and magnetic data, interpreted seismic reflection sections and borehole data combined with re-evaluated geological constraints. Based on petrological analysis of core samples from available six exploration boreholes, the volcanic rocks consist of a series of alkaline trachytic and trachyandesitic volcanoclastic and effusive rocks. The measured magnetic susceptibilities of these samples are generally very low suggesting a deeper magnetic source. The age of the modelled Pásztori volcano, buried beneath a 2xa0km-thick Late Miocene-to-Quaternary sedimentary sequence, is 10.4xa0+/−xa00.3xa0Ma belonging to the dominantly normal C5 chron. Our model includes crustal domains with different effective induced magnetizations and densities: uppermost 0.3–1.8xa0km thick layer of volcanoclastics underlain by a trachytic-trachyandesitic coherent and volcanoclastic rock units of a maximum 2xa0km thickness, with a top situated at minimal depth of 2.3xa0km, and a deeper magmatic pluton in a depth range of 5–15xa0km. The 3D model of the Danube Basin is consistent with observed high ΔZ magnetic anomalies above the volcano, while the observed Bouguer gravity anomalies correlate better with the crystalline basement depth. Our analysis contributes to deeper understanding of the crustal architecture and the evolution of the basin accompanied by alkaline intraplate volcanism.

Seismic expressions of shallow gas in the lacustrine deposits of Lake Balaton, Hungary

Lake Balaton, a large shallow lake in Central Europe (Hungary), has been the site of extensive ultrahigh- nresolution acoustic and multichannel seismic profiling in the period of 1997–2013. These surveys showed the widespread occurrence of shallow gas in the lake sediments and their immediate substrata. We analyzed about 2000 km of two-dimensional profiles and mapped the different gas occurrences in the uppermost 20 m. The anomalies caused by free gas were identified, classified, and assigned to upper, middle and lower levels based on gas signatures and stratigraphic position. Monitoring of the uppermost gas front has revealed temporal variations between surveys from different years and seasons that manifested in the changes of free gas content in the upper two levels. Free gas in the lower part of the lake sediments and at around the base of the mud indicated greater stability. The different nature of the three free gas levels can be explained by vertical changes in quantity, production rate, and solubility of methane and carbon dioxide gases. We suggest that methane was derived from the microbial decomposition of organic matter in the mud and Pleistocene peat at the base of the mud, whereas CO2 is transported to the lower mud layers by upwelling fluids.