Miroslav Bielik

Rheology predictions across the western Carpathians, Bohemian massif and the Pannonian basin: Implications for tectonic scenarios.

On the basis of extrapolation of failure criteria, lithology, and temperature models, we predict the rheology of the lithosphere for several sections through the Carpathians and surrounding regions. Our models show significant lateral variations in rheology for the different tectonic units, with important implications for the tectonic evolution. The rheologically strong lithosphere of the Polish Platform area contrasts with the weak lithosphere of the Pannonian basin, indicating that the arcuate shape of the Carpathian orogen is primarily caused by an inherited curvature of an ancient embayment in the foreland, with the Pannonian units passively filling the space. The Polish Platform and the Moesian Platform exhibit a similar rheological anisotropy caused by NW-SE trending weakness zones paralleling the Tornquist - Teisseyre zone. This anisotropy was the main controlling factor on the behavior of the lithosphere in this area since Cadomian times, as documented by the geological evolution of the Sudety Mountains and the Mesozoic Polish Trough, including the Late Cretaceous Alpine inversion and the Neogene development of the Carpathian foreland. This rheological anisotropy appears to have a major controlling impact on the development of at least the eastern part of the European lithosphere. Rheology predictions for the Bohemian massif support the idea that the rigid lithosphere of the Bohemian massif governed the bending of the Alpine-Carpathian transition zone, expressed in the large-scale wrench movements opening the Vienna basin. In the foreland area, detachment levels are predicted for upper and lower crustal levels, leading to a decoupling of crustal and subcrustal flexure in most areas. Comparison with basin formation models indicates that our predictions for effective elastic thickness (EET) are similar to those derived from flexural models for the foreland area. Also, EET predictions from extensional basin models in the Pannonian region yield values close to our findings.

Gravity modelling of the lithosphere in the Eastern Alpine-Western Carpathian-Pannonian Basin region

Abstract Gravity models illustrate changes in the degree of continental convergence in the Eastern Alpine-Western Carpathian region, and modifications to the lithosphere due to the plate convergence and subsequent Pannonian Basin extension. Analysis of the continental collision zone incorporates a kinematic model of ocean basin closure, whereby gravity anomalies and topography are viewed as part of a continuum of continental crustal shortening, erosion and isostatic rebound. Thick crust and high topography in the Eastern Alps, along with a broad Bouguer anomaly of −140 mGal amplitude, are consistent with about 175 km of crustal shortening, followed by 10 km of isostatic rebound. Eastward, crustal thicknesses and gravity anomaly widths and amplitudes are less, so that only about 50 km of continental crustal shortening and 4 km of rebound occurred in the Western Carpathians. Preservation of thick flysch deposits and small isostatic rebound are attributable to the high-density, shallow mantle of the intact continent-ocean transition zone. Seismic delay time studies have suggested that, relative to the average thickness of the region, the lithosphere thickens by about 70 km beneath the Eastern Alps and thins by about 60 km under the Pannonian Basin. In both regions, gravity anomalies cannot be explained fully without considering this large relief on the lithosphere/asthenosphere boundary. The Eastern Alpine crustal root, which extends 15 km below the average depth for the region, overcompensates the topography and results in gravity anomalies that are 40 mGal lower than those observed; the extra 70 km of lithosphere provides excess mass that achieves isostatic equilibrium and accounts for the 40 mGal difference. Observed gravity anomalies and local isostasy are also consistent with thin crust and thin lithosphere beneath the Pannonian Basin, whereby the 60 km of extra asthenosphere provides a large part of the compensation for the elevated mantle. Regional cross sections suggest that shallowing of the lithosphere/asthenosphere boundary, associated with Pannonian Basin extension, has propagated northward beyond the Carpathians, to within the European Platform. Crustal thinning, however, appears to be confined to exotic terranes of the Carpathian interior, so that crustal structure in the Eastern Alps and Outer Carpathians is a remnant of the earlier collision orogen.

A preliminary stripped gravity map of the Pannonian Basin

Abstract By removing the gravity effects of the known near-surface sources from the gravity map we obtain a ‘stripped gravity map’ that enables a better interpretation of the deep-seated geological structures of the Earths crust. The total gravity effect of anomalous bodies is acquired by summation of the gravity effects of n -sided vertical prisms with horizontal bases. The stripping procedure has a more accurate physical foundation than any other mathematical method of gravity field analysis, provided that the geometry and density of the disturbing bodies are known with sufficient accuracy. This method was applied to the Pannonian Basin.

Neogene and Quaternary development of the Turiec Basin and landscape in its catchment: a tentative mass balance model

Neogene and Quaternary development of the Turiec Basin and landscape in its catchment: a tentative mass balance model The development of the Turiec Basin and landscape evolution in its catchment has been reconstructed by methods of geological research (structural geology, sedimentology, paleoecology, and geochronological data) as well as by geophysics and geomorphology. The basin and its surrounding mountains were a subject of a mass balance study during periods of tectonic activity, accompanied by considerable altitudinal differentiation of relief and also during quiet periods, characterized by a development of planation surfaces in the mountains. The coarse clastic alluvial fans deposited beneath the offshore pelitic sediments document the rapid Middle Miocene uplift of mountains on the margin of the Turiec Basin. The Late Miocene finegrained sedimentation represents the main fill of this basin and its origin was associated with the formation of planation surfaces in the surrounding mountains. The rapid uplift of the western and northern parts of the catchment area during the latest Miocene and Early Pliocene times further generated the deposition of coarse-grained alluvial fans. The Late Pliocene basin inversion, due to uplift of the whole Western Carpathians mountain chain, was associated with the formation of the Early Quaternary pediment and ultimately with the formation of the Turiec river terrace systems.

Geophysical study of the Ota-VF Xira-Lisbon-Sesimbra fault zone and the lower Tagus Cenozoic basin

This paper focuses on the interpretation of seismic reflection, gravimetric, topographic, deep seismic refraction and seismicity data to study the recently proposed Ota–Vila Franca de Xira–Lisbon–Sesimbra (OVLS) fault zone and the lower Tagus Cenozoic basin (LTCB). The studied structure is located in the lower Tagus valley (LTV), an area with over 2 million inhabitants that has experienced historical earthquakes which caused significant damage and economical losses (1344, 1531 and 1909 earthquakes) and whose tectonic sources are thought to be local but mostly remain unknown. This study, which is intended as a contribution to improve the seismic hazard of the area and the neotectonics of the region, shows that the above-proposed fault zone is probably a large crustal thrust fault that constitutes the western limit of the LTCB. Gravimetric, deep refraction and seismic reflection data suggest that the LTCB is a foreland basin, as suggested previously by some authors, and that the OVLS northern and central sectors act as the major thrusts. The southern sector fault has been dominated by strike-slip kinematics due to a different orientation to the stress field. Indeed, geological outcrop and seismic reflection data interpretation suggests that, based on fault geometry and type of deformation at depth, the structure is composed of three major segments. These data suggest that these segments have different kinematics in agreement with their orientation to the regional stress field. The OVLS apparently controls the distribution of the seismicity in the area. Geological and geophysical information previously gathered also points that the central segment is active into the Quaternary. The segment lengths vary between 20 and 45 km. Since faults usually rupture only by segments, maximum expectable earthquake magnitudes and other parameters have been calculated for the three sectors of the OVLS fault zone using empirical relationships between earthquake statistics and geological parameters available from the literature. Calculated slip rates are compatible with previous estimates for the area (0.33 mm yr –1 ). A more accurate estimation of the OVLS throw in the Quaternary

3D gravity interpretation of the pre-Tertiary basement in the intramontane depressions of the Western Carpathians: a case study from the Turiec Basin

Abstract New results related to the thickness and density of the sedimentary fill of the Turiec Basin allowed us to construct the first original stripped gravity map for this typical intramontane Neogene depression of the Western Carpathians. The stripped gravity map of the Turiec Basin represents the Bouguer gravity anomalies corrected for the gravity effect of the density contrast of its Quaternary-Tertiary sedimentary basin fill. It means that the map reflects the gravity effects of the density inhomogeneities which are located beneath the sedimentary basin fill. This map is therefore suitable for the interpretation of the structure and composition of the pre-Tertiary basement. Based on the new data analysis, two different density models of the sedimentary fill were constructed. The 3D density modelling was used to calculate the gravity effect of the density models. The stripped gravity maps were produced by subtracting the density model gravity effects from Bouguer anomalies. The regional trend was also removed from the stripped gravity maps. The residual stripped gravity maps were consequently used for geological interpretation of the pre-Tertiary basement of the Turiec Basin. The pre-Tertiary basement of the Turiec Basin can be divided into northern and southern parts due to its gravity characteristics. Furthermore the northern part can be split into two domains: western and eastern. The crystalline basement of the western domain is probably formed by the Hercynian crystalline basement of the Tatric Unit. In the eastern domain the basement could consist mostly of the Mesozoic complexes of the Fatric Unit. The southern part of the pre-Tertiary basement of the Turiec Basin is built predominantly by Mesozoic complexes of the Hronic Unit. It is suggested that the Hronic Unit also forms the bedrock of the volcano-sedimentary complex of the Kremnické vrchy Mts. The resultant stripped gravity maps and the map of total horizontal gravity gradients have also proven to be very useful for the interpretation of faults or fault systems in the study area. Various faults, particularly of NNE-SSW and NW-SE directions were discovered. The analysis of the faults indicates clearly that the contact of the Turiec Basin with the Malá Fatra Mts and the Veľká Fatra Mts is tectonic.

Lithospheric structure of Central Europe: Puzzle pieces from Pannonian Basin to Trans‐European Suture Zone resolved by geophysical‐petrological modeling

We have analyzed the thermochemical structure of the mantle in Central Europe, a complex area with a highly heterogeneous lithospheric structure reflecting the interplay of contraction, strike slip, subduction, and extension tectonics. Our modeling is based on an integrative 3-D approach (LitMod) that combines in a self-consistent manner concepts and data from thermodynamics, mineral physics, geochemistry, petrology, and solid Earth geophysics. This approach minimizes uncertainties of the estimates derived from modeling of various data sets separately. To further constrain our 3-D model we have made use of the vast geophysical and geological data (2-D and 3-D, shallow/crustal versus deep lithospheric experiments) based on experiments performed in Central Europe in the past decades. Given the amount and the different nature/resolution of the available constraints, one of the most challenging tasks of this study was to consistently combine them, finding a trade-off between all local and regional data sets available in a way that (i) preserves as many structural details as possible and (ii) summarizes those data sets into a single robust regional model. The resulting P/T-dependent mantle densities are in LitMod 3-D calculated based on a given mineralogical composition. They therefore provide more reliable estimates compared to pure gravity models, which enhance modeling of the crustal structures. Our results clearly indicate presence of several lithospheric domains characterized by distinct features, Pannonian Basin being one of the most outstanding ones. It has the thinnest crust and lithosphere in the area modeled, characterized by relatively fertile composition.

Geoelectrical and geological structure of the crust in Western Slovakia

Electrical resistivity of the Earth’s crust is sensitive to a wide range of petrological and physical parameters, and it particularly clearly indicates crustal zones that have been tectonically or thermodynamically disturbed. A complex geological structure of the Alpine nappe system, remnants of older Hercynian units and Neogene block tectonics in Western Slovakia has been a target of recent magnetotelluric investigations which made a new and more precise identification of the crustal structural elements of the Western Carpathians possible. A NW-SE magnetotelluric profile, 150 km long, with 30 broad-band and 3 long-period magnetotelluric sites, was deployed, crossing the major regional tectonic elements listed from the north: Brunia (as a part of the European platform), Outer Carpathian Flysch, Klippen Belt, blocks of Penninic or Oravicum crust, Tatricum and Veporicum. Magnetotelluric models were combined with previous seismic and gravimetric results and jointly interpreted in the final integrated geological model. The magnetotelluric models of geoelectrical structures exhibit strong correlation with the geological structures of the crust in this part of the Western Carpathians. The significant resemblance in geoelectrical and crustal geological structures are highlighted in shallow resistive structures of the covering formations represented by mainly Tertiary sediments and volcanics. Also in the deeper parts of the crust highly resistive and conductive structures are shown, which reflect the original building Hercynian crust, with superposition of granitoids or granitised complexes and lower metamorphosed complexes. Another important typical feature in the construction of the Western Carpathians is the existence of young Neogene steep fault zones exhibited by conductive zones within the whole crust. The most significant fault zones separate individual blocks of the Western Carpathians and the Western Carpathians itself from the European Platform.

Structure of the Lithosphere in the Carpathian–Pannonian Region

This chapter summarizes the present data on the change of the thickness of the lithosphere in the Carpathian-Pannonian region obtained by seismics and seismology, and magnetotelluric and gravity studies, emphasizing the updoming of the asthenosphere below the Pannonian Basin in accordance with its high heat flow. Important conclusions have been drawn from the stripped gravity map concerning the deep crustal structures, including the intrusion in the narrow subbasins. Special attention is paid to the narrow rift zone of the Bekes subbasin with high-density mafic intrusion in its crust and the elevation of Moho and asthenosphere during the rifting as it was stated by reflection seismics and interpretation of magnetotelluric sounding measurements and supported by stripped gravity map and gravity modeling.

Joint interpretation of gravity and magnetic data in the Kolárovo anomaly region by separation of sources and the inversion method of local corrections

Abstract We present a new interpretation of the Kolárovo gravity and magnetic anomalies in the Danube Basin based on an inversion methodology that comprises the following numerical procedures: removal of regional trend, depth-wise separation of signal of sources, approximation of multiple sources by 3D line segments, non-linear inversion based on local corrections resulting in found sources specified as 3D star-convex homogenous bodies and/or 3D contrasting structural contact surfaces. This inversion methodology produces several admissible solutions from the viewpoint of potential field data. These solutions are then studied in terms of their feasibility taking into consideration all available tectono-geological information. By this inversion methodology we interpret here the Kolárovo gravity and magnetic anomalies jointly. Our inversion generates several admissible solutions in terms of the shape, size and location of a basic intrusion into the upper crust, or the shape and depth of the upper/lower crust interface, or an intrusion into the crystalline crust above a rise of the mafic lower crust. Our intrusive bodies lie at depths between 5 and 12 km. Our lower crust elevation rises to 12 km with and 8 km without the accompanying intrusion into the upper crust, respectively. Our solutions are in reasonable agreement with various previous interpretations of the Kolárovo anomaly, but yield a better and more realistic geometrical resolution for the source bodies. These admissible solutions are next discussed in the context of geological and tectonic considerations, mainly in relation to the fault systems.