László Lenkey

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.

Investigating the hydrogeology of a water-supply area using direct-current vertical electrical soundings

The drinking water of Budapest (Hungary) is supplied by wells located along the Danube River. In one of the water-supply areas, groundwater polluted by nitrate flows from the upper terrace of the river toward the lower terrace and the production wells. In order to protect the wells from pollution, the waterworks of Budapest has to know the hydrogeology of the area in detail. Since groundwater flow strongly depends on the type and distribution of the subsurface materials, we made 230 dc vertical electrical soundings (VES) in the lower terrace over an area of 6 km2 to map the subsurface conditions. There is a 200–300-m-wide high-resistivity zone (200–400 ohm-m) running parallel to the river and separated from the upper terrace by a 200–500-m-wide low-resistivity zone (<50 ohm-m). The low resistivity is due to silty and clayey sand, as indicated by monitoring wells. These sediments have low hydraulic conductivity; therefore, they form a hydraulic barrier against the polluted groundwater flow coming from the ...

Two-dimensional modelling of stratigraphy and compaction-driven fluid flow in the Pannonian Basin

Abstract During the Pliocene-Quaternary time interval, the peripheral parts of the Pannonian Basin system have been uplifted and subsidence in the basin centre accelerated, causing a distinctive truncation pattern in the basin stratigraphy. Stress analyses indicate that the Pannonian Basin system, originally formed in an extensional regime, is subjected to a compressive stress since the early Pliocene. Results of forward modelling of basin subsidence and sedimentary filling along a cross-section through the southern part of the Pannonian Basin demonstrate that a change of the basin shape due to the compressive stress can successfully explain the observed pattern of differential uplift and subsidence occurring since the early Pliocene. In addition, the forward modelling of subsidence and fill provides constraints for the depth of lithospheric necking during extension, the palaeowater-depth history and lake-level changes in the southern part of the Pannonian Basin. Compaction-driven fluid flow modelling shows that the first significant overpressures in the southern part of the Pannonian Basin developed during progradation of a large deltaic system, at a time when sedimentation rates increased rapidly. Due to the stress-induced acceleration of subsidence during Pliocene to Quaternary times, sedimentation rates increased again, causing a further increase of overpressure. The Pliocene stress induced uplift of the basin flanks combined with a preceding lake-level fall created a larger gravity potential of the groundwater table, enhancing the influx of meteoric water into the basin. This can explain observed diagenetic patterns in the southern part of the Pannonian Basin.

Quantitative investigation of physical properties of mantle plumes in three-dimensional numerical models

Numerical calculations have been carried out to investigate the physical properties of mantle plumes in highly viscous thermal convection depending on the Rayleigh number (Ra). The Boussinesq approximation was applied in a three-dimensional Cartesian domain filled with isoviscous, purely bottom-heated fluid with infinite Prandtl number. In order to monitor the dynamical behavior of plumes an automatic plume detecting routine was developed based on the temperature between the plume and its surroundings. It was established that as the convection becomes more vigorous with increasing Rayleigh number the average cross-sectional area of an individual plume decreases (∼Ra−2∕3), the vertical velocity in plumes increases (∼Ra2∕3), while the average temperature in plumes is independent of Ra. It means that the volume and the heat transport in an individual plume is independent of the Rayleigh number. The number of plumes forming in the box increases (∼Ra1∕3) which is in accordance with the scale analysis using the...

Modelling of helium transport in groundwater along a section in the Pannonian basin

Underground water flow in sedimentary basins controls the distribution of dissolved salts and gases, and their concentrations may therefore be used as indicators of the flow direction. Recent measurements of the 4He concentration in deep waters of the Pannonian basin have great importance in this respect. This paper presents an example of the simultaneous computation of water flow and helium distribution along a section crossing the Great Hungarian Plain. The model consists of three permeable layers. The boundaries of the layers are prescribed using geologic sections constrained by ample borehole and seismic data. The results of the finite-difference calculations are fitted to the observed helium concentrations using a least-squares algorithm that varies the model parameters. The significance of the model is that it reconstructs the structure and flux of the groundwater flow and estimates the poorly known hydrogeological parameters of the flow regime such as hydraulic conductivities, conductivity anisotropy and dispersion coefficients. The statistical uncertainty of the estimated parameters is around half an order of magnitude. An estimate of the regional average of the incoming helium flux is also obtained. The total helium flux in the Great Hungarian Plain at the surface is within the range observed in old stable continental areas of the Earth.