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The western part of the Bohemian Massif hosts an intersection of two regional fault zones, the SW-NE trending Ohře/Eger Graben and the NNW-SSE trending Mariánské Lázně Fault, which has been reactivated several times in the geological history and controlled the formation of the Tertiary Cheb Basin. The broader area of the Cheb Basin is also related to permanent seismic activity of ML 3+ earthquake swarms. The Eastern Marginal Fault of the Cheb Basin (northern segment of the Mariánské Lázně Fault) separates the basin sediments and underlying granites in the SW from the Krušné Hory/Erzgebirge Mts. crystalline unit in the NE. We describe a detailed geophysical survey targeted to locating the Eastern Marginal Fault and determining its geometry in the depth. The survey was conducted at the Kopanina site near the Nový Kostel focal zone, which shows the strongest seismic activity of the whole Western Bohemia earthquake swarm region. Complex geophysical survey included gravimetry, electrical resistivity tomography, audiomagnetotellurics and seismic refraction. We found that the rocks within the Eastern Marginal Fault show low resistivity, low seismic velocity and density, which indicates their deep fracturing, weathering and higher water content. The dip of the fault in shallow depths is about 60° towards SW. At greater depths, the slope turns to subvertical with dip angle of about 80°. Results of geoelectrical methods show blocky fabric of the Cheb Basin and deep weathering of the granite bedrock, which is consistent with geologic models based on borehole surveys. [ABSTRACT FROM AUTHOR]

Geodynamic activity in the area of West Bohemia is typified by the occurrence of earthquake swarms, Quaternary volcanism and high flux of mantle-derived CO. The highest swarm activity occurs beneath the eastern edge of the Cheb basin, which is delineated by the NW-SE trending morphologically pronounced Mariánské Lázně Fault (MLF) controlling the formation of the basin. The previous trenching survey across the MLF zone has identified several fault strands with possible Quaternary activity. In this paper we present the results of the geophysical survey focused to trace the faults signatures in geophysical sections and to build an image of near surface tectonics. The method of electric resistivity tomography (ERT) along two profiles parallel to the trench identified a strong resistivity contrast between the bodies of sandy gravels in the middle and conductive clayey sands to the west and weathered crystalline basement to the east. The 2-D ground penetration radar (GPR) sections show direct correlation of reflections with lithological boundaries identified in the trench. As expected, the GPR signal amplitudes increase with the resistivities found in the ERT sections. Two of the four faults identified in the trench are indicated in the resistivity and GPR sections. A 3-D GPR measurement has identified a spot of high amplitudes elongated parallel to the MLF trend, which coincides with the high resistivity body. To improve the signal-to-noise ratio of the time slices we stacked the GPR time slices within vertically homogeneous blocks. This provided a contrast image of the sand-gravel body including its boundaries in three dimensions. The detailed analysis of the 3-D GPR cube revealed additional fault that limits the highly reflective sands and appears to be offset by another younger fault. Our results suggest a complex fault pattern in the studied area, which deserves a further study. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the results of two separate geophysical investigations undertaken across the Sudetic Marginal Fault zone in the Bohemian Massif. This fault zone represents one of the most important tectonic features in central Europe. The first, preliminary, investigation used electrical resistivity tomography (ERT) to define the exact position of the main fault at two localities (Kamenička & Bílá Voda). Both profiles revealed a resistivity gradient that divided a complex of crystalline rocks with high resistivities from sedimentary deposits with low resistivities. From these data, sites were selected for palaeoseismic trenching. This trenching provided detailed information regarding the surveyed structures and lithologies. The second, more extensive, investigation used electrical resistivity tomography and gravimetric survey in order to extend the previously recognised features both laterally and to depth. The fault is displayed by the presence of an expressive resistivity gradient, even on profiles undertaken in more homogeneous lithologies. Finally, the most intensive investigations were conducted at Bílá Voda, where highly detailed ERT was undertaken on the floor of the trench. From this, it was possible to trace approximately logged geological structures to far greater depths and assess sedimentary thicknesses. A gravimetric survey was also undertaken adjacent to the trench. From modelling the gravitational effects of the fault zone, it is shown that the fault dips about 75° NE and that the vertical offset of Miocene vs. bedrock on the fault is approximately 200m. The presented study has demonstrated the value of undertaking two phases of geophysical exploration separated by a trenching investigation. It is considered that this methodology is particularly suitable for the study of active tectonics in analogous intraplate settings where fault slip-rate is low. [Copyright &y& Elsevier]

The area of Southern Bohemia between Novohradské hory Mountains and the Třeboň and Budějovice Basins – the Novohradské Foothills (in the south of the Czech Republic) is known for the occurrence of moldavites. These tektites are abundant in fluvial Koroseky sands and gravels. This study focuses on reconstructing the paleostreams, which deposited the moldavite-bearing sediments, and their relationship to present streams. Based on a review of the deposits' occurrences, a connection between moldavite-bearing sediments and deposits of the present watercourses, Vltava, Malše, Stropnice and Svinenský, is proposed. The link between moldavite-bearing deposits and the current streams were further supported by analyses of river geometry (longitudinal profiles, stream gradient) and terrain morphology (valley floor ratio, basin asymmetry). It is suggested that the moldavite-bearing deposits were sedimented during the Pliocene or Pleistocene by predecessors of the present streams. The occurrences of particular fluvial deposits and changes in terrain morphology and river geometry can be traces of changes in the river network and the dynamic evolution of local relief induced by tectonic activity during the Pliocene and Pleistocene. • Used methods can help to solve the problem about the age of moldavite-bearing deposits • Used methods can help to solve the problem about evolution of river network as well • Results suggest the age of moldavite-bearing deposits to Pliocene or early Pliocene • Moldavite-bearing deposits could be sedimented by predecessors of current rivers • The influence of tectonic uplift to evolution of stream geometry is probable [ABSTRACT FROM AUTHOR]