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The Mariánské Lázně Fault (western Bohemia, CZE) is a morphologically, geologically and tectonically prominent structure with that is 150 km long with an NNW-SSE orientation. Its tectonic activity, especially in the NW part and in the neighbouring Cheb basin, is well known and has been proven by present-day earthquake swarms, mantle-derived CO2 emanations, geophysical and paleoseismological research. It seems that other parts of the MLF (especially segments of NNW-SSE and N-S orientation) might also have been active during the Pleistocene, possible even in the Holocene. This study provides a robust set of morphometric analyses – mountain-front sinuosity, basin asymmetry, longitudinal stream profiles, SL-index, which assesses the possibility of recent tectonic activity. The results suggest that the activity of the central and southern part of the MLF could have been very young. A reconstruction of the evolution of the stream network of the Mže River, as a result of different timing of the activity of particular segments of the MLF, is also put forward. The first ideas about the evolution of terrain morphology and the stream network are proposed by this study, however subsequent field research (geophysics, paleoseismology) could prove and date the tectonic activity. The delineation of segments with young activity may also have a great implication for seismic hazard assessment. [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]

Successions of colluvia and loesses covering the faults in the eastern and central parts of the Bohemian Massif were exposed in six trenches (two trenches at each fault, up to 6 m deep) and studied in detail in terms of stratigraphy and deformation. Based on OSL and radiocarbon ages these sediments were dated as Weichselian Early Glacial to Younger Dryas/Holocene (∼120 ka to 11 ka). On the Kosíř fault (NW-SE strike), the offset strata bring evidence for Late Pleistocene oblique slip (normal and strike-slip components) on the main fault plane with minimum slip rate in order of 0.1 mm/a. Although the undisrupted topsoil suggests the absence of significant slip in Holocene and no clear evidence of paleoseismic events was found, this fault structure should be included in seismic hazard assessment. Conversely, on the Hluboká and Diendorf-Boskovice faults (NW-SE and NNE-SSW strikes, respectively), the tectonic slip is contradicted for the last 15–23 ka based on dating of undeformed strata sealing the fault planes. Multiple independent evidence suggests that these two faults were not active in Late Pleistocene at least and their prominent scarps is probably largely due to exhumation by differential denudation. Examples given document the mechanisms of slope-related deformations which may lead to destruction of the records of older faulting and occasionally produce deformation structures resembling the tectonic ones. [ABSTRACT FROM AUTHOR]