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The Czech part of the Outer Western Carpathians (COWC) formed by flysch rocks is among the most landslide-prone areas in Europe. Recent LiDAR-based inventory mapping reveals that the area of the COWC encompasses more than 13,500 landslides, with some geological domains exhibiting > 20% of the surface area covered by landslides. Although mass movements represent crucial geomorphic agents in the COWC, their timing and especially lifespan remain elusive. In this study, we focused on rockslides affecting the flanks of the flysch anticline developed within Paleogene sandstones. The area ("Hradisko" ridge) is well known as the most extensive rock city in the COWC. Structural-geological investigation, kinematic analysis and the application of near-surface geophysics (ERT, GPR and seismic refraction) suggest that rockslides accompanied by toppling and lateral spreads are predisposed by the architecture of bedding planes, joints and inherited faults cross-cutting the anticlinal ridge. Based on 10Be exposure dating of five scarps and rockslide boulders (a total of 25 cosmogenic-dated samples), we were able to reconstruct the long-term history of rock slope failures. Although the obtained ages of headscarp exposures reveal consistent patterns suggesting major mass movement phases at ~ 18 ka and ~ 12 ka, the interpretation of boulder exposure ages is ambiguous. The study area has not been affected by catastrophic slope failure in historic times, but dendrogeomorphic analysis reveals surprisingly strong tree ring signals of mass movements within the last 150 years. These findings suggest that progressive failure is developing within some parts of the rockslide and/or slope portions above the major scarp. We conclude the following: (1) some rockslide spots within the COWC might express a very long history, encompassing the full Late Glacial–Holocene Period; (2) major mass movement activity in the study site temporally coincided with the major climatic changes; (3) rockslides with a very long history still represent potential hazards, although evidence of their active movement is not detected by standard geomorphic mapping techniques and (4) dating of the scarps should be preferred as even a higher number of boulders might be unreliable. [ABSTRACT FROM AUTHOR]

Abstract: Deep-seated gravitational slope deformations (DSGSDs) played an important role during the Quaternary evolution of mountain ridges formed by anisotropic flysch rocks of the Outer Western Carpathians (OWC). Four DSGSD-affected ridges in the highest part of the Czech sector of the OWC have been subjected to multidisciplinary investigation involving mapping, structural and kinematic analysis, geoelectrical imaging, trenching and dating. All the studied DSGSDs are strongly predisposed by regional structural fabric given by the intersection of bedding planes, joint sets and faults. Performed research suggests high diversity of mechanisms leading to the origin of individual DSGSDs. The area of a single DSGSD can be affected by various types of movements such as sackung, lateral spreading, toppling or incipient sliding. Electrical imaging and trenching revealed that besides these mechanisms, anisotropic flysch massifs formed by thick-bedded sandstones are strongly exposed to subsurface opening of air-filled voids related to widened crevices. Such a process is driven by deep-seated creep and translational sliding of sandstone beds upon plastic claystones whose strength has been reduced to residual values of the angle of internal friction. Performed radiocarbon and OSL dating revealed that some of the studied DSGSDs originated in the Holocene period, particularly in the Atlantic and Subboreal chronozones. [Copyright &y& Elsevier]

Abstract: One of the largest long-runout landslides in the modern history of the Czech Republic originated on the southern slopes of the Girová Mountain (Outer Western Carpathians) at the end of extreme rainfalls in May 2010. The structurally predisposed Girová Mountain rockslide occurred very quickly within fault-weakened, deeply-weathered flysch bedrock. Its main phase, during which a wedge-like block collapsed in the upper part of slope, lasted for a few hours. The overloaded slope collapsed and its motion lasted for the following 3days. The May 2010 Girová Mountain rockslide is an excellent case of a recurrent long-runout landslide nested within a deep-seated gravitational slope deformation with a long history of Late Quaternary mass movements. The chronology of slope failures within the studied slope was reconstructed by means of radiocarbon dating and dendrogeomorphological analysis. At least one Holocene long-runout landslide (7.4calka BP) and several smaller failures (~1.5 and ~0.6calka BP) preceded this recent catastrophic failure. Dendrogeomorphological analysis showed that the failure had been preceded by at least 80-year-long creep movement that accelerated over the years due to extreme rainfall events. Despite the fact that extreme precipitation of May 2010 represented the last incremental change before catastrophic collapse of the slope, its main reason was gradual weakening of rock massif and concentrated creeping (with some accelerated phases) in the upper part of the slope deformation. [Copyright &y& Elsevier]

Abstract: Widened discontinuities (among them crevice-type caves) are characteristic phenomena of landslides in anisotropic rock massifs forming the Flysch Carpathians. We tested the reliability of 2D electrical resistivity tomography (ERT) in the detection of well-mapped cave passages and then tried to explore new subsurface cavities in three sites of crevice-type caves in the Polish Flysch Carpathians: the Jaskinia Miecharska, Jaskinia Malinowska and Jaskinia Diabla Dziura caves; each comprising a different stage of mass-movement development and thus being a representative case study. The total of 14 ERT profiles was selected, while each of them was measured by several electrode arrays (dipole–dipole, Wenner and/or Wenner–Schlumberger). Our results show that the selection of an appropriate electrode array and spacing are crucial for the detection of subsurface cavities. Measurement using the dipole–dipole electrode array gave best results in the detection of known structures and its interpretation seems to be most reliable in the extrapolation of non-investigated areas. The application of ERT shows abundant occurrence of air-filled voids in gravitationally deformed flysch massifs and in several places its results enable comparison with open spaces detected by speleological exploration. ERT enables us to identify fractures in deformed slopes before the proper failure of these slopes affected by a landslide as well as in landslides themselves: fissure zones connected with trenches, less dense segments of colluvial ramparts and some lithological-tectonical or gravitationally induced elements of slope structures. The reliability of ERT detection of potential caves was verified by speleological exploration of a newly discovered cave at the Jaskinia Miecharska site following the ERT campaign. [ABSTRACT FROM AUTHOR]