The Outer Western Carpathians characterised by flysch structure are prone to the development of various types of slope deformations. Landslides are typical and abundant phenomena of this mountain relief. The study brings new information on the predisposing factors for landslide evolution in the studied area and it focuses on the time constrains of these slope deformations during the Holocene. The interrelationship between slope deformations and fault-induced weathering as a preparatory factor for the development of sliding has been analysed in several case studies from the Western Carpathians in the Czech Republic. The study area comprises flysch nappes with alternating sandstone and shale of different permeability. These lithological structures are affected by systems of faults. Recurring slope instability is found as associated with zones of deep weathering in tectonically weakened areas. Climatic variability of landslide activity during the Holocene can be identified by means of radiocarbon dating and pollen analysis. The age of landslide evolution is analysed by radiocarbon dating of sediments preserved in selected places of the deposition: near-scarp depressions, inter-colluvial depressions, landslide dammed-lakes, and organic material buried under the landslide body. Using this sampling strategy we were able to establish landslide chronology in the studied area. By means of pollen data sets we can analyse palaeoenvironmental conditions of the studied areas and bring new light into the predisposing and triggering factors. Areas affected by recurring landslides suggest both gradual and cyclic landslide frequency. We determined the following landslide phases within the studied area: Older Dryas – Allleröd, Boreal, AT1, AT2/AT3, AT4/SB1, SB2/SB3, and SA1/SA2. The first very important phase is identified with the turn of the Last Glacial (Kotelnice landslide, 11813±383 14C BP). This phase is connected with crucial environmental changes at the end of the last glacial period. Cyclic landslide frequency was confirmed in the case of complex landslide areas (e.g. Velká Čantoryje landslide). A highly fragmented zone of overthrust sandstone-dominated nappe, which lies on a weathered claystone-dominated sequence, contains numerous multiple rotational slides. Basal peat bog deposits situated in the vicinity of the main scarp depressions of these landslides show minimal ages of 3540±80 14C BP to 3680±350 14C BP (late Subboreal) and a further reactivation in approximately 2400±70 14C BP to 2890±90 14C BP (Subatlantic). Recent (rather minor) mass movements have occurred only sporadically throughout the last 100 years (e.g. during an event in July 1997). Another example is connected with the evolution of a catastrophic rockslide of Mt Ropice. The first event occurred at 1450±100 14C BP (Subatlantic age of a basal sediment layer of a former landslide-dammed lake). Another smaller flow-slide (identified on the basis of a change in sedimentation within the impoundment) probably postdated 310±60 14C BP. Special attention was paid to the research of the palaeoenvironmental significance of landslide dammed-lakes. Radiocarbon dating together with palynological and sedimentological analyses detected repeated changes in depositional conditions connected with the palaeoenvironmental changes during the Holocene. Results of the radiocarbon dating of the basal parts of lake sediments show that landslides causing the valley damming originated throughout the whole Holocene with significant increase in the landslide activity in the Subatlantic chronozone. Linear lake sedimentation rates and minimum average catchments denudation for selected contributing catchments in different time spans were calculated in order to understand the relief development dynamics throughout the Holocene. Minimum mean mechanical denudation of landslide-dammed catchments varies between 2.5- 13.4 mm.ky-1.
Acknowledgements: This study was funded by a project of Czech Science Foundation no. P209/10/0309: “The effect of historical climatic and hydrometeorological extremes on slope and fluvial processes in the Western Beskydy Mts. and their forefield”.