Late Holocene catastrophic slope collapse affected by deep-seated gravitational deformation in flysch: Ropice Mountain, Czech Republic

The region of the Flysch Western Carpathians is characterized by abundant deep-seated gravitational slope deformations (DSGSDs). This study deals with a detailed analysis of Late Holocene catastrophic collapse of a fault-created slope of Mountain Ropice in the Moravskoslezské Beskydy Mountains (eastern Czech Republic). The catastrophic collapse was initialized by huge compound rotational slide/toppling deformation which occurs on the upper part of the studied slope and affects tectonically weakened, subhorizontally inclined sandstone beds. Detailed geomorphological mapping accompanied by morphotectonic analysis and geophysical sounding revealed at least two historic flowslides. The older event (F1) affected 0.5–0.8 Mm3 of bedrock and created a 1000-m-long and up to 150-m-wide accumulation with abundant transverse steps and a carapace-like surface. The landslide toe blocked an adjacent valley, which led to the development of a small impoundment with ∼ 4-m-thick deposits containing lacustrine-peat bog, floodplain and alluvial fan facies. Radiocarbon dating and pollen analysis indicate the valley blockage at ∼ 1.5 ka BP. The other flowslide (F2) of much smaller dimensions (10 000–12 000 m3) overlaid the main landslide body and, according to the dating of minerogenic horizon in the impoundment, may have taken place < 0.3 ka BP. The organisation of landforms, character of slope failures and dynamic back-modelling point to a likely rock avalanche (or combined rock slide–debris avalanche) mechanism of movement. This case study shows that rapid rock avalanches can operate also in mountainous terrain with moderate relief if toppling together with deep-seated gravitational deformation of slopes modifies the original stable position of bedding planes.