Under-Dip Toppling in the Outer Western Carpathians: Insight into Investigation of Slope Failures and Implication of Paleo-Seismic Activity

The Outer Western Carpathians, situated in the central European segment of the Alpine-Himalayan orogenic zone, present an intriguing case of an accretionary wedge. This region is characterized by Mesozoic and Cenozoic flysch sedimentary rocks, comprising massive sandstone benches and intercalated clay layers. These formations have undergone significant deformation, including being thrust over the European foreland during the Paleogene and Neogene periods. The resulting hilly to mountainous terrain exhibits notable slope failures. Here we focused on the phenomenon of under-dip toppling –sandstone beds steeply dipping in the direction of the slope, which were locally overturned along systems of brittle fractures. Utilizing high-resolution LiDAR data, our study investigates the locations, geometrical characteristics, and tectonic settings of these toppling. In the Javorníky Mountains, these topples predominantly occur in SSE-dipping fold limbs, an orientation conducive to under-dip toppling. The mechanism of under-dip toppling, involving the lifting of the center of gravity of the toppled layers, presents a complex geomechanical challenge. Recent field investigations, including structural measurements on faults, electrical resistivity tomography (ERT) profiles, and 10Be dating, have identified active polyphase strike-slip surface ruptures in the region. These findings raise questions about the origins of toppling and their implications for understanding paleo-earthquakes in the area. Our preliminary analysis suggests a two-tiered approach to understanding toppling processes: firstly, exploring the deeper structural implications – could active tectonic faulting be the cause of the under-dip toppling? Secondly, we analyzed the mechanism of toppling near to the surface. By analyzing the geometry of near-surface persistent sandstone slabs and employing Pseudo-static analysis to assess seismic slope response, our results indicate that the layer while overturned may be attributed to strong paleo-seismic events. This study employs comprehensive site investigations and back analyses to understand a range of possible trigger and controlling mechanisms. It elucidates the geological conditions of slopes and performs a geomechanical analysis of under-dip toppling. The research is part of the international bi-lateral project “Earthquake triggered landslides in recently active and stabilized accretionary wedges”, supported by the Czech Science Foundation (GAČR 22-24206J) and the Taiwanese Ministry of Science and Technology (NTSC 111-2923-M-008-006-MY3).