Mobilized Clay-Driven Toppling in Flysch Slopes: Resolving an Apparent Mechanical Paradox and Its Implications for Hazard Reassessment

In flysch terrain worldwide, under-dip slopes, which are slopes where the bedding dips more steeply than the ground surface, are traditionally considered kinematically stable. This assumption is challenged by documented toppling failures, which present an apparent mechanical paradox in engineering geology: the required forward rotation of rock slabs seems to oppose gravity by initially lifting their mass, demanding an external energy source. This study introduces and validates a new mechanism, mobilized clay-driven toppling, that resolves this paradox and has direct implications for slope stability assessment. Based on an integrated investigation in the Outer Western Carpathians combining field mapping, LiDAR analysis, and electrical resistivity tomography (ERT), we propose that weathering transforms interbedded claystone into a pressurized viscoplastic medium. Under lithostatic loading, this mobilized clay subsides and extrudes laterally. The resulting pressure forces actively push against and rotates overlying sandstone slabs. This provides the external energy required for paradoxical toppling. A quantitative geometric model links clay subsidence to sandstone rotation and predicts rotation axis depths of 12–26 meters. These depths are independently confirmed by subsurface ERT imaging. This process produces a characteristic, stepped morphology of sink-like depressions upslope of rotated ridges, offering a diagnostic geomorphic signature. These findings necessitate a reevaluation of slope stability concepts in flysch regions. We demonstrate how relatively affordable reconnaissance tools, such as LiDAR and ERT, can identify surface and subsurface indicators that diagnose this mechanism. Our results reveal that under-dip slopes, typically considered low-hazard areas, can undergo active destabilization due to weathering-induced clay mobilization. This bridges a critical gap between process understanding and practical hazard identification in engineering geology. The research was formally supported by the Grant Agency of the Czech Republic (GC22-24206J) and the Taiwanese Ministry of Science and Technology (MOST 111-2923-M-008-006-MY3), the National Science and Technology Council (NSTC) with the Project Numbers NSTC 114-2123-M-008-003-, and by the conceptual development project RVO 67985891 at the Institute of Rock Structure and Mechanics of the Czech Academy of Sciences.