Evolution of deep-seated gravitational slope deformation in a deep valley of the Czech Flysch Carpathians

Abstract

In the Czech Outer Western Carpathians (OWC), the lower limit of deep-seated gravitational slope deformations (DSGSDs) occur associated with moderate local relief and slope gradient, showing a limited degree of geomorphic development. Here, DSGSDs display a relatively high spatial frequency, despite the limited tectonic and seismic activity, often claimed as major preparatory and triggering factors. Nonetheless, favourable stratigraphic and structural features in flysch successions, together with fluvial downcutting, provide conditions prone to DSGSDs. The study area of Travný Mt. hosts the typical DSGSDs in the highest part of Czech OWC. With the aim of unravelling the controlling and triggering factors, the internal structure, and the timing and kinematics of mass-movement activity, a multidisciplinary investigation was performed. LiDAR-based and field geomorphological mapping allowed to constrain the extent of the of DSGSD, expressed by characteristic landforms such as antislope scarps and grabens. The structural analysis and geophysical surveys (ERT and GPR) provided insight into the internal structure of the DSGSD, reaching a depth of >100 m, and supported its mountain-scale spatial propagation. The instability is controlled by inherited faults, deep-penetrating joints and and the stratigraphic contact between brittle caprock overlying weaker rocks. Morphologically, the most prominent deformation is located in the upper part, which is dominated by toppling of flysch blocks with thick-bedded sandstone. The results of the trenching technique and geochronological analyses point to the episodic kinematics of this portion of the DSGSD, revealing a significant displacement event (ca. 9.9 ka) linked to a major climate change occurred after the Late Glacial/Holocene transition.