Deep-seated gravitational deformation of mountain slopes caused by river incision in the Central Range, Taiwan: Spatial distribution and geological characteristics

Fluvial undercutting can cause the large-scale destabilization of mountain slopes and induce deep-seated gravitational slope deformation (DGSD) that eventually leads to catastrophic failures. This study analyses these processes in the catchment of the Dahan River, northern Taiwan. The area has experienced phases of river incision of up to 600 m since the middle to late Pleistocene, and middle Pleistocene paleosurfaces at higher elevations were incised to form V-shaped valleys. A DGSD inventory was compiled by visual interpretation of high-resolution images, and field surveys were conducted to clarify the geological structures associated with DGSDs and major rockslide-avalanches. The observed relationships between DGSDs and the topography modified by long-term river incision show that about 53% of all DGSDs occurred on slopes along the rims of paleosurfaces, which is approximately 2 and 2.5 times as many as in the incised valleys and on the paleosurfaces, respectively. A comparison of the power-law scaling exponents of cumulative frequency-area distributions shows that the frequency of DGSDs along the rims of the paleosurfaces decreases less rapidly with DGSD area than for DGSDs on other slopes. Geological investigations suggest that there are two dominant types of DGSD: one is flexural toppling of slate and argillite with high-angle cleavage, and the other is buckling of alternating beds of sandstone and mudstone on parallel or underdip cataclinal slopes. Catastrophic landslides observed along or below the rims of paleosurfaces were preceded by buckling of alternating beds of sandstone and shale. These beds dipped at 50 degrees to 58 degrees, and each bed was 10(-1)-10(0) m thick. This study suggests that the peripheral zones of the paleosurfaces may be most susceptible to future catastrophic landslides, particularly on parallel or underdip catadinal slopes comprising alternating beds of sandstone and mudstone dipping at 50 degrees to 60 degrees. Thus, an understanding of the geological characteristics, spatial distribution of DGSD occurrence, and landscape evolution within a particular area can provide important information for landslide hazard-level zoning. (C) 2015 Elsevier B.V. All rights reserved.