River incision and fracture density controls on the style and pattern of slope failure on the northeastern Tibetan Plateau

Hillslopes are important elements of mountain landscapes, but there is still limited understanding of how hillslopes respond to river incision and rock mass properties at the landscape scale. Topographic characteristics at Qingyang Mountain (QYM) on the tectonically active northeastern Tibetan Plateau, which we documented using field observations and geometric measurements of high-resolution DEMs, are the result of differences in river incision over timescales of 100,000 years or more. Hillslope erosion at QYM is achieved by slope failures that increase hillslope angles as rivers incise. Based on the Culmann's method, we consider the slope relief of 221–231 m to be limiting values corresponding to the observed threshold slope angle of ~ 26.3° in eastern QYM. Multi-scale analysis of lidar point clouds, obtained by laser scanner, and Structure from Motion photogrammetry reveal a spatial variation of bedrock fracture density that is consistent with lateral evolution of ridge-valley topography. Areas of higher fracture density correspond to eroded gullies, and areas of lower fracture density are found in ridges separating these gullies. Our results suggest that difference in tectonic fracturing of the rock mass dominantly drives lateral erosion of slopes in the study area.