Beyond stream power: the dominance of landsliding on the active Taiwan orogen

Abstract

Fluvial bedrock incision, which creates topographic relief and influences hillslope stability, has been considered the key process linking denudation and tectonic uplift in non-glaciated mountains. However, taking the Taiwan orogen as an example, this study argues that landslides can dominate over river incision in governing the erosion of active mountains. The Taiwan orogen, reaching 3000 – 4000 m in elevation, is prone to landsliding triggered by heavy rains or large earthquakes. It is shown in the orogen that landslides could drive catchment expansion and bedrock-river avulsion. Also, by determining the yield/caliber of bedload sediment, landslides have controlled river incision/deposition processes and, thus, the morphology of bedrock rivers (width, gradient, sinuosity, and shape of longitudinal profiles). The significant spatial/temporal diversities in landslide sequences, with various magnitudes/frequencies, then account for: (1) the occurrence of tributaries that are atypically wider or gentler than trunk rivers; (2) the wide ranges of bedrock incision rates (ranging from zero to several centimeters per year) over different time spans; (3) the contrasts in terrace sequences (and thus river evolutionary histories) among catchments; (4) the differential bedrock incision along rivers, leading to the creation of knickpoints (including waterfalls). All the observations above challenge the applicability of stream power law (assuming the drive of river incision by hydraulic power) in modelling bedrock river incision in the Taiwan orogen. We further find that the activities of landslides around the major drainage divides > 3000 m in elevation have been much lower than the activities of landslides in the mid-elevation regions (perhaps due to the lower seismicity in the high mountains). Given this and the commonness of low-relief surfaces stranded on the major drainage divides, it is unlikely that erosion of the orogen has offset the tectonic uplift. We believe that the elevation of the orogen has been increasing and it is the glacial erosion that can balance the tectonic uplift in the future.       Landslide; Fluvial bedrock incision; stream power law; low-relief surface; Taiwan orogen