Spatial patterns of erosion rates and topographic steepness in the Three Rivers Region, southeastern Tibet

We conduct a new analysis of the geomorphology, calibrated to basin-averaged erosion rates, for the Three Rivers Region (TRR), the southeastern part of the Tibetan Plateau, drained by three major rivers that flow in parallel from north to south —the Salween, Mekong, and Yangtze. We combined DEM analysis of channel steepness indices of the trunk rivers and the tributaries with cosmogenic nuclide concentrations, measured in modern river sands collected from tributaries of these three major rivers. Our analysis reveals surprisingly low erosion rates for a high-relief mountain region, with an exception of the Meili Mountains, where significantly higher rates correlate with high river steepness. This localized anomaly appears to be related to high rock uplift rates associated with a compressive stepover structure linking the Parlung and Zhongdian strike-slip faults. In addition to this local process, we identify a broader west-to-east gradient of decreasing erosion rate and river steepness. This gradient cannot be explained by tectonic models favoring north-south movement but instead reflects the influence of the Indian Plate and Burma's indentation into South China. To further investigate these dynamics, we developed a kinematic model using GPS velocity data to reconstruct the relative positions of India, Burma, and the TRR over 20 Ma. The model estimates approximately 120 km of maximum TRR shortening, offering insights into the geomorphic evolution of this region.