Tectonic and/or climatic perturbations can drive drainage adjustment. The capture events, significantly changing the river network topology, are the major events in river network evolution. While they could be identified through field observations and provenance analysis, reconstructing this evolution process and pinpointing the capture time remain challenging. Following a capture event, the steady-state elevation of the captor river will be much lower than that of the beheaded river. Then, the newly-formed drainage divide will migrate towards the beheaded river, a process also known as river-channel reversal. The migration of the newly-formed drainage divide provides a new perspective for identifying the reorganization of the river network. Here, we employ numerical modeling to reproduce the characteristic phenomena of drainage-divide migration following capture events and analyze the effects of different parameters on the migration rate. We find that (1) the migration of newly-formed drainage divides can last for tens of millions of years, with the migration rate decreasing exponentially over time; (2) larger captured area, higher uplift rate, and lower erosional coefficient, all of which cause a higher cross-divide difference in steady-state elevation, will cause higher migration rate of the newly-formed drainage divide. This insight was further applied to the Dadu-Anning and Yarlung-Yigong capture events. We predict the present Dadu-Anning drainage divide would further migrate ~65–92 km southward to reach a steady state in tens of millions of years. The Yarlung-Yigong capture event occurred in the early-middle Cenozoic, which implies that the late-Cenozoic increased exhumation rate is not related to the capture event.

Landscape evolution is controlled by tectonic strain, bedrock lithology, and climatic conditions, and is expressed in the spatial and temporal variations in river channel networks. In response to tectonic and climatic disturbance, river networks shift both laterally and vertically to achieve a steady state. Several metrics are available to assess the nature of river network disequilibrium, upon which the direction of drainage divide migration can be interpreted. However, to link this information to other observational, theoretical, and experimental data requires the knowledge of the rate of migration, which is still lacking. Here we develop a modified method based on Gilbert metrics to calculate the transient direction and rate of drainage divide migration from topography. By choosing a high base level, linear or quasi-linear χ-plots are obtained for rivers on both sides of the drainage divide, and the elevation-χ gradient is proportional to the average normalized steepness index (ksn). In turn, the velocity of divide migration can be quantified theoretically from the cross-divide comparison of χ. We applied this method to eastern Tibet and obtained a uniform, westward migration pattern for 29 points along two drainage divides with rates between 0.02 and 0.66 mm/yr, which is consistent with the great river capture events in the region. The ongoing reorganization of the river network in eastern Tibet is caused by the Cenozoic growth and eastward expansion of the Tibetan Plateau, the strengthening of the precipitation and regional extension throughout East Asia, and the local fault activities.

Whether external or internal forces of the Earth control the behaviors of upper-crustal faults in a fold-and-thrust belt has been debated for decades. The Longmenshan thrust belt (LTB) along the eastern margin of the Tibetan Plateau may provide insights into such a debate. In this study, we focus on the central segment of the LTB which has relatively uniform shortening strains yet various fluvial incision capability along the strike. This tectonic setting enables a better assessment of the effects of external forces on fault activities. We analyzed the variations of the topography, fluvial incision intensity, co-seismic slips, and co-seismic landslides along the central LTB. The Longmen sub-segment in the northern half has higher elevation and three times lower fluvial incision intensity than the Hongkou sub-segment in the south. We calculated the topographic stresses on the faults ruptured during the 2008 Wenchuan earthquake and found topographic introduced normal stress increase may explain the co-seismic slip partitioning onto two sub-faults along the Longmen sub-segment. Our results indicate that fluvial incision may have produced the along-strike variations of the topography, which may further produce the different rupture behavior. In addition, the mean hillslope angle along the central LTB prior to the 2008 Wenchuan earthquake appeared to be at the critical condition of this region. Co-seismic deformation reduced the mean hillslope angle significantly, indicating that geomorphic indices may vary with different stages in an earthquake cycle. Therefore, scrutinizing the mean hillslope angle and other geomorphic indices may help identify potential seismic hazards in an active fault system.

Mechanism for fault segmentation in thrust belt is a key to understanding the orogenic process and seismic risks. A ~50 km long aftershock gap emerged between the ruptures of the 2008 Wenchuan and the 2013 Lushan earthquakes along the eastern margin of the Tibetan Plateau. Previous studies suggested that weak materials under ductile deformation cause the gap. Here we propose an alternative explanation: differential erosion drives the along-strike variation in fault activity. To testify the two competing models, we conducted low-temperature thermochronology and fluvial shear stress analyses to depict the spatial distributions of erosion. We obtained eight apatite fission track dates (6-44 Ma) in the gap and deduced erosion rates of 0.5-0.6 mm/yr and 0.3-0.4 mm/yr since ~8 Ma in the hanging -wall and footwall of the Shuangshi-Dachuan fault, respectively. We carried out linear fitting based on an empirical relationship between thermochronology-derived erosion rate and fluvial shear stress, and then calculated the erosion rate for each survey point of fluvial shear stress. Our new data reveal that in the hinterland, the erosion rate at the gap is lower than that of adjacent areas along strike, whereas in the range front, the erosion rate at the gap is greater. This spatial pattern supports the “differential erosion” hypothesis and is at odds with the “weak material” model. This study illustrates that heterogeneous erosion regulates fault segmentation in this thrust belt. Moreover, the aftershock gap acts as a barrier for the past major earthquakes, which poses substantial seismic potential to this region.