River reorganization based on geomorphic indices in the Huashan Mountains, central China

In many mountainous regions on Earth, strong spatial variations in uplift with a fault-bounded transition from uplift to subsidence drive the coevolution of high mountain topography and adjacent low-lying basins. In this study, we investigate which topographic patterns are characteristic for such a geodynamic setting where actively subsiding and uplifting regions in direct vicinity are tightly linked via dynamically evolving drainage systems. The Huashan Mountains, which is part of the Qinling Mountains range, and the adjacent Weihe Graben close to the city of Xi’an (China) are the perfect locations to investigate the formation of topography in an active basin and range system, and this area links directly to the uplift of the Tibet plateau. The Weihe Graben formed in an extensional environment and experienced significant subsidence with up to ∼7000m Cenozoic sediments. Contemporaneously, topography has formed in the Huashan Mountains bordering the Weihe Graben. Major earthquakes in this region (e.g. the M∼8.5 Huaxian earthquake in the year 1556), pristine fault scarps, bedrock fractures, and loess crevices are evidence for recent tectonic activity. The high relief between the Huashan Mountains and the Weihe Graben favors fluvial bedrock incision and related mass wasting at hillslopes as a response to local relief formation. Frequent landslides triggered by both seismic and storm events are distributed throughout the Huashan Mountains. To quantify the impact of gradients in uplift rate on topography and active tectonics, we applied several DEM-based morphological analyses and compared catchments that drain north to the low-lying Weihe Graben with those that drain south, which were not affected by tectonically induced base level lowering. We analyzed longitudinal channel profiles, channel steepness (k_sn), catchment hypsometry, and geophysical relief. To quantify the topographic state of the Huashan Mountains and detect drainage divides that are potentially mobile, we computed χ maps and χ-profiles of these drainage systems. We found that rivers at the northern steep slope of the Huashan Mountains, which is directed towards the Weihe Graben, are in general steeper with a higher valley relief, and feature lower χ value compared to rivers south of the drainage divide. Large across divide gradients in χ could indicate a southward migration of the watershed. Analyzing the drainage pattern close to the watershed, we found strong evidence for two river piracy events (wind gaps, beheaded rivers) suggesting that catchments north of the drainage divide indeed grow at the expense of those in the south. We conclude that the evolution of high, tectonically-driven relief in the Huashan - Weihe region with rising mountain ranges and subsiding basins in direct vicinity causes a state of morphological disequilibrium, where the observed reorganization of the drainage system represents the adjustment towards a morphological steady state. We suggest that strong gradients in uplift rate between Huashan Mountains and adjacent Weihe Graben, and their link via dynamic drainage systems control channel and hillslope morphology, the topology of the drainage system, eventually the overall architecture of the orogen, and to the creation of morphology related to the uplift of the Tibet plateau.