Asymmetrical river valleys and their tectonic significance in the Maxianshan area, NE Tibetan Plateau

Abstract

The geometry of river networks is particularly sensitive to active tectonic deformation. Morphotectonic analyses conducted using geomorphic indices and numerical models are useful methods with which to identify such deformation. The northeastern (NE) margin of the Tibetan Plateau is characterized by active tectonics, easily erodible bedrock and well-developed valleys, and is thus an ideal region for studying the effects of tectonics on the landscape evolution. Here we firstly identify the systemic asymmetrical valleys in the Maxianshan area, NE Tibetan Plateau. To quantitatively analyze the asymmetrical characteristics of these valleys identified in the southern Maxianshan, 33 parallel streams were investigated by using two drainage basin analytical techniques, namely, the transverse topographic symmetry factor (T) and asymmetry factor (AF). Our results show that the mean T value is 0.14 (magnitude) and 319.08° (direction) and the majority of the AF results are <50, indicating a north-westward migration. In addition, we suggest that this north-westward migration could be caused by the variable rates of uplift experienced across the Maxianshan area, an assertion which is also supported by the inversion results of GPS measurements and earthquake distributions. In contrast to previous studies, we conclude that the variable rates of uplift are probably caused by the distribution of stress within the four quadrants of a strike-slip fault. This mechanism is validated by using the landscape evolution model SIGNUM and studying examples from other strike-slip fault zones, such as the Haiyuan Fault Zone on the NE Tibetan Plateau, Qujiang Fault Zone and Daqing Fault Zone on the southeastern (SE) Tibetan Plateau and Tieluzi Fault Zone in central China. By using quantitative methods and modeling, our study appears to demonstrate that asymmetrical valleys are important geomorphic characteristics of strike-slip faults.