Assessment of the relative tectonic activity of the Longxian-Baoji Fault Zone in the northeastern Tibetan Plateau based on geomorphic indices

Since the Late Cenozoic, tectonic deformation has been intense in the Longxian-Baoji Fault Zone (LBFZ), which is at the intersection of the southwest margin of the Ordos Block, the northeast margin of the Tibet Plateau, and the Qinling Orogenic Belt. To evaluate the relative tectonic activity within the LBFZ and discuss the influence of the northeastward expansion of the Tibetan Plateau on the geomorphological evolution of the LBFZ, we extracted data of the Qianhe, Hengshuihe, and Jinlinghe River Basins from the ASTER GDEM and analyzed the geomorphic indices, including the hypsometric integral (HI), standardized stream length-gradient index (SL/K) and Hack profile, elongation ratio (Re), the drainage basin asymmetry factor (AF) and valley floor width-to-height ratio (VF). Through geomorphic indices and field investigations, we found that the LBFZ has experienced relatively high tectonic activity. Combined with the index of relative active tectonics (IAT), we compared the tectonic activity strengths of the four major faults in the study area. Among them, the tectonic activities of the Longxian-Qishan Fault (LQF) and the Taoyuan-Guichuansi Fault (TGF) are the highest. The morphology and AF values of the drainage basin of the southwest side of the study area indicate the influence of northwest-southeast compressive stress. The northeastward expansion of the Tibetan Plateau affected the LBFZ region, and the stress brought about by it controlled the tectonic deformation in the region and sculpted the modern landscape. This study is of great significance for understanding the impact of the northeastward expansion of the Tibetan Plateau on the geomorphological evolution of the LBFZ.