Assessment of the relative tectonic activity of the Longxian&#8211;Baoji Fault Zone in the northeastern Tibetan Plateau based on geomorphic indices
Authors: Zhou Xiaohu , Huang Qi , Xu Shuaishuai , Liu Lushan 
Published: March 2024

The uplift of the Tibetan Plateau&#160;is one of the most important geological events in Asia and is a natural laboratory for&#160;the study of continental dynamics. The Longxian-Baoji Fault Zone (LBFZ) is at the&#160;intersection of the northeast margin of the Tibetan Plateau, the southwest margin of the&#160;Ordos Block, and the Qinling Orogen. It is the leading edge of the northeastward extension&#160;of the Tibetan Plateau that was formed by the collision between the Indian and Eurasian&#160;plates. Since the late Cenozoic, the tectonic deformation of the LBFZ has been intense, and&#160;earthquakes have been repeated in history. To evaluate the relative tectonic activity within&#160;the LBFZ and discuss the influence of the northeastward expansion of the Tibetan Plateau&#160;on the geomorphological evolution of the LBFZ, this paper cambines field surveys, used&#160;remote sensing images, and extracted data of the Qianhe, Hengshuihe, and Jinlinghe River&#160;Basins based the ASTER GDEM, computed six geomorphic indices, including the&#160;hypsometric integral (HI), standardized stream length-gradient index (SL/K) and Hack&#160;profile, elongation ratio (Re), the drainage basin asymmetry factor (AF), valley floor&#160;width-to-height ratio (VF) and transverse topographic symmetry factor (T), and the index&#160;of relative active tectonics (IAT) was obtained. The&#160;following understandings are finally drawn:&#160;Various geomorphic indices indicate that the geomorphological response to the&#160;tectonic activity and relative uplift of the LBFZ include rivers with generally high SL/K&#160;values, drainage basins with relatively high HI and low Re (elongated) values, basins with&#160;different degrees of asymmetry (AF, T), and leading edges of mountains with low VF&#160;values. The LBFZ has experienced relatively high tectonic activity.The calculation results of the AF and T show that the regional tectonic tilt&#160;direction presents obvious zoning on both sides of the fault zone. On the TGF and the&#160;southwest side of the TGF (Longxi block), the drainage basin tilts to the east and southeast.&#160;These indicate that tectonic activity since the&#160;Cenozoic has influenced the evolution of the watershed in this area. The results of the IAT show that the tectonic activity of the LQF is the highest in&#160;the area, followed by that of the TGF; activity of the GGF is weak, and the activity of the&#160;QBF is the lowest. Correlation analysis between the IAT and the frequency and magnitude of&#160;earthquakes in the region shows that the frequency and magnitude of earthquakes are also&#160;higher . It shows that the&#160;IAT has a good correlation with the earthquake frequency and magnitude. At the same time,&#160;the areas with strong tectonic activity in the study area were delineated,&#160;which shows the distribution characteristics along the LQF and the TGF, and mainly the&#160;LQF. This will provide certain reference significance for earthquake risk assessment in&#160;Baoji.&#160;The northeastward expansion of the Tibetan Plateau affected the LBFZ region, and&#160;the stress brought about by it controlled the tectonic deformation in the region and also&#160;sculpted the modern landscape.&#160;Keywords: Geomorphic indices, Longxian&#8211;Baoji Fault Zone, Northeastern Tibetan Plateau, Southwest margin of Ordos, Tectonic activity