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

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

Keywords: Geomorphic indices, Longxian–Baoji Fault Zone, Northeastern Tibetan Plateau, Southwest margin of Ordos, Tectonic activity