Assessment of the relative tectonic activity in the Bailongjiang Basin: insights from DEM-derived geomorphic indices

The Bailongjiang drainage basin is located in the transition zone between the Tibetan Plateau and the Loess Plateau. Active tectonic records demonstrate the interplay between the Tibetan Plateau uplift and the different movement of the faults. To assess the relative tectonic activities in this area, five geomorphic indices derived through geographic information system were analyzed: the stream-gradient index (SL), hypsometric integral (HI), valley floor width-valley height ratio (VF), drainage basin shape, and drainage basin asymmetry (AF). Active faults, seismogenic activity, and river terraces were also integrated into this investigation. The results indicate that four classes of relative tectonic activity were defined: Class 1 (very high relative tectonic activity, 5.5 % in the area); Class 2 (high, 54 %); Class 3 (moderate, 24.1 %), and Class 4 (low, 16.4 %). Sub-basins at the midstream and upstream of Bailongjiang show abnormally high SL, high HI, AF, and convex hypsometric curves and low VF values due to the impact of the east-ward extrusion of the Tibetan plateau and active faults. The downstream shows low SL, HI, and AF and concave hypsometric curves due to the low tectonic activity of the faults inhibited by Minshan. These results are consistent with the geological background.