Evolution of Qian River system and neotectonic movement in the southwest margin of Ordos Block

The neotectonic movement is active in the southwest margin of Ordos Block, and the development process of river terraces and evolution history of drainage system contain abundant information of neotectonic movement.Five river terraces are asymmetrically distributed on the two banks of Qian River system in the southwest margin of Ordos.Through field investigation and drilling, the terrace structure and development characteristics were clarified.By using electron spin resonance (ESR), optically stimulated luminescence (OSL)and correlation dating of loess strata, the formation age and cutting rate of terrace were analyzed.Combined with regional geological background, the dynamic background of Qian River terrace development and river system changes was also discussed.It is found that T5 to T1 terraces were formed in 1.176 Ma, 0.766 Ma, 0.504 Ma, 0.131 Ma and 0.04 Ma, respectively, with the incision rates of 76.6mm/ka, 88.3mm/ka, 111.3mm/ka, 149.6mm/ka and 115mm/ka, respectively.From the Early Pleistocene to the middle of the Late Pleistocene, the tectonic movement in Qian River basin was gradually strengthened, and after the Late Pleistocene, the tectonic movement was slowly diminished.It is inferred that the incision of the Qian River was driven by different tectonic stress fields, which caused the river to migrate in different directions.The variation of tectonic stress field is probably related to the interaction between the Qinghai-Tibet tectonic system and the western Pacific tectonic system.