Research On The Geomorphic Evolution Of The Jinta Nan Shan And The Evolution Of The Heihe River

The formation and evolution of the drainage system is closely linked with tectonic activity. The essential issue is how the tectonic activity affects the evolution of the drainage system. The Heihe River is the largest inland river in the Hexi Corridor and provides the main water resources in Zhangye, Alashan. The Corridor Bei Shan (the Longshou Shan, the Heli Shan and Jinta Nan Shan) and the Corridor Nan Shan (Qilian Shan) are two major tectonic units, which were affecting the formation and evolution of the drainage systems in the Hexi Corridor. To explore how the two tectonic units influence the drainage system evolution is a fundamental issue involved in the ecological hydrology management. Moreover, the drainage system evolution is a basic geological background for the water resource utilization.During the field observation, we find a fluvial gravel layer covering on the summit of the Jinta Nan Shan (from Zhengyi Canyon to the Hei Shan). The features of the gravel layer are consistent with that of the terrace T2around the Zhengyi Canyon. We deduce the gravels came from the Qilian Shan based on measuring the tilts of the "ab" face and accounting the Iithology of the gravels. We also compare the Iithology of the gravels on the top of Jinta Nan Shan with that of rivers the Qilian Shan front. The results suggest the gravels probably came from the upstream of the Heihe River and the Beidahe River in the Qilian Shan.The Heihe River cut through the Zhengyi Canyon to the north. The formation of the Zhengyi Canyon and the Heihe terraces is affected by the uplift of Jinta Nan Shan. We analyzed the topographic features of the Jinta Nan Shan and found the northern and southern flanks of the Jinta Nan Shan are asymmetrical in their reliefs. The southern flank is gently tilted to the Hexi corridor, while the northern flank is relatively steep in contacting with the Jinta basin. These characters reflect that the uplift of the mountain is related to the folding deformation. We also have found four staircases of terraces developed around the Zhengyi Canyon, and all terrace surfaces are obviously deformed. They have the same deformation features with the Jinta Nan Shan. According to the geometry of the folded terrace T2and the dipping of the Cretaceous strata beneath the terrace gravels, we infer that the fold deformation is resulted by a buried thrust fault. Combining our foundation with research results of other studies, we obtain that the fault has three components of a low angle thrust part (in the deep), a listric thrust part and a high angle thrust part (near the surface). We analyzed the geometry of the terraces deformation, and calculated the starting depth and ending depth of the listric thrust, which are～18km and～4km, respectively. Since the T2formed, the crust has shortened～145m with a mean shortening rate of～1.2mm/a.Previous studies suggested the exposure age of T2is～230ka, accordingly, we concluded that the Jinta Nan Shan was not elevated before～230ka even it had started folding. Before～230ka, the Heihe River and the Beidahe River both had been wandering in the Jinta Nan Shan area, or they might join in the front of the Jinta Nan Shan, and then wandering in the Jinta Nan Shan area. After～230ka, the uplift rate of the Jinta Nan Shan accelerated, the channels of Heihe River and Beidahe River started to incise along the canyons of Zhengyi (the Heihe River) and Yuanyang (the Beida River), and the Jinta Nan Shan was continually folded and uplifted.