The Study On The Deformation Of Bailang River Terraces And Deformation Kinematics

The Qilian Shan locates in the northeastern margin of the Tibetan plateau and is an important area for studying on the plateau uplift and expansion process.The uplift and expansion of Qilian Shan is controlled by the North Qilian Shan Fault.The slip rate,kinematics and expansion process of the North Qilian Shan Fault are the basis for analyzing the regional deformation process.Most of the current researches focused on the surface faulting,the downward-extending shape of the fault and the deformation process of its related fold are less understood,and thus the obtained deformation rate is also incomplete.In such research situation,it will impede our comprehensive understanding of the deformation kinematics and the uplift and expansion process of the mountain.In previous studies,the geophysical method is usually used to obtain the fault geometry of the North Qilian Shan Fault.Unfortunately,this method has great uncertainty in the upper crust(depth less than 10 km)and for the region without sedimentary rocks.However,using the deformation of the river terrace combined with the fault-related fold model to calculate the geometry of the fault can obtain more reliable results.In this study,we calculated the geometric characteristic of the thrust fault by using the deformation geometry of the Bilang River terrace in the middle Qilian Shan,and calculated the slip rate of the regional fault based on the deformation geometry and chronological results.The main conclusions are as follows:1.Under the Bailang River anticline,the listric fault is connected with the horizontal detachment fault on the north side at a depth of 2.76 km.The detachment fault on the south side of the anticline is in a form of a southward dip,with a dip angle of 11 ± 8° changing to 34 ± 8° in deeper crust,and this fault is probably connecting to the large thrust of the North Qilian Shan Fault.2.Terraces T5 and T4 of the Bailang River were abandoned at 204 ± 5 ka and 107 ± 3 ka,respectively,and the slip rate of the Listric fault is determined as 0.9 ± 0.3 mm/a,indicating that crustal shortening in this region is shared by folding and faulting.3.The comparison between the geometry of the fault inferred by the geomorphological method and the seismic profile proves that the geomorphological method is reliable in defining the geometry of the active fault.