The Deformation Of Terraces And Thrust Kinematics Along The Zamu River

As one of the most important main boundary faults in the northeastern margin of the Tibet Plateau,the North Frontal Thrusts of Qilian Shan(NFT)has experienced multiple periods of tectonic activity since its development in early Paleozoic and gradually formed,and is still active in the Holocene.At present,there are two different understandings about its activity rate(high rate or low rate),and the two rates indicate two different distribution characteristics of crustal shortening.Therefore,a key breakthrough to solve the dispute is: as the main boundary fault,does the fault deformation in the northern margin of Qilian Shan absorb most of the shortening of Qilian orogenic belt?Based on the above background,taking the eastern segment of the northern margin of Qilian Mountains fault(the southern margin of Wuwei Basin fault and Huangcheng-Ta 'erzhuang fault)as the main research object,this paper systematically studied the sequence,distribution and morphology of Zamu River terrace across the two faults,and analyzed the surface fault and related fold deformation characteristics.Combined with the results of Optically Stimulated Luminescence and Cosmogenic nuclide exposure dating,the deformation rates of the two thrust-fault systems since?116 ka were reliably defined.The deformation process of the Zamuhe terrace under the influence of the regional thrust-fault system was discussed and its kinematic characteristics were studied.The main research progress of this paper is as follows:(1)A reliable chronological sequence of river terraces in the lower reaches of Zamu River is established.Based on the field survey data,the elevation of the terrace below the terrace level is mainly 15 km.According to the regional tectonic location and terrace shape,it is divided into three river sections,and a total of 14 terrace cross sections are measured.The dating results show that the ages of terraces at all levels are: T2: ? 48 ka,T2a: ? 73 ka,T3: 80-86 ka,T4: ?116 ka.(2)Combined with the analysis of deformation characteristics and chronological results of terrace,the kinematic characteristics of regional thrust fault system are obtained: Trishear spreading fault-related folds are developed near the HuangchengTa'erzhuang fault.The simulated annealing algorithm model implemented in Matlab shows that the slip rate of fault is 1.12±0.13 mm/a and the shortening rate is1.01±0.12mm/a since ?116 ka.The faults in the southern margin of Wuwei Basin are listric thrust faults.According to the geometric model of listric thrust faults,the dip angle is 2.08±0.1°,the slip rate is 0.75±0.09 mm/a and the shortening rate is0.68±0.07mm/a.Finally,The two faults in the region are rooted on a deep fault surface,and the total slip rate of the deep fault is 1.87±0.20 mm/a,and the overall shortening rate of the entire fracture system is 1.69±0.19 mm/a.(3)Combined with other geological and tectonic evidences(including long-scale evidences),we find that the fault slip rate of the NFT is consistent in space,and the overall thrust rate is ?1.5 mm/a above the Hexi Corridor.Secondly,the total crustal shortening rate of the Qilian Mountains is distributed in various fault-fold systems,and the shortening rate distributed in the northern margin fault zone is approximately1-2 mm/a,indicating that the deformation mode of the Qilian orogenic belt is the overall uplift mode.From the perspective of fault kinematics,this paper systematically limits the thrust deformation rate,which is of great significance to explore the deformation model of the Qilian orogenic belt.