Late Quaternary Kinematic Characteristics Of The Kumysh Fault In The Eastern Tian Shan

The Tian Shan Mountains are ancient orogenic belts formed since the Late Paleozoic.The India plate subducted northward and the entire Tian Shan has been uplifted once again,in the stage of extrusion,thrust and uplift since the Cenozoic.As a typical continental internal orogenic belt,its deformation mainly distributes in the fault-fold zone of the front edge of the intermountain basin and the large-scale strike-slip faults in the mountain.For the Tian Shan in Xinjiang,it is customary to call the eastern part of Urumqi?roughly 88°E?as the Eastern Tian Shan and the western part as the Western Tian Shan.Tianshan is clamped between the Kazakhstan-Junggar and the Tarim plates.Current GPS observations and geological surveys show that the whole central Tian Shan region moves northeast under shortening in NS direction,and the shortening rate of the Eastern Tian Shan is lower than that of the Western Tian Shan,indicating that the deformation mechanism of the Eastern Tian Shan is not consistent with that of the western part.The pattern of basin and mountain in Tian Shan is a kind of unique geomorphology within Tian Shan region.Many young faults are developed in Cenozoic sedimentary basins,especially in the transitional zone from piedmont to basin,which are usually small in scale.However,the activity of these young faults is stronger than that of the old faults since the late Cenozoic.The research area of this paper,Kumysh Basin,which is located in the eastern part of the Eastern Tian Shan,characterized by an elongated strip-shaped sedimentary basin of NWW-SEE direction.It is a double-layered basement developed from the precambrian crystalline basement and the Paleozoic fold basement.It is adjacent to Baoertu Ura Mountain in the north and Kiziletag Mountain in the south.The Kumysh fault,located in the southern margin of the basin,is a thrust fault inside the eastern Tian Shan.It plays a key role in regulating the internal tectonic deformation and the formation and evolution of the basin.However,the fault activity including geometric characteristics,paleoearthquake events,the distribution of displacements and the slip rate are not well studied.So,how to reasonably evaluate the role of Kumysh fault in the Tian Shan deformation?What is the coordination of strain distribution along the Kumysh fault and its surrounding faults?In order to answer these questions,we study the late Quaternary kinematic characteristics of the Kumysh fault.The data used in this paper are the Worldview-2 panchromatic band stereo imagery of 0.5 m resolution.We used this stereo imagery to generate high resolution DEM and discussed whether this method could meet the needs of quantitative study of active tectonics.Then,using these data to classify the multi-level alluvial fans along the Kumysh fault,and measure the vertical displacement retained on the different levels of the alluvial fans;combining with the cosmogenic nuclides ¹⁰Be dating and Optically Stimulated Luminescence?OSL?dating methods to obtain the ages of multi-level alluvial fans,and comprehensively analyzed the late Quaternary kinematic characteristics of the Kumysh fault.On the basis of reviewing previous results,combining with our field research,indoor comprehensive analysis,the following five points are mainly recognized:?1?Using the 0.5 m resolution Worldview-2 stereo imagery to generate DEM,the vertical accuracy of DEM is 1.81 m when no control points are added;extracting profiles across the fault scarps from the DEM with control points,and then compared them with those measured in the field with the differential GPS?ppGPS?.The shape of the fault scarp can be accurately reconstructed by satellite-based DEM with the elevation deviation of 0.29 m after elevation correction and the accuracy of the height measurement of fault scarps can reach 0.25 m.These findings indicate that the DEM generated from Worldview-2 stereo imagery is capable of measuring relative deformed topographic features,which could be of great interest to professionals exploring the use and accuracy of satellite stereo imagery for active tectonic applications.?2?The lower-mid Pleistocene and the Holocene alluvial fans of the southern margin of the Kumysh Basin are well developed.The age of the alluvial fan is obtained by using the ¹⁰Be cosmogenic nuclide dating and optically stimulated luminescence?OSL?dating:fan 5 is about⁵0 ka,fan 4 is about 30⁴5 ka,fan 3 is about 10 ka,and fan 2 is about 5 ka.Compared with the deep sea oxygen isotope curve,it is found that the climate has been in the rapid transition period between the glacial and interglacial periods since 60 ka.The alternation of cold and warm climate caused the river to cut rapidly and it is the active climate change laid the foundation for the development of the alluvial fans in the southern margin of the basin.?3?We classified the level of every alluvial fan through detailed indoor satellite image analysis and field mapping.After measuring the heights of 195 fault scarps,we found that the height distribution of fault scarps has a great correlation with the positions and levels of alluvial fans.Older level fans normally record the cumulative offsets of several seismic events,however,the distribution characteristics of vertical offsets on the surface can not correspond to paleoseismic events,which indicated that the frequency of geomorphic surface formation may be lower than that of earthquake occurrence.That is to say,the time required for the formation of geomorphic surface is longer than the recurrence interval.Therefore,the cumulative offsets on the geomorphic surface are mostly accumulated offsets of multiple earthquakes.?4?The Kumysh fault is a Holocene thrust fault.Quantitative study of the tectonic deformation on the east and west sides of the southern margin of the Kumysh Basin revealed that the late Pleistocene-Holocene alluvial fans are displaced by the Kumysh fault,which is nearly parallel to the mountain with NWW strike.It has been obvious activiated since the late Quaternary.It is a low-angle south-dipping thrust fault.The fault is divided into east and west sections by using Yushugou as the boundary.The fault activity is not strong near the Yushugou,and there is no obvious dislocation.Combining the heights of fault scarps measured by the multi-level alluvial fans of the east and the west segment,and the corresponding ages of these fans,the vertical slip rate of the western segment of the Kumysh fault is estimated to be⁰.15±0.01 mm/a,and the vertical slip rate of the eastern segment is estimated to be⁰.13_-0⁺⁰..₀₂⁰⁴ mm/a.The results of the paleoseismic study show that there have been three earthquakes in the area with a cumulative offset of 2.33 m during the past 5.64 ka,which supplemented the historical earthquake events in the area.?5?The crustal shortening deformation in western Tian Shan is stronger than that of eastern Tian Shan,the deformation in southern Tian Shan is also stronger than that in northern Tian Shan.This paper holds that the pushing effect of Pamir Plateau in Western Tian Shan results in stronger deformation of the Western Tian Shan than that of the Eastern Tian Shan.Many intermountain basins are the main absorption areas of crustal deformation in Tian Shan,and the compressive stress decreases when it transfers from south to north.The left-lateral strike-slip rate of the Baoertu fault in the northern margin of the Kumysh basin is about 0.56 mm/a.The right-lateral strike-slip rate of the Bolokenu-Aqikekudekue?Bo-A?fault obliquely intersecting with the Baoertu fault is1-1.4 mm/a in the western margin of the Turpan basin.The right-lateral strike-slip rate of the Kaidu River fault in the southwestern side of the Kumishi basin is about 1.2 mm/a.Therefore,we propose that the Kumysh basin is a basin with a pull-torsion character,namely,the Kumysh fault and the movement of the faults is developed under the background of two large dextral strike-slip faults,which is to adjust the structural deformation caused by the imbalance of the whole Tian Shan extrusion to the right,the shortening deformation in Tian Shan is regulated by the shortening rate of 0.13⁰.2mm/a.