Late Quaternary Activity Of The Zhifang Fault In The Northeast Of Xinjiang

The Zhifang fault is located at eastern margin of Junggar basin,Beitashan fault and Fuyun fault in Altay tectonic zone to the north,Wutongquan fault in Gobi Altay tectonic to the east,and East Tianshan tectonic zone to the south with a total length of about 80 km..It is a NW-striking fault in the convergence area of Altay,Gobi Altay,and East Tianshan.It belongs to the boundary zone of active blocks of grade I and II.The study of the late Quaternary activities of the fault is helpful to understand the deformation pattern of the present intracontinental orogenic belt and the influence of the uplift of the Qinghai-Tibet Plateau on the intracontinental orogenic belt.Based on the interpretation from Google Earth,analysis of the UAV DEM and30m-resolution ASTER field geological and geomorphological survey and trench excavation were carried out,the geometric distribution and segmentation of the fault were obtained.Through field geological survey and geological profile,high-precise UAV aerial photos,differential GPS for scarp and dislocation measurement,trench excavation,and OSL and ¹⁴C dating,the late Quaternary activities of the Zhifang fault have been studied.The main conclusions are following:?1?In the northern segement of the Zhifang fault,its strike is 330-345°.The fault plane dips to NE with an inclination of 60-78°,The mountains of Zheland,Ahermona Junk and Zhifang tilt toward NE.The inclination of the fault plane to Ahermona Junk is reversed;and the fault plane to the south dips to SW,The fault basically extends along the front of Takezale Mountain.Asymmetrical fault spreading folds are produced in the southern part of the thrust fault.The mountain body has obvious tilting characteristics.The nature of fault activity changes in the northern and southern fault planes of the fault,The fault is not only its distributed in four quadrants geomorphologically,but also its structural properties changed.The faults act as a pivotal movement.The Zhifang fault can be divided into two segments,namely,Zhiifang-Monrok?F1-1?and Chen Dahan-Zheland?F1-2?.According to the analysis of the geometric distribution of faults and the evidence of Quaternary active geology and geomorphology,the faults was faulted in the Holocene strata.The Late Quaternary active faults are dominated by dextral strike-slip,scarp was developed,and there are certain thrusting components with obvious linearity.?2?The trench1 was excavated in the northern segment of the Zhifang fault.Based on the inversion analysis of the fault events and the relationship between the fault,filling wedge and the cut-off cover of the stratum,three paleoearthquake events were revealed.The latest earthquake event occurred 1730-5170aBP,with a single vertical displacement of about 0.5m.The elapsed time of earthquake should last longer than 1730aBP,and a large earthquake may occur in the future.Strong earthquakes occurred in both the north and south extension of the Zhifang fault.There is no historical record of large earthquakes in the Zhifang fault.It is obviously an earthquake gap,and it is located at the boundary of the?and?order blocks.The fracture zone is about 50 km long.Therefore,it is considered that there is a possibility of strong earthquakes of magnitude 7 above in the future.?3?About 1 km north of the Zhifang city,a right-lateral displaced gully was found;Its maximum displacement is also about 14m,measured by GPS from Zhifang to Ahermona Junk.The gully is also close to the excavation position of the trench 2.So the displacement value may represent the maximum displacement of the fault near Zhifang.The trench 2 was excavated on the scarp of the Zhifang fault,and four samples were collected for OSL dating,The maximum age is 8.07±0.90 ka,which represents approximately the formation age of scarp.The average horizontal slip rate is calculated to be about 1.71-1.75 mm/a.The results of GPS study showed about 4mm/a dextral slip rate in Altai region,which is consistent with the results of this study and is in line with the regional dynamic and seismic background.The scarp height of trench 2 is about 1.6 m,and the cluster distribution of scarp height shows three peaks,0.95 m,1.85 m and 3.1 m,respectively.1.6 m is close to 1.85 m,Considering that the height of the scarp has been reduced after a long period of weathering erosion after the earthquake,it is reasonable to use 1.85 as the calculation height.The age of the sample is 4.79±0.59⁸.07±0.90ka with small variation.It is Holocene Quaternary sediment.The age of the sample basically represents the formation time of the scarp and the average vertical slip of the fault can be calculated.The calculated vertical velocity is is about 0.21-0.39 mm/a,which indicates that,the Zhifang fault has a certain vertical component besides the significant dextral strike-slip.?4?There are two small oases along the Zhifang fault,and the relationship between the formation and active faults is studied.This work studies the relationship between the formation of two small oases along Zhifang Fault and fault activities in the eastern Xinjiang.The distribution characteristics and hydrogeological conditions of oasis were analyzed based on unmanned aerial vehicle?UAV?photos,trenches,and sample dating.The results show that active faults play a decisive role on the formation of the nature of oasis.The latest activities of faults built an impermeable and compact dam at the front of the oasis.It is mainly composed of calcareous cemented clay minerals and other debris.Water accumulate in the dam,which can resist to the destroy and erosion because of the continuous forming of the dam by the fault activities.Fine sand and soil carried into the dam by wind provide suitable conditions for the growth of vegetation,while grasses can fix sand layer.Small oases are formed with the virtuous circle.In addition,the small catchment area of the dam can avoid large seasonal floods to destroy the dam.It is another important factor that the oasis can exist.