The Crustal Shortening Across The Bole Basin During The Late Quaternary And Its Implication In The Deformation Of The Tianshan Rang

Since the Cenozoic,since the far-field effect of the India-Eurasia,the Tianshan Mountains created in the Paleozoic were rejuvenated and formed the current tectonic geomorpholigy.The active tectonic faults in the Tianshan Mountains can be divided into three categories,one is the fault related fold belts at the northern and southern piedmonts of the Tianshan Mountains,such as the Keping anticlines,the Kuqa anticlines,and the anticlines at the northern piedmont.And the second is the the large strike-slip faults within the Tianshan Mountains,such as the Talas-Fergana fault and the Boluokenu-Aqike Kuduke fault(also named Dzhungarian fault),and the third are the thrust faults in the intermountain basins within the Tianshan Mountains,such as the The Hejing fault anticlines in the Yanqi Basin,the thrust faults in the Bayinbuluk Basin,etc.In terms of spatial pattern,the Tianshan Mountains can be divided into the western Tianshan,central Tianshan and eastern Tianshan.The results of geodesy show that the shortening rate of the western Tianshan is ~ 20 mm/a,the central Tianshan is ~ 10 mm/a,and the eastern Tianshan is 3 ~ 5mm/a.The crustal shortening rate of the Tianshan Mountains gradually decreases from west to east.But on the geological time scale,especially on the active tectonic time scale,does the crustal shortening of the Tianshan Mountains also follow the trend of gradual decrease from west to east? Previous studies have quantitatively studied the Late Quaternary slip rates of thrust faults in the intermountain basins of the western Tianshan Mountains and the Keping fault-folds in the southern piedmont,and the intermontane basins of the eastern Tianshan Mountains,including the Turfan Basin and the Yanqi Basin.In the central Tian Shan,previous studies have quantitatively constrained the crustral shortening rates of the Yili Basin,Nalati Basin,Bayinbuluk Basin,and the thrust related fold belts at the southern piedmont of the Tianshan Mountains.Therefore,to constrain the late Pleistocene slip rates of the Bole Basin,it has important implications for understanding the strain distribution in the central Tianshan Mountains and the spatial pattern of deformation in the entire Tianshan Mountains.This paper targets the Kusongmuxiek fault on the southern margin and the Alatao southern fault on the northern margin of the Bole Basin.We used in-suit cosmogenic nuclide,Optically stimulated luminescence(OSL),and 14 C to date the abandonment ages of alluvial fan surfaces and river terraces,combined with high-precision DEM image data obtained from drone aerial photography.The following conclusions were obtained:1.There are six levels of alluvial fan surfaces developed in the Bole Basin.Through the 10 Be cosmogenic nuclide dating method,combined with the previous dating results of the alluvial fan surfaces in the Bole Basin,the age of their abandonment was determined.The abandonment ages of the five levels of alluvial fan surfaces in untectonic area are 117.82 ± 4.45 ka、98.68 ± 6.49 ka、76.40 ± 2.58 ka、55.1 + 7.6 /-3.4 ka 和 2.33 ± 1.90 ka respectively.The abandonment ages of these five levels of alluvial fan surfaces correspond to the transitional ages of glacial-interglacial periods in the climate cycle,respectively.This shows that the abandonment of alluvial fan surfaces is mainly controlled by climate,and the alternation of warm and cold climates causes the increase of provenance transportation and the decrease of water volume,leading to the abandonment of alluvial fan.Besides these fan surfaces,the alluvial fan surface in fold area are controlled by tectonic.2.There are mainly three thrust related anticlines in the southern margin of the Bole Basin,namely the Latgan anticline,the Alashak anticline and the Jinghenan anticline.According to the geomorphological surface deformation and the formation and fault occurrences obtained in the field,the sub-surface geometric structure of the Latgan fault is mainly composed of three parts,which are the lower ramp with a dip angle of 41°,the listric fault area,and the upper ramp with dip angle of 55°.Latgan fault is characterized by listric fault.According to the deformation characteristics of the geomorphic surface and the triangular shear inversion script,the Alashak fault is determined to be a thrust fault with a dip angle of 10°-14°.The Jinghenan anticline is the seismogenic tectonic of the 2017 Mw 6.3 Jinghe earthquake.Combining the coseismic rupture of In SAR inversion,seismological inversion results and detailed geological mapping in the field,the Jinghenan anticline can be inversed as a faultbend anticline.The Jinghenan fault is composed of a lower ramp with a dip angle of 42° and an upper ramp with a dip angle of 46°.The coseismic displacements of the Jinghe earthquake are mainly limited on the lower ramp of the Jinghenan fault,so the fault bend of the Jinghenan fault may have controlled the upward propagation of the coseismic rupture of the Jinghe earthquake.3.Based on OSL dating and the relationship between colluvial wedges and strata in the trenches excavated in the eastern section of the Jinghenan fault,four paleoearthquakes were identified,and the earthquakes occurred at E1(29.8 ± 3.0 ka BP),E2(22.4 ± 1.9 ka BP),E3(14.7 ± 1.9 ka BP)and E4(11.6 ± 0.9 ka BP later).The paleoseismic intervals between E1,E2 and E3 are 7.3 ± 3.5 ka BP and 7.7 ± 2.7 ka BP.Therefore,the elapsed time of the latest paleoearthquake event may have reached the earthquake recurrence cycle of the Jinghenan fault,that is,the Jinghenan fault has reached the late period of the major earthquake cycle,and there may be major earthquakes that break through the surface in the future.4.The late Pleistocene slip rate of the Kusongmuxiek fault and Alatao Southern fault was determined by measuring the surface deformation of the geomorphic surface,cosmogenic nuclides,OSL,and 14 C dating.The crustal shortening rate of the Latgan anticline is 0.4 mm/a,the crustal shortening rate of the Alashak anticline is 3.9 ± 0.7 mm/a or 0.67 ± 0.03 mm/a,and the crustal shortening rate of the western section of the Jinghenan fault is 0.21 ± 0.04 mm/a,the horizontal shortening rate of the middle section was 0.37 ± 0.07 mm/a,and the horizontal shortening rate of the eastern section was 0.19 ± 0.02 mm/a.Based on the uplift model of the back limb,the crustal shortening rate of the lower ramp of the Jinghenan fault overlying the Jinghenan Anticline is 0.16 ± 0.07 mm/a.The crustal shortening rate of the Yongji fault is 0.91 ± 0.08 mm/a.The crustal shortening rate of the western section of the thrust fault on the southern margin of Alatao Southern fault is 0.3 ± 0.1 mm/a,and the horizontal crustal shortening rate of the eastern section is 0.18 ± 0.01 mm/a.5.Comparing the crustal shortening rate of the Bole Basin thrust fault obtained in this paper with the previous results in the northern Tian Shan,it can be concluded that the strain in the northern Tian Shan is mainly concentrated in the Yilianhabierg zone and the Boged zone,the Bole zone is the zone with the lowest crustal shortening rate in the northern Tian Shan,which is mainly due to the counterclockwise rotation of the Bole Basin and the far-field effect of the Tarim Basin and the Bole zone.6.Through the comparison of the research results of this paper and previous research results in the geological time scale,the crustal shortening rate of the western Tian Shan is 15.1 ± 2.9 mm/a,the crustal shortening rate of the central Tian Shan is 12.4 ± 1.6 mm/a,and The crustal shortening rate of eastern Tian Shan is 5.7 ± 0.6 mm/a.9.6% of the crustal shortening in the central Tian Shan was absorbed by the Kusongmuxiek fault,and 12.1% of the crustal shortening was absorbed by the thrust faults in the Bole Basin.Consistent with the results of geodesy,on the geological time scale in the Tian Shan,the crustal shortening rates also decreases gradually from west to east.