Accurate Determination Of Late Quaternary Slip Rate Of The Aksay Segment Within The Altyn Tagh Fault

The deformation of the Tibetan Plateau is obviously influenced and controlled by the regional deep faults during its expansion to the northeastern direction.The activity of large faults in the interior and boundary of the plateau can provide kinematic constraints for the regional dynamics,which is of great significance to understand the strain partitioning and seismic hazard analysis of the tectonic transition region.The analysis of the slip rates at different time scales can help to understand the accumulation and release ways of strain.Comparing slip rates of different faults can reveal the strain regulation in the region.The Altyn Tagh Fault(ATF)is an important large strike-slip fault in the northeastern Tibetan Plateau.Along the uplift of the NW mountians in the south of the Qilianshan,the eastern segment of ATF becomes a very important tectonic transformation area.Accurate determination of the slip rate in time and space is helpful to reveal the tectonic deformation process of the northeastern Tibetan Plateau and the tectonic transtion relationship among different blocks.In this paper,the author determines slip rates of Akasy segment accurately,aiming at exploring the change of the slip rate in time and space,through summarizing the results of previous studies on the slip rate in the eastern section of the ATF,combined with the current GPS rates.The author quantitatively assays the spatial distribution of slip rates in the eastern of the ATF using the north-east crust shortening rates of Tanghe Nan and discusses the role of the Akasy segment in the tectonic transotion of the ATF.The paper is in the 60 km range of Akasy segment in ATF,mainly selecting the west of old county,Akasy old county,Jiaerwuzongcun,Yandantu four research positions.These research ways include high-precision satellite image interpretation,field geological survey,Quaternary chronology,and other methods of quantitative study of the active tectonics.Main achievements of the paper are as follows:The fault displacements of typical offsetted geomorphic markers are obtained by using high-resolution photogrammetry method.The author uses High-resolution DEM,which is obtained by the photogrammetry technique to measure displacements.For the small-scale displacement,the semi-automatic measuring software La Di Caoz_v2 can be used,which can give the different degree displacement recovery.This way can reduce some human error.For large-scale displacement restoration,image data can be used,combined with geomorphic evolution analysis to give reasonable displacement error.the displacements sequence of terraces Ta,Tb,Tc in Akasy old county is 1360±110m,1790±130m,2290±220m.The chronology sequence of regional and different landforms was established.The acquisition of landforms age mainly depends on the mature luminescence(OSL)and the situ cosmogenic nuclides(10Be)dating method.the alluvial fans of Altun mountain are mainly covered by the wind-loess.The author uses the luminous age of the loess bottom samples to determine the ages in different the research position.The OSL age at the bottom of loess in the west of Akasy old county is 9.0±1.2ka,close to the formation age of landform.The age of residual landform in Yandantu is 7.2±0.6ka.There are 8 terraces in Jiaerwuzongcun and the range of these landforms are between 2.8ka and 9.0ka.In Akasy old county site,the ages sequence of Ta,Tb and Tc is 125.7±11.3ka,172.9±15.7ka,226.7(+21.9/-34.6)ka by using 10 Be exposure dating combining the depth profile to fit the inheritance concentration.When calculating slip rate of four research sites,the author uses the displacementage sequence of multiple offsetted terraces in the Akasy old county and the Jiaerwuzongcun.Monte Carlo simulation method is introduced to analyse the error of displacement-time synthetically and 1000 possible slip paths are fitted.The results of the slip rate approach the process of geologic history.The slip rate of the west of old county is 10.9±1.2mm/yr since 9ka.In Akasy old county,the slip rate is 10.2+1.2/-1.1mm/yr since 220 ka,consistent to the previous study of the old county since 7ka and making complement in the time scale,which also shows that the slip rate of the ATF is stable at different time scales.The slip rates of the Jiaerwuzongcun and Yandantu is 7.4+1.1/-0.6mm/yr since 7ka and 5.1±0.8mm/yr since 7ka.The reason and mechanism of the spatial change of slip rates are discussed.By analyzing the spatial variation of the slip rate in east segment of the ATF,the slip rate reduction has occurred in the middle position of the Akasy segment,from the 9~11mm/yr in the Akasy old county to 7.4+1.1/-0.6mm/yr in the Jiaerwuzongcun,which has been lowered to the 5.1± 0.8mm/yr near the Yandantu.According to the results of previous study,the paper analyses the spatial distribution of slip rate in the whole ATF east segment and suggests that the slip rate of the east section of the ATF decreases faster in the Akasy-Subei segment,from the 9~11mm/yr to 5.1±0.8mm/yr,which takes up the ~50% of the main segment slip rate.But the slip rate decreases slowly from the Subei to the east.According to the results of the shortening rate of the middle section of the northern edge of Tanghe Nan shan,the shortening rate of the parallel ATF direction is only 0.8~1.0mm/yr.It is believed that the decrease of the remaining 3.9~4.1mm/yr is absorbed mainly by the uplift of the Tanghe Nan Shan,the crustal shortening of the Cenozoic basins.