Cenozoic Tectonic Activity Of The North Altyn Fault:Evidence From Geomorphological Analysis And Thermochronology

The left-reverse Altyn Tagh fault system(ATFS),with a length of?1600-km,is the northern boundary of the Tibetan Plateau and an important boundary affecting the northward extension of the plateau.At present,the understanding of the Altyn Tagh fault system's kinematic process since Cenozoic is still controversial.One of the main reasons is that the time and mode of the North Altyn Fault,which separates the Tarim Basin from the Altyn Shan,are not clear.Therefore,I used active tectonic geomorphology analysis,bedrock and detrital thermochronologic dating and inversion to systematically study the characteristics of the active structures since Quaternary,initial time and movement mode of the North Altyn Fault during Cenozoic.Then I compares the fault's kinematic history with the uplift and extension characteristics of the whole Tibetan Plateau.The conclusion is as follows:Firstly,we used DEM data to extract the geomorphic characteristics of 18 rivers and related catchment basins flowing across the North Altyn fault.Geomorphic indice,such as horizontal and vertical rivers profiles,standardized river length slope index(SLK),normalized river steepness index(K_sn),area-elevation curves and their integrals(HI)of catchment basins,are analyzed to study the tectonic activity of the North Altyn fault during the Quaternary.The results show that the rivers'vertical profile and the area-elevation curves of the corresponding catchment basins,along the western part of the North Altyn fault,are both concave upward.Moreover,they have many small knickpoints on the river profile,relatively low SLK,K_sn,and HI values.On the contrary,most of the river profiles in the eastern part of the fault are convex or linear,with the large knickpoints on the North Altyn fault's hanging wall,which coincide with high SLK and K_sn values.The associated area-elevation curves are mainly S-shaped and convex,and the HI values are relatively large.The above geomorphological indicators show that the eastern part of the North Altyn fault's geomorphic activity is stronger than that of the western part.I infer that this along-strike variation of geomorphic indice results from intense lateral contraction from the Akato restraining double bend of the South Altyn Tagh fault,south of the eastern part of the North Altyn fault.No systematic sinistral dislocation have been detected on the studied rivers when they pass through the North Altyn fault.A newly discovered frontal fault that folded the late Quaternary alluvial fan to the north of the North Altyn fault shows typical thrust characteristics.We thus infer that the North Altyn fault is dominated by reverse dip-slip in the late Quaternary.Together with published results,our findings imply that the North Altyn fault likely changed from a strike-slip-dominated fault to a reverse-dominated fault and have closely interacted with the South Altyn Tagh fault in the late Cenozoic.Secondly,the bedrock and clastic apatite(U-Th)/He dating and inversion were carried out.It is based on the detailed geomorphological analysis of the well-preserved high-elevation low-relief surfaces in the North Altyn fault's hanging wall.The study revealed that the North Altyn fault started to move during the Cenozoic.It also implies the interaction between tectonic uplift and climate change in this area.The results of bedrock apatite(U-Th)/He depth profile and QTQt thermal history simulation show that the northern margin of Altyn Shan changed from the long-term slow cooling stage(<1°C/Myr)to the rapid cooling stage(?12°C/Myr during?36-31 Ma,?4°C/Myr during<31 Ma).However,the analysis and inversion results of detrital apatite(U-Th)/He indicate that rapid rock exhumation of interior of Altyn Shan occurred after?17 Ma.Considering that most of the bedrock samples are located in front of the Altyn Shan and close to the North Altyn fault,the detrital apatite mainly comes from the Altyn Shan's interior.I think that the bedrock results reflect that the North Altyn fault began to be active at?36 Ma,which led to Altyn Shan's uplift on its hanging wall.However,due to the arid climate,the exhumation mainly occurred in the piedmont area and did not affect Altyn Shan's interior.In the global middle Miocene climate optimum(MMCO)of?17-14 Ma,the climate became humid and enhanced erosion.The river eroded rapidly from the front to Altyn Shan's interior,resulting in the rapid exhumation event of?17 Ma.In addition,the low-temperature thermochronological data of the bedrock in the margin and interior of the Tibetan Plateau,including the results of this paper,are systematically summarized.Furthermore,the exhumation rate of the Tibetan Plateau since the Cenozoic is calculated.The results show that rapid exhumation has occurred along the Tibetan Plateau margin from?45 to 25 Ma(the fastest is?0.9 mm/yr).However,the current rapid exhumation events(more than half are>2 mm/yr)generally occurred after?20 to 15 Ma.This inference is consistent with that of this paper along the northern margin of the Altyn Shan.The tectonic uplift caused by the long-range effect of the India-Tibet collision was transmitted to the edge of the present Tibetan Plateau in the late Eocene Oligocene.The rapid exhumation in the Miocene and later may be caused by climate change based on the topography formed by continuous tectonic uplift,or the rapid increase of river erosion capacity after the surface uplift to a certain extent.