| The slip rate is one of the important kinematic parameters to study the fault,and it is also a necessary parameter to test the regional tectonic deformation model.As one of the northern boundary faults of Qinghai-Tibet Plateau,the Altyn Tagh fault zone(ATF)is a hot pot by researchers at home and abroad.They used different methods to determine the strike-slip rates from 10 mm/a to 30 mm/a,but there is still a controversy.In addition,the past studies mainly focused on the use of stratigraphic and lithological correlation to obtain the cumulative displacement of geological bodies with long time scale(> Ma),and the dislocation displacement of geomorphic units with short time scale(since the late Quaternary),such as gullies and river terraces.Geodetic observations with year-to-decade scale,such as GPS and In SAR crustal deformation,generally lack the fault displacement between them,which leads to an empty area to understand the history of fault evolution in a million-year time scale.In this thesis,the geometric and kinematic characteristics of the ATF in a long time scale are studied by combining tectonic geomorphology,various provenance analysis and cosmogenic nuclide burial chronology.The specific contents of this thesis are as follows:(1)By summarizing the previous studies on strike-slip rate of ATF,we present its motion image from different time and space scale.These results show that the strike-slip rate of ATF didn’t be stable,uniform or reducing from west to east since Miocene,but higher in the middle of the southern ATF,lower at both ends,and gradually reducing from west to east in the northern ATF.The uneven distribution of time and space may be the shift of movement amount from the eastern end of southern ATF to northern ATF,and the absorption and adjustment of the adjacent thrust faults.And we should do more works on the time scale of 100 ka to 10 Ma and on the western segment of ATF,which are benefit to understand the Cenozoic deformation model of the Tibetan plateau clearly.(2)To describe the current geomorphological differences of the Altyn fault zone,using Arc GIS software and Topo Toolbox Matlab program,from the 90 m resolution digital elevation model released by NASA,we explore the possible linkage between the topographic features and Quaternary activities of the ATF by presenting results of topographic indices,including the altimetry,hillslope gradient,local relief,and river channel steepness.We found similar along-strike topographic variations of the ATF and Quaternary deformation.From a spatial distribution perspective,the topographical indices along the ATF are characterized by wave-like changes from the west to the east,with dividing points near the Xorkol Basin and Annanba Mountain.The along-strike variation of the topography is closely correlated with the Quaternary slip rates.Further comparison of the annual average precipitation and bedrock lithology indicates less effect on this observed topographic variation.We conclude that the Quaternary oblique-slip along the ATF and geometry of fault led to the development of differential landscapes.(3)Because strike-slip faults broke the system of alluvial fans and their catchment basins,the residual offset alluvial fans would be distributed unevenly along the fault direction.Therefore,we propose three methods to determine the large-scale cumulative displacement of strike-slip fault based on offset alluvial fans.Firstly,determining whether the ratio of alluvial-fan area to the area of the catchment basin Af=γAc(Af is the alluvial-fan area,Ac is the area of the catchment basin,γ is a constant 0.5±0.35)is abnormal can be used to obtain strike-slip displacement so as to limit the fault rate.Secondly,the strike-slip displacement is obtained by the geomorphic units with the same lithological mineral components distributed at both sides of fault.Thirdly,the strike-slip displacement is estimated by comparing the residual geomorphic unit with the corresponding stream outlet.These methods are also used to measure the strike-slip displacements at the ATF on the million-year timescale,and to estimate the age of these geomorphic units based on exiting fault slip-rate.It is a new and useful method to understand exactly the evolutionary history of strike-slip fault,if we can obtain accurately the age of geomorphic unit.(4)Based on the U-Pb chronology of detrital zircon,heavy mineral composition,gravel composition and other provenance analysis methods,as well as cosmogenic nuclides and optical stimulated luminescence(OSL)dating techniques,the age of the offset geomorphic unit is accurately determined.We think that the Annanba section and Subei section recorded the left-lateral displacement accumulated since Pliocene in the northern margin of ATF.Considering the similarity of sections sediments with adjacent modern channels lithology and detrital zircon age spectrum,it is estimated that the long-term average displacements of ATF are 12.5±2.5 km and 26±3 km,and the corresponding burial ages are 2.9±0.2 Ma and 2.5 ±0.3 Ma,and then the strike-slip rates are 4.3±1.2 mm/a and 10.4±2.4 mm/a respectively.In addition,according to the offset river terrace with the OSL age of 32.65±1.71 ka and the corresponding strike-slip displacement of 120±26 m,the left-lateral strike-slip rate is calculated at Annanba,which is basically consistent with the long-term strike-slip rate.Combined with the strike-slip rate obtained by predecessors in the southern margin of ATF since Holocene,it is considered that the total slip rate in this section of ATF is about 8 mm/a.In a word,when discussing the distribution of fracture strain,it should be noted that the total rate change of fault is closely related to the geometric distribution and deep structure of the fault itself. |
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