Cosmogenic¹⁰Be And²⁶Al Chronology Of Layered Landform Piedmont Of The Kalpin Thrust Belt, SW Tianshan

The Kalpin thrust belt system is located in the front edge of foreland ofsouthwestern Tianshan. Its structural styles and evolution are the key to understandingof the activity characteristics of southwestern Tianshan. Alluvial or diluvial terraceswhich extensively developed in piedmont of Kalpin thrust belt system is provide animportant record of local climate fluctuations and regional tectonic activity.It is difficult to date directly by using routine chronology methods because of thelack of suitable dating material. The lack of reliable geochronological data have aserious impact on the study of structural characteristics of the reverse fault activity inpiedmont of Kalpin fold-and-thrust belts and the progress of Tianshan Mountainsclimate evolution during the late Quaternary.The remarkable progress of cosmogenicnuclide surface exposure dating techniques in recent years has opened a new way forthe study of the geomorphic chronology in the arid region.The main progresses made by this thesis is including that：We have selected8places where most developed alluvial terrace in piedmont of the Kalpin thrust beltas the interesting sites, and obtained21alluvial terraces’“apparent” exposure agesfrom the surface gravel mixed samples by¹⁰Be-²⁶Al exposure dating method; In orderto compare the reliability of result and applicability of two dating strategy, the¹⁰Bedepth profile dating techniques are used on four of the alluvial terraces.Through analyzing the image interpretation, field reconnaissance and cosmogenicnuclides measured results of the study alluvial terraces, the preliminary understandingof relationship between the tectonic activity and regional climate fluctuations duringsame period are obtained as following:There three stages geomorphic alluvial terraces widely developed in piedmont ofthe Kalpin thrust belt. The latest stage is the4⁹ka （Holocene）, then20⁴0ka （LatePleistocene） and the stage of60¹00ka.The Piqiang fault divides Kalpin thrust belt into two parts. In the eastern section,both of the piedmont of the first and second-row fold anticlines has developed threetypical fault scarps. The height of each fault scarp is within a range of0.8¹.2m,1.6²m and3⁴m, respectively. But no active evidence of fault in third row duringthe late Quaternary; And in front of the first two rows on west side of Piqiang faultthere are three scarps with approximate feature as east. The activity of the fault isweak in front of the third row. Deformation in piedmont of fold anticlines in west section of the Kalpin thrust beltis mainly dominated by fault activity. In piedmont the first two anticlines（Xikeer andstudy site in front of Aozigeertage anticline）, fault`s vertical slip rates are0.20<sup>0</sup>.44mm/a, with the corresponding crustal horizontal shortening rate between0.74and1.07mm/a. In east section of the Kalpin thrust belt, vertical slip rates of the fault atSanchakou are0.15±0.02mm/a<sup>0</sup>.29±0.05mm/a, crustal horizontal shortening rateare0.18<sup>0</sup>.46mm/a and0.69mm/a, respectively, which are slightly lower than thepiedmont fault at Saergantawutage. On the contrary to Sanchakou, the latest record ofthe uplift rates of the piedmont fault at Saergantage is significantly increased, indictethat the piedmont fault activity at Saergantawutage fold anticline is graduallyincreased since the Late Quaternary.Crustal horizontal shortening rates of the subsidiary anticline at Sanlangtage andWulangtage are1.149mm/a,1.077mm/a, respectively, since the top terrace wasformed.Then the shortening rate is significantly reduced after the formation of the T1terrace. It means the primary deformative style is gradually reducing anticlinal uplift,and accompanied by fault activity to release the regional compressive stress in eastpart of the Kalpin thrust belt. Overall, the late Quaternary tectonic activity of theKalpin thrust belt is mainly concentrated in the first two anticlinals, and the activity inwestern part is slightly stronger than the eastern section.In mixed spample explosure dating, samples from modern gully are used to assessthe inherited concentrations generated during post-depositional processes inindividual clasts on a terrace. From the comparison of the result and process betweenmixed spample explosure with<sup>10</sup>Be depth-profile dating. It is obvious that mixedspample explosure dating is more suitable for the determinating the formed age ofmicro terraces developed in this research region.Inferred by comparison of the evolution of river terraces and<sup>10</sup>Be depth profileanalysis results, only the fault scarp caused by the reverse fault activity after thealluvial terraces have been abandoned can be retaind.If the terrace exposure age isused to limit time when alluvial terraces have been offset by faults，the mixed clastexposure dating results are slightly earlier than the time of tectonic activity events.This study has compared the terrace explosure age with δ18O records of last0.125Ma from Gurria ice cores, LR04stack global compoiste deep-sea data andNGRIP ice cores. The comparison results show that surface mixed clast sample`s¹⁰Be,²⁶Al exposure ages, especially the¹⁰Be depth-profile Simulated dates suggest that theevolution of the extensively developed three proluvial or alluvial terraces is mainlydriven by climatic factors. There is a good consistency between alluvial terraces` ¹⁰Be exposure age and Gurria ice core δ18O records. It is inferred that the climatefluctuations of the research region’s and the climate evolution of the Tibetan Plateauhave a stronger response relationship.Either the obtained alluvial terrace exposure ages have a well sequence feature，orthe chronology results from different dating program also have a good consistency。But there are a lot of aspects of this study need further improvement:Firstly, the significant differences between¹⁰Be and²⁶Al exposure ages suggest thatthe high impurity content of chalcedony as the target sample for cosmogenic nuclideexposure dating is not ideal yet;Secondly, the estimated surface erosion rates based on the ratio of¹⁰Be and²⁶Alstill need to be determined through long-term field observations;Thirdly, it needs to be further researthed whether the surface apparent age can standfor the real age since the time series of when Alluvial terraces formed is an average oftheoretical value;Fourthly, the microrelief,standed by the piedmont alluvial terraces,can not reflectthe tectonic events and fluctuation of regional climate stably. It needs to be improvedby comparison with more time series of a wider range.Fifthly, the exposion age of alluvial terraces still can not meet the requirements forquantitative study of paleoearthquakes. More precise and specific time series arerequired to solve this problem.