Insuiffcient Thermalization Effects On Determining Fission-track Ages And Detrital Fission-track Thermochronology Study On Cenozoic Strata From The Region Of Huaitoutala

As a low-temperature thermochronology，Fission-Track Dating（FTD） has beendeveloped as a useful technique for geological studies. But in the experiment process,the thermalization of thermal neutron channel can affect the reliability of the results.So this work studied the thermalization and spatial distribution of the thermal neutronin the492reactor, and calculated the influence on the FT age. This work also studiedthe Cenozoic strata of the Huaitoutala section in the northern Qaidam Basin by thedetrital fission track dating method, which revealed the time of tectonic activity in thenorth edge of Qaidam and discussed the uplift mechanism of the Qilian Mountains.1、Insufficient thermalization effects on determining fission-track agesIn fission-track dating, parent elements are measured by counting235U tracksinduced by thermal neutrons. If insufficient thermalization occurs, fission of²³⁸U and²³²Th will be induced, and further measurement error will be produced. Therefore,whether the neutrons are well thermalized or not will affect the FTD results. Due torequirement of safe operation, the101reactor was terminated in2007. By using the492reactor as the new thermal neutron reactor, this work attempted to study itsfeasibility and the potential influence on FTD. By irradiating monitor glass SRM612and CN5in pairs, I studied the thermalization situation of the492reactor. Irradiateddata show that thermal neutrons are not evenly distributed either in horizontal or invertical direction. Especially, horizontal heterogeneity is obvious. But I discoveredthat proper irradiation position in the reactor can meet the requirement of FTD. Underthe current irradiation condition, I calculated and assessed the insufficientthermalization effects on determining fission-track ages. I found that the²³²Th/²³⁸Uratios of samples and standards are the main factors to the experiment results. The results would affected if the²³²Th/²³⁸U value of samples was equal to the standardsamples. However, if the²³²Th/²³⁸U ratio was larger than that of the standards, theresults would be smaller than actual ages. Comparatively, the ages would be morethan expected if the²³²Th/²³⁸U ratio was less. Therefore, to reduce the irradiation error,I suggested either locating the position of irradiation strictly, or minimizing theinfluence of lateral heterogeneity by reducing the amount of each sample package.Additionally, accuracy of the experimental results could be improved by increasingstandard samples to adjust ζ value and using the monitor of standard glasses SRM612and CN5together.2、 Detrital FT thermochronology study on Cenozoic strata from thenorthern edge of QaidamThe Qilianshan Mountains is the most active margin of the Tibet Plateau. Itsuplift mechanism plays a vital role in the study of the Tibetan Plateau’s uplift. But atpresent the studies mainly focus on the the northern Qilianshan Mountains, because ofthe smaller relative height difference （the traditional thermal geochronology samplingis limited）, and the studies on the southern margin are less. The north Qaidam Basinis a typical Late Cenozoic tectonic deformation region, where the thick Cenozoicstrata in the basin provide a foundation for the Qilianshan Mountain uplift study.According to the principle of basin-range coupling, this work studied the LateCenozoic tectonic deformation time and style at the northern edge of the Qaidambasin by using the detrital fission track method and field geological mapping.In the Huaitoutala section, because of rapid decrease of lag time, the detritalfission track results show that during12-15Ma the source region, the Qilian Nan ShanMountains, experienced one rapid erosion event. What’s more, the results show thatthe events of sediment recycling occured at about6-8Ma, which suggests that changesin the source region were caused by the uplift of the Oulongbuluke Mountains. Inaddition, according to the age of growth strata in the section, I could infer that thedeformation began to propagate eastward at the rate of10mm/yr³0mm/yr since 6-8Ma, and reached the Huaitoutala region until3.5-4.5Ma. Meanwhile, the verticaluplift rate of the western Oulongbuluke Mountains was0.73mm/yr-1.43mm/yr, andthe the basement uplift to the surface at about2.5Ma.The mapping of the Huaitoutala reservoir section shows growth strata occurredat about2.5-3.58Ma，which represents the time of the Huaitoutala reservoir anticlineformed by tectonic movement. In addition, the Q1strata was deformed by the thrustfault F2, indicating that this region still existed strong fault activity since2.5Ma.In conclusion, the southern edge of the Qilian Mountains experienced fourtectonic activities and deformation events at least since late Cenozoic, which were at12-15Ma,6-8Ma,2.5-3.58Ma, and <2.5Ma respectively.