| Landslide as one of the highest frequency of occurrence,the most extensive distribution,the one of the greatest harm of geological disasters,which take the great harm to human's life and production.A considerable part of which is due to some of the old giant of the ancient landslide caused by repeated resurrection.The age and the recurrence of the giant fossil landslide can highlight the tectonic process and landscape development in high mountains area,and it is also useful both for determining conditions that lead to the landslide,hazards of future events and understanding Quaternary geologic history in high mountains area.There are always uncertainties in determining the timing of giant landslide events using any dating approaches,because different elements dated within a landslide environment have different problems.The Jishi Gorge reservoir of the upper Yellow River valley in the Northeastern margin of the Qinghai-Tibet Plateau lies in the conversion transition area of tectonic geomorphology of China.It is the one of the regions in China which is most seriously affected by landslides and is selected as the case study area.The Gelongbu large-scale landslide(5*107 m3)in the upper reaches of the Yellow River was located in the reservoir zone of Jishi Valley Hydropower Station and is of important geological engineering significance.Meanwhile,the outburst of the dammed lake of Gelongbu landslide has had an important impact on the cultural environment of Lajia of Guanting Basin and thereout has become a hot research subject.However,the forming ages of the landslide has been controversial.In the last ten years,with the development of accelerated mass spectrometry,the terrestrial in situ cosmogenic nuclides has become one of the effective means of landslide dating.Because of its simple mineral composition,the mineral distribute broad,easy to chemical treatment,dating range is wide,as well as the dating age is the direct occurrence of the landslide.In recent years,the use of cosmic origin nuclides for landslide dating is in rapid development.However,the application of cosmic radionuclides has difficult to estimating the denudation rate of the steep main scarp of the landslide.In this paper,we use the terrestrial in situ cosmogenic nuclides dating method to take exposed the bedrock surface of the landslide wall,and to determine the direct age of the GeLongbu landslide.The landslide age was corrected by the masking factor.The exposed age of the posterior wall of the landslide was calculated quantitatively under different erosion rates.The main conclusions are as follows(1)Quantitative calculation of the landslide scarp age under the the conditions of different erosion rate.The denudation rate of the landslide scarp is 30mm/ka,50mm/ka,110mm/ka,which the age of GLB-1 is 1.79±0.29ka.1.84±0.32ka?2.05±0.39ka,respectively,the age of GLB-2 is 1.33±0.21ka?1.36±0.22ka?1.46±0.26ka,respectively,the age of GLB-2 is 1.33 ±0.21ka?.36±0.22ka?.46±0.26ka,respectively,the age of GLB-3 is 1.31 ± 0.2ka?1.33 ± 0.21 ka?1.43 ± 0.24ka,respectively,the age of GLB-4 is 4.57±0.57ka?4.96±0.68ka?7.09±1.61ka,respectively,the age of GLB-5 is 2.37 ± 0.31ka?2.47 ± 0.32ka?2.86 ±0.46ka,respectively.(2)In combination with the steps on the landslide scarp and the analysis of the measured data,it is found that the upper part is steeper than other part of the landslide scarp,with the addition,the sandstone and conglomerate are interphase distribution,which means the hard and the soft interphase distribution.A secondary collapse event of rock occurred around 1.5 ka-2 ka.(3)When the maximum erosion rate is 110mm/ka,the year of the main scarps of Golongbu is not earlier than 7.09±1.61 ka by means of Terrestrial Cosmogenic Nuclides(TCN);The study establishes a reliable age and provides a new TCN datng evidence for the Golongbu landslide. |
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