Lead isotopic compositions in fault gouges and their parent rocks: implications for ancient fault activities |
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| Institution: | 1. Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan;2. Research Fellow of Japan Society for the Promotion of Science, Japan;3. Department of Geology, Faculty of Science, Niigata University, 2-8050 Ikarashi, Nishi-ku, Niigata 950-2181, Japan;4. Department of Geology, University of Johannesburg, PO Box 524, Auckland Park, 2006, South Africa;5. Department of Geoscience and Technology, Graduate School of Engineering, Tohoku University, Aramaki, Aoba-ku, Sendai 980-9570, Japan;6. Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan |
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| Abstract: | This is the first study on Pb stable isotopes in fault gouges and their parent rocks. We analyzed the composition of Pb isotopes and contents of U and Pb in 10 pairs of fault gouges and their parent rocks collected along several active faults in central Japan. Thorium-232-208Pb ages of two fault systems were determined as pre-Tertiary, which are consistent with the data from K Ar ages and geological considerations.Naturally, the235U207Pb system is of little use for dating because the magnitude of difference in207Pb/204Pb between gouges and parent rocks is too small. It is found that the206Pb/204Pb can indicate the contribution of206Pb resulting from excess supplies of226Ra and222Rn along the fault. The excess206Pb accumulation rate corresponds to the average222Rn concentration in soil gas or groundwater through geological time since the gouge formation. A comparison of Quaternary fault activity and estimated Tertiary activity reveals the characteristics of each fault system. |
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