Lead isotopic compositions in fault gouges and their parent rocks: implications for ancient fault activities

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 Ksingle bondAr ages and geological considerations. Naturally, the235Usingle bond207Pb 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.