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Geological Society, London, Special Publications; 1998; v. 144; p. 261-277;
DOI: 10.1144/GSL.SP.1998.144.01.19
© 1998 Geological Society of London

Dating of Quaternary Fluid Flow Events

ESR isochron dating of the Nojima Fault gouge, southwest Japan, using ICP-MS: an approach to fluid flow events in the fault zone

Tatsuro Fukuchi1 & Noboru Imai2

1 Department of Earth Sciences, Faculty of Science, Yamaguchi University, 1677-1 Yoshida, Yamaguchi 753-8512, Japan
2 Geological Survey of Japan, 1-1-3 Higashi, Tsukuba 305-8567, Japan

The absolute age of the earliest fault movement during the Quaternary period of the Nojima Fault, which moved in the 1995 Southern Hyogo Prefecture Earthquake (SHPE) in Japan, has been determined from clay grains in the fault gouge by election spin resonance (ESR) isochron dating using inductively coupled plasma mass spectrometry (ICP-MS). According to ICP-MS analysis, the concentrations of minor elements tend to increase with decreasing clay grain size, and they may adhere to the surface of clay grains, especially montmorillonite, in the fault gouge. The adherence of minor elements is possibly attributed to electrical charges caused on the surface of montmorillonite by substituting Al for some of the Si in the tetrahedral sites of the Si2O5 sheets. 238U, 232Th and 208Pb also increase with decreasing grain size, whereas the 238U/232Th ratio is almost unchanged within the measurement error. This indicates that a closed system in the 238U- and 232Th- series had pertained in the fault gouge collected since the earliest fault movement. On the other hand, the 208Pb/232Th ratio decreases with the grain size, and the 208Pb/232Th ratio from a grain of <1 µm is about 14% of that from <500 µm. 232Th decays to radioactively stable 208Pb and produces 220Rn in the decay process, but 220Rn which is an inactive and non-ionized gas cannot be absorbed electrically on the surface of montmorillonite. Therefore, most of the 220Rn gases produced on the surface of clay grains may be released from the fault fracture zone. A radon anomaly in ground water observed before the 1995 SHPE may be attributed to radon loss by huge clay mineralization in the Earth’s crust. The ESR isochron age of the fault clay gouge of the Nojima Fault is estimated as 0.84 ± 0.12 Ma to 1.23 ± 0.1 Ma from each clay grain by considering the calibration of radon loss, water content and defect creation efficiency by {alpha}-rays. This age is consistent with a date of about 0.53 –3 Ma that was estimated as the beginning of the Nojima Fault from recent drilling core data.