Assessment of fault activity by direct dating of fault gouge

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Recent studies highlighted the potential to determine the age of faulting by using K-Ar dating of clay minerals in fault zone. However, the challenge remains difficult due to the possibility of contamination by old K-bearing minerals of the host rock.In order to establish a direct dating method for clay minerals in fault zone, precise clay separation combined with mineral characterization of fault gouge samples, and dating by unspiked K-Ar method was conducted. The finer fractions yield younger K-Ar ages, and contain larger amount of low-temperature type clay, suggesting enrichment in more recently grown clay minerals. Because K-Ar dating cannot detect no-potassium clay which formed <100 degree C, the ages mean the latest timing of faulting under >100 degree C temperature environment. Further fundamental studies about fault clay and requirements in dating are needed.