ABSTRACT

Fluids within fault zones can change the physical and chemical properties of fault materials and have a substantial influence on faulting. Therefore it is important to understand the nature and timing of fluid migration through fault zones. In order to evaluate the timing of fluid infiltration into a deep fault zone on Awaji Island in southern Japan, we attempted to measure U-Th radioactive disequilibrium of carbonate minerals from the Nojima fault, which triggered the 1995 Kobe earthquake. Bulk carbonate samples of vein material have U concentrations of 1.2 to 2 ppm and Th concentrations of 1.6 to 6.5 ppm. The high Th abundances derived from detritus requires correction for initial ²³⁰Th in order to acquire an accurate U-Th age. Bulk carbonate samples plot near the equiline in a (²³⁰Th/²³²Th)-(²³⁴U/²³²Th) diagram, suggesting that they precipitated around or before 450 ka. Monte Carlo simulation with finite-age constraint in the IsoPlot program yields a ²³⁰Th/²³⁴U age of 486 (+380, -190) ka. The presence of radioactive disequilibrium in (²³⁴U/²³⁸U) indicates that the calcite precipitation age was younger than 1 Ma. The results of leaching experiments indicate that a leaching procedure after firing causes considerable laboratory fractionation of U and Th and does not provide usable age information.