Geochemical signature of fluid-rock interaction in the fault gouge from the Middle Tectonic Line, Japan

To understand the fluid-rock interaction in the fault zone, we investigated the Middle Tectonic Line, Nagano Prefecture, Japan. We performed geochemical analyses of major- and minor-element concentrations and Sr isotope by using an X-ray fluorescence spectrometer, an inductively coupled plasma mass spectrometer, and a thermal ionization mass spectrometer. Rocks from the black gouge zone show fluid-immobile trace-element compositions that are consistent with fluid-rock interactions at temperatures of approximately 150 °C. This interaction might be resulted from coseismic frictional heating and/or hot-fluid migration from deep region along the fault zone.