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Stretching of fluid inclusions in calcite as an indicator of frictional heating on faults

¹ Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama 236-0001, Japan
² Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
³ Department of Earth System Science, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan

Fluid inclusions trapped in calcite tend to increase in volume (i.e., stretching) when heated to temperatures higher than those of the initial entrapment at the entrapment pressure, and the result is an increase in the homogenization temperature. We examined whether the stretching of fluid inclusions in calcite occurred by frictional heating on the subduction thrust exhumed from seismogenic depths (4–6 km) at which the slip is localized along the few-centimeters-thick ultracataclasite layer. The results indicate that the histograms of the homogenization temperatures of the fluid inclusions in the ultracata-clasite are marked by the pronounced skewness, with the frequency mode near the low-temperature end of the range and a tapering tail toward higher temperatures. These skewed frequency distributions are absent in the surrounding rocks and are best explained by the stretching of the fluid inclusions by the frictional heating associated with the localized slip. Experimental stretching of fluid inclusions in calcite demonstrated that the tapering tail toward high temperatures in the histograms of the homogenization temperatures from the ultra-cataclasite can be reproduced by a 50–150 °C temperature rise along the ultracataclasite with respect to the ambient temperature. Stretching of fluid inclusions in calcite could record a small temperature rise during frictional heating on faults and is potentially useful in identifying the restraint of the temperature rise by fluid pressurization during an earthquake.

Key Words: fluid inclusions • stretching • ultracataclasite • frictional heating • subduction thrust • accretionary complex