TERRAPUB Earth, Planets and Space

Earth Planets Space, Vol. 54 (No. 11), pp. 1127-1132, 2002

Water-rock interaction observed in the brittle-plastic transition zone

Koichiro Fujimoto1, Tomoyuki Ohtani1, Norio Shigematsu1, Yukari Miyashita1, Tomoaki Tomita2, Hidemi Tanaka3, Kentaro Omura4, and Yoji Kobayashi2

1Geological Survey of Japan, AIST, Tsukuba, Ibaraki 305-8567, Japan
2Institute of Geoscience, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
3Department of Earth and Planetary Sciences, University of Tokyo, Hongo, Tokyo 113-0033, Japan
4National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Ibaraki 305-0006, Japan

(Received December 28, 2001; Revised July 3, 2002; Accepted July 29, 2002)

Abstract: Rock alteration and geochemistry of the fault rocks are examined to infer the characteristics of the fluid phase related to the ancient fault activity. The Hatagawa Fault Zone, northeast Japan, is an exhumed seismogenic zone which is characterized by close association of brittlely and plastically deformed fault rocks mostly derived from Cretaceous granitoids. Epidote and chlorite are dominant alteration minerals in both rocks. However, calcite is characteristically developed in the cataclastic part only. Decrease in oxygen isotope ratio and existence of epidote and chlorite, even in weakly deformed granodiorite, is evidence of water-rock interaction. The water/rock ratio is interpreted to be relatively small and fluid chemistry is buffered by host rock chemistry in the mylonite. The occurrence of calcite in brittle structures is explained by changes in water chemistry during shear zone evolution. CO2-rich fluid was probably introduced during cataclastic deformation and increased CO2 concentration resulted in precipitation of calcite.

Corresponding author E-mail: k-fujimoto@aist.go.jp

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