Open Access

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

  • Koichiro Fujimoto1Email author,
  • Tomoyuki Ohtani1,
  • Norio Shigematsu1,
  • Yukari Miyashita1,
  • Tomoaki Tomita2,
  • Hidemi Tanaka3,
  • Kentaro Omura4 and
  • Yoji Kobayashi2
Earth, Planets and Space201454:BF03353313

https://doi.org/10.1186/BF03353313

Received: 28 December 2001

Accepted: 29 July 2002

Published: 26 June 2014

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.