Identification of capable faults using fault rock geochemical signatures; A Case study from offset granitic bedrock on the Tsuruga Peninsula, central Japan

Niwa, Masakazu   ; Shimada, Koji   ; Ishimaru, Tsuneari ; Tanaka, Yoshihiro*

In order to identify specific chemical changes induced by recent, near-surface faulting, we compared two faults of different size and evolutionary history, the active Shiraki-Nyu Fault and a nearby minor, inactive fault in central Japan, in terms of their mineralogical and geochemical features. Our analyses suggest that (1) chemical changes such as depletion of NaO and CaO derived from smectite and illite formation, and that changes such as increase of U, Y and REEs derived from absorption to these clay minerals, occurred during the fault initiation and subsequent retrograde alteration and are identical in the two faults, and that (2) MnO increase accompanied by FeO depletion and a positive Ce anomaly can be a key to identify specific chemical changes due to recent, near-surface faulting, because this could be related to upward migration and oxidation of fault-driven reduced fluids from deep underground to the near-surface environment.