2015 Volume 2015 Issue 43 Pages 1-16
We investigated structural, mineralogical and geochemical characteristics of the fault rocks from the active Atera fault, Gifu Prefecture, Japan. We recognized three dominant structural zones at the Tase outcrop: light gray fault gouge, brown fault gouge and black fault breccia. The light gray fault gouge is mainly composed of clay minerals, and exhibited intense shear foliation. The brown fault gouge shows the clearest linear continuity at the outcrop, cutting other structural zones, but the degree of shearing in this zone is relatively weak. The black fault breccia is composed of rock fragments, originated from surrounding host rocks, and black matrix. Fault rocks in these structural zones show fluid-mobile trace element compositions essentially consistent with those of host rocks, indicating that no fluid–rock interaction at high temperatures above 250 ºC took place in these fault zones.
Based on these features of each zone and their cross-cutting relationship at the outcrop, we inferred the structural evolution of the fault zone. The light grey fault gouge and the black fault breccia formed since the Atera fault became active. Under oxidative condition, water might react with minerals in both zones to form smectite and halloysite. The brown fault gouge formed most recently, possibly by the 1586 Tensho earthquake. Mineralogical and geochemical analyses of the fault rocks used in this study could be useful for understanding the structural and chemical evolution of the active faults.
