Reconstruction of the evolution and deformation history of an active fault – implications from the Atera fault, Central Japan

Abstract

Abstract Atera fault clay gouges were collected for age dating near Kawaue, Nakatsugawa city, central Japan and the results integrated within its complex geological history. The results form an internally consistent data set with constraints from existing geochronological data (AFTA, ZFTA, CHIME) and demonstrate the application of gouge dating in providing new data to constrain timing of brittle deformation in Central Japan. The Atera illite age data complete recently obtained limited, illite fault gouge age data from underground exposure in the Toki granite, and the data are identical within error. The age of the heterogenous welded tuff breccia zone (Atera 1) ranges from 40.6 ± 1.0 Ma to 60.0 ± 1.4 Ma whereas ages of the fault core gouge sample (Atera 2) range from 41.8 ± 1.0 Ma to 52.7 ± 1.2 Ma. The finest < 0.1 µm fraction for the fault breccia and fault core gouge yield ages around 41 Ma, identical within error. The obtained data indicate brittle faulting and a subsequent geothermal event occurring in the Paleogene–Eocene, similar to the nearby Toki granite. The brittle deformation and thermal event were synchronous with the post-intrusive exhumation of the pluton. The Atera fault illite age data provide additional insights into an integrated, regional-scale record of tectonic displacement of central Japan, and can be linked to the large scale tectonic processes such as the Emperor sea mount kink around 55 to 46 Ma with fault initiation around 50 Ma and brittle fault cessation or reactivation around 40 Ma in the Eocene.