Abstract
The Lower to Middle Miocene Nojima Group, NW Kyushu, Japan is divided into the Oya, Fukazuki, and Minamitabira Formations in ascending order. Seven samples were collected for the fission-track dating: two samples from the Oya Formation, two samples from the basal Kojimazaki Tuff Breccia of the Fukazuki Formation, and two samples from the lower Fukazuki Formation, and one sample from the Minamitabira Formation. Fission-track dating was carried out by the external detector method, which was applied to the internal (ED1) and external surfaces (ED2) of zircons. Obtained ages ranges from 21 to 15 Ma and are partly inconsistent with the stratigraphic levels, perhaps due to contamination of exotic zircons. Statistically significant ages obtained are 17.5±1.4 Ma and 17.7±0.9 Ma for the Oya Formation, and 15.3±0.5 Ma (1σ) for the lower Minamitabira Formation. The latter age is consistent with the occurrence of the Kadonosawa Fauna (Its known age is 16.5-15 Ma). These reliable ages allow calculation of the accumulation rate for the Nojima Group to be at least 500 m/m.y., which suggests very rapid rifting of the basin during the climactic stage of the Japan Sea opening.
