K–Ar Geochronology of the Andesite Lavas of the Lower Pleistocene Enrei Formation on the Southwestern Slope of the Suwa Basin, Central Japan

The Suwa Basin is believed to be a typical pull-apart basin formed by the 12 km left-lateral motion on the Itoigawa­Shizuoka Tectonic Line active fault system. The starting age of the basin subsidence is considered to be after the violent volcanisms that formed the Lower Pleistocene Enrei Formation (Fm) distributed in and around the basin. However, the history of the basin development is still controversial mainly because of the insufficient geochronological data on the Enrei Fm in the SW Suwa Basin. The results of K­Ar dating are presented for two samples of andesite lavas of the Enrei Fm exposed on the northeastward-dipping slope of the SW Suwa Basin. One gives an age of 2.03 ± 0.19 Ma and the other an age of 1.51 ± 0.16 Ma. The former age corresponds closely to the oldest age of the Enrei Fm to the NE Suwa Basin, and suggests that the coeval and widespread initial volcanism at both sides of the basin occurred during the early Early Pleistocene (Gelasian) before the basin subsidence. The latter age closely corresponds to the peak periods of volcanic activity of the Enrei Fm. These lines of evidence suggest that the basin started to subside after 1.5 Ma, and that the position of basal unconformity, stratigraphy and geologic structures of the Enrei Fm in the SW Suwa Basin need to be re-examined.