K-Ar dating of andesite from the Middle Miocene Yamamoto Formation on the western side of the Itoigawa-Shizuoka Tectonic Line in the Northern Fossa Magna region

Abstract

Andesite rocks of the Yamamoto Formation unconformably overlying pre-Miocene basement rocks in the western part of "the Itoigawa-Shizuoka Tectonic Line" in the Northern Fossa Magna were dated by the K-Ar method. Aphyric andesite of the lower member of the Yamamoto Formation is estimated to be a lava flow erupted at 16.0 Ma on the land surface. Andesite of the upper member is dated at 15.5 Ma, whereas the andesite occasionally occurs as hyaloclastite. These results and geological setting suggest that tectonic movements related to the Itoigawa-Shizuoka Tectonic Line might have continued along at least several hundred meters even after 16 Ma.