Abstract
Lower to middle Miocene Yatsuo Group distributed in the eastern part of Toyama Prefecture, central Japan, is divided into the Nirehara, Iwaine, Iozen, Kurosedani/Fukuhira, Higashi-bessho Formations in ascending order. The Iwaine, Iozen, and Fukuhira Formations consist mainly of subaqueous volcanic rocks occasionally interbedded with subaerial volcanic rocks in several horizons. The volcanic rocks vary from basaltic andesite to rhyolite. In the studied area, normal faults are developed with N to S and NNE to SSW trends, and are intersected by some faults at almost right angles. Such a fault system must have formed a sedimentary basin in which the Yatsuo Group were deposited. The tectonic setting is similar to that of a continental rift, such as the Rio Grand Rift and the rift zone of the Akita-Yamagata oil field. The K-Ar ages of six volcanic rock samples from the Yatsuo Group range from 16.50 My to 12.19 My. This indicates that the volcanism occurred during the spread of the Japan Sea. In conclusion, the studied area was situated at peripheral parts of the rift zone.
