Abstract
The upper Pliocene to lower Pleistocene Enrei Volcanic Rocks occur upon the Utsukushigahara Plateau and adjacent areas on the northwestern margin of the Chushin Highland area, Central Japan. Upon the Utsukushigahara Plateau, the Enrei Volcanic Rocks unconformably overlie the Pliocene Kotakiyama Group, and comprise the following five units: the Wadabokujo, Karasawagawa, Utsukushigahara, Mitsumine, and Wadatoge volcanics. Previous geochronological data and geological data of the present study reveal that the volcanic activity that produced the Enrei Volcanic Rocks started at around 2.1 Ma. From this time, basaltic andesite to andesite, andesite, dacite to rhyolite, and rhyolite volcanism occurred independently of each other until 1.3 Ma, forming several volcanic bodies upon the Utsukushigahara Plateau, and in the Mitsumine and Wada Pass areas.
The Kotakiyama Group is characterized by intense folding and faulting, suggesting that the strata were deformed in a compressional stress field; however, the overlying Enrei Volcanic Rocks are undeformed. The Karasawagawa volcanics are largely of subaqueous origin, but the Utsukushigahara, Mitsumine, and Wadatoge volcanics are mainly subaerial. The Enrei Volcanic Rocks are distributed within a narrow graben-like depression that trends NW-SE (possibly an extension of the Oiwake Graben); the craters of the Yabashira Volcanoes are aligned with this graben, to the southeast. Thus, in the Yatsugatake-Chushin Highland area during late Pliocene to early Pleistocene, a large magmatic field formed along the NW-SE-trending geologic system that is continuous with the Oiwake Graben, producing about 200 km3 of volcanic ejecta.
