Volcanostratigraphy and geologic structure of the Enrei Formation on the southwest side of Lake Suwa, Nagano Prefecture, Central Japan. A Late Pliocene to Early Pleistocene volcanic history in a junction of island arcs.

Abstract

Voluminous Plio-Pleistocene andesitic volcanism with "flood andesite" is characteristic of the three arc-junctions of the Japanese Islands. This paper aims to clarify the nature of the volcanism and to reconstruct the geometry of volcanic basins in central Japan, the junction of the Northeast Japan, Southwest Japan and Izu-Ogasawara arcs.Surveyed area is situated on the southwest side of the Lake Suwa, where the Plio-Pleistocene Enrei Formation has accumulated up to 900 m in thickness. The Enrei Formation is divided into four stratigraphic units (23 subunits). Individual units become more mafic upward in phenocryst composition (predominant phenocrysts are hornblende→orthopyroxene and clinopyroxene→olivine), volcanism ceases at the end of each unit, and the four units show an overall more felsic tendency upward. Large ratio of lava flows of the formation can be attributed to the high rate of effusion that would have supplied thick lava flows with large potentials of flow distances.The formation has accumulated the three tilted basins arranged subparallel. These were formed through northwestward syn-sedimentary tilting of three basement blocks bounded by antithetic faults. Each sedimentary basin shows that the comparative altitudes of the individual basement blocks tend to decrease northwestward, so that the extents and maximum depths of the titled basins conversely increase in the same direction. This regularity provides important restriction to the formative mechanism of the tilted basins that through forming the Enrei Formation the individual basement blocks tend to tilt and subside northwestward and three main longitudinal faults mutually have antithetic displacement.