Geologic structure and volcanic history of Sunagohara caldera volcano, Fukushima, Japan

Abstract

The Middle Pleistocene Sunagohara caldera volcano is located in the Okuaizu geothermal field, Fukushima, northeast Japan. The Sunagohara caldera is 5 km in diameter and has a central vent, which is filled with lithic tuff and buried by lake sediments of the Sunagohara Formation. This caldera is a Nigorikawa-type caldera without subsided basement blocks. The age of the Sunagohara caldera volcano is estimated at 0.5-0.2 Ma with previous datings. Many boreholes have been drilled in and around the Sunagohara caldera for geothermal exploration. A 150 m thick lithic tuff occurs at the bottom of the Sunagohara Formation in borehole OA-7. This tuff contains fragments of basement rocks and has no structure. It is regarded as vent tuff, because basement rock just under this tuff is fractured by eruption, and this tuff is not found at any other boreholes. Estimated diameter of the vent is less than 2.5 km. Post-caldera stage volcanism produced two ash-flow tuffs, six or more rhyolite lava domes and intrusions. The estimated volume of products of the Sunagohara caldera volcano is 4-5 km3. -from English summary