Geological Structures at the Hohi and the Sengan Geothermal Areas in Japan

Abstract

Many deep bore holes have been drilled at the Hohi and the Sengan geothermal areas for deep geothermal energy exploration by the Ministry of International Trade and Industry in Japan. The geological frameworks and evolutionary histories of the both areas were investigated based on the stratigraphic correlations among these drill holes. The Hohi in Kyushu was made clear that four stages of characteristic volcano tectonism occurred in Pliocene and Pleistocene epochs. Among them, the late Pleistocene magmatism is the most important for the present geothermal system because the NW-SE trending fault system derived from this magmatism plays the role of conduits for upflowing of geothermal fluids. The Sengan area was made clear that caldera collapse occurred with acid basic bimodal volcanism around 15-2.0 Ma, and the intermediate unimodal volcanism succeeded and continued up to the present. This lat andesitic volcanism supplies the heat potential for the present geothermal systems. The fault and dike systems accompanied with young magmatism are very important as the paths and/or reservoirs of geothermal fluids from deeper part, even if the intrusive rocks themselves had already cooled down.