Geothermal system in the West Kirishima geothermal area, Kogoshima prefecture.
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概要
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The geothermal system of the West Kirishima field is discussed on the basis of geological structure, hydrothermal alteration, underground temperature and a circulation system of geothermal field. This field is situated in a volcano-tectonic depression "Kagoshima graben" which trends north-south and relates to the block-faulting after deposition of the Shimanto Group. The volcanic activity began in the Pliocene and continued during the Quaternary, so andesitic lava flows and pyroclastics accumulated in enormous thickness, more than 2, 000m in the depression area. Thermal manifestations such as fumaroles, hot springs and altered zones are aligned along fault systems of NE-SW, NW-SE and partially N-S trends. Quaternary volcanic rocks were extensively subjected to hydrothermal alteration. The alteration zones are classified into various types such as montmorillonite zone-mixed layered clay mineral zone-chlorite zone-chlorite sericite zone in increasing temperature order from ground surface to deeper parts. The upper level of the crystallization of quarts and wairakite corresponds to the present isotherms of about 100°C and 200°C respectively, which suggests that the hydrothermal system of this area is a rather young system. Impermeable zones, which are mainly composed of montmorillonite filling cracks and joints of andesitic rocks, play an important role, especially to prevent the mixing of descending cold ground water on the circulation system of geothermal fluid. The fault systems in Makizono and Ebino-Iino lavas under the impermeable zone play the main geothermal reservoir in this field.
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