地熱系モデリングから見たマグマ溜り : 豊肥・仙岩・栗駒地熱地域を例にして  [in Japanese] Magma reservoirs from the viewpoint of geothermal modelling : Examples from the Hohi, Sengan and Kurikoma geothermal areas in Japan  [in Japanese]

豊肥, 仙岩, 栗駒地熱地域では詳しい地熱調査が行われ, 放射年代測定に基づく第四紀火山層序や, 坑井掘削に基づく地下温度構造などが明かにされている。これらのデータを用いて各地域の地熱系モデルを検討した。その際, 特に地熱熱源とマグマ溜りとの時間的, 空間的関係に力点をおいて検討した。その結果, 地熱熱源としては最も若い第四紀火山活動に伴うマグマ溜りよりも, むしろ火山フロントより背弧側のやや古い第四紀火山活動に伴うマグマ溜りが, 今なお地熱熱源として重要な役割を果たしていることが明かとなった。これは, 従来強調されていたマグマ溜りの単純な冷却過程における残存熱だけでは, 現在活発な地熱熱源を説明することが出来ないと言う事を示している。いわば, マグマ貫入後の熱の付与が, 予想以上に大きく, かつ期間も約30-100万年前後と長時間継続しているということである。その様式については, 複合花崗岩体で認められるような追加的なマグマの付与, または単純な伝導熱だけの付与が推定される。

Many deep bore holes have been drilled in the Hohi, Sengan and Kurikoma geothermal areas for deep geothermal energy exploration formed the Ministry of International Trade and Industry in Japan. The modelling of these geothermal areas have been conducted using integrated data from various kinds of surface surveys and well loggings. In the Hohi area, subsurface temperature contours are concordant with the subsurface relief of the pre-Tertiary basement. The surveyed area can be divided into discharge, recharge and conductive zones according to the patterns of temperature logging curves. The results imply the differences in permeability and extent of the cap rocks. The geothermal heat sources in this area can be assumed to be not only the magma reservoirs under the youngest Quaternary volcanoes (<0.3 Ma), but also slightly older magma bodies intruded around 0.6-0.3 Ma. The older magmatic bodies appear to have been supplying heat to the geothermal system until present time. In the Sengan area, subsurface, high temperature zones are concordant with the distribution of young Quaternary volcanics. The surveyed area can be divided into discharge and conductive zones according to the patterns of temperature logging curves. The geothermal heat sources in this area can be assumed to be not only the magma reservoirs under the youngest Quaternary volcanoes (<0.5 Ma), but also older magmatic bodies intruded around 1.0-0.5 Ma, and a young granitic pluton. In the Kurikoma area, the subsurface thermal structure changes from shallow to deep part. The surveyed area can be divided into discharge and recharge areas, except the surrounding conductive zones, according to the patterns of temperature logging curves. The geothermal heat sources in this area appear to be the erupted magma chamber under the Takahinatayama volcanics of about 0.3 Ma and the unerupted magma chamber under the Fukiage hotspring area in the Onikobe caldera. An additional geothermal heat source seems to be controlled by the deepseated structural belt in NNW-SSE trending. As shown formed these case studies, the slightly older magmatic bodies intruded around 0.3-1.0 Ma, still play a role in supplying heat for the active geothermal systems. This observation indicates that the cooling plutons can supply appreciable heat to the geothermal systems for up to 1.0 Ma.