| Abstract |
A Quaternary granite has been confirmed below 2,860 to 3,729 m depth by the drill hole in the Kakkonda field in the Sengan (Hachimantai) geothermal area, Japan, and is regarded as the heat source of the present geothermal reservoir. The temperature exceeds 350?C around the rim of the granitic body and reaches to more than 500?C at 1 km inside. The thermal gradient of this interval is very steep and constant (30?C/100m). This evidence suggests that the confirmed granite corresponds to the consolidated outer zone of a magmatic pluton which is adjacent to a Pliocene dacite intrusive body and not associated with any extrusive volcanics. The Quaternary granite could be subdivided into cupola-shaped smaller intrusions separated from an underlying big pluton, and the big pluton itself. This expectation arouses new interest in the existence of cupolashaped smaller intrusions at other geothermal fields and an underlying big pluton for the whole area. The Sengan geothermal area includes many geothermal fields such as the Sumikawa, Ohnuma, Toshichi, Matsukawa, and Nyuto fields, besides Kakkonda. The Quaternary granite has not been confirmed at any other fields except Kakkonda, but Tertiary granite has been identified at Sumikawa, a Pliocene dacite intrusive body at Nyuto, and a Pleistocene porphyrite at Matsukawa. Subsurface temperature contour maps are described for whole Sengan area, based on a calculation by the relaxation method using bore hole temperature logging data. These maps indicate that a high-temperature zone (HTZ) forms the shape of an eastward-convex horseshoe. The total area over 200?C at -2 km with reference to mean sea level (msl) reaches 390 km2. The HTZ is generally consistent with the distribution of younger (< 0.5 Ma) Quaternary volcanics. The HTZ is also consistent with a geologic upheaval structure and a shallow gravity basement. These mean that the HTZ has been formed by tectonic uplift associated with younger Quaternary magmatism (0.5 Ma to present), which not only produced extrusive volcanics but also concealed intrusive magma bodies. The expected intrusive bodies must be very extensive and/or composed of many bodies to account for the extensive HTZ. The intrusion of Quaternary granite may be enhanced through fracture zones and/or density contrast around intrusions provided by pre-existent intrusive bodies, because the intrusive bodies tend to swarm in spite of intrusion age. The HTZ seems to have been formed and sustained by multiple intrusions of young Quaternary granite during 0.5 Ma to present. Further deep drill holes are expected to prove the emplacement of the Quaternary granite. |