1985 Volume 7 Issue 3 Pages 201-213
The Matsukawa and Kakkonda geothermal areas are located in the central part of the Hachimantai volcanic region, which occupies northern part of the Tohoku backbone range. The Matsukawa geothermal area is characterized by the vapor dominated system and made up of assemblies of cap rock and reservoir rock. Geologic formations in this area are classified into three units, from oldest to youngest, marine sedimentary rocks of Miocene age, Pliocene-Pleistocene Tamagawa Welded Tuffs, and Quaternary Matsukawa andesite. The geothermal fluids are reserved in fractures of the Miocene rocks and lower Tamagawa Welded Tuffs. In contrast to them, the Matsukawa andesite acts as a cap rock because of its low permeability and causes geothermal manifestations to be poor on the surface. The Kakkonda geothermal area, about 7 km southwest of Matsukawa, is known as a hot water dominated system. Geothermal manifestations such as fumaroles and hot springs are widely distributed in this area because of absense of cap rocks and occurrence of steeply dipping faults. The area is underlain chiefly by the Miocene rocks, which are divided into the Obonai, Kunimitoge, Takinoue-onsen and Yamatsuda formations in ascending order. Geothermal fluids are encountered in high permeable fractures in the trough of a syncline, sheared zone in the wing of a asymmetrical anticline, and major faults.