| Abstract |
Yonezawa district located in the northeast of Japan, has many spring areas. Yonezawa district is geologically composed of the pre-Tertiary basement rocks, Tertiary formations, Pliocene to Pleistocene deposits, and Quaternary volcanoes. Geothermal features in this area are clustered into two groups whether related to quaternary volcanoes. Geothermal resources related to quaternary volcanoes such as Azuma is accompanied by high temperature hot springs and hydrothermal alteration zones on the surface at present. Volcanic activity of Azuma volcano is geologically divided into three stages during the period from 1.3 to 0.08Ma by K-Ar dating. The center of volcanic activity moved from west to east and recent products erupted from fresh craters in the eastern part of the volcano. Azuma volcano is distributed within the up rift zone and volcanic rocks erupted from it are mainly andesitic. The Ubayu depression is also located within the up lift zone and filled with acidic welded tuff. The Ubayu hot spring, situated near the Ubayu depression, is associated with an acidic alteration zone characterized by kaolinite, alunite and pyrophyllite. In Azuma area, maximum temperatures of the exploration wells are above 200 oC. The cold geyser of Hirogawara hot spring, situated in the west of Azuma volcano, is activated by the evolution of carbon dioxide. The other side many non-volcanic hot springs are scattered mainly along the Yonezawa Basin. The high temperature hot springs Akayu is situated on margin of the Yonezawa Basin and in volcanotectonic depressions, which is filled with submarine acidic pyroclastic flow of 1,500m thick. The Akayu depression, corresponding to low gravity anomaly, characterizes the geomorphology of this area and has a suitable structure favorable for thermal waters. |