大阪平野の地下地質と温度構造(予報)

Since the first deep borehole in 1960's for assessment of the land subsidence in Osaka Plain，Southwest Japan，few attempt of geologic borings reaching to the basement rocks and examination of whole covering sediments of this region have not been done．However，recent boom of the hot springs in this big city has brought a chance to obtain a valuable information on the structure of the basement rocks（mainly of the Meso-Paleozoic rocks and the Ryoke metamorphic rocks），stratigraphy of the overlying Plio-Pleistocenc sediments（mainly of Osaka Group），and also geochemical and geothermal study of this plain．This paper presents recent
geologic examination on the borehole data and characteristic feature of the geothermal distribution in the deep Part of the plain．
The Osaka Plain is divided into the eastern and western blocks by the north-south trending Butsunen’ji yama-Uemachi reverse fault，and the eastern block upheaved．The basement depth of the western part exceeds 1300m and higher geothermal gradient occurs in along west of the Butsunen’ji-Uemachi Fault．Large volume of hot water（several hundred tons／day），with a temperature over 35℃ occurs in most spas，but hot water of each varies its characteristics．Temperature of the horizon of 1000m depth is usually over 45℃，and spots of the higher geothermal part occurred in a row of N-S trend．Geothermal gradient is usully over 2.5℃／
100m，and characteristic is the higher gradient in the lowermost part of the Osaka Group，mainly from 1000m to 1300m depth，attaining 5℃／100m．This part consists of sediments of lower geothermal conductivity and is supposed to play a effective role as a“Blanket”．The investigation on the geologic data of the borehole in the Osaka Plain is apparently important for an applicable development of hot springs for new and clean energy．