| Abstract |
The thermal structure of the Japanese Islands is estimated based on geothermal gradients from temperature logging data of 1943 drillholes and a temperature-depthdiagram obtained from a deep drillhole at the Kakkonda geothermal field. The temperature-depthdiagrams in non-volcanic areas represent conductive heat transfer and low geothermal gradient values less than 50 K indicating lack of local and shallow heat sources.On the other hand, a deep drilling at the Kakkonda field reveals a hydrothermal convection of active volcanic areas, which cools local and shallow heat sources. Several small partial melting zones must lie at the depth of several kilometers, generating a complex geothermal structure.A conductive zone appears at an outer-margin of the melting zone. The heat is transferred upward at a high rate through the conductive zone and the overlying hydrothermal convection zone. |