Thermal Gradient Map and Thermal Model of the Japanese Islands

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.