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The reflectance of vitrinite (Ro) is adopted as a parameter to envisage the thermal effect and its geological causes in the studied area. The results have revealed the following facts: records of chan...ging burial depth of the sediments in tilting coal basins are delineated from the regional and stratigraphic variation patterns of Ro; diagenetic reaction in the coalfields had to undergo warmer geothermal condition judging from Ro-depth gradient in boreholes; cause of the warmer geothermal condition is inferred as a result of vigorous igneous activities in the inner zone of the Southwest Japan arc since Miocene Epoch, which is analogically supported by the present high heat flow evidence and various types and bodies of igneous rock with anomalously high Ro values especially in the western part; and the Sasagawa reverse fault can be considered as a result of uplifting movement of a hypabyssal igneous body that accompanies high anomalous Ro haloes above it. These characteristic thermal features are inferred as a manifestation of higher temperature and/or lower pressure organic metamorphism which occurred at the innermost back-arc basins behind the volcanic front of the Southwest Japan arc. The pattern is in striking contrast to that of the fore-arc depressions in the Northeast Japan arc which face the Pacific plate of present low heat flow condition. The former is considered as a typical thermal phenomenon of one of the paired organic metamorphism of the Cenozoic Group in the Japanese Islands.show more
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