Abstract
The paleogeothermal influence on the lowest grade metamorphic and diagenetic alteration of young sediments is discussed with special reference to the present conditions of the island arc system around Japan. Vitrinite reflectance ( R 0) is adopted to detect the thermal influence. The regional variation of R 0 in certain horizons, the stratigraphic variation in steeply dipping successions and the depth variation in boreholes are investigated in Cenozoic sediments of specific tectonic situations of fore-arc and back-arc depressions in the northeastern and the southwestern Japan arcs. The results have revealed a zonal distribution of a contrasting pattern of organic metamorphism that underwent either lower temperature and/or higher pressure conditions or higher temperature and/or lower pressure conditions. The distribution of organic metamorphism seems to correspond to the present heat flow distribution; i.e., the former (low temperature/high pressure) corresponds to the present low heat flow region and the latter (high temperature/low pressure) to the present high heat flow region.
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