Most models of thermal structure within subduction complexes predict the occurrences of small geothermal gradients and low heat flow. Calculated and measured geothermal gradients in the vicinity of Nankai Trough, however, do not support this paradigm. Moreover, abnormally high burial temperatures may have affected the accretionary margin of southern Japan throughout Cenozoic time. Uplifted portions of the Nankai accretionary wedge are well exposed along several of the Japanese Islands; the youngest of these accreted strata (Eocene-Miocene) are assigned to the Upper Shimanto Group. This project will collect thermal-maturity data on samples from the Shimanto accretionary prism, which is being studied by workers at Cornell and Brown Universities. Laboratory methods include vitrinite reflectance and illite crystallinity. Objectives include testing tectonic models using thermal-maturity data and documentation of thermal history within individual units of the complex. The significance of the resulting data will extend well beyond site-specific issues. Data will provide a much-needed empirical check on existing theoretical models of thermal structure. More importantly, the Nankai/Shimanto example appears to represent one end member within a spectrum of thermal regimes. Consequently, results will likely force an expansion and refinement of the fundamental conceptual framework of subduction complexes.
