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The Shimanto Group records a Cretaceous through Miocene history of subduction/accretion along the southwest margin of Japan. We used vitrinite reflectance on over 200 samples of shale and slate to determine regional trends in diagenesis and low-grade metamorphism on the Muroto Peninsula, Shikoku Island. Thermal structure throughout the section overprints all but the latest stage of a complicated polyphase structural evolution. Eocene strata of the Murotohanto subbelt display the highest levels of thermal maturity. Reflectance values range from 1.4 to 5.0%Rm; thermal maturity increases from north to south, and the %Rm values correspond to estimates of paleotemperatures of 180 to 315°C. The Shiina-Narashi fault marks the boundary between Eocene rocks and Oligocene-Miocene strata of the Nabae subbelt. South of this fault, thermal maturity for the Nabae strata ranges from 0.9 to 3.7%Rm, and these values correspond to estimates of peak paleotemperature of 140 to 280°C. Reflectance values increase in proximity to the Maruyama intrusive suite, and the maximum rock temperatures adjacent to the intrusions may have been as high as 500 to 550°C based on comparisons with laboratory heating experiments. Paleotemperatures within the Upper Shimanto Group were unusually high compared to shallow levels of other accretionary complexes. The geothermal gradient was at least 40°C/km on a regional scale and much higher locally. Subduction of juvenile oceanic crust can account for the anomalous levels of thermal maturity documented across the Muroto Peninsula.

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