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The primary purpose of this special publication is to show how Eocene through middle Miocene strata of the Shimanto Belt were affected by the Cenozoic geothermal regime of southwest Japan. This introductory paper synthesizes the regional and local geologic framework, as it applies to detailed studies completed on the Muroto Peninsula of Shikoku. In particular, we focus on the temporal and geometric relations between peak heating events and discrete stages in the deformation history. Rocks of the Shimanto Belt display the effects of a complicated history of polyphase folding, faulting, and cleavage formation. Interpretations of the structural geology follow conventional models for subduction-accretion via offscraping, underplating, and out-of-sequence thrusting. The positions of plate boundaries during the Paleogene phase of plate convergence in southwest Japan are somewhat difficult to substantiate, but that episode of tectonism may have been punctuated by subduction of the Kula-Pacific Ridge near the end of the Eocene epoch. The Neogene, on the other hand, was definitely a time of ridge-trench interaction. Geologic events of the middle Miocene (∼15 Ma) are particularly noteworthy because they included anomalous near-trench magmatism, the cessation of backarc spreading in the Shikoku Basin, incipient collision between the Izu-Bonin and Honshu Arcs, the opening of the Sea of Japan, rapid rotation of crustal blocks in both northwest and southeast Japan, and the formation of high-rank coals within forearc-basin strata. The high ranks of organic metamorphism within the accretionary forearc, together with the anomalous basic and acidic volcanic and plutonic rocks, provide unambiguous and widespread evidence in favor of high geothermal gradients at relatively shallow depths. Furthermore, data from offshore extensions of the Shimanto Belt (Nankai accretionary prism) show that Holocene geothermal conditions are still unusually warm when compared to shallow levels of most “typical” subduction zones. Thus, much of the Cenozoic history of southwest Japan contradicts the paradigm of low-temperature, high-pressure metamorphism within subduction zones, and the exposed geology of the Muroto Peninsula provides some of the world’s best examples of the thermal and structural effects of ridge-trench interactions.

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