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The Shimanto accretionary complex on the Muroto Peninsula of Shikoku comprises two major units of Tertiary strata

the Murotohanto subbelt (Eocene-Oligocene) and the Nabae subbelt (Oligocene-Miocene). Field-based structural analyses and laboratory measurements of thermal maturity show that both subbelts were affected by thermal overprints long after the initial stages of accretion-related deformation. Paleotemperatures for the entire Tertiary section range from about 140 to 315°C, based upon mean vitrinite reflectance values of 0.9 to 5.0%Rm. In general, older rocks of the Murotohanto subbelt display higher levels of thermal maturity and better cleavage than rocks of the Nabae subbelt. The Murotohanto subbelt also displays a spatial decrease in thermal maturity from south to north, and this pattern probably was caused by regional-scale differential uplift following peak heating. Conversely, the paleothermal structure exposed within the Nabae subbelt is fairly uniform, except for the local effects of mafic intrusions at the tip of Cape Muroto. The most important structural discontinuity to display a large component of post-metamorphic vertical offset is the Shiina-Narashi fault; this out-of-sequence thrust forms the boundary between hanging-wall rocks of the Murotohanto subbelt and the younger Nabae subbelt.

Rapid heating of the Shimanto Belt evidently occurred immediately after a middle Miocene reorganization of the subduction boundary of southwest Japan. Whether or not all portions of the inherited (Eocene-Oligocene) paleothermal structure were overprinted during the middle Miocene remains controversial. Subduction prior to middle Miocene time probably involved the Kula, or fused Kula/Pacific, Plate. Hot oceanic lithosphere from the Shikoku Basin back-arc spreading ridge first entered the subduction zone at approximately 15 Ma; this event also coincided with the opening of the Sea of Japan and the rapid clockwise rotation of southwest Japan. Middle Miocene thermal overprints elsewhere in the Shimanto Belt are spatially related to and, therefore, probably synchronous with widespread acidic magmatism, local mafic intrusions, and formation of anthracite coals in the forearc. The Kii Peninsula of Honshu appears to be the hottest spot along the strike of the subduction zone; this locality may well correspond to the collision point between the Shimanto accretionary complex and the central ridge of the Shikoku Basin. However, the spreading ridge probably was unstable and disorganized at the time of the collision, which allowed widespread, strike-parallel flux of mantle heat and magma into the accretionary prism. Apatite fission-track data show that most of the Eocene-Oligocene Shimanto strata experienced differential uplift and cooling through the 100°C isotherm shortly after the ridge-trench collision (11 to 8 Ma).

The Shimanto Belt provides an excellent example of an ancient accretionary complex that does not follow the paradigm of high-pressure, low-temperature metamorphism. Although the thermal history of southwest Japan was punctuated by an unusual burst of middle Miocene thermal and tectonic activity, the residual effects of interaction between the trench and a young subducting slab persist today, as evidenced by high heat flow within submerged portions of the Nankai accretionary prism.

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