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Detailed vitrinite reflectance and illite crystallinity paleothermal data from a portion of the Eocene-Oligocene Shimanto accretionary prism in southwest Japan show that: (1) peak paleotemperatures range from ∼200°C (diagenetic zone) in the northern part of the study area to ∼300°C (anchizone) 10 km to the south. Preliminary bo data from illite (Underwood and others, this volume) suggest that these rocks were buried no more than 10 km. This in turn implies that the Eocene-Oligocene Shimanto Belt experienced a relatively high paleogeothermal gradient of at least 30 to 40°C/km. (2) Shales with the best-developed pressure solution cleavages also record the highest temperatures suggesting that cleavage formation and peak heating were coeval. Structural differences with younger rocks and K-Ar dates on cleavage from correlative rocks in neighboring Ashizuri Peninsula that range from 43 to 18 Ma, with most falling in the range 34 to 26 Ma, suggest that major penetrative deformation of these rocks occurred in the middle to late Oligocene. Peak heating may have commenced in the late Eocene to earliest Oligocene when plate reconstructions suggest subduction of young crust (<10 m.y. old) of the fused Kula/Pacific Plate. An elevated paleogeothermal gradient may also have been maintained during the period of cleavage formation (34 to 26 Ma) by the decrease in subduction rates documented for the middle Oligocene. (3) Significant late Miocene or younger block-faulting, uplift, and flexure of the study area postdated the attainment of peak temperatures and substantially modified patterns of peak paleotemperatures. The most important displacement occurred along the subvertical contact fault between the Eocene-Oligocene (4%Rm) and Oligocene-Miocene (1.5%Rm) Shimanto Belts.

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