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Upper Oligocene to lower Miocene accreted rocks and slope-basin strata of the Shimanto accretionary prism at Cape Muroto, southwest Japan, recently have been interpreted as recording the subduction of the active Shikoku back-arc basin spreading ridge. The Shimanto paleotrench was suborthogonal to the trend of the active spreading ridge and subduction apparently started at ∼15 Ma. This event resulted in the formation of a deep structural embayment into the prism, accompanied by mid-ocean ridge basalt (MORB) magmatism in the core of this embayment. Our investigation of the thermal history of these rocks by means of vitrinite reflectance and illite crystallinity indicates that they have experienced high geothermal gradients compared to other accretionary prisms.

Mean vitrinite reflectance (%Rm) values determined for the accreted strata range from 0.9 to 3.7%, corresponding to a peak paleotemperature range of ∼140 to 280°C; %Rm values for slope-basin strata cluster at ∼1.6%, or paleotemperature of ∼200°C. Illite crystallinity studies support the reflectance data. Locally, high %Rm values associated with clastic dikes along major faults within the accreted strata suggest that the faults served as conduits for high temperature fluids. Based on structural-stratigraphic relationships and porosity measurements, maximum burial of the slope-basin strata is estimated to have been 2 to 4 km; combined with the paleotemperature data, this estimate indicates a minimum paleogeothermal gradient in the slope-basin on the order of 70°C/km. Paleogeothermal gradients were likely higher in the accreted strata.

The regional thermal pattern overprints structures related to initial phase of accretion and subsequent intraprism deformation, but thermal maturity has been affected by the embayment and grades smoothly away from the aureoles of the mafic intrusions in the core of the embayment. The embayment and the intrusions are direct results of spreading ridge subduction. Thus, we consider the unusually high paleogeothermal gradients at Cape Muroto to reflect the thermal imprint of active spreading ridge subduction.

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