Early Tertiary fluid flow and pressure-temperature conditions in the Shimanto accretionary complex of south-west Japan: constraints from fluid inclusions

The pressure and temperature history of the Tertiary Shimanto belt of south -west Japan has been elucidated by analysing fluids trapped in quartz cryst als which grew syn-kinematically along late-stage brittle faults. The sampl es come from three areas that span the Paleogene exposures on the Muroto Pe ninsula of Shikoku Island. Applying microthermometric and laser Raman micro sampling techniques to coeval water-rich and carbonic fluid inclusions, we have constrained the pressure and temperature conditions that accompanied a widespread and kinematically distinct phase of deformation. The results su ggest elevated geothermal gradients during late-stage deformation, conditio ns that are in disaccord with previous plate reconstructions that have depi cted old, thermally mature Pacific crust subducting beneath Eurasia during the early to middle Tertiary. These conditions can most easily be accounted for by including an additional plate boundary in the western Pacific durin g Paleogene time. Plate reconstructions that include the Kula plate in this region are therefore consistent with our findings. In addition, our result s provide clues to the conditions that likely accompany seismogenic deforma tion at active convergent plate boundaries.