Synkinematic quartz crystals collected from late-stage (syn- to post-cleavage) faults in the Paleogene Shimanto accretionary complex contain fluids that provide constraints on fluid flow within the accretionary wedge. In particular, the trapped fluids provide a glimpse of intraprism flow during a phase of widespread prism thickening. Carbonic (CH4 ± CO2 rich) and aqueous (H2O-rich) inclusions occur intermixed within single crystals that nucleated primarily in releasing stepovers along faults that deform well-indurated and/or cleaved wallrock. Invariably, the carbonic inclusions contain an aqueous liquid as a perimeter phase, and the aqueous inclusions contain a carbonic vapor phase, suggesting coeval trapping of unmixed phases from a miscible parent fluid. These observations are interpreted to reflect crystal growth following deformation-driven dilitation and consequent unmixing of the parent fluid. At one location (Cape Oyama), the occurrence of CH4 and H2O in the absence of CO2 enables explicit constraints to be placed on pressure and temperature conditions during crystal growth. These conditions suggest geothermal gradients of 50°C/km during late-stage faulting. Our results suggest local intraprism flow that was driven by deformation. The scale of flow that is suggested is quite restricted, and this leads us to hypothesize that the Paleogene Shimanto accretionary prism acted largely as a barrier to fluid flow during late-stage deformation. As a consequence, the Paleogene rocks probably served to focus fluid escape elsewhere, most likely trenchward and arcward. The combination of the elevated geothermal gradient, which would have contributed to increasing the relative strength of this package of rocks, and the lack of fluid pathways within it would have favored the development of new out-of-sequence thrust faults outboard of the Paleogene prism or the reactivation of older, more landward structures.