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Geophysical observations indicate that patches of localized fracturing occur within otherwise viscous regions of subduction plate boundaries. These observations place uncertainty on the possible down‐dip extent of the seismogenic zone, and as a result the maximum magnitude of subduction thrust earthquakes. However, the processes controlling where and how localized fracturing occurs within otherwise viscous shear zones are unclear. We examined three exposures of exhumed plate boundary on Kyushu, Japan, which contain subducted sediments and hydrated oceanic crust deformed at ∼300 to ∼500°C. These exposures preserve subduction‐related viscous deformation, which in two of the studied exposures has a mutually overprinting relationship with quartz veins, indicating localized cyclical embrittlement. Where observed, fractures are commonly near lithological contacts that form viscosity contrasts. Mineral equilibrium calculations for a metabasalt composition indicate that exposures showing cyclical embrittlement deformed at pressure‐temperature conditions near dehydration reactions that consume prehnite and chlorite. In contrast, dominantly viscous deformation occurred at intervening pressure‐temperature conditions. We infer that at conditions close to metamorphic dehydration reactions, only small stress perturbations are required for transient embrittlement, driven by localized dehydration reactions reducing effective stress, and/or locally increased shear stresses along rheological contrasts. Our results show that the protolith composition of the subducting oceanic lithosphere controls the locations and magnitudes of dehydration reactions, and the viscosity of metamorphosed oceanic crust. Therefore, compositional variations might drive substantial variations in slip style. Plain Language Summary: Along tectonic plate boundaries, with increasing depth, pressure and temperature, plate movement by brittle fracturing is suppressed, and mechanisms allowing steady slip become more efficient. Along subduction plate boundaries, where one plate sinks beneath another, observations indicate that a proportion of slip sometimes occurs by fracturing at pressure‐temperature conditions where steady slip typically dominates. We use outcrops of an ancient, inactive plate boundary exposed on Kyushu, Japan to investigate the reason for this behavior. We found that chemical reactions, triggered by increasing temperature in sinking oceanic crust, produce water, with the effect of locally raising the fluid pressure within the plate boundary. We suggest that locally raised fluid pressures assist fracturing at pressure‐temperature conditions where steady slip typically dominates. In some outcrops, fractures are especially common along contacts between different rock types, suggesting that mixing of different materials along the plate boundary might also favor fracturing. Key Points: Brittle fracturing occurred locally within viscously deforming hydrated oceanic crust and subducted sedimentsFracturing is localized at viscosity contrasts and P‐T conditions of metamorphic dehydration reactionsRocks that deformed at P‐T conditions away from dehydration reactions record dominantly viscous behavior [ABSTRACT FROM AUTHOR]