A comparison of pore fluids and authigenic carbonates sampled by the Ocean Drilling Program from the Nankai, Peru, Barbados, and Cascadia accretionary wedges illustrates significant disequilibria in oxygen isotopes, which in some cases may be related to rapid incursions of fluids along fault conduits in the past. The Peru, Barbados, and Cascadia carbonates have lower than expected δ18OPDB (Peedee belemnite) values for the measured downhole temperatures and isotope compositions of the pore fluids. In the Peru wedge the lower values may have resulted from past incursion of meteoric water, but in the Barbados and Cascadia wedges, the low δ18OPDB values were caused in part by past incursion of fluids with higher temperatures, probably along fault conduits. The temperature of fluids in the Cascadia wedge may have exceeded 100 °C, which is 90 °C higher than measured current downhole temperatures. The required high fluid-flow rates suggest a mechanism of seismically induced fluid migration to bring fluids rapidly from depths of greater than 2 km to the surface.