Stable isotopic results from the Heart Mountain fault in northwestern Wyoming show that fluids migrated along the detachment during faulting. We report herein calcite (δ18O and δ13C values in fault breccias, shifted by as much as −15‰o (relative to SMOW) and −5‰o (relative to PDB), respectively, which demonstrate focusing of meteoric waters along the detachment. The isotopic depletions systematically increase toward the northwestern margin of the fault terrane, where Absaroka intrusive centers may have provided a source region for hydrothermal fluids. In addition, isotopic disequilibrium between vein and wallrock samples and enhanced 18O and 13C depletions in lower-plate calcite veins suggest that the fluids were externally derived and migrated upward to the detachment. We advocate a model in which Heart Mountain faulting and syntectonic fluid flow occurred beneath a continuous allochthon.