The character and origin of Sevier Cordilleran thrust fault fluids was investigated through the analysis of Sr, O, and C isotope ratios in thrust fault breccia matrix. Breccia matrix has depleted oxygen, depleted carbon and more radiogenic strontium when compared to undeformed rocks. This isotope chemistry indicates that meteoric waters descended below the horizon of thrusting and interacted with rocks that released radiogenic Sr to the fluid. The fluids subsequently ascended into the thrust faults where they imparted their unique isotopic signature to the matrix of the recrystallizing thrust fault breccia. The similar trends in isotope chemistry of thrust fault breccias along geographically far-spaced locations of the Cordilleran thrust belt indicates that a common hydrologic regime was operating during Sevier deformation. ¿ 1999 American Geophysical Union