Data from the Sevier hinterland and foreland (Pioneer Mountains, Lost River Range, Beaverhead Range) show that fluid infiltration was heterogeneous at cm-km scales and resulted from positive feedback between deformation and far-traveled surficial fluids. Metasomatic strain softening enhanced deformation zone development, which generated increased permeability as the thrust belt evolved from a porosity-based closed system to a discontinuity-based open system. Increased fluid infiltration and isotopic exchange was associated with negative dilatancy, increasingly prolate strains, and neocrystallization of clays, which define cleavage selvages. Mass transfer was accommodated by diffusion early and advection later in the deformation history. Mesoscopic structures (deformation zones, faults, veins) focused fluid flow and were kinematically related to larger-scale structures (faults, fault-related folds). The inferred addition of surficial fluids to depths of ~10 km in the thrust belt implies a fluid regime involving significant topographically driven recharge. Deformed whole-rocks and microsamples are lower in d¹⁸O than undeformed samples which have O- and C-isotope compositions similar to those of marine carbonates. Veins are even lower in d¹⁸O_V-SMOW with minimum values of +5 to +10‰ reflecting penetration of the crust by ocean-derived precipitation with calculated d¹⁸O consistent with reconstructions of Great Western Interior Seaway storms. Later fluid infiltration lowered the d¹⁸O of carbonates in the hanging wall of the Pioneer Metamorphic Core Complex to as low as +4‰. Calcite veins in hanging wall carbonates and in the igneous/metamorphic footwall have a combined d¹⁸O range of -8.7 to +1.4‰ consistent with equilibration with meteoric waters with d¹⁸O as low as -14‰. A transition from a fluid regime influenced by the adjacent Western Interior Seaway to one influenced by more inland meteoric waters is compatible with the retreat of the seaway during Cretaceous emergence of the thrust wedge and Paleogene extension, uplift, and subaerial volcanism.