Mississippian carbonates in the Sevier thrust belt in Idaho-Montana show shifts in Δ18OV-SMOW, from marine carbonate values to as low as +11?, which are best explained by exchange with externally-derived, low-Δ18O fluids. Late-stage, synkinematic calcite veins are depleted in 18O relative to the host-rocks and earlier-formed veins, many having Δ18O of +5 to +10?. These veins could have equilibrated with H2O with Δ18O of -7.5 to +2.5?, perhaps reflecting infiltration of the Sevier thrust wedge by nearshore meteoric waters to depths of ~10 km. Calcite veins in the hangingwall and footwall of the Pioneer Metamorphic Core Complex, produced during later Paleogene extension, have Δ18O of -8.7 to +1.4? consistent with equilibration with meteoric waters with Δ18O as low as -14?. Transition from a Cretaceous crustal fluid regime influenced by the nearby Western Interior Seaway to one influenced by lower-Δ18O, more inland meteoric waters is consistent with seaway retreat during thrust wedge emergence and Paleogene uplift and subaerial volcanism. ¿ 2001 American Geophysical Union