Synorogenic crustal fluid infiltration in the Idaho-Montana thrust belt

Mississippian carbonates in the Sevier thrust belt in Idaho-Montana show shifts in δ¹⁸O_V-_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.