Synorogenic crustal fluid infiltration in the Idaho-Montana thrust belt

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