Synorogenic crustal fluid infiltration in the Idaho‐Montana Thrust Belt

Abstract

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.