FLUIDS AND THE HEART-MOUNTAIN FAULT REVISITED

Stable isotopic results from the Heart Mountain fault in northwestern Wyoming show that fluids migrated along the detachment during faulting , We report herein calcite delta(18)O and delta(13)C values in fault b reccias, shifted by as much as -15 parts per thousand (relative to SMO W) and -5 parts per thousand (relative to PDB), respectively, which de monstrate focusing of meteoric waters along the detachment. The isotop ic depletions systematically increase toward the northwestern margin o f the fault terrane, where Absaroka intrusive centers may have provide d a source region for hydrothermal fluids. In addition, isotopic diseq uilibrium between vein and wall-rock samples and enhanced O-18 and C-1 3 depletions in lower-plate calcite veins suggest that the fluids were externally derived and migrated upward to the detachment. We advocate a model in which Heart Mountain faulting and syntectonic fluid flow o ccurred beneath a continuous allochthon.