Presented at the 1996 Annual Meeting of the Geological Society of America

BEBOUT, G. E., HOLL, J. E., ANASTASIO, D. J., and DAVIDSON, S. G., Dept. Earth & Environ. Sci., Lehigh Univ., Bethlehem, PA 18015, geb0@lehigh.edu.

    Integrated study of deformation and fluid infiltration in Mississippian carbonates in the Sevier foreland fold-and-thrust belt (Northern Lost River Range) has documented shifts to lower d¹⁸O compatible with exchange with an externally-derived fluid reservoir. A regional tendency exists for texturally later-stage veins to be depleted in ¹⁸O, relative to undeformed carbonates and earlier-stage, smaller vein sets, and to level off in d ¹⁸O_SMOW at minimum values of +6 to +8 per mil. Using peak-T estimates for deformation of ~250 degrees C, these values are compatible with the equilibration of the veins with H₂O-rich fluids with d¹⁸O near 0 per mil. Thus, some access of surficial fluids, in this case seawater, may have been afforded to ~10 km depths along fracture systems and shear zones. Regional buffering of meteoric H₂O d¹⁸O to ~0 per mil by crustal fluid-rock interactions is considered less likely and unnecessary, as Late Cretaceous intracontinental seaways documented in the region would have provided a suitable H₂O reservoir.

    Calcite d¹⁸O values of calcite±epidote±chlorite±actinolite veinlets in brecciated mylonites and intrusive rocks in the Wildhorse detachment fault system of the Pioneer metamor-phic core complex (PMCC), which underwent rapid unroofing at ~45 Ma (Silverberg, 1990), range from -8.7 to -1.9 per mil. Calcite from brecciated and intensely veined upper-plate Mississippian limestones has similarly low d¹⁸O (-5.2 to +1.4 per mil). Calculated H₂O d¹⁸O, using the vein and breccia calcite values and T estimates of ~300 degrees C based on mineral assemblages in the hydrothermal overprint, fall in the range of -14.3 to -4.2 per mil. The lower calculated H₂O values are similar to those documented for Eocene meteoric water in the region through study of nearby Eocene epizonal plutons (~-16 per mil Criss and Taylor, 1983).

    This proposed regional transition from a near-surface crustal fluid regime involving seawater to one in which meteoric waters intruded the shallow crust is consistent with paleogeography involving Late Cretaceous intracontinental seaways followed by emergence and aerial exposure of the thrust wedge by the Paleocene. Documented shallow crustal exchange with surficial waters in the PMCC extends to the south and east the region believed to have experienced regional infiltration by Eocene meteoric waters.