Geochemical constraints on the evolution of the fluid-rock system in the salt-detached Nuncios Fold Complex, Mexico

This is a comprehensive study of the geochemistry, petrography, and structural relations of veins and host rocks from the Nuncios Fold Complex, the west-plunging, frontal fold complex of the Sierra Madre Oriental exposed in the Monterrey Salient near Saltillo, Mexico. Oxygen, carbon, and strontium isotope variations in vein-filling calcite and quartz and their host rocks are used to elucidate the origin, spatial, and temporal evolution and migration pathways of fluids in the evaporite-detached Nuncios Fold Complex.; The folded Mesozoic sedimentary sequence contains two regional paleohydrostratigraphic units separated by a unit of low permeability. The lower paleohydrostratigraphic unit is comprised of the Zuloaga and La Casita Formations. The unit directly overlies the lower detachment of Minas Viejas evaporite and lies beneath the low-permeability rocks of the Taraises shale. The upper paleohydrostratigraphic unit is comprised of the Taraises, Cupido, La Pena, and Aurora Formations. That unit is confined beneath the Parras shale, which also constituted the regional upper detachment in the Monterrey salient.; Two main generations of cements are present in both regional paleohydrostratigraphic units. Early vein-filling cements have delta18O, delta 13C, 87Sr/86Sr, and fluid inclusion temperatures that are distinct between the lower and upper units. This suggests, along with a strong correspondence between early cement and host-rock delta 18O and delta13C values, that early diagenetic fluids were compartmentalized between the lower and upper units. Late vein-filling cements have isotopic compositions and fluid inclusion temperatures that converge to similar values, indicating open fluid flow between the lower and upper units.; Overall, the results of this study suggest that the fluid history of the Nuncios Fold Complex evolved in two main stages: (1) burial diagenesis and early folding, during which fluids were confined within individual units; (2) late stage folding, during which increased deformation associated with fold tightening caused the expulsion of fluids from the lower unit into the upper unit. During the latter stage, the fold hinge evolved as a distinct structural and geochemical environment where hydrocarbons and other fluids accumulated and interacted.