FLUID FLOW, FLUID-ROCK INTERACTION, AND FLUID PROVENANCE IN THE MEXICAN FOLD-THRUST BELT, CENTRAL MEXICO
Stable isotopes of oxygen and carbon were analyzed in calcite from veins, fault rocks, and limestone host rocks. Most veins have δ18O values between 24 and 27‰ VSMOW and δ13C values between +2 and +5‰ VPDB; these values are close to the host rock limestone and consistent with local host-rock buffering of the isotopes. This interpretation is supported by the results of REE analysis.
Along transects across major thrusts, most fault rocks have δ18O values between 22 and 25‰ and δ13C between -2 to + 4‰. The more negative δ13C values are in faults bounding the western, more hinterland margins of two carbonate platforms. Relatively low δ18O values down to approximately +10‰ are present in the western most thrusts consistent with external fluids migrating through the faults during deformation. Aqueous fluid inclusions in syntectonic veins have δD values between -70 and 0‰. The highest value was found in an early formed, most foreland-situated vein in the thrust belt, while the lowest value was measured in a syntectonic vein in the hinterland of the belt. A similar pattern is observed in δD measured in clays from the host rock, though the absolute range in values is more constrained. Together the data indicate that formational fluids were active during the early stages of deformation and as deformation progressed and temperature increased, isotopically lighter fluids were involved. This can be due to a subsequent influence of meteoric water or to release of lighter water from clay minerals.