FLUID-ROCK INTERACTION IN THE MEXICAN FOLD-THRUST BELT (MFTB) OF CENTRAL MEXICO

Fluids play an important role in the deformation of fold-thrust belts. The fluids are essential for enabling solution-mass transfer, typically associated with foliation development, and facilitate fault movement. Information about the nature and effect of fluid-rock interaction in fold-thrust belts and thus on processes of fold-thrust belt development may be obtained by analysis of fault rocks and syn-tectonic veins. We report on the stable isotopic composition of veins associated with thrusts and mesoscopic folds in the fold-thrust belt of Central Mexico (MFTB). The rocks are dominantly carbonates, similar to those of the Canadian and U.S. sections of the Rocky Mountain Fold-Thrust Belt, but the tectonic style is significantly different from the style to the north, where large-scale thrusts and limited intra-thrust deformation is typical. Deformation of the MFTB is characterized primarily by intense folding and less so from displacement on thrusts. Deformation intensity, including tightness of folds, frequency of fold-related veins, and displacement on thrusts all increase from east to west (foreland to hinterland, respectively).

Stable isotopes of oxygen and carbon in calcite were analyzed in fold-related veins, fault-related rocks and veins, and host rocks. Most fold-related veins have δ¹⁸O (VSMOW) and δ¹³C (PDB) values close to those of the host rock: 24 to 27 per mil and 2 to 5 per mil, respectively. These values are consistent with rock buffering of the isotopes during folding and vein formation. Variations in isotopic values were analyzed along meter-scale traverses of 7 major thrusts. Samples from slightly deformed rocks in the foot and hanging-walls have δ¹⁸O values typical of Cretaceous carbonates (varying from 22-25 per mil). On the eastern foreland side of the thrust belt, the δ¹⁸O values for sheared rock and veins in shear zones along thrusts are intermediate between the values found for the hanging wall and footwall. By contrast, for the two westernmost thrusts, theδ¹⁸O values are down to 10 per mil within the thrust zones. These low values are consistent with external fluids, perhaps from siliciclastic sediments or crystalline basement, having circulated along the two western faults. More work is being done to elucidate the initial results of this study.