Progressive deformation, fluid flow and water-rock interaction in the Mexican Fold-Thrust Belt, Central Mexico
An integrated study of the Mexican Fold-Thrust Belt (MFTB) in central Mexico provides information on the evolution of the belt and the processes operating to produce structures and fabrics. Key questions addressed concern to the role of lithology in controlling the style of deformation, the role of fluids in facilitating deformation and the timing and sequence of structural development. Overall, the MFTB is thin-skinned and has properties consistent with the critical tapered wedge model of fold-thrust-belt development: there is a decrease in thickness, intensity of deformation and temperature of deformation towards the toe. Critical wedge theory, however, does not explain how deformation accumulates in the rocks within the wedge. In the MFTB, the deformation mechanisms that operate in different segments of the cross-section are strongly controlled by lithology. The dominant rocks in the sedimentary cover are Cretaceous carbonates that vary in facies laterally, with two large carbonate platforms flanked by more thinly bedded basinal carbonates. Kilometer scale thrusts dominate deformation in the platform carbonates (a more brittle behavior), and mesoscopic buckle folds and associated cleavage dominate deformation in the basinal carbonates (a more ductile behavior). Two phases of deformation are recognized, the first, D1 being more intense than the second, D2. The difference in facies distribution is the main reason for the difference between the MFTB and the fold-thrust belt in the southern Canadian Rocky Mountains, which is characterized by large thrust sheets and relatively few buckle folds. Structural observations on the mesoscopic scale allow distinguishing veins (dominantly calcite) of several generations, emplaced early, during and late/after deformation (V1, V2 and V3 respectively). Analysis of δ13C and δ18O in calcite from veins and host-rock shows that the veins confined within thrust slices are isotopically buffered by the host rock and differ in isotopic composition from veins emplaced along major thrusts or cross cutting thrust-slices. The extent of isotopic buffering seems to be controlled by the amount and composition of the fluid interacting with the carbonate sequences rather than by the temperature, which varies between 100 to 300°C along the cross-section. Analysis of δD in water extracted from fluid inclusions trapped in the veins and in clay minerals strongly suggests rock interaction with meteoric fluids in the west (hinterland) and with fluids close to SMOW in the east (foreland) side of the cross-section. The influence of meteoric water was also more important late during D1 than in the early stages of deformation. Stratigraphic constraints indicate that D1 started about 90 Ma on the western side of the MFTB in central Mexico and finished about 65Ma towards the east. Stratigraphic and structural observations in the foreland indicate that D2 took place after 65Ma. K/Ar ages of illite separated from layers of bentonite intensively sheared during folding of D1 lie between 84-77 Ma for one locality and between 64 and 71 in a second locality. Outcrop scale variations in age indicates that illite transformation and the closure of its K/Ar isotopic system are not only controlled by temperature but also by strain around the folds. The data suggest that it took about 5 to 7 Ma to develop individual folds, and this occurred within the time range constrained by stratigraphy. U-Th/He thermochronological ages determined in 21 zircon grains are overall consistent with stratigraphic ages and K/Ar ages.
Carbonates, Fluid Inclusions, Fold-Thrus Belt, Orogenic Wedge, Stable Isotopes, Vein, Geology
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