DATING OF FOLDS, AN EXAMPLE FROM THE MEXICAN FOLD-THRUST BELT

Dating deformation occurring at low temperature represents a big methodological challenge. This is partly due to the fact that there are not many phases enriched in radiogenic elements that grow under these conditions. One exception is illite, which hosts potassium. Illite is an abundant mineral in sedimentary rocks deformed in fold-thrust belts at very low metamorphic conditions. Illite can be dated by the K-Ar method after clay separation and detailed XRD characterization. Of paramount importance is the distinction between detrital and neoformed illite, but the need to make this distinction is avoided when dating volcanic ashes.

We present an attempt to find by the K-Ar method the age of deformation of folds developed in Cretaceous limestone layers in the Mexican Fold-Thrust Belt in central Mexico. The folded layers are interbedded with bentonite, derived from volcanic ash. These strata were folded by buckling and the main mechanism operating was flexural slip between limestone layers, which accentuates shear in the fold limbs in the bentonite interbeds . A detailed geometric-kinematic analysis that included strain determination, showed that folding by buckling accommodated most of the strain at the metric scale and that this was followed by flattening, which also contributed to the deformation at the grain-scale. Samples from bentonite layers were taken from the limbs and hinges of several folds. Illite crystallinity and Ir index —which gives the abundance of smectite interstratified with illite— show a clear correlation with strain and temperature of deformation. K-Ar ages of 9 illite concentrates taken from these folds make sense with predictions from fold analysis. They are also consistent with the regional geology, with stratigraphic constraints, and with U-Th-He thermochronologic data.