Fold dating: A new Ar/Ar illite dating application to constrain the age of deformation in shallow crustal rocks

We propose a deformation dating method that combines XRD quantification and Ar chronology of submicroscopic illite to determine the absolute ages of folds that contain clay-bearing layers. Two folds in the frontal segment of the Mexican Fold-Thrust Belt (MFTB), which was deformed from Late Cretaceous to Eocene, are used to illustrate the method and its future potential.Variations in mineral composition, illite-polytype, crystallite-size (CS) and Ar total gas ages were analyzed in the limbs and hinge of two mesoscopic folds. This analysis examines potential effects of strain variation on illitization and the Ar isotopic system along folded layers, versus possible regional thermal overprints. The Ar total-gas ages for 9 samples in Fold 1 vary between 48.4 and 43.9 Ma. The % of 2M₁ (detrital) illite vs. Ar total-gas ages tightly constrains the age of folding at 43.5 ± 0.3 Ma. Nine ages from three samples in Fold 2 range from 76.2 to 62.7 Ma, which results in a folding age of 63.9 ± 2.2 Ma. Both ages are in excellent agreement with more broadly constrained stratigraphic timing. The method offers a novel approach to radiometric dating of clay-bearing folds formed at very low-grade metamorphic conditions, and has the potential to constrain dates and rates of regional and local deformation along and across foreland orogenic belts.