DATING OF FOLDS IN THE MEXICAN FOLD-THRUST BELT, CENTRAL MEXICO
We explore a new method to obtain absolute ages of folds by combining structural analysis, illite characterization and K-Ar and Ar- Ar illite dating. This is based on the following premises: 1) illitization may be induced by deformation at low temperature; 2) the presence of argon in illite is due solely to the decay of radiogenic potassium; 3) argon is a decay product of radiogenic potassium that diffuses very slowly in illite at low temperature. Two flexural chevron folds from the Mexican fold-thrust belt were chosen for this purpose because they developed in the appropriate lithology and within the appropriate range of temperatures. K-Ar ages of the clay size fraction (<2μm) range between 84 to 64 Ma in 9 analyzed samples. This range of ages falls within the predicted timing from regional stratigraphical constraints. To evaluate the possible contribution of detrital illite age to the overall K-Ar ages, we separated four different size fractions (<0.05, 0.05-0.2, 0.2-1, and 1-2 μm) in the 5 representative samples from these two folds in which the polytypes were characterized and determined Ar-Ar ages for each size fraction. We compare these ages with the overall K-Ar ages and discuss the implications for obtaining the age of folding. We also discuss the application of this technique to the analysis of the propagation of deformation in the Mexican fold-thrust belt and its potential application to other fold-thrust belts.