DATING OF FOLDS IN THE MEXICAN FOLD-THRUST BELT, CENTRAL MEXICO

An important aspect in understanding the propagation of deformation in fold-thrust belts, and therefore of fold-thrust belt evolution, is the age of deformation. Fold-thrust belts develop in the upper crust at very low metamorphic conditions. The most common phase growing under these conditions containing a radiogenic element (potassium) that has been used to obtain absolute ages of deformation is illite. Illitization (production of illite at the expense of smectite) is a phenomenon that has been broadly documented in fault gouge and seems to be induced by shear. To a lesser extent than in fault zones, shear can also be concentrated in incompetent layers on the limbs of chevron folds and thus illitization can also take place in these locations.

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.