ISOTOPIC DATING OF FOLDS FROM THE UPPER CRUST

Flexural folds are among the most common structures in deformed rocks of low-grade regions. Such folds commonly involve successions of layers with different mechanical responses (e. g. limestone and shale). Limb-rotation during folding involves deformation mechanisms such as localized fracturing and faulting, pressure-dissolution and solution transfer, operating in different parts around the fold, and bed-parallel shear within mechanically weaker layers. Such incompetent layers are typically fine-grained and clay-rich, with bed-parallel shear grinding the rock and changing its fabric, but also promoting intense fluid-rock interaction that assists the neoformation of clays, particularly of illite.

Results from kinematical, mineralogical, textural, stable isotopic and geochronological analyses of a series of flexural folds (with wavelengths from km-to-m) in Cretaceous limestone interbedded with bentonitic shale from the Mexican Fold-Thrust Belt are presented. From these analyses we learned that: 1) analogous to fault zones, bed-parallel shear during flexural folding has the ability to produce illite in clay-rich layers, which is evidenced by the formation of an anastomosing fabric (under SEM) of mostly illite grains with a similar grain size and a characteristic mineral polytype; 2) illite productivity is related to the amount of shear in the fold; 3) in contrast to fault gouges, Ar-Ar illite ages in bentonitic layers are about the same among the grain-size fractions. The lack of detrital illite in these samples suggests there was none to begin with or it was transformed into new illite during folding, as stable isotope signatures imply; 4) comparison of δ²H of illite and water trapped in fold-related veins suggest that both, illite in shale layers and vein-forming minerals in limestone layers precipitated from the same pore water, which was a mixture of meteoric and marine. From our results we conclude that Ar-Ar illite dating of folds provides an excellent estimate of the absolute ages of local deformation, as supported by regional stratigraphic constrains. Illite dating of folds has the potential to become a powerful, complementary tool in regional structural analysis of areas that were deformed under sub-greenschist facies conditions.