The role of folding in the development of the Mexican fold-and-thrust belt

The Mexican fold-and-thrust belt in central Mexico has overall characteristics that fit the critical tectonic wedge model. It is thin-skinned, forward propagating, tapers toward the toe (the east), and displays an overall decrease in deformation toward the toe. The internal structures and heterogeneity of deformation are not typical of fold-and-thrust belts, however, due to the presence of two large carbonate platforms, flanked by more thinly bedded basinal carbonates. Kilometer-scale thrusts dominate deformation in the platform carbonates (a more brittle behavior), and mesoscopic buckle folds and associated cleavage dominate deformation in the basinal carbonates (a more ductile behavior). Total shortening across the belt, including both platforms and basins, is similar to 55%-65%, with higher values in the basins than in the platforms and a concentration of deformation near the platform borders. The dominant mechanism of folding in the basinal rocks is buckling, with thin chert horizons behaving as single layers and limestone and shaly limestone interbeds buckling as multilayers, with a dominant chevron style. A significant shear component of the deformation is indicated by monoclinic fold symmetry, with a consistent sense of vergence of top toward the foreland. We estimated strain and strain history from mesoscopic analysis of fold geometry and internal strain distribution at several locations across the basin and used this information used to assess the overall kinematics and progressive deformation in the basins, which involve both shortening and shear components. The implications of this for the kinematics of the fold-and-thrust belt are discussed.