Architecture of an upper-level weak detachment zone: Mexican Fold and Thrust Belt, central Mexico

Abstract

The mechanical properties of detachment zones significantly influence the geometry, structural style, and amount of shortening in the fold and thrust belts. The shale layers in a sedimentary package can act as a detachment zone at different levels of a fold and thrust belt. Detachment zones are not exposed in the active fold and thrust belts, making the detailed study of these detachment zones impossible. This situation makes it difficult to know the architecture and main deformation mechanisms. In this work, we use detailed structural analysis to investigate deformational mechanisms of an ancient and exhumed detachment zone in the Mexican Fold and Thrust Belt, in central Mexico, as an analogue to active modern-day examples. We describe the architecture of the detachment zone through detailed field mapping, strain measuring, and vorticity analysis. Our observations suggest that the studied shear zone is an upper-level detachment zone developed during the Late Cretaceous-Paleogene orogeny. The upper-level detachment zone accommodates the deformation through brittle-ductile structures related to localized mylonitic foliation, s-c structures, duplexes, sheath folds and reverse faults. The mesoscopic structures described in the shear zone suggest a heterogeneous shear deformation. The kinematic vorticity values (Wk = 0.73 in average) and the strain data (RXZ ~3 in average) suggest a sub-simple shear mechanism of deformation. Our results show the role of weak layers during the contraction of a sedimentary package and demonstrate the presence of simple and pure shear mechanisms in the development of the complex detachment zones in fold and thrust belts.