Inversion tectonics and magnetic fabrics in Mesozoic basins of the Western Tethys: A review

Abstract Positive tectonic inversion of sedimentary basins has been recognized as one of the primary mechanisms of mountain building and intraplate deformation. Reconstructing the tectonic history of basins is relatively easy for the inversion stage but becomes more difficult for the basinal stage, especially when strong deformation involving cleavage development is associated with the subsequent compressional tectonics. Since tectonic markers for the extensional episodes are not commonly well developed, Anisotropy of Magnetic Susceptibility (AMS) has provided recently a tool for analyzing early stages in the evolution of sedimentary basins, even in the absence of other outcrop-scale mesostructures. Here, we expose and discuss the applicability of magnetic fabrics (by means of AMS) to different types of intra-plate sedimentary basins in the Western Tethys region formed under extensional or transtensional regimes and which underwent different inversion styles (total or partial inversion, with or without cleavage development, forming part of compressional thrust sheets, etc.) owing to specific particular p-T conditions and structural controls. Factors such as lithology, magnetic mineralogy, position within the sedimentary pile and deformation intensity are key to interpret the obtained magnetic fabrics in terms of tectonic evolution. A basin classification is proposed according to inversion styles and magnetic fabrics: Where inversion did not involve cleavage development, magnetic lineation is parallel to the stretching direction corresponding to the extensional stage. The transition between non-cleaved to inversion-related cleaved units is marked by the switch of magnetic lineations from parallel to extension to parallel to the intersection lineation between cleavage and bedding. These relationships are enhanced when extension and compression are roughly coaxial, then favoring the clustering of axes of the magnetic ellipsoid. Even when extreme inversion occurs and the early, extensional fabric is obliterated, magnetic fabrics provide information about the interaction between preferred deformation directions associated with the main stages in basin evolution.