The origin of tectonic fabrics in ‘‘undeformed’’ clays: The early stages of deformation in extensional sedimentary basin

An integrated approach was used to detect tectonic effects in apparently undeformed fine-grained clay sediments filling extensional basins in Southern Italy. Results from low and high field anisotropy of magnetic susceptibility and mineral fabric, deduced by neutron texture analysis, show penetrative deformation on a grain-scale. The well-defined tectonic magnetic lineation is parallel to the stretching direction and located at the intersection of the basal planes of phyllosilicates, forming grain-scale folds and crenulations. This spatial configuration reflects basin scale deformation processes and suggests the existence of a nearly uniaxial stress field during the incipient extensional phases. The integrated approach of magnetic and mineral fabric investigations is a valid tool to detect grain scale and regional deformation pattern in weakly deformed extensional basins, where macroscopic evidence of deformation is not visible.