The origin of tectonic fabrics in “undeformed” mudstones: an example from the southern Italy sedimentary extensional basins

During the past few years, the anisotropy of magnetic susceptibility (AMS) has been increasingly considered as a powerful method to study subtle deformation in sediments where conventional strain markers are not available. We investigate the deformation mechanisms that cause magnetic lineation in apparently homogeneous clay sediments from extensional regimes, where deformation is not visible at the outcrop scale and whose texture is difficult to detect by the traditional texture techniques. We integrate low-field, high-field and low-temperature AMS analyses with neutron pole figures measurements as an alternative approach to studying the deformation mechanisms that act in sedimentary basins in the incipient phases of extensional processes. We selected the southern margin of the Tyrrhenian basin (Southern Italy) for this case study. Here, for the first time, neutron texture analysis has been applied to study “undeformed” sediments that don’t show tectonic fabric at the mesoscale. Results show that, on the grain scale, clays are characterised by a penetrative deformation and a well-defined tectonic magnetic lineation that formed in an early stage of deformation. The observed magnetic lineation is sub-parallel to the intersection of the basal planes of phyllosilicates, whose spatial configuration, imaged by neutron pole figure measurements, is correlated with the stretching direction at the basin scale. This study suggest that the integration of magnetic and mineral fabric analysis represent a suitable tool to study strain mechanism on the grain scale and regional deformation pattern in weakly deformed extensional basins, even if macroscopic evidence is not visible at the outcrop scale.