Combined Neutron Texture and Magnetic Analyses on “Undeformed” Clays: the Origin of Tectonic Lineations in Extensional Basins

In extensional sedimentary basins it has been demonstrated that fine-grained sediments that do not appear deformed at the outcrop scale can carry a magnetic fabric consistent with the regional deformation. In this study, carried out on clays from extensional basins in southern Italy, the magnetic lineation is tectonically controlled and oriented perpendicularly to the main normal faults. As the origin of such magnetic lineation is not yet well understood, we present combination of magnetic and textural analyses in order to unravel that phenomenon.A combination of low-field, high-field and low-temperature measurements was used in order to distinguish the ferrimagnetic and paramagnetic contribution to the magnetic susceptibility and its anisotropy. The magnetic anisotropy of studied sediments is predominantly carried by the paramagnetic phyllosilicates. Neutron texture analysis was used to evaluate the spatial distribution of the basal planes of chlorite. Results demonstrate that the orientation of the magnetic lineation is related to the spatial distribution of phyllosilicate minerals, lying parallel to the common axis of differently oriented basal planes. Independent of the chlorite basal planes distribution (from an axial symmetric to a girdle pattern in the bedding plane, due to their progressive “crenulation”), the AMS magnetic lineation is always parallel to the minimum axis of the mineral fabric ellipsoid.In order to make a quantitative correlation between the AMS and rock fabric, the low- and high-field magnetic anisotropy (AMS, HFA) were compared to the theoretical anisotropy calculated from the neutron texture goniometry measurements. Quantitative correlation is presented in terms of the standard deviatoric susceptibility, k’, and the difference shape factor, U, expressing anisotropy degree and shape, respectively. The degrees of the theoretical anisotropy, AMS, and HFP correlate very well implying nearly the same degree of anisotropy for all the employed methods. When ellipsoid shape is considered, the prolate and oblate shapes are well defined between respective methods.