A fast way to estimate the clay fabric from shale fragments. Key example from a strained thrust footwall (Pyrenees).

Guía rápida para el estudio de rocas deformadas a partir del análisis de la Anisotropía de la Susceptiblidad Magnética (ASM)

“…This is because of the influence of the magnetic anisotropy of each mineral, which does not necessarily correspond to the way in which that same mineral is oriented under the prevailing stress field (e.g., Biedermann, 2018). A reliable comparison between the magnetic ellipsoid and the strain parameters has been found in areas with homogeneous lithology and significant changes of strain values within small distances (Boiron et al, 2020;Gracia-Puzo et al, 2021). A typical oblate-to-prolate-to-oblate path is found when tectonic shortening increases, from the compaction to the purely tectonic fabric (Parés and van der Pluijm, 2002a).…”

“…The confrontation between the competing effects of sedimentary, compaction-related and tectonic-related fabrics has been a common issue when comparing different fabric types in compressed sedimentary basins (Parés and van der Pluijm, 2002b;Pueyo-Anchuela et al, 2010;García-Lasanta et al, 2014). In this case, when the magnetic mineralogy is homogeneous (i.e., same mineralogical composition), the analysis of scalar parameters derived from the magnetic ellipsoid axes is a good marker of the strain path for rocks in different parts of the basin and can be used as an indicator of the prevalence of one kind of fabric against the other (Gracia-Puzo et al, 2021). In weakly deformed rocks, the orientation of foliation, rather than that of the lineation (whose direction remains usually parallel to the strike of beds and axes of folds), is the most sensitive feature that can help to define the degree of tectonic deformation.…”