Effect of lithostatic pressure and tectonic deformation on the magnetic fabric (anisotropy of magnetic susceptibility) in low-grade metamorphic rocks

The role of lithostatic pressure and strain in low-grade metamorphic cover rocks is analyzed through the anisotropy of magnetic susceptibility (AMS) of Jurassic and Cretaceous siltstones and limestones from the Cameros Basin (north Spain). For rocks initially characterized by precleavage magnetic fabric the main change in the magnetic fabric produced by the increase of lithostatic pressure is the increment in the degree of anisotropy (P-j). In rocks affected by syncleavage deformation the magnetic fabric depends on the mechanism of internal deformation involved in fabric development. In siltstones, where cleavage is correlated with synkinematic recrystallization of phyllosilicates and rigid-body rotation of grains, the magnetic fabric is coherent with the internal deformation determined by strain analysis on deformed nodules. On the contrary, in limestones we observe a complete reorganization of the magnetic fabric probably due to the discontinuous character of the cleavage formation mechanism. Detailed analysis of the beta parameter (angle between the bedding pole and the minimum susceptibility axis parallel to the cleavage pole) indicates that for rocks characterized by syncleavage magnetic fabric the different variation pattern of T-j and P-j parameters is closely related to the deformation mechanisms responsible for the fabric development.