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) o f Jurassic and Cretaceous siltstones and limestones from the Cameros Basin (north Spain). For rocks initially characterized by precleavage magnetic fa bric the main change in the magnetic fabric produced by the increase of lit hostatic pressure is the increment in the degree of anisotropy (P-j). In ro cks affected by syncleavage deformation the magnetic fabric depends on the mechanism of internal deformation involved in fabric development. In siltst ones, where cleavage is correlated with synkinematic recrystallization of p hyllosilicates and rigid-body rotation of grains, the magnetic fabric is co herent with the internal deformation determined by strain analysis on defor med nodules. On the contrary, in limestones we observe a complete reorganiz ation 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 paralle l to the cleavage pole) indicates that for rocks characterized by syncleava ge magnetic fabric the different variation pattern of T-j and P-j parameter s is closely related to the deformation mechanisms responsible for the fabr ic development.