Variable magnetic fabrics under heterogeneous deformation across a shallow fault zone in the Iberian Chain (Monroyo thrust, N Spain)

Abstract

A Cenozoic, N100E-striking thrust fault located in the Iberian Chain, provides an opportunity for testing the applicability of anisotropy of magnetic susceptibility (AMS) to the study of fault rocks deformation in shallow fault zones. The Monroyo thrust, which is probably a splay of the Castellote–Herbers fault, involves Cretaceous and Paleogene rocks in its hanging wall and footwall, respectively, shows a history of frontal thrusting without a strike-slip component. Fault kinematics are inferred from macro- (mapped trace of the thrust surface and folds in the Cretaceous series of the hanging wall) and meso-structures (pressure-solution foliation in Paleocene conglomerates of its footwall). The studied segment of the Monroyo thrust shows a 20 m thick fault zone consisting of brecciated limestones, marls and clays, showing pressure-solution foliation and oriented phyllosilicates (muscovite and chlorite), that can be identified in X-ray diffraction analysis. Analysis of AMS in fault rocks indicates that magnetic fabrics are dependent on the deformation degree, which is interpreted to vary across this relatively narrow fault zone. With increasing deformation, the magnetic lineation changes from parallel to the intersection lineation between foliation and shear planes in the area closest to the hanging wall, to a scattered pattern within the foliation plane in intermediate areas, and finally it gets parallel to the projection of the transport direction onto the magnetic foliation plane near the contact with the footwall (pitch of 90o within the foliation plane). The results obtained also give clues on the tectonic history of the Iberian Chain, in which frontal, NNE-directed thrusting seems to be the main mechanism for E–W trending basement-involved thrusts, in the linking zone between the Iberian Chain and the Catalonian Range.