Magnetic Anisotropy and Fabric of Some Progressively Deformed Ophiolitic Gabbros

The ophiolites, considered remnants of oceanic seafloor, provide a fruitful source of information on the physical and chemical behavior of the deeper uncored lithosphere. A study of the gabbros in a rather well-preserved ophiolite from the western Alps (Montgenèvre, France) shows strong evidence for intraoceanic plate deformation. To characterize the different steps of deformation observed in the outcrops, a magnetic fabric study was conducted using the anisotropy of initial magnetic susceptibility (Aims) method. The main conclusions are (1) the Aims is controlled by the paramagnetic ferromagnesian minerals, such as hornblende, resulting from the ocean floor metamorphism close to the spreading ridge (partial amphibolitization) and (2) the magnetic lineation and foliation of the gabbros show a deformation path with increasing strain. A good correspondence between petrofabric and magnetic anisotropies is observed; both suggest that the deformation mechanism was simple shear