Magnetic anisotropy, rock fabrics and finite strain in deformed sediments of SW Sardinia (Italy)

Abstract

Although some empirical relationships have been established between the anisotropy of magnetic susceptibility (AMS) and the state of finite strain in studies from specific geologic areas, the mechanisms governing correlations are not well understood. A comparative study has been made in order to elucidate the AMS-strain relationship by investigating mineral preferred orientations and the responsible microstructural deformation processes. Detailed analyses are presented from low-grade metamorphic slates of a Palaeozoic sequence exposed in the Variscan of SW Sardinia. Rock magnetic experiments suggest that paramagnetic minerals dominate the AMS in these rocks. Excellent agreement was found between magnetic fabrics and lattice preferred orientations (001) of mica and chlorite, determined from X-ray texture goniometry measurements. Depending on the oblate or prolate ellipsoidal shape of anisotropy, the minimum principal axes of AMS and X-ray textures can coincide with the poles to slaty cleavage or the maximum principal axes can coincide with the bedding/cleavage intersection, respectively. The measured AMS and X-ray textures only partially reflect the finite strain determined from deformed micro-pebbles and reduction spots. Axial orientations of magnetic fabrics, mica and chlorite preferred orientations, and finite strain ellipsoids are generally in good agreement depending on development of oblate or prolate fabric ellipsoidal shapes. Linear correlations can be established for magnitudes of minimum and intermediate axes of the AMS and finite strain ellipsoids but shape comparisons are complex. Microstructural studies by scanning electron microscopy (SEM) indicate that the preferred orientation of the phyllosilicates is caused by different grain-scale deformation mechanisms, such as kinking, micro-folding and preferential growth. Thus, magnetic fabrics and mineral preferred orientations evidently result from different deformation mechanisms than the finite strain. They reflect the heterogeneous deformation due to micro-mechanical reorientation processes of mineral grains while finite strain reflects the homogeneous deformation accommodated cumulatively by the strain markers.