Overprinting of magnetic fabrics in granites by small strains: numerical modelling

Abstract

Abstract The anisotropy of magnetic susceptibility (AMS) is used to determine weakly defined mineral preferred orientation fabrics in granites. However, the effects of small strains superimposed on the low-intensity AMS fabrics typical of these rocks is not well understood. Numerical simulations are presented which investigate the effects of small superimposed strains on the shapes, magnitudes and orientations of pre-existing AMS fabrics, which are similar to those reported in the literature for undeformed granites. A computer-generated biotite petrofabric is subjected to progressive strain, using March's strain response model, for pure shear (plane strain), axially symmetric shortening and simple shear regimes. Results indicate that a pre-existing AMS fabric might be significantly modified or overprinted by small strains (≈10–15% strain in the pure shear and axially symmetric shortening regimes, 0.5 ⩽ γ ⩽ 1.0 for simple shear). A new AMS fabric may develop with an orientation similar to that expected for shearing of an initially isotropic petrofabric. At low strains, the evolution of the shape of the AMS ellipsoid depends strongly on the orientation of the initial petrofabric with respect to the strain framework; the AMS ellipsoid may evolve towards either oblate or prolate shapes and the evolution in the ellipsoid shape may be reversed during progressive strain. This modelling suggests that small strains affecting a pluton following its emplacement could mask AMS fabrics developed earlier. The results also suggest that, in simple shear, the orientation of the AMS ellipsoid may serve as a shear-sense indicator for small strains even if the earlier fabric is not isotropic.