Strain localisation in bimineralic rocks: Experimental deformation of synthetic calcite-anhydrite aggregates

Deformation of synthetic calcite-anhydrite aggregates to large shear strains (up to γ=12.4 at 600 °C, 300 MPa confining pressure and a constant angular displacement rate corresponding to a shear strain rate of 10^-3 s^-1) resulted in the first experimental observation of strain localisation from initially homogeneous rocks. In contrast to experiments on pure calcite and anhydrite, which deformed homogeneously to large strains (γ ≥5), all experiments on calcite - anhydrite mixtures resulted in heterogeneous deformation at γ > 1 and the formation of narrow localised bands in the microstructures at γ > 4. In these bands, the amount of strain is at least twice as large as in the rest of the sample and individual grains of the same phase cluster and align, thereby forming microstructural layering similar to planar fabrics in natural mylonites. A switch in deformation mechanism in anhydrite from dislocation creep to diffusion creep and/or grain boundary sliding occurs simultaneously with strain localisation. It is concluded that deformation-induced heterogeneous phase distributions cause local strength differences initiating strain localisation in the calcite-anhydrite mixtures. The study suggests that the presence of two phases in combination with a change in deformation mechanism may be responsible for strain localisation in natural poly-mineralic mylonites.