The role of second phase in localizing deformation

Small fractions of second phases may play a major role in localizing deformation in rocks. Second phases inhibit the migration of grain boundaries and hence may prevent grain growth. Since flow laws for many deformation mechanisms show a grain size dependence, second phases may indirectly control deformation rate. To illustrate this, a hypothetical sequence of micritic limestones, two impure layers sandwiched between two pure calcite layers, are metamorphosed and then deformed. The initial grain size is assumed to be less than 5 μm. The thermo-mechanical behavior of this sequence is predicted based on three types of experimental data: (1) calcite grain growth kinetics; (2) calcite grain size as a function of second phase; (3) plastic flow laws on natural and synthetic calcite rocks under conditions for grain-size insensitive (GSIC) and grain-size sensitive creep (GSSC). Extrapolations of the results predict that metamorphism at 400°C will increase the grain size in the pure calcite layers to 780 μm in 10 000 years. In a layer containing 5% of 0.3 μm particles, however, the grain size will increase to only 6 μm. Under a differential stress of 100 MPa, the coarser-grained layers deform by GSIC at a strain rate of 10 −14 s −1 while the 6 μm layer deforms by GSSC at 10 −9 s −1 . This large difference in deformation rate insures that virtually all of the strain will be localized in the fine-grained layer.