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Geological Society, London, Special Publications; 1990; v. 54; p. 175-181;
DOI: 10.1144/GSL.SP.1990.054.01.17
© 1990 Geological Society of London

Instabilities and Localization

The role of second phase in localizing deformation

David L. Olgaard

Geologisches Institut, ETH, Zürich, CH-8092, Switzerland

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.





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