Dislocation Creep in Calcite Rocks with Evolving Microstructure

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Deformation during orogenic events at convergent margins is often localized along faults and shear zones within metasediments that include carbonate rocks. Thus, the microstructures within the rocks record an important part of the strain history of many tectonic events. The Helevetic Nappes are a classic example, but similar shear zones are common in most orogenic belts. Displacement along these features is often 10's of kilometers or more and these localized zones are probably critical in determining the overall strength of a rock mass with similar linear dimensions. Observations of exhumed shear zones show that the rocks are polyphase assemblages exhibiting a complex interplay of metamorphic reactions, grain size refinement, recrystallization, and internal deformation. To provide a detailed mechanical description of these features, constitutive relations must include the effect of evolving structure on strength. In general three or more interlinked laws are needed: an evolution equation describing the rate of change for each structural variable, a kinetic equation relating mechanical and thermodynamic loading and the structural variables to the rate of strain, and a kinematic equation involving a time integral of inelastic strain rate. Structure variables may be explicitly identified or implicitly determined without identification. Appropriate explicit state variables might include aspects of the dislocation microstructure or the grain size, as are common in studies of plasticity in metals. But because natural tectonic situations are more complex, a much broader class of state variables will be needed. Among these additional variables might be crystal lattice preferred orientation, progress variables for metamorphic reactions, solid-solution chemistry, porosity, and pore fluid fugacity. In this work, the evolution of the mechanical properties of calcite rocks deforming by dislocation creep is being measured and correlated with the microstructure. Initial studies indicate that changes in second-phase content, substitional magnesium impurities, and porosity are important parameters. The tests include some natural samples containing nanometer-sized graphite particles from shear zones in the Helvetic Nappes. Two-phase synthetic samples containing calcite + dolomite are also fabricated and deformed in the dislocation creep regime to understand the effect of second-phase dispersions on strength and on dynamic recrystallization. The mechanical tests include conventional triaxial compression and extension, loading in simple shear and loading in torsion. Microstructure observations are made using optical, scanning and transmission microscopes and detailed analyses of the chemistry of grain boundaries. One broad impact of the study is the development of a teaching collection of data and interpretations from mechanical tests, field observations, optical and TEM photomicrographs, and electron microprobe data that will be archived as part of the open courseware initiative at MIT.