Rheology of two-phase systems: A microphysical and observational approach

John P. Platt

Submitted 2014, SCEC Contribution #2059

Ductile shear zones commonly contain distinctive bands of high strain rock characterized by intimately mixed fine-grained two-phase or polyphase material. These ultramylonite bands are likely to be weaker than the surrounding material, and may play a critical role in strain localization. How such zones develop, how the phases become evenly dispersed, the bulk rheology of the polyphase mixture, and the controls on grainsize evolution, are all unclear. The following generic scenario, applicable to two-phase mixtures such as quartz+feldspar, olivine+pyroxene, or calcite+dolomite, may resolve some of these questions.
1) Dislocation creep and dynamic recrystallization of both phases causes grain-size reduction: the stronger phase will generally be more fine-grained as a result.
2) A switch to grain-boundary sliding in the stronger phase is facilitated by grain-boundary diffusion creep of the weaker phase.
3) The weaker phase fills the spaces between the grains of the stronger phase by diffusion; hence the grainsize of the weaker phase is controlled by that of the stronger phase. This leads to mixing and dispersion of the two phases, producing a fine-grained, evenly dispersed two-phase aggregate.
4) The rheology of the aggregate will be controlled primarily by grain-boundary diffusion creep of the weaker phase, with grain-size controlled by dynamic recrystallization of the stronger phase.
Bulk flow laws can be developed for quartz-feldspar and olivine-orthopyroxene ultramylonites based on these concepts and appropriate mixing laws for common volume fractions of the two phases.

Platt, J. P. (2014). Rheology of two-phase systems: A microphysical and observational approach. Journal of Structural Geology, (submitted).

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SCEC Community Rheology Model