A Discussion on natural strain and geological structure - The effects of strain on the microstructures, fabrics, and deformation mechanisms in quartzites

Abstract

The oaxes fabrics and the intracrystalline microstructures of naturally deformed quartz indicate that dislocation deformation mechanisms are important natural deformation processes. Two mechanisms, dynamic recovery and cyclic dynamic recrystallization, can lead to steady state flow which is necessary if large strains are to be attained. Cold working followed by cataclasis can also produce a steady state flow, but is limited to low temperature, high stress environments. Available data indicates that the c-axes preferred orientations and the optical strain features develop progressively as the strain increases. Recrystallization occurs by the continual rotation of sub-grains or by the development of small strain free grains. The new grains develop preferentially in the most misorientated areas of the deformed host grains, especially along deformation bands and in grain mantles. They have c axes orientations similar to the range in orientation of the host grain in deformation bands and mantles Grain growth is inhibited if there is a rapid increase in the dislocation density in the new grains. Grain refinement then accompanies recrystallization and produces a quartz mylonite. The new grains may be subjected to further phases of deformation and grain refinement. Deformation maps for quartz show that Coble creep replaces dislocation creep as the main deformation process when the grain size is less than 100 pm. The change is accompanied by strain softening and the maps imply that fine grained quartz mylonites are superplastic.