Flow stresses from microstructures in mylonitic quartzites of the Moine Thrust zone, Assynt area, Scotland

A. Ord, J. M. Christie

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Recrystallized grain sizes, subgrain sizes and dislocation densities of quartz grains in quartzose mylonitic rocks have been examined using optical and transmission electron microscopy (TEM). The samples come from the Moine Thrust zone in the Assynt district, Scotland. They had been studied previously and described in detail with respect to their structural position in relation to the various thrusts in the region and to their preferred orientation. Stresses were derived from these samples using empirical and theoretical equations relating flow stress to the scale of the microstructures. The stresses determined, 43-244 MPa from recrystallized grain size, 9-13 MPa from etched subgrain size, 50-95 MPa from TEM-scale subgrain size and 75-147 MPa from dislocation density, are not constant in individual samples. Simultaneous formation of the quartz fabrics and of the dynamically recrystallized grains during the period of mylonitization may have occurred under flow stresses ranging from 43 to 244 MPa. A later dynamic recovery event reset subgrain sizes and dislocation densities to a constant-value for each microstructure throughout the area. There is insufficient empirical information available on the flow stress/subgrain size relationship and on the effects of annealing recovery and annealing recrystallization to allow for a more detailed interpretation. Recrystallized grain size is still the most easily measured microstructural feature and the relationship of recrystallized grain size to flow stress has a sounder experimental basis than subgrain size or dislocation density. The effect of chemical environment on the behaviour of all the microstructures is still unknown.

Original languageEnglish
Pages (from-to)639-654
Number of pages16
JournalJournal of Structural Geology
Issue number6
Publication statusPublished - 1 Jan 1984
Externally publishedYes


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