In this study we apply electron tomography to characterize 3D dislocation microstructures in two highly sheared quartz mylonite specimens from the Moine and Main Central thrusts which were deformed extensively by dislocation creep in the presence of water. Both specimens show dislocation activity with dislocation densities of the order of 3-4.1012 m-2 and evidence of recovery from the presence of subgrain boundaries. slip occurs predominantly on pyramidal and prismatic planes ( basal glide is not active). [c] glide is not significant. On the other hand, we observe a very high level of activation of glide in the{10-10},{10-11}, {11-2n} (n=1,2) and even {21-31} planes. Approximately 60% of all dislocations involve climb with a predominance of mixed climb, a deformation mechanism characterized by dislocations moving in a plane intermediate between the glide and the climb planes. This atypical mode of deformation demonstrates comparable glide and climb efficiency under natural deformation conditions. It promotes dislocation glide in planes atypical of quartz structure, probably by inhibiting lattice friction. Our quantitative characterization of the microstructure enables us to assess the strain that dislocations can generate. We show that the contribution of glide produced by the observed dislocations is sufficient to satisfy the von Mises-Taylor criterion. Hence, activation of climb is not necessary to provide additional strain components, but it contributes to the magnitudes of strains achieved. On the basis of this characterization, we propose a numerical modelling approach for attempting to characterize the local stress state that gave rise to the observed microstructure.

The spatial separation of macroscopic rheological behaviours has led to independent conceptual treatments of frictional failure, often referred to as brittle, and viscous deformation. Detailed microstructural investigations of naturally deformed carbonate rocks indicate that both, frictional failure, and viscous mechanisms might operate during seismic deformation of carbonates. Here, we investigate the deformation mechanisms that were active in two carbonate fault zones in Greece by performing detailed slip-system analyses on data from automated crystal-orientation mapping transmission electron microscopy and electron backscatter diffraction. We combine the slip system analyses with interpretations of nanostructures and predictions from deformation mechanism maps for calcite. The nanometric grains at the principal slip surface should deform by diffusion creep but the activation of the (0001)<-12-10> slip system is evidence for a contribution of crystal plasticity. A similar crystallographic preferred orientation appears in the cataclastic parts of the fault rocks despite exhibiting a larger grain size and a different fractal dimension, compared to the principal slip surface. The cataclastic region exhibits microstructures consistent with activation of the (0001)<-12-10> and {10-14}<-2021> slip systems. Post-deformational, static recrystallisation and annealing produces an equilibrium microstructure with triple junctions and equant grain size. We propose that repeated introduction of plastic strain and recrystallisation reduces the grain size and offers a mechanism to form a cohesive nanogranular material. This formation mechanism leads to a grain-boundary strengthening effect resulting in slip delocalisation which is observed over six orders of magnitude (μm–m) and is expressed by multiple faults planes, suggesting cyclic repetition of deformation and annealing.