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.