Evidence of Dislocation Mixed Climb in Quartz from the Moine and Main
Central Thrusts: an Electron Tomography Study
Abstract
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.