Shock-induced crystal-plastic deformation and post-shock annealing of quartz; microstructural evidence from crystalline target rocks of the Charlevoix impact structure, Canada

Two types of quartz microstructure from the Charlevoix, Quebec, impact structure are compared and contrasted. The dominant occurrence of rhombohedral planar deformation features (PDFs) in the type 1 microstructure, in contrast to the numerous mechanical Brazil twins in the type 2 microstructure, indicates that the deviatoric component of the shock wave-associated stress increases relative to the hydrostatic component with increasing distance from the centre of the impact structure. The type 2 microstructure records highly heterogeneous and localized glide-controlled deformation accompanied by mechanical twinning and microcracking. This deformation is suspected to be due to the high deviatoric stresses and high loading rates during shock. Post-shock recovery is shown in type 1 microstructure by the actual microstructure of rhombohedral PDFs, dislocations in climb configuration and well-ordered low angle grain boundaries, as well as in the type 2 microstructure by the occurrence of small elongate sub-grains with low angle grain boundaries parallel to low-index planes. This is suggested to be due to annealing shortly after the impact event in quasi-static conditions and still sufficiently high post-shock temperatures, as opposed to a separate regional thermal event.