Fast stress-loading and -unloading during faulting and shock indicated by recrystallized grains along quartz cleavage cracks

Recrystallized quartz grains are localized along cleavage cracks in coarse original quartz grains within pseudotachylyte-bearing gneisses from the Silvretta basal thrust, Austria, and in shock-vein-bearing gneisses from the Vredefort meteorite impact structure, South Africa.

In the fault rocks of the Silvretta nappe, the recrystallized grains along two sets of {10-11} cleavage cracks at an angle of about 90° occur in rounded quartz clasts with a diameter of several tens of mm to cm embedded within pseudotachylytes. No evidences of shear offset were found in relation to the cleavage cracks. The fine-grained recrystallized grains have diameters of about 10 ± 6 µm or less and are slightly elongated parallel to the cleavage planes. These new grains have similar but also deviating crystallographic orientations to that of the host. As these quartz microstructures occur exclusively in spatial relation to pseudotachylytes, they are interpreted to result from the associated high stress/high strain-rate deformation. Mechanical (-101) twins in amphibole revealed stresses on the order of 400 MPa during formation of the pseudotachylytes. Yet, the new quartz grains do not show evidence of deformation after their growth, i.e., no internal misorientation, no crystallographic preferred orientation related to dislocation glide. Therefore, we suggest that the secondary quartz grains formed during annealing after the pseudotachylyte-forming event localized at the damage zone surrounding the cleavage cracks at quasi-isostatic stress conditions.

Very similar microstructures are found in Archean gneisses of the Vredefort impact structure, South Africa. There, the recrystallized grains with diameters of few µm along {10-11} and (0001) cleavage planes occur in shocked quartz grains related to mm-sized shock veins, characterized by Schlieren-microstructure of secondary feldspar. Also here, no major shear offset of the cleavage cracks is obvious and the secondary quartz grains do not show evidence of deformation. The observation that quartz shock effects are spatially related to both, the shock veins and secondary quartz grains, suggests that they formed during shock loading and subsequent pressure release with high strain rates (ca. 10⁶ s^-1) but minor shearing. Analogous to the Silvretta fault rocks, growth of quartz grains is suggested to occur restricted to the damage zone of the cleavage cracks at quasi-isostatic stresses during post-shock annealing.

In both, the Silvretta fault rocks and shocked gneisses from the Vredefort dome, quartz grains fractured without major shearing at high stresses and subsequently recrystallized localized to the damage zone of cleavage cracks at quasi-isostatic stress conditions. Damage in the process zone surrounding the cleavage cracks must have been large enough for effective grain boundary migration, i.e., growth of grains in orientations weakly controlled by the host orientation. Recrystallization ceased because of the missing driving force during subsequent quasi-isostatic stress conditions. These microstructures indicate quasi-instantaneous loading to high differential stresses of a few hundred MPa and fast unloading to quasi-isostatic stress conditions.