Abstract

Low-pressure shock (<15–20 GPa) indicators are of prime importance for the recognition of ancient impact craters because such craters’ strongly shocked ejecta are usually eroded away and shock effects are obscured through post-shock annealing and weathering. To search for such indicators, we have examined experimentally shocked (5–17.5 GPa) sandstones and quartzites by transmission electron microscopy. Quartz from both rock types displays planar deformation features (PDFs) at all pressures. The typical low-pressure microstructures consist of PDFs parallel to the rhombohedral forumla and to the basal (0001) planes. The forumla PDFs are in all cases amorphous. The (0001) PDFs show fringe patterns and partial dislocations typical of mechanical Brazil twins. The formation of these mechanical defects indicates a high shear component prevailing under low-pressure shock deformation of quartz. Moreover, we observed for the first time that Brazil twins incur a process of amorphization at the higher pressures of our experimental series. Thus, we suggest that the decorated (0001) lamellae identified in naturally shocked quartz grains represent mechanical Brazil twins that subsequently became amorphous upon shock compression and subsequently recrystallized due to post-shock annealing. This modification allows their recognition under the optical microscope.

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