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Quartz microstructures from the Rochechouart impact structure in France, are analysed by optical and electron microscopy (SEM, EBSD, TEM) in order to obtain information on the shock wave-associated stress. The mean stress is generally denoted as shock pressure, which can be up to several tens of GPa. The deviatoric stress conditions during shock, however, are largely unknown. Therefore, shock effects that give evidence of compression at high shock pressures on the one hand and shear deformation at high differential stress on the other, are compiled. Quartz microstructures that reveal localized transformation into a high dense amorphous phase are planar deformation features, mosaicism and annealed diaplectic glass. They indicate high shock pressures (high mean stress) on the order of 20-35 GPa. Target rock fragments with such shock features do not show evidence of shear deformation at high differential stress. Conversely, mechanical Brazil twins and microfaults in quartz, as well as cataclastic zones, which indicate high differential stresses on the order of a few GPa, occur in target rock fragments that experienced relatively low shock pressures of <15 GPa. These observations suggest that high shock pressures are not accompanied by high differential stresses. At attenuated shock pressure, differential stresses become effective to cause shear deformation. As a result of deformation during shock compression, the mean stress decreases with time and distance from the point of impact, whereas the deviatoric components can be expected to increase due to the high anisotropy of rocks and minerals. The extreme loading rates, however, inhibit effective crystal-plastic processes apart from mechanical twinning and localized dislocation glide. These findings might be important for calibrating shock effects and for the modelling of impact cratering.
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