Shock effects in target rocks and impact breccias from the Rochechouart impact structure, France
Trepmann, Claudia A.
The microstructures in crystalline target rocks provide information on the shock wave-associated stress conditions during meteorite impact cratering. Target rock fragments with quartz microstructures indicating localized transformation into a high dense amorphous phase at high shock pressures (high mean stress) on the order of 20-35 GPa 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 observationsreveal that high shock pressures are apparently not accompaniedby high differential stresses. Only at attenuated shock pressure, differential stresses become effective to cause shear deformation. The extreme loading rates, however, inhibit effective crystal-plasticprocesses apart from mechanical twinning and localized dislocation glide.