High-Stress Crystal Plasticity of Calcite – Evidence from Ries Impact Breccias

Abstract: Twinned calcite occurs in calcite-bearing metagranite cataclasites within crystalline megablocks of the Ries impact structure, Germany, as well as in cores from the FBN1973 research drilling. The calcite likely originates from pre-impact veins within the Variscan metagranites and gneisses, while the cataclasis is due to the Miocene impact. Quartz in the metagranite components does not contain planar deformation features, indicating low shock pressures (< 7 GPa). Calcite, however, shows a high density (> 1/µm) of twins with widths < 100 nm. Different types of twins (e-, f- and r-twins) crosscutting each other can occur in one grain. Interaction of r- and f-twins results in a-type domains characterized by a misorientation angle of 35-40° and a misorientation axis parallel to an a-axis relative to the host. Such a-type domains have not been recorded from deformed rocks in nature before. The high twin density and activation of different twin systems in one grain require high differential stresses (on the order of 1 GPa). Twinning of calcite at high differential stresses is consistent with deformation during impact cratering at relatively low shock pressure conditions. The twinned calcite microstructure can serve as a valuable indicator of high differential stresses and sufficient confining pressure to prevent brittle deformation. The stress conditions at relatively low shock pressures (< 7 GPa) during impact-cratering are comparable to those at hypocentral depths during seismic rupturing in the continental crust. Therefore, comparable high-stress crystal plasticity of calcite might likewise be expected from fault rocks deformed at hypocentral depths.