Twinned calcite as an indicator of high differential stresses and low shock pressure conditions during impact cratering

Shock-related calcite twins are characterized 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/mu m) of twins with widths <100 nm. Different types of twins (e-, f-, and rtwins) crosscutting each other can occur in one grain. Interaction of r- and f-twins results in a-type domains characterized by a misorientation relative to the host with a misorientation angle of 35 degrees-40 degrees and a misorientation axis parallel to an a-axis. 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 low shock barometer.