We report on an extensional ductile shear zone in central Crete at the contact area between the high-pressure low-temperature (HP-LT) metamorphic Phyllite-Quartzite unit sensu stricto (PQ s.str.) and the Talea Ori group (Plattenkalk unit). Mapping and microscopic analysis along the 20 km long contact reveal extensional shear bands, shear band cleavages (C′-type) and associated quartz veins in both units, occurring over a width of up to a kilometer. The shear offset along the shear bands is systematically perpendicular to the contact with the hanging block (PQ s.str.) being downfaulted. Abundant discordant quartz veins are associated to shear band boundaries, asymmetric boudinage and foliation boudinage, forming m-wide vein networks. These mesoscopic deformation structures together with related microstructures such as strain shadows, growth rims of albite porphyroblasts and stylolites indicate dissolution-precipitation creep as main deformation mechanism accompanied by vein formation. Temperatures during deformation are indicated to be close to peak metamorphic temperatures (≥ 300-350 °C) by the growth rims of albite porphyroblasts and micas present in the shear band cleavages, which are consistent with quartz vein microstructures showing subgrains and sutured high angle grain boundaries. Peak metamorphic temperatures inferred by the degree of graphitization of carbonaceous material are similar in the hanging wall and the footwall of the shear zone, which is consistent with fast and nearly adiabatic exhumation. Our study demonstrates the importance of ductile shear zones with high strain rate dissolution-precipitation creep and vein formation in HP-LT metamorphic rocks for the early exhumation history.

No abstract

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/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 relative to the host with a misorientation angle of 35°–40° 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.

No abstract