We determine the active deformation mechanisms in two active carbonate fault zones in Greece. Detailed slip-system analyses in combination with high-resolution nanostructural investigations show a throughout contribution of crystal plasticity, evident from subgrain-wall misorientations. Post-deformational static recrystallisation and annealing reduces the grain size and offers an alternative mechanism of nanograin formation. The decrease in grain size results in a grain-boundary strengthening effect and slip localisation which can be observed over several orders of length scales. The data publication contains photographs, electron backscatter diffraction (EBSD) data, transmission electron microscope (TEM) images and automated crystal-orientation mapping (ACOM)-TEM data organised in seven subfolder named Figure x, where x is the number of the figure referring to the full publication. The grain-size distributions obtained from the crystal-orientation mapping techniques are provided as tab-separated files. We provide the raw and binned grain-size distributions together with the calculated, relative frequency for the fractal dimension plot. We also provide one EBSD map of the Arkitsa fault exposure as .cpr and .crc file. MATLAB MTEX scripts for obtaining the EBSD map plots, pole figures and the raw grain-size distribution are provided. Thin sections were prepared from drill cores that were cut parallel to the slip direction and normal to the slip surface. Thin sections were polished with 1 µm and 0.3 µm diamond suspensions and finished with 0.025 µm colloidal silica. To reduce charging during EBSD data acquisition, the thin sections were carbon coated. EBSD maps were acquired with a Philips XL30 field emission scanning electron microscope equipped with an Oxford instruments Nordlys 2 CCD camera and Aztec software at 30 kV accelerating voltage, 9.5 nA probe current, and step size of 0.5 µm for the Arkitsa sample and 20 kV accelerating voltage, 9.5 nA probe current, 0.7 µm step size for the Schinos sample. Nanoscale imaging was carried out with a FEI Talos F200X at 200 kV acceleration voltage and 5 – 10 nA beam current. ACOM-TEM data were acquired on electron-transparent foils prepared with a FEI Helios G3 Nanolab UC focussed ion-beam scanning electron microscope (FIB-SEM). Nanoscale map acquisition was executed with a dedicated NanoMEGAS ASTAR/SPINSTAR system on a FEI Tecnai G2-20 twin at the Unité Matériaux et Transformation in Lille, France. Beam conditions during ACOM for the nanoscale map acquisition were 200 kV accelerating voltage and spot size 11, giving a 1-nm probe diameter, which results in a minimum step size of 2 nm. contact: m.ohl@uu.nl
