An Eocene transtensional shear zone driven by the strain localization along slab break-off: Implications from the sheared syn-kinematic northern Kapıdağ Pluton (NW Anatolia, Turkey)

One of the Eocene plutons in NW Anatolia, the Northern Kapıdağ Pluton (NKP) was syn-kinematically intruded into the Kapıdağ Shear Zone (KSZ). The NKP is granodioritic in composition and displays progressive deformation from south to north. The southern part of the NKP is composed of an isotropic granodiorite without any trace of deformation. Towards the north, it gradually passes into a deformed granodiorite in which the development of ductile and brittle structures is clearly observed. The NKP shows a well-developed mylonitic foliation and stretching lineation and several shear sense indicators such as the S-C fabrics and “σ”-type rotated porphyroclasts which indicate a dextral sense of shear. Microstructural investigations suggest that the NKP underwent continuous deformation at temperatures from 600 °C to below 250 °C. The field, structural and geochronological data collectively indicate that the KSZ was active during the early Eocene. The EBSD and three-dimensional strain analysis results of NKP show that the samples display crystal-preferred orientation and Flinn k values change between 0.92 and 5.32 indicating a transtensional regime. Lode's ratios of the samples vary from −0.64 to + 0.13 and are plotted in the general constrictional field in the Hsu diagram. The kinematic vorticity numbers (Wk) of the samples change between 0.73 and 0.99 indicating a simple shear domination. Collectively, all of these data indicate that the KSZ is a simple shear-dominated dextral transtensional shear zone. We infer that KSZ was most likely developed as a result of strain localization along the break-off of the Tethyan oceanic slab during the Eocene.