Three-dimensional kinematic analysis of Northern Kapıdağ Pluton: Implications for a transtensional deformation in NW Anatolia 

Abstract

The Kapıdağ Shear Zone (KSZ) is located in the Kapıdağ Peninsula (NW Anatolia) and syn-kinematically intruded by Northern Kapıdağ Pluton (NKP) along the northern coastline of the peninsula. The NKP displays a granodioritic composition with a discernible progressive deformation from south to north. The southern part is characterized by an isotropic granodiorite with no trace of deformation. Towards the north, it gradually passes into a deformed granodiorite in which the development of ductile and brittle structures is widely observed. To comprehend the nature and origin of deformation within the KSZ, a thorough analysis of micro- and mesostructural features was undertaken, accompanied by a three-dimensional kinematic analysis of the NKP. The NKP exhibits a well-defined mylonitic foliation and stretching lineation, characterized by the shape-preferred alignment of feldspar, quartz, and biotite crystals. Various shear sense indicators, including S-C fabrics and "σ"-type rotated porphyroclasts, are extensively distributed throughout the NKP, pointing to a dextral sense of shear. Microstructures such as chessboard extinction and Grain-Boundary Migrations (GBM) in quartz, myrmekitic textures, and flame pertites in feldspar, as well as sub-grain rotations and bulging recrystallization of quartz, along with the presence of micro-faults and cracks collectively suggest continuous deformation of the NKP from temperatures starting at 600°C to those below 250°C. Three-dimensional strain analysis was conducted on the Northern Kapıdağ Pluton (NKP) using quartz crystals as shear sense indicators, and various parameters including kinematic vorticity (Wk) numbers, Flinn k values, Lode’s ratio, and octahedral shear strains were computed. The outcomes reveal a range of Flinn k values from 1.1 to 5.32. On Flinn’s diagram, the majority of samples plot above the k=1 line, indicative of a transtensional regime. Lode’s ratios exhibit a variation from -0.64 to +0.13, with the Hsu diagram showing that the majority of samples fall within the general constrictional field. To discern the strain component of the NKP, kinematic vorticity numbers (Wk) were determined, ranging from 0.73 to 0.99. This suggests a dominance of simple shear in the deformation rather than pure shear components. The U-Pb zircon and 40Ar/39Ar biotite dating results show that this deformation has developed between 48-36 Ma.  In summary, both micro/mesostructural data and three-dimensional strain analyses of the NKP collectively suggest that the Kapıdağ Shear Zone (KSZ) is characterized by a dextral transtensional shear zone dominated by simple shear. We hypothesize that the KSZ was likely formed during the Eocene period as a consequence of strain localization along the break-off of the Tethyan oceanic slab.