Extensional v. contractional origin for the southern Menderes shear zone, SW Turkey: tectonic and metamorphic implications

Abstract

The southern Menderes Massif in southwest Turkey consists mainly of orthogneisses and overlying Palaeozoic–Middle Paleocene schists and marbles. The contact between the two distinct rock types is almost everywhere structural, herein named the southern Menderes shear zone: a S-facing, high-angle ductile shear zone that separates metamorphic rocks of differing grade. Although there is a consensus that the shear zone was associated with top-to-the-S–SSW shearing and is of Tertiary age, its origin and nature have been highly debated over the last decade. Some claim the contact is a thrust fault, while others have argued for an extensional shear zone. Integration of field and microstructural data (the identification of different fabrics, associated kinematics and overprinting relationships) with fission-track thermochronology and the P–T paths of the rocks above and below the shear zone, supports the conclusion that the southern Menderes shear zone is an extensional shear zone and not a thrust. The data are consistent with a model that the exhumation and cooling of the southern Menderes Massif occurred after a period of extensional deformation. Pervasive top-to-the-N–NNE high-temperature–medium-pressure ductile shear structures (D₂ deformation) overprint an early HP event (D₁ deformation). The subsequent top-to-the-S–SSW greenschist shear band foliation (D₃ deformation) developed mostly around the orthogniess–schist contact and forms the most characteristic features of the massif. The top-to-the-N–NNE structures are attributed to the main Alpine constructional deformation that developed during back-thrusting of the Lycian nappes during the latest Palaeogene collision between the Sakarya continent and the Anatolide-Tauride platform across the Neotethyan Ocean. The top-to-the-S–SSW structures are interpreted to be the result of the exhumation of the massif during the activity of the southern Menderes shear zone. The presence of these two distinct fabrics with differing kinematics suggests that the southern Menderes shear zone operated as a top-to-the-N–NNE thrust fault during early Alpine contractional deformation but was later reactivated with an opposite sense of movement (top-to-the-S–SSW) during subsequent Oligocene–Miocene extensional collapse.