Metamorphic and age constraints on the Alakeçi shear zone: Implications for the extensional exhumation history of the northern Kazdağ Massif, NW Turkey

The Kazdağ Massif exposes an extensional metamorphic dome in the Biga Peninsula of northwest Turkey, bounded on both flanks by detachments and/or shear zones. The northern flank is marked by the extensional Alakeçi shear zone (ASZ), with poorly known P–T–t path. We therefore focus on metamorphic conditions and temporal history of the ASZ to determine its tectono-metamorphic evolution. ASZ mylonites were derived from both the footwall Kazdağ Massif core metamorphic rocks and the hanging wall Çetmi mélange lithologies. The mylonitic fabrics in the ASZ depict a top-to-the NNE shearing, parallel to the NNE-plunging stretching lineation and NNW-dipping mylonitic foliation. This geometry implies normal sense movement i.e. north-side down-dip extensional displacement along this flank of the Kazdağ Massif. The northward transition from ductile to brittle–ductile shear regime through the ASZ shows that the non-coaxial deformation occurred at decreasing temperatures and degree of metamorphism. The paragenesis in equilibrium of the mylonitic gneisses and schists contains quartz + feldspar + muscovite + biotite + garnet ±staurolite, with late retrogressive chlorite after biotite and garnet. Calculated thermo-barometric conditions for ASZ rocks yielded pressures between 6.9 and 5.7 kbar and temperatures between 706 and 587 °C, which constrain the peak metamorphism. The mylonitic rocks supplied isochron 39Ar/ 40Ar mica ages between 26.97 and 24.19 Ma, which we interpret to date the cooling of the mylonites following the P–T decrease across the ASZ. The metamorphic data support the structural record and strengthen the extensional character of the ASZ. The transition from sillimanite-bearing footwall Kazdağ Massif core gneisses in the deeper structural levels to chlorite schists towards the shallow structural levels at the top of the ASZ, is consistent with an extensional exhumation of the metamorphic pile. These data allow us to determine the peak P–T conditions and the temporal evolution in the northern flank of the Kazdağ Massif, where Late Oligocene extensional exhumation was assisted by NNE-directed ductile–brittle ASZ, which had operated from amphibolite to greenschist facies. The ASZ, together with similar time-constrained ductile–brittle activity of SW-directed detachment along the southern flank of the massif, collectively indicates a bivergent mode of extension in the western Kazdağ Massif. The latter is likely influenced by magmatic activity spatially and tempo