South Menderes Monocline: Low-temperature thermochronology constrains role of crustal extension in structural evolution of southwest Turkey

We report apatite and zircon fission-track data across the contact zones between the Menderes nappes, the Cycladic blueschist unit, and the Ören nappe, in the Anatolide belt of southwest Turkey. These data resolve previous debate on the deformation history of these Cretaceous to Eocene nappe contacts, including whether they were reactivated during late Oligocene to Miocene crustal extension. Apatite fission-track ages range from 18 to 28 Ma in the Menderes nappes, and 31 to 42 Ma in the Ören nappe. Zircon fission-track ages are 29 to 31 Ma in the Menderes nappes, 30 to 33 Ma in the Cycladic blueschist unit, and between 93 Ma and 129 Ma in the Ören nappe. The data reveal that the tectonic contacts within the Menderes nappes, and with the overlying Cycladic blueschist unit have been sealed since ~ 35-30 Ma. In the Ören nappe, zircon fission-track ages are distinctly older (with mixed single grain ages between ~ 300 and 75 Ma) reflecting partial to full resetting during late Cretaceous high-pressure metamorphism. The contact between the Cycladic blueschist unit and the Ören nappe shows no significant tectonic movement in the brittle crust after ~ 70 Ma. These data reveal that the nappe boundaries in the southern Menderes Massif have undergone no significant differential offset relative to the Earth's surface since the Oligocene and therefore were not reactivated during late Oligocene to Miocene continental extension. We interpret the steeply dipping portion of the nappe pile in the southern Menderes Massif as a tilted crustal section, which we name the 'South Menderes Monocline'. We speculate this monocline formed by differential uplift in the Miocene, either at the hinge zone of a plateau, or by unloading of the Simav detachment footwall.