WHEN AND WHY DID THE EXTENSION START IN THE AEGEAN?

A controversial topic is the cause and age of the onset of north-south extension in the Aegean region. Both gravitational spreading and roll-back of the subduction zone have been suggested for the origin of extension. Estimates for the inception of north-south extension range from Late Oligocene to Late Miocene. New data from a metamorphic core complex from northwest Turkey indicate that north-south extension in the northern Aegean region started in the latest Oligocene age and was related to the subduction roll-back. The metamorphic core complex crops out in the Kazdag mountain range in the Biga Peninsula in northwest Turkey. The footwall of the core complex consists of gneiss, amphibolite and marble metamorphosed at 5 ± 1 kbar and 640 ± 50 °C. The average muscovite and biotite Rb/Sr ages from the gneisses are 19 Ma and 22 Ma, respectively, and imply high temperature metamorphism during the latest Oligocene. The hanging wall is made up of an unmetamorphosed Upper Cretaceous oceanic accretionary melange with Senonian eclogite lenses. The hanging-wall and footwall are separated by an extensional ductile shear zone, made up of mylonites of gneissic protolith, two-kilometer thick. Mylonites and underlying high-grade metamorphic rocks show a north-trending mineral lineation with the structural fabrics indicating down-dip, top-to-the-north shear sense. The shear zone, the accretionary melange and the high-grade metamorphic rocks are cut by an undeformed granitoid with a 21 Ma Rb/Sr biotite age, analytically indistinguishable from the Rb/Sr biotite ages in the surrounding footwall gneisses. The estimated pressure of the metamorphism, and that of the granitoid emplacement indicate that the metamorphic rocks were rapidly exhumed at ~ 24 Ma from a depth of ~14 km to ~7 km by activity along the shear zone. The metamorphic rocks of the Kazdag range are surrounded by voluminous calc-alkaline volcanic and plutonic rocks of Late Oligocene-Early Miocene age, which formed above the northward-dipping Hellenic subduction zone. The magmatic arc setting of the core complex and stratigraphic evidence for subdued topography in northwest Turkey prior to the onset of extension suggest that the latest Oligocene regional extension was primarily related to the roll-back of the subduction zone rather than to the gravitational collapse.