Cenozoic pure-shear collapse of the southern menderes massif, Turkey

A transition from construction to collapse of the Alpine orogen in Middle to Late Eocene time is recorded in the structural fabrics of gneissic rocks in the southern Menderes Massif, western Turkey. Augen gneiss in the southern Menderes Massif indicates a spatial switch of shear sense along a north-south section through the complex (top-to-north shear in the north and top-to-south in the south). This 'kinematic hinge' corresponds to a broad zone of dominant pure shear where crust underwent vertical thinning and approximate north-south extension. During this extension, foliation became corrugated with fold axes parallel to lineation, indicating inflow from neighbouring crust to the east and the west. Published geochronologic work suggests a Middle to Late Eocene age for this dominant fabric. Thermal weakening of felsic Menderes crust beneath Alpine nappes may have triggeredthis orogenic collapse. Deformation fabrics developed at temperatures of c. 550°C, and progressive cooling during collapse generated strain localization within a kilometre-thickshear zone at the southern margin of the Menderes Massif. In the southern Menderes, the corrugation of foliation indicates that Middle to Late Eocene flow was constricted into anorogen-perpendicular direction. This behaviour is similar to the collapse and orogen-parallel extension of the European Alps (Lepontine dome, Tauern Window).