Abstract

The central Menderes massif is characterized by an overall dome-shaped foliation pattern and a north-northeast–trending stretching lineation. The asymmetry of shear bands and quartz c-axis fabrics on either side of the structural dome demonstrate a top to the north-northeast shear sense in the northern part and a top to the south-southwest shear sense in the southern part of the submassif, i.e., a bivergent downdip movement. This suggests a symmetric collapse of the Alpine Menderes orogenic belt along two extensional shear zones. Conjugate shear bands and symmetric quartz c-axis fabrics in the east-trending transition zone demonstrate a coaxial deformation between the two extension domains. Bivergent extension in the Menderes massif is in contrast to asymmetric extension in the Aegean Sea. Here, the still-active Hellenic subduction zone evolved from an advancing plate boundary associated with crustal thickening into a retreating plate boundary in Oligocene-Miocene time. Southward rollback of the subducting plate during continuous northward subduction allowed asymmetric top to the north-northeast extension in the back-arc region during the exhumation of the Cycladic core complexes. In western Turkey, the arrival of the thick continental crust of the Menderes massif halted subduction and probably caused the symmetric collapse of the massif because the high potential energy of the thickened crust was no longer supported by subduction.

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