Following the Eocene closure of the Neotethys in the region, the first phase of extension took place in late Oligocene about 30 Ma ago, when a north-dipping shear zone was formed possibly by the Orogenic Collapse of the thermally weakened continental crust. The shear zone becomes a high-angle oblique-slip fault zone along its northeastern termination and marks the eastern boundary of the Aegean Extended Terrain. It is called the Southwestern Anatolian Shear Zone. During this phase (1) a series of Oligocene extensional basins formed adjacent to the shear zone containing only carbonate and ophiolitic rock fragments, but no high grade metamorphic rock fragments, and (2) erosion and extensional unroofing brought high-grade metamorphic rocks of the Central Menderes massif to the surface by the early Miocene.
The second phase of the Cenozoic extension was triggered by slab roll-back and associated back-arc extension in early Miocene and produced the north-dipping Alaşehir, the south-dipping Büyük Menderes detachments of the central Menderes massif and the north-dipping Simav detachment of the northern Menderes massif. The detachments control the Miocene sedimentation in the Alaşehir, Büyük Menderes, and Simav grabens. The basins contain high-grade metamorphic rock fragments eroded from the central and northern Menderes massifs. Recent seismic reflection profiles indicate that the Alaşehir graben contains over 3,000 meters sediments deposited from early Miocene to recent. The profiles show well-developed roll-over structures indicating continuous extension and sedimentation in the Alaşehir graben.
The third phase of the extension was triggered by the lateral extrusion of the Anatolian plate when the North Anatolian fault was initiated at about 5 Ma. This extensional phase produced the high-angle faults in the Alaşehir, Büyük Menderes and Simav grabens and the high-angle faults controlling the Küçük Menderes graben.
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