Western Turkey is located on the eastern side of highly extended Aegean region and contains N and E-W trending basins. Timing of initiation of N-S extensional tectonics and related basin formation is subject of current debate. One group of researchers suggests that the N-S extension began in the Late Miocene and is related to the tectonic escape model. Another group proposes that the N-S extension was initiated in Latest Oligocene - Early Miocene and is related to orogenic collapse or back arc spreading/subduction roll-back. Geochemical evolution of the volcanic rocks in the region is interpreted differently by the two groups to fit the two hypotheses.

The Early - Middle Miocene stratigraphy of N-trending basins is well constrained by isotopic dating of associated volcanic rocks and palynological analyses in the Gordes, Selendi and Usak-Gure basins. It is generally accepted that E-W trending Alasehir and Buyuk Menderes grabens are the outcome of N-S extensional tectonics. Therefore, the age of the sedimentary rocks in the two grabens and their tectonic development is particularly important. The two grabens contain four sedimentary packages. The first two sedimentary packages are dated as Early - Middle Miocene by palynological and magnetostratigraphical methods. A fault system containing three faults controlled the deposition of the four sedimentary packages in the two grabens. The system was developed similar to flexural rotation/rolling hinge models. These data, together with Latest Oligocene isotopic dating of syn-extensional intrusions throughout the Menderes massif, clearly places the commencement of E-W trending grabens and N-S extensional tectonics to the Latest Oligocene Early Miocene.