The timing of the transition from compressional to extensional tectonics in the late Cenozoic evolution of west Turkey has been constrained by K-Ar geochronology from acidic volcanic rocks and tourmaline leucogranite dykes in the central and east margin respectively of the Gördes basin.

Dacites and rhyolites in the centre of the Gördes Neogene sedimentary basin cut both the basement ophiolites of the Izmir-Ankara suture zone and the Neogene sediments. On the basin’s eastern margin the leucogranites cut metamorphic basement along a major NE-SW-trending normal fault that controls the regional structure of the basin. Pebbles of these leucogranites occur in adjacent Neogene turfites and conglomerates.

K-Ar dates on biotites from the central volcanic rocks vary from 18.4 ± 0.8 Ma to 16.3 + 0.5 Ma (early Miocene) whilst muscovite from a leucogranite on the eastern margin of the basin provides ages of 24.2 ± 0.8 Ma and 21.1 ± 1.1 Ma (latest Oligocene to early Miocene).

Geochronological data and field relationships demonstrate that the earlier compressional regime had been replaced by extensional tectonics by latest Oligocene-early Miocene.

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