Late Cenozoic crustal extension, basin formation and volcanism in west Turkey.

The NE - SW trending G?rdes basin developed in the early Miocene as indicated by palynological data and K-Ar dating of igneous rocks. In the basin centre rhyolitic and dacitic volcanic domes cut the Neogene sedimentary filling and ophiolitic basement of the Izmir-Ankara suture zone. Geochronological data and field relationships demonstrate that compressional movement on this suture zone had ceased, at least, before the latest Oligocene. The palynological data from the E-W trending Alasehir and Buyuk Menderes graben fillings indicate that these major extensional structures started to develop during middle Burdigalian-middle Serravallian. Consequently, it is concluded that N-S extensional tectonics in west Turkey commenced in the early Miocene. It is evident from the age determinations of the G?rdes basin and the Alasehir and Buyuk Menderes grabens that the E-W and NE-SW trending grabens / basins developed coevally under a N-S extensional regime during the early Miocene in west Turkey. This joint initiation of basins is inconsistent with both the cross graben and the multi-phased extensional models. In west Turkey, the latest Oligocene-early Miocene calcalkaline intermediate and acid volcanism with an inherited subduction-related geochemical signature occurred under the N-S extensional regime. Due to the continuation of extension after the late Miocene the thickness of the lithosphere would be reduced and consequently alkaline volcanism became dominant in this advanced stage of extension (late Miocene and younger). This change in nature of volcanism is not related to a change of regional tectonic regime as previously thought. It is apparent from the above findings that the tectonic escape and back-arc spreading models cannot explain the commencement of extensional regime due to the younger triggering events, the collision of Eurasia / Arabia in Langhian-Serravallian and inception of the roll-back process in late Serravallian-early Tortonian. Alternatively it is proposed that N-S extensional tectonics is related to spreading and thinning of the crust which commenced in the early Miocene. This occurred immediately after the cessation of compressional regime due to the high thermal profile of the crust in west Turkey.