LARGE SCALE CENOZOIC CRUSTAL EXTENSION IN WESTERN ANATOLIA EXTENDED TERRANE (WAET), TURKEY

Monazite radiometric age determinations conducted within the last ten years suggest that the north – northeast directed post-collisional Cenozoic extension in western Turkey was started in late Oligocene. The first stage of the extension was probably due to Orogenic Collapse. A major extensional shear zone, Southwestern Anatolian Shear Zone (SWASZ) developed during this stage. The Shear Zone separates the Western Anatolia Extended Terrane (WAET) from the Central Anatolia Extended Terrain where timing of the Cenozoic extension is younger and extension rates are probably less than the WAET. Cenozoic extension continued into Miocene and caused formation of E-W trending major Graben systems. The Alaşehir and Büyük Menderes Grabens are two main east-west trending extensional basins that formed during the second stage of the extension in the region. The two grabens join together to the east in the Denizli Graben area. The third stage of the extension occurred due to the westward movement of the Anatolian plate within the last 5 m.y. when the North Anatolian fault developed as a major strike-slip fault zone between the Anatolian plate and Eurasian plate. This caused a) formation of the high angle faults that bound the present flat topography of the Alaşehir and Büyük Menderes Grabens and b) N-S trending strike-slip faults that bound North trending basins of the northern Menderes Massif.

Several samples along the Alaşehir Detachment surface yielded Th-Pb monazite ages ranging from 35.8±3.0 Ma to 20.6±.4 Ma. This suggests that a Cenozoic metamorphism along the Detachment surface occurred as the exhumation of the Taurid Anatolide upper plate rocks (Taurid Carbonates and Ophiolites) continued. The upper plate rocks were eroded and deposited in the basins formed along the SWASZ. This exhumation is most likely caused by slab roll-back along the Hellenic Arc which caused large scale extension in the Aegean region in a back-arc setting.