Postcollisional extensional tectonics and exhumation of the Menderes massif in the Western Anatolia extended terrane, Turkey

The Western Anatolia extended terrane in Turkey is bounded by the North Anatolian fault zone to the north, the Lycian nappes to the south. It contains the Menderes massif, one of the post-collisional Alpine metamorphic core complexes. Field data and available radiometric ages suggest that the north-directed Cenozoic extension in the terrane is the product of three consecutive, uninterrupted stages, and that it is still continuing today. The first stage was initiated in the Late Oligocene along a north-dipping extensional simple-shear zone with a listric geometry at depth. The shear zone is named here as the Southwest Anatolian shear zone and marks the southern and southwestern boundary of the Western Anatolia extended terrane. Evidence for the presence of this shear zone includes (1) the dominant top to the north-northeast shear sense indicators in the Menderes massif and (2) a series of Oligocene extensional basins located adjacent to the shear zone that contain carbonate and ophiolitic rock clasts, but no high-grade metamorphic rock fragments. During this stage, erosion and extensional unroofing brought high-grade metamorphic rocks of the Central Menderes massif to the surface by the early Miocene. The second stage of extension produced the north-dipping Alasehir and the south-dipping Büyük Menderes detachment surfaces in the early Miocene. The detachments control the Miocene sedimentation in the Alasehir and Büyük Menderes grabens, containing high-grade metamorphic rock fragments that were already at the surface in the Central Menderes massif in the early Miocene. The third stage of extension may have started ca. 5 Ma, when the North Anatolian fault was initiated. This extensional phase produced faults within the Alasehir and Büyük Menderes grabens and possibly the Kucuk Menderes graben.