TIMING, CAUSE AND ORIGIN OF CURRENT N-S EXTENSIONAL TECTONICS IN WESTERN TURKEY

Western Turkey is one of the most spectacular regions of the world where continental lithosphere has been stretched following crustal thickening due to orogenic contraction. It has been experiencing N-S-directed extension since latest Oligocene-Early Miocene and is currently under the influence of forces exerted by subduction of the African Plate beneath the southern margin of Anatolian Plate along the Aegean-Cyprus Subduction Zone (ACSZ) and the dextral slip on the North Anatolian Fault Zone (NAFZ). Menderes Massif (MM) and E-W-trending grabens are the most prominent features of the region. The extension in the region has expressed itself in two distinct structural styles: rapid exhumation of the MM under presently low-angle ductile-brittle detachment faults and late stretching of crust and consequent E-W graben formation along high-angle normal faults. It is recently claimed that the change in style of extension from core-complex formation to the graben system represents a single continuous event from latest Oligocene-Early Miocene to Quaternary times. This paper reports the results of recent geological mapping and structural analysis along the southern margin of the Gediz graben on the relationship between the detachment and the graben-bounding normal faults where the former is cut and displaced by the latter in a fault-controlled step-like morphology. The total amount of displacement exceeds 1.0 km. The evolution of the Gediz graben is therefore episodic and involves two distinct stages (core-complex mode, then rift mode), separated by a short-time gap as supported by a regional unconformity between the Lower Miocene and Pliocene-Pleistocene sediments. The first event is attributed to the Early Miocene orogenic collapse while the second phase commenced by Pliocene (~5 Ma) as the result of combined effect of the initiation of dextral motion along the NAFZ and the southward pull of the Anatolian Plate along ACSZ. We argue that (1) the usage of Miocene sediments as simple graben fill to determine the timing of graben formation is therefore misleading and any viable work should distinguish sedimentation associated with detachment faulting from that is linked to younger grabens; and (2) the term neotectonic extension is valid to explain the formation of modern grabens in Western Turkey.