Episodic graben formation and extensional neotectonic regime in west Central Anatolia and the Isparta Angle: A case study in the Aksehir-Afyon Graben, Turkey

Abstract

Central and Western Anatolia form a continental back-arc region related to the Hellenic-Cyprus convergent plate boundary of the Anatolian and African Plates. The Aksehir-Afyon Graben (AAG), the easternmost extension of the west Anatolian horstgraben system, is located at the junction of Central Anatolia and eastern limb of the Isparta Angle. The AAG is 4-20 km wide and 90 km long. It trends west-northwest-east-southeast and is an actively growing rift containing two sedimentary infills of continental fluvio-lacustrine origin bounded on both sides by oblique-slip normal faults. The older infill is folded, thrust faulted and early Late Miocene in age. The younger infill, which is nearly horizontally bedded, is Plio-Quaternary in age and rests on the older infill with angular unconformity. The deformation of the older infill and the angular unconformity indicate a Late Miocene phase of compression, which separates two extensional periods. The second phase of extension has lasted since the Pliocene and is part of the current extensional neotectonic regime of both west Central Anatolia and the Isparta Angle, despite being previously reported as a compressional neotectonic regime. The graben-bounding Aksehir Fault Zone (AFZ) and the Karagoztepe Fault Zone display well-preserved fault surfaces and slickenlines. Although stereographic plots of the fault slip data show that the graben-bounding structures are oblique-slip normal faults, the AFZ has also been described as a single reverse fault. Both the field and seismic data, particularly the 1921 Argitham-Aksehir and 1946 Ilgin-Argitham earthquakes, indicate that the AAG is an active neotectonic structure. However, it can also be interpreted to lie in a seismic gap when its rate of seismicity is compared with that of the Gediz-Simav Graben forming its west-northwestern extension.