Ankara Orogenic Phase, Its Age and Transition From Thrusting-dominated Palaeotectonic Period to the Strike-slip Neotectonic Period, Ankara (Turkey)

The Ankara section of the Izmir-Ankara-Erzincan Suture Zone (IAESZ) is characterized by two majorgroups of contractional structures, namely the Ankara forced folds-monoclines and the southward-verging foreland fold-imbricate thrust to reverse fault zone (AFITFZ). In the Ankara region, one of the areas where the various phases of deformation and related structures are particularly well-preserved and exposed is the Edige (Elmadag) area, 48 km east-southeast of the city of Ankara. The youngest palaeotectonic unit in the AFITFZ, and deformed in the last phase of the palaeotectonic period (the Ankara orogenic phase) is the Edige formation. It consists of a fluvio-lacustrine sedimentary sequence mainly comprising basal conglomerate, shale, marl and a clayey limestone alternation dated as Turolian (MN 11) (Late Tortonian–Early Pliocene), based on its rich mammalian fossil content. The Edige formation was deposited within an extensional tectonic regime characterized by an oblique-slip normalfaulting, in which the localized extension prevailed in approximately WNW–ESE direction. This first phase of extension was later replaced by a contractional tectonic regime and related successive and short-term phases of deformation such as folding, thrust to reverse faulting and finally strike-slip faulting which interrupted sedimentation of the Edige formation and deformed it. Both the stereographic plots of poles to bedding planes and the palaeostress analyses of slip-plane data recorded in the Edige formation and along its margin-boundary faults indicate that maximum horizontal stress operated in NW–SE direction during the phase of contraction. Finally, this last contractional tectonic regime (Ankara Orogenic Phase) was replaced by a strike-slip neotectonic regime. Thisis evidenced by an angular unconformity, which separates the underlying intensely deformed older rockassemblages including also the Turolian Edige formation from the overlying undeformed (nearly flat-lying) Edigekoru formation of Plio–Quaternary age. The strike-slip neotectonic regime in the Edige area initiated in Late Pliocene, has continued since that time under the control of active strike-slip faulting caused by a horizontal approximately N–S compression. This is indicated by three sets of approximately N–S strike-slip faults, namely the Elmadag, Kiliçlar and Hisarköy fault sets in the study area, which comprise northeastern continuations of the Elmadag, Balaban and Küredag strike-slip fault zones, their activity proved recently by both the 2005.07.31, Mw = 5.2 Bala earthquake, and local concentration of numerous small earthquakes epicentres along their northeastern extensions, i.e., along the Elmadag, Kiliçlar and Hisarköy fault sets.