A case study of compression to escape tectonic transition: Tectonic evolution of the Nallıhan Wedge and comparison with the Tercan Wedge (Eastern Mediterranean, Turkey)

Abstract

The Nallıhan and Çayırhan regions in western Anatolia host the stratigraphically and tectonically important features of Nallıhan Wedge. The stratigraphic relationships and structural architecture of the wedge are not well understood, which are essential for better understanding of the timing and tectonic evolution of western Anatolia. Today, the Nallıhan Wedge is bounded by the North Anatolian Fault and the Thrace-Eskişehir Fault in the northwest and southwest, respectively. The existence of Dağküplü Mélange within the wedge represents the closure of the İzmir-Ankara Ocean. The occurrence of the Lower Jurassic shelf, slope, reef and deep sea successions transitioning upward into the Cretaceous shelf and slope successions represent the opening and closing cycles of the Intra-Pontide Ocean. The tight and asymmetric folds (steep in the west, gentle in the east) within this stratigraphic sequence and the development of the Eocene units reflect the closure of the Intra-Pontide and İzmir-Ankara oceans. The geometric structure and evolution of the Nallıhan Wedge is directly related to the final continental collision. W-E and SW-NE striking large-scale oblique thrusts also exhibit notable strike-slip components within the study area. The principal direction of the fold axes are W-E and WNW-ESE oriented for the Eocene and Neogene units. During the Eocene, the depositional environment of the study area was a ramp basin mainly dominated by a braided river, when a portion of the Intra-Pontide Ocean was situated on the northern side of the study area. The compressional tectonic regime continued to the end of the Oligocene, which is followed by a new tectonic regime during the Miocene, when the triangular shape region between the Thrace-Eskişehir Fault and the North Anatolian Fault developed as an intermontane basin. The evolution of this basin can be compared with the Tercan Wedge, bordered by the North Eastern Anatolian Fault and the North Anatolian Fault.In the Nallıhan and Çayırhan regions the N-S-trending compressional regime started during the Upper Cretaceous and continued until the final continental collisional during the Upper Eocene. The compressional structures in the area became reactivated during the post-collisional escape tectonic phase in western Anatolia, leading to the development of the prominent Isparta Angle during the Miocene. During this period, the Thrace-Eskişehir Fault and Burdur-Fethiye Shear Zone also formed as intracontinental transform fault zones in western Anatolia. The Nallıhan Wedge started to form in a wedge-shaped area between the Thrace-Eskişehir Fault and the zone of strike-slip faults with thrust component that later developed into the North Anatolian Fault. Thus, Eocene and pre-Eocene structures obtain strike-slip component by an inversion during the Miocene. During and post Pliocene, the Thrace-Eskişehir Fault lost its tectonic activity as the North Anatolian Fault propagated westward into the region previously controlled by the Thrace-Eskişehir Fault. During this interval the Nallıhan Wedge entered a new stage of deformation, which is also observed today.