From Pure Extension to Transtension: Two-Stage Plio-Quaternary Kinematic Evolution of the Edremit and Bakırçay Basins, NW Turkey

Western Anatolia is a tectonically and seismically active region shaped by the complex interaction among the Eurasian, African, and Arabian plates. Since the Late Cenozoic, it has been dominated by an extensional tectonic regime. This study investigates the morphotectonic, seismotectonic, and kinematic evolution of the Edremit and Bakırçay basins and their surroundings, located at the intersection zone of the right-lateral strike-slip North Anatolian Fault System (NAFS) and the subduction-zone-driven Aegean Extensional System (AES). Within the scope of this research study, paleostress analyses were conducted using slip data obtained from fault planes, and these data were compared with 240 earthquake focal mechanism solutions reflecting the contemporary seismicity of the region. The findings indicate that both basins underwent a synchronized, two-stage tectonic evolution throughout the Plio-Quaternary period. During the first phase, encompassing the Pliocene, back-arc extensional forces related to slab rollback in the Hellenic subduction zone were dominant, and the main graben-forming processes (NW-SE-trending extension) took place. In the second phase, spanning the Quaternary period, an axis shift occurred as the southern branches of the NAFS reached the region, leading to the development of transtensional pull-apart basin mechanisms dominated by NE-SW-trending extension. Also supported by GPS velocity vectors and paleomagnetic block rotation data, this kinematic model demonstrates that the neotectonic evolution of Northwest Anatolia is governed not only by upper-crustal deformations but also by deep asthenospheric/lithospheric processes, such as slab rollback, lithospheric slab tear, and the NAFS-controlled westward differential escape of the Anatolian block.