THE SOUTHERN BALKAN CENOZOIC EXTENSIONAL REGION AND ITS RELATION TO EXTENSION IN THE AEGEAN REALM

The southern Balkan Cenozoic extensional region (SBCER) lies north of the North Anatolian fault zone and its northern boundary is a poorly established zone through the central Balkan area. Unlike Cenozoic extension within the Aegean area, the extensional structures are all on land. The earliest extension is middle or late Eocene in SW Bulgaria, eastern Macedonia, and possibly eastern Rhodopes, central Bulgaria. This early period of NE-SW to E-W extension ended in ~late Oligocene or Early Miocene and consisted of E-tilted half-grabens and detachment faults. This extension we interpret to be related to trench rollback at the Hellenic trench and possible lateral spreading of a hot, thick crust within an Eocene volcanic arc. A short period of local compression in Early to Middle Miocene we relate to arrival of Kruja continental fragment in subduction zone in central Albania. During the second period of extension, Middle Miocene to the present, the pattern of extensional faulting was complex with continued NE-SW extension in SW Bulgaria, and E Macedonia, but changed to N-S extension in central Bulgaria and E Macedonia as roll back along the S Hellenic trench became dominant. E-W extension migrated westward in W Macedonia and E Albania due to roll back on the N Hellenic trench. The strain pattern continued to evolve when the North Anatolian fault propagated into the N Aegean at ~ 6 Ma. At that time the N-S extension in Bulgaria and E Macedonia was controlled by the weak southward pull of Balkan lithosphere as the Aegean was dominated by rapid southward motion as it has today. A narrow zone of convergent activity in coastal Albania changes to E-W extension in E Albania and W Macedonia related to continued trench roll back. The strain history of the SBCER is a complex spatial and temporal interplay of different extensional mechanisms.