Patterns of Cenozoic Extensional Tectonism in the South Balkan Extensional System

Abstract

The present day tectonic pattern of the Balkan Peninsula has evolved from three major periods of mainly extensional tectonism: (i) Paleogene extension that is difficult to characterize because its relation to the closing of the Vardar Ocean remains uncertain; (ii) Early to Late Miocene extension related to rollback at the Hellenic subduction zone; and (iii) Late Miocene to Recent extension related to continued trench rollback, but modified by the development of the North Anatolian fault zone and the rapid movement of the Aegean crust to the SSW. Paleogene extension occurred during and shortly after the time final closure of the Vardar Ocean. The timing of Vardar closing remains uncertain, but is between the early and late Eocene and may be diachronous. Extension occurs from eastern Macedonia through western Bulgaria and northern Greece to northwestern Turkey and lies within a region of thickened crust that may be at least partly undergoing shortening contemporaneous with the earliest stages of extension. However, the exact timing relations between extension and shortening remains unclear, and some evidence suggest they are locally contemporaneous. The tectonic setting appears to be one of transition from subduction related convergence and shortening to subduction rollback convergence with extension. The interpretations of the dynamics for the Paleogene extension are currently controversial. By Early or Middle Miocene time, extension within the Balkans was characterized by WSW-ESE extension in western Bulgaria and eastern Macedonia. The NNW striking normal faults were part of an evolving extensional system that was part of the regional Aegean realm and can be related to trench rollback and rotation of large crustal fragments. Short periods of Early or Middle Miocene local compression occur in western Bulgaria and eastern Macedonia that may be related to the time when small, but thick crustal fragments entered the northern Hellenic subduction zone in Albania. Three belts of folds are present in the Thrace basin and are probably related to local transpression along right-lateral strike-slip faults that may be precursors to formation of the North Antatolian fault zone. The age of this folding is poorly constrained and is bracketed only between earliest Miocene and latest Miocene. Beginning in Late Miocene time the pattern of extension within the Balkans progressively evolved related to complex events in the rollback of the Hellenic subducted slab both to the south and to the west and with the initiation of right-lateral strike slip along the southern branch of the North Anatolian fault zone. E-W extension migrated progressively from western Bulgaria into central Macedonia and Albania, followed to the east by N-S extension that migrated from east to west across central and western Bulgaria into eastern Macedonia. This pattern of faulting dominates the present tectonic pattern in the Balkan Peninsula. The rapid Pliocene development of the northern branch of the North Anatolian fault zone partially decoupled the rapid SSW movement of an Aegean plate from slow N-S extension that increased in magnitude from central Bulgaria and Macedonia into northern Greece. The present active pattern of tectonism is characterized by a narrow zone of shortening east of the northwest part of the Hellenic trench, that rapidly changes to E-W extension in central Albania and western Macedonia. GPS data and only rare geological data suggest the transitional zone may be marked by distributed right-lateral strike-slip. Slow N-S extension dominates eastern Macedonia and Bulgaria with associated local strike-slip faulting. Although weakly developed, the seismicity generally supports this pattern of tectonism. Slip-rates on faults probably increases southward into northern Greece, a testable hypothesis, based on GPS data that show increasing southward velocity relative to Eurasia. GPS data shows the abrupt change to rapid strike-slip along the North Anatolian fault. The slow N-S extension in the southern Balkan Peninsula may be related to the southward pull of the Aegean plate, with possible slow counterclockwise rotation of the western part of the North Anatolian fault zone.