Cenozoic Extension in Bulgaria and Northern Greece: the Northern Part of the Aegean Extensional Regime

Abstract The well-known Cenozoic Aegean extensional regime, initiated at c. 25 Ma, thinned the crust so that most of it now lies submerged. North of the western continuation of the North Anatolian Fault the Aegean extensional regime is present in central and southern Bulgaria, northern Greece, Former Yugoslavian Republic (FYR) of Macedonia and eastern Albania. Here the system is exposed on land and offers an opportunity to reconstruct the extensional evolution of the system. The southern Balkan peninsula forms the northern part of the Aegean extensional system; deformation is not as great as in the Aegean, but reconstruction of this part of the extensional regime will provide important constraints on its dynamics. Following a period of arc-normal extension associated with Late Eocene-Late Oligocene magmatism, major lithospheric extension appears have begun between 26 and 21 Ma in northern Greece, involving east-northeast-west-southwest extension east of Mount Olympos, on the Island of Thasos and near Kavala. This period of extension may have been accompanied by a short period of coeval compression north of the arc during Early Miocene time or perhaps a little earlier in the Thrace Basin of northwestern Turkey. Northeast-southwest directed Middle-Late Miocene extension appears to have developed obliquely to the older magmatic arc and migrated northward into southwestern Bulgaria in the Sandanski Graben (and perhaps also into the Mesta and Padesh Grabens) by 16.3–13.6 Ma, and in the Blagoevgrad and Djerman Grabens by c. 9 Ma. Extension in south-western Bulgaria was reorganized by c. 5 Ma and in northern Greece extension on the Strymon Valley detachment fault ended by c. 3.5 Ma, but extension continued on new fault systems. From limited structural and stratigraphic data, it is speculated here that related extension may have also occurred during this time in FYR Macedonia and eastern Albania. This northeast-southwest extension is interpreted to be related to trench roll-back along the northern part of the subduction boundary in the western Hellenides. North-south extension along east-west striking faults in central Bulgaria began only after extension was well underway in northern Greece and the Sandanski Graben of south-western Bulgaria. Within the Sofia Graben, the Sub-Balkan grabens, and grabens to their east, north-south extension began at c. 9 Ma, and may have begun about the same time in the Plovdiv, Zagore and Tundja Grabens of the northern Thracian Basin: north-south extension has continued to the present in these grabens. The cause of the north-south extension is unclear and may be related to trench roll-back along the central part of the subduction zone in the Hellenides, or more local causes of clockwise and counterclockwise rotation of the western Hellenides and western Turkey, respectively. By Late Pliocene time a major erosion surface, the sub-Quaternary surface, was developed over a large area of central Bulgaria creating a major unconformity that marks the beginning of Quaternary deposition in the basinal areas. Many large and small graben-bounding faults in west-central Bulgaria displace this erosion surface and demonstrate the widespread extent of Quaternary north-south extension. North-south extension extended westward, with probably decreasing magnitude, across the older northwest trending graben of southwest Bulgaria (the Simitli and Djerman Grabens) and into eastern FYR Macedonia. During latest Pliocene(? ) and Quaternary time, northern Greece developed a complex pattern of northeast-southwest extension associated with northeast to east-west striking right-lateral faults forming transfer faults between more local extensional areas. This system of faults overprinted the older northwest trending extensional faults, such as the Strymon Detachment, and may be related to the propagation of the right-lateral North Anatolian Fault into the north Aegean Sea and formation of parallel faults to its north. These two different tectonic regimes extend into FYR Macedonia, where a third regime of east-west extension in western FYR Macedonia and eastern Albania is present, and where extension may represent the continuation of the east-west extensional regime initiated in Middle-Late Miocene time. Active deformation determined from seismicity and Global Positioning System studies suggest northern Greece, and perhaps southwest Bulgaria and FYR Macedonia, is dominated by north-south extension. This pattern of deformation must have developed as recently as perhaps Late Quaternary time. Except for mountains near the Adriatic Sea all of the mountainous topography in the southern Balkan region may be the result of Miocene-Recent extension.