ÇAĞRILI KONUŞMALAR / KEYNOTES Invited Speeches ... - TPJD

The Neogene Dynamics of the Balkan Fold-Thrust Belt. Possible Effect on the
Hydrocarbon Accumulations
Radoslav Nakov
Geological Institute, Bulgarian Academy of Sciences, Sofia, Bulgaria
The Alpine geodynamics of the eastern Balkan Peninsula is determined by several episodes
of extension and compression. After the final closing of the Vardar Ocean during the Late
Cretaceous-Early Paleogene the main episode of compression and deformation in the Balkan FTB
occurred in Middle Eocene time, creating a well expressed fold-thrust pattern. At this time the
belt was overthrusted to the north on the Foreland (Moesian Platform). After this main episode
of compression, excepting local thrusting in its easternmost part during the Oligocene, the
frontal part of the belt was thought to be inactive. Since Late Eocene extension is the main form
of deformation. The extension is spread between the Aegean subduction system to the south
and the Moesian Platform (the southern edge of the Balkan Range) to the north. During the
Neogene, extension has created numerous extensional continental basins (grabens) to the south
of the Balkan Range. They occurred in several well distinguished consecutive stages of extension.
Instead, to the north of the range was created the marine (Miocene) to brakish (Pliocene) basin
which western part belonged to the Central Parathetys (Fore-Carpathian basin) and the eastern
to the Eastern Parathetys (Euxinian) basin.
Recently acquired data clearly show that at least the bordering fault of the western part of the
Balkan fold-thrust belt was an active compressional structure during the Miocene, overthrusting
the Neogene Fore-Carpathian Basin. Here the frontier zone between the Western Fore-
Balkan Zone and the Moesian Platform exhibits 4 episodes of deformation established in the
Maastrihtian sediments. The bordering thrust fault with the Platform (Fore-Balkan Fault), based
on field evidences was still a compressional structure during the Upper Volhynian – Lower
Bessarabian (10-12 m.y. BP). This compression is coeval to the early stages of extension in the
Southern Balkans, part of the Aegean extensional region, withnessing for coeval extension
and compression in the different parts of the orogen. After the establishment of a regional
extensional tectonic environment to the south of the Balkan Range (less than 10 m.y), some of
the frontal bordering compressional faults were reactivated to the south and some fragments
were transformed inто extensional structures. Presently some fragments of the zone have a
geomorphological expression and show evidences to be active Quaternary faults.
The eastern part of the Balkan FTB, near the Black Sea coast do not exhibit direct evidences for
post-Oligocene compression. However, such opportunity is quite possible based on available
data. Further more, the Neogene sediments of the Euxinian basin laying to the north of the FTB
have been compressed.
All known hydrocarbon accumulations in the Balkan FTB as well as in the Moesian Platform
are hosted in sediments older than the Neogene. However, some of them are located close to
active Neogene-Quaternary faults. The displacements along these faults may have sealed or
unselad some of the traps, facilitating migration of the hydrocarbons, forming or destroying
some accumulations. Therefore understanding young tectonics could play an important role in
hydrocarbon exploration in the region.
Keywords: Balkan Fold-thrust Belt, Neogene tectonics, Fore-Balkan Fault
241