The Eocene-Oligocene geodynamic setting of the Thrace Basin (Turkey, Greece, and Bulgaria)

The Thrace Basin is an important hydrocarbon province covering an area in excess of 15,000 sq. km in Turkey, Greece, and Bulgaria. The complex historical vicissitudes of the region have made collaboration among the researchers of the three countries difficult. Consequently, a unified and widely accepted geological interpretation of the Thrace Basin is still missing. Nevertheless, a great wealth of outcrop and subsurface data is already available from both academic and industrial sources. Integration of preexisting data (seismic and oilwell stratigraphy, geological-structural field maps) with new field mapping as well as new stratigraphic, sedimentologic, thermochronologic, petrologic, and radiometric data has provided significant constraints on the evolution of the basin.
   The Thrace Basin developed during the complex transition between the collisional tectonic regime following the closure of Vardar-İzmir-Ankara oceanic realm and the extensional regime characterizing the Neogene evolution of the Aegean and periAegean regions. It was long interpreted as a forearc basin which developed in a context of northward subduction. This interpretation was challenged by more recent data emphasizing the lack of a coeval magmatic arc. The interpretation of the Thrace Basin as a forearc basin was also based on the occurrence, along its southern margin, of a belt of chaotic deposits interpreted as a tectonic mélange formed in an accretionary prism. However, this tectonic mélange may represent olistoliths in an Eocene sequence. All these elements along with the correspondence between subsidence pulses in the basin and lithospheric stretching in the metamorphic core complexes of southern Bulgaria and the northern Aegean region may indicate instead that the Thrace Basin was the result of either (i) post-orogenic collapse after the continental collision related to the closure of the Vardar ocean, or (ii) upper-plate extension related to slab retreat in front of the Pindos remnant ocean. Preliminary data indicate that initial subsidence (Ypresian-early Rupelian) was localized in small depocenters delimited by a system of strikeslip faults, probably during the late stages of collision. Further subsidence over a wider area occurred during the rest of the Oligocene, in agreement with the timing and areal distribution of crustal stretching phenomena evident during this length of time over the entire northern Aegean region. This hypothetical two-stage evolutionary trend might represent a predictive tool in the tectonostratigraphic interpretation of similar sedimentary basins.
   Seismic sections across the central part of the basin and the tectonostratigraphic interpretation of outcrops in the Gelibolu Peninsula and along the Greek-Turkish border show that between the Middle Eocene and the Early Oligocene important east-west-trending transcurrent faults cut the Thrace Basin, generating a series of depocenters and uplifts which deeply influenced sediment dispersal and the areal distribution of paleoenvironments. In addition to the "flower" structures seen on seismic lines, strike-slip tectonism induced also abrupt temporal and areal variations in subsidence rates, as well as dramatic sedimentological facies changes within coeval stratigraphic horizons. Such strike-slip-dominated tectonic scenario during the late- and post-collisional stages related to the closure of the Vardar-İzmir-Ankara ocean is further corroborated by the presence of an important strike-slip shear zone of crustal relevance in the region just south-east of the Marmara Sea. Such shear zone is at least 225 km long, has an horizontal offset of about 100 km, and has a trend similar to the the present-day North Anatolian Fault. A similar shear zone- although poorly studied- occurs in the Kapidaği Peninsula south of Marmara Island. In addition, published thermochronological data demonstrate the existence of a praecursor of the North Anatolian Fault in the area of the present-day southern Thrace Basin active at least from the Oligocene.