Origin and deformation of the Thrace Basin: constraints from fault-slip analysis

The origin and deformation style of the Thrace Basin, NW Turkey represent the target of ongoing debate. Uncertainties are partly due to imprecise knowledge of the stratigraphy of basin-fill sediments. In our contribution we report surface structural data including fault-slip analysis which are important for understanding the origin and structure of the Thrace Basin. Measurements were executed along the northern, north-eastern and south-western margin of the basin. The data from the SW margin also contribute to characterisation of the surface segment of the North Anatolian Fault between the Marmara and Aegean seas.
   The earliest detected deformation occurs just SW from the Thrace basin, in the Gelibolu peninsula, along the shore of the Aegean Sea. Folding and faulting of Palaeocene to early Eocene sequence was associated with very low-grade metamorphism. Middle Eocene succession seals the deformation features. Small-scale syn-sedimetary structures (sedimentary dykes and faults) indicate NE-SW to E-W extension along the NE margin of the basin in the Bartonian to Priabonian. Gradual tilting of the beds could occur in map-scale tilted blocks of extensional origin. The repeated normal faulting is deduced from fractures which formed before, during and after the tilting. This deformation process is reflected by progressive transgression on basin margin and intrabasin block margin.
   Extensional fractures were also observed in Eocene sequence near the SW margin, in the Ganos Mountains. The age of faulting is not precisely constrained but pre-dates the latest Oligocene – early Miocene folding and related uplift.
   All data on early deformation phases point to extensional deformation along both the north-eastern and south-western margin of the basin. Regional geodynamic considerations would agree with fore-arc origin of the Thrace basin.
   Before the folding in the Ganos Mountain a strike-slip type deformation occurred during the Late Oligocene - Early Miocene. The E-W compression could induce dextral faulting along the ENE trending southern margin of the Ganos Mountain, the Ganos fault, a precursor of the North Anatolian Fault. This confirms the suggestion that dextral faulting parallel to the NAF could be active already in the late Oligocene-early Miocene.
   The large-scale folding itself occurred in a slightly different stress field, in NNW-SSE compression. Progressive fracturing and folding happened in a coaxial deformation process. The folding and related uplift is constrained by fission track data as ~16 Ma.
   Post-early Miocene deformation history can be detected south from the Ganos fault, in the Sarköy area. Before the tilting of mid- to late Miocene sediments, a peculiar stress field with SE-NW extension and NNE-SSW compression occurred. This deformation was already observed, but its geodynamic interpretation is not clear. Strike-slip type deformation with EW to NW-SE compression is connected to dextral faulting and folding (transpression) along the Ganos segment of the North Anatolian Fault. This is in agreement with Pliocene-Quaternary dextral faulting along the North Anatolian Fault. A slight change in compression direction can be suspected during the progressive deformation; this non-coaxial character can also be connected to strike-slip faulting.