Geotectonic Setting - Discussion
Molassic-type sedimentation starts in both basins, MHT and THB including the Paleogene sequence of the Axios Basin, simultaneously during Mid-Late Eocene time (Bartonian-Priabonian), but it finishes at different time, at the Mid-Late Miocene for the MHT and the Late Oligocene for the THB and its equivalent part of the Axios Basin. This Tertiary, westwards progressive delay of the sedimentary stoppage in both areas, is compatible with the W-SW-wards orogen migration of the Hellenides during the Tertiary (Kilias et al. 1999). Neogene-Quaternary sediments lie discordantly on the molassic deposits of the basins, forming the last intramontane basins of Hellenides. Furthermore, the THB and its equivalent sequence of the Axios Basin is characterized by abundant volcanic products associated with granitoid intrusions of similar age into the Rhodope basement rocks under syn- to late orogenic extension (Kilias and Mountrakis, 1998; Marchev et al. 2005). Nevertheless, important strike-slip movements of Oligocene-Miocene age associated with transpressional or transtensional structures are common during both basins` evolution, showing the great significance of such strike-slip movements along the Hellenides during the Tertiary. Both basins show further analogous deformational setting during the Neogene-Quaternary time with the development of local compressional structures followed again by a general extension regime. Active faults with about the same kinematics, NNE-SSW for the THB and NNW-SSE for the MHT, dominate also in both areas.
Figure 15. Schematic cross section through the Rhodope metamorphic province and the THB
Schematic cross section through the Rhodope metamorphic province and the THB showing the successive deformational stages related to the basin evolution (Kilias et al. 2013).The Paleogene Thrace basin was formed on top of the low angle, extensional fault system during the Paleogene-Neogene extension of the Rhodope metamorphic province. Basin subsidence took place simultaneously with uplift and exhumation of deep crustal metamorphic units. Legend as in Figure 10. Extension migrates towards SW to SSW.
The MHT was evolved as an intramontane piggy-back basin above the ophiolitic nappe and the higher Pelagonian units, during their westward traveling upon the cold Hellenic accretioniary prism (Figures3, 16). This geotectonic position, on the cold accretionary prism (Lower plate), interprets well the total lack of any magmatic activity during the basin evolution. Initial isostatic crustal flexure associated possibly with back-thrusting toward east (Mid-Upper Eocene), strike-slip faulting (Oligocene-Miocene) and finally normal detachment faulting towards west (Lower-Middle Miocene) were the main motor mechanisms related to the basin evolution (Vamvaka et al. 2006, 2010). This differs from previous interpretations, from Doutsos et al. (1994) that envisaged foreland depression related to continue, from Eocene to Miocene, backthrusting towards east or from Ferriere et al. (2004) who suggest that the MHT originated as a forearc basin during the first stages of a Mid-Late Eocene subduction (Pindos basin) and evolved into a piggy-back basin as a result of Oligocene underthrusting of the large thick-crust Gavrovo-Tripolis domain. In contrast to the MHT, at least the studied part of the Greek THB, including the Paleogene deposits of the Axios Basin, evolved as a Paleogene supra detachment basin above the strongly stretched, during the Eocene-Oligocene, Internal Hellenides (Figures15, 16; Kilias et al. 1999, Bonev et al. 2006). Exhumation of deep crustal levels took place about simultaneously with basin subsidence and migration of deformation towards W-SW, as well as with the progressive change of the tectonic conditions from ductile to brittle during the Paleogene – Neogene (Kilias et al. 1999, 2013).The origin of the Upper Eocene-Oligocene syndepositional magmatic activity could be attributed to the subduction processes evolved during the Paleogene more further to the W-SW in Pindos or Axios ocean(s). So that it is concluded that extension and basin formation in the Rhodope province took place simultaneously with contraction, nappe stacking and crustal thickening as well as HP/LT metamorphism at the more external parts of the Hellenides towards the foreland (Figure22; Schermer et al. 1990;Kilias et al. 1991a,b; Schermer 1993; Ring et al. 2010).
Figure 16. Cross section through the Northern Hellenides
Cross section through the Northern Hellenides showing the different geotectonic positions of the MHT and THB during the Tertiary orogenic processes in Hellenides. Extension acts simultaneously with compression during the orogenic evolution. SW to SSW migration of the dynamic compression/extension pair took place. A - A` cross section in Figure. 1.
However, Marchev et al. (2005) explain the origin of the Paleogene magmatism and the simultaneous extension and crustal thinning of the Rhodope continental curst due to convective removal of the lithosphere and mantle diapirism, while Maravelis et al. (2007) and Tranos (2009) regard the Thrace Basin as a fore-arc basin but without clear evidence about the existence or position of a Tertiary accretionary prism associated to the basin formation. Furthermore, their main investigations were focused in the Limnos island at the southernmost continuation of the THB.