The origin and tectono-stratigraphic evolution of the Meso-Hellenic Trough, northern Greece and Albania

The Meso-Hellenic Trough (MHT) is an elongate NW-SE trending intermontane basin that developed during the Mid-Tertiary within the Hellenide fold-and-thrust belt. It formed in the late Eocene behind the deformation-front as the locus of thrusting migrated towards the foreland in the south-west. This followed collision between the Apulian and Pelagonian micro-plates and final closure of the Pindos Ocean basin (a strand of the Neotethys). The MHT basin-fill is of M. Eocene-M. Miocene age, was predominantly marine in nature and can be sub-divided into the Basin-margin and Meso-Hellenic Groups, which are separated by a basin-wide angular unconformity. The Basin-margin Group (Middle-Upper Eocene) is only preserved in small outcrops around the basin margin and the Meso-Hellenic Group (Oligocene-Middle Miocene) forms the majority of the basin-fill. The Basin-margin Group was deposited on the sub-Pelagonian thrust-sheets and is of identical age to the youngest Pindos foreland basin sediments exposed beneath the same thrust-sheets. However, large-scale folds which deform the basal thrust demonstrably do not deform the overlying Meso-Hellenic Group. The Basin-margin Group can therefore be shown to have been deposited on the thrust sheets as they overthrust the Pindos foreland basin. Coarse ophiolitic breccia up to 600 m thick characterises the south-western margin of the Krania sub-basin (a depocentre of the Basin-margin Group) and is interpreted as having been deposited at the base of an active (? reverse) fault-scarp. In addition, there is evidence that strike-slip faults were active along the northern and southern margins of the sub-basin. Although these structures may have controlled subsidence locally, the overall setting of the Basin-margin Group is interpreted as that of a piggy-back basin, formed above the southwestward propagating thrust stack. The palaeobathymetry of the basin increased during the late Eocene, as marked by the change from shallow-marine limestone, fan delta and lagoonal facies to deeper-water turbidite facies. During this period, sedimentation was locally tectonically controlled, for example where basement uplift along reverse faults to the north of the Krania sub-basin shed limestone-dominated mass-flow deposits southwards in Priabonian times. Further compressional folding and high-angle faulting, interpreted to have resulted from overthrusting of the Olympos seamountl micro-plate, deformed the Basinmargin Group at the end of the Eocene and eventually led to its sub-aerial erosion. Renewed subsidence during the Oligocene was characterised by the development of a basin configuration in which both palaeobathymetry and sediment thickness increased towards the basin centre from its ends. This resulted in a "canoe-shaped' morphology to the basin during the deposition of the Heptahorion and Pentalofon Formations. Facies relationships, palaeocurrent indicators, clast provenance and palaeobathymètric considerations demonstrate that sediment was predominantly derived from the Pelagonian zone in Albania and transported axially towards the basin centre to the south-east. In the early Miocene, the south-western margin of the basin was uplifted. This resulted in both the north-eastward verging monocline observed in the central segment of the MHT and depocentre migration towards the north-east, such that the overlying Tsotillion Formation (Lower-Middle Miocene) onlapped the Pelagonian zone. The subsequent uplift of the Pelagonian zone at the south-eastern end of the basin, also in the Lower Miocene, resulted in the deposition of coarse fluvial and deltaic conglomerates of the Itea and Vlahava Members and the development of local angular unconformities. The uplift is interpreted to be the result of the extension-related uplift of Mt. Olympos along the eastern margin of the Pelagonian zone during the Aquitanian-Langhian (16-23 Ma). This was a period when extension was widespread throughout northern Greece and was possibly linked to the collapse of the Hellenide thrust-stack. In the MHT, syn-sedimentary extension was observed in the Nestorion Formation (Lower-Middle Miocene) and meso-scale NW-SE trending extensional faults occur throughout much of the basin. Integration of field data from the MHT with regional considerations therefore suggests that overall basin development was consistent with that of a piggy-back basin whose morphology was primarily the result of deep-seated thrusting, but which subsequently experienced modification due to extension during the early-middle Miocene. In the late Miocene, the MHT and Pindos Mountains were uplifted by up to 1400 m and deformed by compressional folding and faulting. This included the formation of a partially blind, high-angle reverse fault interpreted as bounding the south-western margin of the MHT in Greece and occurred prior to the deposition of the unconformably overlying Pliocene units of the Grevena Formation. Most recently, localised back-arc extensional deformation is interpreted as having resulted in Pleistocene-Recent movement on the Korce normal fault and resultant uplift of Morave (1800 m) in its footwall in southern Albania.