Evolution of the Mesohellenic Basin (Greece) : a synthesis

Jacky Ferrière, Frank Chanier, Jean-Yves Reynaud, Andreas Pavlopoulos, Pitaksit Ditbanjong, and Isabelle Coutand

This article is an attempt to synthesize the knowledge about the Mesohellenic Basin (MHB), based upon available literature and also unpublished data. We focus on our interpretation but also mention alternative ones. The MHB is an orogenic basin of general importance, because of (i) its large size (300km along strike, and 150km in Greece) ; (ii) its location in the middle part of the Hellenic chain (between the Pindos accretionary prism and the Pelagonian upper unit) ; (iii) its large-scale piggyback setting. It has also a regional interest because of its Late Eocene-Middle Miocene age, a period of the Hellenic orogen which remains poorly understood.

The MHB fill is dominated by siliciclastic submarine deposits emplaced by gravity processes. After two main successive tectonic events, the deposits show a continued deepening during late Eocene (Krania basin) and Oligocene (MHB basin s.s.) times. Then, the Miocene series are characterized by shallower coarser-grained (early Miocene) or more calcareous (middle Miocene) sediments.

Water depth overall increases towards the north. In the Miocene, the southern MHB limit extends beyond the Meteora area, beneath the present-day Trikala plain. We consider that the piggyback setting is a key to the evolution of the MHB. Tectonics primarily controlled subsidence and the regime of sedimentation, therefore overprinting the effect of eustatic changes. The MHB infill reflects the timing and nature of understhrusted tectonic units : (i) in the Late Eocene, during the easy subduction of the thin Pindos basin crust and the development of the Pindos accretionary prism in the external zones, subsidence in the MHB is localized in contrasted and likely small areas inherited from heterogeneities of the internal zones, namely the boundaries of the Pelagonian Indentor (PI); (ii) in the Oligo-Miocene, subsidence is generalized in the strike of the chain, due to collision of the thicker crusted Gavrovo-Tripolitsa block of the external zones.

In the Oligo-Miocene MHB, subsidence is first strong (Eptachorion marls), and then migrates to the east, progressively or stepping over structural highs as the Theopetra-Theotokos Structure (TTS) in front of the PI. In the Miocene, sediment supply is abruptly transferred from the Pindos accretionary prism to the Pelagonian hinterland, in response to a severe uplift of the Pelagonian domain notably the Pelagonian Indentor (Meteora conglomerates).

While the collision is recorded as a major compressional phase at the Eocene-Oligocene boundary, most of the following tectono-sedimentary evolution reflects processes at the subduction plane, which remain hypothetical (tectonic erosion, underplating…). Also, the importance of strike-slip motion of some faults on the basin evolution remain matter of debate.

Our ongoing research on the MHB is focused on the chronostratigraphic assessment of sediment supply, based on thermochronology and basin modeling.