1) Amongst large orogenic, intermontane syntectonic basins, the MHB is an important one, the development of which is at the transition between subduction (fore-arc stage) and collision (piggyback stage). It might be basically controlled by underthrusted units. Its large size (300 km in length), thickness (up to 4.5 km), duration (30 Ma, and 20 Ma for its piggyback stage) and dominantly marine siliciclastic infill are to be related to lithosphere-scale processes. These processes (underplating, tectonic erosion, thermal relaxation) can be quantified based on the stratigraphic record of the basin (paleobathymetry, subsidence) together with the thermochronology of the surrounding source areas (rock uplift). Ongoing numerical modeling using these data will allow reconstructing the evolution of relief of this part of the Hellenic chain.

The sedimentary record of the MHB is dominated by fluvially supplied gravity deposits (fan- deltas, slope and basin-floor fans), the most emblematic being the Meteora conglomeratic Gilbert deltas (Pentalofon Fm).

The “molassic” deposits are resting unconformably on the internal zones of the Hellenides, which were intensely tectonized during the lower to middle Eocene. These deposits are also syntectonic, as they record several extensional but also compressional faulting stages (associated to strike-slip motions, the importance of which remains to be determined), as well as olistolites or sharp coarsening-upward shifts pointing to stages of rejuvenation of the border reliefs.

2) The deposits are organized in at least 5 transgressive-regressive supercycles commonly dominated by conglomerates at the base and sometimes at the top, and by finer-grained, turbiditic or pelagic facies during the maximum flooding. These supercycles correspond to the successive stages of basin tectonic evolution.

The first stage of the MHB corresponds to the first supercycle and dates to latest Eocene (ca 45-34 Ma). It is preserved in the isolated Krania and Rizoma sub-basins. The related syn-tectonic facies are contrasted. Close to the subduction front (Krania), they are dominated by turbidites recording slope instabilities (olistostomes, slumps, mass-wasting unconformities). Backwards, they point to shallower-water, deltaic systems (Rizoma).

The second stage of the MHB, which corresponds to four supercycles, dates to Oligo-Miocene (ca 34-15 Ma). The first super-cycles in this stage are well differenciated. They are, from base to top: the Eptachorion Fm (Oligocene), the Pentalofon Fm (Late Oligocene- Lower Miocene), the Tsotyli Fm (Lower Miocene). These super-cycles are followed by a last one, corresponding to the Ondria-Orlias Fms, only outcropping at the tips of the MHB. Within the super-cycles, smaller-scale sequences are preserved, likely controlled by eustatic sea-level changes. This could be for example the case for the wedges forming the Meteora fan-deltas.

3) These successive depositional stages record synsedimentary deformations from which stress fields can be determined, in order to relate the basin evolution to the changing geodynamic context.

Compression is recorded mostly in late Eocene, bringing about folds, reverse faults and larger- scale structures which we called “faulted-flexures”.

Normal faults are also numerous through the MHB evolution, some being cleraly syn- sedimentary (e.g. Mitropoli), and other ones, less well dated, more likely formed during the generalized Plio-Quaternary extension.

Some authors, based on seismic profiles and horizontal slickensides on plane faults, stress on the role of major strike-slips in the Oligocene to Lower Miocene [i.e. Vamvaka et al., 2006], but the importance of these motions in the basin evolution are not established and thus may be overestimated.

These tectonic deformations control the basin geometry and rather the location of depocentres, for instance the dissymmetry of the MHB syncline (location of late Eocene sub-basins, steeper flank of the syncline to the west, narrower to the south in front of the Pelagonian Indentor) and the migration toward the east of the depocenters.

4) Contrasted interpretations have been published regarding to the geodynamic control of the MHB (Fig. 18). These different successive interpretations have been discussed above. The basin has been first interpreted as a retro-arc basin [Doutsos et al., 1994] but the eastward thrusts are of limited extent and the source areas shift from west to east at the end of Oligocene, then it has been interpreted as a strike-slip hemi-graben or pull-apart basin but no major strike-slip faults have been clearly prooved.

In our interpretation, the MHB is basically a piggyback basin, because sedimentation and accommodation take place during the underthrusting of tectonic units which now outcrop to the east of the basin (Olympos). The two main stages of basin tectonic evolution can be related to two geodynamic stages of the subduction of the external zones of the Hellenides beneath the internal zones:

- During the first stage (fore-arc setting), the maximum subsidence area is located close to the subduction front and the scattering of depocentres reflect heterogeneities of the upper plate. In our interpretation, it is because at this pre-collision time, there is little plate coupling and therefore little influence of the thin-crusted subducting plate (Pindos basin).

- During the second stage (piggyback setting), the maximum subsidence is generalized to the great Albano-Thessalian basin and moves stepply eastward. In our interpretation, this is the consequence of the less easy subduction (collision) of a thicker-crusted plate (the Gavrovo- Tripolitsa platform).

The processes controlling the vertical motion of the upper plate during collision are deep processes and so are relatively speculative. The underthrusting of the Gavrovo-Tripolitsa block would likely promote localized tectonic erosion and underplating (Fig. 19).

5) The final inversion of the MHB is not the end of the intermontane basin system of the Hellenides, as it corresponds to the onset of the Ptolemais basin further eastward, on the western border of the Olympos relief. Finally, the Trikala and Larissa plains still accommodate sediments during the Plio-Quaternary above former Miocene (MHB) and Pliocene (Ptolemais) depocentres.