In this paper, the Hellenic orogenic belt’s main geological structure and architecture of deformation are presented in an attempt to achive a better interpretation of its geotectonic evolution during Alpine orogeny. This study was based not only on recent research that I and my collaborators conducted on the deformational history of the Hellenides but also on more modern views published by other colleagues concerning the Alpine geotectonic reconstruction of the Hellenides. The structural evolution started during the Permo–Triassic time with the continental breaking of the supercontinent Pangea and the birth of the Neotethyan ocean realm. Bimodal magmatism and A-type granitoid intrusions accompanied the initial stages of continental rifting, followed by Triassic–Jurassic multiphase shallow- and deep-water sediment deposition on both formed continental margins. These margins were the Apulian margin, containing Pelagonia in the western part of the Neotethyan Ocean, and the European margin, containing continental parts of the Serbo-Macedonian and Rhodope massifs in the eastern part of the Neotethyan ocean. Deformation and metamorphism are recorded in six main deformational stages from the Early–Middle Jurassic to the present day, beginning with Early–Middle Jurassic Neotethyan intra-oceanic subduction and ensimatic island arc magmatism, as well as the formation of a suprasubduction oceanic lithosphere. Compression, nappe stacking, calc-alkaline magmatism, and high-pressure metamorphic events related to subduction processes alternated successively over time with extension, orogenic collapse, medium- to high-temperature metamorphism, adakitic and calc-alkaline magmatism, and partial migmatization related to the uplift and exhumation of deep crustal levels as tectonic windows or metamorphic core complexes. A S- to SW-ward migration of dynamic peer compression vs. extension is recognized during the Tertiary Alpine orogenic stages in the Hellenides. It is suggested that all ophiolite belts in the Hellenides originated from a single source, and this was the Neotethyan Meliata/Maliac-Axios/Vardar ocean basin, parts of which obducted during the Mid–Late Jurassic on both continental margins, Apulian (containing Pelagonia) and European (containing units of the Serbo-Macedonian/Rhodope nappe stack), W-SW-ward and E-NE-ward, respectively. In this case, the ophiolite nappes should be considered far-traveled nappes on the continental parts of the Hellenides associated with the deposition of Middle–Late Jurassic ophiolitic mélanges in basins at the front of the adjacent ophiolite thrust sheets. The upper limit of the ophiolite emplacement are the Mid–Upper Jurassic time(Callovian–Oxfordian), as shown by the deposition of the Kimmeridgian–Tithonian Upper Jurassic sedimentary carbonate series on the top of the obducted ophiolite nappes. The lowermost Rhodope Pangaion unit is regarded as a continuation of the marginal part of the Apulian Plate (External Hellenides) which was underthrust during the Paleocene–Eocene time below the unified Sidironero–Kerdylia unit and the Pelagonian nappe, following the Paleocene–Eocene subduction and closure of a small ocean basin in the west of Pelagonia (the Pindos–Cyclades ocean basin). It preceded the Late Cretaceous subduction of the Axios/Vardar ocean remnants below the European continental margin and the final closure of the Axios/Vardar ocean during the Paleocene–Eocene time, which was associated with the overthrusting of the European origins Vertiskos–Kimi nappe on the Sidironero–Kerdylia nappe and, subsequently, the final collision of the European margin and the Pelagonian fragment. Subsequently, during a synorogenic Oligocene–Miocene extension associated with compression and new subduction processes at the more external orogenic parts, the Olympos–Ossa widow and the Cyclades, together with the lower-most Rhodope Pangaion unit, were exhumed as metamorphic core complexes.

Knowing the rich presence of active faults in northern Thessaly and the lack of any significant seismic activity since at least the mid-1940s, the 2021 seismic sequence did not surprise us. What did surprise us was the fact that (i) despite the great knowledge of the neotectonic faults in the area, the causative faults were unknown, or almost unknown; (ii) the direction of the 2021 faulting was different than the expected, and given that the focal mechanisms showed almost pure normal dip-slip motion, the extensional main axis was also different than the one we thought we knew for this area; and (iii) besides the co-seismic ruptures that occurred within the Domeniko-Amouri basin and along the Titarissios River valley, there is evidence of rupturing in the alpine basement of Zarkos mountains. After thoroughly reviewing both the alpine and neotectonic structural setting and all the available literature concerning the seismotectonic data and interpretations of the 2021 sequence, including investigations of our own, we end up in a complex tectonic setting with older alpine structures now operating as inherited faults, and we also suggest the possible occurrence of a roughly N-dipping, low-angle, detachment-type fault. This fault runs below Mt Zarkos, reaching at least the Elassona Basin, with splay faults bifurcating upwards from the main fault zone. Following this complexity, rupture of the first mainshock must have chosen a split route reaching the surface through the gneiss rocks of Zarkos and almost (?) reaching the basinal sediments of the local tectonic depressions. This seismic sequence is a perfect case study to shed some light on the tectonic and rupture processes in the context of both geodynamics and seismic hazard assessment.

The Late Jurassic to Early Cretaceous sedimentary succession of the Neochorouda Unit lies unconformably on top of the Oreokastro ophiolites of the Vardar/Axios “suture zone” in northern Greece. This succession consists of turbidites and mass flows and provides an upper limit for ophiolite emplacement. New biostratigraphic and microfacies analysis from the clasts in the mass flows were carried out for a better understanding of the Late Jurassic to Early Cretaceous geodynamic history. Microfacies and organism content prove the onset of Late Jurassic carbonate platforms on top of a Middle to Late Jurassic nappe stack striking from the Eastern Alps to the Hellenides. Middle to Late Jurassic nappe stacking towards WNW to NW followed late Early to Middle Jurassic intra-oceanic thrusting in the Western Vardar/Axios (= Neotethys) Ocean and subsequent ophiolite obduction onto the Pelagonian Units forming a thin-skinned orogen on the lower plate. After ophiolite emplacement Kimmeridgian- Tithonian carbonate platforms sealed widespread this tectonic event. Tithonian extension due to mountain uplift resulted in partial erosion of these platforms and new extensional basins were formed. Late Tithonian to earliest Cretaceous erosion of the uplifted nappe stack including the obducted ophiolites resulted in sediment supply into the newly formed basins also east of the Pelagonian Units.

We present the Hellenic orogenic belt`s main geological structure and architecture of deformation, trying to reach a better interpretation of its geotectonic evolution during the Alpine orogeny. We based on our recent studies and experience about the deformational history of the Hellenides but also on the more modern views, published by other colleagues, concerning the Alpine geotectonic reconstruction of the Hellenides. The structural evolution started during the Permo-Triassic with the continental breaking of the Pangea super-continent and the birth of the Neotethyan ocean realm. Bimodal magmatism and A-type granitoid intrusions accompanied the initial stages of the continental rifting, followed by Triassic-Jurassic multiphase shallow- and deep-water sediment deposition on both formed continental margins. These margins were the Apulian margin, containing the Pelagonia, at the western part of the Neotethyan ocean and the European margin, containing continental parts of the Serbo-Macedonian and Rhodope massifs (upper Vertiskos-Kimi and middle Sidironero-Kerdylia units), at the eastern part of the Neotethyan ocean. Deformation and metamorphism are recorded in six main deformational stages from the Early-Middle Jurassic to the present day, getting started with an Early-Middle Jurassic Neotethyan intra-oceanic subduction and ensimatic island arc magmatism. Compression, nappe stacking, calc-alkaline magmatism and high-pressure metamorphic events related to subduction processes alternated successively through time with extension, orogenic collapse, medium- to high-temperature metamorphism, adakitic and calc-alkaline magmatism and partly migmatization, related to uplift and exhumation of deep crustal levels as tectonic windows or metamorphic core complexes. An S- to SW-ward migration of the dynamic peer compression vs. extension is recognized during the Tertiary Alpine orogenic stages in the Hellenides. In any case extension and crustal uplift follow compression and nappe stacking or compression and exten-sion act simultaneously at different parts of the orogen. We suggest that all ophiolite belts in the Hellenides originated from a single source and this was the Neotethyan Meliata/Maliac-Axios/Vardar ocean basin, parts of which obducted during the Mid-Late Jurassic on both continental margins, Apulian`s (containing Pelagonia) and European`s (containing units of Serbo-Macedonian/Rhodope nappes stack), W-SW-ward and E-NE-ward, respectively. In this case, the ophiolite nappes should be considered as far-traveled nappes on the Hellenides continental parts associated with the deposition of Middle-Late Jurassic ophiolitic mélanges in basins at the front of the adjacent ophiolite thrust sheets. The upper limit of the ophiolite emplacement is the Mid-Upper Jurassic, Callovian-Oxfordian, as shown by the deposition of the Kimmeridgian-Tithonian Upper Jurassic sedimentary carbonate series on the top of the obducted ophiolite nappes. The suture zone between the Pelagonian nappe and the External Hellenides as part of the Apulia plate has been dated about coeval in time with the Paleocene-Eocene to Oligocene SW-ward over thrusting (between 56 Ma and 32 Ma), along the Nestos thrust zone, of the Serbo-Macedonian/Rhodope Sidironero-Kerdylia unit on the tectonically lower-most Rhodope Pangaion unit. In this sense, the Nestos shear zone in the Rhodope province continues until the suturing between Pelagonian and External Hellenides and is the same with this. Therefore, the lower-most Rhodope Pangaion unit could be regarded, as the continuation of the marginal part of the Apulian plate (External Hellenides) that was underthrusted during the Paleocene-Eocene below the unified Sidironero-kerdylia unit and the Pelagonian nappe, following the Paleo-cene-Eocene subduction and closure of a small ocean basin at the western of the Pelagonian (Pindos-Cyclades ocean basin). Was preceded by the Late Cretaceous subduction of the Axios/Vardar ocean remnants below the European continental margin and the final closure of the Axios/Vardar ocean during the Paleocene-Eocene, which were associated with the overthrusting of the European origins Vertiskos-Kimi nappe on the Sidironero-Kerdylia nappe and subsequently the final collision of the European margin and Pelagonian fragment. In this scenario should be assumed, that the true Vardar/Axios suture zone should originate in the deep further to the East between the Rhodope units in the northern Rhodope province, while the Axios/Vardar zone in its today outcrops position should be considered as allochthonous. Subsequently, during synorogenic Oligocene-Miocene extension, the lower-most Rhodope Pangaion unit exhumed as a metamorphic core complex below the Serbo-Macedonian/Rhodope nappes, which were successively exhumed, from the higher to the middle, during Paleocene-Eocene and Eocene-Oligocene extension, respectively. A retreating subduction zone associated with rollback of the subducted lithosphere under the Pelagonian and the other Internal Hellenides nappes stack and/or slap break off and mantel delamination could well explain the Tertiary synorogenic extensional tectonics in the Internal Hellenides, taking place simultaneously with compression in the External Hellenides and the Hellenic foreland or accretionary wedge.