The Alpine Geological History of the Hellenides, From the Triassic to the Present: Compression vs. Extension, the Dynamic Pair for the Orogen Structural Configuration. A Synthesis
Authors: Kilias Adamantios 
Published: November 2023

Abstract: 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 m&eacute;langes 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.