Abstract
Paikon and Tzena terranes are situated in the center part of the Axios zone, between the Pelagonian nappe and the Serbomacedonian/Rhodope metamorphic province. Axios zone comprises the ophiolite - bearing Almopia Subzone in the West, the Paikon and Tzena terranes in the Middle, and the also ophiolite - bearing Paionia Subzone in the East (Mercier 1968). The structural evolution and geotectonic position of Paikon and Tzena terranes have long been a matter of controversy between researchers, with several questions remaining unanswered. We try here to correlate Paikon and Tzena terranes, reconstructing the evolution of deformation from Jurassic till recent times, and for this purpose we carried out geological mapping and detailed structural investigations, combined with all available geochronological and stratigraphic data. Paikon and Tzena terranes are two identical tectonostratigraphic terranes, constituting a complicated tectonic nappe pile with several tectonostratigraphic units. ...
Paikon and Tzena terranes are situated in the center part of the Axios zone, between the Pelagonian nappe and the Serbomacedonian/Rhodope metamorphic province. Axios zone comprises the ophiolite - bearing Almopia Subzone in the West, the Paikon and Tzena terranes in the Middle, and the also ophiolite - bearing Paionia Subzone in the East (Mercier 1968). The structural evolution and geotectonic position of Paikon and Tzena terranes have long been a matter of controversy between researchers, with several questions remaining unanswered. We try here to correlate Paikon and Tzena terranes, reconstructing the evolution of deformation from Jurassic till recent times, and for this purpose we carried out geological mapping and detailed structural investigations, combined with all available geochronological and stratigraphic data. Paikon and Tzena terranes are two identical tectonostratigraphic terranes, constituting a complicated tectonic nappe pile with several tectonostratigraphic units. Paleozoic gneisses and Triassic marbles intercalated with schists (Lower Units) are tectonically overlain by volcanoclastic and carbonate rocks of Jurassic to Early Cretaceous age (Medium Units), as well as the obducted ophiolites traveled westwards from their initial place. Transgressive Upper Jurassic to Lower Cretaceous sediments, as well as Upper Cretaceous limestone and flysch (Upper Units) overlie discordantly all the above mentioned units. The Paleozoic basement rocks do not exhumed in the Paikon terrane. Both terranes have undergone the same polyphase deformation and metamorphism (D1 - D6), from Mid-Jurassic to recent times (Katrivanos et al. 2001, 2012, 2013, 2015, 2016). Shortening alternated with extension and the deformation conditions evolved from ductile to brittle. Compressional events are related to nappe stacking, terrane accretion and crustal thickening, while extensional ones are related to orogenic collapse, terrane dispersion, isostatic rebound, unroofing and crustal thinning. Ophiolite obduction and nappe stacking started during the Mid- to Upper Jurassic (D1-deformation). The structures of D1 event are penetrative (synmetamorphic foliation, S1 and mineral stretching lineation, L1) and affect clearly the lower units of Triassic age, as well as the Mid- to Late Jurassic volcano-sedimentary series. We assume that D1 is responsible for the strong synmetamorphic deformation and internal imbrication of the Triassic basement units and the Mid-Late Jurassic volcano-sedimentary series. A HP-LT metamorphism of Middle Jurassic age predates D1 event and is possibly related to a subduction zone formed in Axios ocean. This HP metamorphism is assumed for Paikon terrane by Baroz et al. (1987). In this study, we present evidence for this HP metamorphism in the Tzena terrane as well. During D2 event in the Early Cretaceous, W- to SW-vergent imbrication and folding took place again, now affecting the Upper Jurassic - Lower Cretaceous sedimentary series, the Jurassic volcano-sedimentary series, the Triassic ones, as well as the ophiolitic bodies. The ophiolites continued to advance westwards during that time, overthrusting the Upper Jurassic - Lower Cretaceous sediments.Upper Cretaceous extension (D3 event) and exhumation of deeper crustal rocks is associated with basin subsidence and deposition of the Upper Cretaceous limestones and flysch (Cenomanian to Maestrichtian), outcropping on the western flanks of Paikon and Pinovon Mts. D4 structures form well developed, NW-SE trending folds and thrust belts, during Paleocene to Eocene and caused a renewed SW-directed imbrication of all units. In the western Paikon, Almopia ophiolites overthrust the Theodoraki Carbonate Formation and Tchouka flysch, along a NE-ward D4 back-thrust. The final exhumation of Paikon and Tzena tectonostratigraphic units, as well as crustal thinning occurred during the Oligocene - Miocene extensional event D5 under brittle conditions. Analogous Tertiary tectonic events with similar geometry and kinematics of the deformation have already been established for several other areas in Greece (Olympos – Ossa Mts., Rhodope Mt., Cyclades islands etc.) where they have caused the exhumation of metamorphic core complex (Kilias and Mountrakis 1990, Sokoutis et al. 1993, Kilias 1995, Dinter 1998, Kilias et al. 1999). During Miocene - Pliocene, the D6 extensional event caused faulting overprinting all the previously described structures. D6 structures include high-angle normal, dip-slip to oblique faults, as well as strike-slip faults. One of the very important D6 faults is the ENE - WSW striking and SSE - dipping Aridea fault zone, which is considered to be originally a dextral strike-slip fault, reactivated during the Pliocene - Quaternary until recent as a normal dip-slip fault (Pavlides et al. 1990). In the study area, this fault separates Tzena from Paikon terrane, forming the Neogene to Quaternary Notia - Periklia depression and leading to post-accretional terrane dispersion. In our view, the primary emplacement of ophiolites upon the Triassic marbles was possibly associated with the overthrusting of the volcanoclastic series. Brown and Robertson (2003) assumed Jurassic, northeastward subduction that created an island arc within the Paikon Massif, coupled with back-arc rifting, generating the Guevgeli Ophiolites and related units behind the Paikon arc. They also propose that overthrusting of those ophiolites upon the Paikon Massif took place firstly during the Late Cretaceous to Early Tertiary. Nevertheless, in the Lower Units of both terranes, feeding dykes of the overlying volcanic formations are not found. Therefore, we suggest that these volcanic rocks of calc-alcaline composition formed somewhere else, during an initial Jurassic intra-oceanic subduction in the ocean basins, as also assumed by Michard et al. (1998). Furthermore, Jurassic magmatic series (Chortiatis series; Mussalam 1991) including reefal limestones of Late Jurassic age (Kimmeridgian - Tithonian) and clastic deposits with spilite, keratophyre, andesite and granodiorite elements (Mussalam 1991, Michard et al. 1998), exposed along the western margin of the Serbomacedonian massif, can be correlated in lithology and age with both Paikon and Tzena volcano-sedimentary formations. This Chortiatis magmatic series and the volcanoclastic series along the western Serbo-Macedonian margin was interpreted by Michard et al. (1998) as having developed in an intra-oceanic island arc setting related to an intra-oceanic subduction predating the obduction of the Axios ophiolites. Such a tectonic setting and place may be assumed also for the Paikon and Tzena volcano-sedimentary units, prior their westwards emplacement onto the Triassic units, owing to the arc - continent collision. Thus, we suggest that ophiolite obduction upon the continental series of Paikon and Tzena terranes took place simultaneously with the ophiolite tectonic emplacement towards west upon the Triassic marbles of the eastern Pelagonian margin, as described by several researches (Brown and Robertson 2003, Kilias et al. 2010). Finally, the nappe pile evolved into a multiple tectonic window, where the Lower Units, possible of Pelagonian origin (Gandatch Unit & Tzena marbles) emerged under the Jurassic volcano-sedimentary series and the Upper Jurassic - Lower Cretaceous carbonate series (Medium Units) and these again under the duplicated, ophiolitic nappe (Almopia/Garefi ophiolites to the west and Guevgeli/Peonias Ophiolites to the east). The tectonic window formed due to successive compressional and extensional events from Jurassic to Tertiary time. The final configuration of the tectonic window resulted from the Oligocene - Miocene extension (D5). An overall westward overthrusting of Paikon units, but of Tertiary age, was also proposed by Godfriaux and Ricou (1991), Ricou and Godfriaux (1995). In this approach, Paikon Massif is considered as a multiple tectonic window of Tertiary age beneath the thrusted slices of the Axios Zone ophiolites and the Pelagonian Nappe of metamorphic rocks. Additionally, Tzena Massif is considered as a Tertiary metamorphic nappe with an origin from the east (Rhodopic hinterland), that was placed tectonically upon both the ophiolites and the Paikon units. The lower unit of the Paikon Massif is correlated with the carbonate unit of Olympos - Ossa, while the overthrusted volcanic rocks with the Ambelakia HP/LT unit. However, we do not share this view, because those two windows show a different structural evolution and tectonic position. The Olympos - Ossa window represents a deeper tectonic unit of Hellenides belonging to the External Hellenides below the Pelagonian nappe pile, which exhumed first during the Tertiary. In conclusion, we suggest that Paikon and Tzena terranes are tectonostratigraphically identical and each one of them is a composite terrane that formed by terrane accretion, i.e. a complicated tectonic nappe pile, including obducted ophiolites rooted in the Axios zone, travelled westward from their initial place. The core of both terranes, probably of Pelagonian origin, is exhumed as a multiple tectonic window under the overthrusted nappe pile.
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