(BEING CONTINUED FROM 3/5/16)
MAJOR PALEOGEOGRAPHIC, TECTONIC AND GEODYNAMIC CHANGES FROM THE LAST STAGE OF THE HELLENIDES TO THE ACTUAL HELLENIC ARC AND TRENCH SYSTEM
Present day location and geometry of the Hellenic arc and trench system is only a small portion of the previously developed Hellenic arc that created the Hellenides orogenic system. The timing of differentiation is constrained in Late Miocene, when the arc was divided in a northern and a southern segment. This is based on: a) the dating of the last compressive structures observed all along the Hellenides during Oligocene to Middle-Late Miocene, b) on the time of initiation of the Kephalonia transform fault, c) on the time of opening of the North Aegean Basin and d) on the time of opening of new arc parallel basins in the south and new transverse basins in the central shear zone, separating
the rapidly moving southwestwards Hellenic subduction system from the slowly converging system of the Northern Hellenides. The driving mechanism of the arc differentiation is the heterogeneity produced by the different subducting slabs in the north (continental) and in the south (oceanic) and the resulted shear zone because of the retreating plate boundary producing a roll back mechanism in the present arc and trench system. The paleogeographic reconstructions of the Hellenic arc and surrounding areas show the shortening of the East Mediterranean oceanic area,
following the slow convergence rate of the European and African plates plus the localised shortening following the rapid Hellenic subduction rate. The result is that the frontal parts of the accretionary prism developed in front of the Hellenic arc have reached the African continent in Cyrenaica whereas on the two sides the basinal parts of the Ionian and Levantine basins are still preserved before their final subduction and closure. The extension produced in the upper plate has resulted in the subsidence of the Aegean Sea and the creation of several neotectonic basins in southern continental Greece in contrast to the absence of new basins in the northern segment since Late Miocene.
Key words: oceanic subduction, continental subduction, upper plate extension, orogenic arc evolution,arc parallel structures, arc transverse structures.
The Hellenides are a segment of the Tethyan Alpine Orogenic Belt developed along the European active margin, resulted from plate convergence between the Eurasian plate in the north and the African plate in the south, with longlasting subduction of the Tethyan basins and platforms underneath the European margin (e.g. Papanikolaou et al, 2004; Van Hinsbergen et al, 2005). Convergence between the two plates has started since Jurassic and produced successive orogenic arcs that gave birth to the orogenic systems of the Hellenides until Miocene (e.g. Aubouin, 1974; Jacobshagen et al, 1978; Papanikolaou 1986;1993). However, convergence is still going on today with an average rate of 1 cm/year (e.g. Reilinger et al, 1997; Kahle et al, 2000) in the eastern Mediterranean, including the Hellenides, whereas collision has occured at the western Mediterranean since early Miocene and at the Arabia – Caucasus transcet since middle Miocene (e.g. Cavazza et al, 2004). Thus, subduction of the last remnant of the oceanic basin of the Eastern Mediterranean, developed at the northern part of the African plate, occurs along the
actual Hellenic arc and trench system, which is limited between the Amvrakikos Gulf in the northwest and the Rhodos transect in the southeast, forming the Aegean microplate (Fig.1). The differentiation of the present day Hellenic Arc and trench system from the previous structure of the Hellenic fold and thrust belt necessitates the distinction of the Northern Hellenides and the Southern Hellenides on both sides of the Amvrakikos Gulf (Papanikolaou & Royden, 2007).
Convergence rate between the present day Hellenic arc and Africa, expressed by the ongoing Hellenic subduction, is about 4 cm/year (e.g. Reilinger et al, 1997; Kahle et al, 2000; Hollenstein et al, 2008),which is several times more than the convergence rate between Europe and Africa. This difference between the Africa – Eurasia plate convergence rate and the Hellenic subduction rate is producing extension in the Aegean upper plate and opening of the North Aegean Basin (Papanikolaou & Royden 2007). North of the Amvrakikos Gulf the convergence rate between Apulia (part of the Adria plate) and the Northern Hellenides is around 8 mm/year and there is no arc and trench system developed other than a compressional seismically active thrust belt (Baker et al, 1997). The lateral differentiation of more than 30 mm/year convergence rate north and south of Amvrakikos is accommodated by the Kephalonia transform fault. In the area of continental Greece and the Aegean the different kinematic motion between the Aegean microplate in the south and the European plate in the north produces a vertical shear zone – the Central Hellenic Shear Zone (CHSZ) (Papanikolaou & Royden 2007) comprising strike-slip, oblique-slip and normal faults (Fig. 1). Another shear zone – the West Anatolian Shear
Zone (WASZ) – is developed at the eastern boundary of the Aegean microplate along the western coastline of Minor Asia, because of its differential motion with respect to the Anatolian microplate.
This paper is focused on a review and discussion concerning: 1) The timing of initiation of the present day geometry, from a previous homogeneous geodynamic regime along the Hellenides across the Kephalonia transform, the CHSZ and the North Aegean Basin. 2) The different tectonic and paleogeographic elements, that are considered as key points for understanding the complex evolution of the area. 3) The driving mechanism(s), that produced the observed present day differentiation of the Aegean microplate from the rest European margin. 4) A synthesis, where the overall conclusions are displayed in a series of paleogeographic sketches of the area over the last 34 million years (Oligocene – Present).
Fig. 1: Tectonic sketch of the Hellenides and the actual Hellenic arc and trench system. Bending of the thrusts in the southern segment is shown by the Pindos thrust and Late Miocene thrusts within the Ionian unit. The Late Miocene volcanic arc is shown in comparison to the Quaternary volcanic arc, developed only in the southern segment. Representative GPS rates are included together with the CHSZ and WASZ. Major detachments and normal faults of Late Miocene – Present are indicated.
2. Timing of differentiation of the Hellenic Arc
The data regarding the timing of differentiation of the Hellenic arc can be grouped in four sets:
1)At the front of the fold and thrust belt along the subduction zone on either side of the Kephalonia transform. 2) Along the CHSZ and on either side of it in the continental part of Greece.
3) In the North Aegean Basin between the northern margin of the Aegean Sea at Macedonia and Thrace and the central and southern Aegean Sea. 4) An overall study of the arc geometry in different periods,before and after the differentiation.
The migration of the Hellenic thrust and fold belt from the internal part of the Hellenides in the Aegean Sea to the external part in the Ionian Sea throughout Eocene – Miocene is well known longtime ago (Philippson, 1959; Aubouin, 1959; 1965; Jacobshagen et al, 1978; Papanikolaou, 1986).
The last compressional structures related to the front of the Hellenic fold and thrust belt have been reported from the Ionian islands of Kephalonia and Zakynthos, involving Pliocene or even Early Pleistocene sediments (Mercier et al, 1972; 1979; Underhill, 1989). However, the above structures are localised and rather exceptional with respect to the dating of the latest structures all along the western coast of continental Greece and the rest Ionian Islands, where the latest sediments involved in the compressional structures are of Late Miocene – Early Pliocene age (around 5 million years) including also the well known, all over the Mediterranean Basin, Messinian evaporites (e.g. Zakynthos and Parga) (IGSR & IFP, 1966; B.P. 1971; Hsu et al, 1978). North of the Amvrakikos Gulf there is no evidence of Plio-Quaternary compressional structures and the tectonic trend of the Miocene folds and thrusts is dextrally offset by several tens of km with respect to its location south of the gulf. The overall dextral offset of the Kephalonia transform fault is about 100 km, judging from the location of the northern edge of the Hellenic trench/plate boundary south of the Kephalonia transform and its approximate location north of the Kephalonia transform in the area west of the Paxos and Corfu islands,
determined on the basis of bathymetric and geophysical data (e.g. Monopolis & Bruneton, 1982).
In continental Greece, the difference of the distribution of Plio-Quaternary basins north and south of the CHSZ is impressive, with almost no Plio-Quaternary basin formed in the Northern Hellenides in contrast to five basins developed in the southern Hellenides along the transect from Western Messinia in southwestern Peloponnese to southern Evia in the central Aegean (Fig. 2). These basins form neotectonic grabens filed with marine and/or continental sediments onshore and actual marine sedimentary basins offshore, developed within the successive gulfs of Messiniakos, Lakonikos, Argolikos, Saronikos and Southern Evoikos. These basins correspond to neotectonic grabens bounded by neotectonic horsts
forming the mountainous regions of Western Messinia, Taygetos/Mani, Parnon, Argolis, Attica and Southern Evia. This alternation of neotectonic horsts and grabens in the NNW-SSE direction shows a WSW-ENE directed extension that forms arc-parallel structures within the Aegean upper plate. The intensity of deformation as expressed by the topographic relief, the sedimentary thickness and the fault displacement values shows a decrease from the external part of the arc in the southwest towards the internal part in the northeast (Papanikolaou et al, 1988). The minimum extension estimated across tHe above profile is 35 km, considering a mean 45o dip of the normal faults forming the marginal faults of the basins.
Within the CHSZ the accomodating structures are mainly the basins of Northern Evoikos Gulf, Beotikos Kiffissos and Corinth Gulf. These basins are transverse or oblique with respect to the arc geometry, with prevailing E-W trend controlled by normal faults which are seismically very active. They are filled with continental sediments mainly of Late Miocene – Pliocene age with some marine influence only during Middle – Late Pleistocene. North of the CHSZ, the latest neotectonic basins / grabens occur east of the Mesohellenic molassic basin with Late Miocene – Pliocene continental deposits. The latest sediments
in the continuous molassic sequence of the Mesohellenic basin are of Late Miocene (Tortonian) age marking the end of marine sedimentation in central-northern continental Greece.
Fig. 2: Schematic neotectonic map and profile across the arc parallel basins/grabens of the Southern Hellenides,developed within Late Miocene – Present under an extension in the ENE-WSW direction (modified after Papanikolaou et al, 1988). A decrease of the deformation is shown by the estimated magnitude of fault throw along the profile.
The opening of the North Aegean Basin has occurred sometime in Early Pliocene judging from its geometry,sedimentary thickness, fault displacements and differences in the present day GPS rates observed on both sides of its margins (Lalechos & Savoyat, 1979; Le Pichon et al, 1984; Armijo et al, 1999; Papanikolaou et al, 2002; 2006). The opening of the basin is more pronounced in the western part (40 km)than in the eastern (20 km) and so is the depth (1600 m in the west and 950 m in the east) (Fig. 3). The location of the basin at the western prolongation of the North Anatolian Fault and its seismotectonic characteristics,implying a dextral strike-slip motion, have been the main argument for an induced basin because of wrench tectonics along the European margin, following the continental collision in the Caucasus area during the Middle Miocene and the subsequent lateral escape of Anatolia (Brunn, 1976; Dewey &Sengor, 1979; LePichon & Angelier, 1979; Armijo et al, 1999; McNeill et al, 2004; Kreemer et al, 2004).
The understanding of the transition period between the last stage of the Hellenides, viewed as a continuous orogenic arc involving all the characteristic parts, and the present day geometry of the Hellenic arc and trench system was based on an analysis of the paleogeographic position of the arc segments by comparing the geometry of the Burdigalian period with that of the Messinian and of the Plio-Quaternary period
(Papanikolaou & Dermitzakis, 1981; Dermitzakis & Papanikolaou, 1979; 1981) (Fig. 4). The main features of the arc that have changed in Messinian are: 1) The uplift and ending of marine sedimentation in the Mesohellenic and the Cycladic molassic basins. 2) The opening of a new molassic basin in the area of the Cretan Sea between the Cyclades and Crete. 3) The termination of the volcanic arc activity in the segment of the Northern Hellenides and the continuation of the arc volcanism only in the segment of the Southern Hellenides. During the Plio-Quaternary, the volcanic arc continued its migration towards the
more external zones of the Hellenic arc until its present location along the southern margin of the Cycladic plateau at a 60-80 km distance away from the location of the Late Miocene volcanic arc (see also Fig. 1).
4) The initiation of opening of the North Aegean Basin in the North Aegean Sea at the western prolongation of the newly formed North Anatolian Fault (see also Fig. 3).
5) The beggining of subduction of the last remnant of the Tethyan oceanic crust of the Eastern Mediterranean Basin along the arcuate segment formed between the Amvrakikos Gulf and Rhodes.
6) The initiation of the Kephalonia transform fault at
the level of the Amvrakikos Gulf.
7) The migration of the fold and thrust belt towards the Ionian islands south of the Kephalonia transform fault, with Plio-Quaternary compressional structures at a distance of
several tens of km away from the Late Miocene structures.
8) The disruption of the Late Eocene – Miocene fold and thrust belt and the relative westward displacement of the Hellenides nappes in the southern segment.
Thus, the Pindos thrust is observed today at the front of the nappe system in southwest Peloponnese and Kythira Island up to Gavdos Island south of Crete, adjacent to the trench, whereas in the Northern Hellenides it rests along the western slopes of the Pindos chain at a distance of 90-120 km away from the plate boundary (see also Fig. 1). 9) The maximum uplift of the nappe pile in the southern Hellenides, producing the arrival at the surface of the higher mountains of south Peloponnese of the basal metamorphic unit of Mani and of the overlying metamorphic unit of Arna. 10) The development of a number of arcparallel
extensional neotectonic basins in the Southern Peloponnese and of transverse neotectonic basins in the Northern Peloponnese and Sterea Hellas (see also Fig. 2).
In conclusion, the timing of differentiation of the Hellenic arc and its subdivision in the Northern and the Southern Hellenides is constrained in Late Miocene. Tortonian is the last period of the previous uniform Hellenic arc and Messinian is the re-organization period whereas already by the beginning of Pliocene the new subdivision and arc geometry has been established.
Fig. 4: Paleogeographic and paleogeodynamic sketches of the Hellenic arc from the last stage of the Hellenides (Oligocene – Early/Middle Miocene to a reorganisation period in Late Miocene and to the actual Hellenic arc and trench system (Plio-Quaternary) (after Papanikolaou & Dermitzakis, 1981).
(TO BE CONTINUED)
1 University of Athens, Department of Dynamic, Tectonic, Applied Geology,
Panepistimioupoli 15784, Athens, Greece, email@example.com
SOURCE Bulletin of the Geological Society of Greece, 2010,Proceedings of the 12th International Congress Patras, May, 2010