Tectonostratigraphy and Plate Tectonics of Crete, Greece

Ioannis Vavassis, Eric Champod, Alessia Vandelli, and Gérard M. Stampfli

A six-day field trip was held in Crete during September 2010 in honor of Prof. Gérard Stampfli who dedicated many years of his research in the study of the Hellenides. During the last decades, the members of the working group headed by Prof. G. Stampfli at the University of Lausanne had been working on plate tectonic reconstruction models based on field geology, especially in the eastern Mediterranean region. These models are constantly evolving through the continuous integration of new geological data, which are fundamental to constrain the reconstructions. The asset of these models is that they are global and controlled by geometric and kinematic constraints (Stampfli & Borel, 2002; Moix et al. 2008).
In the Tethyan orogenic system, the Aegean external arc, from the Dinarides to the Taurides, dominates the tectonics of the eastern Mediterranean. Crete represents the link between the Hellenides and the Lycian Taurus and is mainly constituted by tectonic units of the External Hellenides domain exposing a complex nappe structure. Based on their tectonostratigraphic position and their tectonometamorphic history, the units were subdivided into the upper and the lower nappes, separated by a major detachment fault (e.g. Kilias et al., 1993).
No autochtonous basement units are preserved in the whole island and the nappes are allochtonous or parautochthonous. In a structurally ascending order the units of Crete are: the parautochthonous units of Mani and Ionian (included Trypali), the Phyllite-Quartzites series s.l., the Tripolitza neritic platform, the oceanic Pindos domain, the Arvi unit, the Mélange units, the metamorphic Asteroussia nappe and the uppermost ophiolitic nappe (Papanikolaou et al. 1988). The latter represent the relics of the Vardar Ocean, thus it has to be considered as a part of the Internal Hellenides. The origin and emplacement of the Arvi and the Mélange units remain controversial.
During the last years several studies and research expeditions carried out by students and researchers of the Institute of Geology and Paleontology (UNIL) revealed new data regarding the geological history of Crete during Paleozoic and Mesozoic times thus helping to reconstruct the paleogeography and the geodynamic evolution of the Western Tethyan domain during this period (De Bono 1998, Vavassis et al. 2000, Vavassis 2001, Payer 2001, Matti et Mazzardi 2001, Bellini 2002, Stampfli et al. 2003, Champod et Colliard 2003, Kock 2003, Kock et al. 2007, Vandelli 2010, Vandelli et al. 2010 Vachard et al. 2013).

Day 1 - Talea Ori
During the first day, a visit to the Talea Ori region in central Crete took place. Talea Ori is a small mountain range situated between the northern coast and the central mountains of Psiloritis (or Ida Mt). Both ranges belong to the Plattenkalk Group (also called “Relative Autochtonous” or Mani in Peloponnesus), corresponding to the lowest tectonic unit of the Cretan nappe pile. The “Relative Autochtonous” comprises a thick series of carbonates, sometimes called the External Carbonate platform, ranging between the Late Triassic and the Tertiary (Upper Eocene to Oligocene? flysch). In the Talea Ori (TO), several formations of Late Carboniferous to Late Triassic (totally unknown elsewhere) are regarded as the substratum of this platform, from base to top:
- i) the Galinos Beds, ii) the Fodele Beds and iii) the Sisses Beds
At the base of the Sisses beds, close to the contact with the Fodele beds, a conglomerate with Triassic fauna in limestone pebbles, together with clasts from the Fodele beds, suggests an Upper Triassic erosion stage affecting the whole substratum (Kock, 2003; Kock et al., 2007). The Plattenkalk Group is covered by an allochtonous nappe, called the Phyllite-Quartzite sensu stricto (PQ s.s.). Epting (1972) and Krahl (1988), unravel broadly this nappe, and defined four main sub-units (from base to top):
- The Rogdia Schists
- The Bobias formation
- The Wassilikon marbles
- The Achlada formation
Field and laboratory data of our research (Champod et Colliard 2003, Kock 2003, Kock et al. 2007), on the deep-water Carboniferous to Middle Triassic sequences of western and eastern Crete (PQ s.s. and Tyros–Ravdoucha beds), confirm the initial investigations and proposals regarding the origin and geodynamic significance of the PQ s.s. and the Tyros beds as well as the presence of the Eo-Cimmerian collisional event within these sequences (Vavassis 2001, Vavassis et al. 2000, Matti et Mazzardi 2001 and Stampfli et al. 2003). In particular they represent remnants of the Paleotethys ocean trapped within the accretionary prism during its closure from Late Carboniferous to Late Triassic times. The most external tectonic units of Mani (Talea Ori) represent the northern margin of Gondwana (Paleotethyan south passive margin). After the Eo-Cimmerian (Late Carnian-Early Norian) closing event of Paleotethys, these domains (Tyros, PQ s.s., Mani) where covered unconformably by the Norian platforms of the Ionian and the Tripolitza series.

Day 2 - Mélanges
During the second day we focused on the mélanges outcrops. The study of the mélanges allows to reconstruct the nature, the timing and emplacement of the accreted material. Our work contributed to the definition of the accreted slices in terms of stratigraphy and paleogeography.
Above the Pindos nappe and below the crystalline rocks of the Asteroussia nappe, the Vatos, Arvi, Miamou, Spili and Preveli units were grouped together as part of a tectonic mélange, which is in line with their internal structure and their mode of occurrence (Fassoulas, 1999). Nevertheless the Arvi unit (day 4) has a more coherent structure and no ultramafic rocks, has to be distinguished as a unit.
The ophiolitic mélange units (Vatos, Spili, Miamou and the Ophiolitic nappe) were studied in order to constrain the ages and origins of the reworked material. The geochemical analyses gave variable signatures: arc lavas as well as E-MORB and N-MORB basalts. Ages range from Ladinian to Late Cretaceous and recorded the evolution of the Pelagonian terrane. The main results consist in the identification of pelagic sediments attributed to the flexuration of a lower plate during an obduction events. We could identify Oxfordian-Kimmeridgian radiolarites related to the Vardar obduction, but also Turonian-Santonian pelagic limestones related to the Lycian obduction (Vandelli, 2010). The latter is well known in Turkey but it was not identified in Greece so far.
The Asteroussia nappe has two different aspects: i) it exposes fine-bedded series of alternating quartzites and pelites sometimes calcareous, ii) it shows big lenses of amphibolites and gneisses and some basalt blocks. Some granitic intrusions as well as a paragneiss were dated around 72 Ma (Langosch et al., 2000). The basalts gave E-MORB signatures. We interpreted this unit as the fore-arc basin of the Pindos subduction, in which underplating recycles slices of the subducting plate.

Day 3 - Lentas
The predominant lithology of the Lentas hill in southern Crete are dark grey massive limestones, which were dated as Middle Jurassic-Early Cretaceous. Α thrust-fault must exist and separates this formation from the rest of the overlying series and particularly from highly recrystallized light grey limestones which constitute the base of the series. These limestones are overlain by a polygenic conglomerate reworking latest Early Permian limestones (Vandelli, 2010). In discordance with the underlying formation a basaltic flow covers the conglomerate. These basalts as well as the pillows and the tuffites succeeding in the series have N-MORB signature (Vandelli, 2010). Juxtaposed by recent tectonics there is a series of alternating sandstones and calciturbidites, which yielded late Carnian-Norian conodonts (Davi & Bonneau, 1985; Vandelli, 2010). Because of the great concordance between the facies and ages (Carnian-Norian) in the upper part of the Lentas unit and at the base of the Pindos unit, (“Priolithos Fm”), as well as comparisons with units in Greece and Turkey, we suggest that the Lentas unit forms the base of the Pindos unit.
The discovery of shallow marine latest Kungurian sediments reworked at the base of the Pindos-like Lentas unit (Vachard et al. 2013) is in agreement with the presence of Famennian (Late Devonian) conodonts reworked at the base of the Mangassa series which is recognized as an equivalent of the Pindos series in eastern Crete (Bonneau and Aubouin, 1987). This discovery also suggests that this fauna was derived from the Palaeotethyan active margin (arc/fore-arc series) in agreement with the palaeogeographic reconstructions proposed by Stampfli and Kozur (2006). The shallow marine elements reworked in the Lentas unit are unique in Crete, and in all of Greece.

Day 4 - Arvi and Kalos Potamos
The Arvi unit was divided by Davi and Bonneau (1985) into the following main facies: Late Triassic pink or grey limestones, a flyschoid series (probably Late Jurassic), Late Cretaceous creamy limestones, pillow basalts with their sedimentary cover constituted by red micritic limestones and shales (Senonian?-Maastrichtian) and a flysch. The Arvi unit was investigated by our group members Payer (2001) and Bellini (2002), who tried to define its geochemistry and geotectonic position. Moreover, the section of Kalos Potamos brought a better view about the base of the Pindos series in Crete.
The geochemical analysis of Arvi basalts yielded a within-plate signature (Payer, 2001). According to Bellini (2002), the Arvi unit occupies a structural position wedged between the Tripolitza and the Pindos units. Therefore he proposed that Arvi lavas emplaced between the Tripolitza platform and the Pindos basin in form of rare isolated volcanos. Nevertheless, sometimes this unit is found above the Pindos series. Bellini (2002) evoked the presence of out-of-sequence thrusts to explain this juxtaposition, but our recent works propose a different structural position for this unit, which could be interpreted as a back-arc basin of the Pindos oceanic domain, opening in the front of Pindos subduction under the Pelagonian terrain during Late Cretaceous time (Vandelli, 2010).
The Carnian-Norian “Priolithos Fm” consists of a detrital series called “Détritique Triasique” by Fleury (1980). It consists of an alternation of litharenites, shales and siltstones and nodular limestones. These limestones yielded ammonites of Ladinian (early Carnian) in the NW Peloponnese (Tsoflias, 1969) and are associated with basic volcanic rocks. The Priolithos Fm was studied by Bellini (2002) in the region of Kalos Potamos (SE Crete). There, crops out a turbiditic series resulting from the erosion of the Sitia-Arna shoulders and is interpreted as the combination of two events: the syn-rift stage of the Pindos back-arc opening and the inversed tectonics due to the Eo-Cimmerian collision (Vavassis 2001, Vavassis et al. 2000, Bellini 2002; Stampfli et al. 2003). Alternating limestones and volcanics flows mark the oceanization of the basin. The results of geochemical analysis indicate an evolution from E-MORB to calco-alcaline andesites and a middle Carnian age was established by conodonts found at the top of the formation (Bellini, 2002).

Day 5 - Phylite-Quartzite s. l.
Papanikolaou et al. (1988), recognized that the substratum of the External Hellenides (EH) is inhomogeneous, probably due to changeable paleogeographic conditions, therefore proposed a new, at that time, subdivision for the EH of Crete (from base to top):
1. The Relative Autochthonous (incl. the Mani and Plattenkalk units)
2. The nappe of Western Crete (incl. the Trypali unit)
3. The nappe of the Phyllites-Quartzites s.s. (equiv. of Arna unit of Peloponnesus)
4. The nappe of Tripolitza (incl. the Tyros-Ravdoucha beds and Sitia Variscan Unit)
During the 5th and the 6th day, we studied the whole part of the pile, with a special focus on the substratum of the fourth nappe (Tripolitza). In the Orno Oros (to the south) and along the coast (to the North) the “Relative Autochthonous” cropping out with gypsum, cornieule and dolomites of the Sfaka formation. The evaporitic deposits are Late Triassic (Carnian-Norian) in age, and correlated with the Trypali unit (Western Crete) or the Stromatolithe-dolomites of Central Crete (day 1).
Southwest of Sitia, near Kalavros, Krahl et al. (1986) mapped a little area and define a general stratigraphy. Later on, we confirmed that it is not a mélange, and we have been able to distinguish the following units (Vavassis 2001, Matti et Mazzardi 2001, Stampfli et al. 2003, Champod et Colliard 2003):
- Mirsini unit
- Sitia Variscan unit ] Tyros – Ravdoucha beds
- Skopi unit ] Tyros – Ravdoucha beds
- Tripolitza platform
The Mirsini unit (Krahl et al. 1986), comprises two formations, the Violet schists (at the base) and the Agrilos beds (on top). The Violet schists is a thick series of multicolored siltstones and shales (purple, red, green, brown), which is interpreted as a terrigenous turbiditic sequence. Very scarce pelagic limestone lenses occur on top, with radiolarians of late Early Permian to Middle Permian times (Kozur and Krahl, 1987). The Agrilos Formation lithologies are slightly similar to those of the Violet schists, with rare levels of black shales and also numerous discontinuous beds of thinly laminated dark grey pelagic platty limestone (calcarenites), regarded as channelized calciturbidites. This is confirmed by the occurrence in the same beds of synchronous pelagic and neritic conodontes, indicating a platform propagation. The palynology of the dark shales suggests a continental source, and a distal and pelagic depositional environment. That is confirmed by the conodonts, indicating probably 500 m depth or more. The age of this formation ranges between the late Permian (Djulfian to Dorashamian) to the middle Scythian (base of Olenekien). We found Middle Kungurian conodonts with Paleotethyan basin affinities (Champod, unpublished), Upper Murgabian to Upper Permian fauna has been also found (Doert et al. 1985, Goriss and Zacher 1998). The illite crystallinity analyses, the presence of phengite and chloritoid, the Paleozoic ages and the high values (6-7) of the Conodont Alteration Index, indicate that this unit is much more metamorphic than the overlying Tyros-Ravdoucha beds and probably belongs to the PQ s.s. This pelagic sequence represents probably a Permian forearc basin.
In eastern Crete (Chamezi), we studied the only known Cretan outcrops of Variscan Basement, called Sitia Variscan metamorphic unit. On top of it, a detritic cover of distal terrigenous and platform turbidites (Tyros-Ravdoucha beds also found in Vaï) testifies to the collapse of the margin, because of the Paleotethyan slab roll-back (deposition of slope breccias Late Schythian to Carnian in age). The basal contact with the Mirsini unit is a detachment fault, but the upper contact with the Tyros-Ravdoucha beds is less clear. It is of tectonic nature, but the amount of displacement should not be very high. This is confirmed by rare andesitic dikes with basement xenoliths cutting through the Hercynian rocks near Kalavros (Kopp and Wernado, 1983; Franz et al. 2005). Therefore, this basement unit might once was been the substratum of the Gavrovo-Tripolitza nappe. An alternative explanation was proposed by Romano et al., (2006) who described four sub-complexes of pre-Alpine basement of the Phyllite-Quartzite Unit, with different protolith age, type and age of metamorphism, and postmetamorphic cooling history since Carboniferous to Jurassic time and accreted to the northern margin of Gondwana.

Day 6 - Phylite-Quartzite s. l.
On this day, we visited outcrops of the uppermost series of the former Phyllite-Quartzite (s.l.), the Tyros-Ravdoucha beds. The Tyros-Ravdoucha beds represents a Triassic sequence which is covered by an Upper Triassic–Tertiary platform (Tripolitza), and underlain by the Variscan basement that we have seen yesterday near Sitia. The platform is detached from its substratum, and crops out to the North and the South of the Vaï peninsula. In Vaï, detritic series occur, containing many olistoliths of radiolarites, OIB basalts, andesites and basement blocks.
The volcanogenic deposits of the Chamezi formation, are followed by a thick detritic series of Ladinian to Carnian ages (Tourloti formation) made of limestone debris-flows and abundant olistolites up to 30m wide, volcanincs, lenses of sandstones and breccia. The blocks are mainly pink Hallstatt-limestones and red radiolarites of Anisian age (Matti and Mazzardi, 2001). This indicate appearance of steep faults in the volcanic arc. This is interpreted as a synrift sequence of the Pindos back-arc basin. It can be correlated with Ladinian distal turbidites of the Priolithos formation in South Eastern Crete.
The Vaï formation is a monotonous series of quartzitic sandstones, siltstones, claystones, and rare levels of breccias, with yellow to red colors. The beds exhibit turbiditic structures, with typically decimetric to metric thickness (breccias 5m thick occur at the base). The Vai formation was previously regarded as an Eo-cimmerian flysch, consecutive to the closure of the Paleotethyan ocean (Matti and Mazzardi, 2001; Stampfli et al. 2003). However, due to its position on top of the synrift Tourloti formation, it can be regarded as a post-rift sequence (thermal subsidence).
The Eo-Cimmerian tectonic event also affects the upper plate, as demonstrated in the Tripolitza unit by Cimmerian thrusts, basement conglomerates and detrital sequences. More particularly, the Palekastro formation comprises transgressive conglomerates, red molassic sediments and patch-reefs indicating a radical change of depositional environment between the Tourloti-Vaï formations (slope with turbidites and olistolites) and the Toplou-Palekastro formations (shallow water with emersion, platform, and delta).

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