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New structural and petrological data on the Amasia ophiolites (NW Sevan-Akera suture zone, Lesser Caucasus): Insights for a large-scale obduction in Armenia and NE Turkey)
Les ophiolites du Petit Caucase appartiennent à la ceinture ophiolitique nord-téthysienne. Celles-ci représentent la prolongation orientale de la ceinture ophiolitique anatolienne (suture d’Izmir-Ankara-Erzincan) (Knipper, 1975; Knipper & Khain, 1980; Adamia et al., 1981; Şengör & Yılmaz, 1981; Dercourt et al., 1986; Adamia et al., 1987; Ricou, 1994; Yılmaz et al., 2000; Galoyan 2008). Au nord de l’Arménie, cette suture se prolonge par la celle de Sevan-Akera, orientée globalement NW-SE. Elle marque la frontière entre le SAB, au sud, et la marge active de l’Eurasie, au nord. La mise en place d’une marge active de type andin (Somkheto-Karabakh) durant une grande partie du Mésozoïque (Bajocien à Campanien) et de l’Eocène moyen et supérieur témoignerait en faveur d’une subduction plongeant vers le nord sous l’Eurasie (Knipper, 1975; Adamia et al., 1977; Adamia et al., 1981; Maghakyan et al., 1985; Adamia et al., 1987).
Les travaux précédents sur les ophiolites de cette région dans le cadre de la thèse de Galoyan (2008) montrent que les ophiolites arméniennes des zones de Stepanavan, Sevan et Vedi sont toutes de type LOT avec des caractéristiques litho-structurales et pétro-géochimiques identiques (Galoyan et al., 2007; Galoyan, 2008; Rolland et al., 2010; Sosson et al., 2010). De plus, les âges de formation du plancher océanique sont tous compris entre le Jurassique inférieur et moyen. Les datations ont été obtenus par la méthode 40Ar/39Ar sur amphiboles de gabbro de Sevan et de diorite de Vedi, donnant respectivement 165,3 ± 1,7 Ma et de 178,7 ± 2,6 Ma (Galoyan et al., 2009; Rolland et al., 2010). Ces datations radiochronologiques sont confirmées par les assemblages de radiolaires reconnues dans ces zones (Danelian et al., 2000; 2008; 2012; Asatryan et al., 2009; 2010)
Le massif ophiolitique d’Amasia est situé à 5 km NW de la ville Amasia dans le prolongement ouest de la zone de Sevan-Akera. Sur une surface de ~30 km2 affleurent des peridotites et pyroxénites serpentinisées, des gabbros et plagiogranites (en filons d’une épaisseur de 1cm à 1m), des radiolarites, des basaltes ainsi que des roches métamorphiques (amphibolites à grenat : Sokolov, 1974; Aghamalyan, 1998). Tatevosyan (1950) décrit des gabbros au nord et des roches volcaniques Turoniennes au sud. Toujours d’après Tatevosyan (1950), ces roches volcaniques recouvrent en discordance des schistes et sont surmontées par des calcaires du Crétacé supérieur. Les roches ultrabasiques et gabbroïques du massif ophiolitique d’Amasia ont déjà fait l’objet d’études montrant que les péridotites sont parfois fortement serpentinisées. Toutefois, malgré cette serpentinisation il est toutefois possible de reconnaître qu’il s’agit de lherzolites (Tatevosyan, 1950).
Sur la base de directions structurales différentes, certains auteurs ont proposé que les ophiolites Sevan-Akera et Stepanavan-Amasia proviendraient de différents segments de croûte océanique (Melikyan, 2004). Afin de valider l’hypothèse de Galoyan (2008) concernant la présence d’une seule et même nappe ophiolitique obduite à l’échelle du SAB, et donc afin de lever l’ambigüité concernant l’appartenance, et donc la continuité, du massif ophiolitique d’Amasia à la suture Sevan-Akera, nous avons effectué une étude pétro-structurale incluant des datations 40Ar/39Ar de ce massif.
Notre cartographie a mis en évidence une série comprenant (1) de la croûte océanique gabbroïque non métamorphisée, (2) des serpentinites, (3)mélange tectonique de faciès schiste vert composé basaltes en coussins déformés avec des radiolarites intercalées, et (4) une écaille basale d’amphibolites à grenat. Cette semelle métamorphique montre des caractéristiques géochimiques similaires à celles de l’ophiolite. Ces unités sont découpées par des failles inverses et déformés par des phases post-Eocène de compression de la zone de suture au cours de, et après, la collision du SAB avec l’Eurasie. Les datations 40Ar/39Ar sur les amphiboles des gabbros ont donné des âges de 169,0 ± 4,6 à 175,8 ± 3,9 Ma.
L’ensemble de ces âges ainsi que les compositions des roches ophiolitiques sont similaires à ceux des autres ophiolites arméniennes et de celles de Turquie du NE. Aussi, toutes ces données sont donc bien en accord avec la mise en place d’une seule nappe ophiolitique à l’échelle de l’Anatolie du NE et de l’Arménie au début du Crétacé supérieur (c. 90 Ma). Ces résultats suggèrent également que l’ensemble du domaine obduit provient d’un domaine où la croûte océanique formée est contaminée par la subduction. Il pourrait s’agir d’un bassin arrière-arc, ou d’une zone surplombant la subduction. Ce bassin se serait ouvert au Jurassique, puis obduit par la suite sur le SAB-TAP.
Cette étude a fait l’objet d’une publication parue dans Tectonophysics.
Abstract
The ophiolites of Amasia in the northwestern part of the Sevan-Akera suture zone (Lesser Caucasus, NW Armenia) correspond to a well-preserved example of a major obduction of oceanic lithosphere over the South Armenian continental block. Our mapping evidenced a series of (1) un-metamorphosed gabbroic oceanic crust, (2) serpentinites and a greenschist grade tectonic melange composed of deformed pillow-basalts, radiolarites and cherts, and (3) a basal slice of garnet amphibolites bearing similar compositional features as the ophiolite. These units are sliced and deformed by post-Eocene thrusting related to the shortening of the suture zone after the collision of the South Armenian Block with Eurasia. 40Ar/39Ar dating on gabbro amphiboles yielded ages of 169.0 ± 4.6 to 175.8 ± 3.9 Ma. This age and geochemical composition of ophiolite rocks are similar to those of other ophiolite outcrops in Armenia and NE Turkey. Structural and geochemical analyses undertaken on the garnet amphibolites suggest it to represent the obducted ophiolite metamorphic sole. All these data are in agreement with the presence of a unique ophiolite nappe at the scale of NE Turkey-Armenia originating from a Jurassic intra-oceanic back-arc basin, obducted onto the Armenian-Taurides-Anatolides microblocks in the early Late Cretaceous (c. 90 Ma).
Introduction
In order to better understand the different phases linked with the opening and closing of the Tethyan Ocean leading to the current structure of the Lesser Caucasus, it is important to identify the different units involved in the Tethyan paleosuture s.l. and their corresponding geodynamic context. The evolution of central and northern Neotethys can be deduced from both the geochemical and geochronological study of preserved oceanic crust domains obducted (ophiolites) in the Lesser Caucasus and the metamorphic rocks beneath ophiolites. These studies yield key time and palaeogeographic data from the East Mediterranean area to the NW Himalayan belt (Sengör and Yılmaz, 1981; Ricou et al., 1985; Dercourt et al., 1986; Ricou, 1994; Okay and Tüysüz, 1999; Stampfli et al., 2001; Robertson et al., 2004; Barrier and Vrielynck, 2008; Galoyan et al., 2009; Rolland et al., 2010; Sosson et al., 2010). Ophiolites provide constraints on oceanic opening by the dating of related magmatic rocks and of its closure by the dating of metamorphic rocks and post-accretionary sedimentary complexes unconformably overlying the suture zone. Datings undertaken along the Ankara-Erzincan-Sevan suture zone suggest a similar Lower-Middle Jurassic age of the oceanic crust (Çelik et al., 2011; Galoyan, 2008; Galoyan et al., 2009; Rolland et al., 2009b; 2010). A major difficulty in the Lesser Caucasus Mesozoic geodynamic reconstruction lies in the paucity of outcrops due to thick post-obduction (Eocene to Quaternary) deposit of sediments and volcanites over the ophiolitic nappe (Sosson et al., 2010). Therefore, to link the NE Turkey and Armenia ophiolitic domains, one of the main questions posed in this study is the origin of the ophiolite: “are all the NE Turkey-Armenia ophiolites remnants of the same oceanic lithosphere, obducted over the Armenian-Tauride-Anatolide Block?”
In this paper is reported new geological, petrologic and 40Ar/39Ar chronological data obtained on the Amasia ophiolite (NW Armenia) which strongly suggests a common origin with the other Armenian ophiolites (Sevan, Stepanavan and Vedi ophiolites) and NE Turkey (Refahiye, Şahvelet, Karadağ and Kırdağ).
Geological context
During the Mesozoic, the Southern margin of the Eurasian continent was involved in the closure of the Paleotethys and opening Neotethys Ocean (Adamia et al., 1981). Later, from the Jurassic to the Eocene, subductions, obductions, micro-plate accretions, and finally continent–continent collision occurred between Eurasia and Arabia, and resulted in the closure of Neotethys (Sosson et al., 2010; Figure 7).
The study of Armenian ophiolites allows reconstructing part of this complex history. Previous geological, petrological and geochemical works on these ophiolites were carried out mostly during the 1970’s and 1980’s (Knipper, 1975; Sokolov, 1977; Knipper and Sokolov, 1977; Knipper and Khain, 1980; Adamia et al., 1981; Zakariadze et al., 1983, 1990; Knipper et al., 1986; Knipper et al., 1997; Satian, 2005; Zakariadze et al., 2005). These works showed a mainly Jurassic age for the ophiolite bodies, with varied geochemistry (from tholeiitic to calc-alkaline and alkaline), which was interpreted as a complex oceanic context with varied magmatic sources closed by subduction in the Late Cretaceous. More recent works (Hacker, 1991; Yilmaz et al., 1993; Hacker et al., 1996; Harper et al., 1996; Searle and Cox, 1999; Okay et al., 2001; Stampfli et al., 2001; de Sigoyer et al., 2004; Ding et al., 2005; Galoyan et al., 2009; Rice et al., 2009; Rolland et al., 2009a-b; 2011a-b; Agard et al., 2010; Sosson et al., 2010) evidence processes which include Neotethyan oceanic crust obduction and the collision–accretion of microplates to the Eurasian margin before the final Arabia–Asia collision.
North of the obduction zone, in the Eurasian part of the Lesser Caucasus the subduction of the Tethys is evidenced by a thick and mainly calcalkaline volcanogenic and volcanoclastic series dated as Bajocian to Santonian (e.g. Adamia et al., 1981 for a review). At this period of time the northern Lesser Caucasus was characterized by an island arc domain called the Somkheto-Karabakh Island Arc (Knipper, 1975; Adamia et al., 1977; 1987; Ricou et al., 1986; Sosson et al., 2010). During the Early Cretaceous an active erosion event took place, which resulted in the unroofing of plutons of the magmatic arc. This erosion event is the result of significant uplift and denudation during the Early Cretaceous. The reasons for such a change in the Eurasian active margin strain field could be the subduction of the spreading ridge of the back-arc basin. The basement formations are quite similar to those known all along the Eurasian margin (Sosson et al., 2010 for a review).
The Eastern Pontides are interpreted as a part of the Sakarya Zone (Okay and Şahintürk, 1997) They represent an active continental margin of Eurasia, which was formed as result of northward subduction of Neotethys during Late Cretaceous (Şengör and Yılmaz, 1981; Akıncı, 1984; Okay and Şahintürk, 1997; Parlak et al., 2012). There is no consensus concerning onset of subduction since Jurassic (Adamia et al., 1981; Hess et al., 1995; Nikishin et al., 2003), Cenomanian-Turonian (Yılmaz et al., 1997; Okay and Şahintürk, 1997) or Albian (Okay et al., 2006) ages have been proposed. The lack of consensus equally stands when considering the end subduction and continental collision; end of Eocene (Peccerillio and Taylor, 1976; Şengör and Yılmaz, 1981; Robinson et al., 1995), Middle Eocene (Yılmaz et al., 1997) or Paleocene (Okay and Şahintürk, 1997) have been proposed.
South of the obduction zone, the South Armenian Block (SAB) (Knipper, 1975; Knipper and Khain, 1980) is a microplate also corresponds to the Turkish and Iranian platform (Sengör and Yılmaz, 1981; Figure 7). In Armenia the SAB is represented by a Proterozoic metamorphic basement well exposed north of Yerevan, an incomplete Palaeozoic sedimentary succession (mainly represented by Upper Devonian to Upper Permian carbonates and shales) in the SW (north of the Araks Valley), Triassic limestones and sandstones and some Jurassic sedimentary and volcanogenic formations, Cenomanian to Turonian limestone and flysch (Nalivkin, 1976; Sosson et al., 2010) (Figures 8 and 9).
The East Anatolian Platform (EAP) represents a continental platform between the northern and southern branches of Neotethys (Bozkurt and Mittwede, 2001). As the SAB, the EAP represents a sliver of continental crust having rifted off northern Gondwana and drifted north to collide with Eurasia (Stocklin, 1974; 1977; Adamia et al., 1977; Biju-Duval et al., 1977; Dercourt et al., 1986; Şengün, 2006).
Upper Cretaceous obduction on the SAB is deduced from Upper Coniacian to Santonian flysch (reworking the ophiolites), which conformably covers Cenomanian-Turonian reef limestones and flysch of the SAB (Sokolov, 1977; Sosson et al., 2010; Figure 9). Obduction took place while a magmatic arc occurred along the southern edge of Eurasia (Somkheto-Karabagh island arc, Lesser Caucasus; Eastern Pontides arc, Anatolia; Figure 7), which implies that at least two subduction zones were active at the same time (Rolland et al., 2011a). The onset of collision or the continental subduction of the SAB below the Eurasian margin is dated as Late Cretaceous-Paleocene. This process occurred around 20 Ma later than the obduction (Late Coniacian–Santonian, 88–83 Ma) of the marginal basin over the SAB (Sosson et al., 2010). Oceanic closure is indicated by the late-Middle Eocene unconformity on the SAB, the suture zone and the Eurasia margin. Ending of subduction and subsequent accretion of the SAB to the Eurasian margin results in subduction jump to the south of the SAB and related Tauride-Anatolide Block(s) (TAB) in the same period of time. Evidence for this southward jump in subduction can be found between the Bitlis-Pütürge massifs and SAB. A HP metamorphic evolution bracketed between 74-71 Ma (Göncüoğlu and Turhan, 1984; Hempton, 1985; Oberhänsli et al., 2010). The metamorphic age is in agreement with a continental subduction event that occurred before final closure of the southern Neotethys and Arabian-Eurasian collision. 40Ar/39Ar dates agree for initial subduction of the Eastern Bitlis massif at 74 Ma followed by underthrusting of the Pütürge massif under blueschists conditions at 71 Ma (Rolland et al., 2012).
Figure 8 – Structural map of the Lesser Caucasus modified from Sosson et al. (2010). Location is indicated on figure 7.
Plot of geological section figure 9 and location of figure 10 indicated.
For a compilation of works about the ophiolites of the Lesser Caucasus, the reader is referred to Galoyan et al. (2007, 2009) and Rolland et al. (2009b). These authors have shown the following geochemical tendencies in the ophiolite-related nappes: (1) the basalts and gabbros mainly bear an enriched tholeiitic composition, contaminated by subduction components, (2) above these series, a layer of alkaline basalt lava flows with large pillows is supposed to represent Ocean Island Basalts (OIB) erupted in seamounts or oceanic plateau(s), (3) locally some arc-related basalts have been described. In Armenia, the oceanic gabbros of the tholeiitic series are dated to 170-150 Ma similar to radiolarian ages (Danelian et al., 2010), while the alkaline series were dated at c. 117 Ma (Rolland et al., 2009b).
Figure 9 – Interpretative crustal-scale sketch cross-section of the Armenian-Azerbaijan transect. Location is indicated on Figure 8.
Figure 10 – Structural map of the Amasia ophiolite window. Location is indicated on figure 9. Plot of geological sections of figure 11 along with dated samples by the Ar-Ar method and paleontological identification are shown.
The Amasia ophiolite is aligned within the Lesser Caucasus ophiolite belt (Amasia-Sevan-Akera ophiolites), striking SE–NW in northern Armenia (Figure 9), generally interpreted as representing the suture zone (e.g., Zakariadze et al., 2007) between Eurasia and the SAB. In Stepanavan, East of Amasia, ophiolites have been described in association with blueschists and amphibolite facies metamorphic rocks dated at 94-91 Ma (Pressure peak) to 73-71 Ma (High temperature retrogression; Rolland et al., 2009a). These metamorphic rocks evidence the presence of a subduction zone active at least in the Middle Cretaceous and closing in the Late Cretaceous at 80-75 Ma (Rolland et al., 2011a).
Field and sample observations
According to the field observations and the new geological map with cross sections we have made (Figure 10 and 12), 3 main lithotectonic units have been identified (from top to bottom) (Figure 11);
Figure 11 – Synthetic lithostratigraphic log of the three main units of the Amasia ophiolite window. I, the upper unite corresponding to ophiolite. II, the metamorphic unit comprising of the tectonic melange and the lens of garnet bearing amphibolites. III, the lower unit.
1 – An upper unit (ophiolites) with serpentinite, gabbro, pillow lava and volcanic rocks with interlayered reef limestone. Included in this unit a Coniacian-Santonian detrical deposit, reworking elements from the entire ophiolitic unit.
2 – A tectonic melange of low grade (greenschist facies) meta-basalt, meta-chert and metamorphosed serpentinite which includes lenses of ophiolite and a major garnet bearing amphibolites unit,
3 – A lower unit of basal basalts, overlain by Valanginian-Barremian limestones, which are in turn unconformably covered by late Paleocene flysch to Lower Eocene limestone as well as Mid- to Upper Eocene volcanogenic deposits (as in all of the Lesser Caucasus) (Sosson et al., 2010).
All of these units are unconformably overlain by a Miocene to Quaternary volcanic cover.
Figure 12 – Sketch geological cross sections of Amasia ophiolite. Locations are indicated on figure 10.
The upper unit (ophiolite)
The northern part of the map corresponds to an ophiolitic series (upper unit) generally dipping towards NNW. Sampling for dating of formation and characterization was undergone in this unit (Figure 10). It is composed, from top to bottom, of interbedded reef limestones embedded in volcanic tuffs and lava flows of supposed Cretaceous times, serpentinites comprising lenses of gabbros, scattered outcrops of volcanic rocks, and gabbros with punctual lenses of serpentinites (Figure 13A). These volcanics are linked to arc related volcanism and erosion, including possible OIB volcanism deposited on the seafloor prior to obduction. The lenses of gabbros in the serpentinites are generally well preserved and show a WSW-ENE stretching direction. These lenses are also penetrated by dikes of acidic composition (plagiogranite) and quartz veins. The outcrops of serpentinite in the gabbro do not have neatly defined contacts but appear in patches suggesting a gradational transition from the gabbro, in agreement with a cumulative origin. The northern contact of the gabbro with the serpentinites is deduced by a greater number of outcrops of serpentinites, multi-centimetric amphiboles and an increase in dike thickness and density approaching the contact zone. It is masked by Coniacian-Santonian flysch. Thus, the tectonic intercalation nucleates on previous ocean-floor faults, which illustrates the role of previous anisotropies in the obduction tectonics (Figure 13B and 13C).
A syn-tectonic detrital deposits reworking elements of the ophiolite rests unconformably on top. Along the northern limit of the outcrop it is overthrusted by serpentinite (upper ophiolitic unit) towards the south (Figure 13F). Its southern limit is characterized by deformation by thrusting and scaling onto the metamorphic unit. Its nannofossil age (Table 4) is bracketed to the Coniacian-Santonian (89.3-83.5 Ma). The deformation consequent to this thrust is marked by dissymmetrical folds, with axes aligned with the general strike of the thrust. This fault, characterized by a NNW dip, was subsequently active during or after the deposit of this detrital material, i.e. syn-post Coniacian (89 Ma). Laterally to the east, this formation is unconformably lying along on gabbro. At its extreme NE limit with the gabbro, the detrital deposits are overthrusted by the gabbros along a fault contact with a SE dip.
Table des matières
Introduction
Chapitre 1 – Ophiolite/Obduction : historique et problèmatique de la thèse
I.1 Concepts
1.1 Ophiolite
1.2 Obduction
I.2 Contexte général de la zone d’étude
I.3 Problèmatique avant thèse
Chapitre 2 – Etude géologique, pétrogéochimique et métamorphique des ophiolites nord-est anatoliennes et du Petit Caucase : implication géodynamique.
II.1 Article 1 – New structural and petrological data on the Amasia ophiolites (NW Sevan-Akera suture zone, Lesser Caucasus): Insights for a large-scale obduction in Armenia and NE Turkey)
Abstract
1.1 Introduction
1.2 Geological context
1.3 Field and sample observations
1.4 40Ar/39Ar Dating
1.5 Geochemistry
1.6 Discussion
Acknowledgements
References
II.2 Article 2 – P-T-t history of the Amasia ophiolite “metamorphic sole” (Amasia, Lesser Caucasus): implications for the obduction process of an old oceanic lithosphère
Abstract
2.1 Introduction
2.2 Geological setting
2.3 Field Observations
2.4 Geochemistry
2.5 Petrography and mineral chemistry
2.6 Pressure-Temperature path of the Amasia amphibolites
2.7 40Ar/39Ar Dating
2.8 Discussion
2.9 Conclusion
Acknowledgments
References
Tables
Chapitre 3 – Relations entre les ophiolites du N-E de l’Anatolie et du Petit Caucase : arguments pour une obduction de grande échelle de croute océanique.
III.1 Article 3 – Linking the NE Anatolian and Lesser Caucasus ophiolites: evidence for large scale obduction of oceanic crust and implications for the formation of the Lesser Caucasus-Pontides Arc
Abstract
1.1 Introduction
1.2 Previous works across the NE Anatolia-Lesser Caucasus region
1.3 Structural Continuity
1.4 Discussion and Geodynamic implications
Acknowledgements
References
Chapitre 4 – Métamorphisme du Bloc Sud Arménien (Jurassique Supérieur – Crétacé Inférieur) : subduction à vergence sud de la branche nord de la Néotéthys.
IV.1 Article 4 – Multi-stage metamorphism in the South Armenian Block during the Late Jurassic to Early Cretaceous: tectonics over south-dipping subduction of Northern branch of Neotethys
Abstract
1.1 Introduction
1.2 Geological Setting
1.3 New Field Observations
1.4 Mineralogy and Pressure-Temperature path of metamorphic rocks
1.5 Geochronology and Geochemistry
1.6 Discussion
1.7 Conclusion
Acknowledgements
References
Chapitre 5 – Histoire de la branche nord de Néotéthys avant son obduction.
V.1 Article 5 – From ocean crust geneis to obduction initiation: history of the northern branch of Neotethys prior to the Late Cretaceous obduction event in NE Anatolian and Lesser Caucasus regions
Abstract
1.1 Introduction
1.2 Main tectonic units
1.3 Discussion: what evolution of the geodynamic processes can explain the preobduction
framework?
1.4 Conclusion
Acknowlegements
Table des Matières
References
Chapitre 6 – Mise en place d’ophiolites préservées : une modélisation.
VI.1 Introduction
VI.2 Modélisation numérique
2.1 Configuration
2.2 Modèle 1 – rajeunissement thermique étendu à tout le domaine océanique
2.3 Modèle 2 – rajeunissement thermique du domaine océanique restreint à proximité
de la marge continentale
2.4 Modèle 3 – sans rajeunissement thermique du domaine océanique
2.5 Modèle 4 – sans extension post-obduction
VI.3 Discussion de la modélisation
Conclusions Générale
Références
Annexes
Liste des Annexes
