From paleocontinental reconstructions it can be shown that a large oceanic space (Paleotethys) remained unclosed south of the Variscides until late Paleozoic. The Devonian to mid Carboniferous collisional process in Europe is related to the accretion of the Hun superterrane to Avalonia-Baltica. This terrain presents strong affinities with the Paleotethyan passive margin sequences found for example in northern Iran (Alborz). The latter is regarded as representing the southern margin of this ocean, the Hun superterrane the northern margin. This northern margin was separated from Gondwana in the Ordovician/Silurian times. The collision with the already accreted Avalonia terrane started in the Visean, the subduction was then jammed and jumped into the Paleotethys. Northward subduction of the Paleotethys is responsible for late Carboniferous calc-alkaline intrusions and volcanism found everywhere in the Alpine realm. Closure of the Paleotethys was achieved after the Namurian north of Africa but is diachronous going eastward. East of a paleo-Apulian promontory, subduction continued into the Permian and generated the opening of the Hallstatt-Meliata-Karakaya marginal ocean. Closure of Paleotethys between Iberia and Africa resulted from the westward displacement of parts of the Hun superterrane, i.e: the Intra-Alpine and Cantabrian terranes; its un-displaced portion (the Aquitaine terrane) is found in Spain (Pyrenees, Catalonia) and southern France (Montagne Noire) and is separated from its exotic portion by a marine trough lasting until the latest Carboniferous (Cantabrian, Asturian basin).Concomitant opening of the marginal Meliata back-arc ocean within the Eurasian margin and Neotethys along the Gondwanan margin in late Permian accelerated the closure of the Palaeotethys in the Dinaro-Hellenide region. Late Permian to early Triassic mélanges found in Greece point to a final closure of this Paleozoic ocean at that time (Eocimmerian event). The subsequent intra-oceanic subduction of the Meliata ocean generated the Vardar marginal ocean which was obducted in late Jurassic onto the Dinaro-Hellenic area. Its subsequent NE directed subduction generated the collision of an intraoceanic arc with the Austro-Carpathian and Balkhanide areas in late early Cretaceous times.Therefore, in a SE transect of Europe we can see how the Variscan orogen evolved into early and late Cimmerian deformations. The rather clear situation found in the Appalachians cannot be extrapolated much farther than western Iberia where Laurentia and Gondwana collided in the Carboniferous. In the rest of Europe, the collision never happened as such. There was a collision between the Eurasian active margin and terranes derived from Gondwana. One has to wait for the anticlockwise rotation of Africa in the Cretaceous to see a collision between Europe and Africa, giving birth to the Alpine orogen.
Many Triassic rift-related basalts in the Tethyan area have long been known to have an arc-type geochemical signature, as shown typically by relative depletion in Nb compared to light REEs, (e.g. E Othrys; Atalanti) while other coeval suites are alkali basalts (e.g. Avdela; Mamonia). From the lack of evidence of southward (Gondwana-directed) subduction, the arc signature is generally believed to have been inherited from an earlier subduction-induced metasomatism and to reside in the lithospheric mantle. If true then the arc signature might correlate positively with an enriched 143/144Nd signature if the host lithospheric mantle had not been melted for a long period. In the course of our analytical programme designed to test this hypothesis, our colleague, Godfrey Fitton, demonstrated that Icelandic basalts have a consistently higher ratio of Nb/Y than MORB basalts for a given Zr/Y, i.e. that the plume source has a significant relative enrichment in Nb compared to other incompatible elements and this is evident in the products regardless of the degree of melting, (Fitton et al., 1997). We have used the empirical measure of this enrichment, Nb as an additional test for the origin of an extensive Triassic rift basalt suite extending from Greece through W Turkey to Cyprus. Results were unexpected. Basalts with +ve Nb (=plume related) occur throughout the area. Basalts with this signature show no superimposed arc character and have the normal OIB-type La/Nb ratio of 0.9. Basalts with -ve Nb, lying outside the Iceland array, which are often closely associated spatially with the plume type, may have additional variable arc character superimposed. Isotopically, the results to date show a negative correlation between Nb and 143/144Nd and no relation between La/Nb and isotopic composition. These results are consistent with there being in general two distinct classes of source for rift-related basalts along the Gondwanan margin: one is plume type; it has been immune from arc influence and is relatively enriched isotopically. The other is MORB-type, with a variable or absent arc signature and no clear evidence of derivation from old lithosphere. If the arc signature is inherited, the original metasomatic event may have been relatively recent (Hercynian?) and the mantle host subject to melt extraction at the same time. The important conclusion is that the plume signature measured by Nb is found over the full length of the Tethyan belt so far examined and may well imply that more than one plume was involved. This provides a realistic mechanism to explain the hitherto perplexing Permo-Triassic break-up of the Gondwanan margin, thus supporting and extending a suggestion of Pe-Piper (1998). The coexistence of the plume and non-plume±arc suites suggests that the plume(s) entrained MORB-source mantle which in part had a recent inherited arc siganture, perhaps residing in the thermal boundary layer.
Fitton JG, Saunders AD, Norry MJ, Hardason BS & Taylor RN, EPSL, 153, 197-208, (1997).
Pe-Piper G, Geol. Mag, 135, 331-348, (1998).
Metagranodiorites and granites from northern Evia (Edipsos region) and central Evia (Seta region), are the SE continuation of the Flambouro Unit which belongs to the Pelagonian basement. High precision gechronological data on these rocks using U-Pb on zircons and 40Ar/39Ar analysis of micas gave zircon ages from 308 Ma to 1912 Ma with at least two magmatic episodes during the Late Carboniferous; 40Ar/39Ar plateau ages are comprised between 286 ± 4 Ma and 304.8 ± 1.6 Ma and correspond to cooling ages of the magmatic bodies. To define the geodynamic environment during the genesis of these plutonic rocks, typological studies on zircons were carried out together with geochemical analysis. These analyses show a calc-alkaline setting for the granites of central Evia and an alkaline to calc-alkaline environment for the metagranodiorites from the northern part of the island. This Late Carboniferous magmatism is viewed as a result of northward subduction of Paleotethys under the Eurasian margin. This subduction led to the opening of the Meliata back-arc ocean in Permo-Triassic times and the Eocimmerian gentle docking of Pelagonia with blocks drifting away from Gondwana in Early to Middle Triassic. The Paleotethys suture is located at the boundary between the External and Internal structural domains of the Hellenides. We interpret Pelagonia as an Eurasian (Variscan) drifted fragment (Stampfli 1996; Stampfli et al. 1998) representing a migrating magmatic arc which collided in Early to Middle Triassic with a drifting Gondwana (Cimmerian) fragment. This implies that the Paleotethyan suture zone in Greece should be located in the External/Internal Hellenides boundary and not further north as often proposed. The more internal suture(s) correspond to the Meliata/Vardar oceans. The Neotethys suture is represented by the on-going subduction of the east Mediterranean ocean, therefore not yet a suture...
Stampfli GM, Eclogae geol. Helv., 89, 13-42, (1996).
Stampfli G.M., Mosar J., De Bono A. & Vavassis I., Bull. geol. soc. Greece, 32-1, 113-120, (1998).
The Liri Unit is represented by a Triassic flysch deposit outcropping at the base of the Permo-Jurassic Pelagonian sequence in Evia Island. It is mainly composed by turbiditic sequences bearing olistoliths of different lithologies and ages. It has been affected by epizonal metamorphism, and strong (Alpine?) deformation. The olistoliths are constituted by small blocks of Visean, Bashkirian and Serpukhovian dark limestones, by early Permian (?) limestones, late Carboniferous (309 to 315 U-Pb) granites/granodiorites of calc-alkaline affinityThe matrix is composed by a heterogeneous ensemble of terrigenous sediments: mass flows, proximal to distal turbidites deposits with associated hemipelagic sediments. deposited in a relatively deep marine environment. The Liri unit matrix contains rare beds of coal bearing black shales. In these layers some palynomorph assemblages have been found and allow us to date parts of the matrix from Late Permian (?) to Late Triassic. The granite blocks in the Liri unit display the same ages and geochemical features than the typical Pelagonian basement. Therefore the origin of the Carboniferous limestone blocks cannot be easily placed within a Pelagonian context. The sedimentary cover lying on the Late Carboniferous Variscan basement of the Pelagonian Northern margin starts at least in Early Permian in Hydra (Grant, 1991) and at the same time or slightly later in Evia. Generally "autochthonous sediments" (non-olistoliths) older than Early Permian have never been found in the Pelagonian realm. A possible origin of the Carboniferous blocks can be searched in the External Hellenides (e.g. Crete: Trypali group and Talea Ori group), where a lithotype sequence from the Middle Carboniferous up to the Triassic/Liassic boundary has been established (Krahl et al., 1983).
The origin of the mixed detrital provenance (Pelagonian or "Variscan" and External or "Gondwanian") found in the Liri flysch allow to consider it as a good candidate to represent the Paleotethyan suture zone or, at least, part of it, in a Hellenic transect. The unconformity within the Talea Ori group, can be seen as a record of the final closure of Paleotethys, occurring probably in Middle Triassic time (corresponding to the Early Cimmerian event). This scenario implies an original position of the Liri flysch south of the Pelagonia terrane, at the base of the Pindos sequence. The latter is regarded as an episutural basin sealing the Paleotethys suture zone.
Grant RE, Nestell MK, Baud A, Jenny C, Palaios, 6, 479-497, (1991).
Krahl J, Kauffmann G, Kozur H, Richter D, Förster O, Heinritzi F, Dornsiepen U, Z.dt geol. Ges., 137, 523-536, (1983).
Two tectonic units can be distinguished on Lesvos Island, NE Aegean Sea: 1. The relative autochthonous unit of Lesvos, comprising a passive continental margin sequence of marbles and phyllites with fossiliferous formations from the Carboniferous (Productus and foraminifera) up to the Upper Triassic (Megalodon) and 2. The allochthonous unit of Lesvos, comprising an ophiolite, together with its volcanosedimentary cover of pillow-lavas and associated pelagic sediments of Triassic age, determined by conodonts.
Both units are slightly metamorphosed and intensively deformed. The allochthon is, in general, in overturned position with the thick ultramafic bodies resting on the top of the gabbroic bodies, diabases and lavas, which are affected by penetrative S-structures.
The lack of any younger formations, other than the Lower Miocene volcanics, does not permit to date directly the age of the tectonic emplacement of the Lesvos ophiolites over the passive margin. The only age constraint is that it should be post Late Triassic, which may relate this tectonic event either to the Cimmerides in Lias or to the Internal Hellenides in Late Malm-Early Cretaceous.
Nevertheless, in the neighboring island of Chios there areanother two tectonic units: 1) The Chios allochthon, comprising a Carboniferous clastic sequence, followed by a Permian shallow-water carbonate platform and then, after a hiatus including the whole Triassic period, the Liassic carbonate platform which is developed over a thin formation of red argillaceous sediments (laterites?) and 2) The Chios autochthon, comprising a thick flysch formation with olistholites and olisthostromes of Paleozoic rocks, followed by a volcano-sedimentary sequence of Lower-Middle Triassic, which, in turn, is followed by a shallow-water carbonate platform of Upper Triassic-Upper Jurassic.
The Lesvos autochthon could be part of the same passive margin, as the Chios allochton, which is characterised by the Liassic unconformity. In this case, the Late Triassic-Early Liassic tectonism, characterising the Cimmerides, may have produced the tectonic emplacement of the Lesvos ophiolite over the Lesvos autochthonous passive margin sequence whereas the area of the Chios allochthon may have been only slightly affected by an uplift creating a temporary emergence.
On the contrary, the Chios autochthon should be out of the influence of this Late Triassic-Liassic tectonic event since there is a continuous shallow-water carbonate sedimentation throughout Late Triassic-Late Jurassic. Thus, the Chios autochthon may be included only in the Internal Hellenides domain, tectonised in Late Jurassic-Early Cretaceous.
In conclusion, the Lesvos ophiolite should represent a remnant of a Tethyan oceanic basin, which was opening during Triassic in the area north of the Cimmeride terrane, including the Lesvos autochthon and the Chios allochthon.
Understanding the tectonic evolution of the Tethyan orogen depends critically on unravelling the tectonic relations between Palaeotethys and Neotethys throughout Eurasia. The Eastern Mediterranean region has the advantage of a large data base, but may be atypical in view of its far westerly location. The most long-standing (modern) view is that Palaeotethys (essentially Palaeozoic) was subducted northward beneath the Eurasian continental margin (i.e. a northerly Palaeotethys), while Neotethys (essentially Mesozoic) opened to the south in its wake (Robertson and Dixon, l984; Dercourt et al., l993). Some see Neotethys as a mosaic of microcontinental fragments and small ocean basins (e.g. similar to the Caribbean; Robertson et al., l991). An alternative involves Permo-Triassic (or earlier) opening of a wide southerly basin (i.e. a southerly Palaeotethys; Stampfli et al., l991). Opposed to this are models that involve southward subduction of a northerly Palaeotethys beneath the African continental margin, leading to back-arc basin opening along the North African margin and rifting of a Cimmerian fragment or fragments (Sengor and Yilmaz, l981). How can these diametrically opposed views be resolved?
It is widely assumed by land geologists that polarity of subduction has remained unchanged as an orogen has evolved (e.g. assumed westerly convergence in the Hellenides). However, studies of the present-day SW Pacific region shows that subduction zones within intra-oceanic regions can and do change polarity over time intervals of as little as several million years (e.g. Woolark-Solomon system). Thus, regional thrust polarity in a suture zone may not be a reliable guide to earlier subduction polarity which may have changed regionally or through time.
In the Eastern Mediterranean region, evidence from e.g. the Tauride Mountains points to a North African passive margin setting during Lower Palaeozoic. However, pre-late Permian unts including melanges along the north margin of the Tauride-Anatolide platform are suggestive of southward subduction during the Late Palaeozoic (Late Carboniferous). Yet, northward subduction along the Eurasian margin is widely accepted for the Caucusus-Pontides region in Late Palaeozoic-Early Mesozoic time (Permo-Triassic). Furthermore, to the south the regionally important Late Palaeozoic-Early Mesozoic Karakaya Complex shows evidence of southward subduction/accretion, mainly during Late Triassic (a probable intra-oceanic event). Finally, northward subduction along the Eurasian margin in Late Mesozoic-Early Tertiary time is generally accepted, related to opening of the Black Sea. It is therefore likely that a long-lived Tethyan ocean existed in the heart of the orogen and was consumed by alternating (or even coeval) phases of both southward and northward subduction, especially during Late Palaeozoic-Early Mesozoic time. Evidence of the critical earlier, Palaeozoic history of the axial orogenic zones was largely destroyed, but key fragments remain (e.g. Karaburan-Chios melange; Robertson and Pickett, l998), as will be discussed.
Robertson AHF & Dixon JE, Spec. Publ. Geol. Soc. (London), 17, 1-74, (1984).
Dercourt J, Rico LE & Vrielynck B (eds), Atlas Tethys Palaeoenvironmental Maps, Gauthier-Villars, Paris, 307p, (l993).
Robertson AHF, Clift PD, Degnan P & Jones G, Palaeogeog. Palaeoclimatol. Palaeoecol, 87, 289-344, (l991).
Stampfli G, Marcoux J & Baud A, Palaeogeog. Palaeoclimatol. Palaeoecol, 87, 373-410, (l991).
Sengor, AMC & Yilmaz, Y, Tectonophysics, 75, 81-241, (l981).
Robertson, AHF & Pickett, E, 3rd Internat Turkish Geology Symp, Ankara l998, 23, (1998).
Paleogeographic reconstructions indicate the presence of a large oceanic Tethys between Gondwana and Laurasia from Late Paleozoic to early Tertiary. The age of continental margin assemblages, ophiolites and accretionary complexes in the Eastern Mediterranean region suggest that the Tethys consisted of at least two oceanic plates, one largely of Paleozoic age (Paleo-Tethys) and the other of Mesozoic age (Neo-Tethys). New field and isotopic evidence from the Pontides of Turkey has strong bearing on the often controversial relation between these two former oceans.
Dating of radiolaria have shown the presence of Upper Triassic (Norian) as well as Jurassic and Cretaceous cherts in the Senonian accretionary complexes along the Yzmir-Ankara Neo-Tethyan suture (Bragin and Tekin, 1996). This implies that the age of the Neo-Tethyan ocean extends back at least to the Late Triassic in the Turkish transect. On the other hand, latest Triassic blueschists (Monod et al., 1996) and eclogites (Okay and Monie, 1997) with Early to Mid-Triassic depositional ages in the Paleo-Tethyan accretionary complexes indicate Late Triassic subduction of an ocean as young as Triassic. Lower ages for this ocean is implied by the scarce radiolarian cherts of Permian and Carboniferous age in the Paleo-Tethyan accretionary complexes.
In Anatolia the Neo-Tethyan Izmir-Ankara suture is easy to define as a tectonic line separating two continental blocks (Pontides and Anatolides) with different Mesozoic stratigraphic and tectonic evolutions. In contrast, attempts to map a Paleo-Tethyan suture, and to define a coherent Cimmerian continent, that collided with the Eurasian margin during the Triassic, have been elusive. The Paleo-Tethyan accretionary complexes form a blanket cover in the Sakarya Zone of the Pontides, and are commonly in tectonic contact with the Neo-Tethyan accretionary complexes along the Yzmir-Ankara suture. This suggests that the latest Triassic Cimmeride orogen was not a collisional but an accretionary orogen and probably was caused by the docking of a large oceanic plateau to the Eurasian continental margin. This Triassic oceanic plateau is represented by a thick (> 4 km) pile of basaltic tuff intercalated with limestone and shale, now all metamorphosed in greenschist and blueschist facies. It covers an area of more than 50 000 km2 in the Pontides. The accretionary nature of the Cimmeride orogeny in Anatolia explains the short time span of deformation and metamorphism (Norian-Hettangian, < 20 Ma) compared to that of the collisional Alpide orogeny (mid-Cretaceous-present, > 80 Ma) and implies a semi-continuous Tethyan evolution.
Bragin NY & Tekin UK, The Island Arc, 5, 114-122, (1996).
Monod O, Okay AI, Maluski H, Monie P & Akkok R, 16th Reunion des Sciences de la Terre, Orleans, France, 43, (1996).
Okay AI & Monie P, Geology, 25, 595-598
Paleogeographic reconstructions in the Middle East suggest that a large tract of Late Paleozoic Tethyan oceanic lithosphere was subducted during the early Mesozoic, but the age and duration of this process are controversial. A good evidence for former subduction would be oceanic crustal rocks metamorphosed in blueschist or eclogite facies. Latest Triassic blueschists (Monod et al., 1996) and eclogites (Okay and Monie, 1997), related to the subduction of the Paleo-Tethys has recently been reported from the Sakarya Zone in northern Turkey. In 1998, further fieldwork was carried out in the Triassic blueschists in order to characterize their extent, tectonic relations and petrology.
North of Eskisehir (NW Turkey), the Triassic blueschists form a tectonic inlier 25 km by 7 km. Metabasites predominate, and are intercalated with minor amounts of marble and phyllite and rare lenses of metagabbro and serpentinite. In terms of lithology and metamorphic age they are very similar to the Nilüfer unit of the Karakaya Complex (Okay et al., 1996), and are considered as part of this unit. The depositional age of the Nilüfer unit is Early to Mid-Triassic and may extent down to the Late Permian. Northwest of Eskisehir, the Triassic blueschists are overlain by another slice of the Nilüfer unit, exhibiting the more usual greenschist facies metamorphism. In the northeast, they are in tectonic contact with a late Cretaceous accretionary complex.
The Triassic blueschists present a distinct foliation but mineral lineation is infrequent (N-S to NW-SE). Well preserved blueschists with the mineral assemblage sodic amphibole + garnet + epidote + phengite are rare, as most metabasites show a strong greenschist facies overprint. Phengite and glaucophane Ar-Ar ages from the blueschists range from 205 Ma to 198 Ma (latest Triassic to earliest Jurassic).
The Nilüfer unit has been variously interpreted as a intra-oceanic arc to fore-arc sequence (Okay et al., 1996) or as an oceanic seamount (Pickett and Robertson, 1996). Its large extent in NW Turkey (Sakarya Zone) suggests a third interpretation as an oceanic plateau, which was partially subducted as the Palaeo Tethys closed in latest Triassic.
Monod O, Okay A I, Maluski H, Monié P, Akkok R, 16e Réunion des Sciences de la Terre, Orléans, France, Abs., 43, (1996).
Okay A I, Satir M, Maluski H, Siyako M, Monié P, Metzger R, Akyüz S, Tectonics of Asia (ed. A. Yin & M. Harrison), Cambridge University Press, 420-441, (1996).
Okay A I, Monié P, Geology,, 25, 595-598, (1997).
Pickett EA, Robertson AHF, J. Geol. Soc. London, 153, 995-1009, (1996).
Pre-Late Jurassic E Pontides tectonic belt of N Turkey documents multi-stage crustal extension, basin formation and associated magmatism associated with an active margin. Although similar extensional histories for the same period are well documented in the Central and SW Pontides, the E Pontides differ from these by an absence of ophiolites, ophiolitic melanges and subduction-accretion complexes. The pre-Jurassic basement of the E Pontides in the Yusufeli area comprises four NE-SW trending tectonic units, separated from each other by dominantly high-angle shear zones. A thick (>5 km) sequence of upright, isoclinally folded phyllites, volcaniclastic turbidites and occassional calc-rudites, intercalated with locally pillowed lava flows are exposed in the NW (Irmakyani Group). A fault-bounded sliver of high-grade metamorphics (amphibolites-migmatites)and cross-cutting dykes are exposed in the mid-part of the tectonostratigraphy (Demirkent Intrusive Complex). The dykes range in composition from gabbro to plagiogranite (100% dykes)and cut the foliation of the host rock. The amphibolites crop out only in screens of several tens of metres wide in the dyke complex. A third unit exposed in the SE is a volcanic sequence (the Kinalicam volcanics), made up of pillow lavas and lava breccias with numerous cross-cutting dacitic dykes and associated lava flows. The lava breccias are stratigraphically overlain by shallow marine sandstones. An allocthonous sequence, termed the Karadag Group, is thrust over the above units. The Karadag Group is represented at the base by a sequence of quartz mica schists and metaconglomerates intruded by two-mica meta-granites. The metamorphics are unconformably overlain by fossiliferous black shales and tuffaceous sediments, cut by gabbroic intrusions.The lavas associated with the Irmakyani Basin are of high-aluminum and IAT basalts and andesites. The geochemistry of interbedded volcaniclastic sandstones implies a volcanic arc-type provenance. The extensional dykes were derived from a common source and their geochemical characteristics indicate above subduction zone melt generation, accumulation and fractional crystallisation, with differantiation in a magma chamber in an extensional setting. The Kinalicam lavas show LIL-element enrichment, Nb depletion relative to LREE and Zr depletion relative to Ti. These are also characteristics of supra-subduction zone lavas. I hypothesise that the Irmakyani basin represents a Triassic (?) deep-marine basin, constructed on stretched continental crust. The basement of this basin was the Demirkent intrusive complex and the amphibolites. The Kinalicam volcanics are interpreted as marginal eruptive centers. The Karadag Group is interpreted as a preserved sliver of a continental assemblage, adjacent to the basin. The basin was closed prior to Jurassic. No pre-Late Jurassic Palaeotethyan ophiolite or accretionary complex exist in the Yusufeli area. This implies that: i) the Palaeotethyan active margin was located further to the south, the remains of which should be found in the East Anatolian Accretionary Complex; ii) it was located further N in the N Caucasus; iii) important strike-slip deformation displaced the pre-Late Jurassic accretionary complexes; or iv) the supra-subduction zone signature was inherited from an earlier subduction event and thus the Palaeotethys was not present in the E Pontides. The working hypothesis is that the pre-Jurassic E Pontides tectonic belt of NE Turkey was an Andean-type active margin, with rifting and closure of intra-arc and/or back-arc basins.
Northerncentral Anatolia is a critical region to study interrelationship between the remnants of the Paleo- and, Neo-Tethyan ophiolites, and the present Black Sea. In this region there are three distinctly different tectonic units. These are; the Pontides, and the Sakarya continent, and an ophiolitic suture squeezed between them (The Intra - Pontide Ocean of Sengör and Yilmaz, 1981). The suture is alternatively known as The Armutlu - Ovacik Zone. This zone is a tectonic mosaic, made up of fragments and tectonic slices of the Sakarya and Pontides, embedded tectonically and stratigraphically into a flysch, formed during the late Cretaceous - early Eocene period.
The Pontides as a tectonic unit has two different basement associations, which were amalgamated before Malm. One of them is a thick sedimentary succession, which developed during the Ordovician - Triassic period. This is known as the Istanbul - Zonguldak tectonic unit, representing a south - facing passive continental margin sequence. The second one is an oceanic assemblage, which consists mainly of an ordered metaophiolite together with a metamorphosed mélange association, and the related metasedimentary succession. This type of basement outcrops mainly in the eastern Pontides, and is called here the Ballidag - Küre group. These two distinctly different basements were collectively covered by the continental clastics and shallow marine carbonates, deposited during the Callovian - early Berriasian period. They delimit the terminal closure of the Paleo - Tethyan oceanic realm, and related basins of the Pontides.
During the early Cretaceous period, a new sedimentation began on the amalgamated package producing a thick flysch-like succession, known as the Ulus group, under an extensional tectonic regime. In the northern part of the Western Pontides, the Ulus group passes upward into an andesitic volcanosedimentary assemblage, whereas in the southern part they give way to a distal flysch, and the late Cretaceous mélange association from Turonian onwards. During the Turonian - late Campanian interval, the region underwent a compression and consequently elevated above the sea. A new transgression began on the deformed and eroded units during the late Campanian till the end of the early Eocene, and covered the entire Western Pontides as a vast blanket. A new compressional regime effected the region during the late early Eocene - middle Eocene transition.
Sengor AMC, Yilmaz Y, Tethyan Evolution of Turkey; a Plate Tectonic Approach, Tectonophysics, 75, 181-241, (1981).
The rift, drift, closure history of Neotethys in the Baer-Bassit area of NSyria exemplfies the evolution of Neotethys at the W end of the "ophiolitic crescent". Mesozoic Arabian platform carbonates (Middle Jurassic to Lower Maastrichtian) are overthrust by allochthonous Neotethyan units. Emplaced successions (mainly melange) begin with deep-sea radiolarian cherts and pelagic limestones, and alkaline volcanics of Upper Triassic age. These are overlain by mainly non-calcareous deep-water sediments of distal passive margin affinities. Also present are Late Jurassic-Lower Cretaceous alkaline volcanics that may record regional hot-spot activity adjacent to the Levant-Arabia passive margin. Structurally above is a complete, but thrust deformed, Upper Cretaceous ophiolitic suite of supra-subduction zone-type, based on geochemical characteristics, that includes plutonics, sheeted dykes and volcanics, the latter similar to the upper pillow lavas of the Troodos ophiolite, Cyprus. The plutonics are structurally underlain by a well developed metamorphic sole, 200-300 m thick, for which the protoliths were alkaline extrusives, similar to volcanics within the melange beneath. The entire assemblage of rift/passive margin and ophiolitic units was emplaced onto the Arabian margin in middle Maastrichtian time.
Structures in both the melange (e.g. duplexes) and the metamorphic sole (e.g. stretching lineations) indicate emplacement mainly towards the SE.Regional evidence (e.g. SE Turkey, Oman) indicates that latest Cretaceous ophiolite obduction predated continental collision. Following emplacement and short-lived conglomerate deposition, the area was transgressed by late Maastrichtian-Eary Tertiary marine carbonates deposited on an unstable passive margin. This was followed by Mid-Late Eocene to Miocene deformation related to diachronous suturing of Neotethys, including regional stratigraphic inversion within the Palmyra fold belt, the Euphrates transtensional fault system and, locally updoming of the Jabal Aqraa Mesozoic carbonate platform in Baer-Bassit. Finally, Neotectonic structures in Baer-Bassit relate to the effects of strike-slip (mainly left-lateral) and normal faulting at the eastern end of the Cyprus active margin.
The Kohistan arc terrane was initiated offshore of Asia during the mid-Cretaceous as an intraoceanic island arc above the subducting Tethyan oceanic slab. At ca. 100 Ma it was accreted to the south margin of Asia, and subsequently behaved as a continental margin volcanic arc until being underplated by continental India in the early Tertiary.
Three main volcanic and volcano-sedimentary sequences are recognised from the intraoceanic stage of arc evolution. Modelling of geochemical differences from basaltic rocks from these sequences illustrates significant changes in magma source regions during arc evolution. The oldest sequence, the Kamila Amphibolites, are tholeiitic to calc-alkali basalts, erupted on to thickened, plateau-like oceanic crust. Trace element and REE chemistry have clear arc signatures. Modelling shows the basalts were derived by up to 16% melting of spinel lherzolite at between 30 and 80 km. Field and geochemical data demonstrate an apparent upward transition from this sequence into the Jaglot Group. Jaglot Group basalts are also tholeiitic to calc-alkali, but are interbedded with turbiditic volcaniclastic sediments, the proportion of sediment to volcanic rocks increasing eastward. Basalts in the west are chemically similar to those in the Kamila sequence. Those from the eastern part of the sequence are more enriched in the REE than in the west although both have similar patterns. Modelling shows these basalts were derived by high (>80 km) pressure melting of garnet lherzolite, differences in REE chemistries being consistent with lower percent melting in the west (8%) than in the east (12%). Basalts from the Chalt Volcanic Group in the north of the arc, which include tholeiitic, pillowed, high-Mg basalts and high-Mg andesites and boninites, have signicantly different geochemical signatures from basalts of the other sequences. In contrast to those, the Chalt basalts are HREE-enriched relative to the LREE. Modelling suggests derivation through 10% percent melting of a cpx-depleted spinel-rich mantle at shallow depths, probably during back-arc rifting, above a north-dipping subduction zone.
On the basis of geochemistry and modelling there is no evidence for significant metasomatism of the mantle wedge in the source region. It is hence likely that differences in basalt chemistry reflect differences in depth of melting and of melt percentage, both of which can be related to either magmatic thickening of the arc or to extensional thinning.
Rifting structures of the Alpine Neo-Tethys have been previously recognized and described within meta sedimentary sequences of Permian-Liassic age outcropping in two different areas from the Penninic zone of the Western Alps (Bianchi et al., 1998a, 1998b). Rift-related features can be also found in the external sectors of the Alpine chain, within the cover sequences of the Pelvoux, Grand Rousses and Belledonne basements (i.e. the Bourg d'Oisans area): such sectors underwent a weaker deformative and metamorphic evolution than the internal zones, so that extensional features can be more easily recognized and have been described in detail (e.g. Grand, 1988). A comparison of the internal, strongly deformed zones with the external sectors can hence furnish useful information in attempt to reconstruct the evolution of the distensive phases. Strong similarities are shown by the lithologies of the cover sequences, i.e. upper-Triassic volcanic rocks and breccia horizons marking the Triassic-Liassic boundary. The main difference between the Penninic and the Dauphiné sectors concerns the chemical composition and the geodynamic setting of emplacement of the volcanic rocks: - an alkaline trend is shown by the volcanic rocks of the Dauphiné zone (e.g. Wolff-Boenisch, 1993);- a tholeiitic trend, with a WPB affinity are more typical of the analyzed Penninic sectors (Bianchi et al., 1998b). Even more problematic is a comparison of the directions of the rift-related distensive tectonic planes. The presence of a pull-apart basin has been recognized in the Bourg d'Oisans area (Grand, 1988), due to the presence of well-recognizable extensional features (i.e. the Col d'Ornon fault); similar distensive structures have been described as well in other sectors of the Dauphiné zone (e.g. Basile and Dumont, 1997). In the internal sectors of the chain such extensional features are in places still recognizable (Bianchi et al., 1998b), although the overprint of the collisional events does not allow to define a precise genetic model as in the case of the Dauphiné zone.
Based on these observations, a palinspastic model defined for the Penninic zone can be tentatively extended to the external sectors of the Alps; in this perspective two hypothesis can be made about the different geochemical trend showed by the meta-volcanic rocks:- the Bourg d'Oisans pull-apart basin can be interpreted as one of the Triassic rim-basin previously hypothesized; in such a case the alkaline composition of the volcanic rocks is expression of the different structural position of the rim basin with respect to the main spreading axes, wherein tholeiitic basalts are found;- alternatively a different source for the magma could be hypothesized: in this case the different trends are expression of two distinct basins opening at different times.
Basile C & Dumont T, Quaderni di Geodinamica Alpina e Quaternaria, 4, 10-11, (1997).
Bianchi GW, Martinotti G & Oberhänsli R, Schweiz. Mineral. Petrogr. Mitt, 78, 133-146, (1998).
Bianchi GW, Martinotti G & Oberhänsli R, SMPG 73 Abstracts, 68, (1998).
Grand T, Bull. Soc. géol. France, 4, 613-621, (1988).
Wolff-Boenisch D, Unpublished Diploma Thesis (Mainz), 1-45, (1993).
Tibet is the result of Palaeozoic, Mesozoic and Paleocene accretion of severalcrustal blocks to the southern edge of the Asian continent. Major fault zones, some of which with remnant ophiolites that document oceanic sutures, separate the accreted terranes. To resolve the complexities of this tectonic collage, wehave surveyed a section in southeastern Tibet across the Songpan-Ganze and the Qiangtang terranes separated by the Triassic Jinsha Suture. The Songpan-Ganze terrane consists mainly of intricately folded turbiditic formations and is interpreted as a huge accretionary complex. Magmatic bodies of various compositions intrude the Qiangtang terrane. To understand significance of these plutons (as time markers of geodynamic evolution), we have focused this work on the analysis and high-resolution U/Pb single zircon dating of the Qiangtang plutonic and volcanic rocks.
The magma geochemical parameters point to subduction-related, within-arc magmatism. Precise U/Pb dating yielded reproducible and perfectly concordant 206Pb/238U-207Pb/235U-206Pb/207Pb ages for all carefully selected entirelymagmatic accessory zircons. The best estimate for the age of magmatic crystallisation thus is the combined weighted-mean concordant ages (95% confidence level).
Five plutons of the Qiangtang terrane yield distinctive Triassic ages that range from 248 Ma to 207 Ma. These ages suggest an evolutionary trend from intermediate (monzogabbro-diorites) to acidic rocks (granites) and are very close to the presumed Triassic age of the Songpan-Ganze accretionary complex. This would suggest subduction of the Songpan-Ganze below the Qiangtang terrane at that time. The interpretation is consistent with north verging deformation that intensifies southward, developing locally polyphase deformation marked by crenulation cleavage and lineations. One granitoid yields a concordant Cretaceous (72 Ma) age that reflects either the emplacement of younger, crustally contaminated magmas or recrystallisation during a late tectonic event.
The tectonic onset, cohesive evolving of structrural constituents and driving forces of tectonic development for the circum-Black area and the mega-depression itself represent complex and intriguing geological problem. The idea of historical likeness for geological evolution of the Western Mediterranean and Black Sea, pointed out for the first time 15 years ago (Khain, 1984), now gains more confident contours based on modern exploration data and induces to revise existing tectonic models for the Black Sea evolution from the standpoint of their compliance with the Mediterranean counterparts. Though the basins under comparison have many distinctions and differ each from other due to their linear dimensions (ca 1.6-1.9 times), thickness of a sedimentary pile, lithological spectra for some epochs and so on, they, nevertheless, reveal a striking morphological coincidence and notable genetic analogy in their spatial and temporal, kinematic, palaeogeographic, sedimentological and other features.
Following main geostructural units/terraines of the Western Mediterranean and Black Sea with their surroundings would be proposed for comparative analysis as possible contemporaneous tectonic analogues or homologues:· Gulf of Lion - NW Shelf· Proveance + Tyrrhenian Basins - West Black Sea · Pyrenees Mts., core zone - Dobrogea Orogen· Iberian plate - Moesian plate· Alboran Sea - Sea of Marmara· Balearic Islands - buried volcanic calderas east of Cape Emine· Nevado-Fillabrides - Strandja zone?· Kabylies Massives - Istanbul zone + Kastamonu complex ?· Tell-Rif (Maghrebides) - Western Pontides· Mursia Depression - Burgas Depression ?· Crimean Mts. - Southern Alps ?· Western Alps paleo-basin - Karkinit-Sivash Trough, aborted rift branch ??· Po Basin - Sorokin Trough· Apennines - buried foldbelt of the Andrussov Ridge ?· Adria (Apulian Plate) - Dzirulia/Shatsky Ridge?· Bay of Biscay - Pannonian Basin before Alpine collision· Dinarides - Greater Caucasus· Paleozoic core of the Alps, deformed - Plain (Steppe) Crimea ??· South Atlas Fault - North Anatolian Fault, etc.The above comparison allow to speculate that the Black Sea basin is a contemporaneous to the Western Mediterranean one minor and simplified tectonic copy being evolved in similar way and caused by similar tailoring of its structural constituents that stipulated by uniform stress/fault pattern inherited from the late Paleozoic Paleo-Tethys tectonic realm. For example, minor counterclockwise rotation and eastward drift of the Andrussov ridge could represent equal to the Apulia microplate relative motion (Biju-Duval et al., 1975; Olivet 1996), and obvious manifestations of strike-slip tectonics in the Pyrenees can be scalable observed in the Dobrogea and adjacent shelf (Gradinaru, 1994, Kitchka, 1998).
Khain V.Ye, Regional geotectonics. Alpine Mediterranean Belt, Moscow, Nedra Press, 344, (1984)
Biju-Duval B., Dercourt J., Le Pichon X., Structural history of the Mediterranean basin, 163-164, (1977)
Olivet J.-L., La cinematique de la plaque Iberique, Bull. Elf Aquitaine, 20-1, 131-195, (1996)
Gradinaru, E., Revue roumaine de Geologie, Geophysique et Geographie, 28, 247-262, (1984)
Kitchka A., Abs. 3rd Int Conf. Petroleum geology of the Black and Caspian Seas area, 81-82, (1998)
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