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Cretaceous and Triassic Subduction-Accretion, High-Pressure– Low-Temperature Metamorphism, and Continental Growth in the Central Pontides, Turkey

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Cretaceous and Triassic Subduction-Accretion, High-Pressure– Low-Temperature Metamorphism, and Continental Growth in the Central Pontides, Turkey Cretaceous and Triassic subduction-accretion, high-pressure– low-temperature metamorphism, and continental growth in the Central Pontides, Turkey A.I. Okay† O. Tüysüz‡ Istanbul Teknik Üniversitesi, Avrasya Yerbilimleri Enstitüsü, Maslak, Istanbul 34469, Turkey M. Satır§ Institut für Geowissenschaften, Universität Tübingen, Wilhelmstraße 56, D-72074 Tübingen, Germany S. Özkan-Altıner# D. Altıner†† Middle East Technical University, Department of Geology, Ankara 06531, Turkey S. Sherlock‡‡ Department of Earth Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK R.H. Eren Anadolu Yerbilimleri Ltd. S¸irketi, Perpa Ticaret Merkezi, No. 1955, Okmeydanı, Istanbul, Turkey ABSTRACT ~20 m.y. after the HP-LT metamorphism, ered the result of the collision of the megacon- and ~25 m.y. before the terminal Paleocene tinents Laurasia, Gondwana, India, and a small Biostratigraphic, isotopic, and petrologic continental collision. The Cretaceous sub- number of continental terranes during the Ter- data from the Central Pontides document duction-accretion complex is tectonically tiary (e.g., Dercourt et al., 1986; S¸engör, 1987). major southward growth of the Eurasian overlain in the north by oceanic crustal rocks Here we show extensive precollisional growth continental crust by subduction-accretion accreted to the southern margin of Eurasia of continental crust along the southern margin during the Cretaceous and Triassic Periods. during the latest Triassic–earliest Jurassic. of Laurasia in the Central Pontides by accretion A major part of the accreted material is rep- The Triassic subduction-accretion complex and underplating of oceanic rocks in the Late resented by a crustal slice, 75 km long and is made up of metavolcanic rocks of ensi- Triassic and Late Cretaceous. up to 11 km thick, consisting of metabasite, matic arc origin and has undergone a high A second aim of the paper is to contribute to metaophiolite, and mica schist that represent pressure, greenschist facies metamorphism the evaluation of the mechanism of exhuma- underplated Tethyan oceanic crustal and with growth of sodic amphibole. Most of the tion of high-pressure–low-temperature (HP-LT) mantle rocks. They were metamorphosed at Central Pontides consists of accreted Pha- metamorphic rocks. Most HP-LT metamorphic 490 °C and 17 kbar in mid-Cretaceous time nerozoic oceanic crustal material, and hence complexes occur within the crystalline part of (ca. 105 Ma). The syn-subduction exhuma- is comparable to regions such as the Klamath the orogen associated with ophiolites and other tion occurred in a thrust sheet bounded by Mountains in the northwestern United States metamorphic rocks. In contrast, those in the a greenschist facies shear zone with a normal or to the Altaids in Central Asia. Central Pontides in northern Turkey are closely sense of movement at the top and a thrust fault associated with sedimentary sequences with at the base. A fl exural foreland basin devel- Keywords: Pontides, Turkey, HP-LT metamor- ages that overlap the age of the HP-LT meta- oped in front of the south-vergent high-pres- phism, exhumation, continental growth, orogeny. morphism. Interpretation of the new isotopic, sure–low-temperature (HP-LT) metamor- biostratigraphic, and structural data indicates phic thrust sheet; the biostratigraphy of the INTRODUCTION that the HP-LT metamorphic rocks from the foreland basin constrains the exhumation of Central Pontides were exhumed in a forearc set- the HP-LT rocks to the Turonian–Coniacian, The Pacifi c mountain belts along the western ting during the ongoing oceanic subduction. North American margin have grown oceanward A third aim of the paper is to contribute to †E-mail: [email protected]. by subduction-accretion and by incorporation the regional tectonics of the Tethyan region. ‡E-mail: [email protected]. of oceanic edifi ces, such as island arcs, oceanic Geological data from the Central Pontides §E-mail: [email protected]. #E-mail: [email protected]. islands, and oceanic plateaus (e.g., Coney et al., have been important in models regarding the ††E-mail: [email protected]. 1980; Irwin, 1981; Ernst, 1984). In contrast, the evolution of the Paleo-Tethys (S¸engör et al., ‡‡E-mail: [email protected]. Alpine-Himalayan orogen is generally consid- 1984; S¸engör, 1984; Yılmaz and S¸engör, 1985; GSA Bulletin; September/October 2006; v. 118; no. 9/10; p. 1247–1269; doi: 10.1130/B25938.1; 14 fi gures; 4 tables. For permission to copy, contact [email protected] 1247 © 2006 Geological Society of America Okay et al. Tüysüz, 1990; Ustaömer and Robertson, 1994, Sea in the north. The latter is a backarc basin, these two zones occurred during the early Late 1997; Yılmaz et al., 1997; Robertson et al., which opened during the Cretaceous above Cretaceous (Cenomanian–Turonian) (Tüysüz, 2004). A major part of the oceanic assemblages the northward-subducting Tethyan ocean (e.g., 1999). in the Central Pontides, interpreted in the past Boccaletti et al., 1974; Görür, 1988). The Cen- A major part of the Central Pontides con- to be Triassic or older, is shown here to be of tral Pontides includes two Pontide terranes: sists of metamorphic rocks, known as the Kargı Cretaceous age. This requires a reevaluation the Istanbul and Sakarya zones (Fig. 1). The Massif, occupying an area of 55 km to 120 km, of the Paleo- and Neo-Tethyan history of the Istanbul zone is characterized by an Ordovi- and generally regarded as part of the Sakarya Pontides, and their relation to the opening of cian to Carboniferous sedimentary succession zone (Fig. 2). The Kargı Massif is made up of the Black Sea, a potential hydrocarbon-bearing of passive margin type unconformably overlain northwest-dipping, crustal-scale tectonic slices basin. by Triassic and younger rocks (e.g., Görür et separated by Cretaceous and younger thrust al., 1997). In contrast, the Sakarya zone has faults (Tüysüz, 1990, 1993; Yılmaz et al., 1997; TECTONIC SETTING a basement of Permo-Triassic subduction- Ustaömer and Robertson, 1997, 1999). Previ- accretion complexes unconformably overlain ous studies consider the Kargı Massif to be a The term Central Pontides is a geographi- by a Jurassic and younger sedimentary cover Paleo-Tethyan subduction complex consisting cal term for the arched central part of the Pon- (e.g., Okay and Tüysüz, 1999; Robertson et of accreted pre-Jurassic ophiolite, mélange, tide orogenic belt (Fig. 1). It lies between the al., 2004; Okay and Göncüog˘lu, 2004). In the and magmatic arc sequences. Our fi eld and iso- I˙zmir-Ankara suture in the south and the Black Central Pontides the tectonic juxtaposition of topic data show the presence of a crustal-scale Dobrudja Va r n a SAKARYA ZONE PELAGONIA AN A TOLIDE 0 200 400 km Figure 1. Tectonic map of the Eastern Mediterranean region (modifi ed from Okay and Tüysüz, 1999). Dashed-line box shows location of Figure 2. 1248 Geological Society of America Bulletin, September/October 2006 Cretaceous and Triassic subduction-accretion and HP-LT metamorphism Figure 2. Tectonic map of the Central Pontides (compiled from Tüysüz, 1998; Ug˘uz et al., 2002; Aksay et al., 2002; and this study). Dashed- line boxes show locations of Figures 3 and 11. Geological Society of America Bulletin, September/October 2006 1249 Okay et al. 0 4 8 km Figure 3. Geological map of the central portion of Kargı Massif, based on Eren (1979), Tüysüz (1985), and this study. For location, see Figure 2. 1250 Geological Society of America Bulletin, September/October 2006 Cretaceous and Triassic subduction-accretion and HP-LT metamorphism Cretaceous HP-LT metamorphic unit in the cen- minor marble and metachert (~3%), and irregu- Two generations of penetrative foliation can ter of the Kargı Massif (Fig. 2). lar tectonic lenses of metamorphosed ophiolite be recognized in the Domuzdag˘ Complex. A represented mainly by serpentinite (~12% of steeply dipping fi rst generation of foliation asso- CRETACEOUS HP-LT METAMORPHIC the outcrops). There are no data on the protolith ciated with isoclinal folds, probably coeval with SLICE: THE DOMUZDAG˘ COMPLEX ages of the Domuzdag˘ Complex. Metabasites the HP-LT metamorphism, is preserved in the in the Domuzdag˘ Complex range from eclo- north near the Elekdag˘ lherzolite. The S1 folia- The Domuzdag˘ Complex forms a north- gite through garnet-blueschist to albite-chlorite tion is overprinted toward the south by a shal- west-dipping crustal slice, 75 km long and up gneiss, depending on the degree of retrogression. lower dipping second generation of foliation to 11 km thick, extending in a northeast-south- Metabasites and metaophiolite fragments domi- associated with tight recumbent folds. The inter- west direction and bisecting the Kargı Massif nate the upper parts of the Domuzdag˘ Complex section of S1 and S2 has produced an intersection (Fig. 2). It is tectonically overlain by Permo- in the north, where they make up >70% of the lineation parallel to the S2 fold axis. The dips of Triassic metabasite and phyllite in the north and outcrops, whereas quartz–mica schists are the S1 and S2 are generally to the north (Fig. 5). The by a slightly metamorphosed Cretaceous fl ysch main rock type in the basal parts of the complex shallowly dipping lineation shows a high scatter sequence in the west. In the south it is under- in the south. In previous studies the southern in its trend, indicating subsequent refolding. lain by a Late Cretaceous foreland basin and by part of the Domuzdag˘ Complex was assigned to The oldest sedimentary rocks that lie uncon- the metamorphic rocks of the Kargı Complex a different unit (the Bekirli Formation of Tüysüz formably over the Domuzdag˘ Complex are lat- (Figs. 3 and 4). and Yig˘itbas¸, 1994; Domuzdag˘-Saraycıkdag˘ est Santonian–earliest Campanian pelagic lime- The Domuzdag˘ Complex is a complexly Complex of Ustaömer and Robertson, 1997). stones.
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