İstanbul paleozoyiği'nin yapısı ve tektonik evrimi

Abstract

Türkiye'nin kuzeybatısında, Rodop-Pontid Tektonik Bloğu'nun bir parçası olan İstanbul Zonu, Neoproterozoyik yaşlı (neredeyse tamamen Ediyakaran), orta-yüksek derecede metamorfik kristalin bir temel üzerine geç Ordovisyen'den Karbonifer'e kadar sürekli olarak gelen, iyi gelişmiş transgresif bir çökel istiften oluşmaktadır. Palaeozoyik istif üzerine Alt Triyas yaşlı çökeller uyumsuz olarak gelmektedir. İstanbul tektonik birliği Istıranca Masifi'nin üzerine tektonik olarak yerleşirken, Sakarya tektonik birliği ile sınırı ise Erken-Orta Eosen yaşlı İç Pontid kenedidir. Sakarya tektonik birliği ve Istranca Masifi Geç Triyas ve Geç Jura-Erken Kretase metamorfizması ve deformasyonu sergilerken, İstanbul tektonik birliğinde bu gözlenmemektedir. Ancak Istranca'dan bilinen Geç Kretase magmatizması aynen İstanbul tektonik birliğinde de görülür. Silüriyen yaşlı kırıntılılardan yapılan kırıntılı zirkon yaşları İstanbul Zonu'nun kaynak alan açısından Istranca Masifi ile Sakarya tektonik birliğine nazaran daha çok benzerlik gösterir. Triyas kırıntılılarından elde edilen kırıntılı zirkon yaşları ise İstanbul Zonu'ndaki Triyas ile Istranca Masifi'ndeki Triyas'ın aynı kaynak alandan beslendiğini belirtmektedir. Ayrıca daha önce Sarıyer batısında Silüriyen olarak haritalanan kırıntılıların yaşının Triyas olduğu bu çalışmada ortaya konulmuştur. İstanbul tektonik birliğinin deformasyon tarihçesini ortaya koymak için bölgedeki fay kırıklarından illit mineralleri yaşlandırılmıştır. Bu analizler sonucunda Kretase (Albiyen-Turoniyen), geç Paleosen-erken Eosen ve erken Miyosen dönemlerine denk gelen deformasyonlar tespit edilmiştir. İstanbul'un kuzeyinde ürünleri görülen geç Kretase volkanizması ile dayk sokulumları ve Çavuşbaşı Granodiyoriti aynı volkanik aktivenin ürünleridir. Granodiyorit yerleşimi ana kayaçta deformasyona sebep olurken dayk sokulumlarını da kontrol etmektedir. Bahsedilen volkanik aktivite Boğaziçi Volkanı olarak adlandırılan geç Kretase volkanı ile ilişkilidir. Sahada toplanan yapısal verilerden bölgenin deformasyonuna dair bilgiler elde edilmiştir. İstanbul Zonu'nun İstanbul çevresinde kalan bölümü bütün olarak ele alındığında en az iki farklı deformasyon evresi geçirmiştir. Bunlardan yaşlı olanı, Triyas ve daha genç yaşlı kayaçlarda izine pek rastlanmayan yaklaşık D-B eksenli bir sıkışma evresidir. Genç olan deformasyon evresi ise Alt Kretase ve Triyas yaşlı kayalarda görülen, K-G doğrultulu sıkışmadır. Bu sıkışma yönü İstanbul'un kuzeyinde varlığı iyi bilinen erken Eosen yaşlı kuzeye verjanslı bindirme ile eşleşmektedir. Toplanan normal bileşenli fayların geometrisi bize bölgeyi etkileyen bir diğer deformasyonun Kuzey Anadolu Fay Zonu ile ilişkili olduğunu göstermektedir. Paleozoyik istifi içerisinde hazırlanan yapısal kesitler sonucunda, istifin derinliği 3500 m'ye kadar inen bir dekolman yüzeyi boyunca D-B yönlü yaklaşık %50 daralma ile sıkıştığı ortaya çıkmıştır. İstanbul Paleozoyik istifinin içerisinde metamorfik kayaların bulunmaması, zayıf klivaj gelişimi ve okyanusal kabuk kökenli malzemenin eksik olması, istifin içerisinde geliştiği havzanın, Triyas öncesindeki deformasyon evresi boyunca orojen kuşağının ard-ülkesinde yer aldığını göstermektedir. İstanbul civarının morfolojisini, Kuzey Anadolu Fayı ile ilişkili makaslamaya bağlı ikincil yapıların kontrol ettiği ortaya çıkmıştır. Bu yapılar İstanbul bölgesinde karada meydana gelen depremlerle de uyumludur. İstanbul'da karada da aktif olması muhtemel ve yıkıma sebep olabilecek faylar mevcut olabilir. İstanbul Paleozoyiği'nden toplanan Brakiyopod fosilleri İstanbul Zonu'nun Rhenik okyanusu, yani Avrupa Hersinyeni ile doğrudan bir bağlantısı olduğunu göstermektedir. Toplanan krinoid fosilleri ise İstanbul Zonu'nun erken Emsiyen döneminde Tien Shan, Rudny Altay, Armorikan Masifi, Güney Minusa Havzası, Gorny Altay, Kazakistan, Uzak Doğu ve Salair ile toplanan ammonoid fosilleri ise, Batı Avrupa (Kuzey İspanya, Montagne Noir, Fransa, Almanya) ve Kuzey Afrika ile bağlantılı olduğunu işaret etmektedir. İstanbul Zonu ile Peri-Gondwana ve Armorika arasında denizel bir bağlantı olması gerektiği ortaya konmuştur.

İstanbul is the only city that lies on the boundary between two continents; Europe and Asia and sits on a non-metamorphosed Palaeozoic transgressive passive margin sequence. The Istanbul Palaeozoic sequence is an exotic body compared with the surrounding rocks in the region. The basement of the Istanbul Zone is a middle to high-grade crystalline latest Proterozoic, possibly Pan-African. The Palaeozoic sequence starts with a regressive series from early Ordovician to early-middle Ordovician and then continues transgressively until the early Carboniferous. Kocatöngel Formation is the lowermost unit in the sequence and consists of clastics that are considered as covering a basement that is not exposed within the province limits of Istanbul. The observations in the company of Professor Daniel Bernoulli revealed that this sediment pile consists of a turbidite, possibly marine or lacustrine. The Kurtköy Formation consists of arkosic sediments and began with turbidites but rapidly became shallow water and then terrestrial fanglomeratic clastics. Deposition of these arkosic units may be related to the Ordovician Gondwanian Glaciation. No fossils recorded in the Kocatöngel Formation and the Kurtköy Formation and Ages of both formations are determined by means of stratigraphic correlation as early Ordovician. The Aydos Formation is represented by quartzarenites, quartz conglomerate and shales that are considered to be deposited in shallow marine environment and represents the end of the regression and the beginning of a new transgression in the basin. Yayalar Formation consists of micaceous feldspathic sandstones and overlies the Aydos Formation. The trangression continues with shallow marine limestones of the Pelitli Formation which is rich in fossils such as corals, crinoids, brachiapodes and stramatoporoids. Pendik Formation conformably overly the Pelitli Formation. Pendik Formation consists of micaceous shales and indicates increasing clastic sediment input into the basin. This formation is rich in fossils such as tabulate corals (Pleurodictyum sp.), brachiopodes, trilobites, ammonoids and crinoids. Pendik Formation is deposited during early to medial Devonian. An oxygen rich, open and deepening shelf environment without significant clastic input is represented by the Denizli Köyü Formation and its limestones, shales, nodular limestones and radiolarian cherts with phosphate nodules. Thick turbiditic flysch deposits with limestone intercalations of the Trakya Formation. Age of the Trakya Formation is considered as late Tournasian to late Visean in previous studies. In this study, late Carboniferous fossils are reported. Northwest of Bosphorus, in a very limited area, a Triassic sequence crops out. The base of this sequence consists of Lower Triassic massive basaltic lava flows that are overlain by Upper Triassic (Scythian) white to light grey, thickly-bedded Çiftalan sandstone, which was investigated in this study. The overlying late Scythian-Anisian Köseler limestone passes into Bakırlıkıran Formation, represented by sandstone and shale.Triassic units in northwestern İstanbul and Kocaeli regions correlate with each other.The Triassic units exposed in the northwestern part of İstanbul are thrust by Carboniferous flysch of the İstanbul Zone. In this study the ages of the detrital zircons obtained from the Triassic rocks in northwest İstanbul are examined and correlated with the known age data reported from the İstanbul Zone and the Strandja Massif. According to provenance analyses of the Triassic rocks we came to a conclusion that they were fed from the Strandja Massif rather than the İstanbul Zone. Thus, the Triassic rocks can be attributed to the Strandja massif and the buried contact between these two units in the NW part of İstanbul must be thrust fault as it is in the exposed area. Another result of the present study is the necessity of reexamination of some clastic rocks of the İstanbul Zone attributed to Paleozoic from which there have so far been no paleontological data. We here show that one such allegedly Silurian unit turned out to be at the oldest post-Carboniferous. The data provided from U-Pb dating on detritic zircons from the Silurian Yayalar Formation allow us to corrolate Palaeozoic of İstanbul with Strandja units and it reveals the direct contact between these two units. To reveal the deformation history of the İstanbul Zone, 40Ar/39Ar illites from the fault gouges are dated. Seven samples were suitable for the dating. Results of the analysis can be grouped in three episodes; Albian-Senomanian, Paleogene and Neogene. Neogene faults show the effect of the North Anatolian Fault Zone in the region, while Paleogen aged faults are compatible with the closure of the Intra-Pontide Ocean in this period. Albian-Senomanian deformation ages indicate a deformation of the Kimmerid Orogenesis in this period in the region. In many ancient and active volcanic provinces, dyke systems represent radial and concentric patterns commonly around a central intrusive body. In İstanbul, late Cretaceous dykes, which are emplaced in pre-Cretaceous basement rocks consisting of sedimentary rocks of Palaeozoic and Triassic ages, have both patterns. In the region, late Cretaceous volcanism is represented by three elements, (1) The Çavuşbaşı granitoid, (2) volcano-sedimentary units and (3) dykes. The U-Pb zircon ages of the Çavuşbaşı granitoid is given as 67.91±0.63 to 67.59±0.5 Ma. It is emplaced at a shallow depth and has an indistinct contact aureole with about 500 meters in width. Volcano sedimentary units, preserved mostly to the north of it, were deposited in an intra-arc basin. Three types of dykes are reported in the region: lamprophyre, diabase and intermediate to felsic dykes (72.49±0.79 to 65.44±0.93 Ma). Different petrology and the crystallization depths of the crystals in the dykes and the Çavuşbaşı granitoid suggest two different magma chambers emplaced at two different depths. Upper Cretaceous dykes are concentrated around the Çavuşbaşı granitoid and extend almost as far as 30 km away from the pluton. The intrusion of the plutonic body of the Çavuşbaşı granitoid caused a dome structure in the basement rocks. The formation of this dome structure may have controlled the stress field and the orientation of the dyke system. Similar patterns are observed in the British Tertiary igneous province, Galapagos volcanoes, Boa Vista (Cape Verde), Summer Coon volcano and Spanish Peak Mountain, Dike Mountain (Colorado), Vesuvius, Etna and Stromboli. It is suggested that the Upper Cretaceous volcanic edifice in the İstanbul region is related to an arc volcano similar to the andesitic volcanoes in Sumatra and Java in Indonesia and those in Japan and the Philippines; we name it the Bosphorus Volcano. Main structural components in the İstanbul Palaeozoic sequence are N-S trending thrusts. Vergence of these thrusts are considered to the west and the entire sedimentary sequence to have deformed on a décollment surface Under this deformation thick, competent arkoses folded with long wavelengths, cut by big thrusts and show spaced cleavage. Devonian shales are folded with shorterwavelengths, show evidence of plastic deformation such as similar folds and in some regions shows severe folding probably related to local high strain. In many places the presence of kink folds indicate more brittle behaviour. It seems that the Palaeozoic sequence is squeezed between the Ordovician arkoses and Carboniferous flysch. Carboniferous flysch is characterized by parallel folding and flexural slip along bedding planes which must have resulted from a basal décollement. Lack of metamorphism and penetrative (??) structures show that this Palaeozoic sequence was deformed in a marginal fold-and thrust belt. Because of the small-scale of the entire system and the lack of abundant ophiolitic and other kind of magmatic clasts in the flysch sequence it was located in a hinterland fold and thrust belt. The geomorphology of the Istanbul region is controlled by the shear deformation of the North Anatolian Fault Zone. The existence of faults that can produce earthquakes on land is a strong possibility. Lower Devonian crinoids have been studied based on columnals from carbonate interbeds in the lower part of the Pendik Formation exposed in the Akbaba and Gebze sections (Istanbul Zone). The crinoid assemblage includes 20 species of 16 genera. Of these, nine species are identified in open nomenclature. One new species Hexacrinites? gebzensis Kurilenko sp. nov., is described by Dr. Alena Kurilenko. The crinoids are found in association with small trilobites, brachiopods, gastropods, and ammonoids. The ammonoid faunas of the lower part of the Kartal Formation represent the Erbenoceras-Anetoceras Genozone (Eocostapolygnathus excavatus conodont zone) of the late Emsian, and also show similarities with the European and Northern Gondwanan faunas. The crinoid assemblage shares similar species with the Armorican Massif and Central Europe, and Altai in Siberia. Brachiopods from siltstones and sandstones of the Pendik Formation indicate the provincial affinities are Rhenish-Bohemian which shows İstanbul Zone had a direct contact with Rheic Ocean. Plant fossils are collected from the upper sections of the Trakya Formation and they indicate that the age of the formation extends to the late Carboniferous.