İstanbul paleozoyiğinin paleomanyetik verilerle incelenmesi ve hersiniyen orojenezindeki yeri

Abstract

Rodop-Pontid kıtasının bir parçası olan İstanbul tektonik birliği Neoproterozoyik metamorfik bir temel üzerine yerleşmiş Paleozoyik bir istif ile tanımlanır. İstanbul sınırları içerisinde bu istif Ordovisyen'den erken Karbonifer'e sürekli bir gelişim gösterir. Üst Karbonifer molasına geçen Alt Karbonifer fliş çökelleri ve hemen ardından gelen uyumsuzluk ile Avrupa Hersinidleri'ne benzerlik gösterir. İstanbul Paleozoyik parçası (İstanbul+Çamdağ+Zonguldak) hakkında genel bir tektonik yorum yapılamayacak kadar küçük bir alanı işgal etmektedir. Bu nedenle öncelikle ait olduğu düşünülen Hersiniyen Orojen'i ele alınmıştır. Avrupa Hersinidlerinden elde edilen 1138 adet yüksek kaliteli izotopik yaş verisi Ediyakaran'dan başlayıp geç Karbonifer'e kadar faaliyet göstermiş olan ve Gondwana-Land'ın kuzey kıta kenarını oluşturan bir magmatik yayın ürünleri olarak yorumlanmıştır. Elde edilen veriler bu magmatik yayın Hersiniyen Sistemi'nin bel kemiğini oluşturduğunu göstermektedir. Bu çalışmada bu yaya `Protogonos` (= ilk oluşan) adı verilmiştir. Yaş verilerine ek olarak manyetik anomali haritaları ve kılavuz hatlar kullanılarak bu yayın uzanımı ve gelişimi takip edilmiştir. Bu tez kapsamında yapılan paleomanyetik gözlemler, İstanbul Paleozoyik parçasınının Hersiniyen Orojeni'nin içerisindeki konumu belirlemek amacıyla yapılmıştı. Ancak İstanbul'u oluşturan tüm Paleozoyik kayaçların ikincil mıknatıslanmaya maruz kalmış olmaları Paleozoyik'teki mıknatıslanmalarını kaybetmelerine sebep olmuştur. Yapılan gözlemler yeniden mıknatıslanmanın genellikle Kenozoyik dönemin son evresinde olduğunu göstermiştir. Bu nedenle İstanbul tektonik birliğine ait paleoenlem ya da dönme bilgisi elde etmek mümkün olmamıştır. Bu sonuç, bu tezin önemli sonuçlarından biri olup bundan sonra İstanbul'da paleomanyetik gözlemlerle vakit kaybedilmemesini sağlayacaktır.İstanbul Paleozoyik parçasının Gondwana-Land kökenli olduğu gösterilmiş olan Moezya ve Tepla-Barrandiyum birliği ile birlikte Protogonos yayının ardında, Orta Karbonifer'e kadar Gondwana-Land'a ait olduğu düşünülmektedir. Moezya'nın batısından itibaren meydana gelen Gondwana-Land/Lavrusya çarpışması Avrupa ve Kuzey Amerika'da Hersinid orogenik kuşağını oluşturmuş, Moezya'nın batısında kalan kesim ise Gondwana-Land'ın Paleo-Tetis kenarını oluşturarak tektonik evrimine Kimmeridler olarak devam etmiştir. Bu evrim de Jura'da Kimmerid çarpışması ile son bulmuştur. Bu şekilde Karadeniz çevresinde herhangi bir Hersinyen (veya bir diğer adıyla Varisk) olaylarının olmadığı tespit edilmiştir. Burada sunulan Hersinid modeli tamamen orjinal bir model olup bugüne kadar dünyada ileri sürülen Hersinid evrim modellerinden tamamen farklıdır. Bu modelin eldeki stratigrafik, paleontolojik ve yapısal verilerle diğer modellerden daha uyumlu olduğu görülmüştür.

The Istanbul tectonic unit is a part of a bigger continental fragment called the Rhodope-Pontide Fragment, and it consists, at the base, of a Neoproterozoic crystalline basement. This basement is overlain by a continuous, well-developed sedimentary sequence extending from the Lower Ordovician to the Upper Carboniferous. The Palaeozoic sequence commences with laminated siltstones and shales. The following thick arkoses are covered by Upper Ordovician-Lower Silurian feldspathic quartz arenite representing a low energy open shelf, probably tidal and beach environment. The basin became progressively deeper and more stable during the Silurian and Devonian. Lower-Middle Devonian nodular limestones show a transition from open shelf to a slope setting. Continuous deepening Lower Carboniferous black lydites. The basin, which was tectonically stable from the Ordovician to the end of the Devonian, became a site of turbiditic flysch deposition and tectonically active during the Early Carboniferous. The Carboniferous flysch marks the progress of a collision. That collision created a dominantly (now) west vergent marginal fold and thrust belt on the eastern side of the Bosphorus and what now seems an east vergent (but with many inconsistencies) on the western side as a retrocharriage. The structural style of folds and faults requires a décollement underneath the whole city which thrusts the entire structure westward.The İstanbul Zone has a complicated deformation history related to the Hercynide (or Scythide), Cimmeride and Alpide orogenies. Although the region of Istanbul shows a weak metamorphism and a weak cleavage development, constraining the entire history of the deformation in the İstanbul Zone marginal fold and thrust belt is a difficult task, primarily due to the multiple deformation phases. But yet it is not impossible. The Palaeozoic sequence is cut by late Cretaceous plutonics together with dacitic and andesitic dykes. This arc magmatism is ascribed to the north-dipping subduction of the Neo-Tethyan ocean along the İzmir-Ankara-Erzincan suture. The Palaeozoic sequence is unconformably overlain by Permian and younger sedimentary strata. Istanbul tectonic unit resembles the Hercynides with its abundant Lower Carboniferous flysch deposits passing into Upper Carboniferous molasse and with a sharp unconformity upon its sediments. The Palaeozoic sequences of Istanbul and Zonguldak have been compared and correlated with similar sequences in Europe, including the Moesian platform in Romania and Bulgaria, Moravo-Silesia (Brunovistulian) in the Czech Republic and the Rhenohercynian zone in Germany and Belgium, all deposited on the northern passive margin of the Rheic ocean. The Istanbul Zone is treated as a part of Avalonia. However, continuous transgressive sedimentation and absence of collision related magmatics or volcaniclastic sediments rule out this relationship. By contrast, the Istanbul sequence resembles the Pyrenees, the Carnic Alps, the Bohemian (Saxo-Thuringian) sequences and thus northern Gondwana-Land of the Palaeozoic times. The zircon ages from its Neoproterozoic basement suggest that İstanbul Zone once was located at the north-eastern margin of Gondwana-Land, recent paleontological studies place the İstanbul Zone to about 30–400S for early Ordovician. Devonian fauna shows similarities with Thuringia, Rhenish Massif, Cantabrian Mountains, Pyrenees, Holy Cross Mts. and North Africa.In this study a total of 688 samples were obtained from 54 sites around İstanbul and Kocaeli. 465 samples collected from the Palaeozoic sedimentary rocks and 223 samples belong to the dykes that cut these sediments and lavas, and related ashes, tuffs that overlay these sediments. 624 standard palaeomagnetic specimens were prepared from 688 samples, and 547 of them are successfully completed their demagnetization steps. The specimens were demagnetized in the laboratory by using both AF and thermal treatments depending on their effectiveness. After demagnetization treatments, 290 specimens showed stable demagnetization patterns and majority of these samples have a characteristic remanant magnetization component close to the present day geomagnetic field. Palaeomagnetic data processing were made on RemaSoft 3.0 and IAPD2014 softwares. Demagnetization studies demonstrate variable degrees of overprinting in a large number of samples. After the application of the tilt correction, %70 of the specimens failed the fold test at site level (early Ordovician siltstones; late Silurian-early Devonian limestones; late Devonian limestones; early Carboniferous turbidites). Rest of them clearly got scattered with increasing α95 and decreasing k values (mid Ordovician conglomerates; mid-late Devonian shales; late Ordovician-early Silurian sandstone and siltstones). This secondary magnetization, acquired during or after the folding event, constitutes evidence of pervasive remagnetization that can be caused viscous remanant magnetization.The İstanbul tectonic unit (İstanbul-Çamdağ-Zonguldak) occupies a rather small area that makes its tectonic evolution hard to reconstruct. This raises the necessity of dealing with the European Hercynides. The Hercynides of Europe are part of a large (~1000 km broad and ~8000 km long) Palaeozoic mountain belt, which was formed as a result of diachronic collision between Laurussia±Baltica and Gondwana-Land at the end of the Carboniferous. This system extends from the Caucasus to the Appalachians.In this thesis we mainly focused on European side of this orogenic belt and used methodology of comparative anatomy of orogens. Every orogenic belt has its own organs represented by distinctive rocks. Fore-arc, arc, back-arc basin, a continental shelf (clastic shelf, shallow shelf, and/or carbonate platform) are basic organs of an orogenic belt. These organs may not be present in every orogenic belt but the most common feature among all these organs is the magmatic arc. Magmatic rock types are strongly related to geodynamic environments. Well-typed and well-dated magmatic rocks can be used as indicators of geodynamic environments, and even further as tracers of geodynamic evolution. To identify the magmatic arcs, the intermediate and felsic magmatic rocks, namely granodiorites, diorites, andesites, granites, and rhyolites are used. For this purpose geological maps of the Germany, Spain, England, France, Czech Republic, Austria, Poland were digitized and mapped as one geological map. ~2700 high quality isotopic age data are collected from the literature. 1138 of them are interpreted as products of a single magmatic arc which had been active from the Ediacaran to the late Carboniferous on the northern margin of Gondwana-Land. The magmatic anomaly maps also used to track this magmatic fragments. This magmatic arc is herein named `Protogonos` (=the first born) in this study. Magnetic anomaly maps and structural trend lines are used as supplements in identifying the extent and evolution of Protogonos.Palaeomagnetic studies revealed remagnetization in the Palaeozoic rocks of İstanbul. The timing of this event coincides with the latest Cainozoic. As a result of this remagnetization we are unable to find a paleolatitude or a rotation for the Palaeozoic rocks. The final step of this thesis is reconstructing the tectonic units and find a proper place for İstanbul tectonic unit. The magmatic arc is the key point in this reconstruction. The fragments of this magmatic arc repositioned according to the displacements on the major transform faults at that time. For this purpose, we used Pangaea B reconstruction template to replace the tectonic units. We propose a retro-arc setting for the İstanbul tectonic unit with Moesia and Tepla-Barrandium behind the Protogonos magmatic arc as part of Gondwana-Land until the medial Carboniferous. While the western part of Hercynian orogen went through collision and formed an orogenic belt from North America and North-western Africa to Moesia, the eastern part including the İstanbul tectonic unit remained untouched by this collision and continued its tectonic evolution as a part of Palaeo-Tethys margin. This episode is ended in Jurassic with the Cimmeride collision. This leads us to the conclusion that no Hercynian event is present in the İstanbul tectonic unit.