STRUCTURAL FEATURES AND TECTONIC EVOLUTION OF PALAEOZOIC SEQUENCE IN ISTANBUL ZONE, NW TURKEY

Istanbul 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. This sequence sits on a latest Precambrian basement and starts with an Ordovician deep lake turbiditic clastics which consist of arkoses. Arkose sedimentation suggests possible relationship with Gondwanian glaciation. Over these clastics, continental colluvial conglomerates are seen as a sign of rifting. During Silurian basin shows deeper environment represented by quartz arenites and micaceous siltstone-sandstones. During Lower Devonian thick shelf type limestones with fossil fragments (Brachiopod, Corals) deposited in the basin. Upper Devonian shales which are 650 m thick cover these limestones comformably. Basin continues to deepening during Upper Devonian and Lowest Carboniferous, it is represented by nodular limestones, cherts and shales. On the top of sequence Lower Carboniferous flysch which is considered to be in relationship with Hercynian Orogeny deposited. One of the biggest problems about these rocks is the relation of deformation and Hercynian (Variscan) Orogeny. Structural studies aiming to solve deformation characteristics of this transgressive sequence shows that rock type is in control during deformation process. Vergence of main deformation is considered as to the west with the help of structural cross sections. Under this deformation thick, competent arkoses folded with high wave length, cut by big thrusts and shows spaced cleavage. Devonian shales are folded with lower wave length, shows evidence of plastic deformation such as kink-folds and similar folds and on some regions shows severe folding probably related with strain accumulation. Also it seems that they are squeezed between Ordovician arkoses and Carboniferous flysch. Carboniferous flysch is characterized by parallel folding and flexural slip along bedding planes which refer necessity of a main decollement. Lack of metamorphism and penetrative (??) structures shows that this Palaeozoic sequence was deformed on a hinterland of an orogenic belt.