THE ISTANBUL ZONE: A CONNECTING LINK BETWEEN THE HERCYNIDES AND THE SCYTHIDES

The Istanbul Zone is a part of the Rhodope-Pontide fragment in northern Turkey. It extends from Çatalca in the west, where a buried fault (right-lateral strike-slip?) separates it from the Strandja Nappes, to Kastamonu, where its tectonic klippen lie above the Akgöl Flysch of the subduction-accretion complex of the Cimmerides. The Black sea now delimits it to the north (in places along the coast fragments of a late Cretaceous magmatic arc lie above it), whereas the Intra-Pontide suture lies to its south. In Istanbul proper it contains a formerly-east facing (present geographic coordinates) Atlantic-type continental margin that rifted in the Ordovician and destroyed during an early Carboniferous collision. That collision created a dominantly (now) west-vergent marginal fold and thrust belt. Ordovician arkoses forming a rift fill are thrust westward in two major nappes overriding a Siluro-Devonian carbonate- and shale-dominated shelf. That shelf is folded and thrust eastward in retrocharriage resulting from a triangle zone just west of the Bosphorus. No metamorphism accompanied this deformation. Permian granodiorites, probably related to Cimmeride (i.e. palaeo-Tethyan) subduction, pierce the eastern parts of the deformed structure. The stratigraphic development of the Istanbul zone can be correlated almost one-to-one with that of the Montagne Noire and thus greatly resembles the development of the south margin of the Theic Ocean (=Océan centralien) with Ordovician arkoses possibly reflecting cold Gondwanian conditions. The Middle Devonian fauna, however, is typically Rhenic and thus indicates connexions with the Rheic Ocean. It is clear that eastwards these two oceans came together and merged into one near Istanbul and eastwards continued into the future Palaeo-Tethys, along the northern margin of which the Scythides had long been developing. The Istanbul zone thus provides a direct link from the Hercynides to the Scythides in the then newly-forming late Palaeozoic Pangaea.