Okay an oceanic plateau ? Was the Late Triassic orogeny in Turkey caused by the collision of Geological Society

A belt of Late Triassic deformation and metamorphism (Cimmeride Orogeny) extends east-west for 1100 km in northern Turkey. It is proposed that this was caused by the collision and partial accretion of an Early-Middle Triassic oceanic plateau with the southern continental margin of Laurasia. The upper part of this oceanic plateau is recognized as a thick Lower-Middle Triassic metabasite-marble-phyllite complex, named the Niltifer Unit, which covers an area of 120 000 km 2 with an estimated volume of mafic rocks of 2 x 105 km 3. The mafic sequence, which has thin stratigraphic intercalations of hemipelagic limestone and shale, shows consistent within-plate geochemical signatures. The Niliifer Unit has undergone a high-pressure greenschist facies metamorphism, but also includes tectonic slices of eclogite and blueschist with latest Triassic isotopic ages, produced during the attempted subduction of the plateau. The short period for the orogeny (< 15 Ma; Norian-Hettangian) is further evidence for the oceanic plateau origin of the Cimmeride Orogeny. The accretion of the Niltifer Plateau produced strong uplift and compressional deformation in the hanging wall. A large and thick clastic wedge, fed from the granitic basement of the Laurasia, represented by a thick Upper Triassic arkosic sandstone sequence in northwest Turkey, engulfed the subduction zone and the Niltifer Plateau. An east-west trending belt of latest Triassic deformation and regional metamorphism extends for over 1100 km in northern Turkey. The Early Mesozoic deformation (but not the regional metamorphism) was known previously ($eng6r 1979; Bergougnan & Fourquin 1982) and was referred to as the Cimmeride deformation ($eng6r et al. 1984). The Cimmeride deformation was ascribed to the closure of the Palaeotethys ocean following the collision of a Cimmerian continental sliver with the southern margin of Laurasia ($eng6r 1979; Seng6r et al. 1984). Here, an alternative explanation, involving the collision and partial accretion of an oceanic plateau to the southern margin of Laurasia, is proposed for the origin of the latest Triassic deformation and metamorphism in northern Turkey. A tectonic map of Turkey and the surrounding region is shown in Fig. 1. During the Palaeozoic and Mesozoic, the various continental blocks that make up present-day Turkey were situated on the continental margins of the Tethys Ocean. The Pontides, which comprise the Strandja, istanbul and Sakarya Zones, show Laurasian stratigraphic affinities, while the Anatolide-Tauride Block and the Klr~ehir Massif are tectonically and stratigraphically related to Gondwana ($eng6r & Yllmaz 1981; Okay et aL 1996; Okay & Ttiystiz 2000). The istanbul Zone is a continental fragment, which was translated south from the Odessa Shelf with the Cretaceous opening of the oceanic West Black Sea Basin (Fig. 1; Okay et al. 1994). Its stratigraphy is similar to that of the Scythian and Moesian platforms, with a fully developed Palaeozoic sedimentary sequence unconformably overlain by Triassic and younger sedimentary rocks (Haas 1968; Dean et al. 1997; G6rtir et al. 1997). In the Istanbul Zone, a weak latest Triassic deformation is marked by an unconformity between the Norian siliciclastic turbidites and the overlying Upper Cretaceous carbonates. The Strandja Zone consists of a Late Hercynian metamorphic and granitic basement unconformably overlain by Lower Triassic-Middle Jurassic sedimentary rocks (Chatalov 1988; Okay et al. 1996). The Anatol ide-Tauride Block and the Klr~ehir Massif are also devoid of Triassic metamorphism, and of any significant Triassic deformation. Several well studied Lower Mesozoic stratigraphic sections in the Taurides, including those in the Bornova Flysch Zone (Erdo~an et al. 1990.). and in the central Taurides (Gutnic et al. 1979; Ozgti11997), show a continuous transition between Triassic and Jurassic with no evidence of an intervening deformation phase. The pre-Jurassic thrusting, described by Monod & Akay (1984) from a small locality in the central Taurides, is as yet of unknown significance. Late Triassic deformation and regional metamorphism in Turkey are predominantly found in the Sakarya Zone, which will form the main subject of this paper. From: BOZKURT, E., WINCHESTER, J. A. & PIPER, J. D. A. (eds) Tectonics and Magmatism in Turkey and the Surrounding Area. Geological Society, London, Special Publications, 173, 25-41.1-86239-064-9/00/$15.00 (C) The Geological Society of London 2000.