THE EVOLUTION OF THE INTRA - PONTIDE SUTURE: IMPLICATIONS OF THE DISCOVERY OF LATE CRETACEOUS – EARLY TERTIARY MELANGES

The Intra - Pontide suture is the boundary between the Istanbul and Sakarya zones of the western Pontides, in northwest Turkey. Our new detail palaeontological and stratigraphic data on the sedimentary mélanges of the Intra - Pontide suture show that the subduction of the Intra - Pontide Ocean was still going on between late Cretaceous and early Ypresian. This makes a contrast with the recently reported Santonian closure of the Intra - Pontide Ocean. In order to understand the events during late Cretaceous – early Tertiary in western Pontides, we have gathered much of the earlier published stratigraphic, palaeontological, radiogenic data both on the metamorphic units and unmetamorphosed sedimentary basins along and near to the Intra - Pontide suture. Our analysis showed that two accreationary prism was formed along the suture zone: (1) an Upper Cretaceous blueschist – eclogite facies metamorphic prism in the Biga Peninsula and (2) an Upper Jurassic – Lower Cretaceous greenschist – epidote amphibolite facies metamorphic prism in the Armutlu Peninsula. During late Cretaceous when the Intra – Pontide arc was forming, an Upper Cretaceous - Palaeocene forearc basin had also formed over Istanbul Zone which was the hinterland. At the same time interval Central Sakarya Basin was evolved to a flexural basin south of the Upper Jurassic – Lower Cretaceous accretionary prism. The palaeontological data suggests collision must occured later than early Ypresian because this is the age of the youngest blocky trench flysch. A Cuisian molasse covers all the older units suggesting a closure occured between early Ypresian – Cuisian (~56-53,5 Ma). This closure age also confirmed by previously published fission track uplift ages along the Intra - Pontide suture. The collision related shortening probably ended before Late Bartonian because the post tectonic structures cannot be followed in Upper Bartonian limestone and clastics cropping out 30 km west of Bosphorus of Istanbul. During the Middle Eocene – Oligocene interval a wedge shaped transtensional intra-montaine basin, the Thrace basin, had oppened while the westerly escaping fragments of the Intra - Pontide suture tore a piece of the Strandja system following the pre-Cuisian closure of the Intra - Pontide ocean.