Abstract: The Sarıyer-Şile thrust, which has been known for many years in the geological literature, caused the northward emplacement of the Istanbul Palaeozoic sequence and the unconformably overlying Permo-Triassic rocks onto Upper Cretaceous volcanic and volcanogenic rocks. During investigations of drill cores and surficial geological observations, carried out with in the scope of route research for a highway tunnel being built between Kilyos and Sarıyer, a cataclastic zone that developed along this thrust was identified. While the thickness of this zone is limited at the base of the overlying Palaeozoic sequence, it probably reaches 200 metres in the underlying volcanic-volcanogenic rocks to the south, and gradually thins and disappears towards the north. This compression altectonic regime, which probably developed during the middle Eocene period, caused different deformations inrheologically different hanging and foot wall blocks along this low angle thrust. Carboniferous gray wackes in the hanging block are shortened and thickened by generally fan-shaped thrusts, and less commonly by duplex thrusts, while the volcanic-volcanogenic sequence in the footwall block was affected by intense cataclasis.