The North Anatolian Fault is a 1200-km-long transform fault forming the boundary between the Anatolian and Eurasian plates. Near its western end it strikes through the Marmara Sea, and is responsible for the creation of three deep marine depressions, The North Anatolian Fault system and the linked basins in the eastern Marmara Sea were studied using newly acquired multi-channel seismic reflection data. In the Marmara Sea the North Anatolian Fault consists of a main strand and a few subsidiary branches. The main strand is made up of the Ganos (15 km long), Central Marmara (105 km) and North Boundary (45 km) fault segments. The North Boundary Fault joins in the east to the Izmit segment, which was ruptured during the M(S) 7.4 earthquake on 17.8.1999. The North Boundary Fault is highly oblique to the regional displacement vector, which results in the generation of a symmetrical fault-wedge basin. This Cmarcik Basin is a wedge-shaped deep marine depression south of istanbul. It is about 50 km long, up to 20 km wide and 1250 m deep, and is filled with Pliocene to Quaternary sediments, over 3 km thick. The North Boundary Fault joins in the west to the transpressional Central Marmara Fault. Around the restraining segment boundary the syntransform sediments are being deformed into a large anticlinorium. The Cmarcik Basin started to form when the westward-propagating North Anatolian Fault intersected a northwest-trending pre-existing fault zone during the Pliocene and bifurcated into NW- and SW-trending segments, forming a transform-transform-transform-type triple junction. Dextral strike-slip movement along the arms of the triple junction led to the development of the Cmarcik Basin, The present fault geometry in the Marmara region was achieved later during the mid-Pliocene following the termination of the triple junction geometry. South of the main North Anatolian Fault system there is a second active strike-slip fault in the Marmara Sea made up of several segments. A highly asymmetrical fault-bend basin, comprising Pliocene-Quaternary sediments over 2.5 km thick, is forming along a releasing fault segment of this South Boundary Fault system. (C) 2000 Elsevier Science B.V. All rights reserved.