The Aegean region is one of the most rapidly deforming continental areas in the world. In the Sea of Marmara, interaction between strike-slip and extensional faulting has led to a complicated style of local deformation. To the east, the continental plate containing eastern and central Turkey is moving westward with respect to Eurasia, with little internal deformation. Tectonic deformation is restricted to a narrow zone along the dominantly strike-slip North Anatolian fault. However, in the vicinity of the Sea of Marmara, both surface mapping of faults and earthquake fault plane solutions indicate that extensional motions, presumably related to back arc extension behind the Hellenic subduction zone, create a much wider zone of deformation which reaches hundreds of kilometers in width. We use a regional grid of high-resolution seismic reflection profiles to map the fault systems as they cross the Sea of Marmara and show how they divide the Sea of Marmara into separate deep basins with distinctive sediment sources and depositional styles. In the southern Sea of Marmara these basins are half graben, formed on north dipping fault planes, which have trapped sediment coming into the Sea of Marmara from the south. No evidence is found in the data set for the existence of a single strike-slip fault through the Sea of Marmara or for the existence of a northern boundary fault along the Tekirdağ and Central Marmara basins. We conclude that a single strike-slip fault would be incapable of accommodating the relative motion and extension observed between western Turkey and Eurasia.