On the basis of gravity modelling on the southern Marmara Sea shelf and conventional multichannel seismic reflection data, the tectonic setting of the region was studied. The relation of the middle strand of the North Anatolian Fault Zone with the Thrace-Eskisehir Fault Zone was examined. The southern Marmara Sea region was tectonically uplifted in the Late Pliocene. Gemlik Bay was developed during Late Pliocene-Early Pleistocene under the control of right lateral strike-slip faults along the middle strand of the North Anatolian Fault. The northwest trending Thrace-Eski ehir Fault is the second important tectonic element in this evolution. (C) 2002 Elsevier Science B.V. All rights reserved.