Submarine active tectonism in the Gulf of Gokova located at the southwest Anatolia-southeast Aegean Sea region was investigated by means of multi-channel seismic reflection data. The Gokova basin is filled by the latest Miocene-Pliocene-Quatemary sediments with maximum thickness of about 2.5 km. The Lycian Nappes, which predominantly cover extreme southwestern Anatolia, constitutes the basement rocks for the Gokova province. The gulf was mainly opened by a buried major listric normal fault, so-called Datca a Fault, which has nor been previously discussed in the literature. The north-dipping, mainly E-W-trending Datca Fault is located at the southern part of the gulf, whereas its associated antithetic faults are located at the north. The onset time of the opening of the gulf is possibly in the latest Miocene-Pliocene. In terms of local rather than regional effects, the activity of the Datca Fault has decelerated, possibly since the Pleistocene. The Datca Fault might have gained its curved fault plane as it evolved, beginning as planar and/or using antecedent planes of the Lycian Nappes in the area. As the extension progressed, i.e., as the hanging wall block slipped further north, gravity may have impeded rather than helped the faulting. On the other hand, continuing extension in the area may have initiated a second phase of faulting, i.e., WNW-ESE-oriented subgrabens in the gulf and major E-W normal faulting in the northeast margin. A bathymetric low in the mid-gulf area and a horst-graben system in the eastern part of the gulf are observable from the bathymetric data and are well correlated to the seismic data. Although the main orientation of the gulf is E-W, more recent WNW-ESE structures are remarkable in the mid-gulf and in the eastern part of the gulf. The latest WNW-ESE structures are also in agreement with the results of GPS and SLR studies as well as plate motion modelling by total moment tensor of earthquakes in the western Anatolia-Aegean Sea region, particularly in southwestern Anatolia. The amount of total N-S extension within the gulf is estimated as at least 5.5 km since the latest Miocene-Pliocene with overall constant extension rate of at least 1.1 mm/y where the estimated extension factor is about beta = 1.3, (C) 1999 Elsevier Science B.V. All rights reserved.