The Tekirdagˇ depression within the Marmara Sea in the Mediterranean region is an active, rhomb-shaped strike-slip basin along the North Anatolian fault with a basin floor at a water depth of -1150 m. New multichannel seismic reflection data and on-land geological studies indicate that the basin is forming along a releasing bend of the strike-slip fault and is filled with syntransform sediments of Pliocene-Quaternary age. The basin is bounded on one side by the North Anatolian fault and on the other side by a subparallel normal fault, which forms the steep submarine slope. In cross section the basin is strongly asymmetric with the thickness of the syntransform strata increasing from a few tens of meters on the submarine slope to over 2.5 km adjacent to the North Anatolian fault. Seismic sections also show that the slope-forming normal fault connects at depth to the North Anatolian fault, implying that the basin is completely detached from its substratum. The whole structure can be envisaged as a huge, rather flat, negative flower structure. The releasing bend of the North Anatolian fault, responsible for the formation of the basin, is flanked by a constraining bend. Along the constraining bend, the syntransform strata are being underthrust, implying a recent change in the direction of the regional displacement vector. This thrusting is responsible for the uplift of the submarine slope to a height of 924 m, possibly by a mechanism of elastic rebound. Regional geology suggests that most of the syntransform strata are lacustrine with only the topmost few hundred meters consisting of deep marine clays. The anomalous present depth of the Tekirdagˇ depression is due to reduced Quaternary sedimentation coupled with high rates of displacement along the North Anatolian fault, which amounts to 20 mm/yr in the Marmara Sea region. ¿ 1999 American Geophysical Union