ORIGINAL RESEARCH PAPER Submarine structures in the Gulf of _ Izmit, based on multichannel seismic reflection and multibeam bathymetry Hu ¨lya Kurt Æ Esra Yu ¨ cesoy Received: 16 August 2008 / Accepted: 25 May 2009 / Published online: 12 June 2009 Ó Springer Science+Business Media B.V. 2009 Abstract Multichannel seismic reflection and multi- beam bathymetry data were used to study the active tec- tonic and syn-tectonic stratigraphic setting of the Gulf of _ Izmit in the Marmara Sea (Turkey). The gulf and its near surroundings are deformed by the northern strand of the dextral North Anatolian Fault. Three connected basins of the gulf, the western (Darıca), central (Karamu ¨rsel) and eastern (Go ¨lcu ¨k) basins are formed by active faults, as observed in the stacked and migrated seismic sections, as well as the bathymetry map. The main branch and its surrounding sedimentary strata are confined by normal faults to the north and south. These normal faults converge at depth towards the main fault, forming a negative flower structure in the gulf. The average maximum sedimentation rate is 0.4 mm/year according to the three most recent seismo-stratigraphic units that are located to the south of the main fault branch within the central basin. A 20° south- dipping major discontinuity along the northern shoreline of the gulf represents the top of Paleozoic basement. Keywords Sea of Marmara Á Gulf of _ Izmit Á North Anatolian Fault Á Multichannel seismic reflection data Introduction The Gulf of _ Izmit is located in the eastern part of Marmara Sea. It is, on average, 50 km long and 10 km wide (Fig. 1). The northern strand of the North Anatolian Fault enters the Marmara Sea through the Gulf of _ Izmit. The North Anatolian Fault is an E–W trending right-lateral strike-slip transform fault running across northern Turkey with a length of *1,600 km (S ¸ engo ¨r et al. 1985;S ¸ arog ˘lu 1988). The fault zone is one of the most tectonically active regions of the world. The mechanics of strike-slip faulting and its associated structures have been extensively studied (Tchalenko 1970; Wilcox et al. 1973; Wernike and Bur- chfiel 1982; Sylvester 1988). Eight westward-progressing fault ruptures along the North Anatolian Fault have caused destructive earthquakes (Barka 1996; Stein et al. 1997), the last two of which occurred on 17 August 1999 (M w = 7.4) and 12 November 1999 (M w = 7.2). Stress triggering has been proposed as the cause of the westward progression of earthquakes along the North Anatolian Fault, and has increased the probability of a large earthquake on the western segment of the fault (Barka 1999; Parsons et al. 2000). The Gulf of _ Izmit is a key location for under- standing the geometry of the North Anatolian Fault, because it also includes the offshore continuation of the August 17th fault rupture. Three connected basins, the western (Darıca), central (Karamu ¨rsel) and eastern (Go ¨lcu ¨k) basins, form the Gulf of _ Izmit (Fig. 2). The central basin is connected to the other basins by two shallow sills located north of the Hersek Peninsula and north of Go ¨lcu ¨k at about 55 and 38 m water depths, respectively. The evolution of the Gulf of _ Izmit has been studied by various groups (S ¸ engo ¨r et al. 1999; Go ¨kas ¸an et al. 2001; Alpar and Yaltırak 2002; Kus ¸c ¸u et al. 2002; Polonia et al. 2004; Cormier et al. 2006). It is gen- erally accepted that the geological structures in the gulf are mostly formed by deformation related to the North Ana- tolian Fault, but their distribution and interpretation differ. Conventional bathymetry, coastal morphology, coarse grids of shallow seismic lines, and measured seismicity have been used to infer the geometry of the active faults in H. Kurt (&) Á E. Yu ¨cesoy Department of Geophysics, Istanbul Technical University, 34390 Maslak, Istanbul, Turkey e-mail: kurt@itu.edu.tr 123 Mar Geophys Res (2009) 30:73–84 DOI 10.1007/s11001-009-9068-y