We perform three-dimensional spontaneous dynamic rupture models of potential earthquakes on the geometrically complex North Anatolian fault zone (NAFZ) under the Marmara Sea, Turkey. The NAFZ south of the city of Istanbul consists of a large-scale extensional stepover, with right lateral strike-slip segments linked by an oblique normal segment. We find that earthquakes nucleating near the stepover do not propagate across the entire fault system due to the statically unfavorable stress field on the oblique fault, and lead to moderate-size events only. However, earthquakes initiating a significant distance from the stepover cause significant dynamic unclamping of the oblique fault, and thus generate large through-going ruptures. Dynamic unclamping also produces supershear rupture propagation in the vicinity of fault discontinuities. The results emphasize that estimations of earthquake size, rupture propagation, and slip distribution cannot be decoupled from the location of the hypocenter and the orientation of the geometrically complex fault system within the tectonic stress field.