Despite several offshore data are available, a general model of the central part of the Izmit Gulf is still a challenge and only few measurements have been done on the Hersek peninsula. This area is a key point not only because represents the only onshore segment of the North Anatolian Fault (NAF) within the central part of the Izmit Gulf, but also because the new Istanbul-Izmit highway will cross such gulf in this zone, with a suspension bridge, which will be the second longest bridge of such type in the world. This paper, integrating different analyses and data sets aims verifying at some of the present hypotheses. In particular, we propose the joint interpretation of new seismic reflection profiles from the Hersek Peninsula, as well as geophysical, remote sensing and geological data from the Izmit Gulf (Sea of Marmara, Turkey), close to the epicenter of the 1999, M-w 7.4 Izmit earthquake, to identify shallow-crustal deformations associated with the NAF. Our seismic reflection profiles, which are the first onshore two-components records within the Izmit Gulf, image compressive features that form a structural high cut by the principal deformation zone of the NAF. The observed transpressive stress pattern appears to be the main factor contributing to the physiography of the peninsula, together with the growth northwards of the Yalakdere River deposits. We demonstrate that this pattern is compatible with an 8 degrees bending of the NAF strand. Based on these new data, we propose a kinematic model for the evolution of this structure around the Hersek Peninsula during Holocene, and we test our reconstruction using a finite-elements numerical simulation model of the stress/strain distribution.