Eastern Marmara region consists of three different morphotectonic units: Thrace–Kocaeli Peneplain (TKP) and Çamdağ–Akçakoca Highland (ÇAH) in the north, and Armutlu–Almacık Highland in the south of the North Anatolian Fault Zone (NAFZ). The geologic‐morphologic data and seismic profiles from the Sakarya River offshore indicate that the boundary between the TKP in the west and ÇAH in the east is a previously unrecognized major NNE–SSW‐trending strike‐slip fault zone with reverse component. The fault zone is a distinct morphotectonic corridor herein named the Adapazarı–Karasu corridor (AKC) that runs along the Sakarya River Valley and extends to its submarine canyon along the southern margin of the Black Sea in the north. It formed as a transfer fault zone between the TKP and ÇAH during the Late Miocene; the former has been experiencing extensional forces and the latter compressional forces since then. East–West‐trending segments of the NAFZ cuts the NE–SW‐trending AKC and their activity has resulted in the formation of a distinct fault‐bounded morphology, which is characterized by alternating E–W highlands and lowlands in the AKC. Furthermore, this activity has resulted in the downward motion of an ancient delta and submarine canyon of the Sakarya River in the northern block of the NAFZ below sea level so that the waters of the Black Sea invaded them. The NE–SW‐trending faults in the AKC were reactivated with the development of the NAFZ in the Late Pliocene, which then caused block motions and microseismic activities throughout the AKC. Copyright © 2004 John Wiley & Sons, Ltd.