Tectonic evolution of the Marmara Sea and its surroundings

Abstract

The basins in the Marmara Sea are the products of a superimposed evolutionary history defined by two different-aged fault systems: the early Miocene–early Pliocene Thrace–Eskişehir Fault Zone and its branches, and the late Pliocene–Recent North Anatolian Fault and its branches. The Thrace–Eskişehir fault and its westward branching secondary fault systems define the early neotectonic signature in the region. The late neotectonic period started at the end of the early Pliocene when the North Anatolian Fault divided the Thrace–Eskişehir fault into four parts. During the late neotectonic period, the North Anatolian Fault extended westward as a number of splays by joining with the Ganos, Bandırma–Behramkale and Manyas–Edremit Fault Zones. The branches of the North Anatolian Fault Zone (NAFZ) caused the evolution of a number of basins, which differ in character depending on the trend and past characteristics of the older branches that became connected. Since the northern branch of the North Anatolian Fault is connected to the N80°E-trending Ganos Fault Zone (GFZ) in the west, a single buried fault has developed in the Marmara Sea, causing the well-known troughs and ridges, superimposed onto the negative flower structure formed by the GFZ in the early neotectonic period. The middle strand, which extends from Iznik Lake to Bandırma, is oriented east–west up to the N60°E-trending Bandırma–Behramkale Fault Zone, then turns southward in the vicinity of Bandırma, forming a region dominated by compressional tectonics. This bending caused N30°E-trending tension in addition to the strike-slip motion between the eastern part of Gemlik Bay and Bandırma Bay. The southern branch of the NAFZ, on the other hand, produced three pull-apart basins with different characteristics along the Yenişehir, Bursa and Manyas segments. The southern branch of the NAFZ connected to the Manyas–Edremit Fault Zone, which is oriented N45°E to the south of Manyas, and the associated bending and rotation caused a N15°E-trending extension in addition to the strike-slip regime between Manyas and Uluabat. The branches of North Anatolian Fault cut through the Thrace–Eskişehir fault at three places: the East Marmara Sea region, in Gemlik Bay, and to the east of Bursa, giving lateral offsets of 58–59, 7–8 and 10–11 km, respectively. The cumulative motion is 75–78 km, corresponding to the total lateral offset of the North Anatolian Fault in the region. The correlation of these offsets with Global Positioning System slip vectors and with stratigraphic results implies that the North Anatolian Fault reached into the Marmara Sea region about 3.5 million yr ago. Tectonic processes forming the Marmara Sea and its environs were initiated by the Thrace–Eskişehir fault and its splays have been most recently controlled by the North Anatolian Fault and its splays during the last 3.5 million yr.