Erkan Gökaşan

On the origin of the Bosphorus

Abstract The Palaeozoic-Upper Cretaceous basement palaeomorphology of the Bosphorus (the Strait of Istanbul) bears the evidence of a valley of a palaeostream running to the Black Sea in the north, a palaeobasin deeper than —160 m opening to the Sea of Marmara in the south, and a barrier between these two features. This suggest that the northern part of the Bosphorus was formed mainly by fluvial activity, whereas the southern part developed as a basin by faulting. The recent sediment thickness exceeds 130 m in the basin, indicating that the southern part of the Bosphorus was once essentially depositional rather than an erosional. The present form of the Bosphorus was established in Holocene time by the connection of the basin in the south with the stream in the north. The barrier and the stream valley in the north have been deepened by erosion and faulting to form a strait connecting the Black Sea and the Mediterranean.

The last sea level changes in the Black Sea: evidence from the seismic data

Abstract High resolution shallow seismic data collected from the southwestern shelf of the Black Sea indicate five different seismic stratigraphical units. The lower three of them belong to the Upper Cretaceous–Eocene, Oligocene–Miocene and Early Quaternary (prior to Holocene) sediments, respectively. These units are considered as a basement for the recent sediments deposited related to the latest connection of the Black Sea and the Mediterranean. The surface of these units are truncated to form an etchplain developed before the Flandrian transgression. The fourth unit covers the older units by an onlap. Its contact with the older units seen at −105 m is the shoreline of the Black Sea prior to the last major sea-level change. The fifth unit has been deposited since drowning of the Black Sea shelf. The principal cause of drowning of the Black Sea shelf is not only the last sea level rise as it is at the shelves of the Sea of Marmara but also the opening of the Strait of Istanbul. It is also realised by the comparison of the shelf area and the Catalca–Kocaeli etchplain that, the present continental part of this etchplain has been considerably uplifted with respect to the shelf area along the present shoreline. This uplifting must have also reactivated the faults around the Strait of Istanbul foundering the strait valley and, thus, permitting the Mediterranean waters to pass into the Black Sea, and initiating the sudden drowning of the Black Sea shelf.

Morphologic features of the Marmara Sea from multi-beam data

Abstract The bathymetrical image of the Marmara Sea (NW of Turkey) was prepared, using data from mainly multi-beam surveys and from various other sources. The Marmara Sea presents a unique and complicated morphological system, which is mainly controlled by the active tectonic regime of the North Anatolian Fault Zone (NAFZ). The sub-bottom morphology of the Marmara Sea consists of a wide shelf area (55% of the total area) and the Marmara Trough, which includes the slopes, ridges and deep basins. The four basins separated by ridges constitute the E–W trending Marmara Trough in an elliptical form. Submarine canyons and landslides constitute the secondary morphological units of the Marmara Sea. The angle and the shape of the slopes are variable in different parts of the Marmara Sea. The linear-shaped slopes bounding the Cinarcik Basin (eastern extremity of the Marmara Sea) and off the Ganos Mountain System (west of Marmara Sea) are characteristic with their high gradient, and resulted from strike–slip fault. The curve-shaped slopes with low angles are considered to be associated with normal fault. E–W-aligned lineaments are morphological expressions of strike–slip faults and can be traced from the Ganos Mountain System through Cinarcik Basin, cutting through the basins and ridges in the western Marmara Trough, and bounding the Cinarcik Basin at the slope–basin transitions. These lineaments reflect a new rupture of NAFZ and must have occurred after the formation of the Marmara Trough.

Active tectonics of the İzmit Gulf (NE Marmara Sea): from high resolution seismic and multi-beam bathymetry data

Abstract Several active faults were determined in the Gulf of Izmit, using high-resolution shallow seismic and multi-beam bathymetry data. One of them is a buried fault (Fault 2), following central axis of the gulf, which forms the current morphology within its releasing and restraining bends. Releasing bends control the current morphology of the sub-basins. Also restraining bends form folds in the central basin and a thrust in the western basin. A left step over between two strike-slip faults (Faults 2 and 11) creates a transpressional area. Therefore, Hersek and Catal deltas are uplifted. It is determined that Fault 2 could not be related to the origination of the gulf because it cuts all basins and ridges, thus, present geometry of the Fault 2 is not suitable for the initial formation of the gulf. From this point of view, the Fault 2 is initiated recently compared to the whole North Anatolian Fault Zone (NAFZ) in the Marmara Sea. Therefore, Fault 2 is recognised as a new rupture of the NAFZ probably active since the end of the last glacial ice period.

The effects of the North Anatolian Fault Zone on the latest connection between Black Sea and Sea of Marmara

Abstract The development of the Strait of Istanbul is also one of the principal results of the tectonics which led to the evolution of the North Anatolian Fault Zone (NAFZ) in the Marmara Region 3.7 Ma ago. High resolution seismic profiles from the Marmara entrance of the Strait of Istanbul show a folding which occurred after the deposition of the parallel reflected Tyrrhenian sediments. Over the Tyrrhenian strata, a fondoform zone of a deltaic sequence and marine sediments of the latest sea level rising are present. These sediments also display syn-depositional folding. This situation implies that a local compressional stress field was created over the area probably since the Wurm Glacial age. This recent variation of the tectonic regime in the northern shelf of the Sea of Marmara may indicate a significant change in the development of the NAFZ through the Sea of Marmara. This variation of evolution of the NAFZ affected the latest development of the Strait of Istanbul via clockwise rotation of the Istanbul and Kocaeli peninsulas by right-lateral shearing between two zone bounding faults. This rotation has led to the development of NNE–SSW left-lateral faults in the Strait of Istanbul and local compressional and tensional areas explaining the compressional structures seen in the southern entrance of the Strait of Istanbul. Therefore, the latest Mediterranean–Black Sea connection was established by means of the sufficient deepening of the Bosphorus channel by a variation in the evolution of NAFZ through the Sea of Marmara.

Morphologic and seismic features of the Gulf of Gökova, SW Anatolia: evidence of strike-slip faulting with compression in the Aegean extensional regime

Recently acquired (2005) multi-beam bathymetric and high-resolution seismic reflection data from the E–W-oriented Gulf of Gökova off SW Anatolia were evaluated in order to assess the uneven seafloor morphology and its evolution in terms of present-day active regional tectonics. Stratigraphically, the three identified seismic units, i.e., the basement, deltaic sediments deposited during Quaternary glacial periods, and modern gulf deposits, are consistent with those observed in previous studies. Structurally, the folds and faults with strike-slip and reverse components have been regionally mapped for the first time. Of these, NE–SW-oriented left-lateral strike-slip faults with compressional components forming the so-called Gökova Fault Zone intersect and displace two WNW–ESE-oriented submarine ridges and deep submarine plains. Thus, strike-slip faults are the youngest major structures in the gulf, and control present-day active tectonism. E–W-oriented folds on the inner and outer shelf, which are generally accompanied by reverse faults, delimit the margins of these submarine ridges, and deform the young basin deposits. These features also reveal the concomitant existence of a compressional tectonic regime. The compressional structures probably represent pressure ridges along left-lateral strike-slip fault segments. However, some E–W-oriented normal faults occur on the northwestern and partly also southern shelf, and along the borders of the adjacent deep submarine plains. They are intersected and displaced by the strike-slip faults. The lower seismicity along the normal faults relative to the NE–SW-oriented strike-slip faults suggests that the former are at present inactive or at least less active.

Morpho-tectonic evolution of the Çanakkale Basin (NW Anatolia): evidence for a recent tectonic inversion from transpression to transtension

Onshore and offshore seismic and geologic-morphologic evidence from the wider region of the Çanakkale Basin indicates that this area has been widely exposed to transpressional tectonism, which already commenced in the Pliocene. During this transpressional tectonism, the Gelibolu Fault and the Anafartalar Shear Zone on the Gelibolu Peninsula, as well as the Bozcaada-Biga Shear Zone on the Biga Peninsula were activated. As a consequence, the northern part of the Gelibolu Peninsula, and a broad zone between Bozcaada Island and the Karaburun Peninsula were uplifted to form the northern and southern boundaries of the Çanakkale Basin, respectively. This remained a low-elevation intermontane basin between these two highlands. The original morphology of the Çanakkale Basin may have developed as a coastal and shelf section of the large extensional Marmara Sea Basin at the end of the Late Miocene. This tectonic phase was followed in the Pliocene by the transpressional tectonism of the North Anatolian Fault Zone, which destroyed the initial morphology and formed the present V-shaped basin. The activity of the Gelibolu Fault and the Anafartalar Shear Zone along the northern boundary of the Çanakkale Basin ended in the late Pleistocene with the initiation of the northern segment of the North Anatolian Fault Zone. The tectonism along the northern boundary of the Çanakkale Basin thus shifted from a transpressional to a transtensional regime. Seismic data indicate that the Bozcaada-Biga Shear Zone continues to be active to the present day.