A. M. Celal Şengör

Kinematic history of the opening of the Black Sea and its effect on the surrounding regions

The Black Sea consists of two oceanic basins separated by the mid-Black Sea ridge. The east-west-oriented west Black Sea basin opened as a back-arc rift in the Cretaceous by tearing a Hercynian continental sliver, the Istanbul zone, from the present-day Odessa shelf. The Istanbul zone, which was initially contiguous with the Moesian platform in the west, moved south during the Late Cretaceous-Paleocene with respect to the Odessa shelf along two transform faults: the dextral west Black Sea and the sinistral west Crimean faults. It collided in the early Eocene with a Cimmeride zone in the south, thereby ending the extension in the western Black Sea and deactivating both the west Black Sea and the west Crimean faults as strike-slip faults. The east Black Sea basin opened as a result of the counterclockwise rotation of an east Black Sea block around a rotation pole located north of the Crimea. This block was bounded by the west Crimean fault, the southern margin of the eastern Black Sea, and the southern frontal thrusts of the Greater Caucasus. The rotation of the east Black Sea block was contemporaneous with the rifting of the west Black Sea basin but lasted until the Miocene, resulting in continuous compression along the Greater Caucasus.

Tectonics of an ultrahigh-pressure metamorphic terrane: The Dabie Shan/Tongbai Shan Orogen, China

Ultrahigh-pressure metamorphic rocks with coesite and diamond form a tectonic slice over 20 km thick, called the eclogite zone, within the Dabie Shan complex in the Qinling orogen in central China. The orogen separates the Sino-Korean block in the north from the Yangtze block in the south. The Dabie Shan Complex is a composite terrane made up of eclogite facies and amphibolite facies gneiss slices and represents fragments of the lower continental crust of the Yangtze block. The Dabie Shan Complex is bounded in the south by a Triassic foreland fold-thrust belt and in the north by a greenschist facies metaclastic unit, the Foziling Group, which probably represents the passive continental apron deposits of the Yangtze block. Farther north is a granulite facies gneiss complex, the Qinling Group, which has ultramafic slivers and includes the remnants of an island arc with two bounding suture zones. North of the Qinling Group are early Paleozoic active margin deposits of the Sino-Korean block. The eclogite zone in the Dabie Shan Complex is sandwiched between amphibolite facies gneiss slices. Dating by Sm-Nd, Rb-Sr, and Ar-Ar of two eclogite samples from the eclogite zone gives early to middle Triassic ages (236–246 Ma); the initial eNd values indicate reworking of a 2.11 and 1.55 Ga continental crust. A Himalayan-type tectonic evolution is envisaged for the Qinling orogen with the creation of a 100-km-thick crustal thrust wedge through continuous underplating during the subduction of the Yangtze continental lithosphere. Exhumation of the ultrahigh-pressure metamorphic rocks was chiefly achieved by the southward propagation of the thrust planes, thereby isostatically uplifting and eroding the earlier deeply subducted parts of the orogen. A total of 680 km of southward thrusting in front of Dabie Shan is inferred, based on the abrupt termination of the Tanlu fault. Normal faulting possibly caused by gravitational collapse probably also had a role in the exhumation process.

Evidence for intracontinental thrust-related exhumation of the ultra-high-pressure rocks in China

Coesite- and diamond-bearing ultra-high-pressure metamorphic rocks produced during the Triassic continental collision between the Sino-Korean and Yangtze plates are offset by about 530 km along the sinistral Tanlu fault in eastern China. The Tanlu fault terminates abruptly south of the ultra-high-pressure metamorphic terrain in Dabie Shan and does not extend into the foreland fold belt along the Yangtze River. This suggests that in this region the Mesozoic major sinistral displacement along the Tanlu fault was transferred to southward- directed thrusting, which led to the exhumation of the ultra-high-pressure metamorphic rocks formed earlier. Postcollisional Himalayan-type intracontinental thrusting and concomitant erosion appear to be the main agents in the exhumation of the very high pressure metamorphic rocks in China.

Tectonic Evolution of the Central Anatolian Basins

The central Anatolian basins can be grouped into two basic types—arc-related (forearc and intra-arc) basins and collision-related (peripheral foreland) basins. The former began developing in the Late Cretaceous, whereas the latter started to form at the beginning of the Eocene. Both types of basins were filled until the Oligocene with turbidites overlain by shelf and nonmarine strata. During the Oligocene, all these basins were unified into a large epi-Anatolide molasse basin, in which widespread gypsiferous series were deposited together with abundant clastic deposits and volcanics. After the Oligocene, these basins evolved into an areally more extensive “intra-cratonic” basin that bears no relation to the earlier orogenic structures. This superposition of different types of basins makes the geology of central Anatolia extremely complicated. To date, no reliable evolutionary tectonic model for this region has been formulated. Considering that the hydrocarbon potential of these basins is largely unknown, ...

Investigation of the tectonics of the Main Marmara Fault by means of deep-towed seismic data

Abstract We report a study of the active principal deformation zone (PDZ) of the Main Marmara Fault (MMF) in the sub-basins of the Sea of Marmara by means of deep-towed seismic (Pasisar) as well as multi-beam bathymetry data collected by Ifremers r/v Le Suroit in September 2000. Our main objective is to investigate the active deformation within the uppermost sedimentary layers with a much higher resolution than hitherto has been available. To the west of the Sea of Marmara, the PDZ is located along the southern flank of the Tekirdag Basin where the sediments are affected by steep reverse faults dipping toward the north. E–W strike–slip faults are also observed in the central part of the Tekirdag Basin and thrusting occurs along its N50°-trending margin. We interpret these structures in terms of a slight clockwise rotation in the basin north of the PDZ. To the east, the PDZ enters the Central Basin and follows a steep scarp along the southern flank of a tectonic depression. The scarp consists of an en echelon fault system with a normal component. These faults are combined with small parallel anticlines and synclines that extend along the southern portion of the depression. The northern scarp of the depression is made of a mixed system of faults with both normal and reverse components associated with anticlines and synclines. These faults are best interpreted as right lateral strike–slip faults with a vertical component that is dominantly normal. These faults and the sigmoid shape of the depression are compatible with a clockwise rotation above the PDZ. This recent tectonic structure appears to be superimposed over a pre-existing graben that is now essentially deactivated. The PDZ continues eastwards, out of the Central Basin, as a N50°E-trending NW-verging thrust system toward the Kumburgaz Basin that is located on a restraining bend of the PDZ. This shortening zone consists of two main N60°E-trending branches. The northern one is more pronounced and composed of two successive restraining bends. The southern branch is smoother and forms a gently curved connection between the two segments of the PDZ. This suggests that the PDZ migrates southward to cut through this restraining bend. Further east, the PDZ enters the Cinarcik Basin along its northern scarp. Active deformation observed on the Pasisar profiles along the 290°-trending eastern portion of this scarp consists of N110±5°-trending dextral strike–slip faults connecting short segments of active N130–140° normal faults that control elongated depocenters. Along the southern flank of the Cinarcik Basin, the E–W Izmit fault enters the basin from the east. Pasisar data confirm the extension of the Izmit strike–slip fault into the Cinarcik Basin and the large development of normal faulting along the southern flank of the basin. Some of the normal faulting observed here may be related to horse tail termination of the Izmit fault, while most of it is the expression of strain partitioning.

Tectonic and sedimentary controls on widespread gas emissions in the Sea of Marmara: Results from systematic, shipborne multibeam echo sounder water column imaging

Understanding of the evolution of fluid-fault interactions during earthquake cycles is a challenge that acoustic gas emission studies can contribute. A survey of the Sea of Marmara using a shipborne, multibeam echo sounder, with water column records, provided an accurate spatial distribution of offshore seeps. Gas emissions are spatially controlled by a combination of factors, including fault and fracture networks in connection to the Main Marmara Fault system and inherited faults, the nature and thickness of sediments (e.g., occurrence of impermeable or gas-bearing sediments and landslides), and the connectivity between the seafloor and gas sources, particularly in relation to the Eocene Thrace Basin. The relationship between seepage and fault activity is not linear, as active faults do not necessarily conduct gas, and scarps corresponding to deactivated fault strands may continue to channel fluids. Within sedimentary basins, gas is not expelled at the seafloor unless faulting, deformation, or erosional processes affect the sediments. On topographic highs, gas flares occur along the main fault scarps but are also associated with sediment deformation. The occurrence of gas emissions appears to be correlated with the distribution of microseismicity. The relative absence of earthquake-induced ground shaking along parts of the Istanbul-Silivri and Princes Islands segments is likely the primary factor responsible for the comparative lack of gas emissions along these fault segments. The spatiotemporal distribution of gas seeps may thus provide a complementary way to constrain earthquake geohazards by focusing the study on some key fault segments, e.g., the northern part of the locked Princes Islands segment.