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 Ifremer's r/v Le Suroı̂t 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 Tekirdağ 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 Tekirdağ 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 échelon 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 Çınarcık 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 Çınarcık Basin, the E–W İzmit fault enters the basin from the east. Pasisar data confirm the extension of the İzmit strike–slip fault into the Çınarcık 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 İzmit fault, while most of it is the expression of strain partitioning.