Investigation of the tectonics of the Main Marmara Fault by means of deep-towed seismic data Emin Demirbag ˘ a,b , Claude Rangin c, * , Xavier Le Pichon b , A.M. Celal S ßengo ¨r d a I ˙ stanbul Teknik U ¨ niversitesi, Jeofizik Mu ¨hendislig ˘i Bo ¨lu ¨mu ¨, I ˙ stanbul, Turkey b Colle `ge De France, Paris, France c CNRS, E ´ cole Normale Supe ´rieure, Laboratoire de Geologie, 24 rue Lhomond Paris 75231, France d I ˙ stanbul Teknik U ¨ niversitesi, Avrasya Yerbilimleri Enstitu ¨su ¨, I ˙ stanbul, Turkey Received 18 January 2002; accepted 6 September 2002 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 Suroı ˆt 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 N50j-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 e ´chelon 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 N50jE-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 N60jE-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 C ßınarcık Basin along its northern scarp. Active deformation observed on the Pasisar profiles along the 290j-trending eastern portion of this scarp consists of N110 F 5j-trending dextral strike – slip faults connecting short segments of active N130 – 140j normal faults that control elongated depocenters. Along the southern flank of the C ßınarcık Basin, the E–W I ˙ zmit fault enters the basin from the east. Pasisar data confirm the extension of the I ˙ zmit strike – slip fault into the C ß ınarcık Basin and the large 0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0040-1951(02)00535-8 * Corresponding author. Tel.: +33-144-322-273; fax: +33-144-322-000. E-mail address: claude.rangin@ens.fr (C. Rangin). www.elsevier.com/locate/tecto Tectonophysics 361 (2003) 1 –19