Strain localization in the Sea of Marmara: Propagation of the North Anatolian Fault in a now inactive pull-apart

The Sea of Marmara is presently the site of pure dextral strike-slip faulting that connects the Izmit Fault in the east to the Ganos Fault in the west along a linear strike-slip fault that is part of the northwestern branch of the North Anatolian Fault. This active Main Marmara Fault (MMF) cuts across a succession of depocenters and highs. Analysis of a large data set that includes multichannel seismic lines, sparker and deep towed high definition seismics, shows that most of the N120°E trending normal faults framing these depocenters are now inactive as they are sealed by an average thickness of 300 m of sediments. The highs correspond to a series of NE-SW trending anticlines and reverse faults blanketed by a thin cover of undeformed sediments. We interpret these structures as inverted former transfer faults of the now inactive Sea of Marmara pull-apart system. Depocenters and highs are the relicts of a former pull-apart basins and transfer faults system that accommodated the North Anatolian Fault motion within the Sea of Marmara prior to the formation of the MMF. We propose that a relatively short (a few hundreds of thousands of years) period of basin inversion accompanied the localization of strain along the present Main Marmara Fault, until it was fully formed, 200 ± 100 Kyr ago. The preexisting pull-apart basin system accounts for about 30 km of motion along this fault system and thus probably appeared sometime in late Pliocene or early Pleistocene as a reactivated portion of the old Thrace basin. During the Plio-Quaternary the Sea of Marmara has been the site of progressive localization of strain.