No abstract

The submerged section of the North Anatolian fault within the Marmara Sea was investigated using acoustic techniques and submersible dives. Most gas emissions in the water column were found near the surface expression of known active faults. Gas emissions are unevenly distributed. The linear fault segment crossing the Central High and forming a seismic gap-as it has not ruptured since 1766, based on historical seismicity, exhibits relatively less gas emissions than the adjacent segments. In the eastern Sea of Marmara, active gas emissions are also found above a buried transtensional fault zone, which displayed micro-seismic activity after the 1999 events. Remarkably, this zone of gas emission extends westward all along the southern edge of Cinarcik basin, well beyond the zone where 1999 aftershocks were observed. The long term monitoring of gas seeps could hence be highly valuable for the understanding of the evolution of the fluid-fault coupling processes during the earthquake cycle within the Marmara Sea.

The main Marmara Fault exhibits numerous sites of fluid venting, observed during previous cruises and in particular with R.O.V. VICTOR during the MARMARASCARPS cruise (2002). Long CALYPSO cores were recovered near active vents and at reference sites during the MARMARA-VT cruise (2004), together with echosounder sub-bottom profiles (frequency of 3.5 kHz). We compiled R.O.V. video observations from MARMARASCARPS cruise and show that all known seeps occur in relationship with strike-slip faults, providing pathways for fluid migration. Among the main active sites, a distinction is made between gas seeps and water seeps. At gas seeps, bubble emissions at the seafloor or disturbed echofacies on sounder profiles demonstrate the presence of free methane gas at a shallow depth within the sediment. Most cores displayed gas-related expansion, most intense for cores taken within the gas plumes. On the other hand, authigenic carbonate chimneys characterize the water seeps and visible water outflow was observed at two sites (in the Tekirdağ and Central basins). The pore fluid chemistry data show that the water expelled at these sites is brackish water trapped in the sediment during lacustrine times (before 14 cal kyr BP), in relation with the paleoceanography in the Sea of Marmara. The chimney site in the Tekirdağ Basin is located at the outlet of a canyon feeding a buried fan with coarse sandy turbidites. Pore fluid composition profiles indicate that the sand layers channel the brackish fluids laterally from the basin into the fault zone at less than 20 m depth. However, a deeper gas source cannot be excluded.