Structural and tectonic control of fluid seeps and mud volcanoes in the eastern Mediterranean Sea

Abstract

Fluid vents and mud volcanoes in the eastern Mediterranean are important sources of gas escape from the seafloor. Flows of clast-rich muds at mud volcanoes create a carpet of gassy sediment which slowly de-gas over time and support temporary symbiotic seep communities. A constant and more concentrated flow of gas occurs probably from small localized vents. On the Mediterranean Ridge, fluids emitted include dense brines as well as gases dominated by methane. The brines collect in depressions on the seafloor which can be highly irregular in shape, as well as in circular pools resembling large pockmarks. They are likely a consequence of rising gas-rich fluids reacting with the Messinian evaporites in the Mediterranean Ridge. In the cases studied during recent cruises (MEDINETH in 1999 and PRISMED II in 1998) the pockmarks and brine pools are clearly associated with faults observable in deep-tow sonar data as well as in the bathymetric trends. Not only are the faults thrusts, related to the compressional environment, but normal faults and shears: on the Mediterranean Rise the long shears are created by horizontal movements in the sediments of the accretionary prism while in the Anaximander Mountains and on the Florence Rise they are caused by major adjustments across the crustal blocks. Seafloor observations by submersible during the MEDINAUT programme in 1998 confirm that seeps and brine pools are associated with faults showing seafloor breaks with vertical offsets ranging from decimetres to hundreds of metres in the case of faults in the Anaximander Mountains, northeast of the Mediterranean Ridge. Mud volcanoes and seeps in the Anaximander Mountains and on the Florence Rise do not appear to produce brine pools, because Messinian evaporites are lacking in these areas. Furthermore, the sources of fluids may be below thrust zones in the Anaximander Mountains and are not directly related to accretionary prism sediments.