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Abstract:
This paper reports the first “in situ” seafloor observations of fluid escape structures in a fault-controlled caldera-type depression of about 8 km diameter, named the Menes caldera, in the Eastern Mediterranean sea off Egypt (western province of the Nile Deep Sea Fan). A detailed analysis of seven Nautile dives, performed during the Nautinil cruise in 2003, focused on determining the nature, origin, importance, mechanisms and environmental impacts of the fluid escape processes from the geological observations.
Seafloor maps, constructed from direct visual observations and video records made from the submersible, and sampling results, for three mud volcanoes of the caldera, Cheops, Chephren and Mykerinos show differences in activity but many common characteristics. The three volcanoes show a crater-like depression. The study reveals a strong contrast between Mykerinos, where no seepage activity was found, and the craters of Cheops and Chephren, where an intense activity of brine seepage was discovered.
On Cheops, a dome of dark grey mud raises slightly above the brine surface in the central part of the brine lake, and shows numerous circular vents of 1–2 m diameter that become gradually colonized by microbial mats. Chephren hosts a deep (> 200 m) and warm (45 °C) brine lake. Alternations of areas void of or covered with microbial mats at the surface of the Chephren brine lake indicate large surface disturbances that may result from the occurrence of several active vents at the bottom of the lake. A special feature at Chephren is a brine overflow at the western edge of the crater, also indicative of the very high activity of brine seepage.
The Nautinil observations indicate that the Menes caldera in the Eastern Mediterranean is a highly active site of brine seepage. Growth faults rooted in pre-Messinian sediment layers provide pathways for warm fluids to escape pressurized source levels below the Messinian salt.
