Mud volcanism on the Mediterranean Ridge: Initial results of Ocean Drilling Program Leg 160

Drilling during Ocean Drilling Program Leg 160 (April–May, 1995) revealed important new evidence concerning the internal composition, depositional processes, and age of two mud volcanoes within the Mediterranean Ridge accretionary complex. Holes were drilled at ca. 2000 m water depth on the crest areas, across the flanks of the mud volcanoes, and onto adjacent deep-sea sediments. The main depositional units forming the flanks of both mud volcanoes are debris flows (‘‘mud breccias’’) composed of a volumetrically dominant mud matrix, containing clasts of mainly clay, mudstone, siltstone, sandstone, and limestone.More variable muddy, silty, and sandy sediments were recovered from the crestal areas of both volcanoes. The lowest mud-volcano units drilled include well-sorted mediumto coarse-grained sediments, mainly composed of clay, that were deposited partly by turbidity currents. The mud-volcano sediments are associated with background deep-sea sediments that allow dating by using microfossils. The Milano mud volcano is at least 1.75 Ma, and is apparently now dormant, and the Napoli mud volcano started prior to, or during, 1.5–1.2 Ma and is currently active. Pore fluids at Napoli, and to a lesser extent at Milano, indicate the presence of halite of presumed latest Miocene age beneath the volcanoes. Hydrocarbon gas is venting from the crest of the Napoli mud volcano, and gas was also detected on the crest of the Milano mud volcano. Methane hydrates (clathrates) are also inferred to exist beneath the crest of the Milano mud volcano. The mud volcanoes are located above a shallowly dipping subduction zone in an area where the accretionary complex is apparently being thrust northward over a backstop of continental crust, related to initial collision of the African and Eurasian plates. Mud volcanismmay have begun when backthrusting punctured a seal of latest Miocene evaporites, allowing the escape of overpressured materials.