MUD VOLCANISM ON THE MEDITERRANEAN RIDGE - INITIAL RESULTS OF OCEAN DRILLING PROGRAM LEG-160

Drilling during Ocean Drilling Program Leg 160 (April-May, 1995) revea led important new evidence concerning the internal composition, deposi tional processes, and age of two mud volcanoes within the Mediterranea n Ridge accretionary complex, Holes were drilled at ca. 2000 m mater d epth on the crest areas, across the flanks of the mud volcanoes, and o nto adjacent deep-sea sediments, The main depositional units forming t he flanks of both mud volcanoes are debris flows (''mud breccias'') co mposed of a volumetrically dominant mud matrix, containing clasts of m ainly clay, mudstone, siltstone, sandstone, and limestone. More variab le muddy, silty, and sandy sediments were recovered from the crestal a reas of both volcanoes, The lowest mud-volcano units drilled include w ell-sorted medium-to coarse-grained sediments, mainly composed of clay , that were deposited partly by turbidity currents, The mud-volcano se diments are associated,vith background deep-sea sediments that allow d ating by using microfossils. The Milano mud volcano is at least 1.75 M a, and is apparently now dormant, and the Napoli mud volcano started p rior 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 ha lite 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 Mila no mud volcano, The mud volcanoes are located above a shallowly dippin g subduction zone in an area where the accretionary complex is apparen tly being thrust northward over a backstop of continental crust, relat ed to initial collision of the African and Eurasian plates, Mud volcan ism may have begun when backthrusting punctured a seal of latest Mioce ne evaporites, allowing the escape of overpressured materials.