Mud Volcanism on the Mediterranean Ridge

Abstract

Two different mud-dome structures (the Milano and Napoli Domes) on the northern margin of the Mediterranean Ridge accretionary complex were drilled during Ocean Drilling Program Leg 160 and were found to be active, or recently active, mud volcanoes. A transect of holes was drilled from adjacent hemipelagic deep-sea sediments, across the flanks, to the crestal areas of these structures. The dominant sediment types recovered are variably clast-rich matrix-supported muds (mud breccias) that are interbedded and interfinger with deep-sea hemipelagic sediments. Lower in the sequence, intervals of inclined clastsupported gravel and sand, composed mainly of clay in both mud volcanoes, are overlain by matrix-supported clast-rich muds that are interpreted as multiple debris flows. Variably silty and sandy sediments characterize the crestal areas. Pore fluids at Napoli Dome, and to a lesser extent at Milano Dome, indicate the presence of halite of presumed Messinian age. Hydrocarbon gas was found to be actively venting from the crest of the Napoli mud volcano. Gas was also detected by drilling on the crest of the Milano mud volcano. In addition, methane hydrates (clathrates) were discovered for the first time in the Mediterranean, a few tens of meters beneath the crest of the Milano mud volcano. The Milano mud volcano is at least 1.5 m.y. old and is apparently now dormant, whereas the Napoli mud volcano was initiated during the interval from, or prior to, 1.5-1.2 m.y. and is currently active.