Tectonic setting and processes of mud volcanism on the Mediterranean Ridge accretionary complex: evidence from Leg 160.
Robertson, Alastair H F
Mud volcanism was initiated when overpressured muds rose through the Mediterranean Ridge accretionary prism. Early mud volcanism was marked by eruption of coarse clastic sediments forming small cones of debris flow deposits and turbidites, followed by eruption of large volumes of clast-rich matrix-supported debris flows. Eruption was accompanied by progressive subsidence to form moat-like features. Later, clast-poor mud flows spread laterally and built up a flat-topped cone (Napoli mud volcano). Clasts in the mud volcano sediments are mainly angular and up to 0.5 m in size. These clasts are dominated by claystone, sandstone, and limestone of early-middle Miocene age that were previously accreted. Matrix material of the mud breccias probably originated from Messinian evaporite-rich sediments located within the décollement zone beneath the accretionary wedge, at an estimated depth of 5-7 km. Pressure release triggered hydrofracturing of poorly consolidated mud near the seafloor. Eruption was accompanied by release of large volumes of hydrocarbon gas. Conditions were suitable for gas hydrate genesis at shallow depths beneath the seafloor at the Milano mud volcano. The mud volcanism is probably related to backthrusting concentrated along the rear of the accretionary wedge near a backstop of continental crust to the north. Clasts within the mud breccias were mainly derived from the North African passive margin, but subordinate lithoclastic ophiolite-related material was also derived from the north, probably from now largely obliterated higher thrust sheets of Crete. Comparisons show that in contrast to other mud volcanoes both on the seafloor (Barbados) and on land (Trinidad), which are usually relatively transient features, mud volcanism on the Mediterranean Ridge has persisted for >1 m.y. A revised hypothesis of mud volcanism at the Ocean Drilling Program sites is proposed, in relation to progressive tectonic evolution of the Mediterranean Ridge accretionary complex.