Fate of sediment during plate convergence at the Mediterranean Ridge accretionary complex: volume balance of mud extrusion versus subduction and/or accretion

Drilling results from two mud volcanoes on the Mediterranean Ridge accretionary complex as well as extensive geophysical surveys have provided new insights about the geometry of these domes at depth. Mud extrusion is related to buoyancy and plate convergence between Africa and Eurasia that caused back-thrust faulting of accreted strata containing overpressured mud at depth. The domes mainly consist of mud breccia formed of as much as 65% polymictic clasts embedded in a clayey matrix of presumed late Miocene age. Volumetric estimates of extruded mud in a well-studied area around the Olimpi mud-volcano field were balanced against sediment input at the deformation front. The results demonstrate that only a small fraction of rock mass having entered the subduction zone since the Messinian is needed to compensate for the mud expelled in the study area. Most of the sediment ([approximately]95% or more) is either subducted or incorporated into the accretionary prism. The volume of gas expelled with the liquefied, overpressured mud was estimated to range between 1.68 x [10.sup.6] and 2.85 x [10.sup.7] [m.sup.3]/yr.