Extreme efficiency of mud volcanism in dewatering accretionary prisms

Drilling results from two mud volcanoes on the Mediterranean Ridge accretionary complex as well as bottom sampling and the wealth of geophysical data acquired recently have provided fundamental knowledge of the 3D geometry of mud extrusions. Mud volcanism is generally related to buoyancy (density inversion), and is triggered by the collision of the African and Eurasian blocks, forcing undercompacted clayey sediments to extrude along faults in the central and hinterlandward parts of the prism. Volumetric estimates of extruded mud in several well-studied areas were based on pre-stack depth-migrated seismic profiles across the entire, up to >150 km wide, prism. The resulting volumes of mud were combined with ages from mud dome drilling, so that rates of mud extrusion were obtained. Subtracting the solid rock mass from the bulk mud volume using physical property data, fluid flux as a function of mud volcanism alone has been quantified for the first time. The volume of fluid extruding with the mud is found to be variable, but reaches up to 15 km3 fluid per km trench length and Ma along cross sections with abundant mud volcanoes. Such large fluid quantities in a region some 50¯150 km behind the deformation front exceed estimates from those elsewhere (where undoubtedly the majority of the interstitial fluid is lost due to compaction). Such fluids near the backstop are likely to result predominantly from mineral dehydration and diagenetic reactions at depth, and consequently provide a window to understand deeper processes along the deep décollement. More importantly, the enormous rates with which such fluids and liquified mud escape along the out-of-sequence faults alter fluid budget calculations in subduction zones drastically.