Extreme efficiency of mud volcanism in dewatering accretionary prisms

Drilling results from two mud volcanoes on the Mediterranean Ridge accretio nary 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 in version), and is triggered by the collision of the African and Eurasian blo cks, forcing undercompacted clayey sediments to extrude along faults in the central and hinterlandward parts of the prism. Volumetric estimates of ext ruded mud in several well-studied areas were based on pre-stack depth-migra ted seismic profiles across the entire. up to > 150 km wide, prism. The res ulting volumes of mud were combined with ages from mud dome drilling, so th at rates of mud extrusion were obtained. Subtracting the solid rock mass fr om the bulk mud volume using physical property data, fluid flux as a functi on of mud volcanism alone has been quantified for the first time. The volum e of fluid extruding with the mud is found to be variable, but reaches up t o 15 km(3) fluid per km trench length and Ma along cross sections with abun dant mud volcanoes. Such large fluid quantities in a region some 50-150 km behind the deformation front exceed estimates from those elsewhere (where u ndoubtedly the majority of the interstitial fluid is lost due to compaction ). Such fluids near the backstop are likely to result predominantly from mi neral dehydration and diagenetic reactions at depth, and consequently provi de a window to understand deeper processes along the deep decollement. More importantly, the enormous rates with which such fluids and liquified mud e scape along the out-of-sequence faults alter fluid budget calculations in s ubduction zones drastically. (C) 2001 Published by Elsevier Science B.V.