Water-carrying sediment may intrude (with or without associated extrusion) wherever the fluid pressure exceeds the intrusion strength of the surrounding materials. It has recently been suggested that thrust sheets moving toward continental interiors act as giant squeegees driving fluids through their substrates ahead of them. Active ice sheets and sedimentary prisms can also squeeze fluids through rocks. Such fluids potentially inject hydraulic melanges along their plumbing systems, but these are difficult to distinguish from other melanges and from broken formations or olistostromes because of surficial slumping or extrusion. Miniature examples of melanges pumped through a marine slope by the tidal grounding of an ice sheet of Permian-Carboniferous Dwyka age in South Africa are used to illustrate that there are even more categories of melange than previously suspected. Instead of being clarified by recent work, the problem of deducing the origin of individual melanges becomes more difficult. This is because melanges are formed by the mixing of thick hydrous slurries in any confined conditions and are not diagnostic of subduction. Nonetheless, the results of hydraulic activity within a subglacial marine slope suggest that the sedimentary volcanoes developed on many accretionary prisms overlie relatively cold hydraulic arcs oceanward of the hot magmatic arcs behind subduction zones.

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