Diapiric mud breccias: modern and ancient examples (northern Apennines, Italy).

Composition and architecture of polygenic breccias actually extruded from the Modena-Reggio mud volcanoes have allowed insight into the genesis of similar chaotic deposits associated to ancient cold seep chemoherms in the northern Apennines. Both cold seeps and mud volcanoes result from rising and expulsion of variable amounts of hydrocarbon-rich fluids at the sea floor or in continental areas, and are the locus of abundant brecciated sediments. Subaerial mud volcanoes have a pattern of activity similar to submarine mud volcanoes: they present short eruptive periods with emission of chaotic breccias floating in a viscous mud, thus producing debris flows. Polygenic breccias extruded from their cones and craters show sedimentological features similar to monogenic and polygenic brecciated lithofacies observed in fossil methane-derived carbonates. These affinities suggest that autobrecciated structures and exotic sediments in fossil chemoherms are probably related to the offscraping and chaotic mixtures of sediments during the rapid fluid rise along diapiric conduits or fractures, following the same processes and mechanisms as in chaotic deposits associated with shale diapirs and mud volcanoes. Our data suggest a relationship between extruded sediments and substratum typology: mud breccias with polygenic clasts are associated with a geological substratum including sedimentary mélanges and olistostromes. It is possible that the block-in-matrix textures of these poorly consolidated chaotic deposits favour the offscraping of exotic sediments and also the upward migration of fluids.Similarities between the examined fossil seep carbonates and mud volcanoes include also: - the type of fluids which consist mainly of methane mixed with connate waters and clay mud, and their isotopic signatures strongly depleted in δ13C values (fluid origin can be both thermogenic and biogenic); - the presence of a population of active sulphate reducing bacteria (by comparison with modern seep carbonates); - the occurrence of chaotic deposits; - the existence of thick, fine-grained, plastic sediments in the enclosing sedimentary succession; - the control of tectonic activity, particularly compressional activity.The comparison between recent and fossil diapiric-related structures has been useful for constraining the nature of the fossil seepage pathway, understanding fluid expulsion processes and reconstructing models of chemoherm evolution. In particular, this investigation suggests that brecciated structures and exotic sediments in ancient chemoherms are due to the offscraping and chaotic mixtures of sediments during the rapid rise of methanogenic fluids along diapiric conduits or fractures, following similar processes and mechanisms as in chaotic deposits associated with mud volcanoes.Finally, the strictly association of seep carbonates, autobrecciation related structures and chaotic deposits is a more useful criterium than structural foliations as proposed by various authors (Orange et al., 1990; Orange and Brown, 1993) in order to distinguish diapiric mélanges from other type of chaotic deposits, because original features of sedimentary bodies are usually modified by later sedimentary or tectonic processes