The Modena-Reggio mud volcanoes (northern Italy): an actualistic model for the interpretation of Miocene authigenic carbonates related to fluid expulsion.

The paper examined physical, geochemical and geological features, and fluid expulsion processes of fossil seep carbonates and recent mud volcanoes in the northern Apennines. Chaotic brecciated deposits associated with short “eruptive” periods of the Modena-Reggio mud volcanoes (Regnano) are debris flows made of polygenic breccias floating in a viscous mud. These deposits show a number of analogies with monogenic and polygenic brecciated lithofacies of the Miocene methane derived authigenic carbonates of the northern Apennines. 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. 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 clasts in ancient chemoherms are due to the offscraping and chaotic mixing 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.