THE MIOCENE CHEMOSYNTHETIC COMMUNITIES OF THE NORTHERN APENNINES AND THE MECHANISMS OF FLUID RELEASE FROM A CONVERGENT MARGIN

The Miocene cold seeps communities of the Northern Apennines document sites of fluid release from the sea floor, derived from processes related to the plate convergence. The specialized chemosynthetic communities are associated with sites of tectonically-fractured, compressed or gravitationally-unstable sediments in the Apennine foredeep-thrust belt system. Recent studies on the Apennine cold seeps demonstrate the general similarity in community structures among seeps from differing geologic settings, but characterized by the same fluid expulsion mechanisms. Studies on the 3D structure of the Apennine chemoherms indicate that despite the considerable sedimentological and structural variation existing within each body, they mainly reflect different mechanisms which regulated the fluid flow. Chemoherm autobrecciation, for example, marks sites where methane is episodically and explosively released at shallow depth, as also testified by the exotic sediments offscraped from the walls of diapiric conduits during the rapid fluid rise. Detailed field analysis allowed the reconstruction of the different phases of the evolution of the chemoherms enabling the recognition between concentrated and diffusive fluid episode/es. Concentrated fluid flow produced volcanic apparatus, where overpressure is testified by the injection of unlithified material. The upward decrease of detrital particle size reflects a progressive decrease of energy, until the environment becomes suitable for the chemosynthetic organism life and authigenic carbonate precipitation overcomes the detrital sedimentation. Diffusive fluid flow is associated with widespread carbonate authigenesis and the organisms are present in the entire volume of the chemoherm, lacking any evidence of fluid overpressure. The knowledge of the fluid release phases and their typology are indicators of the advancing deformational front of the Apennine chain accompanying the main tectonic phases. They also directly influenced the gravitational slope instability, which widely affected the Apennine foredeep.