Fluid seepage variability across the external Northern Apennines (Italy): Structural controls with seismotectonic and geodynamic implications.

The relations between fluid seeps and tectonic structures have been targeted in some key areas of the axial sector, and partly at the edge of the exposed Northern Apennines (Pede-Apennine margin). In the axial zone, fluid seepage is dominated by methane venting, which may occur in the form of dry seeps or associated with mineral springs. Surface venting is linked to the presence of foreland-dipping normal faults, or related to reservoirs localised at inactive anticlines. The Pede-Apennine margin is instead dominated by thrusting and mud volcanism. The two different categories of fluid seepage appear strongly coupled to the dissimilar stress fields (compressional or extensional) operating in these sectors. Pressure data inferred from deep wells delineate an overall fluid pressure increase from the axial zone toward the Pede-Apennine margin, possibly as a result of the growth of tectonic compaction in this direction. The increase of fluid pressure at the Pede-Apennine margin is thus interpreted as the primary control on the transition from dry vents to mud volcanism. The intimate association between seepage modes and distinct tectonic structures involves relevant consequences on fluid-fault interactions and seismotectonics, and also shows connections with processes dictating the internal deformation of evolving fold-and-thrust belts.