Interrelations of mud volcanism, fluid venting, and thrust-anticline folding: Examples from the external northern Apennines (Emilia-Romagna, Italy).

The present work aims to provide an example of connection between fluid seepage and fault/fracture zones associated with compressional thrust folds. Surface fluid seepage along the Pede-Apennine margin consists of mud volcanism and methane-rich emissions showing an intimate link with thrust-related folds. Dominant gaseous or CH4-bearing fluid emissions characterize thrust fold settings in which fluid escape from the main reservoir and source rock (i.e., the Marnoso Arenacea flysch) is not impeded by a seal layer. Conversely, remarkable mud volcanism is closely linked to the presence of the impermeable Ligurian units sealing the Marnoso Arenacea reservoir. Structural analyses focused on selected thrust folds exhibiting clear relations between surface seepage and brittle elements associated with the fold. Various scenarios potentially explaining mud volcanism and venting over the crestal region of the thrust-related folds are discussed with respect to fluid pressure state and the development of second-order fault and fracture sets that are observed to control surface fluid expulsion. It is proposed that mud volcanism can be potentially triggered by first-order fault failure cycles during which the overpressured fluid is released during faulting events. Past anomalous eruptions associated with strong earthquakes seem to support this hypothesis. The relatively quiescent but continuous activity of the current Pede-Apennine mud volcanoes could instead reflect a short-lived leakage of overpressured fluids along permeable fractures/faults. Expansion of methane contained in the rising mud may assist this process fundamentally.