Tectonically-driven mud volcanism since the late Pliocene on the Calabrian accretionary prism, central Mediterranean Sea

Mud volcanoes recently discovered on the offshore Calabrian Arc are investigated at two sites 60 km apart, in water depths of 1650--2300 m, using swath bathymetry, 2D&3D multichannel seismic and cores. The seabed and subsurface data provide information on their formation and functioning in relation to tectonic activity during the rapid Plio-Quaternary advance of the accretionary prism. Fore-arc extension and thrust-belt compression are seen to have involved two main phases of activity, separated by a regional unconformity recording a mid-Pliocene (3.5–3.0 Ma) tectonic reorganization. The two sites of mud volcanism lie in contrasting tectonic settings (inner fore-arc basin vs central fold-and-thrust belt) and record differing forms of seabed extrusive activity (twin mud cones and a caldera vs a broad mud pie). At both sites, subsurface data show that mud volcanism took place throughout the second tectonic phase, since the late Pliocene; differing forms of mud extrusion were accompanied by subsidence to form depressions beneath and within extrusive edifices up to 1.5 km thick. The basal subsidence depressions point to sources within the succession of thrusts underlying the inner to central Arc, consistent with microfossils within cored mud breccias from both sites that are derived from strata as old as Late Cretaceous.These results are argued to support a model of mud volcanism in which deeply-rooted fluid conduits drive a process of near-surface sediment mobilization. The onset of extrusive activity is inferred to have been triggered by the mid-Pliocene tectonic reorganization, the change in stress fields (which post-dates the migration of frontal compression across the area by >4 Ma) opening pathways for the release of overpressured fluids from deep within the accretionary prism. The rising fluids interacted with shallow (<2 km) geological structures (thrusts, basins) to cause the formation and progressive evacuation of mud chambers, resulting in the growth of mud volcanoes within subsidence depressions. It is hypothesised that lowering of the base of the extrusive edifices by subsidence and thickening led to the formation of internal mud chambers that remobilized previously extruded material, a process that could account for the common lack of internal structure observed on seismic profiles. Differing forms of mud extrusion (mud pie, mud cones, caldera) are attributed to episodic activity driven by the evolving dynamics of the mud chambers over the last ≥3 Ma.