Submarine Morphology Offshore Crotone (Calabrian Accretionary Prism, Central Mediterranean): Pockmark Fields and a Mud Diapir in a Mobile Shale Domain

Fluid expulsion and sediment mobilization are typical processes in accretionary prisms, where sediments are scraped off the subducting plate and piled up and squeezed to originate a tectonic prism, resulting in fluid venting, mud volcanoes and mud diapirs. The Mediterranean region is characterized by subduction zones where residual portions of the Tethyan oceans have survived the Aftica-Eurasia continental collision. Among these subduction zones, the Calabrian accretionary prism is known to be populated by mud volcanoes. The Ionian offshore of the Crotone promontory offers examples where the expressions of fluid expulsion and sediment mobilization are visible both in the subsurface and at the seafloor. The analysis of a proprietary 3D seismic cube allows to characterize patterns of pockmarks, which are direct expression of fluid expulsion at the seafloor, and to identify a mud diapir which appears at the seafloor as a large mud pool, ca. 1200 m in diameter. The high resolution 3D seismic profiles also allow to infer differences in the  mechanisms of fluid focussing at very shallow depth. Small, closely spaced normal faults, produced by outer arc extension, and dilation in the shallow unconsolidated sediments, due to sharp slope gradient increase, both favour fluid focussing. In some instances it can be shown that fluid venting also contributed to destabilize the uppermost sedimentary strata, triggering small landslides along the slope. A Pliocene extensional system has  developed within a mobile shale domain. The diapir that surfaces as a mud pool has been mobilized along a recent extensional fault, which tapped into the mobile shale domain. Furthermore, a fossil mud pool has also been recognized in the study area. This fossil mud pool is sealed by undeformed sedimentary strata which allow to constrain a minimum age for fluid and sediment mobilization in the accretionary prism. Seismic reflections amplitude suggests that the fossil conduit still acts as a preferential fluid seepage pathway, contributing to destibilize the overlaying slope sediments.