Morphometric analysis of seafloor morphology revealing recent mud and fluid migration around Scoglio d’Affrica islet (Tuscan Archipelago, northern Tyrrhenian Sea)

Mud and fluids migration producing active seepage at the seafloor is a global phenomenon documented in different geodynamic contexts. Scoglio d’Affrica islet is one of the culminations of the Elba-Pianosa Ridge (northern Tyrrhenian Sea), where submarine methane emissions have been studied since the 1960’s, sometimes evolving in violent gas outbursts such as those occurred in 2017. In the study area, the seafloor is punctuated by more than 250 small pockmarks with mean diameter of 10 m and occurring mainly between 20 and 60 m water depth. Pockmarks are characterized by planform shapes from sub-circular to elongated and U/V-shaped cross-sections. They are predominantly arranged as isolated or in clusters or minorly organised in strings-oriented about N-S, running almost parallel to the fault escarpments which represent one of the main structural features of the study area. Pockmarks have been classified on the basis of their size parameters (i.e., depth, mean diameter) according to the recent literature and they resulted to belong mainly to the morphological classes of the "unit pockmark" and minorly to the “normal pockmark”. The complex seafloor morphology of the area is also characterised by several positive features, showing very different shapes and sizes (up to 35 m high and 600 m wide). In this work, we select 67 positive features (named as M1-67) more than 2 meters high and perform on them the first morphometric analysis by means of high-resolution bathymetric data. The obtained morphometric parameters (e.g., flatness value, mean slope), which allow us to classify the positive features as mounded, flat topped and conical features, are compared with those of other submarine mud volcanoes from literature, showing often high similarity. In view of that, we suggest that M1-67 have an origin likely linked to the migration of fluidised mud or mud breccia (a mud matrix with clasts), probably from shallow mud sources, rising through the thick Eocene-Early Miocene siliciclastic succession and overlying sedimentary layers. We interpret as mud volcanoes the larger sub-circular positive features (M1-7) since they are characterised by the occurrence of lobate flows along their flanks, widespread mud-breccia and focused emissions of CH₄ observed on ROV videos. Whilst, with the available data, to avoid any speculation, we propose for M8-67 an alternative and more generic explanation interpreting them as “piercement structures” formed due to the seafloor deformation associated with a rising mud diapirism. Considering the high-magnitude outburst occurred in 2017 and the shallow water setting with evidence of active fluid seepage (as vertical focused gas bubbles) documented by ROV videos, the morphometric analysis of mud and fluids migration morphologies is an important baseline study since it can provide insight for a marine geohazard assessment around Scoglio d’Affrica islet.