Tectonic controls on the submarine Scoglio d’Affrica mud volcanoes (Northern Apennines hinterland)

Abstract

Four submarine mud volcanoes were discovered in the northernmost Tyrrhenian Basin, Italy, following the eruption of one of them in 2017 near the Scoglio d’Affrica Islet. The eruption occurred a few months after a seismic sequence that hit central Italy (Mwmax 6.5–6.6). Although the peak dynamic stresses estimated at the eruption site are not high enough to confidently substantiate a cause-effect link, they possibly contributed to triggering a delayed response of the mud volcano. These mud volcanoes, along with several pockmarks, lie on top and at the margins of a morpho-structural high known as the Elba-Pianosa Ridge (EPR). The structural setting of the EPR plays an important role in the storage and subsequent upward migration of deep-sourced fluids. However, there is no consensus on the tectonic setting of this feature, and different models have been proposed. We have undertaken a structural-geological study integrating data from (1) wellbores drilled in proximity of the mud volcanoes, (2) the CROPM12A seismic reflection profile, which crosses the EPR near the mud volcanoes, and (3) structural data collected on the Island of Pianosa, an emerged portion of the EPR. The results support the hypothesis that the EPR corresponds to an antiformal stack structure bounded by high-angle normal faults at its margins. Wellbore data indicate the presence of significant overpressures associated with an increase in hydrocarbon gases in the core of the antiform, which suggest the existence of a deep-seated fluid reservoir. The pressurised fluids leak from the reservoir and move upwell capitalizing on a complex network of interconnected brittle planar features (i.e., thrust and normal faults, fractures), reaching both the culmination and the margins of the EPR. The lesson from this study is that mud volcanoes can also form in the innermost part of a thrust belt, where compressional stresses have long since faded. Inactive anticlines still have the capacity to collect and pressurise deep-sourced fluids, which can accumulate into the fold core and ultimately generate mud volcanoes.