Abstract
Active fluid seeps have been described in a wide range of geological environments and geodynamic contexts, which include continental shelves of non-volcanic passive margins and accretionary wedges. Fluids seeping in hybrid volcanic-sedimentary basins, characterized by the presence of magmatic intrusive complexes, have always received less attention. We detected and imaged dozens of distinct gas flares, as high as 700 m, on the continental slope of the Paola Basin in the southeastern Tyrrhenian Sea, at 550–850 m water depth. The sedimentary basin is surrounded by Pleistocene active and inactive volcanoes and volcanic-intrusive complexes, which formed in the back-arc basin of the Calabrian subduction zone, in response to subduction-induced mantle flow. Gas flares develop above pockmarks, craters and mud flows that form over and along the scarps of mound structures and correspond to seismic zones of free gas accumulation in the sub-seafloor. Here, methane-derived siderite shows enrichment in δ13C and δ18O isotopes likely related to methanogenesis and intermittent venting of deep-sourced CO2. Multichannel seismic reflection data showed that the gas flares develop in correspondence of doming and diapirism apparently originating from the top of the Messinian evaporites and nearby magmatic sills, that are present in the lower part of the Plio-Quaternary succession. These diapiric structures can be related to seafloor hydrothermal vent complexes fed by the igneous intrusions. Our data suggest that the vent complexes acted as fluid migration pathways and gas conduits, which at times are bounded by deep-rooted normal faults, leading to post-explosive near-surface microbial activity and seep carbonate formation. Fluids being mobilised by magmatism in the study area include: hydrocarbons and hydrothermal fluids generated at depth, interstitial water expelled during formation of polygonal faults. The close spatial correlation between seafloor seep manifestations, fluid migration pathways in the sub-surface involving part of the Messinian units and igneous features indicates that magmatic activity has been the main driver of fluid flow and can have a long-term effect in the southern Tyrrhenian Sea.
Highlights
Some major seismic reflections can be assessed with confidence by correlation with CROP line M36, which passes above the volcanic structure E5c and crosses several multichannel seismic reflection survey (MCS) profiles presented in this study (Figure 2A)
A regional reflector associated with the top of the evaporites deposited during the late Messinian Salinity Crisis (MSC) is recognized by analogy with basins in the Western and Central Mediterranean, where it coincides with the top of the trilogy of units of the MSC (Dal Cin et al, 2016; Camerlenghi et al, 2020)
Using a ship-based multibeam echosounder we detected 15 clusters of flares that are indicative for free gas releases from the seafloor, which are mostly related to seafloor pockmarks, landslide headwall scars and fault scarps
Summary
Evidence is growing that intrusive magmatic bodies, such as sills, can influence the long-term migration of fluids from the deep subsurface to form HTVCs that can be reutilized as focused areas of fluid flow (Lawrence and Cartwright, 2010; Rollet et al, 2012; Roelofse et al, 2021). This influence is largely due to permeability contrasts between the intrusion and the host sediment or to interconnected open fractures and faults within and around intrusions acting as conduits for migrating fluids (Omosanya et al, 2018). Salt diapirs have been connected to seepage of gas to the seabed in different geological contexts (Serié et al, 2012; de Mahiques et al, 2017; Madof, 2018; Müller et al, 2018)
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