GEOCHEMISTRY OF PALEOFLUIDS IN MINERALIZATION ALONG SEISMOGENIC FAULTS IN THE IRPINIA AREA (SOUTHERN ITALY)

The present PhD Thesis Dissertation includes the results of a integrated and multidisciplinary study of fault-related calcite veins from the Contursi Hydrothermal basin (Irpinia area, southern Italy). The whole Contursi hydrothermal activity takes place within a seismically active portion of the southern Apennines of Italy, where destructive earthquakes unfortunately have occurred in recent times. The Contursi village lies close (ca. 1 – 5 km) to the epicentral area of the MW = 6.9, 1980 Irpinia earthquake, and the local fault system is well-exposed along the numerous fault scarps dissecting Mesozoic shallow-water carbonates pertaining to the Apennine Platform. The Contursi hydrothermal activity is characterized by the outgassing of deep sourced CO2 coupled with mantle-derived volatiles such as He (Buttitta et al., 2023). In order to gain news insight into the modalities of crustal-scale fluid circulation along active extensional fault systems, in this work I has been investigated the paleofluids that precipitated calcite minerals within fault-related veins of the aforementioned Contursi Hydrothermal Basin. Specifically, tackle the source of the mineralizing paleofluids and the modalities of fault-related fluid circulation by integrating the results of field structural studies with microstructural, stable isotopes (δ13C and δ18O) and trace elements characterizations of the calcite veins, and both noble gases and δD-H2O of Fluid Inclusion (FI). The studied fault-related calcite veins crosscut the Mesozoic Apennine Carbonate platform exposed in the Contursi area, which is about 40 km2-wide and characterized by the exposure of high-angle fault forming two main sets roughly striking NNW-SSE and ENE-WSW, respectively. There, in close proximity to the main slip surfaces, both comb and slip-parallel calcite veins (labelled as C and SP, respectively) were sampled. In accordance with previous works, both C and SP-veins are interpreted as co-seismic or early post-seismic features, whose formation as brittle extensional fractures occurred during multiple seismic ruptures. Most of the sampled fault-related calcite veins include blocky calcite crystals, whose precipitation from the fault-controlled paleofluids, likely post-dated the occurrence of the aforementioned brittle fracturing. Only two samples show a fibrous texture of the calcite crystals, which are interpreted as syn-kinematics, and formed during small increments of shear-induce dilation and mineral precipitation. Due to their variable sized and inner texture, incremental crack and seal processes are documented for some studied calcite veins samples. Most of calcite veins samples show δ13C and δ18O values similar to those that characterize the Mesozoic marine carbonate field (Apennine Carbonate platform). Such an isotopic signature is consistent with mineral precipitation form paleofluids in an isotopic equilibrium with the encompassing carbonate host rocks. This interpretation is supported by the calculated δ18Oparentalfluids values, δD values measured in Fluid Inclusion (FI) and trace and REE content, which all point out towards the long residence time of the mineralizing fluids within the faulted carbonate host rocks. In addition, since the homogenization temperatures (between 100 °C and 130°C) obtained by FIs analysis are similar to the maximum burial temperatures experienced by the Apennine Carbonate Platform host rock (100°C – 120°C), assuming a geothermal gradient of ~30 °C/km, and taking known values of groundwater temperature into account, the geochemical data are interpreted as due calcite precipitation within the fault-related veins from paleofluids that localized at depths of ca. 3 ~ 4 km, and interacted with the Mesozoic carbonates. Differently, some calcite veins are characterized by δ13C values similar to those reported above, but by a pronounced depletion in δ18O which, together with the high homogenization temperature assessed for the FIs, are consistent with the paleofluids derived from a deep reservoir from depths of ca. 8 ~ 10 km. Accordingly, similar to the conceptual model invoked for crustal-scale fault-related fluid flow in the High Agri Valley of southern Italy, results point out to an ascendance of deep fluids towards the Earth’s surface that likely occurred thanks to the temporary rupture of the tectonic mélange forming the sole thrust of the southern Apennine fold-and-thrust belt, and the main seal of overpressurized fluids entrapped within the Apulian carbonates. Regarding to the results of noble gases analyses performed on the FIs trapped within the calcite veins, most of the investigated samples shows R/Ra values comprised between 0.53 and 1.38 Ra, which is consistent with a prevalent crustal component added to mantle one (up to 20%), and also with a negligible meteoric component. The variability of He isotopic signature in the FIs, that is not consistent with narrower range of He ratios for the current outgassing, can be due to early trapping processes (probably earthquakes occurred in the past). Based on co-seismic dilatancy model, an “early trapping” hypothesis for He isotopic signal, the following model of fault-related fluid circulation in the Contursi Hydrothermal basin is here proposed: the co-seismic dilatancy formed transient of high-permeability pathways through the upper crust, allowing the ascendence of likely overpressured fluids from rock volumes underneath the impermeable tectonic mélange made up of Apulian carbonates. During ascendence towards the Earth’s surface, these overpressured fluids characterized by crustal and mantle derived He (up to 20%), reached shallower crustal volumes (1-5 km), and were either in thermal and isotopic equilibrium or in disequilibrium with the Apennine carbonates generating the two distinct groups of calcite veins and associated FIs. For the fluids in thermal and isotopic equilibrium with the Apennine carbonate host rock, the mixing with the meteoric and/or shallow derived fluid cannot be excluded, as demonstrated through noble gas analysis (i. e. 40Ar/36Ar and relationship with R/Ra values) of some of the study samples. The high-permeability pathways permitting the rapid ascendance of deeper fluids towards shallow crustal conditions is still active in the Irpinia area, as suggested by soil gas measurements indicating a current outgassing of mantle-derived fluids. However, regarding the mantle component measured in the study sample, two possible scenarios are envisioned. First, the mantle-derived fluids derive from outgassing of magmatic intrusion at depth. Second, a through-going structural discontinuity that crosscut the whole crust down to the mantle channeled the deep fluids a depth. In conclusion, the results of this study showed that the Contursi hydrothermal basin forms a great natural laboratory to gaining a new knowledge on modalities of crustal-scale fluid circulation associated with crustal deformation and earthquake faulting. Such investigations are fundamental to understanding the role of deep fluids in crustal deformation over time, and in the processes of earthquakes nucleation and rupture propagation. Moreover, this study provides hints upon long-time multidisciplinary integrated monitoring approach in tectonically active continental regions, so that the new insights can be helpful to improve earthquake forecast.