Crustal deformations and fluid geochemistry: Application of new approaches in some study areas of the Apennines

It has been well established in geological, geochemical and both experimental and theoretical investigations that modification of the chemistry of fluids, which are stored or move through the crust, can occur as a consequence of rock deformation and seismo- genetic processes1–5. Indeed, variations in the flow of volatiles as well as in the fluid chemistry of fluids and water temperatures released from springs and wells have been recorded at seismic events. Additionally, both rock fracturing and seismicity both show the formation and release of volatiles5. A close theoretical and experimental relationship between rock dilatancy and volatile release has also been identified6. In this sense, noble gases (e.g., He and Ar), as non-reactive species, are very valid tools for studying the correlations between rock deformation and volatiles in the study of earthquake physics7. Moreover, noble gases facilitate the identification of origin of fluids, even when not atmospheric: He isotopes, for example, allow recognition of a mantle origin of fluids even in the absence of any expression of magmatism and volcanism on the surface8. Noble gases are known to be powerful tracers of source and if used in association with the geochemistry of major volatiles (CO2, CH4) can provide a means to both identify and quantify water-gas interaction processes9. Given that these processes can be easily modified by crustal deformation and earthquakes, they can provide useful data for understanding the relationships between fluids and tectonics. This PhD project is focused on the study of the fluid geochemistry emitted in some tectonically active areas along the Apennines (Italy), where a relationship between fluid degassing and seismicity is recognized3,10. In particular, two areas of Apennines, have been studied which differ in their geological and geodynamic setting: the Nirano Mud Volcano Field (NMVF), in the foothills of the Northern Apennines, and Irpinia region in the southern Apennines. The surface manifestations in these areas represent these differences. Indeed, the NMVF is characterized by CH4 dominated emissions from mud volcanoes, in Irpinia instead, the fluid emissions are dominated by CO2, which is dissolved in the groundwater or is released as free gas in Mofettes. The two sites are placed at the CO2–CH4 geochemical boundary along the Apennine watershed that seems to mark a boundary limit of the emergence of natural CO2-rich gas vents11. There is a distinct division between the CO2 domain in the peri-Tyrrhenian internal sector and the CH4 domain in the foredeep sections. Gas transfer is mainly associated with mud diapirs in the compressed region of the Apennine orogene, where methane-rich mud volcanoes are found. On the contrary, in the inner part of the Apennines where extension has caused the intrusion of several deep fluid sources (mantle and/or magmatic) CO2 prevails. The enormous amount of CO2 diffused in the shallower regions of the crust and produced in several stratified layers in the deeper crust dilutes methane in the peri-Tyrrhenian sector12. Similar reasoning may be applied to the relative total amount of helium discharged at the surface, which is lower in the Tyrrhenian sector compared to the NE Apennine CH4-rich vents11. This thesis aims to define a model that can explain the origin of fluids in the different areas along the Apennine, the main processes that modify their chemistry in crustal layers and finally the relationships between deformation and fluids geochemistry in Apennines. Therefore, it involves the collection of geochemical data, which must be processed and subsequently integrated with those already collected from previous studies. The specific targets have been:1) to evaluate the origin of fluids in the two areas; 2) to define the role of aquifers in the transfer of non-atmospheric fluids (mantles and crustal) in relation to crustal deformations and seismicity of the areas; 3) to discuss the geochemical data in relation to the tectonics and seismicity of the area in order to define a model for the transfer of fluids through to the crust and therefore can highlight any relationships between seismicity and fluid transfer and/or variations of water-gas interactions. The samples taken will be analysed to define 1) the chemical composition of the gaseous mixtures emitted at the surface, 2) Carbon (CCO2, CCH4) and noble gas (He, Ne, Ar) isotopic composition. The analyses were carried out at the laboratories of the Palermo section of the National Institute of Geophysics and Volcanology of the Palermo section (Italy). The abundance ratios of CO2 or CH4 and noble gases integrated with the isotopic ratios of noble gases will make it possible to investigate a possible role of the aquifers in modifying the chemistry of the gases. These data provided a fundamental contribution to the understanding of the relationships between fluids at depth and the seimo-genetic processes3,13. The data acquired had been elaborated and discussed in order to figure out a unique scenario of the origin of fluids, how water-gas-rock interactions processes modify the fluid chemistry during their transfer trough the crust and it will allow and understanding of possible variations of the geochemical parameters that are acquired at high frequency. In detail, the collected geochemical data collected was discussed and interpreted on the basis of geological-structural knowledge and seismic data in order to build a model that can explain the migration of fluids through the crust and the role of seismically active faults in the transfer of the fluids themselves. Finally, I try to define and quantify the migration of deep fluids in the areas of investigation so as to assess the depth of the tectonic discontinuities active in fluid transfer and the role of the fluids themselves in relation to the seismicity of the area, thus providing answers to questions still open. In conclusion, the collected results are used to create a model defining the origin of the fluids emitted, their transfer through the crust, any water-gas-rock interactions and the relationships between fluids and crustal deformations and seismogenic processes. In addition, the collected data and the model were discussed and compared with the data collected within the geological, structural and seismic network created by the INGV in the Apennines in order to improve the knowledge on the deformation processes active in the study areas.