Exploring the Variscan syn- to post-orogenic tectonics at the Einstein Telescope site of Sardinia (Italy).

The Variscan tectonics of north-eastern Sardinia is defined by syn-orogenic collision-related middle to high-T metamorphic terrains and by post-orogenic intrusions. The aim of this study lies in the understanding of the relationships between faults and intrusions in an area that has been candidate for hosting the Einstein Telescope (ET), a European third-generation underground interferometric detector of gravitational waves. Free of regional faults, the ET layout is currently projected as a triangular infrastructure (10 km long sides), whose location has been proposed because of its present-day geodynamic quietness, very low seismicity and anthropogenic seismic noise. Despite previous maps underestimated the presence of faults in the area, new fieldwork has mapped them in higher detail. The syn-orogenic deformation of the Palaeozoic metamorphic rocks consists of distinguished fold and cleavage generations with at least two ductile phases (D2+3) almost completely transposing the original bedding and the oldest schistosity (S0+1), that is still visible in the south were the thermometamorphic grade is lower. The later brittle fault network affects the metamorphic-plutonic ensemble with faults that mostly run parallel to the orientation of both dykes and plutonic contacts. Fault zones are generally NNW-, and WSW-striking and are associated with either more altered bedrock and/or cataclastic bands that are locally affected by late hydrothermal circulation with thick quartz veins, thin chlorite fibers or pseudotachilites and gouge that can be as thick as a meter each. In the surroundings of the boreholes drilled at two ET vertices, multiscale morphostructural analysis and 2D Electrical resistivity tomographies was carried out. The distribution of fault zone-related morphostructures shows maximum length up to 2.5 kilometres. At depth (ca. 250 m), the tomographies show also a complex internal resistivity stratification, that consists of up to three electrolayers with variable distribution and thickness. This is related to the occurrence of faults in the bedrock corresponding to the larger morphostructural lineaments. In conclusion, these results highlight the interaction between the syn- and the post-orogenic features that seem related to the inherited Variscan crust structure, which has a present-day implication in the groundwater flow. Evidence shows that the post-orogenic Variscan structures guided the post-orogenic dyke and vein injection. Further, the main fault zones were site of later hydrothermal circulation, possibly reactivated during the Oligocene-Aquitanian tectonics. Further studies should constrain the contribute of the eventually current differential uplift into reactivating the inherited Variscan structures to exclude the presence of neotectonics in the area.