Regional tectonic implications on the tectonic contact at Zannone Island, Italy

Zannone is a very important island, located in the Neogene-Quaternary extensional domain of the Tyrrhenian back-arc basin, as it is the unique spot where the Paleozoic (?) crystalline basement is hypotesized to be exposed in central Apennines. The exposure of such hypothetical basement in the Zannone Island is very problematic as it implies very large normal displacements (> 3 km) along surrounding faults. No such displacements are known along faults close to Zannone Island. In this work, we study the hypothetical Paleozoic crystalline basement exposed in the Zannone Island with the main aim of understanding its geological nature and relationships with the surrounding rocks. We use a multidisciplinary approach including 1) field survey; 2) petro-textural observations; 3) petrologic analyses of the host rocks; 4) microthermometry on fluid inclusions; 5) geochemical analyses of stable and clumped isotopes; 6) analyses of minor gaseous species (He, Ne, and Ar concentrations and isotope ratios) in fluid inclusions; 7) U-Pb geochronology of syn-tectonic calcite; 7) K-Ar dating of syn-kinematic clay minerals, and 8) X-ray diffraction (XRD) analysis of the clay size fraction. Our results show that the hypothetical Paleozoic (?) crystalline basement exposed on the Zannone Island is, instead, represented by siliciclastic rocks of very low metamorphic grade. This is testified by the presence of chloritoid and by the observed incipient foliation marked by fine-grained white micas and disposed parallel to the bedding. The contact between such siliciclastic rocks and the overlapping Triassic Dolostones is represented by a low-angle thrust cut by sets of high-angle normal faults with associated calcite mineralizations. K-Ar dating on clay minerals in fault gouge reveals a strong contamination of K- bearing minerals from the protolith. In detail, we obtained an age of ~22 Ma which reveals that at least one event of authigenesis (i.e. fluid-assisted tectonic activity) occurred in Zannone Island <22 Ma ago. U-Pb dating on sin-tectonic calcite mineralizations allowed to constrain the compressional deformation and subsequent normal faulting in the study area at around 7 Ma. This result is consistent with the 1) described emplacement of imbricate thrust sheets onshore close to Zannone Island and 2) syn-tectonic sediments-filling basins observed by seismic reflection studies. Petrographic observations on fluid inclusions in tectonic quartz and calcite mineralizations allowed to determine that such inclusions were entrapped at the same time and therefore at the same depth. Microthermometry on fluid inclusions allowed to constrain a wide range on P-T entrapment conditions. For this reason, we highlighted a transition from lithostatic toward hydrostatic pressure during precipitation of syn-tectonic quartz and calcite mineralizations. Microthermometry on fluid inclusions highlighted also the presence of two fluids during tectonic processes. One characterized by low salinity (as NaCl equivalent) and one by high salinity (as NaCl equivalent). Coupled stable isotopes analyses on the same calcite mineralizations, we highlighted a mixing between meteoric and deep fluids, as testified by the high spread of calculates values of δ18O and δ13C.