Thrusting and faulting in metamorphic and sedimentary units of Ligurian Alps: an example of integrated field work and geochemical analyses

In a sector placed in the SE part of the Alps–Apennine junction, a kilometre-scale shear zone has been identified as the Grognardo thrust zone (GTZ), which caused the NE-directed thrusting of metaophiolites (Voltri Group) and polymetamorphic continental crust slices (Valosio Unit) of Ligurian Alps onto Oligocene sediments of an episutural basin known as “Tertiary Piemonte Basin”. The structural setting of the GTZ is due to syn- to late-metamorphic deformation, followed by a brittle thrusting that occurred in the Late Aquitanian times and can thus be related to one of the main contractional tectonic events suffered by northern Apennines. The GTZ was then sealed by Lower Burdigalian carbonate platform sediments (Visone Formation). Transtensive faulting followed in post-Burdigalian times along NW–SE regional faults and displaced the previously coupled sedimentary and metamorphic units. The GTZ thus underwent a plastic-to-brittle evolution, during which carbonate-rich fluids largely sustained the deformation. In these stages, a complex vein network originated within both the metamorphic and sedimentary rocks. Field data and stable isotopic analyses (¹³C and ¹⁸O) of bulk rocks and veins show that fluid–rock interaction caused the carbonatisation of the rocks in the late-metamorphic stages and the cataclasis and recementation, by the action of isochemical cold carbonate groundwater during the thrusting events. Carbonate veins largely developed also during the transtensive faulting stages, with composition clearly different from that of the veins associated to thrust faults, as indicated by the strong depletion in ¹³C of carbonate fillings, suggesting the presence of exotic fluids, characterised by a high content of organic matter.