Coexistence of low-angle normal and high-angle strike- to oblique-slip faults during Late Miocene mineralization in eastern Elba Island (Italy)

Abstract

In this paper we deal with the kinematic and chronological relationships among low angle normal faults and high angle strike- to oblique-slip faults in an exhumed mineralized area, where shear veins and minor associated structures filled with the same mineral assemblage has been interpreted as indicators of coeval fault activities. The study area is located in the eastern Elba Island, where a mineralized late Miocene-early Pliocene low-angle normal fault (Zuccale fault) and high-angle strike- to oblique-slip faults extensively crop out, the latter giving rise to the Capoliveri-Porto Azzurro shear zone. The field study highlighted that: (a) the damage zones of both fault sets are mineralized by syn-kinematic tourmaline, graphite, Fe-oxides and/or Fe-oxyhydroxides shear veins, thus indicating their coeval activity during the hydrothermal event (5.9–5.4Ma); (b) the Capoliveri-Porto Azzurro shear zone is constituted by a network of fractures, whose geometry and kinematics display the evolution of a NE-trending left-lateral oblique-slip transtensional shear zone; (c) its internal architecture is defined by tourmaline and Fe-oxides and/or Fe-oxyhydroxides mineralized veins, framed in the same kinematic field characterizing the Zuccale fault evolution; for this reason, the Capoliveri–Porto Azzurro shear zone is interpreted as a transfer zone active during the low-angle fault activity; (d) the Capoliveri–Porto Azzurro shear zone played the role of a significant normal fault during the Late Pliocene–Pleistocene, therefore favouring the deepening of the Tyrrhenian Basin with respect to the uplift and exhumation of the mid-crustal rocks of the Elba Island. It is finally argued that the interaction between the low-angle normal fault and the almost vertical shear zone determined an increase of permeability, favouring the mineralizing fluid flow during the hydrothermal stage and, reasonably, the previous emplacement of the Porto Azzurro magmatic body.