Strain localisation driven by marble layers: the Palmi shear zone (Calabria-Peloritani terrane, Southern Italy)

In this study a shear zone transecting the Hercynian basement of the Calabria - Peloritani terrane in the neighbourhood of Palmi is described for the first time. A structural analysis was carried out to outline the tectonic meaning of the shear zone. In addition, thermobarometry and geochronology allowed to define the structural level and age of deformation. The shear zone is localised along layers of calc-silicate bearing marbles near the contact between late Hercynian tonalites and migmatitic paragneisses. Marbles, interpreted as former skarns, still retain the original high temperature Hercynian mineral assemblage including diopside, grossular, anorthite-rich plagioclase ± scapolite ± hornblende. Deformation took place in greenschist facies at about 400 °C and 600 MPa. This event partially affected the peak assemblage in marbles, as documented by coronas of clinozoisite around anorthite-rich plagioclase. Rb-Sr method applied on two biotite separates coming from sheared gneisses gave Eocene ages of 51 and 56 Ma. The shear zone is broadly oriented WNW - ESE and shows nearly vertical foliation planes. Large scale folds undulate the foliation planes whose direction swings from NW - SE to NE-SW. Lineations generally show a shallow plunge with trends mostly spanning from E-W to NW-SE. In some cases lineations show a wide scatter in orientation and locally can be nearly vertical. Shear sense indicators and quartz c-axis distribution are coherent with sinistral kinematics along nearly horizontal lineations, and south side up displacement along steeply plunging lineations. The Palmi shear zone is interpreted as an early Alpine sinistral transpressive belt developed within weak calc-silicate bearing marbles sandwiched between the rigid domains of tonalites and migmatitic paragneisses. Due to sinistral transpression tonalites were uplifted with respect to the migmatitic paragneisses.