C-axis fabrics of quartz-ribbons during high-temperature deformation of syn-tectonic granitoids (Sila Massif, Calabria, Italy)

Late-Hercynian, high temperature (HT) shear deformation affected the zone straddling the contact between the Sila Massif batholith (above) and the host migmatitic paragneisses (below). A segment of the regional HT shear zone was investigated close to the town of Mesoraca, where a natural section allows one to analyse the solidus deformation of syn-tectonic granitoids. Shearing favoured the formation of polycrystalline quartz ribbons in deformed granodiorite and tonalite. Two main c-axis fabrics were found in the quartz-ribbons, different from each other by opening angles around Z between strong point maxima, in accordance with deformation under granulite to amphibolite facies conditions. These fabrics, along with microstructural observations, suggest that temperature of deformation played a key role and that ribboning was accompanied by the activation of (i) prism [c] slip and prism [c] + prism <a> slips, under granulite facies conditions, followed by (ii) basal <a> and prism <a> slips under amphibolite facies conditions.     

Les Montagnes Noires : cisaillement bordier méridional du bassin carbonifère de Châteaulin (Massif Armoricain,France). Caractéristiques structurales et métamorphiques

Résumé

L’évolution structurale et métamorphique de la zone des Montagnes Noires, qui borde vers le sud le bassin carbonifère de Châteaulin, est contrôlée par le fonctionnement d’un cisaillement trans-current dextre. Le caractère unique, mais progressif et inhomogène, de la déformation régionale d’âge carbonifère supérieur conduit à remettre en cause le concept d’une structuration hercynienne polyphasée et celui de « phase bretonne ».