Abstract
Alpine deformation in the meta-ophiolitic Morteda-Farinole nappe of the Lower Oceanic Unit of Corsica occurs under greenschist facies conditions. It is notably characterized by a series of meter-scale shear zones in metagabbros. These ductile faults are cm- to dm- in width and are composed of amphibole ± chlorite schists. This occurrence offers a unique opportunity to study mechanisms of low-grade shear zone formation, extensive fluid/rock interaction and weakening processes controlled by mineralogical changes in the metamorphosed oceanic crust in the shallow part of a subduction system. A detailed sampling of undeformed metagabbros, protomylonitic metagabbros, as well as schistose mylonites allows tackling several finite strain states that involved rare occurrence of serpentinite clasts within the fault zones. Petrological observations combined with thermodynamic and geochemical modelling indicate that the metagabbro to amphibole-chlorite schist transformation involved mass transfer and occurred under open-system and water-saturated conditions. The main chemical features of this metasomatic process are significant gains in MgO, H2O and Fe2O3 accompanied by an almost complete loss in Na2O. Deciphering softening processes controlled by the transformation of metagabbros into low-grade schistose rocks are of major importance to understand the seismogenic behavior of the mafic crust during subduction.