Natural deformation related to serpentinisation of an ultramafic inclusion within a continental shear zone: The key role of fluids

The role of fluids in the deformation of continental serpentinites is investigated from structural, microstructural and petrographic analyses applied to a km-scale porphyroclast mantled in a viscous matrix of amphibolites. The clast is sited within a shear zone of the Palaeozoic Maures massif, France. Syntectonic fluid–rock interactions occurred from km to mm scales, at first on the clast borders (along the main rheological boundaries) then within the clast. They are accommodated macroscopically by slickenfibers faults and microscopically by shear microcracks within crack-seal veins, typifying an intermediate, brittle–ductile behaviour of serpentinites. Three main stages of deformation–serpentinisation processes occurred in relation with the left-lateral movement of the hosted shear zone. They developed under metamorphic conditions evolving from amphibolites to green-schists facies conditions (∼ 400 MPa/550 °C to ∼ 200 MPa/< 300 °C), as inferred from the surrounding sheared amphibolites. Deformation and serpentinisation increase through time although fluid pressure decreases. If the shape of the inclusion and its orientation relative to the shear zone mainly controlled the deformation pattern though time (P then R' shears), fluid pressure is required for starting deformation–serpentinisation processes along inherited anisotropy planes. Whatever the origin of fluids, they play a key role all along the deformation processes by influencing stress states within the shear zone at the onset of deformation and by changing at various scales and through time behaviour of the rock, depending of the intensity of serpentinisation and the rate of deformation.