Shear structures and microstructures in micaschists: the Variscan Cévennes duplex (French Massif Central)

Structural, microstructural and quartz 〈c〉 axis orientations have been investigated in shear zones within the Cévennes micaschist series (southeastern French Massif Central). These shear zones define a hinterland-dipping duplex formed during crustal thickening. They are characterised by superposed cleavages, a well-defined stretching lineation, isoclinal folds, shear planes and abundant quartz lenses. These shear zones were formed under P–T metamorphic conditions of ca. 500 °C and 450 MPa. The dominant deformation mechanism was dissolution–crystallisation. Fluid circulation responsible for element transfer was limited in space and dissolved silicate minerals are now found in the quartz lenses. Study of the syntectonic quartz lenses suggests fluid overpressure that has favoured brittle behaviour of the rock and formation of quartz lenses at different stages of the shearing event. Quartz lenses have accentuated the initial anisotropy of the micaschists and have strongly enhanced folding, boudinage and the development of shear zones. These deformation criteria for shear zone recognition can be applied in understanding the structure and significance of monotonous micaschist series in orogenic belts.