Résumé
The Variscan nappes of Vendée (South Brittany, France) were displaced westward through a mechanism of regional subhorizontal ductile shear involving the entire body of the nappes. Ductile shear also affected the upper part of the Silurian footwall, including the rhyolite of Chapelle-Hermier, which was mylonitized and sheared with upward-increasing intensity, as well as the calcalkaline Vendée porphyroids of volcanic and volcanoclastic origin which form a mylonite sheet outlining the major basal thrust zone of the nappes. Nappe displacement was accompanied by a severe extensional strain, estimated at up to 300–400% in both the porphyroids and the rhyolite, and indicating that the deformational history may have involved a gravitational spreading component. Within the mylonitic rhyolite, accommodation of the bulk strain was mainly by solution-assisted mass-transfer, as shown by (1) growth of quartz fibres in low-pressure zones, (2) development of polycrystalline ribbons through continuous growth of quartz grains in pressure shadows and (3) crystallization of muscovite at the expense of the felsitic groundmass. A similar deformation process was probably active in the mylonitic porphyroids at the sole of the nappe, suggesting that pressure-solution flow was an important mechanism in nappe displacement.