Feedback between fluid infiltration and rheology along a regional ductile-to-brittle shear zone: The East Tenda Shear Zone (Alpine Corsica)

The East Tenda Shear Zone (ETSZ) is themajor Alpine tectonic boundary marking the overthrusting of the ocean-derived Schistes Lustrés nappe onto the Variscan crystalline basement of Corsica. New structural, mineralogical, and geochemical investigations along a transect ranging from the Variscan basement to the contact with the Schistes Lustrés are used to construct a rheological model for the ETSZ during its polyphase deformation history. The progressive transformation of the isotropic granitoid protolith into gneisses, shear zones, and, locally, phyllonites is described. The textural/mineralogical change suggests a concurrent increasing metasomatism associated with structurally controlled fluid flow. The effect of such textural/mineralogical evolution on the bulk rheology of the ETSZ is estimated. Rheological flow laws are obtained using an averaging procedure based on the integration of single-phase rheological behavior. The flow laws are used to infer strain rates and construct strength envelopes for each structural domain during progressive deformation, which highlight the relative rheological differences of the main textural components of the ETSZ and the variations in their brittle/ductile transition depths. Two competing processes are inferred. On the one hand, the combined effects of the feldspar-to-mica reaction and the development of a strong planar fabric induce weakening and strain localization along shear zones. On the other hand, fluid channeling along these shear zones enhances alkali feldspar neoblastesis, inducing strain hardening. Among the possible consequences of such feedback processes between strain localization andfluid-rock interaction are episodes of rheological transitions fromductile to brittle behavior during the polyphase tectonic evolution of the ETSZ.