Nucleation of ductile shear zones in a granodiorite under greenschist facies conditions, Néouvielle massif, Pyrenees, France

The problem of ductile shear zone nucleation under greenschist facies conditions is approached from the example of small-scale shear zones developed in the Néouvielle granodioritic pluton (Pyrenees, France) by means of field and microstructural observations and chemical analyses. These shear zones are not related to pre-existing fractures and exhibit networks of numerous conjugate, fairly parallel and regularly spaced centimetre-scale brittle–ductile shear zones involving diffuse localization mechanisms. Although the mode of deformation depends on the minerals present, deformation in the shear zones is basically controlled by hydration processes. Hydration and consequent fluid-controlled alteration and deformation are related to fluid migration towards developing cracks. All these cracks are extensional and formed on the scale of at most a few grains and frequently single grains. A comparison of chemical compositions of undeformed and sheared granodiorite shows that the shear zones can be interpreted as isochemical and isovolumetric systems. We propose a sequence of mechanisms by which the nucleation of a small-scale brittle–ductile shear zones spreading out within a granodioritic rock may occur. In these mechanisms shear zone nucleation occurs independently of pre-existing fractures and results from the heterogeneous character of the polymineralic rock. In the studied granodiorite the mineral heterogeneities favour focusing of locally derived fluids by processes involving grain-scale hydraulic fracturing. Fluid focusing generates instabilities by local softening and subsequently shear zone nucleation.