The generation of ductile and brittle shear bands in a low-angle mylonite zone

In the Saint-Barthélemy Massif, French Pyrenees, a ductile thrust zone developed in gneisses during retrogression from lower amphibolite facies conditions to the upper greenschist facies. The last major structures formed in the zone are isolated shear bands, divided into three types.Anastomosing, inhomogenous ultramylonitic shear bands (Type I) are subparallel to the mylonitic foliation in the gneiss (S_g). Most of these bands developed by ductile deformation processes only.Planar, homogeneous ultramylonite bands (Type II) are usually oblique to S_g. They generated as pseudotachylyte bands by brittle fracturing and underwent strong subsequent ductile deformation.Type III shear bands are planar and oblique to S_g. They consist of pseudotachylyte, weakly affected by ductile deformation.Type I, II and III bands seem to represent progressively younger structures on a local scale, linked to falling P-T conditions. The systematic variation in orientation of the different shear bands with respect to S_g is interpreted as being due to a different response of brittle and ductile structures to the orientation of the kinematic frame and the rock anisotropy.