Pseudotachylyte and the development of ultramylonite bands in the Saint-Barthélemy Massif,French Pyrenees

In mylonitic gneisses of the Saint-Barthelemy Massif in the French Pyrenees, evidence was found that ultramylonite bands developed by ductile deformation of pseudotachylyte.In the gneiss a mylonitic shape fabric was produced by a continuous structural event during retrogression from amphibolite to upper greenschist facies. Decrease in temperature caused gradual hardening in the gneiss which led to the development of pseudotachylyte bands by intermittent seismic slip. These acted as weak zones in which ductile deformation was concentrated. A microstructure typical of ultramylonite was produced in these deformed pseudotachylyte bands.