DEFORMATION IN LOW-GRADE SHEAR ZONES IN THE OLD RED SANDSTONE, SW WALES

Shear zones in sandstones of the Old Red Sandstone at Marloes Sands, S. W. Wales show three stages in their microstructural evolution. The initial phase is dominated by microfracturing and infilling of dilation sites with secondary quartz. This stage occurs before any significant shear displacement parallel to the shear zone borders. The second phase is associated with increased intracrystalline plasticity accompanying shear displacement in the zones. The third phase is characterized by heterogeneous deformation and recrystallization concentrated in zones marking cleavage planes. The high bubble content of grains influenced the sub-grain formation and recrystallization behaviour. © 1979.