Kink-bands and Related Geological Structures

Abstract

ANDERSON¹ has proposed that kink-bands develop in response to a maximum principal pressure acting within, or close to, a strong planar anisotropy, and that their orientation is controlled by shear movements on the intra-kink-band folia. He has also stated that those shears make an angle of (90 − φ)/2 degrees with the maximum pressure (P_max), while the kink-bands form angles of (90 + φ)/2 degrees with the same stress axis. This theory has been criticized² on the grounds that, in planar aniso-tropic rocks, the maximum principal stress cannot be given unless the kink-bands are truly symmetrical about the anisotropy, that the known variation in the angle formed by kink-bands about P_max was not wholly explained, and that the intra-kink-band movements are a secondary response to a primary direction of shear paralleling the kink-band. It was also stated that Anderson's figured example was not truly conjugate and that the kink-band system in the Ards Peninsula might not be symmetrically arrayed about the cleavage. Anderson's inadequate reply³ to these criticisms merits a further communication.