Shear veins observed within anisotropic fabric at high angles to the maximum compressive stress

Some faults seem to slip at unusually high angles (>45 degrees) relative to the orientation of the greatest principal compressive stress(1-5). This implies that these faults are extremely weak compared with the surrounding rock(6). Laboratory friction experiments and theoretical models suggest that the weakness may result from slip on a pre-existing frictionally weak surface(7-9), weakening from chemical reactions(10), elevated fluid pressure(11-13) or dissolution-precipitation creep(14,15). Here we describe shear veins within the Chrystalls Beach accretionary melange, New Zealand. The melange is a highly sheared assemblage of relatively competent rock within a cleaved, anisotropic mudstone matrix. The orientation of the shear veins-compared with the direction of hydrothermal extension veins that formed contemporaneously-indicates that they were active at an angle of 80 degrees +/- 5 degrees to the greatest principal compressive stress. We show that the shear veins developed incrementally along the cleavage planes of the matrix. Thus, we suggest that episodic slip was facilitated by the anisotropic internal fabric, in a fluid-overpressured, heterogeneous shear zone. A similar mechanism may accommodate shear at high angles to the greatest principal compressive stress in a range of tectonic settings. We therefore conclude that incremental slip along a pre-existing planar fabric, coupled to high fluid pressure and dissolution-precipitation creep, may explain active slip on severely misoriented faults.