Fabric development in cm-scale shear zones in ultramafic rocks, Red Hills, New Zealand

The Red Hills ultramafic massif, the South Island, New Zealand, contains a suite of cm-scale shear zones that are composed of dunite, pyroxenite, and olivine websterite. Offset on these shear zones was measured using displacement of cross-cutting dikes or distinct compositional foliation layers in the host rock, and is interpreted to occur by dominantly simple shear. Shear zones contain microstructures similar to the host rock, including coarse grain sizes (> 1 mm) and dominantly polygonal grains. The olivine lattice preferred orientation (LPO) in the host rocks is consistent with the (010)[100] slip system, active at high-temperature, dry upper mantle conditions. Within the shear zones, the olivine LPO, when plotted relative to the shear zone foliation and lineation, suggests (010)[001] slip. The shear zone LPO is typically more poorly clustered than in host rock samples and double maxima are observed in some samples, with the second maxima suggesting (010)[100] slip.When olivine LPO from both host rock and shear zone samples is plotted within the same (geographic) reference frame, it is apparent that the shear zone LPO retains evidence for the dominant LPO of the host rocks. The LPO in the shear zone rocks reflects the pre-existing fabric; the poorly clustered data and double maxima reflect the changes that developed during shear zone deformation. These rocks demonstrate the influence of pre-existing LPO on textures formed during deformation and the need to consider the LPO of deformed rocks within different reference frames.