PROGRESSIVE OLIVINE LPO REORIENTATION IN A SMALL-SCALE SHEAR ZONE, RED HILLS, NEW ZEALAND

The Red Hills complex, part of the Permian Dunn Mountain ophiolite belt, New Zealand, is a large (100 km2) exposure of fresh mantle periodite. The Porter’s Knob region, located in the southern portion of the Red Hills complex, contains an early, N-dipping compositional foliation (S1) that is locally cross-cut by later stage, S-dipping shear zones (S2). In a few locations, compositional banding (S1) is rotated into parallelism with a shear zone and then discretely offset. We studied the fabric development of one olivine-rich compositional layer (5-8 cm) by sampling this layer at intervals from the shear zone. The shear zone is thin (~1 cm) and discretely offsets the olivine-rich compositional layer by ~50 cm.

The lattice preferred orientation (LPO) of olivine was measured in seven samples by using a universal stage microscope. These data suggest that olivine in the S1 layer has a strong LPO in the sample farthest away (25 cm) from the shear zone. The [100] axis (a-axis) displays a point maximum and the [010] axis (b-axis) and [001] axis (c-axis) show girdle distributions. Closer to the shear zone the [100] axis is a point maximum that is unchanged from its orientation in the S1 compositional layering, despite the relatively high shear strains. The [010] and [001] axes remain girdle distributions, although the maximum orientations within the girdles change with distance from the shear zone. This analysis suggests that the earlier LPO (associated with S1) imparts a preferred orientation that is difficult to change in later deformations (the S2 shear zone fabric). Consequently, interpretation of LPO fabric without knowledge or preservation of prior fabrics must be interpreted cautiously in terms of both temperature of deformation and water content.