STRAIN PARTITIONING AND COMPOSITIONAL CONTROLS ON DEFORMATION STYLE IN THE RED MOUNTAIN PERIDOTITE MASSIF, SOUTH ISLAND, NEW ZEALAND

The Red Mountain massif (RMM) is the southernmost peridotite body in the Dun Mountain Ophiolite Belt (DMOB; South Island, New Zealand), and is composed dominantly of cm- to m-thick bands of pyroxene-rich and pyroxene-poor harzburgite, and minor dunite. Compositional layering in the harzburgites is best developed in the south of the massif, where 10- to 100-meter thick folded domains occur between discrete dextral reverse shear zones. Shear zones are typically subvertical, oriented ENE-WSW, and have a steeply west-plunging lineation (>50-60º) defined by elongated spinel aggregates or the shape preferred orientation of pyroxene. At least four major zones have been identified in the region that are continuous for >1 km along strike; shear zone widths range from 10 cm to 10 m. Toward the north of the complex, structural fabrics show a marked change from discrete to distributed modes of deformation, which correlates broadly with an increase in the abundance of dunite bands within the host harzburgites. Samples from harzburgite shear zones, folded domains, and regions containing dunite that lack mappable localization structures, were collected for microstructural and textural analyses to characterize the conditions and mechanisms of deformation in the upper mantle lithosphere during localization. Samples from within shear zones have a porphyroclastic microstructure and a strong shape-preferred orientation aligned parallel to shear zone margins, which contrasts a dominantly homogeneous microstructure in dunite regions. Patterns of olivine lattice preferred orientation, measured by electron backscatter diffraction, suggest that [100]{0kl} (Type ‘D’, pencil glide) was the dominant mechanism of deformation during shear zone localization under mantle conditions. The change from discrete to distributed deformation across the complex suggests that strain partitioning at the m- to km- scale in the DMOB peridotites was largely controlled, and/or facilitated by, upper mantle compositional variations.