Fabric superposition in upper mantle peridotite, Red Hills, New Zealand

At least four generations of superposed fabrics that formed at upper mantle conditions are recorded by field relations, mineral compositions, and olivine lattice preferred orientation in the Red Hills, an ultramafic massif in South Island, New Zealand. The oldest fabric is found in the Central Domain and contains north-dipping compositional bands of harzburgite, dunite, and minor lherzolite. Expressions of second-generation fabrics that cross-cut Central Domain fabric are cm-scale, south-dipping shear zones; and a >50 m-thick zone of south-dipping shear, the South-dipping Domain. A third, younger, superposed fabric occurs in a ≥1 km-thick package on the west side of the field area, called the West Domain. The West Domain is composed of harzburgite and lherzolite, and is characterized by steeply west-dipping foliation and by isoclinal folds of older compositional banding. The youngest fabric, the East Domain, is preserved in a ∼50 m-thick zone on the east side of the field area. The East Domain is composed of lherzolite and contains cm-scale compositional bands that define a microstructually distinct, shallowly east-dipping foliation. The West and East Domains are interpreted to have formed by transposition of Central Domain fabric. Fabric transposition is generally expressed by an abrupt overprinting or truncation, rather than by a progressive deflection of foliations. This study demonstrates that multiple overprinting fabrics exist in the upper mantle, similar to the fabric superposition commonly observed in mid- to lower-crustal rocks.