MULTIPLE GENERATIONS OF CROSS-CUTTING FABRICS IN NATURALLY-DEFORMED UPPER MANTLE ROCK, RED HILLS, NEW ZEALAND

The Red Hills complex (100 km²), South Island, New Zealand, is a Permian-age ultramafic massif. We mapped in detail a 3 km² area at Porter’s Knob (south-central Red Hills). Olivine microstructures suggest that all deformation took place at mantle conditions.

Four fabrics are observed in the Porter’s Knob area. The center of the mapped area is a lherzolite that contains two foliations: 1) a shallowly north-dipping compositional foliation defined by thin (1-3 cm) bands of olivine- and orthopyroxene-rich lherzolite (S1); and 2) shallowly south-dipping, cm-scale shear zones (S2). These shear zones show apparent normal offset of compositional layering ranging from 3 to 90 cm. The western Porter’s Knob area is composed of olivine-rich harzburgite with a steeply west-dipping foliation defined by thick (6-8 cm) bands of alternating olivine- and orthopyroxene-rich harzburgite and down-dip lineation defined by alignment of spinel (S3). This zone is at least 350 m wide. The eastern portion of Porter’s Knob is a mylonitic lhzerolite (S4); the parallel alignment of elongate spinel defines a moderately east-dipping foliation and down-dip lineation. This zone is at least 200 m wide, and lherzolite of the central region is interpreted to be the protolith of the mylonite.

Cross-cutting relations suggest the oldest fabric is compositional banding (S1), which is cut and overprinted in some locations by abundant shear zones (S2). In the western part of the map area, S1 and S2 fabrics are almost entirely overprinted and are cut by a steeply west-dipping fabric that we interpret to be a younger shear zone (S3). S1 and S2 fabrics were initially present in the western zone, as suggested by a small body of olivine-rich harzburgite that contains these fabrics and occurs on the western edge of the central region. This observation further suggests that strain localized in the western zone because of a difference in original lithology (more olivine). East-dipping (S4) fabric in the mylonitic lhzerolite cuts S1 and S2 fabrics. The S4 fabric was not observed in contact with the S3 zone, so relative timing of these two fabrics is unknown. These results suggest that multiple cross-cutting foliations observed in crustal rocks also occur at mantle conditions.