FABRIC SUPERPOSITION IN UPPER MANTLE PERIDOTITES, RED HILLS, NEW ZEALAND

The Red Hills ultramafic massif, New Zealand contains evidence of at least four generations of superposed fabrics that formed at upper mantle conditions as recorded by field relations, mineral compositions, and olivine lattice preferred orientations (LPO). The Central Domain contains the oldest fabric, north-dipping compositional bands of harzburgite, dunite, and minor lherzolite. The South-dipping Domain is defined by cm-scale, south-dipping shear zones and a ~50 m-thick zone of south-dipping shear that crosscut Central Domain fabrics. A third, younger, superposed fabric occurs in a ≥ 1 km-thick package on the west side of the field area, the West Domain. The West Domain is composed of harzburgite and lherzolite, with steeply west-dipping foliation and 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 shallowly east-dipping foliation. The West and East Domains formed by transposition of Central Domain fabric. The four structural domains suggest that the upper mantle may experience a complex deformational history, and thus acquire a heterogeneous deformational fabric, especially when viewed at the kilometer-scale. The possibility of multiple overprinting fabrics in the upper mantle should be considered when interpreting seismic anisotropy data and when designing models that incorporate upper mantle deformation.

Investigation of the South-dipping cm-scale shear zones reveals that strain is accommodated along zones containing similar microstructures and grain sizes as harzburgitic host rocks (>1 mm polygonal grains), but with different mineralogies (dunites, pyroxenites and olivine websterites). While the LPO in the host rocks is consistent with (010)[100], typical of high-temperature upper mantle conditions, the shear zones instead contain a LPO consistent with slip in the [001] direction. Fourier transform infra-red spectroscopy indicates that shear zone rocks are not fluid-rich relative to the host rock. Rather, we interpret the atypical crystallographic slip direction as a result of reactivation of the pre-existing olivine LPO, similar to the host rock LPO, along local zones of heterogeneous lithologies relative to the host rock.