STRAIN ACROSS A MANTLE SHEAR ZONE, BOGOTA PENINSULA, NEW CALEDONIA

A 3-km wide mantle shear zone is exposed along the Bogota peninsula, part of a larger Eocene-age ophiolite sheet in southern New Caledonia. Field observations are useful for understanding broad patterns of deformation across the shear zone. Field foliations strike N-NNE and dip steeply; lineations where observable, plunge gently both to the N and S. Pyroxene dikes on the margins of the shear zone have a variety of orientations and are often folded or boudinaged whereas those in high strain areas are nearly always parallel to the foliation. To characterize the strain patterns in more detail, we used a variety of laboratory techniques from approximately 10 stations across this mantle shear zone.

The lattice-preferred orientation (LPO) of olivine was measured using a 5-axes universal stage microscope. Most sites have well-defined point distributions for all axes with horizontal a-axes trending NNE to NE, roughly parallel to field lineations, and c-axes plunging moderately to steeply to the NW or SE. Seismic anisotropy calculations based on olivine LPO patterns indicate high shear wave anisotropy values of 5-11%. Orthopyroxene fabrics were characterized by tracing outcrop surfaces in the field and analyzed using digital image analysis techniques to calculate the three-dimensional shape-preferred orientation (SPO) of orthopyroxene across the shear zone. The short-axes of SPO ellipsoids generally parallel the poles to field foliations. The long-axes always plunge steeply, however, and the intermediate-axes are subparallel with field lineations. Low-field AMS fabrics show no consistent pattern across the shear zone and probably reflect later serpentinization of the ophiolite. The paramagnetic component of the AMS, characterized with high-field AMS, is often controlled by the magnetocrystalline anisotropy of olivine and was therefore expected to coincide with olivine LPO patterns. There is significant scatter in the high field AMS (HFAMS) results, preliminarily attributed to the presence of hematite. If hematite is present, the HFAMS results reflect a mixture of paramagnetic and ferromagnetic components.