Seismic and Petrofabric studies from Red Mountain Ophiolites, South Island, New Zealand

We study deformation mechanisms within the crust and upper mantle by comparing seismic and structural measurements of peridotite fabric at Red Mountain. We measure fabric from microscopic through km scales by comparing crystallographic orientations of peridotites with field measurements of seismic anisotropy along lines of 100-200 m length, and with shear-wave splitting of local and teleseismic earthquakes. SKS splitting from stations on and near Red Mountain are similar to previous measurements on South Island. Polarizations of the first-arriving waves (φ) are in the range of 15 to 60 deg with 1.9 - 3.1 s delay times (dt), consistent with previous interpretations of crystal preferred orientation of olivine due to shearing parallel to the Alpine Fault at mantle depths. Preliminary average shear wave splitting measurements for local earthquakes with depth >100 km have φ of 55 deg, consistent with the SKS results, but dt is much smaller, at 0.03 ± 0.01 s (1 σ). Shallower events yield φ of 145 deg with dt=0.32 ± 0.003 s. Similar to shallow S-wave splitting, shallow seismic P-wave speeds for waves traveling in the upper few meters are faster (1.8 ± 0.1 km/s) along a profile oriented at 150 degrees than along the orthogonal profile (1.4 ± 0.15 km/s), yielding anisotropy of 23 ± 9.5 %. The S-wave velocity for the average top 30 m calculated from surface wave analysis on the radial components yielded 14% anisotropy, with a fast direction of 150 degrees. Shear wave splitting measurements from hammer shots yield delay times of 0.01± 0.005 s with the fast orientations in the 120-150 deg range. On Red Mountain, macroscopic foliation attitudes defined by compositional layering and orthopyroxene (opx) shape fabrics strike ~70 deg., and dip ~ 80 NW. Stretching lineations defined by opx prism alignment have a mean pitch of ~50 SW. Near-surface cracks have strikes of ~120 degrees, almost perpendicular to the foliation and lineation. Electron Back-Scattered Diffraction studies of crystallographic orientation of olivine were carried out on three peridotite samples taken from near the seismic lines. Olivine [100] axes are oriented in the lineation direction, [010] axes are normal to the foliation, and [001] is within the foliation plane and perpendicular to lineation. This suggests dislocation creep within a high-temperature [100] (010) slip system. Average S wave anisotropy of these samples based on the single-crystal elastic constant tensors is a maximum of 7.3 % for a wave traveling in the foliation plane normal to the flow direction, and φ is sub-parallel to the olivine [100] axes representing the flow direction marked by lineation. Shallow anisotropy measured from near surface studies and shallow local earthquake shear-wave splitting is parallel to the near-surface cracking, consistent with anisotropy caused by cracks rather than by mineral alignment. This suggests that the NE/SW anisotropy measured from teleseismic phases is not being contaminated by near surface crustal mineral alignment, but instead may be caused by shear-induced olivine alignment at mantle depths.