Pre‐Alpine Fault Fabrics in Mantle Xenoliths From East Otago, South Island, New Zealand

Abstract

In the last century geophysicists discovered that seismic velocity in the Earth's upper mantle is anisotropic and inferred that the crystallographic preferred orientations (CPOs) of olivine in the subcontinental lithospheric mantle (SCLM) contributes significantly to this anisotropy (Hess, 1964; Nicolas & Christensen, 1987). During the last two decades, measurements of the CPOs of natural samples (orogenic mantle slices and mantle xenoliths) and the products of laboratory experiments have established a framework of olivine CPO types and understanding of how these CPOs relate to deformation processes and how they contribute to seismic anisotropy (

Alpine Fault‐Related Microstructures and Anisotropy of the Mantle Beneath the Southern Alps, New Zealand

“…Anisotropy patterns reflect pre‐Alpine Fault CPOs in coarser xenoliths. In East Otago these are consistent in orientation and may relate to an ancient shear zone (Shao et al., 2021). Moving west across the South Island the CPOs are rotated and modified in blocks between protomylonite zones.…”

“…Protomylonites have shear crystallographic preferred orientations (CPOs) of Alpine Fault age that do not contribute significantly to the seismic anisotropy signal. Anisotropy patterns reflect pre-Alpine Fault CPOs in coarser xenoliths.In East Otago these are consistent in orientation and may relate to an ancient shear zone(Shao et al, 2021). Moving west across the South Island the CPOs are rotated and modified in blocks between protomylonite zones.…”

“…Shao et al. (2021) infer that consistent CPOs across the East Otago area, relating to a fossil shear zone, explain the anisotropy data there. We thus speculate that the lower S ‐wave anisotropy in West Otago is because olivine CPO is not aligned on a large scale.…”

“…Electron Backscatter Diffraction (EBSD) data (see Shao et al., 2021 for details) were collected with step sizes between 10 and 50 μm depending on grain size. CPOs are extracted from the EBSD data and are presented as one point per grain in lower hemisphere stereographic projections contoured using a counting cone with 20° half width in the MTEX toolbox (Bachmann et al., 2010).…”

“…We have chosen to plot CPOs in an orthopyroxene reference frame to facilitate comparisons between samples: orthopyroxene maxima of (100) poles parallel to Z, maxima of [001] directions parallel to X (see Figures 4–6). This reference frame is the same as that chosen for work on East Otago xenoliths (Shao et al., 2021) and represents the most common orthopyroxene CPO (Gan et al., 2011; Puziewicz et al., 2020; Yu et al., 2019). The reference frame choice is arbitrary and will be discussed further later in the paper (Section 5.4).…”