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    Pn anisotropy beneath the South Island of New Zealand and implications for distributed deformation in continental lithosphere

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    Date
    2014-10-23
    Author
    Collins, John A.  Concept link
    Molnar, Peter H.  Concept link
    Metadata
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    Citable URI
    https://hdl.handle.net/1912/7014
    As published
    https://doi.org/10.1002/2014JB011233
    DOI
    10.1002/2014JB011233
    Keyword
     Pn; Anisotropy; New Zealand; Lithosphere 
    Abstract
    Pn travel times from regional earthquakes recorded both by stations on New Zealand and by ocean bottom seismographs deployed offshore indicate anisotropy in the uppermost mantle beneath the region. The largest anisotropy of ~8% (±2%, 1σ) lies beneath the deforming part of the South Island to just off its West Coast, a zone roughly 100–200 km wide. The fastest propagation is aligned N60°E (±3°), essentially parallel to the largely strike-slip relative plate motion since 20 Ma, also ~ N60°E. The magnitude of anisotropy decreases abruptly northwest and southeast of this zone, and on the southeast side of the island, the orientation of fastest propagation is between N32°W and N-S. The ~ N60°E orientation of fast propagation is consistent with finite strain within the uppermost part of the mantle lithosphere if the measured 850 km of displacement of the Pacific plate past the Australia plate is spread over a region with a width of 100–200 km. The agreement of this orientation of fast propagation with the orientation or relative plate motion suggests the possibility of but does not require some dynamic recrystallization in rock as cold as 500–800°C, where Peierls creep seems to be the likely deformation mechanism. Such a strain distribution matches deformation of a thin viscous sheet that obeys a constitutive relationship of the form inline image, where inline image is the average strain rate and τ is the operative deviatoric stress, with an average value of n ≈ 3–10. Presumably, the NW-SE fast propagation in the region southeast of the island results from strain that precedes the Cenozoic deformation that has shaped the island.
    Description
    Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 119 (2014): 7745–7767, doi:10.1002/2014JB011233.
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    • Geology and Geophysics (G&G)
    Suggested Citation
    Journal of Geophysical Research: Solid Earth 119 (2014): 7745–7767
     

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