Pn anisotropy beneath the South Island of New Zealand and implications for distributed deformation in continental lithosphere
Collins, John A.
Molnar, Peter H.
MetadataShow full item record
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.
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.
Showing items related by title, author, creator and subject.
S-wave splitting in the offshore South Island, New Zealand : insights into plate-boundary deformation Karalliyadda, Sapthala C.; Savage, Martha K.; Sheehan, Anne F.; Collins, John A.; Zietlow, Daniel W.; Shelley, Adrian (John Wiley & Sons, 2015-08-30)Local and regional S-wave splitting in the offshore South Island of the New Zealand plate-boundary zone provides constraints on the spatial and depth extent of the anisotropic structure with an enhanced resolution relative ...
Zietlow, Daniel W.; Sheehan, Anne F.; Molnar, Peter H.; Savage, Martha K.; Hirth, Greg; Collins, John A.; Hager, Bradford H. (John Wiley & Sons, 2014-02-05)New shear wave splitting measurements made from stations onshore and offshore the South Island of New Zealand show a zone of anisotropy 100–200 km wide. Measurements in central South Island and up to approximately 100 km ...
Constraints on lithosphere net rotation and asthenospheric viscosity from global mantle flow models and seismic anisotropy Conrad, Clinton P.; Behn, Mark D. (American Geophysical Union, 2010-05-13)Although an average westward rotation of the Earth's lithosphere is indicated by global analyses of surface features tied to the deep mantle (e.g., hot spot tracks), the rate of lithospheric drift is uncertain despite its ...