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Lithospheric Deformation at the South Island Oblique Collision, New Zealand

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posted on 09.11.2021, 01:32 by Bourguignon, Sandra

Lithospheric deformation is investigated within the Southern Alps oblique collision zone of the Australian and Pacific plate boundary. Seismological methods and gravity modelling are used to estimate seismic anisotropy, wave-speed anomalies and mass anomalies in the uppermost mantle. While seismic anisotropy is generally interpreted to result from Cenozoic mantle shear, wave-speed and mass anomalies can be explained solely by thermal contraction of mantle rocks that results from the downward deflection of isotherms during mantle shortening. Along the eastern Southern Alps foothills and approximately 15' clockwise from their axis, earthquake Pn waves propagate at 8.54 +/- 0.20 km/s. This high wave speed is attributed to a high average Pn speed (8.3 +/- 0.3 km/s) and Pn anisotropy (7 - 13 %) in the mantle lid beneath central South Island. Two-dimensional ray-tracing suggests that the crustal thickness is 48 +/- 4 km beneath the Southern Alps' southern extent near Wanaka (western Otago). Such a thickness represents an 18 +/- 4 km thick crustal root that is thicker than necessary to isostatically sustain the approximately 1000 m topographic load of this region. A mass excess is proposed in the mantle below the region of over-thickened crust to compensate for the crustal root mass deficit. Assuming that the crustal root represents a -300 kg/m3 density contrast with the mantle lid, this mantle mass excess requires a minimum density contrast of 35 +/- 5 kg/m3, 110 +/-20 km width and 70 +/- 20 km thickness that will impart a downward pull on the overlying crust.

History

Copyright Date

01/01/2009

Date of Award

01/01/2009

Publisher

Te Herenga Waka—Victoria University of Wellington

Rights License

Author Retains Copyright

Degree Discipline

Geophysics

Degree Grantor

Te Herenga Waka—Victoria University of Wellington

Degree Level

Doctoral

Degree Name

Doctor of Philosophy

Victoria University of Wellington Item Type

Awarded Doctoral Thesis

Language

en_NZ

Victoria University of Wellington School

School of Geography, Environment and Earth Sciences

Advisors

Stern, Tim; Savage, Martha