How the Stalled Hikurangi Large Igneous Plateau Forms the Lower Crust Under the Eastern South Island, New Zealand, Revealed by Receiver Functions
Abstract
The Gondwana accretionary margin formed the eastern South Island, amalgamating greywacke and schist terranes over oceanic slab, including a long section of the Hikurangi large igneous plateau (LIP) which now forms sections of the lower crust and uppermost mantle. Subsequent sparsely distributed intraplate volcanism from regional extension has locally disrupted this oceanic lower crust. Previous receiver function studies have inferred varied Moho depths and 8-16 km thick lower crust (Spasojevec and Clayton, 2008). Here we examine receiver functions from the 2014-2015 Otago 40-station network as well as recent permanent network stations to assess the oceanic lower crust and its large igneous plateau component. Midpoint stacking of 2000 earthquakes indicates oceanic lower crust of 8-20 km thickness is required under most of the South Island (Wilson and Eberhart-Phillips, 1998). Previous seismic refraction and reflection studies show variably thick lower crustal reflectivity up to 4 s TWT, consistent with stalled subducted crust (Henrys et al. 2004). The Hikurangi plateau includes a deeper thin, high velocity layer (HVL) which produces high-frequency P-wave precursors observed from earthquakes along the North Island and across much of the South Island (Love et al., 2015) and has been considered either the base of the LIP crust, or the uppermost mantle of the LIP. Hence the eastern South Island inferred Moho observations generally correspond to this LIP HVL, while a more subtle decrease in velocity below the HVL has been less well observed from previous receiver functions. Relevant Ontong Java LIP receiver functions show a layer above the Moho as well as a varied shallower crustal discontinuity and deeper mantle feature (Tonegawa et al., 2019). For the current study we consider receiver functions from both 1 hz and 2.5 hz low-pass cut-off frequencies to infer structure. About half of these Otago network stations indicate both oceanic lower crust and HVL or Moho using cut-off 2.5 hz, and about 20 percent are consistent with deeper velocity reduction. Unfortunately, there are limited teleseismic events available for the 6-month deployments, and we will analyse the sparser permanent stations from a longer time-period. We will also undertake velocity modelling to constrain structure of the LIP lower crust and upper mantle.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMS069...06E
- Keywords:
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- 7299 General or miscellaneous;
- SEISMOLOGY