DI015-0005
Exploring the Lithosphere-Asthenosphere Boundary Zone with Wide-Angle Seismic Reflections

Friday, 11 December 2020
Poster
Pasan Herath1, Tim A Stern1, Martha K Savage2, Dan Bassett3 and Stuart A Henrys3, (1)Victoria University of Wellington, Wellington, New Zealand, (2)Victoria University of Wellington, School of Geography, Environment and Earth Science, Wellington, New Zealand, (3)GNS Science, Lower Hutt, New Zealand
Abstract:
Recent controlled-source, near-vertical seismic reflection surveys have begun to identify the lithosphere-asthenosphere boundary (LAB) as a low-velocity, melt-rich channel. We investigate the fine structure of the LAB system from a wide-angle reflection perspective using offshore airgun sources recorded by onshore seismographs at the southern Hikurangi margin in New Zealand. Here the oceanic Hikurangi Plateau subducts beneath continental North Island, New Zealand. A series of wide-angle reflections are identified in several onshore-offshore common receiver gathers at source-receiver offsets >150 km and travel-times >30 seconds, which arrive after upper mantle refraction (Pn). From forward-model raytracing, these reflections are placed at depths of 70-85 km in the upper mantle. Synthetic seismogram and amplitude variation with offset modelling show that these wide-angle reflections are consistent with a ~5 km thick layer with strong (13-15%) azimuthal anisotropy above a ~8-10 km thick, low P-wave-speed layer with VP/VS>>2. The high VP/VS ratio for this layer suggests that it is fluid rich. It is also the up-dip continuation of a ~10 km thick, low-velocity, melt-rich LAB channel detected by an earlier study about 100 km down-dip of the subduction zone. We interpret the anisotropic layer to mark a change from radial anisotropy characteristic of the oceanic Hikurangi Plateau to azimuthal anisotropy, with a fast azimuth subparallel to the direction of absolute plate motion. This is interpreted to be a result of olivine crystals realigning due to the finite shear strain generated by the differential motion between the lithosphere and the asthenosphere. Our results highlight the importance of wide-angle reflections in illuminating the fine structure at the base a tectonic plate, as the detection of a ~5 km thick azimuthally anisotropic layer would not be possible with passive-source methods, or controlled-source, near-vertical seismic reflections.
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