[S21-1] Lithospheric discontinuities I - LAB

[S21-1-03] Imaging the lithosphere - top to bottom - of the Hikurangi plateau as it subducts beneath North Island, New Zealand

^○Tim Stern¹, Stuart Henrys², Simon Lamb¹, David Okaya³, Brook Tozer⁴ (1.Victoria University of Wellington, Wellington, New Zealand, 2.GNS Science, Lower Hutt, New Zealand, 3.University of Southern California, Los Angeles, USA, 4.University of Oxford, Oxford, UK)

We present the first images, from active seismic source profiling, for both top and bottom of an oceanic plateau as it undergoes subduction. Active continental margins provide a useful platform from which to conduct seismic experiments to image subducting oceanic lithosphere. This is especially so if the subduction is at a low angle (5-20 degrees), as it is beneath North Island, New Zealand. Here we report results of the SAHKE experiment that was designed to image the locked section of the Hikurangi subduction zone , just north of Wellington city. The on-land section of the SAHKE profile was ~ 90 km long, along which 900 seismographs were distributed. 12 x 500 kg explosive charges were detonated in 50m deep bore holes. Structure for the top of the plate, at a depth of 15-30 km beneath the land surface was well imaged and features a suite of strong reflections that are interpreted to represent a ~ 5 km-thick sequence of low–wave speed sediments/and or serpentinites. We also recorded the Moho reflection from the subducted plate. The thickness of the oceanic crust is calculated to be 10.5 ± 1.5 km, which is about 50% greater than regular oceanic crust. This is significantly less than the ~ 35 km thickness of oceanic crust associated with the Ontong Java plateau, which the Hikurangi Plateau was once thought to be connected to.
Coherent reflections within the 8-20 Hz band were recorded at 27-32 s two-way travel time, which turn out to be from a double band of reflectors at depth of ~ 100 km that dip ~ 12 -15 degrees to the NW, parallel to the top of the plate. We interpret these reflections as marking a ~ 10-km-thick channel of ponded melt that defines the base of the mechanical boundary layer for the ~ 74 km thick plate. This is consistent with the LAB of a thin oceanic plateau, but a mid–lithospheric discontinuity from a thick, old, oceanic plate cannot be ruled out.