3-D Velocity Structure of the Northern Hikurangi Margin: Implications for Crustal Growth
Authors:   , Bassett Daniel Graham 
Published: November 2021

<p>Onshore-offshore wide-angle seismic reflection and refraction data are combined with multi-channel seismic reflection profiles to model crustal structure within a 32,000 km2 block of the northern Hikurangi subduction margin, New Zealand. A 3-D onshore-offshore dataset comprising ~250,000 seismic traces is analysed as a suite of 81 receiver-gathers. These receiver-gathers contain reflected and refracted arrivals and constrain a 3-D P-wave velocity forward model. Southwest along the strike of the subduction zone, velocity forward models resolve a > 10 km increase in Moho depth and a > 20 km increase in basement thickness between Raukumara Basin and the Raukumara Peninsula. Beneath the topographic crest of the East Cape Ridge and Raukumara Peninsula, low velocities (3.0-5.0 km/sec) are resolved within a triangular prism (> 200 km3 /km-1 ie per km of strike) positioned on top of the subducting slab. The arcward/inboard edge of this prism coincides with the intersection of the subducting slab with the Moho and southward along East Cape Ridge, both the prism and topographic crest are observed migrating arcward, concomitant with the increase in Moho depth beneath the continental shelf. Second-order seismic-stratigraphic sequences within Raukumara Basin record a southeast-northwest migration of the Neogene depocenter. Internal deformation increases east and south of the central basin at East Cape Ridge and within the continental shelf where strata tilt up toward the topographic crest. Refracted arrivals place the first controlled-source seismic constraints on the thickness of the about-to-be subducted northern Hikurangi Plateau. The Hikurangi Plateau crust is modelled as two layers with velocities of 4.9-6.3 km/sec and 7.1-7.3 km/sec with a combined thickness of 17-18 km. The upper 10 km of the subducting mantle lithosphere is modelled with velocities of 8.1-8.4 km/sec. The geometry of the plate interface is also constrained by wide-angle reflections. Low velocities (3.0-5.0 km/sec) beneath the topographic crest of the East Cape Ridge and the Raukumara Peninsula are interpreted as underplated sedimentary and crustal material and a rate of accretion of 10-25 km3 Myr-1 km-1 is calculated. The migration of the Neogene Raukumara Basin depocenter and what is considered postdepositional internal deformation are associated with uplift and protuberant growth at East Cape Ridge and Raukumara Peninsula in response to lower crustal underplating. The underplating process appears modulated by Moho depth and a cyclical crustal dynamic is proposed to provide a viable means of maintaining or increasing the netvolume of the forearc in the presence of trench slope collapse and subduction erosion. At present, a discrepancy exists between published estimates of continental creation at volcanic arcs and continental destruction at subduction margins. Nd isotopic evidence and continental freeboard arguments require a constant volume of continental crust throughout the Phanerozoic. The crustal dynamic developed in this thesis highlights the importance of lower crustal underplating as a means of retaining subducted sediment within the forearc, thus reducing the flux of continental material to the mantle. The calculated rates at which subducted sediment is underplated and accreted to the upper plate (10-25 km3 Myr-1 km-1) are considered sufficient to solve the discrepancy between the inputs and outputs at subduction margins within analytical uncertainty.</p>