Fore-arc deformation and underplating at the northern Hikurangi margin, New Zealand

Geophysical investigations of the northern Hikurangi subduction zone northeast of New Zealand, image fore-arc and surrounding upper lithospheric structures. A seismic velocity (Vp) field is determined from seismic wide-angle data, and our structural interpretation is supported by multichannel seismic reflection stratigraphy and gravity and magnetic modeling. We found that the subducting Hikurangi Plateau carries about 2 km of sediments above a 2 km mixed layer of volcaniclastics, limestone, and chert. The upper plateau crust is characterized by Vp = 4.9-6.7 km/s overlying the lower crust with Vp > 7.1 km/s. Gravity modeling yields a plateau thickness around 10 km. The reactivated Raukumara fore-arc basin is > 10 km deep, deposited on 5-10 km thick Australian crust. The fore-arc mantle of Vp > 8 km/s appears unaffected by subduction hydration processes. The East Cape Ridge fore-arc high is underlain by a 3.5 km deep strongly magnetic (3.3 A/m) high-velocity zone, interpreted as part of the onshore Matakaoa volcanic allochthon and/or uplifted Raukumara Basin basement of probable oceanic crustal origin. Beneath the trench slope, we interpret low-seismic-velocity, high-attenuation, low-density fore-arc material as accreted and recycled, suggesting that underplating and uplift destabilizes East Cape Ridge, triggering two-sided mass wasting. Mass balance calculations indicate that the proposed accreted and recycled material represents 25-100% of all incoming sediment, and any remainder could be accounted for through erosion of older accreted material into surrounding basins. We suggest that continental mass flux into the mantle at subduction zones may be significantly overestimated because crustal underplating beneath fore-arc highs have not properly been accounted for.