Structural Variation Along the Southern Hikurangi Subduction Zone, Aotearoa New Zealand, From Seismic Reflection and Retro-Deformation Analysis

The southern Hikurangi subduction zone exhibits significant along-strike variation in convergence rate and obliquity, sediment thickness and, uniquely, the increasing proximity of southern Hikurangi to, and impingement on, the incoming continental Chatham Rise, an ancient Gondwana accretionary complex. There are corresponding changes in the morphology and structure of the Hikurangi accretionary prism. We combine widely spaced multichannel seismic reflection profiles with high resolution bathymetry and previous interpretations to characterize the structure and the history of the accretionary prism since 2 Ma. The southern Hikurangi margin can be divided into three segments. A northeastern segment (A) characterized by a moderately wide (∼70 km), low taper (∼5°) prism recording uninhibited outward growth in the last ∼1 Myr. Deformation resolvable in seismic reflection data accounts for ∼20 % of plate convergence, comparable with the central Hikurangi margin further North. A central segment (B) characterized by a narrow (∼30 km), moderate taper (∼8°) prism, with earlier (∼2-∼1 Ma) shortening than segment A. Outward prism growth ceased coincidentally with development of major strike-slip faults in the prism interior, reduced margin-normal convergence rate, and the onset of impingement on the incoming Chatham Rise to the south. A southwestern segment (C) marks the approximate southern termination of subduction but widens to ∼50 km due to rapid outward migration of the deformation front via fault reactivation within the now-underthrusting corner of the Chatham Rise. Segment C exhibits minimal shortening as margin-normal subduction velocity decreases and plate motion is increasingly taken up by interior thrusts and strike-slip faults.