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. Between the North and South Islands of New Zealand, the interaction of the Pacific and Australian tectonic plates is highly complex. Beneath the North Island, the Pacific plate moves westward and slides underneath the Australian plate at the Hikurangi subduction zone, but through the South Island, the two plates slide past each other along the Alpine Fault. The transition between these two zones affects tectonic patterns to the north and south, and the resulting earthquake and tsunami hazard. Using marine seismic reflection technology, we have built detailed cross‐sectional images of the shallowest ∼20 km of the structure of the Earth beneath the seabed, which record the history of plate interactions. By studying how the structure changes from northeast to southwest off the east coast of New Zealand, we can link these changes to tectonic processes. We suggest that changes in fault structure and activity relate to relatively low‐density rocks off the east coast of New Zealand. These lower density rocks were formed during a now extinct phase of tectonic activity, but in the present day they are impinging on the Hikurangi subduction zone. This changes the relative buoyancy of the two tectonic plates and inhibits subduction. The southern Hikurangi subduction margin lies within a subduction to transform transitionWe divide the region into three segments based on prism morphology, structure and tectonic shorteningMargin properties vary due to proximity of buoyant rocks on the incoming plate, decreasing subduction velocities, and strike‐slip faults The southern Hikurangi subduction margin lies within a subduction to transform transition We divide the region into three segments based on prism morphology, structure and tectonic shortening Margin properties vary due to proximity of buoyant rocks on the incoming plate, decreasing subduction velocities, and strike‐slip faults