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1. Seismic images of the accretionary prism of the Hikurangi margin, New Zealand, reveal its structural variations and influences on seismogenic character

   Van_Avendonk, H; Gase, A; Bangs, N (December 2022, AGU)

   The Hikurangi margin of New Zealand exhibits contrasting slip behavior from south to north. Whereas the southern Hikurangi margin has a locked plate boundary that can potentially produce large megathrust earthquakes, the northern section of this margin accommodates plate motion by creep and episodic shallow slow-slip events. To investigate these different modes of slip we examine the geometry of the plate boundary and consolidation state of the materials along the plate interface. We use marine seismic reflection data from the SHIRE project to image the reflectivity and seismic velocity structure along 20 profiles across the accretionary wedge of the Hikurangi subduction zone of New Zealand. These active-source seismic data were gathered in 2017 with the R/V Marcus Langseth using a 6,600 in3 seismic source and 12 km long receiver array. We carried out streamer tomography on the SHIRE profiles where we integrated seismic velocity constraints from stacking the reflection data along all SHIRE transects. The seismic velocity images and prestack depth migrations together characterize the nature of incoming basement, sediment subduction and accretion, and faulting and compaction of the accretionary wedge. Our seismic velocity models show that a layer of sediment,with seismic wavespeeds of ~3.0 km/s, is entrained beneath the accretionary prism in the southern Hikurangi margin, but there is no coherent subducted sediment layer to the north. This is a significant result, because it implies that the sediment layer covers basement roughness and forms a smoother plate boundary in the south. In addition, the deepest sediments on the incoming plate in the southern Hikurangi margin are believed to be quartz-rich turbidites, which are prone to unstable slip along the plate boundary. In contrast, the accretionary prism of the northern Hikurangi margin exhibits more variation in accretionary wedge thrust geometry due to interactions with large seamounts on the downgoing oceanic basement. These findings are consistent with the geodetically locked nature of a smooth, quartz-rich plate boundary along the southern Hikurangi subduction zone, and the creeping nature of a heterogeneous plate boundary along the Hikurangi margin to the north. 

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

   https://doi.org/10.1029/2023TC008212  

   Stevens, D E; McNeill, L C; Henstock, T J; Barnes, P M; Crutchley, G; Bangs, N; Henrys, S; Van_Avendonk, H_J A (July 2024, Tectonics)

   Abstract 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. 

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3. Insights into the style of deformation in the Hikurangi subduction zone from rock physics measurements: elastic properties, permeability, and fracture healing

   Tisato, N; Bland, C; Van_Avendonk, H; Gase, A; Bangs, N (December 2022, AGU)

   The Hikurangi subduction zone exhibits a north-to-south variation in deformation style. The plate interface in the south is locked, and megathrust earthquakes could accommodate the long-term plate convergence. In contrast, the northern megathrust regularly experiences shallow slow-slip events possibly extending into the thrust faults of the sedimentary prism. Understanding such a difference could reveal slip behavior and seismic cycle controls and help earthquake forecasting globally. One hypothesis is that the prism rock properties and fluid pressures affect these different slip behaviors. To test such a hypothesis, we measured the physical properties of rocks from the northern Hikurangi margin, focusing on ultrasonic elastic properties, permeability, and fracture healing. Such lithologies are equivalent to rocks buried to a few km depths within the accretionary prism. We found that all rocks contain >18 vol% of clay minerals. The hydraulic permeability of rock samples that are proxies for the deep part of the prism (i.e., 5 to 10 km depth) is three to four orders of magnitude lower than the values estimated by different authors for the prism as a whole. The results suggest that active faults and fractures in the accretionary prism must play a key role in draining fluids from the base of the prism and potentially from the subducting plate. Tests on a fractured sample show that fractures heal in tens of days, and permeability decreases over a short period relative to slip cycles of just a few weeks. Microphotography and micro-CT images suggest that healing is achieved by clay expansion. The observed healing could be underestimated as achieved under high confining pressure (up to 200 MPa) but at room temperature and humidity. We conclude that slow slip events in the northern Hikurangi margin may have a critical role in briefly increasing permeability at the beginning of the slip cycle, thus regulating pore pressure in the prism and allowing drainage. 

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4. Crustal Structure of the Hikurangi Subduction Zone Revealed by Four Decades of Onshore‐Offshore Seismic Data: Implications for the Dimensions and Slip Behavior of the Seismogenic Zone

   https://doi.org/10.1029/2024JB030268  

   Bassett, Dan; Henrys, Stuart; Tozer, Brook; van_Avendonk, Harm; Gase, Andrew; Bangs, Nathan; Kodaira, Shuichi; Okaya, David; Jacobs, Katie; Sutherland, Rupert; et al (January 2025, Journal of Geophysical Research: Solid Earth)

   NA (Ed.)

   Abstract Four decades of seismic reflection, onshore‐offshore and ocean‐bottom seismic data are integrated to constrain a high‐resolution 3‐D P‐wave velocity model of the Hikurangi subduction zone. Our model shows wavespeeds in the offshore forearc to be 0.5–1 km/s higher in south Hikurangi than in the central and northern segments (V_P ≤ 4.5 km/s). Correlation with onshore geology and seismic reflection data sets suggest wavespeed variability in the overthrusting plate reflects the spatial distribution of Late Jurassic basement terranes. The crustal backstop is 25–35 km from the deformation front in south Hikurangi, but this distance abruptly increases to ∼105 km near Cape Turnagain. This change in backstop position coincides with the southern extent of shallow slow‐slip, most of which occurs updip of the backstop along the central and northern margin. These relationships suggest the crustal backstop may impact the down‐dip extent of shallow conditional stability on the megathrust and imply a high likelihood of near/trench‐breaching rupture in south Hikurangi. North of Cape Turnagain, the more landward position of the backstop, in conjunction with a possible reduction in the depth of the brittle ductile transition, reduces the down‐dip width of frictional locking between the southern (∼100 km) and central Hikurangi margin by up‐to 50%. Abrupt transitions in overthrusting plate structure are resolved near Cook Strait, Gisborne and across the northern Raukumara Peninsula, and appear related to tectonic inheritance and the evolution of the Hikurangi margin. Extremely low forearc wavespeeds resolved north of Gisborne played a key role in producing long durations of long‐period earthquake ground motions. 

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5. How Subduction Margin Processes and Properties Influence the Hikurangi Subduction Zone

   https://doi.org/10.1146/annurev-earth-040523-115520  

   Henrys, Stuart; Bassett, Dan; Ellis, Susan; Wallace, Laura; Barnes, Philip M; Eberhart-Phillips, Donna; Saffer, Demian; Boulton, Carolyn (May 2025, Annual Review of Earth and Planetary Sciences)

   The Hikurangi margin has been an important global focus for subduction zone research for the last decade. International Ocean Discovery Program drilling and geophysical investigations have advanced our understanding of megathrust slip behavior. Along and across the margin, detailed imaging reveals that the megathrust structure varies spatially and evolves over time. Heterogeneous properties of the plate boundary zone and overriding plate are impacted by the evolving nature of regional tectonics and inherited overriding plate structure. Along-strike variability in thickness of subducting sediment and northward increasing influence of seamount subduction strongly influence mega-thrust lithologies, fluid pressure, and permeability structure. Together, these exert strong control on spatial variations in coupling, slow slip, and seismicity distribution. Thicker incoming sediment, combined with a compressional upper plate, influences deeper coupling at southern Hikurangi, where paleoseismic investigations reveal recurring great (M_w> 8.0) earthquakes.▪The Hikurangi Subduction Zone is marked by large-scale changes in the subducting Pacific Plate and the overlying plate, with varied tectonic stress, crustal thickness, and sediment cover.▪The roughness of the lower plate influences the variability in megathrust slip behavior, particularly where seamounts enhance subduction of fluid-rich sediments.▪Variations in sediment composition impact the strength of the subduction interface, with the southern Hikurangi Subduction Zone exhibiting a more uniform megathrust fault.▪Properties of the upper plate influence fluid pressures and contribute to the observed along-strike variations in Hikurangi plate coupling and slip behavior. 

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