The seismic behavior of subduction megathrusts varies spatially and is influenced by the properties of subducting plates, including their sedimentary cover. Characterizing these subduction inputs is essential for understanding the mechanisms behind fault slip variability. The Hikurangi subduction margin exhibits well-documented variations in fault slip ranging from large earthquakes to slow slip events and aseismic creeping. The variations coincide with along-margin changes in convergence rate, trench sediment thickness, and accretionary wedge development. Here, we integrate regional seismic reflection profiles with drill cores from the International Ocean Discovery Program Expeditions 372 and 375, and with other seafloor samples. We find that the Hikurangi Trough hosts some of Earth's thickest trench sediments, at up to 10 km, including pelagic sediments and a >6 km thick siliciclastic trench wedge that is younger than previously thought. Seismic imaging reveals that the shallow megathrust depth increases by 3.5-4.5 km southward along the margin in parallel with increasing trench sediment thickness despite a "step up" of the fault's stratigraphic position to a different assemblage of host rocks. In contrast to the irregular northern megathrust associated with subducting seamounts and heterogeneous volcaniclastic and pelagic host rocks, the southern megathrust is smooth, hosted in calcareous pelagic rocks, and straddles inherited polygonal faults. These along-strike variations in physical properties and megathrust geometry correspond to observed changes in fault slip behavior. Our findings provide new insights into the factors controlling fault slip variability, with implications for seismic hazard assessment at subduction zones worldwide and for understanding subduction margin evolution.

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 (VP <= 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 similar 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 (similar to 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.

Seamounts and basaltic basement can influence deformation and mass fluxes within subduction zones. We examined seamounts and volcanic units across the western Hikurangi Plateau, near the Hikurangi subduction margin, New Zealand, with seismic reflection images. Volcanism at the Hikurangi Plateau occurred in at least three phases that we attribute to (1) Early Cretaceous large igneous province formation, the top of which is marked by laterally continuous and dipping wedges of reflections that we interpret as lava flows; (2) Late Cretaceous seamounts and volcaniclastics that erupted onto the crust of the Hikurangi Plateau and make up the majority of seamount volume and basement relief; and (3) late-stage, Pliocene volcanics that erupted through and adjacent to Cretaceous seamounts and younger sediments of the north-central Hikurangi Plateau. The Pliocene volcanoes do not appear to be strongly welded to the plateau basement and may be petit spot volcanoes that are related to the displacement and accumulation of hydrous transition zone melts. Large seamounts and volcaniclastic units are evenly distributed across most of the Hikurangi Plateau near the Hikurangi margin but are absent from the Pegasus Basin. Although faults are imaged throughout the basement of the Pegasus Basin, contemporary normal faulting of the Hikurangi Plateau is uncommon, except for a zone of Quaternary normal faults near the Pliocene volcanics. These trends indicate that the Hikurangi megathrust may be more influenced by volcanic structures in the north and central Hikurangi margin, where plateau rifting and voluminous seamount eruptions have more substantially overprinted the original Early Cretaceous basement.

Abstract Fluid flow and pore‐pressure cycling are believed to control slow slip events (SSEs), such as those that frequently occur at the northern Hikurangi margin of New Zealand. To better understand fluid flow in the forearc system we examined the relationship between several physical properties of Cretaceous‐to‐Pliocene sedimentary rocks from the Raukumara peninsula. We found that the permeability of the deep wedge is too low to drain fluids, but fracturing increases permeability by orders of magnitude, making fracturing key for fluid flow. In weeks to months, plastic deformation, swelling, and possibly not‐yet‐identified mechanisms heal the fractures, restoring the initial permeability. We conclude that overpressures at the northern HM might partly dissipate during SSEs due to enhanced permeability near faults. However, in the months following an SSE, healing in the prism will lower permeability, forcing pore pressure to rise and a new SSE to occur.

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.

Most conceptual models for how fluids and sediment influence slip behavior and uplift along subduction margins are poorly constrained by geophysical observations. Given the complexity of subduction systems, overcoming this gap in knowledge will require a systems-level approach which uses high quality geophysical constraints. We present wide-angle, onshore-offshore seismic data collected along the northern Hikurangi margin, New Zealand, from which P-wave velocities were calculated using active- and passive-sources. A gravity model and reflection profiles were also assembled to create a complete, ~400 km long transect which images the incoming plate, down going slab, overthrusting forearc, and backarc rift. Velocities and gravity modelling help to constrain the lithology of the forearc basement to ~20 km depth. Upper plate lower crustal velocities and reflectivity point to the presence of underplated sediments immediately above the lithospheric mantle nose, suggesting that underplated sediments are driving uplift of the forearc. Comparing these results to geophysical images from the southern Hikurangi margin, we suggest that the backarc rift influences along-strike changes in the compressional stresses experienced by the forearc, driving changes in bending stresses within the subducting slab.

Abstract Marine multichannel and wide‐angle seismic data constrain the distribution of seamounts, sediment cover sequence and crustal structure along a 460 km margin‐parallel transect of the Hikurangi Plateau. Seismic reflection data reveals five seamount up‐to 4.5 km high and 35–75 km wide, with heterogeneous internal velocity structure. Sediment cover decreases south‐to‐north from ∼4.5 km to ∼1–2 km. The Hikurangi Plateau crust (VP 5.5–7.5 km/s) is 11 ± 1 km thick in the south, but thins by 3–4 km further north (∼7–8 km). Gravity models constructed along two seismic lines show the reduction in crustal thickness persists further east, coinciding with a bathymetric scarp. Gravity data suggest the transition in crustal thickness may reflect spatial variability in deformation and lithospheric extension associated with plateau breakup. Variability in the thickness of subducting crust may contribute to differences in megathrust geometry, upper‐plate stress state and high‐rates of contraction and uplift along the southern Hikurangi margin.

Marine multichannel and wide-angle seismic data constrain crustal structure along a 530 km margin-parallel transect of the Hikurangi subduction zone. The subducting Hikurangi Plateau crust (V-P 5.0-7.4 km/s) is similar to 1 km thicker (11 +/- 1 km) and mantle velocities are similar to 0.2 km/s higher (V-P 8.3-8.5 km/s) beneath south/central Hikurangi relative to north Hikurangi. In the overthrusting plate, an abrupt 0.5 km/s south-to-north reduction in forearc wavespeeds occurs in concert with a change in seismic reflection character. We analyze legacy seismic data to show that the forearc transition likely reflects lateral variability in the updip extent of the Torlesse Backstop. Furthermore, we map this unit along-strike and note a broad correlation between the backstop and down-dip extent of shallow slow-slip. We propose that the geological architecture of the overthrusting plate contributes to spatial variability in the location of shallow frictional transitions along the Hikurangi margin, impacting both seismic and tsunami hazard.

Subduction megathrusts exhibit a range of slip behaviors spanning from large earthquakes to aseismic creep, yet what controls spatial variations in the dominant slip mechanism remains unresolved. We present multichannel seismic images that reveal a correlation between the lithologic homogeneity of the megathrust and its slip behavior at a subduction zone that is world renowned for its lateral slip behavior transition, the Hikurangi margin. Where the megathrust exhibits shallow slow-slip in the central Hikurangi margin, the protolith of the megathrust changes ~10 km downdip of the deformation front, transitioning from pelagic carbonates to compositionally heterogeneous volcaniclastics. At the locked southern Hikurangi segment, the megathrust forms consistently within pelagic carbonates above thickened nonvolcanic siliciclastic sediments (unit MES), which subduct beyond 75 km horizontally. The presence of the MES layer plays a key role in smoothing over rough volcanic topography and establishing a uniform spatial distribution of lithologies and frictional properties that may enable large earthquake ruptures.

&lt;p&gt;Throughout Earth&amp;#8217;s history, the movement, suturing, and rifting of tectonic plates in the Wilson cycle often takes advantage of lithospheric weaknesses and pre-existing plate boundaries. Continental rifting and subduction initiation represent arduous phases of this cycle for plate divergence and convergence, respectively, where strain is not yet focused into a narrow and mature plate boundary. Here we present an analysis of the Puysegur margin to demonstrate how past tectonic regimes create inherited lithospheric structures that facilitate subsequent stages of the Wilson cycle.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;The Puysegur margin is a young subduction zone and forms the northern segment of the Australian-Pacific plate boundary south of New Zealand, which has evolved from divergence to strike-slip and recently to oblique convergence, all in the last ~45 million years. Magnetic anomalies and curved fracture zones located south of the Puysegur segment show the divergent phase involved seafloor spreading and the formation of new oceanic lithosphere. However, these features are not present in the upper Pacific plate at the latitudes of the Puysegur margin, and the lack of quality seismic images in this region hampered our understanding of the local crustal structure, which was assumed to be a northward extension of the oceanic domain. A deep penetrating multichannel reflection (MCS) and ocean-bottom seismometer (OBS) dataset was acquired in 2018 with the R/V Langseth and provided new high-quality seismic images of the crustal structure along the Puysegur margin.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Our seismic images reveal that the overriding Pacific plate contains stretched continental crust with magmatic intrusions, which formed from rifting between Zealandia continental plateaus during AUS-PAC plate divergence. This stretching phase was highly asymmetric and resulted in the opening of the Solander Basin. Rifting was more advanced to the south, yet never proceeded to breakup and seafloor spreading as previously thought. A new southern continent-ocean transition is inferred from potential field data, marking the boundary between stretched continental crust and new oceanic crust formed during the extensional phase.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Along-strike heterogeneity with mixed continental and oceanic domains and asymmetric rift architecture along the Puysegur margin were critical features for following tectonic regimes. Increasingly oblique plate motions sparked strike-slip motion, which localized near the pre-existing spreading center in the south, but along the western edge of the rift zone in relatively unstretched crust at the Puysegur margin in the north. Translational motion juxtaposed weak ~10 Myr old oceanic lithosphere with buoyant continental crust across the strike-slip boundary. Incipient subduction transpired as oceanic lithosphere from the south forcibly underthrust continent lithosphere at an oblique collision zone.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;We suggest that subduction initiation at the Puysegur Trench was enabled by inherited buoyancy contrasts and structural weaknesses that were imprinted into the lithosphere during earlier phases of continental rifting and strike-slip along the plate boundary. In the global evolution of plate tectonics, strike-slip might be the key component to achieving the Wilson cycle, as it is the most efficient mechanism to offset terranes and juxtapose lithospheric domains of contrasting properties across broad regions, thus generating advantageous conditions for subduction initiation and subsequent closure of oceanic basins.&lt;/p&gt;

SUMMARY The southern boundary of the Cayman Trough in the Caribbean is marked by the Swan Islands transform fault (SITF), which also represents the ocean–continent transition of the Honduras continental margin. This is one of the few places globally where a transform continental margin is currently active. The CAYSEIS experiment acquired an ∼165-km-long seismic refraction and gravity profile (P01) running across this transform margin, and along the ridge-axis of the Mid-Cayman Spreading Centre (MCSC) to the north. This profile reveals not only the crustal structure of an actively evolving transform continental margin, that juxtaposes Mesozoic-age continental crust to the south against zero-age ultraslow spread oceanic crust to the north, but also the nature of the crust and uppermost mantle beneath the ridge-transform intersection (RTI). The traveltimes of arrivals recorded by ocean-bottom seismographs (OBSs) deployed along-profile have been inverse and forward modelled, in combination with gravity modelling, to reveal an ∼25-km-thick continental crust that has been continuously thinned over a distance of ∼65 km to ∼10 km adjacent to the SITF, where it is juxtaposed against ∼3–4-km-thick oceanic crust. This thinning is primarily accommodated within the lower crust. Since Moho reflections are only sparsely observed, and, even then, only by a few OBSs located on the continental margin, the 7.5 km s–1 velocity contour is used as a proxy to locate the crust–mantle boundary along-profile. Along the MCSC, the crust–mantle boundary appears to be a transition zone, at least at the seismic wavelengths used for CAYSEIS data acquisition. Although the traveltime inversion only directly constrains the upper crust at the SITF, gravity modelling suggests that it is underlain by a higher density (>3000 kg m–3) region spanning the width (∼15 km) of its bathymetric expression, that may reflect a broad region of metasomatism, mantle hydration or melt-depleted lithospheric mantle. At the MCSC ridge-axis to the north, the oceanic crust appears to be forming in zones, where each zone is defined by the volume of its magma supply. The ridge tip adjacent to the SITF is currently in a magma rich phase of accretion. However, there is no evidence for melt leakage into the transform zone. The width and crustal structure of the SITF suggests its motion is currently predominantly orthogonal to spreading. Comparison to CAYSEIS Profile P04, located to the west and running across-margin and through 10 Ma MCSC oceanic crust, suggests that, at about this time, motion along the SITF had a left-lateral transtensional component, that accounts for its apparently broad seabed appearance westwards.

Subduction zones are fundamental to plate tectonics, yet how they initiate remains enigmatic. Geodynamic models suggest that if horizontal forces dominate, the upper plate experiences compression and uplift followed by extension and subsidence, whereas vertically forced subduction involves only extension. Geologic evidence of past subduction initiation events has been interpreted in terms of these alternatives; however, it is unclear whether they are mutually exclusive or represent different stages of early subduction. Here, we present seismic images of the Puysegur plate boundary south of New Zealand that reveal space–time relations of stress during subduction initiation. Our data show evidence for a stress transition (compression followed by extension) that spread from north to south as the trench nucleated and propagated along the plate boundary. Both the magnitude and duration of compression diminish from north (8 Myr) to south (5 Myr). This indicates that transition to self-sustaining subduction accelerates after nucleation of a downgoing slab increases driving forces and decreases fault strength near the propagating tipline of the nascent trench. Instead of horizontally forced versus vertically forced initiation, we propose a four-dimensional evolution, where horizontal forces initially dominate at the site of nucleation, but with time, vertical forces accelerate, propagate along strike and facilitate the development of self-sustaining subduction.

Critical ingredients and conditions necessary to initiate a new subduction zone are debated. General agreement is that subduction initiation likely takes advantage of previously weakened lithosphere and may prefer to nucleate along pre-existing plate boundaries. To evaluate how past tectonic regimes and lithospheric structures might facilitate underthrusting and lead to self-sustaining subduction, we present an analysis of the Puysegur Margin, a young subduction zone with a rapidly evolving tectonic history. The Puysegur Margin, south of New Zealand, currently accommodates convergence between the Australian and Pacific plates, exhibits an active seismic Benioff zone, a deep ocean trench, and young adakitic volcanism on the overriding plate. Tectonic plate reconstructions show that the margin experienced a complicated transformation from rifting to seafloor spreading, to strike-slip motion, and most recently to incipient subduction, all in the last ~45 million years. Details of this tectonic record remained incomplete due to the lack of high-quality seismic data throughout much of the margin. Here we present seismic images from the South Island Subduction Initiation Experiment (SISIE) which surveyed the Puysegur region February-March, 2018. SISIE acquired 1252 km of deep-penetrating multichannel seismic (MCS) data on 7 transects, including 2 regional dip lines coincident with Ocean Bottom Seismometers (OBS) deployments which extend (west to east) from the incoming Australian plate, across the Puysegur Trench and Puysegur Ridge, over the Solander Basin and onto the continental Campbell Plateau margin. We integrate pre-stack depth migrated MCS profiles with OBS tomography models to constrain the tectonic development of the Puysegur Margin. Based on our results we propose the following Cenozoic evolution: (1) The entire Solander Basin contains thinned continental crust which formed from orthogonal stretching between the Campbell and Challenger plateaus during the Eocene-Oligocene. This phase of rifting was more pronounced to the south, producing thinner crust with abundant syn-rift volcanism across a wider rift-basin, in contrast to the relatively thicker crust, moderate syn-rift volcanism and narrower rift basin in the north. (2) Strike-slip deformation subsequently developed along Puysegur Ridge, west of the locus of rifting and within relatively unstretched continental lithosphere. This young strike-slip plate boundary translated unstretched crust northward causing an oblique continent-collision zone, which led to a transpressional pattern of distributed left-stepping, right-lateral faults. (3) Subduction initiation was aided by large density contrasts as oceanic lithosphere translated from the south was forcibly underthrust beneath the continent-collision zone. Early development of oblique subduction generated modest and widespread reactivation of faults in the upper plate. (4) Present-day, the Puysegur Trench shows a spatiotemporal transition from nearly mature subduction in the north to a recently initiated stage along the southernmost margin, requiring a southward propagation of subduction through time. Our new seismic images suggest subduction initiation at the Puysegur Margin was assisted by inherited buoyancy contrasts and structural weaknesses that were imprinted into the lithosphere during earlier phases of continental rifting and strike-slip along the developing plate boundary. The Puysegur Margin demonstrates that forced nucleation along a strike-slip boundary is a viable subduction initiation model and should be considered throughout Earth’s history.

Seismic anisotropy measurements show that upper mantle hydration at the Middle America Trench (MAT) is limited to serpentinization and/or water in fault zones, rather than distributed uniformly. Subduction of hydrated oceanic lithosphere recycles water back into the deep mantle, drives arc volcanism, and affects seismicity at subduction zones. Constraining the extent of upper mantle hydration is an important part of understanding many fundamental processes on Earth. Substantially reduced seismic velocities in tomography suggest that outer rise plate‐bending faults provide a pathway for seawater to rehydrate the slab mantle just prior to subduction. Estimates of outer‐rise hydration based on tomograms vary significantly, with some large enough to imply that, globally, subduction has consumed more than two oceans worth of water during the Phanerozoic. We found that, while the mean upper mantle wavespeed is reduced at the MAT outer rise, the amplitude and orientation of inherited anisotropy are preserved at depths >1 km below the Moho. At shallower depths, relict anisotropy is replaced by slowing in the fault‐normal direction. These observations are incompatible with pervasive hydration but consistent with models of wave propagation through serpentinized fault zones that thin to <100‐m in width at depths >1 km below Moho. Confining hydration to fault zones reduces water storage estimates for the MAT upper mantle from ∼3.5 wt% to <0.9 wt% H 2 0. Since the intermediate thermal structure in the ∼24 Myr‐old MAT slab favors serpentinization, limited hydration suggests that fault mechanics are the limiting factor, not temperatures. Subducting mantle may be similarly dry globally.

About 25% of the Earth’s mid-ocean ridges spread at ultraslow rates of less than 20 mm/yr. However, most of these ultraslow spreading ridges are located in geographically remote areas, which hamper investigation. Consequently, how the crust forms and ages at such spreading centres, which traditional models predict to be magma-starved and cold, remains poorly understood. CAYSEIS project was proposed to survey the Cayman Trough area in order to obtain new data that constraints the nature of the crust, tectonic structures, lithologies outcropping and hydrothermal processes taking place in this area, which includes the Mid Cayman ultra-slow spreading centre (MSCS) with spreading rates of ~15-17 mm/yr. Understanding the sub-seabed geophysical structure of the MCSC is key to understanding not only the lithologies and structures exposed at the seabed, but more fundamentally, how they are related at depth and what role hydrothermal fluid flow plays in the geodynamics of ultraslow spreading. CAYSEIS was a joint and multidisciplinary programme of German, British and US American top tier scientists designed for the obtaining of a new high-quality dataset, including 3D Wide-Angle Seismic (WAS), magnetic, gravimetric and seismological data. We took leverage of the CAYSEIS dataset to invert a 3D velocity model of the Cayman Trough lithosphere using the Tomo3D code (Meléndez et al., 2015; 2019). This is one of the first times that the Tomo3D code is used for 3D inversion of real datasets. Thus, we are checking our results comparing them with travel time tomographic inversions of 2D lines and testing the different parameters to obtain the more accurate and higher resolution model as possible. The results of this experiment show not only the lithospheric structure along and across the MSCS, including the exhumed Ocean Core Complexes in the surrounding areas, but the 3D lithospheric configuration of the region which is important to understand the crustal formation processes and evolution of ultra-slow spreading settings.

Exploring the structure of convergent margins is key to understanding megathrust slip behavior and tsunami generation. We present new wide‐angle and marine multichannel seismic data that constrain the crustal structure and accretion dynamics of the northern Hikurangi margin. The top of the basement of the Hikurangi Plateau is overlain by a rough, 2–3 km thick layer of volcanic cover with P‐wave velocities (VP) between 3 and 5 km/s. This volcanic cover contributes significantly to seismic reflectivity beneath the shallow subduction plate boundary. The frontal prism structure varies along‐strike from ∼25 km wide with imbricate thrust faults where accretion of trench sediments is undisrupted, to narrower (∼14 km) with slumps and branching, irregular thrust fault geometries, which may reflect lower sediment supply or past seamount collisions. A large thrust fault network in the inner prism with a seismically fast hanging wall indicates a mechanical boundary between a seismically faster deforming backstop and the seismically slower frontal prism. Near the coastline, VP increases between 2.8 and 4 km/s at 2–8 km depth and is 0.5–1.7 km/s slower than the southern Hikurangi margin. Low seismic wavespeeds and low vertical velocity gradients in the inner prism support the hypothesis that a weak overthrusting plate contributes to historic tsunami‐earthquakes and long duration seismic ground motion.

Subduction zones are fundamental to Earth’s plate tectonic history yet details of how they initiate remain enigmatic. Geodynamic models suggest that early stages of subduction depend on whether underthrusting is driven by horizontal or vertical forces. If horizontal forces dominate, the upper plate experiences compression and uplift followed by extension and subsidence, whereas vertically-forced subduction involves only extension. Geologic evidence from the Izu-Bonin-Mariana forearc supports a ~1 Myr rapid transition, whereas observations from Oman indicate a >8 Myr time lag between initial underthrusting and the onset of upper plate extension. We present seismic images of the incipient Puysegur subduction zone south of New Zealand. Our data show evidence for a stress signal (compression followed by extension) that spread from north to south as the trench initiated and propagated along the plate boundary. Both the magnitude and duration of the compressional phase diminish from ~8 Myrs long in the north to ~5 Myrs in the south. This timing indicates that the transition to self-sustaining subduction is more rapid when an adjacent downgoing slab contributes a driving force that aids subduction initiation. We therefore argue for a new framework in which horizontal forces dominate at sites of subduction nucleation and vertical forces gradually strengthen during later propagation as the developing plate boundary weakens and the slab-pull force intensifies. Our findings corroborate evidence for ancient horizontally-forced subduction initiation events and suggest that the geologic record may be biased, since vertically-forced scenarios of subduction propagation are more likely to be preserved than destructive subduction nucleation events.

Solander Basin is characterized by subduction initiation at the Pacific‐Australia plate boundary, where high biological productivity is found at the northern edge of the Antarctic Circumpolar Current. Sedimentary architecture results from tectonic influences on accommodation space, sediment supply and ocean currents (via physiography); and climate influence on ocean currents and biological productivity. We present the first seismic‐stratigraphic analysis of Solander Basin based on high‐fold seismic‐reflection data (voyage MGL1803, SISIE). Solander Trough physiography formed by Eocene rifting, but basinal strata are mostly younger than ca. 17 Ma, when we infer Puysegur Ridge formed and sheltered Solander Basin from bottom currents, and mountain growth onshore increased sediment supply. Initial inversion on the Tauru Fault started at ca. 15 Ma, but reverse faulting from 12 to ca. 8 Ma on both the Tauru and Parara Faults was likely associated with reorganization and formation of the subduction thrust. The new seabed topography forced sediment pathways to become channelized at low points or antecedent gorges. Since 5 Ma, southern Puysegur Ridge and Fiordland mountains spread out towards the east and Solander Anticline grew in response to ongoing subduction and growth of a slab. Solander Basin had high sedimentation rates because (1) it is sheltered from bottom currents by Puysegur Ridge; and (2) it has a mountainous land area that supplies sediment to its northern end. Sedimentary architecture is asymmetric due to the Subtropical Front, which moves pelagic and hemi‐pelagic sediment, including dilute parts of gravity flows, eastward and accretes contourites to the shelf south of Stewart Island. Levees, scours, drifts and ridges of folded sediment characterize western Solander Basin, whereas hemi‐pelagic drape and secondary gravity flows are found east of the meandering axial Solander Channel. The high‐resolution record of climate and tectonics that Solander Basin contains may yield excellent sites for future scientific ocean drilling.