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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. Plain Language Summary: The thickness of crust arriving at subduction zones exerts a major influence on the configuration, structure, and distribution of stresses at plate boundaries. A region of thickened oceanic crust, the Hikurangi Plateau, is subducting beneath the North Island of New Zealand. To understand the structure of this Plateau, we analyze seismic data along a 460 km long transect, which reveals five large seamounts, and a 3–4 km reduction crustal thickness between the southern and northern areas of the Plateau. Gravity data show this difference is persistent across the plateau and reveal structures suggesting the crust may have thinned when the Hikurangi plateau separated from a second plateau (Manihiki), which is thought to have formed at the same time. We propose that the subduction of thicker crustal along the southern Hikurangi margin may contribute to the shallow angle of the subducting plate, and high rates of uplift and deformation in the overthrusting plate. Key Points: Crustal thickness of the Hikurangi Plateau reduces by ∼30% between the southern (10–11 km) and northern (7–8 km) PlateauCoincident contrasts in plateau fabric may attribute the thickness contrast to rifting from the Manihiki PlateauThickness contrasts may impact the geometry of the subducting plate and rates of upper‐plate compression and uplift along the Hikurangi margin [ABSTRACT FROM AUTHOR]

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 (VP 5.0–7.4 km/s) is ∼1 km thicker (11 ± 1 km) and mantle velocities are ∼0.2 km/s higher (VP 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. Plain Language Summary: Some subduction zones produce the largest earthquakes and tsunami on Earth, while others slip freely. To understand what factors impact subduction zone slip behavior, we analyze seismic data along a 530 km long transect spanning a transition from strong (south Hikurangi) to weak (central/north Hikurangi) interseismic locking. From south‐to‐north, we find that seismic wavespeeds in the overthrusting plate undergo an abrupt (∼10%) reduction, which coincides with a reduction in seismic reflectivity. We show that these changes likely reflect differences in the offshore (updip) extent of basement rocks within the forearc crust, which we map using seismic data. These maps also show that the offshore extent of basement rocks is broadly correlated with the maximum depth of shallow slow‐slip events. We propose that geological architecture of the overthrusting plate may contribute to spatial variability in megathrust slip‐behavior, impacting both seismic and tsunami hazard along the Hikurangi margin. Key Points: Coincident reductions in seismic wavespeeds and lower‐crustal reflectivity occur from south‐to‐north along the Hikurangi forearcLegacy seismic reflection data attribute the forearc transition to along‐strike differences in the updip extent of the Torlesse BackstopShallow slow‐slip is focused updip of the backstop, suggesting it may impact the location of shallow frictional transitions at Hikurangi [ABSTRACT FROM AUTHOR]

Relationships between extensional tectonics and magmatism are ubiquitous in continental rifts and oceanic spreading centers. Yet few studies document interactions between extensional faults and mantle melts in volcanic arcs. We constrain the crustal structure of the extensional offshore Taupo Volcanic Zone (TVZ) from a marine multichannel and wide‐angle seismic experiment. The TVZ crust thins from >26 km to ∼18–19 km across ∼50 km in the Bay of Plenty. Elevated P wave velocities in the lower crust indicate mafic additions. Magmatic sills between 4‐ and 15‐km depth lie beneath listric normal faults in a ∼40‐km‐wide active rift zone. P wave velocities in the middle and upper crust along the arc front are ∼0.3–0.5 km/s slower than in the adjacent crust, indicating a possible thermal anomaly imparted by heat from magmatic intrusions. We propose that rifting in the offshore TVZ is partially compensated by intrusions and assisted by thermal weakening of the lithosphere. Plain Language Summary: The offshore Taupo Volcanic Zone is a rifting volcanic arc located in the North Island of New Zealand. We use seismic waves from man‐made sources to map seismic wave speeds and image fault and magma‐related structures beneath the offshore Taupo Volcanic Zone. We find that rifting has stretched the crust from >26 km to ∼18–19 km. Fast seismic wave speeds in the lower crust suggest that magma has partially filled some of the stretched crust to reduce the amount of thinning. Using seismic reflections, we find that the zone with the largest faults coincides with an area of intruded magma located between 4‐ and 15‐km depth. We propose that a feedback relationship influences the locations of faults and magma. Extension helps magma to rise from deep in the Earth. In response, the intruded magma heats and weakens the rigid outer layer of the earth to assist more rifting. The hazard potential of the fault and magma systems in this region is not fully known, motivating the need for further investigations to map and monitor magma bodies and faults in the offshore Taupo Volcanic Zone. Key Points: Magmatic intrusions and thermal weakening may facilitate rifting in the offshore Taupo Volcanic ZoneBay of Plenty crust rifted from >26 km to ∼18–19 km across a 50‐km‐wide zone; magmatic additions compensate ∼20–30% of crustal extensionListric faulting overlies an ∼40‐km‐wide zone of sill complexes and heterogeneous P wave velocities in the upper and middle crust [ABSTRACT FROM AUTHOR]

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 northcentral 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 FROM AUTHOR]