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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]