At convergent margins, the overriding plate responds to processes ranging from accretion to erosion in its rock record. Thus, the structural configurations of overthrusting plates are the products of long-term processes, for example frontal accretion,

The seismogenic (earthquake generating) portion of subduction megathrusts is flanked by regions of stable (aseismic) or conditionally stable fault slip behavior (Lay et al., 2012). The location of these transitions imposes key constraints on tsunami generation, the proximity of strong ground shaking to densely populated coastal communities, and the area, and thus magnitude, of earthquakes rupturing the seismogenic zone. Although frictional transitions are often attributed to thermal, metamorphic or diagenetic processes (Hyndman & Wang, 1993;Moore & Saffer, 2001; Oleskevich et al., 1999), depth dependent variations in the frequency-content, source duration and slip amplitude of earthquakes (Bilek & Lay, 1999;Lay et al., 2012) have recently been linked to variations in rigidity of the overthrusting plate (Sallarès & Ranero, 2019). Overthrusting plate structure played a key role in modulating the distribution of slip in the 2011 Mw 9.0 Tohoku-oki earthquake (Bassett et al., 2016), but there are also many examples of great earthquakes being stalled (Robinson et al., 2006), deflected (Kodaira et al., 2000 or stopped (Bilek, 2010) by subducting topographic relief. Ultimately, both factors are clearly important and at many subduction zones it is not agreed what properties cause some megathrust segments to lock up and accumulate large quantities of elastic strain, while adjacent segments slip with relative ease.

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.

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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.