Description: Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (〈2 km) of well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust.

Description: The developing asymmetry of rifting and continental breakup to form rifted margins has been much debated, as has the formation, mechanics and role of extensional detachments. Bespoke 3D seismic reflection data across the Galicia margin, west of Spain, image in unprecedented detail an asymmetric detachment (the S reflector). Mapping S in 3D reveals its surface is corrugated, proving that the overlying crustal blocks slipped on S surface during the rifting. Crucially, the 3D data show that the corrugations on S perfectly match the corrugations observed on the present-day block-bounding faults, demonstrating that S is a composite surface, comprising the juxtaposed rotated roots of block-bounding faults as in a rolling hinge system with each new fault propagation moving rifting oceanward; changes in the orientation of the corrugations record the same oceanward migration. However, in contrast to previous rolling hinge models, the slip of the crustal blocks on S occurred at angles as low as ∼20°, requiring that S was unusually weak, consistent with the hydration of the underlying mantle by seawater ingress following the embrittlement of the entire crust. As the crust only becomes entirely brittle once thinned to ∼10 km, the asymmetric S detachment and the hyper-extension of the continental crust only developed late in the rifting process, which is consistent with the observed development of asymmetry between conjugate magma poor margin pairs. The 3D volume allows analysis of the heaves and along strike architecture of the normal faults, whose planes laterally die or spatially link together, implying overlaps in faults activity during hyper-extension. Our results thus reveal for the first time the 3D mechanics and timing of detachment faulting growth, the relationship between the detachment and the network of block-bounding faults above it and the key processes controlling the asymmetrical development of conjugate rifted margins. Highlights • The 3D seismic data provide unprecedented details of the mechanisms of breakup. • S detachment is corrugated and made of root zones of successive normal faults. • S rooted steeply but continued to slip at low-angle (down to 20°). • Extensional faulting migrated oceanwards by sets of faults active concurrently. • The asymmetric detachment developed as the crust became entirely brittle.

Description: A large, intermittently active, submarine landslide known as the Hilina slump has been interpreted along the south flank of Kilauea volcano. Seaward-dipping faults on land mark its headwall, and an offshore bench may define its uplifted toe. Geodetic data show that the entire south flank is also moving seaward, by a process referred to as volcanic spreading; this provides an alternative explanation for the bench, i.e., overthrusting along the edge of the sliding flank. The latter interpretation is consistent with new seismic reflection data across the submarine flank. A prominent reflection near the top of the oceanic plate suggests the decollement upon which the mobile flank slides. Landward-dipping reflections rise from this horizon and bound packages of bedded strata faulted and imbricated within the bench. The absence of correlative seaward-dipping faults and rotated strata on the upper flank suggests that the bench is not coupled to a slump. Moreover, kinematic reconstructions of the bench indicate that it has accommodated 15–24 km of displacement. This value is consistent with estimates for rift-zone extension but too high for shortening at the toe of a slump. We interpret the bench to result from overthrusting and accretion of volcaniclastic sediments to the edge of the mobile flank, and suggest that morphologic benches develop preferentially where landslide debris has accumulated near the base of the volcano and can be accreted to its sliding edifice.