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The International Ocean Discovery Program cored five sites during Expeditions 372 and 375 to investigate the Hikurangi margin, offshore Poverty Bay, North Island, New Zealand. Trench‐wedge deposits were recovered at Site U1520, and the frontal accretionary prism was cored at Site U1518. Based on X‐ray diffraction analyses of the clay‐sized fraction, illite and smectite are the dominant clay minerals. Compositional scatter is unusually large, and a well‐defined mixing trend is evident between illite‐rich and smectite‐rich end members. The proportion of smectite ranges from 9 to 59 wt.% at Site U1518 and from 12 to 62 wt.% at Site U1520. Nearby slope deposits at Sites U1517 and U1519 are more homogeneous, with consistently higher proportions of smectite. The East Cape Current is probably responsible for transporting smectite‐rich suspensions towards the south‐west from sources that include the Taupo Volcanic Zone. Illite‐rich muds require different sources and different routing paths to the trench. Some suspended sediment probably originated in the Waipaoa watershed on the North Island, entering the trench via Māhia Canyon. Other gravity flows probably started in Kaikōura Canyon or Cook Strait and moved towards the north‐east, down the trench's axial channel. The influence of turbidity currents on clay composition changes with water depth, as do the effects of reworking by contour currents and the role of passive hemipelagic settling. The broad continuum of clay compositions in the trench is enhanced by variable amounts of mixing among multiple ocean currents and fluctuating durations of suspension within a bottom nepheloid layer. That complexity deviates from the paradigm of turbidite‐dominated trench wedges. The margin's compositional diversity also complicates the goal of predicting or modelling three‐dimensional variations in frictional, geotechnical and hydrogeological properties. Those lithological properties are important because they affect slope stability along strike and changes in fault‐slip behaviour along the subduction interface. [ABSTRACT FROM AUTHOR]

Bottom simulating reflections (BSRs) interpreted in seismic data are one of the most used indicators for the presence of gas hydrates. In numerous hydrate provinces, including the Hikurangi margin, east of New Zealand, distinct and anomalous gaps in reflectivity punctuate otherwise continuous BSRs. We undertake a seismic stratigraphic and structural interpretation of a dense grid of two‐dimensional reflection seismic data to investigate possible causes of widespread BSR gaps that occur near the hinge area of synclines. We explain these BSR gaps with a tectono‐sedimentary model where sedimentation into accretionary wedge synclines leads to an upward migration of the base of gas hydrate stability with respect to stratigraphy, causing dissociation of gas hydrates to water and free gas. A trough‐shaped syncline's radially dipping beds promotes along‐strata, upward migration of gas away from the hinge area, incrementally depleting synclinal hinges of gas and resulting in a BSR gap or muted BSR. This model may be applicable to observed gaps in BSRs in tectonically active slope basins worldwide. [ABSTRACT FROM AUTHOR]

Characterization of the porosity evolution across the sedimentary section entering subduction zones and accreted sediments provide valuable information for understanding the deformation history at accretionary margins and the physico‐chemical processes in and around fault zones. International Ocean Discovery Program Expeditions 372 and 375 drilled, logged, and cored the reference section on the incoming plate (Site U1520) and across the active Pāpaku thrust (Site U1518), which is a <30° westward‐dipping splay fault in the frontal accretionary wedge at the north Hikurangi margin in a region where tsunami earthquakes and recurrent slow slip events are documented. We observe strong variations of physical properties across the thrust fault. In particular, interstitial porosity increases by ∼10% through the fault zone while it exponentially decreases with depth, showing lower values in the hanging‐wall (HW; on average, ∼36%) than in the footwall (∼42%). Based on comparisons of porosity with the compaction curve at Site U1518 with that of reference Site U1520, we infer an overcompaction in the HW and a nearly normal compaction in the footwall of the Pāpaku thrust. We suggest that the porosity pattern across the thrust infers differences in the maximum burial depth of sediments, characterized by an overcompacted HW that has been uplifted, thrusted, and concomitantly tectonically eroded above the footwall. Porosity data indicate hydrostatic conditions around the thrust fault contrasting with commonly assumed excess pore pressure at the plate interface. Key Points: Strong contrasts of physical properties including porosity occur across the Pāpaku thrust fault at IODP Site U1518The hanging‐wall (HW) is overcompacted and shows lower interstitial porosity and higher P‐wave velocity and resistivity than the footwallPorosity suggests differences in maximum burial depth with an uplifted, thrusted, and concomitantly eroded HW above the footwall [ABSTRACT FROM AUTHOR]