No abstract

Many subduction complexes exhumed from seismogenic depths are described as thick, turbidite-dominated sequences disrupted by duplexes and imbricate slices, and containing mélange shear zones defined by a foliated mudstone matrix surrounding clasts of more competent sandstone and/or basalt (e.g., the Chrystalls Beach,

Calcareous‐pelagic input sediments are present at several subduction zones and deform differently to their siliciclastic counterparts. We investigate deformation in calcareous‐pelagic sediments drilled ∼20 km seaward of the Hikurangi megathrust toe at Site U1520 during International Ocean Discovery Program (IODP) Expeditions 372 and 375. Clusters of normal faults and subhorizontal stylolites in the sediments indicate both brittle faulting and viscous pressure solution operated at <850 m below sea floor. Stylolite frequency and vertical shortening estimated using stylolite mass loss, porosity change, and distribution increase with carbonate content. We then use U1520 borehole data to constrain a P‐T‐t history for the sediments and apply an experimentally derived pressure solution model to compare with strains calculated from stylolites. Modeled strains fail to replicate stylolite‐hosted strain distribution or magnitude, but comparison shows porosity, composition, and grain‐scale effects in diffusivity and mass transfer pathway width likely exert a strong influence on pressure solution localization and strain rate. Stylolite and fault clusters concentrate clay in these sediments, creating weak volumes of clay within carbonates, that may localize slip where the plate interface intersects the carbonates at <5‐km depth. Plate interface slip character and rheology will be influenced by the deformation of intermixed phyllosilicates and calcite, occurring by variably stable frictional slip and pressure solution of calcite. Pressure solution of calcite is therefore important at the shallow plate interface, waning at the base of the slow‐slipping zone because calcite solubility is low at temperatures >150°C where frictional (possibly seismic) slip likely predominates.

At the northern Hikurangi margin (North Island, New Zealand), shallow slow slip events (SSEs) frequently accommodate subduction-interface plate motion from landward of the trench to &lt;20 km depth. SSEs may be spatially related to geometrical interface heterogeneity, though kilometer-scale plate-interface roughness imaged by active-source seismic methods is only constrained offshore at &lt;12 km depth. Onshore constraints are comparatively lacking, but we mapped the Hikurangi margin plate interface using receiver functions from data collected by a dense 22 × 10 km array of 49 broadband seismometers. The plate interface manifests as a positive-amplitude conversion (velocity increase with depth) dipping west from 10 to 17 km depth. This interface corroborates relocated earthquake hypocenters, seismic velocity models, and downdip extrapolation of depth-converted two-dimensional active-source lines. Our mapped plate interface has kilometer-amplitude roughness we interpret as oceanic volcanics or seamounts, and is 1–4 km shallower than the regional-scale plate-interface model used in geodetic inversions. Slip during SSEs may thus have different magnitudes and/or distributions than previously thought. We show interface roughness also leads to shear-strength variability, where slip may nucleate in locally weak areas and propagate across areas of low shear-strength gradient. Heterogeneous shear strength throughout the depth range of the northern Hikurangi margin may govern the nature of plate deformation, including the localization of both slow slip and hazardous earthquakes.