How Clay Mineral Assemblages Affect Instability on the Upper Slope of the Hikurangi Subduction Zone, New Zealand.

The International Ocean Discovery Program cored Sites U1517 (Tuaheni landslide) and U1519 (Tuaheni Basin) on the Hikurangi slope, North Island, New Zealand. We employed X‐ray diffraction to document clay mineral assemblages within the muddy sediments, with the aim of testing their potential influences on slope stability. Detrital smectite dominates the clays, with average proportions of 52 wt% at Site U1517 and 53 wt% at Site U1519. Bulk sediment from Site U1517 contains up to ∼29 wt% smectite (average = 21 wt%), high enough to reduce the angle of internal friction (on average) to ∼6°. Stratigraphic sections at both sites are homogeneous; compositional excursions are not evident along inferred slip surfaces or weak layers. Smectite decreases in the "downstream" direction of the East Cape Current, and that spatial trend correlates with lower densities of slide scars. The uniformity of compositional preconditioning, however, points to other factors as determinants of slip nucleation. Plain Language Summary: Expandable clay minerals (smectite) are notorious for weakening sediments and contributing to conditions that promote landslides. The Hikurangi margin offshore North Island, New Zealand, is noted for its abundance of submarine landslides. To assess their possible causes, we analyzed clay mineral assemblages from two sites on the upper trench slope that were sampled by the International Ocean Discovery Program. Cores from Sites U1517 and U1519 display unusual degrees of compositional homogeny, with smectite as the dominant clay mineral. We did not find any compositional anomalies at inferred slip surfaces or along weak layers of the Tuaheni landslide. Instead, the entire stratigraphic succession appears to be relatively weak. With inherited preconditioning, dynamic loading during large earthquakes is probably enough to trigger landslides, especially on steeper slopes. Key Points: Hikurangi trench‐slope sediments, including muds within the Tuaheni landslide complex, contain high concentrations of detrital smectiteSmectite is abundant enough to reduce the bulk sediment's coefficient of friction, with little stratigraphic variabilityThe homogeneous mud composition results from strong currents and does not change significantly along inferred slip surfaces or weak layers [ABSTRACT FROM AUTHOR]