Quantifying stress fields to better understand shallow tectonics of the Hikurangi Subduction Margin, NZ

Quantitative stress data is crucial to understanding the mechanical behaviour of faults and the variation of  interface slip behaviours at subduction zones. The Hikurangi Subduction Margin (HSM), New Zealand is characterized by along-strike variations in subduction interface and fault slip behaviour, changing from shallow slow slip events (SSEs) and creep to interseismic locking and stress accumulation moving south. We quantify the shallow (<3km) HSM stress magnitudes and orientations and utilise this new data to determine tectonic variation along the HSM and discuss how this may relate to the large-scale observation in HSM subduction dynamics. For depths below ~650 mTVD results show σ₃: S_v ratios of 0.92-1 along the entire HSM, and S_Hmax: S_v ratios of 0.95-1.81 in the central HSM, and 0.95-2.15 in the southern HSM. Such ratios infer that below ~650 mTVD a prevalent thrust to strike-slip (σ₁=S_Hmax) faulting regime exists along the entire HSM. Our results also reveal a NE-SW (margin-parallel) S_Hmax orientation in the shallow central HSM, which rotates to a WNW- ESE/NW-SE (margin-perpendicular) S_Hmax orientation in the shallow southern HSM.

In the central HSM, we determine the  NE-SW orientation of S_Hmax= σ₁, which is inconsistent with  NNE/NE striking reverse faults (inferring a NW-SE oriented S_Hmax= σ₁) in the region. This suggests that the stress state evolved over time from a contractional to a strike/oblique-slip state. This temporal change in stress state in the central HSM is likely driven by development of clockwise rotation of the Hikurangi forearc and upper plate overpressures. A contemporary NW-SE oriented S_Hmax in the southern HSM, associated with NNE/NE striking faults, suggests the stress regime here remains contractional over time, and is less effected by forearc rotation. The variation in stress state along the HSM spatially correlates with reported along-strike variation in subduction interface slip behaviour. This spatial correlation suggests that contemporary stresses in the overriding plate above the subduction interface may reflect contemporary elastic strain accumulation processes related to subduction megathrust locking.