Asymmetric Brittle Deformation at the Pāpaku Fault, Hikurangi Subduction Margin, NZ, IODP Expedition 375

Quantifying fault damage zones provides a window into stress distribution and rheology around faults. International Ocean Discovery Program (IODP) Expeditions 372/375 drilled an active thrust splay fault within the Hikurangi subduction margin. The fault, which is hosted in Pleistocene clastic sediments, is surrounded by brittle fractures and faults as well as ductile deformation features. We find that fracture density in the damage zone enveloping the fault is asymmetric, with the hanging wall showing greater overall fracture density and at greater distances from the fault than the footwall. Furthermore, the peak in fracture density occurs within an area of mesoscale folding and localized slip in the hanging wall rather than adjacent to the main fault zone. We attribute the asymmetry in damage to disparate deformation histories between the hanging wall and footwall, greater ductile deformation within the footwall, and/or dynamic stress asymmetry around a propagating rupture. Damage asymmetry is common at shallow depths in subduction zones and influences the mechanical and hydrological properties of the fault, such as channelized fluid flow and fault stability. Finally, we demonstrate that subduction zone faults show similar damage‐displacement scaling as continental faults. Plain Language Summary Fault damage zones form when slip along faults creates off‐fault stresses that bring the surrounding rock to failure. The greatest fracturing usually occurs closest to the fault core. During fault zone drilling of Hikurangi subduction zone east of New Zealand, we found that damage around a large splay fault is asymmetric, and the peak in fracture density occurs ∼25 m above the fault, rather than adjacent to the fault core. We hypothesize that this is caused by folding and fracturing in the hanging wall, more ductile deformation in the footwall, and/or a stress asymmetry around the fault that was created while the fault was slipping. Key Points The Pāpaku fault shows a ≥160 m thick damage zone as defined by fracture density within a drillcore from IODP Exp. 375 Damage is asymmetric around the thrust fault, with a wider and more intensely fractured damage zone in the hanging wall than the footwall Damage decay and damage zone thickness of subduction faults are similar to continental faults