Geochemical and Thermal Constraints on the Hikurangi Subduction Zone Hydrogeologic System and Its Role in Slow Slip

Abstract Fluid generation and migration regulate the development of pore fluid pressure, which is hypothesized to influence the occurrence of slow slip events at subduction zones. Seafloor seep sites present the opportunity to directly sample fluids flowing through the accretionary wedge and assess the hydrogeologic conditions of the outer forearc. We present heat flow measurements and pore water geochemistry from sediment cores collected at fault‐hosted seep sites on the southern and northern Hikurangi margin, offshore the North Island of New Zealand. These measurements span the deformation front to the shelf break. Along the northern margin, heat flow data do not show anomalies that can be obviously attributed to the discharge of warm fluids. Pore fluid compositions indicate that seep fluids originate from compaction within the uppermost wedge. Reactive‐transport modeling of pore water solute profiles produces fluid flow rate estimates ≤2 cm/yr. Shallow fluid sources and low discharge rates at offshore fault‐hosted seeps suggest that the sampled fault zones are characterized by low permeability at depth, preventing efficient drainage of the megathrust and underthrust sediments to the seafloor. These results provide additional evidence that the northern Hikurangi margin plate boundary is associated with high pore fluid pressures that likely act as a control on slow slip activity.