Fluid flowrates and compositions and water–rock interaction in the Hikurangi margin forearc, New Zealand

The decrease in interseismic coupling and slip rate deficit from south to north, from locked to slipping, of the Hikurangi margin is attributed from interpretation of geophysical data to more fluid-saturated sediments in the north. To assess this interpretation, fluid flowrates were measured in springs of the subaerial forearc, structures correlated with flowrates and the fluids geochemically and isotopically evaluated to understand their origins and source depths. Extrapolations from on-ground measurements show that-95% of aqueous fluids and-98% of gas are expelled to the surface in the slipping zone, within the northern subaerial forearc. The relative fluid residence time, from the equilibration of CH4 and CO2 and measured ascent rates, is 3x shorter in the slipping zone suggesting less constricted migration pathways than in the locked zone. Fluids are expelled not only from the sediments that produce Na-Cl fluids and thermogenic gas, but also from the oceanic igneous mafic slab that releases high-Cl fluids and abiotic CH4 from serpentinization, giving rise to Na-Ca-Cl fluids. Chloride-rich sur-face emissions consist of-70% by volume Na-Cl and-30% Na-Ca-Cl fluids. Higher rates of fluid expulsion in the north coincide with higher permeability and greater production of aqueous fluids from clay dehydration, serpentinization and a higher degree of diagenesis in sedimentary rocks. Mantle gas, subduction interface fluids and porewaters have the most direct access to the surface in the slipping Center, where the degree of water-rock interaction and homogeneity of water compositions suggest more recent fluid generation than other sectors in the subaerial forearc. Porewaters and thermogenic gas equilibrate at 110 degrees + 20 degrees C with a slight increase in temperature from south to north. Overall,-120 Tg/a water is subducted in the Hikurangi margin and <0.5 Tg/a CH4 is being emitted to the atmosphere. This study shows the added impact of waters of serpentinization on the overall fluid compositions, aqueous fluid subduction budget, extent of fluid distribution in the oceanic slab and, indirectly, the temperature estimate of 185 + 25 degrees C for fluids ascending from the subduction interface in the Hikurangi margin.