Geochemical Constraints on the Hikurangi Margin Hydrogeologic System – Results from the SAFFRONZ Expedition

Abstract:

Fluid production and pore pressure accumulation are thought to be intimately linked with the occurrence of slow slip events (SSEs) at subduction zones. The primary goal of the SAFFRONZ (Slow Slip And Fluid Flow Response Offshore New Zealand) project is to assess the interrelationships between pore fluid overpressure, fault slip, and fluid flow at the Hikurangi subduction zone. Here we report new hydrogeochemical data from the northern and southern Hikurangi margin, where there are dramatic changes in the along-strike depth, size, duration, and recurrence interval of slow slip. Porewater geochemistry from sediment cores, collected at structurally-controlled seep and non-seep sites from the shelf break to the deformation front, comprise a broad dataset that provides (1) direct information on fluid sources and the rates of fluid flow, and (2) a basis for comparison of the present background state of fluid flow with long-term, continuous measurements from 16 benthic flow meters (to be recovered in early 2021) which recorded the hydrogeochemical response to a large SSE in March-June 2019. The results show that fluids emitted at seeps in the northern margin originate from compaction within the wedge, rather than from underthrust sediment. Modeling of porewater solute profiles indicate flow rates up to a few centimeters per year both into and out of the sediment. Vertical porewater solute profiles with seawater composition in the upper section of most cores indicate a rapid, ongoing mixing process that we interpret as the product of bubble irrigation and gas-driven pumping. This new dataset provides direct constraints on the long-term state of the Hikurangi margin hydrogeologic system and is consistent with previous inferences from seismic reflection data that indicate high pore fluid pressures are developing along the plate boundary at the Hikurangi margin, acting as an important control on slow slip activity.