Probing the nature of the Hikurangi Margin hydrogeologic system - Preliminary results from the SAFFRONZ Expedition

Fluid generation, migration, and pore fluid pressure at subduction zones are hypothesized to exert an important control on the generation of seismicity and slow slip events (SSEs). The SAFFRONZ (Slow slip and fluid flow response offshore New Zealand) project aims to test interrelationships between fluid production, fluid flow, and slow slip at the Hikurangi Margin, where there are dramatic changes in the along-strike distribution of SSEs and their recurrence intervals. During a 38-day expedition on the R/V Revelle in early 2019, we employed a nested approach to constrain the fluid flow distribution and rates along the Hikurangi margin that included hydroacoustic surveys, 93 heat flow measurements, collection of over 80 gravity/piston cores for pore water geochemistry, and deployment of 16 benthic fluid flow meters that will continuously measure fluid flow rates and composition over a 2-year period. Data was collected at seep and non-seep sites from the shelf break to the deformation front at both the southern and northern margin. This comprehensive dataset will provide (1) information on the present background state of fluid flow and how it relates to inferred pore pressure evolution along the plate boundary from seismic and modeling studies, and (2) a continuous 2-year record of fluid flow rates and composition through a large SSE that occurred March-June 2019. Here we present preliminary pore water geochemical data from 25 cores collected across the margin. The solute profiles provide information on fluid sources and are modeled for flow rates, providing critical information on the long-term state of the hydrologic system. These flow rates provide the foundation for comparison with the benthic flow meters (to be recovered in early 2021) that recorded the 2019 SSE. The combined coring and fluid flow meter results from this project will greatly contribute to our understanding of the relationship between pore pressure, fluid flow, and SSEs at subduction zones.