Variation of Bottom-simulating-reflection Strength in a High-flux Methane Province, Hikurangi Margin, New Zealand

Bottom-simulating reflectors (BSRs) represent the base of a gas-hydrate zone underlain by widespread free gas. On the southern Hikurangi margin offshore of the east coast of New Zealand, multichannel seismic data reveal that the gas-hydrate province extends from about 600-m (1968-ft) water depth to the Hikurangi Trench and covers an area of about 50,000 km2 (19,305 mi2). We analyzed BSR strength in a grid of seismic data across this area. Simplified rock-physics models were used to estimate the reflection coefficient of BSRs with a gas concentration above which compressional wave velocity is mostly insensitive to gas saturation. This reflection coefficient was found to be 0.20, resulting from at least 8–10% gas saturation. Four percent of the gas-hydrate stability zone on the southern Hikurangi margin is underlain by strong BSRs with reflection coefficients that are 0.20 or stronger. Mapped variations in BSR and sea-floor reflection amplitude ratios and reflection coefficients reveal a strong correlation, on a regional scale, between the amplitude of BSRs and structures that promote fluid flow. Isotope, geochemical, and geophysical data from previous studies onshore point to a thermogenic origin for methane and suggest that New Zealand east coast fluids are derived from accreted, organic-rich, sedimentary sources overlying the subducting slab and that these sources must have an age of about 70 Ma. We therefore speculate that BSR formation on the Hikurangi margin is supported by the long-term recycling of fluids along faults that penetrate through the oldest sediments in the forearc and sole at the plate interface, as mapped in crustal seismic sections elsewhere on the Hikurangi margin. Squeezed subducting sediments at the plate interface may provide a rich source of water driving fluid recycling.