Both Longitudinal and Transverse Extension Controlling Gas Migration Through Submarine Anticlinal Ridges, New Zealand's Southern Hikurangi Margin

Sub-seabed fluid flow, gas hydrate accumulation and seafloor methane seepage are tightly interwoven processes with implications for marine biodiversity, ocean chemistry and seafloor stability. We combine long-offset seismic reflection data with high-resolution seismic data to investigate shallow structural deformation and its relationship to focused gas migration and hydrate accumulation in the southern Hikurangi subduction wedge. Anticlines, effective traps for focusing free gas, are characterized by both normal faults and vertical zones of hydraulic fracturing within the hydrate stability zone. The normal faults form as a result of sediment layer folding and gravitational collapse of ridges during uplift. We document both longitudinal (ridge-parallel) and transverse (ridge-perpendicular) extensional structures (normal faults and elongated hydraulic fracture zones) in the sub-seafloor of anticlinal ridges. Intriguingly, gas flow through ridges close to the deformation front of the wedge exploits longitudinal structures, while ridges further inboard are characterized by gas flow along transverse structures. This highlights pronounced changes in the shallow deformation of ridges in different parts of the wedge, associated with a switching of the least and intermediate principal stress directions. It is critical to understand these shallow stress fields because they control fluid flow patterns and methane seepage out of the seafloor.Plain Language Summary Methane gas is released naturally from the seafloor at locations known as "gas seeps." Gas seeps often occur on seafloor ridges, because gas tends to flow into folded sediments beneath ridges. In this study, we explored how geological structures (faults and fractures) beneath seafloor ridges control the flow of gas toward the seafloor. We observed that gas flow beneath ridges is aligned along one of two main orientations: either approximately parallel or perpendicular to the long-axis of the ridge. Structures aligned parallel with a ridge axis occur beneath ridges near the toe of the accretionary wedge-that is, at the seaward end of the large wedge of deformed sediments that has formed as a result of one tectonic plate sliding beneath another. Structures aligned perpendicular to the ridge occur further landward within the accretionary wedge. The change in the orientation of structures was unexpected, and shows us there are significant differences in the way ridges deform as they develop through geological time. The results are important because geological structures control how and where gas migrates to the seafloor, where it then acts as a food source for diverse biological communities, while also influencing the chemistry of the ocean.