Description: Highlights • We map out the 3D extent of gas hydrate stability beneath two methane seep sites. • Focused fluid flow has sustained large-scale gas hydrate instability. • The two seeps likely have the same deep fluid source, despite shallow differences. • Fault networks influenced the initiation of advective flow through the hydrate system. • Ongoing flow towards the seeps is likely sustained by networks of hydrofractures. Abstract Fluid flow through marine sediments drives a wide range of processes, from gas hydrate formation and dissociation, to seafloor methane seepage including the development of chemosynthetic ecosystems, and ocean acidification. Here, we present new seismic data that reveal the 3D nature of focused fluid flow beneath two mound structures on the seafloor offshore Costa Rica. These mounds have formed as a result of ongoing seepage of methane-rich fluids. We show the spatial impact of advective heat flow on gas hydrate stability due to the channelled ascent of warm fluids towards the seafloor. The base of gas hydrate stability (BGHS) imaged in the seismic data constrains peak heat flow values to View the MathML source∼60 mWm−2 and View the MathML source∼70 mWm−2 beneath two separate seep sites known as Mound 11 and Mound 12, respectively. The initiation of pronounced fluid flow towards these structures was likely controlled by fault networks that acted as efficient pathways for warm fluids ascending from depth. Through the gas hydrate stability zone, fluid flow has been focused through vertical conduits that we suggest developed as migrating fluids generated their own secondary permeability by fracturing strata as they forced their way upwards towards the seafloor. We show that Mound 11 and Mound 12 (about 1 km apart on the seafloor) are sustained by independent fluid flow systems through the hydrate system, and that fluid flow rates across the BGHS are probably similar beneath both mounds. 2D seismic data suggest that these two flow systems might merge at approximately 1 km depth, i.e. much deeper than the BGHS. This study provides a new level of detail and understanding of how channelled, anomalously-high fluid flow towards the seafloor influences gas hydrate stability. Thus, gas hydrate systems have good potential for quantifying the upward flow of subduction system fluids to seafloor seep sites, since the fluids have to interact with and leave their mark on the hydrate system before reaching the seafloor.

Description: Based on multichannel seismic, geoacoustic, and methane sensor data, four different areas along the Hikurangi Margin show multiple indications for seep activity including bright spots, transparent zones, vertical chimneys, and the occurrence and distribution of bottom simulating reflectors. Locations where these features reach the seafloor are characterised by high backscatter intensity on sidescan sonar images and transparent zones in sediment echosounder profiles, while methane sensors show episodic, elevated methane concentrations near the seep sites. Methane discharge is facilitated by reduced hydrostatic pressure during low tides. The greatest number of seeps at Opouawe Bank correlates with the highest methane activity along the Hikurangi Margin. High heat flow values on flanks of ridges and low heat flow values on anticlines reflect a topographic effect on subsurface temperatures. Elevated heat flow occurs in the vicinity of seeps on Opouawe Bank. We propose that there are two drivers behind methane seepage with respect to the migration pathways of methane through the gas hydrate stability zone (GHSZ) to the seafloor: (1) structurally controlled and (2) stratigraphically controlled. In the structural model, vertical chimneys are the major pathways for methane through the GHSZ. Part of the upwardly migrating methane forms gas hydrate within the chimney. In the stratigraphic model, methane migration is stratigraphically controlled beneath seeps that are located on bathymetric highs and/or where subsurface anticlines occur beneath seeps. The structurally controlled seeps produce higher methane escape at the seafloor than those that are stratigraphically controlled. A combination of both driving mechanisms results in the highest methane seepage rates at the Tui Seep on Opouawe Bank.

Description: The imbricated frontal wedge of the central Hikurangi subduction margin is characteristic of wide (ca. 150 km), poorly drained and over pressured, low taper (not, vert, similar 4°) thrust systems associated with a relatively smooth subducting plate, a thick trench sedimentary sequence (not, vert, similar 3–4 km), weak basal décollement, and moderate convergence rate (not, vert, similar 40 mm/yr). New seismic reflection and multibeam bathymetric data are used to interpret the regional tectonic structures, and to establish the geological framework for gas hydrates and fluid seeps. We discuss the stratigraphy of the subducting and accreting sequences, characterize stratigraphically the location of the interplate décollement, and describe the deformation of the upper plate thrust wedge together with its cover sequence of Miocene to Recent shelf and slope basin sediments. We identify approximately the contact between an inner foundation of deforming Late Cretaceous and Paleogene rocks, in which widespread out-of-sequence thrusting occurs, and a 65–70 km-wide outer wedge of late Cenozoic accreted turbidites. Although part of a seamount ridge is presently subducting beneath the deformation front at the widest part of the margin, the morphology of the accretionary wedge indicates that frontal accretion there has been largely uninhibited for at least 1–2 Myr. This differs from the offshore Hawkes Bay sector of the margin to the north where a substantial seamount with up to 3 km of relief has been subducted beneath the lower margin, resulting in uplift and complex deformation of the lower slope, and a narrow (10–20 km) active frontal wedge. Five areas with multiple fluid seep sites, referred to informally as Wairarapa, Uruti Ridge, Omakere Ridge, Rock Garden, and Builders Pencil, typically lie in 700–1200 m water depth on the crests of thrust-faulted, anticlinal ridges along the mid-slope. Uruti Ridge sites also lie in close proximity to the eastern end of a major strike-slip fault. Rock Garden sites lie directly above a subducting seamount. Structural permeability is inferred to be important at all levels of the thrust system. There is a clear relationship between the seeps and major seaward-vergent thrust faults, near the outer edge of the deforming Cretaceous and Paleogene inner foundation rocks. This indicates that thrust faults are primary fluid conduits and that poor permeability of the Cretaceous and Paleogene inner foundation focuses fluid flow to its outer edge. The sources of fluids expelling at active seep sites along the middle slope may include the inner parts of the thrust wedge and subducting sediments below the décollement. Within anticlinal ridges beneath the active seep sites there is a conspicuous break in the bottom simulating reflector (BSR), and commonly a seismically-resolvable shallow fault network through which fluids and gas percolate to the seafloor. No active fluid venting has yet been recognized over the frontal accretionary wedge, but the presence of a widespread BSR, an extensive protothrust zone (〉 200 km by 20 km) in the Hikurangi Trough, and two unconfirmed sites of possible previous fluid expulsion, suggest that the frontal wedge could be actively dewatering. There are presently no constraints on the relative fluid flux between the frontal wedge and the active mid-slope fluid seeps. Article Outline

Description: The role of methane in the global bio-geo-system is one of the most important issues of present-day research. Cold seeps, where methane leaves the seafloor and enters the water column, provide valuable evidence of subsurface methane paths. Within the New Vents project we investigate cold seeps and seep structures at the Hikurangi Margin, east of New Zealand. In the area of Opouawe Bank, offshore the southern tip of the North Island, numerous extremely active seeps have been discovered. High-resolution seismic sections show a variety of seep structures. We see seismic chimneys either characterised by high-amplitude reflections or by acoustic turbidity and faults presumably acting as fluid pathways. The bathymetric expression of the seeps also varies: There are seeps exhibiting a flat seafloor as well as a seep located in a depression and small mounds. The images of the 3.5 kHz Parasound system reveal the ear-surface structure of the vent sites. While highamplitude spots within the uppermost 50 m below the seafloor (bsf) are observed at the majority of the seep structures, indicating gas hydrate and/or authigenic carbonate formations with an accumulation of free gas underneath, a few seep structures are characterised by the complete absence of reflections, indicating a high gas content without the formation of a gas trap by hydrates or carbonates. The factors controlling seep formation have been analysed with respect to seep location, seep structure, water depth, seafloor morphology, faults and gas hydrate distribution. The results indicate that the revailing structural control for seep formation at Opouawe Bank is the presence of numerous minor faults piercing the base of the gas hydrate stability zone.

Description: Along many active and some passive margins cold seeps are abundant and play an important role in the mechanisms of methane supply from the subsurface into seawater and atmosphere. With numerous cold seeps already known, the convergent Hikurangi Margin east of North Island, New Zealand, was selected as a target area for further detailed, multidisciplinary investigation of cold seeps within the New Vents and associated projects. Methane and temperature sensors (METS) were deployed at selected seep sites on the Opouawe Bank off the southeastern tip of North Island and near the southern end of the imbricate-thrust Hikurangi Margin, together with seismic ocean bottom stations. They remained in place for about 48 h while seismic data were collected. The seeps were associated with seep-related seismic structures. Methane concentrations were differing by an order of magnitude between neighbouring stations. The large differences at sites only 300 m apart, demonstrate that the seeps were small scale structures, and that plumes of discharged methane were very localised within the bottom water. High methane concentrations recorded at active seep sites at anticlinal structures indicate focused fluid flow. Methane discharge from the seafloor was episodic, which may result from enhanced fluid flow facilitated by reduced hydrostatic load at low tides. The strong semi-diurnal tidal currents also contribute to the fast dilution and mixing of the discharged methane in the seawater. Despite dispersal by currents, fluid flow through fissures, fractures, and faults close to the METS positions and tidal fluctuations are believed to explain most of the elevated methane concentrations registered by the METS. Small earthquakes do not appear to be correlated with seawater methane anomalies.

Description: This paper is an introduction to and an overview of papers presented in the Special Issue of Marine Geology “Methane seeps at the Hikurangi Margin, New Zealand”. In 2006 and 2007, three research cruises to the Hikurangi Margin at the east coast of New Zealand's North Island were dedicated to studying methane seepage and gas hydrates in an area where early reports suggested they were widespread. Two cruises were carried out on RV TANGAROA and one on RV SONNE using the complete spectrum of state-of-the-art equipment for geophysics (seismic, sidescan, controlled source electromagnetics, ocean bottom seismometers and hydrophones, singlebeam and multibeam), seafloor observations (towed camera systems, ROV), sediment and biological sampling (TV-guided multi-corer, gravity-corer, grab, epibenthic sled), deployment of in-situ observatories (landers) as well as water column sampling and oceanographic studies (CTD, moorings). The scientific disciplines involved ranged from geology, geophysics, petrography, geochemistry, to oceanography, biology and microbiology. These cruises confirmed that a significant part of the Hikurangi Margin has been active with locally intense methane seepage at present and in the past, with the widespread occurrence of dead seep faunas and knoll-forming carbonate precipitations offshore and on the adjacent land. A close link to seismically detected fluid systems and the outcropping of the base of the gas hydrate stability zone can be found at some places. Pore fluid and free gas release were found to be linked to tides. Currents as well as density layers modulate the methane distribution in the water column. The paper introduces the six working areas on the Hikurangi Margin, and compiles all seep locations based on newly processed multibeam and multibeam backscatter data, water column hydroacoustic and visual data that are combined with results presented elsewhere in this Special Issue. In total, 32 new seep sites were detected that commonly show chemoherm-type carbonates or carbonate cemented sediment with fissures and cracks in which calyptogenid clams and bathymodiolid mussels together with sibloglinid tube worms live. White bacterial mats of the genus Beggiatoa and dark gray beds of heterotrophic ampharetid polychaetes typically occur at active sites. Bubble release has frequently been observed visually as well as hydroacoustically (flares) and geochemical analyses show that biogenic methane is released. All seep sites, bubbling or not, were inside the gas hydrate stability zone. Gas hydrate itself was recovered at three sites from the seafloor surface or 2.5 m core depth as fist-sized chunks or centimeter thick veins. The strong carbonate cementation that in some cases forms 50 m high knolls as well as some very large areas being paved with clam shells indicates very strong and long lasting seep activity in the past. This activity seems to be less at present but nevertheless makes the Hikurangi Margin an ideal place for methane-related seep studies in the SW-Pacific.