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The Tuaheni Landslide Complex (TLC) is characterised by areas of compression upslope and extension downslope. It has been thought to consist of a stack of two genetically linked landslide units identified on seismic data. We use 3D seismic reflection, bathymetry data, and IODP core U1517C (Expedition 372), to understand the internal structures, deformation mechanisms and depositional processes of the TLC deposits. Unit II and Unit III of U1517C correspond to the two chaotic units in 3D seismic data. In the core, Unit II shows deformation whereas Unit III appears more like an in situ sequence. Variance attribute analysis shows that Unit II is split in lobes around a coherent stratified central ridge and is bounded by scarps. By contrast, we find that Unit III is continuous beneath the central ridge and has an upslope geometry that we interpret as a channellevee system. Both units show evidence of lateral spreading due to the presence of the Tuaheni Canyon removing support from the toe. Our results suggest that Unit II and Unit III are not genetically linked, that they are separated substantially in time and they had different emplacement mechanisms, but fail under similar circumstances.

The Pāpaku Fault Zone, drilled at International Ocean Discovery Program (IODP) Site U1518, is an active splay fault in the frontal accretionary wedge of the Hikurangi Margin. In logging‐while‐drilling data, the 33‐m‐thick fault zone exhibits mixed modes of deformation associated with a trend of downward decreasing density, P‐wave velocity, and resistivity. Methane hydrate is observed from ~30 to 585 m below seafloor (mbsf), including within and surrounding the fault zone. Hydrate accumulations are vertically discontinuous and occur throughout the entire logged section at low to moderate saturation in silty and sandy centimeter‐thick layers. We argue that the hydrate distribution implies that the methane is not sourced from fluid flow along the fault but instead by local diffusion. This, combined with geophysical observations and geochemical measurements from Site U1518, suggests that the fault is not a focused migration pathway for deeply sourced fluids and that the near‐seafloor Pāpaku Fault Zone has little to no active fluid flow.

Geophysical data indicate that the Hikurangi subduction margin on New Zealand’s East Coast contains a large gas hydrate province. Gas hydrates are widespread in shallow sediments across the margin, and locally intense fluid seepage associated with methane hydrate is observed in several areas. Glendhu and Honeycomb ridges lie at the toe of the Hikurangi deformation wedge at depths ranging from 2100 to 2800 m. These two parallel four-way closure systems host concentrated methane hydrate deposits. The control on hydrate formation at these ridges is governed by steeply dipping permeable strata and fractures, which allow methane to flow upwards into the gas hydrate stability zone. Hydrate recycling at the base of the hydrate stability zone may contribute to the accumulation of highly concentrated hydrate in porous layers.To improve the characterisation of the hydrate systems at Glendhu and Honeycomb ridges, we estimate hydrate saturation and porosity of the concentrated hydrate deposits. We first estimate elastic properties (density, compressional and shear-wave velocities) of the gas hydrate stability zone through full-waveform inversion and iterative geostatistical seismic amplitude versus angle (AVA) inversion. We then perform a petrophysical inversion based on a rock physics model to predict gas hydrate saturation and porosity of the hydrate bearing sediments along the two ridges.Our results indicate that the high seismic amplitudes correspond to the top interface of highly concentrated hydrate deposit, with peak saturations around 35%. Because of the resolution of the seismic data we assume that the estimated properties are averaged over layers of 10 to 20 meters thickness. These saturation values are in agreement with studies conducted in other areas of concentrated hydrate accumulations in similar geologic settings.

Hydrogeological processes influence the morphology, mechanical behavior, and evolution of subduction margins. Fluid supply, release, migration, and drainage control fluid pressure and collectively govern the stress state, which varies between accretionary and nonaccretionary systems. We compiled over a decade of published and unpublished acoustic data sets and seafloor observations to analyze the distribution of focused fluid expulsion along the Hikurangi margin, New Zealand. The spatial coverage and quality of our data are exceptional for subduction margins globally. We found that focused fluid seepage is widespread and varies south to north with changes in subduction setting, including: wedge morphology, convergence rate, seafloor roughness, and sediment thickness on the incoming Pacific plate. Overall, focused seepage manifests most commonly above the deforming backstop, is common on thrust ridges, and is largely absent from the frontal wedge despite ubiquitous hydrate occurrences. Focused seepage distribution may reflect spatial differences in shallow permeability architecture, while diffusive fluid flow and seepage at scales below detection limits are also likely. From the spatial coincidence of fluids with major thrust faults that disrupt gas hydrate stability, we surmise that focused seepage distribution may also reflect deeper drainage of the forearc, with implications for pore-pressure regime, fault mechanics, and critical wedge stability and morphology. Because a range of subduction styles is represented by 800 km of along-strike variability, our results may have implications for understanding subduction fluid flow and seepage globally., peer-reviewed

It is important to understand how and where concentrated gas hydrates form because the gas hydrate system modulates methane flow through the seafloor and into the oceans. We investigated gas hydrate formation in relation to tectonic folding at New Zealand’s southern Hikurangi subduction margin. Concentrated gas hydrates form preferentially in strata crossing the base of the hydrate stability zone at angles greater than ∼5°. Intriguingly, concentrated deposits are more common in landward-dipping strata than seaward-dipping strata. We explain this asymmetry with a conceptual model for hydrate formation in accretionary wedges. Preferential sedimentation on the landward sides of ridges leads to pronounced gas hydrate recycling in landward-dipping strata. Together with focused fluid flow beneath the hydrate system, gas hydrate recycling favors the development of interconnected gas columns that drive gas back into the hydrate stability zone to form concentrated gas hydrates. Our results advance the understanding of gas hydrate formation in accretionary wedges, which are common global tectonic settings for gas hydrate systems.

We investigate gas hydrate system dynamics beneath a submarine canyon on New Zealand's Hikurangi subduction margin using seismic reflection data and petroleum systems modelling. High seismic velocities just above the base of gas hydrate stability (BGHS) indicate that concentrated gas hydrates exist beneath the canyon. Two-dimensional gas hydrate formation modelling shows how the process of canyon incision at this location alters the distribution and concentration of gas hydrate. The key modelling result is that free gas is trapped beneath the gas hydrate layer and then ‘captured' into a concentrated gas hydrate deposit as a result of a downward-shift in the BGHS driven by canyon incision. Our study thus provides new insight into the functioning of this process. From our data, we also conceptualise two other models to describe how canyons could significantly change gas hydrate distribution and concentration. One scenario is related to deflection of fluid flow pathways from over-pressured regions at the BGHS toward the canyon, and the other is based on relationships between simultaneous seafloor uplift and canyon incision. The relationships and processes described are of global relevance because of considerations of gas hydrate as an energy resource and the influence of both submarine canyons and gas hydrate systems on seafloor biodiversity.

In this study we provide evidence for methane hydrates in the Taranaki Basin, occurring a considerable distance from New Zealand's convergent margins, where they are well documented. We describe and reconstruct a unique example of gas migration and leakage at the edge of the continental shelf, linking shallow gas hydrate occurrence to a deeper petroleum system. The Taranaki Basin is a well investigated petroleum province with numerous fields producing oil and gas. Industry standard seismic reflection data show amplitude anomalies that are here interpreted as discontinuous BSRs, locally mimicking the channelized sea-floor and pinching out up-slope. Strong reverse polarity anomalies indicate the presence of gas pockets and gas-charged sediments. PetroMod™ petroleum systems modelling predicts that the gas is sourced from elevated microbial gas generation in the thick slope sediment succession with additional migration of thermogenic gas from buried Cretaceous petroleum source rocks. Cretaceous–Paleogene extensional faults underneath the present-day slope are interpreted to provide pathways for focussed gas migration and leakage, which may explain two dry petroleum wells drilled at the Taranaki shelf margin. PetroMod™ modelling predicts concentrated gas hydrate formation on the Taranaki continental slope consistent with the anomalies observed in the seismic data. We propose that a semi-continuous hydrate layer is present in the down-dip wall of incised canyons. Canyon incision is interpreted to cause the base of gas hydrate stability to bulge downward and thereby trap gas migrating up-slope in permeable beds due to the permeability decrease caused by hydrate formation in the pore space. Elsewhere, hydrate occurrence is likely patchy and may be controlled by focussed leakage of thermogenic gas. The proposed presence of hydrates in slope sediments in Taranaki Basin likely affects the stability of the Taranaki shelf margin. While hydrate presence can be a drilling hazard for oil and gas exploration, the proposed presence of gas hydrates opens up a new frontier for exploration of hydrates as an energy source.

The bottom simulating reflection (BSR) is widely observed along continental margins and is believed to mark the base of gas hydrate stability zone (BGHSZ). In some regions, double or multiple overlapping BSRs are observed, yet their formation mechanisms and geologic implications are not well understood. Here we present 3D seismic images from the 2018 NZ3D experiment that covers a 14 × 60 km 2 survey area on New Zealand's northern Hikurangi subduction margin. We observe double BSRs in five locations. Beneath the Tuaheni Basin in the mid-slope, a secondary BSR (BSR2) lies ∼100-360 m deeper than the primary BSR (BSR1) and its 3D geometry mimics the unconformity at the base of the basin. At three thrust ridges located 18-38 km from the deformation front, BSR2 lies ∼55-130 m below and is sub-parallel to BSR1. At another thrust ridge ∼14 km from the deformation front, BSR2 forms above the BSR1, and the two BSRs converge towards the peak of the ridge. Through 3D modeling of BGHSZ and analysis of the geometry and reflection characteristics of the double BSRs, we identify three potential mechanisms for their formation (1) rapid sedimentation, (2) tectonic uplift, (3) overpressure/heat advection caused by fluid migration. Our study demonstrates that formation of double BSRs is closely linked to subduction processes along the northern Hikurangi margin, and double BSRs may be used as indicators for areas with recent sedimentation, tectonic and/or fluid activities.

The Hikurangi Margin, east of the North Island of New Zealand, is known to contain significant deposits of gas hydrates. This has been demonstrated by several multidisciplinary studies in the area since 2005. These studies indicate that hydrates in the region are primarily located beneath thrust ridges that enable focused fluid flow, and that the hydrates are associated with free gas. In 2009–2010, a seismic dataset consisting of 2766 km of 2D seismic data was collected in the undrilled Pegasus Basin, which has been accumulating sediments since the early Cretaceous. Bottom-simulating reflections (BSRs) are abundant in the data, and they are accompanied by other features that indicate the presence of free gas and concentrated accumulations of gas hydrate. We present results from a detailed qualitative analysis of the data that has made use of automated high-density velocity analysis to highlight features related to the hydrate system in the Pegasus Basin. Two scenarios are presented that constitute contrasting mechanisms for gas-charged fluids to breach the base of the gas hydrate stability zone. The first mechanism is the vertical migration of fluids across layers, where flow pathways do not appear to be influenced by stratigraphic layers or geological structures. The second mechanism is non-vertical fluid migration that follows specific strata that crosscut the BSR. One of the most intriguing features observed is a presumed gas chimney within the regional gas hydrate stability zone that is surrounded by a triangular (in 2D) region of low reflectivity, approximately 8 km wide, interpreted to be the result of acoustic blanking. This chimney structure is cored by a ∼200-m-wide low-velocity zone (interpreted to contain free gas) flanked by high-velocity bands that are 200–400 m wide (interpreted to contain concentrated hydrate deposits).

The existence of free gas and gas hydrate in the pore spaces of marine sediments causes changes in acoustic velocities that overprint the background lithological velocities of the sediments themselves. Much previous work has determined that such velocity overprinting, if sufficiently pronounced, can be resolved with conventional velocity analysis from long-offset, multichannel seismic data. We used 2D seismic data from a gas hydrate province at the southern end of New Zealand’s Hikurangi subduction margin to describe a workflow for high-resolution velocity analysis that delivered detailed velocity models of shallow marine sediments and their coincident gas hydrate systems. The results showed examples of pronounced low-velocity zones caused by free gas ponding beneath the hydrate layer, as well as high-velocity zones related to gas hydrate deposits. For the seismic interpreter of a gas hydrate system, the velocity results represent an extra “layer” for interpretation that provides important information about the distribution of free gas and gas hydrate. By combining the velocity information from the seismic transect with geologic samples of the seafloor and an understanding of sedimentary processes, we have determined that high gas hydrate concentrations preferentially form within coarse-grained sediments at the proximal end of the Hikurangi Channel. Finer grained sediments expected elsewhere along the seismic transect might preclude the deposition of similarly high gas hydrate concentrations away from the channel.

Highlights • 3D seismic imaging of an entire landslide complex. • Shallow gas accumulation within and underneath Tuaheni Landslide Complex. • Imaging of a basal shear zone within a subaqueous landslide complex. Abstract The Hikurangi margin is an active continental margin east of New Zealand's North Island. It is well recognized as a seismically active zone and is known for the occurrence of free gas and gas hydrates within the shallow sediments. A variety of subaqueous landslides can be observed at the margin, including the Tuaheni Landslide Complex off Poverty Bay. This slide complex has been interpreted previously as a slowly creeping landform, as its morphology and internal deformation is comparable to terrestrial earthflows and rock glaciers. In 2014, we acquired a high-resolution 3D seismic volume covering major parts of the Tuaheni South landslide. The 3D data show a variety of fluid migration indicators, free gas accumulations and manifestations of the base of gas hydrate stability in the pre-slide sedimentary units and the lower unit of the landslide system. The data also show that the landslide system is composed of an upper and lower unit that are separated by an intra-debris negative-polarity reflection. Free gas accumulations directly beneath the landslide units suggest that the debris acts as a boundary for rising fluids and only few migration pathways to the intra-debris reflector are observed in the distal parts of the landslide. Deformation within the landslide's debris is focused in the upper landslide unit, and we interpret the intra-debris reflector as a basal shear zone or ‘glide plane’ upon which the debris has been remobilized. The origin of the intra-debris reflector is unclear, but we suggest it could be a relatively coarse-grained horizon that would be prone to fluid flow focusing and the development of excess fluid pressure. Our seismic study provides one of the most detailed examples of a subaqueous landslide system and reveals insights into the fluid flow system and potential basal shear zone development of the Tuaheni Landslide Complex.

We present an integrated 2D model of thermal and microbial generation of methane, migration into the gas hydrate stability zone (HSZ), and formation of methane hydrates. The model reconstructs the shallow (0–20 km) thermal structure of the subduction interface between the Australian plate and the subducting Pacific plate, and the trench basin (Pegasus Basin). Modelled temperatures of less than 110 °C within Pegasus Basin constrain the generation of oil and gas. Whilst a cool thermal regime is predicted to limit thermogenic generation of gas to a burial depth of >10 km, it extends the interval where prolific microbial gas generation occurs. The modelled rate of microbial generation of methane increases beneath the HSZ and peaks at ~1600 m below seafloor. Diffusive upward migration of microbially generated methane is interpreted to lead to widespread methane hydrate formation and the presence of a semi-continuous bottom simulating reflector (BSR). Predicted average hydrate saturation within the HSZ is 0.9% for a modelled sedimentary organic matter content of 0.5% and 1.6% for 1% organic matter in fine-grained Pegasus Basin sediments. Considerably higher concentrations of methane hydrate of up to 20–70% are predicted to occur where gas migration is focussed within the frontal anticline and proto-thrust zone southeast of the modern accretionary wedge and in channel and basin floor sandstones related to the Hikurangi Channel. The Hikurangi Channel sedimentary system transported coarse clastic sediments eroded from the rising Southern Alps along the eastern margin of the Pegasus Basin since the Miocene. It provides carrier beds specifically for transport of thermogenic gas generated close to the subduction interface. A buried Mesozoic accretionary wedge originating from subduction of the Pacific Plate beneath Gondwana further focusses the migration of gas. Focussed migration of thermogenic gas leads to the highest predicted hydrate concentrations in potential channel sand reservoirs.

We present multichannel seismic data from New Zealand's Hikurangi subduction margin that show widespread evidence for gas migration into the field of gas hydrate stability. Gas migration along stratigraphic layers into the hydrate system manifests itself as highly reflective segments of dipping strata that originate at the base of hydrate stability and extend some distance toward the seafloor. The highly reflective segments exhibit the same polarity as the seafloor reflection, indicating that localized gas hydrate precipitation from gas-charged fluids within relatively permeable layers has occurred. High-density velocity analysis shows that these layer-constrained gas hydrate accumulations are underlain by thick (up to ~500 m) free gas zones, which provide the source for focused gas migration into the hydrate layer. In addition to gas being channeled along layers, we also interpret gas migration through a fault zone into the field of hydrate stability; in this case, a low-velocity layer within the hydrate stability zone extends laterally away from the fault, which might indicate that gas-charged fluids have also migrated away from the fault along strata. At this site, where both dipping strata and faulting seem to influence fluid migration, we observe anomalously high velocities at the base of hydrate stability that we interpret as concentrated gas hydrates. Our results give insight into how shallow fluid flow responds to permeability contrasts between strata, fault zones, and perhaps also the gas hydrate system itself. Ultimately, these relationships can lead to gas migration across the base of hydrate stability and the precipitation of concentrated hydrate deposits.

A TOUGH + HYDRATE flow model analogous of hydrate deposits in the Hikurangi margin has been constructed using the best available estimates of permeability, porosity and hydrate saturation. Seismic velocity and AVO analyses have been used to develop this model. Production rate forecasts were made in a single well model with homogeneous properties. In this forecast, the system was depressurised by producing gas from an underlying free gas zone included in the model. Production of 4.6 Bscf (130 × 106 m3) of gas was achieved from the well over 20 years with average flow rates of 0.65 MMscf/day (0.213 m3/s).

The potential of gas hydrate systems to play a role in submarine slope failure has been well-documented since the late 1970s. Several conceptual models exist for how the gas hydrate-free gas system might weaken submarine sediments, but there is no definitive evidence for gas hydrate-related processes being the primary cause of a particular submarine slope failure. We present a review of coincident gas hydrates and submarine slope instabilities on New Zealand’s active margins. The examples we show represent different failure modes in a range of slope environments, including the upper continental slope and tectonic ridges, with the common factor being that the base of gas hydrate stability approaches the seafloor in these regions. We synthesise several proposed sediment weakening mechanisms and draw comparisons to other global models for gas hydrate-related slope instability. This contribution highlights diverse influences that gas hydrate systems could have on submarine sediment strength, while acknowledging gaps in our understanding of the potential role of gas hydrates, free gas and fluid flow on slope stability.

Gas hydrates have recently been recognised as a class of unconventional petroleum resource and the economic viability of gas production from hydrates is now being viewed as a realistic possibility within the next decade. Therefore, potential offshore hydrate accumulations in the world-class endowed gas hydrate province, the Hikurangi Margin, New Zealand, represent a significant medium- to long-term opportunity to meet the country's future energy requirements. In this paper we delineate a potential gas hydrate reservoir in the East Coast Basin, New Zealand and quantitatively estimate its gas hydrate concentrations from 2D seismic data with no well information available. The target is interesting for exploration since it shows evidence for gas-hydrate bearing sands, in particular, buried channel systems. We use a combined analysis of high-resolution velocity analysis, amplitude-versus-offset (AVO) attribute and AVO inversion to investigate whether we can identify regions that are likely to contain highly concentrated gas hydrates and whether they are likely to be sand-dominated. To estimate hydrate concentrations we apply a rock physics model. Our results indicate the presence of several – up to 200 m thick – zones that are likely to host gas hydrates, with one location predicted to consist of high-permeable channel sands and an inferred gas hydrate saturation of ∼25%. These findings suggest significant amounts of gas hydrates may be present in high-quality reservoirs on this part of the margin.

The quality of reservoir rocks, in particular their permeability, is likely to be a key factor for the economic viability of future gas production from gas hydrates. As for conventional gas resources, high-permeability sands are considered the economically most promising gas hydrate reservoirs. Studies of subsurface lithology however, are difficult without calibration from boreholes. We investigated seismic data from the Hikurangi Margin, a subduction zone east of New Zealand and New Zealand's largest gas hydrate province. We suggest that the strength of bottom simulating reflections (BSRs) from the base of the gas hydrate stability zone may support lithologic interpretations on this margin. BSRs along large parts of this margin are exceptionally weak. Absolute reflection coefficients of a weak BSR on Puke Ridge, a thrust ridge in the accretionary wedge, are roughly between 0.01 and 0.02, an order of magnitude lower than those observed for many BSRs globally. A combination of rock physics modelling and seismic amplitude-versus-offset analysis leads to the conclusion that these weak BSRs are primarily caused by low saturation of gas with patchy distribution, i.e., gas that is only present in pores or fractures of some mesoscopic (i.e., larger than pore sizes but smaller than seismic wavelengths) sediment patches while other patches are fully water saturated. This type of distribution, combined with observed high seismic velocities, is compatible with lithified fine-grained reservoir rocks, similar to indurated mudstones dredged from a submarine outcrop close to the study area. We therefore suggest that weak BSRs may mark fine-grained reservoir rocks with usually low primary permeability. Even though these reservoir rocks may exhibit enhanced secondary permeability from fracturing, they would currently not be considered prime candidates for potential gas production from hydrates. We identified several high amplitude bright spots along weak BSRs. Two possible lithologic explanation for this reflection pattern are that (1) the bright spots mark higher saturation of gas in high-permeability, probably sand-dominated layers, as found in the Gulf of Mexico, and (2) evenly distributed networks of pores may result in gas to be distributed more homogenously in the sediments even though permeability may still be relatively low, as suggested for highly reflective layers beneath the Blake Ridge. Elevated gas hydrate saturations above layers with high saturations of gas would be expected to lead to highly reflective layers in the gas hydrate stability zone and thus, reflection patterns above the BSR may allow distinguishing between both causes.

Highly concentrated gas hydrate deposits are likely to be associated with geological features that promote increased fluid flux through the gas hydrate stability zone (GHSZ). We conduct conventional seismic processing techniques and full-waveform inversion methods on a multi-channel seismic line that was acquired over a 125 km transect of the southern Hikurangi Margin off the eastern coast of New Zealand’s North Island. Initial processing, employed with an emphasis on preservation of true amplitude information, was used to identify three sites where structures and stratal fabrics likely encourage focused fluid flow into and through the GHSZ. At two of the sites, Western Porangahau Trough and Eastern Porangahau Ridge, sub-vertical blanking zones occur in regions of intensely deformed sedimentary layering. It is interpreted that increased fluid flow occurs in these regions and that fluids may dissipate upwards and away from the deformed zone along layers that trend towards the seafloor. At Eastern Porangahau Ridge we also observe a coherent bottom simulating reflection (BSR) that increases markedly in intensity with proximity to the centre of the anticlinal ridge. 1D full-waveform inversions conducted at eight points along the BSR reveal much more pronounced low-velocity zones near the centre of the ridge, indicating a local increase in the flux of gas-charged fluids into the anticline. At another anticline, Western Porangahau Ridge, a dipping high-amplitude feature extends from the BSR upwards towards the seafloor within the regional GHSZ. 1D full-waveform inversions at this site reveal that the dipping feature is characterised by a high-velocity zone overlying a low-velocity zone, which we interpret as gas hydrates overlying free gas. These results support a previous interpretation that this high-amplitude feature represents a local “up-warping” of the base of hydrate stability in response to advective heat flow from upward migrating fluids. These three sites provide examples of geological frameworks that encourage prolific localised fluid flow into the hydrate system where it is likely that gas-charged fluids are converting to highly concentrated hydrate deposits.

We present recently-acquired high-resolution seismic data and older lower-resolution seismic data from Rock Garden, a shallow marine gas hydrate province on New Zealand's Hikurangi Margin. The seismic data reveal plumbing systems that supply gas to three general sites where seeps have been observed on the Rock Garden seafloor: the ‘LM3’ sites (including LM3 and LM3-A), the ‘Weka’ sites (including Weka-A, Weka-B, and Weka-C), and the ‘Faure’ sites (including Faure-A, Faure-B, and Rock Garden Knoll). At the LM3 sites, seismic data reveal gas migration from beneath the bottom simulating reflection (BSR), through the gas hydrate stability zone (GHSZ), to two separate seafloor seeps (LM3 and LM3-A). Gas migration through the deeper parts of GHSZ below the LM3 seeps appears to be influenced by faulting in the hanging wall of a major thrust fault. Closer to the seafloor, the dominant migration pathways appear to occupy vertical chimneys. At the Weka sites, on the central part of the ridge, seismic data reveal a very shallow BSR. A distinct convergence of the BSR with the seafloor is observed at the exit point of one of the Weka seep locations (Weka-A). Gas supply to this seep is predicted to be focused along the underside of a permeability contrast at the BGHS caused by overlying gas hydrates. The Faure sites are associated with a prominent arcuate slump feature. At Faure-A, high-amplitude reflections, extending from a shallow BSR towards the seafloor, are interpreted as preferred gas migration pathways that exploit relatively-high-permeability sedimentary layers. At Faure-B, we interpret gas migration to be channelled to the seep along the underside of the BGHS — the same scenario interpreted for the Weka-A site. At Rock Garden Knoll, gas occupies shallow sediments within the GHSZ, and is interpreted to migrate up-dip along relatively high-permeability layers to the area of seafloor seepage. We predict that faulting, in response to uplift and flexural extension of the ridge, may be an important mechanism in creating fluid flow conduits that link the reservoir of free gas beneath the BGHS with the shallow accumulations of gas imaged beneath Rock Garden Knoll. From a more regional perspective, much of the gas beneath Rock Garden is focused along a northwest-dipping fabric, probably associated with subduction-related deformation of the margin.

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 decollement, 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 decollement, 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 decollement. 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

Regional erosion of the Rock Garden ridge top, a bathymetric high within New Zealand’s Hikurangi Subduction Margin, is likely associated with its gas hydrate system. Seismic data reveal gas pockets that appear partially trapped beneath the shallow base of gas hydrate stability. Steady-state fluid flow simulations, conducted on detailed two-dimensional geological models, reveal that anomalous fluid pressure can develop close to the sea floor in response to lower-permeability hydrate-bearing sediments and underlying gas pockets. Transient simulations indicate that large-scale cycling of fluid overpressure may occur on time scales of a few to tens of years. We predict intense regions of hydro-fracturing to preferentially develop beneath the ridge top rather than beneath the flanks, due to more pronounced overpressure generation and gas migration through hydrate-bearing sediments. Results suggest that sediment weakening and erosion of the ridge top by hydro-fracturing could be owed to fluid dynamics of the shallow gas hydrate system.

SUMMARY Gas hydrates were discovered in a mud mound in the lease block Mississippi Canyon 798, Gulf of Mexico, through piston coring. Subsequently, a seismic experiment was carried out to investigate the dynamics behind the hydrate formation. During the experiment, high-resolution multichannel seismic reflection data using a 24-channel, 240 m long streamer and wide-aperture data using six ocean bottom seismometers were collected along five lines. High-reflectivity zones (HRZs) are present in the reflection data along all lines. To better constrain the interpretation of the reflection data, the traveltimes from the multichannel and wide-aperture data sets were jointly inverted to estimate a 2-D P-wave layered velocity model for each line. A minimum-parameter/minimum-structure modelling approach yielded simple models and a comparison of the models at their intersection points shows they are consistent to within ±10 m s −1 in velocity and ±20 m in depth. In the final P-wave velocity models, the HRZs are associated with a lowering of velocity. In the reflection data, the top of the HRZs show a polarity reversal with respect to the seafloor. Presence of free gas in the HRZs best explains the velocity lowering and polarity reversal. It is speculated that the gas has deeper sources and migrates upwards through conduits formed by salt movement in the vicinity. The upward migrating gas accumulates in the axis of a channel complex and manifests itself as HRZs in the reflection data. The fluids circulating along the conduits push the base of the hydrate stability zone close to the seafloor. From the channel axis, the free gas migrates further upwards and close to the seafloor, and as it comes within the gas hydrate stability zone, it forms hydrates.

Recently acquired seismic reflection data across the southern edge of Ritchie Ridge, a prominent bathymetric high on the Hikurangi margin, display zones of high amplitudes and reflections that crosscut strata. We interpret the latter as bottom‐simulating reflections which are commonly associated with gas beneath gas hydrates. An analysis of reflection strength indicates the high‐amplitude zones are caused by free gas in the pore space of sediments, probably migrating upward along layers. One of the high‐amplitude regions is situated beneath the projected location of a known gas vent site. The seismic data appear to image the conduits that supply this vent site with gas. The seafloor in most of the study area is likely to be within the zone of gas hydrate stability, depending on bottom water temperatures and hydrate composition. Hence, gas appears to be venting through the gas hydrate stability zone, favouring locally high concentrations of gas hydrates.

As offshore petroleum exploration and development move into deeper water, industry must contend increasingly with gas hydrate, a solid compound that binds water and a low-molecular-weight gas (usually methane). Gas hydrate has been long studied in industry from an engineering viewpoint, due to its tendency to clog gas pipelines. However, hydrate also occurs naturally wherever there are high pressures, low temperatures, and sufficient concentrations of gas and water. These conditions prevail in two natural environments, both of which are sites of active exploration: permafrost regions and marine sediments on continental slopes. In this article we discuss seismic detection of gas hydrate in marine sediments. Gas hydrate in deepwater sediments poses both new opportunities and new hazards. An enormous quantity of natural gas, likely far exceeding the global inventory of conventional fossil fuels, is locked up worldwide in hydrates. Ex-traction of this unconventional resource presents unique exploration, engineering, and economic challenges, and several countries, including the United States, Japan, Canada, India, and Korea, have initiated joint industry-academic-governmental programs to begin studying those challenges. Hydrates also constitute a potential drilling hazard. Because hydrates are only stable in a restricted range of pressure and temperature, any activity that sufficiently raises temperature or lowers pressure could destabilize them, releasing potentially large volumes of gas and decreasing the shear strength of the host sediments. Assessment of the opportunities and hazards associated with hydrates requires reliable methods of detecting hydrate and accurate maps of their distribution and concentration. Hydrate may occur only within the upper few hundred meters of deepwater sediment, at any depth between the seafloor and the base of the stability zone, which is controlled by local pressure and temperature. Hydrate is occasionally exposed at the seafloor, where it can be detected either visually or acoustically by strong seismic reflection amplitudes or high backscatter …

Morphological and seismic data from a submarine landslide complex east of New Zealand indicate flow-like deformation within gas hydrate-bearing sediment. This “creeping” deformation occurs immediately downslope of where the base of gas hydrate stability reaches the seafloor, suggesting involvement of gas hydrates. We present evidence that, contrary to conventional views, gas hydrates can directly destabilize the seafloor. Three mechanisms could explain how the shallow gas hydrate system could control these landslides. (1) Gas hydrate dissociation could result in excess pore pressure within the upper reaches of the landslide. (2) Overpressure below low-permeability gas hydrate-bearing sediments could cause hydrofracturing in the gas hydrate zone valving excess pore pressure into the landslide body. (3) Gas hydrate-bearing sediment could exhibit time-dependent plastic deformation enabling glacial-style deformation. We favor the final hypothesis that the landslides are actually creeping seafloor glaciers. The viability of rheologically controlled deformation of a hydrate sediment mix is supported by recent laboratory observations of time-dependent deformation behavior of gas hydrate-bearing sands. The controlling hydrate is likely to be strongly dependent on formation controls and intersediment hydrate morphology. Our results constitute a paradigm shift for evaluating the effect of gas hydrates on seafloor strength which, given the widespread occurrence of gas hydrates in the submarine environment, may require a reevaluation of slope stability following future climate-forced variation in bottom-water temperature.

The Blake Ridge Diapir is the southernmost of a line of salt diapirs along the Carolina trough. Diapirs cause faulting of the superjacent sediments, creating pathways for migration of fluids and gas to the seafloor. We analyzed reflection seismic data from the Blake Ridge Diapir, which is located in a region with known abundant gas hydrate occurrence. A striking feature in these data is a significant shallowing of the base of gas hydrate stability (BGHS) over the center of the diapir: The seafloor is warped up by about 100 m above the diapir, from about 2300 m to about 2200 m. The BGHS, as indicated by a bottom simulating reflection (BSR), is about 4.5 s off the flanks of the diapir, rising to about 4.15 s at the center. Above the diapir, a fault system appears to rise vertically from the BGHS to about 0.05 s below the seafloor (40–50 m); it then diverges into several steeply dipping faults that breach the seafloor and cover an area ∼700 m in diameter. Other secondary faults diverge from the main fault or emerge directly from the BGHS near the crest of the diapir. Gas and other fluids may migrate upward through the faults. We performed complex trace analysis to compare the reflection strength and instantaneous frequency along individual reflections. A low-frequency anomaly over the center of the diapir indicates high seismic attenuation. This is interpreted to be caused by migration of fluids (probably methane) along fault pathways. The migration of gas (i.e. probably mainly methane) through the gas hydrate stability zone is not yet understood. We speculate that pore fluids in the faults may be too warm and too salty to form gas hydrate, even at depths where gas hydrate is stable away from the diapir. Alternatively, gas hydrates may seal the fault walls such that water supply is too low to transform all the gas into gas hydrates. The shallowing of the BSR may reflect increased heatflow above the diapir either caused by the high thermal conductivity of the underlying salt or by advective heat transport along with fluids. High pore water salinity shifts the gas hydrate stability to lower temperatures and may also play a significant role in BSR shallowing. We, therefore, investigated the possible effect of pore water salinity on shallowing of the BSR. We found that BSR shallowing may theoretically be entirely caused by increased salinity over the diapir, although geologically this would not be reasonable. This observation demonstrates the potential importance of pore water salinity for lateral variations of BSR depths, in particular, above salt structures.

The distribution of bottom simulating reflections (BSRs), which mark the base of the gas hydrate zone on marine seismic records across the Peru margin, is examined with respect to tectonic uplift, sedimentation, and carbon content of sediment. Where accretion of only deep ocean pelagic sediment containing less than 1% carbon is seismically imaged, no BSR is observed. No BSR is apparent on seismic records of carbon-rich (4–8%) sediment of rapidly subsiding basins. BSRs are abundant on records of carbon-rich sediment affected by tectonic uplift and/or rapid sedimentation. In areas of uplift, the bottom simulating reflectors contain small amounts of free gas in contact with sediment containing small amounts of gas hydrate. During uplift, free gas is released from gas hydrates by dissociation as the hydrate phase boundary shifts upward with respect to sea level. The strong effects of free gas in reducing seismic velocity and gas hydrate in increasing velocity can explain the relatively small total quantities of gas and hydrate indicated by observations of BSRs off Peru.

Summary Bottom simulating reflectors (BSRs) at the base of the hydrate stability zone are often observed in marine seismic reflection data. The compressional (P-)wave velocity structure of a BSR offshore Costa Rica was analysed by applying a full-waveform inversion technique. The resulting velocity profile indicates that the BSR is caused by a thin low-velocity layer, suggesting the presence of at least a small amount of free gas in the sediment pore space. In undisturbed sediment sections, BSRs offshore Costa Rica are observed over much of the continental margin. They frequently occur at shallow depths beneath the seafloor, usually ~ 100–400 m; at some locations BSRs appear to intersect the seafloor. This was interpreted as an indication that hydrates in the study area form partly from methane which was produced beneath the hydrate stability zone and migrated upwards. In the study area, the vertical movement of the hydrate stability zone relative to a given point in the sediment column, one of the potential factors leading to BSR formation or suppression, is controlled by both sedimentation and vertical tectonism. Both processes may hence play a role in controlling BSR distribution. BSRs are absent in areas which have been affected by slumping, except where the sedimentary section above the BSR remained intact during slumping. This indicates that slope failure can cause the destruction of BSRs.

The convergent margins of Peru and Middle America are dominantly shaped by subduction erosion, which causes tectonic subsidence of large areas. Both margins are largely non-accretionary and most of the incoming trench sediment is being subducted. Gas hydrates are ubiquitous across the Peru margin with its high organic carbon content. Strong bottom simulating reflections (BSRs) are present on the lower slope. A regional lack of BSRs further upslope in Lima Basin may be explained by rapid subsidence leading to a downward movement of the base of gas hydrate stability and absorption of free gas into the gas hydrate stability zone. BSRs in small areas in Lima Basin are perhaps supported by high rates of methane supply offsetting the absorption of gas. Off Costa Rica, seamount subduction promotes slope failure. A general lack of BSRs in sediment sections that were destroyed by slumping suggests no or a very slow re-formation of BSRs after slumping. Above a subducted seamount, sediments containing a BSR remain intact despite inferred strong uplift. The associated pressure decrease should lead to dissociation of gas hydrates, generation of gas, and overpressuring. We speculate that the gas may escape through faults above the BSR. Thermo-hydraulic modeling based mainly on BSR heat flow indicates high frictional heating in the Peru subduction zone causing a landward increase of the thermal gradient. Off Costa Rica, a shallowing of BSR depth reflects locally elevated heat flow coinciding with expected expulsion of fluids at the landward boundary of the frontal prism.

The Hikurangi Margin, east of the North Island of New Zealand, contains a significant gas hydrate province. However, the distribution, concentration and dynamics of hydrate accumulations in the southern portion of the margin (the Pegasus Sub-basin) off the northeastern coast of the South Island are poorly constrained due to a lack of data. In late 2009 and early 2010, a seismic dataset consisting of approximately 3000 km of long-offset 2D seismic data was collected in the Pegasus Sub-basin. Bottom-Simulating Reflections (BSRs) are widespread in the data, and they are supplemented by other features that may indicate the presence of free gas and gas hydrates in zones of high concentration. We present the results of reprocessing and analysis of the data that has focussed on the identification of such specific gas-related features.

Methane is one of the most aggressive greenhouse gases driving climate change. Occurrence of marine gas hydrates depends on temperature, pressure, available gas and fresh water. Therefore changes in pressure and bottom water temperature will influence the formation or dissolution of gas hydrates. In general both parameters vary slowly and hence changes do not result in large Methane contribution to the atmosphere. An exception could be a sudden dissolution of larger quantities of gas hydrate with a related expulsion of Methane. Such focused fluid flow appears as funnel-shaped depressions at the seafloor, so called “pockmarks”. Typical dimensions are within a few hundreds of meters. However, five to twelve kilometre wide “giant pockmarks” (GP) are known as well. The mechanism of formation of GPs is not fully understood. New 2D and 3D multichannel seismic data have been acquired across two fields of giant pockmark-like features to better understand the formation of these depressions and potential for catastrophic methane release in the past.

Pockmarks have been observed on the Chatham Rise east of New Zealand over an area of >20,000 km2. Their formation has been linked primarily to gas hydrate dissociation during glacial-stage sealevel lowstands. We here show first images from a recent high-resolution 3D seismic images targeting the sub-surface structure of a giant pockmarks. Results show in particular that the pockmark may be underlain by a large gas chimney.

Identification of methane sources controlling hydrate distribution and concentrations in continental margins remains a major challenge in gas hydrate research. Lack of deep fluid samples and high quality regional scale seismic reflection data may lead to underestimation of the significance of fluid escape from subducting and compacting sediments in the global inventory of methane reaching the hydrate zone, the water column and the atmosphere. The distribution of concentrated hydrate zones in relation to focused fluid flow across the southern Hikurangi subduction margin was investigated using high quality, long offset (10 km streamer), pre-stack depth migrated multichannel seismic data. Analysis of low P wave velocity zones, bright-reverse polarity reflections and dim-amplitude anomalies reveals pathways for gas escape and zones of gas accumulation. The study shows the structural and stratigraphic settings of three main areas of concentrated hydrates: (1) the Opouawe Bank, dominated by focused periodic fluid input along thrust faults sustaining dynamic hydrate concentrations and gas chimneys development; (2) the frontal anticline, with a basal set of protothrusts controlling permeability for fluids from deeply buried and subducted sediments sustaining hydrate concentrations at the crest of the anticline; and (3) the Hikurangi Channel, with buried sand dominated channels hosting significant amounts of gas beneath the base of the hydrate zone. In sand dominated channels gas injection into the hydrate zone favors highly concentrated hydrate accumulations. The evolution of fluid expulsion controlling hydrate formation offshore southern Hikurangi is described in stages during which different methane sources (in situ, buried and thermogenic) have been dominant.

[1] Multibeam swath bathymetry data from the southwest margin of the Chatham Rise, New Zealand, show gas release features over a region of at least 20,000 km2. Gas escape features, interpreted to be caused by gas hydrate dissociation, include an estimated a) 10 features, 8–11 km in diameter and b) 1,000 features, 1–5 km in diameter, both at 800–1,100 m water depth. An estimated 10,000 features, ∼150 m in diameter, are observed at 500–700 m water depth. In the latter depth range sub-bottom profiles show similar gas escape features (pockmarks) at disconformities interpreted to mark past sea-level low stands. The amount of methane potentially released from hydrates at each of the largest features is ∼7*1012 g. If the methane from a single event at one 8–11 km scale pockmark reached the atmosphere, it would be equivalent to ∼3% of the current annual global methane released from natural sources into the atmosphere.

We evaluate different hypotheses concerning the formation of a peculiar, flat-topped ridge at Rock Garden, offshore of the North Island of New Zealand. The coincidence of the ridge bathymetry with the depth at which gas hydrate stability intersects the seafloor has been previously used to propose that processes at the top of gas hydrate stability may cause seafloor erosion, giving rise to the flat ridge morphology. Two mechanisms that lead to increased fluid pressure (and sediment weakening) have previously been proposed: (1) periodic formation (association) and dissociation of gas hydrates during seafloor temperature fluctuations; and (2) dissociation of gas hydrates at the base of gas hydrate stability during ridge uplift. We use numerical models to test these hypotheses, as well as to evaluate whether the ridge morphology can develop by tectonic deformation during subduction of a seamount, without any involvement from gas hydrates. We apply a commonly-used 1D approach to model gas hydrate formation and dissociation, and develop a 2D mechanical model to evaluate tectonic deformation. Our results indicate that: (1) Tectonics (subduction of a seamount) may cause a temporary flat ridge morphology to develop, but this evolves over time and is unlikely to provide the main explanation for the ridge morphology; (2) Where high methane flux overwhelms the anaerobic oxidation of methane via sulphate reduction near the seafloor, short-period temperature fluctuations (but on timescales of years, not months as proposed originally) in the bottom water can lead to periodic association and dissociation of a small percentage of gas hydrate in the top of the sediment column. However, the effect of this on sediment strength is likely to be small, as evidenced by the negligible change in computed effective pressure; (3) The most likely mechanism to cause sediment weakening, leading to seafloor erosion, results from the interaction of gas hydrate stability with tectonic uplift of the ridge, provided bulk permeability strongly decreases with increasing hydrate content. Rather than overpressure developing from dissociation of hydrates at the base of gas hydrate stability (as previously thought), we found that the weakening is caused by focusing of gas hydrate formation at shallow sediment levels. This creates large fluid pressures and can lead to negative effective pressures near the seafloor, reducing the sediment strength.

[1] P-wave velocity increases in the presence of gas hydrates and decreases in the presence of free gas in the sediments, making it an excellent means to investigate gas hydrate systems. However, seismic velocity is typically derived from surface seismic data without consideration of seismic anisotropy. The presence of anisotropy in the hydrate bearing sediments adds an additional complexity in data analysis; however anisotropy can help reveal the distribution of hydrates. Here we report on the evidence of seismic anisotropy at Hydrate Ridge along the Cascadia convergent margin. We find that the south summit is anisotropic, while the basin side (east of south summit) is isotropic. Anisotropy is likely caused by the hydrate veins. We interpret the anisotropy parameters in terms of the distribution and fabric of gas hydrates.

[1] The dissociation of gas hydrates in sediment pores is thought to decrease seafloor strength potentially facilitating submarine slides because of the generation of overpressured gas and the “melting” of load-bearing or cementing solid hydrate. Here, we present findings that suggest gas hydrates may lead to a previously unknown mechanism of seafloor erosion. Gas-hydrate-bearing sub-sea ridges on the Hikurangi Margin are eroded close to the top of the hydrate stability field in the ocean. We hypothesize that gas hydrate instability may lead to ridge erosion by a combination of two processes; hydrate destabilization caused by depressurization during ridge uplift, and repeated pore volume expansion and contraction from hydrate dissociation and formation triggered by fluctuating water temperatures.

The determination of geotechnical parameters at the sea floor is of growing importance for engineering purposes. The measurement of compressional (P-) and especially shear (S-) wave velocities seems to he an appropriate method for the estimation of other physical properties in marine sediments.

Numerous large pockmark structures found on the slope of the Chatham Rise and Canterbury Bight off the east coast of New Zealand have been examined. Using a variety of techniques, we test a hypothesis of pockmark formation that involves the dissociation of hydrates on a glacial timescale (Davy et al. 2010), estimate the timing of pockmark formation, and investigate the possible link between gas hydrates and hydrocarbon reservoirs in the Canterbury and Great South basins.