Description: Highlights • Large seafloor depressions with diameters of up 10 km across have been mapped on the southern Chatham Rise, New Zealand. • Seismic reflection data show scarce indications for vertical fluid flow but no clear link between fluid flow and depressions. • Methane gas or methane hydrates appear to be absent on the southern Chatham Rise. • Seismic evidence suggests that vertical fluid flow was likely fuelled by polygonal faulting and silica diagenesis • The depressions are the results of erosion and sediment drift deposition of bottom currents associated with the Subtropical Front. Abstract Several giant seafloor depressions were investigated on the Chatham Rise offshore New Zealand using mainly bathymetric and seismic data, supplemented by sediment cores and reported porewater geochemistry data. The depressions have diameters of up to 11 km and occur on the southern flank of the Chatham Rise in water depths between 600 and 900 m, i.e. roughly underneath the location of the strongest thermal gradients of the Subtropical Front (STF) and characterized by eastward flowing currents. With up to 150 m of relief the depressions cut into post-Miocene deposits. Some of the depressions are partially filled with drift deposits that have similar seismic characteristics as the surrounding sediments and consist of alternations of silty muds and silts. Seismic profiles also show completely filled depressions that no longer have a bathymetric expression. Despite several pipe structures indicating vertical fluid flow, neither active fluid seepage nor indications for past fluid seepage are present at the seafloor of the Chatham Rise. Also, both pore water geochemistry and geophysical data do not show indications for an existing or past gas hydrate system in the area. Instead, seismic data suggest widespread polygonal faulting and the presence of silica diagenetic fronts. The release of mineral-bound water during silica diagenesis or fluid expulsion during sediment compaction can explain the presence of vertical fluid flow features but not the giant depressions themselves. Instead, the depressions are interpreted as the result of scouring by strong bottom currents for which fluid venting may have created the nucleation points.

Description: Highlights: • Identify 3 groups of gas migration structures in seismic data from the Danube Fan. • Migration structures related to shallow gas migration and flares at the seafloor. • Gas migration is controlled by lithological heterogeneity and sediment deformation. • Mass transport deposits play a role in controlling vertical migration occurrence. Abstract: A large geophysical dataset, including bathymetry, and 2D and 3D P-cable seismic data, revealed evidence of numerous gas flares near the S2 Canyon in the Danube Fan, northwestern Black Sea. This dataset allows us to investigate potential relationships between gas migration pathways, gas vents observed at the seafloor and submarine slope failures. Vertical gas migration structures as revealed in the seismics appear to be concentrated near submarine slope failure structures. Where these seismically defined features extend upwards to the seafloor, they correlate with the location of gas flares. However, not all these structures reach the seafloor, in some cases because they are capped by overlying sediments. A strong correlation is inferred between gas migration pathways, heterogeneous mass transport deposits and contacts between adjacent units of contrasting lithology. Although missing age constrains prevent a final judgement, we discuss the potential relationship between submarine slope failures and gas migration in order to determine if gas migration is a precursor to failure, or if the presence of slope failures and associated mass transport deposits facilitates the migration of gas. Our observations indicate that lithological heterogeneity, mass transport deposits and minor sediment deformation control gas migration pathways and the formation of gas chimney-like features. Gas migration is focused and gradual, resulting in gas flares where the chimney-like features extend to the seafloor, with no evidence of erosive features such as pockmarks.

Description: Highlights • BSR position does not match BGHS as predicted based on regional TP conditions. • Use steady state and transient models to determine extent of hydrate stability. • Investigate the influence of topographic focusing on hydrate stability. • Variable thermal properties of sediment impact hydrate stability. The Danube Fan in the western Black Sea shows many features indicating the presence of gas and gas hydrates, including a bottom simulating reflection (BSR), high-amplitude anomalies beneath the BSR and the presence of gas flares at the seafloor. The BSR depth derived from 3D P-cable seismic data of an older slope canyon of the fan (the S2 canyon) suggests that the BSR is not in equilibrium with the present-day topography. The Danube Fan was abandoned ∼7.5 ka, and the S2 canyon was likely incised ∼20 ka, suggesting that the gas hydrate system has had at least 7.5 ka years to equilibrate to the present-day conditions. Here we examine the extent and position of the hydrate stability zone through constructing both steady and transient state models of a 2D profile across the S2 canyon. This was done using inputs from mapping of the 3D P-cable seismic data and geochemical analysis of core samples. Using these models, we investigate the effects of different factors including variable thermal properties of heterogeneous sediments in the vicinity of the canyon and, topographic focusing on the geothermal gradient on the extent of the hydrate stability zone. Our results indicate that both factors have a significant effect and that the hydrate system may actually be in, or approaching equilibrium.

Description: A seismic refraction profile across Langeland (Denmark) obtained from land stations recording airgun shots allowed to resolve upper crustal velocities to a depth of 8 km. The profile traverses the proposed Caledonian Deformation Front and the Ringkoebing-Fyn High. The Ringkoebing-Fyn High is about 10 km wide and the top basement lies less than 2 km below the surface. Basement velocities as high as 6.4 km/s, at depths between 6 and 8 km, can be best explained by compositional changes between adjoining basement units to the north and south. South of the Ringkoebing-Fyn High another high velocity basement unit is encountered and most probably represents a basement affected by the Caledonian orogeny. Along this profile on Langeland the positions of the Caledonian Deformation Front and the northern limit of the Zechstein deposits coincide.

Description: Highlights • Elongated fault structures are conduits for focused fluid flow. • Gas migration occurs only along a sub-set of faults across Opouawe bank. • Stress state deduced from 3D fault structures appears partially stratigraphically controlled. Abstract High-resolution 2D and 3D seismic data from Opouawe Bank, an accretionary ridge on the Hikurangi subduction margin off New Zealand, show evidence for exceptional gas migration pathways linked to the stress regime of the ridge. Although the ridge has formed by thrusting and folding in response to a sub-horizontal principal compressive stress (σ1), it is clear that local stress conditions related to uplift and extension around the apex of folding (i.e. sub-vertical σ1) are controlling shallow fluid flow. The most conspicuous structural features are parallel and horizontally-elongated extensional fractures that are perpendicular to the ridge axis. At shallower depth near the seafloor, extensional fractures evolve into more concentric structures which ultimately reach the seafloor where they terminate at gas seeps. In addition to the ridge-perpendicular extensional fractures, we also observe both ridge-perpendicular and ridge-parallel normal faults. This indicates that both longitudinal- and ridge-perpendicular extension have occurred in the past. The deepest stratigraphic unit that we image has undergone significant folding and is affected by both sets of normal faults. Shallower stratigraphic units are less deformed and only host the ridge-parallel normal faults, indicating that longitudinal extension was limited to an older phase of ridge evolution. Present-day gas migration has exploited the fabric from longitudinal extension at depth. As the gas ascends to shallower units it ‘self-generates’ its flow pathways through the more concentric structures near the seafloor. This shows that gas migration can evolve from being dependent on inherited tectonic structures at depth, to becoming self-propagating closer to the seafloor.

Description: The convergent margin of the central Sunda Arc in Indonesia was the target of a reflection and refraction seismic survey conducted in 1998 and 1999. Along two seismic lines across the subduction complex off southern Sumatra and off Sunda Strait, coincident multichannel and wide-angle data were collected, complemented by two refraction strike-lines in the forearc basin off Sumatra. The combined analysis of the acquired data allows us to present a detailed model of the subduction zone where initiation of strain partitioning occurs due to the onset of oblique subduction. The dip of the subducted plate is well defined along both dip-lines and a lateral increase from 5° to 7° from beneath the outer high off Sumatra to Sunda Strait is supported by complementary gravity modelling. The downgoing slab is traced to a depth of more than 30km. On both reflection dip-lines, a clearly developed backstop structure underlying a trench slope break defines the landward termination of the active accretionary prism and separates it from the outer high. Active subduction accretion is supported by laterally increasing velocities between the deformation front and the active backstop structure. Seismic velocities of the outer high are moderate along both lines (〈5.8kms−1 at 20km depth), suggesting a sedimentary composition. Reduced reflectivity beneath a rugged top basement traced along the outer high of both dip-lines supports a high degree of deformation and material compaction. Several kilometres of sediment has accumulated in the forearc domain, although a distinct morphological basin is only recognized off southern Sumatra and is not developed off Sunda Strait. The bathymetric elevation of the Java shelf that is encountered in the southern Sunda Strait corresponds to increased velocities of a basement high there and is connected to extensional structures of the Sunda Strait transtensional basin. Differences observed in the morphology of the forearc domain are also reflected in the lower crustal structure. Off southern Sumatra, the velocity–depth model clearly indicates a continental-type crust underlying the forearc basin, whereas lower velocities are found beneath the Sunda Strait forearc domain. Off Sumatra, some 3-D constraint on the upper plate structure is gained from the refraction strike-lines, which in addition is supported by synthetic data modelling.

Description: Highlights • The Danube deep-sea fan offers best conditions for hydrate production. • Gas production out of a hypothetical methane hydrate reservoir was simulated. • Hazard assessment to investigate the hazard of production-induced slope failures. • Factor of Safety against slope failure is not affected by the production process. • Mobilized mass could hit the production site if landslide were to happen. Methane production from gas hydrate reservoirs is only economically viable for hydrate reservoirs in permeable sediments. The most suitable known prospect in European waters is the paleo Danube deep-sea fan in the Bulgarian exclusive economic zone in the Black Sea where a gas hydrate reservoir is found 60 m below the seafloor in water depths of about 1500 m. To investigate the hazards associated with gas production-induced slope failures we carried out a slope stability analysis for this area. Screening of the area based on multibeam bathymetry data shows that the area is overall stable with some critical slopes at the inner levees of the paleo channels. Hydrate production using the depressurization method will increase the effective stresses in the reservoir beyond pre-consolidation stress, which results in sediment compaction and seafloor subsidence. The modeling results show that subsidence would locally be in the order of up to 0.4 m, but it remains confined to the immediate vicinity above the production site. Our simulations show that the Factor of Safety against slope failure (1.27) is not affected by the production process, and it is more likely that a landslide is triggered by an earthquake than by production itself. If a landslide were to happen, the mobilized sediments on the most likely failure plane could generate a landslide that would hit the production site with velocities of up to 10 m s-1. This case study shows that even in the case of production from very shallow gas hydrate reservoirs the threat of naturally occurring slope failures may be greater than that of hydrate production itself and has to be considered carefully in hazard assessments.

Description: Seafloor elongated depressions are indicators of gas seepage or slope instability. Here we report a sequence of slope-parallel elongated depressions that link to headwalls of sediment slides on upper slope. The depressions of about 250 m in width and several kilometers in length are areas of focused gas discharge indicated by bubble-release into the water column and methane enriched pore waters. Sparker seismic profiles running perpendicular and parallel to the coast, show gas migration pathways and trapped gas underneath these depressions with bright spots and seismic blanking. The data indicate that upward gas migration is the initial reason for fracturing sedimentary layers. In the top sediment where two young stages of landslides can be detected, the slope-parallel sediment weakening lengthens and deepens the surficial fractures, creating the elongated depressions in the seafloor supported by sediment erosion due to slope-parallel water currents.

Description: Submarine currents are a principal factor in controlling seafloor geomorphology. Herein, we investigate the role of dynamic current systems associated with the Subtropical Front in the formation and modification of seafloor depressions off the east coast of New Zealand’s South Island. Seafloor depressions are widespread in this region, with a diverse range of morphologies and sizes. We focus on two ‘end-member’ classes of depressions; densely spaced decametre-scale structures and more isolated ‘giant’ depressions of up to 12 km in diameter. Our results reveal a direct correlation between the dominant current flow direction, and the modification and alignment of depressions. We present a model to illustrate the role of submarine currents in shaping the morphology of these enigmatic seafloor depressions. This model demonstrates how contour currents, and potentially eddy currents, have extensively modified seafloor structures, resulting in elongate, asymmetrical depressions, partially infilled by sediment drift deposits.

Description: 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.