Description: Highlights • Hindon Maar Complex is a new mid-Miocene Fossil-Lagerstätte in New Zealand. • Anoxia in maar lakes allowed exquisite preservation of plant and animal fossils. • The biota is from a lake and Nothofagus/podocarp/mixed broadleaf forest ecosystem. • Fossils record high diversity at humid, warm Southern Hemisphere mid-latitudes. Abstract This paper highlights the geology, biodiversity and palaeoecology of the Hindon Maar Complex, the second Miocene Konservat-Lagerstätte to be described from New Zealand. The Lagerstätte comprises four partly eroded maar-diatreme volcanoes, with three craters filled by biogenic and highly fossiliferous lacustrine sediments. The exceptionally well-preserved and diverse biota from the site is derived from a mid-latitude Southern Hemisphere lake-forest palaeoecosystem, including many fossil taxa not previously reported from the Southern Hemisphere. The most common macrofossils are leaves of Nothofagus, but the flora also includes conifers, cycads, monocots (such as Ripogonum and palms), together with Lauraceae, Myrtaceae and Araliaceae leaves and flowers. The small maar lakes were surrounded by Nothofagus/podocarp/mixed broadleaf forest growing under humid, warm temperate to subtropical conditions. The fossil fauna comprises insects in the orders Odonata, Hemiptera, Thysanoptera, Coleoptera, Diptera, Hymenoptera and Trichoptera, and the fish assemblage includes a non-migratory species of the Southern Hemisphere Galaxias (Galaxiidae) and a significant new record of the freshwater eel Anguilla (Anguillidae). The fossil assemblage also includes the first pre-Quaternary bird feathers from New Zealand and abundant coprolites derived from fish and volant birds, presumably waterfowl. Palynomorph analysis and a 40Ar/39Ar age of 14.6 Ma obtained from basanite associated with the maar complex indicate that the Hindon Maar Complex is of mid-Miocene age (Langhian; New Zealand local stage: Lillburnian). It thus provides a new and unique perspective on Neogene terrestrial biodiversity and biogeography in the Australasian region, around the end of the mid-Miocene thermal optimum and prior to late Miocene–Pleistocene climate cooling episodes when many warm-temperate and subtropical forest components became extinct in New Zealand.

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 • Geostatistical analysis methods applied to multibeam bathymetry and seismic data • Geomorphology of seafloor depressions has been quantitatively characterised. • No direct correlation between gas venting and formation of seafloor depressions • Likely mechanism of depression formation: groundwater flux linked to current flow Abstract Seafloor depressions are widespread on the present-day continental slope along the southeast coast of New Zealand's South Island. The depressions appear to be bathymetrically constrained to depths below 500 m, correlating to the top of the gas hydrate stability zone, and above 1100 m. Similar depressions observed on the Chatham Rise are interpreted to have formed as a result of gas hydrate dissociation, leading to the hypothesis that a similar origin can be applied for the depressions investigated in this study. Our investigation, however, has found limited geophysical or geochemical evidence to support this hypothesis. The objective of this paper is to examine whether a causal relationship can be established between potential mechanisms of depression formation and the present-day seafloor geomorphology. Geostatistical analysis methods applied to multibeam bathymetry and interpretation of 3D seismic data have been used to empirically describe the geomorphology of the seafloor depressions and investigate potential correlations between geomorphology and other processes such as current flow along the shelf and slope in this region and underlying polygonal fault systems. Although the results of our analysis do not preclude that the seafloor depressions formed as a result of gas hydrate dissociation, neither does our geophysical or geochemical evidence support the theory. Therefore, we propose an alternative mechanism that may have been responsible for the formation of these structures. Based on the evidence presented in this study, the most likely mechanism responsible for the formation of these seafloor depressions is groundwater flux related to the interaction of current systems and the complex geomorphology of submarine canyons on the southeast coast of the South Island.

Description: 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.

Description: The southern Hikurangi Subduction Margin is characterized by significant accretion with predicted high rates of fluid expulsion. Bottom simulating reflections (BSRs) are widespread on this margin, predominantly occurring beneath thrust ridges. We present seismic data across the Porangahau Ridge on the outer accretionary wedge. The data show high-amplitude reflections above the regional BSR level. Based on polarity and reflection strength, we interpret these reflections as being caused by free gas. We propose that the presence of gas above the regional level of BSRs indicates local upwarping of the base of gas hydrate stability caused by advective heatflow from upward migrating fluids, although we cannot entirely rule out alternative processes. Simplified modelling of the increase of the thermal gradient associated with fluid flow suggests that funnelling of upward migrating fluids beneath low-permeability slope basins into the Porangahau Ridge would not lead to the pronounced thermal anomaly inferred from upwarping of the base of gas hydrate stability. Focussing of fluid flow is predicted to take place deep in the accretionary wedge and/or the underthrust sediments. Above the high-amplitude reflections, sediment reflectivity is low. A lack of lateral continuity of reflections suggests that reflectivity is lost because of a destruction of sediment layering from deformation rather than gas-hydrate-related amplitude blanking. Structural permeability from fracturing of sediments during deformation may facilitate fluid expulsion on the ridge. A gap in the BSR in the southern part of the study area may be caused by a loss of gas during fluid expulsion. We speculate that gaps in otherwise continuous BSRs that are observed beneath some thrusts on the Hikurangi Margin may be characteristic of other locations experiencing focussed fluid expulsion.

Description: 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.

Description: Sediment weakening due to increased local pore fluid pressure is interpreted to be the cause of a submarine landslide that has been seismically imaged off the southwest coast of New Zealand. Data show a distinct and continuous bottom‐simulating reflection (BSR)—a seismic phenomena indicative of the presence of marine gas hydrate—below the continental shelf from water depths of c. 2400 m to c. 750 m, where it intersects the seafloor. Excess pore fluid pressure (EPP) generated in a free gas zone below the base of gas hydrate stability is interpreted as being a major factor in the slope's destabilisation. Representative sediment strength characteristics have been applied to limit‐equilibrium methods of slope stability analysis with respect to the Mohr‐Coulomb failure criterion to develop an understanding of the feature's sensitivity to EPP. EPP has been modelled with representative material properties (internal angle of friction, bulk soil unit weight and cohesion) to show the considerable effect it has on stability. The best estimate of average EPP being solely responsible for failure is 1700 kPa, assuming a perfectly elastic body above a pre‐defined failure surface in a static environment.

Description: Seismic detection of methane hydrate often relies on indirect or equivocal methods. New multichannel seismic reflection data from the Blake Ridge, located approximately 450 km east of Savannah, Georgia, show three direct seismic indicators of methane hydrate: (1) a paleo bottom‐simulating reflector (BSR) formed when methane gas froze into methane hydrate on the eroding eastern flank of the Blake Ridge, (2) a lens of reduced amplitudes and high P‐wave velocities found between the paleo‐BSR and BSR, and (3) bright spots within the hydrate stability zone that represent discrete layers of concentrated hydrate formed by upward migration of gas. Velocities within the lens (∼1910 m/s) are significantly higher than velocities in immediately adjacent strata (1820 and 1849 m/s). Conservative estimates show that the hydrate lens contains at least 13% bulk methane hydrate within a 2‐km3 volume, yielding 3.2 × 1010kg [1.5 TCF (4.2 × 1010 m3] of methane. Low seismic amplitudes coupled with high interval velocities within the lens offer evidence for possible methane hydrate “blanking.” Hydrate bright spots yield velocities as high as 2100 m/s, with bulk hydrate concentrations predicted as high as 42% in an approximately 15‐m thick layer. Our results show that, under certain circumstances, hydrate in marine sediments can be directly detected in seismic reflections but that quantification of hydrate concentrations requires accurate velocity information. Read More: http://library.seg.org/doi/abs/10.1190/1.1543196

Description: Highlights • Recently acquired high-resolution seismic data and existing low-resolution industry data are presented. • Two large concentrated hydrate deposits are identified beneath Glendhu and Honeycomb ridges. • A novel method involving analysis of seismic velocity and reflectivity is used to obtain estimates of hydrate saturations. • Hydrate saturations peaks of 〉80% are estimated locally. • The main driving mechanism for hydrate accumulations is inferred to be along-strata gas migration. Abstract In the southern Hikurangi subduction margin, widespread gas hydrate accumulations are inferred based on the presence of bottom simulating reflections and recovered gas hydrate samples, mainly associated with thrust ridges. We present a detailed analysis of high- and medium-resolution seismic reflection data across Glendhu and Honeycomb ridges, two elongated four-way closure systems at the toe of the deformation wedge. High-amplitude reflections within the gas hydrate stability zone, coincident with high seismic velocities, suggest the presence of highly concentrated gas hydrate accumulations in the core regions of the anticlinal ridges. A novel method involving combined seismic velocity and reflectivity analysis and rock physics modelling is used to estimate hydrate saturations in localised areas. The effective medium model consistently predicts gas hydrate saturations of ~30% of the pore space at Glendhu Ridge and 〉60% at Honeycomb Ridge, whereas the empirical three-phases weighted equation likely underestimates the amount of gas hydrate present. We note that our estimates are dependent on the vertical resolution of the seismic data (5–14 m), and that the existence of thin layers hosting gas hydrate at higher concentrations is likely based on observations made elsewhere in similar depositional environments. A comparison between the two ridges provides insights into the evolution of thrust related anticlines at the toe of the accretionary wedge. We propose that the main driving mechanism for concentrated hydrate accumulation in the study area is along-strata gas migration. The vertical extent of these accumulations is a function of the steepness of the strata crossing the base of gas hydrate stability, and of the volume of sediments from which fluid flows into each structure. According to our interpretation, older structures situated further landward ofthe deformation front are more likely to host more extensive concentrated hydrate deposits than younger ridges situated at the deformation front and characterised by more gentle folding. The method introduced in this work is useful to retrieve quantitative estimates of gas hydrate saturations based on multi-channel seismic data.

Description: 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.