Description: Modern scleractinian corals are classical components of marine shallow warm water ecosystems. Their occurrence and diversity patterns in the geological record have been widely used to infer past climates and environmental conditions. Coral skeletal composition data reflecting the nature of the coral environment are often affected by diagenetic alteration. Ghost structures of annual growth rhythms are, however, often well preserved in the transformed skeleton. We show that these relicts represent a valuable source of information on growth conditions of fossil corals. Annual growth bands were measured in massive hemispherical Porites of late Miocene age from the island of Crete (Greece) that were found in patch reefs and level bottom associations of attached mixed clastic environments as well as isolated carbonate environments. The Miocene corals grew slowly, about 2-4 mm/yr, compatible with present-day Porites from high-latitude reefs. Slow annual growth of the Miocene corals is in good agreement with the position of Crete at the margin of the Miocene reef belt. Within a given time slice, extension rates were lowest in level bottom environments and highest in attached inshore reef systems. Because sea surface temperatures (SSTs) can be expected to be uniform within a time slice, spatial variations in extension rates must reflect local variations in light levels (low in the level bottom communities) and nutrients (high in the attached reef systems). During the late Miocene (Tortonian–early Messinian), maximum linear extension rates remained remarkably constant within seven chronostratigraphic units, and if the relationship of SSTs and annual growth rates observed for modern massive Indo-Pacific Porites spp. applies to the Neogene, minimum (winter) SSTs were 20°-21°C. Although our paleoclimatic record has a low resolution, it fits the trends revealed by global data sets. In the near future we expect this new and easy to use Porites thermometer to add important new information to our understanding of Neogene climate.

Description: The Chatham Rise is located offshore of New Zealand's South Island. Vast areas of the Chatham Rise are covered in circular to elliptical seafloor depressions that appear to be forming through a bathymetrically controlled mechanism, as seafloor depressions 2-5 km in diameter are found in water depths of 800-1100 m. High resolution P-Cable 3D seismic data were acquired in 2013 across one of these depressions. The seafloor depression is interpreted as a mounded contourite. Our data reveal several smaller buried depressions (〈20-650 m diameter) beneath the mounded contourite that we interpret as paleo-pockmarks. These pockmarks are underlain by a complex polygonal fault system that deforms strata and an unusual conical feature. We interpret the conical feature as a sediment remobilization structure based on the presence of stratified reflections within the feature, RMS amplitude values and lack of velocity anomaly that would indicate a non-sedimentary origin. The sediment remobilization structure, polygonal faults and paleo-depressions are indicators of past subsurface fluid flow. We hypothesize that the pockmarks provided the necessary topographic roughness for formation of the mounded contourites thus linking fluid expulsion and deposition of contouritic drifts.

Description: The Hikurangi Margin off the east coast of the North Island (Te Ika-a-Māui) is a tectonically active subduction zone and the location of New Zealand’s largest gas hydrate province. Faults are internally complex volumetric zones that may play a significant role in the migration of fluids beneath the seafloor. The combined processes of deformation and fluid migration result in the formation of concentrated hydrate accumulations along accretionary ridges. It is not fully understood to what extent faults control fluid migration along the Hikurangi Margin, and whether deep-seated thrust faults provide a pathway for thermogenic gas to migrate up from sources at depth. Using 2D models based on seismic data from the region we investigated the role of thrust faults in facilitating fluid migration and contributing to the formation of concentrated gas hydrates. By altering permeability properties of the fault zones in these transient state models we can determine whether faults are required to act as fluid flow pathways. In this study we focus on two study sites offshore southern Wairarapa, using realistic yet simplified fault geometries derived from 2D seismic lines. The results of these models allow us to start to disentangle the complex relationship between fault zone structure, permeability, geometry, fluid migration and gas hydrate formation. Based on the model outputs we propose that faults act as primary pathways facilitating fluid migration and are critical in the formation of concentrated gas hydrate deposits.

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.

Description: Highlights • Gas hydrate systems modelling reproduces concentrated gas hydrates indicated by high amplitude seismic reflections. • Spatially variable rates in microbial gas generation beneath the hydrate stability zone drive gas hydrate formation. • Gas migration through faults and up-dip migration through permeable layers control gas hydrate distribution within ridges. • Gas hydrate accumulation is enhanced by gas recycling, leading to the formation of concentrated gas hydrates in 〈2 Ma. Abstract Gas hydrates are widespread along convergent margins, but their distribution is highly variable. This variability has been attributed to a range of factors, such as the source of gas and the occurrence of permeable faults and porous or fractured reservoirs. We test these concepts on the Hikurangi Margin, where gas hydrate occurrences of variable character are well-documented by seismic reflection datasets and scientific drilling. We use 3D gas hydrate systems modelling to reconstruct processes of gas generation, migration and gas hydrate formation through time in two thrust ridges at the deformation front (Glendhu and Honeycomb ridges). We compare the results of scenarios using different fault and rock properties with indications for concentrated gas hydrates in reflection seismic data. Gas hydrate distributions are best reproduced by models predicting focussed gas migration through thrust faults and permeable strata. The gas is predominantly sourced from microbial generation beneath the gas hydrate stability zone (HSZ) in sedimentary troughs adjacent to the ridges and migrates up-dip as free gas. During progressive ridge deformation, gas generation shifts to the landward side of the ridges, where strata are rapidly buried, while erosion occurs at the crest of the ridges. A prominent back-thrust in the structurally more mature Glendhu Ridge diverts migrating gas into the HSZ and leads to preferential gas hydrate formation in the landward side of the ridge. Recycling of gas at the base of the HSZ during the past 2 Myrs led to an enrichment of gas hydrates, first in the center of the anticlines and then progressively more in the landward limbs. We propose that this process of diverting gas migration into the HSZ during thrust ridge formation is a common feature not only at the southern Hikurangi Margin, but in many convergent margins with high sedimentation rates and a thick accretionary wedge.

Description: Sub-seabed fluid flow, gas hydrate accumulation and seafloor methane seepage are tightly interwoven processes with implications for marine biodiversity, ocean chemistry and seafloor stability. We combine long-offset seismic reflection data with high-resolution seismic data to investigate shallow structural deformation and its relationship to focused gas migration and hydrate accumulation in the southern Hikurangi subduction wedge. Anticlines, effective traps for focusing free gas, are characterized by both normal faults and vertical zones of hydraulic fracturing within the hydrate stability zone. The normal faults form as a result of sediment layer folding and gravitational collapse of ridges during uplift. We document both longitudinal (ridge-parallel) and transverse (ridge-perpendicular) extensional structures (normal faults and elongated hydraulic fracture zones) in the sub-seafloor of anticlinal ridges. Intriguingly, gas flow through ridges close to the deformation front of the wedge exploits longitudinal structures, while ridges further inboard are characterized by gas flow along transverse structures. This highlights pronounced changes in the shallow deformation of ridges in different parts of the wedge, associated with a switching of the least and intermediate principal stress directions. It is critical to understand these shallow stress fields because they control fluid flow patterns and methane seepage out of the seafloor. Key Points Gas migration through ridges occurs along both longitudinal (ridge-parallel) and transverse (ridge-perpendicular) zones of fracturing Shallow stress fields differ significantly between ridges, reflecting differences in ridge evolution and deformation Seismic reflection images of the base of gas hydrate stability and gas-water contacts are strongly affected by seismic frequency content