A topographic signature of a hydrodynamic origin for submarine gullies

Submarine gullies - small scale, straight, shallow channels formed in relatively high seafloor-slope settings - are ubiquitous features that play an important role in the general evolution of continental margin morphology. The mechanisms associated with the origin and evolution of submarine gullies are, however, still poorly defined. In this paper we present evidence of a topographic signature of gully erosion in the Cook Strait sector of the Hikurangi subduction margin, New Zealand. This signature indicates that submarine gully initiation is a threshold process driven by unconfined, directionally-stable, fluid or sediment gravity flows accelerating downslope. We propose cascading dense water, a type of current that is driven by seawater density contrast, as the source of these flows. The sensitivity of such ephemeral hydrodynamic events to climate change raises questions regarding implications for future variation of the distribution and magnitude of a significant seafloor erosion process.peer-reviewe

Submarine mass movements and their consequences

Submarine spreading is a type of mass movement that involves the extension and fracturing of a thin surficial layer of sediment into coherent blocks and their finite displacement on a gently sloping slip surface. Its characteristic seafloor signature is a repetitive pattern of parallel ridges and troughs oriented perpendicular to the direction of mass movement. We map ~30 km2 of submarine spreads on the upper slope of the Hikurangi margin, east of Poverty Bay, North Island, New Zealand, using multibeam echosounder and 2D multichannel seismic data. These data show that spreading occurs in thin, gently-dipping, parallel-bedded clay, silt and sandy sedimentary units deposited as lowstand clinoforms. More importantly, high-amplitude and reverse polarity seismic reflectors, which we interpret as evidence of shallow gas accumulations, occur extensively in the fine sediments of the upper continental slope, but are either significantly weaker or entirely absent where the spreads are located. We use this evidence to propose that shallow gas, through the generation of pore pressure, has played a key role in establishing the failure surface above which submarine spreading occurred. Additional dynamic changes in pore pressure could have been triggered by a drop in sea level during the Last Glacial Maximum and seismic loading.peer-reviewe

Geomorphic response of submarine canyons to tectonic activity: Insights from the Cook Strait canyon system, New Zealand

Active margins host more than half of submarine canyons worldwide. Understanding the coupling between active tectonics and canyon processes is required to improve modeling of canyon evolution and derive tectonic information from canyon morphology. In this paper we analyze high-resolution geophysical data and imagery from the Cook Strait canyon system (CS), offshore New Zealand, to characterize the influence of active tectonics on the morphology, processes, and evolution of submarine canyons, and to deduce tectonic activity from canyon morphology. Canyon location and morphology bear the clearest evidence of tectonic activity, with major faults and structural ridges giving rise to sinuosity, steep and linear longitudinal profi les, cross-sectional asymmetry, and breaks in slope gradient, relief, and slope-area plots. Faults are also associated with stronger and more frequent sedimentary fl ows, steep canyon walls that promote gully erosion, and seismicity that is considered the most likely trigger of failure of canyon walls. Tectonic activity gives rise to two types of knickpoints in the CS. Gentle, rounded and diffusive knickpoints form due to short-wavelength folds or fault breakouts. The more widespread steep and angular knickpoints have migrated through canyonfloor slope failures and localized quarrying and/or plucking. Migration is driven by base-level lowering due to regional margin uplift and deepening of the lower Cook Strait Canyon, and is likely faster in larger canyons because of higher sedimentary flow throughput. The knickpoints, nonadherence to Playfair"s Law, linear longitudinal profiles, and lack of canyon-wide, inverse power law slope-area relationships indicate that the CS is in a transient state, adjusting to perturbations associated with tectonic displacements and changes in base level and sediment fluxes. We conclude by inferring unmapped faults and regions of more pronounced uplift, and proposing a generalized model for canyon geomorphic evolution in tectonically active margins

Subaqueous mass movements in the context of observations of contemporary slope failure

The consequences of subaqueous landslides have been at the forefront of societal conscience more than ever in the last few years, with devastating and fatal events in the Indonesian Archipelago making global news. The new research presented in this volume demonstrates the breadth of ongoing investigation into subaqueous landslides, and shows that while events like the recent ones can be devastating, they are smaller in scale than those Earth has experienced in the past. Understanding the spectrum of subaqueous landslide processes, and therefore the potential societal impact, requires research across all spatial and temporal scales. This volume delivers a compilation of state-of-the-art papers covering regional landslide databases, advanced techniques for in situ measurements, numerical modelling of processes and hazard

Processes on the precipice : seafloor dynamics across the upper Malta-Sicily escarpment

The Malta-Sicily Escarpment (MSE) is a steep, sediment-undersupplied, carbonate escarpment incised by a series of submarine canyons. In this study we present data acquired from the upper MSE during the Eurofleets-funded CUMECS cruise to document a complex seafloor morphology comprising gullies, canyon heads, mass movement scars, channels, contourites and escarpments. The evolution of the upper MSE has been driven by the interaction of fault activity, sedimentary activity related to hemipelagic, pelagic and contouritic sedimentation, and seafloor incision by bottom current activity. Submarine mass movements play a key role in canyon development – they control the extent of lateral and headward extension, facilitate tributary development, remove material from the continental shelf and slope, and feed sediment into the canyons.peer-reviewe

Deep-seated bedrock landslides and submarine canyon evolution in an active tectonic margin : Cook Strait, New Zealand

The Cook Strait sector of the Hikurangi subduction margin, off south-east central New Zealand, is dominated by a multi-branched canyon system where landslides are widespread. The objective of this study is to determine the character, origin, and influence of these landslides on the evolution of the canyon system. Multibeam bathymetry covering seven submarine canyons is utilised to characterise landslides’ spatial distribution, morphological attributes and area-frequency characteristics. We demonstrate that mass movements within the Cook Strait canyons consist of spatially dense, predominantly retrogressive, small, deep-seated, translational bedrock landslides occurring in Late Cenozoic sequences. These landslides affect up to a quarter of the canyoned area. Concentration of landslides in the shallow canyon reaches (down to 800 m) is attributed to the influence of oceanographic processes originating on the continental shelf such as tide- generated currents, dense shelf water cascading and internal waves. Canyon incision and wall undercutting, locally favoured by underlying lithological control, are proposed as major landslide drivers in Cook Strait. Ground motion during regional earthquakes is considered a secondary cause. Retrogressive landslides are responsible for canyon widening and wall retreat, cross-sectional asymmetry, preconditioning for additional failure, destabilisation of adjacent slopes and delivery of sediment into canyon floors.peer-reviewe

Porewater Geochemical Assessment of Seismic Indications for Gas Hydrate Presence and Absence: Mahia Slope, East of New Zealand’s North Island

We compare sediment vertical methane flux off the Mahia Peninsula, on the Hikurangi Margin, east of New Zealand’s North Island, with a combination of geochemical, multichannel seismic and sub-bottom profiler data. Stable carbon isotope data provided an overview of methane contributions to shallow sediment carbon pools. Methane varied considerably in concentration and vertical flux across stations in close proximities. At two Mahia transects, methane profiles correlated well with integrated seismic and TOPAS data for predicting vertical methane migration rates from deep to shallow sediment. However, at our “control site”, where no seismic blanking or indications of vertical gas migration were observed, geochemical data were similar to the two Mahia transect lines. This apparent mismatch between seismic and geochemistry data suggests a potential to underestimate gas hydrate volumes based on standard seismic data interpretations. To accurately assess global gas hydrate deposits, multiple approaches for initial assessment, e.g., seismic data interpretation, heatflow profiling and controlled-source electromagnetics, should be compared to geochemical sediment and porewater profiles. A more thorough data matrix will provide better accuracy in gas hydrate volume for modeling climate change and potential available energy content

A new depositional model for the Tuaheni Landslide Complex, Hikurangi Margin, New Zealand

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

Submarine mass movements and their consequences

This sixth edition of the Submarine Mass Movements and Their Consequences volume, coincident with the seventh eponymous conference includes 61 papers that span a variety of topics and are organized into nine parts as follows: (1) Submarine mass movement in margin construction and economic significance; (2) Failure dynamics from landslide geomorphology; (3) Geotechnical aspects of mass movement; (4) Multidisciplinary case studies; (5) Tectonics and mass move- ment processes; (6) Fluid flow and gas hydrates, (7) Mass transport deposits in modern and outcrop sedimentology; (8) Numerical and statistical analysis; and, (9) Tsunami generation from slope failure. The breath and quality of this body of work underpins a positive outlook and our enthusiasm for the future direction of research in this area of science as it moves towards ever more detailed analysis and monitoring. We also emphasize in this volume the need to look at mountain-scale outcrops to better understand our seismic imaging, to carry out statistical studies that draw on global data sets to better constrain broad behavioural characteristics, and to undertake numerical modelling to understand the sensitivity of a range of natural slopes.peer-reviewe