152 research outputs found
Shallow Gas and the Development of a Weak Layer in Submarine Spreading, Hikurangi Margin (New Zealand)
No full textSubmarine 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
How Offshore Groundwater Shapes the Seafloor
No full textThe MARCAN project, launched last January, is working to fill a gap in our knowledge of how freshwater flowing underground shapes and alters the continental margins
Time-migrated P-Cable seismic reflection data, Tuaheni Landslide, New Zealand, 2014
No full textThree dimensional (3D) seismic reflection data were collected in 2014 at the Tuaheni Landslide Complex, east of New Zealand's North Island. Data were collected using GEOMAR's 3D P-Cable seismic system, deployed from New Zealand's research vessel Tangaroa during voyage TAN1404. The dataset has been processed through to a 3D Kirchhoff time migration using Globe Claritas processing software at GNS Science. The spacing between successive inlines is 6.25 m and between successive crosslines is 3.125 m. See following publication for details about data processing: doi:10.1029/2017TC004906
Evaluating methane outputs from an area of submarine seeps along the northern Hikurangi Margin, New Zealand
No full textCollated global marine surveys have documented large volumes of gaseous methane able to escape from deeply-buried deposits into global oceans as seeps. Seeps are evident where permeable faults and fracture networks allow for the upward transportation of methane from buried deposits into the water column as plumes of rising bubbles. Seep bubbles dissolve the majority of their constitutive methane into the surrounding water column as they rise; however there is evidence of more-prominent seeps transferring undissolved methane through the water column and into the atmosphere.
Due to the biologic origins of methane, the global distribution of buried methane de-posits is highly varied and difficult to predict. High uncertainties in seep locations have resulted in all previous estimations of the global proportion of atmospheric methane attributed to seeps to have very large associated errors. These are mainly due to large extrapolations over global oceans based on findings from surveyed seep fields.
A 2014 NIWA research voyage saw the discovery of an abundant seep field situated at uncharacteristically shallow water depths (150–300 m below sea level) along the raised continental shelf of the Hikurangi Margin, New Zealand. In comparison to other globally documented seep fields, the Hikurangi Margin seeps are numerous (estimated between 585 and 660 surveyed seeps) and cover a large area (∼ 840 km²). Prior to the discovery of this seep field, there was only evidence of 36 seeps along the entire Hikurangi Margin. Acoustically surveyed bubble-rise paths of newly discovered seeps also show evidence of seeps extending the entire height of the water column. The large number of shallow flares present in the abundant seep field represent the potential for considerable amounts of gaseous methane outputs.
To further investigate these seeps, NIWA voyages TAN1505 and TAN1508 that took place in June and July of 2015 employed a range of scientific equipment to analyse features of the rising seep bubbles. Part of these investigations involved the video recordings of rising seep bubbles from the seafloor as well as acoustically surveying rising bubbles using a singlebeam and multibeam echsounder. We have used video and acoustic data sets to create multiple tools and computational techniques for better assessing features of seeps.
We have developed photogrammetric tools that can be used in Matlab to compute bubble-size distributions and bubble-rise rates from still frames of underwater video footage. These bubble parameters have then been combined with singlebeam recorded flare profiles to calculate the flux of emitted methane at the seafloor. These calculations were carried out using the FlareFlow Matlab module, devised by Mario Veloso.
To assess the number of seeps in a multibeam surveyed region, we have created vertically-summed intensity maps of the obtained water column data. Summed-intensity maps display localised high-amplitude features, indicative of seeps. Seep indicators have been used to (1) map the distribution of seeps of the surveyed Hikurangi Margin, (2) assess the total surveyed seep count, and (3) identify regions where seep concentrations are particularly high.
We have combined methane fluxes from analysed seeps with regional seep-distribution maps to approximate the rate at which gaseous methane is escaping from the seafloor across the seep field. Extrapolating seep emissions over the surveyed area approximates 0.99×10⁵ ±0.64×10⁵ m³/yr of undissolved methane is being released across the seep field. Using models of methane preservation, combined with staggered depth models of flares, we have approximated that ∼ 0.2% of the methane emitted at the seafloor is able to reach the atmosphere
Outer shelf seafloor geomorphology along a carbonate escarpment: The eastern Malta Plateau, Mediterranean Sea
No full textSubmarine carbonate escarpments, documented in numerous sites around the world, consist of thick exposures of Mesozoic shallow water carbonate sequences – primarily limestones and dolomites – with reliefs of >1 km and slope gradients of >70°. Whilst most research efforts have focused on the processes that shaped carbonate escarpments into complex and extreme terrains, little attention has been paid to the geomorphology of shelves upslope of carbonate escarpments. In this study we investigate high resolution geophysical, sedimentological and visual data acquired from the eastern Malta Plateau, central Mediterranean Sea, to demonstrate that the outer shelf of a carbonate escarpment is directly influenced by escarpment-forming processes. We document forty eight erosional scars, six long channels and numerous smaller-scale channels, three elongate mounds, and an elongate ridge across the eastern Malta Plateau. By analysing their morphology, seismic character, and sedimentological properties, we infer that the seafloor of the eastern Malta Plateau has been modified by three key processes: (i) Mass movements – in the form of translational slides, spreading and debris flows – that mobilised stratified Plio-Pleistocene hemipelagic mud along the shelf break and that were likely triggered by seismicity and loss of support due to canyon erosion across the upper Malta Escarpment; (ii) NNW-SSE trending sinistral strike-slip deformation in Cenozoic carbonates – resulting from the development of a mega-hinge fault system along the Malta Escarpment since the Late Mesozoic, and SE-NW directed horizontal shortening since the Late Miocene – which gave rise to NW-SE oriented extensional grabens and a NNW-SSE horst; (iii) Flow of bottom currents perpendicular and parallel to the Malta Escarpment, associated with either Modified Atlantic Water flows during sea level lowstands and/or Levantine Intermediate Water flows at present, which was responsible for sediment erosion and deposition in the form of channels and contouritic drifts
Rock mass defect controlled deep-seated landslides in Tertiary soft rock terrain : implications for landscape evolution
No full textRock mass defect controlled deep-seated landslides are widespread within the deeply incised landscapes formed in Tertiary soft rock terrain in New Zealand. The basal failure surfaces of deep-seated slope failures are defined by thin, comparatively weak and laterally continuous bedding parallel layers termed critical stratigraphic horizons. These horizons have a sedimentary origin and have typically experienced some prior tectonically induced shear displacement at the time of slope failure. The key controls on the occurrence and form of deep-seated landslides are considered in terms of rock mass defect properties and tectonic and climatic forcing.
The selection of two representative catchments (in southern Hawke's Bay and North Canterbury) affected by tectonic and climatic forcing has shown that the spatial and temporal initiation of deep-seated bedrock landslides in New Zealand Tertiary soft rock terrain is a predictable rather than a stochastic process; and that deep-seated landslides as a mass wasting process have a controlling role in landscape evolution in many catchments formed in Tertiary soft rock terrain.
The Ella Landslide in North Canterbury is a deep-seated (~85 m) translational block slide that has failed on a 5 - 10 mm thick, kaolinite-rich, pre-sheared critical stratigraphic horizon. The residual strength of this sedimentary horizon, (C'R 2.6 - 2.7 kPa, and Ѳ'R = 16 - 21°), compared to the peak strength of the dominant lithology (C' = 176 kPa, and Ѳ' = 37°) defines a high strength contrast in the succession, and therefore a critical location for the basal failure surface of deep-seated slope failures. The (early to mid Holocene) Ella Landslide debris formed a large landslide dam in the Kate Stream catchment and this has significantly retarded rates of mass wasting in the middle catchment. Numerical stability analysis shows that this slope failure would have most likely required the influence of earthquake induced strong ground motion and the event is tentatively correlated to a Holocene event on the Omihi Fault. The influence of this slope failure is likely to affect the geomorphic development of the catchment on a scale of 10⁴ - 10⁵ years.
In deeply incised catchments at the southeastern margin of the Maraetotara Plateau, southern Hawke's Bay, numerous widespread deep-seated landslides have basal failure surfaces defined by critical stratigraphic horizons in the form of thin « 20 mm) tuffaceous beds in the Makara Formation flysch (alternating sandstone and mudstone units). The geometry of deep-seated slope failures is controlled by these regularly spaced (~70 m), very weak critical stratigraphic horizons (C'R 3.8 - 14.2 kPa, and Ѳ'R = 2 - 5°), and regularly spaced (~45 m) and steeply dipping (-50°) critical conjugate joint/fault sets, which act as slide block release surfaces. Numerical stability analysis and historical precedent show that the temporal initiation of deep-seated landslides is directly controlled by short term tectonic forcing in the form of periodic large magnitude earthquakes. Published seismic hazard data shows the recurrence interval of earthquakes producing strong ground motions of 0.35g at the study site is every 150 yrs, however, if subduction thrust events are considered the level of strong ground motion may be much higher.
Multiple occurrences of deep-seated slope failure are correlated to failure on the same critical stratigraphic horizon, in some cases in three adjacent catchments. Failure on multiple critical stratigraphic horizons leads to the development of a "stepped" landscape morphology. This slope form will be maintained during successive accelerated stream incision events (controlled by long term tectonic and climatic forcing) for as long as catchments are developing in this specific succession. Rock mass defect controlled deep seated landslides are controlling catchment head progression, landscape evolution and hillslope morphology in the Hawke's Bay study area and this has significant implications for the development of numerical landscape evolution models of landscapes formed in similar strata. Whereas the only known numerical model to consider deep seated landslides as an erosion process (ZSCAPE) considers them as stochastic in time and space, this study shows that this could not be applied to a landscape where the widespread spatial occurrence of deep-seated landslides is controlled by rock mass defects.
In both of the study areas for this project, and by implication in many catchments in Tertiary soft rock terrain, deep-seated landslides controlled by rock mass defect strength, spacing and orientation, and tectonic and climatic forcing have an underlying control on landscape evolution. This study quantifies parameters for the development of numerical landscape evolution models that would assess the role of specific parameters, such as uplift rates, incision rates and earthquake recurrence in catchment evolution in Tertiary soft rock terrain
Development of submarine canyon systems on active margins: Hikurangi Margin, New Zealand.
No full textThe development and activity of submarine canyons on continental margins is strongly influenced by temporal and spatial changes in sediment distribution associated with orbitally-forced sea-level cyclicity. On active margins, canyons are also strongly influenced by tectonic processes such as faulting, uplift and earthquakes. Within this framework the role of mass-wasting processes, including sediment failures, bedrock landslides and sediment gravity flows, are to: 1) transport material across the slope; 2) act as intra-slope sediment sources; and 3) shape seafloor morphology. In this project the seafloor-landscape signatures of tectonic and geomorphic processes are analysed to interpret the development of submarine canyon morphology on active margins. Datasets include high-resolution bathymetry data (Simrad EM300), multichannel seismic reflection data (MCS), high-resolution 3.5 kHz seismic reflection data, sediment cores, and dated seafloor samples. High-resolution bathymetric grids are analysed using techniques developed for terrain-roughness analysis in terrestrial landscapes to objectively map and interpret features related to seafloor mass-wasting processes.
The Hikurangi subduction margin of New Zealand provides world-class examples of the control of tectonic and sedimentary processes on margin development, hosting multiple examples of deeply-incised canyon systems across a range of scales. Two main study sites, in Poverty Bay and Cook Strait, provide examples of canyon formation. From these examples conceptual and representative models are developed for the spatial and temporal relationships between active tectonic structures, geology, sediment supply, slope- and shelf-incised canyons, slope gully systems, and bedrock mass failures.
The Poverty Bay site occurs on the subduction-dominated northern Hikurangi Margin, where the ~3000 km² Poverty re-entrant hosts the large Poverty Canyon system, the only shelf-break-to-subduction-trough canyon on the northern margin. The geomorphic development of the re-entrant is affected by gully development on the upper slope, and multi-cubic-kilometre-scale submarine landslides. From this site the study focuses on the initiation and development of upper-slope gullies and the role of deep-seated slope failure in upper-slope evolution. The Cook Strait site occurs on the southern Hikurangi Margin in the subduction-to-strike-slip transition zone. The 1800 km² Cook Strait Canyon incises almost 50 km into the continental shelf, with a multi-branching canyon head converging to a deeply slope-incised meandering main channel fed by multiple contributing slope canyons. Other medium-sized canyons are incised into the adjacent continental slope. Fluvial sediment supply to the coast is relatively low on the southern margin, but Cook Strait is subject to large diurnal tidal currents that mobilise sediment through the main strait area.
Prior to the morphostructural analysis of the Cook Strait and Poverty study sites a revision of the tectonic structure was undertaken. In Cook Strait a revision of the available fault maps was undertaken as part of a wider, related tectonic study of the central New Zealand region. In Poverty Bay very limited prior information was available, and as part of this study the structure and stratigraphy of the entire shelf and upper slope has been interpreted.
On active tectonic margins submarine canyons respond to tectonics at: 1) margin-setting scales relating to their ability to become shelf incised; 2) regional scales relating to canyon-incision response to base-level perturbations; and 3) local scales relating to propagating structures affecting canyon location and geometry.
Interpretation of the spatial distribution of fluid vent sites, gully development and landslide scars leads to the conclusion that seepage-driven failure is not a primary control on the widespread instances of gully formation and landslide erosion affecting structurally-generated relief across the margin. Rather, the erosion of tectonic ridges is dominated by tectonics by: slope oversteepening; weakening of the rockmass in fault-damage zones; and triggering of slope failure by earthquake-generated cyclic loading.
Deep-seated mass failures affect numerous aspects of submarine landscapes and play a major role in the enlargement of canyon systems. They enable the development of slope gully systems and represent a major intra-slope sediment source. Quantitative morphometric analysis together with MCS data indicate that landslides may evolve to be active complexes where landslide debris is remobilized repeatedly, analogous to terrestrial-earthflow processes. This process has not previously been documented on submarine slopes.
A model is presented for the evolution of active margin canyons that contrasts highstand and lowstand canyon activity in terms of channel incision, sedimentary processes and slope-erosion processes. During sea-level highstand intervals, canyons become decoupled from their terrestrial/coastal sediment-supply source areas, while during sea-level lowstand intervals, canyons are coupled to fluvial and littoral sediment-supply sources, and top-down (i.e. shelf-to-lower-slope) sediment transport and channel incision is active. Canyon-head areas are incision dominated during the lowstand while mid to lower canyon reaches experience both a transient increase in sediment in storage and canyon-fill degradation and incision into bedrock. Tectonics influences the canyon landscape through both uplift-controlled perturbations to canyon base-levels and earthquake-triggering of mass movement. Following sea-level rise the sediment supply to canyon heads will be switched off at a certain threshold sea level. From this point canyon heads become aggradational. Mid to lower canyon reaches continue to incise due to continuing tectonic uplift and earthquake-triggered slope instability. Knickpoints are propagated up channel and excavate canyon and sub-canyon channels from the bottom up. Thus, while top-down infilling of non-coupled canyons occurs during sea-level highstands, the lower reaches of active margin canyons continue to incise due the influence of tectonic processes
Submarine Landslides
No full textRobust interpretation of geomorphology is a primary method of understanding failure modes, emplacement mechanisms and post-failure modification of submarine landslides. Since high-resolution hull-mounted multibeam systems became widely available in the last 20 years, our understanding of submarine landslides has improved dramatically. Techniques such as 3D seismic and cm-resolution seafloor mapping has revealed both surface and sub-surface geomorphology in unprecedented detail, and we are making rapid advancements towards refining our understanding of the processes that lead to specific geomorphological signatures associated with slope failure. One of the greatest challenges in the geomorphological analysis of submarine landslides is in accounting for post-failure modification processes. As erosional processes, such as gullying, erode the easily recognisable landslide geomorphology, or sediment drape smothers landslide features, it becomes increasingly more challenging to identify where landslides have occurred. In some depositional environments (e.g. a slope basin) the landslide debris may be preserved in the stratigraphy and analysed using 3D data. However, in erosional environments, such as submarine canyons, there is often little or no remaining deposit and interpretation of landslide processes must be based solely on the landslide scar, which is often heavily modified due to the dynamic nature of the canyon environment. Accurate interpretation and quantification of landslide parameters becomes important for determining magnitude frequency for landslide populations, which is a key piece of information for hazard studies
A topographic signature of a hydrodynamic origin for submarine gullies
No full textSubmarine 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
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