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Although subaqueous slopes on active continental margins are subject to a variety of failure styles, their movement mechanisms during earthquakes remain poorly constrained. A primary explanation is that few submarine landslides have been directly sampled for detailed investigation. We have conducted a series of dynamic shear experiments on samples recovered from the base of the Tuaheni Landslide Complex, located off the east coast of the North Island of New Zealand, to explore its behaviour during earthquakes. Our experiments suggest that whilst the basal landslide sediments can be prone to liquefaction in certain conditions, this is not a likely failure mechanism at the stress states operating in the low angled shear zone at the base of this landslide system. Instead, episodic landslide movement can occur through basal sliding when pore water pressures increase sufficiently to lower the shear zone effective stress to the material failure envelope. These low effective stress conditions are most likely to be reached during earthquakes that produce large amplitude, long duration ground shaking. The observed behaviour provides a credible mechanism through which subaqueous landslides moving on low angled shear zones in similar materials may be subject to episodic movement during earthquakes without undergoing catastrophic failure.

Although submarine landslides have been studied for decades, a persistent challenge is the integration of diverse geoscientific datasets to characterise failure processes. We present a core-log-seismic integration study of the Tuaheni Landslide Complex to investigate intact sediments beneath the undeformed seafloor as well as post-failure landslide deposits. Beneath the undeformed seafloor are coherent reflections underlain by a weakly-reflective and chaotic seismic unit. This chaotic unit is characterised by variable shear strength that correlates with density fluctuations. The basal shear zone of the Tuaheni landslide likely exploited one (or more) of the low shear strength intervals. Within landslide deposits is a widespread “Intra-debris Reflector”, previously interpreted as the landslide’s basal shear zone. This reflector is a subtle impedance drop around the boundary between upper and lower landslide units. However, there is no pronounced shear strength change across this horizon. Rather, there is a pronounced reduction in shear strength ∼10-15 m above the Intra-debris Reflector that presumably represents an induced weak layer that developed during failure. Free gas accumulates beneath some regions of the landslide and is widespread deeper in the sedimentary sequence, suggesting that free gas may have played a role in pre-conditioning the slope to failure. Additional pre-conditioning or failure triggers could have been seismic shaking and associated transient fluid pressure. Our study underscores the importance of detailed core-log-seismic integration approaches for investigating basal shear zone development in submarine landslides.

The Malta-Sicily Escarpment (MSE) is a steep carbonate escarpment that appears to have largely remained isolated from inputs of fluvial and littoral sediments since the Messinian Salinity Crisis. Mass movement activity has so far only been inferred from sediment cores at the base of the MSE. In this study we use geophysical and sedimentological data acquired from the upper MSE and outer Malta Plateau to: (i) map and characterise the dominant forms of mass movements, and (ii) determine the nature and origin of these mass movements, and their role in the evolution of the MSE. We document 67 mass movement scars across 370 km2 of seafloor. Slope instability entailed translational slides, spreads and debris flows that mobilised Plio-Pleistocene outer shelf hemipelagic/pelagic sediments or carbonate sequences across the upper continental slope. Slope failure events are caused by loss of support associated with the formation of channels, gullies, canyon heads and fault-related escarpments. Mass movements play a key role in eroding the seafloor and transferring material to the lower MSE. In particular, they control the extent of headward and lateral extension of submarine canyons, facilitate tributary development, remove material from the continental shelf and slope, and feed sediment and drive its transport across the submarine canyon system.

Sediment starved passive margins, particularly those of insular slopes, display significant mass transport activity despite the absence of environmental stresses from high sedimentation rates. The “Menorca Channel” represents the up to 120 m deep shelf sector connecting the Menorca and Mallorca Islands (Balearic Islands – Western Mediterranean). South of the Menorca Channel submarine gravitational processes have been mapped and interpreted from swath-bathymetry, TOPAS parametric echosounder and deep-towed videos. The shelf-break is located at an average depth of 140 m, and sediment instability is a widespread phenomenon. The slope region south the Menorca Channel shows a number of submarine canyons disrupting the outer shelf. The north-easternmost canyon is the more active feature, with an incised axis and scars shaping the flanks up to their edges. Headwall scarps, between 140 and 700 m depth, are up to 20 m high. The shallower scarps producing slab-type failures have carved the outer edges of planar sedimentary bodies interpreted as formed in shallow environments during previous glacial stages. Results show that a variety of sediment instability processes extensively shape the southern upper slope of the Menorca Channel. Submarine canyons develop on the Emile Bodout Escarpment (EBE), a passive tectonic feature which bounds the slope region of the study area. A number of knickpoints within the canyons suggest backward erosion control on mass wasting and, at the same time, that slope failure is one of the main drivers for canyon upslope migration. Steep gradients of the upper slope, the presence of weak layers and the action of major storms during lowstand stages are additional factors likely to influence the distribution and frequency of mass wasting processes in this area.

The passive continental margins of the Atlantic Ocean are characterized by thick sedimentary successions, which might become unstable resulting in landslides of various sizes. The type of mass-wasting differs between individual margin sections but the reasons for these differences are not well understood. The NW-African continental margin is characterized by several large-scale but infrequent landslides, while the continental margin in the de la Plata River region (northern Argentina and Uruguay) shows widespread small-scale mass transport deposits. These different styles of mass wasting can be explained by different oceanographic and sedimentary settings. The margin off Northwest Africa is characterized by high primary productivity caused by oceanic upwelling as well as locally focused aeolian input resulting in relatively high sedimentation rates. This setting leads to sediment instabilities arising primarily from underconsolidation of deposited sediments and widespread weak layers. In contrast, the modern ocean margin off Uruguay and northern Argentina is characterized by strong contour currents and a high amount of fluvial sediment resulting in widespread contouritic deposits. These contourites are potentially unstable leading to smaller but more frequent landslides.

Submarine landslides on open continental slopes can be far larger than any slope failure on land and occur in locations worldwide on gradients of <2°. Significantly elevated pore pressure is necessary to overcome the sediment’s shearing resistance on such remarkably low gradients, but the processes causing such overpressure generation are contentious, especially in areas with slow sedimentation rates. Here we propose that the progressive loss of interparticle bonding and fabric could cause such high excess pore pressure. Slow sedimentation may favour the formation of a structural framework in the sediment that is load-bearing until yield stress is reached. The bonds then break down, causing an abrupt porosity decrease and consequently overpressure as pore fluid cannot escape sufficiently rapidly. To test this hypothesis, we implement such a loss of structure into a 2D fully coupled stress-fluid flow Finite Element model of a submerged low angle slope, and simulate consolidation due to slow sedimentation. The results suggest that destructuring could indeed be a critical process for submarine slope stability.

The BIG’95 landslide was emplaced 11,500 years ago and is one of the largest known submarine landslides in the Mediterranean Sea. The simulated landslide dynamics matches the observed run-out and deposited thickness. Water elevation simulated by using a dispersive tsunami model exceed 10 m close to the landslide area and at the nearest shorelines. Modeling further indicates that the tsunami probably had widespread consequences in the Mediterranean. Compared to previous studies, this new simulation provides larger waves. There is, however, still a need to better constrain the landslide dynamics in order to illuminate the uncertainties related to the tsunamigenic power of this, and other, submarine landslides.

Seismic profiles extending from the southern South African shelf into the deep sea reveal a strong erosional activity, which affects large parts of the continental margin. Recent to Oligocene sequences, and in places the whole sedimentary column, appear to have been removed. Mass movements were considered as the origin of this erosion. However, structures indicating slumping could only be identified in a few places, The erosional activity is confined to specific water depths, which correlate well with the activity levels of water masses observed here. We thus suggest that the Agulhas Current, Antarctic Intermediate Water, North Atlantic Deep Water, and Antarctic Bottomwater have intensively shaped the sedimentary sequences for a considerable period. It is difficult to estimate the duration of the erosion. Numerical simulation of sediment transport and erosion is needed to indicate the onset of the erosional activity.

The area south of South Africa is an important gateway for the interoceanic exchange of water masses from the Atlantic, Indian and Southern Oceans. Three major water masses are involved: the Agulhas Current (AC), the North Atlantic Deep Water (NADW), and Antarctic Bottom Water (AABW). The interplay of these currents is widely believed to control the sediment transport pathways along the South African continental slope, the Agulhas Passage as well as the Transkei Basin.During the research cruise SO-182 with R/V SONNE in spring 2005 extensive reflection seismic data were recorded along the Agulhas Passage, the Agulhas Plateau as well as the Transkei Basin to reconstruct sediment transport pattern through time. Thereby, the Agulhas Drift situated in the Transkei Basin, was surveyed by a dense profile grid. This large sediment body could be identified as major sediment deposition center which was developed since mid-Miocene. Furthermore, seismic data indicate that current conditions were highly variable in this region: a north-south directed inflow of AABW dominated the sediment transport and deposition in the Transkei Basin from middle Miocene to early Pliocene times, while after that an eastward flow of NADW responded to this sedimentation and the inflow of AABW must have shifted back to the Agulhas Passage gateway.To get to better understanding of the temporal evolution of the Agulhas Drift and the development of the palaeocirculation south of South Africa, the Regional Ocean Modeling System (ROMS) coupled with a sediment model is applied. This is an advanced open-source model which solves the free surface, hydrostatic, primitive equations over variable topography using stretched, terrain-following coordinates in the vertical, orthogonal coordinates in the horizontal. Based on these current simulations sediment transport pattern have been calculated simultaneously. We started with a 3D model under recent current conditions for a one year model run. Levitus is used as

The southern African continental margin represents a crucial gateway within the oceanic circulation system where Indian-Pacific Ocean and Atlantic Ocean water masses meet and mix. Here, surface Agulhas Current, Antarctic Intermediate Water AAIW and deep North Atlantic Deep Water NADW and Antarctic Bottomwater AABW are fully concentrated and flow into opposing directions. Hence, this is a unique location where the fluctuating strength on interocean circulation may be sensitively recorded by erosional-depositional processes on the subjacent seafloor over an approximately 4 km wide range of depths.Seismic data collected on the southern South African continental margin show strong erosion on the shelf, slope, the Agulhas Passage and the Agulhas Plateau. In parts, the whole sedimentary column has been eroded and basement forms the seafloor. For the shelf, slope and the Agulhas Passage specific depth intervals can be identified, where the erosion is particularly strong. Those intervals span the water depths of 900-1700 m, 2000-2600 m, 2800-3000 m, and 4000-4500 m, and comprise the depth ranges of the Agulhas Current, AAIW, NADW, and AABW. Thus we can identify the paths of those water masses, and distinguish between them, via their erosive activity on the shelf, slope and in the Agulhas Passage.This is not valid for the Agulhas Plateau. There, erosion occurs over the whole depth range of the seafloor. Thicker sediment bodies can be observed in between basement highs, and larger well stratified deposits can only be found on the southern plateau where basement topography is not that rough. On the northern Agulhas Plateau obviously both paths of the Agulhas Retroflection and the AAIW and hence deposition/erosion are primarily influenced by the basement topography.

A numerical model of the ocean circulation coupled with a sediment transport model is proposed to reconstruct the currents evolution in the south of South Africa. This area is an important gateway for the interocean exchange of water masses from the Atlantic, Indian and Southern Oceans. Three major water masses involved are: the warm and saline water of the Agulhas Current at the surface, the cold and saline North Atlantic Deep Water (NADW) in larger depth and the cold Antarctic Bottom Water (AABW) near the bottom. The development of the oceanic currents can be inferred from the record of sediment drifts, because the current system drives the sedimentary system of erosion transport and deposition in oceanic basin. Hence the characteristics of sediment drifts provide the information of the regional paleocirulation patterns. Seismic reflection and refraction data in the region of the Agulhas Drift in the southwest Indian Ocean were measured during the RV SONNE cruise SO-182. It is concluded that the Agulhas Drift formed in mid-Miocene. Based on the Agulhas Drift, we plan to develop a model to simulate the evolution of current system and sediment transport since mid-Miocene times in the selected period in this region. The model will be validated and improved by comparing the results computed from the model with the geological and oceanographic features indicated from the measurements. Within the wide range of numerical models available, the Regional Oceanic Modelling System (ROMS) is selected, because it is an advanced open-source model with high resolution, and the sediment transport algorithms have been incorporated

Der Kontinentalrand des südlichen Afrikas repräsentiert einen wichtigen Seeweg im globalen Zirkulationssystem, da hier Wassermassen aus dem Idik-Pazifik und dem Atlantik auf einander treffen und sich mischen. Hier sind Agulhas Strömung, Antarktisches Zwischenwasser und die tiefen Nordatlantischen Tiefenwasser und Antarktisches Bodenwasser stark konzentriert und fliessen in entgegengesetze Richtungen. Dieser Seeweg ist also eine einzigartige Lokation, wo die Variabilität der interozeanischen Zirkulation durch Erosions-Ablagerungsprozesse am Meeresboden über einen Tiefenbereich von ca. 4 km sensibel aufgezeichnet wird.Seismische Daten, die am südlichen Kontinentalrand Südafrikas aufgezeichnet wurden, zeigen starke Erosion auf dem schelf, am Hang, in der Agulhas Passage und auf dem Agulhas Plateau. Teilweise fehlt die gesamte Sedimentsäule und Basement bildet den Meeresboden.Für den Schelf, Hang und die Agulhas Passage könnenspezifischen Wassertiefenintervalle identifiziert werden, in welchen die Erosion besonders stark ist. Dabei handelt es sich um die Wasertiefenintervalle 900-1700 m, 2000-2600 m, 2800-3000 m und 4000-4500 m, welche die Aktivitätstiefen der Agulhasströmung, des AAIW, NADW und des AABW umfassen. Es ist also möglich, die Pfade der verschiedenen Wassermassen über ihre Erosionsaktivität zu identifizieren.Diese gilt leider nicht für das Agulhas Plateau. Dort lässt sich Erosion über den gesamten Tiefenbereich des Meeresbodens beobachten. Mhtigere Sedimentkörper können zwischen Basementhochlagen gefunden werden, und gut geschichtete Ablagerungen lassen sich erst auf dem südlichen Plateau identifizieren, wo das Basement nicht so rauh ist. Auf dem nördlichen Agulhas Plateau bestimmt also offensichtlich die Basementtopographie die Pfade der Agulhas Retroflektion und des AAIW und damit auch primär die Ablagerung/Erosion.

The southern African continental margin represents a crucial gateway within the oceanic circulation system where Indian-Pacific Ocean and Atlantic Ocean water masses meet and mix. Here, surface Agulhas Current, Antarctic Intermediate Water AAIW and deep North Atlantic Deep Water NADW and Antarctic Bottomwater AABW are fully concentrated and flow into opposing directions. Hence, this is a unique location where the fluctuating strength on interocean circulation may be sensitively recorded by erosional-depositional processes on the subjacent seafloor over an approximately 4 km wide range of depths.Seismic data collected on the southern South African continental margin show strong erosion on the shelf, slope, the Agulhas Passage and the Agulhas Plateau. In parts, the whole sedimentary column has been eroded and basement forms the seafloor. For the shelf, slope and the Agulhas Passage specific depth intervals can be identified, where the erosion is particularly strong. Those intervals span the water depths of 900-1700 m, 2000-2600 m, 2800-3000 m, and 4000-4500 m, and comprise the depth ranges of the Agulhas Current, AAIW, NADW, and AABW. Thus we can identify the paths of those water masses, and distinguish between them, via their erosive activity on the shelf, slope and in the Agulhas Passage.This is not valid for the Agulhas Plateau. There, erosion occurs over the whole depth range of the seafloor. Thicker sediment bodies can be observed in between basement highs, and larger well stratified deposits can only be found on the southern plateau where basement topography is not that rough. On the northern Agulhas Plateau obviously both paths of the Agulhas Retroflection and the AAIW and hence deposition/erosion are primarily influenced by the basement topography.

A numerical model of the ocean circulation coupled with a sediment transport model is proposed to reconstruct the currents evolution in the south of South Africa. This area is an important gateway for the interocean exchange of water masses from the Atlantic, Indian and Southern Oceans. Three major water masses involved are: the warm and saline water of the Agulhas Current at the surface, the cold and saline North Atlantic Deep Water (NADW) in larger depth and the cold Antarctic Bottom Water (AABW) near the bottom. The development of the oceanic currents can be inferred from the record of sediment drifts, because the current system drives the sedimentary system of erosion transport and deposition in oceanic basin. Hence the characteristics of sediment drifts provide the information of the regional paleocirulation patterns. Seismic reflection and refraction data in the region of the Agulhas Drift in the southwest Indian Ocean were measured during the RV SONNE cruise SO-182. It is concluded that the Agulhas Drift formed in mid-Miocene. Based on the Agulhas Drift, we plan to develop a model to simulate the evolution of current system and sediment transport since mid-Miocene times in the selected period in this region. The model will be validated and improved by comparing the results computed from the model with the geological and oceanographic features indicated from the measurements. Within the wide range of numerical models available, the Regional Oceanic Modelling System (ROMS) is selected, because it is an advanced open-source model with high resolution, and the sediment transport algorithms have been incorporated.

During the last decade, higher latitudes, particularly the Antarctic continental margin, were main target areas to investigate climate shifts and global ocean circulation over long-time periods. In such a way during several cruises a unique data base, including numerous multi channel reflection seismic profiles, bathymetric maps, shallow gravity measurements, and in particular core data from ODP Leg 178, was collected along the Antarctic Peninsula. These data monitored a number of asymmetric sediment mounds on the continental slope which were interpreted as sedimentary drifts. These drifts serve as an excellent achieve for the geological environment and current conditions during their formation.In particular, Drift 7 is the best monitored sedimentary mound in this area. Detailed information about spatial extension, internal structures, sedimentary sequences, and grain size distribution exist. These data enable on one hand the identification of sediment sources, the reconstruction of sediment transport pathways and transport mechanisms, as well as current velocities. On the other hand, different theories about controlling parameters for the evolution of drifts could be developed such as: Is an initial topography necessary for the formation of such a sedimentary structure?Therefore, numerical oceanic circulation models enable extensive parameter sensitivity studies combining multidisciplinary datasets as model input parameters to test different hypotheses. Thus, we are using the Regional Ocean Model System (ROMS) - an academic hydrostatic ocean circulation model, based on the finite difference method to investigate the environmental situation and current conditions during the evolution of Drift 7. Major aim of this project is the reconstruction of depositional and re-depositional processes from observed sediment structure of Drift 7. Therefore, ROMS included a complex sediment transport module to compute particle transport within the water column as well as sediment s