Description: A newly identified large-scale submarine landslide on the NW African margin (Agadir Slide) is investigated in terms of its morphology, internal architecture, timing, and emplacement processes using high-resolution multibeam bathymetry data, 2D seismic profiles, and gravity cores. The Agadir Slide is located south of the Agadir Canyon at a water depth ranging from 500 m to 3,500 m, showing an estimated affected area of approximately 5,500 km2. The analysis of the Agadir Slide's complex morphology reveals the presence of two headwall areas and two slide fairways (the Western and Central slide fairways). Volume calculations indicate that ∼340 km3 of sediment were accumulated downslope along the slide fairways (∼270 km3) and inside the Agadir Canyon (∼70 km3). Stratigraphic correlations based on five gravity cores indicate an emplacement age of 142±1 ka for the Agadir Slide. However, its emplacement dynamics suggest that the slide was developed in two distinct, successive stages. The presence of two weak layers (glide planes) is a major preconditioning factor for the occurrence of slope instability in the study area, and local seismicity related to fault activity and halokinesis likely triggered the Agadir Slide. Importantly, the Agadir Slide neither disintegrated into sediment blocks nor was transformed into turbidity currents. The emplacement timing of the Agadir Slide does not correlate with any turbidites cored downslope across the Moroccan Turbidite System.

Description: Highlights • 3D seismic imaging of an entire landslide complex. • Shallow gas accumulation within and underneath Tuaheni Landslide Complex. • Imaging of a basal shear zone within a subaqueous landslide complex. Abstract The Hikurangi margin is an active continental margin east of New Zealand's North Island. It is well recognized as a seismically active zone and is known for the occurrence of free gas and gas hydrates within the shallow sediments. A variety of subaqueous landslides can be observed at the margin, including the Tuaheni Landslide Complex off Poverty Bay. This slide complex has been interpreted previously as a slowly creeping landform, as its morphology and internal deformation is comparable to terrestrial earthflows and rock glaciers. In 2014, we acquired a high-resolution 3D seismic volume covering major parts of the Tuaheni South landslide. The 3D data show a variety of fluid migration indicators, free gas accumulations and manifestations of the base of gas hydrate stability in the pre-slide sedimentary units and the lower unit of the landslide system. The data also show that the landslide system is composed of an upper and lower unit that are separated by an intra-debris negative-polarity reflection. Free gas accumulations directly beneath the landslide units suggest that the debris acts as a boundary for rising fluids and only few migration pathways to the intra-debris reflector are observed in the distal parts of the landslide. Deformation within the landslide's debris is focused in the upper landslide unit, and we interpret the intra-debris reflector as a basal shear zone or ‘glide plane’ upon which the debris has been remobilized. The origin of the intra-debris reflector is unclear, but we suggest it could be a relatively coarse-grained horizon that would be prone to fluid flow focusing and the development of excess fluid pressure. Our seismic study provides one of the most detailed examples of a subaqueous landslide system and reveals insights into the fluid flow system and potential basal shear zone development of the Tuaheni Landslide Complex.

Description: Bottom currents and their margin-shaping character became a central aspect in the research field of sediment dynamics and paleoceanography during the last decades due to their potential to form large contourite depositional systems (CDS), consisting of both erosive and depositional features. A major CDS at the northern Argentine continental margin was studied off the Rio de la Plata River by means of seismo- and hydro-acoustic methods including conventional and high-resolution seismic, parametric echosounder and single and swath bathymetry. Additionally, hydrographic data were considered allowing jointly interpretation of morphosedimentary features and the oceanographic framework, which is dominated by the presence of the dynamic and highly variable Brazil-Malvinas Confluence. We focus on three regional contouritic terraces identified on the slope in the vicinity of the Mar del Plata Canyon. The shallowest one, the La Plata Terrace (similar to 500 m), is located at the Brazil Current/Antarctic Intermediate Water interface characterized by its deep and distinct thermocline. In similar to 1200 m water depth the Ewing Terrace correlates with the Antarctic Intermediate Water/Upper Circumpolar Deep Water interface. At the foot of the slope in similar to 3500 m the Necochea Terrace marks the transition between Lower Circumpolar Deep Water and Antarctic Bottom Water during glacial times. Based on these correlations, a comprehensive conceptual model is proposed, in which the onset and evolution of contourite terraces is controlled by short- and long-term variations of water mass interfaces. We suggest that the terrace genesis is strongly connected to the turbulent current pattern typical for water mass interfaces. Furthermore, the erosive processes necessary for terrace formation are probably enhanced due to internal waves, which are generated along strong density gradients typical for water mass interfaces. The terraces widen through time due to locally focused, partly helical currents along the steep landward slopes and more tabular conditions seaward along the terrace surface. Considering this scheme of contourite terrace development, lateral variations of the morphosedimentary features off northern Argentina can be used to derive the evolution of the Brazil-Malvinas Confluence on geological time scales. We propose that the Brazil-Malvinas Confluence in modern times is located close to its southernmost position in the Quaternary, while its center was shifted northward during cold periods

Description: Analysis of multi-channel seismic reflection and chirp data from Lake Van (eastern Turkey) reveals various shallow gas indicators including seismic chimneys, enhanced reflections, bright spots, mud volcanoes, pockmarks, and acoustic blanking. The enhanced reflections, suggesting the presence of free gas, are most dominant and observed at more than 200 locations. They are characterized by very-high amplitude reflections and occur in both deep and shallow sedimentary sections. Some enhanced reflections are accompanied by very subtle seafloor expressions such as mounds, which may suggest active venting activity. Seismic chimneys or columnar zones of amplitude blanking have been observed in much of the surveyed area. Seismic chimneys in the study area cannot be associated with any known faults that would act as migration pathways for deep fluids. This suggests that the observed structures in Lake Van sediments allow the preferential emission of gases which might be for a large share of biogenic origin. The acoustic blanking, characterized by transparent or chaotic seismic facies, is seen in the eastern part of the lake. The lakeward edge of the acoustic blanking largely coincides with the 100 m water depth contour, indicating that (past) changes of the hydrostatic pressure may be responsible for the distribution of these anomalies. Mound-like features, interpreted as mud volcanoes, occur in a few locations. The presence of these features may suggest active gas emission. Very strong amplitude anomalies or bright spots with negative polarity, indicating gas-charged zones, are also seen in a number of locations. Pockmarks are observed only in the northeastern part of the study area. The scarce occurrence of pockmarks in the study area might be ascribed to a higher permeability of the lake sediments or to the absence of the substrate/reservoir providing the critical mass of gases necessary to produce such features. Turbidites, tephra layers, and deltaic deposits have the potential to provide ideal conditions to allow the sediments to act as a gas reservoir.

Description: Highlights • We map out the 3D extent of gas hydrate stability beneath two methane seep sites. • Focused fluid flow has sustained large-scale gas hydrate instability. • The two seeps likely have the same deep fluid source, despite shallow differences. • Fault networks influenced the initiation of advective flow through the hydrate system. • Ongoing flow towards the seeps is likely sustained by networks of hydrofractures. Abstract Fluid flow through marine sediments drives a wide range of processes, from gas hydrate formation and dissociation, to seafloor methane seepage including the development of chemosynthetic ecosystems, and ocean acidification. Here, we present new seismic data that reveal the 3D nature of focused fluid flow beneath two mound structures on the seafloor offshore Costa Rica. These mounds have formed as a result of ongoing seepage of methane-rich fluids. We show the spatial impact of advective heat flow on gas hydrate stability due to the channelled ascent of warm fluids towards the seafloor. The base of gas hydrate stability (BGHS) imaged in the seismic data constrains peak heat flow values to View the MathML source∼60 mWm−2 and View the MathML source∼70 mWm−2 beneath two separate seep sites known as Mound 11 and Mound 12, respectively. The initiation of pronounced fluid flow towards these structures was likely controlled by fault networks that acted as efficient pathways for warm fluids ascending from depth. Through the gas hydrate stability zone, fluid flow has been focused through vertical conduits that we suggest developed as migrating fluids generated their own secondary permeability by fracturing strata as they forced their way upwards towards the seafloor. We show that Mound 11 and Mound 12 (about 1 km apart on the seafloor) are sustained by independent fluid flow systems through the hydrate system, and that fluid flow rates across the BGHS are probably similar beneath both mounds. 2D seismic data suggest that these two flow systems might merge at approximately 1 km depth, i.e. much deeper than the BGHS. This study provides a new level of detail and understanding of how channelled, anomalously-high fluid flow towards the seafloor influences gas hydrate stability. Thus, gas hydrate systems have good potential for quantifying the upward flow of subduction system fluids to seafloor seep sites, since the fluids have to interact with and leave their mark on the hydrate system before reaching the seafloor.

Description: Large sedimentary deposits consisting of several major contourite drifts were studied by means of high-resolution multichannel seismic data at the middle slope along the Northern Argentina Continental Margin to determine their evolutionary stages as well as to identify and assess the possible impact of Northern Source Deep Water (NSDW) on the slope architecture. The imaged contouritic sediments allow decoding on the regional paleo-oceanographic setting of the last 32 Ma. Earliest contouritic sedimentation can be observed close to the Eocene/Oligocene boundary based on an aggradational stacking pattern with a complex and wavy seismic facies, pointing toward a hydrodynamically turbulent flow pattern. This facies is most likely related to the opening of the Drake Passage associated with global cooling and a strengthening of surface, intermediate and deep ocean currents in the Southern Ocean. During the Middle Miocene plastered drift sequences with an aggradational reflection pattern were deposited. Their depositional style indicates weak, non-turbulent current conditions, which are interpreted to be related to a vertical shift of water mass interfaces caused by the first formation of NSDW during the Mid-Miocene climatic optimum. On top, the formation of plastered drift sequences led to the modern extent of the Ewing Terrace, which was probably controlled by the continuous strengthening and thickening of NSDW until the final closure of the Central American Seaway (CAS). During the Pliocene and Quaternary, after the complete closure of the CAS and under the influence of the full force of the NSDW, mounded plastered drift sequences are built upon the Ewing Terrace generating the modern slope morphology. Therefore, we suggest that deep-water production in the northern hemisphere plays a significant role by controlling the shape of the continental slopes in the southwestern South Atlantic since the Middle Miocene. Highlights ► Slope of northern Argentine Continental margin is current controlled since 32 Ma. ► Variability of Northern Source Deep Water (NSDW) controls sedimentary processes. ► Sedimentary processes are susceptible to changes of the Brazil-Malvinas Confluence. ► Impact of NSDW on slope processes is underestimated in the southern hemisphere.

Description: Newly acquired bathymetric and seismic reflection data have revealed mass-transport deposits (MTDs) on the northeastern Cretan margin in the active Hellenic subduction zone. These include a stack of two submarine landslides within the Malia Basin with a total volume of approximately 4.6 km(3) covering an area of about 135 km(2). These two MTDs have different geometry, internal deformations and transport structures. The older and stratigraphic lower MTD is interpreted as a debrite that fills a large part of the Malia Basin, while the second, younger MTD, with an age of at least 12.6 cal. ka B.P., indicate a thick, lens-shaped, partially translational landslide. This MTD comprises multiple slide masses with internal structure varying from highly deformed to nearly undeformed. The reconstructed source area of the older MTD is located in the westernmost Malia Basin. The source area of the younger MTD is identified in multiple headwalls at the slope-basin-transition in 450 m water depth. Numerous faults with an orientation almost parallel to the southwest-northeast-trending basin axis occur along the northern and southern boundaries of the Malia Basin and have caused a partial steepening of the slope-basin-transition. The possible triggers for slope failure and mass-wasting include (i) seismicity and (ii) movement of the uplifting island of Crete from neotectonics of the Hellenic subduction zone, and (iii) slip of clay-mineral-rich or ash-bearing layers during fluid involvement. (c) 2009 Elsevier B.V. All rights reserved.

Description: More than 1500 km of multi-channel seismic reflection profiles combined with ICDP (International Continental Scientific Drilling Program) drilling data, provide important insights into the stratigraphic evolution of Lake Van, eastern Turkey. Three major basins (Tatvan, Northern and the Deveboynu basins) comprise the main lake basin and are separated by morphological highs (Ahlat ridge and Northern ridge). Moreover, NE–SW faults, parallel to the general tectonic lineament of the area, dominate the entire basin and are in charge of creating graben and half-graben structures. Well-developed prograding deltaic sequences on top of the basement were recognized by seismic stratigraphy analysis. Most likely, they formed during the initial flooding of Lake Van ∼600 ka. The Tatvan basin sediments are dominated by mass-flow deposits of various origins alternating with undisturbed lacustrine sediments including distinct tephra layers. Faulting along the Tatvan basin margins may have triggered margin-wide slope failures. Ahlat ridge started to form between ca 340 ka–290 ka. Since then, Ahlat ridge was sheltered from major mass-flows due to its elevation. Hence, slow lacustrine sedimentation has prevailed throughout lake history on Ahlat ridge, which was the location of the main drill site during the ICDP. Several lake level fluctuations are evident on the eastern slope area but the deep basins were permanently covered by water. A significant lake-level low stand (ca 600 ka BP) was found at ∼610 m below present lake level. The setting of the lake changed at about 30 ka. Tectonic activity appears to have waned significantly as the mass-transport deposition decreased across the Tatvan basin while normal undisturbed lacustrine sedimentation prevailed. A different setting is found in the Northern basin from ca 90 ka to Present, especially due to the strong influx of mostly volcaniclastic turbidites causing sedimentation rates to be about 3.5 times higher (drill Site 1), than at Site 2 (Ahlat ridge).

Description: Highlights • The upper headwall region of Sahara Slide is mapped for the first time. • The upper headwall region comprises multiple slope failures. • Slope failure occurred on pronounced glide planes at different stratigraphic levels. • Failure is young (~ 2 ka) contradicting the hypotheses of a relatively stable continental margin at present. • This young age requires a reassessment of slope instability and associated risks off NW Africa. Abstract The Sahara Slide Complex in Northwest Africa is a giant submarine landslide with an estimated run-out length of ~ 900 km. We present newly acquired high-resolution multibeam bathymetry, sidescan sonar, and sub-bottom profiler data to investigate the seafloor morphology, sediment dynamics and the timing of formation of the upper headwall area of the Sahara Slide Complex. The data reveal a ~ 35 km-wide upper headwall opening towards the northwest with multiple slide scarps, glide planes, plateaus, lobes, slide blocks and slide debris. The slide scarps in the study area are formed by retrogressive failure events, which resulted in two types of mass movements, translational sliding and spreading. Three different glide planes (GP I, II, and III) can be distinguished approximately 100 m, 50 m and 20 m below the seafloor. These glide planes are widespread and suggest failure along pronounced, continuous weak layers. Our new data suggest an age of only about 2 ka for the failure of the upper headwall area, a date much younger than derived for the landslide deposits on the lower reaches of the Sahara Slide Complex, which are dated at 50–60 ka. The young age of the failure contradicts the postulate of a stable slope off Northwest Africa during times of relative stable sea-level highstands. Such an observation suggests that submarine-landslide risk along the continental margin of Northwest Africa should be reassessed based on a robust dating of proximal and distal slope failures.

Description: Highlights • The Fram Slide Complex has been active from late Miocene to late Pleistocene. • Local processes were critical for slope stability in the Fram Strait area. • Toe erosion caused by normal faulting may have led to retrogressive failure. • Low gradient contourite drifts might smooth and stabilize submarine slopes. • Low tsunami potential from the Fram Slide Complex could increase in the future. Abstract The best known submarine landslides on the glaciated NW European continental margins are those at the front of cross-shelf troughs, where the alternation of rapidly deposited glycogenic and hemi pelagic material generates sedimentary overpressure. Here, we investigate landslides in two areas built of contourite drifts bounded seaward by a ridge-transform junction. Seismic and bathymetric data from the Fram Slide Complex are compared with the tectonically similar Vastness area ~ 120 km to the south, to analyze the influence of local and regional processes on slope stability. These processes include tectonic activity, changes of climate and oceanography, gas hydrates and fluid migration systems, slope gradient, toe erosion and style of contourite deposition. Two areas within the Fram Slide Complex underwent different phases of slope failures, whereas there is no evidence at all for major slope failures in the Vastness area. The comparison cannot reveal the distinct reason for slope failure but demonstrates the strong impact of variation in the local controls on slope stability. The different failure chronologies suggest that toe erosion, which is dependent on the throw of normal faults, and the different thickness and geometry of contourite deposits can result in a critical slope morphology and exert pronounced effects on slope stability. These results highlight the limitations of regional hazard assessments and the need for multi-disciplinary investigations, as small differences in local controlling factors led to substantially different slope failure histories.