Sebastian Krastel

Submarine landslides around the Canary Islands

The morphology and structure of the submarine flanks of the Canary Islands were mapped using the GLORIA long-range side-scan sonar system, bathymetric multibeam systems, and sediment echosounders. Twelve young (<2 Ma) giant landslides have been identified on the submarine flanks of the Canary Islands up to now. Older landslide events are long buried under a thick sediment cover due to high sedimentation rates around the Canary Islands. Most slides were found on the flanks of the youngest and most active islands of La Palma, El Hierro, and Tenerife, but young giant landslides were also identified on the flanks of the older (15–20 Ma) but still active eastern islands. Large-scale mass wasting is an important process during all periods of major magmatic activity. The long-lived volcanic constructive history of the islands of the Canary Archipelago is balanced by a correspondingly long history of destruction, resulting in a higher landslide frequency for the Canary Islands compared to the Hawaiian Islands, where giant landslides only occur late in the period of active shield growth. The lower stability of the flanks of the Canaries is probably due to the much steeper slopes of the islands, a result of the abundance of highly evolved intrusive and extrusive rocks. Another reason for the enhanced slope instability is the abundance of pyroclastic deposits on Canary Islands resulting from frequent explosive eruptions due to the elevated volatile contents in the highly alkalic magmas. Dike-induced rifting is most likely the main trigger mechanism for destabilization of the flanks. Flank collapses are a major geological hazard for the Canary Islands due to the sector collapses themselves as well as triggering of tsunamis. In at least one case, a giant lateral blast occurred when an active magmatic or hydrothermal system became unroofed during flank collapse.

Temporal Constraints on Hydrate-Controlled Methane Seepage off Svalbard

What Does It All Mean? Strong emissions of methane have recently been observed from shallow sediments in Arctic seas. Berndt et al. (p. 284, published online 2 January) present a record of methane seepage from marine sediments off the coast of Svalbard showing that such emissions have been present for at least 3000 years, the result of normal seasonal fluctuations of bottom waters. Thus, contemporary observations of strong methane venting do not necessarily mean that the clathrates that are the source of the methane are decomposing at a faster rate than in the past. Seasonal gas hydrate destabilization has been releasing methane from marine sediments near Svalbard for at least 3000 years. Methane hydrate is an icelike substance that is stable at high pressure and low temperature in continental margin sediments. Since the discovery of a large number of gas flares at the landward termination of the gas hydrate stability zone off Svalbard, there has been concern that warming bottom waters have started to dissociate large amounts of gas hydrate and that the resulting methane release may possibly accelerate global warming. Here, we corroborate that hydrates play a role in the observed seepage of gas, but we present evidence that seepage off Svalbard has been ongoing for at least 3000 years and that seasonal fluctuations of 1° to 2°C in the bottom-water temperature cause periodic gas hydrate formation and dissociation, which focus seepage at the observed sites.

An interdisciplinary investigation of a recent submarine mass transport deposit at the continental margin off Uruguay

Assessing frequency and extent of mass movement at continental margins is crucial to evaluate risks for offshore constructions and coastal areas. A multidisciplinary approach including geophysical, sedimentological, geotechnical, and geochemical methods was applied to investigate multistage mass transport deposits (MTDs) off Uruguay, on top of which no surficial hemipelagic drape was detected based on echosounder data. Nonsteady state pore water conditions are evidenced by a distinct gradient change in the sulfate (SO42−) profile at 2.8 m depth. A sharp sedimentological contact at 2.43 m coincides with an abrupt downward increase in shear strength from ∼10 to >20 kPa. This boundary is interpreted as a paleosurface (and top of an older MTD) that has recently been covered by a sediment package during a younger landslide event. This youngest MTD supposedly originated from an upslope position and carried its initial pore water signature downward. The kink in the SO42− profile ∼35 cm below the sedimentological and geotechnical contact indicates that bioirrigation affected the paleosurface before deposition of the youngest MTD. Based on modeling of the diffusive re-equilibration of SO42− the age of the most recent MTD is estimated to be <30 years. The mass movement was possibly related to an earthquake in 1988 (∼70 km southwest of the core location). Probabilistic slope stability back analysis of general landslide structures in the study area reveals that slope failure initiation requires additional ground accelerations. Therefore, we consider the earthquake as a reasonable trigger if additional weakening processes (e.g., erosion by previous retrogressive failure events or excess pore pressures) preconditioned the slope for failure. Our study reveals the necessity of multidisciplinary approaches to accurately recognize and date recent slope failures in complex settings such as the investigated area.

Active tectonics of the South Chilean marine fore arc (35°S–40°S)

The South Chilean marine fore arc (35°S–40°S) is separated into four tectonic segments, Concepcion North, Concepcion South, Nahuelbuta, and Tolten (from north to south). These are each characterized by their individual tectonic geomorphology and reflect different ways of mechanical and kinematic interaction of the convergent Nazca and South American plates. Splay faults that cut through continental framework rock are seismically imaged in both Concepcion segments and the Tolten Segment. Additionally, the Concepcion South Segment exhibits prominent upper plate normal faults. Normal faults apparently relate to uplift caused by sediment underthrusting at depth. This has led to oversteepening and gravitational collapse of the marine fore arc. There is also evidence for sediment underthrusting and basal accretion to the overriding plate in the Tolten Segment. There, uplift of the continental slope has created a landward inclined seafloor over a latitudinal distance of 50 km. In the Nahuelbuta Segment transpressive upper plate faults, aligned oblique to the direction of plate motion, control the seafloor morphology. Based on a unique acoustic data set including >90% of bathymetric coverage of the continental slope we are able to reveal an along‐strike heterogeneity of a complexly deformed marine fore arc which had escaped attention in previous studies that only considered the structure along transects normal to the plate margin.

Submarine mass movements and their consequences : 6th International Symposium

We present a short review of the state-of-the-art numerical tools that have been used for modeling landslide-generated waves. A comparative study is conducted on the physical properties of earthquake- and landslide-generated waves suggesting that both dispersion and nonlinearity effects may be neglected for the former waves whereas they may be considered for the latter ones. We introduce landslide tsunami models and group them into three classes: (1) models treating the moving mass as a fluid, (2) models estimating the initial water surface, and (3) models fed by the transient seafloor deformation. Selection of a particular model from the list of models introduced here depends on: (1) the dimensions of the source, (2) the available computing capacities, (3) availability of fine bathymetric grid, and (4) the purposes of the modeling.

CapTimiris Canyon: A newly discovered channel system offshore of Mauritania

Intensive research over the past decades has greatly improved our understanding of processes operating in the deep ocean. There has been a particular focus on continental margins, as sediments deposited in these areas can provide a high-resolution record of past climatic changes, as well as serve to host some of the worlds major hydrocarbon reservoirs. However, the exploration and understanding of the deep ocean remains one of the great challenges of the 21st century [Stow and Mayall, 2000], and many fascinating features still wait to be found. The potential for new deep-water discoveries was recently highlighted during Meteor cruise M58/1 (depart Dakar, Senegal, 21 April 2003, return Las Palmas, Spain, 12 May 2003) of the Research Center Ocean Margins at the Universitat Bremen in Germany. A spectacular 400-km-long submarine meandering channel system was discovered off Mauritania. In this article, the system is called the Cap Timiris Canyon (Figure 1). Although a series of incisional gullies at the shelf break and uppermost slope have been described before [e.g., Rust and Wienecke, 1973], the enormous size and complex morphology of this submarine channel system were previously unknown.

Crustal structure of northern Gran Canaria, Canary Islands, deduced from active seismic tomography

Seismic P-wave travel times collected during METEOR cruise M24 are inverted to derive a three-dimensional model of the P-wave velocity structure of the northern part of Gran Canaria, Canary Islands. The data consist of 6689 P-wave travel times from 1487 offshore air-gun shots which were recorded by both land-based seismometers and ocean bottom hydrophones. The crustal structure is well imaged by the data set as demonstrated by analysis of the resolution and tests with synthetic data. The volcanic island is characterized by generally high P-wave velocities (>5.5 km/s) and a heterogeneous structure with large lateral velocity variations. High P-wave velocities are found around the centers of the Miocene shield volcanoes in the vicinity of Aguimes, San Nicolas, and Agaete as well as the center of the Pliocene Roque Nublo volcano. The velocity structure suggests a high percentage of dense intrusive rocks. Some of the intrusive rocks were emplaced during the eruption of >1000 km3 of Miocene felsic magmas following the basaltic shield phase. The velocity structure beneath La Isleta peninsula and its submarine continuation is interpreted as a volcanic rift zone with abundant dikes. The velocities decrease to <5 km/s north of the coastline. A high velocity zone thinning away from the central edifice is interpreted as the massive island flank extending up to 50 km off the coast which is underlain by prevolcanic Neogene–Jurassic sediments. The igneous part of the oceanic crust exhibits an anomalous structure with a relatively small thickness (∼3 km) layer 3 and a 2–4-km-thick layer 2, probably reflecting a modification of the crust due to long-lasting magmatic intrusive activity during the evolution of the Canary Islands. The Moho north of Gran Canaria is found at a depth of ∼15 km. The structure of Gran Canaria and the adjacent ocean basin is thought to be the result of a diffuse mantle upwelling under a slowly moving plate.

A 500,000‐year‐long sediment archive drilled in eastern Anatolia

Sedimentary archives host a wealth of information that can be used to reconstruct paleoclimate as well as the tectonic and volcanic histories of specific regions. Long and continuous archives from the oceans have been collected in thousands of locations by scientific ocean drilling programs over the past 40 years. In contrast, suitable continental archives are rare because terrestrial environments are generally nondepositional and/or subject to erosion. Lake sediments provide ideal drilling targets to overcome this limitation if suitable lakes at key locations have existed continuously for a long time.

Sahara Slide: Age, initiation, and processes of a giant submarine slide

The Sahara Slide is a giant submarine landslide on the northwest African continental margin. The landslide is located on the open continental slope offshore arid Western Sahara, with a headwall at a water depth of ~2000 m. High primary productivity in surface waters drives accumulation of thick fine‐grained pelagic/hemipelagic sediment sequences in the slide source area. Rare but large-scale slope failures, such as the Sahara Slide that remobilized approximately 600 km3 of sediment, are characteristic of this sedimentological setting. Seismic profiles collected from the slide scar reveal a stepped profile with two 100 m high headwalls, suggesting that the slide occurred retrogressively as a slab-type failure. Sediment cores recovered from the slide deposit provide new insights into the process by which the slide eroded and entrained a volcaniclastic sand layer. When this layer was entrained at the base of the slide it became fluidized and resulted in low apparent friction, facilitating the exceptionally long runout of ~900 km. The slide location appears to be controlled by the buried headwall of an older slope failure, and we suggest that the cause of the slide relates to differential sedimentation rates and compaction across these scarps, leading to local increases of pore pressure. Sediment cores yield a date of 50–60 ka for the main slide event, a period of global sea level rise which may have contributed to pore pressure buildup. The link with sea level rising is consistent with other submarine landslides on this margin, drawing attention to this potential hazard during global warming.

Pleistocene giant slope failures offshore Arauco Peninsula, Southern Chile

Abstract: Three Pleistocene giant slope failures are observed in high-resolution bathymetric and seismic reflection data off Southern Chile, two of which extend across the full width of the continental slope from the shelf break to the trench. With mobilized volumes between 253 km3 and 472 km3, these slides are among the largest submarine landslides documented at active continental margins so far. Deposits of each of the slides are imaged as chaotic sequences in seismic reflection lines buried beneath well-stratified sediments in the Chile Trench. The ages of the three slides are about 0.25, 0.41 and >0.56 Ma. The main preconditioning factor for the slope instabilities seems to be local uplift of the continental slope that results in peculiarly high slope angles of up to 30°. Uplift of the marine and continental forearc of the study area is the result of shortening across upper plate faults and therefore a long-term continuous process. Slope instability seems to be an iterative process and failure is likely to recur.