1,720,967 research outputs found
Generation and migration of coarse-grained sediment waves in turbidity current channels and channel-lobe transition zones
No full textLarge-scale sediment waves, composed of gravels and sands, have been studied using deep-water sidescan systems. New data are presented from submarine channels off the Canary Islands and from canyon mouths off Portugal. Data from other areas are briefly reviewed, including a re-interpretation of data from Laurentian Fan, in order to summarise the varied morphology and setting of these bedforms. Coarse-grained sediment waves are found in the proximal, dominantly bypassing areas of deep-water turbidite systems, within canyons, channels and channel-lobe transition zones. Wave heights are in the region of 1-10 m, and wavelengths are up to several hundred metres. The distribution of waves, and sparse sedimentological evidence from modern and ancient sediment wave fields, suggests that initial transport and deposition of coarse sediment occurs within a high-density turbidity current, and not as a non-Newtonian debris flow. In some cases the development of pronounced wave asymmetry, and evidence of wave disruption and reworking, suggests that the wave morphology is at least partially controlled by a later phase of low-density turbidity flow. Grain size also appears to exert some control on wave morphology, for example, gravel-rich waves have a greater height for the same wavelength than sand-rich waves. Coarse-grained sediment waves are often difficult to recognise on the seafloor because of reworking or burial by younger turbidity currents, and are equally difficult to recognise in outcrop because of their large siz
Passage of debris flows and turbidity currents through a topographic constriction: seafloor erosion and deflection of flow pathways
No full textPassage of debris flows and turbidity currents through a topographic constriction: seafloor erosion and deflections of flow pathways
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Offshore continuation of volcanic rift zones, El Hierro, Canary Islands
No full textEl Hierro is the youngest and most southwesterly of the seven Canary islands. The established view, based on subaerial geology, is that El Hierro is a classic example of an oceanic island with 120° –– spaced volcanic rift arms (VRZs) forming a "mercedes star". However, new offshore data do not support this simple interpretation. Instead of the discrete ridges of VRZs, we observe (to the NW and NE) broad areas of irregular morphology, which suggest that rifting activity might not be confined to narrow zones. Furthermore, our data suggest that the anomalously long and steep-flanked Southern Ridge could be part of an older, eroded volcanic edifice that predates much of the other submarine flanks of El Hierro. The Southern Ridge has a distinctive gullied morphology, which strongly contrasts with adjacent flanks. There is also a ~400-m-deep saddle in its longitudinal profile 15 km from the coastline, which we interpret as evidence that the Southern Ridge did not form by continuous dyke intrusion from the El Hierro volcanic centre. South of the saddle, mean flank slopes are 10° steeper (~30°) with a sharp slope break at 3700 m between the ridge and smoothly sedimented seafloor. These steeper slopes and lack of landslide scars to the south of the saddle indicate that the Southern Ridge is a stable edifice, relative to the rest of El Hierro. Surrounding sediments to the southeast appear to onlap the Southern Ridge. A large landslide deposit, El Julan (estimated age >200 ka), occurs to the west of the ridge. This landslide appears to have been constrained from spreading southeastwards by the Southern Ridge, resulting in an elevation difference of 300 m for the seafloor on either side of the ridge
The morphology of the submarine flanks of volcanic ocean islands. A comparative study of the Canary and Hawaiian hotspot islands
No full textThe submarine flanks of volcanic islands are shaped by volcanic constructional processes, landslides, erosion, sediment deposition and tectonic movements. We use a newly acquired multibeam sonar dataset from the westerly Canary Islands (El Hierro, La Palma and Tenerife) to develop a comparison with the Hawaiian Islands, which suggests differences in the processes constructing and modifying their flanks. Landslides affect the flanks of both island groups. Debris avalanches (fast-moving shallow landslides) have left smooth chutes and blocky deposits in both cases, but blocks within some Hawaiian avalanche deposits are markedly larger. We attribute the larger block sizes in the Hawaiian Islands to the fact that their avalanches were relatively unconfined, whereas many Canary and Hawaiian avalanches with small block sizes appear to have been constrained down narrow chutes, forcing interactions between blocks within the flows and encouraging disintegration. Furthermore, the Hawaiian avalanches with the largest blocks initiated near sea-level, whereas many of the Canary avalanches initiated above sea-level, so hydraulic resistance of water entering cracks may be an additional factor in resisting block disintegration during flow. Slow-moving deep-seated slumps or volcanic spreading have produced submarine benches and tabular escarpments due to thrust faulting adjacent to several Hawaiian rift zones, but are not well-developed in the Canaries. Although volcanic morphology is partly obscured by sedimentation in the Canaries, we are able to interpret lava terraces around the deep flanks of El Hierro which are similar to those found in the Hawaiian Islands. However, cones rather than terraces are the most common volcanic forms in the Canary Islands, implying that flank eruptions have involved magma with significant volatile contents, assuming that volatile contents dictate whether cones or terraces are formed. These differences may ultimately originate from the different building rates of the two island groups. For example, the lack of evidence for high-level magma chambers in the Canaries, associated with their lower outputs, implies that there is less possibility for degassing of magma below the summit before lateral intrussion down rift zones, hence cones rather than lava terraces are more commonly observed. The apparent lack of slumping or volcano spreading could also reflect a lack of driving pressure from extensive high-level magma chambers in the Canaries. © 2002 Elsevier Science B.V. All rights reserved
Landslides and the evolution of El Hierro in the Canary Islands
No full textSeismic and sonar data have been used to evaluate the extent and characteristics of giant landslides on the flanks of El Hierro in the Canary Islands. As the youngest and most southwesterly of the Canary Islands, El Hierro has experienced rapid growth and destructive events in its 1.12 million year history. At least four giant landslides (El Golfo, El Julan, San Andres, and Las Playas) have modified ~450 km3 of El Hierro during the last 200–300 thousand years, with each landslide event removing around 3% of the total edifice volume. The extent of landsliding indicates that it is the main process of decay. We characterise flank morphology around El Hierro and distinguish between rugged, unfailed flank, failed flank and steep gullied ridge. Flanks affected by landsliding have downslope long profiles with distinctive b coefficients and exponential forms. The El Golfo landslide is the most recent (15 ka), best described and clearly defined landslide in the Canary Islands. The El Julan landslide (SW flank) has an estimated volume of 130 km3, an age of >200 ka and is characterised by gravitational slumping. On the SE flank, two new landslide events are reported. The younger landslide (Las Playas) occurred 145–176 ka, has a narrow, steep-sided embayment and a corresponding blocky debris avalanche deposit. The older landslide (San Andres) is recognised on the basis of a highly chaotic seismic facies offshore and reduced upper flank gradients. Its lack of an upper flank embayment and offshore blocky debris avalanche lead us to interpret that the landslide involved gravitational slumping, possibly a series of events, which reduced upper flank gradients, but did not catastrophically collapse to produce a debris avalanche
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