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A new scenario for the mass transport deposits west Canary volcanic province
Abstract
A new temporal history of mass wasting processes for the west of the Canary volcanic province is presented. Its onset has been estimated in the middle–upper Miocene (∼13.5 ± 1.2 Ma), matching with a critical period of construction for this volcanic province. Seismic profiles show an emplacement longevity (from the Miocene to Quaternary) in multiple events, defined by stacked lobes of debrites, linked to the flank collapses and volcanic avalanches of the volcanic edifices (islands and seamounts). An evolution of pathways and source areas has been detected from east (Miocene) to west (Quaternary); as well as a migration of the activity to the northwest (west of the Canary Islands: e.g. El Hierro and La Palma). Six connected branches (I–VI), three of them described for the first time here, of Quaternary seismic units of mass transport deposits (MTDs) have been characterized. The Pleistocene makes up a huge buried MTDs system, until now unknown, pointing a new mass transport sedimentological scenario. Finally, the two southernmost branches (V–VI), up to now unknown, are a mainly buried system of stacked and terraced lobes of debrites sourced mainly from the flank collapses of the volcanic seamounts of the Canary Island Seamount Province, apparently inactive from upper Cretaceous.
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Derived digital elevation models (DEMs) are high-resolution acoustic technology that has proven to be a crucial morphometric data source for research into submarine environments. We present a morphometric analysis of forty deep seafloor edifices located to the west of Canary Islands, using a 150 m resolution bathymetric DEM. These seafloor structures are characterized as hydrothermal domes and volcanic edifices, based on a previous study, and they are also morphostructurally categorized into five types of edifice following an earlier classification. Edifice outline contours were manually delineated and the morphometric variables quantifying slope, size and shape of the edifices were then calculated using ArcGIS Analyst tools. In addition, we performed a principal component analysis (PCA) where ten morphometric variables explain 84% of the total variance in edifice morphology. Most variables show a large spread and some overlap, with clear separations between the types of mounds. Based on these analyses, a morphometric growth model is proposed for both the hydrothermal domes and volcanic edifices. The model takes into account both the size and shape complexity of these seafloor structures. Grow occurs via two distinct pathways: the volcanoes predominantly grow upwards, becoming large cones, while the domes preferentially increase in volume through enlargement of the basal area.
Volcanic flank collapses and explosive eruptions are among the largest and most destructive processes on Earth. Events at Mount St. Helens in May 1980 demonstrated how a relatively small (<5 km3) flank collapse on a terrestrial volcano could immediately precede a devastating eruption. The lateral collapse of volcanic island flanks, such as in the Canary Islands, can be far larger (>300 km3), but can also occur in complex multiple stages. Here, we show that multistage retrogressive landslides on Tenerife triggered explosive caldera-forming eruptions, including the Diego Hernandez, Guajara and Ucanca caldera eruptions. Geochemical analyses were performed on volcanic glasses recovered from marine sedimentary deposits, called turbidites, associated with each individual stage of each multistage landslide. These analyses indicate only the lattermost stages of subaerial flank failure contain materials originating from respective coeval explosive eruption, suggesting that initial more voluminous submarine stages of multi-stage flank collapse induce these aforementioned explosive eruption. Furthermore, there are extended time lags identified between the individual stages of multi-stage collapse, and thus an extended time lag between the initial submarine stages of failure and the onset of subsequent explosive eruption. This time lag succeeding landslide-generated static decompression has implications for the response of magmatic systems to un-roofing and poses a significant implication for ocean island volcanism and civil emergency planning.
Volcanic island inception applies large stresses as the ocean crust domes in response to magma ascension and is loaded by eruption of lavas. There is currently limited information on when volcanic islands are initiated on the seafloor, and no information regarding the seafloor instabilities island inception may cause. The deep sea Madeira Abyssal Plain contains a 43 million year history of turbidites among which many originate from mass movements in the Canary Islands. Here, we investigate the composition and timing of a distinctive group of turbidites that we suggest represent a new unique record of large-volume submarine landslides triggered during the inception, submarine shield growth, and final subaerial emergence of the Canary Islands. These slides are predominantly multi-stage and yet represent among the largest mass movements on the Earth's surface up to three or more-times larger than subaerial Canary Islands flank collapses. Thus whilst these deposits provide invaluable information on ocean island geodynamics they also represent a significant, and as yet unaccounted, marine geohazard.
IODP Expedition 340 successfully drilled a series of sites offshore Montserrat, Martinique and Dominica in the Lesser Antilles from March to April 2012. These are among the few drill sites gathered around volcanic islands, and the first scientific drilling of large and likely tsunamigenic volcanic island-arc landslide deposits. These cores provide evidence and tests of previous hypotheses for the composition and origin of those deposits. Sites U1394, U1399, and U1400 that penetrated landslide deposits recovered exclusively seafloor sediment, comprising mainly turbidites and hemipelagic deposits, and lacked debris avalanche deposits. This supports the concepts that i/ volcanic debris avalanches tend to stop at the slope break, and ii/ widespread and voluminous failures of preexisting low-gradient seafloor sediment can be triggered by initial emplacement of material from the volcano. Offshore Martinique (U1399 and 1400), the landslide deposits comprised blocks of parallel strata that were tilted or microfaulted, sometimes separated by intervals of homogenized sediment (intense shearing), while Site U1394 offshore Montserrat penetrated a flat-lying block of intact strata. The most likely mechanism for generating these large-scale seafloor sediment failures appears to be propagation of a decollement from proximal areas loaded and incised by a volcanic debris avalanche. These results have implications for the magnitude of tsunami generation. Under some conditions, volcanic island landslide deposits composed of mainly seafloor sediment will tend to form smaller magnitude tsunamis than equivalent volumes of subaerial block-rich mass flows rapidly entering water. Expedition 340 also successfully drilled sites to access the undisturbed record of eruption fallout layers intercalated with marine sediment which provide an outstanding high-resolution data set to analyze eruption and landslides cycles, improve understanding of magmatic evolution as well as offshore sedimentation processes.
Mass-transport deposits (MTDs) are gravity-induced units that represent an important component of modern and ancient deep-water stratigraphic successions. MTDs have been widely documented in the literature , but a comprehensive compilation of quantitative morphometric parameters associated with their external architecture is still lacking. This work presents a mor-phometric database that contains 332 data points that document the length, area, volume , and thickness of MTDs from different geologic ages and a variety of continental margins around the world. The compilation contains data collected from interpretations done by the authors in eastern offshore Trinidad and the Gulf of Mexico as well as from data mining from the peer-reviewed literature. Preliminary results indicate that there is a good correlation between a series of parameters that include the area, length, and volume of MTDs. On the other hand, the correlation between thickness and volume seems to be harder to document mainly due to lateral variations in thickness that are typical within MTDs. Data analysis suggests that previous qualitative classification of attached and detached MTDs can be validated by using a quantitative approach. This validation suggests that morphometric parameters associated with the architecture of MTDs can be used as a hint to link geologic setting , deposit geometry, and potential causal mechanisms. In addition, the defined mor-phometric relationships that were encountered between the different morphometric parameters (e.g., length and area) are useful to predict MTD dimensions in areas of the subsurface where data are limited and/or data quality is low. Likewise, these morphometric relationships can be used in outcrop studies where exposure of the MTD units is also limited.
Volcaniclastic turbidites on the Madeira Abyssal Plain provide a record of large-volume volcanic island flank collapses from the Canary Islands. This long-term record spans 17 Ma, and comprises one hundred and twenty-five volcaniclastic beds. Determining the timing, provenance and volumes of these turbidites provides key information about the occurrence of mass wasting from the Canary Islands, especially the western islands of Tenerife, La Palma and El Hierro. These turbidite records demonstrate that landslides often coincide with protracted periods of volcanic edifice growth, suggesting that loading of the volcanic edifices may be a key preconditioning factor for landslide triggers. Furthermore, the last large-volume failures from Tenerife coincide with explosive volcanism at the end of eruptive cycles. Many large-volume Canary Island landslides also occurred during periods of warmer and wetter climates associated with sea-level rise and subsequent highstand. However, these turbidites are not serially dependent and any association with climate or sea level change is not statistically significant.
New 3·5 kHz profiles and a series of piston cores from the north-west African margin provide evidence that the Saharan debris flow travelled for more than 400 km on a highly fluid, low-friction layer of poorly sorted sediment. Data suggest that the Saharan debris flow is a two-phase event, consisting of a basal, volcaniclastic debris flow phase overlain by a pelagic debris flow phase. Both phases were emplaced on the lower continental rise by a single large debris flow at around 60 ka. The volcaniclastic flow left a thin deposit less than 5 m thick. This contrasts with the much thicker (over 25 m) deposit left by the pelagic debris flow phase. We suggest that pelagic sediment, sourced and mobilized as debris flow from the African continental margin, loaded and destabilized volcaniclastic material in the vicinity of the western Canaries. When subjected to this loading, the volcaniclastic material appears to have formed a highly fluid sandy debris flow, capable of transporting with it the huge volumes of pelagic debris, and contributing to a runout distance extending over 400 km downslope of the Canary Islands on slopes that decrease to as little as 0·05°. It is likely that the pelagic debris formed a thick impermeable slab above the volcanic debris, thus maintaining high pore pressures generated by loading and giving rise to low apparent friction conditions. The distribution of the two debris phases indicates that the volcaniclastic debris flow stopped within a few tens of kilometres after escaping from beneath the pelagic debris flow, probably because of dissipation of excess pore pressure when the seal of pelagic material was removed.
Volcaniclastic turbidites in the Madeira Abyssal Plain provide a record of major landslides from the Western Canary Islands in the last 1.5 Ma. These volcaniclastic turbidites are composed of multiple fining‐upward turbidite sands, known as subunits. The subunits indicate that the landslides responsible for the sediment gravity flows occurred in multiple stages. The subunits cannot result from flow reflection or splitting because the compositions of volcanic glasses from each individual subunit in an event bed are subtly different. This indicates that each subunit represents a discrete failure as part of a multistage landslide. This has significant implications for geohazard assessments, as multistage failures reduce the magnitude of the associated tsunami. The multistage failure mechanism reduces individual landslide volumes from up to 350 km3 to less than 100 km3. Thus although multistage failure ultimately reduce the potential landslide and tsunami threat, the landslide events may still generate significant tsunamis close to source.
Volcanic island landslides can pose a significant geohazard through
landslide-generated tsunamis. However, a lack of direct observations
means that factors influencing tsunamigenic potential of landslides
remain poorly constrained. The study of distal turbidites generated from
past landslides can provide useful insights into key aspects of the
landslide dynamics and emplacement process, such as total event volume
and whether landslides occurred as single or multiple events. The
northern flank of Tenerife has undergone multiple landslide events, the
most recent being the Icod landslide dated at ˜165 ka. The Icod
landslide generated a turbidite with a deposit volume of ˜210
km3, covering 355,000 km2of seafloor off northwest
Africa. The Icod turbidite architecture displays a stacked sequence of
seven normally graded sand and mud intervals (named subunits SBU1-7).
Evidence from subunit bulk geochemistry, volume, basal grain size,
volcanic glass composition and sand mineralogy, combined with
petrophysical and geophysical data, suggests that the subunit facies
represents multistage retrogressive failure of the Icod landslide. The
basal subunits (SBU1-3) indicate that the first three stages of the
landslide had a submarine component, whereas the upper subunits (SBU4-7)
originated above sea level. The presence of thin, non-bioturbated, mud
intervals between subunit sands suggests a likely time interval of at
least several days between each stage of failure. These results have
important implications for tsunamigenesis from such landslides, as
multistage retrogressive failures, separated by several days and with
both a submarine and subaerial component, will have markedly lower
tsunamigenic potential than a single-block failure.
The extent of mass wasting along the north flank of Tenerife has been mapped using swath bathymetry, GLORIA side-scan sonar, and 3.5-kHz echo sounder data. The marine surveys show that, north of Tenerife, a giant landslide is exposed over an area of 5500 km2 of the seafloor, more than twice the surface area of the island. The landslide truncates an older ridge and valley topography that is associated with the shield building basalts on Tenerife. We interpret the ridge and valley topography as the result of subaerial erosion. The landslide is estimated to have a length of 100 km, a width of up to 80 km, and a volume of about 1000 km3. It extends onshore into the Orotava and Icod valleys which have been interpreted as of landslide origin. K-Ar dating of basaltic flows in the steep headwall of Orotava suggests an age of formation for the valley is younger than 0.78 Ma and may even be younger than 0.27 Ma. The Icod valley is located immediately to the north of the most recent volcano on Tenerife, Las Cañadas, and has been associated with the collapse of its caldera, between 1.2 and 0.2 Ma.A young age for the landslide is supported by the 3.5-kHz echo sounder data which show that the landslide is draped by a thin (
Geologic evidence on Tenerife, Canary Islands, indicates six successive north-directed debris avalanche events, including: the Anaga and Teno (ca. 6 Ma) events that affected the old basaltic series, and the Tigaiga (>2.3 Ma), Roques de García (possibly 0.6 0.7 Ma), Orotava (ca. 0.6 Ma), and Icod (1000 km3) inferred for these events can account for the volume of previous estimates of offshore volcanic debris. These repeated flank failures can also account for the present morphology of Las Cañadas caldera wall, which partly bounds a multiepisodic lateral-collapse structure 25 km wide.
A large area of debris avalanche deposits has been discovered on the western submarine flanks of the island of La Palma. Multibeam bathymetry and its derivative backscatter data, Towed Ocean Bottom Instrument (TOBI) sidescan sonar images, and 3.5 kHz and airgun seismic reflection data have been used to identify at least two, and possibly as many as four, major landslide events. The youngest of the events, the Cumbre Nueva Debris Avalanche, extends onshore into the valleys bounded by the Caldera de Taburiente and Cumbre Nueva Ridge, which mark the degraded collapse scars. Radiometric dating of the volcanic flows in the headwall indicate an age of between 536 and 125 ka for this landslide. The debris avalanche covers an area of 780 km2, has a maximum thickness of 500 m, and has an estimated volume of 95 km3. Older deposits, collectively referred to as the Playa de la Veta Debris Avalanche Complex, are probably, as the name indicates, an amalgamation of at least two or three events rather than the result of a single catastrophic failure. The Playa de la Veta Debris Avalanche Complex is associated onshore with an unconformity dated as late Matuyama (1 Ma to 800 ka). It covers an area of 1200 km2, has a maximum thickness of 1300 m, and may represent a total volume of up to 650 km3. The greater thicknesses and limited areas occupied by debris avalanches on the western flank of La Palma, compared to other landslides in the Canary Archipelago, suggest that the La Palma landslide masses have relatively low mobility. The different debris avalanche lobes formed by each landslide event are separated by channels 2-2.5 km wide. The clear relationship between channel position and the boundaries of each debris avalanche lobe indicates that debris avalanches control later channel formation and pathways. The relief of the submarine flanks of the La Palma volcanoes, in the areas of island slope unaffected by landslides, is mainly the result of constructional volcanic processes. However, the older submarine slopes, such as in the northern Taburiente volcano, may also have been modified by smaller-scale submarine mass wasting and sediment flows.
The role of mantle plumes in the formation of intraplate volcanic islands and seamount chains is being increasingly questioned. Particular examples are the abundant and somewhat irregularly distributed island and seamount volcanoes off the coast of northwest Africa. New 40Ar / 39Ar ages and Sr–Nd–Pb isotope geochemistry of volcanic rocks from seamounts northeast of the Madeira Islands (Seine and Unicorn) and northeast of the Canary Islands (Dacia and Anika), however, provide support for the plume hypothesis. The oldest ages of shield stage volcanism from Canary and Madeira volcanic provinces confirm progressions of increasing age to the northeast. Average volcanic age progression of ∼1.2 cm/a is consistent with rotation of the African plate at an angular velocity of ∼0.20° ± 0.05 /Ma around a common Euler pole at approximately 56° N, 45° W computed for the period of 0–35 Ma. A Euler pole at 35° N, 45° W is calculated for the time interval of 35–64 Ma. The isotope geochemistry further confirms that the Madeira and Canary provinces are derived from different sources, consistent with distinct plumes having formed each volcanic group. Conventional hotspot models, however, cannot easily explain the up to 40 m.y. long volcanic history at single volcanic centers, long gaps in volcanic activity, and the irregular distribution of islands and seamounts in the Canary province. A possible explanation could involve interaction of the Canary mantle plume with small-scale upper mantle processes such as edge-driven convection. Juxtaposition of plume and non-plume volcanism could also account for observed inconsistencies of the classical hotspot concept in other volcanic areas.
Holocene and slightly pre-Holocene submarine landslide are found both in high-latitude glacial-dominated margins and in lower latitude, river-dominated margins. This paper constitutes a major assessment on some of the best-studied submarine instabilities in the world. We review and update from original data and literature reports the current state of knowledge of Storegga, Traenadjupet and Finneidfjord slides from the mid-Norwegian margin, Afen Slide from the Faeroe-Shetland Channel, BIG’95 Slide and Central Adriatic Deformation Belt (CADEB) from continental slope and inner continental shelf settings off the Ebro and Po rivers in the Mediterranean Sea, Canary Slide west of the westernmost, youngest Canary Islands and Gebra Slide off the northern tip of the Antarctic Peninsula in the southern hemisphere, i.e. those studied in the Continental Slope Stability (COSTA) project. The investigated slides range in size from the gigantic 90,000 km2 and almost 3000 km3 Storegga Slide to the tiny 1 km2 and 0.001 km3 Finneidfjord Slide. Not only do individual submarine landslides rarely involve processes precisely fitting with pre-established categories, mostly based on subaerial research, but also they display complex mechanical behaviors within the elastic and plastic fields. Individual events can involve simultaneous or successive vertical to translational movements including block detachment, block gliding, debris flow, mud flow and turbidity currents. The need for an in-depth revision of the classification criteria, and eventually for a new classification system, based on the new imaging capabilities provided by modern techniques, is more than obvious. We suggest a new system, which, for the moment, is restricted to debris flows and debris avalanches.
Volume calculation methods are critically reviewed and the relations between some key geomorphic parameters are established for the selected slides. The assumed volume missing from scar areas does not necessarily match the actual volume of sediment remobilised by an individual event since in situ sediment can be remoulded and eventually incorporated during the slide downslope journey. CADEB, a shore-parallel prodelta detached from its source, is the exception to the good correlation found between across slope width and alongslope length with slide area. Height drop measured from the headwall upper rim to its foot correlates with the debris deposit maximum thickness unless the slide moves into restricted areas, which prevent farther forward expansion of the deposit, such as Gebra and BIG’95. In such cases, bover-thickenedQ deposits are found. A particularly loose and fluid behavior can be deduced for slides showing an bover-thinnedQ character, such as the Canary Slide that traveled 600 km.
Scar areas and slip planes have been investigated with particular emphasis. Although slide headwalls might present locally steep gradients (up to 238 for Storegga Slide), the slope gradients of both the failed segment margins and the main slip planes are very low (max. 28 and usually around 18 and less). An exception is the Finneidfjord Slide (208–b58) that occurred in 1996 because of a combination of climatic and anthropogenic factors leading to excess pore pressure and failure. Mechanically distinct, low permeable clayey bweak layersQ often correspond to slip planes beyond the slide headwall. Since not only formation of these bweak layersQ but also sedimentation rates are climatically controlled, we can state that slide pre-conditioning is climatically driven too.
Run-out distances reflect the degree of disintegration of the failed mass of sediment, the total volume of initially failed material and transport mechanisms, including hydroplanning. Commonly, specific run-outs could be attributed to distinct elements, such as cohesive blocks and looser matrix, as nicely illustrated by the BIG’95 Slide. Total run-outs usually correspond to matrix run-outs since the coarser elements tend to rest at shorter distances. Outrunner blocks are, finally, a very common feature proving the ability of those elements to glide over long distances with independence of the rest of the failed mass.
In addition to pre-conditioning factors related to geological setting and sedimentation conditions, a final trigger is required for submarine landslides to take place, which is most often assumed to be an earthquake. In high latitude margins, earthquake magnitude intensification because of post-glacial isostatic rebound has likely played a major role in triggering landslides. Although it cannot be totally ruled out, there is little proof, at least amongst the COSTA slides, that gas hydrate destabilisation or other processes linked to the presence of shallow gas have acted as final triggers.
Submarine volcanic eruptions are frequent and important events, yet they are rarely observed. Here we relate bathymetric and hydroacoustic images from the 2011-2012 El Hierro eruption with surface observations and deposits imaged and sampled by ROV. As result of the shallow submarine eruption, a new volcano named Tagoro grew from 375 to 89 m depth. The eruption consisted of two main phases of edifice construction intercalated with collapse events. Hydroacoustic images show that the eruptions ranged from explosive to effusive with variable plume types and resulting deposits, even over short time intervals. At the base of the edifice, ROV observations show large accumulations of lava balloons changing in size and type downslope, coinciding with the area where floating lava balloon fallout was observed. Peaks in eruption intensity during explosive phases generated vigorous bubbling at the surface, extensive ash,vesicular lapilli and formed high-density currents, which together with periods of edifice gravitational collapse, produced extensive deep volcaniclastic aprons. Secondary cones developed in the last stages and show evidence for effusive activity with lava ponds and lava flows that cover deposits of stacked lava balloons. Chaotic masses of heterometric boulders around the summit of the principal cone are related to progressive sealing of the vent with decreasing or variable magma supply. Hornitos represent the final eruptive activity with hydrothermal alteration and bacterial mats at the summit. Our study documents the distinct evolution of a submarine volcano and highlights the range of deposit types that may form and be rapidly destroyed in such eruptions.
New seismic profiles, bathymetric data and sediment-rock sampling document for the first time the discovery of hydrothermal vent complexes and volcanic cones at 4800-5200 m depth related to recent volcanic and intrusive activity in an unexplored area of the Canary Basin (Eastern Atlantic Ocean, 500 km west of the Canary Islands). A complex of sill intrusions is imaged on seismic profiles showing saucer-shaped, parallel or inclined geometries. Three main types of structures are related to these intrusions. Type I consists of cone-shaped depressions developed above inclined sills interpreted as hydrothermal vents. Type II is the most abundant and is represented by isolated or clustered hydrothermal domes bounded by faults rooted at the tips of saucer-shaped sills. Domes are interpreted as seabed expressions of reservoirs of CH4- and CO2-rich fluids formed by degassing and contact metamorphism of organic-rich sediments around sill intrusions. Type III are hydrothermal-volcanic complexes originated above stratified or branched inclined sills connected by a chimney to the seabed volcanic edifice. Parallel sills sourced from the magmatic chimney formed also domes surrounding the volcanic cones. Core and dredges revealed that these volcanoes, which must be among the deepest in the world, are constituted by OIB-type, basanites with an outer ring of blue-green hydrothermal Al-rich smectite muds. Magmatic activity is dated, based on lava samples, at 0.78±0.05 and 1.61±0.09 Ma (K/Ar methods) and on tephra layers within cores at 25-237 ky. The Subvent hydrothermal-volcanic complex constitutes the first modern system reported in deep-water oceanic basins related to intraplate hotspot activity.
This paper presents a combined onshore-offshore morpho-structural characterization of the El Golfo giant landslide, island of El Hierro (Canary Archipelago, Spain). Offshore data from multibeam echosounders, chirp sub-bottom profiles and multichannel seismic reflection data and onshore data coming from water wells and galleries have been analyzed. The geomorphology and the internal architecture of this giant flank collapse are defined in order to determine its multi-event nature. The subaerial headscarp shows a non-continuous arcuate profile formed by two nested semi-circular amphitheaters that extend offshore along a smooth chute, suggesting the occurrence of at least two large retrogressive events. Channels/gullies and escarpments developed along the submarine sector of the scar also indicate smaller-scale events and predominance of sediment bypass. In the lower slope, two subunits of submarine mass transport deposits (MTDs), debris avalanche, are identified on multichannel seismic reflection profiles with a total estimated volume of 318km³: ~84km³ and ~234km³, for the younger and older MTDs respectively. Data from wells and galleries show abrasion platforms with beach deposits at sea-level (0masl) formed after the landslide scar and buried by the post-collapse El Golfo lavas infill, suggesting an age at least older than 23.5-82.5Ka for the landslide.
The margin of the continental slope of the Volcanic Province of Canary Islands is characterised by seamounts, submarine hills and large landslides. The seabed morphology including detailed morphology of the seamounts and hills was analysed using multibeam bathymetry and backscatter data, and very high resolution seismic profiles. Some of the elevation data are reported here for the first time. The shape and distribution of characteristics features such as volcanic cones, ridges, slides scars, gullies and channels indicate evolutionary differences. Special attention was paid to recent geological processes that influenced the seamounts. We defined various morpho-sedimentary units, which are mainly due to massive slope instability that disrupt the pelagic sedimentary cover. We also studied other processes such as the role of deep bottom currents in determining sediment distribution. The sediments are interpreted as the result of a complex mixture of material derived from a) slope failures on seamounts and submarine hills; and b) slides and slumps on the continental slope.
Landslides are common features in the vicinity of volcanic islands. In this contribution, we investigate landslides emplacement and dynamics around the volcanic island of Martinique based on the first scientific drilling of such deposits. The evolution of the active Montagne Pelée volcano on this island has been marked by three major flank-collapses that removed much of the western flank of the volcano. Subaerial collapse volumes vary from 2 to 25 km3 and debris avalanches flowed into the Grenada Basin. High-resolution seismic data (AGUADOMAR-1999, CARAVAL-2002, and GWADASEIS-2009) is combined with new drill cores that penetrate up to 430 m through the three submarine landslide deposits previously associated to the aerial flank-collapses (Site U1399, Site U1400, Site U1401, IODP Expedition 340, Joides Resolution, March-April 2012). This combined geophysical and core data provide an improved understanding of landslide processes offshore a volcanic island. The integrated analysis shows a large submarine landslide deposit, without debris avalanche deposits coming from the volcano, comprising up to 300 km3 of remobilized seafloor sediment that extends for 70 km away from the coast and covers an area of 2100 km2. Our new data suggest that the aerial debris avalanche deposit enter the sea but stop at the base of submarine flank. We propose a new model dealing with seafloor sediment failures and landslide propagation mechanisms, triggered by volcanic flank-collapse events affecting Montagne Pelée volcano. Newly recognized landslide deposits occur deeper in the stratigraphy, suggesting the recurrence of large-scale mass-wasting processes offshore the island and thus, the necessity to better assess the associated tsunami hazards in the region.
The Cumbre Vieja volcano, on La Palma in the western Canary Islands, is an unstable area that may develop into a future landslide,
generating a tsunami that could cause damage far from the source. However, volcaniclastic turbidites that are directly correlated
with the two most recent Canary Islands landslides, show stacked sub-units within a single turbidite bed. This may indicate
multiple stages of landslide failure. Similar findings have previously been reported from volcaniclastic turbidites linked
to Hawaiian landslides. Consequently the potential tsunami hazard from such failures may be lower than previously predicted.
Active volcanoes are revealed to be dynamically evolving structures, the growth and development of which are characteristically punctuated by episodes of instability and subsequent structural failure. Edifice instability typically occurs in response to one or more of a range of agencies, including magma emplacement, the overloading or oversteepening of slopes, and peripheral erosion. Similarly, structural failure of a destabilized volcano may occur in response to a number of triggers of which seismogenic or magmagenic are common. Edifice failure and consequent debris avalanche formation appears to occur, on average, at least four times a century, and similar behaviour is now known to have occurred at volcanoes on Mars and Venus.
Landslides play an important role in the evolution of many volcanic
islands, producing huge fields of blocky volcanic debris on their
submarine slopes. Sidescan sonar images presented in this paper provide
evidence for a large debris avalanche, El Golfo, on the northern flank
of Hierro Island in the Canary Islands. Angular blocks, as much as 1.2
km across and 200 m high, cover the debris avalanche surface. El Golfo
avalanche is related to both the Canary debris flow and a volcaniclastic
turbidite found in the Madeira abyssal plain 600 km west of the
Canaries. Dating of the turbidite and the failure scarp onshore
indicates that the failure probably occurred between 13 and 17 ka. There
appears to be a general correlation between volcaniclastic turbidites in
the abyssal-plain sequence and landslides in the Canaries during the
past 750 ka. Tentatively, this correlation suggests that seven major
landslides have affected the Canaries in that time.
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.
Multichannel seismic reflection and gravity data define the structure of Mesozoic ocean crust of the Canary Basin, formed at slow spreading rates. Single and multichannel seismics show a transition from smooth to rough basement topography from Jurassic to Cretaceous crust and a coeval change in crustal structure. Internal reflectivity of the rough basement area comprises upper, upper middle or whole crust cutting discrete dipping reflections. Lower-crustal reflectivity is almost absent and reflections from the crust-mantle transition are short and discontinuous or absent for several kilometers. In contrast, crust in the smooth basement area is characterized by sparse lower crustal events and common reflections from the crust-mantle boundary. The crustal structure of fracture zones in the rough basement area is associated with depressions in the basement top and in most cases with thin crust. In the smooth basement area, fracture zones exhibit neither a clear topographic expression nor crustal thinning. We interpret these characteristics as indicative of an increase in extensional tectonic activity and decrease in magmatic activity at the spreading ridge associated with a general decrease of spreading rate from Jurassic to Cretaceous times. In addition, the crust imaged across the path of the Cape Verde Hot Spot in the Canary Basin exhibits a widespread lower crustal reflectivity, very smooth topography and apparently thick crust. Our data document significant changes in the structure of crust formed at slow spreading rates which we attribute to thermal changes in the lithosphere due either to variations in spreading rate or to the presence of a hot spot beneath the Mesozoic Mid-Atlantic Ridge.
This paper describes the sediments of the whole Madeira Abyssal Plain including the MADCAP area to the south. Previous studies have centred on the central sub-basin which occupies most of the plain and which has become one of the most studied areas of seafloor. The plain is made up of a series of large turbidites separated by thin pelagic layers which can be used to provide a high resolution stratigraphy back to about 750,000 years. Many of the turbidites were deposited close to oxygen isotope stage boundaries suggesting a strong relationship to sea-level/climate change, although the exact initiation mechanism for the flows is not known. Data from the northern and southern extensions of the plain shows the effects of different entry points for the four main turbidite groups; volcanic rich turbidites enter from the NE and E, calcareous turbidites from the W, one group of organic rich turbidites enters from the SE and one group from the NE and E. Different turbidite sources operate for limited periods before switching to one of the other source areas. It is very rare for two sources to provide turbidites at the same isotope stage boundary.
The morphology of the source area of the Canary Debris Flow has been mapped using both GLORIA reconnaissance and TOBI high-resolution sidescan sonar systems. West of ≈19°W, the seafloor is characterized by a strongly lineated downslope-trending fabric. This fabric can be interpreted as being caused by streams of debris separated by longitudinal shears. Multiple flow pulses are indicated by a series of asymmetrical lateral ridges which mark the northern boundary of the flow. East of ≈19°W, GLORIA data show only a weak fabric of irregular patches and alongslope lineaments. The TOBI data show the patches to be coherent sediment blocks up to 10 km across, surrounded by debris flow material. These are interpreted as in situ areas of seafloor sediment which have survived the slope failure and debris flow event rather than transported fragments of a failed sediment slab. TOBI data from the best developed area of alongslope lineaments show a series of small faults downstepping to the west. This area of seafloor is interpreted as one of partial sediment failure, where the failure process became ‘frozen’ before total mobilization of the seafloor sediments could occur. The overall morphology of the failure area indicates removal of a slab-like body of sediment, although we cannot distinguish between retrogressive and slab-slide failure mechanisms. If the latter mechanism is applicable, fragmentation of the failing ‘slab’ must have commenced concurrently with the onset of downslope transport. Immediately upslope from the debris flow source area, a seafloor of characteristic rough blocky texture is interpreted as the surface of a debris avalanche derived from the slopes of the island of El Hierro. The debris flow and avalanche appear to be simultaneous events, with failure of the slope sediments occurring while the avalanche deposits were still mobile enough to fill and disguise the topographic expression of the debris flow headwall. Loading of the slope sediments by the debris avalanche most probably triggered the Canary Debris Flow.
The north-east Atlantic continental margin displays a wide range of sediment transport systems with both along-slope and down-slope processes. Off most of the north-west African margin, south of 26°N, upwelling produces elevated accumulation rates, although there is little fluvial input. This area is subject to infrequent but large-scale mass movements, giving rise to debris flows and turbidity currents. The turbidity currents traverse the slope and deposit thick layers on the abyssal plains, while debris flows deposit on the continental slope and rise. From the Atlas Mountains northwards to 56°N, the margin is less prone to mass movements, but is cut by a large number of canyons, which also funnel turbidity currents to the abyssal plains. The presence of a lithospheric plate boundary off SW Iberia is believed to have led to high rates of sediment transport to the deep sea. Even larger quantities of coarse sediments have fed the canyons and abyssal plains in the Bay of Biscay as a result of drainage from melting icecaps. Bottom currents have built sediment waves off the African and Iberian margins, and created erosional furrows south of the Canaries. The Mediterranean outflow is a particularly strong bottom current near the Straits of Gibraltar, depositing sand waves and mud waves in the Gulf of Cadiz. North of 56°N, the margin is heavily influenced by glacial and glaciomarine processes active during glacial times, which built glacial trough-mouth fans, such as the North Sea Fan, and left iceberg scour marks on the upper slope and shelf. Over a long period, especially during interglacials, this part of the margin has been greatly affected by along-slope currents, with less effect by turbidity currents than on the lower latitude margins. Large-scale mass movements are again a prominent feature, particularly off Norway and the Faeroes. Some of these mass movements have occurred during the Holocene, although high glacial sedimentation rates may have contributed to the instability.
The Marnoso Arenacea Formation provides the most extensive correlation of individual flow deposits (beds) yet documented in an ancient turbidite system. These correlations provide unusually detailed constraints on bed shape, which is used to deduce flow evolution and assess the validity of numerical and laboratory models. Bed volumes have an approximately log-normal frequency distribution; a small number of flows dominated sediment supply to this non-channelized basin plain. Turbidite sandstone within small-volume (<0·7 km3) beds thins downflow in an approximately exponential fashion. This shape is a property of spatially depletive flows, and has been reproduced by previous mathematical models and laboratory experiments. Sandstone intervals in larger-volume (0·7–7 km3) beds have a broad thickness maximum in their proximal part. Grain-size trends within this broad thickness maximum indicate spatially near-uniform flow for distances of ∼30 km, although the flow was temporally unsteady. Previous mathematical models and laboratory experiments have not reproduced this type of deposit shape. This may be because models fail to simulate the way in which near bed sediment concentration tends towards a constant value (saturates) in powerful flows. Alternatively, the discrepancy may be the result of relatively high ratios of flow thickness and sediment settling velocity in submarine flows, together with very gradual changes in sea-floor gradient. Intra-bed erosion, temporally varying discharge, and reworking of suspension fallout as bedload could also help to explain the discrepancy in deposit shape. Most large-volume beds contain an internal erosion surface underlain by inversely graded sandstone, recording waxing and waning flow. It has been inferred previously that these characteristics are diagnostic of turbidites generated by hyperpycnal flood discharge. These turbidites are too voluminous to have been formed by hyperpycnal flows, unless such flows are capable of eroding cubic kilometres of sea-floor sediment. It is more likely that these flows originated from submarine slope failure. Two beds comprise multiple sandstone intervals separated only by turbidite mudstone. These features suggest that the submarine slope failures occurred as either a waxing and waning event, or in a number of stages.
The Northwest African slope apron is an interesting modern analogue for deep-water systems with complex seafloor topography. A sediment process map of the Northwest African continental margin illustrates the relative roles of different sedimentary processes acting across the entire margin. Fine-grained pelagic and hemipelagic sedimentation is dominant across a large area of the margin, and is considered to result from background sedimentary processes. Alongslope bottom currents smooth and mould the seafloor sediments, and produce bedforms such as erosional furrows, sediment waves and contourite drifts. Downslope gravity flows (debris avalanches, debris flows and turbidity currents) are infrequent but important events on the margin, and are the dominant processes shaping the morphology of the slope and rise. The overall distribution of sedimentary facies and morphological elements on the Northwest African margin is characteristic of a fine-grained clastic slope apron. However, the presence of numerous volcanic islands and seamounts along the margin leads to a more complex distribution of sedimentary facies than is accounted for by slope apron models. In particular, the distribution and thickness of turbidite sands are controlled by the location of the break-of-slope, which is itself controlled by the pre-existing submarine topography.
Geomorphologic analysis of submarine and subaerial surface features using a combined topographic/bathymetric digital elevation model coupled with onshore geological and geophysical data constrain the age and geometry of giant landslides affecting the north flank of Tenerife. Shaded relief and contour maps, and topographic profiles of the submarine north flank, permit the identification of two generations of post-shield landslides. Older landslide materials accumulated near the shore (<40-km) and comprise ∼700 km3 of debris. Thickening towards a prominent axis suggests one major landslide deposit. Younger landslide materials accumulated 40–70 km offshore and comprise the products of three major landslides: the La Orotava landslide complex, the Icod landslide and the East Dorsal landslide complex, each with an onshore scar, a proximal submarine trough, and a distal deposit lobe. Estimated lobe volumes are 80, 80 and 100 km3, respectively. The old post-shield landslide scar is an amphitheatre, 20–25 km wide, partly submarine, now completely filled with younger materials. Age–width relationships for Tenerife's coastal platform plus onshore geological constraints suggest an age of ca. 3 Ma for the old collapse. Young landslides are all less than 560 ka old. The La Orotava and Icod slides involved failures of slabs of subaerial flank to form the subaerial La Orotava and Icod valleys. Offshore, they excavated troughs by sudden loading and basal erosion of older slide debris. The onshore East Dorsal slide also triggered secondary failure of older debris offshore. The slab-like geometry of young failures was controlled by weak layers, deep drainage channels and flank truncation by marine erosion. The (partly) submarine geometry of the older amphitheatre reflects the absence of these features. Relatively low H/L ratios for the young slides are attributed to filling of the slope break at the base of the submarine edifice by old landslide materials, low aspect ratios of the failed slabs and channelling within troughs. Post-shield landslides on Tenerife correlate with major falls in sea level, reflecting increased rates of volcanism and coastal erosion, and reduced support for the flank. Landslide head zones have strongly influenced the pattern of volcanism on Tenerife, providing sites for major volcanic centres.
Landslides have been a key process in the evolution of the western Canary Islands. The younger and more volcanically active Canary Islands, El Hierro, La Palma and Tenerife, show the clearest evidence of recent landslide activity. The evidence includes landslide scars on the island flanks, debris deposits on the lower island slopes, and volcaniclastic turbidites on the floor of the adjacent ocean basins. At least 14 large landslides have occurred on the flanks of the El Hierro, La Palma and Tenerife, the majority of these in the last 1 million years, with the youngest, on the northwest flank of El Hierro, as recent as 15 thousand years in age. Older landslides undoubtedly occurred, but are difficult to quantify because the evidence is buried beneath younger volcanic rocks and sediments. Landslides on the Canary Island flanks can be categorised as debris avalanches, slumps or debris flows. Debris avalanches are long runout catastrophic failures which typically affect only the superficial part of the island volcanic sequence, up to a maximum thickness of 1 to 2 km. They are the commonest type of landslide mapped. In contrast, slumps move short distances and are deep-rooted landslides which may affect the entire thickness of the volcanic edifice. Debris flows are defined as landslides which primarily affect the sedimentary cover of the submarine island flanks. Some landslides are complex events involving more than one of the above end-member processes.
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