Neil C. Mitchell

Slope failures on the flanks of the western Canary Islands

Abstract 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. Individual debris avalanches have volumes in the range of 50–500 km3, cover several thousand km2 of seafloor, and have runout distances of up to 130 km from source. Overall, debris avalanche deposits account for about 10% of the total volcanic edifices of the small, relatively young islands of El Hierro and La Palma. Some parameters, such as deposit volumes and landslide ages, are difficult to quantify. The key characteristics of debris avalanches include a relatively narrow headwall and chute above 3000 m water depth on the island flanks, broadening into a depositional lobe below 3000 m. Debris avalanche deposits have a typically blocky morphology, with individual blocks up to a kilometre or more in diameter. However, considerable variation exists between different avalanche deposits. At one extreme, the El Golfo debris avalanche on El Hierro has few large blocks scattered randomly across the avalanche surface. At the other, Icod on the north flank of Tenerife has much more numerous but smaller blocks over most of its surface, with a few very large blocks confined to the margins of the deposit. Icod also exhibits flow structures (longitudinal shears and pressure ridges) that are absent in El Golfo. The primary controls on the block structure and distribution are inferred to be related to the nature of the landslide material and to flow processes. Observations in experimental debris flows show that the differences between the El Golfo and Icod landslide deposits are probably controlled by the greater proportion of fine grained material in the Icod landslide. This, in turn, relates to the nature of the failed volcanic rocks, which are almost entirely basalt on El Hierro but include a much greater proportion of pyroclastic deposits on Tenerife. Landslide occurrence appears to be primarily controlled by the locations of volcanic rift zones on the islands, with landslides propagating perpendicular to the rift orientation. However, this does not explain the uneven distribution of landslides on some islands which seems to indicate that unstable flanks are a ‘weakness’ that can be carried forward during island development. This may occur because certain island flanks are steeper, extend to greater water depths or are less buttressed by the surrounding topography, and because volcanic production following a landslide my be concentrated in the landslide scar, thus focussing subsequent landslide potential in this area. Landslides are primarily a result of volcanic construction to a point where the mass of volcanic products fails under its own weight. Although the actual triggering factors are poorly understood, they may include or be influenced by dyke intrusion, pore pressure changes related to intrusion, seismicity or sealevel/climate changes. A possible relationship between caldera collapse and landsliding on Tenerife is not, in our interpretation, supported by the available evidence.

The morphology of the submarine flanks of volcanic ocean islands: A comparative study of the Canary and Hawaiian hotspot islands

The 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 intrusion 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.

A model for attenuation of backscatter due to sediment accumulations and its application to determine sediment thicknesses with GLORIA sidescan sonar

Where the seafloor consists of a highly backscattering surface covered with a drape of fine-grained sediment, and where the sediment does not contribute significantly to the backscattered signal, the sediment thickness can potentially be inferred from the amount of signal attenuation in the sediment layer. A first-order acoustical model is proposed in which the signal strength is reduced by an amount proportional to the sediment thickness and attenuation rate in the sediments. Data collected with Geological Long-Range Inclined Azdic (GLORIA) are used to develop and test this model. At an acoustic frequency of 6.5 kHz, the range of sediment thickness that the sonar can potentially recover is 0–20 m, a range that is often below the resolution of vertical profilers. In the first example, the signal variation with grazing angle over a uniformly buried surface is analyzed for attenuation, which is expected to increase toward shallow grazing angles and longer attenuating paths through the sediment layer. The surface is a lava flow north of Hawaii covered by 1–2 m of fine-grained sediments (Clague et al., 1990), and GLORIA data analyzed with the model suggest an attenuation coefficient of 0.2–0.4 dB/m, a value consistent with results from laboratory and field measurements of sediment attenuation. In the second example, a rocky surface is buried by varying amounts of sediment, which is analyzed at almost constant grazing angle. The surface is the volcanic basement of the Southeast Indian Ridge, and GLORIA data reveal the thickening of the sediment cover with distance away from the spreading center. This is used to derive an average sedimentation rate of 6 mm/ka, which is generally consistent with results from other studies and suggests that this technique may be used to study differences in sedimentation rates between different regions. Furthermore, the model is used to calculate, from image amplitude distributions, the sediment thickness distribution, which represents the accumulation of sediments in ponds and the exposure of abyssal hills. These sedimentary processes are also reflected in the standard deviation of sediment thickness which increases with seafloor age. I explore the various sources of error in estimating sediment thickness using this technique and propose a model for nonuniform sediment drapes (the first-order model assumes a uniform thickness at the subfootprint scale).

Landslides and the evolution of El Hierro in the Canary Islands

Seismic 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.

Quantifying tectonic strain and magmatic accretion at a slow spreading ridge segment, Mid-Atlantic Ridge, 29°N

High-resolution, deep-towed side-scan sonar data are used to characterize faulting and variations in tectonic strain along a segment of the slow spreading Mid-Atlantic Ridge near 29°N. Sonar data allow us to identify individual fault scarps, to measure fault widths and spacing, and to calculate horizontal fault displacements (heave) and tectonic strain. We find that over long periods of time (>1 Myr on average), tectonic strain is ∼10% on average and does not vary significantly along axis. There is a marked asymmetry in tectonic strain that appears to be linked to asymmetric accretion along the whole segment, indicated by ∼50% lower tectonic strain on the east flank than on the west flank. These variations in tectonic strain do not correlate directly with changes in fault spacing and heave. Fault spacing and heave increase from the center of the segment toward the end (inside corner) on the west flank and from the outside to the inside corner across the axis. These parameters remain relatively constant along the segment on the east flank and across the axis at the segment center. Tectonic strain appears to be decoupled from magmatic accretion at timescales >1 Myr, as the decrease in magma supply from the segment center toward the end (inferred from variations in crustal thickness along the axis) is not correlated with a complementary increase in tectonic strain. Instead, tectonic strain remains relatively constant along the axis at ∼7% on the east flank and at ∼15% on the west flank. These results indicate that variations in fault development and geometry may reflect spatial differences in the rheology of the lithosphere and not changes in tectonic strain or magma supply along axis.

Morphologies of knickpoints in submarine canyons

The question of how turbidity currents erode their beds is important for understanding how submarine canyons develop, how they maintain continuity in tectonically active margins to ensure sediment bypass, and for knowing how knickpoints (reaches of anomalously steep gradient) record tectonic information. The problem is potentially more complex than fluvial erosion, because flow vigor is also affected by the flow entraining ambient water and incorporating or depositing suspended load, which can significantly affect its excess density. However, in canyon sections where the total sedimentary mass passing through the canyon is much larger than the locally excavated mass, the solid loads of eroding currents change little during passage down-canyon. Canyon morphology can then potentially reveal how gradient and other factors affect erosion rate. Simple bed erosion models are presented herein, which are analogous to the detachment- and transport-limited erosion models of fluvial geomorphology, which predict that the channel topography should advect or diffuse (smooth out), respectively. Data sets from continental slopes off Alaska, New Jersey, Oregon, Chile, the Barbados accretionary prism, and published maps from other areas show these tendencies. Although knickpoints may arise from spatially varied resistance to erosion, some of those described here lie upstream of faults or anticlines and within uniform turbidites, implying that they can advect upstream. A forward numerical model is developed for knickpoints in the southern Barbados accretionary prism, which appear to have been created in a simple manner by the frontmost thrusts. If the erosion rules are applied continuously, the channel profiles are well represented with both advective and diffusive components. If a boundary condition of nondeposition/erosion is imposed on the base of the knickpoint slope (representing scour associated with a hydraulic jump, for example), the upstream profiles can be reproduced solely by diffusion. In these channels, the threshold stress for transport or erosion is probably small relative to stress imposed by the currents, because modeling shows that a threshold sharpens the knickpoint lip rather than rounds it. For the other, mostly smaller, knickpoints studied, however, the lip varies from sharp to rounded. This varied morphology could arise from a number of influences: effects of flow acceleration, differing threshold stress, differing sediment flux affecting flow power, or depth-varying substrate resistance to erosion. Despite the diversity of forms, upstream migrations imply that erosion can be enhanced where flow is more vigorous on steep gradients, implying that the body rather than the head of turbidity currents is responsible for erosion in those cases. Also discussed is how bed failure, quarrying, and abrasive scour lead to knickpoint evolution in submarine channels that is analogous to that in fluvial channels, but also likely differences are noted.

Fault structure and detailed evolution of a slow spreading ridge segment: the Mid-Atlantic Ridge at 29 degrees N

We present preliminary results of a detailed near-bottom study of the morphology and tectonics of the 29°N “Broken Spur” segment on the slow spreading Mid-Atlantic Ridge, using principally the TOBI deep-towed instrument. The survey covered two-thirds of the segment length, including all of its southern non-transform boundary, and extended off-axis of 40 km (3.3 Ma) on either side. We obtained nearly complete near-bottom sidescan sonar coverage and deep-towed three-component magnetic observations along 2-km-spaced E–W tracks. Sidescan data reveal new details of fault structure and evolution. Faults grow by along-axis linkage. In the inside corner, they also link in the axis-normal direction by curving to meet the next outer (older) fault; this leads to wider-spaced faults compared to segment centre or outside corner. Outward facing faults exist but are rare. The non-transform offset is characterised by faults that are highly oblique, not parallel, to the spreading direction, and show cross-cutting relations with ridge-parallel faults to the north, suggesting along-axis migration of the offset. Almost all volcanic activity occurs within 5 km of the axis. Most fault growth is complete within 15 km of the axis (1.2 Ma), though large scarps continue to be degraded by mass-wasting beyond there. Crustal magnetisation is strongly three-dimensional. The current neovolcanic zone is slightly oblique to earlier reversal boundaries, and its magnetisation rises to a maximum of 30 A m−1 near its southern tip. The central magnetisation high tapers southwards and is asymmetric, with a sharp western but gradual eastern boundary. We infer a highly asymmetric accretion of layer 2 near the segment end. Older magnetic anomalies are kinked and sometimes missing. We interpret these observations as evidence of a rapid, 18 km southward migration of the segment boundary during the past 1.8 Ma, and present a series of reconstructions illustrating this tectonic history.

Classification of seafloor geology using multibeam sonar data from the Scotian Shelf

Abstract We describe a method for classifying multibeam sonar data, and illustrate the method using data collected with a Simrad EM1000 sonar on the Scotian Shelf, Canada. The method involves comparing various attributes of the bathymetry and backscatter with typical examples of specified seafloor types and computing their statistical degree of similarity. Sediment ponds are identified as areas of low echo amplitude and low topographic gradient and curvature. Ridges and troughs are identified by fitting a paraboloid to patches of the bathymetry. Once the sediments, ridges and troughs are located, we are able to use the database to extract orientations and other characteristics of these features. For example, the orientation of the topography can be computed from the paraboloid surface, and rose diagrams of the ridge and trough orientations reflect the fabric of up-turned Cambro-Ordovician sedimentary beds outcropping in this area. The unconsolidated sediments ponded within small basins have low topographic gradients with consistent tilts towards the southeast (mean 133°), which we interpret as due to offshore transport of Holocene sediments, possibly in response to storms.

Quantitative backscatter measurements with a long-range side-scan sonar

It is shown that useful relative backscatter strengths can be calculated from GLORIA long-range side-scan sonar data using a simple acoustic model. The calculation was performed on GLORIA side-scan sonar data collected during 1987 in the southern Indian Ocean. GEOSECS hydrographic information was used to access the effects of refraction (ray bending and aspherical spreading signal losses). Sea Beam bathymetry was used to correct the effective insonified area and compute the grazing angle. A major difficulty in performing this calculation over the terrain chosen (mid-ocean ridge topography) was one of adjusting navigation so that small features in Sea Beam and GLORIA data matched. Preliminary results show a 10-dB falloff in backscatter strength with decreasing grazing angle (10 degrees -40 degrees ) at 6.5 kHz over what must presumably be a rough surface (extruded basalts and breccias). >

Transition from circular to stellate forms of submarine volcanoes

Large volcanic islands and guyots have stellate forms that reflect the relief of radiating volcanic rift zones, multiple volcanic centers, and embayments due to giant flank failures. Small mid-ocean ridge volcanoes, in contrast, are commonly subcircular in plan view and show only embryonic rift zones. In order to characterize the transition between these two end-members the morphology of 141 seamounts and guyots was studied using the shape of the depth contour at half the height of each edifice. Irregularity was characterized by measuring perimeter distance, elongation, and moment of inertia of the contours, assuming an “ideal” edifice is circular. The analysis reveals a general transition over 2–4 km edifice height (best transition estimate 3 km), while some large edifices 4–5 km high show no major embayments or ridges, suggesting considerable variation in the effectiveness of mechanisms that cause flank instability and growth of rift zones. The various origins of the transition are discussed, and the upper limit of magma chambers, many of which lie above the basement of the larger edifices, is proposed to affect the morphologic complexity via a number of mechanisms and is an important factor affecting the mode of growth. The origins of the truncated cone shape of mid-ocean ridge volcanoes are also discussed. Of the eruption mechanisms that have been proposed to explain their flat summits, the most likely mechanisms involve eruption from small ephemeral magma bodies lying within the low-density upper oceanic crust. The discussion includes speculations on factors affecting the depths of magma chambers beneath oceanic volcanoes. Supporting table is available via Web browser or via Anonymous FTP from ftp://kosmos.agu.org, directory “append” (Username = “anonymous”, Password =“guest”); subdirectories in the ftp site are arranged by paper number. Information on searching and submitting electronic supplements is found at http://www.agu.org/pubs/csupp_about.html.