CANARY ISLANDS LANDSLIDES AND TSUNAMI GENERATION: CAN WE USE TURBIDITE DEPOSITS TO INTERPRET LANDSLIDE PROCESSES? R. B. WYNN and D. G. MASSON Challenger Division, Southampton Oceanography Centre, European Way, Southampton, SO14 3ZH, U.K. Abstract 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. Keywords: Canary Islands, landslides, turbidites, tsunamis. 1. Introduction Giant landslides, normally in the form of debris avalanches, have affected all of the Canary Islands at some time in their evolution, although in the last 1 Ma landslide activity has been concentrated on the younger and more volcanically active islands of El Hierro, La Palma and Tenerife (Fig. 1) (Krastel et al., 2001; Masson et al., 2002). Recent studies (Carracedo et al., 1999a; Day et al., 1999; Ward and Day, 2001) have highlighted the Cumbre Vieja volcano on La Palma as a potentially unstable area, which may form the site of a future landslide. It has been suggested that this landslide could generate a ‘mega-tsunami’ with sufficient energy to cross the Atlantic Ocean and inundate the eastern US coast with 20-25 m high waves (Ward and Day, 2001). However, this assumes a ‘worst case’ scenario in terms of the volume and velocity of any displaced block, with suggested values of 500 km3 and 100 m/s-1, respectively. Simply halving these values would reduce the predicted tsunami wave height to about 5-10 m (Ward and Day, 2001). Unfortunately, our knowledge of the initial stages of landslide formation, which largely determines the characteristics of any related tsunami, is limited (Keating and McGuire, 2000; Waythomas, 2000; Ward, 2001). Although numerical modelling of tsunami propagation is a relatively advanced science, models of landslide-related tsunami remain hugely dependent on poorly defined geological parameters, such as slide velocity and mobility. Most models of tsunami generation treat landslides as single, rapidly moving blocks (Harbitz, 1992; Satake, 2001; Tappin et al., 2001; Ward, 2001; Ward and Day, 2001), although there is only limited geological evidence to substantiate this assumption, especially when applied to giant volcanic island slides (e.g. Moore et al., 1994). Understanding the way in which any future landslide might 325 326 Wynn and Masson develop is therefore crucial to establishing its hazard potential, including its ability to generate a tsunami. This understanding may be improved in part by the study of turbidite deposits that are directly linked to volcanic island landslides. In this paper, turbidites associated with the two most recent volcanigenic landslides to have affected the Canary Islands (the El Golfo landslide on El Hierro and the Icod landslide on Tenerife) are investigated, and compared to previously documented examples from the Hawaiian Islands. Figure 1: (a) 3D image of the NW African margin showing location of the Canary Islands, Agadir Basin and Madeira Abyssal Plain. White arrows show landslide directions from Tenerife (T), La Palma (P) and El Hierro (H). Black arrows show schematic turbidity current pathways related to emplacement of turbidites b and g in the Agadir Basin and Madeira Abyssal Plain. Stars mark locations of sediment cores in the Agadir Basin (see Fig. 3). (b) Map of the western Canary Islands (black shading) showing areas affected by landslides. Variations in grey shading serve only to distinguish landslides of different ages. EG = El Golfo, CN = Cumbre Nueva, I = Icod. 2. Turbidite deposits associated with Canary Islands landslides A sequence of turbidites, interbedded with pelagic/hemipelagic sediments, was recovered in giant piston cores from the Agadir Basin (about 300 km north of the western Canary Islands) at a water depth of 4500 m (Fig. 1a). Several of the turbidites can be correlated across the entire Agadir Basin, and are also present in the Madeira Abyssal Plain sequence, 600 km west of the Canary Islands (Weaver et al., 1992; Wynn et al., 2002). Turbidites are dated using micropaleontological data correlated to oxygen isotope stage boundaries, giving dates that are accurate to within a few thousand years (Weaver et al., 1992; Wynn et al., 2002). In the Agadir Basin sequence, the two youngest turbidites rich in volcaniclastic debris, indicating a Canary Island source, can be confidently correlated with the two most recent landslides on the islands. These Canary Islands landslides and tsunami generation 327 correlations are based upon landslide/turbidite age relationships (Masson, 1996; Ablay and Hurlimann, 2000), geochemical data (Pearce and Jarvis, 1995), turbidite sand mineralogy, and analysis of seafloor morphology, which defines likely turbidity current pathways (Wynn et al., 2002). Figure 2: 3D image of El Hierro viewed from the NW showing the El Golfo landslide scar and deposit. The shoreline is shown as a black line, below which is a narrow zone of no data. Note the contrast between the smooth landslide chute and the more rugged un-failed flanks to either side. Large blocks forming the bulk of the landslide deposit can be seen beyond the base of the chute. Location is shown in Fig. 1b. The youngest turbidite (Turbidite b) is dated offshore at about 15 Ka, and correlates with the El Golfo landslide on El Hierro (Fig. 2) (Masson, 1996; Gee et al., 2001). Onshore studies, based on dating within the subaerial collapse scar, have suggested that the main El Golfo failure (El Golfo I) occurred earlier than 100 ka (Carracedo et al., 1999b), and that it was only a more recent, smaller event (El Golfo II) that occurred at 15 ka. However, there is no evidence for an older debris avalanche offshore of the collapse scar, and no turbidite deposits in the surrounding basins that can be correlated with an older event. The next youngest volcaniclastic turbidite to be investigated (Turbidite g) is dated at about 170 Ka and correlates with the Icod landslide on the north flank of Tenerife (Watts and Masson, 1995; 2001; Ablay and Hurlimann, 2000). The basal sections of both turbidites b and g show distinctive sequences of stacked subunits, with the laminated sand/silt-rich bases of each fining-upwards sub-unit separated by up to 10 cm of turbidite mud (Fig. 3). Each sub-unit displays similar colour and composition to those above and below, but generally it is the sub-units towards the base of each turbidite that show the coarsest grain size and greatest thickness (Fig. 3). Turbidites b and g are separated from other turbidites higher and lower in the sequence by well-defined pelagic/hemipelagic sediment units, indicating substantial time intervals 328 Wynn and Masson (> ~1000 yrs) between deposition of individual turbidites (Wynn et al., 2002). In contrast, no pelagic/hemipelagic intervals occur between sub-units within b and g, and bioturbation appears to originate from the top of each turbidite and decreases downwards (Fig. 3). This indicates that the individual sub-units are not separated by substantial time intervals and are therefore all linked to one event. Figure 3: Core photographs showing multiple, stacked sub-units (marked with numbered italics) in turbidites b and g derived from the Canary Islands. The tops and bases of the turbidites are bounded by pelagic/hemipelagic sediments (boundaries marked with dotted line), while the downward decrease in bioturbation from the top of each turbidite indicates that the sub-units are not separated by substantial time intervals, and may therefore represent multiple stages of a single landslide event. Magnetic susceptibility curves highlight the concentrations of volcaniclastic debris, which displays high magnetic susceptibility, in the silty bases of each sub-unit. A similar trend can be observed in P-wave velocity and grain size curves, although in this example magnetic susceptibility is used as a proxy for these parameters as it has greater accuracy at high resolution (sub-cm scale). 3. Turbidite deposits associated with Hawaiian landslides The stacked sub-units shown by turbidites linked to Canary Islands landslides are unique within the Agadir Basin, being very different in character to turbidites derived from the adjacent Moroccan slope and shelf (Wynn et al., 2002). However, they are not unique in a global context, as studies of turbidites linked to volcanic island landslides around the Hawaiian Islands have shown that these turbidites also show evidence of Canary Islands landslides and tsunami generation 329 stacked sub-units (Garcia, 1996). They are described as showing multiple, closely spaced sandy horizons with sharp bases and gradational tops. One of the turbidites contains four sub-units and is 11 cm thick, with each sub-unit consisting of a 0.5-2 cm thick sandy base overlain by a 1-3 cm thick muddy top. The petrology of the turbidite sands indicates that they are sourced from the Hawaiian Islands, and assessment of turbidite ages allows them to be roughly correlated with individual Hawaiian landslides (Garcia and Hull, 1994; Garcia, 1996). The structure and grain size of the turbidite subunits indicates that they were deposited in quick succession by a series of turbidity currents, and there are no obvious seafloor complexities (e.g. channels) separating the source area and the site of deposition. Garcia (1996) therefore suggests that the origin of the sub-units is related to multiple stages of failure within a single catastrophic landslide event, with each stage of failure initiating a single turbidity current. 4. Origin of the sub-units in Canary Island turbidites We tentatively suggest that the stacked sub-units shown in turbidites linked to Canary Island landslides also represent multiple stages of failure within single landslide events. In the Agadir Basin, turbidite g has nine sub-units, potentially corresponding to nine stages of failure, and turbidite b contains three sub-units, suggesting three stages of failure (Fig. 3). Alternative interpretations for the origin of the sub-units, such as flow reflection from basin margins or seamounts (Rothwell et al., 1992), flow surging or eddying (Kneller and McCaffrey, 1999) or transport through multiple flow pathways related to complex channel systems (Masson, 1994) can be ruled out in this area. Flow reflection is eliminated because the core sites in the Agadir Basin are >100 km away from any significant topography (Fig. 1a). Flow surging or eddying in the low-density part of stratified turbidity currents was briefly discussed by Kneller and McCaffrey (1999), but these effects probably involved interaction of flow with a basin margin slope, which is not applicable in this case. In addition, the grain size and thickness of individual sub-units indicates that each would have taken at least two days to deposit, based upon settling velocities of fine-grained sediment (Stow and Bowen, 1980). This time interval is probably far outside the time-scale of flow surging or eddying within a single flow, although data to substantiate this are lacking. Finally, the general absence of submarine channels between the Canary Islands and the Agadir Basin constrains any flow originating on the islands to a single broad pathway (Fig. 1a). This means that multiple sub-units in the Agadir Basin turbidites cannot easily be explained by flow through a number of separate pathways. Further west, in the Madeira Abyssal Plain, turbidite b is much thicker than in the Agadir Basin, and shows up to 8 basal sub-units (Rothwell et al., 1992). However, the plain is fed by a complex channel system (Masson, 1994), and it is therefore impossible to determine whether the sub-units in this particular basin represent multiple flow pathways or multiple stages of failure. It is difficult to assess the exact trigger mechanism for individual turbidity currents responsible for depositing the sub-units. Garcia (1996) suggested that Hawaiian landslides may have ‘evolved’ into turbidity currents at their distal margins. Another possibility is that individual landslides occurred as retrogressive failures. Under this interpretation, the initial failure would have left a steep, unstable scar on the island flank, while emplacement of the landslide deposit upon unconsolidated seafloor 330 Wynn and Masson sediments caused them to fail, initiating a turbidity current. Every successive failure of the over-steepened landslide scar would then see more material flow downslope, displacing the previous deposits and pushing them outwards onto undisturbed seafloor. This would lead to destabilisation of a new section of seafloor and triggering of a further turbidity current. The whole event could last several days in this scenario, with each failure stage separated by periods of a day or more as discussed above (assuming the velocity of individual turbidity currents was similar between the source and the basin floor). Another alternative is that individual turbidity currents were initiated by mixing at the upper interface of each landslide sub-event on its passage downslope, with a muddy suspension cloud on the top of each slide generating a low-density turbidity current. It could be argued that the homogeneous internal appearance of the landslide deposits, as imaged in 2-D seismic profiles (Masson et al., 2002), indicates that they were actually emplaced during a single event. However, field and experimental study of large-scale debris flows has shown that in strongly surging (or even separate successive flows), the final deposit can be homogeneous in character and lacking internal boundaries (Major, 1997). Therefore, although not necessarily directly applicable to the landslides discussed in this paper, which show a range of characteristics from classic debris avalanche through to debris flow (Masson et al., 2002), these experimental results show that at least some multiple-stage mass flows can produce a homogeneous final deposit that contains no evidence of internal boundaries. The interpretations presented above favour a setting where a landslide is emplaced on the seafloor in multiple stages, although we recognise that different stages of failure may have widely varying volumes. We also recognise the possibility that the landslide could have been emplaced as a single unit, and that the stacked turbidite sub-units were formed by a different mechanism to those proposed. Alternative interpretations could include multiple failure of the landslide deposits and/or the slope sediments beyond the toe of the landslide, shortly after the landslide was emplaced. Unfortunately, without direct observation, it is difficult to see how confirmation of one or other of these interpretations may be achieved with certainty. 5. Implications for the tsunami hazard potential Previous studies, based upon direct observation, have shown that relatively small (<3 km3), high-velocity (100 m/s-1) landslides, formed by lateral collapse of small volcanic islands, are capable of generating significant tsunamis upon entering the sea (e.g. Satake 2001; Satake and Kato, 2001). However, the true relationship between giant (>100 km3) volcanic island landslides and tsunamis is poorly understood, as direct observations of landslide velocity and failure style in this particular setting are lacking. In some cases, such as the submarine Storegga Slide offshore Norway, tsunami deposits have been directly linked to the landslide, and can provide useful information on tsunami wave height and run-up (Bondevik et al., 1997). However, in other areas such as Hawaii, data linking tsunami deposits to source landslides has proved more controversial (Felton et al., 2000; Keating and McGuire, 2000; Waythomas, 2000; Ward and Day, 2001). In the Canary Islands, onshore evidence of past landslide-related Canary Islands landslides and tsunami generation 331 tsunamis is completely lacking (Krastel et al., 2001). However, the preservation potential of tsunami deposits is poor in the rugged island environment, and the lack of identifiable deposits should be viewed as a function of low preservation potential, rather than as evidence for the absence of tsunamis. Our offshore geological data may therefore provide new insights into giant volcanic island landslide mechanisms, and may help us to better assess the potential hazard from any future landslide showing similar characteristics to those studied here. The new dataset presented from the Canary Islands complements that already obtained from offshore Hawaii, and reinforces the idea that landslides previously regarded as single catastrophic events could actually result from multiple failures spread over periods of several days. Separate subsidiary failures occurring over this time scale would each generate a discrete tsunami, but these successive failures are too widely spaced in time to have a cumulative effect on tsunami magnitude. The El Golfo and Icod landslides have total volumes of 150-180 and 80-150 km3, respectively (Ablay and Hurlimann, 2000; Masson et al., 2002). As has already been demonstrated by Ward and Day (2001), a failure of this size occurring as a single event and entering the sea at high velocity could be capable of generating a catastrophic tsunami far from the source area, although this has recently been disputed (Mader, 2001; Pararas-Carayannis, 2002). However, if as suggested here, each landslide comprised up to nine stages of failure, the average volume of each subsidiary failure could be as low as 10-25 km3. A failure of this size is much less likely to generate tsunamis that are damaging far from the source. 6. 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