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.

Introduction

It is now firmly established that large-scale landsliding is a key processes in the evolution of oceanic islands. Detailed studies of landslides have been carried out around the Hawaiian Islands Lipman et al., 1988, Moore et al., 1989, Moore et al., 1994, Reunion Cochonat et al., 1990, Labazuy, 1996, Ollier et al., 1998 and the Canary Islands Holcomb and Searle, 1991, Krastel et al., 2001, Masson, 1996, Masson et al., 1998, Teide Group, 1997, Urgeles et al., 1997, Urgeles et al., 1999, Watts and Masson, 1995. Some of the clearest evidence for landsliding, in the form of large fields of blocky landslide deposits, has been reported offshore. Landslide deposits can be transported several hundred kilometres and cover many hundreds of km² of seafloor on the submarine island flanks. Individual landslides can involve up to a few thousand km³ of material, but more typically are a few hundred km³ in volume. Onshore, landslide headwalls are typically expressed as arcuate embayments and steep cliffs Cantagrel et al., 1999, Navarro and Coello, 1989, Ollier et al., 1998, Ridley, 1971.

In the Canary Islands, the relatively recent discovery of landslide deposits offshore Holcomb and Searle, 1991, Masson, 1996, Watts and Masson, 1995 confirms earlier controversial interpretations based on the onshore geology Bravo, 1962, Navarro and Coello, 1989. Prior to the study presented here, all the offshore studies have concentrated on areas of island flank downslope of suspected subaerial landslide scars, in particular the Orotava, Icod and Guimar valleys on Tenerife, the Taburiente Caldera/Cumbre Nueva Arc on La Palma, and the El Golfo embayment on El Hierro Holcomb and Searle, 1991, Masson, 1996, Masson et al., 1998, Teide Group, 1997, Urgeles et al., 1997, Urgeles et al., 1999, Watts and Masson, 1995. Here we present the results of a more comprehensive study of flank collapse processes on Tenerife, La Palmas and El Hierro. The paper is partly a review and summary of previously published data, but also draws on a considerable volume of new material. Much of the discussion, particularly the section on flow processes, is based on a new comparison between landslides on the different islands.

A Simrad EM12 multibeam system was used to map the submarine morphology and backscatter characteristics of large areas of island flank. These data clearly distinguish between unfailed island slopes and those affected by landsliding processes. High resolution, deep-towed, sidescan sonar data, acquired with the TOBI 30 kHz system, was used to examine the surface structure of landslide deposits in greater detail, to gain a better understanding of landsliding processes. Our results show that landsliding on the flanks of the islands is more widespread than previously supposed, and that landslide processes are both variable and complex. At least 14 individual landslides have been identified.

The Canary Islands are a group of seven volcanic islands in the eastern Atlantic Ocean off the northwest African margin (Fig. 1). There is evidence for a general decrease in the age of the islands from east to west, suggesting a hotspot origin for the island chain, although volcanic activity has occurred within historic times on all islands apart from La Gomera (Carracedo et al., 1998). El Hierro and La Palma are the most westerly and youngest of the Canary Islands, and with Tenerife appear to have been the most active, in terms of both volcanic and landslide activity, in the recent past (Urgeles et al., 1997). The most recent large landslide in the Canaries was probably the El Golfo failure on El Hierro, which occurred at about 15 ka (Masson et al., 1996).

The study area covers much of the submarine flanks of Tenerife, La Palma and El Hierro Fig. 1, Fig. 2, Fig. 3. Complete seafloor coverage of the north flank of Tenerife, the west flank of La Palma and all around El Hierro up to water depths of around 4000 m was obtained using an EM12 multibeam system Fig. 2, Fig. 3. Less comprehensive surveys were carried out around the remainder of Tenerife and the eastern flank of La Palma. TOBI 30 kHz sidescan sonar images were obtained north of Tenerife, south of El Hierro and west of both La Palma and El Hierro (Fig. 2). 3.5 kHz profile data were recorded along all survey tracks. Seismic profiles consist of 12-channel sleeve-gun data collected north of Tenerife and 4-channel airgun data collected north of Tenerife, west of La Palma and both southeast and southwest of El Hierro. A complete list of the cruises from which data was used is given in the caption to Fig. 2.