The Trænadjupet Slide, offshore Norway — morphology, evacuation and triggering mechanisms

Marine Geology 171 (2000) 95±114

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The Trñnadjupet Slide, offshore Norway Ð morphology, evacuation and triggering mechanisms J.S. Laberg*, T.O. Vorren Department of Geology, University of Tromsù, N-9037 Tromsù, Norway Received 26 January 2000; accepted 29 August 2000

Abstract The Trñnadjupet Slide, located on the continental slope off Norway, was studied using TOBI (Towed Ocean Bottom Instrument) high-resolution side-scan sonar data together with 7.5 kHz seismic records. The slide extends from the shelf break to more than 3000 m water depth in the Lofoten Basin, implying a slide-affected area of about 14,100 km 2. The slide probably occurred during the mid-Holocene, prior to 4000 14C years BP. The slide scar includes escarpments, detached ridges of sediment, sediment streams, grooves, elongated highs, tabular sediment blocks, pressure ridges, and tension fractures. The initial sediment disintegration produced detached sediment ridges that moved by back-tilting or through basal deformation. Transition to sediment streams comprising more-or-less disintegrated sediments occurred over some kilometres. Movement of consolidated sediments formed the tabular sediment blocks. The initial failure was either located near the present headwall of the Trñnadjupet Slide, or downslope from a large escarpment located at a present water depth of 1800 m. A combination of events led to the slope failures in this area. Sedimentation within the Trñnadjupet Slide area was characterised by high sedimentation rates during the glacial maxima. Periods of high sedimentation rates have promoted instability of the glacigenic sediments themselves, or more important, prevented water and/or gas to escape from the relatively thin layers of interglacial/ interstadial sediments (,10 m), due to the low permeability of the glacigenic sediments. This could have led to build-up of excess pore pressure and the interglacial/interstadial sediments then have acted as planes of weakness. Triggering most likely was caused by one large or a series of small earthquakes associated with postglacial crustal uplift of Fennoscandia. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Norwegian Sea; Holocene; Submarine landslide; Morphology; Triggering

1. Introduction During the Holocene, the continental slope off Norway has been the most unstable part of the European Atlantic continental margin, and three giant failure events have occurred: the Storegga Slide (Bugge et al., 1987), the Andùya Slide (Laberg * Corresponding author. Tel.: 147-776-44409; fax: 147-77645600. E-mail address: [email protected] (J.S. Laberg).

et al., 2000) and the Trñnadjupet Slide. Studies so far have mainly focused on the Storegga Slide (Bugge, 1983; Bugge et al., 1987, 1988; Jansen et al., 1987; Kenyon, 1987; Evans et al., 1996). The Storegga Slide generated a large tsunami wave, affecting the coast of Scotland, Norway, Iceland, and the Faeroe Islands (Dawson et al., 1988; Harbitz, 1992; Bondevik et al., 1997). In this paper, we focus on the Trñnadjupet Slide (Fig. 1), a slide which has so far not been discussed in detail. We present newly acquired Towed Ocean

0025-3227/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0025-322 7(00)00112-2

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Bottom Instrument (TOBI) high-resolution side-scan sonar data together with simultaneously acquired 7.5 kHz seismic records. TOBI is a high-resolution side-scan sonar that produces sea-¯oor images of much higher resolution than has previously been available. Owing to the data coverage we focus on the slide scar area (Fig. 1). Our aim is to discuss: (i) the morphology characterising a submarine slide on a glaciated continental margin, as exempli®ed by the Trñnadjupet Slide; (ii) the sediment disintegration and ¯ow; (iii) the spatial and stratigraphic location of the initiation of mass movement within the slide area; (iv) the triggering mechanism(s) of the Trñnadjupet Slide, and (v) the causes of the instability. This kind of information permits a better assessment of the risk of giant submarine slides in the future, including generation of tsunamis, and is crucial for both the offshore and coastal activities of northern Europe. 2. Physiographic setting The Trñnadjupet Slide is located on the continental slope immediately east and north-east of the Vùring Plateau (Fig. 1). The slide-affected area extends from the shelf break to more than 3000 m water depth in the Lofoten Basin, implying a run-out distance of ca 200 km. The average slope gradient in the slide scar is 1.258. The shelf and slope bedrock in the area comprises sedimentary rocks of Tertiary and Mesozoic age, whereas older crystalline rock occurs along the coast and onshore (Sigmond, 1992). The slide scar is located off Trñnadjupet, a large glacial shelf trough (Fig. 1). The slide affected Quaternary sediment only. The late Quaternary slope sediments comprise mostly glacigenic debris-¯ow deposits and glacimarine sediments. A relatively thin layer (,2 m) of Holocene hemipelagic sediments drapes them (Laberg et al., 2000, submitted). The Quaternary continental shelf succession in the Trñnadjupet area comprises till units interbedded in a complex relationship with strati®ed glacimarine sediments. Within the Trñnadjupet trough, the Quaternary succession is relatively thin,

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increasing in thickness north and south of the trough (King et al., 1987). 3. Database As part of the European Community funded ENAM-II Programme (European North Atlantic Margins Ð Quanti®cation and modelling of largescale sedimentary processes and ¯uxes) high-resolution side-scan sonar data was acquired in 1997. The data was acquired by the TOBI-deep towed side-scan sonar system. The TOBI data covers an area of about 2700 km 2 on the continental slope immediately northeast of the Vùring Plateau, much of it from the area of sediment removal within the Trñnadjupet Slide (Fig. 1). The TOBI side-scan sonar system operates at a frequency of 30 and 32 kHz on starboard and port side, respectively. The swath of the system is 6 km and horizontal resolution is 6 m. In general, a high level of backscatter on the TOBI data is displayed by light grey tones, whereas a low level is indicated by dark grey to black tones. Features characterised by a pronounced relief and which can be followed across more than one swath width (e.g. escarpments) may partly be displayed in light grey tones (facing the sonar) and partly in black tones (shadow) depending on the direction of the sonar signal relative to the feature. High-resolution seismic data (7.5 kHz) were co-registered with the TOBI data. Penetration depths of up to 70 m and a resolution ,1 m were recorded. For further discussion of the TOBI-system and processing of the data, see Le Bas et al. (1995) and Le Bas and Mason (1997). 4. Morphology of the Trñnadjupet slide scar Based on 3.5 kHz pro®les, Damuth (1978) identi®ed an area of sediment removal on the Norwegian continental slope off Trñnadjupet. Based on seismic sparker data, the headwall and upper slide scar area were identi®ed by Bugge (1983). This interpretation

Fig. 1. Location map (inserted) and bathymetric map of the continental slope off Trñnadjupet, north Norway including the location of the Trñnadjupet Slide. The location of the TOBI survey area, the study area (Fig. 2) and morphological zones 1, 2 and 3 are indicated. The bathymetry was adapted from Perry et al. (1980) and the location of the Trñnadjupet Slide was modi®ed from Dowdeswell and Kenyon (1994).

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was con®rmed by Geological Long Range Inclined Asdic (GLORIA) long-range side-scan sonar records showing the upper slide scar (Kenyon, 1987). Later, the slide-affected area was outlined by GLORIA data that revealed the large aerial extent of the failure (Dowdeswell et al., 1996; Vorren et al., 1998). The headwall of the Trñnadjupet Slide is up to 150 m high and 20 km long. The slide scar (de®ned as the area of sediment evacuation) widens downslope and reaches a maximum width of about 70 km at about 2000 m water depth (Fig. 1). The slide scar can be followed downslope to ca 2400 m water depth and covers an area of ca 5000 km 2. The slide deposits cover an area of about 9100 km 2, and the total slide-affected area is about 14,100 km 2. Offshore mass movements can be classi®ed according to the shape of the failure surface (Prior and Coleman, 1979; Mulder and Cochonat, 1996). The h/l-ratio (h ˆ depth, l ˆ length of slide) of the Trñnadjupet Slide is 2.60 km/200 km ˆ 0.013, i.e. ,0.15 which makes the Trñnadjupet Slide a translational slide according to Skempton and Hutchinson (1969) (in Mulder and Cochonat, 1996). In general, translational slides are characterised by a more-or-less planar failure surface, translational sediment displacement, and their common occurrence in mechanically inhomogeneous sediments, i.e. sediments containing bedding planes (Allen, 1985; Hampton et al., 1996). We have studied a small part of the slide scar in detail (Figs. 1 and 2). This study area has been divided into three morphological zones: zones 1, 2 and 3 (Fig. 2a and b). The zones are delimited by major escarpments and/or pronounced changes in backscatter character. Zone 1 is located at the western sidewall (escarpment E1), decreases in width upslope, and is relatively complex morphologically. Zone 2 is characterised by low relief and has a more uniform morphology. Zone 3, bounded by escarpment E2 upslope, is morphologically complex and is characterised by relief with highs up to several tens of metres immediately downslope from zone 1. Away from the slide scar, the relief is smoother. The boundaries between the different zones are described ®rst,

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followed by the morphological features characterising each zone. 4.1. Morphological zone boundaries Escarpments: Two major escarpments, termed escarpments E1 and E2, affecting sediments at different stratigraphic levels, have been identi®ed (Figs. 1 and 2). Smaller escarpments also occur within the area affected by the slide (Fig. 2, see below). Escarpment E1: Escarpment E1 de®nes the southwestern limit of zone 1. It is oriented downslope (NW-SE), is relatively steep and has a height between 50 and 100 m (Fig. 3). From about 1700 to 1800 m water depth the south-western limit of zone 1 is based on a change in acoustic backscatter contrast to a paler grey compared with the area immediately to the west (Fig. 2a and b). Escarpment E2: Escarpment E2 de®nes the upslope limit of zone 3 (Fig. 2b). It is up to 100 m high, alongslope oriented and located at about 1800 m water depth (Figs. 1 and 2). In the study area, escarpment E2 comprises two downslope-oriented, concave escarpments (Fig. 2b). Within the study area, the maximum total depth of erosion of the slide complex is up to 200 m (added heights of E1 and E2). Backscatter contrasts: The transition from zone 1 to 2 is de®ned by a pronounced contrast in acoustic backscatter, from relatively uniform grey with pronounced downslope-oriented lineations (see below) in zone 1, to more irregular, light grey, patchy to blocky fabric with some dark grey areas in zone 2 (Fig. 2a and b). The eastern delineation of zone 2 is unknown due to the absence of TOBI data (Fig. 1). 4.2. Slide scar features Morphological features identi®ed within the studied part of the slide scar include detached sediment ridges, sediment streams, elongate highs, tabular sediment blocks, tension fractures, pressure ridges, and small scale sea-¯oor irregularities. Detached sediment ridges: Immediately downslope from escarpments E1 and E2, the TOBI-data display

Fig. 2. Mosaic of TOBI side-scan images covering the western, upper Trñnadjupet Slide. For location, see Fig. 1. Inserted ®gure shows the three morphological zones identi®ed: zone 1, 2 and 3. The location of Figs. 3, 4, 5, 7, and 8 also is shown.

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Fig. 3. Part of a 7.5 kHz pro®le across detached sediment ridges downslope from escarpment E1. The ridges are up to 30 m high. For location of the pro®le, see Fig. 2.

irregular, elongate features, most of them parallel or subparallel to the adjacent escarpments (Figs. 3±5). These are ridges having a maximum length of up to 1 km (Fig. 4) and rising several tens of meters above the surrounding sea ¯oor (Fig. 3). The ridges decrease in spacing and size downslope (Fig. 4). They represent detached sediment portions of the sea ¯oor that moved for some distance and then stopped. Possible analogue subaerial features have been described by Allen (1985; Fig. 9.15). Sediment streams: A well-developed, downslopeoriented lineated fabric dominates Zone 1. Some of the lineations are located downslope from the areas of detached sediment ridges close to the sidewall of zone 1 (Fig. 5). Further into zone 1, other lineations can be followed downslope across zone 1 (Fig. 2a). In this area, individual subparallel lineations, some of which originate upslope from zone 1, can be followed downslope for several tens of kilometres (Fig. 2a). The lineations correspond to boundaries between sediment streams that are sediment ¯ows, easily identi®able on seismic pro®les (Figs. 5 and 6). The sediment ¯ows are 100 to 500 m wide, some originating from the disintegration of detached sediments within Zone 1, others from upslope of Zone 1. Lineations also correspond to relatively narrow grooves (Fig. 7). Analogues for the observed lineations are described in subaerial slides by Marangunic and Bull (1968); Post (1968; Fig. 6); Brunsden (1984; Fig. 9.7). These probably were formed by longitudinal shear

caused by sediments travelling at different speeds or ªnarrow splitsº caused by divergent motion of the sediments (Marangunic and Bull, 1968 and references therein). Longitudinal shear caused by trains of sediments travelling at different speed has also been described from a submarine slide in a fjord-head setting (Prior et al., 1982). Masson et al. (1993) found the lineated texture of the submarine Sahara debris ¯ow to have been caused by streams of debris of different character and texture. Here, the boundaries between streams were inferred to be shears dividing areas of the ¯ow that moved at different speed and possibly at different times (Masson et al., 1993). Alternatively, grooves may have been formed by erosion into underlying, in situ sediments. Formation of downslope-oriented grooves by erosion of snow has been described from wet-snow avalanches (Mellor, 1978; Fig. 2). Elongated highs: Within Zone 1, large, elongated highs reach a length of ca 1.5 km and a width of 1 km (Fig. 2a). One of them has a ªtear-dropº form and rises more than 100 m above the surrounding sea ¯oor (Fig. 7) and above the nearby sea ¯oor outside the slide scar. By analogy with observations made elsewhere, these highs may represent remnants of the sea-¯oor not affected by sliding (Urgeles et al., 1997; Masson et al., 1998), or they may represent large rafts or displaced blocks of sediments (Macdonald et al., 1993; 1995). For the high displayed on Fig. 7, we favour the latter alternative because it rises above

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Fig. 4. TOBI side-scan sonograph and simpli®ed interpretation illustrating some of the detached ridges of sediment located immediately downslope from escarpment E2. The ridges are sub-parallel to escarpment E1 and seem to decrease in size downslope. See Fig. 2 for location.

the sea ¯oor outside the slide scar. This implies that it originated within a deeper part of the slide scar farther upslope. The lack of a ¯at top on some of these features indicates that they disintegrate rapidly. The downslope-oriented lineations are affected by these highs, because they bend around them before merging farther downslope. Slower

movement compared with the adjacent sediments may explain this. Tabular sediment blocks: Large tabular sediment blocks are prominent in two areas in the northern, downslope part of zone 1 (Fig. 8) and also occur in zone 2 (Fig. 8). The blocks are up to 300 m across, more than 1 km long and rise 10±20 m above the

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Fig. 5. TOBI side-scan sonograph illustrating the pattern of downslope-oriented lineations, probably caused by smaller escarpments delineating streams of sediments, located downslope from an area of detached sediments. Escarpments E1, E2 and a local high are indicated. See Fig. 2 for location.

main sea ¯oor (Fig. 8). Some of the blocks described by Masson (1996) from the El Golfo Debris avalanche off the Canary Islands have comparable dimensions. The blocks probably constitute relatively consolidated sediments as indicated by their fracture pattern, which is parallel and perpendicular to the direction of presumed shear stress (Fig. 8). Movement of the

blocks probably occurred by sediment deformation in a basal zone of high shear stress. Tension fractures: Some of the transverse fractures identi®ed have a length of up to 1 km. They separate large blocks of sediments within the north-eastern part of Zone 1 (Fig. 8). Based on analogy with inferred similar subaerial features (e.g. Shreve, 1968;

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Fig. 6. 7.5 kHz pro®le across the are of sediment streams shown on Fig. 5. Escarpment E1 is indicated. See Fig. 5 for location.

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Fig. 7. TOBI side-scan sonograph (above) and corresponding 7.5 kHz pro®le (below) showing an elongated high, rising c. 100 m above the main sea-¯oor. Downslope-oriented grooves are affected by the high, or they bend around it and merges farther downslope. In this area, the pattern of grooves west of the high seems to truncate the pattern of lineations close to the sidewall (T), illustrating that the latter predates the former. Also, note the change in acoustic properties from zone 1 to 2. See text for further discussion. For location, see Fig. 2.

Marangunic and Bull, 1968), the transverse features probably represent tension fractures. Tension fractures probably form due to extensional stress within the tail of the downslope moving sediment (Prior et al., 1982). Pressure ridges: In zone 2, sets of transverse posi-

tive surface features, bounded by longitudinal shear zones have been identi®ed (Figs. 9 and 10). Their height is dif®cult to estimate but, based on the data available, seems to be relatively uniform, about 50 m (Fig. 9). From their location and morphology, we suggest that these features represent pressure ridges.

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Fig. 8. TOBI side-scan sonograph (above) and corresponding 7.5 kHz pro®le (below) across an area of tabular sediment blocks (B). In this area, the tension-fracture pattern is parallel and perpendicular to the downslope direction of shear stress, indicating blocks of relatively consolidated sediments. Areas of longitudinal shear delineating a larger block, as well as part of escarpment E2, is also indicated. See Fig. 2 for location.

Pressure ridges form due to reduced motion near the slide margins accompanied by continued sediment supply from farther upslope (Prior et al., 1982). Small-scale sea-¯oor irregularities. Most of zone 2 is dominated by small-scale sea-¯oor irregularities, and the TOBI images display a blocky acoustic fabric (Figs. 2a and 7). Smaller areas of more uniform, dark

backscatter correspond with relatively ¯at areas of the sea-¯oor. The blocky acoustic fabric may be due to pervasive mass wasting: zone 2 sediments may have moved farther and thus become more fragmented than zone 1 sediments. Alternatively, zone 2 sediments may be more consolidated compared with zone 1

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Fig. 9. TOBI side-scan sonograph illustrating a set of pressure ridges, bounded by longitudinal shear. The pressure ridges have a length of up to 1 km. For location, see Fig. 2.

sediments. The relatively ¯at sea-¯oor depressions may represent areas that have been completely evacuated (Fig. 8).

as bypass of sediments released from farther upslope (Fig. 1). 5.1. Chronology/age

5. Discussion As a result of the large-scale slope failure at the mouth of Trñnadjupet, sediments were mobilised from an area of 5000 km 2 on the upper continental slope and deposited on the lower slope and in the Lofoten Basin. The study area forms part of the evacuated slope, an area that has experienced sediment mobilisation and formation of escarpments, as well

The age of the Trñnadjupet Slide is discussed in detail by Laberg et al. (2000, submitted). According to them, the Trñnadjupet Slide probably formed during the mid-Holocene, prior to 4000 14C yr. BP. This age estimate is based on: (1) 14C-AMS dating results from cores penetrating the post-slide sediments in the slide scar; (2) the prominent depression of the slide on the present sea-¯oor, and (3) the relatively thin layer of

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Fig. 10. Part of a 7.5 kHz pro®le showing the lobe of sediments characterized by pressure ridges delineated by zones of longitudinal shear. See Fig. 9 for location.

sediments that have been deposited within the slide scar since the failure occurred. 5.2. Slide-scar morphology: evacuation of the slide scar In terms of sea-¯oor morphology, there are marked contrasts between morphological zones 1 and 2. Zone 1 forms a wedge near the western sidewall (escarpment E1). It decreases in width upslope, has a relief of up to 100 m, and is relatively complex. Zone 2 is located farther into the slide-scar. It is, in general,

characterised by low relief and has a more uniform morphology. The low relief of Zone 2 is indicative of a complete sediment evacuation of the slide scar in this area, whereas Zone 1 has been evacuated to a lesser degree. These differences probably are due to the distance from the boundary of the slide scar. Zone 1 represents an area where small displacement have taken place (see below), whereas Zone 2 is located further into the slide scar and represents an area more completely evacuated. Zone 2 is located immediately downslope from the

Fig. 11. Model of initial sediment disintegration and the formation of detached sediment ridges and sediment streams within the Trñnadjupet Slide.

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headwall of the Trñnadjupet Slide (Fig. 1), and sediments successively released from near the slide headwall may have incorporated sediments within Zone 2 and moved them farther downslope. Zone 1 represent a more local embayment in an area where the continental slope starts to turn north toward the Vùring Plateau, protecting Zone 1 sediments from forces associated with sediment that moved from farther upslope. The morphology of Zone 3 is characterised by a relief of up to several tens of meters immediately downslope from Zone 1. This area also is inferred to comprise sediments characterised by a small displacement. More distally into the slide scar the relief is lower, probably due to a more complete evacuation. 5.3. Slide-scar morphology: sediment disintegration and ¯ow The profound downslope pattern of lineations represents an interplay of erosion by the downslope¯owing sediments and the formation of smaller escarpments delineating lobes of sediments. As indicated from the slide morphology near the sidewall, the distance across an area of detached sediment ridges to the area dominated by ¯ows of blocky disintegrated sediment is on the order of some kilometres (Fig. 5). Thus the transition to more-or-less disintegrated sediments probably occurred over a relatively short distance. Initial movement of the detached sediment ridges could occur by back-tilting or deformation in the basal part of the ridge. Stratigraphic inhomogeneous sediments, i.e. sediments containing bedding planes probably promotes movement through basal deformation whereas back-tilting probably is more dominant in homogeneous sediments (Fig. 11). The presence of elongated highs inferred to be sediments characterised by slower movement compared with the adjacent sediments shows that the sediments were not remoulded completely or moved for a longer distance before remoulding. In Zone 1, the TOBI data display a complex interplay of morphological forms. For example, a pronounced downslope pattern of lineations occurs immediately downslope from some of the detached ridges (Fig. 5), whereas this signature apparently is absent in other areas downslope of detached ridges (Fig. 3). The implications in terms of sediment disin-

tegration and downslope movement within relatively short distances is unknown, but we speculate that sediment disintegration leading to ¯ow rather than sliding may be most pronounced in the area of lineations downslope from the detached ridges. This variation could be related to local variations in sea-¯oor gradient, sediment composition and/or physical properties, particularly initial void ratio (Lee et al., 1991). Based on the observation that grooves bend around a local high within the slide scar and merge farther downslope, and the fact that very little piling-up of sediments has occurred upslope from local highs (Fig. 7), at least part of the sediment ¯ow was characterised by a low viscosity or strength. Tension within downslope-moving sediments due to strength is indicated by the presence of densely spaced transverse features inferred to represent tension fractures, probably within relatively consolidated sediments. High strength implies that the ¯ow was not purely viscous. Inferred tension fractures immediately upslope from escarpment E2 suggest succeeding backstepping and retrogressive failure associated with E2. Large, tabular blocks of sediment moved for some distance before they came to rest. These blocks probably were composed of relatively consolidated sediments undisturbed internally, as indicated by the presence of fracturing that has occurred mainly parallel and perpendicular to the downslope-oriented shear stress (Fig. 8). Movement of the blocks immediately upslope from escarpment E2 may have been triggered by the loss of support due to the formation of E2. In the area of sediment accumulation (Fig. 1), GLORIA images indicate small and large blocks (Dowdeswell et al. 1996). Thus some of the failed masses characterised by the longest run-out distance, probably the most-consolidated sediments, were not remoulded completely during the downslope ¯ow. This phenomenon also has been reported from other submarine slides (Bugge et al., 1987; Masson et al. 1996; 1997). 5.4. Initiation of mass movement: geographical location Regarding the geographic location of the initiation of mass movement within the Trñnadjupet Slide, two possibilities are suggested: (1) the initiation of the failure was located near the present headwall of the

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Trñnadjupet Slide, or (2) the initiation of the failure was located downslope from escarpment E2, which represents the earlier headwall. A third alternative, that initiation of the failure occurred at two separate places simultaneously is considered less likely but cannot be excluded. Alternative (1) probably implies that sediments within the upper ca 150 m (height of headwall) of the sea-bed were affected ®rst. The shear stress induced by the downslope moving sediments may then have mobilised sediments at deeper levels. These then failed downslope from a present water depth of ca 1800 m and caused the formation of escarpment E2. Escarpment E2 also may have been formed during a separate event sometime after the formation of the headwall. Alternatively, the initial mass movement was initiated downslope from escarpment E2 and ®rst affected sediments down to ca 250 m below the sea¯oor on this part of the slope. This event then triggered a retrogressive failure characterised by a progressive backstepping of the slide upslope from E2 due to loss of support from the removed sediments. The backstepping terminated with the formation of the present headwall of the Trñnadjupet Slide. In this case, sediments down to 150 m below the sea¯oor were affected in the second phase. The possibility that the headwall was formed sometime after escarpment E2 can, however, not be excluded. The data at hand do neither show which of these alternatives is most likely nor if sliding upslope and downslope of escarpment E2 was coeval or occurred during separated periods. However, as indicated from the TOBI images, there are no major differences in post-slide sediments draping the slide scar between the morphological zones. 5.5. Initiation of mass movement: stratigraphical location Within the slide scar, the TOBI data and the coregistered seismic records display a subparallel sea¯oor pattern upslope and downslope from the escarpment E2, indicating initiation of mass movement within two layers, at about 150 and 250 m below the present sea-¯oor (see also Laberg et al., 2000, submitted). Movement of the large blocks or rafts of sediment also implies sediment deformation

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within a zone or layer of weakness. This suggests that bedding planes, i.e. slope-parallel beds of Pleistocene age (see below) controlled initial sediment disintegration and that such planes probably occurred at several stratigraphic levels. Movement above layers of weakness is consistent with results from other submarine slides on the eastern Norwegian-Greenland Sea continental margin (e.g. Bugge et al., 1987; 1988; Laberg and Vorren, 1993) and characterises translational slides in general (Hampton et al., 1996). 5.6. Factors promoting mass movement Submarine mass movement is initiated when the downslope-oriented shear stress exceeds the shear strength (resisting stress) of the continental slope sediments. Several factors may contribute to the release of slope failures (e.g. Hampton et al., 1996) as will be further discussed below. 5.6.1. Underconsolidation due to high sedimentation rate Sediments affected by the Trñnadjupet Slide comprised the northernmost part of the late Pliocene and Pleistocene depocentre offshore mid-Norway where the sediment thickness reaches 1800 m (v ˆ 2000 m/s) (Henriksen and Vorren, 1996). During this period, the sedimentation rate was relatively high; average Pleistocene sedimentation rates of 1.0 mm/yr. has been estimated (Stuevold and Eldholm, 1996). During the late Pliocene and Pleistocene period, the continental margin is inferred to have been affected by a deteriorating climate, and most of the sediments are probably glacigenic, deposited during glacial periods (Henriksen and Vorren, 1996). Based on analogy with the present interglacial, the sediment input to the continental margin probably was reduced by an order of magnitude during previous interglacial periods (,10 m). If this is correct, actual sedimentation rates during the Pliocene and Pleistocene glacial periods were higher than the average rates presented. The implication is that the sediments were underconsolidated, therefore weak, due to rapid sedimentation. 5.6.2. Liquefaction within plane(s) of weakness From the regional stratigraphy established for the area, the weak layers at about 150 and 250 m below the present sea-¯oor probably were within the

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Pleistocene succession (Henriksen and Vorren, 1996; McNeill et al., 1998). No published drilling results are available from the study area. The nearest drill sites on the continental slope are DSDP sites 338, 340 and 341 and ODP sites 642±644 on the eastern Vùring Plateau. Based on results from the ODP sites, the Pleistocene succession comprises glacigenic sediments; sandy, pebbly mud layers interbedded with interglacial calcareous mud (Henrich, 1989). This stratigraphy probably can be extended into the study area (McNeill et al., 1998). Based on analogy with the Holocene continental slope sediments off northern Norway (Laberg et al., 2000, submitted), the pre-Holocene interstadial/interglacial mud in the study area may have been characterised by relatively high water content and low undrained shear strength. The interbedded late Weichselian diamictons (sandy mud with pebbles) have a relatively low permeability due to the high clay content (30±55%). This could lead to build-up of excess pore pressure within the interstadial/ interglacial sediments because of the sealing capacity of the glacigenic sediments. In addition, the higher organic content of the interglacial mud (ca. 1%) compared to the glacigenic sediments (ca. 0.2±0.3%) due to reduced organic productivity (Laberg et al., 2000, submitted), may cause generation of methane that also could result in build-up of excess pore pressure within the hemipelagic sediments. As a result, the effective shear strength would be reduced and failure potential increased. Also, plasticity tends to increase with greater carbon content, in a clayey silt a 20 and 14% increase in the liquid limit and the plastic limit, respectively, was caused by a 1% increase in organic carbon (Booth and Dahl, 1986). This increase results in a higher plasticity index, an important indicator of the potential instability of the sediment because the higher the plasticity index the less stable are slope materials (Summer®eld, 1997). 5.6.3. Earthquakes Large earthquakes may reduce the slope sediment shear strength, and this probably is the most cited cause for triggering submarine slides, including the large failures identi®ed offshore Norway (Bugge, 1983; Bugge et al., 1987; 1988; Laberg and Vorren, 1993; Evans et al., 1996; Laberg et al., 2000). At

present, the eastern Norwegian-Greenland Sea is an area of relatively high intraplate seismicity (Bungum, 1989; Kvamme and Hansen, 1989), where events above magnitude 5 have been recorded on the continental margin north of the Vùring Plateau (Bungum et al., 1991; Fig. 3b). Based on historical data and instrumentally located earthquakes (from 1878 to 1987), earthquake hazard maps have been produced for the Norwegian continental margin (Dahle and Bungum, 1993). The maps show ground motion for the 10 22 and 10 24 per year exceedence probability, i.e. exceedence probability of 1% per 100 years and 10,000 years, respectively (Dahle and Bungum, 1993). Based on these maps, three areas of potential high ground acceleration have been identi®ed offshore Norway. The northernmost is located on the continental slope immediately north of the Vùring Plateau, extending into the Lofoten Basin (Dahle and Bungum, 1993; Fig. 3). The headwall area of the Trñnadjupet Slide also is located within the marginal area of the postglacial crustal uplift of Fennoscandia where the largest earthquakes are to be expected (Gudmundsson, 1999). The concurrence of large slides and old lineaments along the Norwegian continental margin indicates that the location of the earthquake activity was controlled by the old lineaments in the area including the Bivrost Lineament (Fig. 12). From the above results we suggest that triggering of the large slope failure identi®ed in this study may have been caused by a large earthquake. Even if individual earthquakes were too small to cause slope failure, earthquake activity may have been important for triggering the failure. From the eastern continental margin of north America, O'Leary (1991) concluded that slide triggering may have been due to the accumulated strength reduction following several small earthquakes, remoulding layers of weakness on the continental slope (see below) over a period of time. Such a triggering mechanism also should be considered for the slides on the continental margin off Norway. 5.6.4. Decomposition of gas hydrates From the Trñnadjupet Slide area, indications of gas hydrate have so far not been reported. The only data available from this are is Ocean Drilling Program (ODP) Leg 104. Leg 104 drilled through the Holocene±Pliocene succession on the southern,

J.S. Laberg, T.O. Vorren / Marine Geology 171 (2000) 95±114

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Fig. 12. Submarine slides in the north-eastern Norwegian-Greenland Sea (from Vorren et al., 1998), the marginal area (dotted) of the postglacial crustal uplift of Fennoscandia, and the location of the old lineaments in the area. The headwall area of the Trñnadjupet Slide is located within the marginal area of the postglacial crustal uplift of Fennoscandia where the largest Holocene earthquakes are to be expected (Gudmundsson, 1999). The concurrence of large slides and old lineaments along the Norwegian continental margin indicates that the location of the earthquake activity was controlled by the old lineaments in the area including the Bivrost Lineament. See text for further discussion. SFZ ˆ Senja Fracture Zone, BL ˆ Bivrost Lineament, VFZ ˆ Vùring Fracture Zone, EJMFZ ˆ East Jan Mayen Fracture Zone, JMFZ ˆ Jan Mayen Fracture Zone, WJMFZ ˆ West Jan Mayen Fracture Zone.

inner Vùring Plateau and found a relatively high gas concentration, mainly as methane. In some sediment these gases may have been present in the form of gas hydrates although no gas hydrates were visually observed (Kvenvolden et al., 1989). From the above results, we conclude that triggering by the release of large amounts of free gas from the decomposition of gas hydrate may be a possible triggering mechanism for the Trñnadjupet Slide.

However, so far no conclusive evidence for the presence of gas hydrate within this area has been published. 6. Conclusions 1. The Trñnadjupet Slide extends from the shelf break to more than 3000 m water depth in the Lofoten Basin, implying a slide-affected area of about

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2.

3.

4.

5.

14,100 km . The slide probably occurred during the mid-Holocene, prior to 4000 14C yr BP. The slide scar includes escarpments, detached ridges of sediments, sediment streams, grooves, elongated highs, tabular sediment blocks, transverse ridges, transverse fractures and small scale sea-¯oor irregularities. The initial sediment disintegration produced the detached sediment ridges, which moved by back-tilting or through basal deformation. Transition to sediment streams comprising more-or-less disintegrated sediments occurred over some kilometres. Movement of relatively consolidated sediments formed the tabular sediment blocks. Two possibilities exist for the geographic location of the initiation of mass movement within the Trñnadjupet Slide: the initiation of the failure was located either near the present headwall (escarpment E1) of the Trñnadjupet Slide, or downslope from an escarpment (E2) located at a water depth of 1800 m. Regardless of which alternative is correct, sliding upslope and downslope from escarpment E2 was probably almost coeval. A combination of events paved the way for slope failures in this area. Sedimentation rates within the Trñnadjupet Slide area probably were high during glacial maxima. Periods of high sedimentation rates of glacigenic sediments may have promoted instability in the glacigenic sediments themselves, or probably more important, prevented water or gas escaping from the relatively thin layers of interglacial/interstadial sediments (,10 m) due to the relatively low permeability of the glacigenic sediments. This could have led to a build up of excess pore pressure and the interglacial/interstadial sediments may then have acted as planes of weakness. Triggering was most likely due to one large or a series of smaller earthquakes possibly due to the postglacial crustal uplift of Fennoscandia.

Acknowledgements This work is a contribution to the ENAM II program. The TOBI side-scan sonar data and highresolution seismic were collected by the Universities of Bergen, Tromsù and Oslo and we gratefully

acknowledge the contribution of the cruise leader H. Ha¯idason, D.G. Masson and the crew of RRS Charles Darwin. The cruise was funded through the European Community's Training and Mobility of Researchers Programme (TMR-LSF contract ERBFMGECT 950030) and by the SEABED Project (an industrial oil consortium led by Norsk Hydro). The TOBI data was processed by J.S.L. at the Southampton Oceanography Centre, UK under the supervision of T. Le Bas. We would also like to thank T. Midtun, J.P. Holm and A. Igesund who produced the ®gures. G. Corner corrected the English text and together with D.G. Masson, T. Bugge and D. Evans made valuable suggestions for improvements of the manuscript. Financial support from EC MAST III project MAS3-CT95-0003 and the SEABED Project to the University of Tromsù is gratefully acknowledged. The manuscript bene®ted from constructive reviews by Monty Hampton and an anonymous referee. References Allen, J.R.L., 1985. Principles of Physical Sedimentology. George Allen and Unwin, London 272 pp. Booth, J.S., Dahl, A.G., 1986. A note on the relationships between organic matter and some geotechnical properties of a marine sediment. Mar. Geotech. 6, 281±297. Bondevik, S., Svendsen, J.I., Johnsen, G., Mangerud, J., Kaland, P.E., 1997. The Storegga tsunami along the Norwegian coast, its age and runup. Boreas 26, 29±53. Brunsden, D., 1984. Mudslides. In: Brunsden, D., Prior, D.B. (Eds.). Slope Stability. Wiley, Chichester, pp. 363±418. Bugge, T., 1983. Submarine slides on the Norwegian continental margin, with special emphasis on the Storegga area. , Continental Shelf and Petroleum Research Institute (IKU) Publication 110. Continental Shelf and Petroleum Research Institute, Trondheim, Norway (152 pp.). Bugge, T., Befring, S., Belderson, R.H., Eidvin, T., Jansen, E., Kenyon, N.H., Holtedahl, H., Sejrup, H.P., 1987. A giant three-stage submarine slide off Norway. Geo-Marine Lett. 7, 191±198. Bugge, T., Belderson, R.H., Kenyon, N.H., 1988. The Storegga Slide. Phil. Trans. Royal Soc. London, Ser A 325, 357±388. Bungum, H., 1989. Earthquake occurrence and seismotectonics in Norway and surrounding areas. In: Gregersen, S., Basham, P.W. (Eds.). Earthquakes at North-Atlantic Passive Margins: Neotectonics and Postglacial Rebound. Kluwer Academic Publishers, Dordrecht, Holland, pp. 501±519. Bungum, H., Alsaker, A., Kvamme, L.B., Hansen, R.A., 1991. Seismicity and seismotectonics of Norway and nearby continental shelf areas. J. Geophys. Res. 96 (B2), 2249±2265.

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