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Geometry of a major slump structure in the Storegga slide region offshore western Norway

Roald B. Færseth & Bjørn Helge Sætersmoen

Færseth, R.B. & Sætersmoen, B.H. 2008: Geometry of a major slump structure in the Storegga slide region offshore western Norway. Norwegian Journal of Geology vol 88, pp 1-11. Trondheim 2008. ISSN 029-196X.

A gravitational collapse structure, 155 km long with a maximum width of 35 km, has been released as a single slope failure in the Storegga region off western Norway. The released volume of sediment, which we interpret as a slump, is a part of and of the same age as the Storegga Slide. This means that the slump was released approximately 8200 years ago. Unlike the main Storegga Slide, in which slide sediments have been totally disintegrated by debris flows, the slump was subject to relatively modest down-slope displacement and is an intact body located along the southern margin of the Storegga Slide scar. Rotational movement of the slump sediments released horizontal stress at the head of the slump and at the same time, horizon-tal stress intensified down-slope where contractional structures such as folds, reverse faults and thrusts developed.

Roald B. Færseth, Hydro ASA, Research Centre, Box 7190, N-5020 Bergen, Norway. Present address: Ryenbergveien 75A, N-0677 Oslo, Norway (e-mail: [email protected]); Bjørn Helge Sætersmoen, Petroleum Geo-Services, P.O. Box 290, N-1326 Lysaker, Norway (e-mail: Bjorn.Helge. [email protected])

IntroductionGravitational collapse structures, often termed landslides, possess two essential features: a rupture surface (failure surface, glide surface, detachment, décollement) and a displaced mass of sediment (Hampton et al. 1996). The displaced mass is the material that travelled down-slope. It commonly rests partly on the rupture surface, but it might have moved completely beyond it. Moreover, the displaced mass may remain intact, be slightly to highly deformed, or break up into distinct slide blocks. In some cases, all or part of the mass completely disintegrates, producing a flow. Gravitational collapse structures are common in many settings around the world, both sub-aerially and subaqueously. Such structures are typically associated with areas of instability where some sort of triggering mechanism resulted in tectonic failure (e.g. Hampton et al. 1996).

The Storegga region off western Norway has been the site of several giant submarine slides (Evans et al. 1996; Bryn et al. 2003; Haflidason et al. 2004; Sejrup et al. 2004; Nygård et al. 2005; Solheim et al. 2005). The Storegga Slide (Fig. 1) of Holocene age is one of the largest known submarine slides and has been known since the 1970s (Bugge 1983; Jansen et al. 1987). Over the last decade, a large number of publications have described the Storegga Slide. One main reason for this interest in the area is related to the Ormen Lange gas field, discovered in 1997, which is located within the scar of the Storegga Slide (Fig. 2). This situation raises concerns about the hazard to which the production installations would be exposed. The Storegga Slide scar defines a depression bounded by

an approximately 300 km long backwall along the pres-ent-day shelf edge. A volume of some 3000 km3 of sedi-ment was removed leaving the slide scar (Haflidason et al. 2004; Bryn et al. 2005a). The run-out distance was 450 km for the debris flows and 800 km for the associ-ated turbidity currents (Haflidason et al. 2005). The slide affected an area of 90.000 km2 and was released in an area where the average slope inclination was of the order of 0.6-0.7°. Unstable sediments in the area were removed during one main slide event at approximately 8200 years BP (Haflidason et al. 2004; Bryn et al. 2005a).

The present study is concerned with the morphology and internal structure of a specific gravitational col-lapse structure in the Storegga region, which has not been described previously. Unlike the Storegga Slide proper where slide sediments have, to a large extent dis-integrated, the allochthonous mass of sediment we now describe was subject to relatively modest down-slope displacement and is an intact body located along the southern margin of the Storegga Slide scar (Fig. 2). High- resolution 2D and 3D seismic data and Multibeam Echo Sounder data from the seabed excellently displays the slump-like shape of the body as well as its internal defor-mation structures and their spatial distribution.

Gravitational collapse structuresGravitational collapse structures may range in length from centimetres to hundreds of kilometres and affect both loose sediments and consolidated rocks under subaerial as well as subaqueous conditions (Martinsen

NORWEGIAN JOURNAL OF GEOLOGY Geometry of a major slump structure in the Storegga

� R. B. Færseth & B. H. Sætersmoen NORWEGIAN JOURNAL OF GEOLOGY

T

T

T

Fig. 2. Bathymetric image of the Storegga Slide scar with outline of the slump in its present location along the southern margin of the slide scar (stippled blue line). See Fig. 1 for location of this figure. Late Pliocene-Pleistocene sediments of the North Sea Fan represent the southern boundary of the slump on the sea floor, whereas inliers of the older Tampen Slide (T) are located along the northern boundary. The Ormen Lange gas field (continuous blue line) is located within the Storegga Slide scar.

Fig. 1. Bathymetrical map of the southern Norwegian Sea and the Norwegian margin area with outline of the Storegga Slide.

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1994; Hesthammer & Fossen 1999; Canals et al. 2004 and references therein). There is, however, a significant dif-ference in size between subaerial and subaqueous land-slides (e.g. Hampton et al. 1996). The largest subaerial slides displace a few tens of cubic kilometres of material. By comparison the largest submarine slide known, the post-Pliocene Agulhas slide off South Africa has a vol-ume of 20.000 km3 and is interpreted to have occurred as a single slope failure event (Dingle 1977). Submarine landslides can originate in nearly flat areas. Analyses have shown that the force of gravity alone is not sufficient to be the sole cause of failure (e.g. Ross 1971; Hampton et al. 1978). Gravity failure is generally due to a triggering mechanism provided by seismic shocks, rapid sedimen-tation, over-steepening of slopes, changes in pore pres-sure, or extensional deformation (Morton 1993).

Despite the variation in scale as well as variation in lith-ification of sediments affected, there is a striking similar-ity in the overall geometry of many gravity-related fail-ures (Fig. 3). The area affected is commonly described as spoon-shaped or amphitheatre-like (e.g. Eckel 1958; Var-nes 1958; Lewis 1971; Brundsen 1973; Brundsen & Jones 1976; Martinsen 1994; Hesthammer & Fossen 1999, Canals et al. 2004). In cross section, the rupture surface typically displays a concave upwards geometry. The sur-face may form a flat that is bedding-parallel in its cen-tral part but cuts across bedding planes at the head and in some cases also at the toe to define ramps (e.g. Varnes 1958; Levis 1971; Schwarz 1982). The bedding-parallel part of a rupture surface is typically confined to strati-graphic horizons that appear to be inherently weak either because of low sediment strength, high fluid pressure, or both. The term décollement is generally used only where

the rupture surface is layer-parallel (Ramsay & Huber 1987).

Terminology

The terms “slide” and “slump” are referred to in the lit-erature as features produced by various mass gravita-tional processes, but the use of the terms has often been imprecise. Most classification schemes make a distinc-tion between slide and slump mechanisms although different definitions are proposed (e.g. Hampton 1972; Coates 1977; Woodcock 1979; Allen 1985; Stow 1986; Nemec 1990; Martinsen 1994; Hesthammer & Fossen 1999; Lucente and Pini 2003, Canals et al. 2004). Slide masses are in general said to move on planar rupture surfaces (translational movements) and show no or little internal deformation, whereas slump masses move on an upwards-concave rupture surface (rotational move-ments) and show a wide range of internal deformation mechanisms. Following this definition the term slump should be used for the displaced mass of sediment described in this paper.

Models of gravitational collapse structures

Idealized models of gravitational collapse structures (e.g. Eckel 1958; Lewis 1971; Farrell 1984) show the displaced unit to have a well-defined upper extensional zone (head) and a down-slope contractional zone (toe) (Fig. 3). This is because movement of the mass of sediment releases horizontal stress at the head and at the same time, hori-zontal stress increases towards the toe. Rupture surfaces below slumps may display substantial curvature or sharp bends. As the slump moves down and away from the scarp past irregularities along the substrate, there must be deformation in the sediments above the detachment


Fig. 3. Schematic illustration showing characteristic geometries associated with gravitational collapse structures (after Echel 1958). This idealized model shows the displaced unit to have a well-defined upper extensional zone (head) and a down-slope contractional zone (toe). This is because movement of the mass of sediment releases horizontal stress at the head and at the same time, horizontal stress increases towards the toe.

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(Ramsay & Huber 1987). For this reason, folds may develop within the advancing slump, formed by bend-ing of the sediments as they slip over non-planar rupture surfaces.

Geometry of a large, intact slump in the Storegga regionGeological setting: sediments and slides in the Storegga region

Thick units of biogenic ooze of the Eocene-Oligocene Brygge Formation and Miocene-Early Pliocene Kai Formation (Dalland et al. 1988) were deposited in the deeper parts of the margin offshore Mid-Norway fol-lowing break-up in the North Atlantic. This was followed by deposition of the Mid-Late Pliocene Naust Forma-tion, which is a thick prograding wedge with interbed-ded claystone, siltstone and sand, occasionally with very coarse clastics in its upper part (Riis 1996; Bryn et al. 2003). The Naust Formation has been subdivided into five hemipelagic/glacigenic sequences (Berg et al. 2005; Bryn et al. 2005a, their Fig. 4). An intra Naust bedding-parallel reflector represents the most pronounced failure surface associated with the main Storegga Slide (Haflidason et al. 2003; Bryn et al. 2005a). The sediments involved in this slide were mainly deposited during the last glacial-inter-glacial cycle, and their only consolidation took place in the shelf edge area where they were in close contact with shelf glaciers (Haflidason et al. 2004). The displaced mass or slump we describe here is located along the southern margin of the Storegga Slide scar (Fig. 2), and comprises

sediments of the upper part of the Naust Formation.Sediments in the Storegga region are severely dissected by slides, and repeated sliding during the last 1.7 mil-lion years is well documented (Evans 1996; Bryn et al. 2003; Haflidason et al. 2004; Sejrup et al. 2004; Nygård et al. 2005; Solheim et al. 2005). Five slides predating the Storegga Slide with lateral extent from 7000 to 27.000 km2 have been mapped (Solheim et al. 2005), and they partly underlie the area where the Storegga Slide was later released (Bryn et al. 2003). The Storegga Slide and the older slides display several common features regard-ing slide mechanisms, slide development and their rela-tionship to the regional geology (Solheim et al. 2005). A common characteristic is the layer-parallel slide surfaces in stratified marine clay units (Bryn et al. 2005a; Kvalstad et al. 2005). The most likely triggering mechanism caus-ing the slides was strong earthquake activity following onshore uplift after glaciations. This explains why the slides take place after a glacial period (Bryn et al. 2005b). Earthquake activity was at its highest in Scandinavia between 10 and 7 ka as a response to glacial rebound.

Slump morphology

In map view, the gravitational slump described here has an overall lens-shape (Fig. 2) that covers an area of approximately 3700 km2. It is 155 km long with a maxi-mum width of 35 km, and is bounded to the north by three huge bodies of rocks that are circular to elliptical in map view (Figs. 2 and 4). These are remnants (inliers) of the older (130-150 Ka, Solheim et al. 2005), more com-petent and once continuous Tampen Slide, which is now preserved south of the Storegga Slide scar. The concave

R. B. Færseth & B. H. Sætersmoen NORWEGIAN JOURNAL OF GEOLOGY

Fig. 4. A shaded relief map from multibeam bathymetry shows the mid segment of the slump on the seabed. Irregularities at the upper surface of the slump reflect reverse as well as normal faults. See Fig. 2 for location this figure. The faults strike consistently N-S, i.e. orthogonal to the transport direction of the slump. Inliers of the older Tampen Slide, circular to elliptical in map view, are located along the northern boundary of the slump. Small and large rectangle outline insets in figs 7 and 10, respectively.

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BF

FR

upwards rupture surface below the slump is confined to an intra Naust bedding plane horizon below the mid seg-ment and the western part of the proximal segment (Fig. 5). The intra Naust bedding plane horizon dips gently (0.1°) to the west. To the west, the rupture surface cuts 250 m of the sequence to form a ramp, the distal part of the slump being transported beyond the ramp along a base Tampen Slide detachment (Fig. 5). On a regional scale and in a west-east cross-section, the upper surface of the slump exhibits a convex outline on the seabed, and the maximum thickness of 700 m is associated with the mid segment of the slump. In the proximal segment, the

thickness ranges from 250 to 600 m, whereas the distal segment of the slump immediately west of the ramp has a thickness of 300 m and thins gradually further west.

The heave associated with the breakaway fault at the head of the slump involves approximately 10 km of lateral movement away from the footwall (Fig. 5). Hence, the slump has moved a minimum distance of 10 km down-slope in its proximal part. However, thinning of the slump in the eastern part of the proximal segment indi-cates that total extension may be significantly more than 10 km. To the west, shortening is evident due to a gradual


Fig. 5. Schematic profile of the slump. See Fig. 2 for location of the profile. The slump involves Pliocene sediments of the Naust Formation (dark brown) as well as interlayered, older and more competent sediments of the Tampen Slide (green). The slump is a composite feature characterized by different structural associations that occur in three well-defined segments (proximal, mid and distal). Shortening in the western part of the slump is evident due to folding, reverse faulting and thrusting, which compensated for extension in the proximal part. BF – Breakaway fault; FR – Frontal ramp. The profile is not to scale.

Fig. 6. W-E oriented seismic section across the mid segment of the slump showing the style of deformation.. The segment is characterized by conjugate reverse faulting and folding east of the frontal ramp, followed by normal faults in the easternmost part of this segment. The fold pat-tern displayed by Tampen Slide sediments in the easternmost part of the section attests to contraction and shortening in the western part of the proximal segment. Reflections: Seabed (blue); Top Tampen Slide (yellow); Base Tampen Slide (green). The detachment east of the ramp is confined to an intra Naust bedding plane horizon (light green). At the frontal ramp, the detachment climbs 250 m across the lithological layers to the base of the older Tampen Slide that is interlayered with sediments of the Naust Formation in the footwall.

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thickening of the slump as well as folding, thrusting and reverse faulting, features which compensated for exten-sion in the proximal part.

The slump we describe is a composite feature where dif-ferent structural associations occur in three well-defined segments. The three segments and their characteristic structures are described below. The structures and their spatial distribution throughout the displaced mass of sediment are schematically shown in Figure 5.

The proximal segment of the slump

From the head of the slump and to the west, the proximal segment is approximately 80 km long (Fig. 5). The thick-ness of this segment increases from 250 m at the head to 600 m in its westernmost part. The seismic resolution is generally poor in this area as a result of internal defor-mation, but also due to the presence of relatively thick post-slump sediments within the Storegga Slide scar. The rupture surface, dips approximately 4° immediately west of the fault scarp but flattens down-slope to become sub-horizontal in the westernmost part of the segment. In the western part of the proximal segment, over a distance of some 30 km, folding is evident (Figs. 5 and 6). The fold pattern is typical of ptygmatic folds (Ramsay and Huber 1987), and folding is displayed by the most competent layer, the Tampen Slide, enclosed in the less competent sediments of the Naust Formation. The disharmonic style of ptygmatic folds usually implies layer-parallel contrac-tion, and the lack of folding at some distance away from the competent layer into the over- and underlying rocks implies that the contraction is taken up as homogeneous strain in the less competent sediments (Ramsay and Huber 1987). The western part of the proximal segment has been subject to significant shortening, a process that reduced the length of the slump and increased its thick-ness. An estimate of the shortening can be obtained by comparing the bed length of the folded Tampen Slide sediments with the present length of the folded segment. This indicates that the western part of the proximal seg-ment has been shortened from an original length of 44 km to its present length of 30 km, i.e. a shortening of 32%. We interpret shortening and folding in this part of the slump to result from the rapid down-slope motion in the east when the slump slipped away from the backwall and moved down a relatively steep rupture surface. As the movement of the slump had to adjust to a near horizon-tal rupture surface in a westerly direction, the velocity of the advancing slump slowed down and the most compe-tent sediments above the rupture surface, i.e. the Tampen Slide, started to buckle.

The mid segment of the slump

The rupture surface is sub-horizontal below the mid seg-ment of the slump, indicating translational movement. For slumps and slides where the mid section shows evi-dence of translational movement, the displaced mass of sediment commonly remains nearly undisturbed after

mass movement (e.g. Schwartz 1982; Martinsen 1994). This is in contrast to the mid segment of the slump we describe, which exhibits considerable deformation.

The segment is approximately 60 km long and is bounded to the west by a ramp where the detachment cuts obliquely up-section in the direction of tectonic trans-port (Fig. 5). The mid segment, which is the thickest part of the slump, has a maximum thickness of 700 m. The segment has been shortened horizontally and extended vertically due to lateral compaction, folding and reverse faulting. Within the mid segment, the upper surface of the slump is irregular due to faults that are contractional as well as extensional in origin. The faults strike consis-tently N-S, i.e. orthogonal to the transport direction to form transverse features at the seabed level (Fig. 4). The faults are planar and most of them terminate downwards at the detachment surface, and hence in general transect the total thickness of the slump (Figs. 5 and 6). The faults typically occur in “families”, which means that a number of faults with similar appearance are characteristic for a specific volume of the mid segment. East of the ramp, regularly spaced reverse faults were generated. The con-jugate set of reverse faults created uplifted ridges 450-900 m wide over the upper surface of the slump (Figs. 5 and 6). These faults have maximum vertical offsets from 40 to 70 m, and exhibit dips in the range 50° to 60°.

Bedding is well preserved within the mid segment, which allows exact calculation of vertical offset as well as hori-zontal overlap associated with individual reverse faults. Based on measurements of the bed length of the folded Tampen Slide sediments as well as horizontal overlap for a total of some 45 reverse faults, the shortening is esti-mated to be 3.5 km. Hence, reverse faulting and folding account for 11% horizontal shortening over a part of the mid segment where the present length is 28 km. Apart from folding and reverse faulting, shortening and thick-ening of the mid segment of the slump are also results of lateral compaction. Assuming that the original thick-ness of the slump in the mid segment was the same as the original thickness in the westernmost part of the proxi-mal segment, which we calculate to be 4.5 km, the mid segment has then been shortened from an initial length of some 80 km to its present length of 60 km. Hence, the total shortening of the mid segment is approximately 25%, and this may be subdivided into a component of 11% due to folding and reverse faulting and a compo-nent of 14% due to lateral compaction.

A number of fault blocks, all tilted to the west, occupy the eastern part of the mid segment (Figs. 5 and 6). The fault blocks are typically 1200-1400 m wide, and are bound by normal faults which dip 45-50° east, with fault throws in the range of 100 to 200 m. This sideway collapse of fault blocks to give a domino-style geometry probably resulted from a change in the velocity of dislocation. The normal faults represent an adjustment to the space created in a part of the advancing slump undergoing localized extension.


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Fig. 7. W-E oriented seismic section showing fault-bend folding associated with the frontal ramp and associated reverse faults east of the ramp. Reflections: Seabed (blue); Top Tampen Slide (yellow); Base Tampen Slide (green); Base slump (light green).

Fig. 8. The progressive development of hanging wall fault-bend folds where the slump moves across the frontal ramp. Horizontal and vertical scale is equal. Stage 1: Mid-Late Pliocene Naust Formation (brown colours), and the interlayered older Tampen Slide prior to the release of the slump. Stage 2: The slump moves down-slope along a detachment, which is an intra Naust bedding-parallel horizon. The stepping up of the detachment to form a frontal ramp, results in fault-bend folding as the hanging wall adjust its shape to the form of the underlying fault. As the slip on the detachment continues, the anticline grows in amplitude (Stage 3). The uplifted/overthrust mass of sediment at seabed has been partly removed by post-slump ero-sion. The distance x´ – x (1.5 km), equals the amount of overthrus-ting in this position.


The distal segment of the slump

In the westerly, distal part of the slump, the detachment climbs 250 m, from an intra Naust level and crosses the lithological layers to the base of the older Tampen Slide that is interlayered with sediments of the Naust Forma-tion in the footwall (Figs. 5 and 7). The ramp dips 30° east (Fig. 8) and is classified as a low angle reverse or thrust fault (Ramsay & Huber 1987). The ramp is perpendicular to the main direction of slump movement and accord-ingly is a frontal ramp that allowed the detached mass of sediment to override younger sediments in the footwall. The compatibility constraints that arise when the slump (hanging wall) moves relative to the in situ footwall are resolved by allowing the hanging wall to adjust its shape to conform with the morphology of the underlying foot-wall and, as a result, to undergo fault-bend folding. Such folds form only in the hanging wall as it is forced over the underlying staircase-like step (Fig. 8).

The deformation associated with the fault-bend folding resulted in uplift of the slump and local vertical duplica-tion of horizontal strata. The uplifted/overthrust masses at sea bottom level were partly removed by succeeding erosion (Figs. 7 and 8). Fault-bend folds are typically associated with foreland fold-and-thrust belts on land (e.g. Suppe 1985). However, although most geometric analyses of folds associated with ramp-flat situations have been undertaken in studies of thrust faulting on land, such features are also associated with large-scale gravita-tional collapse structures elsewhere. The folding associ-ated with the frontal ramp of the slump we describe has the ideal kink-like geometry of fault-bend folds related to a single step in the rupture surface, following Suppe (1985). This geometry conserves cross-sectional area and also rock volume in three dimensions (Ramsay & Huber

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Fig. 9. A composite seismic section that illustrates deformation within the distal segment of the slump, west of the frontal ramp. In this area, the detachment below the slump follows the base of the Tampen Slide. Contraction resulted in relatively small asymme-trical folds, thrusts and high angle reverse faults that bifurcate from the underlying detachment level. Folds and thrusts decrease in frequency and size to the west. Reflections: Top Tampen Slide (yellow); Base Tampen Slide (green).

Fig.10. N-S oriented seismic section across the southern lateral margin of the slump. In this area the margin is inclined at a rela-tively low angle to the transport direction of the slump to cause a left-lateral shear sense of transpression. Along this part of the slump margin the detachment climbs from an intra Naust bedding plane horizon (light green) below the slump, across lithological layers to the top of the Tampen Slide in the footwall to the south, to form a steep lateral ramp. Reflections: Seabed (blue); Top Tam-pen Slide (yellow); Base Tampen Slide (green).

1987). The characteristic shape of single-step fault-bend folds is a steeper front dip and a more gentle back dip that is equal to the angle of fault dip (Figs. 7 and 8). As slip on the detachment continues, the anticline grows in amplitude and the fold reaches its maximum amplitude when it equals the height of the step in the detachment, i.e. 250 meters. The distance x´ – x (Fig. 8), equals 1.5 km, and represents the amount of overthrusting in this posi-tion and accordingly the amount of shortening associ-ated with the frontal ramp.

West of the ramp, where the detachment follows the base of the Tampen Slide, contraction is evident over a dis-tance of approximately 15 km. Contraction results in rel-

atively small asymmetrical folds, thrusts and high angle reverse faults that bifurcate from the underlying detach-ment level (Figs. 5 and 9). Asymmetrical folds typically have a westerly vergence and reverse faults dip to the east to give an imbricate structure. The faults cut up through the stratigraphy and some of them may even reach the seafloor. Folds and thrusts decrease in frequency and size to the west as the effects of contraction deteriorate when approaching the toe of the slump.

Deformation along lateral margins of the slump

The slump has a sharp lateral margin to the south (Fig. 2). Lateral margins of slumps in general are likely areas of strike-slip or oblique-slip motion, depending on the


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width variability of the slump scar and also whether they maintained parallelism to the movement direction or are skewed. When the slump described here was released, the overall direction of transport was to the west (Fig. 2). In map view, the lateral margin south of the mid segment is oriented at a small angle to the transport direction of the slump (Fig. 4), and may cause a left-lateral sense of movement (transpression) to take place. Along this part of the slump margin, the detachment is typically a steep ramp against in situ sediments (Fig. 10). The detachment climbs from the intra Naust level below the slump across lithological layering to the top of the Tampen Slide to the south. The amount of thrusting and shortening of the slump along the lateral/oblique ramp is subordinate to that associated with the frontal ramp (Fig. 7). The north-ern slump margin is not well displayed due to post-slump erosion. However, with a westerly directed transport of the slump this lateral margin was most likely subject to overall transtension.

DiscussionThe seabed morphology and seismic facies of the Storegga Slide indicate that the main slide developed by a headwall retreating upslope with unloading at the toe as the main driver (regressive slide) (Bryn et al 2005a; Haflidason et al. 2005; Kvalstad et al. 2005). The slide left a deep scar to the east with headwall slope gra-dients in the range 20-45° (Haflidason et al. 2004). The slide scar associated with the main Storegga Slide is the scar that has eroded deepest into the shelf area (Figs. 1 and 2). The slump we describe here is located along the southern margin of the main Storegga slide scar, and the intra Naust rupture surface below it can be traced to the north where it coincides with the deepest-seated failure surfaces associated with debris lobe phases of the Storegga Slide. Haflidason et al. (2004) reconstructed the continuous retrogressive development of the Storegga Slide. They mapped slide debris lobe phases associated with this c. 8200 years BP slide event, as well as distinc-tive provinces defined by morphological textures on the seabed.. Five major lobe phases were identified by Hafli-dason et al. (2004) and interpreted to have taken place within a very short time interval. The slump we describe was termed lobe 2, identified as Province Type IV (com-pression zone), and interpreted to be a part of and of the same age as the main Storegga Slide.

We have observed increases and decreases in length within the slump, and associated deformation features are both brittle and ductile. The slump shows a gradual thickening from the breakaway fault in the east towards a frontal ramp to the west (Fig. 5). There is no evidence suggesting that the observed changes in bed thicknesses are primary. We interpret the thickening as a result of lat-eral compaction processes taking place in high porosity sediments, which moved down an inclined rupture sur-face. Hence, the displaced mass of sediment was subject to layer-parallel shortening that reduced the length of the

slump and increased its thickness. We have attempted to calculate the amount of shortening in various parts of the slump associated with lateral compaction as well as fold-ing, reverse faulting and thrusting. Applying the present length and thickness of a slump segment and assuming a constant cross sectional area before and after deforma-tion, the original length of the deformed segment can be obtained. A limitation in applying this area balance method is that the original thickness of the deformed segment must be known. As knowledge of this is not available, it introduces some uncertainties in our calcula-tions. However, based on these calculations as well as a comparison of bed lengths before and after deformation in parts of the slump, the slump as a whole is interpreted as having been shortened some 30-35 km in the area represented by the mid segment and the western part of the proximal segment. We interpret shortening to have resulted from the obstacle represented by the ramp at the rupture surface. Following the initial acceleration at the head of the slump, the velocity came to a halt across the ramp, and the mid segment as well as the western part of the proximal segment became subject to overall con-traction. Farell (1984) suggested that if slumps halt first at their down-slope area, a contractional strain wave is likely to propagate up-slope through the slump to form contractional structures such as folds and reverse faults, a model that is consistent with our observations.

We interpret the slump failure as a single event. How-ever, the speed at which the mass movement took place is uncertain. The velocity achieved during gravitational mass movements, which is an important factor with respect to deformation within the displaced mass of sedi-ment, can be influenced by factors such as the volume of sediment dislodged, the dip of the detachment, the cur-vature of the detachment and associated obstacles as well as the rheological properties of the sediments involved. Schwartz (1982) concluded that subaqueous slope fail-ures move down-slope with a velocity mostly less than 5 m/sec, i.e. 18 km/h. By applying this velocity, the slump we describe was displaced in less than one hour. As a comparison, modelling has indicated maximum front velocity of the Storegga Slide to be in the range 10-20 m/sec (Bryn. et al. 2005a; Kvalstad et al. 2005). De Bla-sio et al. (2003), modelled slide velocities of 25-30 m/sec based on run-out studies of the Storegga Slide. Bondevik et al. (2005), compared deposits from the Storegga tsu-nami with numerical simulations for a retrogressive slide development, and obtained a good agreement applying velocities of 25-30 m/sec. For the slump we describe, a lower velocity (< 5 m/sec) seems more likely.Following the initial acceleration at the head of the slump, the velocity slowed down above the sub-hori-zontal part of the rupture surface and across the frontal ramp, to come to a halt some 15 km kilometres west of the ramp. A relatively low velocity may also explain why the slump we describe has remained intact, in contrast to the Storegga Slide proper, in which the sediments have been totally disintegrated.


�0 R. B. Færseth & B. H. Sætersmoen NORWEGIAN JOURNAL OF GEOLOGY

ConclusionsThis study is concerned with the morphology and inter-nal structure of a gravitational collapse structure of some magnitude in the Storegga region, which has not been described previously. The displaced mass of sediment is located along the southern margin of the main Storegga Slide scar and displays internal structures that are consid-ered to be diagnostic for submarine slumps. The slump was transported to the west without loss of cohesion. High-resolution 2D and 3D seismic data and Multibeam Echo Sounder data from the seabed provide an excellent coverage of the slump and allow the internal deforma-tion structures and their spatial distribution within the displaced mass of sediment to be identified. Our obser-vations reflect the post-kinematic situation of the slope failure and demonstrate that the slump is a composite feature where different structural associations character-ize the proximal, mid and distal segments. We interpret the slump to represent a single slope failure event of the same age as the main Storegga Slide, released approxi-mately 8200 years ago.

In map view, the slump has an overall lens-shape that covers an area of approximately 3700 km2. It is 155 km long with a maximum width of 35 km, and the maxi-mum thickness of the displaced mass of sediment is 700 m. The concave upwards detachment is sub-horizontal and bedding-parallel below a major part of the slump. To the west, the detachment cuts obliquely up-section in the direction of tectonic transport to form a 250 m high ramp.

The slump shows a gradual thickening from the break-away fault in the east towards a frontal ramp to the west. As the slump moved to the west, down an inclined rup-ture surface, compaction processes took place in high porosity sediments and the displaced mass of sediment was subject to layer-parallel shortening that reduced the length of the slump and increased its thickness. Calcu-lations demonstrate that the slump was shortened some 30-35 km east of the frontal ramp, due to combina-tions of lateral compaction, folding, reverse faulting and thrusting.

We interpret that the slump moved down-slope with a velocity less than 5 m/sec, i.e. 18 km/h. The sub-horizon-tal detachment below a major part of the slump together with the steep frontal ramp prevented further down-slope mass movement and accounted for the overall modest displacement of the slump. Following the initial acceleration at the head, the velocity slowed down above the sub-horizontal part of the detachment and across the ramp. As the slump came to a halt in the down-slope area, a contractional strain wave propagated up-slope through the slump to form contractional structures such as folds and reverse faults.

Acknowledgements: – We are grateful to Ole Martinsen for suggestions and comments on an earlier version of the manuscript. We also want to thank Reidulf Bøe and Ebbe Hartz for constructive reviews of the article.

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geometry of a major slump structure in the storegga slide ...geometry of a major slump structure in...

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