A Large-Scale Radial Pattern of Seismogenic Slumping towards the Dead Sea Basin
Alsop, G. Ian, Marco, Shmuel, Journal of the Geological Society
Abstract: Although it has been tacitly assumed since the seminal work of Jones in the 1930s that slump folds bear a systematic and meaningful relationship to the slope upon which they were presumably created, there has in reality been very little attempt to objectively verify this association via the collection of regional slump data in a relatively controlled setting. The potential to walk around the intact Dead Sea Basin at c. 425 m below mean sea level provides a perhaps unparalleled opportunity to undertake such verification via the direct examination of slump fold relationships. The collection of slump data in this well-constrained environment, where the seismogenic trigger for slumping is established via earthquake records, and the palaeogeographical controls are also recognizable and clearly link to the present bathymetry and landscape, thereby permits an evaluation of the use of slump folds as indicators of palaeoslope. The Late Pleistocene Lisan Formation cropping out to the west of the Dead Sea contains superb examples of slump folds that systematically face (>95%) and verge (>90%) towards the east. This study employs and evaluates five statistical techniques, including a new mean axial-planar dip (MAD) method, to analyse relationships between the orientation of slump folds and palaeoslopes. We recognize for the first time that the direction of slumping inferred from slump folds and thrusts varies systematically along the entire c. 100 km length of the western Dead Sea Basin. SE-directed slumping is preserved in the north, easterly directed slumping in the central portion and NE-directed slumping at the southern end of the Dead Sea. They are interpreted to form part of a large-scale and newly recognized radial slump system directed towards the depocentre of the precursor to the Dead Sea, and to be triggered by earthquakes associated with seismicity along the Dead Sea Fault.
The use of slump folds and associated soft-sediment structures to determine the orientation of palaeoslopes (i.e. the direction of dip of the slope) has been widely employed in a number of settings since the pioneering work of Jones (1939). However, the debate on both the usefulness and robustness of a variety of techniques used to estimate palaeoslopes continues (e.g. Lewis 1971; Corbett 1973; Woodcock 1976a,b, 1979; Potter & Pettijohn 1977; Rupke 1978; Farrell 1984; Maltman 1984, 1994a,b; Collinson 1994; Bradley & Hanson 1998; Debacker et al. 2001, 2009; Strachan & Alsop 2006; Strachan 2008; Alsop & Marco 2011). In addition, improved offshore seismic lines have led to the recognition that relatively recent sediments are also deformed on a large scale via gravity-driven processes creating mass-transfer complexes (MTCs) in slope settings (e.g. see Gardner et al. 1999; Lee et al. 2007; Bull et al. 2009; Jackson 2011). One of the central problems in recognizing and interpreting slump folds and associated palaeoslopes in ancient settings is that subsequent tectonics has frequently complicated and masked the original structural geometries associated with slumping (e.g. Waldron & Gagnon 2011), and the palaeogeographical setting also becomes more difficult to ascertain in older rocks. To overcome these problems we examine slump folds and structures developed in laminated marls of the Lisan Fm, which were deposited in the Late Pleistocene (70-15 ka) precursor to the Dead Sea (e.g. Marco et al. 1996; Fig. 1).
The Dead Sea Basin forms a sediment 'sink' currently reaching -750 m below mean sea level, with depths of -425 m at present exposed above water level and therefore accessible for fieldwork. As such, it is the lowest point on Earth at which direct field observations and analysis of structures developed within a basin may be made. This has a number of distinct advantages including the fact that the palaeogeographical setting of the Late Pleistocene Lisan Fm is exceptionally well-constrained around the present Dead Sea Basin (Fig. 1). As such, this may be used as an ideal case study and direct 'controlled' test not only of the use of slump folds as indicators of palaeoslope, but also of the relative complexity and variability that may develop around a pronounced, but relatively simple basin on the scale of c. 100 km.
Our study therefore aims to explore a number of factors and fundamental questions pertaining to slumped sediments, including the factors that control the spatial distribution and the transport directions of regional (basin-scale) slump patterns. We thereby evaluate a range of established and new techniques designed to determine the orientation of palaeoslopes from slump fold geometries and orientations. We first present a general overview of fold and fault patterns developed in slump systems, together with a summary of statistical methods used to determine palaeoslope directions from slump folds.
Overview of fold and fault patterns developed in slump sheets
Soft-sediment slump sheets may be considered as deformation cells translating downslope and are typically modelled as displaying extension at the head of the slump balanced by contraction at the lower toe (e.g. Hansen 1971; Lewis 1971; Farrell 1984; Farrell & Eaton 1987; Elliot & Williams 1988; Martinsen 1989, 1994; Martinsen & Bakken 1990; Smith 2000; Strachan 2002; Gilbert et al. 2005; Strachan & Alsop 2006; Alsop & Marco 2011). Contractional folds and thrusts developed at the toe of the slump have attracted the most attention in the literature, perhaps because they typically form the most obvious structures, and are also the focus of the present case study. The reader is referred to Alsop & Marco (2011) for a review and details of extensional faulting associated with slumps.
Slump folds may form above flat-lying detachments, where they can be viewed as a variety of flow perturbation folding, broadly defined as where displacement on an underlying detachment governs the geometry and orientation of the overlying folds (e.g. Coward & Potts 1983; Ridley 1986; Holdsworth 1990; Alsop & Holdsworth 2002, 2004a, 2007). Detachments in which displacement is typically uniform along the strike of the slope will create layer-parallel shearing (LPS), which generates folds at high angles to, and verging towards, the downslope flow direction (e.g. Alsop & Holdsworth 2007; Fig. 2). Conversely, detachments marked by pronounced gradients in displacement along the strike of the slope will create layer-normal shearing (LNS), which initiates folding oblique or subparallel to the flow direction (e.g. Strachan & Alsop 2006; Alsop & Holdsworth 2007; Debacker et al. 2009; Alsop & Marco 2011).
As a result of variations in the LPS and LNS components of flow in slump sheets, it may be found that slump folds display broadly arcuate traces about the flow direction. In addition, slump folds may subsequently undergo hinge and axial-planar rotations during continued downslope movement to create curvilinear sheath fold geometries (e.g. Alsop & Holdsworth 2004b, 2006; Alsop & Carreras 2007; Alsop et al. 2007; Fig. 2). Further complications may include local backthrusts and folds verging up the palaeoslope, together with attenuation of fold limbs (Fig. 2; for further details see Alsop & Marco 2011). We now summarize and evaluate the main statistical methods of determining palaeoslopes from slump fold geometries before applying these to the Dead Sea Basin.
Statistical methods to determine palaeoslope directions from slump folds
A variety of techniques have been developed to deduce the direction of a palaeoslope from the orientation of slump folds. These techniques have recently been reviewed by Strachan & Alsop (2006), Alsop & Holdsworth (2007) and Debacker et al. (2009) and we therefore only provide brief descriptions below and summaries in Table 1.
Fold facing directions
Fold facing is defined as the direction, normal to the fold hinge and along the axial plane, in which younger rocks are encountered (see Holdsworth 1988). As with fold vergence it is given a directional notation (west, east, etc.), but it is also noted if the facing direction has an upward or downward component (Fig. 3). The facing direction represents a line that plots as a point on a stereonet (Fig. 3). Downward-facing lines intersect the standard lower hemisphere of a stereonet and are directly plotted as points (see Holdsworth 1988), whereas upward-facing lines intersect the upper hemisphere of the stereonet and are simply projected vertically down to plot on the lower hemisphere as chordal points (Fig. 3; see Alsop & Marco 2011). Woodcock (1976a) recognized that slump folds will typically face upwards and verge towards the downslope flow direction. Facing therefore represents a valuable tool in deciphering slump folds, as downward-facing folds are atypical and may represent critical areas of refolding (Woodcock 1976a).
The mean axis method (MAM)
It has been recognized since the work of Jones (1939) that fold hinges may form at right angles to the downslope direction and form statistical groupings normal to the flow or slump direction. This mean axis method (MAM) has been recently reviewed by Strachan & Alsop (2006), Alsop & Holdsworth (2007) and Debacker et al. (2009). Although this simple fold-slope relationship may be complicated by differential LNS, resulting in a range of both transport subparallel and transport-normal fold hinges, it is apparent that such complications may be readily identified via the detailed examination of geometric relationships associated with fold vergence and asymmetry (see Alsop & Holdsworth 2002, 2004a,b, 2007; Table 1). In areas of marked differential movement associated with LNS, fold hinges may initiate subparallel to transport and display asymmetries in both directions (S and Z folds) to define bimodal vergence patterns (Table 1) (for a full review see Alsop & Holdsworth 2007). Conversely, in areas of marked LPS but only limited LNS, fold hinges initiate at right angles to transport and display a consistent sense of asymmetry to define unimodal vergence patterns (Table 1). Clearly, the MAM is most suited to areas where layer-parallel shear dominates and little subsequent fold rotation has occurred.
The mean axial plane strike method (MAPS)
This technique is a development of the mean axis method noted above and simply employs the mean strike of axial planes (rather than the trend of associated fold axes) to determine the transport direction (Table 1). The relationship of axial-planar strike with the transport direction in both LPS- and LNS-dominated extensional and contractional systems has been discussed at length by Alsop & Holdsworth (2007). In systems governed by LNS, axial planes will strike normal to the trend of the palaeoslope and parallel to the downslope direction (Table 1). Within systems dominated by LPS, the strike of axial planes will form a statistical grouping parallel to the trend of the palaeoslope and normal to the downslope direction (Table 1). The strike of slope-parallel and slope-normal axial planes will vary little during progressive deformation in LPS and LNS settings respectively, as rotation of axial planes into the shear plane will simply affect the dip of these surfaces (see the section below).
The mean axial-planar dip method (MAD)
This new method utilizes the trend of fold hinges together with the angle of dip of associated axial planes and is based on methods used by Strachan (2002) (Table 1). Statistical fanning of axial planes about the strike of the palaeoslope, with the majority of planes dipping in an upslope direction, has been long recognized (e.g. Woodcock 1979). This fanning of axial planes is typically considered to represent the effects of shearing and rotation of earlier upright folds during progressive deformation, and is consistent with overall layer-parallel shear (Alsop & Holdsworth 2004a; Strachan & Alsop 2006; Alsop & Carreras 2007; Lesemann et al. 2010). More steeply dipping axial planes are considered to have undergone less significant rotation during subhorizontal shearing, and the fold hinges associated with these steep axial planes therefore more closely preserve the original fold trend. Restricting the use of fold hinges to those associated with steeper axial planes (typically >45° relative to non-deformed beds) statistically removes fold hinges that may have undergone more significant rotation during progressive deformation. Thus, the most steeply dipping axial planes are least rotated, and the associated fold hinges should form normal to the downslope transport direction in LPS-dominated settings (Table 1). In LNS-dominated settings associated with large amounts of differential shear, the steeper axial planes are typically considered to develop subparallel to the downslope transport direction and be associated with tighter interlimb angles (e.g. Alsop & Holdsworth 2007; Debacker et al. 2009). Therefore a clear understanding of whether a system is dominated by LPS or LNS is essential.
The separation arc method (SAM)
The separation arc method (SAM) of Hansen (1971) is based on the observation that folds may display highly variable hinge orientations about the palaeoslope direction. This technique assumes that the flow direction will symmetrically bisect the acute angle between groups of folds that display opposing asymmetry or vergence (Table 1). Thus, folds that are developed clockwise of the downslope flow direction will typically be associated with differential sinistral shear, whereas anticlockwise folds are marked by differential dextral shear (for a full review see Alsop & Holdsworth 2007). The main weakness of the method is that it relies solely on extreme end-member fold orientations to constrain the arc of separation, and is thus heavily dependent on overall sampling issues (see Woodcock 1979). The method becomes non-applicable when (1) folds with opposing vergence define statistically overlapping distributions, thereby rendering the 'separation arc' with which to constrain flow inoperable, or (2) only one sense of fold vergence is observed owing to either real or apparent gross fold asymmetry, thereby once again negating any separation arc with which to constrain the flow direction (Table 1).
The axial-planar intersection method (AIM)
The axial-planar intersection method (AIM) was originally devised by Alsop & Holdsworth (2002, 2004a,b) for flow perturbation folds and subsequently applied to slumps by Strachan & Alsop (2006) and Debacker et al. (2009). The AIM utilizes the statistical orientation of fold axial planes to determine both the trend and direction of flow associated with slumping. Within LNS-dominated slump systems, axial planes will trend subparallel to the flow direction, and define a statistical fan with the mean intersection of axial planes parallel to the downslope direction (Table 1). Within LPS-dominated slump systems, axial planes will trend at high angles to the transport direction. They will define a statistical fan with the majority of axial planes variably dipping upslope (Woodcock 1979), and relatively few dipping in the downslope direction (as a result of subsequent warping and rotations). The general trend of variably dipping axial planes parallel to the strike of the palaeoslope will result in the calculated intersections between fanning axial planes also forming parallel to the strike of the palaeoslope (Woodcock 1979; Table 1).
Regional setting of the Dead Sea Basin case study
The Dead Sea Basin represents an exceptional area to analyse slump folds as it is located on the Dead Sea transform, which is marked by two parallel fault strands that generate numerous earthquakes with which to trigger slumping (e.g. Marco et al. 1996, 2003; Ken-Tor et al. 2001; Migowski et al. 2004; Begin et al. 2005; Kagan et al. 2011; Fig. 1a). This transform is thought to have been active since the Miocene (e.g. Bartov et al. 1980; Garfunkel 1981). In addition, the pronounced bathymetry of the present Dead Sea (reaching -750 m below sea level) mirrors that of the Late Pleistocene Lake Lisan (which acts as the precursor to the Dead Sea) and is the focus of the current study (Fig. 1b). As both the seismic triggering mechanism for slumping and palaeogeographical control on slump directions are equally well constrained, this provides an ideal opportunity to evaluate slump folds as indicators of palaeoslope.
The Lisan Fm comprises a sequence of finely laminated annual varve-like repetitions of aragonite and clastic-rich pairings considered to represent dry summers and winter flood events respectively (Begin et al. 1974). The Lisan Fm was deposited between 70 and 15 ka (Haase-Schramm et al. 2004) and preserves depositional dips of less than 1° indicating an absence of subsequent tilting and tectonism. The end of the last ice age brought about a sharp drop of water level at c. 15 ka, forming the hypersaline Dead Sea in the lowest place, which had also acted as the depocentre of Lake Lisan (Fig. 1b and c).
Within the Lisan Fm, folding and thrusting in distinct layers that are capped by overlying undeformed horizontal beds is relatively common (see Alsop & Marco 2011). These intraformational deformed horizons are subsequently cut by sedimentary injections, proving the syndepositional nature of the 'soft-sediment' deformation. The folded horizons are frequently capped by breccia layers, which also abut syndepositional faults indicating that the breccia layers are associated with deformation of the sea floor and are seismites (Marco & Agnon 1995; Agnon et al. 2006). The breccia layers are considered to be the final stage in a progressive sequence of deformation that initiated with laminar folds, which evolved into asymmetric billows and finally turbulent chaotic breccias (Heifetz et al. 2005; Wetzler et al. 2010). We now present new slump fold data from along the entire western shore of the Dead Sea with which to directly test if the geometry and orientation of the folds accurately reflects downslope slumping of sediments towards the Dead Sea Basin.
Detailed observations of slump fold geometries and orientations within the Lisan Fm
We have documented and recorded >350 representative folds from all the available outcrops of the Lisan Fm on the western shore of the Dead Sea from Jericho in the north to Peratzim in the south (Fig. 1c). For these measurements we utilized natural outcrops on numerous canyon walls that are incised into the Lisan Fm. To determine the true geometry of the fold hinges, we carefully excavated them in three dimensions in the little consolidated sediments, and then labelled and photographed each of them. In all photographs, west is on the left and east is on the right (unless stated otherwise), and the hammer (30 cm long), paper ruler (20 cm long) and coin (15 mm diameter) act as scales. The global positioning system (GPS) coordinates for each of the stations are provided in Table 2.
A description of slump folding from each site within the Lisan Fm down the western shore of the Dead Sea is provided below, including the orientation, geometry, scale, tightness of folds and typical fold vergence directions (Figs 1c and 4). We apply the various techniques of palaeoslope analysis described above, before summarizing the regional slump patterns along the c. 100 km transect of the western Dead Sea Basin. Results from the five techniques are analysed via mean slump directions, and also by calculating the spread of slump transport directions and taking the bisector of this range with appropriate errors noted (see Debacker et al. 2009).
Slump folds at Jericho by the northern Dead Sea are <10 cm amplitude, 15-20 cm wavelength, upright to recumbent folds, which typically tighten as they become recumbent (Fig. 5a). Upright folds are generally slightly more angular with some evidence of brittle deformation in the hinge associated with limb failure (Fig. 5b). Fold hinges are NE-SW-trending with axial planes displaying a range of shallow to steep NW-dipping orientations, although the NE-SW strike remains relatively constant (Figs 4a and 6a, Table 2). Fold facing directions are consistently up towards the SE, subparallel to the calculated palaeoslope direction towards 134° (Table 2). The range of transport directions calculated from the five techniques varies between 127° and 140° with an overall mean bisector towards 134 ± 7° (Table 2).
Slump folds at Almog by the northern Dead Sea are <1 m amplitude, <1 m wavelength, upright to recumbent folds, which tighten as they become overturned (Fig. 5c and d). Exposures at Almog also preserve the highly irregular top surface of a slump, which is draped by overlying undeformed sediments that display pronounced thickening and thinning over the slumped horizon (Fig. 5c). Fold hinges are typically north-south-trending, although they display an arc of orientations (Figs 4b and 6a). Axial planes show a range of shallow to steep west-dipping orientations, although the north-south strike remains relatively constant (Figs 4b and 6a, Table 2). Fold facing is marked by a range of NE to SE directions, although the mean is parallel to the calculated palaeoslope direction towards 091° (Table 2). The range of transport directions calculated from the five techniques varies between 085° and 096° with an overall mean bisector towards 091 ± 6° (Table 2).
Slump folds at En Gedi by the central Dead Sea are <0.5 m amplitude, <0.5 m wavelength, recumbent folds with rounded hinges that display distinct asymmetry with thinning of fold limbs relative to the hinge (Fig. 5e and f). Fold hinge orientations are widely dispersed, although they are generally SE-NW- to SW-NE-trending and typically ENE-verging (Figs 4c and 6b, Table 2). Axial planes are SSEstriking and dip gently towards the west, with associated fold facing up and towards the ENE (068°), subparallel to the calculated palaeoslope direction (Figs 4c and 6b, Table 2). The range of transport directions calculated from the five techniques varies between 043° and 080° with an overall mean bisector towards 062 ± 19° (Table 2). The larger spread (37°) in calculated slope directions is considered to reflect the relatively small dataset at this site.
Slump folds at Masada by the central Dead Sea are up to 2 m amplitude and 2 m wavelength, although more typically <0.25 m (Fig. 5g). They display a range of upright to sub-recumbent attitudes marked by well-defined asymmetry and vergence (Fig. 5g). Largerscale thrusts associated with easterly directed thrusting display up to c. 5 m displacement with back-rotation of the thrust slice being accommodated by folding of up to 5 m of overlying sediment (Fig. 5h). These observations suggest that, in some cases, significant thicknesses and volumes of sediment may be affected by single slump events. Fold hinges are concentrated around a NNW-SSE trend and verge consistently towards the east (Fig. 4d, Table 2). Axial planes are SSE-striking and dip gently towards the west, with associated fold facing up and towards the ENE (076°), subparallel to the calculated palaeoslope direction (Figs 4d and 6b, Table 2). The range of transport directions calculated from the five techniques varies between 073° and 083° with an overall mean bisector towards 078 ± 5° (Table 2).
Slump folds at Peratzim by the southern Dead Sea are typically <1 m wavelength and form in packages less than 1 m thick, although some thrust sequences can be 2 m thick. Folds are upright to more typically recumbent with rounded hinges (Fig. 5i and j). Fold hinges are statistically distributed around a broad arc, although fewer hinges are recorded in a NE-SW orientation (Figs 4e and 6c, Table 2). Axial planes are SE-NW-striking and typically dip gently towards the SW, although steeply SW-dipping axial planes are also recorded (Figs 4e and 6c, Table 2). This range of fold hinge and axial-planar orientations is considered a consequence of their rotation towards the downslope transport direction during progressive deformation associated with downslope slumping (see Strachan & Alsop 2006). A minority of axial planes are gently NE-dipping and may be associated with marginally (typically <10°) downward-facing folds, possibly related to gentle warping and refolding during slumping. The vast majority (>94%) of folds face up towards the NE (Table 2). Fold axes associated with axial planes that dip moderately to steeply SW (>45°) are all SE-NW-trending (mean 124°), suggesting that the palaeoslope direction would be towards 034° (using mean axial-planar dip) (Fig. 6c and d, Table 2). The general facing direction, although displaying an arc of orientations, is broadly unimodal towards 042° and subparallel to the assumed palaeoslope direction (Figs 4e and 6c, Table 2). The range of transport directions calculated from the five techniques varies between 034° and 046° with an overall mean bisector towards 040 ± 6° (Table 2).
Regional patterns of radial slumping towards the Dead Sea Basin
On a regional scale, our results show that the Late Pleistocene Lisan Fm on the western side of the Dead Sea displays SE-directed slumping in the north (Fig. 6d and e), easterly directed slumping in the central portion and NE-directed slumping at the southern end of the Dead Sea (Fig. 6d and f). The direction of slumping inferred from the fold and thrust geometries therefore systematically varies along the entire length of the western Dead Sea (Fig. 6d).
Seilacher (1984) and El-Isa & Mustafa (1986) discussed softsediment deformation structures from within the Lisan Fm exposed along the eastern side of the Dead Sea. They described folds with amplitudes up to 15 cm and wavelengths of 17 cm, which are fairly constant within single localities. On the eastern side of the Dead Sea, El-Isa & Mustafa (1986) described the inclination of detachments as 'mostly, if not always, westwards' and minor folds as displaying 'systematic westward inclination' relating to westdirected vergence (Fig. 6d). Such folded horizons are capped by undeformed beds both above and below, and were attributed by El-Isa & Mustafa (1986) to earthquakes triggering slumps on very gentle westerly dipping slopes. Collectively, the directions of slumping ascertained from folds and thrusts within the Lisan Fm exposed around the western and eastern shores of the Dead Sea define a pronounced radial pattern of slumping towards its depocentre (Fig. 6d).
Regional fold vergence and facing patterns
Lake Lisan was bounded by the same fault system that bounds the Dead Sea, with the studied outcrops of the Lisan Fm typically within 5 km of the Dead Sea Fault (Figs 1 and 6d). The present bathymetry, which is similar to that of Lake Lisan, is shown in Figures 4 and 6d. These maps highlight the position of the studied localities on very gentle slopes, with the main depocentre located further to the east in the northern part of the Dead Sea. This depocentre, which is marked by a sharp bathymetric break, is considered to be fault bounded (e.g. Lazar et al. 2006). Bedding dips within the Lisan Fm are <1° and reflect original depositional slopes within Lake Lisan. Slopes of 1° are typically considered sufficient to develop slumping within many subaqueous settings (e.g. Lewis 1971; Almagor & Garfunkel 1979; Canals et al. 2004), and seismicity has also been recognized as a trigger for slumping within lake sediments (e.g. Doig 1991; Doughty et al. 2010). The direction of palaeoslope inferred from slump folding defines a radial pattern directed towards this depocentre. A coherent regional slump pattern across the Dead Sea indicates a lack of secondary tectonic control on slope directions such as associated with locally tilted fault blocks.
Fold vergence on the western shore of the Dead Sea is systematically (>90%) towards the east. Furthermore, mean fold facing directions display a unimodal distribution (>95% face up towards the east) and are consistently subparallel to the mean transport direction calculated from the five techniques at each site. This marked consistency in both fold vergence and facing directions is to be expected in areas where LPS dominates (Alsop & Holdsworth 2004b, 2007). Within LPS systems, the strike of fold axial planes will parallel the trend of the palaeoslope and dips are typically directed upslope (Fig. 6e and f). Axial-planar dips directed to the east (downslope) may be associated with subsidiary west-vergent folds, or subsequent rotations through the horizontal with continued movement and contraction. The small percentage (<10%) of folds verging towards the west up the regional slope reflect secondary structures generated by local slumping off the bathymetric 'highs' generated by the crests of underlying antiforms and thrust culminations (Alsop & Marco 2011). In areas where differential layer-normal shear operates, folds with opposing vergence are typically developed in equal proportions around classic flow perturbations. The strike of fold axial planes in such systems will parallel the transport direction. The extreme asymmetry of fold vergence (>90% verging east) and axial-planar dip directions indicates the dominance of layer-parallel shear in the present case study (Figs 4-6, Table 2).
Wetzler et al. (2010) suggested that the Kelvin-Helmholtz instability, where folding is triggered by shear between stably stratified layers, is a plausible mechanism for folding in the Lisan Fm as it requires only a small perturbation in the interface between sheared layers. This shear instability is considered to grow and evolve through the folding stages up to a turbulent breccia (see Wetzler et al. 2010; Alsop & Marco 2011). Wetzler et al.'s model does not attempt to explain all of the folds and those workers did not analyse fold vergence or transport directions. Our work clearly now demonstrates that the sense of fold vergence and associated transport direction is dictated by the palaeoslope.
The use of slump folds as indicators of palaeoslope
Relationships between slump folds and palaeoslope orientations may be complicated by a number of factors including the following: (1) variable angles of fold initiation; (2) variable amounts of fold hinge and axial-planar rotation towards the transport direction; (3) variable overprinting relationships between adjacent slumps; (4) variable slope or transport directions that may evolve with time. These complications are very similar to many of those encountered in ductile metamorphic shear zones, where the concepts of progressive shearing coupled with flow perturbations have allowed meaningful deformation analyses to be undertaken (e.g. see review by Alsop et al. 2010).
Within this case study, there is little evidence of folds initiating at variable angles to the downslope transport direction. This indicates that deformation is dominated by LPS, perhaps reflecting the layer-cake stratigraphy of the Lisan Fm, which lacks lateral facies changes that would encourage along-strike partitioning of flow (see discussion by Alsop & Marco 2011). Thus, fold hinges and associated axial planes typically initiate at high angles to the downslope direction. In addition, fold hinge and axial-planar rotation towards the transport direction during progressive deformation appears relatively limited, and mostly restricted to intensely deformed horizons at Peratzim where some curvilinear folding is observed (Figs 4e and 6f). We attribute the development of curvilinear folds at Peratzim as reflecting an increased component of mud-rich units that are relatively weak and would encourage more intense folding to form (see Alsop & Marco 2011, p. 453, for full details). Overall, fold hinges and axial planes maintain high angles to the slope direction, although rotation of axial planes about their strike allows fanning geometries to be utilized in slope analysis. There is also little evidence of overprinting relationships between laterally adjacent slumps that may otherwise result in reworking and reorientating of slump folds.
Recent slump analogues from seismic reflection and sea-floor data
Recent submarine examples of slumped zones include the Humboldt Slide, where large bowl-like depressions or 'ampitheatres' are imaged (e.g. Lee et al. 2007). In this case, the bathymetric ridges trend directly across the basin and do not parallel the geometry of the bowl. However, these ridges form at wavelengths of hundreds of metres and are several orders of magnitude larger than folds described from the Lisan Fm. In addition, they may relate to sediment wave fields rather than landslides or slumping processes (see Gardner et al. 1999, and discussion by Lee et al. 2007).
Detailed analysis of seismic reflection data from sub-lacustrine landslides reveals radial patterns of slope failure that converge towards the centre of a c. 2.5 km diameter volcanic crater lake (Moernaut & De Batist 2011). Slumped units extend for up to 500 m and display extensional faulting towards the upslope margins and contractional folds and thrusts in the toe of the slumps towards the downslope centre of the basin. This pattern is repeated at different stratigraphic levels with palaeoslope dips of just c. 2° to form a coherent radial system. Angles of palaeoslope, coupled with the extent of single slumps and the overall radial pattern within a confined basin setting dominated by lacustrine sediments, allow analogies to be drawn with the Lisan Fm case study. Although details of folding and faulting are limited by seismic resolution, our direct observations of slump folds and their associated regional patterns allow greater controls and confidence to be placed on such seismically imaged systems.
In summary, many of the factors that create complexity within slump systems are absent or minimized within the Dead Sea Basin, thereby allowing original and pristine relationships to be observed and investigated. The clear control exerted by the geometry of the pronounced Dead Sea Basin from the Late Pleistocene, when the Lisan Fm was deposited, to the present day, when bathymetry reaches -750 m, has allowed us to evaluate the viability of slump folds as indicators of slope direction. Importantly, we can directly observe how transport directions vary around a relatively simple and recent system, where the palaeogeographical control is still evident. Our direct field observations also allow greater confidence to be placed in seismically imaged structures defining convergent and radial systems of slumping.
Detailed fieldwork in the Dead Sea Basin has allowed us to objectively test if slump folding accurately reflects palaeoslope orientations over a regional scale. The results of this work clearly indicate that the sediments along the present western shore of the Dead Sea slumped downslope towards the Dead Sea Basin at c. 18 ka. We employ five methods of slump transport analysis and note that they all provide consistent results that are typically within 15° of one another at each site. A greater appreciation of the variability that may develop around simple modern basins, such as described here, may permit a better understanding of the complexities observed in ancient and only partially exposed basins where the palaeogeographical setting may be poorly understood, and may also be masked by subsequent tectonics.
This case study represents one of the few regional analyses of slump fold datasets and allows us, for the first time, to recognize a large-scale radial slump system directed towards the depocentre of the Dead Sea as clearly illustrated by bathymetric maps (Fig. 6d). SE-directed slumping is preserved in the north, easterly directed slumping in the central portion, and NE-directed slumping at the southern end of the Dead Sea. In detail, >90% of fold hinges verge broadly towards the east, >95% of fold hinges face upwards towards the east, and >90% of axial planes dip towards the west. These consistent relationships support flow perturbation models in which transport-normal fold hinges are generated by layer-parallel shearing during broadly east-directed slumping. This radial slump pattern highlights not only the reliability and robustness of techniques, but also the variability of slump directions, which vary by more than 90° along the c. 100 km western shore of the Dead Sea. The pristine depositional dips of <1° within the Lisan Fm demonstrate that local tectonics has not significantly affected transport directions with time. Thus, although slumps within the Lisan Fm exposed along the entire length of the Dead Sea Basin may be of slightly different (Late Pleistocene) ages, they portray an overall coherent radial pattern because of the absence of local tilting. Assuming that the slumps were triggered by earthquakes that were distributed all over the basin (much like the recent activity), the pattern proves that the shape, and in particular the vergence, of deformed layers is governed by palaeoslope.
We thank J. Levy, together with the Carnegie Trust and the Royal Society of Edinburgh, for travel grants to I.A., and the Israel Science Foundation for grant 1539/08 to S.M. We would like to thank N. Woodcock and T. Debacker for detailed and constructive reviews of this paper, and J. Marshall for careful editing. S.M. would like to acknowledge the Department of Earth Sciences at Durham University for hosting him and facilitating completion of this paper.
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Received 8 March 2011; revised typescript accepted 5 July 2011.
Scientific editing by John Marshall.
G. IAN ALSOP1* & SHMUEL MARCO2
1Department of Geology and Petroleum Geology, School of Geosciences, University of Aberdeen, Aberdeen AB 3UF, UK
2Department of Geophysics and Planetary Sciences, Tel Aviv University, Tel Aviv 69978, Israel
*Corresponding author (e-mail: firstname.lastname@example.org)…
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Publication information: Article title: A Large-Scale Radial Pattern of Seismogenic Slumping towards the Dead Sea Basin. Contributors: Alsop, G. Ian - Author, Marco, Shmuel - Author. Journal title: Journal of the Geological Society. Volume: 169. Issue: 1 Publication date: January 2012. Page number: 99+. © Geological Society Publishing House Jan 2009. Provided by ProQuest LLC. All Rights Reserved.