Salt Tectonics at Passive Margins: Geology Versus Models

Marine and Petroleum Geology 28 (2011) 1123e1145

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Marine and Petroleum Geology

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Review Article Salt tectonics at passive margins: Geology versus models

Jean-Pierre Brun a,*, Xavier Fort b a Géosciences Rennes, Rennes, France b G.O. Logical Consulting, Rennes, France article info abstract

Article history: Salt tectonics at passive margins is currently interpreted as a gravity-driven process but according to two Received 4 September 2010 different types of models: i) pure spreading only driven by differential sedimentary loading and ii) Received in revised form dominant gliding primarily due to margin tilt (slope instability). A comparative analysis of pure 1 March 2011 spreading and pure spreading is made using simple mechanics as well as available laboratory experi- Accepted 7 March 2011 ments and numerical models that consider salt tectonic processes at the whole basin scale. To be Available online 12 March 2011 effective, pure spreading driven by sedimentary loading requires large differential overburden thicknesses and therefore significant water depths, high sediment density, low frictional angles of the sedi- Keywords: fl Salt tectonics ments (high uid pore pressure) and a seaward free boundary of the salt basin (salt not covered by fi Gravity-driven deformation sediments). Dominant gliding does not require any speci c condition to be effective apart from the dip Pure spreading on the upper surface of the salt. It can occur for margin tilt angles lower than 1 for basin widths in the Dominant gliding range of 200e600 km and initial sedimentary cover thickness up to 1 km, even in the absence of Angolan Margin abnormal fluid pressure. In pure spreading, salt resists and sediments drive whereas in dominant gliding Gulf of Mexico both salt and sediments drive. In pure spreading, extension is located inside the prograding sedimentary Model wedge and contraction at the tip. Both extension and contraction migrate seaward with the sedimentary Tilted blocks progradation. Migration of the deformation can create an extensional inversion of previously contrac- Diapir tional structures. In pure spreading, extension is located updip and contraction downdip. Extension Rollover migrates downdip and contraction updip. Migration of the deformation leads to a contractional inversion of previously extensional structures (e.g. squeezed diapirs). Mechanical analysis and modelling, either analogue or numerical, and comparison with margin-scale examples, such as the south Atlantic margins or northern Gulf of Mexico, indicate that salt tectonics at passive margins is dominated by dominant gliding down the margin dip. On the contrary, salt tectonics driven only by differential sedimentary loading is a process difficult to reconcile with geological evidence. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction origins of differential stresses, namely margin tilting and/or differential sedimentary loading. Therefore, passive margins are Numerous passive margins display thick layers of evaporites, favourable natural laboratories to study the dynamics of salt among which halite (salt) dominates, that are most often deposited tectonics, in the absence of crustal-scale tectonics. during and/or immediately after continental rifting. They When the only tools available to study salt-related structures commonly display deformations over periods as long as 100 Ma or were surface geology and drilling, attention was focused on salt more, as recorded by syntectonic sedimentation in extension or diapirs and salt tectonics was mostly perceived as a deformation contraction (Fig. 1). Most often, these deformations are still active. involving dominantly vertical displacements e i.e. salt upwelling Such salt basins are extremely unstable because salt is a very weak from a source layer buried below a sedimentary cover. The explo- material that is able to ﬂow under very low differential stresses, ration of passive margins became possible with the fast techno- even at surface temperature conditions. In many salt margins logical development of seismic imaging, thereby allowing the where no regional tectonic effects are recorded, the observed identiﬁcation of “tectonic rafts” in the Angolan margin (Burollet, deformations are obviously gravity driven with two often-invoked 1975). The nearly horizontal seaward displacements of these tectonic rafts on top of salt have controlled rollover-type sedimentary depocentres (Fig. 1). Today, this discovery appears to have * Corresponding author. been a breakthrough in the understanding of salt-related defor- E-mail address: [email protected] (J.-P. Brun). mation processes. “Raft tectonics” is the most extreme form of

Fig. 1. Composite regional-scale cross-section of the north Angolan margin showing the extensional and contractional deformation domains and the type of individual structures. See location of the two sub-lines 1 and 2 in the lower left insert (modified after Fort et al., 2004a). The two enlargements of the cross-section show the growth character of structures since 100 Ma, in extension to the right and contraction to the left. thin-skin extension, in which horizontal seaward displacements Peel et al., 1995; Hall, 2002), in conceptual models (Schuster, 1995; dominate (Duval et al., 1992). It is a gravity-driven process corre- Rowan, 1995) as well as in experimental and numerical dynamic sponding to the gliding of the saltesediment package primarily models (Talbot, 1992, 1993; Vendeville, 2005; Gaullier and stimulated by margin tilt, as modelled in laboratory experiments Vendeville, 2005; Gradmann et al., 2009). The concept of salt (Cobbold and Szatmari, 1991; Demercian et al., 1993; Gaullier et al., tectonics dominantly driven by differential sedimentary loading, 1993; Mauduit et al., 1997; Fort et al., 2004a,b). Rafts form at early which was elaborated upon in the GoM, has been tentatively stages of extension by brittle faulting of thin sedimentary layers, applied to other passive margins including those of western Africa generally less than 1 km thick, deposited on top of the salt (Lundin, (e.g. Fig. 10 in Hudec and Jackson, 2002; discussion in Hudec and 1992; Liro and Cohen, 1995; Spathopoulos, 1996; Rouby et al., Jackson, 2004), and Brazil (e.g. Szatmari et al., 1996; Fig. 17 in 2002). Seismic data in the ultra-deep part of the margins has Ge et al., 1997). shown that updip extension was coeval with either thrusting and Consequently, two different types of dynamic models are pres- buckling of the sedimentary cover downdip or flowing of frontal ently invoked to explain gravity-driven salt tectonics at passive salt nappes on top of the abyssal plain (Cobbold and Szatmari, 1991; margins: i) gravity spreading driven only by differential sedimen- Cobbold et al.,1995; Marton et al., 2000; Cramez and Jackson, 2000; tary loading and ii) gravity gliding primarily due to margin tilt. The Tari et al., 2001; Gottschalk et al., 2004; Brun and Fort, 2004; Fort difference between the two types of explanations or the choice et al., 2004a). Even if sedimentary loading participates in the between one over the other are not purely issues of academic gliding process, its role is not a major one as salt is able to flow interest because the two types of processes have significantly downdip without any sedimentary loading. different patterns of displacement and different modes of rela- The northern Gulf of Mexico (GoM) presents a dual situation tionships between deformation and sedimentation, with obvious (Worrall and Snelson, 1989). The northwest margin (Texas) shows consequences for hydrocarbon exploration. a whole picture that is somewhat comparable to south Atlantic Our aims in the present paper are first to compare the two types margins with an updip extension characterised by long and straight of models on the basis of simple mechanics and modelling results normal faults and a downdip contraction characterised by a well and second to discuss their applicability to salt tectonics at passive developed fold belt (Peel et al., 1995). The north margin (Louisiana) margins. We identify structures, or associations of structures, that displays a more complicated situation with short and curved can be considered as diagnostic of each of the two processes as well normal faults, dipping in various directions, and a massive extru- as those that are ubiquitous and therefore not significant for any of sion of allochtonous salt bodies, often several kilometres thick, on them. Finally, we discuss the reasons why the GoM is most often top of which Neogene minibasins are deposited, sometimes considered as being different. reaching depths of 6000 m. In the slope area, this results in shallow salt structures whose southern border, against the abyssal plain, 2. Salt basins displays a spectacular lobate pattern. For almost 20 years, this major structural difference in salt 2.1. Deposition of thick salt layers tectonic patterns between the GoM north margin and South Atlantic margins has drawn attention. In terms of processes, Thick accumulation of evaporites requires specific conditions. whereas salt tectonics in Atlantic-type margins primarily corre- Seawater evaporation progressively concentrates brines whose spond to gliding stimulated by margin tilt, almost all (if not all) volume reduction is responsible for a drop of the water surface authors argue that spreading due to sedimentary loading is the below the worldwide sea level. Consequently, the salt basin must main driving force of salt tectonics in the Gulf of Mexico at once in be isolated from the ocean, at least temporarily. The evaporation of regional reviews (e.g. Worrall and Snelson, 1989; Diegel et al., 1995; 1000 m of seawater does not give more than 15 m of evaporites and J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145 1125 consequently, the evaporation of at least 66,000 m of seawater is sub-basins separated by basement highs but which can remain necessary to produce a 1000 m thick layer of evaporites. As at connected e e.g. Angola: Hudec and Jackson, 2004 (see Fig. 2). Such passive margins, evaporite layers sometimes reach several thou- basement highs are likely inherited from normal faulting and sand metres and salt basins cannot result only from the single variations in the amount of crustal stretching related to rifting. evaporation of one body of seawater, regardless of its depth. However, some basement deformation can also occur after salt A quasi-constant seawater refill of the basin is needed. For a full basin deposition e e.g. extensional in the North Sea (Nalpas and discussion of depositional environments leading to several kilo- Brun, 1993; Stewart and Coward, 1995; Stewart et al., 1997). metre-thick accumulations of evaporites, see the recent syntheses by Rouchy and Blanc-Valleron (2006) and Warren (2006). 2.3. Initial sedimentary cover above salt Among the modes of lithosphere necking that can give birth to passive margins, depth-dependent stretching (see review by Davis In all salt margins where the seaward salt pinch-out is clearly and Kusznir, 2004) leads to an extremely attenuated crust with imaged, the most distal part of the salt basin appears to be covered thermal subsidence histories that favour shallow water deposition by sediments soon after the end of evaporitic sedimentation, as of evaporites under appropriate climatic conditions (Huismans and exemplified in most Atlantic passive margins (e.g. West Africa: Beaumont, 2008). Marton et al., 2000; Tari et al., 2001; Brazil: Cobbold and Szatmari, 1991; Demercian et al.,1993; Davison, 2007) as well as in the Gulf of 2.2. Salt basin size and geometry Mexico (e.g. Peel et al., 1995; Trudgill et al., 1999; Grando and McClay, 2004). This indicates that an initial sedimentary cover is The mean width of salt basins in the margin dip direction is in generally deposited on top of the salt at the scale of the whole the range of 200e400 km, as illustrated in the South Atlantic (see basin, soon after salt deposition. Fig. 1 in Davison, 2007). However, some salt basins could have In many salt margins, the initial position of the top salt surface initially been narrower e e.g. in Nova Scotia: possibly less than can be deduced from the inferred location of landward and seaward 100 km (Ings and Shimeld, 2006; Albertz et al., 2010). Other basins salt pinch-outs, as illustrated in Figure 2a. As the top salt surface is are obviously larger e e.g. Gulf of Mexico: between 500 and 800 km initially close to horizontal, the straight line joining the two pinch- (Worrall and Snelson, 1989). outs allows a direct and accurate calculation of the margin tilt (e.g. The initial basin geometry is often not well assessed. This is in 1.17 in Fig. 2c). It also provides a good estimate of the initial salt particular due to the difficulty to clearly image the base salt, but basin geometry and volume, especially if the geometry of the base also applies to narrow basins like in Nova Scotia and large basins salt is known. In the particular case of the Angolan margin, it is like in the Gulf of Mexico. Some attempts to reconstruct the initial interesting to note that the initial salt surface passes slightly above geometry of salt basins have led to bulk double-wedge shapes at a central basement high, indicating that the salt basin at the margin the margin scale, with the salt pinching out landward and seaward scale was initially divided into two connected sub-basins and that and with a maximum salt thickness located toward the seaward both sub-basins have been tilted by the same angle (Fig. 2a). In basin termination e e.g. Angola: Marton et al., 2000; Brazil: Quirk addition, the comparison between the initial top salt surface and et al., 2011. In detail, the basin geometry can be more compli- final saltesediment interface help check the conservation (as in cated with the general double-wedge shape divided into several Fig. 2c) or non-conservation of the salt volume and the amount of

Fig. 2. Regional section across the Kwanza Basin in South Angolan Margin. (a) Vertically exaggerated section. (b) Same section with no vertical exaggeration (a and b modiﬁed after Hudec and Jackson, 2004). (c) Simpliﬁed section showing the relative amounts of sediments and salt below and above initial top salt surface. 1126 J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145 salt transfer at the margin scale. Using a similar argument, the salt initial sedimentary layer, often called a prekinematic layer, whose volume also seems to be conserved on the Brazilian margin thickness is often around several hundred metres, but which can (Davison et al., 2011). reach a kilometre or more locally. In such circumstances, the frontal (seaward) part of the salt basin cannot be easily modelled as a free 3. Modelling of salt tectonics at passive margins boundary.

The geometrical and geological features of margin salt basins 3.2. Boundary conditions summarised in the previous section provide important constraints for the setting up of the initial and boundary conditions of salt In the absence of tectonic displacements applied to the salt tectonic models applicable to passive margins (Fig. 3). basin basement, two types of boundary conditions must be considered: i) margin tilt and ii) differential sedimentary loading. 3.1. Initial conditions Margin tilt is a consequence of either continuing rifting or post- rift thermal subsidence and depends on the margin width. At the At passive margins, the salt basin width is commonly in the end of subsidence, the tilt can reach values between 1 and 2 for range of 200e400 km. As salt is most often pinched out landward margin widths of several hundred kilometres (e.g. Fig. 2). For nar- and seaward, the salt thickness is not constant and can reach rower margins, it can reach values up to 3e4 (Mauduit et al., 1997). several thousand metres inside the basin. A shallow water envi- Synkinematic sedimentation (sedimentary loading) cannot ronment of thick salt deposition implies a horizontal top salt exceed water depth and therefore is directly dependant on margin surface, close to the global sea level. tilt. This implies that the role of sedimentary loading is minimum at In very speciﬁc salt deposition conditions e e.g. in the Medi- the onset of the process and increases with progressive tilting. terranean during the Messinian salt crisis, in continental rifts or in the Red Sea e diachronous salt deposition can occur in several 3.3. Two categories of models small basins with different levels of top salt (see reviews in Rouchy and Blanc-Valleron, 2006; Warren, 2006). In such basins e e.g. Available numerical and physical models of salt tectonics at Messinian e the local sea level can be well below the global sea passive margins fall into two categories (Fig. 3b and c): i) pure level. In addition, small and separated salt basins with different gravity spreading driven only by sedimentary loading and ii) levels of top salt are too small to result in salt tectonics processes dominant gravity gliding primarily driven by margin tilt. Because like those observed at the scale of whole passive margins. dominant gravity gliding is also accompanied by sedimentation, On the Brasilian margin, salt appears to have deformed prior to margin tilt is the main difference between the two categories of the deposition of the sedimentary cover indicating a margin tilt at models, in terms of boundary conditions. A single ductile layer an early stage of evolution (Fiduk, 2010; Davison et al., 2011; Quirk (Brun and Merle, 1985) or a brittle-ductile two-layer system such as et al., 2011). More commonly, the salt basin is entirely covered by an sediments lying on top of salt (Mauduit et al.,1997) that ﬂows down

Fig. 3. Setting of salt basins at lithosphere scale at the end of rifting (a) and the two categories of salt tectonic models: Pure spreading (b) and dominant gliding (c). J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145 1127

Fig. 4. Set-up of gravity spreading models driven only by differential sedimentary loading. The salt basin is overlain by an initial sedimentary cover of thickness h. s1 and s3 are the principal stresses at the tip of the prograding sedimentary wedge of thickness Dh (overburden). a slope under its own weight undergoes a combination of gliding McClay et al., 1998; Vendeville, 2005; Gaullier and Vendeville, and spreading. For simplicity, this is here referred to as “dominant 2005; Krézsek et al., 2007) or numerical modelling (Cohen and gliding” instead of “gliding-spreading”. Hardy, 1996; Gemmer et al., 2004; Ings et al., 2004; Ings and Before any detailed analysis is presented, it must be noted that Shimmeld, 2006; Gradmann et al., 2009) have been carried out to in pure spreading, salt starts to flow seaward as it is expulsed from characterise the mechanical behaviour of salt tectonic systems below the prograding sedimentary wedge whereas in gliding, the driven by the progradation of a sedimentary wedge on top of a salt entire salt layer flows downdip (Fig. 3b and c). In other words, salt basin, either covered by or not covered by sediments from the resists in spreading whereas it drives in gliding. This major differ- outset. In all these models (Fig. 4), a horizontal layer of viscous ence requires a separate treatment and justifies the making of material e i.e. salt e is submitted to the progressive loading of a clear distinction between pure spreading and dominant gliding. a prograding sedimentary wedge. Salt is expulsed frontward and See Appendix 1 for further details. the wedge spreads. Figure 5 presents an experiment of this type. The initial model is 4. Pure spreading models: salt tectonics only driven by 150 cm long and made of a 2 cm thick layer of silicone putty differential sedimentary loading covered by a 1 cm thick layer of sand. Both layers are horizontal and have a constant thickness. The initial geometry of the model 4.1. Laboratory experiments represents a margin salt basin prior to the arrival of a prograding sedimentary wedge. Sedimentation starts at the “landward” model As already mentioned, the concept of salt tectonics only driven end and is done at regular time intervals with a step by step pro- by differential sedimentary loading was inspired by studies carried gradation toward the “seaward” model end. At the end of experi- out in the Gulf of Mexico (Worrall and Snelson, 1989; Diegel et al., ment, the model is wetted and vertical sections are cut to examine 1995; Peel et al., 1995; Rowan, 1995; Schuster, 1995) where the salt the internal structures (Fig. 5a). Top views of the model surface is very thick and sedimentation rates were rather high in the (Fig. 5b) show the progressive evolution of deformation during Neogene. Different types of laboratory experiments (Ge et al., 1997; prograding sedimentation. The general principles of small-scale

Fig. 5. Experimental model of pure spreading simulating the progradation of a sedimentary wedge on top of a salt basin initially covered by sediments (see set-up in Fig. 4). The four top views show the progressive evolution of structures at surface during sedimentary progradation; Photographs were taken at 0, 20, 40 and 70 h. The section shows the ﬁnal state of structures in the middle of the model. Numbers indicate the sequence of development of contractional structures. 1128 J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145

Fig. 6. Experimental models of pure spreading simulating the progradation of a sedimentary wedge on top of a salt basin initially covered by sediments in two main stages. Sections show contractional structures inverted in extension during the progradation of a second sedimentary wedge. Upper and lower diagrams modified after Vendeville (2005) and McClay et al. (1998), respectively. modelling and the physical properties of the materials used to a function of the thickness h of sediments deposited on top of salt model the salt tectonic systems are given in Appendix 2. that is required to reach system failure. To keep the calculations as After 20 h, extension starts to develop inside the wedge with simple as possible, we assume that salt behaves like an inviscid a contractional domain formed at its tip (Fig. 5b). The top views at fluid e i.e. negligible salt viscosity. By ignoring the resisting effect of 40 h and 70 h show the progressive frontward migration of both salt ductile flow, the effect of sedimentary loading is slightly extension and contraction. The model section (Fig. 5a) shows that overestimated. both extensional and contractional structures were progressively At the future site of contractional failure (Fig. 4), at the wedge sealed by the newly deposited sediments and that extension and tip, the horizontal stress sh depends on the pressure gradient that contraction propagated frontward with sedimentary progradation. results from the difference between the density of sediments rs and This experiment, as well as those previously done by other water rw. Considering that the principal stresses are either close to authors (McClay et al., 1998; Vendeville, 2005)(Fig. 6a and b), vertical or horizontal, the horizontal stress at the wedge tip sh ¼ s1, shows that extension develops coeval with contraction at the tip of with s1 s2 s3, is: the prograding wedge. The sedimentary wedge deposited at one model end exerts a differential loading on top of the initial sand- s ¼ s ¼ðr r Þ D : h 1 s w g h (1) silicone two-layer system. At the tip of the sedimentary wedge, the In the sedimentary cover the vertical stress s ¼ s is: initial sand cover is submitted to a horizontal stress sh that is v 3 proportional to the loading applied by the wedge on top of the s ¼ s ¼ r D þ r : silicone layer. With ongoing sedimentation, sh progressively v 3 wg h sgh (2) increases until it becomes large enough to overcome the strength of The MohreCoulomb relation between the principal stresses s the initial sedimentary cover. This triggers a zone of contraction at 1 and s and the angle of friction f in compression being (for details, the wedge tip and a simultaneous wedge collapse backward. In 3 see Appendix 2 in Nalpas and Brun, 1993): other words, the sedimentary wedge spreads under its own weight if the sedimentary loading is large enough to overcome the s ¼ Ks ; (3a) strength of the sedimentary cover. This experiment, like those 1 3 shown in Figure 6, illustrates the basic mechanisms by which where sediments deposited on top of a horizontal salt basin could be able to drive the gravity spreading of a saltesediment pile. K ¼ð1 þ sin fÞ=ð1 sin fÞ; (3b) The experiments shown in Figures 5 and 6 illustrate that sedi- the combination of equations (1)e(3) gives: mentary progradation leads to a frontward (seaward) migration of contraction. During progradation, contractional structures are first D ¼ r =½r r ð þ Þ: h K sh s w K 1 (4) sealed by newly deposited sediments and can be later reactivated D ¼ (inverted) in extension (Fig. 6a and b). Equation (4) shows that in absence of water h Kh. In laboratory experiments where the viscous layer representing the salt is 4.2. Stresses at failure entirely covered by a constant thickness layer of sand with a frictional angle f ¼ 30, Dh ¼ 3 h. The experiments (Figs. 5 and 6) effectively show that failure occurred for an overburden that was Experiments (Figs. 5 and 6) show that the thickness of the D sedimentary overburden Dh necessary to activate gravity spreading always more than three times thicker ( h) than the initial sand is several times thicker than the initial sedimentary cover h (Fig. 4). cover thickness (h). D > In order to apply this to natural systems, because sedimentation In addition, for h 0 in equation (4), the right hand term must requires a certain water depth, the effects of water must be taken also be positive: into account. This can be easily done in numerical models but not in r r ðK þ 1Þ > 0; (5) laboratory experiments. As pointed out by Gemmer et al. (2005), s w the presence of water has several effects among which, in partic- or: ular, is the increase of solid and fluid pressure in the sediments. In < ðr =r Þ : the following, we consider the minimum value of overburden Dh as K s w 1 (6) J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145 1129

f f e Density (kg/m3) eff is taken as a simple reduction of the angle of friction e.g. 2000 2200 2400 2600 feff ¼ 20 in Ings et al. (2004) e or it is calculated using an arbitrary 0 value of fluid pressure l e e.g. sin feff ¼ (1 l)sin f where l ¼ pf/sv, Salt density pf is the pore pressure ratio of Hubbert and Rubey (1959),in Gradmann et al. (2009). Using the recent compilation of sediment density made in the Gulf of Mexico (Fig. 7)(Hudec et al., 2009), it is possible to calculate that the mean density of a sedimentary 1000 column is around 2000 and 2220 kg m 3 at depths of 1000 and 3 4000 m, respectively. For rs ¼ 2200 kg m , equations (3b) and (6) give f 5.2. Taking a slightly higher value of f ¼ 6, equation (4) Mean gives Dh ¼ 30.8 h. This indicates that extremely thick amounts of sediments are required to stimulate spreading above a horizontal 2000 sediment salt basin that is entirely covered by sediments under the sea level, density even if the frictional angle is very low. Moreover, as the shear traction at the sedimentesalt interface is not taken into account in the above calculations e i.e. salt is considered as an inviscid fluid e 3000 the sedimentary loading Dh must be even larger than the calculated value. In agreement with this conclusion, it is noteworthy that the most recent numerical models consider an initial salt basin to Depth (m below mudline) either not be covered by sediments at all or to be covered by an extremely thin sedimentary cover (few 10 m thick). An interesting 4000 exception is found in Gradmann et al. (2009) where the salt is initially covered by 4.5 km of sediments but with an extremely high 3 sediment density rs ¼ 2500 kg m and an extremely low coeffi- cient of friction feff ¼ 5 . Before further considering geological arguments, the above 5000 results and comments show that salt tectonics driven only by differential sedimentary loading is a process that is rather difficult to activate. It requires very thick sedimentary overburden and/or fi Fig. 7. Variation of sediment density with depth in the Gulf of Mexico. Modi ed after anomalously high sediment densities and/or very strongly reduced Hudec et al. (2009). sediment friction e i.e. high fluid pressure values.

Condition (6) is verified only if the sediment density rs is high and/or the frictional angle f is low. In agreement with this, avail- 4.3. The hypothesis of a seaward free boundary able numerical models of salt tectonics driven only by sedimentary loading tend to use high sediment densities and low frictional As mentioned above, analogue experiments that consider a salt 3 angles (feff) e e.g. rs ¼ 2300 kg m and feff ¼ 16.42 in Gemmer basin as being entirely covered by sediments (Figs. 5 and 6) confirm 3 et al. (2005) and rs ¼ 2500 kg m and feff ¼ 5 in Gradmann that in absence of water, the sedimentary overburden must i) be at et al. (2009). Low values of the effective angle of friction feff least three times thicker than the initial cover e i.e. Dh ¼ 3he and assume high to very high fluid pressures. According to the models, ii) display a deformation pattern, contractional at the sedimentary

Fig. 8. Sections of pure spreading models simulating the progradation of a sedimentary wedge on top of a salt basin initially covered by an extremely thin sedimentary cover e i.e. free frontal boundary (modiﬁed after Vendeville, 2005). The series of sections shows the progressive development of rafts and diapirs inside the wedge and the progressive extrusion of a salt nappe at the front. 1130 J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145

shortening, as indicated by the crenulation of the thin sand cover. Its proportional thickening leads to a progressive frontal overflow, forming a ductile nappe (Fig. 8c and d). A similar result was also obtained numerically by reducing the sedimentation rate (see Model 4 “sediment starvation” in Gemmer et al., 2005). These models show that extensional diapirs can form inside a sedimentary wedge if i) the salt basin is not initially covered by sediments (also see the models in Figs. 4e6inVendeville, 2005) and ii) enough time separates each sedimentation interval (“sediment starvation”). Reciprocally, high and constant sedimentation rates forbid the development of diapirs within the sedimentary wedge, fi Fig. 9. Type of set-up used for most numerical models of pure spreading (modi ed even when the salt basin is not initially covered by sediments (see after Ings et al., 2004). Before the arrival of the prograding sedimentary wedge, the salt e fi basin is initially horizontal, non-covered by sediments and seated at a significant water models 1 3inGemmer et al., 2005). Finally, when a signi cant depth (commonly 4500 m). thickness of sediments is deposited on top of the salt layer, prior to the arrival of a prograding sedimentary wedge, extensional diapirs wedge tip and extensional inside the wedge, that migrates front- cannot form inside the wedge because extension, which is ward with sedimentary progradation. restricted by cover shortening at the wedge tip, never reaches Several experiments have been carried out to simulate the amounts large enough to allow diapirism to take place, as exem- differential sedimentary loading of a salt basin only covered by an plified in Figures 5 and 6. extremely thin layer of sediments, assuming implicitly that the Numerical models that take into account the presence of seaward end of the salt basin is a free boundary (Fig. 9 in Rowan seawater also consider salt basins covered by only few 10s of metres et al., 2004, Figs. 4e6inVendeville, 2005; Gaullier and of sediments prior to the deposition of the prograding sedimentary Vendeville, 2005). In such experiments, the sedimentary wedge, wedge (Gemmer et al., 2005)(Fig. 9). In such models, the frontal as soon as it is deposited, starts to spread with the development of boundary can be considered as free, as stated by Ings and Shimeld distributed extension (Fig. 8). The wedge sand layer is therefore cut (2006). Such an initial condition, which is considered necessary to into several rafts that progressively separate with ongoing sedi- allow the development of minibasins (Gemmer et al., 2005), is mentation. Each new sand deposit leads to proportional amounts of somewhat difficult to reconcile with the fact that in these models, raft subsidence and separation. Consequently, triangular shape the water depth is around 4500 m at the onset of sedimentation. diapirs progressively grow between the rafts as long as a source How could a thick salt basin have reached a depth of around layer is available. At the model front, outside the sedimentary 4500 m without receiving more than few 10s of metres of sedi- wedge, the layer of silicone putty undergoes a rather homogeneous ments while in addition, keeping a horizontal position?

Fig. 10. Experimental model simulating the dominant gliding of a salt basin initially covered by sediments (see set-up in Fig. 11). The four top views show the progressive evolution of structures at surface during downdip gliding and synchronous sedimentation. Photographs were taken at 0, 20, 40 and 70 h. 1 h in the model is in the order of 1 Ma in nature. The section shows the ﬁnal state of structures in the middle of the model. J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145 1131

5. Dominant gliding models: salt tectonics primarily driven shortening, the squeezing of salt-cored anticlines produces a series by margin tilt of small zones of salt extrusion whose progressive connexion leads to the complete failure of the contractional front and the formation 5.1. Laboratory experiments of a large salt nappe (Day 10). The domain that was previously underformed, located on the updip side of the contractional The concept of salt tectonics driven by dominant gliding down domain, is then submitted to a backward extension that reaches the an inclined basal surface of saltesediment two-layer systems was previous domain of the updip extension. This illustrates that when mostly inspired by studies carried out in the South Atlantic salt nappes form after a long period of frontal contraction, the salt (Western Africa: Duval et al., 1992; Spathopoulos, 1996; Marton that thickened in the downdip contractional domain and which is et al., 2000; Tari et al., 2001; Brazil: Cobbold and Szatmari, 1991; suddenly liberated by the frontal failure allows the whole downdip Demercian et al., 1993) and North Sea (Nalpas and Brun, 1993; domain to spread seaward at a fast rate on top of the abyssal plain. Coward, 1995; Stewart and Coward, 1995). Different types of labo- In this process, salt is massively extruded in two ways: i) by the ratory experiments (Cobbold and Szatmari, 1991; Gaullier et al., frontal salt nappe and ii) by diapirism in the backward zone of 1993; Mauduit et al., 1997, 1998; Mauduit and Brun, 1998; Fort extension. et al., 2004a,b; Brun and Mauduit, 2008, 2009) or numerical modelling (Ings et al., 2004; Albertz et al., 2010; Quirk et al., 2011) 5.2. Stresses at failure have been carried out to characterise the structures produced by gliding above salt. When it is inclined at an angle a, the whole saltesediment two- In the dominant gliding experiment shown in Figure 10, a thin layer system tends to glide downslope, creating a contractional sand layer is deposited on top of a double-wedge shaped layer of state of stress in the vicinity of the downdip salt pinch-out. But silicone putty with a length l, representing a salt basin (Fig. 11). The gliding can start if the resistance opposed by the sedimentary cover model is then inclined at an angle a, to simulate the tilting of is overcome, leading to folding and/or thrusting. Considering that a passive margin during post-rift thermal subsidence (Fig. 3c). As the principal stresses are either close to vertical or horizontal, the soon as it is inclined, the sand-silicone two-layer model starts to maximum horizontal stress sh ¼ s1, with s1 s2 s3, exerted by glide downdip. The model displays extension updip and contrac- the two-layer system at the downdip end of the salt basin is: tion downdip, giving the typical succession of structural domains s ¼ s ¼ ðr þ r Þ a; observed at many passive margins (e.g. Fig. 1). In the downdip h 1 s e gl sin (7) direction, these domains are primarily: i) sealed tilted blocks, where re is the density of salt. grabens, rollovers and diapirs in the updip extensional domain and The vertical stress at the base of the sedimentary cover sv ¼ s3 is: ii) folds, thrusts and contractional diapirs (squeezed silicone-cored s ¼ s ¼ r a þ r : anticlines) in the downdip contractional domain. All these struc- v 3 wgl sin sghs (8) tures develop synchronously with sedimentation and therefore, e s s they display a growth character in extension e e.g. rollovers e as With the Mohr Coulomb relation between 1 and 3 in s ¼ s well as in contraction e e.g. growth synclines. Contraction occurs compression 1 K 3, the combination of equations (7) and (8) a due to a downdip constraint e e.g. edge of the silicone and/or gives the critical value of the angle necessary to reach failure as r decreasing slope e and starts at the downdip end of the salt basin, a function of cover thickness hs, salt basin length l and densities s, r r progressive shortening migrates in the updip direction, ultimately e and W of the materials involved: reaching the extensional domain and inverting in contraction a ¼ arc sin½ðh =lÞðKr =ðr þ r Kr ÞÞ: (9) structures that were previously extensional e e.g. squeezed diapirs s s s e W (Fig. 10a). In addition, it can be noted from the top view taken at Figure 13 is a graphical illustration of equation (9) showing a as 20 h that the initial contraction zone does not directly initiate a function of basin width l for different values of sedimentary cover 3 against the downdip salt pinch-out but at a small distance from it. thickness hs using typical density values (rs ¼ 2000 kg m , 3 3 Consequently, during progressive deformation, contraction also re ¼ 2200 kg m and rw ¼ 1000 kg m ). The two sets of curves migrates downdip down to the salt pinch-out, forming a frontal correspond to a sedimentary cover both without (in red: f ¼ 30 e thrust. More details on this type of experiments can be found in i.e. K ¼ 3) and with (in blue: feff ¼ 20 e i.e. K ¼ 2) fluid pressure, Brun and Fort (2004) and Fort et al. (2004a,b). respectively. The diagram shows that gliding occurs for tilt angles The formation of frontal salt nappes is also obtained in this type lower than 1 for basin widths in the range of 200e600 km and of experiment when the sedimentation rate in the downdip sedimentary cover thicknesses up to 1 km, even in absence of fluid contractional domain remains moderate (Fort and Brun, 2011). In pressure. With a fluid pressure l ¼ 0.31 giving feff ¼ 20 and K ¼ 2, the model shown in Figure 12, the contractional domain starts to even if a 2 km thick sedimentary cover is present, gliding occurs for develop (Day 6) in the same way as in Figure 10. With ongoing tilt angles of 1 or less. In other words, gliding is a very efficient

Fig. 11. Set-up of dominant gliding models primary due to margin tilt. The salt basin of length l is overlain by an initial sedimentary cover of thickness h. a is the margin tilt angle. s1 and s3 are the principal stresses at the downdip salt pinch-out. 1132 J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145

Fig. 12. Dominant gliding model showing the formation of a frontal salt nappe. The three top views illustrate two main periods of deformation. From day 1 to day 6, deformation occurs in two domains of updip extension and downdip contraction, like in the model of Figure 10. From day 6 to day 10, the downdip contraction leads to a frontal failure and the emplacement of a frontal salt nappe accompanied by backward extension. Simultaneously, in the updip domain, the shelf migrates rapidly downdip with formation of transfer zones parallel or slightly oblique to the salt flow direction. The section shows the structure of the downdip part of the model: the frontal salt nappe and the associated backward extension. process of salt tectonics that in most common geological cases can the incision of the Colorado River (McGill and Stromquist, 1979; be easily stimulated by margin tilt angles smaller than 1, even in Trudgill and Cartwright, 1994). In this small-scale example of the absence of abnormal fluid pressure. dominant gravity gliding without frontal buttress, the sedimentary layer deforming on top of the salt décollement is entirely submitted to extension down to the Colorado River. 5.3. The effect of sedimentary loading on gliding On planet Mars, over a distance of approximately 3000 km, the superficial cryosphere of the Thaumasia plateau displays a domain Sedimentary loading is not a necessary condition of salt of frontal contraction and a domain of backward extension inter- tectonics at tilted margins because the salt layer can even start to preted as a “mega-slide” on top of the mega-regolith made of ice flow downdip if tilting occurs before the deposition of the sedi- and impure salts dipping approximately 1 due to arching above mentary cover. However, as the sedimentary pile thickens the a mantle plume (Montgomery et al., 2008). sedimentary pile above the salt layer, the vertical stress increases Interestingly, these two examples are one order of magnitude and tends to accelerate the rate of gliding, although only slightly, as smaller (dwarf) and larger (giant) than the dominant gravity gliding illustrated in experiments by Mauduit et al. (1997) and Fort et al. systems observed at passive margins. A supplementary loading was (2004b). The rate of gliding primarily depends the differential not added during gliding in either cases. height from updip edge to downdip edge of the basin and not on layer thickness (eq. (7)). 5.5. Dominant gliding in the North Sea and above the Messinian 5.4. Dwarf and giant examples of dominant gravity gliding above salt salt in the Mediterranean

The Needles area of the Canyonlands National Park in Utah The Zechstein salt basin of the North Sea was submitted to provides a 12 km wide natural model of gliding above a sequence of regional-scale extension from the Triassic to the late Jurassic evaporites forming a décollement layer with a 1e2 dip, on top of (Bishop et al., 1995). Basement normal faulting was responsible for which a stepped series of grabens has formed within a 460 m thick a local salt basin tilt of 1e3 that induced dominant gliding of the brittle layer of upper Palaeozoic sandstone and shale, in response to salt-sedimentary cover with synsedimentary updip extension and J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145 1133

depends on the thickness of the salt. In a salt layer with thickness h and viscosity m undergoing a simple shear with a gliding velocity v, the shear strength is s ¼ vm/h. Therefore, the salt layer strength increases as a direct function of the salt layer thinning. In other words, dominant gliding slows down progressively with salt thinning but the salt layer almost never disappears (Waltham, 1996; Fort and Brun, 2005). On the other hand, the system tends toward equilibrium at the margin scale as the system surface is approaching the horizontal and dominant gliding velocity approaches zero. This is likely the case for the section shown in Figure 2. However, it is interesting to note that active updip and downdip faulting and downdip folding affect the topography of the seabed, indicating that the system is still not in equilibrium 110 Ma after salt deposition.

6. Comparison between pure spreading and dominant gliding models

The two previous sections show that pure spreading and dominant gliding are two extremely different driving mechanisms of salt tectonics at passive margins, both in dynamic terms as well as from the point of view of their structural response. It is therefore useful to summarise here the most striking dynamic (Table 1) and

Fig. 13. Diagram showing the minimum margin tilt angle a necessary to activate the structural (Table 2) features that help distinguishing between the dominant gliding of a salt basin of width l. The two sets of curves have been calculated two types of models. for thicknesses of the initial sedimentary cover h ¼ 0.5, 1.0 and 2.0 km and for a normal f ¼ f ¼ frictional angle 30 (red) and a reduced frictional angle e 20 (blue) of the 6.1. Setting of models sediments. Pure spreading driven only by differential sedimentary loading considers a salt margin that is not affected by tilting (Figs. 3be9). diapirism and downdip folding and thrusting (Nalpas and Brun, Models show that even with a free frontal boundary e i.e. salt not 1993; Stewart and Coward, 1995). In the passive margins consid- covered by sediments e in such circumstances, dominant spreading ered in the present paper, the margin tilt occurs mostly after salt requires significant water depths to allow the deposition of a several deposition, as a consequence of thermal subsidence following kilometre-thick sedimentary wedge, high sediment densities to lithosphere stretching and thinning e i.e. post-rift salt basins. In the increase loading for a given sedimentary thickness and significant North Sea, the salt basin is pre-rift and, contrary to Atlantic-type reduction of sediment friction, assuming high fluid pore pressures, passive margins, it is not tilted as a whole but is locally affected by in order to decrease the strength of the sediments (Table 1). the tilting of underlying basement blocks. Nevertheless, North Sea Dominant gliding corresponds to a slope instability stimulated salt tectonics display comparable organisation in extensional and by margin tilt. It does not require any other particular condition in contractional domains controlled by dominant gliding above terms of water depth, sediment density or friction. Even in the a basal slope. absence of sediments that would increase the driving force, the In the Mediterranean, the deposition of Messinian salt occurred entire salt basin flows downdip. in two main stages: first between 5.75 and 5.60 Ma, marked by Sedimentation and deformation are strongly related in both a sea level fall of a few hundred metres, and second, between 5.60 model types, but in pure spreading, the sedimentation drives and 5.32 Ma, marked by a sea level fall possibly as much as 1500 m deformation whereas in dominant gliding, the sedimentation is (Clauzon et al., 1996). Despite their young age and local regional trapped inside lows created by the deformation. tectonic effects, the Messinian evaporites and their Pliocenee Pleistocene sedimentary cover display extensionalecontractional 6.2. Structural response salt tectonic systems that obviously result from dominant gliding above very low angle basal slopes (Cyprus Arc: Bridge et al., 2005; The location of extension and contraction as well as their Levant Sea: Cartwright and Jackson, 2008; Gradmann et al., 2005; migration in time and space at the salt basin scale differ signifi- Nile Delta: Loncke et al., 2006). The origin of tilting is not formally cantly (Table 2). In pure spreading, extension occurs within the identified but it could likely result from lithosphere flexure due to prograding wedge and contraction occurs at the wedge tip; the a reloading of the Mediterranean lithosphere by seawater and salt. latter only occurs if the salt basin is initially fully covered by sedi- The salt tectonic examples briefly described above (Canyonlands ments. Both extension and contraction migrate seaward with National Park, Utah; Thaumasia plateau, Planet Mars, North Sea; sedimentary progradation. Consequently, extension can lead to the Messinian salt, Mediterranean) show that dominant gravity gliding inversion of previously contractional structures. In dominant that corresponds to a slope instability is i) not restricted to tilted gliding, extension occurs updip and contraction downdip. The passive margins and ii) can occur at various scales, from several contraction migrates updip, progressively reaching the domain of tens to several thousands of kilometres. updip extension and leading to the contractional inversion of previously extensional structures e e.g. squeezed diapirs. 5.6. Duration of margin tilt effectiveness The formation of salt nappes at the basin front can occur progressively in both pure spreading (Fig. 8) and dominant gliding Once created, margin tilt remains and, as long as salt is present, (Figs. 1 and 2) or can be sudden, but only in gliding (Fig. 12). Sudden dominant gliding can be effective. The effectiveness of gliding frontal failure that gives birth to a salt nappe generally follows a long 1134 J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145

Table 1 Comparison of pure spreading and dominant gliding models considering dynamic parameters, initial conditions and the place and role of salt and sediments in the process.

Model Pure spreading Dominant gliding Driving mechanism Differential sedimentary loading Slope instability

Specific requirements Significant water depths (to allow the deposition of thick sedimentary wedges) None High sediment density (to increase loading for a given sedimentary thickness) Low sediment friction/high fluid pore pressure (to decrease sediment strength) No initial sedimentary cover (to eliminate the frontal buttress)

Role of sedimentation No deformation without sedimentation Salt can flow even without sediments Role of salt Resisting Driving Salt flow at the onset of process Salt is expulsed seaward from below the sedimentary overburden The entire salt basin flows downdip Relation between sedimentation Deformation results from sedimentation (loading) Deformation controls the location and geometry and deformation of sedimentary depocentres

period of frontal contraction and salt thickening (sometimes called spreading by differential sedimentary loading requires an “salt inflation”; e.g. Hall, 2002). Simultaneously, the downdip domain extremely thick sedimentary overburden that fixes a minimum that was previously contractional is submitted to extension (Fig. 12). value for water depth (approximately 4500 m in most numerical models). This is rather unrealistic as the salt basin should have 7. Geological testing of models reached a water depth of approximately 4500 m without margin tilt before the onset of sedimentation. The density of sediments (v) 7.1. Applicability of models to nature is most often available from well data. Sediment friction (v) is rather well known from laboratory measurements but the effective fl A salt tectonic model, either experimental or numerical, is based friction also depends on pore uid pressure, something that can be on a particular concept (working hypothesis) of the physical extremely variable and generally poorly known at the basin scale. processes involved. The model has its own coherence that links any combination of the input parameters to a specific structural 7.2. Systems and sub-systems response with two important implications. The model response to variations in the input parameters i) helps to understand the Models show that a salt basin submitted to gravity-driven mechanical behaviour of the system defined by the working deformation in spreading or gliding is a dynamic system that can be hypothesis and ii) can be compared to natural examples in order to divided into extensional and contractional sub-systems (Figs. 5, 10 test the applicability of the working hypothesis to nature. In other and 12) that can in turn be divided into sub-sub-systems such as words, a model does not directly demonstrate that a given process tilted blocks, rollovers, diapirs, etc. (Fig. 10). As detailed above, both exists in nature. It is only a tool to study the dynamics of a system the pure spreading and dominant gliding models display a number based on a physical concept of a natural system. of similar growth-like structures such as rollovers, extensional and There are two different concepts of salt tectonics leading to two contractional diapirs, minibasins, folds, thrusts or salt nappes. different categories of models (Fig. 3, Table 1) with significantly These structures, either considered individually or grouped different types of responses (Table 2). To apply any of these models together, can provide a direct diagnostic of pure spreading versus to nature, it is therefore critical to examine the geological relevance dominant gliding. Other structures are rather ubiquitous and are of the initial and boundary conditions used for building each model not specific to any type of model. More generally, it is the organi- type. In particular, they are: i) the initial salt basin size and geom- sation of the structure in space and their evolution in time at the etry, ii) the margin tilt, iii) the presence or absence of an initial whole basin scale that can be used to discern between pure sedimentary cover on top of salt, iv) the water depth and v) sedi- spreading and dominant gliding, as illustrated by basin-scale ment physical properties, particularly density and strength. As experiments (Figs. 5, 10 and 12; Table 2): i) distribution in domains reviewed in Section 2, well and seismic data often provide the of extension and contraction, ii) migration of the extensional and necessary information to set up the initial conditions (i, ii and iii). contractional domains in space and time and iii) type of structural Water depth (iv) is more conjectural. As discussed in Section 2, the inversion to which the migration of extension and contraction deposition of thick evaporites would normally require shallow might lead. Therefore, the identi fication of the driving mechanism water conditions. This is in agreement with the initial conditions of salt tectonics in a given basin e i.e. pure spreading vs. dominant for gliding but not with those for spreading. The activation of gliding (Table 1) e requires a study of the relationships between

Table 2 Comparison of pure spreading and dominant gliding considering the patterns of deformation observed in analogue and numerical modelling.

Model Pure spreading Dominant gliding Extension Initiation Landward Updip Migration Seaward together with sedimentary progradation Inversion Extensional inversion of previous contractional structures

Contraction Initiation At sedimentary wedge tip if salt is initially covered by sediments Downdip Migration Seaward together with sedimentary progradation Landward/Updip Inversion Contractional inversion of previous extensional structures

Frontal salt extrusion Progressive ramping of salt, as a function of More generally sudden failure after a long period of (salt nappe) sedimentary progradation frontal contraction J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145 1135

Fig. 14. Sealed tilted blocks in nature (Angolan margin) (a) and experiment (b). Modiﬁed after Brun and Fort (2008). sedimentation and deformation at the whole salt basin scale thickness of sediments on top of salt at their time of development is (Table 2). Whole basin cross-sections showing the evolution of too small to be able to activate pure spreading. As block size is deformation since salt deposition are essential tools for this a direct function of sedimentary thickness (Brun and Fort, 2008), purpose. Unfortunately, not enough well documented and updated this deformation pattern clearly indicates that deformation started cross-sections of this type are yet available for most passive at very early stage of deformation and sedimentation. In addition, margins. because all normal faults dip in the same sense as the basal sole, salt ﬂow occurred with a strong component of downdip shear. The sealing of blocks by ongoing sedimentation demonstrates that the 8. Diagnostic structures of pure spreading vs. dominant salt layer has thinned enough to allow stabilisation of the shelf. gliding Consequently, sealed tilted blocks provide an excellent diagnostic feature of dominant gliding. 8.1. Sealed tilted blocks

Many margins display wide series of small tilted blocks in the 8.2. Early frontal contraction updip part of the margin, close to the salt pinch-out (Fig.14a). These structures, which are well reproduced in gliding models (Fig. 14b), Evidence for early contraction in the downdip part of the salt cannot result from differential sedimentary loading because the basin (Figs. 1 and 2) is the best evidence that demonstrates

Fig. 15. Structures that provide a diagnostic of pure spreading vs dominant gliding even considered individually. Sealed tilted blocks in the updip margin and early folds and thrusts in the downdip margin are excellent diagnostic of dominant gliding (a). Landward (updip) migration of contraction is diagnostic of dominant gliding (b). Seaward migration of contraction is diagnostic of pure spreading if the salt basin is initially covered of sediments (c). 1136 J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145

Graben / Rafts an early stage of the salt basin evolution (Burollet, 1975; Duval et al., 1992). Because the sedimentary thickness is low, it could be tempting to directly conclude that differential sedimentary loading Raft A Raft B cannot be responsible for the deformation observed. However, Salt rafting is obtained by differential sedimentary loading at the early a stages of spreading experiments when the salt basin is not overlain by an initial sedimentary cover of signiﬁcant thickness e i.e. frontal Diapir Rollover free boundary (cf. Section 4.3) e (Fig. 8). This shows that early rafting and simultaneous diapirism alone cannot be used to tell the difference between pure spreading and dominant gliding. There- fore, an essential additional test consists in examining if sediments of the same age are present or not at the seaward end of the salt basin (Fig. 17b). A lack of sedimentary cover thicker than a few tens of metres would favour the hypothesis of pure spreading driven Low sedimentation rate High sedimentation rate only by differential sedimentary loading. The presence of a sedi- b c mentary cover, especially if it is affected by contractional deformation, would argue in favour of dominant gliding stimulated by

Fig. 16. Relation between rafting (a) and the development of diapirs (b) vs rollover (c) margin tilt (Fig. 17a). as a function of sedimentation rate (modified after Brun and Fort, 2008). dominant gliding (Fig. 15a), even when considered by itself. Labo- 9.2. Diapirism ratory experiments (Figs. 10 and 12) show that they are the first structures to develop at the basin scale, prior to updip extension Diapirism results from the rise of salt in the footwalls of normal faults (Vendeville and Jackson, 1992; Quirk and Pilcher, 2011) 8.3. Migration of contraction where the sedimentation rate remains low (Fig. 16a and b). In the absence of sedimentation, salt can even reach the surface and flow In dominant gliding, the progressive downdip thickening of salt horizontally on top of the bounding rafts. and sediments leads to the updip (backward) migration of In dominant gliding, diapirs generally form in the downdip part contraction, toward the extensional updip domain. This results in of the extensional domain where sufficient salt is present (Figs. 1,10 the inversion of previously extensional structures e e.g. squeezed and 12). According to the models (see Section 4), two situations diapirs e (Figs. 10 and 15b). must be considered in pure spreading. In the absence of an initial In pure spreading, when salt is initially covered by sediments, sedimentary cover, the prograding sedimentary wedge can spread both extension and contraction migrate seaward with the sedi- and form rafts allowing the development of diapirs (Fig. 8). This mentary progradation (Figs. 5 and 15c). During migration, the explanation for diapirism requires that the frontal basin boundary extensional domain progressively reaches domains that were is free and therefore, the same test as that used for rafting can also previously contractional. This can result in extensional inversions be used here (Fig. 17b). If an initial sedimentary cover overlies the of previously contractional structures (Fig. 6). whole salt basin, extensional diapirs cannot form inside the pro- These contrasted migration patterns of extension and contrac- grading wedge because of insufficient extension (Fig. 6). Therefore, tion and related structural inversions provide a good diagnostic of diapirism requires the analysis of synchronous deformation in the dominant gliding (Fig. 15b) versus pure spreading (Fig. 15c). downdip domain to discriminate between pure spreading and dominant gliding. 9. Diagnostic of pure spreading vs. dominant gliding provided by structure associations 9.3. Frontal salt nappes 9.1. Early rafting Many passive margins show salt nappes that formed at the Rafting of the thin sedimentary cover deposited directly on top seaward front of salt basins. According to models, they can develop of salt at the landward basin end indicates that extension started at in two ways: progressive or sudden.

Fig. 17. Structures that provide a diagnostic of pure spreading vs dominant gliding when grouped together. The development of rafts and diapirs landward is diagnostic of dominant gliding if it is simultaneous to frontal contraction seaward (a) and could be diagnostic of pure spreading when the salt is not initially covered by sediments (b). J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145 1137

identify small sedimentary basins only a few tens of kilometres wide that have deeply subsided into salt. The term was so appealing that it has been further used in most salt basins, in various extensional or contractional settings. For almost four decades, it has been commonly believed that minibasins sink into salt under their own weight. However, as argued by Hudec et al. (2009), this explanation would hold true only if the average density of the basin fill exceeds Fig. 18. Experimental model of rollover structure produced by differential sedimentary that of salt, which in the Gulf of Mexico needs at least 2300 m of loading (prograding wedge) on top of a salt basin initially covered by sediments fi (modified after Ge et al., 1997). This rollover does not result from extension. siliciclastic ll to ensure enough compaction. Hudec and his co- workers identified several alternatives to the density-driven In pure spreading models in which the salt basin is not initially subsidence of minibasins, including extension, contraction, top- covered by sediments, the salt nappe develops progressively as the ographicebathymetric effects or sub-salt flow. In other words, amount of salt flowing out of the initial limits of the salt basin is minibasins do not represent a category of structures with a single a direct function of the amount of sediments deposited on top of mechanical origin and they are not characteristic of early salt the salt basin. This is well illustrated in Figure 8 (also see models tectonics driven by differential sedimentary loading. 1e3inGemmer et al., 2005). In pure spreading models where salt is initially covered by sediments, salt nappes do not form easily (see the models in Figs. 8e13 in Gemmer et al., 2004). 10.2. Rollovers and stepped counter-regional systems In dominant gliding models, the development of frontal salt nappes is more generally sudden, as illustrated in Figure 12. During Contrary to diapirs, rollovers result from a permanent filling up a long first stage, the downdip part of the salt basin undergoes of the surface depression created by extension between rafts contraction and salt inflation. When the frontal failure occurs, the (Mauduit and Brun, 1998)(Fig. 16c). They can be either synthetic or thickened salt flows rapidly on top of the abyssal plain and carries antithetic according to the seaward or landward dip of the main rafts of the sedimentary cover. The whole downdip part of the basin growth fault, respectively (Brun and Mauduit, 2008). Both synthetic that was previously contractional is submitted to extension. and antithetic rollovers develop in pure spreading (Fig. 6; see also The two models apparently produce similar salt nappes but in models 1e6inGemmer et al., 2005) as well as in dominant gliding pure spreading, nappes develop progressively and only if the models (Fig. 10). seaward part of the salt basin is not covered by sediments whereas Ge et al. (1997) have experimentally shown that rollover-like in dominant gliding, salt nappes form suddenly and involve the structures could result from differential sedimentary loading sedimentary cover. These differences can be used to discriminate without extension. Figure 18 shows a rollover that developed by the between pure spreading and dominant gliding. frontward expulsion of salt without extensional faulting of the initial sedimentary cover. The initial sedimentary cover undergoes 10. Ubiquitous structures a migrating rolling-hinge flexure at the tip of the sedimentary progradation, accommodating frontward salt expulsion. In this 10.1. Minibasins experimental model, the overburden forming the rollover is more than seven times thicker than the initial sedimentary cover and no In their review of salt tectonics in the Gulf of Mexico, Worrall sediment is deposited in front of it. Therefore, the applicability of and Snelson (1989) introduced the general term “minibasin” to this process to natural rollovers is extremely restrictive.

a b

Fig. 19. Conceptual models of “stepped counter-regional systems” formed by upward expulsion of a salt body driven by differential sedimentary loading. (a) The expulsion of the salt body is accommodated by salt dissolution (modified after Schuster, 1995). (b) Salt flow occurs through alternating periods of expulsion and sub-horizontal flow (modified after Rowan, 1995). In a and b, the base of the rollovers is not a fault but a salt weld and the rollovers do not result from extension. 1138 J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145

Fig. 20. Models of rollover development. (a) The classical model with two blocks separated by a listric fault. (b) The three-block model of rollover development in salt tectonics (modiﬁed after Brun and Mauduit, 2008). The rollover shown in b is antithetic, for the purpose of the present paper.

In the Gulf of Mexico, antithetic rollovers have been interpreted provide arguments either against extension or in favour of differ- as structures resulting from the expulsion of salt by the differential ential sedimentary loading. sedimentary loading of a salt sheet by a prograding sedimentary In this regard, the spectacular antithetic rollover of the Cabo Frio wedge, called “stepped counter-regional systems” (Diegel et al., structure in the Santos Basin of the Brazil margin (Fig. 21) 1995; Rowan, 1995; Schuster, 1995; Hudec and Jackson, 2006) (Demercian et al., 1993; Mohriak et al., 1995) is especially inter- (Fig. 19). The footwall of the rollover is considered fixed in reference esting. The Cabo Frio structure can be mapped along the strike for to the underlying basement and the concave-shape rollover base is hundreds of kilometres but has been interpreted either as a result interpreted as a salt weld instead of an antithetic normal fault. As of differential sedimentary loading (Szatmari et al., 1996; Ge et al., stated by Schuster (1995) “Interpretation as a detachment fault 1997; Mohriak and Szatmari, 2001) or as an antithetic rollover would require northward extension and gliding of the overlying strata. related to downdip rafting (Mauduit and Brun, 1998). However, However, there are no known toe structures updip to have accom- a recent detailed seismic study by Quirk et al. (2011) shows that the modated such extension; furthermore, extension updip would violate rollover development is controlled by the landward dipping Cabo the general rule of gravity tectonics”. This structure was therefore Frio fault and that remnants of the Albian rafts are present at the qualified as a “pseudo-extension” as neither of the two blocks has rollover base. Quirk and his co-workers conclude that: “it is an moved horizontally (Fig. 19a and b). In Figure 19a, the upward extensional feature caused by salt draining downdip (seaward) as the expelled salt is removed by salt dissolution. Figure 19b, displays passive margin tilted with thermal subsidence”. a double sequence of expulsion and sub-horizontal salt flow giving a staircase-type salt weld. It is the application of the classical model 11. What kind of exception is the Gulf of Mexico? of a listric fault separating two blocks (Fig. 20a) could be regarded as violating the general rule of gravity tectonics. However, in salt As stated in the introduction, for a long time now, the GoM has tectonics, the development of a rollover requires a three-block been a natural laboratory where many concepts of salt tectonics system: an updip block and a downdip block both moving downdip have been elaborated upon. The northern GoM displays a rather and that separate from each other on top of an extending complicated situation with a massive extrusion of allochthonous decollement layer lying above a basement block (Brun and salt bodies during the Cenozoic, as much as 8 km thick in some Mauduit, 2008). As shown in Figure 20b, an antithetic rollover places, on top of which Neogene minibasins are deposited, some- can develop between two blocks moving downdip. Laboratory times very deeply. The presence of minibasins and stepped experiments have shown that a synthetic rollover can pass laterally counter-regional systems (see Section 9), among other structures, to an antithetic one (see Fig. 9 in Brun and Mauduit, 2008), indi- have strengthened the impression that salt tectonics was mainly cating that they developed synchronously between the two same driven by differential sedimentary loading. For more than 30 years, rafts and that they both result from “true extension”. Consequently, salt tectonics in the GoM has therefore been considered to be the evidence for counter-regional or antithetic rollovers does not different from what is observed and/or described in other margins.

Fig. 21. The Cabo Frio antithetic rollover in the Santos Basin of the Brazil margin (modiﬁed after Quirk et al., 2011). J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145 1139

Fig. 22. Kinematics of salt flow in the northern Gulf of Mexico as recorded by the shelf migration (up) and seafloor deformation in the slope (down) (modified after Fort and Brun, 2011). Successive positions of the shelf break are identified by their age in Ma. Thick lines represent transfer zones that principally developed from Miocene to present. In the slope domain (Bathymetric data from NOAA), orange arrows indicate the mean salt flow direction deduced in four domains from the statistical analysis of the senses of shear in the deformation zones separating the minibasins and yellow bars indicate the direction of stretching in zones of co-axial deformation.

Interestingly, Diegel et al. (1995, p. 145) concluded their review of 11.1. Salt flow at regional scale the tectono-stratigraphic framework by asking the following question: “Are Gulf Coast style allochthonous salt structures more We recently mapped the regional direction of salt flow (Fort and common, but unrecognized, or are the scale and complexity of salt- Brun, 2011) using two types of data (Fig. 22): i) shelf break related structures in the Gulf Coast unique?”. It is beyond the scope of migration and ii) seafloor deformation in the slope domain. For the the present paper to enter into the details of the GoM salt tectonics. purpose of the present paper, we will summarise below the major However, in the framework of this present paper, which is devoted conclusions. The full data analysis can be found in Fort and Brun to salt tectonic models and their applicability to passive margins, it (2011). seems necessary to pay some attention to this question that still The shelf break has migrated over approximately 400 km since remains: Is the GoM a unique case in salt tectonics? Or, said the Cretaceous (Galloway et al., 2000). In a salt basin, a shelf can differently, what kind of exception is the GoM? form and stabilise only if the salt layer underlying the sediments

Fig. 23. Line drawings of regional seismic sections showing the major structures in the slope area in the northern margin of the Gulf of Mexico (modiﬁed after Fort and Brun, 2011). (a) Keathley Canyon salt nappe. (b) Section of Walker Ridge showing the four-layer structure. 1140 J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145

which have locally been folded during sedimentation and later stretched, (3) a layer of allochthonous salt fed by diapirs cutting through the CretaceouseLower Miocene sediments in the Walker Ridge or a salt nappe extruded from a ramp, and (4) a layer of Late Miocene to Quaternary sediments forming minibasins. The allochthonous salt nappe was mostly fed by a zone of salt extrusion above a frontal ramp, now located at the nappe rear. The nappe salt base rests on top of the Lower MioceneePliocene sediments at very low angle, over more than 100 km, indicating that salt was flowing very quickly during sedimentation. In Walker Ride, at the back of the salt nappe, the allochthonous salt layer was fed by a series of salt walls located between large Cretaceous to Early Miocene rafts that sunk down to the base of the autochthonous Louann salt. Salt was vertically extruded from a series of salt walls and then spread horizontally, forming convergent salt sheets on top of rafts. Synchronous sedimentation Fig. 24. Relation in time and space between the shelf migration, the frontal ramp is cast in downward-tapering bodies whose apex corresponds to e fi failure and the salt nappe advance along a NE SW trending pro le (see location in fl Fig. 21) (modified after Fort and Brun, 2011). the sutures between convergent owing sheets (S in Fig. 23b). The flat attitude of the allochthonous salt base envelope, over distances stops flowing (Fig. 14a) or is flowing at a very low rate. Therefore, at larger than 100 km, indicates that the extruding salt in Walker fl a given moment of basin evolution, a shelf break contour represents Ridge has been able to ow along a near-horizontal surface. This fl the boundary between the seaward part of the basin still deforming implies that the salt ow was very fast and/or the sedimentation above the flowing salt and the landward part where salt has rate very low. Because the rate of sediment supply in the Ear- e stopped flowing and where previous structures are sealed by ly Middle Miocene is in the medium range of values observed in sediments being deposited. If the rate of the salt flow varies on the the Gulf of Mexico (Galloway, 2001), it is more likely that salt fl seaward side of the shelf break, and in particular when sedimentary owed at a rather fast rate as observed for the Keathley Canyon salt progradation is oblique to the salt flow direction, transfer zones nappe. Sedimentary units have been deposited on top of develop parallel to the salt flow during shelf break migration. In the allochthonous salt since the Late Miocene but are dominantly of e northern GoM, where sedimentary progradation is dominantly Pliocene Pleistocene age and form minibasins with variable directed to the south (Galloway et al., 2000), transfer zones trend in maximum depths. a mean NEeSW direction indicating that salt has been flowing For more than approximately 400 km and over 100 Ma, salt has fl toward the SW since the Cretaceous (Fig. 22). In other words, there owed in a SW direction at a high angle to the mainly south- is a mean obliquity of around 55 between the directions of sedi- directed sediment supply in the area. The sudden onset of shelf ment progradation and salt flow in the northern GoM. Figure 22 migration in the Middle Miocene can be correlated with a frontal shows that most transfer faults are well constrained by offsets of ramp failure in Keathley Canyon (Fig. 24), without an obvious some tens of kilometres, and even reaching 80 km at few places. temporal relationship with a particular sedimentary event fl In the slope area, the analysis of seafloor structures over (Galloway, 2001). The ramp failure to liberate salt ow at the basin a 450 250 km area has been carried out using Seabeam-type front increased the rate of stretching and the thinning of the salt digital bathymetric data (NOAA). These structures mostly corre- updip, triggering shelf migration. This sequence of events can be spond to shear zones on top of salt ridges located between mini- directly compared with the experiment in Figure 12 as well as the basins. They indicate relative displacements between minibasins. relationship between the formation of the frontal nappe (Fig. 23a) A detailed mapping of senses of shear with a resolution of and backward extension and salt extrusion (Fig. 23b). The lack of an approximately 100 m has been processed statistically, providing obvious temporal relationship with a particular sedimentary event fl a mean direction of allochtonous salt flow for four domains and a direction of salt ow that is oblique to the direction of sedi- covering the whole studied area (orange arrows in Fig. 22). They all ment supply excludes the idea that sedimentary loading could have indicate a SW directed salt flow in the slope domain bound by the been the main driving force of salt tectonics in the GoM, as fl shelf break to the north and the salt basin border to the southeast. generally advocated. The permanence of the SW directed salt ow, In addition, local directions of stretching have been directly irrespective of the sedimentation rate, suggests that the salt obtained from zones of co-axial deformation that also trend in tectonics in the northern GoM corresponds to dominant gliding aNEeSW direction (yellow bars in Fig. 22). controlled by a SW dipping margin. On the above basis, our answer ’ These different types of data (Fig. 22) indicate that salt flow was to Diegel et al. s (1995) question would be the following: If salt dominantly directed toward the SW since the Cretaceous. tectonics in the GoM can be considered as an exception, it is, before any other aspect, related to allochtonous salt and the dynamics of its extrusion from the mother Louann salt and not to differential 11.2. Frontal salt nappe and backward salt extrusion sedimentary loading.

Recent seismic imaging in the slope area helped to understand 11.3. A methodological remark the extrusion processes of allochtonous salt. The two nearly parallel sections trending NEeSW, shown in Figure 22, parallel to the mean Since several decades, salt tectonics in the GoM has been direction of salt ﬂow at the regional scale, illustrate the four-layer considered to be driven by differential sedimentary loading. This structure of the salt basin in the slope area. The overall structure is was suggested not only by high values of sedimentation rates, in strikingly “ﬂat” and, from base to top, consists of: (1) the middle particular in the Neogene, but also by some spectacular and Jurassic autochthonous Louann salt that pinches out to the south- intriguing local structures like deep minibasins or stepped counter- west of Keathley Canyon, (2) a comprehensive pile of “lower” regional systems. But, as discussed above, minibasins do not result sediments, from the JurassiceCretaceous to the lower Miocene, from subsidence under the weight of sediments (Section 9.2) and J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145 1141

“stepped counter-regional systems” do not provide arguments Reminder: the presence of an initial sedimentary cover on top either against extension or in favour of differential sedimentary of salt is a serious obstacle to pure spreading. loading (Section 9.3). Therefore, it is noteworthy that in a region (5) Water depth. The deposition of thick evaporite layers would where local-scale structures are often complicated in 3D, a kine- normally require shallow water conditions. This is in agree- matic analysis at regional scale provides a rather consistent direc- ment with initial conditions for dominant gliding but not with tion of salt flow with two major outcomes. First, this kinematic those for pure spreading. The activation of spreading by signal consistency at large scale (over 400 km) is such that it must differential sedimentary loading requires an extreme differ- be controlled by a physical cause at the same scale e i.e. dominant ential thickness of the overburden that fixes a minimum value gliding stimulated by margin tilt. Second, from a practical point for water depth (about 4500 m in most numerical models). This of view: i) the regional-scale direction of salt flow provides implies that the salt basin has reached a water depth of around a dynamic reference for the interpretation of 3D structural 4500 m without tilt, before the onset of sedimentation. From complexities at local scales and ii) regional-scale cross-sections and a geological point of view, this is rather unrealistic. section balancing that are essential tools to understand the tectonic (6) Sediment physical properties. Density is most often available history of the salt basin must be constructed parallel to the regional from well data. Sediment friction is rather well known from salt flow. laboratory measurements but the effective friction also depends on pore fluid pressure that can be extremely variable and generally poorly known at basin scale. 12. Conclusions An analysis of the organisation of structures in space and their Two types of gravity-driven processes are invoked to explain salt evolution in time at the scale of the entire basin is, in general, tectonics at passive margins: i) pure spreading driven only by necessary to identify pure spreading from dominant gliding. differential sedimentary loading and ii) dominant gliding primarily However, few structures either considered individually or grouped due to margin tilt (slope instability). Their comparative analysis together can provide a direct diagnostic of pure spreading versus using simple mechanics and available analogue and numerical dominant gliding. models leads to the following conclusions. (7) Diagnostic structures (1) Dynamics: Good diagnostic structures of dominant gliding include Pure spreading driven by sedimentary loading requires the sealed tilted blocks in the updip extensional domain and early following to be effective: i) a thick sedimentary overburden folding and/or thrusting in the downdip contractional and therefore significant water depths, iii) high sediment domain. density, iv) low frictional angles of the sediments (high fluid Migration of the contraction is updip (landward) in dominant pore pressure), v) a seaward free boundary of the salt basin gliding and seaward in pure spreading. (salt not covered by sediments). Early rafting associated to extensional diapirism could indicate Dominant gliding has no specific requirement to be effective pure spreading only if the seaward part of the salt basin is provided the strength of the overburden can be overcome. It free of sediments. It indicates dominant gliding if it is can occur for margin tilt angles lower than 1 for basin synchronous with downdip contraction. widths in the range of 200e600 km and for sedimentary Frontal salt nappes form progressively in pure spreading, as cover thickness up to 1 km, even in the absence of abnormal a direct function of the amount of sediments deposited on fluid pressure. top of the salt basin, but only if the seaward part of the salt (2) Role of salt and sediments basin is free of sediments. They form suddenly, after a long Salt resists and sediments drive in pure spreading whereas period of downdip contraction, in dominant gliding. both salt and sediments drive in dominant gliding. (8) Ubiquitous structures Sedimentation generates deformation in pure spreading and Minibasins do not result from subsidence under the weight fills the bathymetric lows created by deformation dominant of sediments and do not represent a category of structures gliding. with a single mechanical origin. They cannot be used to (3) Deformation identify pure spreading from dominant gliding. In pure spreading, extension is located inside and contrac- Both synthetic and antithetic rollovers develop in pure tion at the tip of the prograding sedimentary wedge. Both spreading as well as in dominant gliding. Antithetic rollovers extension and contraction migrate seaward with the sedi- that are so-called “stepped counter-regional systems” and mentary progradation. Migration of the deformation can qualify as a “pseudo-extension” do not provide arguments create an extensional inversion of previously contractional either against extension or in favour of differential sedi- structures mentary loading. In pure spreading, extension is located updip and contraction downdip. Extension migrates downdip and contraction updip. Migration of the deformation leads to a contractional Acknowledgements inversion of previously extensional structures (e.g. squeezed diapirs) We are extremely grateful to Statoil for support over several years and for authorisation to publish this work. We benefited from To apply one of these two types of models to nature it is critical a considerable freedom of action, large access to data and positive to previously examine the geological relevance of the initial and interaction with numerous people in Bergen, Houston and Oslo. We boundary conditions used for each type of model: want to address our special thanks to Nicholas Ashton, Eric Blanc, Torbjorn Fristad, Rob Hunsdale, Bard Krokan, Tommy Mogensen (4) Deformation, initial salt basin size and geometry, margin tilt and Egebjerg, Bjorn Rasmussen and Christian Zwach, for their confi- presence or absence of an initial sedimentary cover on top of salt dence and help at various stages of this work. At Géosciences can most often be checked using well and seismic data. Rennes, we want to thank Frédéric Gueydan for stimulating 1142 J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145 discussions on mechanical aspects of this work and Jean-Jacques sediments lying on top of salt (Mauduit et al., 1997). In salt tectonics, Kermarrec for so many years of invaluable technical assistance in gravity sliding can be omitted as saltesediment systems undergoing the experimental laboratory. Thanks to Dave Quirk who authorized gravity-driven deformation always display internal strain. In domi- us to use a line drawing of the Cabo Frio structure. Great thanks to nant gliding the maximum displacement can be large as it primarily Ian Davison and Dave Quirk for constructive reviews and to Sara depends on the slab length. In spreading, the maximum displace- Mullin for careful text editing. ment is limited by the volume of material that must displace for the initially inclined surface to come to horizontal (Fig. 25c). Figure 26 Appendix 1. Displacements and strain in gravity-driven illustrates the three types of strain patterns that characterise deformation a viscous body undergoing true gliding (theoretical model) or a combination of gliding and spreading (dominant gliding), above Salt passive margins display a broad spectrum of gravity-driven an inclined base, and pure spreading, above a horizontal base. True deformations even in the absence of extensional or contractional gliding displays strong slab-parallel shearing whose intensity crustal-scale tectonics. In salt tectonics studies it has become rather decreases upward (Fig. 26a). Pure spreading displays surface-parallel common to consider that all types of deformations result from co-axial stretching in the upper part and base-parallel shear in the gravity spreading of layered saltesediment systems. Consequently, lower part whose intensity increases frontward (Fig. 26c). In terms of the term “gravity spreading” progressively became understood as strain pattern, a combination of gliding and spreading (dominant an equivalent (synonymous) of “gravity-driven deformation”. gliding) (Fig. 26b) is closer to true gliding (Fig. 26a) than to pure Beyond any semantic consideration, this shortcoming, which is spreading (Fig. 26c). In summary, a major difference exists between partly true, is responsible for the obvious misunderstanding of the pure spreading above a horizontal base and dominant gliding above processes involved and their consequences in terms of strain and an inclined base in terms of strain and displacement. Therefore, it is displacement. important to define two categories of salt tectonic models: i) pure Gravity-driven deformation can result either from the spreading spreading only driven by differential sedimentary loading and under its own weight or the gliding down a slope of a deformable ii) dominant gliding primarily driven by margin tilt. body. This is well known in glaciology where the gliding of glaciers Schultz-Ela (2001) attempted to combines boundary conditions down a slope (Ch. 6 in Paterson, 1972) is opposed, in terms of (base dipping or not) and rheology (slab deforming or not) in order displacement and strain, to the spreading of ice caps above a hori- to compare spreading and gliding: “The concepts of gravity gliding zontal rigid base (Ch. 9 in Paterson, 1972). In geology, similar and gravity spreading are typically defined by the general rigidity and approaches have been developed to establish the mechanics of the orientation of an underlying detachment”. In using the term nappes and thrusts driven by gravity (Ch. 7 in Price and Cosgrove, gliding instead of sliding, Schulz-Ela introduced a potential misun- 1990; Merle, 1994). Three situations are distinguished: sliding, derstanding of the basic concepts concerned. The term “gravity dominant gliding and pure spreading. In sliding (Fig. 25a), a rock slab sliding”, as used here in Figure 25a, was introduced and defined long moves downslope along a décollement layer without significant ago by Schardt (1898) (quoted by Hubbert and Rubey, 1959). The internal deformation (e.g. Fig. 7 in Hubbert and Rubey, 1959). In sliding of a rigid block above an inclined décollement layer is obvi- dominant gliding (Fig. 25b), a slab moves downslope but deforms ously not applicable to salt tectonics. In addition, as pure spreading internally with a strong slab-parallel shear component. In pure and dominant gliding have significantly different strain patterns spreading (Fig. 25c), the rock body flows under its own weight above (Fig. 26) a difference is required for the analysis of salt tectonics. a base that can be either horizontal or dipping opposite to the Whether or not an inclined base is present is a decisive factor. displacement (Ch. 9 in Ramberg,1981). The term “dominant gliding” The term “salt drainage”, which has recently emerged (Davison is used here instead of “gliding” because as the slab is deformable it et al., 2011; Quirk et al., 2011), describes the early flow of a salt basin necessarily undergoes a certain amount of spreading. This has been non-covered by sediments. This corresponds to the dominant exemplified in laboratory experiments inwhich a single viscous layer gliding of a viscous slab described above (Figs. 25b and 26b). A full (Brun and Merle,1985) or a brittleeductile two-layer system such as analysis of strain variations in space and time can be found in Brun and Merle (1985).

Appendix 2. Analogue modelling of salt tectonics systems

For a small-scale model to be representative of a natural a example (a prototype), a dynamic similarity in terms of distribution Décollement of stresses, rheologies, and densities between the model and the prototype is required (Hubbert, 1937; Ramberg, 1981). Following Sliding the above principle, it can be shown (Brun, 1999) that in sand- silicone models, the dynamic similarity is respected when the b ratios of stresses and lengths are nearly equal. To represent the sediments, we use Fontainebleau sand with a density r of about 1400 kg/m 3 and an angle of friction F in the range 30e31, but Dominant Gliding without signiﬁcant cohesion c. To represent the salt, we use a silicone putty (transparent gum SGM36, Rhone Poulenc, France) with a Newtonian viscosity m ¼ 104 Pa s and a density r ¼ 1000 kg/m 3. Pure Spreading The density contrast between silicone putty and sand (Drm ¼ 1.4) is slightly higher than might be expected between salt and sediments Dr ¼ e c in nature ( p 1.05 1.18 according to Weijermars et al., 1993). However, as pointed out by Weijermars et al. (1993) and Vendeville Fig. 25. Comparison between the three main processes of gravity-driven deformation: (a) and Jackson (1992), this disparity is acceptable because the density sliding, (b) dominant gliding and (c) pure spreading. In sliding the rock body is rigid and contrast between salt and sediments is not the primary factor does not deformed. In dominant gliding and pure spreading the rock body is deformable. responsible for the rise of diapirs. J.-P. Brun, X. Fort / Marine and Petroleum Geology 28 (2011) 1123e1145 1143

Fig. 26. Strain pattern (deformed grid/left and principal stretch l1/right; with l1 l2 l3) in a viscous slab undergoing gravity-driven deformation: (a) pure gliding, (b) dominant gliding (gliding þ spreading) and (c) pure spreading. Modiﬁed after Brun and Merle (1985).

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