Compressional salt tectonics (Angolan margin)

Transcription

1 Tectonophysics 382 (2004) Compressional salt tectonics (Angolan margin) Jean-Pierre Brun*, Xavier Fort Géosciences Rennes, UMR 6118 CNRS, France Institut de Geologie, Campus de Beaulieu, University de Rennes1, Ave du General Leclerc, Bat Rennes cedex, France Received 2 April 2003; accepted 25 November 2003 Abstract We present an analysis of compressional deformation at the front of a gravity spreading system above salt using seismic data from the Angolan margin and laboratory experiments. The geological setting and structural zonation is briefly reviewed and illustrated with two cross sections parallel to the margin slope in the Kwanza Basin. Experiments are carried out using sand and silicone putty to represent sediments and salt, respectively. The silicone layer was double-wedge-shaped to simulate more precisely the initial geometry of the Aptian salt basin of the Angolan margin. Models based on the Angolan margin example display the same structural zonation consisting of an upslope domain of extension, a downslope domain of compression and an abyssal undeformed domain. We present three different models, with different input parameters, showing the lateral variability of compressional structures in the downslope compressional domain. Models show two main stages of compression, which first appears in a domain located at some distance from the toe of the ductile wedge and then propagates both downslope to the distal salt pinchout and upslope in the formerly extensional domain. The initial zone of compression evolves into a domain of strong shortening characterised by folds, thrusts and squeezed diapirs. Synclines undergo strong pinching and can become detached as pod-like structures encapsulated within the underlying ductile layer. Anticlines are also pinched, thus isolating blobs of ductile material forming compressional diapirs that can extrude up to the surface. Unfolded layers develop into pop-up-type anticlines flanked by growth synclines. The propagation of compression, both downslope and upslope, creates domains of moderate shortening on each sides. Close to the domain of strong shortening, double-wavelength folds form a transition to a domain where compression is superposed onto the lower part of the upslope extensional domain, leading to extensional diapir squeezing. In the Angolan margin, propagation of compression downslope is characterised by recent folding affecting a sedimentary sequence of constant thickness and even the seafloor. Characteristic structures identified in the models are compared with seismic examples. We tentatively apply the mechanisms of sediment incorporation within the underlying ductile layer, as demonstrated in models, to the zone of apparently thick massive salt of the Angolan margin. D 2004 Elsevier B.V. All rights reserved. Keywords: Angolan margin; Kwanza Basin; Salt tectonics; Scale modelling; Gravity spreading; Migration of compression; Pod-like-type structure; Compressional diapir; Growth folding * Corresponding author. Géosciences Rennes, Campus de Beaulieu, University de Rennes1, Ave du General Leclerc, Bat Rennes cedex, France. Tel.: ; fax: addresses: Jean-Pierre.Brun@univ-rennes1.fr (J.-P. Brun), Xavier.Fort@univ-rennes1.fr (X. Fort). 1. Introduction At passive margins, gravity spreading above salt leads to the development of domains of upslope extension and downslope contraction (Wu et al., 1990; Demercian et al., 1993; Letouzey et al., 1995; /$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi: /j.tecto

2 130 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) Peel et al., 1995). This has been demonstrated recently in the Kwanza and Lower Congo basins of the Angolan margin (Spathopoulos, 1996; Marton et al., 2000; Cramez and Jackson, 2000; Kolla et al., 2001). While the processes responsible for the development of synsedimentary structures in the upslope extensional domain are rather well understood, the mechanisms occurring in the downslope compressional domain remain a matter of debate. This is partly due to the lack of well data and the presence of apparently thick massive salt, which often obscures the seismic images. Discussions concern in particular: (i) the nature of the downslope buttress necessary to induce downslope compression during gravity spreading (Duval et al., 1992; Spathopoulos, 1996), (ii) the downslope vs. upslope direction of compression propagation (Jackson et al., 1998; Cramez et al., 2000), (iii) the dynamic role of sedimentation and its possible interaction with deformation (Jackson et al., 1998) and (iv) the mechanisms of salt extrusion and resulting geometries (e.g. salt canopies: Marton et al., 2000). Laboratory experiments simulating gravity-driven deformation of models, made up of sand and silicone putty to represent sediments and salt, respectively, have greatly contributed to the understanding of extensional salt tectonics. Conversely, only few attempts have been made to simulate the development of synsedimentary structures in compression either (i) in experiments where displacements are applied at model boundaries (diapirs: Koyi, 1998; folds and thrusts: Koyi, 1998; Cobbold et al., 1995) or (ii) in gravity-spreading-type experiments (folds and thrusts: Cobbold et al., 1989; McClay et al., 1998; compressional diapirs: Ge et al., 1997; compression of extensional diapirs: Mauduit, 1998). Only the latter type of study examines possible relationships between upslope extension and downslope compression, but with a limited range of possible input parameters. In the present paper, we first review the structural zonation of salt tectonics at the scale of the Angolan margin and the structural characteristics of the downslope compressional domain. A series of gravity spreading experiments of brittle ductile models are used here to study the development of compressional structures and interactions between deformation and synchronous sedimentation. Finally, we compare the characteristic structures identified in models with some seismic examples. 2. The compressional domain of the Angolan margin In the context of the opening of the South Atlantic Ocean (Nürnberg and Müller, 1991), rifting of the Angolan margin starts at around Ma (Tesseirenc and Villemin, 1989; Guiraud and Maurin, 1992) and comes to an end at around Ma (Brice et al., 1982; Tesseirenc and Villemin, 1989; Guiraud and Maurin, 1992; Karner and Driscoll, 1998). Neocomian to Barremian synrift deposits of lacustrine type lie unconformably on top of Precambrian fault blocks. The end of rifting occurred during the late Barremian to early Aptian, accompanied by the filling of a so-called sag basin whose base is interpreted as a breakup unconformity (Uchupi, 1992; Jackson et al., 2000; Marton et al., 2000). The Aptian marine transgression led to the deposition of a massive salt formation. Fig. 1 presents a reconstruction of the initial geometry of the salt basin (Marton et al., 2000). Salt thickness is around 1700 m in the middle of the basin and wedges out both landward and seaward. From Albian times to the present day, post-salt sedimentation was entirely marine. Three main formations are classically recognised. From Albian to Late Cretaceous, a carbonate platform was followed by sedimentation increasingly Fig. 1. Restoration of the initial geometry of the Aptian salt basin of the Angolan margin (modified after Marton et al., 2000).

3 Fig. 2. Structural zonation of post-salt sediment of the Angolan margin. (a) Slope-parallel section crosscutting the upslope extensional domain and the upper part of the downslope compressional domain. (b) Slope-parallel section crosscutting the downslope compressional domain and the abyssal undeformed domain. Location of the sections is given in the insert. At the frontal part of section b (between marks 137 and 187.5), tertiary formations from upper Cretaceous to upper Miocene are too thin to be represented. J.-P. Brun, X. Fort / Tectonophysics 382 (2004)

4 132 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) dominated by marls and clays (Walgenwitz et al., 1990; Lavier et al., 2001). From the Late Cretaceous to Eocene, sedimentation was dominantly siliciclastic, but with a significant proportion of carbonates. Most of these deposits are located on the platform, pinching out seaward into condensed sections indicating a long-term highstand during the Late Cretaceous (Anderson et al., 2000). Post-Oligocene times were characterised by lowstand conditions producing strong erosional events at regional scales both on the shelf and of the coastal plain margin. From Oligocene to the present day, a large prograding clastic wedge is directly linked to the combined effect of the above-mentioned eustatic change and Tertiary coastal uplift (Lunde et al., 1992). The two sections shown in Fig. 2 illustrate the slope-parallel structural zonation of post-salt sediments on the Angolan margin. For the present study, it is convenient to distinguish three major domains at margin scale from east to west: extensional upslope, compressional downslope and undeformed seaward. The upslope extensional domain is made up of a subdomain of tilted blocks sealed by post-cretaceous sedimentation and a subdomain of rollovers whose development is controlled by still-active normal faults. The following domain of diapirs is affected by late compression. In fact, these diapirs developed in an extensional regime as demonstrated by normal faults, located in rafts between diapirs at lower stratigraphic levels. The downslope compressional domain, which is the object of the present study, starts with a subdomain of squeezed diapirs, superposed on the lower end of the extensional domain. The second subdomain is characterised by double-wavelength folds, while the third exhibits a strong structural complexity, with apparently thick salt obscuring the seismic images. However, folds, thrusts and compressional diapirs can be identified locally. The compressional domain is bounded downslope by a subdomain of small-wavelength folds (around 2.5 km), which developed recently and affected the seafloor. To the west of the compressional domain, salt is absent and sediments of the abyssal plain are undeformed. Fig. 3. Laboratory experiments. (a) Apparatus, model and procedure of synkinematic sedimentation. (b) Cross section of a model showing the structural zonation induced by gravity spreading. (c) Initial geometry of the three models presented in the paper.

5 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) Even if the general lines of the above structural zonation are accepted by most recent studies on the Angolan margin, a number of questions remain open, especially concerning the compressional domain. First of all, structural analysis is made difficult by the presence of salt at various levels in the sections, which often obscures the seismic image. The timing and relative chronology of compressional events are extremely difficult to establish, in particular due to a lack of well data, except in the case of DSDP Hole 364 located ca. 200 km to the south. Most recent interpretations are made on the basis of geometrical correlations on seismic lines, using the stratigraphic data available in the upslope extensional domain. Consequently, the relationship between extension and compression in space and time, as well as the dynamic interpretation of compressional structures, vary according to different authors. Nevertheless, all authors agree that both extension and compression result from gravity-driven deformation on top of the salt layer (Duval et al., 1992; Spathopoulos, 1996; Jackson et al., 1998; Cramez and Jackson, 2000; Cramez et al., 2000; Marton et al., 2000). The compressional structures observed on seismic lines can be interpreted in terms of strong vs. moderate shortening. At the scale of the compressional domain, this allows us to identify a central subdomain of strong shortening bounded on the downslope and upslope sides by domains of moderate shortening, corresponding to the above-described structural zonation (Fig. 2). This analysis is strongly supported by the modelling results presented below. 3. Analogue experiments Since the early contributions of Vendeville (1987) and Vendeville and Cobbold (1987), gravity-spreading experiments with brittle ductile systems have proven extremely useful in studying salt tectonic interactions between deformation and synchronous sedimentation. However, the emphasis was mostly placed on extensional deformation leading to grabens, rollovers and diapirs (Cobbold and Szatmari, 1991; Vendeville and Jackson, 1992a,b; Gaullier et al., 1993; Jackson and Vendeville, 1994; Mauduit et al., 1997a,b; Mauduit, 1998; Mauduit and Brun, 1998). Compressional deformation at the front of spreading systems has also been considered (Cobbold et al., 1989; Mauduit, 1998; Ge et al., 1997; McClay et al., 1998), but no detailed study has yet been provided of the resulting structures Materials and scaling The experiments presented here are carried out using the same materials, techniques and scaling principles used in the literature (Vendeville and Fig. 4. Top views of model 3 showing the evolution of compression during spreading.

6 134 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) Fig. 5. Serial sections in the compressional domain of model 1.

7 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) Fig. 6. Serial sections in the compressional domain of model 2.

8 136 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) Fig. 7. Serial sections in the compressional domain of model 3.

9 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) Jackson, 1992a; Weijermars et al., 1993). The experimental procedure requires a dynamic similarity of stress distribution, rheology and density between the model and the prototype (Hubbert, 1937; Ramberg, 1981). To represent the sediments, we used a pure aeolian quartz sand (Fontainebleau sand) with a mean grain size of 0.5 mm, a density of about kg/m 3 and an angle of friction in the range 30 33j, but without significant cohesion. To represent the salt, we used a silicone putty (transparent gum SGM36, Rhone Poulenc, France) with a Newtonian viscosity l =10 4 Pa s and a density q = kg/m 3. The density contrast between silicone putty and sand (Dq m = 1.4) is slightly higher than might be expected between salt and sediments in nature (Dq p = according to Weijermars et al., 1993). However, as pointed out by Weijermars et al. (1993) and Vendeville and Jackson (1992a), this disparity is acceptable because the density contrast between salt and sediments is not the primary factor responsible for the rise of diapirs. From the general equation of dynamics, it can be shown (e.g. Brun, 1999) that the ratio of stresses (r*) is related to the ratios of density (q*), acceleration ( g*) and length (L*) by the equation: r*=q*g*l*. As our experiments are carried out under normal gravity, the gravity ratio is g* = 1. The densities of model materials range from to kg/m 3 and rocks from to kg/m 3. Because model and prototype densities are of the same order of magnitude, the density ratio is q*c 1. Therefore, the previous equation simplifies to r*c L*. In other terms, the ratio of stresses becomes nearly equal to the ratio of lengths Experimental procedure A layer of silicone putty representing salt is deposited on a rigid base and overlain by sand layers representing sediments. The silicone layer is doublewedge-shaped, thinning both upslope and downslope, and is entirely overlain by the 1.0-cm-thick prekinematic layer (Fig. 3a). Such an initial geometry simulates the initial shape of salt basins at passive margins (Fig. 1). As soon as the prekinematic layer is deposited, the rigid base is inclined and spreading starts. Synkinematic sand layers are Fig. 8. (a) Schematic diagram summarising the evolution of folding in the zone of strong shortening. (b) Detail of two model sections showing the relationships between pop-up-type structures and growth synclines. A thick white bar indicates the thickness of the prekinematic layer. The same symbol is used in a number of the following figures.

10 138 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) progrades toward the front of the model during the experiment (Fig. 3a). The total experiment duration is 70 h. At the end of the experiment, models are wetted and then cut into serial vertical sections that are photographed to study the internal structures. Photographs of the model surface are also taken at regular time intervals to study the progressive development of structures. Fig. 9. Lateral variations of structures within the zone of strong shortening. (a b) Neighbouring sections from model 1; (c d) neighbouring sections from model 3. Distance between sections a and b or c and d is approximately 5 cm. built up in two steps at regular time intervals of 5.0 h. First, local topographic depressions related to deformation are filled. Second, a thin layer is deposited using a funnel to simulate a sedimentary progradation at model scale. Each new layer is wedgeshaped, with a thickness of 2 mm at the back, and Fig. 10. Pop-up-type structures associated with growth synclines in model (a) and seismics (b and c).

11 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) Fig. 3c shows the initial geometry and size of the models. The slope angle of the prekinematic layer is 1.8j in models 1 and 2, and 3.8j in model 3. The ductile wedge geometry is similar in models 2 and 3, which differ only in inclination. Beneath the frontal ductile wedge, the basal slope dips backward by 1j in model 1, by 0.5j in model 2 and towards the front by 1j in model Structure of models and downslope compression Fig. 3b shows the typical structural zonation obtained in the spreading type experiments, regardless of the initial conditions. The upslope domain is characterised by extensional structures: grabens, tilted blocks, rollovers and extensional diapirs. The downslope domain is characterised by folding, thrusting and compressional diapirs. This section shows a zonation that is directly comparable to the pattern observed at the regional scale on the Angolan margin (Figs. 2 and 3). The present paper focuses on the structural patterns of the downslope compressional domain obtained in three experiments, for two different slope angles of the prekinematic layer and two different geometries, i.e. frontal wedge angle, of the silicone layer (Fig. 3c). Fig. 11. Salt extrusion between a growth syncline and a tilted slab in model (a) and seismics (b and c).

12 140 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) Evolution of compression in time and space Three top views of model 1 (Fig. 4) illustrate the evolution of the downslope compressional domain in time and space. Compression starts at some distance from the toe of the ductile wedge, in a domain of initial width Wo (Fig. 4a). Compression remains localised in this domain during the initial stages of evolution, while the width Wo decreases to Wi. This stage ends when thrust faults are initiated in the cover above the ductile wedge toe (Fig. 4b). Then, compression migrates both downslope and upslope while the compressional domain widens with time, mostly on the upslope side (Fig. 4c). In terms of bulk shortening, the compressional domain can thus be subdivided into an inner domain of strong shortening bounded upslope and downslope by domains of moderate shortening. However, the domain resulting from downslope migration can itself be affected by strong shortening and become indistinguishable from the inner domain Overall structure of the compressional domain The serial sections of models 1 to 3 (Figs. 5 7) display a domain of strong shortening with pinched synclines, pop-up-type anticlines and compressional diapirs. One particularly striking deformation feature, common to all models, is syncline pinching leading to pod-like structures. Serial sections of model 2 Fig. 12. Growth syncline located in a thrust footwall in model (a) with a sketch of evolution (b) and seismics (c and d).

13 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) after total encapsulation of the pod within the ductile layer. Models display a large variety of structures resulting from this process. Model 1 (Fig. 5) shows two domains of moderate shortening with folds and thrusts that result from downslope and upslope migration of compression. In the downslope domain of moderate shortening, thrusts are directed forward in the center of the model and backwards at the sides due to lateral boundary shear. Models 2 and 3 (Figs. 6 and 7) show only one domain of moderate shortening; this results from the upslope migration of compression. In these models, frontal propagation also leads to a strong shortening with narrow-spaced folds and thrusts, while the cover above the ductile wedge toe is thrusted on top of the undeformed domain (Fig. 4b and c). In other words, at the end of deformation, the toe of the ductile wedge, i.e. the salt basin, is displaced towards the front. This is not the case in model 1. Generally speaking, all the models display a strong along-strike variation of structures (i.e. noncylindricity). This results from interactions between mechanical instabilities of the brittle ductile system during shortening, amplified by the effects of synchronous Fig. 13. Compressional diapir between growth synclines. show the lateral evolution of one of these structures through the full range of possible configurations. From top to base of Fig. 6, a growth syncline becomes pinched and progressively isolated from the source layer, giving a pod-like structure incorporated into the ductile layer. This along-strike variation illustrates the role of sedimentation within synclines. Slow pinching gives rise to a growth syncline, whereas fast pinching allows isolation of a pod at an early stage. The pod-like structure observed on the right of the lower section is located at a distance d from its pinching point in the prekinematic layer. This gives a minimum estimate of the downslope displacement of the sand layer Fig. 14. Schematic diagram summarising the evolution of pinched synclines.

14 142 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) sedimentation. Consequently, the bulk structural pattern of the compressional domain can be expressed as a combination of elementary structures, but not always with the same spatial arrangement. The next section attempts to (i) identify few basic structural patterns, observed in experiments, and (ii) discuss their application to the ultradeep area of the Angolan margin. 5. Deformation in zones of strong shortening Fig. 4 shows zones of strong shortening that are mostly concentrated in the domain of initial compression and possibly involving the domain of downslope migration. Deformation starts with folding in which the wavelength is chiefly controlled by the thickness of the prekinematic layer. However, as all folds do not develop instantaneously at the scale of the deforming domain, fluctuations occur in the final fold spacing. This leads to lateral variations in the evolution of folding and to interactions with sedimentation. Small-wavelength folding evolves into pinching of synclines and compressional diapirs, whereas locally unfolded or slightly folded layers evolve into pop-up-type structures surrounded by growth synclines (Fig. 8). However, narrow anticlines flanked by pop-down synclines are also observed locally (e.g. Fig. 6) Syncline pinching and compressional diapirs Fig. 9 presents two pairs of neighbouring sections from models 1 and 3 which demonstrate the strong lateral variability of pinched synclines and compressional diapirs. The distance between the sections of Fig. 9a b and c d is broadly equal to the model thickness. Fig. 15. Nearly detached pinched growth syncline in seismics (a) and model (b).

15 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) In model 1 (Fig. 9a), a series of small-wavelength folds affect the prekinematic layer. Among these folds are four pinched synclines plus one podlike structure that is detached and rotated into the silicone putty. The anticlines are also strongly pinched, some of them isolating a blob of silicone putty (cf. compressional diapir) that may be extruded up to the surface. In contrast to the prekinematic layer, the synkinematic layers on top of the folded layer are not strongly deformed. Apart from the overall synclinal curvature, evidence of compressional deformation is provided by diapir extrusion, wedge inversion and thrusts cutting through into the synkinematic section. On the next section (Fig. 9b), we can easily recognise the diapir crosscutting the synkinematic syncline. In the prekinematic layer, folds appear to evolve in a different way. In particular, to the right of the diapir root, the prekinematic layer forms a large recumbent syncline almost entirely encapsulated within the silicone putty. This structure corresponds to a lateral variation of the detached pod-like structure as observed in Fig. 9a. Similar strong lateral variations of structures are observed in model 3 (Fig. 9c and d). Small-wavelength folding of the prekinematic layer (Fig. 9d) also yields pinched synclines and compressional diapirs. Synkinematic layers above the fold train are nearly undeformed, except up against the diapirs. In the neighbouring section (Fig. 9c), the large diapir of the previous section becomes a forward-directed thrust fault associated with a pod in the silicone layer. In the thrust footwall, the series of folds develops into two extremely pinched synclines, one forming a pod within the silicone Growth synclines and pop-up anticlines Fig. 16. Onlaps and unconformities in model (a) and seismics (b). Pop-up-type structures surrounded by growth synclines develop where layers are unaffected by small-wavelength folds as shown in Fig. 10a. The roof of the pop-up is slightly folded and bounded laterally by thrust faults. The thrust footwalls evolve into growth synclines that are progressively pinched during amplification. These model structures compare very closely with a seismic example (Fig. 10b and c). During progressive shortening, the pop-up roof can be tilted to form an inclined slab. Such an

16 144 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) Fig. 17. Growth folding in the zone of moderate shortening. evolution facilitates salt extrusion, as observed in both experiments and nature (Fig. 11). Growth synclines are not necessarily associated with popup-type structures (i.e. conjugate thrusts) but can also develop in the footwall of a single thrust (Fig. 12) or close to each other, being only separated by compressional diapirs (Fig. 13). In both experiments and seismics, all growth synclines in the domain of strong shortening undergo progressive pinching during their development/ deformation. Pinching is a direct consequence of compression as portrayed in Fig. 14. Here, it should be noted that such structures cannot simply result from downloading, as previously proposed by Khele (1988). The pinching of growth synclines can also be extreme, leading to the development of pod-like structures incorporated within the salt layer (Fig. 15). Onlaps and unconformities are common features of such compressional salt tectonic environments. Onlaps appear mostly on syncline limbs. They form during the deposition of new sediments within Fig. 18. Schematic diagram summarising the evolution of double-wavelength folds.

17 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) seafloor depressions above growing synclines (Fig. 13). During syncline pinching and subsidence, onlaps are tilted and even sheared, thus appearing as uplaps. Unconformities occur above pinched anticlines or over the thrusted limbs of broken anticlines (Fig. 16). They are due to vigorous upward bending of fold limbs and thrust hangingwalls in the domain of strong shortening. It is noteworthy that such unconformities have no particular significance in the classical terms of deformation phases, the lifetime of a particular compressional structure being purely local. 6. Deformation in zones of moderate shortening As already pointed out, moderate shortening mostly characterises the upslope part of the compressional domain. Compressional structures result from two different types of deformation history. Adjacent to the strong shortening domain, structures result entirely from compression. In contrast, during upslope migration, compression reaches the extensional domain and leads to tectonic inversion and squeezing of extensional diapirs Folding and thrusting Fig. 17 shows a series of short wavelength folds associated with thrust faults and a large growth syncline (see location between marks 125 and 145 in Fig. 2a). Folding started in Paleocene times and remains active as indicated by the nearly constant thickness of Cretaceous sediments and seafloor relief. A nearly continuous deformation is suggested by the growth of the succession from Paleocene to present day, but the lack of stratigraphic data does not allow a detailed analysis of the amplification rate or possible variations in time. On the left, thrust faults are located in fold limbs. A local unconformity occurs on top of the seaward-directed thrust unit. On the right, a large-wavelength growth syncline does not show any sign of pinching, contrary to the synclines described in the previous section. The existence of a large slab of Cretaceous sediments at the base of the syncline suggests that folding did not start at the onset of compression but only when compression migrated upslope, likely in upper Cretaceous times. Double-wavelength folding (between marks 65 and 95 in Fig. 2b) is a lateral equivalent of the series of short-wavelength folds on the left-hand Fig. 19. Series of squeezed diapirs in seismics (a) and model (b).

18 146 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) side of Fig. 17. Small wavelengths of ca. 3 6 km correspond to an early stage of folding controlled by a thin sedimentary cover above salt (Fig. 18). Synkinematic sedimentation increases the strength of the folded layer, leading to a progressive widening of wavelength up to ca. 30 km. Small thrust faults also are initiated, at an early stage, coeval with folding. Thickness variations of post-cretaceous layers reflect the synchronicity of sedimentation and folding. Comparable double-wavelength folding has also been described in the Santos basin (Demercian et al., 1993). At the front of the compressional domain (between marks 160 and 190 in Fig. 2b), folding develops at a late stage with deformation affecting the sea bottom itself and a sedimentary series with a nearly constant thickness, from the Cretaceous to the present day, as described by Cramez et al. (2000). This indicates that early compression was localised at some distance from the salt wedge tip of the Angolan margin, and that compression propagated seaward quite recently. This is comparable to the evolution of compression observed in experiments (Fig. 4) Squeezing of extensional diapirs During upslope migration, compression reaches the downslope part of the extensional domain where reactive diapirism is triggered by early extension between rafted blocks (Fig. 3b). In the Angolan margin, as already pointed out by Marton et al. (2000), nearly all extensional diapirs have undergone compression. Fig. 19 compares a series of squeezed diapirs from the Angolan margin with an experimental model. In both cases, diapirs are located between undeformed or slightly deformed rafts. In the Angolan example, sedimentary layers remain flat lying in the central part of rafts, from Cretaceous to the present day, and are bent upward near diapirs. However, in the model, the prekinematic and early synkinematic layers are bent downward or stay flat. Diapir amplitude and volume increase downslope, as previously observed in the Gulf of Mexico (Wu et al., 1990). On the downslope side (diapirs 1 and 2 in Fig. 19a), diapir crests are close to the surface and display large overhangs. On the upslope side (diapirs 3 and 4 in Fig. 19a), diapir crests are overlain by older and thicker sedimentary sequences indicating an earlier cessation of diapir ascent. But all diapirs are associated with a seafloor relief indicating recent vertical displacements. On the downslope side, this could be interpreted as due to the negative buoyancy of salt instead of squeezing, but on the upslope side, the thick tilted diapir roof and associated thrust faults clearly indicate compression and late salt rise. It is thus likely that the whole domain of diapirs shown in Fig. 19a underwent compression. The same situation is observed in the experiment where diapirs initiate during extension forming silicone walls. The stems of diapirs undergo lateral shortening, with consequent width reduction and extrusion of silicone. Where stems shortening reaches a complete welding, the vertical extrusion of silicone ceased before the end of the experiment (diapir 3 in Fig. 19b), unless silicone moves along strike into the plane of section. The downslope diapirs that do not obviously appear squeezed in this particular section have in fact also undergone compression, with lateral welding of their trunks, as observed at model surface during experiment. 7. Discussion and conclusions We draw the following conclusions from combining the analysis of seismic data, from the compressional domain of the Angolan margin, with laboratory experiments, of synsedimentary gravity spreading above a ductile material: (1) Downslope compression starts within a domain located at some distance from the salt wedge toe and later propagates both downslope and upslope. (2) The domain of compression displays a central subdomain of strong shortening bounded on downslope and upslope sides by domains of moderate shortening. (3) The domain of strong shortening is characterised by (i) pinched synclines which can detach and become incorporated as pods within salt, (ii) pinched anticlines giving rise to compressional diapirs and possible salt extrusion and (iii) thrust faults yielding, in some cases, pop-up-type anticlines. Pod-like structures incorporated within the ductile layer have never been observed on seismic lines. However, in the zone of moderate shortening, we observe nearly detached pods both

19 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) Fig. 20. Interpretation of the zone of apparently thick massive salt. (a) Uninterpreted seismic section. (b) Salt canopy-type interpretation (after Marton et al., 2000). (c) Interpretation based on laboratory experiments. (d) Model example showing the type of structures possibly existing within the zone.

20 148 J.-P. Brun, X. Fort / Tectonophysics 382 (2004) in experiments and seismics (Fig. 15). We therefore suggest that pods should be rather common in zones of strong shortening. Moreover, as strong shortening characterises the domain of early compression, it should affect a still thin sedimentary cover. As a consequence, this would lead to short-wavelength folding and facilitate syncline pinching and the formation of small pods. In the future, improvements in seismic processing should help in recognising the occurrence of pods in salt. (4) The upslope domain of moderate shortening results from upslope migration of compression. Close to the domain of strong shortening, the downslope side is characterised by doublewavelength growth folding. In the upslope direction, compression affects the lower part of the upslope extensional domain, leading to squeezing of the diapirs and tightening of adjoining synclines. (5) The downslope domain of moderate shortening results from late downslope migration of compression. On the Angolan margin, smallwavelength folds affect a sedimentary succession of nearly constant thickness. As the seafloor itself is folded, the compression must be recent. Evidence for sediment incorporation within salt and the resulting structures, as described here from laboratory experiments, has never been observed on seismic lines. At least four main reasons can be given: (i) such structural complexity is difficult to image using conventional seismic methods, (ii) due to compressional diapirism, the presence of salt at various levels in the sections leads to strong perturbations of the seismic signal, (iii) imaging sediments incorporated within salt will probably require specific developments in seismic processing, and (iv) well data are not yet available in the ultradeep area affected by compression. The compressional domain of the Angolan margin presents a large zone of apparently thick salt (Fig. 20a) covered by a thin layer of recent sediments (between marks 110 and 155; Fig. 2c). However, this zone is bounded on the upslope and downslope sides by sedimentary formations with Cretaceous layers at the base. The lack of Cretaceous sediments on top of the large and apparently thick salt zone suggests that parts of a previous sedimentary cover could have been incorporated within the salt. On the other hand, deep reflectors within the apparently thick massive salt could correspond to complex imbrications of salt and sediments. As proposed by Marton et al. (2000), salt may have been extruded to the surface leading to nearly connected salt canopies that conceal sedimentary layers beneath (Fig. 20b). The experiments presented here demonstrate other possible scenarios combining folding and thrusting to obtain simultaneous salt extrusion and sediment incorporation within salt. On such a basis, we propose an alternative interpretation (Fig. 20c). It is, however, interesting to quote here that the central Kwanza Basin (Hudec and Jackson, 2002) displays a compressional domain dominated by the Angola salt nappe rather than a fold and thrust belt. There, the late shortening instead appears to have entirely propagated upslope from the Angola Escapement after the seaward translation of the Angola salt nappe became blocked by sedimentation in the abyssal plain seaward of the salt pinchout. Acknowledgements This work was funded by Norsk Hydro (Norway). Thanks are due to Norsk Hydro and Western Geophysical for permission to use the seismic data presented in this paper. We also thank Jean-Jacques Kermarrec (Géosciences Rennes) for his constant help and advice during the experiments and Laurence Rioche and Jeroen Smit for correcting the English style. Comments and suggestions of improvement by the referees, I. Davison and M.P.A. Jackson, were greatly appreciated. References Anderson, J.E., Cartwright, J., Drysdall, S.J., Vivian, N., Controls on turbidite sand deposition during gravity-driven extension of a passive margin: examples from Miocene sediments in Block 4, Angola. Marine and Petroleum Geology 17, Brice, S.E., Cochran, M.D., Pardo, G., Edwards, A.D., Tectonics and sedimentation of the South Atlantic rift sequence: Cabinda, Angola. American Association of Petroleum Geologists Memoir 34, 5 18.

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