compressional salt tectonics (angolan margin)

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Tectonophysics 382 (2004) 129–150

Compressional salt tectonics (Angolan margin)

Jean-Pierre Brun*, Xavier Fort

Geosciences Rennes, UMR 6118 CNRS, France

Institut de Geologie, Campus de Beaulieu, University de Rennes1, Ave du General Leclerc, Bat 15. 35042 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

0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tecto.2003.11.014

* Corresponding author. Geosciences Rennes, Campus de

Beaulieu, University de Rennes1, Ave du General Leclerc, Bat

15. 35042 Rennes cedex, France. Tel.: +33-2-23-23-60-94; fax:

+33-2-23-23-26-93.

E-mail addresses: [email protected]

(J.-P. Brun), [email protected] (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;

J.-P. Brun, X. Fort / Tectonophysics 382 (2004) 129–150130

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 extension-

al 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 prop-

agation (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 result-

ing 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; compres-

sional diapirs: Ge et al., 1997; compression of exten-

sional 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.

Fig. 1. Restoration of the initial geometry of the Aptian salt basin

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 down-

slope 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 (Nurnberg and Muller, 1991), rifting

of the Angolan margin starts at around 144–140 Ma

(Tesseirenc and Villemin, 1989; Guiraud and Maurin,

1992) and comes to an end at around 127–117 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 Precam-

brian 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 deposi-

tion 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 recog-

nised. From Albian to Late Cretaceous, a carbonate

platform was followed by sedimentation increasingly

of the Angolan margin (modified after Marton et al., 2000).

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/Tecto

nophysics

382(2004)129–150

131


dominated by marls and clays (Walgenwitz et al.,

1990; Lavier et al., 2001). From the Late Cretaceous

to Eocene, sedimentation was dominantly siliciclas-

tic, but with a significant proportion of carbonates.

Most of these deposits are located on the platform,

pinching out seaward into condensed sections indi-

cating a long-term highstand during the Late Creta-

ceous (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 Oligo-

cene 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 sedi-

ments 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

Fig. 3. Laboratory experiments. (a) Apparatus, model and procedure of sy

structural zonation induced by gravity spreading. (c) Initial geometry of t

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 strati-

graphic levels.

The downslope compressional domain, which is

the object of the present study, starts with a subdo-

main 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-wave-

length folds (around 2.5 km), which developed re-

cently and affected the seafloor. To the west of the

compressional domain, salt is absent and sediments of

the abyssal plain are undeformed.

nkinematic sedimentation. (b) Cross section of a model showing the

he three models presented in the paper.

J.-P. Brun, X. Fort / Tectonophysics 382 (2004) 129–150 133

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. moder-

ate 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,

Fig. 4. Top views of model 3 showing the evo

corresponding to the above-described structural zona-

tion (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-spread-

ing 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.

3.1. Materials and scaling

The experiments presented here are carried out

using the same materials, techniques and scaling

principles used in the literature (Vendeville and

lution of compression during spreading.

Fig. 5. Serial sections in the compressional domain of model 1.



Jackson, 1992a; Weijermars et al., 1993). The exper-

imental 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

1.400 kg/m3 and an angle of friction in the range

30–33j, but without significant cohesion. To repre-

sent the salt, we used a silicone putty (transparent

gum SGM36, Rhone Poulenc, France) with a New-

tonian viscosity l = 104 Pa s and a density q = 1.000

kg/m3. The density contrast between silicone putty

and sand (Dqm = 1.4) is slightly higher than might be

expected between salt and sediments in nature

(Dqp = 1.05–1.18 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*)

Fig. 8. (a) Schematic diagram summarising the evolution of folding in the

the relationships between pop-up-type structures and growth synclines. A t

same symbol is used in a number of the following figures.

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 1.000 to 1.400 kg/m3 and

rocks from 2.300 to 3.000 kg/m3. Because model and

prototype densities are of the same order of magni-

tude, 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.

3.2. 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 double-

wedge-shaped, thinning both upslope and down-

slope, and is entirely overlain by the 1.0-cm-thick

prekinematic layer (Fig. 3a). Such an initial geome-

try simulates the initial shape of salt basins at

passive margins (Fig. 1). As soon as the prekine-

matic layer is deposited, the rigid base is inclined

and spreading starts. Synkinematic sand layers are

zone of strong shortening. (b) Detail of two model sections showing

hick white bar indicates the thickness of the prekinematic layer. The

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.

Fig. 10. Pop-up-type structures associated with growth synclines in

model (a) and seismics (b and c).


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 depos-

ited using a funnel to simulate a sedimentary pro-

gradation at model scale. Each new layer is wedge-

shaped, with a thickness of 2 mm at the back, and

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 devel-

opment of structures.


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 3.

4. Structure of models and downslope compression

Fig. 3b shows the typical structural zonation

obtained in the spreading type experiments, regardless

Fig. 11. Salt extrusion between a growth syncline and

of the initial conditions. The upslope domain is

characterised by extensional structures: grabens, tilted

blocks, rollovers and extensional diapirs. The down-

slope 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 pat-

terns 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).

a tilted slab in model (a) and seismics (b and c).

nophysics 382 (2004) 129–150

4.1. 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, com-

pression 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

J.-P. Brun, X. Fort / Tecto140

Fig. 12. Growth syncline located in a thrust footwall in model

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.

4.2. 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

(a) with a sketch of evolution (b) and seismics (c and d).

Fig. 13. Compressional diapir between growth synclines.

Fig. 14. Schematic diagram summarising the evolution of pinched

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 incorpo-

rated into the ductile layer. This along-strike varia-

tion 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

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. noncylindric-

ity). This results from interactions between mechani-

cal instabilities of the brittle–ductile system during

shortening, amplified by the effects of synchronous


sedimentation. Consequently, the bulk structural pat-

tern of the compressional domain can be expressed as

a combination of elementary structures, but not al-

ways 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 com-

pression and possibly involving the domain of

downslope migration. Deformation starts with fold-

ing 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

Fig. 15. Nearly detached pinched growth sy

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 in-

to 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 anti-

clines flanked by pop-down synclines are also ob-

served locally (e.g. Fig. 6).

5.1. 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 compres-

sional diapirs. The distance between the sections of

Fig. 9a–b and c–d is broadly equal to the model

thickness.

ncline in seismics (a) and model (b).

Fig. 16. Onlaps and unconformities in model (a) and seismics (b).


In model 1 (Fig. 9a), a series of small-wave-

length folds affect the prekinematic layer. Among

these folds are four pinched synclines plus one pod-

like 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 extrud-

ed 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 compres-

sional 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 varia-

tion 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-wave-

length 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.

5.2. Growth synclines and pop-up anticlines

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 com-

pare 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

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 pop-

up-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 under-

go progressive pinching during their development/

deformation.

Fig. 18. Schematic diagram summarising the

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

evolution of double-wavelength folds.

J.-P. Brun, X. Fort / Tectonophy

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 anti-

clines 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 life-

time of a particular compressional structure being

purely local.

6. Deformation in zones of moderate shortening

As already pointed out, moderate shortening most-

ly 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 migra-

tion, compression reaches the extensional domain and

leads to tectonic inversion and squeezing of exten-

sional diapirs.

Fig. 19. Series of squeezed diapirs i

6.1. 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 re-

lief. 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 Cre-

taceous 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

sics 382 (2004) 129–150 145

n seismics (a) and model (b).


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). Syn-

kinematic 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).

6.2. Squeezing of extensional diapirs

During upslope migration, compression reaches the

downslope part of the extensional domain where reac-

tive 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 over-

hangs. 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 displace-

ments. On the downslope side, this could be inter-

preted 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 partic-

ular 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 combin-

ing the analysis of seismic data, from the compres-

sional 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

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.



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 occur-

rence 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 double-

wavelength 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, small-

wavelength folds affect a sedimentary succes-

sion 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 com-

pressional diapirism, the presence of salt at various

levels in the sections leads to strong perturbations of

the seismic signal, (iii) imaging sediments incorporat-

ed within salt will probably require specific develop-

ments in seismic processing, and (iv) well data are not

yet available in the ultradeep area affected by com-

pression. 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 sedimen-

tary layers beneath (Fig. 20b). The experiments pre-

sented here demonstrate other possible scenarios

combining folding and thrusting to obtain simulta-

neous salt extrusion and sediment incorporation with-

in 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 Escape-

ment after the seaward translation of the Angola salt

nappe became blocked by sedimentation in the abys-

sal 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 (Geosciences 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.

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