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The Guinea continental margin: an example of a structurally complex transform margin
TE~OIIOFIffSI~ ELSEVIER
Tectonophysics 248 (1995) 117-137
The Guinea continental margin: an example of a structurally complex transform margin Jean Benkhelil a, Jean Mascle b, Pierre Tricart
c
a Laboratoire de SEdimentologie et Gdochimie Marines, CNRS URA 715, Universitd de Perpignan, Avenue de Villeneuve, 66 860 Cedex Perpignan, France b Laboratoire de G~odynamique Sous-Marine, CNRS URA 718, B.P. 48, 06230 l,Tllefranche-sur-Mer, France c Laboratoire de Gdodynaraique des chatnes alpines, CNRS URA 718, Institut de Gdologie de l'Universitd Joseph Fourier, Rue Maurice Gignoux, 38031 Grenoble Cedex, France
Received 3 June 1994; accepted 24 October 1994
Abstract
The Guinean continental margin corresponds to a wide hinge zone which developed between the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic. The Guinea Marginal Plateau that characterizes the southern border of this margin is a wide triangular morphological feature bounded to the west by a typical rifted margin slope segment, while its southern edge corresponds to a narrow and morphologically complex continental slope probably resulting from intracontinental transcurrent motion during the Early Cretaceous. On the basis of seismic profiles recorded in two selected areas located along the southern Guinea margin, a seismic stratigraphy is established. At depth, the plateau is underlain by a thick Mesozoic and Cenozoic sedimentary wedge made of two main ensembles. A lower ensemble (or sequence 1) consists of Early Cretaceous clastics. An upper ensemble includes a series of four distinct sequences attributed to Albian to Tertiary deposits. Eastward, the sedimentary cover of the southern plateau edge is locally pierced by volcanic bodies of Paleocene age. Beneath the southern margin of the Guinea Plateau, the sedimentary sequences appear deformed by faults and folds resulting from, at least, two main tectonic episodes. A first event only affected sequence 1, which is cut by a set of extensional faults. A second tectonic phase including folding, reverse faulting and transcurrent faulting is responsible for the deformation of sequences 1 and 2. During this tectonic episode, former normal faults have been reactivated as reverse faults resulting in a structural inversion. A third tectonic phase was responsible for structures transverse to the general E - W trend of the southern Guinea margin and consisting of N-S-trending normal faults and associated volcanoes. The spatial distribution of the tectonic trends clearly substantiates a polyphase tectonic activity that can be related to plate motion changes during early stages of the continental separation. A first stage is characterized by a divergent rifting leading to N-S-trending synsedimentary normal faults. During the Cretaceous, the tectonic activity increased as a general transform motion occurred between the Guinea Plateau and its American conjugate margin (the Demerara Plateau). I ~ c a l tensional stresses were responsible for a splay fault system active near the tip of the Guinea Plateau. In late Albian times, a slight adjustment of the Africa plate, with respect to the South American one, resulted in local squeezing of the whole sedimentary wedge. After this short compressive event, final continental separation occurred in an extensional regime characterized by a general collapse along two major scarps and volcanic activity.
0040-1951/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0040-1951 (94)00246-0
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.I. Benkhelil et al. / Tectonophysics 248 (1995) 1 1 7 - 1 3 7
I. Introduction
torial Atlantic appears as a key area, and according to the different models, the consequences on the surroundings continental margins should have been recorded through different tectonic regimes. During the Mesozoic, the Equatorial Atlantic area acted as a wide strip of shearing where margin segments experienced transform motion and associated rifting. Three main shear zones (Fig. 1) are either direct consequences of transform evolution of the oceanic basins, or have accommodated parts of the motion if the hypothesis of intraplate discontinuities is accepted. The Romanche Fracture area was, and is still the major transform boundary between the Africa and South America plate; this fracture zone has controlled the formation of the transform C6ted'Ivoire-Ghana margins as well as of the C6ted'Ivoire deep divergent basin and Brazilian equiv-
Off West Africa the Mesozoic evolution of the continental margin is tightly controlled by the variations of the African and the American plates relative motion (Le Pichon and Hayes, 1971: Mascle, 1977; Rabinowitz and Labrecque, 1979; Jones and Mgbatogu, 1982; Klitgord and Schouten, 1986; Jones, 1987). The diachronous opening of the Central and South Atlantic has been evaluated in terms of plate kinematics (Bullard et al., 1965; Le Pichon, 1968; Le Pichon and Hayes, 1971; Sibuet and Mascle, 1978) and different predrift reconstructions were based either on a rigid behaviour of plates (Pindell and Dewey, 1982) or on potential intraplate deformations (Curie, 1984; Unternehr et al., 1988; Fairhead and Binks, 1991). In both hypotheses, the Equa-
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J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
alents (Mascle and Blarez, 1987; Basile, 1990; Basile et al., 1993). The Chain and the Charcot fracture zones may connect onshore with features in the Benue Trough, itself interpreted as a major intracontinental discontinuity (Mascle, 1977; Benkhelil, 1982) where deformations have been correlated with plate adjustment during the initial stages of the Atlantic opening. A third major shear zone corresponds to the southern Guinea margin connected with the Guinea Fracture Zone (Mascle et al., 1988). In this paper, we intend to synthetize the seismo-stratigraphic and structural characteristics of the southern Guinea margin in order to evaluate the tectonic environment in which this margin
has experienced shearing during early stages of the continental separation. Today, the southern Guinea margin comprises two main segments, trending, respectively, north-south and east-west and bounding the Guinea Marginal Plateau (Fig. 2): the western segment can be interpreted as a typical rifted margin (of Jurassic age) (Marinho, 1985); the southern segment is interpreted as a complex transform margin related to the activity of the Guinea Fracture Zone and was created during Early Cretaceous times (Marinho, 1985). The southern Guinea margin has been surveyed during several cruises, particularly during the Equamarge I (1983) and Equamarge II (1988) cruises (Fig. 3). The data (Marinho, 1985; ~
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120
.I. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
Benkhelil et al., 1989) have revealed a complex structural pattern. During these cruises, attention was focused on the western tip of the Guinea Marginal Plateau, a wide triangular deep plateau, which lies at the boundary between two areas of the Atlantic created at different periods and according to different rift geometries (Mascle et al., 1986).
gins were generated chiefly through transform movements and display a series of either rifted or sheared segments. The Guinea continental margin is itself characterized by the presence of a wide marginal plateau that forms a triangular wedge protruding into the Atlantic Ocean. The plateau is a 250kin-wide, gently seaward-dipping platform without any significant morphological features (Fig. 3). Westward, at about 1500 m of water depth, the plateau is interrupted by a wide N N E - S S W trending slope where gravity sliding is common. Its southern slope exhibits a more complex morphology reflecting a complex internal structure. The slope includes a triangular-shaped basin (or mid-slope basin) wedged between two sub-linear scarps (Fig. 4; Jones and Mgbatogu, 1982). This basin widens toward the northwest where it connects progressively with the N-S-trending margin segment and is cut in its central part by a N - S trending valley. Eastward, the basin becomes narrower and the slope grades into a single scarp
2. Main morphological characteristics of the West African continental margin Along West Africa, the morphology of the different margin segments reflects the geodynamic processes responsible for their formation and for their subsequent evolution. From Morocco to GuinEe Bissau, the margin can be interpreted as a typical divergent one resulting from an early Jurassic rifting. From southern Guinea to the Niger Delta, the Equatorial Atlantic mar-
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J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
where several sub-circular volcanic seamounts emerge. South of the Guinea margin, the Sierra Leone Rise is a large aseismic elevation (Fig. 2), trend-
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ing in a SW direction and interrupted near the St. Paul Fracture Zone (Emery et al., 1975). This still poorly known oceanic rise is a complex volcanic and l~ectonic feature characterized by a rugged
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122
J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
t o p o g r a p h y d u e to n u m e r o u s p r o m i n e n t seamounts (Perfil'yev et al., 1987; Jones et al., 1991).
3. Seismic stratigraphymtentative dating During the Equamarge II cruise, about 3900 km of continuous seismic profiling were recorded in two selected areas (G1 and G2) located along the southern Guinea margin (Figs. 3 and 4). The proposed seismic stratigraphy, based on a detailed analysis of the seismic lines, includes five acoustic sequences that we integrated into two main ensembles exhibiting different seismic reflection patterns, and volcanic bodies. 3.1. The lower e n s e m b l e
The lower ensemble corresponds to the deepest sequence (sequence 1) that can be detected
beneath both the marginal plateau border and the mid-slope basin. Beneath the plateau, sequence 1 shows high-amplitude reflectors, slightly continuous in the upper part (Fig. 5, G14); internal reflections are mostly parallel, although locally slightly divergent. This sequence is clearly deformed by faulting and folding and its top is capped by strong reflectors of regional extent delineating a clear unconformity. Along the upper scarp, this sequence is more difficult to identify and often affected by vertical fault displacements; in this area sequence 1 may locally crop out. Within the mid-slope basin, the sequence is well expressed and can be subdivided into two sub-sequences. A lower sub-sequence is characterized by parallel reflectors (sometimes divergent or curved) often covered in strong toplap by an upper sub-sequence; however, toward the basin centre both sub-sequences are in conformity (Fig.
N 1 sdwtt
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Fig. 5. Seismic reflection profiles (lines 12 and 14, see Fig. 4) across the southern Guinea continental margin. The upper ensemble (2-5) includes flat-lyingsequences resting unconformablyupon the folded sequences (1) of the lower ensemble (G14). Section G12 cuts across the mid-slope basin showing the distinct sequences 1 to 5. sdwtt = second double way travel time.
J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
5, G12). The upper sub-sequence is only detected towards the basin centre where it consists of discontinuous reflectors becoming more continuous westward. The base of sequence 1 has not been detected seismically, but it is greater than 2 s of two-way seismic travel time. This sequence has been assigned to the Lower Cretaceous (Mascle et al., 1986) and samples cored just below the unconformity have yielded upper Albian calcareous sandstones (Moullade et al., 1993).
Sequence 4
3.2. The upper ensemble
Sequence 5
The upper ensemble includes four distinct seismo-stratigraphic sequences being from bottom to top:
Sequence 2 This constitutes the bulk of the infilling of the mid-slope basin where it reaches a thickness of 0.6 s (Fig. 5, G12). In this area it is made of a weakly reflecting lower part (2a), in concordance with the underlying ensemble, and of a strongly reflective upper part (2b). Beneath the plateau, sequence 2 lies unconformably on sequence 1 and corresponds to a series of sub-horizontal continuous reflectors. Its base has also been cored and consists of black shales of Cenomanian-early Turonian age (Moullade et al., 1993). Sequence 2b is attributed to Upper Cretaceous marine sediments.
Sequence 3 In the mid-slope basin sequence 3 exhibits a very weak seismic layering, with rare discontinuous reflections. It corresponds to lenticular bodies, narrowing along a N120°E direction and overlying an erosional surface locally characterized by strong reflectors. On the marginal plateau, sequence 3 shows a chaotic facies and its base is concordant while its top shows slight onlaps. This sequence, which may be locally missing, has a highly variable thickness. A Middle to Late Eocene age has been assigned to sequence 3 by Marinho (1985) according to correlations with DSDP site 367 (Lancelot et al., 1977).
123
On the marginal plateau, sequence 4 contains continuous well-layered reflectors. Within the mid-slope basin, this sequence is made of parallel (sometimes chaotic) reflectors reaching a maximal thickness of 0.5 s. Its specific acoustic character is interpreted as indicative of alternating sandy and clayey beds and may correspond to prograding deltaic series. A speculative Eocene to middle Neogene age (Marinho, 1985) is assigned to this sequence by correlation with DSDP 367.
Well developed in the mid-slope basin, sequence 5 is affected by numerous syn-sedimentary phenomena. The few discontinuous reflectors of this almost acoustically transparent sequence show various amplitudes and frequencies. On the plateau, its basal boundary is concordant (or slightly onlapping), and its thickness is less than 0.25 s. This sequence correlates with recent soft sediments shaped by bottom currents (Rossi et al., 1992), that have built up a series of slopeparallel sedimentary mega-waves in the western area and moats infillings in the eastern volcanic area. The age of the sequence ranges from Pliocene to Present times (Marinho, 1985).
3.3. Volcanic-type features Both the southernmost Guinea margin slope and the northern Sierra Leone abyssal plain are pierced by subcircular seamounts interpreted as volcanic bodies on the basis of their high-amplitude magnetic anomalies (Jones and Mgbatogu, 1982) or by sampling (Bertrand et al., 1988). These seamounts lie along an E-W-trending belt running for more than 300 km from 19 to 15°W approximately on strike with the Guinea Fracture Zone. Westward, this line of seamounts connects with the northern edge of the Sierra Leone Rise. East of 17° 10'W, the magmatic belt clearly intersects the slope trend. Across both the continental slope and the northern Sierra Leone Basin the seismic reflection data show two volcanic-type features (Fig. 6): --volcanic cones, directly exposed on the sea floor, or hidden below a thin sedimentary cover;
J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
124
GuineaMarginal Plateau N sdwlt 11
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Fig. 6. Interpreted seismic section of the volcanic area (Nadir Mount; location in Figs. 4 and 7). The numbering corresponds to the sedimentary sequences.
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--sills (or lava flows) interbedded in the sedimentary cover (mainly within sequence 2 and sometimes in sequences 3 and 4). Interbedded lava flows, characterized by a typical diffraction pattern may be identified within the eastern mid-slope basin area and near the border of the Sierra Leone Basin. Seismic profiles indicate lateral continuity between the volcanic cones and interbedded horizons. In the eastern volcanic province a main volcano forms a huge seamount (Fig. 4; about 20 km in diameter and 2.5 km in elevation) characterized by an
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Fig. 7. Structural map of part of the southern Guinea continental margin. The margin is segmented by a major N-S-trending transcurrent fault zone defining two main areas with different structural trends ( A - A ' = line section of Fig. 6).
J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
E-W-trending crest that rises at 840 m below sea level. Other volcanic cones of smaller sizes (ranging between 2 and 5 km in diameter) exist and are interpreted as secondary vents. Five secondary volcanoes constitute with the main one, two, N15 ° E- and E-W-trending alignments. During the Equamarge cruises and more recently, during the Equanaute diving cruise (1992), magmatic rocks have been collected from the main volcano. The rock types consist of alkali basalts and polygenic breccia made of basalts, basaltic hyaloclastics and scarce ultramafic xenoliths. A Paleocene age was obtained on biotite phenocrysts (58.6 Ma) from a massive basalt (Bertrand et al., 1988,1989,1993).
4. Structuralunits of the southern Guinea margin The main structural characteristics of this margin were previously outlined (Marinho, 1985). The new set of closely spaced seismic lines (Fig. 3), has enabled a more detailed analysis of the various tectonic features and a relatively precise structural mapping. According to Marinho (1985) the southern Guinean Margin can be subdivided into seven main morphostructural units arranged in ESEWNW-trending strips and roughly parallel to the main trend of the continental slope (Fig. 7): (a) The marginal plateau corresponding to a wide deep triangular platform extending from the shelf to about 300 km seaward. This plateau
125
shows a gentle regular slope, underlain by almost fiat-lying horizons, resting unconformably upon the lower ensemble (Fig. 8). (b) An upper scarp roughly bounded upward by the 1500 m isobath and corresponding to an important slope break (Fig. 9). It is made of two E-W-trending segments, changing westward to an almost N-S strike. This scarp, that varies in slope from 10 to 15%, is thickly covered by recent sediments; locally, steeper slopes display lower ensemble outcrops. (c) A mid-slope basin that extends at the foot of the upper scarp, consisting of a triangular rugged area about 20 km wide at its centre. Eastward, the basin thins out running in E - W direction and westward it widens while curving towards a NW-SE direction (Fig. 7). Seismic lines illustrate that the basin can be interpreted as a wide synclinorium structure separated into two areas by a N170 ° E-trending fault zone (Figs. 7 and 10), detected over a distance of 60 km and coinciding with a cross-slope valley along 17° 30'W (Fig. 4). (d) A southern bordering marginal ridge corresponds to the topographic expression of a wide tilted block (Figs. 7 and 9). This ridge, disrupted by the N-S-trending valley (Fig. 4), is oriented N125°E to the west and trends east-west to the east of the valley. This change of direction occurs on both sides of the N170 ° E-trending fault zone with an apparent horizontal offset of about 10 km of the ridge. East of the fault zone, the crest of the ridge which correlates with the top of the half-graben shows a clear inflexion, from N125 ° E
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Fig. 8. Interpreted seismic section (line 14, see Fig. 4) showing a set of normal faults affecting sequence 1 unconformably covered by sequences 2 to 5.
126
.L Benkhelil et aL / Tectonophysics 248 (l 995) 117-137
to N80 ° E. Such a sharp change in trend may be correlatable with a dragging effect related to a dextral transcurrent movement. (e) A lower scarp, which corresponds to the southern slope of the previous feature, underlines a second major slope break; it becomes steeper east of the N170 ° E-trending fault zone (Fig. 7). (f) The foot of the continental rise correlates with an elongated, 10 km wide basin where most of the seismic sequences can be identified (Fig. 7). (g) Finally eastward, a volcanic area is characterized by steep reliefs corresponding to a major sub-circular volcano rising to 840 m below sea level and lateral vents dissected by radial valleys (Fig. 6).
5. Structural analysis Regional seismic profiles indicate that the continental margin of the Guinea Plateau displays a complex structural pattern, especially along its southern border. The two main seismo-stratigraphic ensembles described above show evidence of deformation which reflects various tectonic events that have affected the Guinea continental margin during the Mesozoic. Within an
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E - W - t r e n d i n g zone running along the southern margin continental slope, the deformation includes fracturing (at many scales) and gentle folding. 5.1. Fracturing
Fracturing is best expressed at the level of the lower ensemble which appears strongly cut by a grid of high-angle normal faults. Minor faulting is expressed by sets of conjugate faults, resulting in a pattern of tilted blocks. Beneath both the marginal plateau (Figs. 10 and l l a ) and the slope basin, this extensional pattern determines a system of small horsts and accompanying grabens and within the central mid-slope basin, several faults are clearly characterized by thickening of the sedimentary cover near the fault plane within the downthrown block; this is interpreted as a direct result of synsedimentary tectonic activity. Below both areas, normal faults vanish near the post-sequence 1 unconformity and die out within the base of sequence 2 (Fig. llb). The horizontal and vertical extents of normal faults vary from some tens to a few hundred meters; their azimuthal distribution varies from northwestsoutheast to east-west. Both the upper and lower scarps are themselves topographic expressions of major normal
MID-SLOPE BASIN
SSE i II
Fig. 9. Interpreted seismic section (line 3, see Fig. 4) across continental slope showing the different morphostructural ensembles. Sequences 2 to 5 underline the synclinorium shape of the mid-slope basin enbedded within the underlying sequence 1. OB = Oceanic Basement.
127
J. Benkhelil et aL /Tectonophysics 248 (1995) 117-137
fault systems. On seismic sections, the upper scarp corresponds to an area cut by a complex set of steep faults that chiefly affect sequence 1. This scarp seems to have resulted and recorded successive collapses of the mid-slope basin relative to the plateau. The lower scarp, which flanks the marginal ridge to the south, appears as an asymmetrical horst-like block border cored by sequence 1 (Fig. 9). A N170 ° E-trending depression underlain by a major fault zone cuts the upper scarp fault system. It is associated with a series of N30 ° E-trending faults located on the eastern side of the N170 ° E-trending fault zone interpreted as associated shears (synthetic strike slip fault; Fig. 10). In the western mid-slope basin, reverse faulting affects both sequences 1 and 2. The faults bound an anticlinal-like structure capped by the base of sequence 2, forming an upward-diverging splay pattern (Fig. llb). At depth, the faults seem to converge while upward they gradually flatten, dying out into the basal sequence 2. A reverse offset is clearly attested by an upward displacement of the inter-sequence 1 / 2 boundary. A thickening of sequence 1 against the fault plane is interpreted as an inheritance of former synsedimentary normal fault motion (Tricart et al., 1991). The reverse faults are closely associated with folding in the process of structural inversion. Among the major structures, a N170 ° E-trending fault zone (running approximately along 17° 30'W), cuts across the entire continental slope
MID-SLOPE BASIN W
(Fig. 7). Southward, this feature correlates with a lower scarp indentation; it is, however, best expressed within the mid-slope basin where it runs along the "median valley" (Fig. 4). This fault zone corresponds to a 3-km-wide positive flower structure (Fig. l l d ) well documented on E - W trending profiles (Fig. 10). The faults are nearly vertical and sealed by sequence 3 (Fig. 1 lc). The structure is flanked by two footwall troughs, filled with a thick sequence onlapping its western flank. This flower structure can be followed over more than 60 km from the southern border of the mid-slope basin to the marginal plateau (Fig. 7) where, although attenuated, it can still be identified. Defined as an uplifted and tilted block, the lower-slope marginal ridge, that appears fault bounded on its seaward side (along the lower scarp), is cut along its northern flank by a fault system progressively grading to a flexure westward. Eastward, this may evolve to a rupture particularly near the N170 ° E-trending fault zone (Fig. 7). Dragging along this border of a parallellayering pattern attests to a permanent tectonic activity of the marginal ridge during sequence 2 deposition. Beneath the marginal plateau itself, faulting is clearly sealed by the basal sequence of upper ensembles (likely sequence 2). Within the midslope basin, the fault system mainly affects the lower ensemble; some of the reverse faults may, however, have been active during deposition of
MARGINAL PLATEAU -.-E sdw.tt
Fig. 10. Interpreted seismic section (line 36, see Fig. 4) across the continental slope showing the transcurrent fault zone at the centre of the basin and a graben structure across the upper scarp.
.L Benkhelil et a L / Tectonophystcs 248 (1995) 117-137
128
5.2. Folding
the basal sequence 2. The activity of the marginal ridge border fault died out progressively during deposition of sequence 2, and finally ceased during deposition of sequence 3.
Along the southern margin of the Guinean Plateau, folding has been recognized by Marinho S
N sdwtt
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b Fig. 11. Interpreted seismic sections. (Only parts of sections G3, GI2, G31 and G36 are shown. On Fig. 4 the full trace of the original section is indicated.) (a) Folding and faulting beneath the Guinea Plateau (G12). (b) Reverse faults and folds associated with a flower structure in the mid-slope basin. The horizon (bold line) between sequences 2 and 3 corresponds to an unconformity which bounds the lower compressive structures. The dashed line corresponds to an internal differentiation of sequence 2 (G3). (c) Seismic reflection profile (G3) across the flower structure. (d) Both sections show the structure associated with the N-S-trending transcurrent fault: a flower structure flanked by two depressions, the western one being the deepest (G31 and G36).
J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
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J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
130
(1985) and analyzed in more detail by Tricart et al. (1991). At a regional scale the mid-slope basin can be interpreted as a wide synclinorium feature squeezed between two scarps and interpreted as a first-order folding structure. This syncline, about 40 km wide in its western sector, is progressively narrowing eastward where it reaches only 15 km, east of 17°30'W. Further to the east the basin, almost concealed by volcanic cones and associated lava flows, connects progressively with a monoclinal slope (Marinho, 1985). A second large-scale "synclinal" type feature corresponds to the elongated basin that lies at the foot of the lower scarp. This basin extends only west of the N170°E-trending fault zone where it contains internal second-order undulations. Locally, in the central mid-slope basin, the syncline appears flat bottomed and double hinged. Such a morphology is interpreted as a direct inheritance from deep-seated faults. Tricart et al. (1991) proposed that the former primary structures were extensional blocks superimposed on a wide graben. This structure, that later on grew during the short tectonic phase that occurred
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after deposition of sequence 1, has acted as a wide depocenter for sequence 2. The persistent activity of some of the faults and their bearing on the syncline is attested to by onlaps of sequence 2 along its flanks. The seismic lines show that the mid-slope basin is also affected by slight undulations, a few kilometres in amplitude, interpreted as second-order folds. These features are also expressed by wide, open, symmetrical deformations which are best expressed in sequence 1, and die out at the base of sequence 2. Second-order folding is not restricted to the mid-slope basin, and can be detected northward beneath the marginal plateau where it appears truncated by an erosional surface (Fig. 1 la). Finally, small-scale folds (about one kilometre in length) appear to have affected sequence 1 and part of sequence 2. They correspond to asymmetric and rounded isolated anticlines, particularly well displayed in the western part of the mid-slope basin, where they are generally associated with either second-order folds or reverse faulting (Fig. llb). They may either constitute a tight associa-
S
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Fig. 12. Block diagram illustrating the deformation of sequence 1 in the mid-slope basin. The overall synclinorial shape of the basin is interrupted by a flower structure, reverse faults and secondary folds. Open arrows indicate normal displacement, solid arrows indicate reverse/transcurrent movement.
J. Benkhelil et aL / Tectonophysics 248 (1995) 117-137
tion due to a stacking of anticlinal hinges, or correspond to isolated features separated by open box-shaped synclines. A synthetic block diagram showing the various deformations affecting sequence 1 is shown in Fig. 12. The sequence 1/2 interface is also affected by folding and especially by a flower structure flanked by two reverse faults. This structure is though to result from a tectonic inversion, the uplifted block being an older graben structure flanked by normal faults reactivated as reverse ones.
6. Distribution and relationship between folds and faults
As a whole the fold pattern and most of the fault systems constitute a set splaying out towards the northwest and pinching out against the N-Strending transcurrent 17°30'W fault zone (Fig. 13). The first-order syncline structure disappears both towards the north-northwest and eastward. The syncline axis splays out towards northwest and north, and progressively connects with the
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N-S-trending western Plateau border. The southern marginal ridge progressively smooths out westward and abuts against the N-S-trending segment of the Guinea Plateau. East of the 17°30'W fault zone, the mid-slope syncline dies out, grading progressively to a monoclinal structure (Fig. 13). The major fault systems control most of the main topographic features of the margin. They form a series of E-W-trending segments truncated by two oblique sets, a N170° E and a N3040°E direction. The small-scale faults (both normal and reverse) contribute to N90°E to N140 ° E-trending systems. The N170 ° E-trending fault zone, and a few similarly trending secondary faults, are contemporaneous with the folding. Both the trend of the fault relative to the folds, and the associated flower structure, are indicative of a transcurrent motion. Secondary fault arrangements about this major structural feature are in agreement with such an interpretation (Fig. 7). The structural fabric of the southern Guinea margin segment is, in our opinion, consistent with
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Fig. 13. Structural sketch of the southern Guinea margin. The strongest deformations are located to the west of the N-S-trending fault zone. Magmatic activity is restricted to an E - W - t r e n d i n g strip in the southernmost side of the margin (to the east of 17° W data are from Marinho, 1985). Note the offset of the oceanic basement boundary on both sides of the transcurrent fault. D = Demerara Plateau; G = Guinea plateau.
132
J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
an E - W direction superimposed to former N - S structural trends presently represented by the main N-S-trending shear zone (Fig. 7). The present segmentation of the margin arises from two main fracturing phases. A first one, associated with a compressional event acting on margin segment corresponds to the transcurrent reactivation of previous N-S-trending extensional features. Transverse normal faults and volcanoes alignment, attributed to a Late Cretaceous extensional episode, has also segmented the margin following N15 ° E and N30 ° E directions (volcanic area; Figs. 7 and 13).
7. Geodynamic evolution of the southern Guinea margin
7.1. The pre-rift reconstruction (Fig. 14a) The conjugate American margin of the Guinea Plateau is the Demerara Plateau, a broad submarine plateau north of French Guyana (Fig. 2). In several reconstructions of the central Equatorial Atlantic (Klitgord and Schouten, 1986; Mascle et al., 1986) it has been proposed that both plateaus were part of the southern border of the Central Atlantic margin during Jurassic times prior to becoming conjugate margin segments. Around 180 Ma (Mascle et al., 1988) this area was believed to be under a prevailing tensional stress regime (rifting of the Central Atlantic) which resulted in a series of faulted blocks and grabens where clastics were trapped in significant amounts (Jones and Mgbatogu, 1982). At that time, the prevailing structural trends were trending north-south, indicating a dominantly E-W-oriented extension. Today we have no evidence of such an early structural pattern the only exception being the N170 ° E-trending fault zone that could represent a reactivated structure related to this early event. In terms of plates, the West African and South American blocks were still attached forming a single block probably already subjected to tensional stresses.
7.2. The rifting phase (Fig. 14b) In Early Cretaceous times, both margins ( G u i n e a - D e m a r a r a ) started to split along the Guinea Fracture, probably as a result of pre-existing Pan-African lines of weakness. The Demera r a / G u i n e a plateaus, still in contact, were sliding one past the other along a continental transform boundary. The transform motion occurred mainly under a transtensional regime in the sense of Harding (1985) and generated tensional features recorded on the newly created southern Guinea margin area. The corner-like shape of the Guinea margin shape led to the development of a pinnate set of tensional fractures. The NW-SE-trending faultbounded blocks, were oblique to the previous N-S-trending Jurassic trends. Their arrangement can be interpreted as splay faults within a fault system termination created parallel to the transcurrent fault zone; they splay out westward and curve to connect with the N-S-trending Jurassic faults. The two major fault zones that have shaped the present slope scarps, are thought to have been created during this transtensive event and thus to have directly initiated the mid-slope basin. This rifting is also marked by the occurrence of synsedimentary normal faults which bound a system of tilted blocks and associated grabens filled with clastics. They are the oldest tectonic structures (Early Cretaceous) detected below the Guinea Plateau. Along the northern Demarara escarpment, this event seems to have been differently recorded. Beneath the northern Demerara plateau, the pre-Albian sedimentary sequences are deformed by folds and flower structures. The folded area forms an E-W-running belt with an "en 6cheion" fold arrangement interpreted as dragging effects resulting from a dextral motion along the Guinea Fracture Zone (Gouyet, 1988). The eastern margin of the Demerara Plateau is a rifted segment characterized by steep slopes and escarpments, facing the Sierra-Leone/Liberian margins with evidence for associated volcanic activity. This episode corresponds to the break up of the West African/South American continental
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block into two blocks which began to slide past one on other along the Guinea Fracture zone. Separation occurred between the eastern Demerara Plateau and the continental margin off Sierra Leone and Liberia with probable creation of oceanic crust.
7.3. The compressional phase (Fig. 14c) A second deformation episode corresponds to a short compressional event that has affected the previous structures. The resulting compressive deformation with locally reactivated Early Cretaceous tensional fractures leading to structural inversion, are dearly post-rift. Since both extensional and compressive structures are comparable in direction, normal faults were easily reactivated as reverse faults associated with folding. In this stress regime, the N-Strending faults of Jurassic age were also reactivated but as dextral transcurrent zones. All the compressional trends are consistent with a N30 ° E-oriented compression with a maximum horizontal shortening located near the southwestern tip of the Guinea margin. Eastward, the major syncline structures correspond rather to basinal synclines than to tectonic derived folds. Such deformation, restricted both in time and space, suggests plate boundary adjustments rather than a continuous transform motion. The timing of this event is constrained by the striking angular unconformity detected on the Guinea margin (Figs. 8 and lla). The recovery of sandstones and black shales, located, respectively,
135
just below and just above the unconformity, provides a post-middle Albian to pre-late Cenomanian age (Moullade et al., 1993), although declining tectonism locally persisted during deposition of sequence 2, since part of this sequence bears evidence of deformation, also detected in the vicinity of the N170°E transcurrent area. The base of sequence 3, characterized across the margin by regular reflecting horizons, sealed the compressive deformation. On the Guinea margin such a local compression cannot be related to dragging effects due to the transcurrent Guinea Fracture Zone as suggested for the Demerara Plateau (Gouyet, 1988). We believe that local mechanisms such as block re-adjustments, maybe linked to a pole of rotation shift, are more likely. Such mechanisms were predicted in the kinematic reconstruction of Rabinowitz and Labrecque (1979) for the late Aptian. A subsequent slight anticlockwise rotation of the Guinea Plateau relative to the Demerara Plateau, may explain the NE-SW-oriented compression deduced from the mapping of compressive deformation. Such a scenario may account for a slight squeezing of Lower Cretaceous sediment against the Guinea Fracture Zone, and for a dextral reactivation along the N170 ° E-trending transcurrent fault zone (Fig. 14c). In our opinion, this is also in good agreement with the relative confinement of the deformation to the apex of the Guinea margin; the N170 ° E-trending transcurrent fault may have acted as a major structural barrier between a western, strongly deformed, and an eastern, less deformed domain. Compressional structures have been reported by Gouyet (1988) along the northern segment of the Demerara plateau. When compared with the Guinea margin, and while showing a comparable overall structure, we note discrepancies between the tectonic processes and the timing that have governed the evolution of both marginal plateaus. Concerning, the "en 6chelon" folds arrangement on the Demerara plateau, we note a significant difference from the fold trends as detected on the Guinea margin. The attribution of these "en ~chelon" folds to a dragging effect during the rifting phase is based on the age of the unconformity that postdates the tectonic event. The age
136
.L Benkhelil et al. / Tectonophysics" 248 (1995) 117-137
of the unconformity beneath the Guinea Plateau is well constrained to the interval middle-late Albian (Moullade et aI., 1993). Beneath the Demerara plateau a double unconformity is reported from the sedimentary succession being, respectively, late Aptian and late Albian in age (Gouyet, 1988). The first unconformity may be correlated with the formation of the "en 6chelon" folds, while the second one may be an equivalent of the main compressive phase on the Guinea Plateau.
7.4. The tensional phase (Fig. 14d) After minor recurrence of compressive tectonics, a clear change in the tectonic regime occurred during the Late Cretaceous. An extensional regime, characterized by the accentuation of the mid-slope basin and by a general collapse of the margin along the upper and lower faults scarps, prevailed; deformation during this period is likely to be related to the final continental parting between the African and South American continental plates, and the creation of an oceanic crust between the Demerara and Guinea plateaus. Along the southern Guinea margin extensional tectonism was then accompanied by widespread magmatic activity, located near the ocean/continent boundary; the E-W-trending belt of volcanoes is interpreted as injections along normal faults and locally interrupted by the N170°E trend. Along the southern Guinea margin tectonic deformation vanished near the end of Cretaceous times. Since that time the region has been subject to more normal cooling subsidence; strong bottom currents closely controlled sedimentation.
Acknowledgements We thank the reviewers E.S.W. Jones and R. Scrutton for their critics and suggestions to improve the manuscript. GEMCO (URA-CNRS) contribution No. 655. References Basile, C., 1990. Analyse structurale et mod~lisation analogique d'une marge transformante: l'exemple de la
marge profonde de C6te-d'lvoire-Ghana. Ph.D. Thesis, Univ. P. et M. Curie, Paris, 205 pp. Basile, C., Masele, J., Popoff, M., Bouillin, J.P. and Mascle, G., 1993. The Ivory Coast-Ghana transform margin: a marginal ridge structure deduced from seismic data. Tectonophysics, 222: 1-19. Benkhelil, J., 1982. Benue Trough and Benue Chain. Geol. Mag., 119: 155-168. Benkhelil, J., Mascle, J., Villeneuve, M., Tricart, P., Auroux, C., Basile, C., Ciais, G. and the Equamarge Group, 1989. La marge transformante sud-guin6enne: premiers r6sultats de la campagne Equamarge II (F~vrier 1988). C. R. Acad. Sci. Paris, 308: 655-661. Bertrand, H., Mascle, J,, Marinho, M. and Villeneuve, M., 1988. Volcanics from the Guinea Continental Margin: geodynamic implications. J. Aft. Earth Sci., 7: 181-188. Bertrand, H., Mascle, J., Villeneuve, M., Robert, C., Cousin, M. and the Equamarge Group, 1989. Le volcanisme de la marge sud-guin~enne, implications pour l'ouverture de ['Atlantique Equatorial: r6sultats de la campagne Equamarge. C. R. Acad. Sci. Paris, 309: 1703-1708. Bertrand, H., Feraud, G. and Mascle, J., 1993. Alkaline volcano of Paleocene age on the Southern Guinean Margin: Mapping, petrology, 4°Ar-3'~Arlaser probe dating, and implications for the evolution of the Eastern Equatorial Atlantic. Mar. Geol., 114: 251-262. Bullard, E.C., Everett, J.E. and Smith, A.G., 1965. The fit of the continents around the Atlantic. In: P.M.S. Blacken, E. Bullard and S.K. Runcorn (Editors), Symposium on Continental Drift. Philos. Trans. R. Soc. London, Set. A, 258: 41-51. Curie, D., 1984. Ouverture de l'Atlantique Sud et discontinuit,s intra-plaques: une nouvelle analyse. Thesis 3rd cycle, Univ. Bretagne Occidentale, Brest, 192 pp. Emery, K.O., Uchupi, E., Phillips, J., Bowin, C. and Mascle, J., 1975. The continental margin off Western Africa: Angola to Sierra Leone. Am. Assoc. Pet. Geol. Bull., 59: 2209-2265. Fairhead, J.D. and Binks, R.M., 1991. Differential opening of the Central and South Atlantic Oceans and the opening of the West African rift system. Tectonophysics, 187: 191-203. Gouyet, S., 1988. Evolution tectono-s6dimentaire des marges guyannaise et Nord-Br~silienne au cours de l'ouverture de I'Atlantique Sud. Ph.D. Thesis, Univ. Pau et des Pays de l'Adour, 374 pp. Harding, T.P., 1985. Seismic characteristics and identification of negative flower structures, positive flower structures and positive structural inversions. Am. Assoc. Pet. Geol. Bull., 69: 582-600. Jones, E.J.W., 1987. Fracture zones in the Equatorial Atlantic and the breakup of Pangea. Geology, 15: 533-530. Jones, E.J.W. and Mgbatogu C.C.S., 1982. The structure and evolution of the West african continental margin off Guin~ Bissau, Guinfe, and Sierra Leone. In: R.A. Scrutton and M. Talwani (Editors), The Ocean Floor. Wiley, New York, NY, pp. 165-202. Jones, E.J.W., Goddard, D.A., Mitchell, J.G. and Banner, F.T., 1991. Lamprophyric volcanism of Cenozoic age on
J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137 the Sierra Leone Rise: implications for regional tectonics and the stratigraphic time scale. Mar. Geol., 99: 19-28. Klitgord, K.D. and Schouten, H., 1986. Plate kinematics of the central Atlantic. In: P.R. Vogt and B.E. Tuckolke (Editors), The Western North Atlantic Region. (The Geology of North America, Vol. M.) Geol. Soc. Am., Boulder, CO, pp. 351-377. Lancelot, Y., Seibold, E. et al., 1977. Site 367 Cape Verde basin, Init. Rep. DSDP, 41: 163-232. Le Pichon, X., 1968. Sea floor spreading and continental drift. J. Geophys. Res., 73: 3661-3697. Le Pichon, X. and Hayes, D.E., 1971. Marginal offsets, fracture zones and the early opening of the South Atlantic. J. Geophys. Res., 76: 6283-6293. Marinho, M.O., 1985. Le plateau marginal de Guin6e: transition entre Atlantique central et Atlantique 6quatorial. Pd.D. Thesis 3rd cycle, Univ. P. et M. Curie, Paris, 183 pp. Mascle, J., 1977. Le Golfe de Guin6e: un exemple d'6volution de marges atlantiques en cisaillement. M6m. Soc. G6ol. Fr. 128, 104 pp. Mascle, J. and Blarez, E., 1987. Evidence for transform margin evolution from the Ivory Coast-Ghana continental margin. Nature, 326: 378-381. Mascle, J., Marinho, M. and Wannesson, J., 1986. The structure of the Guinean continental margin: implications for the connection between the central and the South Atlantic Oceans. Geol. Rundsch., 75: 57-70. Mascle, J., Blarez, E. and Marinho, M., 1988. The shallow structures of the Guinea and Ivory Coast Ghana transform margins: Their bearing on the Equatorial Atlantic Mesozoic evolution. Tectonophysics, 155: 193-209.
137
Moullade, M., Mascle, J., Benkhelil, J., Cousin, M. and Tricart, P., 1993. Occurrence of marine mid-Cretaceous sediments along the Guinean slope (Equamarge II cruise): their significance for the evolution of the central Atlantic African margin. Mar. Geol., 110: 63-72. Perfil'yev, A.S, Pushcharovskiy, Yu.M., Yel'nikov, Ye.N., Kadela, K.G. and Kogan, L.I., 1987. Tectonic structure of the Sierra Leone Rise in the Equatorial Atlantic Ocean. Geotectonics, 21: 547-559. Pindell, J. and Dewey, J., 1982. Permo-Triassic reconstruction of Western Pangea and the evolution of the Gulf of Mexico-Caribbean domain. Tectonics, 1: 179-211. Rabinowitz, P.D. and Labrecque, J.C., 1979. The Mesozoic South Atlantic Ocean and evolution of its continental margin. J. Geophys. Res., 84: 5973-6002. Rossi, S., Westall, F. and Mascle, J., 1992. The geomorphology of the Southwest Guinea Margin: tectonic, volcanic, mass movement and bottom current influences. Mar. Geol., 105: 225-240. Sibuet, J.C. and Mascle, J., 1978. Plate kinematic implications of Atlantic Equatorial fracture zones trends. J. Geophys. Res., 83: 3401-3421. Tricart, P., Mascle, J., Basile, C., Benkhelil, J., Ciais, G. and Villeneuve, M., 1991. La tectonique d'inversion m6diocr~tac~e de la marge sud-guin~enne (campagne EQUAMARGE II). Bull. Soc. G6ol. Fr., 162: 91-99. Unternehr, P., Curie, D., Olivet, J.L., Goslin, J. and Beuzart, P., 1988. South Atlantic fits and intraplate boundaries in Africa and South America. Tectonophysics, 155: 169-179.
Tectonophysics 248 (1995) 117-137
The Guinea continental margin: an example of a structurally complex transform margin Jean Benkhelil a, Jean Mascle b, Pierre Tricart
c
a Laboratoire de SEdimentologie et Gdochimie Marines, CNRS URA 715, Universitd de Perpignan, Avenue de Villeneuve, 66 860 Cedex Perpignan, France b Laboratoire de G~odynamique Sous-Marine, CNRS URA 718, B.P. 48, 06230 l,Tllefranche-sur-Mer, France c Laboratoire de Gdodynaraique des chatnes alpines, CNRS URA 718, Institut de Gdologie de l'Universitd Joseph Fourier, Rue Maurice Gignoux, 38031 Grenoble Cedex, France
Received 3 June 1994; accepted 24 October 1994
Abstract
The Guinean continental margin corresponds to a wide hinge zone which developed between the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic. The Guinea Marginal Plateau that characterizes the southern border of this margin is a wide triangular morphological feature bounded to the west by a typical rifted margin slope segment, while its southern edge corresponds to a narrow and morphologically complex continental slope probably resulting from intracontinental transcurrent motion during the Early Cretaceous. On the basis of seismic profiles recorded in two selected areas located along the southern Guinea margin, a seismic stratigraphy is established. At depth, the plateau is underlain by a thick Mesozoic and Cenozoic sedimentary wedge made of two main ensembles. A lower ensemble (or sequence 1) consists of Early Cretaceous clastics. An upper ensemble includes a series of four distinct sequences attributed to Albian to Tertiary deposits. Eastward, the sedimentary cover of the southern plateau edge is locally pierced by volcanic bodies of Paleocene age. Beneath the southern margin of the Guinea Plateau, the sedimentary sequences appear deformed by faults and folds resulting from, at least, two main tectonic episodes. A first event only affected sequence 1, which is cut by a set of extensional faults. A second tectonic phase including folding, reverse faulting and transcurrent faulting is responsible for the deformation of sequences 1 and 2. During this tectonic episode, former normal faults have been reactivated as reverse faults resulting in a structural inversion. A third tectonic phase was responsible for structures transverse to the general E - W trend of the southern Guinea margin and consisting of N-S-trending normal faults and associated volcanoes. The spatial distribution of the tectonic trends clearly substantiates a polyphase tectonic activity that can be related to plate motion changes during early stages of the continental separation. A first stage is characterized by a divergent rifting leading to N-S-trending synsedimentary normal faults. During the Cretaceous, the tectonic activity increased as a general transform motion occurred between the Guinea Plateau and its American conjugate margin (the Demerara Plateau). I ~ c a l tensional stresses were responsible for a splay fault system active near the tip of the Guinea Plateau. In late Albian times, a slight adjustment of the Africa plate, with respect to the South American one, resulted in local squeezing of the whole sedimentary wedge. After this short compressive event, final continental separation occurred in an extensional regime characterized by a general collapse along two major scarps and volcanic activity.
0040-1951/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0040-1951 (94)00246-0
118
.I. Benkhelil et al. / Tectonophysics 248 (1995) 1 1 7 - 1 3 7
I. Introduction
torial Atlantic appears as a key area, and according to the different models, the consequences on the surroundings continental margins should have been recorded through different tectonic regimes. During the Mesozoic, the Equatorial Atlantic area acted as a wide strip of shearing where margin segments experienced transform motion and associated rifting. Three main shear zones (Fig. 1) are either direct consequences of transform evolution of the oceanic basins, or have accommodated parts of the motion if the hypothesis of intraplate discontinuities is accepted. The Romanche Fracture area was, and is still the major transform boundary between the Africa and South America plate; this fracture zone has controlled the formation of the transform C6ted'Ivoire-Ghana margins as well as of the C6ted'Ivoire deep divergent basin and Brazilian equiv-
Off West Africa the Mesozoic evolution of the continental margin is tightly controlled by the variations of the African and the American plates relative motion (Le Pichon and Hayes, 1971: Mascle, 1977; Rabinowitz and Labrecque, 1979; Jones and Mgbatogu, 1982; Klitgord and Schouten, 1986; Jones, 1987). The diachronous opening of the Central and South Atlantic has been evaluated in terms of plate kinematics (Bullard et al., 1965; Le Pichon, 1968; Le Pichon and Hayes, 1971; Sibuet and Mascle, 1978) and different predrift reconstructions were based either on a rigid behaviour of plates (Pindell and Dewey, 1982) or on potential intraplate deformations (Curie, 1984; Unternehr et al., 1988; Fairhead and Binks, 1991). In both hypotheses, the Equa-
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Fig. 1. G e o l o g i c a l sketch m a p of W e s t Africa. 1 = Mesozoic and Cenozoic; 2 = u p p e r P r e c a m b r i a n to Paleozoic; 3 = H e r c y n i a n belt ( M a u r i t a n i d e s ) ; 4 = P a n - A f r i c a n M o b i l e belt ( D a h o m e y i d e s ) ; 5 = P a n - A f r i c a n M o b i l e belt (Nigeria); 6 = A r c h e a n and B i r r i m i a n b a s e m e n t . L O S refers to a C r e t a c e o u s Syenite Complex.
J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
alents (Mascle and Blarez, 1987; Basile, 1990; Basile et al., 1993). The Chain and the Charcot fracture zones may connect onshore with features in the Benue Trough, itself interpreted as a major intracontinental discontinuity (Mascle, 1977; Benkhelil, 1982) where deformations have been correlated with plate adjustment during the initial stages of the Atlantic opening. A third major shear zone corresponds to the southern Guinea margin connected with the Guinea Fracture Zone (Mascle et al., 1988). In this paper, we intend to synthetize the seismo-stratigraphic and structural characteristics of the southern Guinea margin in order to evaluate the tectonic environment in which this margin
has experienced shearing during early stages of the continental separation. Today, the southern Guinea margin comprises two main segments, trending, respectively, north-south and east-west and bounding the Guinea Marginal Plateau (Fig. 2): the western segment can be interpreted as a typical rifted margin (of Jurassic age) (Marinho, 1985); the southern segment is interpreted as a complex transform margin related to the activity of the Guinea Fracture Zone and was created during Early Cretaceous times (Marinho, 1985). The southern Guinea margin has been surveyed during several cruises, particularly during the Equamarge I (1983) and Equamarge II (1988) cruises (Fig. 3). The data (Marinho, 1985; ~
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Fig. 2. Location m a p of the study area (box) in the Central Atlantic (after Klitgord and Schouten, 1986). N u m b e r s refer to magnetic anomalies. Selected lineations and fractures zones (FZ) are shown.
120
.I. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
Benkhelil et al., 1989) have revealed a complex structural pattern. During these cruises, attention was focused on the western tip of the Guinea Marginal Plateau, a wide triangular deep plateau, which lies at the boundary between two areas of the Atlantic created at different periods and according to different rift geometries (Mascle et al., 1986).
gins were generated chiefly through transform movements and display a series of either rifted or sheared segments. The Guinea continental margin is itself characterized by the presence of a wide marginal plateau that forms a triangular wedge protruding into the Atlantic Ocean. The plateau is a 250kin-wide, gently seaward-dipping platform without any significant morphological features (Fig. 3). Westward, at about 1500 m of water depth, the plateau is interrupted by a wide N N E - S S W trending slope where gravity sliding is common. Its southern slope exhibits a more complex morphology reflecting a complex internal structure. The slope includes a triangular-shaped basin (or mid-slope basin) wedged between two sub-linear scarps (Fig. 4; Jones and Mgbatogu, 1982). This basin widens toward the northwest where it connects progressively with the N-S-trending margin segment and is cut in its central part by a N - S trending valley. Eastward, the basin becomes narrower and the slope grades into a single scarp
2. Main morphological characteristics of the West African continental margin Along West Africa, the morphology of the different margin segments reflects the geodynamic processes responsible for their formation and for their subsequent evolution. From Morocco to GuinEe Bissau, the margin can be interpreted as a typical divergent one resulting from an early Jurassic rifting. From southern Guinea to the Niger Delta, the Equatorial Atlantic mar-
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13°00
J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
where several sub-circular volcanic seamounts emerge. South of the Guinea margin, the Sierra Leone Rise is a large aseismic elevation (Fig. 2), trend-
Wl 8
17o50
17040
17030
121
ing in a SW direction and interrupted near the St. Paul Fracture Zone (Emery et al., 1975). This still poorly known oceanic rise is a complex volcanic and l~ectonic feature characterized by a rugged
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Fig. 4. Sea-beam bathymetric map of the two selected area (G1 and G2) surveyed during the Equamarge II cruise. Locations of seismic lines cited in the text are shown.
122
J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
t o p o g r a p h y d u e to n u m e r o u s p r o m i n e n t seamounts (Perfil'yev et al., 1987; Jones et al., 1991).
3. Seismic stratigraphymtentative dating During the Equamarge II cruise, about 3900 km of continuous seismic profiling were recorded in two selected areas (G1 and G2) located along the southern Guinea margin (Figs. 3 and 4). The proposed seismic stratigraphy, based on a detailed analysis of the seismic lines, includes five acoustic sequences that we integrated into two main ensembles exhibiting different seismic reflection patterns, and volcanic bodies. 3.1. The lower e n s e m b l e
The lower ensemble corresponds to the deepest sequence (sequence 1) that can be detected
beneath both the marginal plateau border and the mid-slope basin. Beneath the plateau, sequence 1 shows high-amplitude reflectors, slightly continuous in the upper part (Fig. 5, G14); internal reflections are mostly parallel, although locally slightly divergent. This sequence is clearly deformed by faulting and folding and its top is capped by strong reflectors of regional extent delineating a clear unconformity. Along the upper scarp, this sequence is more difficult to identify and often affected by vertical fault displacements; in this area sequence 1 may locally crop out. Within the mid-slope basin, the sequence is well expressed and can be subdivided into two sub-sequences. A lower sub-sequence is characterized by parallel reflectors (sometimes divergent or curved) often covered in strong toplap by an upper sub-sequence; however, toward the basin centre both sub-sequences are in conformity (Fig.
N 1 sdwtt
3~
[q sdwtt "
S
Fig. 5. Seismic reflection profiles (lines 12 and 14, see Fig. 4) across the southern Guinea continental margin. The upper ensemble (2-5) includes flat-lyingsequences resting unconformablyupon the folded sequences (1) of the lower ensemble (G14). Section G12 cuts across the mid-slope basin showing the distinct sequences 1 to 5. sdwtt = second double way travel time.
J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
5, G12). The upper sub-sequence is only detected towards the basin centre where it consists of discontinuous reflectors becoming more continuous westward. The base of sequence 1 has not been detected seismically, but it is greater than 2 s of two-way seismic travel time. This sequence has been assigned to the Lower Cretaceous (Mascle et al., 1986) and samples cored just below the unconformity have yielded upper Albian calcareous sandstones (Moullade et al., 1993).
Sequence 4
3.2. The upper ensemble
Sequence 5
The upper ensemble includes four distinct seismo-stratigraphic sequences being from bottom to top:
Sequence 2 This constitutes the bulk of the infilling of the mid-slope basin where it reaches a thickness of 0.6 s (Fig. 5, G12). In this area it is made of a weakly reflecting lower part (2a), in concordance with the underlying ensemble, and of a strongly reflective upper part (2b). Beneath the plateau, sequence 2 lies unconformably on sequence 1 and corresponds to a series of sub-horizontal continuous reflectors. Its base has also been cored and consists of black shales of Cenomanian-early Turonian age (Moullade et al., 1993). Sequence 2b is attributed to Upper Cretaceous marine sediments.
Sequence 3 In the mid-slope basin sequence 3 exhibits a very weak seismic layering, with rare discontinuous reflections. It corresponds to lenticular bodies, narrowing along a N120°E direction and overlying an erosional surface locally characterized by strong reflectors. On the marginal plateau, sequence 3 shows a chaotic facies and its base is concordant while its top shows slight onlaps. This sequence, which may be locally missing, has a highly variable thickness. A Middle to Late Eocene age has been assigned to sequence 3 by Marinho (1985) according to correlations with DSDP site 367 (Lancelot et al., 1977).
123
On the marginal plateau, sequence 4 contains continuous well-layered reflectors. Within the mid-slope basin, this sequence is made of parallel (sometimes chaotic) reflectors reaching a maximal thickness of 0.5 s. Its specific acoustic character is interpreted as indicative of alternating sandy and clayey beds and may correspond to prograding deltaic series. A speculative Eocene to middle Neogene age (Marinho, 1985) is assigned to this sequence by correlation with DSDP 367.
Well developed in the mid-slope basin, sequence 5 is affected by numerous syn-sedimentary phenomena. The few discontinuous reflectors of this almost acoustically transparent sequence show various amplitudes and frequencies. On the plateau, its basal boundary is concordant (or slightly onlapping), and its thickness is less than 0.25 s. This sequence correlates with recent soft sediments shaped by bottom currents (Rossi et al., 1992), that have built up a series of slopeparallel sedimentary mega-waves in the western area and moats infillings in the eastern volcanic area. The age of the sequence ranges from Pliocene to Present times (Marinho, 1985).
3.3. Volcanic-type features Both the southernmost Guinea margin slope and the northern Sierra Leone abyssal plain are pierced by subcircular seamounts interpreted as volcanic bodies on the basis of their high-amplitude magnetic anomalies (Jones and Mgbatogu, 1982) or by sampling (Bertrand et al., 1988). These seamounts lie along an E-W-trending belt running for more than 300 km from 19 to 15°W approximately on strike with the Guinea Fracture Zone. Westward, this line of seamounts connects with the northern edge of the Sierra Leone Rise. East of 17° 10'W, the magmatic belt clearly intersects the slope trend. Across both the continental slope and the northern Sierra Leone Basin the seismic reflection data show two volcanic-type features (Fig. 6): --volcanic cones, directly exposed on the sea floor, or hidden below a thin sedimentary cover;
J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
124
GuineaMarginal Plateau N sdwlt 11
Volcanoarea ~)(~)
S
z ~
ier;aL~one
A
A'
Fig. 6. Interpreted seismic section of the volcanic area (Nadir Mount; location in Figs. 4 and 7). The numbering corresponds to the sedimentary sequences.
-- 9°30
I 17°30
F
--sills (or lava flows) interbedded in the sedimentary cover (mainly within sequence 2 and sometimes in sequences 3 and 4). Interbedded lava flows, characterized by a typical diffraction pattern may be identified within the eastern mid-slope basin area and near the border of the Sierra Leone Basin. Seismic profiles indicate lateral continuity between the volcanic cones and interbedded horizons. In the eastern volcanic province a main volcano forms a huge seamount (Fig. 4; about 20 km in diameter and 2.5 km in elevation) characterized by an
I
GUINEAN
i
] 17°
9°30 " ~
MARGINAL
PLATEAU
\
A
9°00
LEGEND
ABYSSA L
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V
V
V
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fault Extensionof v o l c a n i s m
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fault
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V
Volcaniccone
20 km ['~
%,
Volcanic rocks Oceanic b a s e m e n t
17o
L
L______I
.L
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Fig. 7. Structural map of part of the southern Guinea continental margin. The margin is segmented by a major N-S-trending transcurrent fault zone defining two main areas with different structural trends ( A - A ' = line section of Fig. 6).
J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
E-W-trending crest that rises at 840 m below sea level. Other volcanic cones of smaller sizes (ranging between 2 and 5 km in diameter) exist and are interpreted as secondary vents. Five secondary volcanoes constitute with the main one, two, N15 ° E- and E-W-trending alignments. During the Equamarge cruises and more recently, during the Equanaute diving cruise (1992), magmatic rocks have been collected from the main volcano. The rock types consist of alkali basalts and polygenic breccia made of basalts, basaltic hyaloclastics and scarce ultramafic xenoliths. A Paleocene age was obtained on biotite phenocrysts (58.6 Ma) from a massive basalt (Bertrand et al., 1988,1989,1993).
4. Structuralunits of the southern Guinea margin The main structural characteristics of this margin were previously outlined (Marinho, 1985). The new set of closely spaced seismic lines (Fig. 3), has enabled a more detailed analysis of the various tectonic features and a relatively precise structural mapping. According to Marinho (1985) the southern Guinean Margin can be subdivided into seven main morphostructural units arranged in ESEWNW-trending strips and roughly parallel to the main trend of the continental slope (Fig. 7): (a) The marginal plateau corresponding to a wide deep triangular platform extending from the shelf to about 300 km seaward. This plateau
125
shows a gentle regular slope, underlain by almost fiat-lying horizons, resting unconformably upon the lower ensemble (Fig. 8). (b) An upper scarp roughly bounded upward by the 1500 m isobath and corresponding to an important slope break (Fig. 9). It is made of two E-W-trending segments, changing westward to an almost N-S strike. This scarp, that varies in slope from 10 to 15%, is thickly covered by recent sediments; locally, steeper slopes display lower ensemble outcrops. (c) A mid-slope basin that extends at the foot of the upper scarp, consisting of a triangular rugged area about 20 km wide at its centre. Eastward, the basin thins out running in E - W direction and westward it widens while curving towards a NW-SE direction (Fig. 7). Seismic lines illustrate that the basin can be interpreted as a wide synclinorium structure separated into two areas by a N170 ° E-trending fault zone (Figs. 7 and 10), detected over a distance of 60 km and coinciding with a cross-slope valley along 17° 30'W (Fig. 4). (d) A southern bordering marginal ridge corresponds to the topographic expression of a wide tilted block (Figs. 7 and 9). This ridge, disrupted by the N-S-trending valley (Fig. 4), is oriented N125°E to the west and trends east-west to the east of the valley. This change of direction occurs on both sides of the N170 ° E-trending fault zone with an apparent horizontal offset of about 10 km of the ridge. East of the fault zone, the crest of the ridge which correlates with the top of the half-graben shows a clear inflexion, from N125 ° E
N
S
sdwtt
.
5
.|
.~ 5
Fig. 8. Interpreted seismic section (line 14, see Fig. 4) showing a set of normal faults affecting sequence 1 unconformably covered by sequences 2 to 5.
126
.L Benkhelil et aL / Tectonophysics 248 (l 995) 117-137
to N80 ° E. Such a sharp change in trend may be correlatable with a dragging effect related to a dextral transcurrent movement. (e) A lower scarp, which corresponds to the southern slope of the previous feature, underlines a second major slope break; it becomes steeper east of the N170 ° E-trending fault zone (Fig. 7). (f) The foot of the continental rise correlates with an elongated, 10 km wide basin where most of the seismic sequences can be identified (Fig. 7). (g) Finally eastward, a volcanic area is characterized by steep reliefs corresponding to a major sub-circular volcano rising to 840 m below sea level and lateral vents dissected by radial valleys (Fig. 6).
5. Structural analysis Regional seismic profiles indicate that the continental margin of the Guinea Plateau displays a complex structural pattern, especially along its southern border. The two main seismo-stratigraphic ensembles described above show evidence of deformation which reflects various tectonic events that have affected the Guinea continental margin during the Mesozoic. Within an
-'-- M A R G I N A L PLATEAU
NNW Sdwtt 4
I
E - W - t r e n d i n g zone running along the southern margin continental slope, the deformation includes fracturing (at many scales) and gentle folding. 5.1. Fracturing
Fracturing is best expressed at the level of the lower ensemble which appears strongly cut by a grid of high-angle normal faults. Minor faulting is expressed by sets of conjugate faults, resulting in a pattern of tilted blocks. Beneath both the marginal plateau (Figs. 10 and l l a ) and the slope basin, this extensional pattern determines a system of small horsts and accompanying grabens and within the central mid-slope basin, several faults are clearly characterized by thickening of the sedimentary cover near the fault plane within the downthrown block; this is interpreted as a direct result of synsedimentary tectonic activity. Below both areas, normal faults vanish near the post-sequence 1 unconformity and die out within the base of sequence 2 (Fig. llb). The horizontal and vertical extents of normal faults vary from some tens to a few hundred meters; their azimuthal distribution varies from northwestsoutheast to east-west. Both the upper and lower scarps are themselves topographic expressions of major normal
MID-SLOPE BASIN
SSE i II
Fig. 9. Interpreted seismic section (line 3, see Fig. 4) across continental slope showing the different morphostructural ensembles. Sequences 2 to 5 underline the synclinorium shape of the mid-slope basin enbedded within the underlying sequence 1. OB = Oceanic Basement.
127
J. Benkhelil et aL /Tectonophysics 248 (1995) 117-137
fault systems. On seismic sections, the upper scarp corresponds to an area cut by a complex set of steep faults that chiefly affect sequence 1. This scarp seems to have resulted and recorded successive collapses of the mid-slope basin relative to the plateau. The lower scarp, which flanks the marginal ridge to the south, appears as an asymmetrical horst-like block border cored by sequence 1 (Fig. 9). A N170 ° E-trending depression underlain by a major fault zone cuts the upper scarp fault system. It is associated with a series of N30 ° E-trending faults located on the eastern side of the N170 ° E-trending fault zone interpreted as associated shears (synthetic strike slip fault; Fig. 10). In the western mid-slope basin, reverse faulting affects both sequences 1 and 2. The faults bound an anticlinal-like structure capped by the base of sequence 2, forming an upward-diverging splay pattern (Fig. llb). At depth, the faults seem to converge while upward they gradually flatten, dying out into the basal sequence 2. A reverse offset is clearly attested by an upward displacement of the inter-sequence 1 / 2 boundary. A thickening of sequence 1 against the fault plane is interpreted as an inheritance of former synsedimentary normal fault motion (Tricart et al., 1991). The reverse faults are closely associated with folding in the process of structural inversion. Among the major structures, a N170 ° E-trending fault zone (running approximately along 17° 30'W), cuts across the entire continental slope
MID-SLOPE BASIN W
(Fig. 7). Southward, this feature correlates with a lower scarp indentation; it is, however, best expressed within the mid-slope basin where it runs along the "median valley" (Fig. 4). This fault zone corresponds to a 3-km-wide positive flower structure (Fig. l l d ) well documented on E - W trending profiles (Fig. 10). The faults are nearly vertical and sealed by sequence 3 (Fig. 1 lc). The structure is flanked by two footwall troughs, filled with a thick sequence onlapping its western flank. This flower structure can be followed over more than 60 km from the southern border of the mid-slope basin to the marginal plateau (Fig. 7) where, although attenuated, it can still be identified. Defined as an uplifted and tilted block, the lower-slope marginal ridge, that appears fault bounded on its seaward side (along the lower scarp), is cut along its northern flank by a fault system progressively grading to a flexure westward. Eastward, this may evolve to a rupture particularly near the N170 ° E-trending fault zone (Fig. 7). Dragging along this border of a parallellayering pattern attests to a permanent tectonic activity of the marginal ridge during sequence 2 deposition. Beneath the marginal plateau itself, faulting is clearly sealed by the basal sequence of upper ensembles (likely sequence 2). Within the midslope basin, the fault system mainly affects the lower ensemble; some of the reverse faults may, however, have been active during deposition of
MARGINAL PLATEAU -.-E sdw.tt
Fig. 10. Interpreted seismic section (line 36, see Fig. 4) across the continental slope showing the transcurrent fault zone at the centre of the basin and a graben structure across the upper scarp.
.L Benkhelil et a L / Tectonophystcs 248 (1995) 117-137
128
5.2. Folding
the basal sequence 2. The activity of the marginal ridge border fault died out progressively during deposition of sequence 2, and finally ceased during deposition of sequence 3.
Along the southern margin of the Guinean Plateau, folding has been recognized by Marinho S
N sdwtt
2
a N sdwtt
S
3
b Fig. 11. Interpreted seismic sections. (Only parts of sections G3, GI2, G31 and G36 are shown. On Fig. 4 the full trace of the original section is indicated.) (a) Folding and faulting beneath the Guinea Plateau (G12). (b) Reverse faults and folds associated with a flower structure in the mid-slope basin. The horizon (bold line) between sequences 2 and 3 corresponds to an unconformity which bounds the lower compressive structures. The dashed line corresponds to an internal differentiation of sequence 2 (G3). (c) Seismic reflection profile (G3) across the flower structure. (d) Both sections show the structure associated with the N-S-trending transcurrent fault: a flower structure flanked by two depressions, the western one being the deepest (G31 and G36).
J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
N sdwtt
129
S
"
~'~
~
-
,3
4
5
C
W sd~'t
d Fig. 11 (continued).
J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
130
(1985) and analyzed in more detail by Tricart et al. (1991). At a regional scale the mid-slope basin can be interpreted as a wide synclinorium feature squeezed between two scarps and interpreted as a first-order folding structure. This syncline, about 40 km wide in its western sector, is progressively narrowing eastward where it reaches only 15 km, east of 17°30'W. Further to the east the basin, almost concealed by volcanic cones and associated lava flows, connects progressively with a monoclinal slope (Marinho, 1985). A second large-scale "synclinal" type feature corresponds to the elongated basin that lies at the foot of the lower scarp. This basin extends only west of the N170°E-trending fault zone where it contains internal second-order undulations. Locally, in the central mid-slope basin, the syncline appears flat bottomed and double hinged. Such a morphology is interpreted as a direct inheritance from deep-seated faults. Tricart et al. (1991) proposed that the former primary structures were extensional blocks superimposed on a wide graben. This structure, that later on grew during the short tectonic phase that occurred
N
I
after deposition of sequence 1, has acted as a wide depocenter for sequence 2. The persistent activity of some of the faults and their bearing on the syncline is attested to by onlaps of sequence 2 along its flanks. The seismic lines show that the mid-slope basin is also affected by slight undulations, a few kilometres in amplitude, interpreted as second-order folds. These features are also expressed by wide, open, symmetrical deformations which are best expressed in sequence 1, and die out at the base of sequence 2. Second-order folding is not restricted to the mid-slope basin, and can be detected northward beneath the marginal plateau where it appears truncated by an erosional surface (Fig. 1 la). Finally, small-scale folds (about one kilometre in length) appear to have affected sequence 1 and part of sequence 2. They correspond to asymmetric and rounded isolated anticlines, particularly well displayed in the western part of the mid-slope basin, where they are generally associated with either second-order folds or reverse faulting (Fig. llb). They may either constitute a tight associa-
S
Sequences 1/2 interface
•
Sequence 1 0
5 km
I
I
Fig. 12. Block diagram illustrating the deformation of sequence 1 in the mid-slope basin. The overall synclinorial shape of the basin is interrupted by a flower structure, reverse faults and secondary folds. Open arrows indicate normal displacement, solid arrows indicate reverse/transcurrent movement.
J. Benkhelil et aL / Tectonophysics 248 (1995) 117-137
tion due to a stacking of anticlinal hinges, or correspond to isolated features separated by open box-shaped synclines. A synthetic block diagram showing the various deformations affecting sequence 1 is shown in Fig. 12. The sequence 1/2 interface is also affected by folding and especially by a flower structure flanked by two reverse faults. This structure is though to result from a tectonic inversion, the uplifted block being an older graben structure flanked by normal faults reactivated as reverse ones.
6. Distribution and relationship between folds and faults
As a whole the fold pattern and most of the fault systems constitute a set splaying out towards the northwest and pinching out against the N-Strending transcurrent 17°30'W fault zone (Fig. 13). The first-order syncline structure disappears both towards the north-northwest and eastward. The syncline axis splays out towards northwest and north, and progressively connects with the
.
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-.:
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!
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./..18 ~
ii
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N-S-trending western Plateau border. The southern marginal ridge progressively smooths out westward and abuts against the N-S-trending segment of the Guinea Plateau. East of the 17°30'W fault zone, the mid-slope syncline dies out, grading progressively to a monoclinal structure (Fig. 13). The major fault systems control most of the main topographic features of the margin. They form a series of E-W-trending segments truncated by two oblique sets, a N170° E and a N3040°E direction. The small-scale faults (both normal and reverse) contribute to N90°E to N140 ° E-trending systems. The N170 ° E-trending fault zone, and a few similarly trending secondary faults, are contemporaneous with the folding. Both the trend of the fault relative to the folds, and the associated flower structure, are indicative of a transcurrent motion. Secondary fault arrangements about this major structural feature are in agreement with such an interpretation (Fig. 7). The structural fabric of the southern Guinea margin segment is, in our opinion, consistent with
1~o
LEGEND Undifferentiated fault I ~
i~
,,,, ".
"..
131
'..
GUINEA MARGINAL
Normal fault Reverse fault Syncline
Transcurrent fault zone Volcanic rocks
~
Oceanic basement
Fig. 13. Structural sketch of the southern Guinea margin. The strongest deformations are located to the west of the N-S-trending fault zone. Magmatic activity is restricted to an E - W - t r e n d i n g strip in the southernmost side of the margin (to the east of 17° W data are from Marinho, 1985). Note the offset of the oceanic basement boundary on both sides of the transcurrent fault. D = Demerara Plateau; G = Guinea plateau.
132
J. Benkhelil et al. / Tectonophysics 248 (1995) 117-137
an E - W direction superimposed to former N - S structural trends presently represented by the main N-S-trending shear zone (Fig. 7). The present segmentation of the margin arises from two main fracturing phases. A first one, associated with a compressional event acting on margin segment corresponds to the transcurrent reactivation of previous N-S-trending extensional features. Transverse normal faults and volcanoes alignment, attributed to a Late Cretaceous extensional episode, has also segmented the margin following N15 ° E and N30 ° E directions (volcanic area; Figs. 7 and 13).
7. Geodynamic evolution of the southern Guinea margin
7.1. The pre-rift reconstruction (Fig. 14a) The conjugate American margin of the Guinea Plateau is the Demerara Plateau, a broad submarine plateau north of French Guyana (Fig. 2). In several reconstructions of the central Equatorial Atlantic (Klitgord and Schouten, 1986; Mascle et al., 1986) it has been proposed that both plateaus were part of the southern border of the Central Atlantic margin during Jurassic times prior to becoming conjugate margin segments. Around 180 Ma (Mascle et al., 1988) this area was believed to be under a prevailing tensional stress regime (rifting of the Central Atlantic) which resulted in a series of faulted blocks and grabens where clastics were trapped in significant amounts (Jones and Mgbatogu, 1982). At that time, the prevailing structural trends were trending north-south, indicating a dominantly E-W-oriented extension. Today we have no evidence of such an early structural pattern the only exception being the N170 ° E-trending fault zone that could represent a reactivated structure related to this early event. In terms of plates, the West African and South American blocks were still attached forming a single block probably already subjected to tensional stresses.
7.2. The rifting phase (Fig. 14b) In Early Cretaceous times, both margins ( G u i n e a - D e m a r a r a ) started to split along the Guinea Fracture, probably as a result of pre-existing Pan-African lines of weakness. The Demera r a / G u i n e a plateaus, still in contact, were sliding one past the other along a continental transform boundary. The transform motion occurred mainly under a transtensional regime in the sense of Harding (1985) and generated tensional features recorded on the newly created southern Guinea margin area. The corner-like shape of the Guinea margin shape led to the development of a pinnate set of tensional fractures. The NW-SE-trending faultbounded blocks, were oblique to the previous N-S-trending Jurassic trends. Their arrangement can be interpreted as splay faults within a fault system termination created parallel to the transcurrent fault zone; they splay out westward and curve to connect with the N-S-trending Jurassic faults. The two major fault zones that have shaped the present slope scarps, are thought to have been created during this transtensive event and thus to have directly initiated the mid-slope basin. This rifting is also marked by the occurrence of synsedimentary normal faults which bound a system of tilted blocks and associated grabens filled with clastics. They are the oldest tectonic structures (Early Cretaceous) detected below the Guinea Plateau. Along the northern Demarara escarpment, this event seems to have been differently recorded. Beneath the northern Demerara plateau, the pre-Albian sedimentary sequences are deformed by folds and flower structures. The folded area forms an E-W-running belt with an "en 6cheion" fold arrangement interpreted as dragging effects resulting from a dextral motion along the Guinea Fracture Zone (Gouyet, 1988). The eastern margin of the Demerara Plateau is a rifted segment characterized by steep slopes and escarpments, facing the Sierra-Leone/Liberian margins with evidence for associated volcanic activity. This episode corresponds to the break up of the West African/South American continental
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block into two blocks which began to slide past one on other along the Guinea Fracture zone. Separation occurred between the eastern Demerara Plateau and the continental margin off Sierra Leone and Liberia with probable creation of oceanic crust.
7.3. The compressional phase (Fig. 14c) A second deformation episode corresponds to a short compressional event that has affected the previous structures. The resulting compressive deformation with locally reactivated Early Cretaceous tensional fractures leading to structural inversion, are dearly post-rift. Since both extensional and compressive structures are comparable in direction, normal faults were easily reactivated as reverse faults associated with folding. In this stress regime, the N-Strending faults of Jurassic age were also reactivated but as dextral transcurrent zones. All the compressional trends are consistent with a N30 ° E-oriented compression with a maximum horizontal shortening located near the southwestern tip of the Guinea margin. Eastward, the major syncline structures correspond rather to basinal synclines than to tectonic derived folds. Such deformation, restricted both in time and space, suggests plate boundary adjustments rather than a continuous transform motion. The timing of this event is constrained by the striking angular unconformity detected on the Guinea margin (Figs. 8 and lla). The recovery of sandstones and black shales, located, respectively,
135
just below and just above the unconformity, provides a post-middle Albian to pre-late Cenomanian age (Moullade et al., 1993), although declining tectonism locally persisted during deposition of sequence 2, since part of this sequence bears evidence of deformation, also detected in the vicinity of the N170°E transcurrent area. The base of sequence 3, characterized across the margin by regular reflecting horizons, sealed the compressive deformation. On the Guinea margin such a local compression cannot be related to dragging effects due to the transcurrent Guinea Fracture Zone as suggested for the Demerara Plateau (Gouyet, 1988). We believe that local mechanisms such as block re-adjustments, maybe linked to a pole of rotation shift, are more likely. Such mechanisms were predicted in the kinematic reconstruction of Rabinowitz and Labrecque (1979) for the late Aptian. A subsequent slight anticlockwise rotation of the Guinea Plateau relative to the Demerara Plateau, may explain the NE-SW-oriented compression deduced from the mapping of compressive deformation. Such a scenario may account for a slight squeezing of Lower Cretaceous sediment against the Guinea Fracture Zone, and for a dextral reactivation along the N170 ° E-trending transcurrent fault zone (Fig. 14c). In our opinion, this is also in good agreement with the relative confinement of the deformation to the apex of the Guinea margin; the N170 ° E-trending transcurrent fault may have acted as a major structural barrier between a western, strongly deformed, and an eastern, less deformed domain. Compressional structures have been reported by Gouyet (1988) along the northern segment of the Demerara plateau. When compared with the Guinea margin, and while showing a comparable overall structure, we note discrepancies between the tectonic processes and the timing that have governed the evolution of both marginal plateaus. Concerning, the "en 6chelon" folds arrangement on the Demerara plateau, we note a significant difference from the fold trends as detected on the Guinea margin. The attribution of these "en ~chelon" folds to a dragging effect during the rifting phase is based on the age of the unconformity that postdates the tectonic event. The age
136
.L Benkhelil et al. / Tectonophysics" 248 (1995) 117-137
of the unconformity beneath the Guinea Plateau is well constrained to the interval middle-late Albian (Moullade et aI., 1993). Beneath the Demerara plateau a double unconformity is reported from the sedimentary succession being, respectively, late Aptian and late Albian in age (Gouyet, 1988). The first unconformity may be correlated with the formation of the "en 6chelon" folds, while the second one may be an equivalent of the main compressive phase on the Guinea Plateau.
7.4. The tensional phase (Fig. 14d) After minor recurrence of compressive tectonics, a clear change in the tectonic regime occurred during the Late Cretaceous. An extensional regime, characterized by the accentuation of the mid-slope basin and by a general collapse of the margin along the upper and lower faults scarps, prevailed; deformation during this period is likely to be related to the final continental parting between the African and South American continental plates, and the creation of an oceanic crust between the Demerara and Guinea plateaus. Along the southern Guinea margin extensional tectonism was then accompanied by widespread magmatic activity, located near the ocean/continent boundary; the E-W-trending belt of volcanoes is interpreted as injections along normal faults and locally interrupted by the N170°E trend. Along the southern Guinea margin tectonic deformation vanished near the end of Cretaceous times. Since that time the region has been subject to more normal cooling subsidence; strong bottom currents closely controlled sedimentation.
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