The stratigraphy of the Ifni/Tan-Tan margin is given by the Ifni-1, Tan-Tan-1, MO-4, Chebeika-1 and El Amra-1 wells (Fig. 1) and from neighbouring western Anti-Atlas outcrops and Tarfaya and Tindouf basins [2,3,5–7,12,14–17,21,25–29] (Fig. 2).

The Precambrian shows alternating conglomerates, sandstones, limestones and clays with volcanic rocks and granitic intrusions. The Cambrian is represented by limestones, clays and sandstones. The Ordovician is represented by clays, sandstones and limestones. The Silurian consists of shales and the Devonian shows alternating clays and sandstones, with limestones intercalations. The Carboniferous shows a series of clays and sandstones toped by limestones.

The Triassic sediments are essentially siliciclastic, with minor evaporate horizons and basaltic sills. By Lower Jurassic times, a carbonate platform was established in the area and persisted up to the end of this period whose sediments reflect eustatic fluctuations and show oolithic and bioclastic limestones, laterally changing into marls or sandstones with favourable periods of build-up in the Upper Jurassic (Fig. 2). During the Cretaceous, shallow water conditions prevailed again, resulting in a series of sandstones and shales. The Tertiary is mainly composed of clays and silts with few limestone and sandstone interbeds.

We divided the post-Palaeozoic formations of the Ifni/Tan-Tan margin into nine seismostratigraphic units (Tr1 to Tr4, J1–2, J3, Cr1, Cr2 and Ce), most of which represent seismic sequences (Figs. 3 and 4). The Palaeozoic, however, was not sufficiently resolved in our profiles to be stratigraphically subdivided, even though we could characterize its structural style.

The Triassic is bounded by two major regional unconformities: the top Palaeozoic and the break-up unconformities (Fig. 2). It was subdivided into four sequences (Tr1 to Tr4), which, except for the most-upper one, were obviously deposed in half-graben and graben structures that opened during the Triassic rifting phase. In absence of well data, we tried to infer lithological facies from seismic facies with respect to our knowledge from similar basins [8,9,13]. Tr1, which is bounded by two limits cutting underlying reflections in top-laps, shows three main seismic facies (Figs. 3 and 4): (i) a chaotic facies seen only in the down-thrown blocks near the fault planes, interpreted as representing the alluvial fan deposits sourced from the hanging wall highs, and (ii) a continuous and parallel facies with very high amplitudes, interpreted, by comparison with other basins where the Triassic section was totally penetrated [9,13,20,22,23], as representing the basaltic flows, and (iii) a facies showing more or less continuous reflections diverging towards the fault planes and containing some chaotic areas. This facies is interpreted as representing fluvial and lacustrine deposits that are distal with respect to facies (i). Tr2 is also bounded by an upper limit with top-laps. It is characterized by a gradual westward lateral facies change from a chaotic, to a well stratified and energetic facies (Figs. 3 and 4). This may reflect the normal lithological change from alluvial deposits near the shoulder of the rift to more stratified fluvial and deltaic deposits in the West. Tr3 shows essentially the same reflection patterns as Tr2, which we thus interpret in the same manner. Tr4, however, differs from the underlying sequences by its being essentially non faulted and showing continuous and parallel reflections that we interpret as being deposited in playas and lakes that formed in the area during the transitional period from the rifting to the drifting phases. A similar sequence was defined in another segment of the margin [13].

The seismostratigraphy of the Jurassic, the Cretaceous and the Cenozoic sections, which is relatively more studied [10,11,16,17,27] is not the object of this paper. For the sake of correlation and structural analysis, we however, subdivided it into five units (Fig. 2).

Seismo-structural analysis of the Ifni/Tan-Tan margin seismic lines indicates the presence in this area of three differently deformed groups of reflections (Figs. 3–5): (i) a lower group (P) corresponding to all reflections below the top Palaeozoic unconformity and characterized by folding that resulted in the northwestward-verging imbricate ramps bounded by low-angle faults; (ii) an intermediate group formed by sequences Tr1 to Tr4 and which is, in contrast, characterized by distensive graben and half-graben structures, and (iii) an upper group that contains J1–2, J3, Cr1 and Cr2 sequences and that shows either a westward-dipping monoclinal structure or a broad syncline. This group is truncated at the edge of the platform by the basal Cenozoic unconformity (base Ce).

In the light of regional geological data, the three above-described groups can be respectively linked to the three main stages of the geodynamic evolution of the Ifni/Tan-Tan margin. Thus the lower group corresponds to the long pre-rifting Precambrian and Palaeozoic stage whose structural features are well studied immediately to the east of our study area, in the western Anti-Atlas. The intermediate group corresponds to the rifting phase of the central Atlantic Ocean and the upper group to its drifting phase.

Except for an opposite vergence, the structures of the lower group described from our sections show a similar style as that reported from the western Anti-Atlas Palaeozoic and Proterozoic outcrops [2,3,5,6,12,14,15,21,25,26,28,29]. These works indicate that the Bas-Draâ-Ifni onshore zone represents the foreland of the Hercynian Anti-Atlas folded belt, whose geological evolution underwent many phases resulting in present-day mostly NE–SW-trending southeast-verging structures. Two structurally different domains are differentiated in this area by Soulaimani [26]: (i) an eastern domain containing the Precambrian outcrops and their Palaeozoic cover, characterized by thick-skin deformation, and (ii) a western domain formed by Palaeozoic outcrops showing thin-skin deformation. Likewise, Belfoul et al. [2] suggest the presence of these two styles of deformation, but assign them to two different tectonic events, of which an early one (D1) was generalized to the entire Anti-Atlas belt and was of a thin-skin type resulting in imbricate southeast-verging ramps.

Our seismic data show within the ante-Triassic basement of the Ifni/Tan-Tan margin a structural style similar to the that suggested by Soulaimani et al. [26] for his western domain and by Belfoul et al. [2] for the entire belt, with the difference that here there is western vergence instead of a southeast one (Figs. 3–5). Preliminary mapping of these structures shows them with a NE–SW direction comparable to that mapped in the outcrops [2,5,14,15,21,25,26] (Fig. 1). Cover deformation in the Anti-Atlas is mostly decoupled from the underlying basement along thick incompetent horizons such as the Lower Cambrian ‘Lie de Vin’ and Silurian shales [5,15,21]. Our data seem also to corroborate the two-domain subdivision of Soulaimani [26]. We, however, suggest that, in the light of our subsurface data and of other previous surface works, the limit between these two domains separates rather a western domain with thick-skin northwest-verging structures from an eastern domain with thick-skin southeast-verging structures (Fig. 1). This change of vergence can be understood in the framework of a double-verging Anti-Atlas folded belt resulting from the Hercynian-Appalachian collision.

In the northern extremity of the Tarfaya basin, we see below 4 s (TWT) a system of tilted blocks that we interpreted as delimited by east-facing normal faults that could be similar to those suggested by Caritg et al. [5] in the Precambrian to Cambrian basement. Above these blocs, reflections are organised in differently tilted piles that we interpreted as delimited by low angle reverse faults that are rooted in the faults delimiting the block bellow: we could have thus Precambrian extensive structures that are partially inverted by the Hercynian Compression (laramide style) as suggested by these authors [5].

Using seismic data, we mapped, major normal-fault trends related to the syn-rift stage. These faults show a NE–SW direction, which is parallel to that of the gravity and magnetic anomalies (Fig. 6). The lateral shift shown by adjacent clusters of these faults coincides with a similar shift shown by the axis of these anomalies and is interpreted as caused by strike-slip movement along WNW–ESE-striking transfer faults. This type of faults is reported from other segments of the margin [9,13,19].

Two sections allow us to visualise the subsurface geometry of these syn-rift structures (Figs. 3 and 4). Fig. 4 illustrates a westward diachronous opening of the Ifni half-grabens: the eastern half-graben opened first and accumulated Tr1 sequence before its bounding fault is sealed by the lower limit of Tr2, which accumulated within the western half-graben. A similar westward migration of rifting, which is seen farther to the south in Tan-Tan graben (Fig. 3), is also reported from other Moroccan basins [9,13,18,20,22,23]. Our four sequences Tr are thus tectonically controlled and are comparable to those defined in the Fundy and the Argana basins [22].

Furthermore, seismic data illustrate the role of the pre-existing basement faults in the location of rifting fractures. This role is suggested by the fact that both of these groups of faults have the same NE–SW direction and seismic sections show that most of the Triassic normal-faults are rooted in older Hercynian reverse faults (Figs. 3 and 4). We thus have a negative inversion, which was also reported from other Moroccan basins [9,13,18,20,23,24].

Les profils étudiés montrent, au niveau du socle anté-triasique de la marge d'Ifni/Tan-Tan, des structures en nappes imbriquées, délimitées par des failles inverses plus au moins plates (Figs. 3 et 4) suggérant un style tectonique de type thin-skin. La cartographie sismique de ces failles (Fig. 1) montre (i) qu'elles ont la même direction NE–SW que celle des structures du socle affleurant dans l'Anti-Atlas occidental [2,3,5,14,15,21,26,28] et que (ii) leur vergence vers l'ouest est, en revanche, opposée à celle de ces structures affleurantes.

En effet, les terrains du socle précambriens et paléozoïques affleurant à l'est de la ligne de rivage ont fait l'objet de nombreuses études [2,3,5,6,12,14,15,21,25,26,28,29]. Ce socle a subi un raccourcissement régional de direction NW–SE à est–ouest, qui l'a structuré en deux domaines distincts [26] : (i) un domaine autochtone caractérisé par une tectonique de socle thick-skin constituant la majeure partie de cette zone et où affleurent les boutonnières précambriennes, et (ii) un domaine allochtone caractérisé plutôt par une tectonique tangentielle thin-skin à vergence est, située à l'extrême ouest de l'Anti-Atlas, dans la zone du Bas-Draâ atlantique. La présence de ces deux styles de déformation est également suggérée, avec une généralisation du dernier style thin-skin à l'ensemble du socle de l'Anti-Atlas occidental [2].

D'après nos données sismiques, la limite NE–SW placée entre les deux domaines allochtone à l'ouest et autochtone à l'est [26] correspondrait à une limite qui séparerait plutôt deux domaines, que nous avons nommés occidental et oriental, de même style tectonique, mais à vergences opposées (Fig. 1a). Le changement de la vergence des structures hercyniennes compressives, de part et d'autre de cette limite, serait lié à un double déversement hypothétique de l'Anti-Atlas occidental, chaîne issue de la collision entre l'Afrique de l'Ouest et l'Amérique du Nord, mais jusqu'ici considérée comme uniquement déversée au sud-est. Les données sismiques actuellement disponibles ne nous permettent pas de préciser la géométrie de ce déversement.

Au nord du bassin de Tarfaya, les sections sismiques montrent, au-delà de 4 s (temps double), un système de blocs basculés délimités par des failles normales à vergence est, similaires à celles décrites par Caritg et al. [5] dans le socle du Précambrien et du Paléozoïque inférieur de l'Anti-Atlas central. Au-dessus, les réflexions sont organisées en nappes empilées, que nous interprétons comme étant délimitées par des failles inverses enracinées au niveau des failles délimitant ces blocs sous-jacents : il s'agirait donc de structures distensives anciennes (PIII à Cambrien ?), qui seraient partiellement inversées par la compression hercynienne [5]. Plus au nord-est du bassin de Tarfaya, deux phases de plissement hercyniennes, caractérisées par une tectonique de socle thick-skin ayant réactivé les structures synrift du PIII à Cambrien (style laramien) ont été identifiées.

La carte (Fig. 6) montre les principales failles triasiques cartographiées par sismique et les principaux axes des hauts magnétiques et gravimétriques de la marge d'Ifni/Tan-Tan. Ces failles sont de direction NE–SW et sont parallèles à la plupart des axes magnétiques et gravimétriques. Le décalage latéral entre les failles normales coïncidant avec celui de ces axes, nous l'interprétons comme étant dû au jeu dextre de failles transverses WNW–ESE. De telles failles de transfert ont été mises en évidence dans d'autres bassins de la marge marocaine [9,13,19,23].

Les coupes des Figs. 3 et 4 nous permettent d'analyser les modalités d'ouverture de ces structures distensives. En effet, il existe, au sud-est et au nord-ouest, de l'emplacement du puits Ifni-1 (Fig. 4), deux demi-grabens dont l'ouverture est diachrone : d'après le découpage sismo-stratigraphique détaillé plus haut (§3), le demi-graben nord-occidental serait plus récent que le demi-graben sud-oriental. Cette même propagation de la distension du sud-est au nord-ouest est observable plus au sud, au niveau du graben de Tan-Tan (Fig. 3). Ce diachronisme, également signalé dans d'autres bassins marocains [9,13,18–20,22,23], caractériserait une tectonique saccadée du rift. Nos quatre séquences seraient ainsi des séquences tectono-stratigraphiques comparables à celles définies dans les bassins de Fundy et Argana [22].

La direction NE–SW commune aux structures hercyniennes et triasiques (Figs. 1 et 6) définirait un lien génétique entre ces deux groupes de structures. En effet, les profils sismiques montrent que les failles normales triasiques, le long desquelles se sont développés les dépocentres triasiques de la marge d'Ifni/Tan-Tan, sont toutes enracinées en profondeur au niveau d'anciennes failles inverses hercyniennes (Figs. 3 et 4) : il s'agirait ici d'une inversion négative. Ce mécanisme a été avancé également par plusieurs auteurs à propos d'autres segments de la marge atlantique marocaine [8,9,13,23] ou d'autres bassins atlasiques [18,20,24].