Variscan deformation at the northern border of the West African Craton, eastern Anti-Atlas, Morocco: Compression of a mosaic of tilted blocks

Variscan deformation at the northern border of the West African Craton, eastern Anti-Atlas, Morocco: Compression of a mosaic...

Article in Bulletin de la Societe Geologique de France · September 2007

DOI: 10.2113/gssgfbull.178.5.343

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Variscan deformation at the northern border of the West African Craton, eastern Anti-Atlas, Morocco: compression of a mosaic of tilted blocks

Y

RADDI

, L

BAIDDER

, M

TAHIRI

and A

MICHARD

Keywords. – Foreland fold belts, Variscan, Paleofaults, Inversion tectonics, Anti-Atlas, Morocco

Abstract. – North of the Saharan cratonic domain, the Anti-Atlas mountains correspond to the foreland, external fold

belt of the Variscan orogen which extends in the Meseta block to the north, and Mauritanides to the southwest. The Anti-Atlas was uplifted during the Mesozoic-Cenozoic, and display several basement culminations (“boutonnières”) amidst the folded Palaeozoic cover. Recent studies in western Anti-Atlas emphasized the basement implication in the shortening process (thick skinned structure). Hereafter we investigate the cover-basement relations in eastern Anti-Atlas south of the Ougnat culmination, based on mapping at scale 1:50,000. The Palaeozoic sequence is much thinner than in the west, and the décollement levels are less important. Flexural slip folds are concentrated along the faults (en échelon folds) and within some rhombic domains crushed between major faults (e.g. Angal-Gherghiz Lozenge), whereas other areas are monoclinal. The main shortening direction deduced from the fold axes trend is directed ~N045

E as in the Ou- garta range further to SE. At a regional scale, this shortening direction interferes with a N-S trending one. A sketch map of the top of the basement makes visible a mosaic of S- to SE-ward tilted blocks. The faults between these blocks are in- herited from paleofaults which formed during extensional events during the Cambrian, late Ordovician, and (mainly) Middle-Late Devonian. The paleofault array is indicative of a proximal passive margin setting at the northern border of the metacratonic domain. The fault inversion and their dominant strike-slip throw occurred during a late Variscan (Ste- phanian-Permian) compression event, postdating the NNW-SSE collision of the Meseta block.

La déformation varisque à la bordure septentrionale du craton Ouest-africain, Anti-Atlas oriental, Maroc: compression d’une mosaïque de blocs basculés

Mots-clés. – Chaînes d’avant-pays, Varisque, Paléofailles, Tectonique d’inversion, Anti-Atlas, Maroc

Résumé. – En bordure du domaine saharien, l’Anti-Atlas correspond à l’avant-pays plissé de l’orogène varisque repré-

senté au nord par le bloc mésétien et au sud-ouest par les Mauritanides. Soulevé au Mésozoïque-Cénozoïque, l’Anti-Atlas montre de nombreux massifs ou « boutonnières » de socle précambrien affleurant sous la couverture paléo- zoïque plissée. Des études récentes dans l’Anti-Atlas occidental ont montré que le socle est impliqué dans le raccourcis- sement varisque (structure thick skinned). Notre étude concerne les relations socle-couverture dans l’Anti-Atlas oriental au sud de la boutonnière de l’Ougnat, et s’appuie sur une cartographie au 1:50 000

. La série paléozoïque est beau- coup plus mince qu’à l’ouest, et les niveaux de décollement y sont moins importants. Les plis de flambage et glissement flexural sont concentrés le long des failles et à l’intérieur de quelques domaines rhombiques écrasés entre des failles majeures (cas du losange Angal-Gherghiz). Les autres secteurs sont monoclinaux. La direction principale de raccourcis- sement déduite de l’orientation des axes de plis est dirigée ~N045

E, comme dans la chaîne de l’Ougarta plus au SE. A l’échelle régionale, cette compression interfère avec une compression de direction N-S. La carte du toit du socle fait ap- paraître une mosaïque de blocs basculés vers le sud ou le sud-est. Les failles limitant ces blocs sont héritées de paléo- failles normales, formées durant plusieurs événements extensionnels, d’âge cambrien, ordovicien supérieur, et surtout dévonien moyen-supérieur. Le réseau de paléofailles correspond à un dispositif de marge passive proximale en domaine métacratonique. L’inversion des paléofailles et leur jeu en décrochement se sont produits pendant un événement com- pressif tardi-varisque (Stéphano-Permien), postérieur à la collision NNW-SSE entre le bloc mésétien et son avant-pays.

INTRODUCTION

The West African Craton (WAC) is bounded by two supe- rimposed orogens, namely the Pan-African and Variscan orogens of Neoproterozoic and Palaeozoic age, respectively (fig. 1). The Pan-African orogen crops out in the culmina- tions (“boutonnières”) of the Anti-Atlas mountains, whereas the Variscan orogen makes up the basement of the Meseta

block (Meseta and Atlas domains), and additionally forms the Mauritanides at the southwestern fringe of the Saharan domain. The Anti-Atlas corresponds to the external, fore- land fold belt of the Variscan orogen [Michard et al., 1982;

Piqué and Michard, 1989; Piqué, 2001; Simancas et al., 2005; Hoepffner et al., 2005, 2006; Villeneuve et al., 2006], uplifted during the Mesozoic and Cenozoic times. The wes- tern Anti-Atlas structure was recently deeply studied by

Bull. Soc. géol. Fr., 2007, t. 178, n^o5, pp. 15-24

o 1. Ministère de l’Energie et des Mines, BP 6208, Rabat Agdal, Maroc, e-mail : [email protected].

2. Département de Géologie, Faculté des Sciences Aïn Chok, BP 5366, Casablanca Maarif, Maroc, e-mail [email protected].

3. Laboratoire de Géologie, Ecole Normale Supérieure, 24 rue Lhomond, 75231 Paris cedex 05, e-mail : [email protected].

Manuscrit déposé le 18 septembre 2006 ; accepté après révision le 13 mars 2007.

Caritg et al. [2004], Helg et al. [2004] and Burkhard et al.

[2006]. These authors emphasize the implication of the Pre- cambrian basement in the shortening process which causes the polyharmonic folding of the thick Palaeozoic cover.

Thus, they describe the tectonic style as a dominantly thick skinned style. Reclined folds, thrust and duplex structures are lacking except in the westernmost part of the belt [Sou- laimani et al., 1997; Belfoul et al., 2002]. Moreover, accor- ding to Burkhard et al. [2006], the Anti-Atlas Palaeozoic basin may not represent the Paleo-Gondwana continental margin, but rather the northern part of the intracratonic Tin- douf basin.

In the present paper, we consider the eastern Anti-Atlas case study around the Ougnat culmination. As far as Varis- can deformation is concerned, the interest of this area with respect to western Anti-Atlas is twofold: i) its Palaeozoic cover sequence is much thinner than that of the western re- gions [Destombes et al., 1985; Destombes and Hollard, 1986; Du Dresnay et al., 1988], allowing us to more easily recognize the basement structure underneath, and ii) the Ougnat area corresponds to the junction of the E-trending Anti-Atlas axis with the NW-trending Ougarta belt [Don- zeau, 1974; Haddoum et al., 2001; Fabre, 2005]. Moreover, the development of a multidirectional paleofault system throughout the Palaeozoic, and particularly during the Middle-Late Devonian can be evidenced south of the Ou- gnat massif [Baidder et al., 2007]. In the following, we show that folding essentially occurs in relation with paleo- fault inversion and strike-slip displacements of the base- ment blocks. Our study is based on two years of field work during the elaboration of the geological map of Morocco,

scale 1:50,000, sheets Oukhit [Raddi et al., 2006] and Bouadil (Bou’Adil) [Raddi et al., in prep.].

GEOLOGICAL SETTING

The Palaeozoic sequences of eastern Anti-Atlas overlie a Precambrian basement which crops out widely in the Jebel (J.) Saghro and Ougnat massifs, in some restricted areas east of Erfoud, and further south in the Ougarta range in Algeria (figs. 1, 2). This continental basement formed du- ring the Pan-African orogeny then was converted into an ex- tending metacratonic domain by the latest Neoproterozoic times [Ennih and Liégeois, 2001; Gasquet et al., 2005]. The whole area was flooded by the Palaeozoic seas from the Early-Middle Cambrian to the late Early Carboniferous (next section). Cambrian-Ordovician sediments are exposed around the Precambrian culminations and make up a SE-trending folded ridge (Ougnat-Ouzina ridge) between two areas where Devonian-Carboniferous formations are widely exposed, i.e. the Tafilalt and Maider basins to the east and west, respectively [Hollard, 1974, 1981; Wendt, 1985; Baidder et al., 2007]. The Saghro-Ougnat axis is overthrust along its northern border by the Tineghir and Tis- dafine Ordovician-Carboniferous slivers [Michard et al., 1982; Soualhine et al., 2003]. These units belong to the strongly deformed, epimetamorphic South Meseta Zone of the Meseta block, which collided with the metacraton du- ring the Late Carboniferous [Houari and Hoepffner, 2003;

Hoepffner et al., 2005, 2006].

The eastern Anti-Atlas and Ougarta Palaeozoic belts are progressively uplifted during the Late Westphalian then deeply eroded during the Stephanian-Autunian as indicated by the sedimentation of coal measures, then continental red beds in the neighbouring Kenadza-Bechar and Abadla ba- sins [Fabre, 2005]. Late Triassic rifting dominantly affects the Atlas trough north of the eastern Anti-Atlas rift

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FIG. 1. – The Variscan fold belts of northwestern Africa, after Fabre [2005], Hoepffner et al. [2005], and Villeneuve et al. [2006]. Framed: loca- tion of figure 2. SAF: South Atlas Fault.

FIG. 1. – Les chaînes varisques d’Afrique nord-occidentale, d’après Fabre [2005], Hoepffner et al. [2005] et Villeneuve et al. [2006]. Encadré : loca- lisation de la figure 2. SAF : Faille sud-atlasique.

FIG. 2. – Sketch map of eastern Anti-Atlas, after the geological maps of Morocco, scale 1/200,000, sheet Todhra-Maider [Du Dresnay et al., 1988]

and Tafilalt-Taouz [Destombes and Hollard, 1986]. Framed : location of the mapped area (FIG. 4).

FIG. 2. – Carte schématique de l’Anti-Atlas oriental, d’après les cartes géologiques du Maroc, échelle 1/200 000, feuille Todhra-Maider [Du Dresnay et al., 1988] et Tafilalt-Taouz [Destombes et Hollard, 1986].

Encadré : localisation du secteur cartographié au 1/50 000 (FIG. 4).

shoulder, although the occurrence of Triassic normal faults in the latter domain is arguable [Robert-Charrue, 2006].

The folded Palaeozoic sediments are intruded by the 200 Ma-old sills and dykes of the Central Atlantic Magma- tic Province [Hollard, 1973; Knight et al., 2004]. The Pa- laeozoic fold belt is surrounded to the east and south by virtually horizontal Cretaceous-Neogene plateaus (hama- das), whereas to the north the unconformable Creta- ceous-Eocene sediments are tilted and possibly folded close to the South Atlas Fault. Recent uplift of the Saghro-Ougnat axis is related on the one hand to the Late Eocene-Pliocene Atlas compression event [Giese and Jacobshagen, 1992;

Frizon de Lamotte et al., 2000], and on the other hand to the influence of a large scale mantle anomaly [Missenard et al., 2006].

STRATIGRAPHY

The Ougnat basement rocks include, i) folded Neoprotero- zoic meta-turbidites intruded by Pan-African granitoids, and ii) a ~2 km-thick, unconformable late Neoproterozoic cover mostly consisting of felsic volcanics intruded by late calc-alkaline dykes and high-K plutons [Ouarzazate Super- group; Thomas et al., 2004; Fekkak et al., 2003; Gasquet et al., 2005; Raddi et al., 2006b]. These late Precambrian rocks are disconformably overlain by the Palaeozoic sedi- ments (fig. 3). The whole Palaeozoic sequence hardly reachs 4 km thick in the south Ougnat area, contrasting with the 10 km thick cover of western Anti-Atlas. In particular, the thick carbonate formations which initiate the Lower Cambrian sequence in western Anti-Atlas are lacking here.

In the Bouadil area, the Early Cambrian deposits consist of reddish conglomerates and silts equivalent to the western Anti-Atlas “Lie-de-vin” Formation. They are followed up- ward by shallow water sandstones which correspond to the

“Grès Terminaux” Formation, now dated from the earliest Middle Cambrian [Geyer and Landing, 2004]. Going further east and north, the “Lie-de-vin” séries and the “Grès termi- naux” progressively vanish. Apart from the extreme reduc- tion of the Early Cambrian sediments, another peculiarity of the Oukhit area is the occurrence of alkaline volcanism with trachybasalt and trachy-andesite flows and sills intercalated within the Middle Cambrian Tabanit sandstones, whereas pipes and dykes are also shown within the underlying

“Schistes à Paradoxides” Fm.

The Ordovician sequence corresponds to alternating pe- litic and sandy platform deposits similar to, although thin- ner than those of western Anti-Atlas. Silurian black shales correspond to high sea level sediments related to the mel- ting of the Saharan inlandsis [Destombes et al., 1985]. Car- bonate sedimentation becomes more and more frequent during the latest Silurian-early Emsian interval, and finally dominates during the late Emsian-early Frasnian. In the South Ougnat region, the Paleozoic sedimentation ends with unconformable, shallow water Famennian sediments follo- wed upward by Lower Carboniferous rythmic deposits. At larger scale (from North Ougnat to Tafilalt and Maider), si- gnificant variations indicative of protracted extensio- nal/transtensional faulting occur in the Ordovician-Visean sequences (see Interpretation section).

STRUCTURE Folds and faults

In the south Ougnat area, deformation of the Palaeozoic se- quences is strongly heterogeneous (fig. 4). Large monocli- nal areas, tilted by 20-30

S- or SE-ward over tens of square kilometres coexist with more restricted folded and faulted zones. Metamorphism is lacking (except in the Tineghir sli- vers), consistent with the thin stratigraphic overburden (< 4000 m). Therefore, the Ougnat basement is essentially deformed by brittle faulting, and the stratified Palaeozoic cover by flexural slip folds, as illustrated below.

The larger folds are observed in the Angal-Guerghis Lo- zenge (AGL), a broadly lozenge-shaped, E-W elongated zone jammed between four large monoclinal blocks, i.e. the Tinchraramine and Taghwilest-Tamjout blocks to the west, and the Tawjint n’Tibiren and Mecissi blocks to the east. At the southwestern side of the AGL, the J. Angal fold is a

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VARISCAN DEFORMATION (EASTERN ANTI-ATLAS, MOROCCO)

17

FIG. 3. – Synthetic stratigraphic column of the eastern Anti-Atlas Palaeo- zoic formations. The informal lithostratigraphic units are labelled on the right-hand side, with their age after Destombes et al. [1985], Destombes and Hollard [1986], and Du Dresnay et al. [1988]. Potential detachment le- vels are shown in black. Age of the “Grès terminaux” Fm after Geyer and Landing [2004].

FIG. 3. – Colonne stratigraphique synthétique des formations paléozoïques de l’Anti-Atlas oriental. L’âge et les noms traditionnels des formations sont donnés d’après Destombes et al. [1985], Destombes et Hollard [1986], et Du Dresnay et al. [1988]. Les niveaux de décollement potentiels sont en noir. L’âge des “Grès terminaux” est d’après Geyer et Landing [2004].

massive, cylindrical box fold formed by the “Grès termi- naux” calcareous sandstones, beneath the “Schistes à Para- doxides” and Tachilla-Fezzouata décollement levels (fig. 5A). Minor dysharmonic folds are observed in the Ta- banit sandstones in between the latter incompetent forma- tions. The Precambrian crops out as a “mini-boutonnière” at the fold crest, which implies dense brittle shear zones be- neath the flanks, and suggests an axial pop-up structure.

The competent Cambrian layers are detached from the brittle basement along the “Lie-de-vin” marly-argilaceous sediments (fig. 6A). A basic sill likely of Triassic-Liassic age emplaced in the mechanical discontinuity between the Precambrian footwall and the Cambrian hanging-wall. The axis of this large fold is slightly curved in map view, with some transverse normal-wrench faults, and it nearly paral- lels the major fault bounding the NE corner of the Taghwi- lest block (North Taghwilest Fault, NTF). This probably accounts for the close minor fault spacing which has to be assumed in the basement beneath the fold itself.

E-trending folds occur in the E-W axis of the AGL zone (Temsermass area), showing remarkable tightness changes along strike. At the western tip of the zone, which is pin- ched between the Taghwilest and Tinchraramine blocks, folds are rather tight (fig. 5B), and intense shearing must be inferred within the underlying basement. Conversely, in the

central part of the lozenge zone (Lalla Mimouna area), large open folds crosscut by longitudinal reverse faults occur (fig. 5C). Eventually, at the eastern tip of the lozenge, the J.

Gherghiz displays a set of NW-trending, SW-reclined faul- ted folds (fig. 5D). These folds affect the Devonian layers preserved in the northern, downthrown side of the N-Mecis- si fault (NMF), and they are limited eastward by another uplifted block, i.e. the Tawjit n’Tibirene block (fig. 4). The short wavelength of the Devonian folds with respect to the folds in the underlying Cambrian-Ordovician layers illus- trates an example of polyharmonic folding permitted by the occurrence of the Silurian-Lower Devonian detachment le- vel.

Outside the AGL zone, the Ighil n’Ighiz block also shows a conspicuous folding of the Premier Bani sandsto- nes detached from the Tabanit sandstones along the Tachil- la-Fezzouata pelites. The Tabanit slab itself is detached from the “Grès Terminaux” and Precambrian basement along the “Schistes à Paradoxides” pelites and forms heavy anticlines north and south of the Ighil n’Ighiz downthrown block. Most of the other folds observed in the south Ougnat area correspond to small size, en échelon folds associated to the N070-N110 and N020-N045 faults bounding the mono- clinal blocks. The dominant trend of both the major and

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FIG. 4. – Structural map of the Palaeozoic terranes of the South Ougnat area (sheets Oukhit and Bouadil of the geological map of Morocco, scale 1:50,000). J. BI.: Jebel Bou Iblane; JG: Jebel Gherghiz; LM: Lalla Mimouna; NIIF: North-Ighil n’Ighiz fault; NMF: North-Mecissi fault; NTF:

North-Taghwilest fault; OSF: Oued Smile fault; SIIF: South-Ighil n’Ighiz fault. A-D: location of cross-sections figure 5. Asterisks : location of figu- res 6A, B and figures 8A-C. Stereograms (Wulf stereonet, lower hemisphere) a: J. Angal ; b: North Angal-Gherghiz fault; c : North-Mecissi fault; d: Tizi N’Tarhatine fault.

FIG. 4. – Carte structurale du Paléozoïque du secteur Sud Ougnat (feuilles Oukhit et Bouadil de la carte géologique du Maroc, échelle 1:50,000. A-D : lo- calisation des coupes figure 5. Astérisques : localisation des figures 6A, B et 8A-C. Stéréogrammes (canevas de Wulf, hémisphère inférieur).

minor fold axes is ESE to SE (fig. 4), which indicates a re- gional shortening direction close to NE-SW.

Fault kinematics

Fault kinematics obviously change according to the fault orientation, either N070-110 or N020-N045, but also accor- ding to the location of the faults, either in the western or eastern part of the studied area.

In the east and south-central parts of the map (fig. 4), the folds associated with the dominantly N070-N110 faults yield evidence of left-lateral throw along these faults, often associated with a reverse component of displacement. The Guerghiz folds, for example, display a left-lateral en éche- lon pattern along the N090-striking N-Mecissi fault, consis- tent with a NE-trending direction of shortening. The nearly vertical NE limb of the northeastern syncline (fig. 5D) indi- cates that the neighbouring Tawjit n’Tibirene block

overthrust the AGL downthrown tip. The N100-110 stri- king, N-dipping South Ighil n’Ighiz fault (SIIF, fig. 4) is an example of a paleofault inherited reverse fault; the early normal throw is documented by the downthrow of the Ordo- vician of the Ighil n’Ighiz block with respect to the Cam- brian of the Tawjit n’Tibirene block, whereas a minor reverse throw is documented by S-vergent chevron folds lo- cated along the northern side of the fault (fig. 6B).

In contrast, the west-southwest part of the studied area displays a succession of NNE-trending normal faults which bound SE-ward tilted blocks without being inverted. One of these normal faults (N-Bou Iblah fault) connects eastward with a left-lateral, N-trending horse tail, whereas the S-Bou Iblah normal fault branches into the N110 left-lateral wrench fault north of the Tinchraramine block. The latter fault in turn connects with the Oued Smile fault (OSF, fig. 4) through a minor, left-lateral pull-apart structure.

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VARISCAN DEFORMATION (EASTERN ANTI-ATLAS, MOROCCO)

19

FIG. 5. – Cross-sections of typical fold structures from the Angal-Gherghiz Lozenge (AGL). For loca- tion, see figure 4. kmb: base of Middle Cambrian (“Grès terminaux” Fm); km1: schistes à Paradoxides;

km2 : Tabanit sandstones; or2/or3: Tachilla/Fezzouata silts; or4: 1^stBani quartzites; or5: Ktaoua sandstones;

or6b: Ashgill; Si: Silurian; D: Devonian; hv: Early Carboniferous.

FIG. 5. – Coupes dans les plis du Losange Angal-Gherghiz (AGL). Localisation : figure 4. kmb : Cambrien moyen basal (“Grès terminaux”); km1 : schistes à Paradoxides; km2 : grès du Tabanit;

or2/or3 : schistes Tachilla/Fezzouata; or4 : grès du Premier Bani; or5 : grès du Ktaoua; or6b : Ashgill;

Si : Silurien; D : Dévonien; hv : Carbonifère infé- rieur.

Basement blocks

The surface structure (fig. 4) strongly suggests that the Pre- cambrian basement itself is divided into large tilted blocks and minor lozenge or triangle-shaped, intensely faulted zo- nes between the larger blocks. It results in the possibility to produce a schematic map of the top of the Precambrian ba- sement based on the following assumptions: i) the major faults observed at the surface continue downward into the basement almost vertically; ii) the thickness of the Palaeo- zoic formations remain approximately constant over areas at least as large as the block where it is observed. Based on these assumptions, and on the measured dip and thicknesses of the Palaeozoic layers, the map of the basement isobaths makes apparent a mosaic of tilted blocks (fig. 7). Most of the blocks southeast of the Ougnat culmination dip S- to SE-ward, whereas west and north of the culmination axis, they dip west and north, respectively. The Central Ougnat block itself is more than 2 000 km

large, whereas in con- trast, the basement of the Angal-Gherghis lozenge is less than 700 km

large and dissected by several faults associa- ted with intensely sheared zones (see above, fig. 5A, B).

INTERPRETATION AND DISCUSSION Origin of the fault system

We noted above, based on geometric arguments, that the N090-110 S-Ighil n’Ighiz fault (figs. 4, 6B) corresponds to a partly inverted N-dipping normal fault, changed into a left-lateral reverse fault during the folding episode. We also observed that a group of NNE-striking minor faults is pre- served in the eastern part of the studied area between SE-ward tilted blocks involved in large dextral strike-slip faults. Indeed, the stratigraphic record documents several extensional events from (at least) the Early Cambrian on- ward, as follows.

The “Lie-de-Vin” and “Grès Terminaux” thickness abruptly changes from c. 400 m southwest of the Ougnat

massif to c. 150 m going eastward across the Oued Smile fault (OSF), and the whole sequence entirely vanishes north of the Oukhit faults (fig. 7), suggesting that the N110 W.

Smile fault was active at that time. Likewise, conspicuous slumpings are observed in the Tabanit sandstones close to the S-Oukhit fault (fig. 8A). By the end of the Middle Cam- brian, the alkaline mafic volcanism developed between the Oued Smile and Oukhit faults strongly suggests an exten- sional setting there. After the Upper Cambrian sedimentary gap (likely of eustatic origin), the Tremadoc-Llandeilo iso- pachs parallel the Saghro-Ougnat axis. Then, the Caradoc isopachs turn SE, i.e. parallel to the Ougnat-Ouzina ridge [Destombes, 2006a, 2006b]. Chaotic conglomerates of Ca- radocian age (fig. 8B) are indicative of active fault scarps along the ridge. Likewise, the Lower Silurian pelites are only preserved within a NE-trending faulted “channel”

along the same ridge [Destombes, 2006a, 2006b].

A major extensional crisis occurs during Middle-Late Devonian, as clearly evidenced by the dramatic facies and thickness variations in the Tafilalt-Maider regions [Hollard, 1974, 1981; Wendt, 1985, 1988]. As early as the Emsian, the large, and yet undifferentiated Tafilalt-Maider platform is progressively replaced by a restricted Tafilalt platform boun- ded by two subsiding areas in the Maider and South Tafilalt.

Synsedimentary faulting responsible for this paleogeographic differenciation is particularly active during the Famennian. A system of ENE- and SE-trending normal faults controls these sedimentary contrasts [Baidder et al., 2007]. In the South Ougnat area, the N-Mecissi fault is an example of such ENE-striking, N-dipping Late Devonian faults: this can be inferred from the unconformity of the Famennian ferruginous conglomerates and calcareous siltites onto the Frasnian to Praguian levels within the tilted Guerghiz-Tamjout block (fig. 9). Visean deposits also show important N-S changes:

Tineghir-Tisdafine olistostromes and Ben Zireg chaotic brec- cias [Michard et al., 1982; Soualhine et al., 2003]; Maider and Erfoud area fine grained turbidites [Destombes and Hol- lard, 1986; Du Dresnay et al., 1988]; J. Begaa (south Tafilalt) reef or mud mounds [cf. in Michard, 1976, p. 66-67]. The

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FIG. 6. – A : Detachment of the basal Middle Cambrian calcareous sandstones on top of the late Neoproterozoic volcanics at the crest of the J. Angal anti- cline (see figs. 4 and 5A for location); the cataclastic level derives from the thin “Lie-de-vin” conglomeratic silts.– B : Minor reclined chevron folds deve- loped during the partial inversion of the South-Ighil n’Ighiz paleofault (see figure 4 for location); wavelength close to 1.50 m.

FIG. 6. – A : Décollement des « Grès Terminaux » (base du Cambrien moyen) au dessus du socle néoprotérozoïque terminal (« P III » à la crête anticlinale du J. Angal (voir figs. 4 et 5A pour la localisation); le niveau cataclastique dérive de la mince formation « Lie-de-vin » argilo-conglomératique.– B : pli mineur en chevron formé dans l’Ordovicien par inversion de la paléofaille Sud-Ighil n’Ighiz (localisation : voir figure 4); longueur d’onde ~ 1,50 m.

distribution of these contrasting Visean facies was likely con- trolled by strike-slip movements along the E-W major faults north and south of eastern Anti-Atlas, i.e. the SAF and AMA, respectively [Baidder, in prep.]

Therefore, we argue that the fault system which control- led the Variscan deformation is inherited from a paleofault system dominantly parallel to the Saghro-Ougnat axis or to

the Ougarta trough (“Sillon de la Saoura”; Fabre [2005]), respectively. Combined with NNE-trending intersecting faults, these faults defined triangle-shaped or rhombic blocks dominantly tilted to the south or south-east. This portraits a Palaeozoic proximal margin domain at the northern, metacra- tonic rim of the WAC, and south of the Meseta distal domain [Hoepffner et al., 2005, 2006]. It is worth recording that both the Anti-Atlas and Ougarta belts are superimposed onto the

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FIG. 7. – Sketch map of the top of the Proterozoic base- ment in the Ougnat area, showing the thickness changes of the Lower Cambrian-basal Middle Cambrian depo- sits around the culmination (numbers in parentheses), and the approximate trace of cross-section figure 10.

The map is based on the measured dip and stratigraphic thickness of the Palaeozoic sequence over each block, and on the assumption that the basement block pattern roughly corresponds to the major fault pattern observed in the Palaeozoic cover (FIG. 4).

FIG. 7. – Carte schématique du toit du socle précam- brien dans le secteur Sud Ougnat, montrant les varia- tions d’épaisseur du Cambrien inférieur-base du Cambrien moyen (nombres entre parenthèses) et la lo- calisation de la coupe figure 10. La carte est dressée d’après la mesure des pendages et des épaisseurs des strates paléozoïques dans chaque bloc, en supposant que les failles dans le socle ont à peu près la même po- sition que les failles majeures de la couverture (FIG. 4).

FIG. 8. – Sedimentary recordings of Cambrian and Ordovician faulting, Oukhit sheet. For location, see figure 4.– A: slumped beds in the Tabanit Fm (late Middle Cambrian), SW of Ouinguigui.– B: Upper Ordovician chaotic conglomerates (Imzizoui conglomerates; Destombes et al. [1985], Destombes [2206a, 2006b]). Light colored blocks are Lower Cambrian limestones; darker ones are Lower Ordovician Fe-oolithic sandstones.

FIG. 8. – Enregistrements sédimentaires de mouvement de faille au Cambrian et à l’Ordovician, feuille Oukhit. Localisation : voir figure 4.– A : slumping dans la Fm Tabanit (partie supérieure du Cambrien moyen), au SW de Ouinguigui.– B : conglomérats chaotiques de l’Ordovicien supérieur (conglomérats Imzizoui; Destombes et al. [1985], Destombes [2006a, 2006b]). Les blocs de couleur claire sont des calcaires du Cambrien inférieur; les blocs sombres sont des grès à oolithes ferrugineuses de l’Ordovicien inférieur.

Pan-African belt around the West African Craton; in other words, at least part of the Palaeozoic paleofault system is ul- timately inherited from the late Neoproterozoic extensional structures [Soulaimani et al., 2003].

Direction and age of compression

The paleofault inversion as reverse strike-slip faults occur- red in the frame of a dominant NE-trending direction of shortening, as shown by the fold axes orientation (fig. 4).

Accordingly, the sketch cross-section (fig. 10) was drawn along a NE direction, although this direction is oblique to many of the faults. As deformation is not plane, the amount of shortening cannot be calculated, but it is certainly weak, less than 10% at the scale of the whole foreland section, al- though it can reach ~ 20% by place (cf. J. Angal; fig. 5A).

This NE-SW maximum stress direction compares with the Stephanian-Autunian shortening direction of the Ougarta chain [Donzeau, 1974; Haddoum et al., 2001; Fabre, 2005]

and Meseta domain [Saber, 1994; Saidi et al., 2002], which suggests a similar age for this event in the studied area.

However, a SSE-directed compression is recorded by the Tineghir-Tisdafine imbrications along the northern bor- der of the Saghro-Ougnat axis [Michard et al., 1982; Hoepf- fner et al., 2006], the Tamlelt massif of eastern High Atlas [Houari and Hoepffner, 2003], and the Djebel Bechar struc- tures farther east [Fabre, 2005, p. 358]. Although the NNW-SSE “Anti-Atlas compression” clearly interferes with the NE-SW “Ougarta compression” at the regional scale [Robert-Charrue, 2006; Baidder et al., 2007], the former was not recognized in the Bouadil-Oukhit area, where the Ougarta compression dominates. The NNW-SSE shortening was probably older than the NE-SW one as it compares with the S-verging, last shortening event of western Anti-Atlas [Caritg et al., 2004; Hoepffner et al., 2006], and also with the late Westphalian event recognized in the Meseta Block [Hoepffner et al., 2006]. It is worth noting that the late Na- murian-Westphalian ESE-directed compression event reco- gnized in western Anti-Atlas [Soulaimani et al., 1997;

Belfoul et al., 2002; Caritg et al., 2004; Burkhard et al., 2006] and western Meseta [Hoepffner et al., 2006] is lac- king in eastern Anti-Atlas.

Mesozoic evolution

Robert-Charrue [2006] recently argued that the Triassic ex- tension was able to reactivate as normal faults the N-dipping

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FIG. 9. – An example of Famennian paleofault in the South Ougnat area:

the North-Mecissi fault.– A: stratigraphic columns of the Gherghiz massif (1) and Lalla Mimouna-Oued Smile area (2) a few kilometres further north.

At both localities, the Famennian ferruginous conglomerates overlie the ol- der deposits with an unconformity angle of ~10^o.– B: restoration of the til- ted Gherghiz block in the hanging wall of the N-Mecissi fault during the Famennian [after Baidder et al., 2007, modified].

FIG. 9. – Exemple de paléofaille du Faménnien dans le secteur Sud Ou- gnat : la faille Nord-Mecissi.– A : colonnes stratigraphiques du J. Gherg- hiz (1) et du secteur Lalla Mimouna-Oued Smile (2) quelques kilomètres plus au nord. Dans les deux localités, les conglomérats ferrugineux du Fa- ménnien recouvrent les formations antérieurs avec un angle de ~10^o.– B : reconstitution du bloc du Gherghiz au dessus de la faille N-Mecissi pen- dant le Faménnien [d’après Baidder et al., 2007, modifié].

FIG. 10. – Generalized cross-section of the Ougnat culmination area. See figure 7 for location. Cross pattern: Neoproterozoic basement. Folded layers: Pa- laeozoic sediments (Anti-Atlas domain). Grey: Palaeozoic metasediments (South Meseta frontal units). White arrows: synsedimentary movements. Black arrows: synorogenic inversion. The post-Variscan uplift of the Saghro-Ougnat axis integrates the Triassic-Jurassic uplift of the Atlas rift shoulder and the Cenozoic uplift of the Atlas belt foreland, with an additional mantle contribution. STF/NTF: South/North Taghwilest faults; SBF: South Bouadil fault;

OSF: Oued Smile fault; SOF/NOF: South/North Oukhit faults.

FIG. 10. – Coupe généralisée de la culmination de l’Ougnat. Voir figure 7 pour la localisation. Croix : socle précambrien. Couches plissées : sédiments paléozoïques (Anti-Atlas). Gris : métasédiments paléozoïques (unites frontales Sud Mésétiennes). Flèches blanches : mouvement synsédimentaires. Flè- ches noires : inversion synorogénique. Le soulèvement post-hercynien de l’axe Saghro-Ougnat intègre le soulèvement de l’épaulement du rift atlasique au Trias-Jurassique et le soulèvement de l’avant-pays de la chaîne atlasique au Cénozoïque, avec une contribution mantélique. STF/NTF failles Sud/Nord Taghwilest; SBF : faille Sud Bouadil; OSF : faille Oued Smile; SOF/NOF : failles Sud/Nord Oukhit.

Cambrian paleofaults inverted during the Variscan compres- sion. We agree that extension was strong enough in the area to allow Triassic-Liassic gabbroic magmas to emplace in the crust and folded Palaeozoic sediments, but we consider that rifting deformation was concentrated north of the Sag- hro-Ougnat axis, i.e. in the Atlas trough. We did not find clear indications of extensional reactivation of the inverted Palaeozoic paleofaults, except, possibly, the Oukhit faults at the northern border of the Ougnat culmination. Triassic-Lias- sic sediments are, indeed, concentrated in the Atlas domain, and virtually lacking in the Anti-Atlas and Saharan domain, except the upper red beds of the Abadla basin [Fabre, 2005].

In contrast, we emphasize that paleofaults not only develo- ped during the Cambrian, but also in the Ordovician, Silu- rian, and above all during the Devonian crustal dislocation event (figs. 8, 9). In our opinion, these faults account for most if not all the dislocation of the metacratonic crust of eastern Anti-Atlas.

CONCLUSION

Detailed mapping of the South Ougnat area allowed us to present the following conclusions:

– during the Palaeozoic, this region belongs to the proximal margin of northwestern Gondwana, at the junction with the Ougarta aulacogen;

– a paleofault system develops in the already faulted Pan-African basement from Early Cambrian to Late Devo- nian times, resulting in a mosaic of rhombic, S– to SE-tilted blocks;

– during the Variscan orogeny, deformation occurs by partial or total inversion of the former normal faults, and strike-slip relative displacements of the tilted blocks;

– folding of the detached Palaeozoic cover is concentra- ted along the reactivated faults, and within some crushed minor blocks;

– during this orogenic event, the dominant, horizontal compression axis is directed NE-SW, as in the Ougarta range;

– the “Ougarta compression” can be dated from the Ste- phanian-Autunian;

– this late Variscan event interferes at a regional scale with the “Anti-Atlas compression” directed NNW-SSE and referable to the late Westphalian collision of the Meseta block against Gondwana.

Acknowledgments. – A. M. acknowledges grants from the Bureau de Re- cherches Géologiques et Minières (France)??????????, programme Géo- forma, and from the Ministère de l’Energie et des Mines, Rabat, Programme National de Cartographie géologique. The authors are indebted to J.-P. Prian (BRGM), L. Tabit, A. El Khlifi and A. Charik (MEM) for technical support at Rabat, and to D. Frizon de Lamotte and A. Soulaimani for very constructive reviews.

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