1 Introduction
The Palaeozoic of North Africa is part of the peri-Atlantic Palaeozoic chains, and more particularly of the Variscan belt, built up during the Late Palaeozoic by collision between Gondwana and Laurussia [77].
Outside of the Rifian–Kabylian Alpine allochthons (not considered in this paper), the Variscan terranes of North Africa crop out widely in Morocco and much lesser in westernmost Algeria; they form the Palaeozoic inliers and massifs of the Moroccan and West Algerian Mesetas, and of the Atlas chain (Fig. 1). These elements constitute the Variscan orogenic domain or ‘Mesetan’ domain, which is opposed to the weakly deformed Saharan domain (Anti-Atlas, Ougarta). The relationships between these two fields are not perfectly clear. Their respective positions during most of the Palaeozoic are in debate, except at the end of Carboniferous, where the Anti-Atlas area can be delineated as the southern foreland of the chain.
Structural domains of North Africa and location of the Palaeozoic terranes (after [60]). 1: Rif–Tell = Maghrebide belt, a: southern front of the belt; b: Palaeozoic allochthonous terranes of the Maghrebide internal zones. 2: Atlasic belt (HA: High Atlas, MA: Middle Atlas, SA: Saharan Atlas, TA: Tunisian Atlas). 3: Palaeozoic terranes, a: deformed by the Variscan orogeny, b: undeformed. 4: West-African craton (Archean and Eburnian). 5: Mauritanides allochthons.
Domaines structuraux de l'Afrique du Nord et localisation des terrains paléozoïques (d'après [60]). 1 : Chaîne Rifo-Tellienne = Maghrébides, a : front sud de la chaîne, b : terrains paléozoïques allochtones des Maghrébides internes. 2 : chaîne atlasique (HA : Haut Atlas, MA : Moyen Atlas, SA : Atlas saharien, TA : Atlas tunisien). 3 : Terrains paléozoïques, a : déformés par l'orogenèse varisque, b : non déformés. 4 : Craton Ouest-Africain (Archéen et Éburnéen). 5 : Unités allochtones des Mauritanides.
The Moroccan Variscan orogen was the object of several synthesis and attempts of interpretation [39,60,81,83,93,94,96]. According to these authors, during the Palaeozoic, North Africa was located at the margin of Gondwana, and went through an intracontinental evolution. There is no geological argument indicating the existence of an oceanic lithosphere during the Palaeozoic. The crustal thinning events recorded during various periods, from Cambrian to Carboniferous, did not result in the formation of any documented oceanic crust in the area. Some geodynamical models, mainly based on the study of the Carboniferous magmatism, postulate, however, the existence of an oceanic crust [20,72,101]. A more or less broad oceanic domain would have separated the Meseta domain and the Saharan platform during the Early Palaeozoic [52] or the Devonian [106]. However, the palaeomagnetic data in support of these models are controversial [73]. In the present paper, we show that the hypothesis of exotic Mesetan terranes before the Eovariscan events is not supported by undeniable field data.
This note focuses on the tectonics of the Moroccan Meseta Domain during the Palaeozoic, and more particularly during the Variscan orogeny from Late Devonian to Late Permian. Based on stratigraphic, structural and radiometric arguments, it is possible to distinguish several events or phases of deformation: (i) Eovariscan events (370–360 Ma), (ii) Mesovariscan or Intravisean events (330–320 Myr), (iii) Neovariscan events developed at 300–290 Ma (main Variscan phase), (iv) Tardi-Variscan events between Early Permian and Late Triassic.
2 Structural framework
According to the different synthesis quoted above, it is possible to subdivide the Variscan belt of North Africa into five structural zones (Fig. 2).
Structural zones of the Variscan domain of Morocco and Western Algeria (Mesetan domain) with the main Palaeozoic massifs delineated (after [60]). 1: Sehoul Zone (a) and its likely prolongation to the west (b). 2: Coastal Block. 3: Central Zone, subdivided into Western Subzone (a) and Eastern Subzone (b). 4: Eastern Zone. 5:. Southern Zone. 6: Internal Zones of the Rif belt. Main structural limits: RTFZ: Rabat–Tiflet Fault Zone; WMSZ: Western Meseta Shear Zone; SOFZ: Smaala–Oulmès Fault Zone; TBBFZ: Tazekka–Bsabis–Bekrit Fault Zone; APTZ: Atlas Palaeozoic Transform Zone; SMVF: South Moroccan Variscan Front; TTFZ: Tizi n'Test Fault Zone.
Les zones structurales du domaine varisque au Maroc et en Algérie occidentale (domaine mésétien) et principaux massifs paléozoïques (d'après [60]). 1 : Zone des Sehoul (a) et son prolongement probable vers l'ouest (b). 2 : Bloc côtier. 3 : Zone centrale divisée en sous-zone occidentale (a) et sous-zone orientale (b). 4 : Zone orientale. 5 : Zone sud. 6 : Zones internes du Rif. Principales limites structurales : RTFZ : zone de failles de Rabat–Tiflet ; WMSZ : zone de cisaillement de la Meseta occidentale ; SOFZ : zone de failles Smaala–Oulmès ; TBBFZ : zone de failles Tazekka–Bsabis–Bekrit ; APTZ : Zone transformante paléozoïque de l'Atlas ; SMVF : front varisque sud-marocain ; TTFZ : zone de failles du Tizi n'Test.
(1) The Sehoul Zone or ‘Caledonian’ zone is located at the northwestern corner of western Meseta, and extends under the foredeep basin of the Rif belt, and offshore in the Atlantic margin. Cambrian deposits were folded, metamorphosed and granitized between 450–430 Ma. This terrane was attached to the main Meseta terranes between Upper Silurian and Upper Devonian along the Rabat–Tiflet Fault Zone (RTFZ).
(2) The Coastal Block, which corresponds to the Atlantic lowlands consists of Lower to Middle Palaeozoic deposits. The Variscan deformation is generally weak, but its intensity increases towards the Eastern limit of the Coastal Block, i.e. the Western Meseta Shear Zone (WMSZ).
(3) The Central Zone includes the larger massifs of western Moroccan Meseta. All the Palaeozoic systems are represented from Cambrian to Permian. An Early–Middle Palaeozoic, more or less subsident, marine platform is replaced during the Late Devonian–Carboniferous by syn-orogenic basins with basic magmatism in a context of crustal thinning. Variscan deformation is strong and polyphased (between 330 and 300 Ma), with Barrovian-type metamorphism along the WMSZ, and scattered syn- or late-orogenic granitoid bodies. This zone is crosscut by an important thrust zone, i.e. the Smaala–Oulmès Fault zone (SOFZ) and limited towards the East by the Tazekka–Bsabis–Bekrit Fault Zone (TBBFZ).
(4) The Eastern Zone includes the small inliers of the Moroccan Eastern Meseta and Algeria (Midelt, Debdou, Mekkam, Beni Snassen, Traras, North of Tamlelt). The Palaeozoic series (Ordovician to Westphalian) are characterized during Lower and Middle Devonian by turbiditic deposits, and during Lower Carboniferous by volcanoclastic rocks with calc-alkaline magmatism. Deformation is polyphased with two main events at 370 and 300 Myr followed by emplacement of late orogenic granitic plutons.
(5) The Southern Zone marks the limit between the Mesetan and Saharan domains; it is located south of the Atlas Palaeozoic Transform Zone (APTZ). The Palaeozoic series are similar to that of the Anti-Atlas from Cambrian to Devonian, whereas the Carboniferous is characterized by subsident detrital basins. Variscan deformation is somewhat stronger than in the Anti-Atlas, but metamorphism is virtually lacking (very low-grade greenschist facies in the main shear zones) and magmatism is not recorded. This zone is limited to the south by the folds and overthrusts of Tamlelt and Tineghir which correspond to a South Morocco Variscan Front (SMVF), which connects towards the west with the Tizi n'Test Fault Zone (TTFZ).
3 Prevariscan events
3.1 Sehoul Zone
The stratigraphic sequence of this zone consists of Cambrian pelites and greywackes, the primary sedimentary structures of which are indicative of a deltaic environment [37]. A tectono-metamorphic episode, with southward increasing intensity, corresponds to east–west-trending, south-verging overturned folds with spaced to slaty cleavage generated in a very low- to low-grade regional metamorphism. The Cambrian meta-sediments are intruded by granitic bodies. The metamorphism is dated at (K/Ar method on micas, [38]) and the granites at (Rb/Sr, [25], recomputed). According to the radiometric data, these events correspond to the Caledonian orogeny [79]. Likewise, in the Atlantic offshore of El Jadida, Lower Cambrian granodiorite has been mylonitized at 455 Ma [102]. Therefore, it is likely that a ‘Caledonian’ belt wraps around the northwestern Meseta [13,34] (Fig. 2); the convergence between these two domains was probably realized by ductile wrenching and/or thrusting of the Sehoul Zone upon the main Meseta domain prior to the Late Silurian as within the RTFZ, Silurian deposits unconformably overlay a Caledonian mylonitized granite tectonically inserted into unmetamorphosed Lower Ordovician shales [37].
3.2 Main Mesetan domain
From Neoproterozoic until Middle Devonian, the sedimentation is organized in a positive megasequence [93]: Cambrian and Ordovician silicoclastics, Silurian black shales, Lower and Middle Devonian limestones. An extensional tectonic context is well documented during the Cambrian with the Middle Cambrian rift trending NNE–SSW in the Coastal Block [15]. Basic magmatic rocks of alkaline affinity accompany this intraplate extension, essentially during Middle Cambrian and Silurian [34,41,87–89]. Tilted blocks are described in the Devonian [40]. A turbiditic basin opened during Devonian in eastern Morocco and Algeria; the clastic and deep water characters of the sedimentation distinguishes this zone from the western Meseta carbonate shelf.
The tectono-metamorphic and magmatic Caledonian events documented in the Sehoul Zone are not observed in the main Meseta domain. However, contemporaneous ‘epirogenic’ movements are described in the Coastal Block and in the RTFZ, where the detrital, continental deposits (‘Old Red Sandstones’ type) of Upper Silurian and Lower Devonian overlay directly (locally with angular unconformity) the Cambrian and Lower Ordovician strata [16,37,81,92].
4 Eovariscan and Mesovariscan (Intra-Visean) events
4.1 Eastern Zone
4.1.1 Eovariscan phase (370–360 Ma)
In the Meseta Eastern Zone (Figs. 2 and 3), the earliest deformations of the Lower Palaeozoic and Devonian deposits are attributed to the Eovariscan phase, based on isotopic datings of the metamorphic rocks. Metamorphism is important at Midelt and North Tamlelt (greenschist to amphibolite facies with biotite and garnet), whereas it is weak elsewhere (very low- or low-grade greenschist facies). This tectono-metamorphic event has been dated at 366 Ma at Midelt (Rb/Sr method, [26]), and 370 Ma at Debdou-Mekkam (K/Ar on white micas, [63]).
Structural map of the Mesetan Domain. 1: Sehoul Zone; 2: Coastal Block (Lower to Middle Palaeozoic); 3: Lower to Middle Palaeozoic of Western Meseta and Eastern and Southern Zones; 4: Carboniferous; a (triangles): volcanoclastic series of the Eastern Zone, b: Upper Westphalian redbeds, c: Late Stephanian–Lower Permian redbeds; 5: Variscan granitoids; 6: Eovariscan structures (cleavage, fold axis); 7: Variscan structures, a: cleavage trajectories and main vergence, b: anticlines, c: synclines; 8: gravity nappes; 9: main Variscan thrusts (b: subsurface data); 10: strike-slip faults. RTFZ, WMSZ, SOFZ, TBBFZ, APTZ (see Fig. 2). SBB: Sidi Bettache Basin; AKB: Azrou-Khénifra Basin; FB: Fourhal Basin.
Carte structurale du domaine mésétien. 1 : Zone des Sehoul ; 2 : bloc côtier (Paléozoïque inférieur et moyen) ; 3 : Paléozoïque inférieur et moyen de Meseta occidentale et des Zones orientale et méridionale ; 4 : Carbonifère ; a (triangles) : séries volcanoclastiques de la Zone orientale, b : couches rouges du Westphalien supérieur, c : couches rouges du Stéphanien terminal–Permien inférieur ; 5 : granitoïdes varisques ; 6 : structures éovarisques (schistosités, axes de plis) ; 7 : structures varisques, a : trajectoires de schistosité et vergence dominante ; b : anticlinaux ; c : synclinaux ; 8 : nappes gravitaires ; 9 : principaux chevauchements varisques (b : d'après les données de subsurface) ; 10 : décrochements. RTFZ, WMSZ, SOFZ, TBBFZ, APTZ (voir Fig. 2). SBB : Bassin de Sidi Bettache ; AKB : bassin d'Azrou–Khénifra ; FB : bassin de Fourhal.
Two stages of deformation (D1 and D2) are distinguished; they correspond to a progressive deformation during the Eovariscan phase [57,109]. The D1 and D2 structures are well exposed in Midelt and North Tamlelt, where the Lower Palaeozoic metamorphic series crop out. Elsewhere, it is difficult to separate D1 and D2. The earliest recognizable structures (D1) consist of a gently dipping, bedding-parallel foliation or slaty cleavage associated with isoclinal, recumbent folds trending east–west or NW–SE at Midelt and NW–SE to NNE–SSW in North Tamlelt. The foliation is generally refolded during the D2 stage by NW–SE to east–west trending, overturned folds with crenulation cleavage or differentiated layering. Fold vergence is toward the west or the southwest, rarely toward the east. It seems that D2 is associated with horizontal shearing (S/C structures, sheath folds, dispersion of the folds axis) probably related to top-to-the-west or south-west thrusts. A stretching and/or mineral lineation is locally well developed (Midelt, Tamlelt), generally gently dipping and parallel to the fold hinges. Locally, finite strain ellipsoid measurements indicate flattening or plane strain. A recent work [53] proposed that in the Midelt area, all the structures (D1 and D2) and metamorphism would be Intravisean events related to the ascent of the granitoids between 330 and 320 Ma. However, this hypothesis does not explain the structures observed elsewhere in weakly metamorphic or unmetamorphosed rocks, nor fits the reported isotopic datings of the metamorphic rocks at 366–370 Ma.
4.1.2 Intravisean phase (330–320 Ma)
This period corresponds in the Eastern Zone to the opening of volcano-clastic basins (Debdou-Mekkam, Jerada, Traras). The Lower Carboniferous deposits (conglomerates, limestones) dated from the Late Visean and Early Namurian [14,67,80] unconformably overlain the older rocks, weakly deformed by the Famenno-Tournaisian episode. These sediments are associated with pyroclastites and calc-alkaline lavas, e.g., andesites, dacites, rhyolites and ignimbrites [14,24,71]. South of Oujda, volcanism is coupled with chaotic facies sedimentation localised along ENE–WSW faults. This indicates that the volcanism is contemporaneous with extensional or transtensive tectonics [35,65,108]. Simultaneously, the oldest granitoids emplaced in the Midelt metamorphic rocks (333–319 Myr; UPb on zircons [91]), and in more superficial levels southwest of Oujda (328–321 Myr, RbSr [85]). Petro-structural studies carried out in the Midelt granitoids have led to ambiguous conclusions, suggesting that the plutons emplaced in their country rocks either in a low-angle crustal shear-zone [30] or along normal NE–SW faults [42], or else in the form of diapirs [53]. As well as the opening of the volcano-clastic basins, the emplacement of the granitoids can be integrated in an extensional or more probably transtensive context.
4.2 Tazekka–Bsabis–Bekrit Fault Zone (TBBFZ)
The TBBFZ encompasses the whole Tazekka massif and corresponds to the kilometre-wide contact zone between the Eastern Zone and Central Meseta [18,57,84] (Figs. 2 and 3). The Ordovician, Silurian and Devonian rocks are involved in west-vergent thrust sheets, either metamorphic or displaying pervasive slaty cleavage or else unmetamorphosed. Stacking of tectonic slivers resulted from folding and westward thrusting in a compressive context. The very low- or low-grade metamorphism has been dated at 320 Myr (KAr on white micas, [63]). Just after the Intravisean episode, a small granite stock emplaced in the Ordovician slates of the Tazekka massif [66]. On the eastern side of the massif, the Ordovician basement is unconformably covered by a volcano-clastic complex of Late Visean–Early Namurian age [24,57]. This complex is contemporaneous with the volcano-clastic basins of the Eastern Zone, which display volcanic rocks of similar calc-alkaline affinity [71]. The opening model of the Tazekka basin is controversial: (i) extension or transtension relaying the former Intravisean compression [24,51] or (ii) persistence of compression [11].
To conclude, the Variscan evolution of the TBBFZ in the Tazekka massif seems very similar to that of the Eastern Zone, but the isotopic ages suggest that compression is more recent here, 320 Ma instead of 370 Ma. This could indicate a westward ‘migration’ of the regional shortening between Late Devonian and Late Visean [93,96], or else that the Eastern Zone collided with the Tazekka and neighbouring western Meseta during the Visean.
4.3 Central Zone
4.3.1 East Central Subzone (Azrou–Khénifra–Zaian, eastern Jebilet, Ait Tamelil)
The development of the Variscan orogeny in this subzone is characterized by the synchronism of deformations with Lower Carboniferous sedimentation. This is recorded at map scale by olistostromes and gravity-driven nappes [4,18,64] sliding in a wide, syn-orogenic basin formed after the Late Visean transgression.
Two types of markers characterize the Early Variscan evolution in this area: (i) sedimentary markers, related to the filling up of a foreland basin (turbiditic deposits, gravitational slides, chaotic deposits near active faults), (ii) tectonic markers, related to the shortening of the basement and westward thrusting of the internal zones of the basin (folds with axial plane cleavage, and thrust with stacking of tectonic slivers). The distribution of these markers is, on the one hand, chronological, related to the successive tectonic events, and, on the other hand, spatial, related to the structural sub-zones or structural levels as follows.
4.3.1.1 Eovariscan (Famennian–Tournaisian) phase.
This phase has been recognized in the northeast part of Central Meseta (Azrou and Mrirt areas), where Devonian rocks, at the bottom of the basin, registered an extensional deformation before and during the Tournaisian. Tilted blocks and decametric slumps are described in the Devonian layers [19]. The facies and thickness of the Upper Tournaisian series reveal the occurrence of a half-graben system during this period, and the tilted block structures indicate a SW–NE direction of extension.
4.3.1.2 Intravisean phase (330–320 Ma).
The age of folding is determined by isotopic dating at 330–320 Ma (KAr on white micas, [63]) obtained from the Cambrian and Ordovician rocks forming the bottom of the basin, from the Ordovician allochthonous units and from the Carboniferous autochthonous deposits. This Intravisean (Meso-Variscan) phase is marked by a heterogeneous deformation. At the eastern margin of the basin, the allochthonous terranes display northwest to NNE trending isoclinal folds with spaced or slaty cleavage and very low- to low-grade metamorphism. Folding is followed by gently dipping thrusts with top-to-the-west motion. In the pre-Visean autochthonous rocks of the ‘Zaian Block’, the Intravisean structures consist of a subhorizontal mylonitic fabric in the Neoproterozoic rhyodacites, and a slaty or spaced cleavage in the overlying Cambro-Ordovician series, with a well-developed east–west to NW–SE-trending stretching lineation [4,18,23]. The associated submeridian folds are recumbent or overturned toward the west. Upper Visean sequences unconformably cover these pre-Visean deformed rocks. In the Visean autochthonous deposits, the Intravisean deformation is marked by map-scale NE–SW-trending folds overturned to the northwest. The pelitic rocks display spaced cleavage associated with very low-grade metamorphism, and superimposed by shear surfaces subparallel to the fold axial planes.
The Intravisean phase is accompanied by the emplacement of allochthonous units or nappes of the Azrou–Khénifra [4,18], eastern Jebilet [5,64] and Ait Tamelil areas [69,70]. Two types of nappes are distinguished: (i) large allochthonous units including Ordovician, Silurian, Devonian and Visean rocks, moderately deformed before their emplacement by gravity sliding and, locally, back compression mechanisms. They originate from the eastern uplifted margins of the basin (TBBFZ and Eastern Zone), (ii) weakly metamorphic units involved in westward thrusts that overlay the autochthonous Carboniferous rocks and previous gravity nappes. This overthrusting tectonics represents the latest stage of the Intravisean deformation in the East Central subzone.
These data can be integrated into a model of evolution of the Carboniferous Basin from the Azrou–Khénifra to eastern Jebilet and Ait Tamelil areas. The basin opening begins during the Tournaisian and Early Visean in a transtensive context controlled, in the Azrou–Khenifra basin, by NE–SW dextral transcurrent faults (TBBFZ and SOFZ, Fig. 5B). Blocking of this transcurrent movement during the Late Visean changes the TBBFZ into a zone of reverse faults with westward and southwestward thrusting. The eastern margins are folded and uplifted while gravity nappes are detached from the substratum and slide into the basin. Finally, the eastern part is sheared and tectonic slices are thrust onto the central part of the basin. In other words, from sedimentation to deformation, the Azrou–Khénifra basin evolves from a transtensive to a transpressive foreland-type basin [19]. Some pillow lavas and doleritic sills and dykes are described in the Upper Visean and Namurian sequences of the western part of the basin (Fourhal Basin); the magma shows a transitional to tholeiitic affinity [71,99], but locally, a calc-alkaline trend is evidenced [101].
Geodynamical model for the Mesetan orogen. 1: Caledonian block; 2: Eovariscan structures; 3: Magmatic melting; 4: Granitic bodies; 5: Chaotic deposits; 6: Gravity nappes; 7: Carboniferous alkaline and tholeiitic volcanism (lava flows, dykes and sills); 8: Carboniferous calc-alkaline volcanic flows; 9: Eovariscan and Variscan NE–SW dextral strike-slip; 10: Late Variscan east–west dextral strike-slip; 11: Hypothetical remnants of oceanic crust.
Modèle géodynamique de l'orogène mésétien. 1 : Bloc calédonien ; 2 : structures éovarisques ; 3 : fusion partielle ; 4 : granites ; 5 : dépôts chaotiques ; 6 : nappes gravitaires ; 7 : volcanisme alcalin et tholéiitique carbonifère (coulées, dykes et sills) ; 8 : volcanisme calco-alcalin carbonifère ; 9 : décrochements dextres NE–SW ; 10 : décrochements dextres est–ouest tardi-varisques ; 11 : reliques de croûte océanique (hypothétique).
From the Late Visean to the Early Westphalian, the whole East Central subzone belonged to a compressional foreland basin system. Similar models were recently proposed [9,10,19], where the Azrou–Khénifra and Fourhal basins are the depocentres resulting from the northwestward propagation of synsedimentary thrust and fold system. Shortening and sedimentation migrated progressively toward the west from Late Visean to Early Westphalian. From the point of view of their evolution and age, these basins are similar to the Carboniferous foreland basins of the south European Variscan belt [28,44,54,68].
4.3.2 West Central subzone (Central Morocco, Rehamna, Central Jebilet, Marrakech High Atlas)
In the West Central Sub-Zone of the Moroccan Meseta, the Eovariscan phase corresponds to the opening of a Late Devonian–Carboniferous basin, i.e. the Sidi Bettache Basin (SBB) and its prolongations in the Rehamna, Jebilet and Haouz massifs. The SBB is interpreted as a transtensive basin [92], controlled by dextral motion along NNE–SSW fault zones, mainly the WMSZ and the SOFZ. Near the margins, some isotopic datings at 320 Ma (KAr on micas [38,63]) indicate a very low-grade metamorphic event associated with Intravisean deformation contemporary with the opening of the basin. The palaeogeographical pattern would involve emerged ridges or uplifted blocks (Zaers–Oulmès, Coastal Block and Sehoul Zone) and subsident depocentres that would coincide with the regional anticline and syncline structures resulting from the final Variscan shortening, respectively [92]. According to this model, the geological units are essentially autochthonous. The same transtensive context was recently used to explain the opening of the Carboniferous basin of the Rehamna [40] and Central Jebilet [47]. This model has been discussed, because the synsedimentary kinematics along the fault zones is not well documented and the Intravisean emersion of the Zaers–Oulmès ridge is controversial [98], but it remains the more likely explanation for the sedimentary facies distribution during Devono-Carboniferous times.
The basin development is accompanied by a moderate mafic magmatism with lava flows interbedded in the Carboniferous sedimentary rocks, and intrusive bodies like sills and dykes. The geochemical affinity is alkaline to transitional-tholeiitic [1,49,71,100], consistent with an extensive pre-orogenic context. In this context, a model of tilted blocks is often evoked to explain the rising of the ridges [107,111]. Thus, during the Devono-Carboniferous period, the West Central Sub-Zone could be regarded as an area evolving in an extensional or transtensional context, whereas the East Central Sub-Zone recorded compressional events. However, there are also compressive deformations which could be related to the basin formation in the western sub-zone. In the pre-Carboniferous rocks of the Zaers–Oulmès ridge, folds with spaced cleavage [107] and imbricate thrusts [21] have been described, and thus, the ridges could be interpreted as antiformal stacking of tectonic slivers associated with westward-verging thrusts and folds [9], or with positive flower structures in a transpressive context [60]. In the Carboniferous rocks of the Central Jebilet, structural studies [47–49] show the broadly syntectonic character of the bimodal magmatism. The latter is dated at 330 Myr (U/Pb on zircons, [50]), i.e. Late Visean, which shows that the Devono-Carboniferous basins of West Central Sub-Zone recorded Intravisean events and also evolved in a broadly compressive context.
4.4 Coastal Block
During the Late Devonian, the central part of the Coastal Block rose up and constituted a NNE-trending ridge. On the western side of this ridge, under the recent coastal basins (Doukkala, Chaouia), an Upper Devonian–Lower Carboniferous basin is known from subsurface data [6,32]. The eastern side of the uplifted Coastal Block is interpreted as the western limit of the Carboniferous basins (Sidi-Bettache, central Jebilet) [17,78,96]. As in the West Central Sub-Zone, these Eovariscan events can be integrated in a transtensive or extensive context. However, the recent reinterpretation of seismic data in the Doukkala basin [32] shows that NNE-trending folds and west-verging thrusts are unconformably covered by weakly deformed Upper Devonian and Carboniferous rocks. This interpretation would indicate that at least part of the structures of the Coastal Block have developed during the Eovariscan compressive episode.
5 Neo- and Tardi-Variscan events
Late Carboniferous deformations affected the whole Mesetan Domain. This classical Hercynian phase also labelled the ‘main’ phase [81] or Westphalian phase [96] began during the Westphalian, and the resulting structures are sealed by unconformable Upper Stephanian or Lower Permian continental redbeds. Moreover, the micas neoformed during the deformation yield K/Ar ages between 300–290 Ma [63], i.e. close to the Westphalian–Stephanian limit. The structures resulting from the Neovariscan events superimposed to the previous events are described hereafter along two complementary transects (Fig. 4).
Simplified geological sections across the eastern (4A) and western parts (4B, after [60], modified) of the Mesetan Domain (see location in Fig. 3). P3: Neoproterozoic; Cb: Cambrian; O: Ordovician; Si–D: Silurian to Middle Devonian; UD–LC: Upper Devonian, Lower Carboniferous; C: Carboniferous; V–N: Visean–Namurian; W: Upper Westphalian; A: Autunian; γ : granitoids. S1: composite cleavage (Eovariscan and Intravisean phases), S2: Late Variscan cleavage. SBB: Sidi Bettache Basin, AKB: Azrou–Khénifra Basin, M: Mesozoic cover (High Atlas). RTFZ, WMSZ, SOFZ, TBBFZ, APTZ (see Fig. 2).
Coupes géologiques simplifiées dans les parties orientale (4A) et occidentale (4B, d'après [60], modifié) du domaine mésétien (localisation : Fig. 3). P3 : Néoprotérozoïque ; Cb : Cambrien ; O : Ordovicien ; Si–D : Silurien–Dévonien moyen ; UD–LC : Dévonien supérieur–Carbonifère inférieur ; C : Carbonifère ; V–N : Viséen–Namurien ; W : Westphalien supérieur ; A : Autunien ; γ : granitoïdes. S1 : schistosité (phases éovarisque et intraviséenne) ; S2 : schistosité (phase varisque). SBB : Bassin de Sidi Bettache ; AKB : bassin d'Azrou–Khénifra ; M : couverture mésozoïque (Haut Atlas). RTFZ, WMSZ, SOFZ, TBBFZ, APTZ (voir Fig. 2).
5.1 Neovariscan events in the eastern transect
This transect (Fig. 4A) crosscuts the Eastern and Southern Zones which are separated one from each other by the Atlas Palaeozoic Transform Zone (APTZ), widely exposed in the northern part of the Tamlelt inlier. In the Eastern Zone, the Lower Palaeozoic rocks, previously deformed during the Eovariscan events, constitute a basement covered by Carboniferous deposits. This generates a marked contrast between the Variscan deformation in the basement and cover, respectively. The Eovariscan basement is moderately deformed during the Neovariscan stage. East- to ENE-trending folds (often kink-bands) with generally steeply dipping axial planes are overprinted by brittle dextral shear zones parallel to the fold axial planes. The Carboniferous cover displays east- to ENE-trending overturned folds. Their most usual vergence is to the north, as exemplified by the Ez–Ziroug syncline in the Mekkam inlier [57] and the Jerada syncline [45]. Folding is followed by northward thrusting. Some localized décollements are observed near the basement-cover contact. Spaced or crenulation cleavages are associated with a very low-grade metamorphic evolution. Small granitic bodies emplaced after these deformations at about 286 and 247 Ma [85]. Their petrological and geochemical characteristics indicate a post-collisional emplacement [36,56].
In the Southern Zone, which is represented along this transect by the Tamlelt inlier, the Variscan deformation results in a combination of southward thrusting and dextral strike-slip faulting. ENE-trending, upright or overturned folds are associated with east–west dextral, ductile shear zones [61]. Spaced or continuous axial plane cleavage is well developed, coeval with a low-grade metamorphic evolution. S/C structures and subhorizontal stretching lineations parallel to the fold axes characterise a wrench dominated, transpressive fabric. Thus, during the main Variscan stage, the Southern Zone evolved in a transpressive context corresponding to the eastward motion of the Eastern Zone along the APTZ, and more generally to the ongoing eastward translation of the Meseta domain along the African margin [62]. All the Variscan structures observed along this transect indicate a NW–SE to north–south shortening in a transpressive context, controlled by dextral translation along ENE–WSW and east–west fault zones, especially the APTZ. In comparison with the western Meseta (see below), ductile shortening seems relatively weak. This difference is probably due to the presence of the more or less rigid basement, previously deformed during the Eovariscan stage.
5.2 Neovariscan events in the western transect
This composite transect (Fig. 4B) crosscuts the Coastal Block, the Central Zone (Moroccan Central Massif), the Eastern Zone (Midelt), and the Southern Zone (Tineghir). In the Coastal Block, the Variscan deformation is weak. The kilometric-scale, NNE-trending folds are either upright or overturned toward the west, especially in the western Rehamna and Jebilet where folding is followed by westward thrusting. The intensity of deformation and the associated metamorphism increase from west (spaced cleavage) to east (slaty cleavage and schistosity) [95]. The eastern limit of the Coastal Block corresponds to the Western Meseta Shear Zone (WMSZ) [97], with complex movements combining west-verging folds and thrusts and dextral wrench-faults.
The transect crosses the northern part of the Meseta Central Zone (Moroccan Central Massif), where the Variscan deformation is complex and polyphased. The main structures are NNE- to northeast-trending folds. The regional antiformal and synformal structures could correspond respectively to the ridges and depocentres formed during the Eovariscan episodes [92], but this interpretation is controversial [98]. The orientation of the former structures and the displacement along their limits could explain the local disturbances of the Variscan structures (cleavage, fold axis), especially in the Sidi Bettache Basin (SBB), where the Devono-Carboniferous series display north–south to N140-trending folds related to dextral shearing of the WMSZ, and N70 and N110 folds related to the dextral motion of the Sehoul Block along the RTFZ (Fig. 3). Elsewhere in central Morocco, the folds associated to brittle–ductile thrusts (locally nappes) are generally overturned to the southeast, rarely towards the northwest [22]. These opposite verging directions can be interpreted either as the result of polyphased tectonics, with the southeast-facing structures being more recent as they affect Upper Westphalian deposits discordant upon northwest-vergent folds [59,98], or else as forming a system of coeval, antithetic folds and thrusts in a globally northwest-facing regional structure [9,10,12,76]. The deformation intensity is highly variable, very weak in the Carboniferous synclines where cleavage is not observed and metamorphism absent or very low grade, and much more intense elsewhere, with a conspicuous slaty cleavage coeval with low-grade greenschist-facies metamorphism.
In the southern part of the Central Zone (Rehamna, Jebilet, Marrakech High Atlas), the Variscan deformation results in NNE-trending, WNW-facing folds associated with ductile westward thrusting and dextral wrenching, well marked along the WMSZ. A subhorizontal stretching lineation parallel to the fold axes is well developed. Southeast-facing folds associated with ductile dextral shear zones are observed near the eastern limit of the Palaeozoic massif of the Atlas of Marrakech [33]. As in the Moroccan central massif, the cleavage trajectories and fold axis trends display some variations, especially in the Carboniferous series: N70 in Eastern Jebilet, N110–N140 in the Haouz and Ait Tamelil inliers [55,70] (Fig. 3). The most complex, polyphased Variscan deformations are found in the southern–central Rehamna, where they are associated to prograde Barrovian metamorphism ranking from low grade to medium grade (staurolite–kyanite–garnet sub-facies). The culmination of metamorphism and deformation has been ascribed to a high thermal gradient during the Variscan shortening with combined ductile thrusting and wrench faulting [58,75,96]. However, stacking of synmetamorphic nappes or slices has been also evoked to explain the burial of the metamorphic rocks [27,29] before their subsequent exhumation in an extensional context likely resulting in the negative inversion of the ductile thrusts [2,8].
The transect (Fig. 4B) then crosses the Meseta Eastern Zone through the TBBFZ (projected from the Tazekka massif), and the Midelt and Mougueur inliers (Figs. 3 and 4B). There, the Variscan deformation displays the same characteristics as in the north–south transect (Fig. 4A). Finally, the Southern Zone is represented by the Tineghir area, where Carboniferous detrital deposits fill up a subsident basin on the northern margin of the Saharan platform. The Variscan structures are represented by east-trending, south-facing fold-and-thrust systems associated with east-trending dextral strike-slip faults. This structure is typical of a foreland zone of deformation associated with an orogen [82,105]. So, from Tineghir to Tamlelt, the Southern Zone is deformed in a transpressive context that can be related to the eastward motion of the Mesetan domain along the APTZ. Towards the west, the Southern Zone would connect with the ENE-trending Tizi–n'Test Fault Zone (TTFZ) through the N120 Skoura Fault [90]. The TTFZ–APTZ system makes up the limit between the orogenic domain (Meseta) and the pericratonic domain (Anti-Atlas).
Scattered, syn- to post-tectonic granitoids emplaced in the Central Zone during and after the Late Carboniferous tectonic events. Kinematic studies of some of these massifs evidence a ductile–brittle deformation resulting from the interference between emplacement and regional deformation, the granitic magma being transferred through the crust via the NNE–SSW and ENE–WSW shear zones, and emplaced in extensional zones up to shallow crustal levels [29,48–50,74]. The transpressive–transtensive context corresponds to the late compressional events and perhaps to the beginning of a post-thickening extension. Most of these granitoids emplaced between 305 and 270 Ma [8,56,85], thus they are contemporaneous with the Stephanian and Lower Permian redbeds that unconformably overlay the Variscan structures without reworking granitic or metamorphic pebbles. These continental deposits and the associated calc-alkaline volcanism fill up small pull-apart basins that can be considered as the surface expression of the tectonic processes which controlled the granitoids emplacement at depth [8,31,43,110].
5.3 Tardi-Variscan events
The latest compressive deformations affect the Upper Stephanian and Lower Permian (Autunian) molasses of the Variscan belt. The continental redbeds are by place moderately folded, but deformation is principally accommodated by brittle, reverse and strike-slip faults. The direction of compression is initially oriented east–west, then NW–SE to north–south [3,46,86].
6 Discussion and conclusion
6.1 New insights on the Eovariscan events
In the previous models of the Moroccan Variscan belt, the Eovariscan and Intravisean compressive events were supposed to be virtually limited to the Eastern Zone and East-Central Sub-Zone, whereas the western zones (West-Central Sub-Zone, Coastal Block) had evolved supposedly in an extensive or transtensive context [93,96]. However, Eovariscan compressive structures were described in the western Meseta and even in the Coastal Block, being probably responsible for the initial structuration of the basins and the development of uplifted ridges and depocentres [60]. Accordingly, a model of generalized and continuous shortening from Late Devonian until Namurian has been proposed [10,101]. The mafic magmatism within the Devono-Carboniferous basins of West-Central Sub-Zone is alkaline to transitional-tholeiitic, but its geochemical signature remains ambiguous, anorogenic or orogenic, corresponding either to intracontinental extension or back-arc basin formation [47,71]. To conclude, the geodynamic context of the chain during the Eovariscan period is characterized by compressive and extensive tectonic events that can be contemporaneous or more or less separated in time, but most likely controlled by, or at least involving dextral movements along NNE-trending fault zones in a transtensive–transpressive context. Such a tectonic regime fits a general model of oblique convergence between Gondwana and Laurussia [103].
6.2 A revised tectonic model
In the tectonic interpretation proposed here (Fig. 5), we assume that most of the Meseta was located at the margin of Gondwana during Cambrian–Ordovician times. At the end of Silurian, an allochthonous terrane deformed during tectono-metamorphic events spanning Ordovician to Early Silurian (450–430 Ma) collides with the still undeformed Meseta. This terrane would form a part of a ‘Caledonian belt’, perhaps discontinuous, belonging to North Africa. The Late Devonian–Tournaisian period (Fig. 5A) corresponds to the Eovariscan phase of the Variscan orogeny. Tectono-metamorphic events dated at 370–360 Ma are well recorded in the Eastern Zone. Shortening would extend towards the west up to the Coastal Block, although with a decreasing intensity. The resulting deformations are contemporaneous with the opening of the Devono-Carboniferous basins. The geodynamic context is transtensive–transpressive, controlled by dextral motions along NNE-trending crustal faults. Localized compressive deformations were probably responsible for uprising of the ridges. During the Early Carboniferous (Fig. 5B), the Intravisean or Mesovariscan phase corresponds to the opening and filling up of detrital and volcanic-detrital basins. Crustal thinning was more important in the Central Zone (transitional-tholeiitic magmatism) than in the Eastern Zone (‘orogenic’ calc-alkaline magmatism), which was in situation of post-thickening extension. The geodynamical context is still transtensive–transpressive. A metamorphic event dated at 330–320 Ma is associated with compressive deformations like folding and uprising of the TBBFZ and eastern margin of the East Central Sub-Zone from where gravity nappes originated and slid towards the basins (Azrou, Jebilet, Ait Tamelil). The major, Neovariscan phase (Fig. 5C) spans from Late Westphalian to Early Permian (300–290 Myr). The whole Mesetan Domain, which is now entirely emerged, suffered a generalized compression. The shortening is important in the Central Zone with polyphased tectonic–metamorphic events, the intensity of which reaches the medium grade (amphibolite facies). The progressive and complex deformations are controlled by northeast- to NNE-trending ductile shear zones combining westward thrusting and dextral wrenching, and by dextral motion along ENE-trending strike-slip faults like the RTFZ and APTZ. This Variscan phase results in the final suturing of the Meseta Domain and West African Craton through dextral transpression and southward thrusting, with the Anti-Atlas acting as the southern Variscan foreland (see Burkhard et al., this issue).
6.3 Drifted versus para-autochthonous Meseta?
In our model, the North-African Variscan Belt corresponds to a tectonic prism involving a continental crust moderately thinned during Early Palaeozoic extensional events, then shortened during the Laurentia–Gondwana collision (Fig. 6). Thus, the North-African Variscides clearly belong to the Periatlantic Palaeozoic chains, and more particularly to the southern branch of the Variscan Belt [7,77,93,104]. The similarity between western Morocco and the South Portuguese Zone has been noted, especially with respect to the development of Devono-Carboniferous basins with bimodal magmatism [96]. However, given the lack of suture zone in Morocco, it seems that the Variscan Belt of North Africa would be rather prolonged in the Central Iberian Zone [104]. The limit with Laurentia is not clear because the contact between the Moroccan Meseta and the Gondwanan terranes (Avalon, Meguma) has been obscured by the opening of the Atlantic Ocean. This limit is probably underlined by Caledonian terranes like the Sehoul Zone (6.3B). By contrast, the limit with the West African Craton crops out in Morocco and corresponds, at least during the Neovariscan phase to a major, transcurrent dextral shear zone (APTZ–TTFZ). According to most previous studies, it seems that the Mesetan Domain evolved at the margin of the West African Craton, and that the various extensional events recorded in the Palaeozoic formations never reached the stage of oceanization [39,93]. In the lack of ophiolites and of high-pressure, low-temperature metamorphism in the North African Variscides, it is difficult to argue for a model of mountain building by a classical subduction–collision process [20,101]. The shortening of the Palaeozoic cover and Late Proterozoic basement likely was mainly controlled by crustal-scale ductile shear zones like the WMSZ and APTZ, combining dextral motion and overthrusts, compatible with an oblique convergence between Laurentia and Gondwana.
The North African Variscides in the frame of the Periatlantic Palaeozoic belts at the end of Permian. A: Restored map, after [62,77]; B: schematic cross-section showing the possible situation of the Mesetan Domain between the North American and West African cratons.
La chaîne varisque d'Afrique du Nord dans le cadre des chaînes paléozoïques péri-atlantiques, à la fin du Permien. A : Carte d'après [62,77] ; B : coupe schématique montrant une situation possible du domaine mésétien entre les deux cratons, nord américain et ouest africain.