The Mesozoic–Cenozoic Atlas belt (North Africa): an overview

Geodinamica Acta 15 (2002) 185–208 www.elsevier.com/locate/geoact

Original article

The Mesozoic–Cenozoic Atlas belt (North Africa): an overview Alain Piqué a,*, Pierre Tricart b, René Guiraud c, Edgard Laville d, Samir Bouaziz e, Mostafa Amrhar f, Rachid Ait Ouali g a

Institut Universitaire Européen de la Mer, Université de Bretagne occidentale, 29280 Plouzané, France b Géodynamique des Chaînes alpines, LGCA, Bâtiment IRIGM, BP 53, 38041, Grenoble, France c Géophysique et Tectonique, Case 060, 34095 Montpellier cedex 5, France d Département de Géologie, BP 5186, 14032 Caen, France e ENIS, Département de Génie géologique, BP W, 3038 Sfax, Tunisia f Département de Géologie, Faculté des Sciences Semlalia, Université Cadi Ayad, BP 2390, 40 000, Marrakech, Morocco g IST-USTHB, BP 32, El Alia, 16110 Bab-Ezzouar-Alger, Algeria Received 15 March 2001; accepted 14 April 2002

Abstract The Atlas domain extends in North Africa (= Maghreb) from the Atlantic (Moroccan Atlas) to Algeria and the Pelagian Sea (Tunisian Atlas), north of the Saharan platform. On top of a Palaeozoic basement affected by the Hercynian orogeny in Morocco and, at least, in western Algeria, the Early Mesozoic transgressions deposited a variably, thick sedimentary cover. After a Triassic episode of aborted rifting in the western Maghreb, related to the opening of Central Atlantic, the distribution of the sedimentary facies suggests that an Atlasic trough established during the Late Liassic, trending WSW–ENE, from Morocco to northern Tunisia. This trough was filled then affected by a transpressive deformation during the Mid-Jurassic in Morocco, the Late Eocene in Algeria and at a poorly defined period in northern Tunisia. Thereafter, a Cenozoic shortening event overprinted the previous folds in the Atlas series, particularly along the edges of the chain and uplifted the orogenic belt. The thick-skin vs. thin-skin style of the Cenozoic deformation is not surely determined. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Atlas; Cenozoic; Maghreb; Mesozoic; Morocco; North Africa; Tunisia

1. Introduction From Morocco to Tunisia, all along North Africa (= Maghreb), a mountain range extends parallel to the northern limit of Africa, between the Mediterranean Sea and the Saharan platform (Fig. 1). Significant differences in the structural style lead to distinguish two belts in this system: to the north, the Rif–Tell chain plunges abruptly in the Mediterranean. Its mains characters, i.e. the presence of Mid–Late Mesozoic and Early Cenozoic flyschs, the HP metamorphism in the internal zones and a general structural vergence to the south and southwest, clearly expressed by recumbent regional folds and nappes, are those of the Alpine belt of western Europe to which it is related [1–4]. It is not considered in this paper. The Atlas belt is located between * Corresponding author. E-mail address: [email protected] (A. Piqué). © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 0 9 8 5 - 3 1 1 1 ( 0 2 ) 0 1 0 8 8 - 4

the Rif–Tell chain to the north and the Saharan platform to the south. It differentiates from the Rif–Tell by a weaker general shortening, the overall steep dip of the outcropping structures, and the general lack of a conspicuous metamorphism. It is separated from the Saharan platform by a clear physiographic boundary often referred to as the Saharan Flexure (Flexure saharienne) or South-Atlasic fault (Accident Sud Atlasique). In western Maghreb, the Rif–Tell and the Atlas are mostly separated from each other by the Moroccan or Western Meseta and by the Oran Meseta, or Eastern Meseta, or High-Plateaus, whereas they come into contact to the east, in Tunisia. From the Atlantic Ocean to the Pelagian Sea, the Atlas belt extends over more than 2500 km. As a whole, its elevation decreases from west [Djebel (= Mount) Toubkal 4165 m in Moroccan Atlas] to east [Djebel Chambi, 1544 m in Tunisia] (Fig. 1A). As a consequence, the Precambrian and Palaeozoic basement of the chain crops out largely in

186

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

Fig. 1. The Maghreb Atlas: General location: A and B. Details: (a) Moroccan High Atlas; (b) Algerian Atlas; (c) Tunisian Atlas.

Morocco, weakly in Algeria and is not visible in Tunisia. Within the Atlas belt, several segments are distinguished. The Moroccan Atlas includes the N70°E trending High Atlas and the N45°E trending Middle Atlas. Both belts contain Palaeozoic inliers and are physiographically higher than the Hercynian Western Meseta. They are bordered by depressions filled up by recent clastic, continental sediments. In Algeria, the N60°E trending Saharan Atlas and the

Aurès are separated by the nearly E–W Zibane zone. To the north, the pre-Atlasic zones make a transition with the allochthonous Tellian units. In Tunisia, the northernmost part of the Atlas is the Tunisian Trough or “Diapir zone”, an NE–SW trending narrow zone which parallels the front of the Tellian units. The Tunisian Atlas s.str. is a complex fold belt that comprises the Central Atlas and the Southern Atlas. The dominant structures trend NE–SW but major faults and

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

187

Fig. 2. The Hercynian belt in North Africa with regard to western Europe. (1) Moroccan Western Meseta shear zone; (2) Marrakech–Oujda lineament; (3) Atlas Palaeozoic transform fault.

narrow isolated folds also display other trends: E–W, N–S and NW–SE. To the east, the Tunisian Atlas is separated by the “North–South Axis” from the Sahel which belongs to the Pelagian block. So far, various parts of the Atlas have been studied by national teams and the available syntheses [5–7] only concern the national territories of Morocco, Algeria and Tunisia, respectively. Now, development of studies on the geodynamics of Africa and its relations with Eurasia [8] and more generally the evolution of the Tethyan domain [9,10] call for a higher scale of synthesis which is allowed by the progression of the knowledge on the whole Atlas belt. Recently published papers [11,12] deal with the structural aspect of the Atlas development, principally determined from the study of the marginal zones of the belt where Cenozoic and more recent sedimentary sequences crop out. These papers, which will be discussed in the following section, conclude that the main shortening occurred during the Late Cenozoic. On the contrary, our purpose in the present paper is to present a general view of the palaeogeographic and structural evolution of the whole Atlas, including the stratigraphic filling of the Atlasic basins and the structural development of various parts of the belt since the Mesozoic times. Considering the whole sequence of the development of the Atlas, we will try to ask if its structural development, i.e. the Atlasic shortening, was polyphased and began early, since the Jurassic in Morocco.

2. Stratigraphy and magmatism 2.1. The Palaeozoic basement In Morocco, the basement crops out in several inliers of the High and Middle Atlas, and is made up of Precambrian and mostly Palaeozoic rocks, deformed during the Hercynian orogeny. This orogeny, which is extensively represented in the Moroccan Meseta, is characterized by the inhomogeneous nature of the deformation, usually concentrated in elongate and narrow shear zones that often developed along former palaeogeographic limits [13] and acted as weakness zones by the end of the Palaeozoic and even later as it will be shown below. Two main orientations are noticeable: the N20–45°E trending zones (e.g. the Western Meseta Shear Zone, WMSZ, and the Marrakech–Oujda Lineament, MOL) dip to the southeast; the N70–90°E trending zones (Atlas Palaeozoic Transform Zone, ATPZ are steeply dipping (Fig. 2). To the east, the Palaeozoic basement of the Atlas is shown in restricted areas of westernmost Algeria [14], where it presents very strong similarities with respect to the sequences and deformations, with that of eastern Morocco. Elsewhere in Algeria, the frequent presence of deformed and metamorphosed Palaeozoic rocks in the core of unmetamorphic, diapiric Atlasic anticlines testifies that the Hercynian orogeny affected the Saharan Atlas domain. The Tellian

188

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

Kabylia does not belong to this Atlasic domain, but it is important to emphasize that it exhibits evidences of a strong Hercynian deformation, in the prolongation of the Hercynian belt of eastern Morocco [15] and Fig. 2. Farther to the east, the Atlas basement is not visible in Tunisia where the oldest exposed rocks are the tilted (25°) Permian marine sediments of southernmost Tunisia (Djebel Tebaga), outside the Atlasic domain. Elsewhere, subsurface data suggest that the Palaeozoic sequence remained unmetamorphosed [16]. Very mild deformation was at the origin of the large-scale E–W Dahar arch [17]. Consequently, the Hercynian belt does not comprise eastern Maghreb, switching around a “Palaeo-Apulia” promontory (Fig. 2). 2.2. Triassic South of the Atlasic domain, in southernmost Tunisia, the Triassic strata were deposited after a short stratigraphic hiatus upon the marine Permian rocks. The Lower Triassic sediments are sandstones, clays, mudstones and dolomites; they are covered by Middle and Late Triassic carbonates and evaporites grading eastward to marine carbonates. The Triassic sedimentation was controlled by an NNW–SSE extension expressed by the roughly E–W trend of the synsedimentary flexures and normal faults [18–20]. In eastern Morocco, the oldest Triassic rocks are Late Ladinian in age [21]. As in Algeria, Carnian sandstones are restricted to the Atlasic domain, whereas the Norian and Rhetian sequences were deposited more largely upon northwestern Morocco. The sedimentation in the Atlasic domain was syntectonic [22,23]. It was associated to a thermal flow, even perceptible in the northern Sahara [24], dated around 200 Ma [25–27] and a volcanic activity [28–30]. This allowed Laville and Piqué [31] and Piqué and Laville [32] to describe an NE–SW trending “Atlasic rift”, extending from the Middle Atlas to the central High Atlas, parallel to the “Atlantic rift”, bordered by NE–SW trending normal faults (Figs. 3B and 4A), which are Hercynian faults reactivated in a pure extensional regime, and limited in the south by a sinistral accommodation zone system that corresponds to an old fracture zone reactivated with a strong left-lateral component (Fig. 3C). However, whereas the Atlantic rift succeeded, giving way to oceanic accretion, the Atlasic rift aborted at the end of the Triassic times (Fig. 3D). 2.3. Jurassic 2.3.1. Western and central Maghreb The Early Jurassic Sea flooded the Maghreb, covering first Tunisia and eastern Algeria. The sea reached the Oranese Meseta at the end of the Sinemurian [33]. In Morocco, the transgression proceeded southwestward, but the sea remained probably separated from the Central Atlantic seaway (Fig. 4B). An extended carbonate shelf, postrift with regard to the Triassic rifting, established all over the Maghreb in Early Liassic times. It was character-

ized by tidal and inner shelf environments and by sediments, mainly dolomitic, devoid of silicoclastics. From the Middle Liassic to the Early Bathonian (Fig. 4C,D), more open shelf conditions developed in the future Atlas of Morocco and Algeria in response to the persistence of the thermal subsidence. Hard grounds and Ammonitico rosso facies indicate local sedimentary hiatuses. A new extensional episode fragmented the Early–Middle Liassic platform during the Late Domerian–Early Toarcian, and created two first-order troughs (Fig. 4C) in Morocco [6,34] and Algeria [5,23,35], composed themselves of second-order, kilometric-scale basins. The tectonic activity coincided with a weak eustatic high level of the sea water during the Mid-Toarcian [36]. As a whole, the Atlas trough was characterized by a subsidence ratio more important than in the neighbouring shelves (Fig. 5). The total thickness recorded in the northern Saharan domain bordering the Mesozoic Atlas trough is about 2500–3000 m, sharply contrasting with the 7000 m locally represented in the Saharan Atlas trough itself (e.g. AMI1 well, Djebel Bou Lerhfad, in [5]). Inside the Atlas trough, very rapid changes of thicknesses from the elevating ridges, themselves rapidly eroded, and the more subsident depocentres do not allow to draw precise isopaque maps of the successive sedimentary sequences deposited in the trough. The trough borders were marked by vertical or steeply dipping faults with an important normal component. In Morocco, an N–S section across the Atlas trough shows, from north to south (Fig. 6): • a carbonate shelf that prolongates into the Oranese Meseta, with inter- to supratidal flats; • bioherms [37]; • limestones, marls and turbidites with sedimentary structures pointing to a south-dipping slope [38]; • mudstones in the trough axis; • a symmetrical repartition of the sedimentary facies up to the Saharan platform [39]. The maximum depth was reached during the Late Liassic (Toarcian–Aalenian: Fig. 4C). In the centre of the Moroccan Atlas trough, the monotonous, 1000 m thick Boulemane (or Talsint) marls were deposited at that time. Then, a decrease of the subsidence rate led to the progressive up filling of the trough with the deposition of Late Bajocian limestones (“Calcaire corniche” of the Moroccan literature) followed by Bathonian–Callovian silico-clastics, Callovian (?)–Oxfordian gypsum, marls, lignite and sandstones and, finally, Late Jurassic fluviatile sandstones and conglomerates [40–42] (Fig. 4D). In western Algeria (Ksour Mts), the Atlas trough was progressively filled by fine, then coarse clastic sediments (“Grès des Ksour”) representing the northward progression of the Palaeo-Niger delta [43] by which the detrital influx from the Saharan shield transited to the Tethys. The Ksour sandstones sedimentation lasted up to Mid-Berriasian, and their thickness is over 2000 m thick by place. Within the sandy formation, each major sedimentary sequence registers, first, a rapid, tectonic deepening of the

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

189

Fig. 3. The Atlasic rift in western Maghreb; B and D, from Beauchamp [158], modified; C, from Laville et al. [22].

delta floor concomitant with an acceleration of the down warping of the Tethys southern margin, and second, the filling of the basin leading to the progressive development of limestones. Eustatic variations interfered with tectonics in controlling the sedimentation, with the highest sea level during the Early Kimmeridgian [36]. At the end of the Jurassic, and during the Cretaceous (cf. Fig. 4E,F), the western and central Maghreb were emerged, except their northeastern and southwestern parts, connected to the Tethys [44,45] and the Atlantic [46], respectively. In the Oranese Meseta, the Late Jurassic–Berriasian interval is represented by: (i) the Oxfordian Nador sandstones [47] and Saïda mudstones [14]; (ii) Early Kimmeridgian marls and limestones; (iii) the Late Kimmeridgian–Berriasian Nador limestones, Tlemcen dolomites and Remaïlia limestones [48]. Detailed mapping and subsurface analysis performed in Algeria [23,49] and Morocco [50,51], show that the Atlasic trough was characterized by a pattern of N60°E and N70–90°E trending ridges separated from each other by

rhombic depocentres. A distinctive character of the central part of the Moroccan High Atlas is the presence of plutonic, mainly gabbroic intrusions [52–55]. These massifs are elliptical, 10–20 km long and 2–5 km wide, or circular. They are hosted in synsedimentary, NE–SW trending anticlines formed by steeply dipping, normal or overturned Jurassic strata. The magmatic structures of the extensively studied Tasraft and Talmest intrusions suggest a diapiric emplacement [53,56], although their very weak metamorphic aureole would rather suggest a forcefully emplacement of “cold” and rigid magmatic rocks. 2.3.2. Eastern Maghreb In the Aurès Mountains, the Ksour sandstones grade laterally to thick carbonates and marls preserved in N110–140°E tilted blocks [57]. Northward, in the Djebel Bou Taleb (Hodna Mts) [58], the contemporary facies are thicker and more monotonous than those of the Oran Meseta.

190

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

Fig. 4. Palaeogeographic maps: distribution of the Mesozoic sedimentary facies throughout the Maghreb Atlas.

In Tunisia, the Jurassic sedimentary facies evolve from south to north [59]. The southern part of the country was rather stable during Jurassic times, only submitted to local tilting and faulting [60]; the detrital sedimentation with south-derived clasts indicates continental influences even during the transgressive pulsations. Southwest of the Gafsa fault, the Tataouine and Chotts highly subsident zones were controlled by E–W normal faults. To the north, the Central Tunisia Atlas was occupied by a shallow sea where pelagic sequences were deposited nevertheless during the Late Jurassic–Berriasian. An important subsidence resulted from block tilting along north-dipping listric faults [61]. Argillaceous and carbonate series exhibit important thinning and evidences of submarine erosion at the top of the tilted blocks. Here too, the tectonic subsidence interfered with eustatic variations, particularly well expressed on this shallow shelf [59]. The NE–SW trending “Tunisian Trough” sharply contrasts with the Central Tunisia Atlas. Its evolu-

Fig. 5. Tectonic subsidence curves of the Saharan Atlas compared to its adjacent domains, from Kazi Tani [5].

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

191

Fig. 6. Northern border of the Moroccan High Atlas trough, from Letsch [38]. Location in Fig. 4C (dotted line).

tion can be summarized as follows: (i) building of a carbonate shelf during Early–Middle Liassic times; (ii) dislocation of the shelf from the Late Liassic to the Callovian and development of a horst and graben structure; (iii) deposition of pelagic and deep sediments (Ammonitico rosso and radiolarites), unconformably overlying the inactive faults [62]. This evolution, very similar to the evolution of the western Maghreb Atlasic troughs, is interpreted as the development of a rift, aborted during the Late Jurassic [63]. This extensional tectonics have been accompanied by the emplacement of doleritic gabbros [64] which would correspond to the High Atlas coeval intrusions. Finally, whatever the region of the Tunisian Atlas, the Jurassic and earliest Cretaceous sedimentation appears to have been mainly controlled by normal faulting, resulting in a general N–S extension [65]. The major faults are oriented E–W and represent reactivated deep-seated pre-existing faults. Other inherited faults, oriented NE–SW, NW–SE and N–S, were also active at that time. Normal movement along all these faults favoured halokinesis which, in turn, contributed to accentuate the opposition between the uplifted and the subsiding areas [61,66–69].

unconformably covered the folded structures in the Moroccan central High Atlas. In the Algerian Ouled Nail and Aurès Mts, the Atlasic trough remained marine and a thick sedimentary pile (2000 m) was deposited, composed of marls and limestones intercalated during the Barremian with the most distal part of the Ksour sandstones. This subsident trough contrasts with the Oran Meseta and Hodna areas, characterized by thinner sequences of prodeltaic mudstones, limestones and sandstones [58]. Farther to the east, the palaeogeography of Tunisia remained unchanged since the Late Jurassic and the sedimentation was still controlled by E–W normal faults [71]. In the southern part of the country, the sedimentary sequences are mainly clastic and indicative of subcontinental to littoral environments passing northward to the marine carbonate shelf of central Tunisia. This central platform was submitted to a persisting N–S extension which was responsible for the individualization of the Kairouan island, an uplifted block bordered by a shallow marine shelf where bioclastic and reefal limestones registered both the tectonic activity and several eustatic cycles. To the north, the deeper Tunisian Trough received marls and limestones [72].

2.4. Cretaceous–Eocene 2.4.1. Valanginian–Barremian In the western High Atlas, the Early Cretaceous was marked by a transgressive episode issuing from the Atlantic [70]. From there, toward the Algerian Ksour Mts, the continental regime initiated during the Mid-Jurassic persisted with the deposition of fluviatile sandstones which

2.4.2. Aptian–Eocene This period was marked by the development of the worldwide “Mid-Cretaceous” transgression that proceeded here both from the Atlantic and the Tethys. In the central High Atlas, the continental Late Jurassic–Early Cretaceous redbeds are covered by the Aït Tafelt formation dated from the Aptian by Ammonites, deposited in a marine gulf

192

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

connected to the Atlantic. In the Middle Atlas, the unconformable, fluviatile base of the Sidi Larbi conglomerate is undated, but marine influences are stronger and stronger toward its top which is dated from the Aptian. Faunal similarities with the coeval Tethyan sediments [73] suggest connections to the NE with open sea. Finally, a transgressive sea spread over the country during the Late Cretaceous, overflowing the Atlas axis and parts of the Meseta. In this axis, Cenomanian–Turonian limestones indicative of open sea environments grade to more restricted sea facies (bituminous sediments) during the Senonian [74,75] and lagoonal afterward. Eocene marls, dolomites and gypsum are the last marine deposits. Two magmatic alkaline small massifs, Taourirt and Tamazert, respectively, in northeastern Middle Atlas and northern High Atlas, are dated at about 57 Ma (Taourirt) [76] and 40 Ma (Tamazert [77]: Rb–Sr, K–Ar). The Mid-Cretaceous transgression also affected Algeria and its maximum extension occurred at the Cenomanian–Turonian limit. The Late Cretaceous regression started there earlier than in Morocco; it proceeded from west to east: while the Senonian is represented by a sedimentary hiatus in the Ksour Mts and the Oran Meseta at the eastern prolongation of the Moroccan “Idrissides Land” [78], the Ouled Nail and Aurès Mts underwent marine sedimentation until the Maastrichtian. In the Aurès, the transition from pelagic and benthic sediments to continental deposits likely resulted from an eustatic fall. At a larger scale, however, the uneven subsidence of the Atlasic domain was controlled by structural factors: synsedimentary ridges separated subsident depocentres which received clasts issued from the contemporary erosion of the ridges. Diapirs were emplaced in the eastern Saharan Atlas [79]. Separating the western–central Saharan Atlas (Ksour Mts and Ouled Nail Mts) and the eastern Atlas (Aurès), the Zibane zone [5], or Biskra Promontory [58] is an important boundary marked by the deposition of thin carbonate sediments, a synsedimentary NW–SE faulting and a hydrothermal activity. The end of the marine conditions is exactly dated in the Aurès–Nementcha where a Late Eocene formation, dated by small aerial mammalians [80] unconformably overlies folded marine strata dated from the Mid-Eocene. Since the Late Eocene, the entire Saharan Atlas was submitted to continental conditions. In central Tunisia, continental fine clastics (Bou Hedma and Sidi Aïch formations) and marine limestones and evaporites (Orbata and Serdj formations) were deposited during the Albo–Aptian southward transgression. Sedimentation was controlled by an extensional stress regime oriented NE–SW to NNE–SSW, different from the N–S extension that had prevailed in Tunisia from the Triassic to the Barremian. The “Albo–Aptian crisis” [81] activated the Sbiba, Kasserine and Gafsa NW–SE trending normal faults and marked an increasing extension. In central Tunisia, motion along the NW–SE normal faults was responsible for block tilting and development of reefal buildings along

uplifted ridges. Similarly, along the North–South Axis, kilometric-scale blocks were separated and tilted by ENEdipping listric faults [82]. East of the North–South Axis, the Sahel area and the Pelagian Sea underwent a strong subsidence. In this area, from Berriasian to Maastrichtian, tholeiitic, the alkaline magmatic rocks were emplaced along NW–SE directions [64]. Toward the north, in the Tunisian Trough, the diapir ascent went on. The mechanism of emplacement of the salt, either as “classical” diapirs [83] or “salt glaciers” [84,85], is still debated. Whatever this mechanism will be, the salt mobility was probably enhanced by an abnormal heat flow dated from the Aptian [86,87]. The coincidence of crustal extension, tholeiitic and alkaline magmatism and abnormal heat flow at the Aptian–Albian boundary suggests the occurrence of a rifting episode in Tunisia at that time. From the Late Maastrichtian onward, large areas in Central Tunisia Atlas (Kasserine island) and in Southern Tunisia Atlas were emerged. Beside the phosphoric layers characterizing shallow environments, shelf carbonates (Nummulites facies) were deposited around the emerged zone. The pelagic carbonates (Globigerine facies) make up the transition to the Tellian domain with deeper and deeper conditions toward the northeast [18,88,89]. All over the shelf, the diapiric and/or transtensive, or extensive tectonic activities interfere with eustatic variations of the sea level [90]. Several regressive episodes took place, at the Ypresian–Lutetian limit and at the Priabonian–Oligocene limit [90]. 2.5. Oligocene to Present Along the High Atlas axis, some continental sequences attributed to the Miocene rest unconformably over the older series. Along the Middle Atlas axis, and above the regressive Eocene deposits, the Djebel Hayane conglomerates and lacustrine limestones are attributed to the Oligocene and the Miocene, respectively. Only in some restricted areas of the Middle Atlas (e.g. the Skoura basin), connected to the south-Rifan seaway, the Neogene is represented by a marine cycle [91,92] which ended with Miocene (?) Ostrea marls. A similar pattern is described in Algeria, with unconformable, Latest Eocene–Oligocene continental deposits on top of the folded Mesozoic–Middle Eocene strata, followed upward by clastic marine Miocene strata starting from the Burdigalian. These marine strata are connected to contemporary deposits in southern Tunisia, through the Aurès region [58]. In the Tunisian Atlas, the Oligocene–Middle Miocene deposits correspond to fluviatile and deltaic clastics, originated from the southwest and accumulated in a basin which was segmented by N140°E, N40°E and E–W trending normal faults. The corresponding multidirectional extension was registered during the Langhian marine transgression–regression cycle [93]. Younger sedimentary rocks are Late Miocene fluviatile conglomerates and Plio–Quaternary silts. In central Tunisia, progressive unconformities indicate that the main folding initiated during the Serraval-

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

lian and lasted up to the Tortonian. During the Quaternary, the deformation moved southward to the North-Saharan fault [7,18]. Oligocene–Neogene subsident troughs fringe the Moroccan, and locally the Saharan Atlas. The Moroccan Ouarzazate basin, at the southern margin of the High Atlas, is a good example of such “molassic” troughs. It is filled with clastic sediments which originated from the south (Anti Atlas) during the Oligocene and from the north (High Atlas) since the Miocene [94,95]. Laterally, the Toundoute allochthon is described as a synsedimentary nappe emplaced in the basin from the Eocene [96], or a duplex [12]. On the northern side of the Atlas, the piedmont exhibits continental clastic sequences attributed to the Mio-Pliocene and to the Quaternary. Contemporaneously, a Late Neogene–Early Quaternary volcanism is recorded along an NE–SW axis, from the Oujda region to the Djebel Siroua [97].

3. Deformations The reported sedimentary evolution in the different Atlas segments indicates that the period of the major compressive deformation varies from one segment to the other: in the Moroccan Atlas axis, the Late Jurassic–Early Cretaceous redbeds rest unconformably upon the main anticlines. In the Saharan Atlas, the Latest Eocene series postdate the folding. In central Tunisia, the main shortening episode took place during the Late Miocene–Quaternary sedimentation. This diachrony explains why the corresponding segments are hereafter studied separately. 3.1. The Moroccan Atlas 3.1.1. The Atlas axes The structural style is the same in the High and the Middle Atlas: wide S-shaped synclines where Mid-Jurassic strata crop out, with a subhorizontal bottom, separated by narrow and often faulted anticlines (Fig. 7a). The core of the anticlines is occupied either by Early Jurassic or Triassic sedimentary rocks (N70°E and NW–SE anticlines), or by magmatic intrusions in the central High Atlas (N45°E anticlines). These folds are characterized by the reduction of the sedimentary thicknesses and the development of intraformational truncations from the syncline to the anticline. In addition, the limbs of the anticlines contain progressive unconformities. This general disposition argues for a Jurassic, synsedimentary early folding, the synclines corresponding to former depocentres and the anticlines to former positive ridges. This also suggests a reactivation of basement fractures during the Jurassic. A similar pattern occurs in the NE–SW anticlines, the core of which is occupied by magmatic rocks (Fig. 6b). Here too, the structure suggests a synsedimentary uplift of the anticlinal ridge above the ascending magma. In the Middle Atlas, several anticlines display the same disposition. For example, the Djebel

193

Tichoukt anticline, in the central Middle Atlas, [51,75,92] exhibits on its flanks a continuous thickness reduction of the Toarcian to Early Bathonian formations which shows that the anticline overprints a synsedimentary ridge uplifted with regard to the adjacent El Mers and Skoura depocentres/synclines. Postdating the synsedimentary folding, the anticline was thrust northward upon the Skoura syncline and southward onto the El Mers syncline. The thrusting was accompanied by a left-lateral component and eventually sealed by the Skoura conglomerates, attributed to the Late Pliocene. An incipient cleavage [98–101] and a very low-grade syntectonic and hydrothermal metamorphism [102–104] are depicted along several structures of the High Atlas, i.e. the N70°E trending anticlines and particularly the NE–SW anticlines where the heat flow accompanying the gabbroic intrusion softened the rocks. The age of the folding in the High and Middle Atlas axes (not along its borders, see below) is deduced from field observations (Fig. 7b): the penetrative deformation, including the development of an incipient cleavage, developed before the deposition of the unconformable Late Jurassic–Early Cretaceous redbeds. The Jurassic (end of MidJurassic?) age of this deformation is corroborated by that of the magmatic rocks, often spatially associated to metamorphism and axial plane cleavage development: these magmatic rocks are locally reworked in the unconformable redbeds and more largely in the Late Cretaceous marine limestones. The main and penetrative shortening in the Moroccan Atlas axis was therefore pre-Late Cretaceous and probably Jurassic in age, an opinion which is also shared, among others, by Froitzheim et al. [105]. Due to the general lack of Cretaceous and Cenozoic rocks in the Central Atlas, it is always difficult to evaluate the importance of subsequent episodes of deformation. Locally, however, open folds of undetermined age affecting the Late Cretaceous strata have been described in the High Atlas ([50] and Fig. 7b). This example, at least, shows that the amount of the pre-Upper Cretaceous shortening is much more important than the shortening revealed by the bending of the Upper Cretaceous beds. In the Middle Atlas, some Pliocene thrusts are known [92]. In the western High Atlas, the Late Jurassic marine series registered a very similar evolution, with a synsedimentary development of the Atlasic folds. Here, however, N20°E “Atlantic” directions like the axial orientation of the Djebel Tidsi anticline interfere with the N70°E “Atlasic” directions [46,106]. 3.1.2. The Atlas borders By contrast with the Atlas axis, the presence of Late Cretaceous–Neogene strata allows the youngest deformation episodes to be stratigraphically dated in the Atlas borders. The limit between the High Atlas (and at a lesser degree the Middle Atlas) and the adjacent domains (Saharan platform, Meseta) is very sharp; at the longitude of Marrakech, it is marked by a difference of elevation of about

194

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

Fig. 7. Tectonic style of the Central Atlas: (a) Moroccan High Atlas, from [111]; (b) Central High Atlas, from [50]; (c) Algerian Aurès, from [58]; (d) Tunisian Atlas, from [123].

4000 m, no doubt, therefore, for a recent uplift of the belt. Along the southern limit of the High Atlas, here taken as an example, the folded chain is separated from the Souss basin

by the Tizi n’Test fault and from the Ouarzazate basin by the South-Atlasic fault. The Tizi n’Test fault was active as a dextral strike-slip fault during the Palaeozoic and possibly

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

earlier. Its movement became sinistral during the Triassic [107] and the Jurassic [108]. Later on, and since the Cretaceous, the small-scale structures (striae, stylolites, etc.) associated to the fault indicate a strong reverse component. The South-Atlasic fault initially described as a unique structure [109] is rather a set of anastomosing faults which first controlled the Mesozoic sedimentation in the Atlasic trough and later on realized the southward thrust of the Mesozoic series upon the Cretaceous–Cenozoic strata of the Ouarzazate basin [94,110,111]. Locally, allochthonous units may complicate the structural pattern. The above-mentioned Toundoute nappe (Fig. 8b) propagated southward, emplaced within the Ouarzazate basin and was subsequently folded in a complex and polyphase development that lasted from the Eocene to the Pliocene [12,96]. Presently, active blind thrusts have been recently demonstrated from subsurface data [112]. Their extension to the north, below the Atlas, is still conjectural. A similar and contemporaneous, but symmetrical pattern characterizes the northern border of the High Atlas [113,114]. The faults which limit the Middle Atlas to the NW [115, 116] and the SE [117] are sinistral transpressive structures. 3.1.3. The Moroccan Atlas polyphase deformation In summary, the shortening in the Moroccan Atlas occurred in two stages: • During the first half of the Mesozoic (Early–MidJurassic), N70°E trending basement faults were reactivated. This episode is responsible for a relatively important amount of the deformation in the Mesozoic cover, that occurred during the sedimentation and ended locally with the development of a cleavage. By the end of Mid-Jurassic and, moreover, during the Late Jurassic and the Early Cretaceous, the opening of the Atlasic trough progressively stopped and the regime became transpressive, individualizing the major folds along the Atlas axis. • Since the beginning of the Miocene, the Atlasic trough has been inverted. Its former limits acted as reverse faults (High Atlas) or sinistral transpressive faults (Middle Atlas). The Atlas was uplifted and individualized as a high mountain belt. • One important point is the lack of compressive events dated from the Eocene, sharply contrasting with the Saharan Atlas. 3.2. The Saharan Atlas 3.2.1. The Atlas axis Compared to the Moroccan Atlas, the Saharan Atlas presents many similarities, but some significant differences. 3.2.1.1. Similarities with the Moroccan High and Middle Atlas • Both segments present the same structural style: large synclines with a subhorizontal bottom are separated by

195

narrow anticlines, often faulted and sometimes thrust upon the adjacent syncline (Fig. 7c). The faults are parallel to the fold axis and formed contemporaneously with the folding, but there are also evidences of a Late, Post-Miocene reactivation. • In the Saharan Atlas of Algeria, like in the High Atlas of Morocco, the finite structures result from a long-life, synsedimentary folding. The Jurassic and, in Algeria, the Cretaceous depocentres evolved to the present-day synclines. Symmetrically, the positive ridges, or zones of weaker subsidence, sometimes eroded, changed into the present-day anticlines. 3.2.1.2. Differences with the Moroccan Atlas • The first difference, which is discussed hereafter, lies in the timing of the main deformation. In the Saharan Atlas, folding is dated from the Late Eocene, and more precisely, from the Bartonian–Priabonian transition, i.e. much later than in the High Atlas axis where it started as early as the Jurassic. • The late structures of Miocene to Early Quaternary age which formed in the Saharan Atlas are often coaxial with the Eocene ones. These late structures develop eastward in central Tunisia, where, on the contrary, the effects of the Eocene shortening decrease. • Two particularities concern the Zibane zone [58] and the Aurès [117]. The first one is the importance of the NW–SE trending faults, which are either visible in the field or depicted in the basement, as they controlled the deformation of the overlying cover. Those which constitute the western limit of the Aurès Mountains are composite structures and show a set of NW–SE dextral or normal faults shifted by pre-Miocene E–W faults. The NW–SE trend of basement segmentation is a distinctive character of eastern Algeria (and Tunisia) with regard to western Maghreb. The second particularity, which also concerns Tunisia, is the important role played by halokinesis in the Atlasic deformation [35]. 3.2.2. Other Algerian Atlasic regions (Fig. 1B) Between the Atlas axis and the Tell, the variably wide Oran Meseta is also referred to as the “High Plateaux”, suggesting a stable area with regard to the Atlasic deformation. As its Mesozoic cover is folded, it has been renamed Pre-Atlasic zone by Guiraud [58]. On the basis of its sedimentary facies and of the intensity of its deformation, this zone has been subdivided into two subzones: (i) the Pre-Atlasic trough is characterized by mildly folded sequences nearly similar to those of the Atlas axis; (ii) the Pre-Atlasic belt presents a relatively thin cover, but now intensely folded. 3.2.2.1. Pre-Atlasic trough The Chott Chergui depression corresponds to Jurassic– Cretaceous sequences, folded into a large N50°E trending

196

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

Fig. 8. Tectonic style of the southern Atlas limit: (a) the Tunisian Gafsa fault, from [112]; (b) the Moroccan Toundout nappe, from [96]; (c) the Algerian South-Atlasic fault, from [134].

syncline. The main folding is attributed to the Atlasic Eocene compression and Mio-Pliocene thick sequences, almost unfolded, crop out in the centre of the topographic depression. A similar scheme is known eastward in the Zahrez area, although a Mio-Pliocene compression is often depicted. Eastward, in the Hodna, the N70–80°E Djebel Bou Taleb, for instance [118], presents evidences of a progressive deformation where Guiraud [58] described

successively: (i) sedimentary hiatuses and instability from the Liassic to the Senonian; (ii) a first compression during Senonian times with Triassic salt intrusions; (iii) “Atlasic” Eocene folding; (iv) fold accentuation during the Miocene; (v) important shortening, post-Late Pliocene and pre-MidQuaternary. Along its northern limit, the region is affected by south-vergent folds and thrusts [58,119,120] of Eocene age, overprinted by a post-Miocene shortening.

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

3.2.2.2. Pre-Atlasic belt In the Tlemcen Mts of the western pre-Atlasic belt, the Hercynian basement crops out. The Atlasic structures initiated in the Late Santonian. They are brittle and represented by N45–70°E compressive horsts and grabens. In the centre of the region, the folds and the reverse faults are southvergent. Many of them are intruded by Triassic salt. These structures affect the Miocene, but several angular unconformities at the base of the Miocene suggest the existence of pre-Neogene deformations, probably related to the “Atlasic” Late Eocene deformation [57]. The Constantine block (Môle constantinois), with its Mesozoic and Early–Middle Eocene carbonates, formerly considered as autochthonous [121,122], has been thrust southward upon a plastic Triassic sole [58,120]. Its tectonic displacement is tentatively dated from the Eocene; it was followed during the Burdigalian by the emplacement of the Tell nappes above the parautochthonous, and by a Tortonian southward thrusting. 3.2.2.3. South-Atlasic fault As in Morocco and Tunisia, the South-Atlasic fault is a set of south-directed thrusts, often blind and then revealed by subsurface analysis (Fig. 8c). 3.3. The Tunisian Atlas 3.3.1. Northern Tunisia In the NE–SW oriented zone Tunisian Trough, the deformation is relatively intense, affecting thick and incompetent sedimentary sequences, detached from their substratum in the Triassic evaporitic levels. Deformation started during the Jurassic with the initiation of rhombic sedimentary basins and blocks limited by E–W sinistral and N–S dextral transcurrent faults. Locally, submeridian folds accompanied by an axial plane cleavage and a low-grade metamorphism are observed [123]. This folding event occurred during the Cenozoic orogeny [72] and was followed during the Miocene by the development of NE–SW oriented fold-thrust structures. An important particularity of this zone is the importance of the Triassic salt intrusions (zone des diapirs) which are frequently located at the intersection of two faults. 3.3.2. Central Tunisia As everywhere in the Maghreb Atlas, the structural pattern of central Tunisia is made of large synclines and narrow anticlines. The fold axis directions in the Central Atlas are NE–SW, N–S and very locally NW–SE. Folding is dated from the Late Miocene–Early Quaternary, following an Oligocene–Middle Miocene extensional tectonics. An Early Cenozoic deformation has also been postulated [124]. One could argue for the allochthony of the Mesozoic cover with regard to its basement above the Triassic salt horizons. Indeed, numerous thrusts affect the cover series and were actually favoured by the presence of the salt (e.g. Fig. 7d).

197

However, two objections may be made against a wide allochthony of the sedimentary cover. The first objection comes from the absence of a limit of the alleged fold-andthrust belt at the regional scale, and the second is inferred from the fact that the folds were initiated by a synsedimentary deformation. Their location was therefore controlled by deep-seated faults cutting through the Palaeozoic sediments and the underlying basement. Note that neotectonic evolution supports the existence of a mosaic of rigid blocks. In central Tunisia, the coexistence of two axial directions, NE–SW and N–S for the folds, has been explained either as the result of two successive episodes of compression (e.g. [125]), or as the consequence of a local adaptation of the NE–SW trending Atlasic structures, on N–S trending Panafrican lineaments [123]. The North–South Axis is a good example of such submeridian structures [18,126]. It represents a positive flower structure that combines left-lateral and thrust-to-the-east components and is contemporaneous with the right-lateral E–W fault system. This prominent regional structure separates the Central Tunisia Atlas to the west, deformed during the Late Miocene then during the Quaternary, from the more stable Pelagian block to the east (Fig. 1B). In the western part of the Pelagian block (Sahel), a weak compression resulted in open (partly diapiric?) anticlines associated to conjugate wrench faults; farther to the east (Pelagian Sea), this Early Quaternary to current compression disappears and coeval transtensional and extensional deformation prevail [127]. Another example of very recent structures is given by the Teboursouk fault (T on Fig. 1B). This E–W trending fault accommodates the thrusting of Triassic gypsum upon Villafranchian series. The maximum main stress σ1 is oriented at about N150°E, with σ2 ≈ σ3 [128]. Elsewhere in central Tunisia, roughly N–S to NW–SE sedimentary troughs [129] may be interpreted as grabens that parallel the direction of the same recent to current compression [130,131]. 3.3.3. Southern Tunisia By contrast with central Tunisia, the Southern Atlas is dominated by approximately E–W trending folds. The en-echelon fold pattern suggests a link with dextral transcurrent NW–SE faults (e.g. Gafsa fault (Fig. 8a)). The reconstitution of the successive stress fields [20] suggests variations of the regional compressive direction: probably N–S during the Late Eocene (a transpressive event has been evidenced by Zouari [132]), N130°E during the Tortonian and N–S again during the Quaternary, when the Gafsa fault acted as a dextral fault [133] and when the Chott area was slightly folded. Near the northern limit of the Saharan platform, the South-Atlasic front corresponds to pellicular thrusts which do not affect directly the basement [112,134]. In the Saharan platform itself, extension was continuous during the Mesozoic–Cenozoic but the corresponding deformations remained weak and essentially developed along synsedimentary faults [65]. The NW–SE compression is

198

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

recorded in the Mesozoic strata but the N–S compression cannot be evidenced [20].

4. Discussion 4.1. The Atlas belt: a recent fold-thrust belt with Mesozoic and Cenozoic folding episodes Doubtless, the Atlas is a young chain; its elevation, particularly in its western part, the correlative importance of the erosional activity from the Late Miocene onward, the widespread evidences of neotectonic compressive structures, and the pervasive seismic activity (e.g. [135]) testify a recent and current accentuation of the shortening in the chain with regard to the adjacent domains, especially the Saharan platform. Now, examination of many Atlasic structures from Morocco to Tunisia reveals that they are not only recent folds and faults. In the Atlas axis, most of the folds only affect part of the stratigraphic sequence, being erosionally truncated by unconformable deposits. The age of the latter deposits changes from Late Jurassic–Early Cretaceous in Morocco, to Latest Eocene in Algeria and northern Tunisia, to Late Tortonian in central Tunisia and even Quaternary in southern Tunisia. Moreover, the folded sequences present evidences of synsedimentary fold growth suggesting that folding and sedimentation in the Atlasic trough were controlled by the activity of basement faults as early as the Late Liassic. These basement faults display several directions and probably represent inherited deep-seated structures. In western Maghreb, the important NE–SW and WSW–ENE trends parallel Hercynian or Late Hercynian structures like, for instance, the Moroccan Marrakech–Oujda and Atlas Palaeozoic transform faults. In eastern Maghreb (Tunisia), the NW–SE trend could be related to the Tibesti lineament [136] and the N–S trend would correspond to the Panafrican structures exposed in the Hoggar [18]. In fact, we might schematically distinguish two diachronic compressional stages in the building of each Atlas segment. The first stage corresponds to the development of the main plicative, sometimes penetrative structures and it was associated to a general emersion of the area. The second stage is characterized by a reactivation of the first structures, usually in a different stress field, and by the development of thrusts and/or steeeply dipping reverse and/or transcurrent faults. In Morocco, the first episode occurred during the Jurassic. The deformation was locally accompanied by incipient cleavage development and very low-grade metamorphism. It did not result in an important uplift of the chain as the Atlasic domain was subsequently covered by the Early–Late Cretaceous Sea. The second stage began during the Miocene and is not achieved. The two stages are well separated from each other [101]. In Algeria and westernmost Tunisia, the first stage, i.e. the “Atlasic” deformation, occurred during the Late Eocene. It was

followed by another folding-thrusting stage during the Neogene. Farther east, in central Tunisia, the main Atlasic folding is Tortonian in age, and it is followed by a strong accentuation of the previous folds during the Late Villafranchian [18,137]. To the east (Sahel) and to the south of central Tunisia, the folding phase is Late Villafranchian [127]; it is the only place where the folds are essentially related to a single shortening stage. In the various segments of the Atlas the ages of the folding and thrusting events differ. 4.2. The Atlasic trough: pure shear vs. transtensive/transpressive opening and inversion – During the Mesozoic, the Atlas trough comprises several segments, namely, from west to east the High Atlas, Ksour Mts, Ouled Nail, Aurès and Tunisian Trough, limited by transverse zones, Tamlelt and Zibane, and separated by shallower and less subsident areas, like the Mesetas or the Saharan platform. Except the Tunisian Trough, the main part of the Tunisian Atlas derives from a Jurassic shelf [59,138] and not from the trough. – Two mechanisms were called for the Atlasic trough opening. A first mechanism was proposed for the Moroccan Atlas by Mattauer et al. [139], then developed by Laville [50] and Laville and Fedan [6]. It is based upon the geometry of the depocentres and the distribution of the magmatic massifs. The rhomboidal shape of the depocentres and their disposition with regard to the N70°E sedimentary ridges, interpreted as flower structures, are compatible with the control of the basin development by a sinistral motion along the N70°E transcurrent faults (Fig. 9a). An alternative model dwells upon the normal component of movement along the border faults of the Atlasic trough in Morocco ([140] and Fig. 9b) and Algeria [35]. According to this model, the Atlasic trough opening results from a pure shear mechanism responding to an N–S (or NW–SE [34]) extension. In other words, the Atlasic trough should be a classical rift. However, the term “Atlasic rift” is misleading because it represents either the NE–SW trending basins of the Moroccan Middle Atlas–central High Atlas, opened during the Late Triassic and aborted at the beginning of the Liassic (see above), or the N60–70°E trending basins of the Maghreb, active during the main part of the Mesozoic. To avoid any confusion, the Maghreb Jurassic faulted basins are described in the present paper as parts of the “Atlasic trough”. In Morocco, the Atlasic trough established later than the Late Triassic–Earliest Liassic Atlasic rift and its High Atlas part crosscuts it. Indeed, the two models just quoted are end members among varied interpretations, possibly more realistic. On the one hand, the presence of a normal component for the movement along the border faults of the trough is proven by many observations showing Early–Middle Jurassic collapses to the north along the southern border of the trough and to the south along the northern border (Figs. 6 and 9b).

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

199

Fig. 9. Crustal interpretations of the Atlas trough, based upon the High Atlas trough. (A) from Laville [50]; (B) from Warme [140].

On the other hand, the sigmoidal geometry of the Moroccan gabbroic massifs is satisfactorily explained by an enechelon tension gashes model connected to sinistral N70°E transcurrent faulting (Fig. 9a). Accordingly, the Atlasic trough opening should be attributed to a transtensive mechanism combining a left-lateral and a vertical displacement along the N70°E trending faults. – Symmetrically, two models have been proposed for the folding of the Atlas sedimentary pile. The first model [6,50] proposes that in the Atlas axis, the Jurassic folding developed in response to a transcurrent regime along the inherited N70°E basement faults. Sinistral motion along these faults created: (i) N70°E anticlines succeeding to the former ridges, at the vertical of positive flower structures. In these anticlines, a cleavage developed prior to the Callovian, indicating a sinistral component along the N70°E trending faults; (ii) opening of en-echelon N45°E tension gashes, or normal faults, which favoured the development of “anticlines” at the vertical of the ascendant alkaline magmas; (iii)

true NW–SE anticlines. A second model favours a classical inversion of the Atlasic trough [35], with a diapiric origin for the anticlines, developed in the N70°E trending Atlasic rift [141,142]. The inversion of the former normal faults would result from permutation of σ1 and σ3 axes and change from a pure extensional to a pure compressional regime. In the Saharan Atlas, the change would have occurred rapidly, during the Late Eocene. An intermediate model with a transpressive deformation following the transtensive opening, should be favoured, since it could result from a slight variation of the regional stress field. 4.3. Western and eastern Maghreb – Many features oppose western and eastern Atlas. Western Maghreb (i.e. Morocco and the main part of Algeria from the Ksour Mts to the Zibane zone) shows: • deep and subsident basins, individualized since the Liassic;

200

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

• N45°E to N60–70°E trending major structures developed during the Mesozoic sedimentation in response to an NW–SE regional extension and reactivated according to their orientation as transpressional faults. Eastern Maghreb (central and southern Tunisia and eastern Algeria) contrasts with western Maghreb by: • shelf conditions with a relatively less subsident platform during the Jurassic and the Early Cretaceous until the Aptian. Synsedimentary tectonic activity is dominated by sub-N–S extension; • NW–SE and N–S structures, in addition to the dominant NE–SW structures. The NW–SE faults were extensional during the Mid–Late Cretaceous, then dextral transpressive from the Cenozoic onward. The northern part of the Tunisian Atlas (Tunisian Trough), distinguishes from the Central Tunisia Atlas by a transtensive opening of the Jurassic trough. In that sense, it extends eastward of the Moroccan–Saharan Atlas, to the north of a different domain represented by the Central and Southern Tunisia Atlas. – We propose that the differences between western and eastern Maghreb are primarily related to the different orientations taken by the reactivated basement faults: in Morocco, the main part of Algeria and perhaps northern Tunisia, they are N45°E to N70°E, corresponding to Hercynian directions. From the Algerian Zibane to the central and eastern Tunisia, the NE–SW structural grain is present, although attenuated by NW–SE and N–S directions. Southern Tunisia remains outside the domain of reactivation of the Hercynian structures. 4.4. The Atlas and Africa geodynamic evolution During the Late Triassic–Earliest Liassic, the Moroccan Atlasic rift developed parallel to the Atlantic rift. Its border faults were reactivated Hercynian faults, variably dipping to the southeast. The closure of the Atlasic rift was marked by a postrift unconformity and the development of a widespread shallow marine carbonate shelf which lasted during the main part of Liassic times. The Early–Mid-Liassic shelf was disrupted during the Late Liassic, when the oceanic accretion began in the Central Atlantic, allowing the eastward drifting of Africa and its decoupling from Eurasia. The Africa southeastward drift was responsible for the transform nature of the northern limit of the Africa plate. It was also registered in the Atlasic domain, where N70°E faults were reactivated in a dominantly sinistral transcurrent regime under an NW–SE extension. These faults, inherited from the Hercynian orogeny, are identified in western Maghreb and depicted in the Tunisian Trough. During the Jurassic, the western Maghreb Atlasic domain belonged to the northern Africa transform margin, which extended northward up to the Rif–Tellian External Zones. On the contrary, the lack of sinistral motion along the N70°E trending faults, and above all, the roughly N–S extension in

Central–Southern Tunisia suggests that the Central and Southern Tunisia Atlas domain were not segmented by the eastward drift of Africa and remained outside the Africa margin. If so, the Africa margin was switched northeasterly, outside central Tunisia. In other words, the Central–Southern Tunisia was an African salient, probably part of the Apulian Promontory. The geometrical relationships between the Palaeo–Apulia promontory, delimited by a virgation of the southern Hercynian front (Fig. 2) and the Atlas chain, is compatible with the control of the western Atlasic trough by Hercynian basement faults. The transform nature of the North Africa margin vanished progressively during the Late Jurassic–Early Cretaceous; from the Late Cretaceous onward, its evolution has been driven by the convergence of Africa and Eurasia which is at the origin of the Tell and Atlas compressive structures. The early folding demonstrated in the Moroccan part of the Atlas could be related to the early juxtaposition of the western part of North Africa and the Iberian salient, subsequently forcefully pulled-apart from western Europe during the Early Cretaceous. Development of Central Africa rift structures [143–146] resulted from the South Atlantic initial rifting that occurred during the Early Cretaceous [147]. In the main part of North Africa, the extension was close to N–S as evidenced by pure extension along E–W trending faults in Tunisia and the general downwarping to the north of the Algerian domains, revealed by the northward progression of the Ksour delta. The dextral motion along the Guinea Gulf transform fault [144] since the Mid-Cretaceous induced the separation of Africa and South America and the persistence of a generalized extensional regime in Africa. More precisely, it gave way to the development of an NE–SW extension, well recorded in eastern Maghreb by pure extensional movement along the NW–SE trending faults of Tunisia. At 84 Ma, the abrupt change of Africa trajectory with regard to Eurasia [10,148] induced the frontal convergence of the two plates and initiated the Senonian inversion of many African Mesozoic basins [149]. In the Atlasic domain, the inversion began with vertical movements in the Saharan Atlas, the emersion of the Ksour Mts, and the initiation of the sinking of the Tunisian Sahel, accompanied by a magmatic activity. By the end of the Eocene, the Africa–Europe convergence was fully active, clearly expressed in the internal parts of the Rif–Tell chain and also in the Atlasic domain. The regression which took place in the Moroccan Atlas axis preceded its uplift. In the Saharan Atlas, the main Atlasic shortening is dated at that time. It is noticeable that, with the possible exception of the northern Tunisian Trough, no regional compression affected the Tunisian Atlas, only subjected to halokinetic deformation. At that time, no direct collision was effective between Europe and the Tunisian segment of North Africa, as they were separated from each other by the Adriatic salient.

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

The following deformation events recorded in the Atlas belt are related to the Alpine belt building, then to the opening of the western Mediterranean basins (e.g. [150,151]). The Atlas current structure results from several compressional events having alternated through time with extensional events responsible for large-scale collapses in the internal Rif–Tell chain, in relation with the opening of the Provence and Algerian basins. The first compressional event occurred during the Burdigalian in Algeria, where it was responsible for the thrust of the Tellian nappes over the Constantine block. The second one occurred during the Tortonian and was particularly developed in northern and central Tunisia, as a consequence of back-arc basin opening and island arc drifting in the Central Mediterranean domain [127]. By the Early Quaternary, a new compression accentuated the Atlasic structures in Algeria and northern–central Tunisia, then reached the still unfolded southern Tunisia. All along the borders of the Atlas, principally along the south Atlas front [12,112], active thrusts testify the persistence of a roughly N–S compression. The current stress field at the scale of North Africa characterizes the Maghreb indenter. In this indenter, Algeria is in a frontal collisional position with Western Mediterranean and Europe, and is decoupled from the western and eastern parts of Maghreb, which experience lateral escape to the SW and SE, respectively [106]. 4.5. Unsolved questions Among the striking questions which are still in debate, are the age and the amount of the Atlasic shortening, considered at the crustal or the lithospheric scale. Many workers who recently studied the deformation of the Atlas, concentrated on the marginal zones of the belt, especially the south Atlas front (e.g. [11,12]). This choice results from the facts that subsurface data are available there and that the presence here of Cenozoic sequences allow a precise evaluation of the Atlasic shortening and the dating of its uplift. If one considers only the data which concern the south Atlas front, it is clear that the folding and thrusting of this part of the Atlas developed during the Cenozoic, more precisely during two separate phases in Late Eocene and Pleistocene–Lower Quaternary [12]. However, Laville [152] recalled to mind very recently that an ante-Cretaceous compressive deformation occurred in the Moroccan Atlas axis and that even along the southern High Atlas front, the shortening began as early as the Senonian. Consequently, the High Atlas shortening (15–36 km according to Gomez et al. [11]) calculated on the basis of a Miocene and postMiocene deformation, should also integrate the results of the preceding shortening episodes. Laville [152] and Gomez et al. [153] resume these different points of view. Now, what about the geodynamic significance of the Atlas shortening? It is undisputable that the Atlas, in particular its Moroccan segment, is bordered by divergent thrusts all along its length (Fig. 10). A discussion of these

201

structures initiated since long ago. For instance, Froitzheim et al. [105] asked: “how do the High Atlas faults continue to depth?” Mainly from the geometry of the highly symmetrical structure of the belt, they concluded that its border faults steepen at depth. On the contrary, Jacobshagen et al. [111] presented a section of the Atlasic domain of Morocco where deep flat-lying structures revealed by geophysical surveys [154] are interpreted as traces of crustal thrusts (Fig. 11a). More recently, Frizon de Lamotte et al. [12] examined the Atlas mountain building. They gave an excellent description of the folds and faults visible at the surface and in subsurface along the Southern Atlas limit. All of these thrusts are thought to merge along a flat intracrustal detachment. Fig. 11b, from Bracène et al. [155] shows a similar pattern, drawn for the Saharan Atlas, in which the Atlas domain is thrust upon the Saharan platform, the Oranese Meseta itself being limited at depth by a flat detachment surface. The intracrustal Atlasic detachment postulated [12] prolongates northward over a distance of about 400 km, reaching the Tell. The problem can also be aborded in terms of thin-skin vs. thick-skin tectonics [156] (Fig. 11c): do the orogenic boundaries of the Atlas belt correspond to the old palaeogeographic limits of the Atlasic trough, subsequently reactivated, or are they new independent features? In the first case, they are deep, crustal faults indicative of a basementinvolved tectonics, whereas in the second case, they are just shallow structures like those developed in the orogenic forelands (thin-skin tectonics). It is clear that the border thrusts do not exactly coincide all over their extent with the palaeogeographic limits. This is the case, for instance, with the Moroccan South-Atlasic fault which is often made up of flat-lying thrusts developed lately during the Neogene shortening without relationships with the basin boundaries. However, on the other hand, the rough and regional correspondence between the Atlasic belt and the Atlasic trough calls for a reactivation of major synsedimentary faults (Fig. 11d). Anyway, if the thrusts can be followed under the Atlas marginal zones over a distance of about 10 km, the available subsurface data do not allow to prolongate them below the central part of the chain. The problem is still open to discussion and cannot be solved without a detailed geophysical study including highangle seismic profiles in the crust below the Atlas belt.

5. Conclusions The Atlas belt is an intracontinental chain, where the formation of the regional sedimentary basins was accompanied by a moderate crustal stretching. The Atlasic domain represents a transitional zone between the undeformed craton to the south, and the very mobile Rifian–Tellian zone to the north. The restricted mobility of the Atlas resulted in extensive (or transtensive), then compressive (or transpressive) reactivation of deep-seated faults. These are inherited faults,

202

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

Fig. 10. General section of the central High Atlas.

mainly oriented NE–SW (Hercynian) in western Maghreb, NW–SE and N–S (Panafrican) in eastern Maghreb. These reactivations never gave way to a decoupling of the Atlas domain from the African shield. In other words, the Atlasic domain never became a microplate. This could be due to the fact that the southern limit of the Atlasic trough was never subjected to the same tectonic sollicitation (i.e. stress fields) along its full length; for instance, during the Mesozoic, eastern Maghreb escaped the effects of the Central Atlantic opening and the eastward drifting of Africa. On the contrary, the Atlas domain in eastern Maghreb was more strongly sollicitated during the Neogene–Quaternary Africa–Europe convergence. During the Jurassic, the Atlasic domain, especially in its western part, registered the sinistral motion along the northern limit of the African plate with regard to the Iberian microplate. This mobility was at the origin of Mesozoic synsedimentary structures upon which the Cenozoic structures, themselves related to the Africa–Europe convergence, were subsequently moulded. In the finite structure of the chain, it is often difficult to make a clear distinction between both kinds of deformations. The best example is given by the Tunisian Atlas. As a part of North Africa, the Atlasic domain registered the geodynamic successive episodes relative to the movement of the African plate with regard to the adjacent plates: (i) its separation from the North America plate during the Late Triassic–Early Jurassic contemporaneous with the development of the Moroccan Atlas rift; (ii) its decoupling and lateral motion with regard to the European plate during the Mesozoic, responsible for the opening of transtensive sedimentary basins and the emplacement of mafic magmas; (iii) its convergence with the European plate from the Late Santonian evidenced by the progressive shortening of the belt. All these geodynamic episodes were marked by the development of extensional or compressional structures in

the Atlasic domain. Corresponding structures should also have developed along the northern limit of the Africa plate, i.e. the present external Rif and Tell belt. There, however, the strong Cenozoic shortening obscured the Mesozoic sedimentary and structural pattern which is better preserved in the Atlasic domain. The strong opposition between western and eastern Maghreb may be partly explained by the pre-Mesozoic, i.e. Hercynian structuration of the Maghreb domain. The virgation of the Hercynian belt delimited a Palaeo–Apulian salient in eastern Maghreb. During its eastward drift, eastern Maghreb, part of the Apulian promontory, was part of the stable Africa and suffered only very weak reactivations, while western Maghreb was submitted to the effects of the transform motion along the northern limit of the Africa plate. Finally, in spite of the apparent singleness of the Atlas, there are a wide variety of evolutions and structures along the chain, especially but not only between its western and eastern parts. These discrepancies result from the various dispositions of the Atlasic segments inside the Atlantic– Tethyian frame, then the Alpine–Mediterranean frame. Although it remained outside these mobile regions, the Atlasic area allows, in its different segments, to recognize the evolution of the Atlantic and Mediterranean Oceans and the Alpine chain.

Acknowledgements The works in the field and in the labs that are at the origin of the present paper have greatly benefited from various bilateral cooperation programmes between several universities of France, Morocco, Algeria and Tunisia. The help of the governmental and academic offices of these countries is therefore gratefully acknowledged.

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

Fig. 11. Crustal and lithospheric interpretations of the Atlas structure (a) from [157]; (b) from [155]; (c) from [156]; (d) inspired from [114].

203

204

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

References [1] W. Wildi, La chaîne tello-rifaine (Algérie, Maroc, Tunisie): structure, stratigraphie et évolution du Trias au Miocène, Rev. Geol. Dyn. Geogr. Phys. 24 (1983) 201–297. [2] M. Durand-Delga, Mise au point sur la structure du nord-est de la Berbérie, Publ. Serv. Geol. Algerie N. Ser. 39 (1969) 89–131. [3] J.P. Bouillin, La transversale de Collo et d’El Milia (Petite Kabylie): une région-clef pour l’interprétation de la tectonique alpine de la chaîne littorale d’Algérie, Mem. Soc. Geol. Fr. N. Ser 57 (135) (1979) 84. [4] A. Chalouan, A. Michard, H. Feinberg, R. Montigny, O. Saddiqi, The Rif mountain building (Morocco): a new tectonic scenario, Bull. Soc. Geol. Fr. 172 (2001) 603–616. [5] N. Kazi Tani, Evolution géodynamique de la Bordure nord-africaine: le Domaine intraplaque nord-algérien. Approche megaséquentielle, D. Sci. Thesis, Université de Pau, France, 1986, pp. 871. [6] E. Laville, B. Fedan, Le système atlasique marocain au Jurassique, évolution structurale et cadre géodynamique, Sci. Geol. Mem. 84 (1989) 3–28. [7] P. Burollet, Structures and tectonics of Tunisia, in: R. Freeman, M. Huch, S. Mueller (Eds.), The European Geotraverse, Part 7, Tectonophysics, 195, 1991, pp. 359–369. [8] R. Guiraud, Y. Bellion, Late Carboniferous to recent geodynamic evolution of the west Gondwanian Tethyan margins, in: E. Nairn, et al. (Eds.), The Tethys Ocean, The Ocean Basins and Margins, 88, Plenum Press, New York, 1995, pp. 101–124. [9] J.F. Dewey, M.L. Helman, E. Turco, D.H. Hutton, S.D. Knott, Kinematics of the western Mediterranean, in: M.P. Coward, D. Dietrich, R.G. Park (Eds.), Alpine Tectonics, Geological Society, London, Special Publication, No. 45, 1989, pp. 265–283. [10] J. Dercourt, L. Ricou, Vrieliynk B-Atlas of the Tethys Paleoenvironmental Maps, Gauthier-Villars, Paris, 1993, pp. 307. [11] F. Gomez, W. Beauchamp, M. Barazangi, Role of the Atlas mountains (northwest Africa) within the African–Eurasian plateboundary zone, Geology 28 (2000) 775–778. [12] D. Frizon de Lamotte, B. Saint Bezar, R. Bracène, E. Mercier, The two main steps of the Atlas building and geodynamics of the western Mediterranean, Tectonics 19 (2000) 740–761. [13] A. Piqué, A. Michard, Moroccan Hercynides: a synopsis. The Paleozoic sedimentary and tectonic evolution at the northern margin of West Africa, Am. J. Sci. 289 (1989) 286–330. [14] G. Lucas, Bordure nord des Hautes Plaines dans l’Algérie occidentale: Primaire, Jurassique, Analyse structurale, Publ. 19ème Congr. Geol. internat., Alger, Monogr. rég., 1ère Série, vol. 21, 1952, pp. 139. [15] A. Piqué, G. Bossière, J.P. Bouillin, A. Chalouan, C. Hoepffner, The southern margin of the Variscan belt. The northwestern Gondwana mobile zone (Eastern Morocco and Northern Algeria), Geol. Rundsch. 82 (1993) 432–439. [16] P. Legrand, Lower Paleozoic rocks of Tunisia, in: H. Holland (Ed.), Lower Paleozoic Rocks of North-western and West Central Africa, Wiley, New York, 1985, pp. 1–3. [17] L. Memmi, P. Burollet, I. Viterbo, Lexique stratigraphique de la Tunisie. Précambrien et Paléozoïque, Notes Serv. Geol. Tunisie 53 (1986) 66. [18] P.F. Burollet, Contribution à l’étude stratigraphique de la Tunisie centrale, Ann. Mines Geol. Tunis 13 (1956) 350. [19] J. Marie, P. Trouvé, G. Desforges, P. Dufaure, Nouveaux éléments de paléogéographie du Crétacé de Tunisie, Notes Mem. C. F. P. Paris 19 (1984) 37. [20] S. Bouaziz, Etude de la tectonique cassante dans la plate-forme et l’Atlas sahariens (Tunisie méridionale): évolution des paléo-champs de contraintes et implications géodynamiques, Peri-Tethys Rep. 4 (1995) 485.

[21] M. Oujidi, L. Courel, N. Benaouiss, M. El Moustaïne, M. El Youssi, M. Et Touhami, N. Jalil, D. Ouahrache, A. Sabaoui, A. Tourani, La marge péri-téthysienne marocaine pendant le Trias, Les Marges téthysiennes d’Afrique du Nord, Séance spécialisée de la Société géologique de France 16-17 decembre 1997, 1997, pp. 34 (abstract). [22] E. Laville, A. Charroud, B. Fedan, M. Charroud, A. Piqué, Inversion négative et rifting atlasique: l’exemple du bassin triasique de Kerrouchène (Maroc), Bull. Soc. Geol. Fr. 166 (1995) 364–374. [23] R. Aït Ouali, Le Rifting des Monts des Ksour au Lias: Organisation du Bassin, Diagenèse des Assises carbonatées, Place dans les Ouvertures mésozoïques au Maghreb, Thèse D.Sci., Université de Pau, 1991, pp. 302. [24] P. Logan, I. Duddy, An investigation of the thermal history of the Ahnet and Reggane basins, Central Algeria, and the consequences for hydrocarbon generation and accumulation, in: D.S. Macgregor, R.T. Moody, D.D. Clark-Lowes (Eds.), Petroleum Geology of North Africa, Geological Society, London, Special Publication, No. 132, 1998, pp. 131–155. [25] L. Fiechtner, H. Friedrichsen, K. Hammerschmidt, Geochemistry and geochronology of Early Mesozoic tholeiites from Central Morocco, Geol. Rundsch. 81 (1992) 45–62. [26] S. Huon, J.J. Cornée, A. Piqué, N. Rais, N. Clauer, N. Liewig, R. Zayane, Mise en évidence au Maroc d’événements thermiques d’âge triasico-liasique liés à l’ouverture de l’Atlantique, Bull. Soc. Geol. Fr. 164 (1993) 165–176. [27] N. Rais, Les Roches triasico-liasiques du Maroc septentrional et leur Socle hercynien: Caractérisation pétrologique, minéralogique, cristallochimique et isotopique K–Ar de leur Évolution thermique post-formationnelle, Thèse de Doctorat ès-Sciences, Université de Fès, Morocco, 2002, pp. 344. [28] H. Bertrand, J. Dostal, C. Dupuy, Geochemistry of early Mesozoic tholeiites from Morocco, Earth Planet. Sci. Lett. 58 (1982) 225–239. [29] H. Lapierre, C. Mangold, S. Elmi, M. Brouxel, Deux successions volcano-sédimentaires dans le « Trias » d’Oranie (Algérie occidentale); témoins de la fracturation d’une plate-forme continentale, Rev. Geol. Dyn. Geogr. Phys. 25 (1984) 361–373. [30] M. Wilson, R. Guiraud, Late Permian to recent magmatic activity on the African–Arabian margin of Tethys, in: D. Macgregor, R. Moody, D. Clark-Lowes (Eds.), Petroleum Geology of North Africa, Geological Society, London, Special Publication, No. 132, 1998, pp. 231–263. [31] E. Laville, A. Piqué, La distension crustale atlantique et atlasique au Maroc au début du Mésozoïque: le rejeu des structures hercyniennes, Bull. Soc. Geol. Fr. 162 (1991) 1161–1171. [32] A. Piqué, E. Laville, L’ouverture initiale de l’Atlantique central, Bull. Soc. Geol. Fr. 166 (1995) 725–738. [33] S. Elmi, Evolution dynamique et paléostructure de l’Oranie (Algérie) dans le cadre maghrébin pendant le Jurassique inférieur, Les Marges téthysiennes d’Afrique du Nord, Séance spécialisée de la Société géologique de France, 16-17 décembre 1997, 1997, pp. 54 (abstract). [34] A. El Kochri, J. Chorowicz, Oblique extension in the Jurassic trough of the central and eastern High Atlas (Morocco), J. Can. Earth Sci. 33 (1996) 84–92. [35] R. Vially, J. Letouzey, F. Bénard, N. Haddad, G. Desforges, H. Askri, A. Boudjemaa, Basin inversion along the North African margin. The Saharan Atlas (Algeria), in: F. Roure (Ed.), Peri-Tethyan Platforms, Technip Pub., 1994, pp. 79–118. [36] B. Haq, J. Hardenbol, P. Vail, Chronology of fluctuating sea-levels since the Triassic, Science 235 (1987) 1156–1167. [37] R. du Dresnay, Development and paleogeographic extent of buildups and organic reefs during Jurassic time in Atlas mountains of Morocco, northwest Africa, Am. Assoc. Petrol. Geol. Bull. 60 (1976) 667–668.

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208 [38] D. Letsch, The northern margin of the central and eastern High Atlas, Evolution of the Jurassic High Atlas Rift, Morocco: Transtension, Structural and Eustatic Controls on Carbonate Deposition, Tectonic Inversion, Field Trip 9, AAPG Mediterranean Basin Conference and Exhibit, 1988, pp. VII.1–VII.28 (Guidebook compiled by J. Warme). [39] P. Crevello, Carbonate platform facies mosaic of the southern margin of the central High Atlas rift, Evolution of the Jurassic High Atlas Rift, Morocco: Transtension, Structural and Eustatic Controls on Carbonate Deposition, Tectonic Inversion, Field Trip 9, AAPG Mediterranean Basin Conference and Exhibit, 1988, pp. X.1–VII.42 (Guidebook compiled by J. Warme). [40] M. Monbaron, Sédimentation, tectonique synsédimentaire et magmatisme basique: l’évolution paléogéographique et structurale de l’Atlas de Beni-Mellal (Maroc) au cours du Mésozoïque; ses incidences sur la tectonique tertiaire, Ecl. Geol. Helv. 74 (1981) 625–638. [41] J. Jenny, A. Le Marrec, M. Monbaron, Les couches rouges du Jurassique moyen du Haut Atlas central (Maroc): corrélations lithostratigraphiques, éléments de datations et cadre tectonosédimentaire, Bull. Soc. Geol. Fr. 7 (23) (1981) 627–639. [42] A. Souhel, J. Canerot, Polarités sédimentaires téthysienne puis atlantique: l’exemple des couches rouges jurassico-crétacées du Haut-Atlas central (Maroc), Sci. Geol. Mem. 84 (1989) 39–46. [43] J. Delfaud, K. Zellouf, Existence, durant le Jurassique et le Crétacé inférieur, d’un Paléo-Niger coulant du sud vers le nord au Sahara occidental, 118ème Congr. nat. Soc. hist. et scient., Pau, et 4 ème Coll. de Géologie africaine, 1993, pp. 93–104. [44] Y. Hervouet, Géodynamique alpine (Trias–Actuel) de la Marge septentrionale de l’Afrique au Nord du Bassin de Guercif, Thèse D.Sci., Université de Pau, 1985, pp. 376. [45] G. Cattaneo, Les Formations du Jurassique supérieur et du Crétacé inférieur de l’Avant-pays rifain oriental (Maroc), Thèse D.Sci., Université de Dijon, 1987, pp. 327. [46] M. Amrhar, Tectonique et Inversions géodynamiques postrift dans le Haut Atlas occidental. Structures, Instabilités tectoniques et Magmatismes liés à l’Ouverture de l’Atlantique central et la Collision Afrique–Europe, Thèse D. Sci., Université de Marrakech, 1995, pp. 253. [47] C. Caratini, Evolution paléogéographique et structurale de la région de Chellala–Reibell (Départements de Medea et Tiaret, Algérie), Bull. Soc. Geol. Fr. 9 (1967) 850–858. [48] D. Auclair, J. Biehler, Etude géologique des Hautes plaines oranaises entre Tlemcen et Saïda, Publ. Serv. Geol. Algerie N. Ser. 34 (1967) 3–45. [49] R. Aït Ouali, M. Laadila, J. Delfaud, Le Lias de l’avant-pays atlasique du Maghreb occidental: le rifting des Atlas, Les Marges téthysiennes d’Afrique du Nord, Séance spécialisée de la Société géologique de France, 16-17 décembre 1997, 1997, pp. 52 (abstract). [50] E. Laville, A multiple releasing and restraining stepover model for the Jurassic strike-slip basin of the Central High Atlas (Morocco), in: W. Manspeizer (Ed.), Triassic–Jurassic Rifting. Continental Breakup and the Origin of the Atlantic Ocean and Passive Margins, 1988, pp. 499–523. [51] B. Fedan, Evolution géodynamique d’un bassin intraplaque sur décrochements: le Moyen Atlas (Maroc) durant le Méso–Cénozoïque, Trav. Inst. Sci. Rabat 18 (1989) 142. [52] E. Berraouz, B. Bonin, Magmatisme alcalin intracontinental en contexte de décrochement; le masssif plutonique mésozoïque de Tirrhist, Haut Atlas (Maroc), C. R. Acad. Sci. Paris 317 (1993) 647–653. [53] A. Rahimi, B. Bougadir, A. Saidi, I. Reuber, J. Karson, Mécanisme de mise en place d’intrusion alcaline en niveau structural peu profond retracé par les structures d’écoulement magmatique: exemple du Haut Atlas central (Maroc), C. R. Acad. Sci. Paris Ser. II 313 (1991) 571–577.

205

[54] A. Saidi, B. Bougadir, A. Rahimi, A. Saquaque, M. Aarabe, Etude de la différenciation magmatique des roches plutoniques alcalines du Haut Atlas central. Exemple des massifs de Tasraft et Tassent, Rev. Fac. Sci. Marrakech 7 (1993) 137–143. [55] R. Zayane, A. Essaifi, R.C. Maury, A. Piqué, E. Laville, M. Bouabdelli, Cristallisation fractionnée et contamination crustale dans la série magmatique jurassique transitionnelle du Haut Atlas central (Maroc), C. R. Geosci. 334 (2) (2002) 97–104. [56] B. Bougadir, A. Rahimi, Les structures magmatiques de l’intrusion d’Aqqa n’Ouanine (ride de Tassent, Haut Atlas central, Maroc): mécanisme de mise en place, Rev. Fac. Sci. Marrakech 7 (1993) 129–136. [57] D. Bureau, Approche sédimentaire de la Dynamique structurale: Évolution mésozoïque et Devenir orogénique de la Partie septentrionale du Fossé saharien (SW constantinois), Aurès, Algérie, Thèse, Université de Paris VI, 1986. [58] R. Guiraud, Evolution post-triasique de l’avant-pays de la chaîne alpine en Algérie d’après l’étude du Bassin du Hodna et des régions voisines, Pub. Office Nat. Geol. Algerie Mem. 3 (1990) 259. [59] M. Soussi, R. Enay, C. Mangold, M. Turki, The Jurassic events and their sedimentary and stratigraphic records on the southern Tethyan margin in central Tunisia, in: S. Crasquin-Soleau, E. Barrier (Eds.), Peri-Tethys Memoir, 5, Mémoires du Museum d’Histoire naturelle de Paris, 182, 2000, pp. 57–92. [60] S. Bouaziz, M. Turki, H. Zouari, E. Barrier, Tectonique en extension et failles de transfert jurassiques dans la région du Tebaga de Medenine (Tunisie méridionale), Ann. Soc. Geol. Nord Ser. II 4 (1996) 65–69. [61] C. Gourmelen, Serrage polyphasé de Paléostructures distensives dans l’Axe Nord–Sud tunisien: le Segment Bouzer-Rheouis, Thèse de 3ème Cycle, Université de Grenoble, 1984, pp. 216. [62] J. Rais, S. Gaya, R. Alouani, M. Mouguina El, S. Tlig, Le sillon tunisien: structuration synsédimentaire jurassique. Rift avorté et cicatrisé au Dogger supérieur–Malm, à la marge SE de la Téthys maghrébide, C. R. Acad. Sci. Paris Ser. II 312 (1991) 1169–1175. [63] R. Alouani, J. Rais, S. Gaya, S. Tlig, Les structures en décrochement au Jurassique de la Tunisie du Nord: témoins d’une marge transformante entre Afrique et Europe, C. R. Acad. Sci. Paris Ser. II 315 (1992) 717–724. [64] N. Laridhi-Ouazaa, Etude minéralogique et géochimique des Épisodes magmatiques mésozoïques et miocènes de la Tunisie, Thèse, Université de Tunis II, 1994, pp. 464. [65] S. Bouaziz, E. Barrier, M. Turki, P. Tricart, La tectonique permomésozoïque (anté-Vraconien) dans la marge sud-téthysiene en Tunisie méridionale, Bull. Soc. Geol. Fr. 170 (1999) 45–56. [66] P. Haller, Structure profonde du Sahel tunisien. Interprétation géodynamique, Thèse de 3ème Cycle, Université de Besançon, 1983, pp. 163. [67] J. Ouali, Structure et évolution géodynamique du chaînon Nara–Sidi Khalif (Tunisie centrale), Thèse de 3ème Cycle, Université de Rennes, 1984, pp. 119. [68] C. Soyer, Inversions structurales le Long de la Direction atlasique en Tunisie centrale; le Jebel Boudinar, Thèse, Université de Grenoble, 1987, pp. 280. [69] A. Snoke,, S. Schamel, R. Kavasek, Structural evolution of Djebel Debabib anticline: a clue to the regional style of the Tunisian Atlas, Tectonics 7 (1988) 497–516. [70] J. Rey, K. Taj-Eddine, Eustatisme et tectonique distensive au passage Jurassique–Crétacé: leur enregistrement sédimentaire dans le bassin du Haha (Haut-Atlas occidental, Maroc), C. R. Acad. Sci. Paris Ser. II 308 (1989) 101–106. [71] N. Ben Ayed, A. El Ghali, C. Bobier, H. Chekhma, M Rabhi, Déformations synsédimentaires du Barrémo-Aptien dans l’Atlas tunisien: conséquences paléogéographiques, Les Marges téthysiennes d’Afrique du Nord, Séance spécialisée de la Société géologique de France, 16-17 décembre 1997, 1997, pp. 9 (abstract).

206

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

[72] H. Rouvier, Géologie de l’Extrême Nord Tunisien: Tectoniques et Paléogéographies Superposées à l’Extrémité orientale de la Chaîne Nord Maghrébine, Thèse D.Sci., Université de Paris VI, 1977, pp. 703. [73] A. Charrière, J.M. Vila, Découverte d’Aptien marin à foraminifères dans le Moyen-Atlas (Maroc): un golfe mésogéen à travers la « Terre des Idrissides » ? C. R. Acad. Sci. Paris Ser. II 313 (1991) 1579–1586. [74] N. Benalioulhaj, Les Formations à Phosphates et à Schistes bitumineux du Bassin des Oulad Abdoun et du Bassin de Timahdit (Maroc): Pétrographie, Minéralogie, Géochimie et Environnement de Dépôt, Thèse D. Sci., Université L. Pasteur, Strasbourg, 1991, pp. 238. [75] M. Charroud, A. Ait Slimane, B. Fedan, Répercussions de la tectonique locale et des transgressions–régressions atlantiques sur l’évolution crétacée et paléogène de la bordure sud-ouest du Moyen Atlas (Maroc), The 14th International Meeting of Sedimentology, Marrakech, April 1993, 1993, pp. 97–98 (abstract). [76] R. Charlot, G. Choubert, A. Faure-Muret, C. Hamel, Age des aïounites du Maroc nord-oriental, C. R. Somm. Soc. Geol. Fr. (1964) 401–402. [77] R. Charlot, D. Tisserant, P. Vidal, F. Vidal, Rapport technique et quelques résultats, Unpublished Compte rendu d’activité 1967, Service de la carte géologique du Maroc, 1967, pp. 126–137. [78] G. Choubert, A. Faure-Muret, Evolution du domaine atlasique marocain depuis les temps paléozoïques, Livre à la Mémoire de P. Fallot, Mémoire hors série de la Société géologique de France, 1961, pp. 447–527. [79] M. Aoudjehane, A. Bouzenoune, H. Rouvier, J. Thibieroz, Halocinèse et dispositifs d’extrusion de Trias dans l’Atlas saharien oriental (NE algérien), Geol. Mediterr. 19 (1992) 273–287. [80] P. Coiffait, B. Coiffait, J.J. Jaeger, M. Mahboubi, Un nouveau gisement à Mammifères fossiles d’âge Eocène supérieur sur le versant sud des Nementcha (Algérie orientale): découverte des plus anciens Rongeurs d’Afrique, C. R. Acad. Sci. Paris 299 (1984) 893–898. [81] C. Soyer, P. Tricart, La crise aptienne en Tunisie centrale: approche paléostructurale aux confins de l’Atlas et de l’Axe nord-sud, C. R. Acad. Sci. Paris 305 (1987) 301–305. [82] C. Gourmelen, J. Ouali, P. Tricart, Les blocs basculés mésozoïques dans l’Axe nord–sud de Tunisie centrale: importance et signification, Bull. Soc. Geol. Fr. 8 (5) (1989) 117–122. [83] V. Perthuisot, A. Bouzenoune, N. Hatira, B. Henry, E. Laatar, A. Mansouri, H. Rouvier, A. Smati, J. Thibieroz, Les diapirs du Maghreb oriental: part des déformations alpines et des structures initiales crétacées et éocènes dans les formes actuelles, Bull. Soc. Geol. Fr. 170 (1999) 57–65. [84] J.M. Vila, M. Ben Youssef, A. Charrière, M. Chikhaoui, M. Ghanmi, F. Kamoun, B. Peybernès, J. Saadi, P. Souquet, M. Zarbout, Découverte en Tunisie, au SW du Kef, de matériel triasique interstratifié dans l’Albien: extension du domaine à « glaciers de sel » sous-marins des confins algéro-tunisiens, C. R. Acad. Sci. Paris Ser. II 318 (1994) 1161–1167. [85] J.M. Vila, M. Ben Youssef, M. Chikhaoui, M. Ghanmi, Un grand « glacier de sel » sous-marin albien moyen du Nord-Ouest tunisien (250 km2): le matériel salifère triasique du « diapir » de Ben Gasseur et de l’anticlinal d’El Kef, C. R. Acad. Sci. Paris Ser. II 322 (1996) 221–227. [86] H. Bellon, V. Perthuisot, Ages radiométriques (K–Ar) de felspaths potassiques et de micas néoformés dans le Trias de Tunisie septentrionale, Bull. Soc. Geol. Fr. 7 (19) (1977) 1179–1184. [87] H. Bellon, V. Perthuisot, Données radiométriques 40K–40Ar sur les minéraux potassiques néoformés dans le Trias de la « zones des nappes » (Tunisie septentrionale): mise en évidence de phases thermiques crétacées et tertiaires, C. R. Acad. Sci. Paris Ser. II 296 (1983) 1539–1544.

[88] D. Fournié, Nomenclature lithostratigraphique des séries du Crétacé supérieur au Tertiaire de Tunisie, Bull. Centre Rech. Explor. Prod. Elf–Aquitaine 2 (1978) 97–148. [89] W. Bishop, Eocene and Upper Cretaceous carbonate reservoirs in East Central Tunisia, Oil Gas J. (1988) 137–142. [90] K. Ben Ismaïl Lattrache, C. Bobier, Les événements eustatiques enregistrés dans les dépôts éocènes de la Tunisie centrale et nord-orientale, Les Marges téthysiennes d’Afrique du Nord, Séance spécialisée de la Société géologique de France, 16-17 décembre 1997, 1997, pp. 19 (abstract). [91] A. Charrière, J.P. Saint-Martin, Relations entre les formations récifales du Miocène supérieur et la dynamique d’ouverture et de fermeture des communications marines à la bordure méridionale du sillon sud-rifain (Maroc), C. R. Acad. Sci. Paris Ser. II 309 (1989) 611–614. [92] A. Charrière, Héritage hercynien et Evolution géodynamique alpine d’une Chaîne intracontinentale: le Moyen-Atlas au SE de Fès (Maroc), Thèse Sci., Université de Toulouse, 1990, pp. 589. [93] T. Blondel, Les Séries à Tendance transgressive marine du Miocène inférieur à Moyen de Tunisie centrale, Publications du Département de Géologie et de Paléontologie de l’Université de Genève, 1991, pp. 409. [94] C. Fraissinet, M. Zouine, J.L. Morel, A. Poisson, J. Andrieux, A. Faure-Muret, Structural evolution of the southern and northern central High Atlas in Paleogene and Mio–Pliocene times, in: V. Jacobshagen (Ed.), The Atlas System of Morocco, Springer, Berlin, 1988, pp. 273–291. [95] K. Görler, F.F. Helmdach, P. Gaemers, K. Heissig, W. Hinsch, K. Mädler, W. Schwarzhaus, M. Zucht, The uplift of the central High Atlas as deduced from Neogene continental sediments of the Ouarzazate province, Morocco, in: V. Jacobshagen (Ed.), The Atlas System of Morocco, Springer, Berlin, 1988, pp. 361–404. [96] E. Laville, J.L. Lesage, M. Seguret, Géométrie, cinématique (dynamique) de la tectonique atlasique sur le versant sud du Haut Atlas marocain. Aperçu sur les tectoniques hercyniennes et tardihercyniennes, Bull. Soc. Geol. Fr. 7 (19) (1977) 527–539. [97] M. El Azzouzi, Volcanisme alcalin et calco-alcalin en Contexte de Post-collision continentale Exemple du Maroc, Thèse D. Sci., Université de Rabat, 2002, pp. 289. [98] M. Studer, Tectonique et Pétrographie des Roches sédimentaires, éruptives et métamorphiques de la Région de Tounfite–Tirrhist (Haut Atlas central, Maroc), Thèse, Université de Neuchâtel, 1980, pp. 95. [99] E. Laville, C. Harmand, Mise en place synsédimentaire du massif anorogénique de Talmeste (Haut Atlas central, Maroc), C. R. Acad. Sci. Paris 292 (1980) 1025–1028. [100] R. Bernasconi, Géologie du Haut Atlas de Rich (Maroc), Thèse, Université de Neuchâtel, 1983, pp. 107. [101] E. Laville, B. Fedan, A. Piqué, Déformation synschisteuse jurassique, orogenèse cénozoïque: deux étapes de la structuration du Haut Atlas (Maroc), C. R. Acad. Sci. Paris Ser. II 312 (1991) 1205–1211. [102] J.P. Schaer, F. Persoz, Aspects structuraux et pétrographiques du Haut Atlas calcaire de Midelt (Maroc), Bull. Soc. Geol. Fr. 7 (18) (1976) 1239–1250. [103] Y. Brechbühler, R. Bernasconi, J.P. Schaer, Jurassic sediments of the Central High Atlas of Morocco: deposition, burial and erosion history, in: V. Jacobshagen (Ed.), The Atlas System of Morocco, Springer, Berlin, 1988, pp. 201–217. [104] E. Laville, R. Zayane, J. Honnorez, A. Piqué, Le métamorphisme jurassique du Haut Atlas central (Maroc); épisodes synschisteux et hydrothermaux, C. R. Acad. Sci. Paris Ser. II 318 (1994) 1349–1356. [105] N. Froitzheim, J. Stets, P. Wurster, Aspects of western High Atlas tectonics, in: V. Jacobshagen (Ed.), The Atlas System of Morocco, Springer, Heidelberg, 1988, pp. 219–244.

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208 [106] A. Piqué, L. Ait Brahim, M.H. El Azzouzi, R. Maury, H. Bellon, B. Semroud, E. Laville, Le poinçon maghrébin: contraintes structurales et géochimiques, C. R. Acad. Sci. Paris 326 (1998) 575–581. [107] E. Laville, J.P. Petit, Role of synsedimentary strike-slip faults in the formation of Moroccan Triassic basins, Geology 12 (1984) 424–427. [108] F. Proust, J.P. Petit, P. Tapponnier, L’accident du Tizi n’Test et le rôle des décrochements dans la tectonique du Haut Atlas occidental, Bull. Soc. Geol. Fr. 7 (19) (1977) 541–551. [109] P. Russo, L. Russo, Le grand accident sud-atlasien, Bull. Soc. Geol. Fr. 5 (4) (1934) 375–384. [110] H. Gauthier, Contribution à l’étude géologique des formations post-liasiques des bassins du Dadès et du Haut-Todra (Maroc méridional), Notes Mem. Serv. Geol. Maroc 119 (1957) 212. [111] V. Jacobshagen, R. Brede, R.M. Hauptmann, W. Heinitz, R. Zylka, Structure and post-Paleozoic evolution of the central High Atlas, in: V. Jacobshagen (Ed.), The Atlas System of Morocco, Springer, 1988, pp. 245–271. [112] F. Outtani, B. Addoum, E. Mercier, D. Frizon de Lamotte, J. Andrieux, Geometry and kinematics of the South Atlas front, Algeria and Tunisia, Tectonophysics 249 (1995) 233–248. [113] J.P. Petit, S. Raynaud, J.P. Cautru, Microtectonique cassante lors du plissement d’un conglomérat (Mio–Pliocène du Haut Atlas, Maroc), Bull. Soc. Geol. Fr. 8 (1) (1985) 415–421. [114] J. Lowell, Mechanics of basin inversion from worldwide examples, in: J. Buchanan, P. Buchanan (Eds.), Basin Inversion, Geological Society, London, Special Publication, No. 88, 1995, pp. 39–57. [115] J.L. Morel, E.M. Zouine, A. Poisson, Relations entre la subsidence des bassins moulouyens et la création des reliefs atlasiques (Maroc): un exemple d’inversion tectonique depuis le Néogène, Bull. Soc. Geol. Fr. 164 (1993) 79–91. [116] H.G. Herbig, Synsedimentary tectonics in the northern Middle Atlas (Morocco) during the Late Cretaceous and Tertiary, in: V. Jacobshagen (Ed.), The Atlas System of Morocco, Springer, Berlin, 1988, pp. 321–337. [117] R. Laffite, Etude géologique de l’Aurès, Bull. Carte Geol. Algerie 1 Ser. 15 (1939) 484. [118] J. Savornin, Le massif de Guetiane (S de Setif). Remarques sur la structure parallélogrammique de certains massifs à deux temps principaux d’orogénie, Congrès de l’Association française pour l’Avancement des Sciences, Tunis, 248, 1913. [119] J. Glaçon, Recherches sur la géologie et les gîtes métallifères du Tell sétifien, Publ. Serv. Geol. Algerie N. Ser. 32 (1967) 751. [120] J.M. Vila, La Chaîne alpine d’Algérie orientale et des Confins algéro-tunisiens, Thèse D.Sci., Université P. et M. Curie (1980) 665. [121] J. Savornin, Etude géologique de la région du Hodna et du Plateau sétifien, Bull. Serv. Carte Geol. Algerie 2 Ser. 7 (1920) 499. [122] A. de Spengler, Etude géologique de la région de Bordj bou Arreridj (Plateau sétifien), Rapp. Int. S.N. Repal, Algeria (1954) 89. [123] C. Martinez, M. Turki, R. Truillet, La signification des plis d’orientation subméridienne dans l’Atlas tunisien centroseptentrional, Bull. Soc. Geol. Fr. 6 (5) (1990) 843–852. [124] M. Rabhi, Contribution à l’Etude stratigraphique et Analyse de l’Evolution géodynamique de l’Axe nord–sud et des Structures avoisinantes (Tunisie centrale), Thèse, Université de Tunis II (1998). [125] J.P. Richert, Mise en évidence de quatre phases tectoniques successives en Tunisie, Notes Serv. Geol. Tunisie 34 (1971) 115–125. [126] M. Boccaletti, G. Cello, L. Tortorici, Structure and tectonic significance of the north–south axis of Tunisia, Ann. Tecton. 2 (1988) 12–20. [127] P. Tricart, L. Torelli, A. Argnani, F. Rekhiss, N. Zitellini, Extensional collapse related to compressional uplift in the Alpine chain off northern Tunisia (Central Mediterranean), Tectonophysics 238 (1994) 317–329.

207

[128] S. Rebaï, Recent tectonics in Northern Tunisia: coexistence of compressive and extensional structures, Ann. Tecton. 7 (1993) 129–141. [129] H. Philip, J. Andrieux, M. Dlala, L. Chihi, N. Ben Ayed, Evolution tectonique mio–plio–quaternaire du fossé de Kasserine (Tunisie centrale): implications sur l’évolution géodynamique récente de la Tunisie, Bull. Soc. Geol. Fr. 8 (2) (1986) 559–568. [130] L. Chihi, Les Fossés néogènes à quaternaires de la Tunisie et de la Mer pélagienne: leur Etude structurale et leur Signification dans le Cadre géodynamique de la Méditerranée centrale, Thèse de Doctorat, Université de Tunis II (1995). [131] M. Dlala, M. Hfaiedh, Le séisme de Metlaoui: une tectonique active en compression, C. R. Acad. Sci. Paris Ser. II 317 (1993) 1297–1302. [132] H. Zouari, Evolution géodynamique de l’Atlas centro-méridional de la Tunisie: Stratigraphie, Analyse géométrique, cinématique, Thèse de Doctorat, Université de Tunis II (1995) 378. [133] F. Zargouni, Tectonique de l’Atlas méridional de Tunisie. Evolution géométrique et cinématique des Structures en Zone de Cisaillement, Thèse D.Sci., Université de Strasbourg (1985) 296. [134] D. Frizon de Lamotte, H. Ghandriche, I. Moretti, La flexure saharienne: trace d’un chevauchement aveugle post-Pliocène de flèche plurikilométrique au Nord du Sahara (Aurès, Algérie), C. R. Acad. Sci. Paris Ser. II 310 (1990) 1527–1532. [135] M. Dlala, S. Rebai, H. Philip, Approche sismotectonique des caractères de la sismicité en Tunisie, C. R. Acad. Sci. Paris Ser. IIa 319 (1994) 573–579. [136] R. Guiraud, J.C. Doumnang, S. Carretier, S. Dominguez, Evidence for a 6000 km length NW–SE-striking lineament in northern Africa: the Tibesti Lineament, J. Geol. Soc. London 157 (2000) 897–900. [137] G. Creuzot, E. Mercier, J. Ouali, P. Tricart, La tectogenèse atlasique en Tunisie centrale: apport de la modélisation géométrique, Ecl. Geol. Helv. 86 (1993) 609–627. [138] F. Kamoun, B. Peybernès, P. Fauré, Evolution paléogéographique de la Tunisie saharienne et atlasique au cours du Jurassique, C. R. Acad. Sci. Paris Sci. Terre Planetes 328 (1999) 547–552. [139] M. Mattauer, P. Tapponnier, F. Proust, Sur les mécanismes de formation des chaînes intracontinentales. L’exemple des chaînes atlasiques du Maroc, Bull. Soc. Geol. Fr. 7 (19) (1977) 521–526. [140] J. Warme, Jurassic carbonate facies of the central and eastern High Atlas rift, Morocco, in: V. Jacobshagen (Ed.), The Atlas System of Morocco, Springer, Berlin, 1988, pp. 169–199. [141] J. Canérot, A. Souheil, K. El Hariri, A. Bouchouata, A. Gharib, Geodynamic evolution of the central High Atlas (Morocco), 30th International Geological Congress, Peking, 1996, pp. 190 (F1, abstract). [142] A. Bouchouata, J. Canérot, A. Souhel, A. Gharib, Le rift atlasique sur la transversale de Beni Mellal (Haut Atlas central), 14th International Meeting of Sedimentology, Marrakech, April, 1993, pp. 78–79 (abstract). [143] J. Fairhead, Mesozoic plate tectonic reconstructions of the Central South Atlantic Ocean: the role of the West and Central rift system, Tectonophysics 155 (1988) 181–191. [144] J. Mascle, E. Blarez, M. Marinho, The shallow structures of the Guinea and Ivory Coast–Ghana transform margins: their bearing on the Equatorial Atlantic Mesozoic evolution, Tectonophysics 155 (1988) 193–209. [145] R. Guiraud, J.C. Maurin, Le rifting en Afrique au Crétacé inférieur: synthèse structurale, mise en évidence de deux étapes dans la genèse des bassins, relations avec les ouvertures océaniques péri-africaines, Bull. Soc. Geol. Fr. 162 (1991) 811–823. [146] R. Guiraud, J.C. Maurin, Early, Cretaceous rifts of Western and Central Africa: an overview, Tectonophysics 213 (1992) 153–168. [147] P. Rabinowitz, J. Labrecque, The Mesozoic south Atlantic ocean and evolution of its continental margins, J. Geophys. Res. 84 (1979) 5973–6002.

208

A. Piqué et al. / Geodinamica Acta 15 (2002) 185–208

[148] L. Savostin, J.C. Sibuet, L. Zoneshain, X. Le Pichon, M.J. Roulet, Kinematic evolution of the Tethys belt from the Atlantic ocean to the Pamirs since the Triassic, Tectonophysics 123 (1986) 1–35. [149] R. Guiraud, W. Bosworth, Senonian basin inversion and rejuvenation of rifting in Africa and Arabia: synthesis and implications to plate-scale tectonics, Tectonophysics 282 (1997) 39–82. [150] E. Carminati, M. Wortel, W. Spakman, R. Sabadini, The role of slab detachment processes in the opening of the western-central Mediterranean basins: some geological and geophysical evidence, Earth Planet. Sci. Lett. 160 (1998) 651–665. [151] J. Platt, J.I. Soto, M. Whitehouse, A. Hurford, S. Kelley, Thermal evolution, rate of exhumation, and tectonic significance of metamorphic rocks from the floor of the Alboran extensional basin, western Mediterranean, Tectonics 17 (1998) 671–689. [152] E. Laville, Comment to “Role of the Atlas mountains (northwest Africa) within the African–Eurasian plate-boundary zone” by F. Gomez, W. Beauchamp, M. Barazangi, Geology 28 (2000) 775–778. [153] F. Gomez, W. Beauchamp, M. Barazangi, Reply to Laville’ comment, Geology 96 (2002) 96.

[154] G. Schwarz, H.G. Mehl, F. Ramdani, V. Rath, Electrical resistivity structure of the eastern Moroccan Atlas system and its tectonic implications, Geol. Rundsch. 81 (1992) 221–235. [155] R. Bracène, A. Bellahcène, D. Bekkouche, E. Mercier, D. Frizon de Lamotte, The thin-skinned style of the South Atlas front in Central Algeria, in: D. Macgregor, R. Moody, D. Clark-Lowes (Eds.), Petroleum Geology of North Africa, Geological Society of London, Special Publication, No. 132, 1998, pp. 395–404. [156] W. Beauchamp, R. Allmendiger, M. Barazangi, A. Demnati, M. El Alji, M. Dahmani, Inversion tectonics and the evolution of the High Atlas mountains, Morocco, based on a geological–geophysical transect, Tectonics 18 (1999) 163–184. [157] V. Jacobshagen, K. Görler, P. Giese, Geodynamic evolution of the Atlas system (Morocco) in Post-Paleozoic times, in: V. Jacobshagen (Ed.), The Atlas System of Morocco, Springer, Berlin, 1988, pp. 481–499. [158] J. Beauchamp, Triassic sedimentation and rifting in the High Atlas (Morocco), in: W. Manspeizer (Ed.), Triassic–Jurassic Rifting. Continental Breakup and the Origin of the Atlantic Ocean and Passive Margins, 1988, pp. 477–497.