Tectonics of the Anti-Atlas of Morocco
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Tectonics of the Anti-Atlas of Morocco

By Martin Burkhard, Séverine Caritg, Urs Helg, Charles Robert-Charrue,

Abderrahmane Soulaimani

Institut de géologie, université de Neuchâtel, rue Émile-Argand 11, CP 2, 2007 Neuchâtel, Switzerland

BUWAL, Bundesamt für Umwelt, Wald und Landschaf, 3003 Bern, Suisse

Faculté des sciences Semlalia, université Cadi-Ayyad, av. Moulay-Abdellah, BP S20, Marrakech, Maroc

Received 4 October 2005; accepted after revision 17 November 2005

Available online 4 January 2006

Written on invitation of the Editorial Board

Abstract

The Anti-Atlas is reviewed and examined in the light of its geodynamic significance as a Palaeozoic basin and fold belt. Shortening is accommodated by polyharmonic buckle folding of the cover in a thick-skinned fashion without the development of any significant thrust/duplex systems. The Anti-Atlas is heavily inverted deep intracratonic basin, rather than a former passive margin of the Palaeo-Tethys Ocean. Inversion took place in Late Carboniferous to Early Permian times. Main shortening directions changed from NW–SE to north–south and maybe NE–SW through time, leading to the development of dome and basin patterns on scales from 100 m to 10 km. To cite this article: M. Burkhard et al., C. R. Geoscience 338 (2006).

© 2005 Académie des sciences. Published by Elsevier SAS. All rights reserved.

Résumé      

Tectonique de l’Anti-Atlas marocain. L’Anti-Atlas est revu et examiné sous l’angle de sa signification géodynamique comme bassin paléozoïque et comme chaîne plissée paléozoïque. Le raccourcissement est accommodé par le plissement polyharmonique de la couverture, avec une nette implication du socle. Aucun système significatif de chevauchement ni duplex ne s’est développé.

L’Anti-Atlas est un bassin intracratonique fortement inversé plutôt qu’une partie de la marge passive de la Paléotéthys. L’inversion doit dater du Carbonifère tardif/Permien précoce. La direction du raccourcissement a changé au cours du temps depuis une direction NW–SE vers une direction nord–sud et peut-être même NE–SW, ce qui conduit à la formation de figures d’interférences de plis en dômes et bassins aux échelles allant de 100 m à 10 km. Pour citer cet article : M. Burkhard et al., C. R. Geoscience 338 (2006).

© 2005 Académie des sciences. Published by Elsevier SAS. All rights reserved.

1. The Anti-Atlas of Morocco – Introduction

The Anti-Atlas fold belt of the south-western Moroccan desert (Figs. 1 and 2) offers vast expanses of beautifully exposed bare outcrops. This is due to a recent phase of uplift and concomitant erosion that led to a rejuvenation of relief with summits of 2500 m and higher [35,47]. Geomorphologists have coined the French term relief appalachien to characterize the pattern of deeply eroded fold trains, and there is indeed a striking similarity between the geomorphic expression of the Appalachian Valley and Ridge and the Anti-Atlas.

Both chains are external parts of the larger Variscan Appalachian–Ouachita–Mauritanides orogen (Fig. 1).

The relationship between the Anti-Atlas fold belt and the internal parts of this orogen remains to be elucidated in terms of tectonic style, timing and geodynamics.

At first sight, the Anti-Atlas shares many common features with its American counterpart of the Valley and Ridge in general and with the Alleghany Basin in particular.

Both are located on the craton side of the orogen involving a thick and fairly regular layer cake of mostly shallow marine Palaeozoic sediments. Both fold provinces also have their non-folded time-equivalent intracratonic basins further inland: Michigan and Illinois on the American side, Taoudenni, Tindouf and others on the African side. On closer inspection, however, and in stark contrast to the frontal Appalachian chain, the Anti-Atlas fold belt does not easily conform to the standard anatomy of foreland fold-and-thrust belts worldwide [77]. The most striking difference is the existence of major basement domes at a very short distance behind the deformation front [76]. Similarities exist with Wind River-style basement uplifts of the frontal Rocky Mountains, but in the Anti-Atlas, the basement uplifts occur amidst a tightly folded thick Palaeozoic cover series.

The style of cover folding is quite unique too, with a dominance of upright detachment folds and a complete absence of any thrusting and duplex structures [47], with the exception of the westernmost parts of the Anti-Atlas along the Atlantic coast [8,65]. There is no thin skinned basal décollement level and the western Anti-Atlas does not conform to a foreland fold and thrust belt system in the sense of Boyer and Elliott [15].

An exhaustive review of the geology of the Anti-Atlas has been presented by Michard [64]. While many authors have interpreted the Anti-Atlas fold belt in terms of a predominance of strike-slip movements [43, 61, 74, 99], recent structural analyses depict the western Anti-Atlas folds as highly cylindrical frontal folds related with a ‘head-on’ collision in a NNW–SSE direction [20, 47, 91].

In this paper the Anti-Atlas system is revisited in light of recent data regarding the Precambrian basement, the Palaeozoic cover, structural observations within the Anti-Atlas belt as well as plate tectonic reconstructions on a global scale. Questions of particular interest concern the Palaeozoic sedimentary basin history and subsidence mechanisms as well as the evolution of this Anti-Atlas basin near the border of the West-African Craton through time.

Fig. 1. The Anti-Atlas is shown in its larger context at the End of the Palaeozoic [80]. Isopach contours for total sedimentary thickness are given in kilometres for those Palaeozoic basins which have not or only weakly been involved in inversion tectonics [102]. The same colour shades are schematically superimposed onto the Anti-Atlas fold belt in order to illustrate the estimated depth of >10 km of this basin prior to inversion.

Alleghenian basement uplifts are shown in blue (inspired by the Appalachian ‘Blue ridge’). Internal, metamorphic and in part older portions of the Applachian–Mauritanides–Moroccan Meseta are coloured in green and pink tones [44, 73].

Fig. 1. Situation de l’Anti-Atlas par rapport à la chaîne des Appalaches à la fin du Paléozoïque [80]. Les contours isopaques en kilomètres sont donnés pour les bassins sédimentaires paléozoïques qui n’ont subi que peu ou pas de déformation [102]. Le même code couleur est superposé à la chaîne plissée de l’Anti-Atlas afin d’indiquer la profondeur estimée de ce bassin avant l’inversion. Les massifs de socle alléghaniens sont indiqués en bleu

(Inspiré du Blue Ridge appalachien). Les parties internes, métamorphiques et plus anciennes, de la chaîne des Appalaches–Mauritanides–Méséta marocaine sont colorées en vert et en rose [44,73].

Fig. 2. Geologic overview map of the Anti-Atlas, compiled from the geologic map series (1:200 000) of the ‘Service géologique du Maroc’ [105–114]. NW–SE transects, used for the compilation of Fig. 4 are indicated along the southern border. A cross section through the Adrar Zouggar anticlinorium is shown in Fig. 3.

Fig. 2. Carte géologique de l’Anti-Atlas, synthétisée à partir de la série des cartes 1:200 000 du Service géologique du Maroc [105–114]. Les transects NW–SE, utilisés pour la compilation de la Fig. 4, sont indiqués le long de la bordure sud. Une coupe à travers l’anticlinorium de l’Adrar Zouggar est donnée sur la Fig. 3.

1.1. The basement

The Anti-Atlas basement is a complex assemblage of crystalline, metamorphic and sedimentary rocks. Note that we use the term basement here in the sense of the petroleum geologists [58], including all rocks older than the Gondwana Megacycle. The oldest rocks of

The West-African Craton (WAC) is granitoids, gneisses and a complex series of metamorphic rocks, attributed to the Eburnean orogeny at around 2000 Ma [2, 96, and 98]. The final assembly of most of the African continental crust takes place during the Panafrican orogeny, lasting roughly from 700 to 600 Ma [46]. Remnants of a Panafrican suture zone are present as a dismembered ophiolite series in the Bou Azzer inlier of the central Anti-Atlas [45, 55, 81, 96] – a structure recently reinterpreted as an aulacogen within the WAC [32].

Elsewhere in the Anti-Atlas, the Panafrican event left a more subtle imprint in the form of strike-slip shear zones and thrusts [43]. Post-Panafrican continental extension is well documented for the entire Anti-Atlas region [72, 89] where indications for synsedimentary tectonics are found in the clastic series of the Saghro-group (PII3) [92], the Ouarzazate group (PIII) and, progressively fading upward within lowermost Cambrian series.

The interpretation of events during the Late Neoproterozoic (600–540 Ma) is still a matter of debate, due in part, at least, to the scarcity of reliable absolute ages. The significance of the basement/cover relationships and the geodynamic context are also still open to discussion: interpretations range from syn-orogenic, Late Panafrican molasse series shed in a collision context, to post-orogenic extension and collapse with the formation of tilted blocks and halfgrabens [55, 87, and 91].

In a most recent interpretation [90], based on the type and volumes of volcanic rocks associated with the PIII Ouarzazate series, the Late Proterozoic extension event is compared with a basin and range extensional setting, with an important production of lower crustal melts.

Soulaimani and Piqué [88] even go so far as to propose the present-day Anti-Atlas boutonnières as being reactivated former metamorphic core complexes.