Chronolige et modalités du rifting triasico-liasique à la marge sud-ouest de la Téthys alpine (Moyen Atlas et Haute Moulouya, Maroc): corrélations avec le rifting atlantique : simultanéité

• Autores: Driss Ouarhache, André Charriere, Françoise Chalot Prat, Mohamed El Wartiti
• Localización: Bulletin de la Société Géologique de France, ISSN 0037-9409, Vol. 183, Nº. 3, 2012, págs. 233-249
• Idioma: francés
• Títulos paralelos:
  • Triassic to early Liassic continental rifting chronology and process at the southwest margin of the Alpine Tethys (Middle Atlas and High Moulouya, Morocco): correlations with the Atlantic rifting, synchronous and diachronous
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• Resumen
  • English

    In Morocco from Triassic to Early Liassic times, continental Atlantic and Tethys rifting processes have interfered (fig. 1). On the future southwest tethyan margin, the "Atlasic", continental rift formation was recorded in the "Middle Atlas" and "High Moulouya" basins (MAHM) within the central part of the Moroccan Atlasic domain. The study of their sedimentary infilling in relationships with tectonic and volcanism, from Upper Trias to Lower Lias, enables to distinguish several major steps of the rifting process on this tethysian margin segment.

    Successive rifting steps (fig. 2 and 5) R1, the Carnian-Norian first generation sedimentary basins (G1). The opening of the Triassic first intracontinental basins (SE of Tazekka, Kerrouchen,) resulted in extensive processes along some tectonic sutures (NMAF and SMAF). These first half grabbens consist in syn-rift series made of continental detritic deposits (see plate I, ph. 1-2): piedmont conglomerates and arkoses, fluvial sandstones and argillaceous siltstones. Later, the whole domain was covered with rather homogeneous lagunal-marine sediments corresponding to a slight sea water rising and/or a slight subsidence associated with a first post-rift relaxation. Nevertheless, throughout this period, a number of faults remained active generating more subsiding zones with clays and evaporites and more stable zones with clays and siltstones (fig. 3 and 6A).

    R2, the Triassic-Liassic basaltic trapps (fig. 5C), synchronous with major crustal fractures. The emplacement of basaltic trapps all over the atlasic domain was coeval with the beginning of the formation of the Central Atlantic magmatic province. In the studied area, the trapp sequence (up to 150 m), (pl. I, ph. 3-4) include about twenty aerial to subaqueous (fresh water, exceptionally sea water) lava-flows (5 m to 10 m in thickness).

    R3, the Early Liassic volcano-sedimentary basins of second generation (G2). At that time, the whole domain emerged. Continental erosion became active close to some old and new faults. A new extensive or transtensive tectonic event lead to the dislocation of basaltic trapps and the formation of intracontinental grabbens along four major faults (fig. 5D). This "second generation" basins (fig. 6B) either superposed to the "first generation" basins (Boufekrane; fig. 4B) with a lagunal--marine sedimentation, or could be independent and remained strictly continental (Taggounite, Bou Tkoubay, Ahouli). The corresponding series formed within new syn-rift basins. They always included a continental positive megasequence with proximal deposits, then fluvio-lacustrine deposits and, at last, supratidal carbonates (pl. I, ph. 5 to 7). Within this megasequence, when the sedimentation was detritic and fine, pyroclastic products interstratified. They consisted in ash and lapilli tufs and tuffites (pl. II), which means that volcanic explosions were subaerial while deposits were either subaerial or subaquatic and sometimes reworked in the fluvio-lacustrine sediments (fig. 4A). The volcanic craters were proximal, rather localized and probably formed at some fault crosscutting. These volcano-sedimentary basins formed new syn-rift basins. The following marine post-rift event (fig. 5E) began during the Upper Sinemurian to Lower Carixian times.

    Correlations (fig. 7) At the scale of Morocco, this three-step rifting evolution is dramatically different from this of the southwestern Atlasic domain, which includes both the westwards Atlasic rift (Telouet, Haut Atlas de Marrakech) and the edge of the Atlantic rift (Argana, Haut Atlas occidental).

    Indeed the eastwards MAHM pre-trapp basins (G1) recorded only one sedimentary megasequence (probably Upper Trias), whereas several sedimentary sequences (Upper Permian to Upper Trias) formed successively westwards. The R1 rifting occurred later eastwards than on both the western Tethyan and Atlantic domain.

    The post-trapp basins (G2) du MAHM, where the only one sedimentary megasequence was synchronous with explosive basaltic volcanism, are known only in this area. By the fact, whereas the western sites were the witnesses of a tectonic relaxation, a second generation of syn-rift basins, synchronous with explosive eruptions, was individualizing eastwards. Thus the Early Liassic evolution was also dramatically different eastwards, as the rifting (R3) went on but below sea level.

    Conclusions Our study evidences that in the Middle Atlas and the High Moulouya domains, a second continental rifting phase associated with explosive basaltic eruptions occurred after the enormous fissural basaltic eruptions and before the post-rift marine transgression. The Atlasic rift formation is necessarily polyphased from Trias to Early Lias with two sedimentary syn-rifts periods R1 and R3 separated by a dramatic eruptive phase R2.

    The comparison of the Trias-Lias geological evolution of the MAHM with that of the western zones of the Atlasic domain highlights the following points:

    1-- the genesis of the Atlantic and Tethys continental rifts probably began synchronously westwards during Upper Permian times. The continental rifting process went on eastwards more probably from Upper Triassic times;

    2-- the fact that this continental rifting process hold on after the huge effusive event and until the Jurassic marine transgression, is a specificity of the central and eastern parts of the Atlasic rift. Besides, this rifting activity went on but below sea level and from time to time until Middle Jurassic.

    It comes that the Atlasic continental rifting was active a more long time eastwards (Middle Atlas and High Moulouya) than westwards close to the Central Atlantic margin.

  • français

    Apparu à l'est du rift continental atlantique, un rift continental téthysien s'est développé sur le domaine atlasique du Maroc au cours du Trias et du début du Jurassique. Dans le transect étudié du Moyen Atlas et de la Haute Moulouya (MAHM), ce début du rifting téthysien, le "rifting initial", comporta 3 étapes majeures.

    Au Trias supérieur, apparut une première génération de bassins syn-rift (sr1) continentaux et détritiques, associés au rejeu extensif de certaines sutures hercyniennes ; cette phase se poursuivit avec une transgression laguno-marine généralisée dans un stade post-rift (pr1) au Carnien supérieur-Norien.

    Au voisinage de la limite Trias-Lias, se produisit une effusion de trapps basaltiques aériens à subaquatiques ; la région subit ensuite une phase d'érosion.

    Au début du Lias, se différencia une deuxième génération de bassins syn-rift (sr2) continentaux ou laguno-marins, extensifs ou transtensifs, associés à un volcanisme explosif localisé sur quatre zones faillées (faille d'Adarouch, accident du Tizi n'Trettène, accident Sud Moyen atlasique, faille de Ksabi-Ahouli) ; cette phase se clôtura avec l'arrivée de la mer dans un stade post-rift (pr2) débutant au Sinémurien supérieur-Carixien inférieur.

    La zone du MAHM eut une histoire différente des régions occidentales du domaine atlasique, qu'il s'agisse d'un autre segment du rift atlasique (Haut Atlas de Marrakech) ou d'une bordure du rift atlantique (Haut Atlas occidental).

    Pendant le rifting ante-trapp, alors que ces bassins occidentaux ont été structurés par plusieurs séquences tectono-sédimentaires successives étagées du Permien supérieur au Trias supérieur, les bassins du MAHM n'enregistrèrent qu'une seule mégaséquence sédimentaire triasique. Ainsi, les rifts continentaux atlantique et Ouest atlasique furent initiés, simultanément (?), dès le Permien supérieur, tandis que le rift continental de l'Atlas central et oriental n'apparut vraisemblablement qu'au Trias supérieur. La déchirure du rift atlasique migra ainsi vers l'est de la fin du Primaire au début du Secondaire.

    Durant la période post-trapp, alors que les bassins occidentaux étaient le siège d'un calme tectonique relatif, une fracturation majeure affecta le MAHM, générant de nouveaux bassins sédimentaires associés à un second épisode volcanique, de nature explosive cette fois. Cette poursuite du rifting, après les dernières effusions basaltiques et avant l'arrivée de la mer jurassique, préfigura les découpages tectoniques qui continuèrent de se manifester dans les parties centrale et orientale du rift atlasique jusqu'au Jurassique moyen.