Etude géologique de la partie occidentale du massif hercynien des Rehamna septentrionales (Meseta marocaine). Lithostratigraphie, plissements et métamorphisme, chevauchements et nappes.

Abstract

The northern Rehamna massif, halfway between Marrakech to the south and Casablanca to the north, constitutes part of the Hercynian segment of the coastal Meseta, in which it is exposed as a nucleus surrounded by post-orogenic sediments of Autunian (?), PermoTriassic (?) and Cretaceous age. The massif is composed of two great structural domains separated by an important thrust zone. The western domain is a little-deformed Cambro-Ordovician foreland ; it is considered to represent a relatively autochthonous terrain. Its western margin is one of imbricated wedges, the so-called "wedges of Oued Tarfa". The eastern domain is composed of a thick sequence of superposed thrust sheets, including those of Sidi Abdallah (with Ordovician to Devonian rocks), Skikirat (with Devonian ta Visean rocks), and Gada Jenabia (with Visean rocks only). These units have undergone a polyphase tectonic-metamorphic evolution which stands in strong contrast with that of the western domain.The western domain comprises 6 sedimentary units of detrital character, dated Middle Cambrian to Arenigian. Their paroxysmal deformation resulted in the formation of Fl-folds characterized by their approximately N-S orientation, their km-scale amplitude and half-wave length, their upright cylindrical nature and law angle of plunge, and the contemporaneous development of cleavageThe wedges of Oued Tarfa are composed of rocks identical to those of the western domain. The F1 structures are of the same type, but were truncated and thrust NW-ward during the same period of tangential deformation. The width of caver is 2-6 km, depending on the wedge.The Sidi Abdallah thrust sheet is composed of 10 formations ranging in age from Ordovician (?) to Devonian. Their paroxysmal deformation gave rise to a symmetric to overturned F1-folds with NW vergence and steep NE plunge, formed contemporaneous with metamorphism and the development of schistosity. The intensity of deformation and metamorphism increases toward the E, where F2 and F3-folds make their appearance. The first thrust episode (φ1) placed the Sidi Abdallah thrust unit on top of the western domain and imbricated the unit. The displacement was ta the NW and amounted to a minimum of 8 km. A second thrust episode (φ2) with S-ward displacement in turn gave rise to minor imbrications.The Skikirat thrust sheet is composed of nonmetamorphic rocks of Lower to Middle Devonian and upper Visean age. The first tectonic event appears to have led to the development of olistoliths ; the subsequent paroxysmal deformation gave rise to F1-folds whose orientations are at present highly variable and which are characterized by a weak-S1 cleavage. The first episode of tangential deformation caused displacement towards the NW, but is not widely observed. The second thrust event carried the Skikirat unit S-ward on top of the preceding units including the western domain. The displacement amounts to several km, locally as much as tens of km.The evidence of two distinct thrust periods in the northern Rehamna massif, following their discovery in the Jebilet massif and the southeastern Rehamna massif, demonstrates that these events played a highly important role in the late Hercynian tectogenesis of the Moroccan Meseta.Le massif hercynien des Rehamna septentrionales est constitué de deux grands domaines structuraux actuellement séparés par un important contact de recouvrement. A l’Ouest s’observe le domaine occidental autochtone au moins relatif. C’est un avant-pays à matériel cambro-ordovicien puissant, simplement déformé en vastes plis droits. La marge orientale de ce domaine est écaillée (écaille de l’oued Tarfa). A l’Est s’étend un domaine oriental, constitué de plusieurs grandes unités chevauchantes dont les deux premières ont été étudiées : l’unité de Sidi Abdallah à matériel ordovico(?) – dévonien et au-dessus l’unité de la Skikirat à matériel dévono-viséen.Ces unités ont subi une évolution tectono-métamorphique polyphasée qui contraste très fortement avec le domaine occidental.Le serrage hercynien a d’abord donné lieu à des épisodes de plissement P1 synschisteux et synmétamorphes, P2 et P3 (kinks). Ensuite un premier épisode de tectonique tangentielle a amené sur le domaine occidental l’unité de Sidi Abdallah entraînant les écailles de l’oued Tarfa para-autochtone. Les déplacements, du Sud-Est vers le Nord-Ouest sont au minimum décakilométriques.Un second épisode de tectonique tangentielle mais à déplacement du Nord vers le Sud, avec une flèche minimale également décakilométrique, conduit à la superposition de l’unité de la Skikirat à la fois sur le domaine occidental, les écailles de l’oued Tarfa et de l’unité de Sidi Abdallah.Après avoir été mise en évidence dans le Jebilet et dans les Rehamna méridionales, des deux épisodes tangentiels sont ainsi également connus dans le Rehamna septentrionales. Il s’agit donc là de phénomènes importants, voire majeurs, dans la structuration hercynienne de la Meseta marocain