The intracontinental High Atlas belt: geological overview and pending questions

Abstract

The High Atlas intracontinental belt is an inverted rift formed along the Northwest African Craton, between the Central Atlantic Ocean and the Africa-Eurasia plates’ boundaries. Due to its particular location at this triple junction, the Moroccan High Atlas is a good example to investigate the tectonic and geodynamic complexity within continental interiors. The present paper aims to give an overview of the actual understanding of the post-Variscan tectonic and structural history of the High Atlas belt and to shed light on some of the most actively debatable questions relating to the geodynamic context of its uplifts. The High Atlas basin Alpine history started with a period of passive rifting during the Middle-Late Triassic-Early Liassic resulting in a complex rift system actuated by the reactivation of Variscan NNE to ENE-trending structures in the framework of a global NW-SE extension. The Late Triassic rift basins were filled by thick salt-bearing red-beds whose post-rift extensive halokinetics has a considerable impact on the structural architecture of the Western and Central High Atlas. Extensional deformation was renewed in the Central High Atlas at the Late Liassic-Early Bajocian associated with the drowning of Liassic carbonate platform coevally with salt mobilization which led to the initiation of NE to ENE-elongated narrow diapiric ridges. By the Middle Jurassic, a major part of the High Atlas basin was submitted to a widespread post-rift epeiorogenic upward motion that lasted up until the Lower Cretaceous, and to which is associated the occurrence of transitional to moderately alkaline magmatism that was emplaced within the established ridges across basement faults. The driving geodynamic mechanisms and the tectonic setting of this uplift are poorly known but the large extent of the exhumed lands in both margins of the Central Atlantic and the synchronicity between the upward movements and the drastic increase of the Central Atlantic Ocean spreading rate suggests a possible causal link between the Central Atlantic dynamics and the Middle Jurassic-Lower Cretaceous post-rift exhumation. The Late Cretaceous signals the onset of the convergence between Africa and Eurasia and the consequent Alpine orogeny. The effective inversion of the Atlas paleo-rift begun during the Eocene and accentuated during the Neogene in an intra-continental environment. The western segment of range witness a basement-involved thick-skinned faulting and folding style, whereas thin-skinned deformation detached mostly on Upper Triassic-Early Liassic evaporitic layers is focused in external forelands. In the Central High Atlas, the salt-involved Cenozoic deformation triggered the evaporites squeezing and extrusion and the inversion of the pre-structured magmatic cored ridges into compressional anticlines. During the Alpine orogeny, the High Atlas orogen underwent weak crustal shortening with the development of a moderate crustal root that resides over a highly elevated Lithosphere-Asthenosphere Boundary. The most prevailing view predicts the occurrence of a hot mantle anomaly, superimposed to the collisional deformation, beneath the Atlas range that maintains the high topography featuring the High Atlas orography. Although advanced studies have been performed, several questions regarding the kinematic history, the origin of the vertical movements, the role played by the week salt layers during both extensional and compressional deformation stages, and the factors controlling the selective inversion of the paleo-rift normal faults and the deep geometry of the range’s major faults, are still not well resolved and deserve to be deeply re-investigated.