Alpine tectono-thermal evolution of the North African passive paleo-margin incorporated in the Western Rif belt (Northern Morocco)

Located in Northern Morocco, the Rif fold and-thrust belt is mainly made by the remnant of the north African paleo-passive margin and its sedimentary infill. We present in this contribution new field observations combined with paleo-thermal analysis to investigate the formation of the Rif orogenic wedge. Three structural domains are recognized from north to south, namely, the Alboran domain assigned to a meso-Mediterranean continental terrane, the Maghrebian flysch domain that corresponds to the sedimentary cover of the Maghrebian Tethys and the External domain (namely Intrarif, Mesorif and Prerif) that belongs to the former north African margin. The Rif fold and-thrust belt suffered an important Cenozoic Alpine compressional deformation starting from the Late Oligocene-Early Miocene, as a consequence of the closure of the Maghrebian Tethys and the westward translation and docking of the Alboran Domain onto the African margin.

To define the evolution and geometry of thrust sheet stacking and their burial-exhumation paths, a NE-SW regional transect crossing the Maghrebian flysch and the External domains is presented and discussed. A set of 32 samples have been collected for paleo-thermal analysis. The methodological approach consists in combining petrography and Raman micro-spectroscopy on dispersed organic matter, X-ray diffraction of clay minerals and 1D thermal modelling with viable cross section reconstruction and field structural survey.

The highest thermal maturity values along the section (1.00 and 1.15 Ro%) are concentrated in the Cretaceous Intrarif sub-domain (Loukkos and Tangier Intrarifain sub-units) that are structurally squeezed between the Maghrebian flysch domain and the Mesorifain sub-domain. The relationship between organic and inorganic paleo-thermal indicators plotted on Hillier diagram show a thermal signature for the Intrarifain sub-domain typical of continental rift thermal regime. The thermal evolution of the Tangier sub-unit, tectonically overlain by the Numidian-like sandstones has been modelled. The model shows a thermal jump between the two juxtaposed rock units indicating an allochthonous origin of the Numidian-like sandstones, probably detached from the Maghrebian Flysch domain. In the Mesorif sub-domain, data plots on Hillier diagram indicate a continental rift heating regime except for the Lower Miocene Zoumi siliciclastics at the top of it, cropping out between Intrarif and Mesorif sub-domains that falls in a very cold thermal regime, typical of synorogenic basins. The structural relationships between the Cenozoic Zoumi basin and its substratum (Upper Jurassic-Lower Eocene) shows an unconformity where the Paleocene-Eocene is missing, probably indicating a pre-Oligocene compressive phase.

These evidences constrain the geological timing of the Rif belt structuration. According to new models, the whole external Rif deformed between the Early Langhian and Late Tortonian with the front of the chain placed at the boundary between Intrarif and Mesorif, where the Zoumi basin developed during the Late Serravallian-Early Tortonian times.