The structure of the Central-Eastern External Rif (Morocco); Poly-phased deformation and role of the under-thrusting of the North-West African paleo-margin

Abstract

The Rif belt (Northern Morocco) forms the western edge of the Alpine-Himalayan orogenic system, resulting from the progressive closure of the Tethys oceanic domain between the Africa and Eurasia plates. The External Rif, topic of this paper, comprises three paleogeographic zones, namely the Prerif, Mesorif and Intrarif, inherited from the zonation of the North-West African paleo-margin from proximal to distal domains, respectively. Its current architecture displays tectonic windows (belonging to the Mesorif) at depth supporting far-travelled thrust-sheets (namely, the Senhadja and Higher Nappes of Mesorif and Intrarif origin, respectively). A puzzling aspect of the Mesorif structure is the coexistence of metamorphic and non-metamorphic units within the tectonic windows and Senhadja Nappes. In particular, the latter presents a chaotic structure, which origin remains under debate. We integrate previous studies and new data (field observations, geological mapping and thermochronology results) to reassess the structure of the External Rif and propose an up-dated interpretation of its Cenozoic evolution. In the Prerif and Mesorif, we highlight a poly-phased tectonic evolution occurring before and after a major unconformity (the “Mesorif Unconformity”). This unconformity seals a pre-Miocene compressional event that we relate to the under-thrusting of the North-African paleo-margin below the Maghrebian Tethys (the former Tethys branch located between South Iberia and North Africa). The MP-LT metamorphism, recorded in the metamorphic tectonic window of the Temsamane Massif, is a direct vestige of this process. After the “Mesorif Unconformity”, i.e. during the Miocene, the regional geodynamics is ruled by the westward translation of the Alboran Domain and the coeval out-of-sequence deformation of the “oceanic” units in front of it (Flyschs and Intrarif Units, the most internal parts of the External Rif). This process directly results from the subduction of the Maghrebian Tethys below the Alboran Domain. The docking of the Intrarif Units against the already exhumed Temsamane Units and Senhadja Nappes generated an uplift and the subsequent detachment of the top of the Intrarif lithostratigraphic pile, forming the Higher Nappes. We show that the chaotic structure of the Senhadja Nappes is only apparent, being mostly inherited from the Mesozoic rifting. Results of apatite fission track, apatite helium dating and thermal modeling from the gabbroic intrusions of the Senhadja Nappes confirm an overburden by the Higher Nappes and their subsequent exhumation during the Upper Miocene. Finally, all these elements are integrated in a coherent model incorporating the External Rif in the geodynamics of the West Mediterranean.