Cenozoic synsedimentary tectonics in the External Rif Zone (Maghrebian Chain, Northern Morocco)

Abstract

The Cenozoic tectono-sedimentary evolution of the External Rif Zone (ERZ) has been studied based on an integrated analysis of twenty-two representative stratigraphic successions grouped in seven sectors from N to S: Tangier, Asilah, Chaouen, Zoumi, Ouezzane, Ourtzarh and the Prerifian Ridges. The ERZ is divided classically, from N to S, into Intrarif, Mesorif and Prerif sub-domains. Each sub-domain is subdivided further in to internal and external. The Cenozoic stratigraphic record of the ERZ can be roughly separated into five main stratigraphic intervals bounded by five main unconformities corresponding to the Cretaceous-Paleogene, Eocene-Oligocene, Oligocene-Miocene, Burdigalian-Langhian and middle-late Mioceneboundaries. Each unconformity can be related to a local or regional tectonic events: (1) the Cretaceous-Paleogene boundary unconformity to the tectonic inversion (from extension to compression) occurring in the alpine Tethys domain in the upper Cretaceous ; (2) the Eocene-Oligocene boundary to a flexure phase in the Atlas front; (3) the unconformity that marks the Oligocene-Miocene boundary can result from the starting of the nappes stacking phase in the Internal Zone; (4) the Burdigalian-Langhian boundary unconformity to the end of structuring of the Internal Zone; and (5) the middle-late Miocene boundary unconformity to the nappes stacking phase in the ERZ. The Paleogene evolution can mainly be correlated with the so-called Eo-alpine orogenic phase, while the Miocene one is related to the Mio-Alpine, both recognized in the western Mediterranean area. As a fundamental part of this research, the analysis of synsedimentary tectonics have been performed, considering tectofacies, unconformity implications and subsidence analysis. Tectofacies (such as, turbidites, slumps, mass flow deposits, synsedimentary folds and faults) are checked from the upper Ypresian succession onward, but more frequently during the Oligocene and Miocene, which point out an upward increase in the tectonic activity. Considering the ERZ as a foreland basin, the Eocene foredeep area would correspond to the Internal Mesorif and Internal Prerif sub-domains. This foredeep was represented by a complex of two “sub-geosynclines” separated by a relative bulge located in the External Mesorif. In this way, the Intrarif could represent the relative orogenic front (advanging on the Internal Rif Zone). The Eocene forebulge was located in the External Prerif, while the Gharb Basin was the backbulge of the system. During the Oligocene the depocentral area migrated southward favoring a homogenization of subsidence in the whole ERZ. In this new configuration, the foredeep would be located in the External Mesorif (formerly a relative bulge) while the External Prerif and the Gharb Basin continued to act as the forebulge and the backbulge of the system, respectively. During the early Miocene a new diversification of depocenter took place with the main foredeep in the Internal Mesorif and secondary foredeeps areas in the externalmost and internalmost Intrarif. In this period, the forebulge should be located in the middle Intrarif. Finally, during middle Miocene foredeep were located in the externalmost Intrarif and Internal Prerif while in the late Miocene depocenter migrates southward to the Extenal Prerif-Gharb areas (formerly forebulge and backbulge areas).