Drainage rearrangement and landscape evolution: Insights from the Moroccan Massif Central

Abstract

The Moroccan Massif Central, which is the largest massif of Paleozoic bedrock of the Western Meseta, is located in the foreland of the Alpine Rif orogenic belt. The massif underwent little Cenozoic deformation and represents a peripheral plateau between the Atlantic Ocean and the Cenozoic Middle Atlas chain.On the basis of field observations combined with topographic and fluvial analysis, we show that the upstream part of the Oum Rbia River, which originates from the Middle Atlas Mountains, was once flowing to the northwest in continuity with the Bou Regreg River, which drains the waters of the Moroccan Massif Central to the Atlantic Ocean. A Late Pliocene diversion occurred at the transition between the Middle Atlas and the Massif Central leading the Oum Rbia River to change its course to the southwest into the Tadla Basin and leaving the Bou Regreg River isolated and starved of upstream drainage area. The diversion is documented by a fluvial terrace preserved in a paleostream near the drainage divide between the Oum Rbia and the Bou Regreg catchments, as well as by river profiles and drainage divide instability.Owing to a loss in drainage area, the beheaded Bou Regreg River system could no longer equilibrate the sustained dynamic uplift, which is indicated by lithospheric thinning and Quaternary alkaline volcanism, while the neighboring catchments keep incising. Ultimately, this would have led to a steady decrease in surface relief of the Moroccan Massif Central with the formation of low-relief surfaces that characterize the area as expected by an area-loss feedback model. The bedrock uplift in the Moroccan Massif Central could be related to lithospheric folding associated with the Rif belt.