Drainage network as an indicator of tectonic evolution of mountain belts: insight from the Middle Atlas (Morocco).

In actively deforming regions, the geometry and evolution of fluvial systems are sensitive to surface uplift, style of deformation and erosion processes. The uplift influences drainages via base level changes, drainage reversals, and capture processes. Although drainage development and reorganization might be complex in some cases, it can be used to unravel the tectonic evolution of a region.

The Middle Atlas is an intracontinental fold-and-thrust belt that results from the tectonic inversion of a Triassic to Jurassic continental rift basin. The compressional regime leading to basin inversion has produced limited crustal shortening and thickening in association with the growth of mountain ridges with a wavelength of few km.  These topographic features have been superimposed by a long-wavelength, mantle-driven surface uplift, occurred since the late Cenozoic.

Here, we carry out a topographic and fluvial analysis to investigate at which extent the geomorphic features, mainly the drainage network, reflect the tectonic evolution of the Middle Atlas. Two main drainage divides can be defined in the Middle Atlas: 1) a longitudinal divide that separates an eastern flank draining into the Mediterranean Sea through the Moulouya river from a western flank draining into the Atlantic Ocean through the Sebou and Oum Rbia rivers; 2) a transverse divide that sets apart the catchments of the Sebou and Oum Rbia  rivers. In the eastern flank, where the slopes are steep, the tributaries of the Moulouya river show a parallel pattern and are transversal to the trend of the orogen, whereas in the western flank the rivers are longitudinal and controlled by the tectonic structures. Our results indicate that the topography and drainage are in a disequilibrium condition and in an early stage of evolution. The discrepancy in the rivers network between the two flanks, suggests an asymmetric tectonic uplift history. Specifically the eastern flank of the orogen appears to have accommodated a higher magnitude of late Cenozoic contractional deformation than the western flank