Active and passive tectonic controls for transverse drainage and river gorge development in a collisional mountain belt (Dades Gorges, High Atlas Mountains, Morocco)

Transverse drainages are discordant river patterns whose flow directions cut across geological structures such as faults, folds and the regional tectonic fabric (strike) of mountain belts. Such drainages can form spectacular geomorphological features, with reaches often occupying narrow and deeply dissected canyons/gorges that cut through prominent topographic barriers. Within collisional plate margins (e.g. foreland basin systems), studies have frequently examined transverse drainage development in response to active tectonics (e.g. thrusting). Here, transverse drainage will develop via incision or become deflected and fixed in response to oblique fault/fold growth structures, particularly within wedge-top, thrust front and foredeep basin settings. Within this paper the development of transverse drainage within the little studied fold-thrust component of an orogen is examined using a spectacular series of river gorges along the River Dades from the High Atlas Mountains of Morocco. Integrated field-derived geological and geomorphological data are used to explore the interplay between active tectonics, lithology and structural configuration (passive tectonics) of bedrock for the creation of transverse drainage and associated river gorge reaches. At the southern edge of the High Atlas fold-thrust belt, the River Dades passes through the deeply dissected Tarhia n' Dades and Main Dades Gorges before emerging into the Ait Seddratt wedge-top basin. Fluvial incision is principally controlled by Plio-Quaternary uplift of the High Atlas Mountains. The larger Main Dades Gorge has formed a deeply dissected antecedent course via drainage inheritance into structurally thickened Jurassic bedrock via uplift between two key structural components of the foreland basin system (the fold-thrust belt and the wedge-top basin). The smaller Tarhia n' Dades Gorge involves a more complex interplay between active tectonics and the passive structural arrangement of an asymmetric plunging syncline combined with the stratigraphic arrangement and strength of the Jurassic bedrock. The drainage network is configured to the fold and associated fault-joint structures to form respective NW-SE and NE-SW orientated dip slope and strike drainage. Progressive incision by tributary dip slope streams, cutting down through resistant limestone bedrock into underlying weaker mudstone bedrock, has resulted in accelerated headward erosion and drainage network expansion by river captures. This has resulted in a minor re-routing of the River Dades as a transverse drainage through a former dip slope tributary stream to form the Tarhia n' Dades Gorge. (C) 2008 Elsevier B.V. All rights reserved.