STRUCTURAL AND GEOCHRONOLOGICAL CONSTRAINTS ON THE EARLY VARISCAN EVOLUTION OF THE EASTERN MOROCCAN MESETA

The Moroccan Variscides crop out as a series of isolated deformed Paleozoic massifs in the Moroccan Meseta and the High Atlas in North Africa. To the South, the slightly folded Anti-Atlas region represents the transition to the Gondwana foreland (West African Craton). Our work focuses on the structure of the Debdou-Mekkam (DM) Massif in the Eastern Moroccan Meseta, where a pre-Late Visean thick flysch succession (DM metasedimentary rocks) is intensely folded. DM sandstones contain a population of Precambrian detrital zircons compatible with a North Gondwanan source, but the presence of a noticeable Late Devonian population calls for a juvenile and exotic source.

DM rocks underwent a first (D1) main folding event associated with a penetrative axial-planar foliation (S1). D1 folds can be restored to an original NE-SW trend with SE regional vergence. Later deformations refolded and crenulated the previous structure; these include variably oriented upright folds, which are the only ones recognized in the unconformably overlying Upper Visean rocks.

The age of D1 is constrained to the Tournaisian-Lower Visean, since the maximum depositional age of the DM rocks is Late Famennian and the Upper Visean rocks are not affected by D1. Our preferred tectonic model locates the area in the Devonian passive margin of North Gondwana, separated from the Laurussia continent by the closing Rheic Ocean. At Late Devonian time, an approaching Avalonian salient gave way to the sedimentation of the DM flysch onto the former subduction trench. Thus, the DM sediments sourced from a supra-subduction Avalonian margin, which included Precambrian basement and juvenile Late Devonian arc-derived detritus. The continued SE-directed pushing of the Avalonian spur onto the suture zone detached and deformed the DM rocks, forming the collisional D1 folded structure in the Gondwanan margin. Lateral correlatives of the suggested Avalonian salient are the Caledonian Sehoul Block in the Western Moroccan Meseta and the South Portuguese Zone in the SW Iberian Massif.