Tectonic Evolution of the Eastern Moroccan Meseta: From Late Devonian Forearc Sedimentation to Early Carboniferous Collision of an Avalonian Promontory

Abstract

The deformed Paleozoic succession of the Eastern Moroccan Meseta crops out in relatively small and isolated inliers surrounded by Mesozoic and Cenozoic rocks. Two of the largest inliers (Mekkam and Debdou) are characterized by a monotonous succession of slates and greywackes affected by polyphasic folding that occurred at low‐ to very low grade metamorphic conditions. New U‐Pb ages on detrital zircon grains from the Debdou‐Mekkam metasediments constrain the maximal depositional age as Late Devonian, interpreted to be close to the true sedimentation age. Furthermore, the εHf values of the Devonian detrital zircons, together with the presence of a series of scattered zircon grains with ages between c. 0.9 and c. 1.9 Ga, suggest provenance from a subduction‐related magmatic arc located on the Avalonian margin. The Debdou‐Mekkam massif is characterized by an Early Carboniferous first deformational event (D1), which gave way to a pervasive cleavage (S1) associated with plurikilometric‐scale, tight to isoclinal, overturned to recumbent folds. Later events (Dc) occurred at Late Carboniferous time and generated variably developed crenulation cleavages (Sc) associated with variously oriented metric‐ to kilometric‐scale folds, which complicate the pattern of both D1 intersection lineations (L1) and axial traces. The restoration of this pronounced curved pattern yields originally SW‐NE‐oriented D1 fold axes with regional SE‐vergence. This important Early Carboniferous shortening and SE‐directed tectonic transport can be explained by closure of the Rheic Ocean and the first phases of the collision between the northern passive margin of Gondwana and an Avalonian promontory.

Syn-collisional detrital zircon source evolution in the northern Moroccan Variscides

“…In terms of detrital zircon populations, the magmatic input of Cambrian-Ordovician Rheic Ocean rifting (Linnemann et al, 2008;Nance et al, 2012Nance et al, , 2010 in the Moroccan Variscides was scarce or negligible (Accotto et al, 2019(Accotto et al, , 2020Ghienne et al, 2018;Letsch et al, 2018), except in a few, very localized areas (Letsch et al, 2018). In our samples, Cambrian-Ordovician zircon grains are rare (0-8% of the data).…”

“…1.0 Ga) detrital zircon population is identified in the Moroccan Variscides (up to 30% of the data) although it is not present everywhere. This population was found in Ediacaran-Late Devonian samples from the Anti-Atlas (Avigad et al, 2012) and the Moroccan Mesetas (Accotto et al, 2019(Accotto et al, , 2020Ghienne et al, 2018). To explain the occurrence of these ages, the source of which is unknown in the WAC (Figure 8), a distal and probably intermittent NE African source (namely the Sahara Metacraton; Figure 8) was invoked.…”

“…Unlike the WMM, where the Devonian period was characterized by a carbonatic platform that was progressively fragmented, the EMM (Figure 1D) experienced more or less continuous sedimentation (e.g. Debdou and Mekkam inliers) with a Devonian-Tournaisian thick siliciclastic turbiditic succession of shales, greywackes, and sandstones (Figure 2; Hoepffner, 1989), intensely deformed by a Tournaisian-Early Visean tectonic event induced by the collision between the northern margin of Gondwana and an Avalonian promontory (inferred as the eastern prolongation of the Sehoul Block; Accotto et al, 2020).…”

“…Geochronological studies on detrital zircon grains from Late Ediacaran-Early Carboniferous samples of the Moroccan Variscides have been carried out by several authors (Abati et al, 2010(Abati et al, , 2012Accotto et al, 2019Accotto et al, , 2020Avigad et al, 2012;Ghienne et al, 2018;Letsch et al, 2018;Pérez-Cáceres et al, 2017) with the aim of finding out the main sources of sediments that fed the northern Gondwanan margin during the Paleozoic. All of these authors agreed that the main source area of sediments in the Moroccan Variscides is the West African Craton (WAC; Figure 8), based on: (i) the recurrent presence of important Cryogenian-Ediacaran (ca.…”

“…In the Moroccan transects of the Variscan orogen, detrital zircon investigations have been focused on the pre-orogenic evolution (e.g. Abati et al, 2010;Accotto et al, 2019Accotto et al, , 2020Avigad et al, 2012;Ghienne et al, 2018;Letsch et al, 2018;Pérez-Cáceres et al, 2017), but to date, no data have been reported on the syn-orogenic sediments, with the exception of a recent work on the Eastern Moroccan Meseta (Accotto et al, 2020). The present work provides a wide-scale study of U-Pb geochronological and Hf isotope detrital zircon analyses carried out on syn-collisional J o u r n a l P r e -p r o o f Journal Pre-proof Variscides (after Hoepffner et al, 2006;Michard et al, 2010b).…”